MIL-HDBK-419,
MIL-HDBK-419A
29 DECEMBER 1987
SUPERSEDING
MIL-HDBK419
21
JANUARY
1982
MILITARY HANDBOOK
GROUNDING, BONDING, AND SHIELDING
FOR
ELECTRONIC EQUIPMENTS AND FACILITIES
VOLUME 1 OF 2 VOLUMES
BASIC THEORY
AMSC N/A
EMCS/SLHC/TCTS
DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited
DEPARTMENT OF DEFENSE
WASHINGTON DC 20301
MIL-HDBK-419A
GROUNDING, BONDING, AND SHIELDING FOR ELECTRONIC EQUIPMENTS AND FACILITIES
1.
established
2.
This standardization handbook was developed by the Department of Defense in accordance with
procedure.
This publication was approved on 29 December 1987 for printing and inclusion in the military
standardization handbook series.
Vertical lines and asterisks are not used in this revision to identify changes
with respect to the previous issue due to the extensiveness of the changes.
3.
This document provides basic and application information on grounding, bonding, and shielding
practices recoin mended for electronic equipment.
It will provide valuable information and guidance to
personnel concerned with the preparation of specifications and the procurement of electrical and electronic
equipment for the Defense Communications System. The handbook is not intended to be referenced in purchase
specifications except for informational purposes, nor shall it supersede any specification requirements.
4.
Every effort has been made to reflect the latest information on the interrelation of considerations
of electrochemistry , metallurgy, electromagnetic, and atmospheric physics.
It is the intent to review this
handbook periodically to insure its completeness and currency. Users of this document are encouraged to report
any errors discovered and any recommendations for changes or inclusions to: Commander, 1842 EEG/EEITE,
Scott AFB IL 62225-6348.
5.
Copies of Federal and Military Standards, Specifications and associated documents (including this
handbook) listed in the Department of Defense Index of Specifications and Standards (DODISS) should be
obtained from the DOD Single Stock Point:
Tabor Avenue, Philadelphia PA 19120.
Commanding Officer, Naval Publications and Forms Center, 5801
Single copies may be obtained on an emergency basis by calling
(AUTOVON) 442-3321 or Area Code (215)-697-3321.
obtained from the sponsor.
Copies of industry association documents should be
Copies of all other listed documents should be obtained from the contracting
activity or as directed by the contracting officer.
MIL-HDBK-419A
PREFACE
This volume is one of a two-volume series which sets forth the grounding, bonding, and shielding theory for
communications
electronics
(C-E)
equipments
and
facilities.
Grounding, bonding, and shielding are complex
subjects about which in the past there has existed a good deal of misunderstanding. The subjects themselves are
interrelated and involve considerations of a wide range of topics from electrochemistry and metallurgy to
electromagnetic field theory and atlmspheric physics.
These two volumes reduce these varied considerations
into a usable set of principles and practices which can be used by all concerned with, and responsible for, the
safety and effective operation of complex C-E systems. Where possible, the Principles are reduced to specific
steps. Because of the large number of interrelated factors, specific steps cannot be set forth for every possible
situation.
However, once the requirements and constraints of a given situation are defined, the appropriate
steps for solution of the problem can be formulated utilizing the principles set forth.
Both volumes (Volume I, Basic Theory and Volume II, Applications) implement the (Grounding, Bonding, and
Shielding requirements of MIL-STD-188-124A which is mandatory for use within the Department of Defense.
The purpose of this standard is to ensure the optimum performance of ground-based telecommunications
equipment by reducing noise and providing adequate protection against power system faults and lightning
strikes.
This handbook emphasizes the necessity for including considerations of grounding, bonding, and shielding in all
phases of design, construction, operation, and maintenance of electronic equipment and facilities. Volume 1,
Basic Theory, develops the principles of personnel protection, fault protection, lightning protection,
interference reduction, and EMP protection for C-E facilities.
In addition, the basic theories of earth
connections, signal grounding, electromagnetic shielding, and electrical bonding are presented. The subjects are
not covered independently, rather they are considered from the standpoint of how they influence the design of
the earth electrode Subsystem of a facility, the selection of ground reference networks for equipments and
structures, shielding requirements, facility and equipment bonding practices, etc. Volume I also provides the
basic background of theory and principles that explain the technical basis for the recommended practices and
procedures; illustrates the necessity for care and thoroughness in implementation of grounding, bonding, and
shielding; and provides supplemental information to assist in the solution of those problems and situations not
specifically addressed.
In Volume II, Applications, the principles and theories, including RED/BLACK protection, are reduced to the
practical steps and procedures which are to be followed in structural and facility development, electronic
engineering, and in equipment development,
These applications should assure personnel equipment and
structural safety , minimize electromagnetic interference (EMI) problems in the final operating system; and
minimize susceptibility to and generation of undesirable emanations. The emphasis in Volume II goes beyond
development to assembly and construction, to installation and checkout, and to maintenance for long term use.
Four appendices are provided as common elements in both volumes. Appendix A is a glossary of selected words
and terms as they are used herein, If not defined in the glossary, usage is in accordance with Federal Standard
1037, Glossary of
Telecommunication
Terms.
Appendix B is a supplemental bibliography containing selected
references intended to supply the user with additional material. Appendix C - contains the table of contents for
the other volume. Appendix D contains the index for the two-volume set.
MIL-HDBK-419A
TABLE OF CONTENTS
CHAPTER 1- FACILITY GROUND SYSTEM
Paragraph
Page
1.1
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1
1.2
APPLICATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1
1.3
DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1
1.4
REFERENCED DOCUMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1
1.5
DESCRIPTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
1.5.1
Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
1.5.2
Grounding and Power Distribution Systems . . . . . . . . . . . . . . . . . . . . .
1-3
1.5.3
Electrical Noise in Communications Systems . . . . . . . . . . . . . . . . . . . .
1-4
1.6
BONDING, SHIELDING, AND GROUNDING RELATIONSHIP . . . . . . . . . . .
1-5
1.7
GROUNDING SAFETY PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . .
1-5
CHAPTER 2- EARTHING AND EARTH ELECTRODE SUBSYSTEM
2.1
OBJECTIVES
.
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2-1
Discharge
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2.1.1
Lightning
.
2-1
2.1.2
Fault Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-2
2.1.3
Noise
2.1.4
Summary
.
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2-2
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.
2-2
RESISTANCE REQUIREMENTS.. . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
2.2.1
General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
2.2.2
Resistance to Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
2.2.2.1
National Electrical Code Requirements . . . . . . . . . . . . . . . . . . . . .
2-5
2.2.2.2
Department of Defense Communications Electronics Requirements . . . . . . . . .
2-5
Lightning Require meats . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
SOIL RESISTIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-7
2.3.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-7
2.3.2
Typical Resistivity Ranges . . . . . . . . . . . . . . . . . . . . . . . .
2-7
2.3.3
Environmental Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-7
2.2
2.2.3
2.3
2.4
Reduction
of
.
.
.
Requirements
.
.
.
.
.
.
.
.
.
MEASUREMENT OF SOIL RESISTIVITY . . . . . . . . . . . . . . . . . . . . . . .
2-8
2.4.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-8
2.4.2
Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-8
2.4.2.1
One-Electrode Method . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-8
2.4.2.2
Four-Terminal Method . . . . . . . . . . . . . . . . . . . . . . . .
TYPES OF EARTH ELECTRODE SUBSYSTEMS . . . . . . . . . . . . . . . . . . . .
2-13
2-15
2.5.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-15
2.5.2
Ground Rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-15
2.5.3
Buried Horizontal Conductors. . . . . . . . . . . . . . . . . . . . . . . . . . .
2-15
2.5
i
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-15
Metal Frameworks of Buildings . . . . . . . . . . . . . . . . . . . . . . . . . .
Water Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-16
Incidental Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Well Casings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-16
RESISTANCE PROPERTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Simple Isolated Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-17
Driven Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other Commonly Used Electrodes . . . . . . . . . . . . . . . . . . . . . . . .
2-17
2-23
2.6.2.1
Resistance of Multiple Electrodes . . . . . . . . . . . . . . . . . . . . . . . . .
Two Vertical Rods in Parallel . . . . . . . . . . . . . . . . . . . . . . . , . .
2-23
2.6.2.2
Square Array of Vertical Rods . . . . . . . . . . . . . . . . . . . . . . . . .
2-27
2.6.2.3
Horizontal Grid (Mesh). . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Rods Connected by a Grid . . . . . . . . . . . . . . . . . . . . . . .
2-29
2.5.4
2.5.5
2.5.6
2.5.7
2.5.8
2.5.9
2.6
2.6.1
2.6.1.1
2.6.1.2
2.6.2
2.6.2.4
2-15
2-16
2-16
2-17
2-23
2-30
2.6.3
Transient Impedance of Electrodes .
. . . . . . . . . . . . . . . . . . . . . . .
2-32
2.6.4
2-32
2.6.4.2
Effects of Nonhomogeneous (Layered) Earth . . . . . . . . . . . . . . . . . . . .
Hemispherical Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-33
2.6.4.3
Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-33
MEASUREMENT OF RESISTANCE-TO-EARTH OF ELECTRODES. . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-35
2.6.4.1
2.7
2.7.1
2.7.2.1
Fall-of-Potential Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Probe Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2.2
Extensive Electrode Subsystems. . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2.3
Test Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three-Point (Triangulation) Method . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2
2.7.3
2.8
2.8.1
2.8.1.1
2.8.1.2
2.8.1.2.1
2.8.1.2.2
2.8.1.2.3
2.8.1.3
2.8.2
2.8.2.1
2.8.2.2
2.8.2.3
OTHER CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Voltages Above Earth Electrodes . . . . . . . . . . . . . . . . . . . . .
2-32
2-35
2-35
2-36
2-42
2-45
2-46
2-47
2-47
Step Voltage Safety Limit . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step Voltages for Practical Electrodes . . . . . . . . . . . . . . . . . . . . . .
2-47
Flush Vertical Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Buried Vertical Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Buried Horizontal Grid. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-49
Minimizing Step Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-56
Heating of Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Steady State Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-57
Transient Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minimum Electrode Size . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-57
ii
2-49
2-53
2-55
2-57
2-59
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
2.9
Page
ELECTRODE ENHANCEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-59
2.9.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-59
2.9.2
Water Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-60
2.9.3
Chemical Salting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-60
2.9.4
Electrode Encasement . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-62
2.9.5
Salting Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-63
CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-63
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-63
2.10.2
Protection Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-65
2.10.3
Sacrifical Anodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-65
2.10
2.10.1
2.10.4
Corrosive Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-66
GROUNDING IN ARCTIC REGIONS . . . . . . . . . . . . . . . . . . . . . . . . .
2-66
2.11.1
Soil Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-66
2.11.2
Improving Electrical Grounding in Frozen Soils . . . . . . . . . . . . . . . . . . .
Electrode Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-70
2-7 1
2.11.2.2.1
Installation and Measurement Methods . . . . . . . . . . . . . . . . . . . . . .
Electrode Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.11.2.2.2
Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-71
2-75
2.11
2.11.2.1
2.11.2.2
2.12
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-71
2-71
CHAPTER 3- LIGHTNING PROTECTION SUBSYSTEM
3.1
THE PHENOMENON OF LIGHTNING . . . . . . . . . . . . . . . . . . . . . . . .
3-1
3.2
DEVELOPMENT OF A LIGHTNING FLASH . . . . . . . . . . . . . . . . . . . . . .
3-3
3.3
INFLUENCE OF STRUCTURE HEIGHT . . . . . . . . . . . . . . . . . . . . . . .
3-3
3.4
STRIKE LIKELIHOOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-4
3.5
ATTRACTIVE AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-10
3.5.1
Structures Less Than 100 Meters High. . . . . . . . . . . . . . . . . . . . . . .
3-10
3.5.2
Cone of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-11
LIGHTNING EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-13
3.6
3.6.1
Flash Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-13
3.6.2
Mechanical and Thermal Effects . . . . . . . . . . . . . . . . . . . . . . . . .
3-15
3.6.3
Electrical Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-17
3.6.3.1
Conductor Impedance Effects. . . . . . . . . . . . . . . . . . . . . . . . . .
3-17
3.6.3.2
Induced Voltage Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-18
3.6.3.3
Capacitively-Coupled Voltage . . . . . . . . . . . . . . . . . . . . . . . . .
3-21
3.6.3.4
Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-21
3.7
BASIC PROTECTION REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . .
3-25
3.8
DETERMINING THE NEED FOR PROTECTION . . . . . . . . . . . . . . . . . . . .
3-26
Strike Likelihood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-26
3.8.1
iii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
3.8.2
Type of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-26
Criticalness to System Mission . . . . . . . . . . . . . . . . . . . . . . . . . .
3-27
3.9
APPLICABLE CODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-27
3.10
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-28
3.8.3
CHAPTER 4 - FAULT PROTECTION SUBSYSTEM
FAULT PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
4.1.1
Power System Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
4.1.2
4.2
Ground-Fault-Circuit-Interrupter
(GFCl)
. . . . . . . . . . . . . . . . . . . . . .
EARTH CONNECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3
4.3
AC POWER LINE GROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3
4.4
TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-5
4.5
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-6
4.1
.
.
4-3
CHAPTER 5- GROUNDING OF SIGNAL REFERENCE SUBSYSTEM
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONDUCTOR CONSIDERATIONS. . . . . . . . . . . . . . . . . . . . . . . . . .
5-1
5.2.1
Direct Current Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-1
5.2.2
Alternating Current Impedance .
. . . . . . . . . . . . . . . . . . . . . . . . .
5-1
5.2.2.1
Skin Effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-3
5.2.2.2
AC Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Proximity Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-5
5.1
5.2
5.2.2.3
5.2.2.4
5.2.3
5.2.4
Resistance Properties vs Impedance Properties . . . . . . . . . . . . . . . . . . .
Effects of Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-1
5-7
5-10
5-10
5-12
5-13
5.2.4.2
Stranded Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rectangular Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.3
Tubular Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-13
5.2.4.4
Structural Steel Members . . . . . . . . . . . . . . . . . . . . . . . . . . .
SIGNAL REFERENCE SUBSYSTEM NETWORK CONFIGURATIONS . . . . . . . . . . .
5-15
5-15
5.3.2
Floating Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Single-Point Ground (for Lower Frequencies) . . . . . . . . . . . . . . . . . . . .
5.3.3
Multipoint Ground (for Higher Frequencies) . . . . . . . . . . . . . . . . . . . . .
5-24
. . . . . . . . .
5-26
Types of Equipotential Planes. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . .
Floating System . . . . . . . . . . . . . . . . . . . . . .
5-27
5.2.4.1
5.3
5.3.1
5.3.3.1
5.3.3.2
5.3.4
Equipotential Plane . . . . . . . . . . . . . . . . . . . .
iv
5-13
5-15
5-19
5-28
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
SITE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-28
5.4.1
Lower Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-29
5.4.2
Higher Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-30
Frequency Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-31
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-32
5.4
5.4.3
5.5
CHAPTER 6- INTERFERENCE COUPLING AND REDUCTION
6.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-1
6.2
COUPLING MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-5
6.2.1
Conductive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-5
6.2.2
Free-Space Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-6
6.2.2.1
Near-Field Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-6
6.2.2.2
Inductive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-8
6.2.2.3
Capacitive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-11
6.2.2.4
Far-Field Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-14
COMMON-MODE NOISE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-17
Basic Theory of Common-Mode Coupling . . . . . . . . . . . . . . . . . . . . . .
6-19
6.3
6.3.1
6.3.2
6.4
6.4.1
Differential Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-23
MINIMIZATION TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-23
Reduction of Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-23
6.4.1.1
Reference Plane Impedance Minimization
. . . . . . . . . . . . . . . . . . . .
6-23
6.4.1.2
Spatial Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-24
6.4.1.3
Reduction of Circuit Loop Area . . . . .
. . . . . . . . . . . . . . . . . . . .
6-24
6.4.1.4
Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-24
6.4.1.5
Balanced Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-24
Alternate Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-24
6.5
FACILITY AND EQUIPMENT REQUIREMENTS . . . . . . . . . . . . . . . . . . . .
6-25
6.6
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-25
6.4.2
CHAPTER 7 - BONDING
7.1
DEFINITION OF BONDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-1
7.2
PURPOSES OF BONDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-1
7.3
RESISTANCE CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-3
7.4
DIRECT BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-4
Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-6
7.4.1
7.4.1.1
Surface Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-7
7.4.1.2
Surface Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-7
7.4.1.3
Contact Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-7
v
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
7-8
Bond Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Direct Bonding Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-10
7.4.2.1
Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-10
7.4.2.2
Brazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soft Solder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-14
7.4.1.4
7.4.2
7.4.2.3
7.4.2.4
7.4.2.5
7.4.2.6
Rivets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conductive Adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-11
7-14
7-15
7-16
Comparison of Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . .
INDIRECT BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-16
7.5.1
Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-19
7.5.2
Frequency Effects .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-19
Skin Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bond Reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-19
7-23
7.6.1
Stray Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SURFACE PREPARATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6.2
Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-26
7.6.3
Platings and Inorganic Finishes . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion By-Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPLETION OF THE BOND . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-29
BOND CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical Basis of Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrochemical Series . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galvanic Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-30
7-31
7.8.2
Relative Area of Anodic Member . . . . . . . . . . . . . . . . . . . . . . . . .
7-34
7.8.3
Protective Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
WORKMANSHIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-34
SUMMARY OF GUIDELINES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-36
7.4.2.7
7.5
7.5.2.1
7.5.2.2
7.5.2.3
7.6
7.6.4
7.7
7.8
7.8.1
7.8.1.1
7.8.1.2
7.9
7.10
7.11
7-16
7-19
7-25
7-26
7-29
7-29
7-30
7-31
7-34
7-37
CHAPTER 8- SHIELDING
8.1
FUNCTION OF AN ELECTROMAGNETIC SHIELD. . . . . . . . . . . . . . . . . . .
8-1
8.2
BASIC SHIELDING THEORY . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-2
8.2.1
Oppositely Induced Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-2
8.2.2
Transmission Line Analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-2
8.2.3
Nonuniform Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-4
vi
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
SHIELDING EFFECTIVENESS OF CONTINUOUS SINGLE-THICKNESS SHIELDS . . . . . .
8-4
8.3.1
Absorption Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-5
8.3.2
8.3
Reflection Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-6
8.3.2.1
Low Impedance Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-10
8.3.2.2
Plane Wave Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-13
8.3.2.3
High Impedance Field . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-15
8.3.3
Re-Reflection Correction Factor . . . . . . . . . . . . . . . . . . . . . . . . .
8-19
8.3.4
Total Shielding Effectiveness. . . . . . . . . . . . . . . . . . . . . . . . . . .
8-19
Measured Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-27
8.3.4.1
8.3.4.2
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-27
SHIELDING EFFECTIVENESS OF OTHER SHIELDS . . . . . . . . . . . . . . . . . .
8-31
8.4.1
Multiple Solid Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-31
8.4.2
Coatings and Thin-Film Shields. . . . . . . . . . . . . . . . . . . . . . . . . .
8-32
8.4
8.4.3
Screens and Perforated Metal Shields . . . . . . . . . . . . . . . . . . . . . . .
8-33
SHIELD DISCONTINUITY EFFECTS (APERTURES) . . . . . . . . . . . . . . . . . .
8-41
8.5.1
Seams Without Gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-42
8.5.2
Seams With Gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-45
8.5.3
Penetration Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-45
8.5.3.1
Waveguide-Below-Cutoff . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-50
8.5.3.2
Screen and Conducting Glass . . . . . . . . . . . . . . . . . . . . . . . . . .
8-52
8.6
SELECTION OF SHIELDING MATERIALS . . . . . . . . . . . . . . . . . . . . . .
8-53
8.7
USE OF CONVENTIONAL BUILDING MATERIALS . . . . . . . . . . . . . . . . . .
8-56
8.7.1
Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-56
8.7.2
Reinforcing Steel (Rebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-56
CABLE AN D CONNECTOR SHIELDING . . . . . . . . . . . . . . . . . . . . . . .
8-59
8.8.1
Cable Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-59
8.8.2
Terminations and Connectors. . . . . . . . . . . . . . . . . . . . . . . . .
8-63
8.5
.
Page
8.8
8.9
SHIELDED ENCLOSURES (SCREEN ROOMS) . . . . . . . . . . . . . . . . . . . . .
8-63
8.9.1
Remountable (Modular) Enclosures . . . . . . . . . . . . . . . . . . . . . . . .
8-66
8.9.2
Custom Built Rooms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-70
8.9.3
Foil Room Liners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-71
TESTING OF SHIELDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-72
Low Impedance Magnetic Field Testing Using Small Loops . . . . . . . . . . . . . .
8-73
8.10
8.10.1
8.10.2
Additional Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-74
8.11
PERSONNEL PROTECTION SHIELDS . . . . . . . . . . . . . . . . . . . . . . . .
8-74
8.12
DETERMINATION OF SHIELDING REQUIREMENTS . . . . . . . . . . . . . . . . . .
8-74
8.12.1
Equipment Disturbances . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-76
8.12.2
Electromagnetic Environmental Survey . . . . . . . . . . . . . . . . . . . . . .
8-76
8.12.3
Equipment EMI Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-77
vii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
8.13
8.13.1
SYSTEM DESIGN CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . .
Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
8-77
8-78
. . . .
8-78
. . . .
8-78
. . . .
8-78
. . . .
8-79
ELECTRIC SHOCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Levels of Electric Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9-1
Shock Prevention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
STATIC ELECTRICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9-3
9-5
9-5
9.5
RADIO FREQUENCY (RF) RADIATION HAZARDS . . . . . . . . . . . . . . . . . .
LASER HAZARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-RAY RADIATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.6
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9-6
8.13.2
Layout . . . . . . . . . . . . . . . . . . .
8.13.3
Signal Properties. . . . . . . . . . . . . . .
8.13.4
8.14
Cost . . . . . . . . . . . . . . . . . . . .
REFERENCES. . . . . . . . . . . . . . . . .
CHAPTER 9-PERSONNEL PROTECTION
9.1
9.1.1
9.1.2
9.2
9.3
9.4
9-1
9-3
9-6
CHAPTER 10- NUCLEAR EMP EFFECTS
10.1
10.2
10.2.1
10.2.1.1
10.2.1.2
10.2.1.3
10.2.2
10.2.3
10.2.4
10.3
10.3.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EMP GENERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-1
High-Altitude EMP (HEMP). . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-1
Early-Time HEMP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Late-Time HEMP (MHDEMP) . . . . . . . . . . . . . . . . . . . . . . . . . .
10-1
Intermediate-Time HEMP . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface-Burst EMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0ther EMP Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-3
Comparison With Lightning. . . . . . . . . . . . . . . . . . . . . . . . . . . .
HEMP INTERACTION WITH SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . .
10-5
Current in Long Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-6
10-1
10-3
10-3
10-4
10-5
10.3.1.1
Long Overhead Lines . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
10-6
10.3.1.2
Long Buried Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-7
HEMP Interaction With Local Structure . . . . . . . . . . . . . . . . . . . . . .
10-9
10-9
10.3.2.2
Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Penetrating Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-10
10.3.2.3
Apertures . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
10-11
10.3.1.3
10.3.2
10.3.2.1
viii
.
. .
10-9
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
10.4
Page
PROTECTION AGAINST HEMP . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-13
HEMP Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-13
10.4.1.1
Shield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-13
10.4.1.2
Penetrating Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-13
10.4.1
Apertures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-15
Allocation of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-15
10.4.2.1
Amount of Protection Needed . . . . . . . . . . . . . . . . . . . . . . . . .
10-15
10.4.2.2
Where Protection is Applied . . . . . . . . . . . . . . . . . . . . . . . . . .
10-17
10.4.2.3
Terminal Protection Devices . . . . . . . . . . . . . . . . . . . . . . . . . .
10-17
10.4.1.3
10.4.2
10.4.2.3.1
Spark Gaps and Gas Tubes . . . . . . . . . . . . . . . . . . . . . . . . . .
10-17
10.4.2.3.2
Metal-Oxide Varistors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-18
10.4.2.3.3
Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-18
10.4.2.3.4
Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-18
Waveguide Penetration of Facility Shield . . . . . . . . . . . . . . . . . . . . .
10-19
10.4.2.4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-19
10.4.2.4.2
In-Line Waveguide Attachment . . . . . . . . . . . . . . . . . . . . . . . .
10-21
Sleeve and Bellows Attachment . . . . . . . . . . . . . . . . . . . . . .
10-21
10.4.2.4.2.2
Braided Wire Sleeve . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-23
10.4.2.4.2.3
Stuffing Tube for Waveguide . . . . . . . . . . . . . . . . . . . . . . .
10-24
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-25
10.4.2.4
10.4.2.4.2.1
10.5
CHAPTER 11-NOTES
11.1
SUBJECT TERM (KEY WORD) LISTING . . . . . . . . . . . . . . . . . . . . . . .
11-1
APPENDICES
A
GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A-1
B
SUPPLEMENTAL BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . .
B-1
BI
SUBJECT CROSS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . .
B-1
BII
LISTINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B-2
C
TABLE OF CONTENTS FOR VOLUME II . . . . . . . . . . . . . . . . . . . . . . .
C-1
D
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D-1
ix
MIL-HDBK-419A
LIST OF FIGURES
Page
Figure
2-1
Voltage Differentials Arising from Unequal Earth Electrode Resistances and Unequal
Stray Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-3
2-2
Voltage Differentials Between Structures Resulting from Stray Ground Currents. . . . . .
2-4
2-3
Typical Variations in Soil Resistivity as a Function of Moisture, Temperature, and
Salt Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-9
2-4
Current Flow From a Hemisphere in Uniform Earth . . . . . . . . . . . . . . . . . .
2-11
2-5
Idealized Method for Determining Soil Resistivity . . . . . . . . . . . . . . . . . . .
2-14
2-6
Effect of Rod Length Upon Resistance . . . . . . . . . . . . . . . . . . . . . . . .
2-18
2-7
Effect of Rod Diameter Upon Resistance. . . . . . . . . . . . . . . . . . . . . . .
2-18
2-8
Earth Resistance to Shell Surrounding a Vertical Earth Electrode . . . . . . . . . . . .
2-20
2-9
Resistance of Buried Horizontal Conductors . . . . . . . . . . . . . . . . . . . . .
2-24
2-10
Resistance of Buried Circular Plates. . . . . . . . . . . . . . . . . . . . . . . . .
2-25
2-11
Ground Rods in Parallel.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-26
2-12
Ratio of the Actual Resistance of a Rod Array to the Ideal Resistance of N Rods
in Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-13
2-28
Transient Impedance of an Earth Electrode Subsystem as a Function of the Number
of Radial Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-31
2-14
Current Distribution in Nonuniform Soil . . . . . . . . . . . . . . . . . . . . . . .
2-34
2-15
Fall-of-Potential Method for Measuring the Resistance of Earth Electrodes . . . . . . . .
2-37
2-16
Effect of Electrode Spacing on Voltage Measurement . . . . . . . . . . . . . . . . .
2-38
2-17
Resistance Variations as Function of Potential Probe Position in Fall-of-Potential
Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-41
Earth Resistance Curves for a Large Electrode Subsystem . . . . . . . . . . . . . . .
2-19
Earth Resistance Curve Applicable to Large Earth Electrode Subsystems . . . . . . . . .
2-44
2-45
2-20
Intersection Curves for Figure 2-18 . . . . . . . . . . . . . . . . . . . . . . . . .
2-47
2-21
Triangulation Method of Treasuring the Resistance of an Earth Electrode . . . . . . . . .
2-48
2-22
Variation of Surface Potential Produced by a Current Flowing Into an Isolated
2-18
Ground Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-52
2-23
Surface Potential Variation Along a Grid . . . . . . . . . . . . . . . . . . . . . . .
2-54
2-24
Effect of Chemical Treatment on Resistance of Ground Rods . . . . . . . . . . . . . .
2-61
2-25
2-26
Seasonal Resistance Variations of Treated and Untreated Ground Rods . . . . . . . . . .
2-61
Trench Method of Soil Treatment . . . . . . . . . . . . . . . . . . . . . . . . . .
2-64
2-27
Alternate Method of Chemical Treatment of Ground Rod . . . . . . . . . . . . . . . .
2-64
2-28
Relative Depths of Unconsolidated Materials, Subarctic Alaska . . . . . . . . . . . . .
2-67
2-29
Typical Sections Through Ground Containing Permafrost . . . . . . . . . . . . . . . .
2-30
Illustration Showing Approximate Variations in Substructure . . . . . . . . . . . . . .
2-68
2-69
2-31
Installation of an Electrode During the Process of Backfilling . . . . . . . . . . . . . .
2-32
Apparent Resistivity for Two Soils at Various Moisture and Soil Contents . . . . . . . . .
2-72
2-73
2-33
Configuration of Nearly Horizontal Electrodes Placed in the Thawed Active Layer . . . . .
2-73
x
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
2-34
Resistance-to-Ground Curves for an Electrode Driven Into Ice-Rich Silt . . . . . . . . .
2-35
Resistance-to-Ground Curves for an Electrode Surrounded by Backfill of Saturated Silt. . .
2-36
Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated
Salt-Soil Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-37
2-74
Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated
Salt-Soil Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-38
2-73
2-74
2-74
Resistance-to-Ground Curves for Electrodes Placed in Holes Modified by Spring
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-74
3-1
Charge Distribution in a Thundercloud . . . . . . . . . . . . . . . . . . . . . . . .
3-2
3-2
Mean Number of Thunderstorm Days Per Year for the United States . . . . . . . . . . .
3-5
3-3
Worldwide Isokeraunic Map... . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
3-4
Attractive Area of a Rectangular Structure . . . . . . . . . . . . . . . . . . . . .
3-12
3-5
Effective Height of a Structure. . . . . . . . . . . . . . . . . . . . . . . . . . .
3-12
3-6
Zones of Protection Established by a Vertical Mast and a Horizontal Wire. . . . . . . . .
3-14
3-7
Some Commonly Used Lightning Shielding Angles . . . . . . . . . . . . . . . . . . .
3-14
3-8
Illustration of Processes and Currents Which Occur During a Lightning Flash to Ground . . .
3-15
3-9
Inductive Coupling of Lightning Energy to Nearby Circuits . . . . . . . . . . . . . . .
3-19
3-10
Normalized Voltage Induced in a Single-Turn Loop by Lightning Currents . . . . . . . . .
3-20
3-11
Capacitive Coupling of Lightning Energy . . . . . . . . . . . . . . . . . . . . . . .
3-22
3-12
Coupling of Lightning Energy Through an Interconnected Facility . . . . . . . . . . . .
3-23
3-13
Step-Voltage Hazards Caused by Lightning-Induced Voltage Gradients in the Earth. . . . .
3-24
4-1
Grounding for Fault Protection . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-2
4-2
Single-Phase 115/230 Volt AC Power Ground Connections . . . . . . . . . . . . . . .
4-4
4-3
Three-Phase 120/208 Volt AC Power System Ground Connections . . . . . . . . . . . .
4-5
4-4
Connections for a Three-Phase "Zig-Zag" Grounding Transformer . . . . . . . . . . . .
4-6
5-1
Surface Resistance and Skin Depth for Common Metals . . . . . . . . . . . . . . . .
5-4
5-2
Resistance Ratio of Isolated Round Wires . . . . . . . . . . . . . . . . . . . . . .
5-6
5-3
Nomograph for the Determination of Skin Effect Correction Factor . . . . . . . . . . .
5-8
5-4
Low Frequency Self Inductance Versus Length for 1/0 AWG Straight Copper ‘Wire . . . . .
5-9
5-5
Self Inductance of Straight Round Wire at High Frequencies . . . . . . . . . . . . . .
5-9
5-6
Resistance Ratio of Rectangular Conductors . . . . . . . . . . . . . . . . . . . . .
5-14
5-7
Resistance Versus Length for Various Sizes of Copper Tubing . . . . . . . . . . . . . .
5-14
5-8
AC Resistance Versus Frequency for Copper Tubing . . . . . . . . . . . . . . . . . .
5-16
5-9
Resistance Ratio of Nonmagnetic Tubular Conductors . . . . . . . . . . . . . . . . .
5-17
5-1o
Inductance Versus Frequency for Various Sizes of Copper Tubing . . . . . . . . . . . .
5-18
5-11
Floating Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-19
xi
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
1-148
1-72
Bonding of Equipment Cabinets to Cable Tray. . . . . . . . . . . . . . . . . . . . .
1-73
Bonding to Flexible Cable and Conduit . . . . . . . . . . . . . . . . . . . . . . . .
1-149
1-74
Bonding to Rigid Conduit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-149
1-75
Connection of Bonding Jumpers to Flat Surface . . . . . . . . . . . . . . . . . . . .
1-150
1-76
Bolted Bond Between Flatcars. . . . . . . . . . . . . . . . . . . . . . . . . . .
1-151
1-77
Bracket Installation (Rivet or Weld) . . . . . . . . . . . . . . . . . . . . . . . . .
1-151
1-78
Use of Bonding Straps for Structural Steel Interconnections. . . . . . . . . . . . . . .
1-152
1-79
Direct Bonding of Structural Elements . . . . . . . . . . . . . . . . . . . . . . . .
1-153
1-80
Connection of Earth Electrode Riser to Structural Column . . . . . . . . . . . . . . .
1-153
1-81
Measured Electromagnetic Shielding Effectiveness of a Typical Building at 6 Feet
Inside Outer Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-82
1-155
Measured Electromagnetic Shielding Effectiveness of a Typical Building at 45 Feet
Inside Outer Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-155
1-83
Shielding Effectiveness of Rebars . . . . . . . . . . . . . . . . . . . . . . . . . .
1-156
1-84
Shielding Effectiveness of a Grid as a Function of Wire Diameter, Wire Spacing, and
Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-158
Shield Absorption Loss Nomograph . . . . . . . . . . . . . . . . . . . . . . . . .
1-161
1-86
Nomograph for Determining Magnetic Field Reflection Loss . . . . . . . . . . . . . .
1-165
1-87
Nomograph for Determining Electric Field Reflection Loss . . . . . . . . . . . . . . .
1-166
1-88
Nomograph for Determining Plane Wave Reflection Loss . . . . . . . . . . . . . . . .
1-167
1-89
Shielding Effectiveness of Aluminum Foil Shielded Room . . . . . . . . . . . . . . . .
1-168
1-90
Shielding Effectiveness of Copper Foil Shielded Room . . . . . . . . . . . . . . . . .
1-168
1-91
Formation of Permanent Overlap Seam . . . . . . . . . . . . . . . . . . . . . . .
1-169
1-92
Good Corner Seam Design... . . . . . . . . . . . . . . . . . . . . . . . . . .
1-169
1-93
Pressure Drop Through Various Materials Used to Shield Ventilation Openings . . . . . . .
1-170
1-94
Typical Single-Point Entry for Exterior Penetrations (Top View). . . . . . . . . . . . .
1-174
1-95
Entry Plate Showing Rigid Cable, Conduit, and Pipe Penetrations . . . . . . . . . . . .
1-175
1-96
Effect of Rod Length on Ground Resistance . . . . . . . . . . . . . . . . . . . . .
1-180
1-97
Grounding of 120/208V 3-Phase, 4-Wire Wye Power Distribution System . . . . . . . . .
1-181
1-98
Grounding of Single-Phase, 3-Wire 110/220V Power System . . . . . . . . . . . . . . .
1-183
1-99
Grounding of 28 VDC 2-Wire DC Power System . . . . . . . . . . . . . . . . . . . .
1-184
1-100
Connecting Ground Subsystems for Collocated Shelters Greater than 20 Feet Apart . . . .
1-189
1-101
Method of Grounding a Fence. . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-192
2-1
Transmitter Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-2
2-2
Communication Center/Receiver Building Expansion. . . . . . . . . . . . . . . . . .
2-3
2-3
Earth Resistance Measurement at a Typical Facility . . . . . . . . . . . . . . . . . .
2-7
2-4
Resistance Measurement Worksheet . . . . . . . . . . . . . . . . . . . . . . . .
2-8
2-5
Sample of a Completed Resistance Measurement Work Sheet . . . . . . . . . . . . . .
2-9
1-85
xii
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
True Equivalent Circuit of a Bonded System . . . . . . . . . . . . . . . . . . . . .
7-24
7-15
Measured Bonding Effectiveness of a 9-1/2 Inch Bonding Strap . . . . . . . . . . . . .
7-27
7-16
Measured Bonding Effectiveness of 2-3/8 Inch Bonding Strap . . . . . . . . . . . . . .
7-28
7-17
Basic Diagram of the Corrosion Process . . . . . . . . . . . . . . . . . . . . . . .
7-30
7-18
Anode-to-Cathode Size at Dissimilar Junctions . . . . . . . . . . . . . . . . . . . .
7-35
7-19
Techniques for Protecting Bonds Between Dissimilar Metals . . . . . . . . . . . . . .
7-35
8-1
Electromagnetic Transmission Through a Slot . . . . . . . . . . . . . . . . . . . .
8-3
7-14
8-2
Transmission Line Model of Shielding . . . . . . . . . . . . . . . . . . . . . . . .
8-4
8-3
Absorption Loss for One Millimeter Shields . . . . . . . . . . . . . . . . . . . . . .
8-9
8-4
Wave Impedance Versus Distance from Source . . . . . . . . . . . . . . . . . . . .
8-10
8-5
Reflection Loss for Iron, Copper, and Aluminum With a Low Impedance Source . . . . . .
8-12
8-6
Universal Reflection Loss Curve for a Low Impedance Source . . . . . . . . . . . . . .
8-13
8-7
Plane Wave Reflection Loss for Iron, Copper, and Aluminum (r>2 ). . . . . . . . . .
. . . .
8-14
8-8
Universal Reflection Loss Curve for Plane Waves . . . . . . . . . . . . . . . . . . .
8-15
8-9
Universal Reflection Loss Curve for High Impedance Field . . . . . . . . . . . . . . .
8-16
8-10
Reflection Losses for Iron, Copper, and Aluminum With a High Impedance Source . . . . .
8-17
8-11
Graph of Correction Term (C) for Copper in a Magnetic Field . . . . . . . . . .
. . . .
8-22
8-12
Absorption Loss and Multiple Reflection Correction Term When r = 1 . . . . .
. . . .
8-22
8-13
Theoretical Attenuation of Thin Copper Foil . . . . . . . . . . . . . . . . . . . . .
8-26
8-14
Theoretical Attenuation of Thin Iron Sheet . . . . . . . . . . . . . . . . . . . . . .
8-26
8-15
Measured Shielding Effectiveness of High Permeability Metals . . . . . . . . .
. . . .
8-29
8-16
Measured Shielding Effectiveness of High Permeability Material as a Function
of Measurement Loop Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-29
8-17
Measured Shielding Effectiveness of Two Sheets of High Permeability Metal . . . . . . .
8-32
8-18
Measured and Calculated Shielding Effectiveness of Copper Screens to Low
Impedance Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-37
8-19
Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Size . . . . . .
8-40
8-20
Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Spacing. . . . .
8-40
8-21
Slot Radiation (Leakage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-43
8-22
Shielding Effectiveness Degradation Caused by Surface Finishes on Aluminum . . . . . . .
8-44
8-23
Influence of Screw Spacing on Shielding Effectiveness . . . . , . . . . . . . . . . . .
8-46
8-24
Shielding Effectiveness of AMPB-65 Overlap as a Function of Screw Spacing Along
Two Rows, l.5 Inches Apart . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-46
Shielding Effectiveness of an AMPB-65 Joint as a Function of Overlap . . . . . . . . . .
Typical Mounting Techniques for RF Gaskets . . . . . . . . . . . . . . . . . . . . .
8-47
8-26
8-49
8-27
Enlarged View of Knitted Wire Mesh . . . . . . . . . . . . . . . . . . . . . . . . .
8-50
8-28
Shielding Effectiveness of Conductive Glass to High Impedance Waves . . . . . .
. . . .
8-54
8-29
Shielding Effectiveness of Conductive Glass to Plane Waves . . . . . . . . . . . . . .
8-55
8-25
...
xiii
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
8-30
Light Transmission Versus Surface Resistance for Conductive Glass . . . . . . . . . . .
8-55
8-31
Shielding Effectiveness of Some Building Materials . . . . . . . . . . . . . . . . . .
8-57
8-32
Center Area Attenuation of Induced Voltage by 15 Foot High Single-Course
Reinforcing Steel Room... . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-58
8-33
Surface Transfer Impedance.. . . . . . . . . . . . . . . . . . . . . . . . . . .
8-62
8-34
Shielding Effectiveness of Various Types of RF Cables as a Function of Frequency . . . . .
8-62
8-35
Connector for Shield Within a Shield . . . . . . . . . . . . . . . . . . . . . . . . .
8-65
8-36
RF-Shielded Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-65
8-37
Effectiveness of Circumferential Spring Fingers for Improving the Shielding of a
Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-66
8-38
Use of Finger Stock for Door Bonding . . . . . . . . . . . . . . . . . . . . . . . .
8-69
8-39
Coaxial Loop Arrangement for Measuring Shield Effectiveness . . . . . . . . . . . . .
8-75
8-40
Coplanar Loop Arrangement for Measuring Shield Effectiveness . . . . . . . . . . . . .
8-75
10-1
10-2
10-2
EMP From High Altitude Bursts . . . . . . . . . . . . . . . . . . . . . . . . . .
Schematic Representation of High-Altitude EMP Generation . . . . . . . . . . . . . .
10-3
Surface-Burst Geometry Showing Compton Electrons and Net Current Density, J cnet . . . .
10-4
10-4
Short-Circuit Current Induced at the End of a Semi-Infinite Above-Ground Wire
By an Expodential Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-5
10-6
Short Circuit Current Induced at the Base of a Vertical Riser by a Vertically
10-7
Polarized Incident Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shield to Exclude Electromagnetic Fields . . . . . . . . . . . . . . . . . . . . . . .
10-9
10-10
10-7
The Normalized Current Waveform for Various Valves of the Depth Parameter p
(Expodential Pulse) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-8
10-2
Electromagnetic Penetration Through Small Apertures . . . . . . . . . . . . . . . . .
Shielding Integrity Near Interference - Carrying External Conductors . . . . . . . . . .
Magnetic Field Penetration of Apertures . . . . . . . . . . . . . . . . . . . . . . .
10-8
10-9
10-11
10-12
1o-1-1
10-16
Exclusion of Waveguide Current From Interior of Facility . . . . . . . . . . . . . . .
Waveguide Feedthroughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-19
10-22
10-14
Bellows With Slitted Sleeve Waveguide Attachment . . . . . . . . . . . . . . . . . .
Braided Wire Sleeve Clamped to Waveguide . . . . . . . . . . . . . . . . . . . . . .
10-15
Stuffing Tube for Waveguide . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-24
10-11
10-12
10-13
xiv
10-20
10-23
MIL-HDBK-419A
LIST OF TABLES
Page
Table
2-1
Facility Ground System: Purposes, Requirements, and Design Factors . . . . . . . . . .
2-6
2-2
2-3
Approximate Soil Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistivity Values of Earthing Medium . . . . . . . . . . . . . . . . . . . . . . . .
2-9
2-10
2-4
Resistance Distribution for Vertical Electrodes . . . . . . . . . . . . . . . . . . . .
2-21
2-5
Simple Isolated Electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-22
2-6
Resistance
C2 Spacing . . . . . . . . . . . . . . . . . . . .
2-43
2-7
Step Voltages for a Buried Vertical Ground Rod . . . . . . . . . . . . . . . . . . . .
2-50
2-8
Methods of Reducing Step Voltage Hazards . . . . . . . . . . . . . . . . . . . . . .
2-56
2-9
Effect of Moisture Content on Earth Resistivity . . . . . . . . . . . . . . . . . . . .
2-66
2-10
Effect of Temperature on Earth Resistivity . . . . . . . . . . . . . . . . . . . . . .
2-66
3-1
Range of Values for Lightning Parameters . . . . . . . . . . . . . . . . . . . . . .
3-16
5-1
Properties of Annealed Copper Wire . . . . . . . . . . . . . . . . . . . . . . . . .
5-2
5-2
Parameters of Conductor Materials . . . . . . . . . . . . . . . . . . . . . . . . .
5-3
5-3
DC Parameters of Some Standard Cables. . . . . . . . . . . . . . . . . . . . . . .
5-11
5-4
Sixty-Hertz Characteristics of Standard Cables . . . . . . . . . . . . . . . . . . . .
5-11
5-5
One-Megahertz Characteristics of Standard Cables . . . . . . . . . . . . . . . . . .
5-12
5-6
Impedance Comparisons Between #12 AWG and 1/0 AWG . . . . . . . . . . . . . . . .
5-12
Accuracy
Versus
Probe
7-1
DC Resistance of Direct Bonds Between Selected Metals . . . . . . . . . . . . . . . .
7-8
7-2
Ratings of Selected Bonding Techniques . . . . . . . . . . . . . . . . . . . . . . .
7-18
7-3
Calculated Inductance of a 6 Inch (15.2 cm) Rectangular Strap . . . . . . . . . . . . .
7-20
7-4
Calculated Inductance (µH) of 0.05 Inch (1.27 mm) Thick Straps . . . . . . . . . . . . .
7-20
7-5
Calculated Inductance (µH) of Standard Size Cable . . . . . . . . . . . . . . . . . .
7-21
7-6
Standard Electromotive Series . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-32
7-7
Galvanic Series of Common Metals and Alloys in Seawater . . . . . . . . . . . . . . .
7-33
8-1
Electrical Properties of Shielding Materials at 150 kHz . . . . . . . . . . . . . . . . .
8-7
8-2
Absorption Loss, A, of 1 mm Metal Sheet . . . . . . . . . . . . . . . . . . . . . .
8-8
8-3
Coefficients for Magnetic Field Reflection Loss . . . . . . . . . . . . . . . . . . . .
8-11
8-4
Calculated Reflection Loss in dB of Metal Sheet, Both Faces . . . . . . . . . . . . . .
8-18
8-5
Coefficients for Evaluation of Re-Reflection Correction Term, C . . . . . . . . . . . .
8-20
8-6
Correction Term C in dB for Single Metal Sheet . . . . . . . . . . . . . . . . . . . .
8-21
8-7
Calculated Values of Shielding Effectiveness . . . . . . . . . . . . . . . . . . . . .
8-23
8-8
Measured Shielding Effectiveness in dB for Solid-Sheet Materials . . . , . . . . . . . .
8-28
8-9
Summary of Formulas for Shielding Effectiveness . . . . . . . . . . . . . . . . . . .
8-30
8-10
Magnetic Material Characteristics . . . . . . . . . . . . . . . . . . . . . . . . .
8-31
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MIL-HDBK-419A
LIST OF TABLES (Continued)
Page
Table
8-11
Calculated Values of Copper Thin-Film Shielding Effectiveness in dB Against
8-33
8-12
Plane-Wave Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effectiveness of Non-Solid Materials Against Low Impedance and Plane-Waves . . . . . .
8-13
Effectiveness of Non-Solid Shielding Materials Against High Impedance Waves
. . . . . .
8-39
8-14
Comparison of Measured and Calculated Values of Shielding Effectiveness for
8-15
No. 22, 15 mil Copper Screens . . . . . . . . . . . . . . . . . . . . . . . . . . .
Characteristics of Conductive Gasketing Materials . . . . . . . . . . . . . . . . . .
8-48
8-16
Shielding Effectiveness of Hexagonal Honeycomb Made of Steel With 1/8-Inch
8-51
8-17
Openings l/2-Inch Long.... . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of Cable Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-64
8-19
Connector Application Summary . . . . . . . . . . . . . . . . . . . . . . . . . .
Characteristics of Commercially Available Shielded Enclosures . . . . . . . . . . . . .
9-1
Summary of the Effects of Shock . . . . . . . . . . . . . . . . . . . . . . . . . .
9-2
10-1
Shielding by Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-10
8-18
xvi
8-38
8-41
8-60
8-67
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MIL-HDBK-419A
CHAPTER 1
FACILITY GROUND SYSTEM
1.1 GENERAL.
1.1.1 This handbook addresses the practical considerations for engineering of grounding systems, subsystems,
and other components of ground networks.
installation of ground systems.
Electrical noise reduction is discussed as it relates to the proper
Power distribution systems are covered to the degree necessary to understand
the interrelationships between grounding, power distribution, and electrical noise reduction.
1.1.2 The information provided in this handbook primarily concerns grounding, bonding, and shielding of fixed
plant telecommunications-electronics facilities; however, it also provides basic guidance in the grounding of
deployed transportable communications/electronics equipment.
1.1.3 Grounding, bonding, and shielding are approached from a total system concept, which comprises four
basic subsystems in accordance with current Department of Defense (DOD) guidance. These subsystems are as
follows:
a.
An earth electrode subsystem.
b.
A lightning protection subsystem.
c.
A fault protection subsystem.
d.
A signal reference subsystem.
1.2 APPLICATION. This handbook provides technical information for the engineering and installation of
military communications systems related to the background and practical aspects of installation practices
applicable to grounding, bonding, and shielding. It also provides the latest concepts on communications systems
grounding, bonding, and shielding installation practices as a reference for military communications installation
personnel.
1.3 DEFINITIONS. A glossary of unique terms used in this handbook is provided in Appendix A. All other
terms and definitions used in this handbook conform to those contained in Joint Chiefs of Staff Publication No.
1. (JCS Pub 1), FED-STD-1037, MIL-STD-463, and the Institute of Electrical and Electronics Engineers (IEEE)
dictionary.
1.4 REFERENCED DOCUMENTS. Publications related to the subject material covered in the text of this
handbook are listed in Appendix B.
The list includes publications referenced in the text and those documents
that generally pertain to subjects contained in the handbook but are not necessarily addressed specifically.
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MIL-HDBK-419A
1.5 DESCRIPTION.
The ground system serves three primary functions which are listed below. A good ground
system must receive periodic inspection and maintenance to retain its effectiveness. Continued or periodic
maintenance is aided through adequate design, choice of materials, and proper installation techniques to ensure
that ground Subsystems resist deterioration or inadvertent destruction and thus require minimal repair to retain
their effectiveness throughout the life of the facility.
a.
Personnel safety.
Personnel safety is provided by low-impedance grounding and bonding between
equipment, metallic objects, piping, and other conductive objects, so that currents due to faults or lightning do
not result in voltages sufficient to cause a shock hazard.
b.
Equipment and facility protection. Equipment and facility protection is provided by low-impedance
grounding and bonding between electrical services, protective devices, equipment, and other conductive objects,
so that faults or lightning currents do not result in hazardous voltages within the facility. Also, the proper
operation of overcurrent protective devices is frequently dependent upon low-impedance fault current paths.
c.
ensuring
Electrical noise reduction. Electrical noise reduction is accomplished on communication circuits by
that
(1)
minimum
voltage
potentials
exist
between
communications-electronics
equipments,
(2)
the
impedance between signal ground points throughout the facility to earth is minimal, and (3) that interference
from noise sources is minimized.
1.5.1 Facility Ground System.
All telecommunications and electronic facilities are inherently related to
earth by capacitive coupling, accidental contact, and intentional connection. Therefore, ground must be looked
at from a total system viewpoint, with various subsystems comprising the total facility ground system. The
facility ground system forms a direct path of known low impedance between earth and the various power,
communications,
and other equipments that effectively extends in approximation of ground reference
throughout the facility.
The facility ground system is composed of an earth electrode subsystem, lightning
protection subsystem, fault protection subsystem, and signal reference subsystem.
a.
Earth electrode subsystem. The earth electrode subsystem consists of a network of earth electrode
rods, plates, mats, or grids and their interconnecting conductors. The extensions into the building are used as
the principal ground point for connection to equipment ground subsystems serving the facility. Ground
reference is established by electrodes in the earth at the site or installation. The earth electrode subsystem
includes the following:
(1) a system of buried, driven rods interconnected with bare wire that normally form, a
ring around the building; or (2) metallic pipe systems, i.e., water, gas, fuel, etc., that have no insulation joints;
or (3) a ground plane of horizontal buried wires.
electrode subsystem.
Metallic pipe systems shall not be used as the sole earth
Resistance to ground should be obtained from the appropriate authority if available or
determined by testing. For EMP considerations, see Chapter 10.
b.
Lightning protection subsystem. The lightning protection subsystem provides a nondestructive path
to ground for lightning energy contacting or induced in facility structures.
mast, tower, or similar self-supporting objects from lightning
To effectively protect a building,
damage, an air terminal (lightning rod) of
adequate mechanical strength and electrical conductivity to withstand the stroke impingement must he
provided.
An air terminal will intercept the discharge to keep it from penetrating the nonconductive outer
coverings of the structure, and prevent it from passing through devices likely to be damaged or destroyed. A
1-2
MIL-HDBK-419A
low-impedance path from the air terminal to earth must also be provided.
These requirements are met by
either (1) an integral system of air terminals, roof conductors, and down conductors securely interconnected to
provide the shortest practicable path to earth; or (2) a separately mounted shielding system, such as a metal
mast or wires (which act as air terminals) and down conductors to the earth electrode subsystem.
c.
Fault protection subsystem.
The fault protection subsystem ensures that personnel are protected
from shock hazard and equipment is protected from damage or destruction resulting from faults that may
develop in the electrical system. It includes deliberately engineered grounding conductors (green wires) which
are provided throughout the power distribution system to afford electrical paths of sufficient capacity, so that
protective devices such as fuses and circuit breakers installed in the phase or hot leads can operate promptly.
If at all possible the equipment fault protection conductors should be physically separate from signal reference
grounds except at the earth electrode subsystem. The equipment fault protection subsystem provides grounding
of conduits for signal conductors and all other structural metallic elements as well as the cabinets or racks of
equipment.
d.
Signal reference subsystem.
The signal reference subsystem establishes a common reference for
C-E equipments, thereby also minimizing voltage differences between equipments. This in turn reduces the
current flow between equipments and also minimizes or eliminates noise voltages on signal paths or circuits.
Within a piece of equipment, the signal reference subsystem may be a bus bar or conductor that serves as a
reference for some or all of the signal circuits in the equipment.
Between equipments, the signal reference
subsystem will be a network consisting of a number of interconnected conductors. Whether serving a collection
of circuits within an equipment or serving several equipments within a facility, the signal reference network
will in the vast majority of cases be a multiple point/ equipotential plane but could also, in some cases, be a
single point depending on the equipment design, the facility, and the frequencies involved.
1.5.2 Grounding and Power Distribution Systems.
For safety reasons, both the MIL-STD-188-124A and the
National Electrical Code (NEC) require the electrical power systems and equipments be intentionally grounded;
therefore, the facility ground system is directly affected by the proper installation and maintenance of the
power distribution systems. The intentional grounding of electrical power systems minimizes the magnitude and
duration of overvoltage on an electrical circuit, thereby reducing the probability of personnel injury, insulation
failure, or fire and consequent system, equipment, or building damage.
a.
Alternating currents in the facility ground system are primarily caused as a result of improper ac
wiring, simple mistakes in the ac power distribution system installation, or as a result of power faults. To
provide the desired safety to personnel and reduce equipment damage, all 3-phase wye wiring to either fixed or
transportable communication facilities shall be accomplished by the 5-wire or conductor distribution system
consisting of three phase or “hot” leads, one neutral lead and one grounding (green) conductor. A single building
receiving power from a single source requires the ac neutral be grounded to the earth electrode subsystem on
the source side of the first service disconnect or service entrance panel as well to a ground terminal at the
power source (transformer, generator, etc.). This neutral shall not be grounded at any point within the building
or on the load side of the service entrance panel. The grounding of all C-E equipment within the building is
accomplished via the grounding (green) conductor which is bonded to the neutral bus in the source side of the
service entrance panel and, in turn, grounded to the earth electrode subsystem. In addition to the three phase
or "hot" leads and the neutral (grounded) conductor, a fifth wire is employed to interconnect the facility earth
electrode subsystem with the ground terminal at the power source.
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MIL-HDBK-419A
To eliminate or reduce undesired noise or hum, multiple facilities supplied from a single source shall ground the
neutral only at the power source and not to the earth electrode subsystem at the service entrance point. Care
should be taken to ensure the neutral is not grounded on the load side of the first disconnect service or at any
point within the building. The grounding (green) conductor in this case is not bonded to the neutral bus in the
service disconnect panel.
It is, however, bonded to the facility earth electrode subsystem at the service
entrance panel. The fifth wire shall be employed to interconnect the earth electrode subsystem with the ground
terminal at the power source.
The secondary power distribution wiring for a 240 volt single phase system consists of two phase or “hot” leads,
a neutral (grounded) and a grounding (green) conductor while the three conductor secondary power distribution
system is comprised of one phase, one neutral, and one grounding lead. In both cases, the neutral shall not be
grounded on the load side of the first service disconnect. It shall, however, be grounded to the ground terminal
at the power source and to the earth electrode subsystem if one power source supplies power only to a single
building.
The ac wiring sequence (phase, neutral, and equipment fault protection) must be correct all the way from the
main incoming ac power source to the last ac load, with no reversals between leads and no interconnection
between neutral and ground leads. Multiple ac neutral grounds and reversals between the ac neutral and the
fault protection subsystem will generally result in ac currents in all ground conductors to varying degrees. The
NEC recognizes and allows the removal or relocation of grounds on the green wire which cause circulating
currents. (Paragraph 250-21(b) of the NEC refers.) Alternating current line filters also cause seine a C currents
in the ground system when distributed in various areas of the facility, this is due to some ac current passing
through capacitors in the ac line filters when the lines are filtered to ground. Power line filters should not
induce more than 30 milliamperes of current to the fault protection subsystem.
b.
Dc power equipment has been found to be a significant electrical noise source that can be minimized
through proper configuration of the facility, the physical and electrical isolation of the dc power equipment
from communications equipment, and filtering of the output. Certain communications equipment with inverter
or switching type power supplies also cause electrical noise on the dc supply leads and the ac input power leads.
This noise can be minimized by the use of decentralizing filters at or in the equipment. The location, number,
and termination of the dc reference ground leads are also important elements in providing adequate protection
for dc systems and, at the same time, minimizing electrical noise and dc currents in the ground system.
1.5.3 Electrical Noise in Communications Systems.
Interference-causing signals are associated with
time-varying, repetitive electromagnetic fields and are directly related to rates of change of currents with
time.
A current-changing source generates either periodic signals, impulse signals, or a signal that varies
randomly with time. To cause interference, a potentially interfering signal must be transferred from the point
of generation to the location of the susceptible device. The transfer of noise may occur over one or several
paths.
There are several modes of signal transfer (i.e., radiation, conduction, and inductive and capacitive
coupling).
1-4
MIL-HDBK-419A
1.6 BONDING, SHIELDING, AND GROUNDING RELATIONSHIP.
a.
The simple grounding of elements of a communications facility is only one of several measures
necessary to achieve a desired level of protection and electrical noise suppression. To provide a low-impedance
path for (1) the flow of ac electrical current to/from the equipment and (2) the achievement of an effective
grounding system , various conductors, electrodes, equipment, and other metallic objects must be joined or
bonded together.
Each of these bonds should be made so that the mechanical and electrical properties of the
path are determined by the connected members and not by the interconnection junction. Further, the joint
must maintain its properties over an extended period of time, to prevent progressive degradation of the degree
of performance initially established by the interconnection.
Bonding is concerned with those techniques and
procedures necessary to achieve a mechanically strong, low-impedance interconnection between metal objects
and to prevent the path thus established from subsequent deterioration through corrosion or mechanical
looseness.
b.
The ability of an electrical shield to drain off induced electrical charges and to carry sufficient
out-of-phase current to cancel the effects of an interfering field is dependent upon the shielding material and
the manner in which it is installed. Shielding of sensitive electrical circuits is an essential protective measure
to obtain reliable operation in a cluttered electromagnetic environment.
Solid, mesh, foil, or stranded
coverings of lead, aluminum, copper, iron, and other metals are used in communications facilities, equipment,
and conductors to obtain shielding. These shields are not fully effective unless proper bonding and grounding
techniques are employed during installation. Shielding effectiveness of an equipment or subassembly enclosure
depends upon such considerations as the frequency of the interfering signal, the characteristics of the shielding
material, and the number and shapes of irregularities (openings) in the shield.
1.7 GROUNDING SAFETY PRACTICES.
a.
It is essential that all personnel working with Cormmunications-Electronics (C-E) equipment and
supporting systems and facilities strictly observe the rules, procedures, and precautions applicable to the safe
installation, operation, and repair of equipment and facilities.
All personnel must be constantly alert to the
potential hazards and dangers presented and take all measures possible to reduce or eliminate accidents.
b.
Safety precautions in the form of precisely worded and illustrated danger or warning signs shall be
prominently posted in conspicuous places, to prevent personnel from
making accidental contact with
high-voltage sources such as power lines, antennas, power supplies, or other places where uninsulated contacts
present the danger of electrical shock or short circuits. Signs shall also warn of the dangers of all forms of
radiation hazards, acids, and chemical inhalation, plus all other potential sources of personnel danger. Power
cutoff features built into the equipment must be used in strict adherence to the intended use.
c.
During the installation of equipment, warning tags are used to note the existence of potential danger
when individual circuits or stages are being checked out. The tags should contain appropriate information to
alert all personnel of the dangers involved and specific restrictions as to the use of the equipment. The
equipment being installed shall be appropriately tagged in accordance with the directives of the local safety
officer, equipment manufacturer, or other responsible agent.
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MIL-HDBK-419A
d.
Installation personnel, when working with equipment having high-voltage devices, must ensure that
the devices are grounded and that the high-voltage circuits have been disconnected or turned off. Do not rely
solely on the presence of interlock switches for protection from electrical shock.
1-6
MIL-HDBK-419A
CHAPTER 2
EARTH ELECTRODE SUBSYSTEM
2.1 OBJECTIVES.
Earth grounding is defined as the process by which an electrical connection is made to the earth. The earth
electrode subsystem is that network of interconnected rods, wires, pipes, or other configuration of metals which
establishes electrical contact between the elements of the facility and the earth. This system should achieve
the following objectives:
a.
Provide a path to earth for the discharge of lightning strokes in a manner that protects the
structure, its occupants, and the equipment inside.
b.
Restrict the step-and-touch potential gradient in areas accessible to persons to a level below the
hazardous threshold even under lightning discharge or power fault conditions.
c.
Assist in the control of noise in signal and control circuits by minimizing voltage differentials
between the signal reference subsystems of separate facilities.
2.1.1 Lightning Discharge.
A lightning flash is characterized by one or more strokes with typical peak current
amplitudes of 20 kA or higher. In the immediate vicinity of the point of entrance of the stroke current into the
earth, hazardous voltage gradients can exist along the earth’s surface.
that such gradients are more than adequate to cause death.
Ample evidence (2-l)* exists to show
It is thus of great importance that the earth
electrode subsystem be configured in a manner that minimizes these gradients. The lower the resistance of the
earth connection, the lower the peak voltage and consequently the less severe the surface gradients. Even with
low resistance earth electrode systems, the current paths should be distributed in a way that minimizes the
gradients over the area where personnel might be present.
* Referenced documents are listed in the last section of each chapter.
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MIL-HDBK-419A
2.1.2 Fault Protection.
In the event of transformer failure (e.g., disconnect between neutral and ground or
line to ground faults) or any failure between the service conductor(s) and grounded objects in the facility, the
earth electrode subsystem becomes a part of the return path for the fault current. A low resistance assists in
fault clearance; however, it does not guarantee complete personnel protection against hazardous voltage
gradients which are developed in the soil during high current faults. Adequate protection generally requires the
use of ground grids or meshes designed to distribute the flow of current over an area large enough to reduce the
voltage gradients to safe levels.
The neutral conductor at the distribution transformer must therefore be
connected to the earth electrode subsystem to ensure that a low resistance is attained for the return path.
(Paragraph 5.1.1.2.5.1 of MIL-STD-188-124A refers.) Ground fault circuit interrupters on 120 volt single phase
15 and 20 ampere circuits will provide personnel protection against power faults and their use is therefore
highly recommended.
2.1.3 Noise Reduction.
The earth electrode subsystem is important for the minimization of electromagnetic
noise (primarily lower frequency) within signal circuits caused as a result of stray power currents. For example,
consider a system of two structures located such that separate earth electrode subsystems are needed as shown
in Figure 2-1.
If stray currents (such as may be caused by an improperly grounded ac system, dielectric
leakage, high resistance faults, improperly returned dc, etc.) are flowing into the earth at either location, then
a voltage differential will likely exist between the grounding networks within each facility.
Currents originating from sources outside the structures can also be the cause of these noise voltages. For
example, high voltage substations are frequent sources of large power currents in the earth. Such currents arise
from leakage across insulators, through cable insulation, and through the stray capacitance which exists
between power lines and the earth. These currents flowing through the earth between the two sites will
generate a voltage difference between the earth connections of the two sites in the manner illustrated by
Figure 2-2.
Any interconnecting wires or cables will have these voltages applied across the span which will cause currents
to flow in cable shields and other conductors.
common-mode
noise
As shown in Chapter 6, such intersite currents can induce
voltages into interconnected earth electrode subsystems.
2.1.4 Summary of Requirements. Table 2-1 summarizes the purpose, requirements, and resulting design
factors for earth connections of the lightning protection subsystem, the fault protection subsystem, the signal
reference subsystem, and the ac distribution system neutral (grounded) conductor and safety ground (grounding)
conductor. Refer to Article 100 - Definitions of the NEC for additional information on grounding and grounded
conductors (2-2).
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MIL-HDBK-419A
Figure 2-1.
2-3
MIL-HDBK-419A
Figure 2-2.
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MIL-HDBK-419A
2.2 RESISTANCE REQUIREMENTS.
2.2.1 General. The basic measure of effectiveness of an earth electrode is the value in ohms of the resistance
to earth at its input connection. Because of the distributed nature of the earth volume into which electrical
energy flows, the resistance to earth is defined as the resistance between the point of connection and a very
distant point on the earth (see Section 2.4). Ideally, the earth electrode subsystem provides a zero resistance
between the earth and the point of connection. Any physically realizable configuration, however, will exhibit a
finite resistance to earth. The economics of the design of the earth electrode subsystem involve a trade-off
between the expense necessary to achieve a very low resistance and the satisfaction of minimum system
requirements. This subsystem shall also interconnect all driven electrodes and underground metal objects of the
facilities including the emergency power plant.
Underground metallic pipes entering the facility shall also be
bonded to the earth electrode subsystem.
2.2.2 Resistance to Earth.
Metal underground water pipes typically exhibit a resistance to earth of less than
three ohms. Other metal elements in contact with the soil such as the metal frame of the building, underground
gas piping systems , well casings, other piping and/or buried tanks, and concrete-encased steel reinforcing bars
or rods in underground footings or foundations generally exhibit a resistance substantially lower than 25 ohms.
2.2.2.1 National Electrical Code Requirements.
For the fault protection subsystem, the NEC (2-2) states in
Article 250 that a single electrode consisting of a rod, pipe or plate which does not have a resistance to ground
of 25 ohms or less shall be augmented by one additional made electrode.
Although the language of the NEC
clearly implies that electrodes with resistances as high as 25 ohms are to be used only as a last resort, this 25
ohm limit has tended to set the norm for grounding resistance regardless of the specific system needs. The 25
ohm limit is reasonable or adequate for application to private homes and other lower powered type facilities.
2.2.2.2 Department of Defense Communications Electronics Requirements. The above criteria however, is not
acceptable for C-E facilities when consideration is given to the large investments in personnel and equipment.
A
compromise of cost versus protection against lightning, power faults, or EMP has led to establishment of a
design goal of 10 ohms for the earth electrode subsystem (EES) in MIL-STD-188-124A. The EES designed in
MIL-STD-188-124A specifies a ring ground around the periphery of the facility to be protected. With proper
design and installation of the EES, the design goal of 10 ohms should be attained at reasonable cost. At
locations where the 10 ohms has not been attained due to high soil resistivity, rock formations, or other terrain
features, alternate methods listed in Paragraph 2.9 shall be considered for reducing the resistance to earth.
2.2.3 Lightning Requirements.
For lightning protection, it also is difficult to establish a definite grounding
resistance necessary to protect personnel. The current which flows in a direct lightning stroke may vary from
several hundred amperes to as much as 300 thousand amperes.
Such currents through even one ohm of
resistance can theoretically produce hazardous potentials. It is impractical to attempt to reduce the resistance
of a facility to earth to a value low enough to absolutely prevent the development of these potentials.
Techniques other than simply achieving an extremely low resistance to ground must therefore be employed to
protect personnel and equipment inside a structure from the hazards produced by a direct stroke. Experience
has shown that a grounding resistance of ten ohms gives fairly reliable lightning protection to buildings,
transformers, transmission lines, towers, and other exposed structures. At some sites, resistances as low as one
ohm or less can be achieved economically.
The lower the resistance, the greater the protection; therefore,
attempts should be made to reduce the resistance to the lowest practical value.
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MIL-HDBK-419A
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2.3 SOIL RESISTIVITY.
2.3.1 General.
The resistivities of the soil and rock in which the earth electrode subsystem is buried,
constitute the basic constraint on the achievement of a low resistance contact with earth. The resistance of an
earth electrode subsystem can in general be calculated with formulas which are based upon the general
resistance formula.
(2-1)
where
is the resistivity of the conducting material,
is the length of the path for current flow in the earth, A
is the cross-sectional area of the conducting path, I is the current into the electrode, and E is the voltage of the
electrode measured with respect to infinity. It will be shown later in this chapter that if the soil resistivity is
known, the resistance of the connection provided by the more common electrode configurations can be readily
determined.
The soils of the earth consist of solid particles and dissolved salts. Electrical current flows through the earth
primarily as ion movement; the ionic conduction is heavily influenced by the concentration and kinds of salts in
the moisture in the soil. Ionic disassociation occurs when salts are dissolved, and it is the movement of these
ions under the influence of electrical potential which enable the medium to conduct electricity.
Resistivity is defined in terms of the electrical resistance of a cube of homogeneous material. The resistance
of a homogeneous cube, as measured across opposite faces, is proportional to the resistivity and inversely
proportional to the length of one side of the cube. The resistance is
(2-2)
where
= resistivity of the material, ohms - (unit-of-length);
L = length of one side of the cube, (unit-of-length), and
A = area of one face of the cube, (unit-of-length) 2.
Common units of resisitivity are ohm-cm and ohm-m.
2.3.2 Typical Resistivity Ranges.
A broad variation of resistivity occurs as a function of soil types, and
classification of the types of soils at a potential site for earth electrodes is needed by the designer. Table 2-2
permits a quick estimate of soil resistivity, while Table 2-3 lists measured resistivity values from a variety of
sources. Tables 2-2 and 2-3 indicate that ranges of one or two orders of magnitude in values of resistivity for a
given soil type are to be expected.
2.3.3 Environmental Effects. In addition to the variation with soil types, the resistivity of a given type of soil
will vary several orders of magnitude with small changes in the moisture content, salt concentration, and soil
temperature.
It is largely these variations in soil environment that cause the wide range of values for each soil
type noted in Tables 2-2 and 2-3.
Figure 2-3 shows the variations observed in a particular soil as moisture,
salt, and temperature were changed. The curves are intended only to indicate trends -- another type of soil
would be expected to yield curves with similar shapes but different values.
2-7
MIL-HDBK-419A
The discontinuity in the temperature curve (Figure 2-3(b)), indicates that at below freezing temperatures the
soil resistivity increased markedly.
This undesirable temperature effect can be minimized by burying earth
electrode subsystems below the frost line.
2.4 MEASUREMENT OF SOIL RESISTIVITY.
2.4.1 General.
It is not always possible to ascertain with a high degree of certainty the exact type of soil
present at a given site.
locations.
Soil is typically rather nonhomogeneous; many types will be encountered at most
Even with the aid of borings and test samples and the use of Table 2-3, the resistivity estimate can
easily be off by two or three orders of magnitude. When temperature and moisture variations are added to the
soil type variations, it is evident that estimates based on Table 2-3 are not sufficiently accurate for design
purposes. The only way to accurately determine the resistivity of the soil at a specific location is to measure
it.
2.4.2 Measurement Techniques. The most commonly used field methods for determining soil resistivity employ
the technique of injecting a known current into a given volume of soil, measuring the voltage drop produced by
the current passing through the soil, and then determining the resistivity from a modified form of Equation 2-1.
2.4.2.1
One-Electrode
Method.
To illustrate the principles of this technique, first visualize a metal
hemisphere buried in the earth as shown in Figure 2-4. In uniform earth, injected current flows radially from
this hemispherical electrode.
Equipotential surfaces are established concentric with the electrode and
perpendicular to the radial directions of current flow.
(Regardless of the shape of an electrode, it can be
approximated as a hemispherical electrode if viewed from far enough away.) As the current flows from the
hemisphere, the current density decreases with distance from the electrode because the areas of successive
shells become larger and larger. The current density within the earth, at a given distance x from the center of
the electrode is
amperes per unit area,
(2-3)
where
I = current entering the electrode and
2
= area of the hemispherical shell with radius x.
At the point x the electric field strength can be obtained from Ohm's law:
(2-4)
volts per unit length.
where
is resistivity of material.
2-8
MIL-HDBK-419A
Table 2-2
Approximate Soil Resistivity (2-3)
Type of Soil
(ohm-m)
Resistivity
(ohm-cm)
(ohm-ft)
Wet Organic Soil
10
1 03
33
Moist
1 02
1 04
330
Dry Soil
1 03
1 05
3300
Bed Rock
1 04
1 06
33000
Soil
(a) MOISTURE
(b) TEMPERATURE
(c) ADDED SALT
Figure 2-3. Typical Variations in Soil Resistivity as a Function of Moisture,
Temperature and Salt Content (2-4)
2-9
MIL-HDBK-419A
Table 2-3
Resistivity Values of Earthing Medium (2-5), (2-6), (2-7)
Medium
Resistivity
Minimum
Average
Maximum
(ohm-cm)
(ohm-cm)
(ohm-cm)
Surface soils, loam, etc.
10 2
5 x 103
Clay
2 x 102
10 4
Sand and gravel
5 x 103
10 5
Surface limestone
10 4
106
Limestones
5 x 102
4 x 105
Shales
5 x 102
10 4
Sandstone
2 x 103
2 x 105
Granites, basalts, etc.
1 06
Decomposed gneisses
5 x 103
5 x 104
Slates, etc.
1 03
10 4
2 x 104
Fresh Water Lakes
Tap Water
10 3
Sea Water
20
5 x 1 03
1 02
Pastoral, low hiIls, rich soil, typical
of Dallas, Texas; Lincoln, Nebraska
areas
Flat country, marshy, densely wooded
typical of Louisiana near Mississippi
River
2 x 107
2 x 102
3 x 103
2 x 102
Pastoral, medium hills and forestation,
typical of Maryland, Pennsylvania, New
York, exclusive of mountainous territory
and seacoasts
10 4
2 x 104
Rocky soil, steep hills, typical of New
England
10 3
5 x 104
10 5
Sandy, dry, flat, typical of coastal
count r y
3 x 104
5 x 104
5 x 105
10 5
10 6
City, industrial areas
2-10
MIL-HDBK-419A
Table 2-3 (Continued)
Resistivity Values of Earthing Medium (2-5), (2-6), (2-7)
Medium
Resistivity
Minimum
Average
Maximum
(ohm-cm)
(ohm-cm)
(ohm-cm)
Fills, ashes, cinders, brine, waste
6 x 102
2.5 x 103
7 x 103
Clay, shale, gumbo, loam
3 x 102
4 x 103
2 x l04
Same-with varying proportion of
sand and gravel
10 3
1.5 x 104
10 5
Gravel, sand stones with little clay
or loam, granite
5 x 104
1 05
10 6
—
Figure 2-4. Current Flow From a Hemisphere in Uniform Earth
2-11
MIL-HDBK-419A
The voltage from the surface of the electrode to the point x is the line integral of ex with the lower limit equal
to the sphere's radius, r, and the upper limit equal to the distance, x:
(2-5)
As x becomes very large, E is closely approximated as
(2-6)
The resistance to the earth of the electrode is the resistance between the electrode and a very distant point;
therefore
(2-7)
where:
E = the voltage drop between the electrode and a point infinitely distant,
I = the current entering the electrode,
= earth resistivity, and
r = radius of hemisphere.
Rewriting Equation 2-7 as
(2-8)
shows that the resistivity can be determined by knowing r, E, and L
2-12
MIL-HDBK-419A
2.4.2.2 Four-Terminal Method. In the four-terminal method developed by the U.S. Bureau of Standards (2-8),
four electrodes are inserted into the soil in a straight line with equal spacings. A known current is injected into
the soil through the end electrodes and the voltage drop between the two inside electrodes is measured.
Consider four deeply buried spheres placed in a straight line, separated by a distance, a, as shown in Figure 2-5.
Connection is made to the spheres by insulated conductors. Assume that a current, I, is introduced into one of
the outermost spheres (No. 1) and flows out of the earth through the other (No. 4) outermost sphere. The
voltage from the left hand (No. 2) to the right hand (No. 3) inner sphere can be viewed as resulting from a
current flowing to infinity and another returning from infinity. The two resulting components of the voltage
are (2-8)
(2-9)
where
Ii = input current,
and
(2-10)
where
But since
1 0 = output current.
I0 =
Ii ,
the total potential V is
v = V 1 + V2
2-13
(2-11)
MIL-HDBK-419A
If the probe depth, h, is less than the probe separation distance, a, the potential drop measured between the
inner electrodes divided by the current measured into (or out of) one of the outer electrodes is (2-8):
(2-12)
where:
a = distance between four, equally spaced, in-line probes, and
h = depth of burial of probes (insulated leads to surface).
If h << a, Equation 2-12 simplifies to
(2-13)
or
(2-14)
Short rods provide an effective approximation to the buried sphere, particularly at distances large with respect
to the depth of insertion.
The typical earth resistance test set contains a hand powered generator which can generate an ac signal at
frequencies of 40 to 100 hertz or so. (Fifty or sixty hertz should not be used because errors may be produced
by stray power currents in the soil.
Direct current is not usually used because of polarization effects.) By
adjusting the resistance of an internal double balanced bridge, the instrument provides a direct indication of the
R required in Equation 2-14.
Figure 2-5. Idealized Method of Determining Soil Resistivity
2-14
MIL-HDBK-419A
2.5 TYPES OF EARTH ELECTRODE SUBSYSTEMS.
2.5.1 General. Earth electrode subsystems can be divided into two general types, the most preferable being a
ring ground with 10-foot (3-meter) minimum length ground rods every 15 feet (4.5 meters). A second and less
preferable type consists of a system of radials or grounds used when soil is rocky or has extremely high
resistivity.
At sites where soil resistivity varies from high to very high and frequent electrical storms are
common, a combination of the two is recommended, i.e., a ring ground around the building (worst case-grid
under building) extending 2 to 6 feet (0.6 to 1.8 meters) outside the drip line with radials or horizontal
conductors extending to 125 feet (37.5 meters). With either system, resistance to earth and danger of arc over
can be greatly reduced by bonding any large metal objects in the immediate area to the earth electrode
subsystem. These include metal pipes, fuel tanks, grounded metal fences, and well casings.
2.5.2 Ground Rods.
Vertically driven ground rods or pipes are the most common type of made electrode.
Rods or pipes are generally used where bedrock is beyond a depth of 3 meters (10 feet). Ground rods are
commercially manufactured in 1.27, 1.59, 1.90 and 2.54 cm (1/2, 5/8, 3/4 and 1 inch) diameters and in lengths
from 1.5 to 12 meters (5 to 40 feet). For most applications, ground rods of 1.90 cm (3/4 inch) diameter, and
length of 3.0 meters (10 feet), are used.
Copper-clad steel ground rods are required because the steel core
provides the strength to withstand the driving force and the copper provides corrosion protection and is
compatible with copper or copper-clad interconnecting cables.
2.5.3 Buried Horizontal Conductors.
Where bedrock is near the surface of the earth, the use of driven rods is
unpractical. In such cases, horizontal strips of metal, solid wires, or stranded cables buried 0.48 to 0.86 meters
(18 to 36 inches) deep may be used effectively. With long strips, reactance increases as a factor of the length
with a consequent increase in impedance. A low impedance is desirable for minimizing lightning surge voltages.
Therefore, several wires, strips, or cables arranged in a star pattern, with the facility at the center, is
preferable to one long length of conductor.
2.5.4 Grids.
Grid systems, consisting of copper cables buried about 15.24 cm (6 inches) in the ground and
forming a network of squares, are used to provide equipotential areas throughout the facility area.
system usually extends over the entire area.
Such a
The spacing of the conductors, subject to variation according to
requirements of the installation, may normally be 0.6 to 1.2 meters (2 to 4 feet) between cables. The cables
must be bonded together at each crossover.
Grids are generally required only in antenna farms or substation yards and other areas where very high fault
currents are likely to flow into the earth and hazardous step potentials may exist (see Section 2.8.1.2.3) or soil
conditions prohibit installation of other ground systems.
Antenna counterpoise systems shall be installed in
accordance with guidance requirements of the manufacturer.
2.5.5 Plates.
Rectangular or circular plate electrodes should present a minimum of 0.09 square meters (2
square feet) of surface contact with the soil. Iron or steel plates should be at least 0.64 cm (1/4 inch) thick and
nonferrous metals should be at least 0.15 cm (0.06 inches) thick.
feet) below grade should be maintained.
A burial depth of 1.5 to 2.4 meters (5 to 8
This system is considered very expensive for the value produced and
generally not recoin mended.
2-15
MIL-HDBK-419A
2.5.6 Metal Frameworks of Buildings.
The metal frameworks of buildings may exhibit a resistance to earth of
less than 10 ohms, depending upon the size of the building, the type of footing, and the type of subsoil at a
particular location.
Buildings that rest on steel pilings in particular may exhibit a very low resistance
connection to earth. For this low resistance to be used advantageously, it is necessary that all elements of the
framework be bonded together.
2.5.7 Water Pipes.
Metal underground pipes have traditionally been relied upon for grounding electrodes. The
resistance to earth provided by piping systems is usually quite low because of the extensive contact made with
soil. Municipal water systems in particular establish contact with the soil over wide areas. For water pipes to
be effective, any possible discontinuities must be bridged with bonding jumpers. The NEC requires that any
water metering equipment and service unions be bypassed with a jumper not less than that required for the
grounding connector.
However, stray or fault currents flowing through the piping network into the earth can present a hazard to
workmen making repairs or modifications to the water system.
For example, if the pipes supplying a building
are disconnected from the utility system for any reason, that portion connected to the building can rise to a
hazardous voltage level relative to the rest of the piping system and possibly with respect to the earth. In
particular, if the resistance that is in contact with the soil near the building happens to be high, a break in the
pipe at even some distance from the building may pose a hazardous condition to unsuspecting workmen. Some
water utilities are inserting non-conductive couplings in the water mains at the point of entrance to buildings to
prevent such possibilities.
For these reasons, the water system should not be relied upon as a safe and
dependable earth electrode for a facility and should be supplemented with at least one other ground system.
2.5.8 Incidental Metals.
There may be a number of incidental, buried , metallic objects in the vicinity of the
earth electrode subsystem. These objects should be connected to the system to reduce the danger of potential
differences during lightning or power fault conditions: their connection will also reduce the resistance to earth
of the earth electrode subsystem.
Such additions to the earth electrode subsystem should include the rebar in
concrete footings, buried tanks, and piping.
2.5.9 Well Casings.
Well casing can offer a low resistance contact with the earth. In some areas, steel pipe
used for casing in wells can be used as a ground electrode.
Where wells are located on or near a site, the
resistance to earth of the casing should be measured and, if below 10 ohms, the well casing can be considered
for use as a ground electrode.
2-16
MIL-HDBK-419A
2.6 RESISTANCE PROPERTIES.
2.6.1 Simple Isolated Electrodes.
2.6.1.1 Driven Rod. The resistance to earth of the vertical rod in homogeneous earth can be developed by
approximating the rod as a series of buried spherical elements (2-3). When the contributions of the elemental
spheres are integrated along the length of rod and its image, the resistance to earth of the vertical rod is
computed to be:
(2-15)
where
d = rod diameter, in cm,
= earth resistivity in ohm-cm,
= rod length, in cm.
An inaccuracy in the derived result arises from the assumption that equal incremental currents flow from the
incremental spheres. Actually, more current per unit length flows into the soil near the earth’s surface than at
the lower end of the rod. It has been found empirically that the expression
(2-16)
is a better approximation to the resistance to ground for a driven vertical rod. The net difference in resistance
as given by Equations 2-15 and 2-16 is about 10 percent.
The resistance of the rod is directly affected by changes in the length of the rod and by the logarithm of the
length. Changes in the diameter only show up as slight changes in the logarithm in Equation 2-15 and 2-16.
Figures 2-6 and 2-7 show the measured changes in resistance that occurs with rod length and rod diameter. It
is evident that effects of rod length do predominate over the effects of rod diameter.
2-17
MIL-HDBK-419A
Figure 2-6. Effect on Rod Length Upon Resistance. (2-6)
Figure 2-7. Effect of Rod Diameter Upon Resistance (2-6)
The earth surrounding the rod can be depicted conveniently as consisting of shells of earth of uniform thickness,
as shown in Figure 2-8. The incremental resistance (in the direction of current flow) of each shell is given by
(2-17)
2-18
MIL-HDBK-419A
which is a special form of Equation 2-1.
The soil resistivity is
and dr is the incremental path length in the
direction of current flow. The shell of earth nearest the electrode has the smallest area and thus exhibits the
highest incremental resistance.
This fact has two practical ramifications.
First, lowest earth resistance is
obtained with electrode configurations which have largest areas in contact with the earth. Second, changes
which occur in the soil adjacent to the conductor have a significant effect on the electrode-to-earth contact
resistance.
For example, lightning discharge currents may heat the soil adjacent to the conductors, drying the
soil or converting it to slag and thus increasing the electrode resistance to earth. One reason for providing a
large contact area between the electrode and the earth is to minimize the current density in the soil
immediately adjacent to the electrode, thus reducing the heating of the soil.
The current which flows into the ground rod flows outward through each equipotential shell, and the potential
on the earth’s surface at a distance, x, from the rod is (2-3)
(2-18)
The ratio Ex/I is equivalent to Rx, that portion of resistance-to-ground of the rod which lies between the point
X and infinity:
(2-19)
The ratio of R x to R o i s
(2-20)
where
, d, and x are in the same units.
2-19
MIL-HDBK-419A
Figure 2-8. Earth Resistance Shells Surrounding a Vertical Earth Electrode
Equation 2-20 permits the area of influence of a single rod to be determined. For example, consider a 10-foot
long, l-inch diameter rod at distance x =
:
= 0.15
The ratio of 0.15 indicates that 85 percent of the total resistance to earth of a 10-foot long ground rod is
established within 10 feet of the rod. For a 100-foot rod, 89 percent of the grounding resistance is obtained
within 100 feet of the rod.
2-20
MIL-HDBK-419A
At a distance equal to two ground rod lengths, x = 2
= 0.08
Thus 92 percent of the resistance of a 10-foot by l-inch rod is obtained in a 20-foot radius cylinder. Similarly,
94 percent of the resistance of a 100-foot by l-inch rod is obtained in a 200-foot radius cylinder. The
resistance distribution for representative vertical electrodes is tabulated in Table 2-4.
Table 2-4
Resistance Distribution for Vertical Electrodes
Approximate
Type of Rod Electrode
3/4-inch pipe,
Total Resistance
Distance from Rod
(W
(feet)
90
6
driven 3-feet
95
12
deep
98
31
99
61
90
9
driven 5-feet
95
18
deep
98
46
99
92
3/4-inch pipe,
1-1/4-inch
90
18
driven 19-feet
95
35
deep
98
88
99
176
90
25
2-1/2-inch
pipe,
pipe,
driven 20-feet
95
69
deep
98
173
99
345
2-21
MIL-HDBK-419A
Table 2-5
2-22
MIL-HDBK-419A
2.6.1.2 Other Commonly Used Electrodes.
Table 2-5 lists a number of simple isolated earthing electrodes
along with approximate formulas for their resistance to earth. The plate and spherical electrodes are extensive
in area, whereas the vertical rod, the horizontal rod (or wire), the star, and the circle are extensive in length.
The electrodes in Table 2-5 have been ranked after being normalized for equal surface area in contact with the
earth. The order of ranking is such that the lowest resistance-to-earth electrode (the most effective) heads the
list. As an example, a circular plate lying on the earth’s surface is a more effective electrode (has a lower
resistance to earth) than a buried, horizontal rod which has the same area in contact with the earth, assuming
that the rod is buried at a depth less than 40 percent of its length.
The resistance to earth provided by horizontal conductors as a function of length is shown in Figure 2-9 for two
depths of burial.
Note that as the length is doubled, the resistance is approximately halved. The curves of
Figure 2-9 assume that the conductors are laid out in a straight line.
If the strips are coiled or curved, the
resistance tends to be higher because the cross-sectional area of the soil affected is less.
The resistance of a plate ground is dependent upon the area of the plate.
The variation of resistance as a
function of the radius of a circular plate is illustrated in Figure 2-10 for three depths of burial. These curves
are calculated for a plate in soil of uniform resistivity of 10,000 ohm-cm. Similar relationships hold for
rectangular plates; the curves as shown should be considered to indicate the behavior of resistance as a function
of area rather than as a prediction of the resistance of plate of a given area.
2.6.2 Resistance of Multiple Electrodes.
The theoretical resistance of an electrode, such as given by Equation
2-16, is obtained only at an infinite distance from the electrode. As shown in Section 2.6.1.1, however, most of
the resistance of a single electrode is obtained within a reasonable distance from the electrode. (For a vertical
rod, better than 90 percent is realized within two rod lengths.) If two or more electrodes are closely spaced,
however, the total effective resistance of neither is realized.
This interaction prevents the resistance of N
electrodes connected in parallel from being l/N times the resistance of one of the electrodes. For this reason,
the crowding of multiple vertical rods is not as beneficial in terms of dollar cost per ohm as is achievable with
fewer rods properly spaced.
If the electrodes in a multiple electrode installation are separated by adequate
distances, the interactive influence is minimized.
The separation between driven vertical ground rods in a
group of rods should not be less than the length or greater than twice the length of an individual rod.
2.6.2.1 Two Vertical Rods in Parallel.
Expressions for the resistance of multiple electrodes are more complex
than those for isolated electrodes. To illustrate, consider two rods driven into the earth with their tops flush
with the surface as shown in Figure 2-11. The two rods are electrically in parallel, but the presence of one rod
affects the resistance of the other. The resistance-to-earth of two rods (2-9) is
(2-21)
where s = spacing between rods.
2-23
MIL-HDBK-4l9A
CONDUCTOR LENGTH IN FEET
Figure 2-9. Resistance of Buried Horizontal Conductors
2-24
MIL-HDBK-419A
Figure 2-10.
2-25
MIL-HDBK-419A
Figure 2-11. Ground Rods in Parallel
2-26
MIL-HDBK-419A
For the condition of s >
For s <
,
For s =
,
,
(2-24)
If a number, N, of equal length vertical ground rods (with tops flush with the surface) are separated equally
along a straight line and connected together by an insulated conductor at the tops of the rods, the resultant
resistance will be somewhat greater than l/N times the resistance of single isolated rod. For N rods of length
at spacing s, the total resistance R N is given by
(2-25)
where r is the radius of each rod.
2.6.2.2 Square Array of Vertical Rods.
The resistance of a square array of rods is
R
r
Resistance of one rod
Number of rods in array, N
=
=
Rone rod
N
x Resistance ratio, K
(2-26)
K.
Figure 2-12 shows the value of K for a square array of N equally spaced, equal length rods at spacings up to 10
times a rod length. The distance from a rod to its closest neighbor in the array is s, and the various curves in
Figure 2-12 correspond to values of s, stated as integral multiples of rod length. To illustrate the use of Figure
2-12, consider a 5 by 5 array of 25 rods, each spaced one length from its closest neighbor. From the s =
curve, it is found that the resistance ratio is 2.8 for a 25-rod group. The parallel resistance of the 25 rods is
therefore 2.8 times one twenty-fifth (l/N) of the resistance that one of the rods would exhibit if isolated.
2-27
MIL-HDBK-419A
NUMBER OF RODS IN ARRAY, N
Figure 2-12. Ratio of the Actual Resistance of a Rod Array to the Ideal Resistance of N Rods in Parallel
2-28
MIL-HDBK-419A
2.6.2.3 Horizontal Grid (Mesh).
Earth electrode subsystems for electric power stations and substations must
be designed both to provide low resistance to earth and to minimize voltage gradients at the earth’s surface (see
Section 2.8.1). A common electrode design for such applications is a grid, or mesh, of horizontal rods or wires
connected at each crossing. The resistance to earth for a square or a rectangular grid can be calculated from
the following Equation (2-3):
(2-27)
where
= earth resistivity,
Lt o t
= total length of conductors used,
= A = area covered by grid, and
De
= effective diameter of grid.
As an example, consider a square grid that has dimensions of 30.5 m x 30.5 m (100 feet by 100 feet) with
conductors spaced 3.05 m (10 feet) apart. Thus there are 100 meshes with a total conductor length of 670 m
(2200 feet). The area of the array is 929 square meters (10,000 square feet) with an effective diameter of
= 113 feet
= 3440 cm
Thus the resistance to earth, by Equation 2-27, is
R =
(2-28)
=
1 . 4 5 x 1 0 -4 + 0.15 x I0 - 4
=
1.6 x 10 -4 o h m s
2-29
MIL-HDBK-419A
2.6.2.4 Vertical Rods Connected by a Grid.
The resistance of a bed of vertical rods, interconnected with a
wire grid is (2-10)
(2-29)
where
= resistance of wire grid as given by Equation 2-27
R
L
tot
= length of conductors in grid
R = resistance of bed of rods, as found from Figure 2-12
r
R
m
= mutual resistance
of rods on grid
which
accounts
for
interaction
(2-30)
(2-31)
where
rg =
radius
of
grid
wire,
h = depth of grid, if buried, and
= length of rod,
if
the
grid
2-30
is
near
surface.
MIL-HDBK-419A
MICROSECONDS
Figure 2-13. Transient Impedance of an Earth Electrode Subsystem as a Function
of the Number of Radial Wires
2-31
MIL-HDBK-419A
2.6.3 Transient Impedance of Electrodes.
The expressions given for electrode resistance assume perfect
conductivity for the conductors of an electrode.
Such an assumption introduces very little error in the
calculation of the electrode dc resistance, but if the electrode must dissipate the impulsive energy of a
lightning stroke, its impedance as a function of time must be considered.
When a single star electrode,
containing 305 meters (1000 feet) of conductor, is subjected to a surge of lightning current, the initial value of
its effective impedance is about ten times the dc resistance (2-11).
This initial value is termed the surge
impedance. As the wave of energy propagates through the electrode system, more and more of the wire of the
electrode makes effective contact between the propagating energy and the medium which dissipates the energy.
It is clear that a given length of wire will couple lightning energy more efficiently into the earth if the
electrode is in the form of a star than if it were a single conductor. This is illustrated in Figure 2-13 where it
is indicated that as the energy surges down an electrode (at a velocity in the neighborhood of 100 meters (333
feet) per microsecond), the transient impedance of the electrode decreases and approaches the dc resistance
value.
2.6.4 Effects of Nonhomogeneous (Layered) Earth.
The previous derivations assumed homogeneous earth. A
qualitative understanding of the effects of non-uniform earth resistivity can be deduced from Figure 2-14
which illustrates the electric equipotential surfaces and current flow in layered earth when the earthing
electrode is a small hemisphere.
current.
The lines radiating outward from the earth electrode indicate the flow of
Not surprisingly, if the resistivity of the deeper layer is high, relative to the upper layer, nearly all of
the current is confined to the upper layer of earth.
2.6.4.1 Hemispherical Electrode.
An approximate expression (2-3) for the resistance to earth of a small
hemispherical electrode in layered earth is
(2-32)
where
r = hemisphere radius (assumed less than h),
h = thickness of superficial layer,
= resistivity of superficial layer,
= resistivity of deep layer.
An interesting example is the case of a superficial layer of low resistivity soil (p = 10 3 ohm-cm) over granite
(
= 106 o h m - c m ) :
(2-33)
2-32
MIL-HDBK-419A
where r and h are measured in centimeters. If h < 6.2 r, the resistance to earth will be greatly influenced by
the resistivity of the granite underlayment; if h > 6.2 r, the resistance approaches that for homogeneous earth
with
resistivity,
.
2.6.4.2 Vertical Rod.
When a vertical rod is driven through a high resistivity superficial (upper) layer into a lower resistivity subsoil,
an adjustment can be made to the resistance to earth expression for homogeneous soil by substituting a reduced
“effective length” of the ground rod. Letting
’ be the effective length (2-3)
(2-34)
where
= physical length of rod,
= resistivity of upper layer,
= resistivity of subsoil, and
h = depth of upper layer.
Note
that
if
, the effective length of the rod is reduced to
resistivity than the top layer of soil (
- h.
When the subsoil has a higher
), the current discharged through a slender vertical rod with length
equal to the thickness of the superficial layer of soil will tend to remain in the superficial layer of soil. The
“mean path” of the superficial layer current, that is the radial distance at which half the discharge current has
entered the deeper soil, is approximately (2-3)
(2-35)
If the dimensions of the earth electrode subsystem are large compared to the thickness of the upper stratum,
the upper layer becomes insignificant and the resistance to earth can be computed as through the soil were
homogeneous with resistivity equal to
, the resistivity of the subsoil.
2.6.4.3 Grids.
A useful approximation for the resistance-to-earth of a horizontally extensive electrode system is given by
Equation 2-27.
If the soil has a superficial layer with resistivity
and a subsoil with resistivity
, the resistance to earth of
a grid in the superficial layer is given by (2-3)
(2-36)
2-33
MIL-HDBK-419A
Figure 2-14. Current Distribution in Nonuniform Soil
2-34
MIL-HDBK-419A
when
the earthing resistance is approximately
(2-37)
and when
, it is approximately
(2-38)
, of the superficial layer
If, for example, the diameter, D e , of the grid equals 500 meters, the resistivity,
equals 10,000 ohm-meters, the resistivity,
, of the subsoil equals zoo ohm-meters, and the length, Ltot , of
the conductors in the grid equals 4,000 meters, then
R = 2.7 ohms
Burying the grid within the lower resistivity subsoil would reduce the resistance-to-earth to about 0.4 ohms.
Conversely, if the
= 10,000 ohm-meters, and
= 200 ohm-meters, then
R = 10 ohms
regardless of the depth of the grid.
2.7 MEASUREMENT OF RESISTANCE-TO-EARTH OF ELECTRODES.
2.7.1 Introduction. The calculated resistance of a given electrode system is based on a variety of assumptions
and approximations that may or may not be met in the final installation.
Because of unexpected and
uncontrolled conditions which may arise during construction, or develop afterward, the resistance of the
installed electrode must be measured to see if the design criteria are met.
In an existing facility, the
resistance of the electrode system must be measured to see if modifications or upgrading is necessary. Two
commonly used methods for measuring the resistance to earth of an electrode are the triangulation method and
the fall-of-potential method.
2.7.2 Fall-of-Potential Method.
This technique involves the passing of a known current between the
electrode under test and a current probe, C 2, as shown in Figure 2-15(a). The drop in voltage between the earth
electrode and the potential electrode, P 2, located between the current electrodes is then measured; the ratio of
the voltage drop to the known current gives a measure of the resistance.
(By using a voltage measuring
device - a null instrument or one having a high impedance - the contact resistance of the potential electrode
will have no appreciable effect on the accuracy of the measure merit.) Several resistance measurements are
taken by moving the potential probe, P 2, from the position of the earth electrode, along a straight line to the
Current probe, C 2, which is left in position.
The data obtained is then plotted as resistance versus distance
from the earth electrode as illustrated in Figure 2-15(b).
This is the test method recoin mended for
measurement of single rod or multi-rod earth electrode subsystems.
2-35
MIL-HDBK-419A
2.7.2.1 Probe Spacing.
Current flow into the earth (see Figure 2-8) surrounding an electrode produces shells
of equipotential around the electrode.
A family of equipotential shells exists around both the electrode under
test and the current reference probe, C 2. The sphere of influence of these shells is proportional to the size of
each respective electrode. (See, for example, Section 2.6.1.1. ) The potential probe, P 2, in Figure 2-15 provides
an indication of the net voltage developed at the earth's surface by the combined effect of these two families
of shells. If the electrode under test and the current reference probe are so close that their equipotential shells
overlap, the surface voltage variation as measured by P 2 will vary as shown in Figure 2-16(a). Since the current
flowing between the electrodes is constant for each voltage measurement, the resistance curve will have the
same shape as the voltage curve.
For close electrode spacings, the continuously varying resistance curve does
not permit an accurate determination of resistance to be made.
By locating the current reference probe, C 2, far enough away from the electrode under test to ensure that the
families of equipotential shells do not overlap, a voltage curve like that shown in Figure 2-16(b) will be obtained
to produce the type of resistance curve shown in Figure 2-15.
When the distance, D, between the electrode under test and the current reference probe is very large compared
to the dimensions of the earth electrode subsystem under test, the latter can be approximated as a hemisphere
and interaction between the two electrodes is negligible.
When these assumptions are met, the potential at a
point at distance x from the electrode under test is:
(2-39)
where
is the average soil resistivity; the minus sign indicates that the current, I, flows into C 1, and out from
C 2.
Assume that the electrode under test is equivalent to a hemisphere with radius, r. At the surface of this
hemisphere, the potential is found by letting x = r:
(2-40)
The potential difference between C 1 and P2 is the voltage that is being measured and is:
(2-41)
when x = r
2-36
MIL-HDBK-419A
POTENTIAL PROBE POSITIONS
(b)
Figure 2-15. Fall-of-Potential Method for pleasuring the Resistance of Earth Electrodes
2-37
MIL-HDBK-419A
DISTANCE
P2 FROM EARTH
ELECTRODE
(a) CLOSELY SPACED ELECTRODES
DISTANCE P2 FROM EARTH ELECTRODE
(b) WIDELY SPACED ELECTRODES
Figure 2-16. Effect of Electrode Spacing on Voltage Measurement
2-38
MIL-HDBK-419A
If the r2 is the radius of the hemisphere that is equivalent to the current probe, C 2 , and r is the equivalent
radius of the electrode under test, it is seen that when x = D - r
2
(2-42)
If D >> r2 or r
(2-43)
But the true value of resistance corresponds to
(2-44)
which is found when 0 < x < D - r 2.
In order for the measurement of
to yield the correct value of resistance to earth; it can be seen that the
error term in Equation 2-41 must be zero, i.e.,
(2-45)
x(D-r)
- (D-r) (D-x) - x (D-x) = 0
Again if D >> r
X 2 + DX - D 2 = 0
(2-46)
2-39
MIL-HDBK-419A
which can be solved as follows:
(2-47)
Thus the true value of resistance to earth corresponds to the ratio of the potential difference to the measured
current when x is 62 percent of the distance, D, from the electrode under test to the current probe, C 2. It is
important to remember that D is measured from the center of the electrode under test to the center of the
current probe and that D is large relative to the r adius of the electrode under test.
Figure 2-17 shows an example of data taken with the fall-of-potential method. The correct resistance of
13 ohms corresponds to the potential probe locati on of 27.4 meters (90 feet) which is 62 percent of the distance
to the current probe.
Resistance of the electrode under test with respect to infinity (the true definition of the resistance to earth) is
(2-48)
Thus any value of D less than infinity causes the measured resistance to be in error. The error can be estimated
by observing that
2-40
MIL-HDBK-419A
Figure 2-17. Resistance Variations as Function of Potential Probe Position in Fall-of-Potential Method (2-12)
2-41
MIL-HDBK-419A
Remembering that
is the true resistance, it is evident that if D = 5r, the error will be 25 percent, if D = 11r, the error is 10
percent; if D = 26r, the error is 4 percent, etc.
The equivalent radius of a large electrode system can be determined from
(2-49)
where
A = the area covered by the system.
Consider a rectangular grid 10 meters by 10 meters. Its effective radius is
= 5.64 meters.
For an accuracy of 90 percent, the probe C2 should be positioned at
D = 11 x 5.64
= 62 meters or 203 feet away.
A conservative estimate which leads to improved accuracy of the effective radius is that it is equal to one half
the longest diagonal dimension (D d) of the array. Thus for an accuracy of 90 percent, the location for C2 should
be
11 x (0.5 Dd) or 5.5 D d,
which is the basis for the frequently quoted rule of thumb of 5 times the longest diagonal of the area of the
electrode under test. Table 2-6 gives the percentage accuracies obtained at probe locations up to 50 times the
longest diagonal.
2.7.2.2 Extensive Electrode Subsystems (2-13).
When the earth electrode subsystem is extensive, it is
frequently difficult to locate the current probe at a distance of even five times the largest dimension and
measurements of resistance to earth are subject to large errors.
center of the subsystem may not be possible.
In addition, a connection to the electrical
Figure 2-18 shows a set of resistance curves for an extensive
earth electrode subsystem obtained at current probe spacings of up to 304 meters (1000 feet). Each curve
corresponds to a particular distance, C k , of the current probe from the point of connection to the earth
electrode subsystem. The potential probe spacing, P, is the independent variable.
2-42
MIL-HDBK-419A
Table 2-6
Resistance Accuracy Versus Probe C 2 Spacing
Accuracy
Probe Spacing
(percent)
90
5 x diagonal under test
95
10 x diagonal under test
98
25 x diagonal under test
99
50 x diagonal under test
On each curve the points corresponding to 62 percent of the distance to the current probe have been connected.
It is evident that as the current probe location is moved farther out, the 62 percent value is decreasing. The
true value of resistance can be estimated by extrapolating the connecting line to its asymptotic value. Because
none of the curves in Figure 2-18 level out, even the largest spacing of the current probe is evidently too small
for a direct reading of the resistance. Basic assumptions for the fall-of-potential measurement are that (1) the
electrode to be measured can be approximated as a hemisphere and (2) the connection to the earth electrode is
made at its electrical center.
Since the location of the electrical center may not be known or may be
inaccessible, the connection is usually made at a convenient point at a distance X (Figure 2-19) from the
electrical center, D. The distance from the true center of the electrode to the current probe (assuming the
measurements are made on a radial from the electrical center) is C k + X. The use of 62 percent point on the
curves of Figure 2-18 to determine the resistance of the earth electrode should in reality correspond to a
position of the potential probe that is 0.62 (C k + X) from the true center (D). This means that the distance, P t,
from the point of actual connection (0) to the system to the location at which the correct resistance to earth
exists will be
P t = 0.62 (C k + X ) - X
(2-50)
= 0.62 C k -0.38 X
where
P t = Distance of potential probe from point of connection to electrode when the measured
resistance is the true value of resistance-to-earth for the electrode,
2-43
MIL-HDBK-419A
C k = Current probe distance from point of connection, for the kth set of probe measurements,
and
x = Distance from electrical center of electrode system to point of connection to the
electrode system.
Figure 2-18. Earth Resistance Curves for a Large Electrode Subsystem
2-44
MIL-HDBK-419A
To determine the true resistance of the earth electrode, X is allowed to assume convenient increments from
zero to Ck. For each Ck, the value of measured resistance corresponding to the resultant P t (calculated with
Equation 2-50) is read from the curves of Figure 2-18 and plotted against X. For example, if X and C k b o t h
equal 305 M (1000 feet), considering only the right hand curve in Figure 2-18, the value of P t is 240, and R is
0.08 ohms. Next let X be 244 m (800 feet). The corresponding value of Pt is 96 m (316 feet) and r is 0.1 ohms.
In this manner, estimates of the 62 percent values can be taken from Figure 2-18 and replotted as “true”
resistance versus X, as shown in Figure 2-20.
At the region of intersection of the curves in Figure 2-20, the
value of X = 122 m (400 feet) corresponds to the electrical center of the electrode, and the corresponding value
of resistance (0.13 ohms) is the true value of resistance-to-earth of the electrode system. It is recommended
that the distance to the current probe, “C”, from the point of connection to the earth electrode, “O”, (see
Figure 2-19) be between one and two times the length of the longest side of the electrode system.
Furthermore, failure to obtain a well defined region of intersection of the curves can result if the probe
measurements are not taken on a radial from the electrical center, in that case, new probe directions will be
required.
2.7.2.3 Test Equipments. Test equipments are presently available which will permit the accurate
measurement of ground resistances of earth electrode subsystems from 0.01 to 20,000 ohms and above. Most
equipments used in conducting these measurements are designed to utilize ground test currents other than dc or
60 Hz to avoid or eliminate the effects of stray ac or dc currents in the earth.
Figure 2-19. Earth Resistance Curve Applicable to Large Earth Electrode Subsystems
2-45
MIL-HDBK-419A
2.7.3 Three-Point (Triangulation) Method.
In this method, illustrated in Figure 2-21 the resistances of the
electrode under test (Rx) and the auxiliary electrodes (R a, Rb ) are measured two at a time.
The unknown
resistance is then computed from the formula.
(2-51)
where the terms in the parenthesis are the following measured resistances:
(2-52)
=
voltage drop from test electrode, X, to electrode A, divided by
current entering test electrode, X,
(2-53)
=
voltage drop from test electrode to electrode B, divided by
current into test electrode, X,
(2-54)
=
voltage drop from electrode A to electrode B, divided by current
entering electrode A.
For best accuracy, it is important to use auxiliary electrodes with resistances of the same order of magnitude
as the unknown. The series resistances may be measured either with a bridge or with a voltmeter and ammeter.
For the three-point
Either alternating or direct current may be used as the source of test current.
measurement, the electrodes must be at some distance from each other; otherwise absurdities such as zero or
even negative resistances may arise in the calculations. In measuring a single 3 meter (I0-foot) driven ground
rod, the distance between the three separate ground electrodes should be at least 5 meters (15 feet), with a
preferable spacing of 8 meters (25 feet) or more. For larger area grounds, which are presumable of lower
resistances, spacing on the order of the dimensions of the ground field is required as a minimum. This method is
most effective for measurement of single rods and is not recommended for multi-rod earth electrode
subsystems.
2-46
MIL-HDBK-419A
Figure 2-20. Intersection Curves for Figure 2-18
2.8 OTHER CONSIDERATIONS.
2.8.1 Surface Voltages Above Earth Electrodes.
Very large currents can be conducted into earth electrodes
whenever power line faults or lightning strikes occur.
As a result, there is a substantial voltage developed at
the surface of the earth near the electrode; this voltage varies significantly with distance from the electrode
connection point. The voltage difference between two points about three feet apart on the surface is the “step
voltage”, i.e., it is the voltage level between the feet of a person standing or walking on the surface.
2.8.1.1 Step Voltage Safety Limit. The maximum safe step voltage depends upon the duration of the
individual’s exposure to the voltage and upon the resistivity of the earth at the surface. The maximum safe step
voltage for a shock duration of from 0.03 to 3.0 seconds has been expressed (2-3) as
V
step (safe) =
2-47
(2-55)
MIL-HDBK-419A
Figure 2-21. Triangulation Method of Measuring the Resistance of an Earth Electrode
2-48
MIL-HDBK-419A
where
=
surface
=
t
=
earth
resistivity,
(ohm
-
meters),
10 for a minimum value,
duration of shock (see).
For a 30 millisecond or shorter duration, the maximum safe step voltage is 1000 volts, and for durations greater
than 3 seconds, it is 100 volts.
2.8.1.2 Step Voltages for Practical Electrodes.
The expressions for step voltage estimates in homogeneous
soil for both flush and buried vertical rod electrodes and for buried grid electrodes are given in the following
paragraphs.
It should be noted that step voltages depend upon electrode geometry as well as upon earth
resistivity and current magnitude.
2.8.1.2.1 Flush Vertical Rod.
The potential on the earth at a distance x from the top of a single, isolated
flush-driven vertical rod is (2-3)
(2-56)
and the potential of the rod itself is
(2-57)
The step potential at the ground rod (where p is equal to a pace, or step, length from the rod) is therefore
(2-58)
2-49
MIL-HDBK-419A
When the step length is much less than the rod length, i.e., when
, the step voltage can be approximated
as
(2-59)
The step potential can be expressed as a fraction of the ground rod potential as follows:
(2-60)
The fractional step voltages for ground rods of various length are given in Table 2-7. For this Table, rod
diameter is assumed to be one inch (2.54 cm) and the pace length is assumed to be three feet (0.91 m).
Table 2-7
Step Voltages for a Buried Vertical Ground Rod
Rod Length
Ratio of Step Voltage
(Ft)
To Electrode Potential
5
0.75
10
0.68
20
0.61
50
0.53
100
0.48
2-50
MIL-HDBK-419A
The step voltage near the 10-foot by l-inch (3.05 m x 2.54 cm) rod in 1000 ohm-cm soil is about 68 percent of
the voltage between the rod and a point approaching an infinite distance away. Step voltage near a ground rod
will be between 80 percent of the rod potential (for very short rods) and 50 percent (for very long rods).
The step voltage on the surface of the earth near an isolated 10-foot by l-inch (3.05 m x 2.54 cm) ground rod (
= 1000 ohm-cm) carrying a lightning current of 20,000 amps could be fatal since the step voltage would be
v
o
–
v =
p
(O.366)
(l0)
(1000)
(12)
(20,000)
(2.54)
(2-61)
x
=
( 2 . 4 x 1 04 )
=
41,352
(1.723)
volts,
which is 41 times higher than the safe step voltage derived above.
The resistance of the 10-foot by l-inch (3.05 m x 2.54 cm) rod in 1000 ohm-cm soil is
(2-62)
366)
(103 )
(10) (12) (2.54)
= (O.
log
36,
= 1.2 log 360,
=
(1.2)
(2.556),
= 3.1 ohms.
Higher values of earth resistivity would cause the step voltage near the rod to be even higher than the
calculated 41,400 volts. For a three second duration shock condition, the requirement that the step voltage not
exceed 100 volts means that the single 10-foot by l-inch (3.05 m x 2.54 cm) rod would produce an unsafe step
voltage with a fault current greater than about 50 amperes, even in low resistivity (1000 ohm-cm) soil.
2-51
MIL-HDBK-419A
Figure 2-22. Variation of Surface Potential Produced by a Current
Flowing Into an Isolated Ground Rod
2-52
MIL-HDBK-419A
2.8.1.2.2 Buried Vertical Rod.
If the single isolated vertical rod is driven so that the top of the rod is below
the surface, the maximum step voltage on the surface of the earth is reduced. Figure 2-22 shows the surface
voltage variation for a flush driven rod compared with that for a rod with its top below the surface. Maximum
gradient for the flush driven rod is at the vicinity of the rod. Maxi mum gradient for the rod sunk into the earth
to a depth of h feet occurs at a distance of 3 h to 4 h from the rod (2-3). The step voltage for the rod driven so
that its top is h feet below the surface is:
For
= 1 03 o h m - c m ,
20,000
I0 =
amperes,
= 10 feet (3.05 m),
x = 3h feet, and
P = 3 feet (0.91 m)
If
h
=
3
feet,
the
maximum
step
voltage
= ( 2 . 4 X 1 04 )
=
instead
rod.
of
41,400
1504
volts, which
is
approximately
0.063
volts,
was
characteristic
2-53
of
the
flush–driven
MIL-HDBK-419A
Figure 2-23.
2-54
MIL-HDBK-419A
2.8.1.2.3 Buried Horizontal Grid.
Section 2.6.2.3.
An expression for the resistance to earth for a buried grid was presented in
Equations 2-27 and 2-28 are the sum of a resistance of a superficial plate ( / 2 D e) and a
resistance term representing the per unit diffusion resistance of the earth electrode material (pI/L). A voltage
pI/L which is proportional to the per unit average current flowing from the conductors of the mesh into the
earth represents an approximation of the potential difference between the conductors of the mesh and the
center of the open space with each mesh.
The sketch of Figure 2-23 shows the resultant voltage distribution
across a section of a grid. Note that the approximation used here would predict that
(2-65)
is the minimum voltage (with respect to infinity) at the edge of the grid, so that the grid simply translates the
dangerous voltage gradient to the periphery of the grid (2-3).
If the value of earth resistivity is moderately high--say 104 ohm-cm--and if the lightning current is 2 x 10 4
amperes, the grid in the example of Section 2.6.2.4 would exhibit
(2-66)
= 3000 volts
over a five-foot (1.5 m) distance. This would exceed the safe step voltage of 1000 volts, developed earlier.
If the grid is made of conductors spaced one foot apart for a total conductor length of 20,200 feet (6157 m)
there would be 10,000 meshes on the 10,000 square foot (929 m2) area. The effective diameter would still be
113 feet (34.4 m), and the computed resistance would be
(2-67)
The maximum step potential difference over the grid of the latter case, again assuming
effective lightning current of 20,000 amperes, would be
2-55
is 104 is ohm-cm and an
MIL-HDBK-419A
=
( 1 0 4 ) ( 2 x 1 04 )
(62)
=
322
( 1 04 )
volts
This would be a safe value of step voltage for transients shorter than 30 milliseconds, if the transient, or surge,
Impedance of the line does not greatly exceed its steady state resistance.
2.8.1.3 Minimizing Step Voltage.
Table 2-8 lists several design approaches to reducing the potential hazards
of step voltage. The most effective method is the reduction of the resistance to earth of the earth electrode
system to as low a value as is economically feasible.
Table 2-8
Methods of Reducing Step Voltage Hazards
Remarks
Design Approach
1.
2.
3.
4.
Minimize resistance to earth of electrode
Resistance to earth is directly proportional to soil
system.
resistivity.
Bury earth electrode to reduce maximum
Connection to earth electrode must be insulated to
gradient on surface of earth.
withstand 5 x 10 4 R O volts.
Bury a grid beneath the earth, surrounding
Tends to equalize the surface potential over area of
the earth electrode.
grid.
Erect barricade so that personnel cannot
Fence must be grounded
enter area of danger.
2-56
MIL-HDBK-419A
It is necessary to use enough material in an earth electrode to prevent excessive
2.8.2 Heating of Electrodes.
local heating when large currents flow in the electrode.
2.8.2.1 Steady State Current.
The presence of fault current in the earth electrode subsystem must be limited
to a value which will not raise the temperature of the soil above the boiling point of water. The tolerable
steady state ac current into an earth electrode is (2-3).
(2-68)
where
= earth resistivity,
R
For
= electrode resistance to earth, and
T
= permissible temperature rise (°C).
T
= 60°C, the permissible steady state current is limited by
when
= 106 ohm-cm, and by
when
= 103 ohm-cm. Since the voltage at the earth electrode is equal to the product I R, the corresponding
voltage limits are
and
E ss < 38 volts,
2.8.2.2 Transient Current.
= 10 3 o h m - c m
The permissible transient current density for a temperature rise that does not
exceed 60°C is found from the transient temperature time expression (2-3):
(2-69)
2-57
MIL-HDBK-419A
Where
t = duration of the transient, in second,
= temperature rise,
= soil resistivity, ohm-cm, and
i = transient current density.
Letting
T = 60 0C , o n e h a s
The current density, i, at the surface of a short ground rod is approximately constant over the lengh of the rod
and is given by
(2-70)
where
d = rod diameter (cm),
= rod length (cm), and
I = input current (amperes).
For a 10- foot by 1-inch rod (3.05 m x 2.54 cm), the peak transient current which can be handled without causing
greater than 60 0 C temperature rise is:
(2-71)
2-58
MIL-HDBK-419A
2.8.2.3 Minimum Electrode Size.
The necessity to hold the surface temperature below boiling temperatures
establishes a minimum amount of electrode material.
The minimum length of a single ground rod is
(2-72)
The value of I
is approximately 1000 for both lightning stroke currents and power system fault currents, so
for satisfactory energy dissipation the minimum rod length is specified by
(2-73)
If the earth is moist soil with a
of approximately 104 ohm-cm, the limit becomes
(2-74)
In granite with a
of approximately 10 6 ohm-cm, the limit becomes
(2-75)
If 2-cm rods are used, the safe dissipation of heat in granite would require at least 80 rods, each 2 meters long.
For moist earth, only 8 rods, each 2 meters long, would be required for heat dissipation.
2.9 ELECTRODE ENHANCEMENT.
2.9.1 Introduction.
Sites may be encountered where acceptable and practical numbers of driven rods, buried
cables, and other available materials will not achieve the desired low resistance to earth for special
communication systems, i.e., HF transmitters. In such situations, enhancement of the resistivity of the soil
around the electrodes may be necessary to lower the resistance to the desired value. While enhancement of the
resistivity may be required in certain situations, discretion of its use should be exercised due to the reduced life
span of the earth electrode subsystem.
The resistance to earth of an electrode is directly proportional to soil resistivity and inversely proportional to
the total area of contact established with the soil. For fixed land areas, additional vertical rods or horizontal
cables produce diminishing returns because of increased mutual coupling effects. The most straight forward
enhancement method is to reduce soil resistivity.
The parameters which strongly affect soil resistivity are
moisture content, ionizable salt content, and porosity; the latter determining the moisture retention properties
of the soil. Thus two recommended techniques for reducing earth resistivity are water retention and chemical
salting.
2-59
MIL-HDBK-419A
2.9.2 Water Retention.
Overdrainage of soil leaches away salts that are necessary for high conductivity and
dries out the deeper layers, thereby increasing their rcsistivity. Planting of appropriate ground covers, such a S
legumes, to retard runoff and to enhance the natural production of salts in the soil is useful. Surface drainage
should be channeled so as to keep the earth electrode subsystem moist. Maintaining moist earth over the extent
Drainage water which is high
of the earth electrode subsystem will keep soil salt in solution as conductive ions.
in salt content can be useful for continuous salting of the earth electrode.
A porous clay, bentonite (also known as well drillers mud) can absorb water from surrounding soil and has
hydration as well as water retention properties. When placed around ground rods and their interconnecting
cable, it greatly increases the effective area of the rod and cable which in turn reduces the resistance of the
earth electrode subsystem to earth (2-14, 2-15). Bentonite is generally available in dry (powder) form, must be
saturated with water after initial installation and should be topped with a 12-inch layer of excavated soil.
Caution is urged when using bentonite in areas that will ultimately be paved as it can expand to several times
its dry volume when saturated.
This can also prove to be a disadvantage of bentonite since it expands and
contracts so much with moisture content, it can pull away from the ground rod and surrounding soil when
moisture is lost.
A much better backfill around ground rods is a mixture of 75 percent gypsum, 20 percent
bentonite clay, and 5 percent sodium sulfate. The gypsum, which is calcium sulfate, absorbs and retains
moisture and adds reactivity and conductivity to the mixture. Since it contracts very little when moisture is
lost, it will not pull away from the ground rod or surrounding earth.
The bentonite insures good contact
between ground rod and earth by its expansion, while the sodium sulfate prevents polarization of the rod by
removing the gases formed by current entering the earth through the rod.
This mixture is available from
cathodic protection distributors as standard galvanic anode backfill and is relatively inexpensive. The backfill
mixture should be covered with 12 inches of excavated soil. This mixture is superior to chemical salts since it
is much more enduring.
2.9.3 Chemical Salting.
Reduction of the resistance of an electrode may also be accomplished by the addition
of ion-producing chemicals to the soil immediately surrounding the electrode. The better known chemicals in
the order of preference are:
a.
Magnesium sulphate (MgS0 4) - epsom salts.
b.
Copper sulphate (CuS0 4) - blue vitriol.
c.
Calcium chloride (CaC1 2).
d.
Sodium chloride (NaCl) - common salt.
e.
Potassium nitrate (KNO 3) - saltpeter.
Magnesium sulphate (epsom salts), which is the most common material used, combines low cost with high
electrical conductivity and low corrosive effects on a ground electrode or plate. The use of common salt or
saltpeter is not recommended as either will require that greater care be given to the protection against
corrosion. Additionally, metal objects nearby but not related to grounding will also have to be treated to
prevent damage by corrosion. Therefore, salt or saltpeter should only be used where absolutely necessary.
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Large reductions in the resistance to earth of the individual ground electrodes may be expected after chemical
treatment has been applied to the earth. The initial effectiveness of chemical treatment is greatest where the
soil is somewhat porous because the solution permeates a considerable volume of earth and increases the
effective size of the electrode.
In compact soils, the chemical treatment is not as immediately effective
because the material tends to remain in its original location for a longer period of time.
The effectiveness of chemical treatment in lowering the resistance of a ground rod is illustrated by Figures
2-24 and 2-25. Chemical treatment achieves a significant initial reduction of resistance and further stabilizes
the resistance variations.
It also limits the seasonal variation of resistance and, additionally, lowers the
freezing point of the surrounding soil.
Figure 2-24. Effect of Chemical Treatment on Resistance of Ground Rods
Figure 2-25. Seasonal Resistance Variations of Treated and Untreated Ground Rods
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Chemical treatment is limited in its effectiveness, however.
100 ground rods of length
Consider, for example, a square array of
with spacings of twice the length of a rod.
The resistance to earth (using an
extrapolated value of 3 for K) from Figure 2-12 is (see also Equations 2-16 and 2-26)
Assuming that
d
=
1 06 ohm-cm (gravel sand stone),
=
100 feet (30.5 m) per rod, and
=
1 inch (2.54 cm),
then
= 12.81 ohms
The upper bound on the effectiveness of chemical enhancement can be illustrated by determining tile resistance
to earth of a metal electrode which would completely fill the volume of earth (1800 x 1800 x 100 ft., i.e., 550 x
550 x 30 m) occupied by the above array of ground rods. The effective diameter, D e, of the equivalent plate
would be 2030 feet (619 m), and its resistance to earth would be (2-3):
= (2)
10
(2030)
6
(2-76)
(12)
(2.54)
= 8 ohms
The most that chemical enhancement could reduce the resistance of this large array would be by a factor of
1.58.
2.9.4 Electrode Encasement.
The calculations of resistance of earth electrodes invariably assume zero
contact resistance between the electrode elements and the earth. In reality,
however, the interface between
the surface of the rod and the earth is far from uniform except when the earth is tamped clay or its equivalent.
Granular earth (gravel, etc.) makes very poor contact.
Reduction of this contact resistance should have a
strong effect on reducing the electrode resistance because it is close to the electrode where current density is
high.
Encasing the electrode in conductive mastic or conductive concrete is one approach to improving the
contact between the electrode and the earth.
Effects of local variations or moisture content will also be
reduced and stabilized, if the encasement material absorbs and holds moisture.
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2.9.5 Salting Methods. The trench method for treating the earth around a driven electrode is illustrated in
Figure 2-26. A circular trench is dug about one foot deep around the electrode. This trench is filled with the
soil treating material and then covered with earth. The material should not actually touch the rod in order to
provide the best distribution of the treating material with the least corrosive effect.
Another method for treating the earth around a driven electrode, using magnesium sulphate and water, is
illustrated in Figure 2-27. A 2-foot length (approximately) of 8-inch diameter tile pipe is buried in the ground
surrounding the ground electrode. This pipe is then filled with magnesium sulphate to within one foot of grade
level and watered thoroughly.
The 8-inch tile pipe should have a wooden cover with holes and be located at
ground level.
None of the aforementioned chemical treatments permanently improve earth electrode resistance.
The
chemicals are gradually washed away by rainfall and through natural drainage. Depending upon the porosity of
the soil and the amount of rainfall, the period for replacement varies. Forty to ninety pounds of chemical will
initially be required to maintain effectiveness for two or three years.
Each replenishment of chemical will
extend the effectiveness for a longer period so that the future treatments have to be done less and less
frequently.
Another method of soil treatment or electrode enhancement involves the use of hollow made electrodes which
are filled with materials/salts which absorb external atmospheric moisture. These electrodes (generally 8-feet
long) must be placed in holes drilled by an earth auger making sure the breather holes at the top are above
grade level. Moisture from the atmosphere is converted to an electrolyte which in turn seeps through holes in
the electrode into the surrounding soil.
electrode to earth.
This keeps the soil moist and thereby reduces the resistance of the
These electrodes should be checked annually to ensure sufficient quantities of
materials/salts are available and that good continuity exists between the rod and interconnecting cable.
2.10 CATHODIC PROTECTION.
2.10.1 Introduction. When two metals of different types are immersed in wet or damp soil, a basic electrolytic
cell is formed.
A voltage equal to the difference of the oxidation potentials of the metals will be developed
between the two electrodes of the cell.
If these electrodes are connected together through a low resistance
path, current will flow through the electrolyte with resultant erosion of the anodic member of the pair.
Unfortunately, those factors that aid in the establishment of low resistance to earth also foster corrosion. Low
resistance soils with a high moisture level and a high mineral salt content provide an efficient electrolytic cell
with low internal resistance.
Relatively large currents can flow between short-circuited electrodes (such as
copper ground rods connected to steel footings or reinforcing rods in buildings) and quickly erode away the more
active metal (see Section 7.8.1.2) of the cell. In high-resistance cells, the current flow is less and the erosion is
less severe than in low-resistance cells.
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SOIL TREATING MATERIAL
PLACED IN CIRCULAR TRENCH
AND COVERED WITH EARTH
Figure 2-26. Trench Method of Soil Treatment
REMOVABLE
COVER
Figure 2-27. Alternate Method of Chemical Treatment of Ground Rod
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2.10.2 Protection Techniques.
Three basic techniques can be used to lessen the corrosion rate of buried metals. The obvious method is to
Insulate the metals from the soil by the use of protective coatings. This interrupts the current path through the
electrolyte and stops the erosion of the anode.
for earth electrodes.
metals at a site.
Insulation, however, is not an acceptable corrosion preventive
The second technique for reducing galvanic corrosion is avoiding the use of dissimilar
For example, if all metals in contact with the soil are of one type (such as iron, lead or
copper), galvanic corrosion is minimized.
Each of these materials, however,
has unique properties such as
weight, cost, conductivity, ductility, strength, etc., that makes its use desirable, and thus none can be
summarily dismissed from consideration for underground applications.
Copper is a desirable material for the
earth electrode subsystem; apart from its high conductivity, the oxidation potential of copper is such that it is
relatively corrosion resistant.
Since copper is cathodic relative to the more common structural metals, its
corrosion resistance is at the expense of other metals.
Iron electrodes would, of course, be compatible with
water pipes, sewer lines, reinforcing rods, steel pilings, manhole covers, etc., but iron is subject to corrosion
even in the absence of other metals. In addition, the conductivity of iron is less; however, steel grounding rods
are sometimes used by electric utilities for grounding associated with their transmission lines. Because of the
greater conductivity and corrosion resistance of copper, it is normally used for the grounding of buildings,
substations, and other facilities where large fault or lightning currents may occur and where voltage gradients
must be minimized to ensure personnel protection.
The third technique for combating the corrosion caused by stray direct currents and dissimilar-metal unions is
commonly called cathodic protection. Cathodic protection may be implemented through the use of sacrificial
anodes or the use an an external current supply to counteract the voltage developed by oxidation. Sacrificial
anodes containing magnesium, aluminum, manganese, or other highly active metal can be buried in the earth
nearby and connected to an iron piling, steel conduit, or lead cable shield. The active anodes will oxidize more
readily than the iron or lead and will supply the ions required for current flow. The iron and lead are cathodic
relative to the sacrificial anodes and thus current is supplied to counteract the corrosion of the iron or lead.
The dc current is normally derived from rectified alternating current, but occasionally from photovoltaic cells,
storage batteries, thermoelectric generators, or other dc sources.
Since the output voltage is adjustable, any
metal can be used as the anode, but graphite and high silicon iron are most often used because of their low
corrosion rate and economical cost. Cathodic protection is effective on either bare or coated structures. If the
sacrifical anodes are replenished at appropriate intervals, the life of the protected elements is significantly
prolonged.
2.10.3 Sacrificial Anodes. Sacrificial anodes provide protection over limited areas. Impressed current cathodic
protection systems use long lasting anodes of graphite, high silicon cast iron or, to a lesser extent, platinum
coated mobium or titanium. The protection of long cable or conduit runs can be provided by biasing the metal
to approximately -0.7 to -1.2 volts relative to the surrounding soil.
The external dc source supplies the
ionization current that would normally be provided by the oxidation of the cable sheath or conduit. This dc
current is normally derived from rectified ac and occasionally from photovoltiac cells, storage batteries,
thermoelectric generators, or other dc sources. A layer of insulation such as neoprene must cover the metal to
prevent direct contact with the surrounding soil.
Therefore, the technique is not appropriate for protecting
foundations, manholes, or other structural elements normally in contact with the soil. It is most appropriate for
supplying the leakage current that would normally enter the soil through breaks in the insulation caused by
careless installation, settling, lightning perforation, etc.
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2.10.4 Corrosive Atimospheres.
In regions exhibiting low soil resistivity, in corrosive atmospheres such as
might be encountered near seashores, or near sources of large direct currents such as electroplating facilities,
cathodic protection may be necessary to prolong the life of foundations, underground cable facilities, or other
elements of a facility in contact with the soil.
For additional information on the galvanic series of common
metals see Table 7-7.
2.11 GROUNDING IN ARCTIC REGIONS.
2.11.1 Soil Resistivity.
The problem of electric earth grounding in cold regions is primarily one of making
good contact with high resistivity soils.
Where frozen high resistivity materials are encountered, optimum
grounding of power and communication circuits can only be accomplished by special attention to both surface
and subsurface terrain. The fact is that resistance of frozen soils can be ten to a hundred times higher than in
the unfrozen state.
resistance.
Seasonal changes in temperature and moisture will therefore extensively affect the soil
Table 2-9 provides information on the effect of moisture content on earth resistivity, while Table
2-10 provides the effect of temperature on earth resistivity (2-16, 2-17).
Table 2-9. Effect of Moisture Content on Earth Resistivity
Moisture Content
% By Weight
Resistivity, ohm-cm
Top Soil
1,000 x 104
0
Sandy Loam
1,000 x 10 6
2.5
250,000
150,000
5
165,000
43,000
10
53,000
18,500
15
17,000
10,500
20
12,000
6,300
30
6,400
4,200
Table 2-10. Effect of Temperature on Earth Resistivity *
Resistivity
Temperature
°C
20
10
°F
ohm-cm
68
7,200
50
9,900
0
32 (water)
13,800
0
32 (ice)
30,000
-5
23
79,000
-15
5
330,000
*For sandy loam, 15.2% moisture.
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Permafrost occurs in various degrees throughout much of the arctic and subarctic regions and is defined as that
part of the lithosphere (upper crust of the earth) in which a naturally occurring temperature below 0°C (32°F)
has existed continuously for two or more years. The "annual frost zone" is the zone of annual freezing and
thawing. Where permafrost occurs, the thickness of this surface layer varies from less than afoot in the arctic
to depths in excess of 12 feet in the subarctic. The seasonal thaw zone remains unfrozen only during the short
During this period, it is possible to recognize terrain features which can be located in the
summer months.
spring and fall if there is little or no snow cover.
Willow groves or aspen generally point to the absence of permafrost and to the presence of groundwater which
freezes only for a short time. River bottoms and lake bottoms are usually frost-free. Generally, slow moving
rivers and streams freeze from the top down (surface ice). Clear, fast moving rivers and streams usually freeze
from the bottom up (anchor ice).
Mountains, valleys, lake bottoms, streambeds, tree-covered slopes, tundra
plains, swamplands, ice glaciers, silty estuaries, permafrost areas, and seasonably frozen ground, each will be
found to affect soil resistivity.
suitable for good grounding.
Consequently, it is easily seen how one area versus another might be more
Basic illustrations of variations, layering and asymmetrical contouring can be
found in Figures 2-28, 2-29, and 2-30.
Resistance to ground and configuration of electrodes are further parameters that must be considered.
The
conductivity of cables and overhead wire systems are relatively high in comparison to the earth. Without the
presence of minerals, dissolved salts, and moisture, clean dry soil can be classified as an insulator and possesses
the intermediate characteristics of a poor conductor.
Figure 2-28. Relative Depths of Unconsolidated Materials, Subarctic Alaska
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Seasonal freezing accounts for a reduced conductivity as illustrated in Table 2-10. If frozen soil or earth has a
low conductivity, then providing larger effective electrodes will reduce the ground resistance. In northern
arctic areas generally having very shallow surface thaw Iayers, horizontal rods or wires might be easier to
install than driven rods and still provide optimum resistance values to earth or ground. Whether to install
multiple electrodes or single deep, driven rods , or horizontal wires, the decision will usually be dependent on
soil types and the economics of placement.
Figure 2-29. Typical Sections Through Ground Containing Permafrost
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Figure 2-30. Illustration Showing Approximate Variations in Substructure
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2.11.2 Improving Electrical Grounding in Frozen Soils.
High electrical resistance of grounding sites is
common in areas where the ground freezes. The performance of grounding installations can, however, often be
increased through site selection and various electrode installation schemes.
depend on the local existence and accessibility of conductive soils.
The degree of improvement will
The most common conductive sites are
associated with thaw zones or clay-rich soils. The greatest grounding problems usually occur where bedrock,
coarse-grained soil, or cold, ice-rich soil is found near the surface.
In temperate regions, small field installations can usually be adequately grounded by driving a simple vertical
electrode into the soil.
This technique has been unsuccessful in areas of frozen ground because: (1) driving
electrodes is difficult, (2) frozen materials tend to be electrically resistive, and (3) high contact potentials can
develop between a rod and the frozen soil because a thin ice layer can form around the cold rod.
Installation procedures can be modified in some frozen ground settings to eliminate some of these problems,
permitting order-of-magnitude reductions in the resistance to ground. However, in many regions of the Arctic,
electrical resistivity of the frozen ground is extremely high, and grounding may not be significantly improved by
local modification or treatment of the soil surrounding the electrode. Achieving “low” resistance grounds of
less than several ohms will often require that the site be selected in a zone of conductive material and is
described in paragraph 2.11.1.
Other factors such as accessibility to water, power, roads, real estate, siting requirements, electromagnetic
compatibility, etc, may however require that a site be located in an area of low soil conductivity. This
establishes the rather high probability of not being able to attain a low resistance to ground without
considerable cost and effort.
Studies (2-17) conducted to determine methods to obtain low or acceptable
resistances in areas of low soil conductivity in turn raised additional questions:
a.
What is the influence of ground temperature, material type and associated variations in unfrozen
water content on the performance of an installation?
b.
What is the influence of material type and associated differences in permeability and saturation on
salt solutions added to the soil surrounding an electrode?
c.
What is the effectiveness of using more than one electrode for lowering resistance to ground?
d.
What is the long-term influence of conductive backfills and what is the suitability of various
materials for backfill around electrodes placed in holes of larger diameter than the electrodes?
The main procedure which can be used to reduce resistances to ground is to place the ground rod or electrode in
open holes having diameters greater than the electrodes thereby making emplacement easier and permitting the
usc of conductive backfill. The holes can be made by drilling or blasting with shaped charges. Another
procedure which may be used in limited situations is to lay or drive an array of horizontal rods into an active
layer.
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2.11.2.1 Electrode Resistance.
and radius
The resistance of ground, R, of a single vertical electrode of length
in cm, emplaced in homogeneous soil of resistivit
in cm,
(ohm - cm) is found from:
(2-77)
This equation may be used to estimate the penetration depth of conductive salt solutions in the soil adjacent to
the treated backfill. Since the backfill is conductive, the electrode radius therefore is not just that of the
metallic electrode, but initially the diameter of the hole filled with treated backfill. This large composite
electrode is referred to as the effective electrode.
For a constant ground temperature, any reduction in
electrode resistance of a frozen saturated soil with time should be related to an increase in effective electrode
diameter, presumably through salt movement. This increase can be determined by the soil resistivity from
equation 2-77 using the resistance to ground of the test electrode and the effective electrode radius measured
at the time of installation. Periodically, after installation, the resistance to ground should be remeasured and
the effective electrode radius can be calculated using the following form of equation 2-77 and using the soil
resistivity calculated earlier:
(2-78)
2.11.2.2 Installation and Measurement Methods.
2.11.2.2.1 Electrode Installation.
Holes can be drilled with augers designed for use in frozen ground with hole
diameters ranging from 3.8 cm (1-1/2 in.) to 91.4 cm (3 ft) and depths seldom greater than 2 m (6 ft).
Hand-
held equipment, consisting of an electric drive or a 5-hp gasoline-powered drill can also be used for most of the
shallow, smaller-diameter holes. Both units could be used with a coring auger to drill holes up to 11 cm (4 in.)
in diameter in fine-grained frozen soils.
A truck-mounted auger can be used for the larger-diameter vertical
holes drilled in coarse-grained materials. The horizontal electrodes can be hand-pushed and then driven into the
thin seasonally thawed layer.
Military 6.8 kg (M2A3) shaped charges (used only by qualified personnel) can also be employed to produce
vertical holes. Their similar performances in a range of frozen materials, with penetration approaching the
length of standard electrodes, make this charge size ideal for electrode installation. The volume of several of
the drilled holes can also be expanded by using C-4 block explosives.
2.11.2.2.2 Backfill.
Reduction of contact potential is important in establishing a good electrical ground. In
frozen soil, ice can form around the electrode, causing high contact resistance. Ice formation on the rod
surface is likely since the rod is easily chilled by exposure of the upper end to low air temperatures.
The
beneficial effect of pouring untreated water around an electrode will only be short-term in cold environments.
Therefore, the use of conductive backfill with a low freezing point becomes paramount to attain good ground or
earth contact.
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The backfill can be prepared by mixing salt and local soil or by saturating the soil backfill with a salt-water
solution as shown in Figure 2-31.
Backfill other than soil can also be used because soil is not always easily
recovered from some drilled or blasted holes and because unfrozen material is difficult to find during the
winter.
Absorbent paper saturated with a salt solution and compacted in the hole around the electrode can also
be used as a soil substitute.
The amount of salt added to the backfill is determined by preliminary laboratory conductivity measurements of
several salt-soil mixtures. Salt may be added to silt and to a fine sand to obtain mixtures of from 0 to 20% salt
based on the weight of the air-dried soil.
Distilled water can be added to the salt-soil mixtures to obtain
several soil moisture levels up to saturation for both materials. The soils should be compacted into a cylindrical
plexiglass ring, which is clamped between electrodes for resistivity measurements at 1 kHz. Figure 2-32 shows
the resistivity for two soils as a function of salt concentration at several volumetric moisture contents. A saltsoil mixture containing 1% salt results in a dramatic decrease in resistivity, with little effect after 5% salt for
most moisture levels.
Therefore, a 5% salt by weight is recommended for backfill as it produces a very
conductive salt-soil mixture with the least amount of salt.
Figure 2-31. Installation of an Electrode During the Process of Backfilling with a Salt-Soil Mixture
bait solution may also be poured around shallow-driven horizontal electrodes to minimize contact resistance
during freezeback.
These salt solutions in general may have concentrations on the order of 50-100%.
Figure 2-33 shows a configuration of such horizontal electrodes placed in a thawed active layer.
Curves showing resistance-to-ground for metallic electrodes having various backfills are shown in Figures 2-34
through 2-38. Large seasonal variations are noted in electrode performance due to variations in unfrozen water
content in both thawed and frozen materials.
In some situations the improvement in grounding conditions
during thaw periods can be extended by use of conductive backfill. The lower freezing point of the backfill will
also reduce electrode contact resistance caused by freezing around the metallic electrodes.
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.
MIL-HDBK-419A
Over a period of time, salt very likely will move into the soil adjacent to the electrode backfill and therefore
will increase the effective area of the ground electrode and in turn reduce the resistance values. The level of
the backfill should be checked annually to insure adequate levels are maintained to replenish this loss due to
seepage.
Figure 2-32. Apparent Resistivity for Two Soils at Various Moisture and Salt Contents
Figure 2-33.
Configuration of Nearly Horizontal
Figure 2-34.
Electrodes Placed in the Thawed Active Layer
Resistance-to-Ground Curves for
an Electrode Driven into Ice-Rich Silt
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Figure 2-35.
Figure 2-36.
Resistance-to-Ground Curves for an
Resistance-to-Ground Curves for
Electrode Surrounded by a Backfill of Saturated
an Electrode Surrounded by a Water-Saturated
Silt
Salt-Soil Backfill
Figure 2-38.
Resistance-to-Ground Curves for
Figure 2-37. Resistance-to-Ground Curves for an
Electrode Surrounded by a Water-Saturated
3 Electrodes Placed in Holes Modified by Spring
Salt-Soil Backfill
Charges and Filled with a Salt-Water Solution.
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2.12 REFERENCES.
2-1. N.M. Towne, "Lightning -- Its Behavior and What To Do About It." United Lightning Protection Assn, Inc,
Ithaca NY 1956.
2-2. National Electrical Code,
1987 Edition, NFPA No. 70-1987, National Fire Protection Association,
Battery march Park, Quincy MA 02269.
2-3. Guide for Safety in Alternating-Current Substation Grounding, IEEE Std 80-1961, IEEE, New York NY,
(R 1971).
2-4. R. Rudenberg, "Fundamental Considerations on Ground Currents," Electrical Engineering, Vol 64, pp 1-13,
January 1945.
2-5. “Earthing,” British Standard Code of Practice CP 1013: British Standards Institution, London.
2-6. O.S. Peters, “Ground Connections for Electrical Systems,” Technological Paper No. 108, US National
Bureau of Standards, 20 June 1918.
2-7. "Getting Down to Earth . . ." Manual 25T, James G. Biddle Co, Plymouth Meeting PA, October 1970.
2-8. F. Wenner, “A Method of Measuring Earth Resistivity, " Bulletin of the Bureau of Standards, Vol 12,
pp 469-478, 1915-1916.
2-9. H. B. Dwight, “Calculation of Resistance to Ground,” Elec. Engr, Vol 55, December 1936, pp 1319-1328.
2-10. E. D. Sunde, Earth Conduction Effects in Transmission Systems, Dover Publications, Inc, New York NY,
1968.
2-11. W. W. Lewis, The Protection of Transmission Lines Against Lightning, John Wiley, New York, 1950.
2-12. Installation Practices:
Communications Systems Grounding, Bonding and Shielding, Army FM 11-487-4,
Air Force T.O. 31-10-24, Dept of Army and Air Force, September 1978.
2-13. G. I3. Tagg, “Measurement of the Resistance of an Earth-Electrode System Covering a Large Area,” Proc.
IEEE, Vol 116, March 1969, pp 475-479.
2-14. Warren R. Jones, "Bentonite Rods Assure Ground Rod Installation in Problem Soils,” IEEE Transactions on
Power Apparatus and Systems, Vol PAS-99 No. 4, July/Aug 1980.
2-15. Lloyd B. Watts, “Improved Grounding Systems for Mountain Top Radio Sites.”
2-16. P.V. Sellman, A.J. Delaney, and S.A. Arcone, Conductive Backfill for Improving Grounding in Frozen Soils,
Special Report 84-17, June 1984. US Army Corps of Engineers - CRREL.
2-17. A.J. Delaney, P.V. Sellman, and S.A. Arcone, Improving Electrical Grounding in Frozen Materials, Special
Report 82-13, June 1982, US Army Corps of Engineers - CRREL.
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CHAPTER 3
LIGHTNING PROTECTION SUBSYSTEM
3.1 THE PHENOMENON OF LIGHTNING.
Cumulonimbus clouds associated with thunderstorms are huge, turbulent air masses extending as high as
15 to 20 kilometers (9 to 12 miles) into the upper atmosphere.
Through some means, not clearly understood,
these air masses generate regions of intense static charge.
These charged regions develop electric field
gradients of hundreds, or perhaps thousands, of millions of volts between them. When the electric field strength
exceeds the breakdown dielectric of air (= 3 x 10 6 volts/meter), a lightning flash occurs and the charged areas
are neutralized.
Electric field measurements indicate that the typical thundercloud is charged in the manner illustrated by
Figure 3-1 (3-l).
A strong, negatively charged region exists in the lower part of the cloud with a
counterbalancing positive charge region in the upper part of the cloud.
In addition to these major charge
centers, a smaller, positively charged region exists near the bottom of the cloud. Due to the strong negative
charge concentration in the lower portion of the cloud, the cloud appears to be negatively charged with respect
to earth -- except in the immediate vicinity underneath the smaller positive charge concentration.
Breakdown can occur between the charged regions within the cloud to produce intracloud lightning. It can also
occur between the charged regions of separate clouds to produce cloud-to-cloud lightning. Intracloud and
cloud-to-cloud discharges do not present a direct threat to personnel or structures on the ground and thus tend
to be ignored in the design and implementation of lightning protection systems. However, calculations of the
voltages which could be induced in cross-country cables by such discharges (3-2) indicate that they present a
definite threat to signal and control equipments, particularly those employing solid state devices.
The cloud-to-ground flash is the one of primary interest to ground-based installations. By definition, such
flashes take place between a charge center in the cloud and a point on the earth. This point on earth can be a
flat plain, body of water, mountain peak, tree, flag pole, power line, residential dwelling, radar or
communications tower, air traffic control tower, or multi-story skyscraper. In a given area, certain structures
or objects are more likely to be struck by lightning than others; however, no object whether man-made or
natural feature, should be assumed to be immune from lightning.
The high currents which flow during the charge equalization process of a lightning flash can melt conductors,
ignite fires through the generation of sparks or the heating of metals, damage or destroy components or
equipments through burning or voltage stressing, and produce voltages well in excess of the lethal limit for
people and animals. The objective of all lightning protection subsystems is to direct these high currents away
from susceptible elements or limit the voltage gradients developed by the high currents to safe levels.
3-1
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Figure 3-1.
3-2
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3.2 DEVELOPMENT OF A LIGHTNING FLASH.
As the charge builds up in a cloud, the electric field in the vicinity of the charge center builds up to the point
where the air starts to ionize. A column of ionized air, called a pilot streamer, begins to extend toward earth
at a velocity of about 160 kilometers per hour (100 miles per hour) (3-3). After the pilot streamer has moved
perhaps about 30 to 45 meters (100 feet to 150 feet), a more intense discharge called a stepped leader takes
place. This discharge lowers additional negative charge into the region around the pilot streamer and allows the
pilot streamer to advance for another 30 to 45 meters (100 to 150 feet) after which the cycle repeats. The
stepped leader progresses towards the earth in a series of steps with a time interval between steps on the order
of 50 microseconds (3-4).
In a cloud-to-ground flash, the pilot streamer does not move in a direct line towards the earth but instead
follows the path through the air that ionizes most readily. Although the general direction is toward the earth,
the specific angle of departure from the tip of the previous streamer that the succeeding pilot streamer takes is
rather unpredictable. Therefore, each 30 to 45 meter (100 to 150 foot) segment of the discharge will likely
approach the earth at a different angle.
This changing angle of approach gives the overall flash its
characteristic zig-zag appearance.
Being a highly ionized column, the stepped leader is at essentially the same potential as the charged area from
which it originates. Thus, as the stepped leader approaches the earth, the voltage gradient between the earth
and the tip of the leader increases. The increasing voltage further encourages the air between the two to break
down.
The final stepped leader bridges the gap between the downward programing column and the earth or an
extension of the earth such as a tree, building, or metal structure that is equipotential with the earth. While
the stepped leader is approaching the earth, a positive charge equivalent to the negative charge in the cloud is
accumulating in the general region underneath the approaching leader. Once the stepped leader cent acts earth
(or one of its extensions), the built-up positive charge in the earth flows rapidly upward through the ionized
column established by the stepped leader to neutralize the strong negative charge of the cloud. This return
current constitutes what is generally referred to as the lightning stroke. If additional pockets of charge exist in
the cloud, these pockets may discharge through the ionized path established by the initial stroke. Continuous
dart leaders proceed from a remaining charge pocket toward the earth down this path. Once the dart leader
reaches the earth, another return stroke of positive charge propagates up the channel to neutralize the
secondary charge in the cloud. This cycle may be repeated several times as succeeding charge centers in the
cloud are neutralized.
3.3 INFLUENCE OF STRUCTURE HEIGHT.
Flashes to earth are normally initiated by a pilot streamer from the cloud. As the charged leader approaches
the ground, the voltage gradient at the surface increases. Ultimately the voltage becomes high enough for an
upward-moving leader to be induced. Over flat, open terrain, the length of the upward leader does not exceed a
few meters before it unites with the downward leader to start the return stroke. However, structures or other
extensions from the earth’s surface experience intensified electric field concentrations at their tips.
Consequently the upward leaders are generated while the downward leader is some distance away; the upward
3-3
MIL-HDBK-419A
leader can be several hundred meters long before the two meet. For very tall buildings, the upward leaders
begin to form even before the downward leaders have begun to form within the cloud; such incidents are
generally described as triggered lightning. Triggered lightning is not very common for structures less than 150
meters (500 feet) in height; as the height increases above this threshold, the proportion of triggered strikes
increases rapidly (3-5).
3.4 STRIKE LIKELIHOOD.
The number of total flashes to which the structure is exposed is related principally to local thunderstorm
Local thunderstorm activity can be projected from isokeraunic maps similar to those shown in
activity.
Figures 3-2 and 3-3. These maps show the number of thunderstorm days per year for various regions of the
United States and the world.
Additional maps of worldwide keraunic levels can be obtained from the World
Meteorological Association (3-6).
A thunderstorm day is defined as a local calendar day on which thunder is heard irrespective of whether the
lightning flashes are nearby or at some distance away.
To an observer at a specific location, the average
distance at which lightning may occur and thunder will be heard is about 10 km (6 miles) (3-5). Therefore, a
thunderstorm day means that at least one lightning discharge has occurred within an area of about 300 square
km (120 square miles) surrounding the position of the observer. The actual number of strikes in the immediate
vicinity of the observer may be considerably higher or lower than the number of thunderstorm days might
indicate, depending upon the duration and intensity of a specific storm or series of storms.
In spite of the relative inexactness of a prediction of a lightning strike to a specific object that is based on the
keraunic level, the thunderstorm day is the only parameter related to lightning incidence that has been
documented extensively over many years. Its primary value lies in the qualitative information which it
provides. This information can be used to assist in the deter ruination of whether lightning protection should be
provided in those situations where there is serious doubt as to the relative need for such protection. For
example, a particular facility may not be essential to the safety of aircraft, but the loss of the facility may
cause traffic delay. In an area of frequent thunderstorms such as the west coast of Florida, for example, the
number of outages in areas where there was no protection could be so high as to be unacceptable; in an area of
few thunderstorms; e.g., Southern California or Alaska, the expected outage from lightning might be once every
few years (which could be significantly less than outages for routine maintenance).
The number of lightning flashes per unit earth surface area increases with the number of thunderstorm days per
year, though not linearly. Empirical evidence indicates that the number of flashes per square kilometer, ó y, can
be reasonably predicted from (3-5):
ó y = 0.007 T Y 2
3-4
(3-1)
MIL-HDBK-419A
Figure 3-2.
3-5
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Figure 3-3. (1 of 4)
3-6
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3-7
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Figure 3-3. (3 of 4)
3-8
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Figure 3-3. (4 of 4)
3-9
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where T is the number of thunderstorm days per year. Out of the total number of flasshes per unit area, the
Y
number of discharges increases with increasing geographical latitude (3-7). The proportion, p, of discharges
that go to ground in relation to the geographical latitude,
, can be represented (3-8) as:
(3-2)
Thus in a given location the flash density,
i.e., the number of discharges to earth per square kiIometer per
year, is:
(3-3)
for a specific location, first determine T y from the isokeraunic map of Figure 3-2 to
To calculate
Figure 3-3.
For estimation purposes, the number of thunderstorm days at points between lines may be
determined by interpolation. Using this value of T y, calculate the total flash density with Equation 3-1. Next
obtain the geographical latitude of the site from a map of the area and calculate p from Equation 3-2. Then
determine the number of strikes to earth per year per square kilometer with Equation 3-3.
3.5
ATTRACTIVE AREA.
The concept of attractive area reflects the principle that an object extending
above its surroundings is more likely to be struck by lightning than its actual cross-sectional area might
otherwise indicate.
For example, thin metallic structures such as flag poles, lighting towers, antennas, and
overhead wires offer a very small cross-sectional area relative to the surrounding terrain but ample evidence
exists to show that such objects apparently attract lightning.
3.5.1 Structures Less Than 100 Meters High.
For structures less than 100 meters (330 feet) in height, and which therefore do not normally trigger lightning,
the number of strikes increases according to a power of h, the structure height.
An expression that represents
the attractive radius, r a, in meters of a structure is (3-5) ;
(3-4)
where h is in meters. For a structure 10 meters high, Equation 3-4 given an attractive radius of 57.7 meters;
similarly, the attractive radius for a 100-meter high structure is 356 meters. The attractive area, A a,
Thus A a for a 10-meter structure is approximately 0.01 square kilometer, while the attractive area of a
100-meter structure is 0.4 square kilometer.
Equation 3-4 has been found to adequately describe the number of strikes to objects which are not tall enough
to trigger lightning. For taller structures, a multiplication factor (3-5)
FT= 1 + 2
(9-1500/h)
3-10
(h in meters)
(3-5)
MIL-HDBK-419A
should be applied to Equation 3-4.
The experimental data to justify the use of Equation 3-5 for structures
greater than 400 meters (1300 feet) is sketchy. However, since structures even approaching this height are not
expected to be of primary concern, Equations 3-4 and 3-5 are expected to be adequate for most design
purposes.
Large flat buildings that do not extend above the median treetop level in the general area will have an
attractive area that is essentially the area of the roof (assuming the roof covers the entire structure). If the
building is several stories high such that it appreciably extends above the prevailing terrain, then its attractive
area is its roof area plus that portion of the attractive area not already encompassed by the roof. Figure 3-4
illustrates the method for calculating the attractive area of a rectangular structure of length,
The roof area is given by
x w.
and width, w.
The additional attractive area resulting from the height of the building is
readily determined by recognizing that the areas contributed by the four corners of the building equal a circle
of radius, ra. Both ends of the structure (dimension w) contribute the area of 2 wr a; the sides contribute
The total attractive area is the sum of the roof area
(
(
w), the corners (
), the ends (2 wr a), and the sides
) to produce a total of
(3-6)
Figure 3-5 indicates that the height to be used in calculating the attractive area of a tall structure should be
the height that the structure extends above the effective (i. e., the level that earth charges would rise to if the
building were not there) levels of the earth. On open, level terrain the height, h, would be the full height of the
roof from grade level.
The number of flashes which can be expected to strike a given structure is equal to the product of the flash
density
times the attractive area, A a, of the structure. For example, suppose the relative likelihood of a
lightning strike to a low, flat structure 100 meters on a side, located in Nashville, TN, is desired. From Figure
3-2, T y is determined to be approximately 54 thunderstorm days per year. The flash density as given by
Equation 3-1 is 20.4 flashes/km 2/year.
The proportion of those flashes that are discharges to earth is 24.4
percent (from Equation 3-2) since the latitude is 36 degrees. Thus approximately 5 flashes/km 2/year to earth
can be expected.
Within the area of the structure (0.01 km 2) there will be only 0.05 strikes per year on the
average, or there is a 1 in 20 chance of being struck by lightning in a given year. For the same structure in
Southern California, only a 1 in 330 likelihood of a strike would be expected in a given year.
3.5.2 Cone of Protection.
This ability of tall structures or objects to attract lightning to themselves serves to protect shorter objects and
In effect, a taller object establishes a protected zone around it.
With this protected zone, other
shorter structures and objects are protected against direct lightning strikes.
structures.
As the heights of these shorter
objects increase, the degree of protection decreases.
Likewise, as the separation between tall and short
structures increases, the protection afforded by the tall structure decreases. The protected space surrounding a
lightning conductor is called the zone (or cone) of protection.
3-11
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Figure 3-4. Attractive Area of a Rectangular Structure
Figure 3-5. Effective Height of a Structure
3-12
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The zone of protection provided by a grounded vertical rod or mast is conventionally defined as the space
enclosed by a right circular cone with its axis coincident with the mast and its apex at the top of the mast as
illustrated by Figure 3-6(a). Similarly, the zone protected by a grounded horizontal overhead wire is defined as
a triangular prism with its upper edge along the wire as illustrated in Figure 3-6(b). In either case, the zone (or
cone) of protection is expressed as the ratio of the horizontal protected distance, D, to height, H, of the mast
or wire. This ratio is the tangent of the shielding angle,
Some commonly recommended zones of protection
and the associated shielding angles are illustrated in Figure 3-7.
The NFPA Lightning Protection Code (3-9) recommends that a 1:1 zone of protection
important areas while a 2:1 zone (
(
= 45°) be provided in
= 63°) is acceptable for less important areas. The British Standard Code of
Practice (3-10) states that a shielding angle of 45 degrees provides an acceptable degree of protection for
ordinary structures, but that for structures with explosive or high flammable contents the shielding angle should
not exceed 30 degrees.
Although the existence of a 1:1 zone of protection does not absolutely guarantee immunity to lightning,
documented cases of the 1:1 zone being violated are very few. Thus for all facilities except those associated
with the storage of explosives or fuels, a 1:1 zone of protection can safely be used as a basis of design of
lightning protection systems. As such, C-E facilities or equipments (antennas, etc.) located entirely within the
1:1 zone of protection generally are not required to have separate air terminals. This does not eliminate the
need to ground metal shelters or to meet the grounding requirements of the subsystems which comprise the
facility ground system. If more than one rod or wire is used, the protected zone is somewhat greater than the
total of all of the 1:1 zones of the rods or wires considered individually.
For adjacent structures, the Codes
specify that a 2:1 zone of protection may be assumed for the region between the structures.
Large structures with flat or gently sloping roofs do not lend themselves to the straightforward application of
the 1:1 or 2:1 zone of protection principles.
exceptionally tall air terminals would be required.
are not needed for effective protection.
To establish even 2:1 type coverage on large buildings,
Experience, however, shows that extremely tall terminals
Both the NFPA Lightning Protection Code and UL Master Labeled
Protection System (3-11) specify air terminals that extend from 10 to 36 inches above the object to be
protected. (The British Standard Code of Practice does not require the use of air terminals at all.)
3.6 LIGHTNING EFFECTS.
3.6.1 Flash Parameters.
During the short interval of a lightning flash, several discharges occur. The sequence of events in a multiplestroke flash is illustrated in Figure 3-8. The initial path for the discharge is established in 50 microseconds.
Intermediate return stroke currents of about 1 kA follow the initial return stroke and last for a few
milliseconds. Subsequent strokes occur at intervals of 50 to 60 milliseconds. The return stroke interval may
include a continuing current of 100 A or so which flows for several milliseconds or until the start of the next
return stroke.
*The shielding angle is defined as the angle between the surface of the cone and a vertical line through the
apex of the cone, or between the side of the prism and the vertical plane containing the horizontal wire.
3-13
MIL-HDBK-419A
(a)
(b)
CONE OF PROTECTION PROVIDED BY A VERTICAL
GROUNDED CONDUCTOR OF HEIGHT H.
ZONE OF PROTECTION PROVIDEDBY A HORIZONTAL
AERIAL GROUND WIRE AT HEIGHT H.
Figure 3-6. Zones of Protection Established by a Vertical Mast and a Horizontal Wire
Figure 3-7. Some Commonly Used Lightning Shielding Angles
3-14
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Figure 3.8. Illustration of Process and Currents Which Occur
During a Lightning Flash to Ground
The lightning discharge involves the transfer of large amounts of electric charge between the cloud and the
earth.
The typical flash transfers 15 to 20 coulombs (C) (1 coulomb equals 6.2 x 10 18 electrons) with some
flashes involving as much as 400 coulombs of charge. The energy per flash of lightning has been estimated to be
as high as 108 watt-seconds. Table 3-1 summarizes the range of values for selected lightning parameters.
3.6.2 Mechanical and Thermal Effects.
The fast rise time, high peak amplitude current of the stroke can produce severe mechanical, thermal, and
electrical effects. The damage caused by these currents to objects in the discharge path is closely related to
the relative conducting power of the object.
For example, metals generally receive a discharge with little
damage. In most cases, even slender conductors such as telephone and electric power cables handle the current
without fusing (melting) except at the point where the current enters or leaves the metal (where severe damage
may occur). Very strong discharges of high peak current (> 40 kA) and high coulomb values (>200 C), however,
can melt or burn holes in solid metal plates. This burning effect is not usually of primary concern for a typical
building or structure because, if an adequate protection system is installed, the principle effect will be a small
deformation at the tip of a lightning rod or a small melted area on the intercepting cable. Such effects are of
more concern where flashes to airplanes occur because such burning can perforate the fuselage to cause loss of
pressurization or penetrate the skin of fuel tanks and possibly ignite fuels. The burning or melting also presents
a threat to exposed tanks of volatile gases or fuels on the ground.
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Table 3-1
Range of Values for Lightning Parameters (3-5)
Parameter
Minimum
Typical
Maximum
Number of return strokes per flash
1
2 to 4
Duration of flash(s)
0.03
0.2
Time between strokes (ms)
3
40 to 60
Peak current per return stroke (kA)
1
10 to 20
250
Charge per flash (C)
1
15 to 20
400
26
2
100
Time to peak current (µs)
<0.5
1.5 to 2
Rate of rise (kA/µs)
<1
20
210
30
Time to half-value (µs)
10
40 to 50
250
Duration of continuing current (ms)
50
150
500
Peak continuing current (A)
30
150
1600
Charge in continuing current (C)
3
25
330
Because of the duration of the currents that flow for the extended intervals between return strokes, they are
most likely to cause damage by melting or igniting solid materials. In contrast, the short-duration high-current
peaks tend to tear or bend metal parts by the electromagnetic forces that develop in proportion to the square
of the instantaneous current,
Though potentially hazardous, the damage caused by mechanical forces in
metallic conductors is generally of secondary importance in most situations. However, because of the presence
of these mechanical forces, it is necessary that lightning rods, down conductors, and other elements of the
protection system be securely anchored.
On the other hand, when insulating or semi-insulating material receives a discharge, an explosive reaction may
occur with severe damage. Trees, for instance, whether dry or green, are in many cases split or stripped of
their bark, and the damage can extend underground to their roots.
Related damage may occur to other
unprotected wooden structures or objects such as flag poles, masts, or light supports, and electric and telephone
poles. When lightning strikes a wooden building, the stroke seeks out the lowest impedance path to earth which
is probably through the electric wiring or water pipes.
Often in order to reach these metallic paths, the
discharge must pass through some type of wooden barrier.
In penetrating such barriers, extensive explosive
damage usually results.
Brick, concrete, marble, and other masonry materials are also frequently shattered or broken loose at the point
where the discharge passes through them. Such damage will occur where structural steel support members or
steel reinforcing rods are encased in concrete or sheathed in brick or marble and the structure has an
inadequate protective system.
The explosive effect can dislodge materials with considerable force -- force
sufficient to hurl relatively large pieces several meters. One explanation of the explosive force is that it is the
result of the virtually instantaneous vaporization of the water present in the wood or entrapped in the masonry
materials.
3-16
MIL-HDBK-419A
3.6.3 Electrical Effects.
Lightning discharges to or near the buildings and structures frequently cause damage to electrical and
electronic equipment.
Melting or burning of conductors occurs at the point of interception of the stroke. The
voltages developed by the fast risetime, high amplitude current pulse are frequently high enough to break down
insulation, pose personnel hazards, and cause component and device failure. These voltages are produced by:
a.
IZ (current x impedance) drop resulting from the lightning pulse traveling down power lines or signal
lines, through structural members, along down conductors or overhead ground wires or through the resistance of
the earth connection;
b.
Magnetic induction; and
c.
Capacitive coupling.
Lightning surges in power, signal, and control circuits are generally the result of some combination of these
three components.
3.6.3.1 Conductor Impedance Effects.
Because of the fast risetime (1 to 2 µsec) and high amplitude (10 to 20 kA) characteristics of the current pulse
produced by the lightning discharge, the inductance and resistance of even relatively short conductors causes
extremely high voltages to be developed on the conductor. The voltages frequently are high enough to exceed
the breakdown potential of air or other insulation materials and cause flashover to other conductors or
breakdown of insulation.
The resistive IR drop generated by 20 kA in a 30 meter (100 feet) run of down
conductor conforming to NFPA-78 (2.88 x 10 -4 /m) will be
V = 2 x 104 x 2.88 X 10 -4 X 30 = 173 volts
(3-7)
which is not sufficient to cause flashover or to pose a serious threat to personnel.
For a down conductor length of 30 meters (100 feet), the smallest copper conductor meeting the minimum
requirements of the Lightning Protection Code or the UL Master Labeled Lightning Protection System has a
diameter of 0.894 cm (0.352 inches). Assuming that the conductor is a straight round wire, the inductance can
be determined from (see Section 5.2.2.3):
(2-8)
where L is the total inductance in microhenries,
is the length in cm, and d is the diameter in cm. A 30-meter
length of conductor will exhibit an inductance of 52.5 microhenries.
3-17
MIL-HDBK-419A
The voltage, V, developed across an inductance is given by
V = L di/dt,
(3-9)
where L, is the inductance in henries and di/dt is the rate of change of the current through the inductor in
amperes per second.
From Table 3-1, the rate of rise of the typical lightning stroke is 20 kA/ µs which
corresponds to a di/dt of 2 x 10 10 amps/second.
Thus the voltage developed by the discharge pulse through the
30-meter (100 foot) downconductor is
v = 5.25 x 10-5 x 2 x 10 10 = 1.05 x 10 6 volts.
(3-10)
Although the duration of this voltage is typically less than 2 microseconds, the voltage generated is high enough
to cause flashover to conducting objects located as much as 35 cm (14 in.) away from the down conductor. It is
for this reason that metallic objects within 6 feet of lightning down conductors should be electrically bonded to
the down conductors.
3.6.3.2 Induced Voltage Effects.
In addition to the lightning effects discussed above, circuits not in direct contact with the lightning discharge
path can experience damages even in the absence of overt coupling by flashover. Because the high current
associated with a discharge exhibits a high rate of change, voltages are electromagnetically induced on nearby
conductors. Experimental and analytical evidence (3-12) shows that the surges thus induced can easily exceed
the tolerance level of many components, particularly solid state devices.
Surges can be induced by lightning
current flowing in a down conductor or structural member, by a stroke to earth in the vicinity of buried cables,
or by cloud-to-cloud discharges occurring parallel to long cable runs, either above ground or buried (3-2).
Consider a single-turn loop parallel to a lightning down conductor such as that shown in Figure 3-9. The
voltage E magnetically induced in the loop is related to the rate of change of flux produced by the changing
current in the down conductor (see Section 6.2.2.1).
dimensions of the loop (
The voltage induced in the loop is dependent upon the
, r2 - rl), its distance from the down conductor (r l), and the time rate of change of the
discharge current (di/dt). Figure 3-10 is a plot of normalized voltage per unit length that would be developed in
a single turn loop of various widths.
These results suggest the steps that should be taken to minimize the voltage induced in signal, control, and
power lines by lightning discharges through down conductors. First, since no control can be exercised over di/dt
because it is determined by the discharge itself, E must be reduced by controlling
, r 1 and r2. The variable
is a measure of the distance that the loop runs parallel to the discharge path; thus, by restricting
, the induced
E can be minimized. Thus cables terminating in devices or equipments potentially susceptible to voltage surges
should not be run parallel to conductors carrying lightning discharge currents if at all possible. If parallel runs
are unavoidable, Figure 3-10 also shows that the distance, r l, between the loop and the lightning current path
should be made as large as possible.
Another observation to be made from Figure 3-10 is that r 2 minus r l should be as close as possible to zero. In
other words, the distance between the conductors of the pickup loop should be minimized. One common way of
reducing this distance is to twist the two conductors together such that the average distance from each
conductor to the discharge conductor is the same.
3-18
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Figure 3-9. Inductive Coupling of Lightning Energy to Nearby Circuits
3-19
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Figure 3-10. Normalized Voltage Induced in a Single-Turn Loop by Lightning Currents
3-20
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Another protective measure is to reduce the flux density within the pickup loop by providing magnetic shielding.
Because the coupling field is primarily magnetic in nature, a shielding material having a high permeability such
as iron or nickel should be used. For shielding against lightning-produced fields, steel conduit or cast iron pipe
are much more effective than aluminum or other non-ferrous materials.
3.6.3.3 Capacitively-Coupled Voltage.
Prior to the lightning discharge, an electric charge slowly accumulates on earth-based objects in the vicinity of
the electrified clouds.
This increase in charge occurs slowly enough so that the potential of grounded
conductors does not change appreciably with respect to the earth, even when the impedance to ground is high.
When the lightning stroke terminates on a structure or other point having contact with the earth as illustrated
in Figure 3-11, the charge on all grounded objects nearby suddenly becomes redistributed. The redistribution of
charge produces a current flow through the grounding impedance of the grounded objects and produces a voltage
across that impedance.
Referring to Figure 3-11, the voltage between the conducting objects and the ground can be expressed as
(3-11)
where Q is the stored charge in coulombs, C is the total capacitance to ground in farads, R is the effective
resistance to ground in ohms, and t is the elapsed time in seconds from the occurrence of the stroke.
Equation 3-11 shows that if the product RC is small, the exponential term will be large (for a time t on the
order of 10 µs), thus making the voltage capacitively induced on any reasonably well-grounded object quite
small for a typical lightning stroke.
3.6.3.4 Earth Resistance.
Consider a facility such as the one illustrated in Figure 3-12, that has more than one possible electrical path to
earth. For example, a ground rod is driven into the earth at the transformer pole or at the service entrance to
Building 1. The resistance, R G 1 , of this rod could be 25 ohms or higher and still conform to NEC requirements.
Metal utility pipes such as water lines generally offer a relatively low resistance (labeled R G 2) to earth. (In
soils of high resistivity the point of effective contact between utility pipes may be an appreciable distance from
the facility.) Empirical data indicates that the grounding resistance offered by water pipes is on the order of 1
to 3 ohms. If the electrical ground is not connected to the water pipe, a lightning strike to the ground wire of
the electrical distribution system could produce a potential difference high enough to possibly produce an arc
between the electrical ground (including the equipment cabinet and the building’s structure, if connected) and
the utility piping.
A definite personnel hazard would then exist because of the high voltage that would be
developed between the equipment and building ground and pipes.
Because of this reason as well as the
requirement to prevent analogous hazards from existing during power system faults, MIL-STD-188-124A
requires electrical safety grounds be connected to the metallic water system in the building and recommends
they also be directly connected to the ground rod at the transformer.
3-21
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If R G1 is 25 ohms while R G2 is only 1 ohm or so, then a lightning strike as indicated could easily cause the
potential of the overhead ground wire to become high enough to produce an arc across the transformer windings
and insulators. Since the low voltage secondary side offers a lower impedance to earth, it is the preferred path
for the discharge,
Figure 3-11. Capacitive Coupling of Lightning Energy
This type of lightning threat can be minimized by (1) reducing R G1 to approximately the magnitude of R G 2, (2)
the installation of appropriate lightning arresters at the transformer to keep the potential difference between
the power conductors and the ground wire and between the primary and secondary windings to within the stress
ratings of the transformer, and (3) interconnecting the earth electrode subsystem (to include the water and
ether utility pipes) with a 1/0 or larger buried copper cable as illustrated by the dotted line in Figure 3-12.
Interconnecting the ground electrodes of the building and transformer pole to form one effective earth contact
does not eliminate the lightning threat to the buried cable between the two buildings. A S shown, the cable
shield is connected to the cabinet, i.e., the building ground. In the event of a lightning strike as shown, Building
1 and its power supply system will be elevated in potential relative to Building 2. In particular, if the distance
between the two buildings is more than just a few meters, the inductance, primarily, of the cable shield will
prevent the cable from providing the low impedance necessary to keep the two buildings at the same potential.
In addition if the shield of the cable is insulated from the earth, as is usually the case, the potential of the cable
shield can become high enough with respect to the earth to exceed the breakdown of the insulation.
3-22
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3-23
Figure 3-12.
MIL-HDBK-419A
Figure 3-13. Step-Voltage Hazards Caused by Lightning-Induced Voltage Gradients in the Earth
3-24
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Assume for the moment that Building 1 has an earth electrode subsystem consisting of ground rods
interconnected with the cold water system with a net resistance to earth of 3 ohms. With a lightning discharge
of 20 kA, the voltage of the complex will rise to 60 kV with respect to Building 2 and that portion of the earth
not in the immediate vicinity of Building 1. At Building 1, the cable shield voltage will rise along with that of
the building. This voltage pulse will travel down the cable, successively raising the shield potential to as much
as 60 kV with respect to the surrounding earth. Such high voltages cause insulation breakdown in the form of
tiny pinholes where the lightning energy punches through.
As the lightning pulse travels down the cable, its amplitude diminishes due to cable resistance and dielectric
losses. However, the amplitude of the pulse can still be sufficient to damage circuit components in terminating
equipment in Building 2. To minimize this damage, surge arresters compatible with the terminating components
and hardware should always be provided on such cables.
Further information on the use of surge arresters is
presented in Volume II, Section 1.3.3.5.
In the event of a lightning stroke, there is a definite personnel hazard posed by the voltage gradient in the soil
in the vicinity of the point where the lightning discharge enters the earth.
rapidly leaves the electrode.
In homogeneous soil, the current
The current density is highest near the electrode and rapidly decreases with
distance from the electrode. In soil of uniform resistivity, a significant voltage gradient will exist between two
points that are differing distances from the electrode.
Figure 3-13 illustrates the nature of this voltage
variation and shows the hazard encountered by personnel walking (or standing) in the area.
The voltage
difference across the span of a step can be sufficient to be lethal. As shown earlier, the degree of the hazard is
determined by the magnitude of the stroke current, the grounding resistance of the earth electrode, and the
distance away from the electrode.
No control can be exercised over the current; the threat, however, can be
lessened by achieving a low common ground resistance and by minimizing the step potential as discussed in
Section 2.8.1.3.
3.7 BASIC PROTECTION REQUIREMENTS.
To effectively protect a structure such as a building, mast, tower, or similar self-supporting object from
lightning damage, the following requirements must be met:
a.
An air terminal of adequate height, mechanical strength and electrical conductivity to withstand the
stroke impingement must be provided to intercept the discharge to keep it from penetrating any nonconductive
outer coverings of the structure or to prevent it from terminating on antennas, lighting fixtures, transformers,
or other devices likely to be damaged or destroyed.
b.
A low impedance path from the air terminal to earth must be provided.
c.
The resistance of the connection between the discharge path and the earth must be low.
These requirements are met by either (1) an integral system of air terminals, roof conductors, and down
conductors, securely interconnected to provide the shortest practicable path to earth, or (2) a separately
mounted shielding system such as a metal mast which acts as an air terminal, and a down conductor or an
overhead ground wire terminated at the ends (and at intermediate locations, if needed) with down leads
connected to earth ground electrodes. Specific design practices are contained in Volume II.
3-25
MIL-HDBK-419A
3.8
DETERMINING THE NEED FOR PROTECTION.
The degree to which lightning protection is required, is a subjective decision requiring an examination of the
relative criticalness of the structure location and its contents to the overall mission of the facility. Those
structures containing elements vital to the operational mission such as air traffic control towers, radar
installations, navigational aids, and communications centers are examples of facilities which obviously must be
protected.
Installed.
However, every building or structure does not require that a lightning protection system be
For example, buildings primarily used for the storage of nonflammable materials do not have a
critical need for protection.
Three of the factors to consider in ascertaining whether a given structure should have a lightning protection
system installed or in determining the relative comprehensiveness of the system are the relative threat of being
struck by lightning, the type of construction, and the nature of the facility.
3.8.1 Strike Likelihood.
The relative likelihood of a particular structure being struck by lightning is a function of the keraunic level,
i.e., the thunderstorm activity of the locality, the effective height of the structure and its attractive area.
Average thunderstorm activity can be determined from the isokeraunic maps shown in Figures 3-2 and 3-3.
Then using the techniques described in Section 3.4, estimate the frequency with which strikes to the structure
may occur. Use this estimation as one of the inputs to the decision process.
3.8.2 Type of Construction.
Steel frame buildings with metal outer coverings offer the greatest inherent protection against lightning
damage. Steel towers also exhibit a high immunity to structural damage. Additional protection for these type
buildings will probably be required only for very critical facilities in highly exposed locations. Steel frame
buildings with nonconductive, but nonflammable, outer coverings (like brick or other masonry) also offer a high
degree of protection against lightning damage.
The greatest hazard is posed by pieces of masonry being
dislodged by stroke currents passing through the outer coverings to reach the structural steel underneath.
Minimal protection consisting of interconnected air terminals to down conductors and steel support columns will
be sufficient to prevent this type of structural damage.
Buildings constructed of nonconductive materials such as wood, concrete blocks, or synthetic materials are the
most susceptible to destructive damage.
A complete auxiliary protection system will be required to prevent
lightning damage to buildings utilizing this type of construction.
3-26
MIL-HDBK-419A
3.8.3 Criticalness to System Mission.
If a strike to the facility poses a threat to human life, either to the occupants of the structure or to those
persons whose safety is dependent upon reliable performance of the equipment and people inside the structure,
comprehensive lightning protection should be definitely provided even in areas of low thunderstorm activity. At
the other extreme, the need for the protection of buildings used primarily to store nonflammable or
nonexplosive items is doubtful unless the stored items are critical to system operation, the building is usually
exposed, etc.
In between these extremes are those structures whose incapacitation would cause an
inconvenience or present other difficulties short of life-and-death situations. With these structures, a careful
analysis must be made to determine the relative likelihood of outages from lightning in comparison to normal
equipment failures, downtime for maintenance, and other routine occurrences.
Though not directly related to the protection of electrical or electronic installations, Reference 3-10 is
recommended for further guidance in performing the tradeoff analyses to determine the degree of lightning
protection required for specific facilities.
3.9 APPLICABLE CODES.
The Lightning Protection Code, NFPA No. 78, issued by the National Fire Protection Association (3-9) contains
the basic requirements for the minimization of personnel hazards in the event of a lightning strike to the
structure.
The requirements of NFPA No. 78, however,
are not sufficient to protect the electrical distribution system,
signal and control cables, or sensitive electronic equipment from surges produced by either direct or indirect
strokes.
Thus additional steps such as providing lightning arresters on power lines and on outside signal and
control cables, providing counterpoise cables for overhead and underground cables, providing comprehensive
electromagnetic shielding on sensitive cables, and installing fast response surge protection devices on circuits
exposed to lightning discharges should be taken. MIL-STD-188-124A refers.
3-27
MIL-HDBK-419A
3.10
REFERENCES.
3-1. M.A. Uman, Lightning, McGraw-Hill, New York NY (1969).
3-2. S.R. Crawford, et. al., "Final Report on Investigation of Lightning Hazard and Vulnerability at NHS,"
WDL-TR5103, Contract No. F04701-72-C-0024, Philco-Ford Corporation, Palo Alto CA 94303, 12 March
1973.
3-3. W.W. Lewis, The Protection of Transmission System Against Lightning, Dover Publications, Inc,
New York, (1965).
3-4. “Lightning Protection for Saturn Launch Complex 39,” Contract NASW-410, General Electric Company,
Daytona Beach, Florida, 10 September 1963.
3-5. N. Cianos and E. T. Pierce,
“A Ground-Lightning Environment for Engineering Usage,” Contract
L.S.-2817-A3, Stanford Research Institute, Melo Park CA, August 1972.
3-6. “World Distribution of Thunderstorm Days, "SMO/OMN No. 21, World Meteorological Association, Geneva,
Switzerland, 1953.
3-7. E.L. Maxwell, et. al., “Development of a VLF Atmospheric Noise Prediction Model,” Final Report,
Contract N00014-69-C-1050, Westinghouse Georesearch Laboratory, Boulder CO 1970.
3-8. E.T. Pierce, “Latitudinal Variation of Lightning Parameters,” J. Appl. Meterol., Vol 9, 1970, pp 194-195.
3-9. Lightning Protection Code, NFPA 78-1986, National Fire Protection Association, Battery march Park,
Quincy MA 02269.
3-10. “The Protection of Structures Against Lightning,” British Standard Code of Practice CP 326: 1965, British
Standards Institution, London.
3-11. G. Comber, “A Transistorized Video Line System,” IEEE Conference Publication, Vol 46, Pt 1, September
1968.
3-28
MIL-HDBK-419A
CHAPTER 4
FAULT PROTECTION SUBSYSTEM
4.1 FAULT PROTECTION.
For effective fault protection, a low resistance path must be provided between
the location of the fault and the transformer supplying the faulted line. The resistance of the path must be low
enough to cause ample fault current to flow and rapidly trip breakers or blow fuses.
The necessary low
resistance return path inside a building is provided by the grounding (green wire) conductor and the
interconnected facility ground network.
An inadvertent contact between energized conductors and any
conducting object connected to the grounding (green wire) conductor will immediately trip breakers or blow
fuses.
In a building containing a properly installed third-wire grounding network, as prescribed by
MIL-STD-188-124A, faults internal to the building are rapidly cleared regardless of the resistance of the earth
connection.
4.1.1 Power System Faults.
A power system fault is either a direct short or an arc (continuous or intermittent) in a power distribution
system or its associated electrical equipment. These faults are hazardous to personnel for several reasons:
a.
Fault currents flowing in the ground system may cause the chassis of grounded equipment to be at a
hazardous potential above ground.
b.
The energy in a fault arc can be sufficient to vaporize copper, aluminum, or steel. The heat can
present a severe burn hazard to personnel.
c.
There is a fire hazard associated with any short circuit or arc.
d.
Burning insulation can be particularly hazardous because of the extremely toxic vapors and smoke
which may be produced.
Some common causes of electrical system faults are:
a.
Rodents getting between ground and phase conductors.
b.
Water infiltration.
c.
Moisture in combination with dirt on insulator surfaces.
d.
Breakdown of insulation caused by thermal cycling produced by overloads.
e.
Environmental contaminants.
f.
Damage during installation.
g.
System age deterioration.
4-1
MIL-HDBK-419A
Figure 4-1 illustrates how personnel hazards are developed by improper installation and fault conditions.
Suppose that one phase of the 230-volt line accidentally contacts the motor frame. If the motor is not
grounded, its frame will rise to 133 volts, and anyone coming in contact with it would be subject to a lethal
shock if simultaneous contact is made with a grounded object. To prevent this situation from arising, the motor
frame must be grounded via the green wire. The resistance of the fault path must be low enough to permit the
fault current to trip the overload protector and interrupt the fault.
If the resistance of the fault path is too
large, the fault current will not be enough to trip the overload protectors. Thus to minimize both shock and fire
hazards, the resistance of the fault path must be as low as possible. However, the fault protection subsystem
normally does not depend on the earth electrode subsystem to trip overcurrent devices.
The fault current
normally flows through the green wire (grounding conductor) to the source side of the first service disconnect
means where the green wire and the neutral are tied together. The fault current then flows through the neutral
to the transformer to complete the circuit.
This path functions completely independent of the connection to
the earth electrode subsystem.
Figure 4-1. Grounding for Fault Protection
4-2
MIL-HDBK-419A
Fault clearance in power distribution systems is normally provided by circuit breakers, fuses, or overload relays
in each phase.
These devices provide personnel protection only if the fault current is sufficient to trip the
over-current device. They generally however do not have response times which are adequate to protect the
individual if he happens to be in direct contact with the energized object.
4.1.2 Ground-Fault-Circuit-Interrupter (GFCI). High resistance faults (low and moderate currents of 5
milliamperes or more) can be cleared rapidly with a device called a ground-fault-circuit-interrupter (GFCI).
The GFCI contains an electronic circuit which continuously monitors the difference between the current
supplied to the load and the current returned from the load. If this difference is not zero, some current must be
leaking to ground. When this leakage current exceeds a preset value, the GFCI will act to interrupt the power
to the circuit.
GFCI’s are so sensitive that they can be set to interrupt power fault currents as low as 2
milliamperes.
Experiments with dogs have shown that trip currents of 5 milliamperes or less will prevent
electrocution.
(GFCI’s have proven so effective as protection against electric shock that the National
Electrical Code requires that all 15 and 20 ampere bathroom, garage, and outdoor receptacles in family
dwelling units and in circuits set up at construction sites be protected with a GFCI. MIL-STD-188-124A also
recommends they be installed on 120 volt single phase 15 and 20 ampere receptacles of C-E facilities.)
4.2 EARTH CONNECTION.
Historically, grounding requirements arose from the need to protect personnel, equipment, and facilities from
lightning strokes and from industrially generated static electricity. Structures, as well as electrical equipment,
were connected to earth, i.e., grounded, to provide the path necessary for lightning and static discharges. As
utility power systems developed, grounding to earth was found to be necessary for safety.
All major
components of the system such as generating stations, substations, and distribution systems are earth grounded
to provide a path back to the generator for the fault currents in case of transmission line trouble. The path to
earth should have as low a resistance as possible. A low resistance minimizes the potential difference between
equipments connected to the earth electrode subsystem when fault currents flow. Thus personnel who come in
contact with two or more pieces of equipment at one time are protected.
Ideally, the earth connection should exhibit zero resistance between the earth and the equipment and facilities
connected to it. Any physically realizable connection, however, will exhibit a finite resistance to earth. The
economics of the design of the earth electrode subsystem involves a trade-off between the expense necessary
to achieve a low resistance and the satisfaction of minimum subsystem requirements. The 10 ohm design
objective of MIL-STD-188-124A is considered such a trade-off.
4.3 AC POWER LINE GROUND.
The grounding conductor (green wire) in a single-phase 115/230 volt ac power distribution system in a facility is
one of four leads, the other three being the two phase or “hot” leads (black/red) and the neutral lead (white
wire). The green wire is a safety conductor designed to carry current only in the event of a fault. The "hot"
leads are connected from the first service disconnect to the high sides of the secondary of the distribution
transformer and the neutral is connected to the center tap which is grounded to a ground terminal at the
transformer.
When a single transformer supplies power to only one communications building, for fault
4-3
MIL-HDBK-419A
protection the grounding conductor shall be grounded on the source side of the first service disconnect to the
earth electrode subsystem and also to the ground terminal at the distribution transformer. For 3-phase wye
systems a five-wire service entry cable consisting of one neutral, one grounding, and three phase conductors
shall be employed.
In either case, when a single transformer supplies power to a single building, the safety
ground (green wire) shall be grounded to the earth electrode subsystem at the supply side of the first service
disconnect of the facility as well as at the distribution transformer as shown in Figure 4-2. The neutral shall
also be grounded at both locations.
When a single transformer supplies power to more than one C-E building and if noise or hum is encountered in
C-E circuits or equipments, the neutral should be lifted or removed from ground at each service disconnect. In
this case the neutrals from each building are grounded at the distribution transformer only (see Figure 4-3).
To protect personnel from exposure to hazardous voltages , all exposed metallic elements of electrical and
electronic equipment are connected to ground with the green wire. Then, in the event of inadvertent contact
between the hot lead and chassis, frame, or cabinet through human error , insulation failure, or component
failure, a direct fault clearance path is established to quickly remove the hazard.
Grounding of a 3-phase wye power distribution system is accomplished similarly to the single phase system.
The connections for a typical system are shown in Figure 4-3.
As in single phase systems, the neutral lead is
bonded to the green wire at the supply side of the first service disconnecting means and grounded to the earth
electrode subsystem as well as to the ground terminal at the distribution transformer. If one transformer
supplies power to more than one C-E building, the neutral is lifted from ground at the service disconnect.
A 3-phase system served by a transformer with a delta connected secondary will require the use of a grounding
transformer to ground the system and establish a neutral. The grounding transformer may be either a “zig-zag”
or “wye-delta” type, both of which have leads which are attached to each of three phases and a fourth lead
which is grounded and serves as the neutral. The typical connections for a grounding transformer are shown in
Figure 4-4.
Figure 4-2. Single-Phase 115/230 Volt AC Power Ground Connections
4-4
MIL-HDBK-419A
4.4 TEST EQUIPMENT.
Test equipments are available to measure the resistances and impedances of the fault
protection subsystems including the grounding (green) conductor as well as the signal reference subsystem
(equipotential plane) which may at times become part of the fault protection subsystem. These equipments can
measure the impedances (at 60 Hz) of each path from the equipment having the fault to the first service
disconnect means and therefore assist in determining the value of the fault current over each path. The
information will in turn be beneficial in determining or predicting the degree of interference which may be
anticipated should a fault current be superimposed on the signal reference subsystem. (4-1 and 4-2)
NOTE:
Lift when single transformer supplies power to more than one building or
because of objectionable current, noise or interference.
Figure 4-3. Three-Phase 120/208 Volt AC Power System Ground Connections
4-5
MIL-HDBK-419A
Figure 4-4. Connections for a Three-Phase "Zig-Zag" Grounding Transformer
4.5 REFERENCES.
4-1. A Practical Approach to Establish Effective Grounding for Personnel Protection, IEEE Conference Paper,
Chris C. Kleronomos and Edward Cantwell, 1979.
4-2. Some Fundamentals of Equipment Grounding Circuit Design, R.H. Kaufmann, 1954.
4-6
MIL-HDBK-419A
CHAPTER 5
GROUNDING OF SIGNAL REFERENCE SUBSYSTEM
5.1 INTRODUCTION.
Signal circuits are grounded and referenced to ground to (1) establish signal return paths between a source and a
load, (2) control static charge, or (3) provide fault protection. The desired goal is to accomplish each of these
three grounding functions in a manner that minimizes interference and noise.
If a truly zero impedance ground reference plane or bus could be realized, it could be utilized as the return path
for all currents -- power, control, audio and rf -- present within a system or complex. This ground reference
would simultaneously provide the necessary fault protection, static discharge, and signal returns. The closest
approximation to this ideal ground would be an extremely large sheet of a good conductor such as copper,
aluminum, or silver underlying the entire facility with large risers extending up to individual equipments. The
impedance of this network at the frequency of the signal being referenced is a function of conductor length,
resistance, inductance, and capacitance.
When designing a ground system in which rf must be considered,
transmission line theory must be utilized.
5.2 CONDUCTOR CONSIDERATIONS.
5.2.1 Direct Current Resistance.
The resistance, R d c, of a conductor of uniform cross section is proportional to the length and inversely
proportional to the cross-sectional area, that is
R
where
dc
=
is the resistivity of the conductor material,
(5-1)
/A ohms,
is the length of the conductor in the direction of current
flow, and A is the cross-sectional area of the conductor. Values of R dc for the standard sizes of wire and cable
are given in Table 5-1. (For data on wire sizes not shown in this table, consult References 5-1 and 5-2.)
At dc, the resistance of the conductor is the controlling factor.
Except for very unusual situations (such as
when the signal to be processed is very low in amplitude or where the interfacing equipments are very far apart
physically), an adequate ground can generally be realized for dc in a relatively economical manner utilizing low
resistivity materials such as copper and aluminum.
Most systems, however, employ other than dc signals.
Therefore, the frequency-dependent properties of the conductors become important.
5.2.2 Alternating Current Impedance.
The ac impedance of a conductor is composed of two parts: the ac
resistance and the reactance. Both the ac resistance and the reactance of a conductor vary with frequency as a
result of skin effect.
5-1
MIL-HDBK-419A
Table 5-1
Properties of Annealed Copper Wire
AWG
Diameter
Cross - Sectional Area
2
Resistance in Ohms
No.
mils
mm
cmil
mm
4/0
460.0
11.7
211600
107.2
0.049
0.161
3/0
409.6
10.4
167800
85.0
0.062
0.203
2/0
364.8
9.3
133100
67.4
0.078
0.256
1/0
324.9
8.3
105500
53.4
0.098
0.322
1
289.3
7.3
83690
42.4
0.124
0.407
2
257.6
6.5
66370
33.6
0.156
0.512
4
204.3
5.2
41740
21.1
0.248
0.814
6
162.0
4.1
26250
13.3
0.395
1.296
8
128.5
3.3
16510
8.4
0.628
2.060
10
101.9
2.6
10380
5.3
0.999
3.278
12
80.8
2.1
6530
3.3
1.588
5.210
14
64.1
1.6
4 107
2.1
2.525
8.284
16
50.8
1.3
2583
1.3
4.016
13.176
18
40.3
1.0
1 624
0.8
6.385
20.948
20
31.9
0.8
1022
0.5
10.150
33.300
5-2
per 1000 ft
per k m
MIL-HDBK-419A
5.2.2.1 Skin Effect.
Whereas a direct current is uniformly distributed over the cross-sectional area of a conductor, alternating
current tends to concentrate near the surface of the conductor.
The higher the frequency, the greater the
concentration near the surface. This physical phenomenon is called skin effect. A measure of the degree of
penetration of the currents into the conductor is given by the skin depth,
the current density is attenuated to
is defined as the depth at which
l/ = 1/2.718 = 0.37 of its value at the conductor surface. Skin depth may
also be interpreted as the equivalent thickness of a hollow conductor carrying a uniform distribution over its
cross-sectional area, having the same external shape as the actual conductor, and having a dc resistance exactly
the same as the ac resistance of the conductor.
For conductors whose thickness is at least three times the skin depth, this depth is given by (5-3).
(5-2)
where
is the resistivity of the material in ohm-cm, f is the frequency in hertz, and µ r is the relative
permeability of the material. The skin depth for various metals is given in Table 5-2 and Figure 5-1. Note that
copper has a skin depth of 0.34 inch (8.63 mm) at 60 Hz but only .00026 inch (0.066 mm) at 1 MHz.
Table 5-2
Parameters of Conductor Materials (5-4)
Material*
( -cm)
(cm)
RS
( )
1.62 X 10-6
6.41/
2.52 x 10 -7
Copper
1.73 x 10
-6
6.62/
2.61 X 10 -7
Aluminum
2.69 X 10-6
8.25/
3.26 X 10 -7
Brass
6.37 X 10
-6
12.70/
5.01 x 10 -7
Solder
14.2 X 10-6
18.96/
7.48 X 10 -7
Silver
* µr =
1
5-3
MIL-HDBK-419A
Figure 5-1.
5-4
MIL-HDBK-419A
5.2.2.2 AC Resistance.
The ac resistance of a conductor of any shape can be determined from the skin depth if both the thickness and
the radius of curvature of the conductor are much greater than the skin depth and if the radius of curvature
does not vary too rapidly around the conductor’s perimeter. For a conductor meeting these conditions, the ac
resistance per unit length is
ohms/meter, or
(5-3)
ohms/meter
(5-4)
where P is the circumference of the conductor and R s is the surface resistance of the conductor. The surface
resistance is defined as the ac resistance of a surface of equal length and width and is given by
(5-5)
The surface resistance for various metals is also shown in Figure 5-1 and Table 5-2.
The ratio of the ac resistance to the dc resistance is called the resistance ratio of a conductor. Skin effect
causes the resistance ratio to be greater than unity. The resistance ratio for straight cylindrical wires is given
in Figure 5-2 in terms of a parameter X defined as
(5-6)
where µ r is the relative permeability of the conductor, f is the frequency in hertz, and R dc is the dc resistance
in ohms for 1 cm of conductor.* In the case of copper wire, Equation 5-6 becomes
(5-7)
where d m is the wire diameter in mils, or becomes
(5-8)
where d m is diameter in mm.
*It should be noted that Equation 5-6 applies at all frequencies, whereas Equations 5-3 and 5-4 apply only under
the conditions stated.
5-5
—
v
MIL-HDBK-419A
Figure 5-2. Resistance Ratio of Isolated Round Wires (5-6)
5-6
MIL-HDBK-419A
5.2.2.3 Reactance
The reactance of the conductor is generally inductive and is given by the product of the radian frequency,
,
and the self-inductance, L, of the conductor. The self–inductance of a conductor is a measure of that property
which causes an opposition to a change in the current flowing in the conductor.
Because skin effect
redistributes the current within a conductor with changes in frequency, the inductance of the conductor does
vary with frequency.
The self-inductance of a straight round wire is given (5-6) by
(5-9)
where
is the length in inches, d is the diameter in inches, and
be determined (for copper) from Figure 5-3. For
K
is a skin effect correction factor which may
and d in centimeters, Equation 5-9 becomes
(5-lo)
For materials other than copper,
frequency f, where
K
can be obtained from Figure 5-3 by using f ' = f(
is tile resistivity of the material and
) instead of the actual
is the resistivity of copper. For low frequencies
where the current flow can be assumed to be uniform across the conductor cross-section, the inductance of a
round straight wire of length
, diameter d, and relative permeability µ r (if surrounded by air) is
(5-11)
where all the dimensions are in inches.
As the frequency increases, a limiting value of inductance, L H F, is
approached:
(5-12)
5-7
MIL-HDBK-419A
Figure 5-3. Nomograph for the Determination of Skin Effect Correction Factor (5-6)
In Equations 5-11 and 5-12, the constant 0.00508 becomes 0.002 when
and d are in cm.
Figure 5-4 gives the value of LLF for a 1/0 AWG solid round copper conductor as a function of length, and L H F
for various wire lengths and diameters is given in Figure 5-5.
5-8
MIL-HDBK-419A
Figure 5-4. Low Frequency Self-Inductance versus Length for 1/0 AWG Straight Copper Wire (5-7)
Figure 5-5. Self-Inductance of Straight Round Wire at High Frequencies (5-6)
5-9
MIL-HDBK-419A
5.2.2.4 Proximity Effect.
When two or more conductors are in close proximity, the current flowing in one
conductor is redistributed because of the magnetic field produced by the current in the other conductor. This
effect is an extension of skin effect and is called proximity effect. The proximity effect tends to increase the
ac resistance of a conductor to a value greater than that due to simple skin effect.
5.2.3 Resistance Properties vs Impedance Properties.
Although skin effect exists at all frequencies, it becomes more significant as the frequency increases. The
reactance of a conductor also increases with frequency to further increase the conductor impedance above its
dc value. To design an effective ground system one must consider the relative effects of the dc resistance, the
ac resistance, and the inductance upon the total impedance of a ground conductor.
Using Equation 5-1, the dc resistance of round wire conductors can be calculated. The dc resistance per
1000 feet for four standard size copper cables is given in Table 5-3. Table 5-4 gives the dc resistance and (for
60 Hz) the ac resistance, the inductance and the total impedance of various size and length conductors as
determined from Table 5-3 and from Equation 5-12. At a frequency of 1 MHz, these same characteristics for
30-meter (100-foot) lengths are given in Table 5-5 as calculated from Equations 5-3 and 5-12. Note that for
the larger wires (No. 2 AWG or larger) the inductance of the long (> 100 feet) cables determines the magnitude
of the impedance. Also note that for the same length cables there is not as much difference in the impedance
magnitudes of a small and a large cable as there is in the resistance of the two cable sizes. For example, the
ratio of the dc resistance of a 30-meter (100-foot) length of No. 12 AWG copper cable to the dc resistance of a
30 meter (100 feet) of 1/0 AWG copper cable is 0.15880/0.0098 = 16.20. Since the ac resistance at 60 HZ i s
approximately the same as the dc resistance, the ratio of the 60 Hz ac resistance of the two cables is also
16.20. At a frequency of 1 MHz the ratio of the ac resistance becomes 1.23/0.307 = 4.01. However, the 60 Hz
impedance ratio is only 0.1605/0.0226 = 7.10 and the 1 MHz impedance ratio is only 382.65/329.49 = 1.16. These
ratios are tabulated in Table 5-6 for comparison.
From Tables 5-3 through 5-6 and the above example, the
following conclusions are made:
a.
Because of the inductance, the advantages offered by a large cable such as 1/0 AWG are less than
they might appear to be from a comparison of the dc resistance values.
The advantage offered by a large cable, e.g., 1/0 AWG, will be somewhat more pronounced for
b.
relatively short conductor lengths than for long conductor runs. This is true because inductance increases more
rapidly with length than does resistance (see Equations 5-1 and 5-9).
c.
Because of the lack of dramatic improvement in ac impedance of large cables over smaller cable
sizes for long runs, consideration of materials and labor costs are relatively important and may be the deciding
factor.
d.
Since even 1/0 AWG cables exhibit impedances from 22.6
to 115.8
for lengths of 30 meters
(100 feet) and 137 meters (450 feet), respectively, the control of stray currents should be an essential objective
in any signal grounding system.
5-10
MIL-HDBK-419A
Table 5-3
DC Parameters of Some Standard Cables
Size
Diameter
DC Resistance
(AWG)
(mils)
(Ohms/1000 ft)
No. 12
80.81
1.588
No. 8
128.5
0.6282
No. 2
257.6
0.1563
1/0
324.9
0.09827
Table 5-4
Sixty-Hertz Characteristics of Standard Cables
Size
(AWG)
Length
(Ft)
Ra
c
L
—
(µH)
X
—L
( )
Z
—
( )
No. 12
30
0.04764
16.532
0.00623
0.0480
No. 12
100
0.15880
62.447
0.02354
0.1605
No. 8
30
0.01885
15.684
0.00591
0.0197
No. 2
30
0.00469
14.411
0.00543
0.0072
No. 2
100
0.01563
55.379
0.02088
0.0261
No. 2
150
0.02344
86.777
0.03271
0.0402
1/0
30
0.00294
13.987
0.00527
0.0060
1/0
100
0.00980
53.964
0.0226
0.0060
1/0
150
0.01470
84.654
0.03191
0.0351
1/0
300
0.02940
181.987
0.06861
0.0746
1/0
450
0.04410
284.105
0.10710
0.1158
5-11
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Table 5-5
One-Megahertz Characteristics of Standard Cables
Size
AWG
R
Length
(Ft)
d c
( )
(µH)
X
—L
( )
( )
RAC
L
( )
Z
No. 12
100
0.1588
1.23
60.9
382.65
382.65
No. 2
100
0.0156
0.387
53.8
338.03
338.03
1.0
100
0.0098
0.307
52.44
329.49
329.49
Table 5-6
Impedance Comparisons Between No. 12 AWG and 1/0 AWG
R dc (No. 12 AWG)
Frequency
Lenght
R
(1/0 AWG)
R ac (No. 12 AWG)
R ac (1/0 AWG)
Z (No. 12 AWG)
Z (1/0 AWG)
d c
60 H Z
30 ft.
16.20
16.20
9.23
60 Hz
100 ft.
16.20
16.20
7.99
100 ft.
16.20
4.01
1.16
1 MHz
5.2.4 Effects of Geometry.
Many conductor shapes can be used in the signal ground network. As is the case
for the solid round conductor, the impedance of other configured conductors is dependent upon the current
distribution in the conductor and hence upon the signal frequency.
5-12
MIL-HDBK-419A
5.2.4.1 Stranded Cable.
A stranded cable consists of a number of wires in close proximity twisted about each other: it is more flexible
than a solid conductor of the same cross-sectional area. Because of the close proximity of the wires, the skin
effect within the cable redistributes most of the current to outer wires. These outer wires are in the form of a
coil (due to the lay of the strand), thus increasing the self-inductance of the cable. Skin effect also increases
the ac resistance as the frequency is increased.
For a given cable size, both the ac resistance and the self-inductance of a stranded conductor are greater than
those of a solid round conductor.
Because of their ineffectiveness at higher frequencies, it has been
recoin mended that stranded cables not be used at frequencies over 1200 Hz (5-7). However, in many situations,
large cables are required to safely carry currents produced by power faults and lightning discharges; in addition,
solid wires larger than approximately 0.6 cm (0.25 in.) may be difficult to obtain.
5.2.4.2 Rectangular Conductors.
At frequencies high enough to make the skin effect noticeable, the resistance ratio of a flat rectangular
conductor will be lower than that of a solid round wire with the same cross-sectional area if the
width-to-thickness ratio exceeds approximately 2:1.
The resistance ratios for several sizes of nonmagnetic
( µr = 1) rectangular conductors are plotted in Figure 5-6.
The self-inductance at lower frequencies of a rectangular conductor is (5-6)
(5-13)
where
is the length, b is the width and c is the thickness, and all the dimensions are in inches.
For the
dimensions in cm, Equation 5-13 is
(5-14)
If
is Iarger than 50 (b+c), the last term in each equation may be neglected.
The sharp edges on rectangular conductors tend to radiate energy into space and a flat conductor may become
an efficient antenna.
To reduce the efficiency of the antenna and minimize this radiation, the edges of the
rectangular conductor can be rounded to form an elliptical shape.
5.2.4.3 Tubular Conductors.
Tubular conductors provide the best compromise
between factors such as availability, cost weight,
cross-sectional area, skin effect, resistance ratio and inductance.
By using the actual cross-sectional area of
the conductive material, the dc resistance of tubular conductors can be determined from Equation 5-1; it is
given for three different diameter copper tubes in Figure 5-7.
5-13
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Figure 5-6. Resistance Ratio of Rectangular Conductors (5-3)
Figure 5-7. Resistance versus Length for Various Sizes of Copper Tubing (5-7)
5-14
MIL-HDBK-419A
Both the dc and the ac resistances of a tubular conductor are greater than those of a solid conductor with the
same outside diameter.
However, the ac resistance does not increase as much as the dc resistance, and,
therefore, the resistance ratio of a tubular conductor is always less than that of a solid conductor. The ac
resistance for four sizes of copper tubing is given in Figure 5-8, and the resistance ratio for isolated
nonmagnetic tubular conductors of various sizes is given in Figure 5-9. For a given length of conductor, the ac
resistance per unit weight (i. e., per given amount of copper) is less at high frequencies for tubular conductors
than for any other shape.
The self-inductance of a conductor is reduced by the absence of a conductive medium in the center (5-7).
Therefore, the self-inductance of a tubular conductor will be less than that of a solid conductor with the same
diameter. The self-inductance of three representative sizes of copper tubes is given in Figure 5-10.
5.2.4.4 Structural Steel Members.
The steel I-beam in the structural framework of a building is another
conductor that is frequently used as a ground conductor. The resistivity of steel is approximately ten times
that of copper; however, the skin depth of steel is greater than 3 times that of copper. This increased skin
depth in steel increases the conducting area for high frequency currents.
For example, in comparing a 0.3
meter (12-inch) I-beam with a 4/0 AWG copper cable, the perimeter of the I-beam is about 30 times as great
and with a factor of 3 increase in the skin depth, the conducting area for high frequency currents in the steel
I-beam is close to 90 times larger. This advantage is offset somewhat by the fact that the current tends to
flow in the edges of the I-beam and by the surface roughness. The ac resistance will be increased by a factor of
4 because of this surface roughness and current distribution. Even so, the ac resistance of a 4/0 AWG copper
cable is 4.25 times as great as that of a 0.3 meter (12-inch) I-beam. In addition, the building framework usually
offers many paths in parallel, thus lowering both the ac resistance and the inductance between any two points
(5-8).
5.3 SIGNAL REFERENCE SUBSYSTEM NETWORK CONFIGURATIONS.
Within a piece of equipment the
signal reference subsystem may be a sheet of metal which serves as a signal reference plane for some or all of
the circuits in that equipment.
Between equipments, where units are distributed throughout the facility, the
signal ground network usually consists of a number of interconnected wires, bars or a grid which serves an
equipotential plane.
Whether serving a collection of circuits within an equipment or serving several equipments
within a facility, the signal reference subsystem will be a floating ground, a single-point ground, or a
multiple-point ground known as a multipoint or equipotential plane. Of the aforementioned signal reference
subsystems, the equipotential plane is the optimum ground for communications-electronics facilities.
For
existing facilities where the presence of equipment prohibit the installation of an equipotential plane beneath,
on, or in the floor, the plane may be installed overhead and the equipment connected to it. It is desirable, but
not mandatory, to retrofit existing C-E facilities with equipotential planes.
5.3.1 Floating Ground.
A floating ground is illustrated in Figure 5-11. In a facility, this type of signal ground system is electrically
isolated from the building ground and other conductive objects. Hence, noise currents present in the building's
ground system will not be conductively coupled to the signal circuits.
The floating ground system concept is
also employed in equipment design to isolate the signal returns from the equipment cabinets and thus prevent
noise currents in the cabinets from coupling directly to the signal circuits.
5-15
MIL-HDBK-419A
The effectiveness of floating ground systems depends on their true isolation from other nearby conductors, i.e.,
to be effective, floating ground systems must really float. In large facilities, it is often difficult to achieve a
completely floating system, and even if complete isolation is achieved it is difficult to maintain such a system
(5-9).
Figure 5-8. AC Resistance versus Frequency for Copper Tubing [5-7)
5-16
MIL-HDBK-419A
Figure 5-9. Resistance Ratio of Nonmagnetic Tubular Conductors (5-3)
5-17
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Figure 5-10
5-18
MIL-HDBK-419A
In addition, a floating ground system suffers from other limitations. For example, static charge buildup on the
isolated signal circuits is likely and may present a shock and a spark hazard. In particular, if the floated system
is located near high voltage power lines, static buildup is very likely.
Further, in most modern electronic
facilities, all external sources of energy such as commercial power sources are referenced to earth grounds.
Thus, a danger with the floating system is that power faults to the signal system would cause the entire system
to rise to hazardous voltage levels relative to other conductive objects in the facility. Another danger is the
threat of flashover between the structure or cabinet and the signal system in the event of a lightning stroke to
the facility. Not being conductively coupled together, the structure could be elevated to a voltage high enough
relative to the signal ground to cause insulation breakdown and arcing.
This system generally is not
recommended for C-E facilities.
Figure 5-11. Floating Signal Ground
5.3.2 Single-Point Ground.
(For lower frequencies, 0-30 kHz up to 300 kHz)*
A second configuration for the signal ground network is the single-point approach illustrated in Figure 5-12.
With this configuration, the signal circuits are referenced to a single point, and this single point is then
connected to the facility ground. The ideal single-point signal ground network is one in which separate ground
conductors extend from one point on the facility ground to the return side of each of the numerous circuits
* Refer to 5.4.3 for definition of frequency limits.
5-19
MIL-HDBK-419A
located throughout a facility. This type of ground network requires an extremely large number of conductors
and is not generally economically feasible. In lieu of the ideal, various degrees of approximation to single-point
grounding are employed.
Figure 5-12. Single-Point Signal Ground (For Lower Frequencies)
The configuration illustrated by Figure 5-13 closely approximates an ideal single-point ground.
It uses
individual ground buses extending from an earth electrode subsystem to each separate electronic system. In
each system, the various electronic subsystems are individually connected at only one point to this ground bus.
Another frequently used approximation to the ideal is illustrated in Figure 5-14. Here the ground bus network
assumes the form of a tree. Within each system , each subsystem is single-point grounded. Each of the system
ground points is then connected to a tree ground bus with a single insulated conductor (usually yellow).
5-20
MIL-HDBK-419A
Figure 5-13. Single-Point Ground Bus System Using Separate Risers (Lower Frequency)
5-21
MIL-HDBK-419A
Figure 5-14.
5-22
MIL-HDBK-419A
The single-point ground accomplishes each of the three functions of signal circuit grounding mentioned at the
beginning of this chapter. That is, a signal reference plane is established in each unit or piece of equipment and
these individual reference planes are connected together and to the earth electrode subsystem. An important
advantage of the single-point configuration is that it helps control conductively-coupled interference. As
illustrated in Figure 5-15, closed paths for noise currents in the signal ground network are avoided, and the
interference voltage, V N , in the facility ground system is not conductively coupled into the signal circuits via
the signal ground network. Therefore, the single-point signal ground network minimizes the effects of lower
frequency noise currents which may be flowing in the facility ground.
Single-point grounds, however, also become transmission lines at higher frequencies with earth being the other
side of the line. In addition, every piece of equipment bonded to this transmission line will act as a tuned stub.
In the presence of digital signals (square waves) the tuned circuits will ring at the specific frequencies to which
they are resonant.
Since single-point grounds behave as transmission lines at rf frequencies, they will have
different impedances as a function of frequency, i.e., they may appear as inductors, capacitors, tuned circuits,
insulators or pure resistance, and therefore become extremely poor grounds.
In a large installation, another
major disadvantage of the single-point ground configuration is the requirement for long conductors. The long
conductors (1/8
at the highest frequency of concern) prevent the realization of a satisfactory reference for
EQUIPMENTS
Figure 5-15. Use of Single-Point Ground Configuration to Minimize Effect of Facility Ground Currents
5-23
MIL-HDBK-419A
higher frequencies because of large self-impedances.
Further, because of stray capacitance between
conductors, single-point grounding essentially ceases to exist as the signal frequency is increased (5-10).
Because of the aforementioned reasons, single-point grounds are not recommended for use in communications
electronics facilities.
5.3.3 Multipoint Ground.
(For higher frequencies, 30-300 kHz and above)
The multipoint ground illustrated in Figure 5-16 is the third configuration used for signal ground networks. The
multipoint ground utilizes many conductive paths from the earth electrode subsystem to various electronic
systems or subsystems within the facility. Within each subsystem, circuits and networks are multiply connected
to this ground network. Thus, in a facility, numerous parallel paths exist between any two points in the ground
network as shown in Figure 5-17.
Multipoint grounding frequently simplifies circuit construction inside complex equipments; it is the only
realistic method for the grounding of higher frequency signal circuits.
This method of grounding permits
equipments employing coaxial cables to be more easily interfaced since the outer conductor of the coaxial cable
does not have to be floated relative to the equipment cabinet or enclosure.
The multipoint grounding has the
disadvantage of exhibiting transmission line characteristics at rf frequencies. To be effective, a multipoint
ground system requires an equipotential ground plane whenever the conductors exceed 1/8
frequency of concern (5-11).
EQUIPMENTS
EARTH ELECTRODE SUBSYSTEM
Figure 5-16. Multipoint Ground Configuration
5-24
at the highest
MIL-HDBK-419A
Figure 5-17.
5-25
MIL-HDBK-419A
Care must also be taken to ensure sixty hertz power currents and other high amplitude lower frequency currents
flowing through the facility ground system do not conductively couple into signal circuits and create intolerable
interference in susceptible lower frequency circuits.
5.3.3.1 Equipotential Plane.
The importance of equipotential ground planes cannot be overemphasized for proper equipment operation, as
well as for EMI and noise/static suppression.
An equipotential ground plane implies a mass, or masses of
conducting material which, when bonded together, offers a negligible impedance to current flow. Connections
between conducting materials which offer a significant impedance to current flow, can place an equipotential
plane at a high potential with respect to earth.
High impedance interconnections between metallic members
subject to large amounts of current due to power system faults can be extremely hazardous to personnel and
equipment. The RFI effect of an equipotential plane or system must however be carefully considered, and it is
important to understand that grounding may not, in and of itself, reduce all types of RFI. On the contrary,
grounding a system may in some instances increase interference by providing conductive coupling paths or
radiative or inductive loops.
Many of the deficiencies of the wire distribution system can be overcome by embedding a large conducting
medium, in the floor under the equipments to be grounded. For existing facilities this system may be installed
above the equipment to be grounded.
A large conducting surface presents a much lower characteristic
impedance than that of wire because the characteristic impedance (Z o) is a function of L/C, hence as capacity
The capacity of a metallic sheet or grid to earth is much higher than that of
to earth increases, Z O decreases.
wire. If the size of the sheet is increased and allowed to encompass more area, the capacitance increases.
Also, the unit length inductance decreases with width, which further decreases Z o . If the dimensions of a
metallic sheet increase extensively (as in the case of conducting floor), the characteristic impedance
approaches a very low value. In this case, the characteristic impedance would be quite low throughout a large
portion of the spectrum.
This, in turn, would establish an equipotential reference plane for all equipments
bonded to it.
Although it is not necessary from a functional point of view, terminating the surface to an earth connection
presents the following advantages:
a.
Personnel safety is not dependent on long cable runs for protection against power faults.
b.
Low impedance is provided for power and radio frequencies.
Grounding buses in a communication facility where higher frequencies are present, act as lossy transmission
lines and therefore must be treated as such. Due to this phenomena single-point grounds and multipoint grounds
employing ground buses are high impedance grounds at higher frequencies. To be effective at the higher
frequencies,
the multipoint ground system requires
the existence of an equipotential ground plane.
Equipotential Planes are sometimes considered to exist in a building with a metal floor or ceiling grid
electrically bonded together, or in a building with the ground grid embedded in a concrete floor connected to
the structural steel and the
equipotential plane.
facility ground system.
Equipment cabinets are then connected to the
Chassis are connected to the equipment cabinets and all components, signal return leads,
5-26
MIL-HDBK-419A
etc., are connected to the chassis. The equipotential plane is then terminated to the earth electrode subsystem
and to the main structural steel via multiple connections, to assure personnel safety and a low impedance path
for all frequencies and signals.
It is again emphasized, however, that care must be taken not to create loops
which can couple signals from one system to another.
The equipotential plane also offers the following additional advantages;
Any “noisy” cable or conductor connected to the receptor, i.e., receivers, modems, etc., through or
a.
along such a ground plane will have its field contained between the conductor and the ground plane. The noise
field can be “shorted out” by filters and bond straps because the distance between these “transmission line”
conductors is very small. Shorting out the noise field has the desirable effect of keeping noise current from
flowing over the receptor case and along any antenna input cables.
Filters at the interface terminals of equipment can operate more effectively when both terminals of
b.
their equivalent “transmission line” are available. A S in a, above, a large conducting surface makes it possible
to contain the field carried by the offending conductor, in such a way that it can be more easily prevented from
traveling further.
A large conducting surface may also shield or isolate rooftop antennas from noisy cables below it.
c.
5.3.3.2
Types of Equipotential Planes.
Conducting materials that can be utilized for equipotential planes are
(a) a copper grid embedded in the concrete floor such as a computer floor, (b) a subfloor of aluminum, copper,
phospher bronze screen or sheet metal laid underneath the floor tile or carpet or (c) a ceiling grid above the
equipment.
Additional data and information on each of these planes can be found in para 1.5.1.1.1 of Vol II.
5-27
MIL-HDBK-419A
5.3.4 Floating System.
The floating ground system is completely insulated from the building or from any wiring that may be a source of
circulating currents. The effectiveness of floating ground systems depends on their true isolation. In large
systems, it is difficult to provide required isolation to maintain a good quality floating ground. Insulation
breakdown occurs easily because static charges, fault potentials and lightning potentials may accumulate
between the floating ground and other accessible grounds, such as external power line neutrals, water pipes,
etc.
Due to the personnel hazards from the difference of potential between the floating ground and building
ground, this system is not recommended.
The preferred grounding method is to have an equipotential plane bonded to the earth electrode subsystem and
building structure steel at multiple points with the structural steel also bonded to the earth electrode
subsystem.
In those facilities which do not have structural steel, multiple copper downleads should be
connected from the equipotential plane to the earth electrode subsystem.
5.4 SITE APPLICATIONS.
Because of the interference threat that stray power currents present to audio, digital, and control circuits (or
others whose operating band extends down to 60 Hz or below), steps must be taken to isolate these large
currents from signal return paths.
Obviously, one way of lessening the effects of large power currents is to
configure the signal ground system so that the signal return path does not share a path common with a power
return. This can be accomplished by making sure that the grounding conductor (green wire) of the power system
is always run in the same cable, conduit, duct, or raceway with the phase and neutral conductors to the first
service disconnect and then bonded to the earth electrode subsystem.
The first step in the development of an interference-free signal reference subsystem for an equipment or a
facility is to assure that the ac primary power return lines are interconnected with the safety grounding
network at only one point. Isolation of ac power returns from the signal reference subsystem is a major factor
toward reducing many noise problems.
Additional steps should also be taken to minimize other stray ac
currents such as those resulting from power line filters.
(one way of reducing these currents is to limit the
number of filter capacitors in an installation by using common filtered ac lines wherever possible or by locating
the filters as near as possible to the power service entry of the facility.)
To meet the safety requirements while minimizing the effects of power currents flowing with signal currents
through a common impedance, a single connection* between the power distribution neutral and the earth
electrode subsystem is necessary.
This single connection eliminates conductive loops in which circulating
(power) currents can flow to produce interference between elements of the signal reference network.
*This connection to the earth electrode subsystem should be made from the first service disconnect. Care
should be taken to ensure that the signal reference, fault protection, and lightning protection subsystems are
bonded to the earth electrode subsystem at separate ground rod locations.
5-28
MIL-HDBK-419A
5.4.1 Lower Frequency Network (0-30 kHz, and in some cases up to 300 kHz). The lower frequency grounding
network for the facility should conform to the following principles:
a.
It should be isolated from other ground networks including structural, safety, lightning and power
grounds, etc.
The purpose of this isolation is to prevent stray currents (primarily 50/60 Hz power) from
developing voltage differentials between points on the ground network.
b.
The inter-equipment or facility ground system should not be expected to provide the primary return
path for signal currents from the load to the source. Figure 5-18 illustrates a way of discriminating against
those extraneous signals which may inductively or capacitively induce currents into the grounding network and
develop differential voltages between the source and the load. For example, Figure 5-19 illustrates a practice
that is not recommended.
If only one source and one load constitute the entire system or if the various
source-load pairs within the system are essentially noninterfering in nature of their operation, this grounding
arrangement may be acceptable.
c.
The lower frequency grounding network must be connected to the earth electrode subsystem at only
one point.
Figure 5-18. Recommended Signal Coupling Practice for Lower Frequency Equipment
5-29
MIL-HDBK-419A
Figure 5-19. Ground Network Used as Signal Return
(Practice Not Generally Recommended)
d.
The network must be configured to minimize conductor path lengths. In facilities where the
equipments to be connected to the ground network are widely separated, more than one network should be
installed.
e.
Finally, the conductors of the network are to be routed in a manner that avoids long runs parallel to
primary power conductors, lightning down conductors, or any other conductor likely to be carrying high
amplitude currents.
5.4.2 Higher Frequency Network (> 300 kHz, and in some cases down to 30 kHz).
The higher frequency (equipotential) network provides an equal potential plane with the minimum impedance
between the associated electronic components, racks, frames, etc.
This plane shall be used at facilities or
areas within facilities where interface frequencies are over 300 kHz and may be used at sites where interface
frequencies are as low as 30 kHz. In higher frequency systems, equipment chassis are frequently used as the
signal reference. The chassis in turn is usually connected to the equipment case at a large number of points to
achieve a low impedance path at the frequencies of interest. See Para. 5.4.3.
5-30
MIL-HDBK-419A
The National Electrical Code requires that equipment cases and housings be grounded to protect personnel from
hazardous voltages in the event of an electrical fault.
Stray currents in the fault protection network can
present an interference threat to any signal system whose operating range extends down into the lower
frequency range and should be eliminated.
Where such problems exist, it is advisable to attempt to reduce the
impedance of the reference plane as much as possible.
A practical approach is to interconnect equipment
enclosures with the equipotential plane, via building structural steel, cable trays, conduit, heating ducts, piping,
etc., into the earth electrode subsystem to form as many parallel paths as possible. It should be recognized that
because of the inductance and capacitance of the network conductors, such multi point ground systems offer a
low impedance only to the lower frequency noise currents; however, these currents can be the most troublesome
in many facilities. Higher frequencies find a much lower impedance to ground through the distributed capacity
of the equipotential plane.
5.4.3 Frequency Limits.
The question remaining concerns the frequency below which signals can be considered as lower frequency.
Certainly the dividing line between the lower and higher frequency should be high enough to include all audio
communications signals. Since digital systems employ frequencies which extend from dc up to several hundred
MHz, a decision based on pulsed-signal considerations is more appropriate. To minimize the possibility that the
ground bus conductors will form antennas,
the lengths should not exceed 0.02 wavelength which is
approximately 21 meters (70 feet) at 300 kHz. Since the grounding buses in medium to large sized facilities
may extend 21 meters (70 feet), 300 kHz appears to be the maximum frequency for which a single-point
grounding system should be used. At frequencies up to 30 kHz, conductor lengths up to 210 meters (700 feet)
can be approached without exceeding the 0.02 wavelength criteria.
MIL-STD-188-124A establishes the lower frequency network range from dc to 30 kHz and in some cases
(depending on the interface frequency) up to 300 kHz. The higher frequency network range extends above 300
kHz and may in some cases be used at sites where the interface frequencies are as low as 30 kHz. The
frequency range from 30 kHz to 300 kHz is a mutual area and may be considered as either higher or lower
depending upon the interface frequency.
5-31
MIL-HDBK-419A
5.5 REFERENCES.
5-1. Reference Data for Radio Engineers, Sixth Edition, Howard W. Sams and Co, Inc, New York, 1975.
5-2. D.G. Fink and J.M. Carroll, Standard Handbook for Electrical Engineers, Tenth Edition, McGraw-Hill Book
Company, Inc, New York, 1968.
5-3. F.E. Terman, Electronic and Radio Engineering , McGraw-Hill Book Company, Inc, New York, 1955.
5-4. S. Ramo and J.R. Whinnery, Fields and Waves in Modern Radio, John Wiley and Sons, Inc, New York, 1943.
5-5. H.A. Wheeler, "Formulas for the Skin Effect," Proc. of IRE, September 1942, pp 412-424.
5-6. F.E. Terman, Radio Engineer’s Handbook, McGraw-Hill Book Company, Inc, New York, 1943.
5-7. “Electromagnetic Compatibility Principles and Practices,” NHB 5320.3, National Aeronautics and Space
Administration, Washington DC, October 1965.
5-8. S.L. Crawford, et. al., “Final Report on the Development of Bonding and Grounding Criteria for
John F. Kennedy Space Center, “WOL-TR-4201 (3 volumes), Contract NAS10-6879, Philco-Ford Corp,
Palo Alto CA, 30 June 1970.
5-9. H.W.
Denny,
et.
al.,
“Electronic
Facility
Bonding,
Grounding,
and
Shielding
Review,”
Report No. FAA-RO-73-51, Contract No. DOT-FA72WA-2850, Engineering Experiment Station, Georgia
Institute of Technology, Atlanta GA, November 1972, AD 760639.
5-10.. F.E. Barline, D.H. O'Bryhim, and C.F. Thompson, "A Study of Factors that Affect the Signal-to-Noise
Ratio at U.S. Naval Shore Receiving Facilities,” Contract No. BSR 71118, 13 Bureau of Ships, Department of
the Navy by Cooke Engineering Company, Alexandria VA.
5-11. P.V. Roberts, “Proposed Grounding and Bonding Criteria for Naval Shore Based Facilities,” Naval
Electronics Systems Test and Evaluation Facility, St. Inigoes MD, Project No. 68-78, 20 January 1970.
5-32
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CHAPTER 6
INTERFERENCE COUPLING AND REDUCTION
6.1 INTRODUCTION.
A large number of diverse equipments are usually present in an electronics complex. The various systems and
subsystems making up the complex may be concentrated in a small area such as a single room or they may be
distributed over a wide geographical area and be located in several buildings.
Whether the distances between
individual equipments are large or small, the entire system must function as an integral unit. Each equipment
must supply its designated output -- whether it be audio or rf, or analog or digital -- to some terminal point
such as an antenna, land line, or another piece of equipment. Both primary and backup power must be supplied.
Critical points in the system must be monitored both locally and remotely. To perform all the required tasks
and functions, many control, power distribution, and signal transmission networks are necessary.
Within the interconnected complex, many potentially incompatible signals are present. For example, at one
extreme are the large power sources (primarily dc and 60 Hz) supplying the various subsystems. At the other
extreme, low level dc and very low frequency signals from monitors, indicators and other specialized devices
are present. A lSO in the low frequency range are audio signals used for communications and control functions.
in the higher frequency region of the spectrum are the rf signals ranging from hf to microwaves used for
communications, surveillance and tracking, and other functions.
These signals range in amplitude from the
microwatt levels typical at communications receiver inputs to the kilowatt and megawatt levels transmitted by
some radar systems.
Ranging from audio frequencies into the rf region are the broadband display and
communications systems, both analog and digital, which may span from a few hertz to several megahertz in
frequency and may range in amplitude from a few millivolts to a few volts. Falling in overlapping frequency
ranges, these various signals present within the complex may interact in an undesirable manner to cause
Interference (generally manifested as annoying “noise”).
Interference is any extraneous electrical or electromagnetic disturbance that tends to interfere with the
reception of desired signals or that produces undesirable responses in electronic systems. Interference can be
produced by both natural and man-made sources either external or internal to the electronic system. The major
objective of interference reduction in modern electronic equipments and facilities is to minimize and, if
possible, prevent degradation in the performance of the various electronic systems by the interactions of
undesired signals, both internal and external.
The correct operation of complex electronic equipments and facilities is inherently dependent on the
frequencies and amplitudes of both the signals utilized in the system and the interference signals present in the
facility. If the frequency of an undesired signal is within the operating frequency range of the system, errors in
the system response may be obtained. The extent of the system response is a function of the amplitude of the
undesired signal relative to that of the desired signal.
For example, in systems operating with high level
signals, undesired signals with amplitudes on the order of volts may be tolerable, while in low level systems a
few microvolt may produce intolerable errors in the response of the system. An important element in the
control of unwanted interactions between signals is the proper grounding of the system.
6-1
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An ideal signal system is a simple signal generator-load pair as shown in Figure 6-1.
voltages present within the loop, this simple pair is free of interference.
With no extraneous
Consider, however, what happens
when the current return path is non-ideal and sources of noise are present as shown in Figure 6-2. Unless noise
voltages V N1 and V N2 are identical, a voltage difference will exist between the low side of the generator (Node
1) and the low side of the load (Node 2). As shown in Figure 6-3, this voltage difference effectively appears in
the signal transfer loop in series with the signal generator and produces noise currents in the load. Four ways of
combating this noise problem are as follows:
a.
Isolate the source-load pair from the noise sources; i.e., float the system and provide the necessary
shielding and filtering to prevent coupling by other means.
b.
Connect the low side of the loop to the reference plane at either Node 1 or Node 2 but not at both.
c.
Reduce the impedance, Z return , of the path connecting the two noise sources.
d.
Reduce the magnitudes of V N1 and V N2 through the control of the currents producing them by
lowering the impedance through which these currents flow.
Practical electronics circuits typically are a collection of several source-load combinations such as shown in
Figure 6-4. These various source-load combinations may be functionally dependent on each other. Hence each
individual source-load pair can not operate in isolation; there must be coupling between pairs. For example, one
source may be driving several loads; one load may be receiving signals from several sources; or the load for one
signal source may serve as the source for another load.
At the circuit level, numerous sources and loads are
connected in an interrelated fashion and the use of individual return paths for each source-load pair becomes
impractical. It is more realistic to establish a common ground or reference plane which serves as the return
path for several signals. The control of undesired network responses, particularly in high gain and/or higher
frequency circuits, often requires the establishment of a common signal reference to which functional grouping
of components, circuits, and networks can be connected. Ideally, this common reference connection offers zero
impedance paths to all signals for which it serves as a reference.
The several signal currents within the
network can then return to their sources without creating unwanted conductive coupling between circuits.
Figure 6-1. Idealized Energy Transfer Loop
6-2
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Figure 6-2. Energy Transfer Loop With Noise Sources in Ground System
Figure 6-3. Equivalent Circuit of Non-Ideal Energy Transfer Loop
6-3
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( a ) ONE SOURCE DRIVING MULTIPLE LOADS
( b ) CASCADED PAIRS
( c ) SEPARATE PAIRS C0NNECTED
TO COMMON GROUND PLANE
Figure 6-4. Practical Combinations of Source-Load Pairs
6-4
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At the equipment level, the individual signal reference planes for the various networks must be connected
together to prevent personnel shock hazards (see Chapters 5 and 9) and to provide as near as possible, the same
signal reference for all networks.
Thus, the signal reference plane may extend over large distances within a
facility. The assumption that this large reference plane provides zero impedance paths is not valid; the series
inductance and resistance of the conductors forming the signal reference plane and the shunt capacitance to
nearby conductive objects must be considered.
Currents flowing in the signal reference plane will develop
voltages across this impedance and will produce electric and magnetic fields around the conductors.
6.2 COUPLING MECHANISMS.
Coupling is defined as the means by which a magnetic or electric field produced by one circuit induces a voltage
or current in another circuit.
Interference coupling is the stray or unintentional coupling between circuits
which produces an error in the response of one of the circuits. The possible sources of spurious signals and the
mechanisms by which this interference is coupled into a susceptible circuit must be understood in order to guard
against interference pickup by sensitive signal circuits.
The techniques for reducing pickup depend on the type of interference present.
classified by its coupling means; i.e., as either being conductive or free-space.
Interference is broadly
Conductive coupling occurs
when the interfering and the interfered-with circuits are physically connected with a conductor and share a
common impedance.
Free-space coupling occurs when a circuit or source generates an electromagnetic field
that is either radiated and then received by a susceptible circuit or that is inductively or capacitively coupled
(near-field) to a susceptible circuit.
6.2.1 Conductive Coupling.
Power lines entering a facility provide good conductive coupling paths from interference sources external to the
facility. This interference can easily be conducted into a particular unit or piece of equipment via the power
lines entering the equipment.
Also, interference can conductively couple between various circuits inside the
equipment on the common dc power lines. If one dc power supply is utilized with several circuits operating over
various signal voltage and frequency ranges, the operation of one circuit may adversely affect the operation of
other circuits. For example, if both the preamplifier and the power amplifier sections of an audio amplifier are
supplied from a single dc power supply, variations in the relatively large current drawn by the power amplifier
can appear as supply voltage variations at the preamplifier.
These variations can be large compared to the
operating signal levels in the preamplifier; the unwanted variations are amplified along with the desired signals
and may produce distortion in the output of the amplifier.
Another set of paths for conductive coupling of interference is offered by the signal lines. In general, signal
lines enter each facility and each unit or piece of equipment; i.e., such signal lines are usually necessary for
interfacing electronic circuits. Interference can be conductively coupled into facilities, equipment, and circuits
as readily by signal lines as by power lines.
6-5
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The signal reference plane is another potential coupling path for unwanted signals between equipments and/or
circuits.
Since practical signal reference planes do not exhibit a zero impedance, any currents flowing in a
signal reference plane will produce potential differences (voltages) between various points on the reference
plane. Interfacing circuits referenced to these various points can experience conductively coupled interference
in the manner illustrated by Figure 6-5. The signal current I1, flowing in Circuit 1 of Figure 6-5 returns to its
source through signal reference impedance Z R producing a voltage drop E N1 in the reference plane.
impedance Z R is common to Circuit 2, hence E NI appears in Circuit 2
signal voltage source, E S 2.
The
as a voltage in series with the desired
This undesired source produces an interference voltage, V N 2, across the load of
Circuit 2; similarly, the desired current, I 2 , in Circuit 2 will produce interference in Circuit 1.
In a facility, the conductive coupling of interference through the signal reference plane of interfaced equipment
can occur in a manner similar to that described above for internal circuitry.
If Circuit 1 in Figure 6-5
represents two interfaced equipments and if Circuit 2 represents a different pair of interfaced equipments, then
a current flowing in either circuit can produce interference in the other circuit as described. Even if the pairs
of equipments do not use the signal reference plane as the signal return, the signal reference plane can still be
the cause of coupling between equipments. Figure 6-6 illustrates the effect of a stray current, I R , flowing in
the reference (or ground) plane.
I R may be the result of the direct coupling of another pair of equipments to
the signal reference plane, or it may be the result of free-space coupling to the signal reference plane. In
either case, IR produces a voltage E N in the reference plane impedance, Z R . This voltage produces a current in
the interconnecting loop which in turn develops a voltage across Z L , in Equipment B. Thus, it is evident that
interference can conductively couple via the signal reference plane to all circuits and equipments connected
across the non-zero impedance elements of that reference plane.
6.2.2 Free-Space Coupling.
Free-space coupling is the transfer of electromagnetic energy between two or more circuits not directly
interconnected with a conductor.
Depending on the distance between the circuits, the coupling is usually
defined as either near-field or far-field.
Near-field coupling can be subdivided into inductive and capacitive
coupling, according to the nature of the electromagnetic field.
In inductive coupling, a magnetic field linking
the susceptible device or circuit is set up by the interference source or circuit. Capacitive coupling is produced
by an electric field between the interference source and the susceptible circuit.
Radiation of energy by electromagnetic waves is the principle coupling mechanism in far-field coupling. The
term “radiated coupling” is sometimes used to describe both near-field (inductive and capacitive) coupling and
far-field coupling. However, radiated coupling is generally accepted to mean the transfer of energy from a
source to a susceptible circuit by means of electromagnetic waves propagating through space according to the
laws of wave propagation.
6.2.2.1 Near-Field Coupling.
When two or more wires or other conductors are located near each other, currents and voltages on one wire will
be inductively and capacitively coupled to the other wires. The wire acting as the interference source for this
near-field coupling may be any conductor such as a high level signal line, an ac power line, a control line,or
even a lightning down conductor.
The currents or voltages induced into the other wires can further be
conductively coupled into susceptible circuits.
6-6
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Figure 6-5. Coupling Between Circuits Caused by Common Return Path Impedance
Figure 6-6. Conductive Coupling of Extraneous Noise into Equipment Interconnecting Cables
6-7
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6.2.2.2 Inductive Coupling.
The magnetic field surrounding a long, straight, current-carrying wire as shown in
Figure 6-7 is the means for inductive coupling. This field can be determined from Ampere’s law (6-1):
(6-1)
where H is the magnetic field strength and
d— is a small element of length along the path of integration (any
closed loop around the current i(t)). Choosing a circle of radius r for the integration path in Equation 6-1 allows
one to derive an expression for the magnetic field:
(6-2)
Thus the magnetic field strength surrounding a long straight wire carrying a current i is inversely proportional
to the distance r from the wire, i.e., H decreases as the distance from the wire increases.
Figure 6-7. Magnetic Field Surrounding a Current-Carrying Conductor
6-8
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This magnetic field will induce a voltage into a nearby signal circuit loop as illustrated in Figure 6-8.
According to Faraday’s law (6-1), the induced voltage is
(6-3)
where v i (t) is the induced voltage and
is the total magnetic flux linking the susceptible circuit loop. This
magnetic flux is given by
(6-4)
where B = µH
— is the magnetic flux density, µ is the absolute permeability of the medium, and d s is a small
element of the loop area. Substituting Equation 6-2 into Equation 6-4 and integrating over the area of the loop
in Figure 6-8 gives
(6-5)
where r l and r 2 are the distances from i(t) to the two sides of the loop which are parallel to i(t), and
length of each of these sides (in meters). This equation gives the total magnetic flux linking a susceptible
Figure 6-8. Illustration of Inductive Coupling
6-9
is the
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circuit loop in terms of the current flowing in a nearby conductor parallel to the sides of the loop. Substituting
the total magnetic flux from Equation 6-5 into Equation 6-3 gives the voltage vi(t), induced in the circuit loop:
(6-6)
In free space, µ = 4
x 10-7 henrys/meter, and Equation 6-6 reduces to
(6-7)
If the interference current in the nearby conductor is sinusoidal, i.e.,
(6-8)
then the maximum value of di/dt is
(6-9)
and the maximum value of the induced voltage in Equation 6-6 is
(6-10)
where f is the frequency of i(t).
In Equations 6-6 and 6-7, the induced voltage vi (t) in a circuit loop with sides parallel to a current i(t) is
expressed in terms of the dimensions of the loop, the distance of the loop from the current i(t) and time rate of
change of i(t). As can be seen from these equations, the induced voltage in a susceptible circuit loop increases
with an increase in the loop area (either an increase in
or r 2 or both), the frequency, f, or the time rate of
change, dl/dt, of the interfering sources, and increases with a decrease in the distance, r l, between the
interfering source and the loop.
The preceding equations indicate that the induced voltage is independent of the impedance of the susceptible
circuit loop; i.e., the amplitude of the induced voltage is the same in a high impedance circuit as it is in a low
impedance circuit. The desired signal voltages in low impedance circuits, however, are generally much lower
than in high impedance circuits. Therefore, in low impedance circuits the induced voltage can be high relative
to the signal voltage and thus more likely to produce significant interference in the circuit load. In high
impedance circuits the saline induced voltage may be small compared to the circuit signal voltages and thus not
create any problems.
For these reasons, 1o W impedance circuits are usually more susceptible to inductive
coupling than are high impedance circuits.
6-10
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6.2.2.3 Capacitive Coupling.
When signal conductors of two circuits are near each other as shown in
Figure 6-9, a capacitance, Cc exists between the conductors. The value of this capacitance is a function of the
geometry of the signal lines. For parallel wires, C C is given by (6-2).
farads/meter,
(6-11)
where r c is the distance between the two lines and d is the diameter of the wires. In a similar manner, a
capacitance exists between each signal line and its return.
If the signal line is parallel to its return, these
capacitances can be calculated using Equation 6-11 by replaci ng r c with r l and with r 2 (see Figure 6-9).
The interference source voltage, v S(t) produces a current flow
through the mutual capacitance, Cc, between the
two signal conductors and develops an induced voltage, vi(t), in the susceptible circuit. The equivalent circuit
for Figure 6-9 is given in Figure 6-10(a) where the parallel combination of Z S2 and Z L2 has been replaced by
the equivalent impedance
(6-12)
Figure 6-9. Illustration of Capacitive Coupling
6-11
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and time-varying voltages have been replaced by their ac steady state phasors. The induced voltage (ac steady
state assumed) in the susceptible circuit is
(6-13)
where
(6-14)
and
(6-15)
Substitution of Equations 6-14 and 6-15 into Equation 6-13 yields
(6- 16)
where
If
j
Z 2 C 2 << 1, Which is generally true at lower frequencies, the equivalent circuit of Figure
6- 10(b) is applicable and
(6-17)
At higher frequencies, the equivalent circuit of Figure 6-10(c) is applicable and
(6-18)
These equations illustrate the induced voltage, V i, which is capacitively coupled into a susceptible signal circuit
from a nearby signal conductor, is dependent on the amplitude and frequency of the interference source
voltage, VS, the values of the coupling capacitance, Cc, the stray capacitance in the susceptible circuit, C 2, and
on the magnitude of the impedance of the susceptible circuit. At low frequencies, Equation 6-17 indicates that
the induced voltage increases with either an increase in the coupling capacitance or an increase in the
impedance of the susceptible loop. Similarly, at high frequencies the induced voltage as given in Equation 6-18
increases with either an increase in the coupling capacitance or a decrease in the stray capacitance of the
susceptible circuit. It should also be noted that the value of the interference source voltage, V S, depends upon
the stray capacitance in the interference source circuit, C 1 in Figure 6-9.
6-12
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If the impedance of the susceptible circuit is totally resistive, i.e., if Z2 = R2, the induced voltage is given by
Equation 6-16 as
(6-19)
and the magnitude of the ratio of the induced voltage to the interference (sinusoidal) voltage is
( a ) TRUE EQUIVALENT CIRCUIT
( b ) LOW FREQUENCY APPROXIMATION
( c ) HIGH FREQUENCY APPROXIMATION
Figure 6-10. Equivalent Circuit of Network in Figure 6-9
6-13
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(6-20)
This ratio increases almost linearly with R 2 until R 2 approaches the value
i.e., the reactance of
C 2 and C C in parallel. For larger values of R2, the ratio asymptotically approaches C C /(C 2 + CC ). The behavior
of this voltage ratio with frequency is illustrated in Figure 6-11. The ratio is zero at dc and asymptotically
approaches C C /(C 2 + CC ) as the frequency is increased. Equation 6-20 and Figure 6-11 illustrate again that the
voltage capacitively coupled into the susceptible circuit increases with an increase in the total resistance of the
circuit and with an increase in frequency. Resonances can occur and change the amount of capacitive coupling
if the impedance of the susceptible circuit contains inductive reactance, but such resonances usually only
produce noticeable effects at higher frequencies.
6.2.2.4 Far-Field Coupling.
Radiation is the means by which energy escapes from a conductor and propagates into space. The conductor
does not have to be specifically designed to radiate energy; it may be any current carrying conductor, e.g., a
signal line, a power line, or even a ground lead.
Algebraic expressions for the electromagnetic fields surrounding a current carrying conductor are usually
expressed as the sum of three terms.
Each term is inversely proportional to a power of the distance, r, from
the conductor, i.e., each term is proportional to either 1/r, l/r 2 , or l/r3 .
2
Close to the conductor (near field),
3
the l/r a n d l / r c o m p o n e n t s d o m i n a t e a n d t h e e l e c t r o m a g n e t i c e n e r g y o s c i l l a t e s b e t w e e n t h e s p a c e
surrounding the conductor and the conductor itself; zero average energy is propagated by the near field terms.
Outside the near field region, the l/r term predominates.
In this far field region, radiated energy that has
escaped is propagating away from the “antenna” through space.
The mechanism of energy radiation can be
visualized (6-3) by considering the finite time required for the electromagnetic fields to propagate between two
points in space. Current flows through an antenna at the frequency of the applied signal, and the polarity of the
field produced by this current is reversed at this same frequency. When a positive charge is present at one end
of the antenna, an equal but, negative charge is present at the other end and an electric field in the vicinity of
the antenna will be established between the charges. As the current changes direction, the charges will reverse
positions; the electromagnetic field will collapse and be re-established in the opposite direction.
If the
frequency of the applied signal is low, sufficient time will exist between reversals for practically all the energy
stored in the field to be returned to the circuit and very little radiation will occur. If, however, the frequency
i S high and the charges reverse quickly, a field in the opposite direction is formed near the wire before a
substantial amount of the field energy can return to the circuit. This part of the field is thus separated from
the antenna and propagates outward through space as an electromagnetic wave.
6-14
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Figure 6-11. Characteristic Voltage Transfer Curve for Capacitive Coupling
This method of visualizing radiation from a wire or antenna is illustrated for a dipole antenna in Figure 6-12.
Figure 6-12(a) shows a dipole when the charges are maximum at the ends of the antenna. As the current flow
reverses directions and the charges move back toward the center, the electric field lines collapse as in (b).
Since the field moves with a finite velocity, there is not enough time for all the field lines to return to the
antenna. When the ends of these lines meet at the center of the antenna and the charge on the antenna is zero,
the field lines that have not collapsed will close on themselves and continue to exist as closed loops as
illustrated in (c).
The antenna charges move in the opposite direction as shown in (d), and the oppositely
directed electric field pushes away the previously detached loop as shown in (e). This procedure continues with
the fields in the opposite direction, and a cycle is completed when the fields near the antenna return to their
original state.
These cycles repeat at the frequency of the applied signal, and an electromagnetic field
propagates outward from the antenna at the speed of light. Although only the electric field is illustrated, there
is an associated magnetic field in accordance with Maxwell’s equations (6-1). The magnetic field consists of
concentric circles surrounding the antenna and expanding radially as the electric field propagates outward.
These outward propagating electric and magnetic fields represent energy flowing away from the antenna.
Therefore, the antenna radiates energy into the surrounding space.
In a reciprocal manner , wires and conductors located in a radiated field have currents induced in them and act
as receiving antennas for incident electromagnetic energy.
These induced currents in the wires can then be
conducted into associated signal circuitry as interference (see Section 6.2.1). The amplitude of the resulting
interference depends on the strength of the electromagnetic field in the vicinity of the wire and on the
efficiency of the wire as an antenna. The strength of this field is a function of the distance from the radiating
wire, the efficiency of the radiating wire as an antenna, and the amplitude and frequency of the signal on the
radiating wire.
The efficiency of a wire or other conductor as either a receiving or a radiating antenna is a
function of the length of the wire relative to the wavelength of the signal.
6-15
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Figure 6-12. Electric Field Patterns in the Vicinity of a Radiating Dipole
6-16
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One way of evaluating the efficiency of a wire as an antenna is to compare its radiation resistance with the
radiation resistance of a quarter-wave ( /4) antenna. The radiation resistance of an antenna is the resistance
which would consume the same amount of power as is radiated by the antenna. Thus the radiation resistance is
a direct measure of the energy radiated from the antenna.
A monopole antenna one-quarter of a wavelength
long has a radiation resistance of 36.5 ohms (6-4). An antenna which transmits or receives ten percent or less
of the energy that would be transmitted or received by a
/4 monopole can be considered relatively inefficient.
Thus an inefficient antenna would exhibit a radiation resistance of 3.65 ohms or less. Monopoles of length
meet this criterion (6-4). Greater convenience in calculations results if /10 is choosen instead of /11. Thus
10 is chosen to represent the length below which a conductor does not perform effectively as an antenna.
6.3 COMMON-MODE NOISE.
Common-mode noise is an unwanted noise voltage which appears identically on both sides of a signal line when
measured from the system ground or common point.
It, like normal-mode noise, can be caused by resistive
coupling, capacitive coupling, or magnetic coupling from the unwanted source.
In addition, many measuring
transducers intentionally have a dc or ac common-mode voltage present on both output lines, the presence of
which is necessary for proper operation of the transducer. Although not a noise source, these common-mode
voltages require careful design and use of data and instrumentation amplifiers to prevent interference with the
desired signal components.
The source of most common-mode noise is resistive coupling between separate ground points in a circuit or
system. A simple example of this is illustrated in Figure 6-13. An oscilloscope probe is used to couple a signal
from some point in a circuit to the oscilloscope terminals.
The probe ground is connected to circuit ground
which is in turn referenced through the facility ground system. Since there are generally currents flowing in
the facility ground system (these are primarily at the 60 Hz power line frequency), it follows that the ground
reference potential for the circuit is different from that for the oscilloscope.
This difference in potential is
produced by the flow of the stray ground currents through the impedance of the facility ground system. Thus,
both the ground reference for the circuit and the signal point in the circuit have identical noise voltages
impressed on
them with respect to the ground reference for the oscilloscope.
This noise is called
common-mode noise by virtue of the fact that is common to all points in the circuit, including the circuit
ground.
Not only do these noise sources introduce measurement errors but they also produce interference
between interconnected equipments.
Resistively coupled coimmon-mode noise can also occur in a single equipment rather than between equipments.
The coupling arises from multiple signal currents and power frequency currents flowing in a common ground
lead, chassis, or ground plane.
6-17
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Figure 6-13.
6-18
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6.3.1 Basic Theory of Common-Mode Coupling.
The mechanism of common-mode coupling can be explained with reference to Figure 6-14. In this figure, V S
represents some signal voltage from an unbalanced source, i.e., the output signal of some transducer or
measuring amplifier, and R S is the output impedance of this source.
The source is connected to the input
terminals of some electronic device which is modeled as a two-terminal pair amplifier in the figure. R 1 and R 2
are the series resistances in the interconnecting cables between the source and amplifiers. The voltage source
V cm with output resistance Rein represents a common-mode noise voltage source which causes the signal
source to be at some voltage when measured with respect to the ground reference of the amplifier output. In
Figure 6-14, the impedances Z 1 and Z 2 represent the input impedances of the two amplifier terminals. In a
differential amplifier, these impedances are normally very high, however, in a single ended amplifier, one is
high and the other is very low since it is tied directly to the ground reference terminal.
The analysis of the circuit in Figure 6-14 is complicated enough to make it difficult to reach conclusions
without excessive algebra. Normally,
Rcm is small and can be neglected.
With this approximation, it can be
shown that the output voltage of the amplifier is given by
(6-21)
where K is the voltage gain of the amplifier.
There are two signal contributions to the output signal V O in Equation 6-21:
the desired signal and the
undesired common-mode noise. There are three ways in which the common-mode noise term can be reduced.
These are as follows:
a.
Decrease V c m - By decreasing V c m, the common-mode noise voltage at the output terminals
decreases proportionally.
6-19
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.
Figure 6-14. Common-Mode Noise in Unbalanced Systems
6-20
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b.
Balance the Two Amplifier Inputs - If R 1 and R2 are manipulated such that
Z2
R2 +
Z1
R
S +
Rl +
Z1 =
Z2 ’
(6-22)
the common-mode noise voltage at the amplifier output terminals can be made to vanish.
c.
Increase Z 1 and Z 2 - If Z1 is sufficiently large compared to R S + R l, and Z2 is sufficiently large
compared to R 2, then the common-mode noise voltage at the amplifier output terminals will be
diminished.
This approach normally requires a differential amplifier with carefully shielded input
signal lines.
In the case of balanced signal sources or transducers, the basic circuit and equations differ from those given in
Figure 6-14 and by Equation 6-21. Figure 6-15 shows a balanced source with an output voltage V S and output
In this case, the center tap of the source is
resistance R S connected to the two inputs of an amplifier.
connected to the ground reference terminal. As before, if it is assumed that R cm is small, it can be shown that
V O is given by
V O = KV X ,
(6-23)
The same conclusions regarding the minimization of the common-mode noise component at the amplifier output
apply in this circuit as for the unbalanced source. However, in this case the amplifier must have a differential
input stage. Otherwise, one-half of the source would be shorted out. In Figure 6-14, the amplifier can have
single-ended or differential inputs.
The common-mode rejection (CMR) ratio of an amplifier is the gain of the amplifier (K) multiplied by the
common-mode noise voltage ( V c m) and divided by the amplifier output due to V c m. The CMR ratio describes a
circuit’s ability to avoid converting common-mode noise to normal-mode noise. Expressed as a positive
quantity, the CMR ratio is given by
C M R
=
KV
____cm
o
6-21
V
s
= 0
(6-24)
MIL-HDBK-419A
Figure 6-15. Common-Mode Noise in Balanced Systems
6-22
MIL-HDBK-419A
Ideally, the CMR of an amplifier should be infinite, or as large as possible. Under the worse case conditions,
CMR = 1. As it is defined, the CMR conveys a measure of how well the amplifier can reject a common-mode
noise signal at its input. Typical values for a good differential amplifier with balanced input impedances are in
the vicinity of CMR = 1000. Often this is expressed in decibels which, in this case, would be CMR = 60 dB.
The CMR for the amplifier in Figure 6-14 is easily derived from Equation 6-21 to be
1
CMR =
Z1
R
6.3.2 Differential Amplifier.
S
+
Rl +
Z1 =
(6-25)
Z2
R2 + Z2
A differential amplifier is designed to make Z 2 large compared to R 2 and Z 1
large compared to R 1 + RS . Since Z 1 and Z 2 are normally functions of frequency, it can be seen that the CMR
will also be a function of frequency. Typically Z 1 and Z 2 are resistors shunted by capacitors. Thus, it can be
seen that the CMR will inevitably decrease with increasing frequency when the capacitive reactance becomes
smaller than the resistor. Consequently, a high CMR is difficult to achieve at high frequencies.
6.4 MINIMIZATION TECHNIQUES. Signal interaction, i.e., interference, can be minimized by reducing the
coupling between the signal systems by modifying the signal systems in such a manner that interaction between
the systems does not produce interference in either one, by eliminating the source of the interference, and by
filtering the interference out of the susceptible signal system.
6.4.1 Reduction of Coupling.
The techniques for reducing coupling include minimizing the impedance of the
reference plane, increasing the spatial separation between the signal systems, shielding the systems from each
other, reducing the loop area of each signal system, and balancing the signal lines in each system.
6.4.1.1
Reference Plane Impedance Minimization.
Minimizing the impedance of the signal reference plane
lowers the potential difference between any two points in the reference plane, thereby reducing the conductive
coupling of interference in susceptible circuits referenced to these points.
The impedance of the signal
reference plane is reduced by minimizing both the resistance (R) and the series reactance (X) of the conductors
forming the reference plane. The resistance decreases with a decrease in either the length of the conductors or
the signal frequency (because of skin effect - see Section 5.2.2.1) and with an increase in conductor crosssectional area. The reactance also decreases with a decrease in the signal frequency and with a decrease in the
inductance of the conductors; the inductance is a function of both the conductor length and cross-sectional
area. The impedance of the signal reference plane can be reduced by making the reference plane conductors as
short as possible and by using conductors with cross-sectional areas as large as practical.
The overall
impedance of the signal reference plane also depends upon the establishment of low impedance bonds between
ground conductors. (The various aspects of bonding and bond resistance are discussed in Chapter 7.)
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MIL-HDBK-419A
6.4.1.2 Spatial Separation.
Inductive or capacitive coupling can be reduced by increasing the physical
distance between signal circuits. As can be seen from Equation 6-6 and Equations 6-11 and 6-16, increasing the
separation between the interfering circuit and the susceptible circuit exponentially decreases the voltage
coupled into the susceptible circuit.
6.4.1.3 Reduction of Circuit Loop Area.
Reducing the loop area of either the interference source circuit or
the susceptible circuit will decrease the inductive coupling between the circuits. Equation 6-6 shows that the
inductively coupled voltage can be minimized by reducing the length ( ) or the with (r 2 - rl) of the susceptible
circuit. This width can be minimized by running the signal return adjacent to the signal conductor and, hence,
reducing the loop area of the susceptible circuit. A preferable approach is to twist the signal conductor with its
return.
The use of twisted wires reduces the inductively coupled voltages since the voltage induced in each
small twist area is approximately equal and opposite to the voltage induced in the adjacent twist area.
6.4.1.4 Shielding.
Another effective means for the reduction of coupling is the use of shields around the
circuits and around interconnecting lines. Principles of shielding are presented in Chapter 8.
6.4.1.5 Balanced Lines.
In situations where signal circuits must be grounded at both the source and the load, and hence, establish
conductive coupling paths, the use of balanced signal lines and circuits is an effective means of minimizing the
conductively coupled interference.
respect to ground.
In a balanced circuit, the two signal conductors are symmetrical with
At equivalent points on the two conductors the desired signal is opposite in polarity and
A common-mode voltage will be in phase and will exhibit equal
amplitudes on each conductor and will tend to cancel at the load. The amount of cancellation depends upon the
equal in amplitude relative to ground.
degree to which the two signal lines are balanced relative to ground.
If the signal lines are perfectly balanced, the cancellation would be complete and the coupled interference
voltage at the load will be zero. If the source and load are not normally or cannot be operated in a balanced
mode, balanced-to-unbalanced transformers or other coupling devices should be used at both the source and load
ends of the signal line.
6.4.2 Alternate Methods.
Several alternate methods exist for minimizing interference besides the reduction of coupling. The first
technique consists of actual circuit modification. For example, the signal frequency of either the interfering
source or the susceptible circuit can be changed such that the signals do not interfere with one another.
Similarly, the desired signal can be transposed to another frequency range or to a type of signal not affected by
the noise.
An example of the former is the conversion of the desired signal to VHF/UHF or microwave while an
example of the latter is the use of acoustic coupling and electro-optical transmission. Through the use of one
of these techniques, the frequency of transmission over that portion of the path susceptible to pickup is such
6-24
MIL-HDBK-419A
that the receiving and detection devices do not respond to the noise signal. As another option, the amplitude of
the interference source or the sensitivity of the susceptible circuit can be decreased to reduce the interaction
between the two circuits.
Further, the type of modulation used in one or both circuits can be changed to
minimize the interference.
Another technique is the elimination of the interference source. Although this may seem like a trivial solution,
it is a valid alternative in many situations. For example, the source of interference may be a rusty joint which
can be eliminated by proper bonding.
A third alternative is the use of filters. The majority of interfering signals, even if they are free-space coupled
to the signal and power lines, are conductively coupled into the susceptible circuit. The proper application of
filters to both the signal and power lines can reduce this coupling.
6.5 FACILITY AND EQUIPMENT REQUIREMENTS.
The interference rejection principles identified in this
chapter are responsible, in part, for many of the recommendations contained elsewhere in this volume and in
Volume II. For example, intersite or interbuilding common-mode noise voltages in the earth contribute to the
need for a low resistance of 10 ohms to earth at each facility. Even a resistance to earth of as low as 10 ohms
may not, however, alleviate all common mode noise on a data cable connecting two separate locations or
buildings.
While a low resistance may help, there will always be potential differences between any two rods in
the ground.
The use of shielded, balanced twisted pair for all lower frequency equipment interfaces
recommended in Volume II, is intended to provide additional common-mode rejection to those unavoidable noise
voltages which exist in any facility.
This is not to say that the sources of noise in a facility cannot be
controlled. In fact, much can be done by equalizing the load between the phases of the ac distribution system;
by insuring that the neutral is grounded only at the service disconnecting means as recommended in Volume II;
and by limiting the quantity of leakage current from power line filter capacitors by using tile smallest
acceptable value of capacitance or by sharing common filtered lines with several pieces of equipment.
6.6 REFERENCES.
6-1. J. D. Krause, Electromagnetics, McGraw-Hill Book Company, Inc, New York, (1953).
6-2.
F. E. Terman, Radio Engineers’ Handbook, McGraw-Hill Book Company, Inc, New York, (1943).
6-3. L.V. Blake, Antennas, John Wiley and Sons, Inc, New York NY, (1966).
6 - 4 . E . C . Jordan, Electromagnetic Waves and Radiating Systems, Prentice-Hall, Inc, Englewood Cliffs NJ,
(1950).
6-25/6-26
MIL-HDBK-419A
CHAPTER 7
BONDING
7.1 DEFINITION OF BONDING.
As used in these Volumes, bonding refers to the process by which a low
impedance path for the flow of an electric current is established between two metallic objects. Other types of
bonding which involve simply the physical attachment of one substance or object to another through various
mechanical or chemical means are not treated.
7.2 PURPOSES OF BONDING.
In any realistic electronic system, w hether it be only one piece of equipment or an entire facility, numerous
interconnections between metallic objects must be made in order to provide electric power, minimize electric
shock hazards, provide lightning protection, establish references for electronic signals, etc. Ideally, each of
these interconnections should be made so that the mechanical and electrical properties of the path are
determined by the connected members and not by the interconnection junction.
Further, the joint must
maintain its properties over an extended period of time in order to prevent progressive degradation of the
degree of performance initially established by the interconnection. Bonding is concerned with those techniques
and procedures necessary to achieve a mechanically strong, low impedance interconnection between metal
objects and to prevent the path thus established from subsequent deterioration through corrosion or mechanical
looseness.
In terms of the results to be achieved, bonding is necessary for the:
a.
protection of equipment and personnel from the hazards of lightning discharges,
b.
establishment of fault current return paths,
c.
establishment of homogeneous and stable paths for signal currents,
d.
minimization of rf potentials on enclosures and housings,
e.
protection of personnel from shock hazards arising from accidental power grounds, and
f.
prevention of static charge accumulation.
With proper design and implementation, bonds minimize differences in potential between points within the fault
protection, signal reference, shielding, and lightning protection networks of an electronic system. Poor bonds,
however, lead to a variety of hazardous and interference-producing situations. For example, loose connections
in ac power lines can produce unacceptable voltage drops at the load, and the heat generated by the load
current through the increased resistance of the poor joint can be sufficient to damage the insulation of the
wires which may produce a power line fault or develop a fire hazard or both. Loose or high impedance joints in
7-1
MIL-HDBK-419A
signal lines are particularly annoying because of intermittent signal behavior such as decreases in signal
amplitude, increases in noise level, or both.
dangerous.
Poor joints in lightning protection networks can be particularly
The high current of a lightning discharge may generate several thousand volts across a poor joint.
Arcs produced thereby present both a fire and explosion hazard and may possibly be a source of interference to
equipments.
The additional voltage developed across the joint also increases the likelihood of flashover
occurring to objects in the vicinity of the discharge path.
A degradation in system performance from high noise levels is frequently traceable to poorly bonded joints in
circuit returns and signal referencing networks.
As noted previously, the reference network provides low
impedance paths for potentially incompatible signals.
Poor connections between elements of the reference
network increase the resistance of the current paths. The voltages developed by the currents flowing through
these resistances prevent circuit and equipment signal references from being at the same reference potential.
When such circuits and equipments are interconnected, the voltage differential represents an unwanted signal
within the system.
Bonding is also important to the performance of other interference control measures. For example, adequate
bonding of connector shells to equipment enclosures is essential to the maintenance of the integrity of cable
shields and to the retention of the low loss transmission properties of the cables. The careful bonding of seams
and joints in electromagnetic shields is essential to the achievement of a high degree of shielding effectiveness.
Interference reduction components and devices also must be well bonded for optimum performance. Consider a
typical power line filter like that shown in Figure 7-1.
If the return side of the filter (usually the housing) is
inadequately bonded to the ground reference plane (typically the equipment case or rack), the bond impedance
Z B may be high enough to impair the filter’s performance. The filter as shown is a low pass filter intended to
remove high frequency interference components from the power lines of equipment. The filter achieves its goal
in part by the fact that the reactance, Xc, of the shunt capacitors is low at the frequency of the interference.
Interfering signals present on the ac line are shunted to ground along Path 1 and thus do not reach the load. If
Z B is high relative to Xc, however, interference currents will follow Path 2 to the load and the effectiveness of
the filter is compromised.
Figure 7-1. Effects of Poor Bonding on the Performance of a Power Line Filter
7-2
MIL-HDBK-419A
If a joint in a current path is not securely made or works loose through vibration, it can behave like a set of
intermittent contacts. Even if the current through the joint is at dc or at the ac power frequency, the sparking
which occurs may generate interference signals with frequency components up to several hundred megahertz.
Poor bonds in the presence of high level rf fields, such as those in the immediate vicinity of high powered
transmitters, can produce a particularly troublesome type of interference.
Poorly bonded joints have been
shown to generate cross modulation and other mix products when irradiated by two or more high level signals
(7-1). Some metal oxides are semiconductors and behave as nonlinear devices to provide the mixing action
between the incident signals. Interference thus generated can couple into nearby susceptible equipments.
7.3 RESISTANCE CRITERIA.
A primary requirement for effective bonding is that a low resistance path be established between the two joined
objects. The resistance of this path must remain low with use and with time. The limiting value of resistance
at a particular junction is a function of the current (actual or anticipated) through the path. For example,
where the bond serves only to prevent static charge buildup, a very high resistance, i.e., 50 kilohms or higher, is
acceptable.
Where lightning discharge or heavy fault currents are involved, the path resistance must be very
low to minimize heating effects.
Noise minimization requires that path resistances of less than 50 milliohms be achieved. However, noise
control rarely ever requires resistances as low as those necessary for fault and lightning currents.
Bond
resistance based strictly on noise minimization requires information on what magnitude of voltage constitutes
an interference threat and the magnitude of the current through the junction.
These two factors will be
different for every situation.
A bonding resistance of 1 milliohm is considered to indicate that a high quality junction has been achieved.
Experience shows that 1 milliohm can be reasonably achieved if surfaces are properly cleaned and adequate
pressure is maintained between the mating surfaces. A much lower resistance could provide greater protection
against very high currents but could be more difficult to achieve at many common types of bonds such as at
connector shells, between pipe sections, etc.
However, there is little need to strive for a junction resistance
that is appreciably less than the intrinsic resistance of the conductors being joined.
Higher values of resistance tend to relax the bond preparation and assembly requirements. These requirements
should be adhered to in the interest of long term reliability. Thus, the imposition of an achievable, yet low,
value of 1 milliohm bond resistance ensures that impurities are removed and that sufficient surface contact
area is provided to minimize future degradation due to corrosion.
A similarly low value of resistance between widely separated points on a ground reference plane or network
ensures that all junctions are well made and that reasonably adequate quantities of conductors are provided
throughout the plane or network.
control.
In this way, resistive voltage drops are minimized which helps with noise
In addition, the low value of resistance tends to force the use of reasonably sized conductors which
helps minimize path inductance.
7-3
MIL-HDBK-419A
It should be recognized that a low dc bond resistance is not a reliable indicator of the performance of the bond
at higher frequencies. Inherent conductor inductance and stray capacitance, along with the associated standing
wave effects and path resonances, will determine the impedance of the bond. Thus, in rf bonds these factors
must be considered along with the dc resistance.
7.4 DIRECT BONDS.
Direct bonding is the establishment of the desired electrical path between the interconnected members without
the use of an auxiliary conductor. Specific portions of the surface areas of the members are placed in direct
contact.
Electrical continuity is obtained by establishing a fused metal bridge across the junction by welding,
brazing, or soldering or by maintaining a high pressure contact between the mating surfaces with bolts, rivets,
or clamps.
Examples of direct bonds are the splices between bus bar sections, the connections between
lightning down conductors and the earth electrode subsystem, the mating of equipment front panels to
equipment racks, and the mounting of connector shells to equipment panels.
Properly constructed direct bonds exhibit a low dc resistance and provide an rf impedance as low as the
configuration of the bond members will permit.
Direct bonding is always preferred; however, it can be used
only when the two members can be connected together and can remain so without relative movement. The
establishment of electrical continuity across joints, seams, hinges, or fixed objects that must be spatially
separated requires indirect bonding with straps, jumpers, or other auxiliary conductors.
Current flow through two configurations of a direct bond is illustrated in Figure 7-2. The resistance, R c, of the
path through the conductors on either side of the bond is given by
(7-1)
where
is the resistivity of the conductor materials,
is the total path length of the current through
conductors, and A is the cross-sectional area of the conductors (assumed equal). Any bond resistance at the
junction will increase the total path resistance.
Therefore, the objective in bonding is to reduce the bond
resistance to a value negligible in comparison to the conductor resistance so that the total path resistance is
primarily determined by the resistance of the conductors.
Metal flow processes such as welding, brazing, and silver soldering provide the lowest values of bond resistance.
With such processes, the resistance of the joint is determined by the resistivity of the weld or filler metal which
can approach that of the metals being joined. The bond members are raised to temperatures sufficient to form
a continuous metal bridge across the junction.
For reasons of economy, future accessibility, or functional requirements, metal flow processes are not always
the most appropriate bonding techniques.
It may then be more appropriate to bring the mating surfaces
together under high pressure. Auxiliary fasteners such as bolts, screws, rivets or clamps are employed to apply
and maintain the pressure on the surfaces. The resistance of these bonds is determined by the kinds of metals
involved, the surface conditions within the bond area, the contact pressure at the surfaces, and the crosssectional area of the mating surfaces.
7-4
MIL-HDBK-419A
(a) BUTT JOINT
( b ) LAP JOINT
Figure 7-2. Current Flow Through Direct Bonds
7-5
MIL-HDBK-419A
7.4.1 Contact Resistance.
No metallic surface is perfectly smooth.
In fact, surfaces consist of many peaks and valleys. Even the
smoothest commercial surfaces exhibit an RMS roughness of 0.5 to 1 millionth of an inch (7-2); the roughness
When two such surfaces are
of most electrical bonding surfaces will be several orders of magnitude greater.
placed in contact, they touch only at the tips of the peaks - so called asperities. Thus the actual area of
contact for current flow is much smaller than the apparent area of metallic contact.
An exaggerated side view of the actual contact surfaces at a bond interface is shown in Figure 7-3.
Theoretically, two infinitely hard surfaces would touch at only three asperities. Typically, however, under
pressure elastic deformation and plasticity allows other asperities to come into contact.
Current passes
between the surfaces only at those points where the asperities have been crushed and deformed (7-3) to
establish true metal contact. The actual area of electrical contact is equal to the sum of the individual areas
of contacting asperities.
This actual area of contact can be as little as one millionth of the apparent (gross
surface) contact area (7-4).
Figure 7-3. Nature of Contact Between Bond Members
7-6
MIL-HDBK-419A
7.4.1.1 Surface Contaminants.
Surface films will be present on practically every bond surface. The more active metals such as iron and
aluminum readily oxidize to form surface films while the noble metals such as gold, silver, and nickel are less
affected by oxide films.
Of all metals, gold is the least affected by oxide films. Although silver does not
oxidize severely, silver sulfide forms readily in the presence of sulfur compounds.
If the surface films are much softer than the contact material, they can be squeezed from between the
asperities to establish a quasi-metallic contact. Harder films, however, may support all or part of the applied
load, thus reducing or eliminating the conductive contact area. If such films are present on the bond surfaces,
they must be removed through some thermal, mechanical, or chemical means before joining the bond members.
Even when metal flow processes are used in bonding, these surface films must be removed or penetrated to
permit a homogeneous metal path to be established.
Foreign particulate matter on the bond surfaces will further impair bonding.
Dirt and other solid matter such
as high resistance metal particles or residue from abrasives can act as stops to prevent metallic contact.
Therefore, all such materials must be thoroughly removed from the surfaces prior to joining the bond members.
7.4.1.2 Surface Hardness.
The hardness of the bond surfaces also affects the contact resistance. Under a
given load, the asperities of softer metals will undergo greater plastic deformation and establish greater
metallic contact. Likewise, at a junction between a soft and a hard material, the softer material will tend to
conform to the surface contours of the harder material and will provide a lower resistance contact than would
be afforded by two hard materials. Table 7-1 shows how the resistance of 6.45 square cm (1 square inch) bonds
varies with the type of metals being joined.
7.4.1.3 Contact Pressure.
The influence of mechanical load on bond resistance is illustrated by Figure 7-4.
This figure shows the resistance variation of a 6.45 square cm (1 square inch) bond held in place with a 1/4-20
steel bolt as a function of the torque applied to the bolt.
The resistance variation for brass is lowest clue to its
relative softness and the absence of insulating oxide films. Even though aluminum is relatively soft, the
insulating properties of aluminum oxide cause the bond resistance to be highly dependent upon fastener torque
up to approximately 40 in. -lb torque (which corresponds to a contact pressure of about 1200 psi). Steel, being
harder and also susceptible to oxide formations, exhibits a resistance that is dependent upon load below
80 in.-lb or about 1500 psi (for mild steel). Above these pressures , no significant improvement in contact
resistance can be expected.
7-7
MIL-HDBK-419A
Table 7-1
DC Resistance of Direct Bonds Between Selected Metals
Resistance (Micro+ohms)
Bond Composition
6
Brass-Brass
25
Aluminum-Aluminum
50
Brass-Aluminum
150
Brass-Steel
300
Aluminum-Steel
1500
Steel-Steel
Notes: Apparent Bond Area: 1 in.2 (6.45 cm2)
Fastener Torque: 100 in -lb
Source: Adapted from Reference (7-5)
7.4.1.4 Bond Area.
Smaller bond areas with the same loadings would produce higher contact pressure which would decrease the
resistance.
However, as shown in Figure 7-4, an increase in pressure over 1500 psi for steel and 1200 psi for
aluminum produces relatively slight changes in bond resistance. Further, the improvement in resistance due to
increased pressure is offset by the smaller overall bond area. In a similar fashion, a larger bond area (with no
change in fastener size) under the same torque results in a lowered pressure at the bond surfaces. The reduced
pressure would be counterbalanced to some extent by the increased bond area, but the net effect can be
expected to be an increase in bond resistance.
Thus, when larger bond areas are used, larger bolts at
correspondingly higher torques should be used for fastening. (See Para 7.4.2.4)
7-8
MIL-HDBK-419A
Figure 7-4. Resistance of a Test Bond as a Function of Fastener Torque (7-5)
7-9
MIL-HDBK-419A
Bond mating surfaces with areas as large as practical are desirable for several reasons. Large surface areas
maximize the cross-sectional area of the path for current and correspondingly maximizes the total number of
true metallic contacts between the surfaces. In addition to the obvious advantage of decreased bond resistance,
the current crowding which can occur during power fault conditions or under a severe lightning discharge is
lessened. Such current crowding produces a higher effective bond resistance than is present during low current
flow. The increased bond resistance raises the voltage drop across the junction to even higher values and adds
to the heat generated at the junction by the heavy current flow. Large bond areas not only lessen the factors
which contribute to heat generation, they also distribute the heat over a larger metallic area which facilitates
its removal. A further advantage of a large bond is that it will probably provide greater mechanical strength
and will be less susceptible to long term erosion by corrosive products because only a small portion of the total
bond area is exposed to the environment.
7.4.2 Direct Bonding Techniques.
Direct bonds may be either permanent or semi-permanent in nature.
Permanent bonds may be defined as those
intended to remain in place for the expected life of the installation and not required to be disassembled for
inspection, maintenance, or system modifications.
Joints which are inaccessible by virtue of their location
should be permanently bonded and appropriate steps taken to protect the bond against deterioration.
Many bonded junctions must retain the capability of being disconnected without destroying or significantly
altering the bonded members. Junctions which should not be permanently bonded include those which may be
broken for system modifications, for network noise measurements, for resistance measurements, and for other
related reasons. In addition, many joints cannot be permanently bonded for cost reasons.
All such connections
not permanently joined are defined as semipermanent bonds. Semipermanent bonds include those which utilized
bolts, screws, rivets, clamps and other auxiliary devices for fasteners.
7.4.2.1 Welding.
In terms of electrical performance, welding is the ideal method of bonding.
The intense heat (in excess of
4000° F) involved is sufficient to boil away contaminating films and foreign substances. A continuous metallic
bridge is formed across the joint: the conductivity of this bridge typically approximates that of the bond
members. The net resistance of the bond is essentially zero because the bridge is very short relative to the
length of the bond members.
The mechanical strengh of the bond is high: the strength of a welded bond can
approach or exceed the strength of the bond members themselves.
Since no moisture or contaminants can
penetrate the weld, bond corrosion is minimized. The erosion rate of the metallic bridge should be comparable
to that of the base members; therefore, the lifetime of the bond should be as great as that of the bond
members.
Welds should be utilized whenever practical for permanently joined bonds.
Although welding may be a more
expensive method of bonding, the reliability of the joint makes it very attractive for bonds which will be
inaccessible once construction is completed. Most metals which will be encountered in normal construction can
be welded with one of the standard welding techniques such as gas, electric are, Heliare and exothermic.
7-10
MIL-HDBK-419A
Conventional welding should be performed only by appropriately trained and qualified personnel. Consequently,
increased labor costs can be expected. In many instances, also, the welding of bonds can be much slower than
the installation of fasteners such as bolts or rivets. In such cases, the added costs of welding may force the use
of alternate bonding techniques.
An effective welding technique for many bonding applications is the exothermic process.
In this process, a
mixture of aluminum, copper oxide, and other powders is held in place around the joint with a graphite mold.
The mixture is ignited and the bent generated (in excess of 4000° F) reduces the copper oxide to provide a
homogeneous copper blanket around the junction. Because of the high temperatures involved, copper materials
can be bonded to steel or iron as well as to other copper materials.
Two examples of exothermic bonds are shown in Figure 7-5. The top photograph shows a 4/0 copper clad cable
bonded to a steel plate. The bottom photograph shows two 4/0 copper clad cables axially bonded together. The
tight mechanical bond established by this process is evident from these photographs.
Figure 7-6 shows
examples of the various bond configurations for which molds are readily available.
This process is advantageous for welding cables together, for welding cables to rods, or for welding cables to Ibeams and other structural members. It is particularly attractive for the bonding of interconnecting cables to
ground rods where the use of conventional welding techniques might be awkward or where experienced welders
are not available.
Because of the cost of the molds (a separate mold is necessary for each different bond
configuration), this process is most economical when there are several bonds of the same configuration to be
made.
When using this process. the manufacturer’s directions should be followed closely. The mold should be dried or
baked out as specified, particularly when the mold has not been used for several hours and may have absorbed
moisture. The metals to be bonded should be cleaned of dirt and debris and should have the excess water dried
off. Water, dirt and other foreign materials cause voids in the weld which may weaken it or may prevent a low
resistance joint from being achieved.
A further requirement is that the mold size must match the cable or
conductor cross sections; otherwise, the molten metal will not be confined to the bond region.
7.4.2.2 Brazing.
Brazing to include silver soldering is another metal flow process for permanent bonding. In brazing, the bond
surfaces are heated to a temperature above 800° F but below the melting point of the bond members.
A filler
metal with an appropriate flux is applied to the heated members which wets the bond surfaces to provide
intimate contact between the brazing solder and the bond surfaces.
As with higher temperature welds, the
resistance of the brazed joint is essentially zero. However, since brazing frequently involves the use of metal
different from the primary bond members , additional precautions must be taken to protect the bond from
deterioration through corrosion.
7-11
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Figure 7-5. Typical Exothermic Connections
7-12
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COURTESY OF ERICO PRODUCTS, INC., CLEVELAND, OHlO
Figure 7-6. Typical Bond Configurations Which Can Be Implemented With The Exothermic Process
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7.4.2.3 Soft Solder.
Soft soldering is an attractive metal flow bonding process because of the ease with which it can be applied.
Relatively low temperatures are involved and it can be readily employed with several of the high conductivity
metals such as copper, tin and cadmium. With appropriate fluxes, aluminum and other metals can be soldered.
Properly applied to compatible materials, the bond provided by solder is nearly as low in resistance as one
formed by welding or brazing. Because of its low melting point, however, soft solder should not be used as the
primary bonding material where high currents may be present.
For this reason, soldered connections are not
permitted by MIL-STD-188-124A or the National Electrical Code in grounding circuits for fault protection.
Similarly, soft solder is not permitted for interconnections between elements of lightning protection networks
by either the Military Standard, the National Fire Protection Association’s Lightning Protection Code or the
Underwriter's Master Labeled System.
In addition to its temperature limitation, soft solder exhibits low
mechanical strength and tends to crystallize if the bond members move while the solder is cooling. Therefore,
soft solder should not be used if the joint must withstand mechanical loading.
The tendency toward
crystallization must also be recognized and proper precautions observed when applying soft solder.
Soft solder can be used effectively in a number of ways, however. For example, it can be used to tin surfaces
prior to assembly to assist in corrosion control. Soft solder can be used effectively for the bonding of seams in
shields and for the joining of circuit components together and to the signal reference subsystem associated with
the circuit. Soft solder is often combined with mechanical fasteners in sweated joints. By heating the joint hot
enough to melt the solder, a low resistance filler metal is provided which augments the path established by the
other fasteners; in addition, the solder provides a barrier to keep moisture and contaminants from reaching the
mating surfaces.
7.4.2.4 Bolts.
In many applications, permanent bonds are not desired.
For example, equipments must be removed from
enclosures or moved to other locations which require that ground leads and other connections must be broken.
Often, equipment covers must be removable to facilitate adjustments and repairs. Under such circumstances, a
permanently joined connection could be highly inconvenient to break and would limit the operational
of the system.
flexibility
Besides offering greater flexibility, less permanent bonds may be easier to implement, require
less operator training, and require less specialized tools.
The most common semipermanent bond is the bolted connection (or one held in place with machine screws, lag
bolts, or other threaded fasteners) because this type bond provides the flexibility and accessibility that is
frequently required.
The bolt (or screw) should serve only as a fastener to provide the necessary force to
maintain the 1200-1500 psi pressure required between the contact surfaces for satisfactory bonding. Except for
the fact that metals are generally necessary to provide tensile strength, the fastener does not have to be
conductive.
Although the bolt or screw threads may provide an auxiliary current path through the bond, the
primary current path should be established across the metallic interface.
Because of the poor reliability of
screw thread bonds, self-tapping screws are never to be used for bonding purposes. Likewise, Tinnernman nuts,
because of their tendency to vibrate loose, should not be used for securing screws or bolts intended to perform a
bonding function.
7-14
MIL-HDBK-419A
Figure 7-7. Nomograph for Torque on Bolts (7-6)
The size, number and spacing of the fasteners should be sufficient to establish the required bonding pressure
over the entire joint area. The pressure exerted by a bolt is concentrated in the immediate vicinity of the bolt
head. However, large, stiff washers can be placed under the bolt head to increase the effective contact area.
Because the load is distributed over a larger area, the tensile load on the bolt should be raised by increasing the
torque. The nomograph of Figure 7-7 may be used to calculate the necessary torque for the size bolts to be
used. Where the area of the mating surfaces is so large that unreasonably high bolt torques are required, more
than one bolt should be used. For very large mating areas, rigid backing plates should be used to distribute the
force of the bolts over the entire area.
7.4.2.5 Rivets.
Riveted bonds are less desirable than bolted connections or joints bridged by metal flow processes. Rivets lack
the flexibility of bolts without offering the degree of protection against corrosion of the bond surface that is
achieved by welding, brazing or soldering.
The chief advantage of rivets is that they can be rapidly and
uniformly installed with automatic tools.
7-15
MIL-HDBK-419A
The bonding path established by a rivet is illustrated in Figure 7-8.
The current path through a rivet is
theorized to be through the interface between the bond members and the rivet body. This theory is justified by
experience which shows that the fit between the rivet and the bond members is more important than the state
of the mating surfaces between the bond members. Therefore, the hole for the rivet must be a size that
provides a close fit to the rivet after installation. The sides of the hole through the bond members must be free
of paint, corrosion products, or other non-conducting material.
For riveted joints in shields, the maximum spacing between rivets is recommended to be approximately 2 cm
(3/4 inch) or less (7-7). In relatively thin sheet metal, rivets can cause bowing of the stock between the rivets
as shown by Figure 7-9. In the bowed or warped regions, metal-to-metal contact may be slight or nonexistent.
These open regions allow rf energy to leak through and can be a major cause of poor rf shield performance. By
spacing the rivets close together, warping and bowing are minimized.
For maximum rf shielding, the seam
should be gasketed with some form of wire mesh or conductive epoxy to supplement the bond path of the rivets.
7.4.2.6 Conductive Adhesive.
Conductive adhesive is a silver-filled, two-component, thermosetting epoxy
resin which when cured produces an electrically conductive material. It can be used between mating surfaces
to provide low resistance bonds. It offers the advantage of providing a direct bond without the application of
heat as is required by metal flow processes. In many locations, the heat necessary for metal flow bonding may
pose a fire or explosion threat.
When used in conjunction with bolts, conductive adhesive provides an effective
metal-like bridge with high corrosion resistance along with high mechanical strength. In its cured state, the
resistance of the adhesive may increase through time. It also tends to adhere tightly to the mating surfaces and
thus an epoxy-bolt bond is less convenient to disassemble than a simple bolted bond. In some applications, the
advantages of conductive adhesive may outweigh this inconvenience.
7.4.2.7 Comparison of Techniques. Table 7-2 shows comparative ratings of the most commonly used bonding
methods. In this table a rating from zero to 10 is assigned to each method for each performance parameter. A
rating of 10 means that the method is suitable from the standpoint of the specific parameter listed in the
extreme left hand column of the table.
Lower ratings mean that the method is less suitable. A zero rating
implies the method is a poor choice, while the dash means it does not apply. One-hundred percent consistency
in ratings is impossible because any given method may vary widely in workmanship.
expertly performed, will work better than a high-rated poorly performed method.
A low-rated method
When using the table assume
that all methods are equally well implemented.
7.5 INDIRECT BONDS. The preferred method of bonding is to connect the objects together with no
intervening conductor.
Unfortunately, operational requirements or equipment locations often preclude direct
bonding. When physical separation is necessary between the elements of an equipment complex or between the
complex and its reference plane, auxiliary conductors must be incorporated as bonding straps or jumpers. Such
straps are commonly used for the bonding of shock mounted equipment to the structural ground reference.
They are also used for by-passing structural elements, such as the hinges on distribution box covers or on
equipment covers, to eliminate the wideband noise generated by these elements when illuminated by intense
radiated fields or when carrying high level currents. Bond straps or cables are also used to prevent static
charge buildup and to connect metal objects to lightning down conductors to prevent flashover.
7-16
MIL-HDBK-419A
D = RIVET DIAMETER
T = THICKNESS OF BOND MEMBER
I = INDICATES CURRENT PATH THROUGH
RIVETED BOND
Figure 7-8. Bonding Path Established by Rivets
RIVETS
BOND
MEMBERS
OPENINGS IN JOINT DUE
TO DISTORTION
Figure 7-9. An Improperly Riveted Seam
7-17
MIL-HDBK-419A
Table 7-2
7-18
MIL-HDBK-419A
7.5.1 Resistance.
The resistance of an indirect bond is equal to the sum of the intrinsic resistance of the
bonding conductor and the resistances of the metal-to-metal contacts at each end. The resistance of the strap
is determined by the resistivity of the material used and the dimensions of the strap. With typical straps, the
dc bond resistance is small.
For example with a resistivity of 1.72 x 10 -6 ohm-cm, (6.77 x 10 -7 ohm-inches), a
copper conductor 2.5 cm, (1 inch) wide, 40 roils thick, and 0.3 meters (1 foot) long has a resistance of 0.2
milliohms. To this resistance will be added the sum of the dc resistances of the direct bonds at the ends of the
strap. With aluminum, copper, or brass straps, these resistances should be less than 0.1 milliohm with properly
made connections. If long straps are required, however, the resistance of the conductor can be significant (see,
for example Table 5-1).
7.5.2 Frequency Effects.
7.5.2.1 Skin Effect.
Because high conductivity materials attenuate radio frequencies rapidly, high frequency
currents do not penetrate into conductors very far, i.e., they tend to stay near the surface. At frequencies
where this effect becomes significant the ac resistance of the bond strap can differ significantly from its dc
value. For a detailed discussion of skin effect, see Section 5.2.2.1.
7.5,2.2 Bond Reactance.
The geometrical configuration of the bonding conductor and the physical relationship between objects being
bonded introduce reactive components into the impedance of the bond. The strap itself exhibits an inductance
that is related to its dimensions.
For a straight,
flat strap of nonmagnetic metal, the inductance in
microhenries is given by
(7-2)
or, for a wire of circular cross section, by
7-3
where
= length in cm,
b = width of the strap in cm
c
= thickness of the strap in cm, and
d = diameter of the wire in cm.
Table 7-3 shows the calculated inductance, using Equation (7-2), of a nonmagnetic rectangular strap, 6 inches
(15.2 cm) long. Table 7-4 compares the inductance of 6, 12, and 36 inch lengths of 0.05 inch (1.27 mm) thick
straps while Table 7-5 tabulates the inductance of 6, 12, and 36 inch lengths of selected standard size cables
from No. 14 AWG to 4/0 AWG. The inductive reactance of the straps tabulated in Tables 7-4 and 7-5 is plotted
in Figure 7-10 for frequencies up to 100 MHz.
7-19
MIL-HDBK-419A
Table 7-3
Calculated Inductance of a 6 inch (15.2 cm) Rectangular Strap
Width, b
Thickness, c
L
(in.)
(in.)
(µ H)
0.5 (12.7 mm)
0.01 (0.25 mm)
0.112
0.5
0.05 (1.27 mm)
0.110
0.5
0.10 (2.54 mm)
0.107
1.0 (25.4 mm)
0.01
0.092
1.0
0.05
0.091
1.0
0.10
0.089
2.0 (50.8 mm)
0.01
0.072
2.0
0.05
0.071
2.0
0.10
0.071
Table 7-4
Calculated Inductance (µH) of 0.05 Inch (1.27 mm) Thick Straps
Length
Width (in. )
6 in. (15.2 cm)
12 in. (30.4 cm)
36 in. (91 cm)
0.5 (12.7 mm)
0.110
0.261
0.984
1.0 (25.4 mm)
0.091
0.222
0.866
2.0 (50.8 mm)
0.071
0.183
0.745
7-20
MIL-HDBK-419A
Table 7-5
Calculated Inductance (µH) of Standard Size Cable
Length
AWG NO.
6 in.
12 in.
36 in.
4/0
0.098
0.238
0.914
1/0
0.108
0.259
0.977
2
0.115
0.273
1.020
4
0.122
0.287
1.063
6
0.129
0.301
1.105
10
0.144
0.329
1.189
14
0.158
0.358
1.274
Even at relatively low frequencies, the reactance of the inductive component of the bond impedance becomes
much larger than the resistance (7-5), (7-9). Thus, in the application of bonding straps, the inductive properties
as well as the resistance of the strap must be considered.
The physical size of the bonding strap is important because of its effect on the rf impedance. As the length,
of the strap is increased its impedance increases nonlinearly for a given width; however, as the width, b,
increases, there is a nonlinear decrease in strap impedance. Figure 7-11 shows that the relative reactance of a
) ratio decreases. The curve shows that a strap with an
strap decreases significantly as the length to width
/b ratio of 5 to 1 has an inductive reactance that is 45 percent that of a thin wire (i. e., very high ratio of
b); a 3 to 1 ratio decreases this percentage to 38 percent.
to
Because of this reduction in reactance, bonding
straps which are expected to provide a path for rf currents are frequently recommended to maintain a
length-to-width ratio of 5 to 1 or less, with a ratio of 3 to 1 preferred.
In many applications, braided straps are preferred over solid straps because they offer greater flexibility.
Figure 7-12 compares the measured impedance properties of a braided copper strap with those of a solid copper
strap and shows that no significant difference exists between the impedance of the braided or solid strap for
frequencies up to 10 MHz. Other tests (7-9) confirm that there is no essential difference in the rf impedance
properties of braided and solid straps of the same dimensions and made of the same materials. Because the
strands are exposed they are more susceptible to corrosion; braided straps may be undesirable for use in some
locations for these reasons.
Fine braided straps also are generally not recommended because of higher
impedances at the higher frequencies as well as lower current carrying capacities.
7-21
MIL-HDBK-419A
Figure 7-10.
7-22
MIL-HDBK-419A
7.5.2.3 Stray Capacitance.
A certain amount of stray capacitance is inherently present between the bonding jumper and the objects being
bonded as well as between the bonded objects themselves.
Figure 7-13 shows an equivalent circuit for the
bonding strap alone. R S represents the ac resistance of the strap; L S is the inductance which may be calculated
with either Equation 7-2 or 7-3; and C S is the stray capacitance between the jumper and the two members
being bonded. Except for extremely short straps, the magnitude of the inductive reactance of the strap will be
significantly larger than the resistance and, at frequencies above approximately 100 kHz, the R S term can be
ignored. Thus, not considering R S, the equation for the magnitude of impedance, Z S, of the equivalent circuit is
(7-4)
The equivalent circuit of Figure 7-13 does not take into account the effects of the equipment enclosure or
other objects being bonded. Figure 7-14 shows the true equivalent circuit of an indirectly bonded system. The
bonding strap parameters are again represented by R S, CS, LS.
Figure 7-11. Relative Inductive Reactance versus Length-to-Width Ratio of Flat Straps (7-10)
7-23
MIL-HDBK-419A
Figure 7-12. Frequency Variation of the Impedance of Simple Conductors (7-5)
Figure 7-13. Equivalent Circuit for Bonding Strap
Figure 7-14. True Equivalent Circuit of a Bonded System
7-24
MIL-HDBK-419A
The inherent inductance of a bonded object e.g., an equipment rack or cabinet is represented by L c and the
capacitance between the bonded members, i.e., between the equipment and its reference plane, is represented
by Cc. In most situations, L s >> L c, C c >> Cs, and Rs can again be ignored. Thus, the primary (i. e., the lowest)
resonant frequency is given by
(7-5)
These resonances can occur at surprisingly low frequencies -- as low as 10 to 15 MHz (7-5) in typical
configurations.
In the vicinity of these resonances, bonding path impedances of several hundred ohms are
common. Because of such high impedances, the strap is not effective. In fact, in these high impedance regions,
the bonded system may act as an effective antenna system which increases the pickup of the same signals which
bond straps are intended to reduce.
Figures 7-15 and 7-16 show the measured effectiveness of two different
lengths of bonding straps in the reduction of the voltage induced by a radiated field on an equipment cabinet
above a ground plane.
The bond effectiveness indicates the amount of voltage reduction achieved by the
addition of the bonding strap.
voltage.
Positive values of bonding effectiveness indicate a lowering of the induced
At frequencies near the network resonances, the induced voltages are higher with the bonding straps
than without the straps. Figures 7-15 and 7-16 show that:
a.
at low frequencies where the reactance of the strap is low, bonding straps will provide effective
bonding;
b.
at frequencies where parallel resonances exist in the bonding network, straps may severely enhance
the pickup of unwanted signals and
c.
above the parallel resonant frequency, bonding straps do not contribute to the pickup of radiated
signals either positively or negatively.
In conclusion, bonding straps should be designed and used with care with special note taken to ensure that
unexpected interference conditions are not generated by the use of such straps.
7.6 SURFACE PREPARATION. To achieve an effective and reliable bond, the mating surfaces must be free
of any foreign materials, e.g., dirt, filings, preservatives, etc., and nonconducting films such as paint, anodizing,
and oxides and other metallic films.
Various mechanical and chemical means can be used to remove the
different substances which may be present on the bond surfaces. After cleaning, the bond should be assembled
or joined as soon as possible to minimize recontamination of the surfaces.
After completion of the joining
process the bond region should be sealed with appropriate protective agents to prevent bond deterioration
through corrosion of the mating surfaces.
7-25
MIL-HDBK-419A
7.6.1 Solid Materials.
Solid material such as dust, dirt, filings, lint, sawdust and packing materials impede metallic contact by
providing mechanical stops between the surfaces. They can affect the reliability of the connection by fostering
corrosion.
Dust, dirt, and lint will absorb moisture and will tend to retain it on the surface. They may even
promote the growth of molds, fungi, and bacteriological organisms which give off corrosive products. Filings of
foreign metals can establish tiny electrolytic cells (see Section 7.8) which will greatly accelerate the
deterioration of the surfaces.
The bond surface should be cleaned of all such solid materials. Mechanical means such as brushing or wiping are
generally sufficient. Care should be exercised to see that all materials in grooves or crevices are removed. If a
source of compressed air is available, air blasting is an effective technique for removing solid particles if they
are dry enough to be dislodged.
7.6.2 Organic Compounds.
Paints, varnishes, lacquers, and other protective compounds along with oils, greases and other lubricants are
nonconductive and in general, should be removed.
Commercial paint removers can be used effectively.
Lacquer thinner works well with oil-based paints, varnish, and lacquer.
If chemical solvents cannot be used
effectively, mechanical removal with scrapers, wire brushes, power sanders, sandpaper, or blasters should be
employed.
When using mechanical techniques, care should be exercised to avoid removing excess material from
the surfaces. Final cleaning should be done with a fine, such as 400-grit, sandpaper or steel wool. After all of
the organic material is removed, abrasive grit or steel wool filaments should be brushed or blown away. A final
wipe down with denatured alcohol, dry cleaning fluid or lacquer thinner should be accomplished to remove any
remaining oil or moisture films.
WARNING
Many paint solvents such as lacquer thinner and acetone are highly
flammable and toxic in nature. They should never be used around open
flames and adequate ventilation must be present.
Inhalation of the
fumes must be prevented.
Oils, greases, and other petroleum compounds should be wiped with a cloth or scraped off. Residual films
should be dissolved away with an appropriate solvent. Hot soapy water can be used effectively for removing
any remaining oil or grease. If water is used, however, the surfaces must be thoroughly dried before completing
the bond. For small or intricate parts, vapor decreasing is an effective cleaning method. Parts to be cleaned
are exposed to vapors of trichlorethylene, perchlorethylene, or methylene chloride until the surfaces reach the
temperature of the vapor. In extreme cases, further cleaning by agitation in a bath of dry chromic acid, 2 lbs
per gallon of water, and sulfuric acid, 4 oz per gallon of water, (7-7) may be necessary. The average dip time
should be restricted to less than 30 seconds because prolonged submersion of parts in this bath may produce
severe etching and cause loss of dimension. This bath must be followed by a thorough rinse with cold water and
then a hot water rinse to facilitate drying.
7-26
MIL-HDBK-419A
Figure 7-15. Measured Bonding Effectiveness of a 9-1/2 Inch Bonding Strap (7-5)
7-27
MIL-HDBK-4l9A
Figure 7-16. Measured Bonding Effectiveness of a 2-3/8 Inch Bonding Strap (7-5)
7-28
MIL-HDBK-419A
7.6.3 Platings and Inorganic Finishes.
Many metals are plated or coated with other metals or are treated to produce surface films to achieve
improved wearability or provide corrosion resistance.
Metal platings such as gold, silver, nickel, cadmium, tin,
and rhodium should have all foreign solid materials removed by brushing or scraping and all organic materials
removed with an appropriate solvent. Since such platings are usually very thin, acids and other strong etchants
should not be used. Once the foreign substances are removed, the bond surfaces should be burnished to a bright
shiny condition with fine steel wool or fine grit sandpaper. Care must be exercised to see that excessive metal
is not removed.
Finally, the surfaces should be wiped with a cloth dampened in a denatured alcohol or dry
cleaning solvent and allowed to dry before completing the bond.
Chromate coatings such as iridite 14, iridite 18P, oadkite 36, and alodine 1000 offer low resistance as well as
provide corrosion resistance. These coatings should not be removed. In general, any chromate coatings meeting
the requirements of MIL-C-5541 (7-11) should be left in place.
Many aluminum products are anodized for appearance and corrosion resistance. Since these anodic films are
excellent insulators, they must be removed prior to bonding. Those aluminum parts to be electrically bonded
either should not be anodized or the anodic coating must be removed from the bond area.
7.6.4 Corrosion By-Products.
Oxides, sulfides, sulfates, and other corrosion by-products must be removed
because they restrict or prevent metallic contact. Soft products such as iron oxide and copper sulfate can be
removed with a stiff wire brush, steel wool, or other abrasives.
generally adequate.
Removal down to a bright metal finish is
When pitting has occurred, refinishing of the surface by grinding or milling may be
necessary to achieve a smooth, even contact surface. Some sulfides are difficult to remove mechanically and
chemical cleaning and polishing may be necessary.
Oxides of aluminum are clear and thus the appearance of
the surface cannot be relied upon as an indication of the need for cleaning. Although the oxides are hard, they
are brittle and roughening of the surface with a file or coarse abrasive is an effective way to prepare aluminum
surfaces for bonding.
7.7 COMPLETION OF THE BOND.
After cleaning of the mating surfaces, the bond members should be assembled or attached as soon as possible.
Assembly should be completed within 30 minutes if at all possible. If more than 2 hours is required between
cleaning and assembly, a temporary protective coating must be applied.
Of course, this coating must also be
removed before completing the bond.
The bond surfaces must be kept free of moisture before assembly and the completed bond must be sealed
against the entrance of moisture into the mating region. Acceptable sealants are paint, silicone rubber, grease,
and polysulfates.
Where paint has been removed prior to bonding, the completed bond should be repainted to
match the original finish. Excessively thinned paint should be avoided; otherwise, the paint may seep under the
edges of the bonded components and impair the quality of the connection. Compression bonds between copper
conductors or between compatible aluminum alloys located in readily accessible areas not subject to weather
exposure, corrosive fumes, or excessive dust do not require sealing.
This is subject to the approval of the
responsible civil engineer or the local authorized approval representative.
7-29
MIL-HDBK-419A
7.8
BOND CORROSION.
with its environment.
Corrosion is the deterioration of a substance (usually a metal) because of a reaction
Most environments are corrosive to some degree.
Those containing salt sprays and
industrial contaminants are particularly destructive. Bonds exposed to these and other environments must be
protected to prevent deterioration of the bonding surfaces to the point where the required low resistance
connection is destroyed.
7.8.1 Chemical Basis of Corrosion.
The basic diagram of the corrosion process for metals is shown in Figure 7-17. The requirements for this
process to take place are that (1) an anode and a cathode must be present to form an electrochemical cell and
(2) a complete path for the flow of direct current must exist.
These conditions occur readily in many
On the surface of a single piece of metal anodic and cathodic regions are present because of
impurities, grain boundaries and grain orientations, or localized stresses. These anodic and cathodic regions are
environments.
in electrical contact through the body of metal. The presence of an electrolyte or conducting fluid completes
the circuit and allows the current to flow from the anode to the cathode of the cell.
Figure 7-17. Basic Diagram of the Corrosion Process
7-30
MIL-HDBK-419A
Anything that prevents the existence of either of the above conditions will prevent corrosion. For example, in
pure water, hydrogen gas will accumulate on the cathode to provide an insulating blanket to stop current flow.
Most water, however, contains dissolved oxygen which combines with the hydrogen to form additional molecules
of water. The removal of the hydrogen permits corrosion to proceed. This principle of insulation is employed in
the use of paint as a corrosion preventive. Paint prevents moisture from reaching the metal and thus prevents
the necessary electrolytic path from being established.
7.8.1.1 Electrochemical Series.
The oxidation of metal involves the transfer of electrons from the metal to
the oxidizing agent. In this process of oxidation, an electromotive force (EMF) is established between the metal
and the solution containing the oxidizing agent.
A metal in contact with an oxidizing solution containing its
own metal ions establishes a fixed potential difference with respect to every other metal in the same condition.
The set of potentials determined under a standardized set of conditions, including temperature and ion
concentration in the solution, is known as the EMF (or electrochemical) series. The EMF series (with hydrogen
as the referenced potential of 0 volts) for the more common metals is given in Table 7-6. The importance of
the EMF series is that it shows the relative tendencies of metals to corrode.
more readily and are thus more prone to corrosion.
The series also indicates the magnitude of the potential
established when two metals are coupled to form a cell.
higher the voltage between them.
Metals high in the series react
The farther apart the metals are in the series, the
The metal higher in the series will act as the anode and the one lower will
act aS the cathode. When the two metals are in contact, loss of metal at the anode will occur through oxidation
to supply the electrons to support current flow.
This type of corrosion is defined as galvanic corrosion. The
greater the potential difference of the cell, i.e., the greater the dissimilarity of the metals the greater the
rate of corrosion of the anode.
7.8.1.2 Galvanic Series.
The EMF series is based on metals in their pure state -- free of oxides and other films -- in contact with a
standardized solution. Of greater interest in practice, however, is the relative ranking of metals in a typical
environment with the effects of surface films included. This ranking is referred to as the galvanic series. The
most commonly referenced galvanic series is listed in Table 7-7. This series is based on tests performed in sea
water and should be used only as an indicator where other environments are of concern.
Galvanic corrosion in the atmosphere is dependent largely on the type and amount of moisture present. For
example, corrosion will be more severe near the seashore and in polluted industrial environments than in dry
rural settings. Condensate near the seashore or in industrial environments is more conductive even under equal
humidity and temperature conditions due to increased concentration of sulfur and chlorine compounds, The
higher conductivity means that the rate of corrosion is increased.
7-31
MIL-HDBK-419A
Table 7-6
Standard Electromotive Series (7-12)
Metal
Electrode Potential*
(volts)
Magnesium
2.37
Aluminum
1.66
Zinc
0.763
Iron
0.440
Cadmium
0.403
Nickel
0.250
Tin
0.136
Lead
0.126
Copper
-0.337
Silver
-0.799
Palladium
-0.987
Gold
-1.50
NOTE: *Signs of potential are those employed by the American Chemical Society.
7-32
MIL-HDBK-419A
Table 7-7
Galvanic Series of Common Metals and Alloys in Seawater (7-13)
(ANODIC OR ACTIVE END)
Magnesium
Magnesium Alloys
Zinc
Galvanized Steel or Iron
1100 Aluminum
Cadmium
2024 Aluminum
Mid Steel or Wrought Iron
Cast Iron
Chromium Steel (active)
Ni-Resist (high-Ni cast iron)
18-8 Stainless Steel (active)
18-8 Mo Stainless Steel (active)
Lead-tin Solders
Lead
Tin
Nickel (active)
Inconel (active)
Hastelloy B
Manganese Bronze
Brasses
Aluminum Bronze
Copper
Silicon Bronze
Monel
Silver Solder
Nickel
Inconel
Chromium Steel
18-8 Stainless Steel
18-8 Mo Stainless Steel
Hastelloy C
Chlorimet 3
Silver
Titanium
Graphite
Gold
Platinum
(CATHODIC OR MOST NOBLE END)
7-33
MIL-HDBK-419A
7.8.2 Relative Area of Anodic Member.
When joints between dissimilar metals are unavoidable, the anodic
member of the pair should be the largest of the two.
For a given current flow in a galvanic cell, the current
density i S greater for a small electrode than for a larger one.
The greater the current density of the current
leaving an anode, the greater i S the rate of corrosion as illustrated by Figure 7-18. As an example, if a copper
strap or cable is bonded to a steel column, the rate of corrosion of the steel will be low because of the large
anodic area.
On the other hand, a steel strap or bolt fastener in contact with a copper plate will corrode
rapidly because of the relatively small area of the anode of the cell.
7.8.3 Protective Coatings.
Paint or metallic platings used for the purpose of excluding moisture or to provide
a third metal compatible with both bond members should be applied with caution.
When they are used, both
members must be covered as illustrated in Figure 7-19. Covering the anode alone must be avoided. If only tile
anode is covered then at imperfections and breaks in the coating, corrosion will be severe because of the
relatively small anode area. All such coatings must be maintained in good condition.
7.9 WORKMANSHIP.
Whichever bonding method is determined to be the best for a given situation, the mating surfaces must be
cleaned of all foreign material and substances which would preclude the establishment of a low resistance
connection. Next, the bond members must be carefully joined employing techniques appropriate to the specific
method of bonding. Finally the joint must be finished with a protective coating to ensure continued integrity of
the bond. The quality of the junction depends upon the thoroughness and care with which these three steps are
performed. In other words, the effectiveness of the bond is influenced greatly by the skill and conscientiousness
of the individual making the connection.
Therefore, this individual must be aware of the importance of
electrical bonds and must have the necessary expertise to correctly implement the method of bonding chosen
for the job.
Those individuals charged with making bonds must be carefully trained in the techniques and procedures
required. Where bonds are to be welded, for example, work should be performed only by qualified welders. No
additional training should be necessary because standard welding techniques appropriate for construction
purposes are generally sufficient for establishing electrical bonds. Qualified welders should also be used where
brazed connections are to be made.
Exothermic welding can be effectively accomplished by personnel not specifically trained as welders. Every
individual doing exothermic welding should become familiar with the procedural details and with the
precautions required with these processes.
Contact the manufacturers of the materials for such processes for
assistance in their use. By taking reasonable care to see that the bond areas are clean and free of water and
that the molds are dry and properly positioned, reliable low resistance connections can be readily achieved.
Pressure bonds utilizing bolts, screws, or clamps must be given special attention. Usual construction practices
do not require the surface preparation and bolt tightening necessary for an effective and reliable electrical
bond. Therefore, emphasis beyond what would be required for strictly mechanical strength is necessary. Bench
of this type must be checked rigorously to see that the mating surfaces are carefully cleaned, that the bond
members are properly joined, and that the completed bond is adequately protected against corrosion.
7-34
MIL-HDBK-419A
Figure 7-18. Anode-to-Cathode Size at Dissimilar Junctions
Figure 7-19. Techniques for Protecting Bonds Between Dissimilar Metals
7-35
MIL-HDBK-419A
7.10 SUMMARY OF GUIDELINES.
•
Bonds must be designed
interconnections not only
into the system.
Specific
attention should be directed to the
n power lines and signal lines, but also between conductors of signal
ground bus networks, between equipments and the ground bus networks, between both cable and
component or compartment shields and the ground reference plane, between structural members,
and between elements of the lightning protection network.
In the design and construction of a
facility, signal path, personnel safety, and lightning protection bonding requirements must be
considered along with mechanical and operational needs.
•
Bonding must achieve and maintain intimate contact between metal surfaces. The surfaces must be
smooth and clean and free of nonconductive finishes.
Fasteners must exert sufficient pressure to
hold the surfaces in contact in the presence of the deforming stresses, shocks, and vibrations
associated with the equipment and its environment.
•
The effectiveness of the bond depends upon its construction, the frequency and magnitude of the
currents flowing through it, and the environmental conditions to which it is subjected.
•
Bonding jumpers are only a substitute for direct bonds. If the jumpers are kept as short as possible,
have a low resistance and low
/w ratio, and are not higher in the electrochemical series than the
bonded members, they can be considered a reasonable substitute.
•
Bonds are always best made by joining similar metals. If this is not possible, special attention must
be paid to the control of bond corrosion through the choice of the materials to be bonded, the
selection of supplementary components (such as washers) to assure that corrosion will affect
replaceable elements only, and the use of protective finishes,
Ž
Protection of the bond from moisture and other corrosive elements must be provided.
•
Finally, throughout the lifetime of the equipment, system, or facility, the bonds must be inspected,
tested, and maintained to assure that they continue to perform as required.
7-36
MIL-HDBK-419A
7.11 REFERENCES.
7-1. R. Henkel and D. Mealey, "Electromagmetic Compatibility Operational Problems Aboard the Apollo
Spacecraft Tracking Ship,” 1967 IEEE Electromagnetic Compatibility Symposium Records, IEEE 27C80,
July 1967, p 70.
7-2. D.D. Fuller, Theory and Practice of Lubrication for Engineers, Wiley, New York NY (1956).
7-3. J.C.
Bailey,
"Bolted Connections in Aluminum Bushbars," The Engineer, Vol 199, pp 551-554,
April 22, 1955.
7-4. L. Jedynak, "Where the (Switch) Action Is," IEEE Spectrum, October 1973, pp 56-62.
7-5. H.W. Denny and W.B. Warren, "RF Bonding Impedance Study," RADC-TR-67-106, Contact AF30
(602)-3282, Engineering Experiment Station,
Georgia Institute of Technology, Atlanta, Georgia,
March 1967.
7-6. "Nomogram for Torque on Bolts," Design News, May 22, 1972.
7-7. "Electroinagnetic Compatibility,"
AFSC Design
Handbook 1-4,
Air Force Systems Command,
2 March 1984.
7-8 J.H. Whitley, "Which Permanent Electrical Connection Should You Use?" Electronics, January 25, 1963,
pp 50-51.
7-9. R.W. Evans, "Metal-to-Metal Bonding for Transfer of Radio Frequency Energy," IN-R-ASTR-64-15,
NASA Marshall Space Flight Center, Huntsville AL, June 25, 1964.
7-10. R.J. Troup and W.C. Grubbs, "A Special Research Paper on Electrical Properties of a Flat Thin
Conductive Strap for Electrical Bonding,"
Proceedings of the Tenth Tri-Service Conference on
Electromagnetic Compatibility, IITRI, Chicago IL, November 1964, pp 450-474.
7-11. "Chemical Conversion Coatings on Aluminum and Aluminum Alloys," MIL-C-5541C, 14 April 1981.
7-12. American Institute of Physics Handbook, Second Edition (1 963).
7-13. G.D. Roessler, “Corrosion and the EMI/RFI Knitted Wire Mesh Gasket,” Frequency Technology, Vol 7,
No. 3, March 1969, pp 15-24.
7-37/7-38
MIL-HDBK-419A
CHAPTER 8
SHIELDING
8.1 FUNCTION OF AN ELECTROMAGNETIC SHIELD.
Groups of equipment or subsystems may be made electromagnetically compatible by any combination of three
fundamental
approaches: (1) the interfering signal source level may be reduced, (2) the receptor susceptibility
may be reduced, or (3) the attenuation of the path or paths over which interference is transmitted from source
to receptor may be increased.
Radiated interference signals generated by electromagnetic fields may be
attenuated effectively by electromagnetic shielding,
either at the source or at the receptor.
An
electromagnetic (EM) shield reduces the strength of electric and/or magnetic fields on the side of the shield
away from an interfering EM source. When a shield encloses an EM source, the field strength outside the shield
will be reduced; when the shield is used to enclose a sensitive (susceptible) assembly located near an external
EM source, the field strength inside the enclosure is substantially reduced.
Shielding, when properly designed
and implemented, offers significant wideband protection against EM radiation where source and receptor are
not sufficiently separated for adequate free space radiation attenuation. It is relatively easy to obtain 40 dB of
shielding effectiveness in a frequency range above 100 kHz with a single shield, and values as high as 70 dB are
readily obtained with careful single-shield construction.
For higher values of shielding effectiveness, double
shields are normally used, yielding shielding values as high as 120 dB.
Radiated energy may still be coupled into a susceptible device through a shield of inadequate thickness, through
holes provided for ventilation and other purposes, and through imperfectly joined shield sections.
Precise
calculation of shielding effectiveness, even for perfectly joined solid shields, depends on the form of the shield
and the type field for which the shielding is to be used.
Both electric and magnetic coupling can occur, but
normally it is relatively easy to provide electric shielding. Magnetic shielding, however, is more difficult to
provide, particularly at frequencies below 100 kHz. To avoid uncertainties in critical situations, tests should be
performed to check shielding effectiveness.
Such tests require the establishment of a known field and the
measurement of insertion loss introduced by the shielding.
In the construction of a facility, the installation designer should take advantage of all the inherent shielding
which the installation and its individual equipments and terrain have to offer. Items such as building walls,
partitions, towers and other similar structures may be used to advantage. The shielding effectiveness afforded
by these items can be used to isolate EM radiation generating equipment from potentially susceptible devices,
personnel, flammable mixtures, and other items.
In addition, equipments used in a console or rack may be
placed to take advantage of the inherent shielding of that rack.
Shielding, although an important technique for reducing EM interference effects, is not the only technique
available for this purpose.
Application of shielding techniques should not be made without due regard to the
roles which filtering, grounding, and bonding play in the interference suppression program.
8-1
MIL-HDBK-419A
8.2 BASIC SHIELDING THEORY.
The shielding effectiveness of an equipment or subassembly enclosure depends upon a number of parameters,
the most notable of which are the frequency and impedance of the impinging wave, the intrinsic characteristics
of the shield materials, and the numbers and shapes of shield discontinuities. The effectiveness of a shield is
specified in terms of the reduction of EM field strength caused by the shield. The shielding effectiveness (SE) is
defined as the ratio of the field strength without the shield present to the field strength with the shield in
place. Because of the wide ranges in this ratio, it is common practice to express the shielding effectiveness in
decibels
SE = 20 log (E1/E 2),
(8-1)
SE = 20 log (H1/H 2).
(8-2)
or
The variables E l and H 1 are the electric and magnetic field strengths without the shield present, and E 2 and H 2
are those with the shield in place.
8.2.1 Oppositely Induced Fields.
A shielding action occurs whenever an electromagnetic wave encounters a
metal surface. Part of the wave energy is reflected back toward the source, part is dissipated in the metal, and
the remainder propagates beyond the metal. This shielding effect can be visualized as being the result of the
incoming electric and magnetic fields inducing charges at the surface of the shield and a current flow within
the shield, respectively.
The induced charges and currents are of such a polarity and direction that their
associated electric and magnetic fields oppose the incident fields, thus reducing the EM fields beyond the
shield. Although this concept of the shielding theory does not lend itself to efficient calculation of the degree
of shielding provided by a particular shield, it does provide a useful physical picture of shielding. For example,
it can be seen from this viewpoint that shielding effectiveness would be reduced more if the shield were cut so
as to interfere with the induced current flow than if it were cut along the line of current flow. Thus, if a plane
EM wave is incident upon a conducting shield with a very long slit, more energy will be transmitted through the
slit if the electric field vector is perpendicular to the slit than if it is parallel to the slit (see Figure 8-1). This
is true because the EM boundary conditions (8-1) require that the induced shield current flow be perpendicular
to the incoming magnetic field vector (and thus parallel to the electric field vector).
8.2.2 Transmission Line Analogy.
The shielding theory most applicable to engineering calculations is based
upon an analogy with transmission line theory.
(8-1),
an
electromagnetic
shield
transmits
EM
According to the planewave theory developed by Schelkunoff
waves whose fronts coincide with the shielding boundary
configuration in a manner mathematically analogous to that in which a two-wire transmission line transmits
electrical current and voltage.
flat shield as in Figure 8-2.
Consider an incident EM wave with a power of Pin watts/m 2 impinging upon a
When the wave encounters the first surface of the shield, a portion (p rl) of the
8-2
MIL-HDBK-419A
incident power is reflected back toward the source; the remainder (P tl) penetrates the shield and begins to
propagate through the shield.
The ratio of reflected power to incident power (P r1 / Pin ) depends upon the
intrinsic impedance of the shield material and the wave impedance* of the incident wave in the same manner as
at the junction of two transmission lines of different characteristic impedances.
A portion of the power
transmitted into the shield (P t2) is converted into heat as the wave moves through the shield; this energy loss is
referred to as absorption loss and is analogous to the dissipated energy within a lossy transmission line. Of the
power which propagates through the shield to reach the second surface of the shield, a portion is reflected back
into the shield and the remainder (P out ) is transmitted through the surface and beyond the shield. If the
absorption loss within the shield is small (less than 10 dB), a significant part of the power reflected at the
second surface (P r2 ) propagates back to the first surface where a portion is reflected back into the shield,
propagates back to-and-through the second surface, and contributes to the power propagated beyond the shield.
Shielding effectiveness, then, depends upon three factors:
(1) reflection loss, (2) absorption loss, and (3) a
re-reflection factor which is significant when the absorption is small.
(A)
(B)
Figure 8-1. Electromagnetic Transmission Through a Slot
*Wave impedance is defined as the ratio of the electric field strength to the magnetic field strength in the
plane of interest. For further information, see Chapter 18 of Everett (8-2).
8-3
MIL-HDBK-419A
Figure 8-2. Transmission Line Model of Shielding
8.2.3 Nonuniform Shielding.
through defects.
Nonuniform shielding theory has been developed to deal with wave transmission
It treats the defect as a transmission path in parallel with that representing transmission
through the shielding material itself. The net shielding effectiveness of any practical enclosure is calculated as
the result of all such parallel transmission paths, carefully considering transmission phase differences. The
equipment design process, regardless of the theory utilized, consists of establishing undesired signal levels on
one side of the proposed shielding barrier, estimating tolerable signal levels on the other side, and trading off
shield design options to achieve the necessary effectiveness level.
8.3 SHIELDING EFFECTIVENESS OF CONTINUOUS SINGLE-THICKNESS SHIELDS.
The plane wave theory (or transmission line theory) of shielding is the basis of the most commonly used
shielding design data. The resulting set of design equations, graphs, tables, and nomography is based upon the
separation of the shielding effectiveness into three additive terms:
correction term to account for re-reflections within the shield.
8-4
absorption loss, reflection loss, and a
MIL-HDBK-419A
The shield effectiveness (in decibels) of a large, plane sheet of metal with an EM wave arriving along a path
perpendicular to the sheet has been shown (8--2) to be:
(8-3)
A
R
c
where
= thickness of the shield,
= propagation constant of the shield,
= transmission coefficient,
and
r
= reflection coefficient.
The shielding equation is often written as
SE = A + R + C
(8-4)
where A, R, and C are the indicated three terms in Equation 8-3 and represent respectively the Absorption
Loss, the Reflection Loss, and the Correction Term for re-reflections as discussed earlier.
shielding application, the values of the constants
and permittivity
and
of the shielding material. The values of
depend upon the conductivity
and
In a particular
, permeability (µ),
depend also upon the wave impedance of
the EM wave impinging upon the shield.
For convenience in the use of the shielding effectiveness equation, the individual terms A, R, and C have been
expressed in more readily usable forms as functions of the EM wave’s frequency (f) and of the shield’s thickness
( ), relative permeability (µ r), and conductivity relative to copper (g r) .
have been derived for the reflection and correction terms.
Simplified approximate expressions
The selection of the appropriate approximate
expression
will depend upon whether the wave impedance is low (Z w < < 377
( Zw = 377
; plane wave), or high (Z w > > 377
magnetic field), medium
electric field). Low impedance fields are found in the proximity
of loop antennas, high impedance fields are found near dipole antennas, and plane waves exist away from the
near fields of source antennas.
8.3.1 Absorption Loss.
The absorption loss of an EM wave passing through a shield of thickness
8-5
can be shown (3-2) to be given by:
MIL-HDBK-419A
where
K 1=
131.4 if
3.34 if
f
µ
r
g
r
is expressed in meters,
is expressed in inches,
=
shield thickness,
=
wave frequency, Hz,
=
relative permeability of shield material, and
=
conductivity of shield material relative to copper.
Note that the absorption loss (in decibels) is proportional to the thickness of the shield and also that it increases
with the square root of the frequency of the EM wave to be shielded against. As to the selection of the
shielding material, the absorption loss is seen to increase with the square root of the product of the relative
permeability and conductivity (relative to copper) of the shield material.
Table 8-1 contains a tabulation of electrical properties of shielding materials (g r and µ r); since µr is frequency
dependent for magnetic materials, it is given for a typical shielding frequency of 150 kHz.
The last two
columns of Table 8-1 evaluate Equation 8-5 to give the absorption loss at 150 kHz for both a one millimeter and
a one mil (0.001 inch) thick sheet for each of the listed materials. The absorption loss for other thickness can
be calculated by simply multiplying by the shield thickness in millimeters or roils. Shield thicknesses are
commonly expressed in either millimeters (mm) or mini-inches (roils); these two units are related as follows:
1 mm = 39.37 roils or 1 mil = 0.0254 mm
The variation of absorption loss with frequency, as well as a comparison of the absorption loss of three common
shielding materials one mm thick, can be seen in Table 8-2.
Also included is a listing of the relative
permeability, as a function of frequency, for iron. Figure 8-3 presents the data of Table 8-2 in graphical form.
Remember that the absorption loss is just one of three additive terms which combine to give the attenuation
(shielding efficiency) of the shield. At this point, the absorption loss has been presented in equation form
(Equation 8-5), tabular form (Tables 8-1 and 8-2), and graphical form (Figure 8-3). The tabular and graphical
forms are easy-to-use sources of accurate results when the shield material and frequency of interest are
included in those tables and graphs.
Quick results for almost any material and frequency combination can be
obtained from an absorption nomograph (see Vol II), but the results are generally less precise; nomography are a
good source of data for initial design purposes. Once a shielding material and thickness are tentatively
selected, one may wish to compute a more precise value of the absorption loss by evaluation of Equation 8-5.
8.3.2 Reflection Loss.
According to Equation 8-3, the reflection loss portion, R, of the shielding effectiveness, SE, is given by:
R = -20 log
8-6
dB,
(8-6)
MIL-HDBK-419A
where
is the transmission coefficient for the shield.
The reflection loss includes the reflections at both
surfaces of the shield (see Figure 8-2) and is dependent upon the wave impedance and frequency of the
impinging EM wave as well as upon the electrical parameters of the shielding material. It is independent of the
thickness of the shield.
Table 8-1
Electrical Properties of Shielding Materials at 150 kHz (8-3)
Metal
Relative
Relative
Conductivity
Permeability
gr
µr
Absorption Loss
(dB)
1 mm thick
1 mil thick
Silver
1.05
1
51.96
1.32
Copper, annealed
1.00
1
50.91
1.29
Copper, hard-drawn
0.97
1
49.61
1.26
Gold
0.70
1
42.52
1.08
Aluminum
0.61
1
39.76
1.01
Magnesium
0.38
1
31.10
.79
Zinc
0.29
1
27.56
.70
Brass
0.26
0.23
1
25.98
.66
Cadmium
1
24.41
.62
Nickel
0.20
1
22.83
.58
Phosphor-bronze
0.18
1
21.65
.55
Iron
0.17
1,000
665.40
16.90
Tin
0.15
1
19.69
.50
Steel, SAE 1045
0.10
1,000
509.10
12.90
Beryllium
0.10
1
16.14
.41
Lead
0.08
1
14.17
.36
Hypernick
0.06
80,000
Monel
0.04
1
3484.00*
10.24
88.50*
.26
Mu-metal
0.03
80,000
2488.00*
63.20*
Permalloy
0.03
80,000
2488.00*
63.20*
Steel, stainless
0.02
1,000
224.40
5.70
* With no saturation by incident field.
8-7
MIL-HDBK-419A
In a manner analogous to the classical equat ions (8-1) describing reflections n transmission lines, the
shield reflection loss can be expressed as:
(8-7)
where S is defined as the ratio of the wave impedance to the shield’s intrinsic impedance and is analogous to the
voltage standing wave ratio in transmission line practice.
While the shield’s intrinsic impedance is easily
determined from the electrical properties of the shield material, the wave impedance is highly dependent upon
the type and location of the EM wave source, as indicated in Figure 8-4.
In order to present practical methods for determination of the reflection loss, three separate classes of EM
waves are considered and approximations for the reflection loss relationships applicable to the three classes are
presented. Since wave impedance is the ratio of electric to magnetic field strengths, a predominantly magnetic
field will have a low impedance and a predominantly electric field will have a high impedance. The three wave
impedance classes to be considered are low, medium, and high and are commonly referred to as the magnetic,
plane wave, and electric field, respectively.
Table 8-2
Absorption Loss, A, of 1 mm Metal Sheet [8-2)
µ
µr
A
r
A
1,000
µ
A
r
(dB)
(dB)
(dB)
60.0 Hz
Aluminum
Copper
Iron
Frequency
13
1
1
1
0.8
1.0 kHz
1,000
54
1
4
1
3.0
10.0 kHz
1,000
171
1
13
1
10.0
150.0 kHz
1,000
663
1
56
1
40.0
1.0 MHz
700
1,430
1
131
1
103.0
3.0 MHZ
600
2,300
1
228
1
178.0
10.0 MHz
500
3,830
1
416
1
325.0
15.0 MHz
400
4,200
1
509
1
397.0
100.0 MHz
100
5,420
1
1,310
1
1,030.0
1.0 GHz
50
12,110
1
4,160
1
3,250.0
1.5 GHz
10
6,640
5,090
1
3,970.0
10.0 GHz
1
5,420
1
13,140
1
10,300.0
Relative Conductivity, g r: Iron - 0.17, Copper - 1.0, Aluminum - 0.61.
8-8
MIL-HDBK-419A
Figure 8-3. Absorption Loss for One Millimeter Shields
8-9
MIL-HDBK-419A
r = DISTANCE BETWEEN SOURCE AND MONITORING
POINT, IN SAME UNITS AS WAVELENGTH,
Figure 8-4. Wave Impedance versus Distance from Source
8.3.2.1 Low Impedance Field.
A loop, or magnetic dipole, antenna produces an EM wave which is predominantly magnetic in the near field (r <
), where r is the distance from the antenna and
is the wavelength of the EM field. For such magnetic
(1ow impedance) EM fields, the reflection loss can be approximated as follows:
(8-8)
where
8-10
MIL-HDBK-419A
r
=
distance from EM source to shield,
f
=
frequency (Hz),
g
µ
=
conductivity of shield material relative to copper,
=
relative permeability of shield material,
r
r
and the constants Cl and C 2 depend upon the choice of units for the distance, r, as given in Table 8-3.
Table 8-3
Coefficients for Magnetic Field Reflection Loss
Units for Distance (r)
Coefficient
Meters
C
l
C2
.0117
5.35
Millimeters
11.7
.0053
Inches
Mils
0.462
462
0.136
136
As with absorption loss, the reflection loss for low impedance fields depends upon electrical properties of the
shield material and upon the EM wave frequency. However, the reflection loss depends upon the distance from
the source to the shield rather than upon the shield thickness.
Figure 8-5 shows the reflection loss as a function of frequency for iron, copper, and aluminum shields at
distances of one inch (2.54 cm) and ten inches (25.4 cm) from the low impedance EM field source. For a given
separation distance, the reflection loss is seen to be greater for copper and aluminum than for iron except at
the lower frequencies where iron has a better reflection loss. The curves cannot be extended to higher
frequencies for these separation distances since the approximations used in the derivation of Equation 8-8
assume that the separation distance, r, is less than
For higher frequencies at these distances, the EM
fields are more closely approximated by plane waves rather than by low impedance fields. Figure 8-6 is a
universal curve of the reflection loss for low-impedance sources as a function of the parameter
8-11
MIL-HDBK-419A
Figure 8-5.
8-12
MIL-HDBK-419A
8.3.2.2 Plane Wave Field.
The EM field at a distance of more than a few wavelengths from its source is essentially a plane wave with a
wave impedance equal to the intrinsic impedance of the propagation media
(377
for air). A plane wave has its
electric field and magnetic field vectors, E and H , perpendicular both to each other and to the direction of
propagation.
Unlike the low- and high-impedance fields associated with the near-fields of magnetic dipole and electric
dipole sources, the plane wave field reflection loss is independent of the distance between the source and shield.
The reflection loss for a plane wave impinging upon a uniform shield is given by
(8-9)
Figure 8-6. Universal Reflection Loss Curve for a Low Impedance Source (8-3)
8-13
MIL-HDBK-419A
Figure 8-7.
8-14
MIL-HDBK-419A
where g r, µ r, and f are as defined with Equation 8-8. The plane wave reflection loss is seen to decrease as the
wave frequency increases, and to be better for shielding materials with lower µ r/ gr ratios. Figure 8-7 shows
the plane wave reflection loss as a function of frequency for iron, copper, and aluminum shields. The curve for
iron, unlike those for copper and aluminum, is not a straight line because iron’s relative permeability is
frequency dependent. Figure 8-8 provides a universal curve for plane wave reflection loss as a function of the
parameter
Figure 8-8. Universal Reflection Loss Curve for Plane Waves (8-3)
8.3.2.3
High Impedance Field.
The EM field in the proximity of an electric dipole antenna has a high electric field-to-magnetic field strength
ratio (high wave impedance). The reflection loss for such a field encountering a shield is given by
(8-10)
where
C
C
3
3
=
=
322, if r is in meters
354, if r is in inches,
8-15
MIL-HDBK-419A
and r, gr, µr5 and f are as identified as in Equation 8-8. The high impedance EM wave reflection loss is seen to
depend upon the separation distance, r, between the EM source and the shield, as does the low impedance case.
The reflection loss is seen to decrease as the frequency increases and to be better when the ratio g r/ µr i s
higher.
Figure 8-9 is a universal curve for the high impedance reflection loss; the upper line is for the
parameter range.
and the lower line covers the range
Figure 8-10 shows a plot of the high impedance EM wave reflection loss as a function of frequency for iron,
copper, and aluminum for source-to-shield separation distances of one and ten inches. Separate curves for
copper and aluminum are not shown since the high impedance reflection loss for aluminum is only 2 dB below
that of copper.
The reflection losses for iron, copper, and aluminum shields at representative frequencies for magnetic,
electric, and plane waves are given in Table 8-4. The source-to-shield distance for the magnetic and electric
wave cases is one foot (30.5 cm).
Figure 8-9. Universal Reflection Loss Curve for High Impedance Fields (8-3)
8-16
MIL-HDBK-419A
Figure 8-10. Reflection Losses for Iron, Copper, and Aluminum with a High Impedance Source
8-17
MIL-HDBK-419A
Table 8-4
8-18
MIL-HDBK-419A
8.3.3 Re-Reflection Correction Factor.
For shields in which the absorption loss (A) is reasonably large, say at least 10 dB, the energy reflected back
into the shield at the second surface does not contribute significantly to the wave propagated through and
beyond the shield. However, when the shield’s absorption loss is low, a significant amount of energy is reflected
at the second surface and finally propagated into the area to be shielded.
Accordingly, for shields with low
absorption, the shielding effectiveness is calculated as the sum of (1) the absorption loss, A, (2) the reflection
loss, R, and (3) a re-reflection correction factor, C. The correction factor is
C =
20
log
[1
-
l0 - A / l O ( c o s
[0.23A]
where A is the shield’s absorption loss (see Equation 8-5) and
-
jsin
[0.23A])],
(8-11)
is the two-boundary reflection coefficient;
is
dependent upon both the shield characteristic impedance and the wave impedance of the impinging EM wave.
Equations for the reflection coefficient,
are given in terms of a precalculation parameter, m, for each of
three wave impedance classes in Table 8-5.
Values of the re-reflection correction terms for iron and copper sheets of various thicknesses and typical
frequencies are given in Table 8-6.
The correction term is seen to approach zero for thick shields or high
frequencies since these conditions correspond to large absorption losses in the shield. The larger absorption loss
of iron (compared with copper) for fixed frequency and thickness is also seen to result in a smaller correction
term,
Figure 8-11 presents the correction term in graphical form for copper in a magnetic (low impedance)
field. Figure 8-12 presents a universal absorption loss curve (Equation 8-5).
Recall that the correction term
(Equation 8-11) depends upon the absorption loss, A, and that the reflection coefficient,
whenever the approximation
is essentially unity.
= 1 is valid, the correction term depends only upon the value of the absorption
loss. For such conditions, the sum of the absorption loss and the re-reflection correction term is given by the
dashed line on the universal curve in Figure 8-12.
8.3.4 Total Shielding Effectiveness.
The item of interest for any shield is the (total) shielding effectiveness,
i.e., the sum of the absorption loss (A), reflection loss (R), and the multi-reflection correction term (C). The
terms, A, R, and C are of significance only as a means of predicting the shielding effectiveness. Table 8-7
contains the individual terms and the total shielding effectiveness for various shield thicknesses and EM wave
frequencies for copper, iron, and aluminum shields.
impedance classification:
The entries under “SOURCE” designate the EM wave
L indicates a loop antenna and designates a predominantly magnetic field, D
indicates an electric dipole antenna and designates a predominantly electric field, and P indicates a plane wave
( Zw = 377
). All entries except the plane waves are for a source-to-shield separation distance of one foot.
Figures 8-13 and 8-14 illustrate the total theoretical shielding performance which one may expect to obtain
from enclosures constructed from copper foil and iron sheet to the electric, magnetic and plane wave
propagation modes, although the effect of doors, ventilation apertures, and power line penetrations has not been
considered; in many applications these penetrations, together with techniques used for joining the shield
materials, markedly reduce the overall practical insertion loss of a shielded enclosure.
8-19
MIL-HDBK-419A
Table 8-5
8-20
MIL-HDBK-419A
Table 8-6
Correction Term C in dB for Single Metal Sheet (8-2)
Frequency
Thickness*
(mils)
60 Hz
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
-20.0
-7.0
-3.0
+0.1
+0.6
0.0
-10.0
-0.6
+0.6
+0.1
0.0
-3.0
+0.1
0.0
Copper, µ r = 1, gr = 1, near field of loop
1
5
10
20
30
50
100
200
300
-22.2
-21.7
-19.2
-15.6
-13.0
-9.0
-4.0
-0.8
+0.3
-24.00
-22.00
-19.00
-14.00
-11.00
-7.00
-3.00
+0.50
+0.50
-28.00
-16.00
-10.00
-5.00
-3.00
-0.60
+0.50
0.00
Copper, µ r = 1, gr = 1, plane waves and near field of electric dipole
-42.0
-28.0
-22.0
-16.0
-13.0
-9.0
-4.0
-0.6
+0.1
1
5
10
20
30
50
100
200
300
-40.00
-25.00
-20.00
-14.00
-11.00
-7.00
-3.00
+0.10
+0.60
-30.00
-16.00
-10.00
-5.00
-3.00
-0.60
+0.50
0.00
-20.0
-7.0
-3.0
+0.1
+0.6
+0.1
0.0
-10.0
-0.6
+0.6
+0.1
0.0
Iron, µ r = 1000, g r = 0.17, near field of loop
1
5
10
20
30
50
1.0
0.9
0.8
0.4
0.06
0.00
Iron,
1
5
10
20
30
50
1.00
0.90
0.50
0.08
0.06
0.00
-1.60
-0.60
+0.06
0.00
-1.8
0.0
µ r = 1000, gr = 0.17, plane waves and near field of electric dipole
-20.00
-7.00
-3.00
+0 .20
+0.60
+0.10
-17.00
-5.00
-1.30
+0.50
+0.40
0.00
-8.0
+0.2
+0.4
0.0
*1 mil equals 0.0254 mm
8-21
-1.3
0.0
-3.0
+0.1
0.0
MIL-HDBK-419A
Figure 8-11. Graph of Correction Term (C) for Copper in a Magnetic Field (8-4)
Figure 8-12. Absorption Loss and Multiple Reflection Correction Term when
8-22
= 1 (8-2)
MIL-HDBK-419A
Table 8-7
Calculated Values of Shielding Effectiveness (8-2)
Thickness
Frequency
(mils)
(Hz)
Source*
R
—
(dB)
A
—
(dB)
C
SE= A + R + C
(dB)
(dB)
Copper
1
60
L
22.4
0.026
-22.20
0.23
10
60
L
22.4
0.260
-19.20
3.46
300
60
L
22.0
7.800
+0.32
30.12
10
l k
L
34.2
1.060
-10.37
24.89
10
10 k
L
44.2
3.340
+2.62
50.16
10
10 k
D
212.0
3.340
-2.61
212.73
10
10 k
P
128.0
3.340
-2.61
128.73
30
10 k
L
44.2
10.020
+0.58
54.80
10
150 k
L
56.0
12.900
0.50
69.40
10
150 k
D
176.8
12.900
0.50
190.20
10
150 k
P
117.0
12.900
0.50
130.40
10
l M
L
64.2
33.400
0.00
97.60
10
1M
D
152.0
33.400
0.00
185.40
10
l M
P
108.0
33.400
0.00
141.40
10
15 M
L
76.0
129.00
0.00
205.00
10
15 M
D
116.0
129.00
0.00
245.00
10
15 M
P
96.0
129.00
0.00
225.00
10
100 M
L
84.0
334.00
0.00
418.00
10
100 M
D
92.0
334.00
0.00
426.00
10
100 M
P
88.0
334.00
0.00
422.00
8-23
MIL-HDBK-419A
Table 8-7 (Continued)
Calculated Values of Shielding Effectiveness
Thickness
Frequency
(mils)
(HZ)
Source*
R
—
(dB)
A
—
(dB)
C
—
(cm)
SE= A + R + C
(dB)
Iron
1
60
L
-0.9
0.33
+0.95
0.38
10
60
L
-0.9
3.30
+0.78
3.18
300
60
L
-0.9
100.00
0.00
99.10
10
l
k
L
0.9
13.70
+0.06
14.66
10
10
k
L
8.0
43.50
0.00
51.50
10
10
k
D
174.0
43.50
0.00
217.50
10
10
k
P
99.5
43.50
0.00
143.00
30
10
k
L
8.0
130.50
0.00
138.50
10
150
k
L
19.0
160.00
0.00
179.00
10
150
k
D
139.0
169.00
0.00
308.00
10
150
k
P
79.0
169.00
0.00
248.00
10
l
M
L
28.0
363.00
0.00
391.00
10
l
M
D
116.0
363.00
0.00
479.00
10
l
M
P
72.0
363.00
0.00
435.00
10
15 M
L
42.0
1060.00
0.00
1102.00
10
15 M
D
83.0
1060.00
0.00
1143.00
10
15 M
P
63.0
1060.00
0.00
1123.00
10
100 M
L
56.0
1370.00
0.00
1426.00
10
100 M
D
64.0
1370.00
0.00
1434.00
10
100 M
P
60.0
1370.00
0.00
1430.00
8-24
MIL-HDBK-419A
Table 8-7 (Continued)
Calculated Values of Shielding Effectiveness
Thickness
Frequency
(mils)
(Hz)
Source*
R
—
(dB)
Aluminum
A
—
(dB)
C
—
(dB)
SE= A + R + C
(dB)
10
l
M
L
62.0
26.0
0.00
88.0
10
l
M
D
150.0
26.0
0.00
176.0
10
l
M
P
10
15 M
L
79.0
100.0
0.00
179.0
10
15 M
D
115.0
100.0
0.00
215.0
10
15 M
P
10
100 M
L
82.0
260.0
0.00
342.0
10
100 M
D
90.0
260.0
0.00
350.0
10
100 M
P
*L = near field of loop or magnetic dipole, r = 30 cm from shield.
D = near field of electric dipole, r = 30 cm from shield.
P = plane wave.
8-25
MIL-HDBK-419A
Figure 8-13. Theoretical Attenuation of Thin Copper Foil (8-5)
Figure 9-14. Theoretical Attenuation of Thin Iron Sheet (8-5)
8-26
MIL-HDBK-419A
8.3.4.1 Measured Data.
In contrast to the theoretical shielding effectiveness presented thus far, Table 8-8 and Figures 8-15 and 8-16
present actual measured data.
Figure 8-15 illustrates representative shielding effectiveness data taken for a
variety of high-permeability sheet materials.
Loop sensors were located 0.3 cm (1/8”) from each sheet. The
figure shows the typical leveling off in shielding effectiveness as frequency is decreased, with the breakpoint
occurring in the l-kHz range.
Low frequency magnetic shielding is essentially achieved by establishing a low
reluctance path in which the magnetic field is contained. The variation of shielding effectiveness as a function
of loop sensor separation is shown in Figure 8-16 for one of the materials plotted in the previous figure. A
change in effectiveness of about 5 dB over the range of the test at a particular frequency is indicated.
A difficulty with most magnetic shielding materials is their tendency to change permeability when formed,
machined, subjected to rapid or extreme temperature changes, or dropped.
These processes change the
orientation of the magnetic domains in the material, and it is necessary to reorient the domains by annealing to
restore the initial magnetic properties.
A typical annealing process involves heating the material to about
2000° F (sometimes in an inert gas environment), holding it at that temperature for approximately two hours,
and letting it slowly cool to room temperature.
8.3.4.2 Summary.
The shielding effectiveness in dB for a shield is calculated as the sum of three terms: absorption loss (A),
reflection loss (R), and a correction term (C). The absorption loss is independent of the distance from the EM
source. It depends upon the shield thickness and the shielding material’s conductivity and permeability, as well
as upon the frequency of the incident EM wave.
However, the reflection loss (like that of a junction of two
types of transmission lines) depends upon the ratio of the EM wave impedance to the shield impedance and is
therefore dependent upon both the EM source type and the distance between the source and shield. It is also
dependent upon the EM source frequency and the shield material’s conductivity and permeability but does not
depend upon the thickness of the shield. The multi-reflection correction term is essentially zero for shields
with absorption losses greater than 10 dB; for shields with less absorption loss the correction factor should be
used.
It is dependent upon the EM wave impedance classification and the absorption loss, as well as the
frequency, conductivity, and permeability. Table 8-9 summarizes the shielding equations.
Equations, tables, and graphs, have been presented for evaluation of the components of the shielding
effectiveness. The choice of which form to use will be influenced by the time available to the user and the
accuracy to which the data is needed.
8-27
MIL-HDBK-419A
Table 8-8
8-28
MIL-HDBK-419A
Figure 8-15. Measured Shielding Effectiveness of High Permeability Metals (8-6)
Figure 8-16. Measured Shielding Effectiveness of High Permeability
Material as a Function of Measurement Loop Spacing (8-6)
8-29
MIL-HDBK-419A
Table 8-9
8-30
MIL-HDBK-419A
8.4 SHIELDING EFFECTIVENESS OF OTHER SHIELDS.
8.4.1 Multiple Solid Shields.
There are cases when it is appropriate to consider using two or even three layers of shielding material rather
than a single sheet to obtain particular total shielding characteristics.
The most frequently encountered
circumstances are when good protection against both electric and magnetic fields is desired, although other
situations also occur.
Although Mumetal and similar types of high-permeability alloys provide good shielding for low-frequency weak
magnetic fields, they tend to be less effective under the saturating effects of high-level fields.
Where
magnetic shielding in strong signal environments is necessary, it is often desirable to use a multiple shield,
where the outer material has a lower permeability but a higher saturation level than the inner material. Such a
structure might be constructed with materials having the characteristics given in Table 8-10.
Table 8-10
Magnetic Material Characteristics
Property
Inner Material
Outer Material
(Co-Netic AA)
(Netic S 3-6)
Initial Permeability
20,000.00
300.0
Permeability at 0.02 tesla
80,000.00
500.0
0.75
2.2
Saturation Inductance (tesla)
The material thickness necessary would be dictated by the unexpected levels of external fields and the desired
suppression.
When much of the usefulness of shielding is due to reflection loss, two or more layers of metal separated by
dielectric materials and yielding multiple reflections, will provide greater shielding than the same thickness of
metal in a single sheet. The separation of the two layers of metal is necessary to provide for the additional
discontinuous surfaces.
A similar advantage has been noted with magnetic sheet materials (see Figure 8-17).
For the special case where two metallic sheets of the same material and thickness are separated by an air
space, the penetration and reflection losses are each twice of those of a single sheet. However, the correction
factors differ from double the value of a single sheet. One term in the correction factor is negative over much
of the frequency range.
8-31
MIL-HDBK-419A
Figure 8-17. Measured Shielding Effectiveness of Two Sheets of a High Permeability Metal (8-6)
Consequently, a double shield is considerably less effective than the sum of two single shields. However, it is
considerably more effective than a single shield of the same total thickness.
8.4.2 Coatings and Thin-Film Shields.
Thin shielding* has been employed in a variety of ways, ranging from metallized component packaging for
protection against RF fields during shipping and storing, to vacuum deposited shields for microelectronics
applications, and to wallpaper-like shielding material for shielded enclosures.
Solid material shielding theory is applicable to thin-film shields.
For shields much thinner than
the
absorption loss is very small, but the multiple reflection correction term C, is fairly large and negative, thus
offsetting a portion of the reflection loss. The implication of the negative term is that the various reflections
have additive phase relationships, and thus reduce the effectiveness of the shield. The shield effectiveness is
essentially independent of frequency. When the shield thickness exceeds /4, the multiple reflection term
becomes negligible, and there is no offsetting effect to the other losses.
Thus the material shielding
effectiveness increases and is frequency dependent.
*
The thickness of a thin-film shield is often expressed in Angstroms. This unit is related to roils by
1 Angstrom (Å) = 3.937 x 10-6 mils.
8-32
MIL-HDBK-419A
Table 8-11 provides representative calculations of the shielding effectiveness of thin-film cover for different
thicknesses and frequencies. One-quarter wavelength in copper is approximately 0.13 mils at 1 GHz, and it can
be seen that shield effectiveness changes significantly above this thickness.
Table 8-11
Calculated Values of Copper Thin-Film Shielding Effectiveness in dB Against Plane-Wave Energy (8-7)
Thickness
Frequency
0.0041 Mils
1 MHz
0.049 Mils
0.086 Mils
0.86 Mils
1 CHz
1 MHz
1 GHz
1 MHz
1 GHz
1 MHz
0.014
0.44
0.16
5.2
0.29
9.2
2.9
92
109
79
109
79
109
79
109
79
-47
-17
-26
-6
-21
0.6
-3.5
0
62
62
83
78
88
90
108
171
1 GHz
Absorption
Loss, A
Single
Reflection
Loss, R
Multiple
Reflection
Correction
Term, C
Shield
Effectiveness,
SE
8.4.3 Screens and Perforated Metal Shields.
There are many applications in which the shield cannot be made of a solid material. Screens and perforated
materials must be employed if an enclosure must be transparent (e. g., a meter face) or ventilated.
The
shielding effectiveness of solid metal shields has been treated from the viewpoint of classical transmission line
theory in the preceding sections.
To obtain an expression for shielding effectiveness which is applicable to
screens and perforated metal sheets, it is necessary to account for the following
a.
The attenuation effects of
the individual
shield
apertures
acting as
many
stacked
waveguides-below-cutoff (see Section 8.5.3.1).
b.
Reflection losses, considering the geometry of the openings.
c.
Area of the opening when the test antenna is far from the shield in comparison to the distance
between holes in the shield.
d.
Skin depth effects.
e.
Coupling between closely spaced openings.
8-33
MIL-HDBK-419A
The shielding effectiveness, in decibels, is expressed as follows (8-8):
(8-12)
where, as with solid shields, 4 a represents the absorption or attenuation term, R a the reflection loss term, and
C a the multi-reflection correction term. The additional terms K l, K 2 , and K3 approximate the effects of
items c, d, and e above. Detailed expressions for the screen and perforated metal sheet shielding effectiveness
terms are given as follows for single layer wire cloth or screening:
A
a
=
aperture attenuation in dB,
=
27.3 D/W for rectangular apertures, and
(8-13)
=
32 D/d for circular apertures,
(8-14)
D
=
depth of aperture in inches,
W
=
width of rectangular aperture in inches (measured perpendicular to the E-Vector),
d
=
diameter of circular aperture in inches,
Ra
=
aperture reflection loss in dB,
where
= 20 log
Ca
(1 + k)2
4k
(8-15)
, and
——
correction factor for aperture reflections (negligible when A a is greater than 10 dB)
=
20 log
( k - 1 )2
1 - (k + 1)2
x10
- Aa / 1 0
(8-16)
In Equations 8-15 and 8-16,
=
ratio of aperture characteristic impedance to incident wave impedance, or
—
W/3.142r for rectangular apertures and magnetic fields
(8-17)
—
d/3.682r for circular apertures and magnetic fields
(8-18)
=
jfW x 1.7 x 10-4 for rectangular apertures and radiated fields
(8-19)
——
jfd x 1.47 x 10-4 for circular apertures and radiated fields
(8-20)
f
=
frequency in MHz
r
=
distance from signal source to shield in inches
j
=
k
K 1 = correction factor for number of openings per unit square (applicable when test antennas are far from the
shield in comparison to distance between holes in the shield),
= 10 log
8-34
1
an
(8-21)
MIL-HDBK-419A
where
a = area of each hole in square inches
n = number of holes per square inch
K 2 = correction factor for penetration of the conductor at low frequencies
(8-22)
where
p = ratio of the wire diameter to skin depth,
, where
(8-23)
K 3 = correction factor for coupling between closely spaced shallow holes
1
20 log tanh (Aa/8.686)
—
(8-24)
As an example, determine the shielding effectiveness of a No. 22, 15 mil copper screen when subjected to a
predominantly magnetic field from a loop source 1.75 inches away and operating at a frequency of 1 MHz. For
such a screen, there are 22 meshes per linear inch; the center-of-wire to center-of-wire distance is 1/22
(0.045) inch and the opening width is smaller by an amount equal to the wire diameter, 0.015 inches. The depth
of the apertures is assumed equal to the wire diameter.
Thus
A a = (27.3)D/W = (27.3) (.015)/ (0.045 - 0.015)
= 13.65 dB
The impedance ratio for the magnetic wave and rectangular apertures is given by
k= W/ r
=
(0.045 - 0.015) /
(l.75)
= 0.00546
and the reflection term is
Ra
= 20 log
(1 + k)2
4k
=
33.3 dB
8-35
MIL-HDBK-419A
The multi-reflection correction term is
C
a
=
20
log
1-
( k - 1 )2
(k + 1)2 x 10 A a / 1 0
= -0.4 dB.
The number of openings correction factor is
1
10 log a n
K1 =
= 10 log
1
(0.045 - 0.015)2 (22)
2
= 3.5 dB.
The skin depth correction factor is
K 2 = -20 log ( 1 + (35/ 203) ).
=
0.015
= 5.77
2.6 x 10 - 3
K 2 = -20 log ( 1 + (35/56.3) ) = -4.2 dB.
Finally, the hole-coupling correction factor is given by
K 3 = 20 log ( l/tanh (A a/8.686) )
= 0.8 dB.
The screen’s shielding effectiveness, SE, is the sum of the six factors:
SE = 13.5 + 33.2 -0.4 + 3.5 -4.2 + 0.8
= 46.4 dB.
Figure 8-18 presents both calculated and measured values of shielding effectiveness for several types of copper
screen located 1.75 inches from a loop antenna.
Representative non-solid sheet shielding effectiveness
measurements are shown in Tables 8-12 and 8-13. The two tables provide data on a variety of material forms,
including meshes, perforated sheets, and cellular structures against low-impedance, high-impedance, and plane
waves.
Figures 8-19 and 8-20 illustrate how the effectiveness of perforated sheet material changes with
changes in hole size and hole separation. Table 8-14 contains both calculated and measured values of shielding
effectiveness for the No. 22, 15 mil copper screen of the example for magnetic, plane, and electric waves of
several frequencies.
The shielding effectiveness of the screen is seen to increase with the frequency for
magnetic fields, to decrease with increasing frequency for plane waves , and to be largely independent of
frequency for electric fields.
8-36
MIL-HDBK-419A
Screen shields should use a single or double layer of copper or brass mesh of No. 16 or 22 gauge wire with
openings no greater than 1/16 inch. A mesh less than 18 by 18 (wires to the inch) should not be used. The mesh
wire diameter should be a minimum of 0.025 inch (No. 22 AWG). If more than a nominal 50 dB of attenuation is
required, the screening should have holes no larger than those in a 22 by 22 mesh made of 15 mil of copper
wires. The attenuation of an electromagnetic wave by a mesh is considerably less than that afforded by a solid
metal screen. The principal shielding action of a mesh is due to reflection. Tests have shown that mesh with
50 percent open area and 60 or more strands per wave length introduces a reflection loss very nearly equal to
that of a solid sheet of the same material.
FREQUENCY IN MHz
FREQUENCY IN MHz
FREQUENCY IN MHz
Figure 8-18. Measured and Calculated Shielding Effectiveness of
Copper Screens to Low Impedance Fields (8-8)
8-37
MIL-HDBK-419A
Table 8-12
8-38
MIL-HDBK-419A
Table 8-13
8-39
MIL-HDBK-419A
Figure 8-19.
8-40
Figure 8-20.
MIL-HDBK-419A
Table 8-14
Comparison of Measured and Calculated Values
of Shielding Effectiveness for No. 22, 15 Mil Copper Screens (8-8)
Measured
Calculated
Frequency
Effectiveness
Effectiveness
(MHz)
(dB)
(dB)
Test
Type
Magnetic
0.085
31
29
field
1.000
43
46
10.000
43
49
0.200
118
124
(r = 1.75”)
Plane
Wave
Electric
1.000
106
110
5.000
100
95
100.000
80
70
0.014
65
**65
field
**The value assumes a wave impedance equal to that of a 30-inch square waveguide.
The mesh construction should have individual strands permanently joined at points of intersection by a fusing
process so that a permanent electrical contact is made and oxidation does not reduce shielding effectiveness. A
screen of this construction will be very effective for shielding against electric (high-impedance) fields at low
frequencies because the losses will be primarily caused by reflection. Installation can be made by connecting a
screen around the periphery of an opening.
8.5 SHlELD DISCONTINUITY EFFECTS (APERTURES).
An ideal shielded enclosure would be one of seamless construction with no openings or discontinuities.
However, personnel, powerlines, control cables, and/or ventilation ducts must have access to any practical
enclosure.
The design and construction of these discontinuities become very critical in order to incorporate
them without appreciably reducing the shielding effectiveness of the enclosures. Since most mechanically
suitable metal enclosures will give enough shielding above 1 MHz, EMI leakage above 1 MHz is due primarily to
discontinuities. EMI leakage (the amount of EM energy that will leak from a discontinuity) depends mainly on:
a.
maximum length (not area) of the opening,
8-41
MIL-HDBK-419A
b.
the wave impedance, and
c.
the wavelength of the EM energy.
Maximum length rather than width of an opening is important because the voltage will be highest wherever the
“detour” for the currents is longest. This is at the center of the slot and the voltage increases as the length of
the slot increases. The width has almost no effect on “detour” length and as a consequence has little effect on
the voltage.
Wavelength controls how much the “slot antenna” radiates. If the slot happens to be 1/4 wavelength or longer,
it will be a very efficient radiator; if it is less than 1/100 wavelength, it will be a rather inefficient radiator.
Therefore, slots only .001" to .005" wide but 1/100 wavelength or more long can be responsible for large leaks.
Figure 8-21 shows wavelength and 1/100 wavelength vs frequency for 0"-6" slot lengths typical in normal metal
Combinations of frequency and slot lengths to the right of the 1/100 wavelength line would tend to
enclosures.
be leaky.
This figure shows why discontinuities in shields, even if very narrow but a few inches long, will
severely reduce the shielding capacity of an enclosure above 100 MHz.
Some types of discontinuities commonly encountered include:
a.
Seams between two metal surfaces, with the surfaces in intimate contact (such as two sheets of
material that are riveted or screwed together),
b.
Seams or openings between two metal surfaces that may be joined using a metallic gasket, and
c.
Holes for ventilation or for exit or entry of wire, cable, light, film, water, meter faces, etc.
8.5.1 Seams Without Gaskets.
Seams or openings in enclosure or compartment walls that are properly bonded will provide a low impedance to
rf currents flowing across the seam.
When good shielding characteristics are to be maintained, permanent
mating surfaces of metallic members within an enclosure should be bonded together by welding, brazing,
sweating, swagging, or other metal flow processes. To insure adequate and properly implemented bonding
techniques, the following recommendations should be observed:
a.
All mating surfaces must be cleaned before bonding.
b.
All protective coatings having a conductivity less than that of the metals being bonded must be
removed from the contact areas of the two mating surfaces before the bond connection is made.
c.
When protective coatings are necessary, they should be so designed that they can be easily removed
from mating surfaces prior to bonding.
Since the mating of bare metal to bare metal is essential for a
satisfactory bond, a conflict may arise between the bonding and finish specifications. From the viewpoint of
shielding effectiveness, it is preferable to remove the finish where a compromise of the bonding effectiveness
would occur.
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MIL-HDBK-419A
d.
Certain protective metal platings such as cadmium, tin, or silver need not in general be removed.
Similarly, low-impedance corrosion-resistant finishes suitable for aluminum alloys, such as alodine, iridite,
oakite, turco and bonder rite, may be retained.
Most other coatings, such as anodizing, are nonconductive and
should be removed. See Figure 8-22 for shielding effectiveness degradation data on selected surface finishes.
e.
oxidation.
f.
Mating surfaces should be bonded immediately after protective coatings are removed to avoid
Refinishing after bonding is acceptable from the standpoint of shielding effectiveness.
When two dissimilar metals must be bonded,
metals that are close to one another in the
electrochemical series should be selected in order to reduce corrosion.
g.
strength.
h.
Soldering may be used to fill the resulting seam, but should not be employed to provide bond
The most desirable bond is achieved through a continuous butt or lap weld. Spot welding is less
desirable because of the tendency for buckling, and the possibility of corrosion occurring between welds.
Riveting or pinning is even less desirable because of the greater susceptibility of bond degradation with wear.
i.
An overlap seam, accompanied by soldering or spot welding, provides a relatively effective bond.
Other types of crimped seams may be employed so long as the crimping pressure is uniformly maintained.
Figure 8-21. Slot Radiation (Leakage) (8-9)
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MIL-HDBK-419A
Figure 8-22. Shielding Effectiveness Degradation Caused by Surface Finishes on Aluminum (8-4)
There are often occasions when good temporary bonds must be obtained.
clamp and slide fasteners have been used for this purpose.
Bolts, screws, or various types of
The same general requirements of clean and
intimate contact of mating surfaces, and minimized electrolytic (cathodic) effects apply to temporary bonds as
well.
Positive locking mechanisms that ensure consistent contact pressure over an extended period of time
should be used.
Bolts, nuts, screws, and washers that must be manufactured with material different from the surfaces to be
bonded should be higher in the electromotive series than the surfaces themselves so that any material migration
erodes replaceable components.
A critical factor in temporary bonds (and in spot-welded permanent bonds as well) is the linear spacing of the
fasteners or spot welds. Figure 8-23 provides an indication of the sensitivity of this parameter for a 1.27 cm
(1/2-inch) aluminum lap joint at 200 MHz.
The shielding effectiveness shown in 2.54 cm (l-inch) spacing is
about 12 dB poorer than an identical configuration incorporating a 1.27 cm (1/2-inch) wide monel mesh gasket;
the effectiveness at 25.4 cm (l0-inch) spacing is about 30 dB poorer than that with the same gasket. Use of
conductive gaskets for this and other applications is discussed in the next section.
Similar techniques to those just described can be employed in connection with seams in magnetic materials.
Permanent seams can be butt or lap, continuous or spot welded using an electric arc in an argon or helium
8-44
MIL-HDBK-419A
atmosphere, recognizing that a final material heat treatment will be necessary. Temporary seams are usually
Figures 8-24 and 8-25 indicate the change in shielding effectiveness of an
screwed or bolted together.
AMPB-65 seam at various frequencies as a function of screw spacing and lap joint width, respectively.
8.5.2 Seams With Gaskets.
Considerable shielding improvement over direct metal-to-metal mating of shields used as temporary bonds can
be obtained using flexible, resilient metallic gaskets placed between shielding surfaces to be joined. Clean
metal-to-metal mating surfaces and a good pressure contact are necessary.
The major material requirements for rf gaskets include compatibility with the mating surfaces, corrosion
resistance,
appropriate
electrical properties,
resilience
(particularly when
repeated compression
and
decompression of the gasket is expected), mechanical wear, and ability to form into the desired shape. On this
basis, monel and silver–plated brass are generally the preferred materials, with aluminum used only for
gasketing between two aluminum surfaces. Beryllium-copper contact fingers are also employed, with a variety
of platings available, if desired. Mumetal and Permalloy have been used when magnetic shielding effectiveness
is of concern.
Gaskets are manufactured with rubber or neoprene to provide both fluid and conductive seals, or to sustain a
pressure differential, as well as provide an rf barrier.
They are also made using sponge silicon for high
temperature applications and are made with both nonconductive or conductive pressure sensitive adhesives. A
few of the gasket design approaches that have been employed are summarized in Table 8-15. Typical gasket
mounting techniques are given in Figure 8-6. The most frequently used gasket configuration is the knitted wire
mesh; the structure of this mesh is shown in Figure 8-27.
The necessary gasket thickness is dependent on the unevenness of the joint to be sealed, the compressibility of
the gasket, and the force available. The shape required depends on the particular application involved, as well
as the space available, the manner in which the gasket is held in place, and the same parameters that influence
gasket thickness. Gaskets may be held in place by sidewall friction, by soldering, by adhesives, or by positioning
in a slot or on a shoulder.
Soldering must be controlled carefully to prevent its soaking into the gasket and
destroying gasket resiliency. Adhesives (particularly nonconductive adhesives) should not be applied to gasket
surfaces that mate for rf shielding purposes; auxiliary tabs should be used. A recommended pressure is about 20
psi.
8.5.3 Penetration Holes.
One effective method of neutralizing the shielding discontinuities created by
planned holes (e.g., for air ventilation and circuit adjustment) in a shield is to use cylindrical and rectangular
waveguide-below-cutoff slots or tubes.
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MIL-HDBK-419A
SCREW SPACING
Figure 8-23. Influence of Screw Spacing on Shielding Effectiveness
SCREW SPACING (INCHES)
Figure 8-24. Shielding Effectiveness of AMPB-65 Overlap
as a Function of Screw Spacing Along Two Rows, 1.5 Inches Apart (8-6)
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MIL-HDBK-419A
Figure 8-25. Shielding Effectiveness of an AMPB-65 Joint as a Function of Overlap (8-6)
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MIL-HDBK-419A
Table 8-15
Characteristics of Conductive Casketing Materials
Chief Advantages
Material
Compressed knitted wire
Chief Limitations
Most resilient all-metal gasket (low
Not available in sheet (Certain
flange pressure required). Most
intricate shapes difficult to make).
points of contact. Available in
Must be 0.40 inch or thicker.
variety of thicknesses and
resiliencies, and in combination
with neoprene and silicon.
Brass or beryllium copper
Best break-thru of corrosion
Not truly resilient nor generally
with punctured holes
protection films.
reusable.
Oriented wires in rubber or
silicon
Combines fluid and rf seal. Can
May result in larger size gasket
be effective against corrosion
for same effectiveness.
films if ends of wires are sharp.
Aluminum screen impregnated
Combines fluid and conductive
Very low resiliency (high flange
with neoprene
seal. Thinnest gasket. Can be cut
pressure required).
to intricate shapes.
Soft Metals
Cheapest in small sizes.
Cold flows, low resiliency.
Metal over rubber
Takes advantage of the resiliency
Foil cracks or shifts position.
of rubber.
Generally low insertion loss yielding
poor rf properties.
Conductive rubber
Contact Fingers
Combines fluid and conductive
Practically no insertion loss, giving
seal.
very poor rf properties.
Best suited for sliding contact.
Easily damaged. Few points of
contacts.
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Figure 8-26.
8-49
MIL-HDBK-419A
Figure 8-27. Enlarged View of Knitted Wire Mesh
8.5.3.1
Waveguide-Below-Cutoff.
A properly designed waveguide-below-cutoff opening will act like a high-pass filter. The cutoff frequency is a
function of the cross-section of the waveguide. For a cylindrical waveguide, the cutoff frequency of the
dominant TE mode is
(8-25)
The cutoff frequency for the TE mode of rectangular waveguide is
(8-26)
In these equations,
fc
= cutoff frequency for the dominant mode in gigahertz,
d
= inside diameter of a cylindrical waveguide in inches, and
b
= greatest dimension of rectangular waveguide in inches.
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MIL-HDBK-419A
At any frequency, f a, considerably less than cutoff (i.e., f a < 0.1f c ), the attenuation, a, in dB per inch for
cylindrical waveguides is approximated by the relation
(8-27)
For rectangular waveguides, the attenuation,
in dB per inch is
(8-28)
The equations given above are valid for air-filled waveguides with length-to-width or length-to-diameter ratios
of 3 or more.
In many cases, shielding screens introduce excessive air resistance (See Vol II) and may provide inadequate
shielding effectiveness. In such cases, openings may be covered with specially designed ventilation panels (such
as honeycomb) with openings that operate on
the waveguide-below-cutoff principle.
The shielding
effectiveness of honeycomb panels is a function of the size and length of the waveguide and the number of
waveguides in the panel. Table 8-16 indicates the shielding effectiveness of a honeycomb panel constructed of
steel with l/8-inch hexagonal openings l/2-inch long.
Table 8-16
Shielding Effectiveness of Hexagonal Honeycomb Made of Steel
with l/8-inch Openings l/2-Inch Long (8-10)
Shielding
Frequency
Effectiveness
(MHz)
(dB)
0.1
45
50.0
51
100.0
57
500.0
56
2,200.0
47
Honeycomb-type ventilation panels in place of screening:
a.
allow higher attenuation that can be obtained with mesh screening over a specified frequency range,
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MIL-HDBK-419A
b.
allow more air to flow with less pressure drop for the same diameter opening,
c.
cannot be damaged as easily as the mesh screen and are therefore more reliable, and
d.
are less subject to deterioration by oxidation and exposure.
All non-solid shielding materials, such as perforated metal, fine mesh copper screening, and metal honeycomb,
present an impedance to air flow. Metal honeycomb is the best of these materials because it enables very high
electric field attenuations to be obtained through the microwave band with negligible drops in air pressure (see
Volume II).
However, honeycomb has the disadvantages of occupying greater volume and costing more than
screening or perforated metal.
Further, it is often difficult to install honeycomb paneling because flush
mounting is required. Thus, screening and perforated sheet stock sometimes find application for purely physical
design reasons, although honeycomb panels can achieve attenuations greater than 100 dB for frequencies below
10 MHz.
The waveguide attenuator is also of considerable value where control shafts must extend through an enclosure.
By making use of an insulated control shaft passing through the waveguide attenuator, the control function can
be accomplished with little likelihood of radiation. However, where a metallic control shaft is required, it must
be grounded to the case by a close-fitting gasket or metallic fingers.
Fuseholders, phone jacks, panel connectors not in use, and other receptacles can be fitted with a metallic cap
that provides an electrically continuous cover and maintains case integrity.
The waveguide attenuator approach may also be considered where holes must be drilled in the enclosure. If the
metal thickness is sufficient to provide a “tunnel” with adequate length, a waveguide attenuator is effectively
produced. For example, a metal wall 0.5 cm (3/16-inch) thick would permit a 0.16 cm (1/16-inch) hole to be
used without excessive leakage. This technique definitely should be considered where it is necessary to confine
extremely intense interference sources.
8.5.3.2 Screen and Conducting Glass.
Often it is necessary to provide rf shielding over pilot lights, meter faces, strip chart recorders, oscilloscopes,
or similar devices that must be observed by the equipment user. The alternatives available include:
a.
Use of a waveguide attenuator,
b.
Use of screening material,
c.
Providing a shield behind the assembly of concern, and filtering all leads to the assembly, or
d.
Use of conducting glass.
A waveguide attenuator is a practical approach for rf shielding of lamps. The technique has the advantage of
not introducing light transmission loss.
However, it is not particularly suitable for most meter openings or
larger apertures because of the space requirements involved.
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MIL-HDBK-419A
Use of screens over meter faces and other large apertures has often been employed for shielding purposes. A
typical screen introduces a minimum of 15%-20% optical loss which can create difficulties in reading meters.
If the device being shielded has a scale (such as an oscilloscope graticule), bothersome zoning patterns can
result. However, these potential deficiencies are counterbalanced by good shielding efficiencies at a fairly low
cost.
Glass coated with conducting material such as silver can provide shielding across viewing surfaces with some
loss in light transmission. Conductive glass is commercially available from a number of glass manufacturers.
Figure 8-28 provides shielding effectiveness data on 50 and 200 ohms per square silver-impregnated glass
against electric arc discharges.
Figure 8-29 indicates shielding effectiveness as a function of surface
resistance for plane waves in the frequency range from 0.25 to 350 MHz. The light transmission characteristics
of this type of glass as a function of surface resistance is presented in Figure 8-30. For effective shielding,
good contact to the conducting surface of the glass must be maintained around its periphery.
8.6 SELECTION OF SHIELDING MATERIALS.
The selection of the material should be based on its ability to drain off induced electrical charges and to carry
sufficient out-of-phase currents to cancel the effects of the interfering field. The inherent characteristics of
the metal to consider are its relative conductivity, g r, and its relative permeability,
r. The thickness of the
shield and the frequency of the signal to be attenuated are also important.
The selection of proper materials for shielding should be made in accordance with the following basic rules:
a.
At low frequencies (LF), only magnetic materials can furnish appreciable shielding against magnetic
fields.
b.
For a given material, magnetic fields require a greater shield thickness than do electric fields.
c.
At higher frequencies, smaller shield thickness is required for a given material.
d.
At sufficiently high frequencies, nonferrous materials such as copper and aluminum will give
adequate shielding for either electric or magnetic fields.
e.
The electric field component for frequencies from 60 to 800 Hz (i.e., ac power) can readily be
shielded with thin sheets of conducting materials such as iron, copper, aluminum, and brass.
For a detailed description of the procedure for selecting a shield material for a facility, see Volume II. Care
must be used when adding a shield to a subsystem. For example, a shield placed too close to a circuit in which
the circuit Q is a critical factor can cause degradation of performance because the losses in the shield will
appear as an effective resistance in the critical circuit, thereby lowering the circuit Q.
8-53
MIL-HDBK-419A
Figure 8-28.
8-54
MIL-HDBK-419A
Figure 8-29. Shielding Effectiveness of Conductive Glass to Plane Waves [8-ii)
SURFACE RESISTANCE (OHMS/SQUARE)
Figure 8-30. Light Transmission Versus Surface Resistance for Conductive Glass (8-7)
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MIL-HDBK-419A
Conventional building materials are not normally
8.7 USE OF CONVENTIONAL BUILDING MATERIALS.
selected on the basis of their electromagnetic shielding properties however most materials do provide some
limited degree of shielding.
Some documented evidence of the shielding provided by common construction
materials iS available (8-12). Though the data is sketchy, enough does exist to give a prel iminary indication of
what can be expected from a building made of various materials.
8.7.1 Concrete.
Figure 8-31 shows that the shielding effectiveness of ordinary concrete is very low. (It may
be assumed that the properties of brick are similar to concrete.) The addition of coke and other forms of
carbon to concrete can greatly enhance shielding properties. Approximately 30 dB shielding effectiveness from
1 GHz to 10 GHz can be achieved by using concrete and carbon.
A concrete-coke aggregate apparently can
provide shielding in excess of 30 dB above about 20 MHz and can offer more than 100 dB above 300 MHz.
8.7.2 Reinforcing Steel (Rebar).
Limited shielding to low frequency fields can be provided by the reinforcing steel or wire mesh in concrete. For
maximum shielding, the conductors must be welded at all joints and intersections to form many continuous
conducting loops or paths around the volume to be shielded.
The degree of shielding will depend on the
following parameters:
a.
The size and shape of the volume to be shielded.
b.
The diameter of the bars and spacing (the distance between bar centers).
c.
The electrical and magnetic characteristics of the reinforcement steeI materials (conductivity and
relative permeability).
d.
The frequency of the incident wave.
The family of curves shown in Figure 8-32 describes the attenuation at approximately 10 kHz for an enclosure
whose height is 4.5 meters (15 feet), and other dimensions vary over a 5 to 1 range. Bar diameters are 4.30 cm
(1.692 inches) with a spacing of 35.56 cm (14 inches) on centers. The room dimensions, bar spacing, and
diameters shown in Figure 8-32 are typical and cover most situations encountered in practice. The values of
attenuation indicated are those obtainable at the center of the room.
edges of the room.
There will be less shielding near the
For more detailed design information on the use of reinforcing bars as shields, consult
Reference 8-13.
Welded wire fabric imbedded in the walls of a room or building can provide effective shielding if the individual
wires of the fabric are joined to form a continuous electrical loop around the perimeter of the area to be
shielded,
At each seam where the mesh meets, each wire must be welded or brazed to the corresponding wire,
or the meshes may be connected by a continuous strap.
Additional attenuation may be obtained by use of a
double layer of welded wire fabric separated by the thickness of a regular wall.
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MIL-HDBK-419A
Figure 8-31. Shielding Effectiveness of Some Building Materials (8-12)
8-57
MIL-HDBK-419A
Figure 8-32
8-58
MIL-HDBK-419A
8.8 CABLE AND CONNECTOR SHIELDING.
Electromagnetic shielding is required not only for equipment
containers but also for many of the cables which connect the equipment units since interference may be
transferred from one circuit or location to another by interconnecting cable. The interference may be radiated
from a cable or transferred into a cable from external fields.
Once interference has been transferred by
radiation or common-impedance circuit elements into a cable circuit of an electronic or electrical complex, it
can be conducted through interconnecting cables to the other elements of the complex.
proximity
in cable runs or elsewhere,
Because of cable
intra- and/or inter-cable crosstalk may occur as a result of
electromagnetic transference between cables.
8.8.1 Cable Shields.
The effectiveness of a cable shield is a function of two basic interference mechanisms: (1) EM wave shielding
effectiveness and (2) surface transfer impedance, Z t.
As with other shields,
the EM wave shielding
effectiveness results from attenuation and reflections and is dependent upon such factors as the type and
thickness of the material used and the number and size of openings in the shield. In addition, cable shields
frequently are connected in such a manner as to carry relatively large currents themselves.
Although the
Interfering currents generally flow on the outer surfaces of the shields (skin depth effects), an electric field and
resulting axial voltage gradient is developed along the inner (shielded) conductor (see Figure 8-33). The ratio of
the induced conductor-to-shield voltage per unit length to the shield current is defined as the surface transfer
impedance, Z t.
The effectiveness of a shield is a function of the conductivity of the metal, contact resistance between strands
in the braid, angle and type of weave, strand sizes, percentage of coverage, and size of openings.
Analytical
expressions which define Z t in terms of these parameters are available (8-14). For uniform current distribution
along a cable shield, the resulting Z t can be used to predict the shield effectiveness of the cable knowing the
terminating impedances of the cable. Typically, the cable is several wavelengths long at the frequency of the
impinging field. Thus, the current distribution on the cable sheath varies with length and is a function of its
orientation to the incident wave and to the surroundings.
Since the current distribution will be essentially
unpredictable for other than very specialized conditions, the ability to predict shielding effectiveness of the
cable shield through the use of Z t is severely limited.
There are several methods for shielding cables. These include: (a) braid, (b) flexible conduit, (c) rigid conduit,
and (d) spirally-wound shields of high permeability materials. The principal types of shielded cables that are
available include shielded single wire, shielded multi-conductor, shielded twisted pair, and coaxial. Cables are
also available in both single and multiple shields in many different forms and with a variety of physical
characteristics. The general properties of five classes of cable shields are given in Table 8-17.
Braid, consisting of woven or perforated material, is used for cable shielding in applications where the shield
cannot be made of solid material. Advantages are ease of handling in cable makeup and lightness in weight.
However, it must be remembered that for radiated fields the shielding effectiveness of woven or braided
materials decreases with increasing frequency and increases with the density of the weave (9-14). The relative
shielding effectiveness of single and double braided cables as a function of frequency is shown in Figure 8-34.
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MIL-HDBK-419A
Table 8-17
Comparison of Cable Shields
Single
+
Multiple
+
Flexible
Layered
Layer
Foil
++
++
Braid
Braid
Good
Good
Exe.
Exe.
Good
Good
Exe.
Exe.
Exe.
Poor
60-95%
95-97%
100%
100%
90-97%
Fatigue Life
Good
Good
Fair
Poor
Fair
Tensile Strength
Exe.
Exe.
Poor
Exe.
Fair
Conduit
Conduit
Shield
Effectiveness*
(Audio Frequency)
Shield
Effectiveness*
(Radio Frequency)
Normal Coverage
*Poor < 20 dB; Fair, 20-40 dB; Good, 40-60 dB; Exe. > 60dB.
+Effectiveness against magnetic fields is poor.
++For effective magnetic shield, high permeability material must be used.
Conduit either solid or flexible, or zippered tubing may also be used to shield system cables and wiring from the
rf environment. The shielding effectiveness of solid conduit is the same, for rf purposes, as that of a solid sheet
of the same thickness and material. Linked armor or flexible conduit may provide effective shielding at lower
frequencies, but at higher frequencies the openings between
individual links can take on slot-antenna
characteristics, seriously degrading the shielding effectiveness. If linked armor conduit is required, all internal
wiring should be individually shielded.
Degradation of conduit shielding is usually not because of insufficient
shielding properties of the conduit material but rather the result of discontinuities in
discontinuities usually result from poor splicing or
the cable.
These
from improper termination of the shield. Zippered tubing
may provide greater than 60 dB of shielding to frequencies below 1 GHz.
For protection against primarily magnetic fields, shielding materials with high permeability are necessary. For
example, iron or steel conduit offers better protection against magnetic fields than does aluminum conduit. In
lieu of ferrous conduit, annealed high permeabiliy metal strips wrapped around the cable are sometimes used.
Multiple layers of counterspiral-wound nickel-iron or silicon-iron alloys, or low carbon steel frequently prove
effective. High permeability tape is also available with or without adhesive backing.
Also, combination high
permeability, high conductivity tape is available which provides both electric and magnetic shielding.
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MIL-HDBK-419A
The proper installation of cables is essential if interference difficulties are to be avoided. Assuming proper
grounding techniques have been employed, the following are suggested as guidelines for good signal cable
practice.
a.
Choose the cable to be utilized according to the characteristic impedance desired, amount of signal
attenuation permitted, environment within which the cable must exist, and characteristics of the signal to be
transmitted.
Where a high degree of shielding is needed, cables with multiple shields separated by insulation
b.
should be used. Double shielding is not effective unless each shield is insulated from each other.
c.
Overall shields of multipair cables should not be used for signal return paths.
d.
Individually shielded cables, where used, should have insulating sleeves or coverings over the shields.
Balanced signal circuits should use twisted pair or a balanced coaxial line with a common shield. A coaxial line
with a shield is commonly called a triaxial cable. Where multiconductor twisted pair cables that have individual
shields as well as a common shield are used, all shields should be insulated from one another within the cable.
e.
Coaxial cables should, in all cases, be terminated in their characteristic impedances.
f.
Coaxial cables carrying high-level energy signals should not be bundled with unshielded cables or
shielded cables carrying low-level signals.
Grounding a number of conductor shields by means of a single wire to a connector ground pin should
g.
be avoided, particularly if the shield-to-connector, connector-to-ground lead length exceeds one inch, or where
different circuits that may interact are involved.
Such a ground lead is a common impedance element across
which interference voltages can be developed and transferred from one circuit to another.
Great care should be taken at connectors if impedance characteristics and shielding integrity are to be
maintained.
A shielding shell should be used to shield the individual pins of a connector; a well-designed
connector has a shielding shell enclosing its connecting points.
The shell of multiple connectors should be
connected to the shield. Coaxial lines should terminate in shielded pins.
The use of pigtail connections for
coaxial lines is undesirable since it permits rf leakage.
Serious interference problems arise when shielded wires or coaxial cables are not properly terminated at the
connector. It is important that the connector be properly grounded. The direct bonds for this ground can be
achieved by maintaining clean metal-to-metal contact between the connector and equipment housing. In those
cases where a large number of individual shields from shielded wires must be connected to ground, it is
recommended that the halo technique be used. The exposed unshielded leads should be as short as physically
possible to reduce electrical coupling between conductors. Interference is caused when a shielded cable is run
into a completely sealed box, but is grounded internally. The correct way to install a shielded rf cable is to run
the shield well inside the connector and bond it around the connector shell.
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Figure 8-33. Surface Transfer Impedance
FREQUENCY IN MEGAHERTZ
Figure 8-34. Shielding Effectiveness of Various Types of RF Cables as a Function of Frequency (8-15)
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MIL-HDBK-419A
8.8.2 Terminations and Connectors.
If the effectiveness of a shield is to be maintained, the cable shield must be properly terminated. In an
otherwise adequately shielded system, rf currents that are conducted along shields can be coupled to the system
wiring from the point of an improper cable termination. This is a particularly important consideration in the
case of cables exposed to high power rf fields.
In a properly terminated shield, the entire periphery of the shield is grounded to a low impedance reference,
minimizing any potentials at the surface of the termination. MIL-E-45782B (8-16) recommends against use of
soldering to terminate shields because of the danger of damaging conductor insulation, and suggests a variety of
termination methods, all involving crimping operations. The use of silver epoxy or other synthetic conducting
material has been found to be unacceptable for shield bonding because of lack of mechanical strength necessary
for this application.
Cable connectors are made in many styles for a multitude of power, signal, control, instrumentation,
transducer, audio, video, pulse, and rf applications.
They are made to fulfill special functions and may be
required to be hermetically sealed, submersion proof, and weatherproof. They are manufactured in the straight
type, angle type, screw-on type, bayonet twist-and-lock type, bayonet screw-on-type, barrier type, straight
plug-in type, and push-on types (see Table 8-18).
Figure 8-35 illustrates the type of connector that should be used when a shielded cable assembly contains
individually shielded wires. The practice of pigtailing these shields and connecting them to one of the pins is
not recommended.
The individual shields should be connected to coaxial pins specifically adapted for this
purpose, with the shields of the mating surfaces making contact before the pins.
When maintaining the shielding integrity of a connector pair (i. e., two interconnecting connectors), a good
method to employ (see Figure 8-36) is to place spring contacts inside one portion of one connector so that
positive contact is made along the circumference of the mating parts. These contacts are extended so that the
shell of the connector mates before the pins make contact on assembly of the connector and breaks after the
pins on disassembly. A connector which meets these requirements is available under MIL-C-27599 (8-17) and is
the preferred type to be used in rf-proof designs.
The advantages gamed using circumferential spring fingers over bayonet coupling is dramatically illustrated in
Figure 8-37. In this case, the spring contacts were of silver-plated beryllium copper.
8.9 SHIELDED ENCLOSURES (SCREEN ROOMS). Screen rooms are specially constructed enclosures designed
to provide an electromagnetically quiet area.
In very high level signal environments or where very sensitive
equipments must be protected, screen rooms may be necessary.
significant features of twelve different types of screen rooms.
Table 8-19 summarizes some of the more
These same rooms with carefully engineered
apertures and openings can be expected to provide at least 100 dB attenuation to electric and plane wave fields.
When the installation of a shielded room is required, a number of alternatives must be considered. The most
important of these alternatives is whether to shield an existing or future room or building, or whether to
provide a remountable enclosure which may be relocated quite simply when the need arises.
8-63
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Table 8-18
8-64
MIL-HDBK-419A
Figure 8-36.
RF-Shielded Connector
8-65
MIL-HDBK-419A
FREQUENCY IN MHz
Figure 8-37. Effectiveness of Circumferential Spring Fingers
for Improving the Shielding of a Connector (8-18)
8.9.1 Remountable (Modular) Enclosures.
The basic construction of a demountable enclosure might be a 1.27 cm (1/2 inch) thick plywood panel faced on
both sides with an electro-galvanized steel sheet of nominal 0.56 mm (0.022”) thickness. For non-isolated
double shields, the double facing of the walls makes panel-to-panel joining a considerably more certain process
as each bonding joint is duplicated.
The joining between wall panels is effected by a specially formed metal
section, and tile design of this requires a fairly precisely controlled blend of resilience and rigidity to establish
continuous contact without gaps throughout the length of each bonding member.
The most critical part of any shielded enclosure is the door; with some modern installations doors sizes of 1.86
square meters (20 square feet) and above are required. In general, two types of door bonds are used: these are
referred to as the “wedge” and the “knife edge” design.
The most commonly used is the wedge door (Figure
8-38) which takes the form of a standard casement type hinged opening leaf or leaves with the frame and the
door leaf edges shaped to form a wedge entry, and beryllium copper finger stock affixed in a double layer
around the complete periphery of the door leaf. The reason for adopting the wedge design is that, by correctly
choosing the angle of wedge, contact pressure on tile finger stock can be made high without the risk of tearing
and breaking of the spring fingers when the door is opened; it has been found that this type of construction can
achieve an overall performance on the order of 125 dB attenuation.
The second type of door which has been
used for special applications is the knife edge design in which the door leaf is provided with a flanged edge
made to enter between two sets of finger stock contacts, enclosed within a channel section fixed upon the door
frame.
An advantage of this construction is that finger stock is completely protected and the performance is
better than obtainable with a wedge door, especially at low frequencies.
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Table 8-19
Characteristics of Commercially Available Shielded Enclosures (8-13)
Magnetic
Shielding
Room Description
Copper screen cell type
Styrofoam core, sheet metal skin.
Braided gasket material on door.
Frequency
Effectiveness
(kHz)
(dB)
MAX
MIN
15
61
56
200
97
96
15
90
54
150
87
63
Hollow core construction. Piano
15
100
81
hinge on door with finger stock.
200
118
108
1000
100
80
29 mil sheet metal bonded to 3/4” plywood base
panel (2 sides) with bolted seam clamps. Three
point suspension of personnel door. 20/50 foot
overhead door with double row finger stock.
Construction similar to above,
18
93
64
150
120
95
26 gauge steel with folded and soldered seams
14
58
34
between panels. Commercially available door
280
75
58
14
70
65
except no overhead door.
with double row of beryllium copper finger stock.
All power lines provided with filters.
Continuously soldered 20 gauge sheet metal with
1.25 oz/ft 2 zinc electroplate. Two commercial
100
90
doors with finger stock (2 rows). Power line
filtering installed. Room size = 20 x 20 x 8 feet.
Continuously inert-gas welded sheet steel, 12 gauge
with overlapping seams. Standard commercial
shielded room door with double row finger stock.
8-67
14
90
74
200
130
106
MIL-HDBK-419A
Table 8-19 (Continued)
Magnetic
Shielding
Room Description
Similar to above in construction features.
Frequency
Effectiveness
(kHz)
(dB)
MAX
MIN
15
111
50
100
99
81
25
0.1
Double shielding of 10 gauge continuously inert-gas
1
welded low carbon sheet steel, 2" spacing between
62
52
15
108
92
doors (no gasket).
100
120
107
Room partitioned into three separate rooms; two
0.5
104
73
are 12' x 12' x 10' and the third is 12' x 12' x 14'.
1.0
122
All seams continuously inert-gas welded 16 gauge
5.0
80
sheet steel. Doors have pneumatic bladder with
10
39
triple row of finger stock.
15
20
Room divided into three cells. Single shielding
1
115
sheet steel continuously inert-gas welded, with
15
114
104
pneumatic bladder, and expanding panel sliding
100
140
114
doors (EMI gasket for contact surface). Total
10,000
119
61
walls, pneumatic bladder, expanding panel sliding
room size 30 x 70 x 12 feet.
8-68
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Figure 8-38. Use of Finger Stock for Door Bonding
8-69
MIL-HDBK-419A
Additional door bonding may be incorporated with either woven Mumetal gasketing (for very low frequencies),
or flexible microwave absorber (for very high frequencies).
To attenuate signals below 50 MHz, waveguide hallways can be used (8-19).
The cutoff frequency is
proportional to the largest lateral dimension of the hallway; therefore, a tradeoff is generally necessary
between hallway size and required attenuation.
As shown in Section 8.5.3.1, the amount of attenuation of
frequencies below cutoff is a function of hallway length.
The waveguide hallway may be constructed of 20
gauge, or thicker, low carbon steel supported by any structurally sound, but nonconductive material.
In all types of door design intended for use at frequencies above a few hundred megahertz, it is desirable to
avoid metallic penetration of the door. A special locking catch has been designed which enables full retention
of the door leaf and release of the latch from both sides of the door without the need for any metallic
penetration of the shield.
This lack of metallic penetration is important since even with the most adequate
bonding any operating shaft severely increases the risk of shield degradation at frequencies where the shaft’s
length becomes resonant.
It is also important to ensure that even insulating penetrations through the shield
which pass through waveguide-below-cutoff tubes are correctly designed.
Although the cutoff frequency of a
waveguide in air can be easily calculated, the inclusion of insulating material of high dielectric constant in the
waveguide considerably reduces the cutoff frequency.
A further requirement for shielded enclosures is adequate ventilation.
Honeycomb structures provide a
virtually unimpeded passage for air flow and are normally incorporated in ventilation ducts, ventilation
openings, and fans or air conditioner systems.
It is essential to avoid signal penetration via power and signal wiring.
This demands that filters achieving
adequate insertion loss are installed in all incoming cables; it is fairly normal to have three-phase power
circuits and several hundred signal lines going into a large enclosure. It is essential that the filters provide the
specified attenuation under full-load conditions at all frequencies. Unless the filter attenuation is maintained
at all frequencies and load currents, the overall shield attenuation will be degraded by the signal penetration via
the filters. Shield penetrations may also be provided for air , gas, and water lines; these can be achieved either
by the use of waveguide-below-cutoff tubes carrying insulating piping or by welding metal pipework to the
shield. It is essential that all input circuits and penetrations occur in a localized area.
It is necessary that the shield be grounded adequately for safety purposes.
Although an external ground
connection has no effect on the equipment placed within an ideal shield since the shield itself forms its own
private world, an external ground is essential to prevent the enclosure from reaching dangerous potentials
relative to its surroundings.
8.9.2 Custom Built Rooms.
In spite of the wide range of use of remountable modular enclosures, a considerable demand exists for
specialized custom built shielded areas.
These are employed either where the insertion loss requirements are
markedly different from those obtainable from modular rooms or where the area to be enclosed is exceptionally
large and economy dictates that some other design be adopted.
Many forms of construction are used and these
include enclosures made from woven copper or steel mesh, from pierced and expanded metal, from aluminum or
copper foils, from high permeability materials such as Mumetal, and from all–welded steel sheet.
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MIL-HDBK-419A
The use of mesh and open work materials is only employed where a very economical construction is required and
only a low shielding performance is necessary. Likewise the high permeability foils are not normally employed,
although the low frequency performance of these can be extremely good when related to the foil thickness. A
more economical construction often results from the use of welded steel in thicker gauges, although high
permeability materials are required where the shield must provide high attenuation to extremely low frequency
or constant magnetic fields.
The most efficient practical shielding is provided by a continuously welded steel sheet clad enclosure. Standard
practice in Great Britian is to employ a 1.2 mm (0.048”) thick electrogalvanized mild steel sheet continuously
seam welded along all edges using an inert gas shrouded electric arc welding process.
achieve the highest performance realizable at an economical price.
This approach may
Construction may either be supported by
the walls and ceiling of the parent room, or the shield may incorporate its own independent steel framework.
The shielding effectiveness of a shielded enclosure can be improved with the use of double shields. As indicated
in the earlier section on the theory of shielding, the shielding effectiveness of two parallel (but slightly
separated) shields is better than that of one double thickness shield but not twice as effective as a
single-thickness shield.
The actual improvement in shielding efficiency is dependent upon the degree of
electrical isolation maintained between the two shields.
At least one manufacturer (8-20) of shielded rooms maintains that the isolated double shielded room is
substantially more effective than either the single-shielded or the “not isolated double shielded” room.
The
same types of doors, ventilation apertures, and filters described for the modular rooms are used except that in
many cases an rf-proof access lock is provided; this may combine interlocks between the doors and completely
automatic operation either by electric, hydraulic, or pneumatic systems.
8.9.3 Foil Room Liners.
When the shielding requirement does not justify an all-welded steel room or a separate screen room, it may be
possible to use metal foils.
For example, a copper foil nominally 5 mils thick with continuous soft soldered
seams may be employed. This copper foil can be glued to the walls, floor, and ceiling to provide a complete
lining to an existing room.
If this construction is used in conjunction with gasketed metal doors, properly
designed vents, and electrical filters, performance, while not being good for low frequency magnetic fields, can
be comparable to welded steel at the higher frequencies. To achieve this performance, it is essential that all
seams and joints be carefully soldered to establish continuous bonds. The cost of construction is not as low as it
might first appear, especially when the additional complications which result from the need to provide fixtures
for internal decorative finish and equipment mounting within the shielded area is considered. In general, this
form of construction is only used where a relatively unsophisticated enclosure is required, e.g., in certain
electro-medical work.
If even more economy is required, it is possible to omit the soldering of the joints
between the copper foils and use a conductive adhesive tape which is less expensive to install. If only electric
fields are present at low frequencies, then a copper foil shield constructed in this manner will probably be
adequate.
When shielding is required only for microwave frequencies, a very economic shield may be constructed using
aluminum foil of approximately 5 mils thick glued to the walls, floor, and ceiling. An overlap between adjacent
foil sheets of approximately 5 cm (2 inches) should be allowed; these overlaps should be secured with
aluminum-backed contact adhesive tape. This type of shield is most effective at frequencies above several
8-71
MIL-HDBK-419A
hundred megahertz; its shielding effectiveness increases with frequency since the bond between adjacent sheets
is primarily capacitive.
The normal application for this type of shield is for the protection of computers and
data processing installations operating in the vicinity of high power radars.
Where shields of this type are
intended to work only at very high frequencies, it may be possible to dispense with the shielding over part of the
central floor area in ground level installations.
8.10 TESTING OF SHIELDS.
Shield testing may be categorized as (1) the testing of shielding materials to determine their shielding
properties, and (2) the testing of shield designs (such as shielded enclosures) to determine whether or not the
design and construction are satisfactory.
The first category of testing results in design data such as that
described earlier in this chapter, and is usually performed by the shielding material manufacturer rather than
the equipment designer or user. Methods for performing these and related tests can be found in Reference 8-21
and are not discussed further here.
On the other hand, the second category (the testing of equipment shields
and shielded rooms for verification of sufficient shielding effectiveness) is a necessary part of equipment
development and/or acceptance and is therefore discussed in the following.
The testing of constructed shields is necessitated by the somewhat unpredictable effects of both intentional and
unintentional openings and seams in the shield. Localized testing can point out the location of electromagnetic
(EM) leaks such as those resulting from welding faults in seams and from poorly fitting gaskets. Such testing is
frequently necessary for the successful construction of shielded enclosures. Uniform field (as opposed to local)
testing is useful for acceptance testing of a shield.
Methods have been developed for both localized and
uniform shielding tests for variable-frequency EM fields of low impedance (magnetic), high impedance
(electric), and plane waves.
The variety of test methods available for evaluating shielding effectiveness are due, at least in part, to the
many different factors that can affect material shielding capabilities. These factors include the configuration
of the shield (Is it a sheet of material, or is it a box?), the frequency range of concern, whether or not the
impinging wave is planar, the wave impedance, and others. This section will discuss some frequently employed
and generally applicable shielding effectiveness tests. Frequently employed tests include:
a.
Low Impedance Magnetic Field Testing Using Small Loops,
b.
Low Impedance Magnetic Field Testing Using a Helmholtz Coil,
c.
High Impedance Electric Field Testing Using Rod Antennas,
d.
High Impedance Electric Field Testing Using a Parallel Line Radiator,
e.
Plane Wave Testing Using Antennas,
f.
Plane Wave Testing Using a Parallel Plate Transmission Line, and
g.
MIL-STD-1377 Testing (8-22).
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MIL-HDBK-419A
A number of the above tests are very similar to tests designed to measure equipment and system EMC in
accordance with MIL-STD-462 (8-23). They also are similar to tests performed to evaluate EM effectiveness of
shielded enclosures used for testing purpose in accordance with MIL-STD-285 (8-24). One who is concerned
with the measurement of shielding properties should become familiar with both of these standards.
The MIL-STD-1377 tests represent procedures for evaluating the shielding (and filtering) effectiveness of
systems.
The specification contains a unique approach to shielding measurements; its cable effectiveness
evaluation methods are good illustrations of how cable and connector performance tests should be performed.
It should be pointed out that a high degree of measurement accuracy cannot generally be expected for shielding
tests. Typically, wave impedances are not established when the tests are performed, antenna correction factors
used for calibration purposes are based on plane-wave assumptions even though the test condition may not
warrant this assumption, the degree of radiated field distortion by proximal structures is not known, and other
factors limit the accuracy of the measurement. However, the tests can be expected to provide guidance on the
shielding design approaches and the general effectiveness to be expected of those approaches.
8.10.1 Low Impedance Magnetic Field Testing Using Small Loops.
This test is designed to indicate the shield’s effectiveness in reducing the intensity of predominantly magnetic
field radiation. It employs two small loop antennas and evaluates loop coupling with and without an intervening
shield.
MIL-STD-285 incorporates a similar magnetic field small loop measurement procedure to evaluate the
shielding effectiveness of shielded enclosures used for electronic testing purposes.
In this test, a pair of identical small loop antennas are used, one on one side of the shield and one on the other,
spaced equidistant from the shield. If an enclosure is being tested, the usual practice is to have the test signal
source within the enclosure and the receiving loop and detector outside the enclosure.
Figures 8-39 and 8-40 show the two basic loop orientations. In Figure 8-39 the loops are coaxial, that is, both
loops are normal to a common loop axis. In Figure 8-40 the loops are coplanar, that is, the loop surfaces lie on
the same plane.
Tests using at least these two orientations should be employed, but orientations that may
result in a lower effectiveness figure should not be ignored. Both the loop diameters and the loop separations
should be significantly less than the shortest dimension of the box, container, or enclosure being tested. Since
this will result in only a small section of the shield being illuminated at one time, it will be necessary to move
the loop over the entire surface of the shield to establish the effectiveness of the shield.
The frequency range over which this test can be performed is a function of the level of shielding effectiveness
that must be measured (measurement system dynamic range), the sensitivity of the test equipment, the
available power to drive the test transmitting loop, and the loop-to-shield separations. The limiting factors are
usually the areas of the loops and the number of turns in the loops, since these establish the self-resonance
frequency of the loop. Loop-to-loop separation should not be closer than the loop diameter.
The small loop-to-loop setup specified in MIL-STD-285 is shown in Figure 8-40 with the following parameter
values employed:
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MIL-HDBK-419A
Loop diameters (d): 12 inches (30.48 cm)
Loop-to-shield separations (r/2): 12 inches
Loops: One turn of No. 6 AWG Copper Wire
The test setup in this specification is intended to provide a minimum of 70 dB measurement range.
The corresponding test for a uniform magnetic field requires placing the test shielded enclosure within a
Helmholtz coil (large loop), with a small detection loop inside the test enclosure. The use of a Helmholtz coil
enables a large portion of the enclosure to be illuminated at one time.
Various orientations of the sample
relative to the loop should be tried. The frequency range is limited by the test sample size, which affects the
size of the Helmholtz coil. Increasing coil size increases its inductance, reduces its self-resonant frequency,
and decreases the frequency range over which its magnetic field strength remains constant. The coil diameter
should be at least two and preferably three times the longest test sample dimension. The upper frequency limit
is typically 100-500 kHz.
8.10.2 Additional Test Methods.
Although additional test methods for medium and high impedance incident
waves exist, they are less frequently applied since the most difficult problem is the shielding at low frequency,
magnetic fields.
8.11 PERSONNEL PROTECTION SHIELDS.
Shields for the reduction of EMI are also appropriate for protecting people from potentially hazardous radiation,
either ionized or nonionized in nature. For most situations where sensitive electronic apparatus is present, the
facility shielding required to prevent EMI is more than adequate to provide personnel protection. However, for
high level incident fields, the need for personnel protection alone should not be overlooked. Areas of particular
concern are those near high voltage vacuum devices which may emit X-rays, near high power rf sources or
emitters such as acquisition and search radars, or near other sources of potentially damaging emanations such
as laser emissions encountered during maintenance of fiber optics containing laser diodes.
Shields for protection against contact with hazardous voltages at very low frequencies, i.e., dc and 50/60 Hz,
are not generally of the same type as those which protect against radiated fields. Personnel protection may be
Metal
nonconducting and function more as a simple physical barrier which prevents accidental contact.
electromagnetic shields may also establish a physical barrier; however, the barrier is an incidental byproduct
and should not be considered to be the primary purpose of the shield.
8.12
DETERMINATION
OF
SHIELDING
REQUIREMENTS.
Comprehensive
shielding of a
structure,
particularly a large one, can be very expensive. Fortunately, if the threat signal environment is known or can
be predicted, an appropriate choice of available or existing materials can accomplish the necessary shielding
with minimum costs.
Methods available for establishing the amount of shielding required in a given location
include analyzing equipment malfunctions or disturbances, performing an electromagnetic site survey to obtain
power density levels, and performing electromagnetic susceptibility and emissions tests of the equipments
which are to be located in the facility.
Shielding requirements can then be determined by comparing the
susceptibility levels of the equipment against the power density levels measured in the area where the
equipment is to be located.
8-74
MIL-HDBK-419A
Figure 8-39. Coaxial Loop Arrangement for Measuring Shield Effectiveness
Figure 8-40. Coplanar Loop Arrangement for Measuring Shield Effectiveness
8-75
MIL-HDBK-419A
8.12.1 Equipment Disturbances.
A reliable indicator of the need for shielding of an equipment is the degree of interference that it experiences
or causes. Recognizing that interference can be the result of one of the four different coupling modes, it must
be determined that coupling will occur through one of the modes which can effectively be combatted by
shielding. For example if the interfering signal is coupled into the equipment or system on a power or signal
line, shielding the equipment may accomplish little. The line picking up the disturbing signal may be made less
susceptible to interfering signals by careful shielding of the line itself. If inductive, capacitive, or radiated
coupling is the cause of the problem, then shielding of the cable either alone or along with the equipment will
be effective.
If the equipment is going into a new facility and the decision to be made is whether or not shielding is
necessary, the behavior of that equipment in other similar environments should be considered.
performance of the specific equipment is not known,
If the
the behavior of equipments of similar types or
construction should be studied. The most reliable method of determining shielding requirements is to compare
known susceptibility levels of the equipment or system with known measured power density levels in the area
where the equipment or system is being installed.
8.12.2 Electromagnetic Environmental Survey.
The most effective way of determining the power densities at the location where the equipment or the
structure is to be located is by conducting an electromagnetic environmental survey. This survey is performed
using calibrated antennas with special field strength meters or spectrum analyzers. These instruments permit
the strength of radiated fields to be determined in terms of volts per meter or in power density, i.e., watts per
square centimeter or square meter.
For personnel hazard determination, commercially available rf radiation
monitors may be used.
The spectrum survey should attempt to identify the presence of all potentially interfering fields. Of particular
concern is the field strength of the signals emitted by readily identifiable sources such as commercial radio and
television stations, and radar and communications transmitters. Other possible sources of interference include
rf heating units, rf welders, microwave ovens, and, in locations near medical facilities, diathermy and
electrocautery
machines. Desk top evaluations can also be employed to calculate power density/signal strength
levels in a given area if all local emitters (including output power, locations, etc) can be identified.
The electrical power system can also be a source of interference.
particular, frequently generate noise
insulators.
High voltage transmission systems, in
through corona discharge and arcing across dirty connectors and
The frequency spectrum of this noise generally extends well into the HF region (3-30 KHz) or above
and can be a cause of severe problems. The routing, either existing or planned, of power lines should be noted
carefully. If long runs of signal and control cables in parallel with power lines, either overhead or underground,
are unavoidable, shielding of the signal and control cables may be necessary.
In addition to the above identifiable sources of energy against which shielding may be required, other less
obvious sources exist. For example, ignition noise from internal combustion engines can be troublesome. Also,
office machines,
vending machines, and
fluorescent lights have been frequently observed to produce
interference in digital computers, measuring systems and other sensitive equipments.
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MIL-HDBK-419A
8.12.3 Equipment EMI Properties.
Different equipments will exhibit different emission and susceptibility properties depending upon the job to be
performed, the method of design, the type construction, the type components used, and a variety of other
The best indicator as to how much shielding is going to be required for a given piece of equipment or
factors.
for an entire complex is provided by the measured level of emissions or the susceptibility level of the equipment
or system.
These properties are determined by operating the equipment in an electromagnetically controlled
environment and by (1) measuring the frequency and amplitude of the signals radiated or produced by the
equipment or (2) irradiating or otherwise subjecting the equipment to a known field or given signal and noting
the minimum level to which the equipment or system responds.
Under field conditions, neither of these
procedures should be expected to provide precise detailed data because reradiation and mutual coupling effects
can cause wide variations in the measured results.
However, with a reasonable sampling of the fields or with
illuminations provided at various locations and different orientations, an order-of-magnitude estimate of the
relative susceptibility or threat posed by the equipment or system should be possible. If precise data is needed,
test procedures in accordance with accepted standards, such as MIL-STD-461 and MIL-STD-462 should be
performed.
Unfortunately
because of the expense of performing detailed and accurate emission and
susceptibility tests of equipments (even the ability to perform these tests on large complexes in a meaningful
manner is doubtful), and because a decision is frequently required on structural shielding before the specific
equipment population is known, it is generally necessary to direct attention only to the most critical equipments
or systems expected to be installed in the facility. Shielding requirements can also be determined by comparing
the susceptibility levels (MIL-STD-461) of the equipment being installed with the measured or calculated power
density levels in the area where the equipment is being installed.
If
it
is
simply
not
possible
to anticipate or project the shielding requirements, then the resultant
electromagnetic environment in which equipment will be required to perform must be measured or calculated
and the information provided to the equipment supplier so that appropriate steps can be taken to assure that the
equipment or system will function in that environment.
8.13 SYSTEM DESIGN CONSIDERATIONS.
The total area or volume of a facility to be shielded and the physical configuration of the shield is a function of:
a.
the size of the equipment or system requiring shields;
b.
the physical layout including orientation between sources and receptors;
c.
the amplitude and frequency of the interfering signals; and
d.
the cost of materials.
These factors typically interact and, although in a given situation one will predominate, all must be considered.
8-77
MIL-HDBK-419A
8.13.1 S i z e .
If a very sensitive piece of equipment or small system is to be located in a large structure,
shielding the entire structure to protect that one small element is probably not cost effective. The cost of
shielding is closely related to the size of the enclosed volume, assuming all other factors equal. Thus, a more
economical approach would perhaps be to shield only the room in which the equipment is to be located,
construct a shielded cage just for the susceptible (or offending) equipment, or upgrade the shielding of the
particular equipment cabinet or enclosure.
If, on the other hand, the susceptible element is a fairly large
system, e.g., a communications center or a large scale computer, then incorporating appropriate shielding
materials into the walls, floor, and ceiling of the room or structure may be necessary. If this requirement is
recognized early in the design stage of the facility, the required shielding may be provided by properly-installed
conventional structural materials.
Also, supplemental shields can frequently be installed with greater economy
if done during construction rather than later.
8.13.2 Layout.
If a susceptible equipment or system is to be located in a building and some choice exists as to position, special
effort should be made to take advantage of the inherent shielding properties of the structure. The existence of
metal walls, decorative screens, and other conductive objects may provide all the shielding necessary. Further,
equipments frequently are more sensitive to radiated signals impinging from only one or two directions. Thus,
orienting the equipment such that the susceptible side is facing away from the incident signal can lessen the
shielding
requirements.
Signal and control cables deserve special mention.
Because the voltage (or current) in the receptor wire is
inversely dependent upon the distance from the source wire and directly proportional to the length of the path,
every effort should be made to avoid long runs in parallel.
8.13.3 Signal Properties.
The shielding effectiveness of practically all materials is frequency dependent. The type of shield which will
protect against an X-band radar signal will not necessarily be effective against a commercial broadcast
transmitter. In choosing a shield for a particular purpose, compare the attenuation properties of the material
with the frequency of the threat signal.
The amplitude of the signal to be shielded indicates the amount of field attenuation the shield must provide.
For most fields, the attenuation provided by the shield is not influenced by the magnitude of the field, i.e., a
shield which will attenuate a low level field 60 dB will likewise attenuate a high level field 60 dB. Very strong
magnetic fields, however, can cause saturation effects and the attenuation of the shield will generally decrease
under very strong magnetic fields.
EMP for instance.
This phenomenon is very important in choosing shields to protect against
Where saturation effects are likely, thicker shields are required in order to maintain the
attenuation needed to protect against the very strong fields.
8.13.4 C o s t .
The impact of size on cost was noted previously in Section 8.13.1 above. Other cost factors to consider include
those associated with providing input and output ports for wiring and cabling, ventilation, and physical and
visual access (doors, windows, meter openings, etc.) while maintaining the effectiveness of the shield.
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MIL-HDBK-419A
8.14
REFERENCES.
8-1. S. A. Schelkunoff, Electromagnetic Waves, D Van Nostrand Co, Inc (1943).
8-2. W.W. Everett, Jr, Topics in Intersystem Electromagnetic Compatibility , Holt, Rinehart, and Winston, Inc,
1972.
8-3. R.B. Shultz, et. al., “Shielding Theory and Practice,” Proceedings to the Ninth Tri-Service Conference on
Electromagnetic Compatibility, IITRI, Chicago IL, October 1963, pp 596-635, AD 434850.
8-4. “Electromagnetic Compatibility,” AFSC Design
Handbook DH 1-4, Air Force Systems Command,
2 March 1984.
8-5. A.P.
Hale,
“Electromagnetic
Shielding,”
1973 IEEE International Electromagnetic Compatibility
Symposium Record, New York NY, 20-22 June 1973, pp 330-339.
8-6. R.B. Schultz, “ELF and VLF Shielding Effectiveness of High Permeability Materials,” IEEE Transactions
on Electromagnetic Compatibility, Vol EMC-10, No. 1, March 1968, pp 95-100.
8-7. “Electromagnetic Compatibility Design Guide for Avionics and Related Ground Support Equipment,”
NAVAIR AD1115, Department of the Navy, Washington DC.
8-8. W. Jarva, “Shielding Efficiency Calculation Methods for Screening, Waveguide Ventilation Panels, and
Other Perforated Electromagnetic Shields,” Proceedings of the Seventh Conference on Radio Interference
Reduction
and
Electronic
Compatibility,
Armour
Research
Foundation (IITRI), Chicago IL,
November 1961, pp 478-498.
8-9. ITEM - 1973 R & B Enterprises. PO Box 328. Plymouth Meeting PA 19462.
8-10. “Electromagnetic Compatibility and Electromagnetic Radiation Hazards,” NAVELEX 0101,106,
Department of the Navy, Washington DC, August 1971.
8-11. H. M. Sachs, et. al., “Evaluation of Conductive Glass in Fluorescent Light Shielding Applications,”
Proceedings of the Sixth Conference on Radio Interference Reduction and Electronic Compatibility,
Armour Research Foundation (IITRI), Chicago IL, October 1960, pp 281-294.
8-12.
“Architectural
Interference
Data,”
RADC-TR-63-312,
Contract AF
30(602)-2691,
White
Electromagnetics Inc, Bethesda MD, 20 August 1963.
8-13. “EMP Protection for Emergency Operating Centers,” TR-61A, Defense Civil Preparedness Agency,
Washington DC, July 1972.
8-14. E.F. Vance, “Shielding Effectiveness of Braided-Wire Shields,” IEEE Transactions on Electromagnetic
Compatibility, Vol EMC-17, No. 2, May 1975, pp 71-75.
8-79
MIL-HDBK-419A
8-15. “RF Transmission Line Catalogue and Handbook, TL-6,” Times Wire and Cable Co, 385G Hall Ave,
Wallingford CT 06492.
8-16. “Electrical Wiring, Procedures for, “MIL-E-45782B(1), 15 December 1980.
8-17. “Connector, Electrical, Miniature, Quick Disconnect (For Weapons Systems) Established Reliability,”
MIL-C-27599A(5) Supp 1A, 20 October 1971.
8-18. F.W. Schor, “Measurement of RF Leakage in Multipin Electrical Connectors,” IEEE Transactions on
Electromagnetic Compatibility , Vol EMC-10, No. 1, March 1968, pp 135-141.
8-19. W.D. Peele, et. al., “Electromagnetic Pulse (EMP) Protection Study,” FAA RD 72-68, Rome Air
Development Center, Rome NY, June 1972.
8-20. E.A. Lindgren, Contemporar y RF Enclosures, 1967, Erik A. Lindgren and Associates, Inc, Chicago IL.
8-21. “Measurement of Shielding Effectiveness of High-Performance Shielding Enclosures,” IEEE Recommended
Practice, No. 299, June 1969.
8-22. “Effectiveness of Cable, Connector, and Weapon Enclosure Shielding and Filters in Precluding Hazards of
Electromagnetic Radiation to ordnance, Measurement of, “MIL-STD-1377 (NAVY).
8-23. “Electromagnetic Interference Characteristics, Measurement of, “MIL-STD-462, 31 July 1967.
8-24. “Attenuation Measurements for Enclosures, Electromagnetic Shielding, For Electronic Test Purposes,
Method of, “MIL-STD-285, 25 June 1956.
8-80
MIL-HDBK-419A
CHAPTER 9
PERSONNEL PROTECTION
9.1 ELECTRIC SHOCK.
Electric shock occurs when the human body becomes a part of an electric circuit.
It most commonly occurs
when personnel come in contact with energized devices or circuits while touching a grounded object or while
standing on a damp floor. The major hazard of electric shock is death. Fatalities from shock total about 1,000
annually.
In addition, numerous injuries occur each year due to involuntary movements caused by reaction
currents.
The effects of an electric current on the body are principally determined by the magnitude of the current and
the duration of the shock. The current is given by Ohm’s Law, which, stated mathematically, is I= V/R where V
is the open circuit voltage of the source and R is the resistance of the total path including the internal source
resistance, and not just the body alone. In power circuits, the internal source resistance is usually negligible in
comparison with that of the body. In such cases, the voltage level, V, is the important factor in determining if
a shock hazard exists.
At the commercial frequencies of 50-60 Hz and at voltages of 120-240 volts, the contact resistance of the body
primarily determines the current through the body. This resistance may decrease by as much as a factor of 100
between a completely dry condition and a wet condition. Thus, perspiration on the skin has a great effect on its
contact resistance.*
important.
At voltages higher then 240 volts, the contact resistance of the skin becomes less
At the higher voltages, the skin is frequently punctured, often leaving a deep localized burn. In this
case, the internal resistance of the body primarily determines the current flow.
9.1.1 Levels of Electric Shock (9-l) (9-2).
The perception current is that current which can just be detected by an individual. At power frequencies, the
perception current usually lies between 0 and 1 milliamps for men and women, the exact value depending on the
Individual.
Above 300 Hz, the perception current increases, reaching approximately 100 milliamps at 70 kHz.
Above 100-200 kHz, the sensation of shock changes from tingling to heat. It is believed that heat or burns are
the only effects of shock above these frequencies.
The reaction current is the smallest current that might cause an unexpected involuntary reaction and produce
an accident as a secondary effect. The reaction current is 1-4 milliamps. The American National Standards
Institute (9-3) limits the maximum allowable leakage current to 0.2 milliamps for portable two-wire devices and
0.75 milliamps for heavy movable cord-connected equipment in order to prevent involuntary shock reactions.
*For calculation purposes, the resistance of the skin is usually taken to be somewhere between 500 and 1500
ohms.
9-1
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Shock currents greater than the reaction current produce an increasingly severe muscular reaction.
certain level, the shock victim becomes unable to release the conductor,
Above a
The maximum current at which a
person can still release a conductor by using the muscles directly stimulated by that current is called the
“let-go” current. The “let-go” current varies between 4-21 milliamps, depending on the individual. A normal
person can withstand repeated exposure to his “let-go” current with no serious after effects when tile duration
of each shock lasts only for the time required for him to release the conductor.
Shock currents above about 18 milliamps can cause the muscles of the chest to contract and breathing to stop.
If the current is interrupted quickly enough, breathing will resume. However, if the current persists, the victim
will loose consciousness and death may follow.
Artificial respiration is frequently successful in reviving
electric shock victims.
Above a certain level, electric shock currents can cause an effect on the heart called ventricular fibrillation.
For all practical purposes, this condition means a stoppage of the heart action and blood circulation.
Experiments on animals have shown that the fibrillating current is approximately proportional to the average
body weight and that it increases with frequency.
In Table 9-1, the various hazardous current levels for ac and dc are summarized along with some of the physical
effects of each.
Table 9-1
Summary of the Effects of Shock (9-1) (9-2)
Effects
Alternating Current (60 Hz)
Direct Current
(mA)
(mA)
0-1
0-4
Perception
1-4
4-15
Surprise (Reaction Current)
4-21
15-80
Reflex Action (Let-Go Current)
21-40
80-160
Muscular Inhibition
40-100
160-300
Respiratory Block
Over 100
Over 300
9-2
Usually Fatal
MIL-HDBK-419A
9.1.2 Shock Prevention.
Most shock hazards can be divided into two categories: unsafe equipment and unsafe acts. The most common
hazards in each category can be controlled as follows:
a.
Power cords and drop cords with worn and/or broken insulation should be routinely replaced.
b.
All spliced cords should be removed from service.
c.
Exposed conductors and terminal strips at the rear of switchboards and equipment racks should be
enclosed and warning labels installed.
d.
Rubber mats should be installed on the floor of all enclosures containing exposed conductors and on
the floor in front of high voltage switches.
e.
High voltage switches should be of the enclosed safety type.
f.
All wiring should comply with recognized electrical codes and it should be large enough for the
current being carried.
g.
Temporary wiring should be removed as soon as it has served its purpose.
h.
The noncurrent-carrying metal parts of equipment and power tools should be grounded.
i.
The main power switch to all circuits being worked on should be locked open and tagged.
j.
Power switches should be opened before replacing fuses and fuse pullers should be used.
k.
Fuse boxes should be locked to prevent bridging or replacing with a heavier fuse.
1.
Care should be taken to prevent overloading of circuits.
9.2 STATIC ELECTRICITY.
Static electricity is produced when two bodies, particularly of unlike materials, are brought into intimate
contact and then separated. When in contact, there is a redistribution of charge across the area of contact and
an detractive force is established.
When the bodies are separated, work is done in opposing these attractive
forces. This work is stored in the electrostatic field which is set up between the two surfaces when they are
separated. If no conducting path is available to allow the charges to bleed off the bodies, the voltage between
the surfaces can easily reach several thousand volts as they are separated.
Static electricity is an annoyance to many individuals. Static shock can result in discomfort and even injury to
workers due to involuntary reaction. A far more dangerous aspect of static electricity is the fire and explosion
hazard. This hazard can occur in situations where a vapor-air , gas-air, dust-air, or combinations of these
9-3
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mixtures exist in the proper ratio, In order for static to be a source of ignition of the mixtures, four conditions
must be fulfilled. These conditions are:
a.
There must first be an effective means of static generation.
b.
There must be a means of accumulating the separate charges and maintaining a suitable difference
of electrical potential.
c.
There must be a spark discharge of adequate energy.
d.
The spark must occur in an ignitable mixture.
The most common sources of static electricity are:
a.
Steam, air, or gas flowing from any opening in a pipe or hose, particularly when the stream is wet or
when the air or gas stream contains particulate matter.
b.
Pulverized materials passing through chutes or pneumatic conveyors.
c.
Nonconductive power or conveyor belts in motion.
d.
Moving vehicles.
e.
All motion which involves changes in relative position of contacting surfaces (usually of dissimilar
substances, either liquid or solid), of which one or both must be a poor conductor of electricity.
Static electricity can be controlled in a variety of ways. The most effective means are:
a.
Bond all metallic parts of a system to prevent the existence of a statically-induced potential
difference between any two metallic objects in the system.
b.
Ground all metallic systems to prevent the accumulation of static charges.
c.
Increase the relative humidity to 60% - 70% to increase the moisture content and thus the
conductivity of insulating materials such as fabric, wood, paper, concrete, or masonry.
d.
Use ionizing devices to ionize the surrounding air so that it becomes sufficiently conductive to bleed
off static charges.
e.
Use conductive materials for rugs, flooring, belts, etc. where nonconductive materials might
otherwise be used.
9-4
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9.3
RADIO FREQUENCY (RF) RADIATION HAZARDS.
The effect of rf radiation on living tissue is thought to be primarily thermal in nature. The most vulnerable
parts of the human body are the eyes and the testes. However, other parts which can be affected are the brain,
nerves, skin, and muscles.
The thermal effects can range from a mild heating of the skin or organs to fatal
damage.
Below 1000 MHz, rf energy penetrates deeply into the body. These frequencies are extremely hazardous since
the radiation is not detected by the nerve endings located in the skin. The power absorbed in the body tissues
can be as high as 40% of the incident power.
The urinary bladder, gall bladder, and parts of the gastro-
intestinal tract are particularly vulnerable since they are not cooled by an abundant flow of blood.
Also,
stainless steel and platinum bone implants and fillings in teeth can increase in temperature when subjected to rf
radiation, resulting in burning of tissues.
In the 2-5 GHz region of the rf spectrum, the eyes and the testes are the most vulnerable organs to rf radiation
damage. Damage to the eyes is generally irreversible and can result in blindness from cataracts or loss of lens
transparency.
Animal experiments have shown that damage to the testes from low levels of exposure does not
differ from that caused by common forms of heat applied to the testes, and that the reduction in testicular
function due to heating appears to be temporary. It is not known if rf radiation produces any genetic damage.
To minimize possible
hazards from rf radiation, Dept of Defense Instruction 6055.11 (9-4) provides
recommendations to prevent possible harmful effects in human beings exposed to radio frequency radiation.
9.4 LASER HAZARDS (9-5).
Biological damage from laser radiation is caused by photochemical, thermal, and pressure effects, acoustic and
ultraviolet shock waves, plasma generation , ultrasonic emission, and even the generation of free radicals. Of
these, the first three are the most hazardous to tissues, organs, and eyes. The damages include tissue
ionization, molecular rearrangements, blood vessel occlusion, corneal opacity, retinal lesion, blindness, and even
death.
Lasers are divided into four classes:
Class 1 is non-hazardous; Class 2 depends on blink reflex for the
person to turn away to prevent a hazard; Class 3 is a direct or specular reflection hazard; Class 4 is all other
high energy lasers. See ANSI Z136.1 for further information concerning safe use of lasers.
It is believed that damage to eye tissue by visible and infrared light is mainly thermal in nature. The lens of the
eye is practically transparent in these regions, thus increasing the susceptibility to retinal burn or lesion. The
susceptibility is enhanced by the fact that the power density of light converging on the retina is concentrated
by a factor of 105 when it passes through the pupil and lens of the eye.
At near ultraviolet, the eye responds in nearly the same way as it does to visible light, the exception being a
marked decrease in vision between 380 nm (10 -9 meters) and 420 nm.
absorption at the lens of the eye.
This decrease is caused by the strong
Extreme exposure to these wavelengths may lead to the development of
cataracts. In the B and C ultraviolet bands, radiation is absorbed by the cornea and its outer layer. Excessive
exposure to these wavelengths can cause a condition called “welder’s flash,” an effect similar to snow blindness.
9-5
MIL-HDBK-419A
Serious skin injury can occur at high power levels in the near infrared and visible regions. The skin becomes
Increasingly sensitive in the ultraviolet region. Energy at these wavelengths penetrates deeply and can cause
severe burns. In the near infrared range, the skin becomes relatively transparent, making the internal organs
particularly susceptible.
9.5 X-RAY RADIATION.
X-rays are generated when electrons are accelerated to a sufficiently high velocity before colliding with an
appropriate target.
In addition to being produced by specifically designed equipment, X-rays can also be
produced by high-voltage (> 15 kV) tubes used for other applications. It is important that all sources of X-ray
radiation in equipments be identified and shielded so that they will not present a personnel hazard.
The maximum safe exposure to X-ray radiation is considered to be 100 milli-Roentgens per week (9-6). Based
on a 40-hour work week, this limit translates into a maximum hourly rate of exposure of 2.5 milli-Roentgens
per hour.
9.6 REFERENCES.
9-1. C.F. Dalziel, “Electric Shock Hazard,” IEEE Spectrum, Vol 9, No. 2, February 1972, pp 41-50.
9-2. “Standard General Requirements for Electronic Equipment,” MIL-STD-454J, 30 June 1985.
9-3. “Leakage Current for Appliances,” C101.1-1973, American National Standards Institute, 1430 Broadway,
New York NY 10018.
9-4
Dept of Defense Instruction 6055.11, 20 August 1986, “Protection of DOD Personnel from Exposure to
Radio Frequency Radiation.”
9-5. Marce Eleccion, “Laser Hazards,” IEEE Spectrum, Vol 10, No.8, August 1973, pp 32-38.
9-6. Dept of Defense Instruction 6055.8, 3 January 1983, “Occupational Radiation Protection
9-6
Program.”
MIL-HDBK-419A
CHAPTER 10
NUCLEAR EMP EFFECTS
10.1 INTRODUCTION.
In addition to the blast, thermal effects, and radioactive fallout, a nuclear detonation
produces an intense electromagnetic effect.
Under the proper circumstances, a nuclear detonation generates a
high-intensity electromagnetic pulse (EMP) whose frequency spectrum may extend from below 1 Hz to above
300 MHz. This high-intensity EMP can disrupt or damage critical electronic facilities over an area as large as
the continental United States, unless protective measures are taken in the facilities. The development of such
protective measures involves grounding, bonding, and shielding and requires an understanding of the EMP itself.
10.2
EMP GENERATION.
10.2.1 High-Altitude EMP (HEMP).
The high-altitude EMP (HEMP) produced by an exoatmospheric nuclear
explosion is the form of EMP commonly of most interest because of the large area covered by a single bomb.
The HEMP is also the form for which interaction and protection are most advanced. The standard HEMP
waveforms to be used for tests and analyses of hardened systems are given in DoD-STD-2169 (SECRET-RD). A
brief description of the three parts of the standard waveform is given below.
10.2.1.1 Early-Time HEMP.
The detonation of a nuclear weapon produces high-energy gamma radiation that travels radially away from the
burst center. When the detonation occurs at high altitudes where the mean free path of the gamma photons is
large, these photons travel great distances before they interact with another particle. As illustrated in Figure
10-1, gamma rays directed toward the earth encounter dense atmosphere where they interact with air
molecules to produce Compton recoil electrons and positive ions.
The Compton recoil electrons also travel
radially away from the burst center initially, but these moving charged particles are acted upon by the Earth’s
magnetic field, which causes them to turn about the magnetic field lines (10-1).
The Earth’s magnetic field accelerating the Compton recoil electrons causes them to radiate an electrodynamics
field. Thus, the early-time HEMP is produced by this charge acceleration (electron turning) phenomenon that
occurs in the atmosphere in a region about 20 km thick and 30 km above the Earth’s surface (sea level). This
source region covers the Earth within the solid angle subtended by rays from the burst point that are tangent to
the surface of the Earth, as illustrated in Figure 10-2.
To an observer on the ground, the incoming wave
appears to be a plane wave propagating toward him from the burst point.
The amplitude, duration, and
polarization of the wave depend on the positions of the burst and the observer, relative to the Earth’s magnetic
field lines. Peak electric field strengths of over 50 kV/m with risetimes of a few nanoseconds and decay times
of less than 1 µs are typical for this early-time portion of the HEMP (10-2}.
10-1
MIL-HDBK-419A
h= HEIGHT OF BURST = 400 km = 250 MILES
s= DISTANCE TO HORIZON =2,250 km = 1,400 MILES
A HIGH ALTITUDE BURST ILLUMINATES LARGE GEOGRAPHICAL REGIONS
WITH GAMMA RAYS.
Figure 10-1. EMP from High Altitude Bursts
Figure 10-2. Schematic Representation of High-Altitude EMP Generation
10-2
MIL-HDBK-419A
10.2.1.2 Late-Time HEMP (MHDEMP).
Much later (0.1 to 100 s), currents are induced in the ground by the
effects of the expanding and rising fireball constituents.
These effects are called the magnetohydrodynamic
EMP (MHDEMP). They arise from the motions of the rapidly expanding bomb debris and hot ionized gases in the
Earth’s magnetic field.
MHDEMP has two phases produced by two principal effects. The first effect is an
ionospheric blast wave that deforms the geomagnetic field lines and produces an early phase of the MHDEMP
that reaches the Earth’s surface in 2 to 10 seconds and can be seen worldwide. The second effect is the
“atmospheric heave, " in which hot debris and air ions are moved across geomagnetic field lines to cause large
circulating currents in the ionosphere. These currents induce image currents in the ground over a period of 10
to 100 seconds. Although the field strengths produced at the surface by the MHDEMP are small (tens of volts
per kilometer), they occur over long times.
Thus, the NHDEMP is a consideration for long power and
communications lines and, because of its duration, for the energy it can deliver to protective devices.
10.2.1.3 Intermediate-Time HEMP.
Between the early-time HEMP and the MHDEMP, transitory phenomena
produce what is called intermediate-time HEMP.
This HEMP lasts from about 1 µs to about 0.1 s. The
intermediate-time HEMP observed at the Earth’s surface has a peak electric field strength of a few hundred
volts per meter and is predominantly vertically polarized.
10.2.2 Surface-Burst EMP.
When a nuclear weapon is detonated at or near the surface of the Earth, neutrons
and gamma rays are ejected radially outward from the burst center. The gamma ray photons emitted by the
bomb, and others produced by neutron inelastic collisions with air, ground, and water, interact with air
molecules to produce Compton recoil electrons.
At or near sea level, however, the Compton recoil electrons
quickly collide with air molecules to provide a copious supply of low-energy secondary electrons and ions. Thus,
the Compton recoil electrons account for a large charge separation and, because of the secondary ionization, a
fairly conductive air.
As illustrated in Figure 10-3, the charge displacement is asymmetrical because of the
Earth’s surface. The initial dipole charge is discharged by current through the ionized air and soil. From a
large distance, the EMP from a surface burst appears to emanate from a dipole source; it is vertically polarized
and attenuated as l/r with distance, r, from the burst point.
Thus, the surface-burst EMP is a more localized
source than the HEMP. however, within the source region where the Compton electrons, secondary ionization,
and relaxation currents occur, the fields are large, and long conductors, such as power lines and communication
cables, may have large currents induced on them. These currents may be propagated along the conductors for
great distances from their source. Therefore, this source-region EMP (SREMP) may be important to systems
far outside the source region if they are connected to the source region through wires, cables, or other
conductors.
10-3
MIL-HDBK-419A
Figure 10-3.
Surface-Burst Geometry Showing Compton Electrons and Net Current
D e n s i t y , Jc n e t .
Radiated Fields are Approximately Proportional to
d Jnetc /dt (Electric-Dipole Fields)
10.2.3 Other EMP Phenomena.
The high-altitude EMP (HEMP) is, by far, the most important form of EMP for communication facilities because
of its large area of coverage.
However, in addition to the HEMP and the surface-burst EMP, a few other
electrical effects should be mentioned.
System-generated EMP (SGEMP) is produced when the high-energy
particles (mostly gamma- and X-ray photons) produced by the bomb interact directly with the system structure.
These interactions knock electrons out of the structure, which causes current on the structure and potential
gradients between the structure and the removed charge.
The structure of interest may be system wiring or
cable shielding; the current and potential differences are then on system circuits. (Because this EMP is often
generated inside the system, it is sometimes called internal EMP (IEMP).)
SGEMP is of major concern for
satellites and other space vehicles because the gamma- and X-rays from the high-altitude bomb can travel
great distances without colliding with another particle or structure. SGEMP is also a consideration for surface
systems whose blast and thermal resistance permits them to operate inside the source region.
Another important electrical effect is known as transient radiation effects on electronics (TREE).
The
radiation emitted by the nuclear explosion can interact with components of electronic circuits to produce
ionization or atomic displacements in the semiconductor and insulating materials.
The effects range from
momentary changes in conductivity to permanent changes in crystal lattices. Semipermanent effects, such as
trapped charges in insulating materials, may also occur.
TREE may upset memories, produce spurious circuit
responses (logic errors), drive circuits into abnormal states, or cause permanent damage.
As with most other
EMP forms, damage caused by TREE can also occur through secondary effects. Self-inflicted damage may be
triggered by abnormal conductivity in a junction that allows stored energy to be released. In addition, one
circuit may be caused to instruct another circuit or another part of the system to perform some forbidden act
that destroys the circuit or even the system.
10-4
MIL-HDBK-419A
10.2.4 Comparison with Lightning.
Lightning and the EMP are often compared because they are both large electromagnetic phenomena and
because more people have experienced lightning in some form.
Though they are generated by different
mechanisms, some aspects of their effects on systems are similar, Both can produce large electrical transients
in systems. Both interact with power lines and communication cables to excite systems served by these cables.
However, lightning and HEMP have important differences in their electromagnetic properties and in the way
they interact with systems.
Lightning can deliver greater energy to a moderate impedance load, such as a
power transmission line, than can the HEMP.
On the other hand, the HEMP has a larger rate of change of field
and induced currents and voltages than lightning, so that coupling phenomena that depend on dE/dt and dB/dt
(where E and B are the electric field intensity and magnetic flux density, respectively) are more important for
the HEMP excitation than they are for lightning. Because the HEMP appears to be a plane wave at the Earth’s
surface, its interaction with long insulated conductors, such as overhead lines, can include a “bow wave” effect
in which the inducing wave propagates along the line synchronously with the induced current wave, building up
very large induced currents. The field produced by lightning decreases as l/r with distance, r, from the source,
so that the bow-wave effect is much less prominent for lightning than it is for HEMP.
Perhaps the most important difference between lightning and the HEMP is their area of coverage. Lightning
strikes one point in a large system such as a continental communication network, while the HEMP excites the
entire network almost simultaneously. Large networks have been designed to cope with single-point outages,
such as those that may occasionally occur because of lightning.
We have no experience to assist us in
determining the effect of a large number of simultaneous outages that might accompany HEMP, and it is
virtually impossible to test hypotheses of system reactions with network-scale experiments. Furthermore, the
system is not exposed to the HEMP during peacetime; we get no feedback from a “protected” system on the
effectiveness of the protection. Thus, protecting large networks from the HEMP usually involves conservative
protection of individual parts of the network in the hope that network hardness will follow from component
hardness.
10.3 HEMP INTERACTION WITH SYSTEMS.
effects and local effects.
HEMP interaction with systems may be separated into long-line
Long-line effects are the currents and voltages induced on long power lines,
communication cable links, or even other conductors, such as pipelines.
Some of these HEMP effects may be
induced far away and guided to the facility along the conductor. Local effects are the currents and voltages
induced directly on the facility shield, building structure, wiring, equipment cabinets, etc. These local effects
are very difficult to evaluate analytically because of the complexity of the facility structure, the lack of
information on the broadband electrical properties of many of the structural materials, and the extremely large
number of interaction paths, facility states, and other complicating factors (10-2), (10-3). On the other hand,
the local interactions can be evaluated experimentally with simulated HEMP fields that envelop the facility.
The full length of the long lines connected to a facility can rarely be illuminated with simulated HEMP fields;
the HEMP interaction with the long lines must usually be estimated analytically and simulated as an external
excitation driving at the long-line port.
10-5
MIL-HDBK-419A
10.3.1 Current in Long Lines.
10.3.1.1 Long Overhead Lines.
The currents induced on long straight overhead lines parallel to the Earth’s surface by HEMP-like events have
been analyzed thoroughly (10-4), (10-5), (10-6). If the line is over a perfectly conducting ground plane, the
current has a waveform similar to the HEMP early-time waveform , except for a slightly longer risetime for
lines more than a few feet high.
For imperfectly conducting ground, such as soil, the imperfect reflection of
the wave from the ground allows the line to be driven more strongly and for a longer time than if the ground
were a good conductor.
The short-circuit current induced in a semi-infinite line (one extending from the observer to infinity) over soil
for an exponential pulse of incident field is shown in Figure 10-4. The current is shown for horizontal
is the
polarization (dashed line) and vertical polarization (solid line) of the incident field. The curve
current that would be induced in a wire over a perfectly conducting ground; this current is proportional to line
height (h), decay time constant
, and incident field strength (Ei). The current in Figure 10-4 is normalized by
containing the characteristic impedance of the line, the peak field (E o ) and decay time constant
of the
incident field, the speed of light (c), and a directivity function (D). The directivity function (D) depends on the
azimuth angle
between the wire and the vertical plane containing the Poynting vector of the incident wave,
and on the elevation angle
of the Poynting vector of the incident
wave.
The correction for finitely
conducting ground is proportional to the incident field strength, the 3/2 power of the decay time constant, and
the inverse square root of the soil conductivity (u).
For a 300
line 10 meters (33 feet) above soil having 10 -3 siemans/meter (S/m) conductivity, an incident 15
kV/m exponential pulse with 250 nanoseconds (ns) decay time-constant will induce a short-circuit current of
about 10kA on the line.
Vertically polarized waves induce larger currents than horizontally polarized waves,
but in the latitudes of the mainland United States, the HEMP fields are predominantly horizontally polarized.
Thus, only 15 kV/m was used in this example, even though the peak HEMP field may be much larger than 15
kV/m.
More sophisticated analyses that take into account the burst point, the observer point, and their effect
on HEMP polarization and waveform give peak short-circuit currents between 5 and 10 kA for the early-time
HEMP. The open-circuit voltage induced at the end of the semi-infinite line is the product of the short-circuit
current and the characteristic impedance (Z c) .
For the 300
would be 3 megavolts (MV).
10-6
line in this example, the open-circuit voltage
MIL-HDBK-419A
Figure 10-4.
Short-Circuit Current Induced at the End of a Semi-Infinite
Above-Ground Wire by an Exponential Pulse
10.3.1.2 Long Buried Lines.
As noted in 10.3.1.1, imperfectly conducting soil does not completely reflect the incident field; some of the
incident wave is transmitted into the soil. This field in the soil can induce current in underground cables, pipes,
and other conductors. However, because the velocity of propagation of a wave is much less in soil than in air,
the bow-wave effect is almost negligible on buried conductors.
Furthermore, the attenuation on buried
conductors is greater than on overhead lines because of the proximity of the lossy soil to the buried conductor.
For conductors in contact with the soil (i. e., buried bare conductor), the current at any observation point is
determined primarily by coupling within one skin-depth of the observation point. Current induced at points
farther away is so strongly attenuated by the soil that it adds little to the total current at the observation
point.
The current induced in a long buried cable is shown in Figure 10-5 for various depths of burial, as given by a
depth parameter
where d is the depth of burial. The current is normalized to the inductance per unit
length (L) of the cable, the magnitude of the incident exponential pulse (E o), the decay time-constant
soil time-constant
and a directivity function (D). The induced current is proportional to the incident
field strength (E o ), the 3/2 power of the decay time-constant,
conductivity
the
and the inverse square root of the soil
For a horizontally polarized, vertically incident pulse having E o = 50 kV/m and
= 250 ns, a
long cable buried near the surface (d = 0) in soil of conductivity 10-3 S/m will have about 2.8 kA induced in it.
10-7
MIL-HDBK-419A
Figure 10-5.
The Normalized Current Waveform for Various Values of the Depth
Parameter p (Exponential Pulse)
10-8
MIL-HDBK-419A
10.3.1.3 Vertical Structures.
The HEMP interacts with vertical structures, such as radio towers, waveguides,
and cables to overhead antennas, and downleads from power and communication lines in much the same manner
as it interacts with horizontal lines, except that it is the vertical component of the electric field that drives the
vertical structures. The current induced in a downlead from an overhead power line is shown in Figure 10-6.
Because the line is short and the angle of incidence is only 300, little bow-wave effect is observable. The peak
current is also limited by the line height in this example; for taller structures, the leading edge of the current
wave will continue to rise as the integral of the incident wave. The current will increase with structure height
for structures up to a few hundred feet high.
Figure 10-6.
Short-Circuit Current Induced at the Base of a Vertical Riser by a
Vertically Polarized Incident Wave
10.3.2 HEMP Interaction with Local Structure.
10.3.2.1 Shields.
The HEMP fields incident on a closed shield induce surface currents and charge
displacements on the outer surfaces of the shield.
If the shield is continuous metal (i. e., it has no opening or
discontinuities in its surface) and about 1 mm thick, the voltage induced in circuits inside the shield by the
HEMP will be very small.
Table 10-1 shows the voltage induced in the largest single-turn loop that can be
placed inside a spherical shield of 10 meters radius by a zero-rise-time 50 kV/m incident exponential pulse
having a 250 ns decay time-constant (10-7). Note that even for a shield as thin as 0.2 mm, the induced voltage
is less than 1 V; shields made of workable thicknesses of common metals do not allow significant HEMP
interaction with internal circuits.
Possible exceptions to this conclusion are those shields that are long and of
small cross section, such as the shields on intrasite cables.
10-9
.
.
MIL-HDBK-419A
Table 10-1
Shielding by Diffusion
Internal Voltage Induced in Loop *
Shield
Thickness
Copper
Aluminum
Steel
(mm)
5.8 x 107 S/m
3.7 x 107 S/m
6 x 106 S / m
( µr = 200)
0.2
0.34V
0.85 V
0.076 v
1.0
2.6 mV
6.4 mV
1.1 mV
5.0
21.0 µV
51.0 µV
15.0 µV
* Peak voltage induced in a loop by radius 10 m inside a spherical shield or radius 10 m illuminated by a
high-altitude EMP (by diffusion through walls only).
10.3.2.2 Penetrating Conductors.
Conductors, such as power and signal wires, that pass through the shield, as
illustrated in Figure 10-7 , may allow very large currents and voltages to be delivered to internal circuits. The
current on the wire just inside the shield is about equal to the current just outside the shield; the wire is a 0 dB
compromise of the shield. (At high frequencies, the capacitance between the wire and the shield wall may
cause some attenuation of the wire current, but this effect is negligible at frequencies such that that
where C is the wire-to-shield capacitance and Z c is the load impedance on the wire. (For nominal values of
C = l0 pF and Z c = 200
f = 80 MHz for a 6-dB loss.) Thus a major concern for HEMP interaction is the
penetrating conductor that can guide HEMP-induced waves through shield walls. As discussed above and
illustrated in Figure 10-7, the shield is effective in excluding the incident electromagnetic waves, but it has
little effect on the waves guided through it on insulated penetrating conductors (10-8].
10-10
MIL-HDBK-419A
Figure 10-7. Shield to Exclude Electromagnetic Fields
l0.3.2.3 Apertures.
Apertures in the shield surface allow the external HEMP-induced fields to penetrate through the shield and
interact with internal wiring or other conductors, as illustrated in Figure 10-8. The external electric field (En)
associated with the surface charge density (q =
can induce charge on internal cables as illustrated in
Figure 10-8a. The external magnetic field (H t) which has the same magnitude as the surface current density (J)
can penetrate through the aperture to link internal circuits, as illustrated in Figure 10-8b. Since the current
induced by the electric field is proportional to dE n /dt and the voltage induced by the magnetic field is
proportional to dH t/dt, the aperture coupling emphasizes the fast-changing parts, or high-frequency spectrum,
of the HEMP-induced transient. However, it is important to recognize that it is in the rate of rise that the
HEMP stress is dominant over the other external sources.
The maximum open-circuit voltage induced by a rate of change of external magnetic field of 10 11 A m-1 s -l
penetrating a circular aperture 5 cm in radius is over 600 V. This rate of change of the field is characteristic of
the HEMP, and the analysis leading to the induced voltage is based on a wire directly across the aperture just
inside the shield.
Thus, aperture coupling is an important consideration in HEMP interaction analysis.
Apertures in facility shields take many forms; they range from open doors and windows to the discontinuities at
riveted or bolted joints in the shield.
10-11
MIL-HDBK-419A
Figure 10-8. Electromagnetic Penetration Through Small Apertures
10-12
MIL-HDBK-419A
10.4 PROTECTION AGAINST HEMP.
There are important considerations in designing this protection that affect the value that can be placed on
HEMP protection. The HEMP protection adds cost to the facility, and the value received for the added cost is
confidence that the facility will survive HEMP.
This implies that (1) the protection against HEMP can be
verified, and (2) this protection is retained and can be maintained throughout the life of the facility. The
protection has low value when it is designed in such a way that it is difficult to verify or maintain. The
protection may be difficult to verify when the HEMP-induced stresses inside the facility are large enough to
cause spurious arcing or other insulation breakdown.
It may also be difficult to verify when it depends on
unknown or uncontrolled electromagnetic properties of materials used in the facility.
Finally, hardness
verification will be difficult if the number of features that must be tested is very large. For example, if the
HEMP-induced stress is large deep inside the facility, the number of system states, modes of excitation, stress
waveforms, etc., that must be evaluated may be enormous.
Since HEMP does not ordinarily occur during peacetime, degradation of the protection is not evident from
peacetime operation of the facility.
Therefore, the HEMP protection has greatest value if it is durable. The
protection should not be degraded by normal use and maintenance of the facility. The protection should not
depend on extraordinary configuration control.
It must accommodate facility growth and modification.
Components critical to the protection should be few in number, accessible, and testable.
Protecting communication facilities against the HEMP typically consists of developing a closed HEMP barrier
about the facility.
The barrier consists of a shield to exclude the incident space waves and various barrier
elements on the essential penetrating conductors and in the apertures required for personnel and equipment.
The number of penetrating wires, apertures, and other features that must be evaluated to verify the HEMP
protection is kept as small as possible.
In addition, attention is given to the number of system states or
configurations for which the protection must be determined.
Durability and accessibility of the protection
elements are also important.
10.4.1 HEMP Barrier.
10.4.1.1 Shield.
The facility-level shield used for protection against HEMP is typically fabricated from
welded sheet steel. The thickness is usually selected for ease of fabrication, but in areas where exceptional
mechanical abuse is likely, mechanical strength, as well as workability, may be a consideration.
Shield
assembly is typically accomplished by continuous welding, brazing, hard soldering, or other fused-metal process
to minimize the number of discontinuities in the shield (a weld or other fused-metal pint is considered
continuous metal).
10.4.1.2 Penetrating Conductors.
Concepts for penetrating conductor treatment are illustrated in Figure 10-9. Penetrating conductors that can
be grounded, such as plumbing, waveguides, grounding cables, and cable shields, are bonded to the shield wall at
their point of entry by peripherally welding them to the wall or by the use of clamps, collets, etc., that
peripherally bond the penetrating conductor to the shield with little or no discontinuity.
Signal and power wires that need not penetrate the shield should not penetrate the shield. Wires that must
penetrate the shield must be treated with a barrier element, such as a filter or surge arrester, that closes the
barrier above a voltage threshold or outside the passband required for signal or power transmission.
10-13
MIL-HDBK-419A
(a) GROUNDING CONDUCTORS
(b) “GROUNDABLE” CONDUCTORS
(c) INSULATED CONDUCTORS
Figure 10-9. Shielding Integrity Near Interference-Carrying External Conductors
10-14
MIL-HDBK-419A
10.4.1.3 Apertures.
No unnecessary openings or discontinuities in the shield should be allowed.
Those
openings necessary for personnel and equipment loading and for ventilation should be designed to limit
electromagnetic field penetration.
Openings that must permit air flow or light passage can be made more
opaque to HEMP waves by covering them with mesh or, preferably, honeycomb waveguide-beyond-cutoff, as
illustrated in Figure 10-10. High traffic entryways can use waveguide-beyond-cutoff tunnels with doors at each
end (without the doors, the highest frequencies in the HEMP spectrum can penetrate through a tunnel large
enough for personnel to walk through).
Where possible, discontinuities in the shield should be eliminated by
continuous welding or a similar process. Those that are necessary for equipment installation and maintenance
should be electromagnetically sealed with durable bonding techniques, such as resilient RFI gaskets and small
bolt spacing.
Where access is required infrequently, it might be practical to weld the equipment entry door
shut; large cargo doors are large compromises to shielding integrity and difficult to seal effectively and
durably.
10.4.2 Allocation of Protection.
10.4.2.1 Amount of Protection Needed.
The amount of protection needed depends to some extent on how failure is defined for the system. For
communications facilities, the threshold for failure, or the minimum acceptable performance, may be defined
by a maximum allowable outage time or error rate. In some cases, the principal requirement is that the system
not damage itself so that it can be restarted and restored to service after an attack involving HEMP. The
definition of system failure, or operating requirements, should be prescribed in the system specification; it will
be determined by many factors in addition to HEMP.
In determining the amount of HEMP protection required, it is important to be able to define a transient
tolerance or susceptibility level for the facility or the equipment in the facility. Since most communications
equipment have no transient “withstand” requirement, except perhaps on the power terminals, we cannot obtain
the required tolerance from the equipment specification.
Nevertheless, it is possible to define a transient
stress at or below which the equipment performance will be unaffected.
definition, but more practical values can be found.
Zero stress certainly satisfies this
For example, the equipment tolerates its operating signal
levels, and it tolerates the peacetime transient stress inside the facility. Neither of these is a trivial value of
stress, and we can be assured that if the HEMP-induced stress is made small compared to either, the presence
of HEMP will not cause the equipment to malfunction.
Additional information on transient withstand
requirements may be found in MIL-STD-461C.
The equipment or internal circuit threshold defined in terms of known peacetime tolerances has several
advantages (10-9)
(1)
It takes advantage of known equipment “withstand” capability; no more HEMP protection is necessary
than that required to reduce HEMP transients to a safe margin below this known tolerance.
(2)
It is not necessary to determine the HEMP response of circuits and structural elements inside the
equipment; this greatly reduces the complication of hardness verification and maintenance.
(3) It is possible to place all HEMP requirements at the facility barrier, so that concern for interior
configuration control and internal states are alleviated.
10-15
MIL-HDBK-419A
(a)
(b) MANY SMALL
APERTURES
SINGLE APERTURE
(c) ARRAY OF WAVEGUIDES
BEYOND CUTOFF
Figure 10-10. Magnetic Field Penetration of Apertures
10-16
MIL-HDBK-419A
10.4.2.2 Where Protection is Applied.
As noted above, HEMP protection must be designed to accommodate hardness verification procedures. The
most easily verified protection requires the least number of tests and the least number of assumptions to
establish the integrity of the protection.
linking it to other parts of a network.
For example, suppose the facility is a node with a 100-pair cable
Because of unbalanced and nonlinear terminations, there may be 200 2
two-wire stresses and susceptibilities to evaluate at the cable penetration of the facility barrier. Inside the
facility, the 200-wire cable may branch into 1000 or more wires and equipment terminals. Thus if the HEMP
stress is allowed to be dominant (larger than known peacetime stresses) inside the barrier, 1 0 0 0 2 transient
stresses and susceptibilities must be evaluated (or assumed unimportant). In addition, in the latter case, all the
internal interactions between the 1000 wires and other internal circuits must be assessed (or assumed
unimportant). Typically, both the number of features to be evaluated and the number of assumptions necessary
increase with the depth into the system at which hardness verification is attempted.
Therefore, for facilities whose protection against HEMP has high value (i.e., where confidence in the protection
is important), the protection is placed at the system-level barrier, and the protection at this level is sufficient
that the HEMP-induced stress is not dominant inside this barrier.
10.4.2.3 Terminal Protection Devices.
Problems from HEMP are expected to arise from the antennas and
connecting cables, long interconnecting leads and cables between equipments, and the ac power lines.
Antennas, connecting cables, and the front-end of the associated communications equipments in particular will
be subjected to very large voltages and currents.
The protective technique or device must protect the
equipment without adversely affecting its performance, and must be capable of withstanding the effects of both
EMP-induced transients and other transients in the system.
The latter two considerations may severely limit
applications of many of the protective devices at rf unless they are modified or used in conjunction with other
components.
10.4.2.3.1 Spark Gaps and Gas Tubes.
Spark gaps are one of the oldest forms of surge arrester. A spark gap is
a pair of electrodes, insulated by air or other gas , spaced so that the gap will break down when the voltage
exceeds a specified level.
The insulating gas pressure varies from a fraction of an atmosphere to several
atmospheres , and the electrode spacing varies from a few millimeters in carbon blocks to several inches in large
lightning arresters used for power equipment. Firing voltages range from about 1 kV for some carbon blocks to
hundreds of kV for large lightning arresters.
Large spark gaps can handle large charge transfers (many
coulombs). In the nonconducting state spark gaps behave as open circuits or small capacitances. The spark-gap
firing voltage increases with the rate-of-rise of the applied voltage.
Thus, for the large rates-of-rise
encountered in EMP-induced voltages, the firing voltage may be several times as large as the rated static firing
voltage. When spark gaps are used on energized lines, some provision must be made to assure that the discharge
will be extinguished. Frequently, a metal-oxide varistor (MOV) is used in series with the spark gap to ensure are
extinction after the surge.
Gas tubes are spark gaps with a low-pressure gas so that lower firing voltages can be achieved. Firing voltages
below 100 V are available for commercial gas tubes. The gas tubes are generally more limited in their peak
current and charge transfer capability than the spark gaps.
Gas tubes are used primarily for secondary
protection of wire pairs entering a facility from a long external shielded cable, or for exposed intrafacility
10-17
MIL-HDBK-419A
wiring.
Balanced two-wire models are available that allow ionization from the first discharge to cause
immediate conduction of both halves of the tube so that circuit imbalance is minimized. Coaxial models are
also available for use on coaxial lines such as antenna feed cables.
virtually no loss in the nonconducting states.
Gas tubes have small capacitances and
The glow state occurs in circuits whose impedance limits the
discharge current to less than about 100 mA; the voltage across the tube in this state is about 100 V. The arc
state occurs when large currents are caused to flow; the voltage across the tube in the arc state is usually 10 to
20 V. Gas tubes should not be used on energized lines that can sustain the arc or glow discharge.
Spark gaps and gas tubes display a negative dynamic resistance at the firing point, where a decrease in voltage
across the device is accompanied by an increase in current through it. This property of spark gaps and gas tubes
sometimes leads to unpredicted instabilities in the protected circuits.
In addition, the discharge is a sudden
change in voltage and current that may shock-excite the protected circuit.
It is usually recommended that a
linear filter be placed between the device and the protected circuits to minimize the effects of the negative
dynamic resistance and shock excitation.
10.4.2.3.2 Metal-Oxide Varistors.
MOVs are capable of diverting currents up to tens of kiloamperes and, when
packaged and installed to minimize terminal and lead inductance, they are effective for large rate-of-rise
transients.
Although they are nonlinear, MOVs do not display the negative dynamic resistance and shock
excitation characteristics of the spark gaps and gas tubes.
Their nonlinearity may produce inter modulation
effects in RF circuits. The MOV stops conducting when the applied voltage decreases below the “knee” of the
V-I curve. It is ideal for protecting energized lines, since it has no current-extinguishing problems. The MOV
typically adds nanofarads of shunt capacitance and megohms of shunt resistance to the protected circuit. It
should be used with caution on high-frequency circuits and high-impedance circuits.
The maximum energy
dissipation capability for large MOVs is tens of kilojoules. Just above the failure threshold, they usually fail as
a short circuit or low resistance.
However, for energies well above the failure threshold, the devices may be
physically destroyed, sometimes explosively.
10.4.2.3.3
Semiconductors.
A number of avalanche devices are available for use as surge limiters. The
semiconductor devices limit at lower voltages (1 to 100 V) than the MOVs and gas tubes, but they are less
tolerant of large peak currents and large energies than the other devices.
Peak current ratings up to about
100 A are available. Because the devices themselves may be damaged by transients arriving on external wires
and cables, they are not recommended for facility-level use. They may be used to protect equipment inside the
facility and circuits that are entirely inside the shielded facility. The semiconductor devices add nanofarads of
shunt capacitance to the protected circuit and may aggravate inter modulation problems.
10.4.2.3.4 Filters.
Linear filters may also be used as barrier elements on penetrating wires, but at the outer
(facility-level) barrier, filters are always used in combination with surge arresters. On power lines, for
example, the line filter usually cannot tolerate the peak voltages, so a spark-gap surge arrester is used to limit
the voltage, and the filter isolates the interior circuits from the negative dynamic resistance and shock
excitation of the spark-gap discharge. The shunt input capacitance of the filter may also be used to reduce the
rate-of-rise of the voltage, so that the firing voltage of the surge arrester will be lower. A variety of low-pass,
bandpass, and high-pass filters is available for power and signal line protection.
10-18
MIL-HDBK-419A
10.4.2.4 Waveguide Penetration of Facility Shield.
10.4.2.4.1 Introduction.
Waveguides, like other external conductors that penetrate the facility shield, can allow transients to propagate
into the facility if they are not made continuous with the shield in the manner illustrated in Figure 10-11.
Ideally, the waveguide wall should make continuous contact with the facility shield around the entire periphery
or the waveguide combination.
All of the waveguide current would then flow onto the outer surface of the
facility shield; the external transients could only penetrate to the interior by diffusion through the waveguide or
facility wall.
In practice thi s continuous peripheral contact between the waveguide and the shield can be achieved by welding
or soldering the waveguide to the wall. Two ways of implementing this connection are illustrated in Figure 1012, where waveguide feedthrough sections are installed in the facility shield wall (or in a panel that is welded or
bolted to the wall).
In both cases, a waveguide section with two flange joints is used to allow the internal
waveguide signal to pass through the wall but keep the external transient interference outside the facility. This
method of treating the waveguide allows some flexibility in the waveguide plumbing inside and outside the
facility, since only the feedthrough section is permanently attached to the wall.
For microwave receiving systems operating with very small signals, the fraction of a dB loss in the joints and
the possibility of additional loss from distortion about the weld may be undesirable, although the weld
distortions can be eliminated by machining or reforming the welding operation.
Where these losses are
intolerable, some alternate methods of attaching the waveguide to the facility shield are available. In the
following sections, two of these methods are described.
Although these methods can be used satisfactorily,
they are generally less rugged and more susceptible to corrosion and other degradation than the welded
feedthrough sections of Figure 10-12.
Figure 10-11. Exclusion of Waveguide Current from Interior of Facility
10-19
MIL-HDBK-419A
Figure 10-12. Waveguide
10-20
Feedthroughs
MIL-HDBK-419A
10.4.2.4.2 In-Line Waveguide Attachment.
Connecting the waveguide to the shield without the feedthrough
section and flange joints requires an in-line connection.
In-line connections are somewhat inconvenient,
because the waveguide penetration hole in the facility must be fairly accurately located so that it is aligned
with the waveguide ports on the internal equipment and the external plumbing. In addition, the hole in the wall
must be large enough to pass a waveguide flange, yet must be effectively closed by the attachment mechanism.
Finally, the attachment to the waveguide must accommodate misalignment of the waveguide with the axis of
the hole.
If we further prohibit welding or brazing because of the potential distortion and damage to the
internal finish of the waveguide , we are limited to soft soldering, mercury wetting, and clamping to make the
electrical connection to the waveguide.
Because of its environmental problems and its tendency to dissolve
waveguide materials, mercury wetting has not been proposed to make the connection. The use of soft solder
bonds also is prohibited by MIL-STD-188-124A on conductive paths subject to lightning or power fault currents.
The following procedures are acceptable for bonding waveguides or cables to a designated RF shield, barrier or
entrance plate.
10.4.2.4.2.1 Sleeve and Bellows Attachment. In this method, illustrated in Figure 10-13, the connection to the
waveguide is made with a snug-fitting sleeve over the waveguide.
The sleeve may be installed on the guide
before the end fittings are installed, or a split sleeve may be used so that it can be installed at any time. The
preferred method of attaching the sleeve to the waveguide is to soft solder the sleeve to the guide with a
eutectic lead-tin alloy. For split sleeves, however, it will probably be necessary to provide mechanical support
with a clamp, as illustrated in the figure. If even eutectic soldering cannot be tolerated, a clamping alone may
suffice, if the sleeve is slitted to allow it to grip the waveguide and if the sleeve and waveguide are both clean
and protected so that they remain clean.
To help prevent distortion of the waveguide by the clamp, it is
recommended that a neoprene or other resilient cushion be used between the clamp and the sleeve. The flange
on the sleeve and the bellows and its flanges can be welded together without damaging the waveguide. Details
of their design are optional, but the bellows and flanges must be large enough to pass the waveguide flange if
the bellows assembly is to be installed in the field after the waveguide is assembled.
10-21
MIL-HDBK-419A
Figure 10-13. Bellows with Slitted Sleeve Waveguide Attachment
10-22
MIL-HDBK-419A
10.4.2.4.2.2 Braided Wire Sleeve.
A somewhat less effective, but usually adequate attachment to the waveguide can be made with a braided wire
sleeve.
As illustrated in Figure 10-14, the braided wire sleeve is necked down and soldered to the waveguide
and flared out over a collar on the facility shield wall, where it is also soldered or welded. For mechanical
strength, both of these attachments should be reinforced with a hose clamp and cushion, as was used with the
rigid sleeve. And as with the rigid sleeve, the clamp may be used without solder at the waveguide if soldering
cannot be tolerated, but, as before, both the braid and the waveguide must be clean when assembled and remain
clean after assembly.
The braided wire sleeve must expand into a large enough hoop to enable the waveguide end fittings to pass
through (unless the sleeve is installed before the fittings are installed).
sleeve have an optical coverage of at least 85%.
In addition, it is desirable that the
Thus, the sleeve design is fairly stringent because large
expansion is usually accompanied by low coverage.
For both the bellows and the braided wire sleeve attachments, it is recommended that the attachment
mechanism be placed inside the facility wall and that a weatherproof boot or other external seal be installed to
keep moisture and other foreign matter out of the attachment.
Figure 10-14. Braided Wire Sleeve Clamped to Waveguide
10-23
MIL-HDBK-419A
10.4.2.4.2.3 Stuffing Tube for Waveguide.
In this method, illustrated in figure 10-1.5, the connection to the
waveguide i S made with a highly compressed stainless steel wool placed between rigid conduit and the bare
waveguide.
The conduit must be installed over the waveguide before the end fittings are installed. The
follower plugs serve to compress the steel wool and also aid in weatherproofing and protecting the bond from
corrosion. After all weather proofing has been completed, the rigid conduit should be bonded to the entry panel
or facility shield by welding or brazing.
NOTES:
1. Steel wool lightly compressed by follower plugs.
2. Steel wool and waveguide surface must be protected against corrosion.
Figure 10-15. Stuffing Tube for Waveguide
10-24
MIL-HDBK-419A
10.5 REFERENCES.
10-1.
Longmire, C. L., “On the Electromagnetic Pulse Generated by Nuclear Explosions,” IEEE Trans.
Electromagn. Compat., Vol EMC-20, pp 3-13, February 1978.
10-2.
Baum, C. E., “H OW to Think About EMP Interaction,” Proceedings of the 1974 Spring FULMEN Meeting
Air Force Weapons Laboratory, Kirtland AFB New Mexico, pp 12-23, 16-17, April 1974.
10-3.
Tesche, F. M., “Topological Concepts for Internal EMP Interaction,” IEEE Transactions of
Electromagnetic Compatibility, Vol EMC-20, No. 1, pp 60-64, February 1978.
10-4.
Lee, K. S. H., “EMP interaction:
Principles, Techniques, and Reference Data,” Air Force Weapons
Laboratory, Kirtland AFB NM, December 1980.
10-5.
Vance, E., “Coupling to Cables,” DNA Handbook Revision, Ch 11, Stanford Research Institute, Menlo
Park, California, December 1974.
10-6.
Vance, E., Coupling to Shielded Cables, John Wiley and Sons, New York, 1978.
10-7.
Defense Nuclear Agency Report 5433F-1, Unification of Electromagnetic Specifications and Standards
Part I - Evaluation of Existing Practices, by E.F. Vance, W. Graf, and J.E. Nanevicz, 31 October 1980.
10-8.
Bly, Robert T., Jr and Evangelos, Tonas, The Inside and the Outside Are Not the Same-Experiments
Investigations of Ground and Shield Topology. Proceedings of the 1982 IEEE Symposium on EMC.
10-9
Defense Nuclear Agency Report 5433F-2, Unification of Electromagnetic Specifications and Standards
Part II - Recommendations for Revisions of Existing Practices, by W. Graf, J.M. Harem, and E.F. Vance.
10-25/10-26
MIL-HDBK-419A
CHAPTER 11
NOTES
11.1 SUBJECT TERM (KEY WORD) LISTING.
Key words contained in Volume I and Volume II of this
handbook include:
Grounding
Bonding
Shielding
Facility Ground System
Lightning Protection Subsystem
Fault Protection Subsystem
Signal Reference Subsystem
Earth Electrode Subsystem
Single-Point Grounding
Multipoint Grounding
Equipotential Ground Plane
Air Terminal (Lightning Rod)
Electromagnetic Pulse (EMP)
Lower Frequency Ground
Higher Frequency Ground
Phase Conductor
Grounding Conductor (Green Wire)
Grounded Conductor
Neutral Conductor
Ring Ground
Fall-of-Potential Ground Test
Shielding
Effectiveness
Zone (Cone) of Protection
Power System Grounding
Signal Grounding
Facility Shielding
Equipment Shielding
Corrosion
Down Conductor, Lightning
Cathodic Protection
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APPENDIX A
GLOSSARY
ABSORPTION LOSS -- The attenuation of an electromagnetic wave as it passes through a shield. This loss is
primarily due to induced currents and the associated I 2R loss.
AIR TERMINAL -- The lightning rod or conductor placed on or above a building, structure, tower, or external
conductors for the purpose of intercepting lightning.
APERTURE -- An opening in a shield through which electromagnetic energy passes.
BALANCED LINE -- A line or circuit using two conductors instead of one conductor and ground (common
conductor). The two sides of the line are symmetrical with respect to ground.
Line potentials to ground and
line currents are equal but of opposite phase at corresponding points along the line.
BOND -- The electrical connection between two metallic surfaces established to provide a low resistance path
between them.
BOND, DIRECT -- An electrical connection utilizing continuous metal-to-metal contact between the
members being joined.
BOND, INDIRECT -- An electrical connection employing an intermediate electrical conductor or jumper
between the bonded members.
BOND, PERMANENT -- A bond not expected to require disassembly for operational or maintenance purposes.
BOND, SEMIPERMANENT -- Bonds expected to require periodic disassembly for maintenance, or system
modification, and that can be reassembled to continue to provide a low resistance interconnection.
BONDING -- The process of establishing the required degree of electrical continuity between the conductive
surfaces of members to be joined.
BUILDING -- The fixed or transportable structure which houses personnel and equipment and provides the
degree of environmental protection required for reliable performance of the equipment housed within.
CABINET -- A protection housing or covering for two or more units or pieces of equipments. A cabinet may
consist of an enclosed rack with hinged doors.
CASE -- A protective housing for a unit or piece of electrical or electronic equipment.
CHASSIS -- The metal structure that supports the electrical components which make up the unit or system.
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CIRCULAR MIL -- A unit of area equal to the area of a circle whose diameter is one mil (1 mil = 0.001
-6
or 78.54 percent of a square mil (1 square mil = 10
square inch). The
inch). A circular mil is equal to
area of a circle in circular mils is equal to the square of its diameter in mils.
CIRCUIT -- An electronic closed-loop path between two or more points used for signal transfer.
COMMON-MODE VOLTAGE -- That amount of voltage common to both input terminals of a device.
COMMON-MODE REJECTION -- The ability of a device to reject a signal which is common to both its input
terminals.
CONDUCTED INTERFERENCE -- Undesired signals that enter or leave an equipment along a conductive
path.
COPPER CLAD STEEL -- Steel with a coating of copper bonded on it.
COUPLING -- Energy transfer between circuits, equipments, or systems.
COUPLING, CONDUCTED -- Energy transfer through a conductor.
COUPLING, FREE-SPACE -- Energy transfer via electromagnetic fields not in a conductor.
CUTOFF FREQUENCY -- The frequency below which electromagnetic energy will not propagate in a
waveguide.
DEGRADATION -- A decrease in the quality of a desired signal (i.e., decrease in the signal-to-noise ratio or
an increase in distortion), or an undesired change in the operational performance of equipment as the result of
interference.
DOWN CONDUCTOR, LIGHTNING -- The conductor connecting the air terminal or overhead ground wire to
the earth electrode subsystem.
EARTH ELECTRODE SUBSYSTEM -- A network of electrically interconnected rods, plates, mats, or grids
installed for the purpose of establishing a low resistance contact with earth.
ELECTRIC FIELD -- A vector field about a charged body. Its strength at any point is the force which would
be exerted on a unit positive charge at that point.
ELECTROMAGNETIC COMPATIBILITY (EMC) -- The capability of equipments or systems to be operated in
their intended operational environment, within designed levels of efficiency, without causing or receiving
degradation due to unintentional EMI.
life cycle of equipment.
EMC is the result of an engineering planning process applied during the
The process involves careful considerations of frequency allocation, design,
procurement, production, site selection, installation, operation, and maintenance.
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ELECTROMAGNETIC INTERFERENCE (EMI) -- Any electrical or electromagnetic phenomenon, manmade or
natural, either radiated or conducted, that results in unintentional and undesirable responses from, or
performance degradation or malfunction of, electronic equipment.
ELECTROMAGNETIC PULSE (EMP) -- A large impulsive type electromagnetic wave generated by nuclear or
chemical explosions.
EQUIPMENT, UNIT OR PIECE OF -- An item having a complete function apart from being a component of a
system.
EQUIPMENT GROUNDING -- Attained by the grounding conductor of the fault protection subsystem, and/or
bonding to the signal reference subsystem or the structural steel elements of the building.
EQUIPOTENTIAL PLANE -- A grid, sheet, mass, or masses of conducting material which, when bonded
together, offers a negligible impedance to current flow.
(serves as signal reference subsystem for new
facilities)
FACILITY -- A building or other structure, either fixed or transportable in nature, with its utilities, ground
networks, and electrical supporting structures.
All wiring, cabling as well as electrical and electronic
equipments are also part of the facility.
FACILITY GROUND SYSTEM -- The electrically interconnected system of conductors and conductive
elements that provides multiple current paths to earth. The facility ground system includes the earth electrode
subsystem, lightning protection subsystem, signal reference subsystem, fault protection subsystem, as well as
the building structure, equipment racks, cabinets, conduit, junction boxes, raceways, duct work, pipes, and other
normally noncurrent- carrying metal elements.
FAR FIELD -- The region of the field of an antenna where the radiation field predominates and where the
angular field distribution is essentially independent of the distance from the antenna.
FAULT -- An unintentional short-circuit, or partial short-circuit, (usually of a power circuit) between
energized conductors or between an energized conductor and ground.
FIRST SERVICE DISCONNECT -- The necessary equipment (circuit breakers, switches, fuses etc.) located at
the point of entrance of power conductors to a building or other structure.
GROUND -- The electrical connection to earth primarily through an earth electrode subsystem. This
connection is extended throughout the facility via the facility ground system consisting of the signal reference
subsystem, the fault protection subsystem, the lightning protection subsystem and the earth electrode
subsystem.
GROUNDED CONDUCTOR -- (Neutral) The circuit conductor that is intentionally grounded (at first service
disconnect or power source).
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GROUNDING CONDUCTOR -- (Green Wire) A conductor used to connect equipment or the grounded circuit
of a power system to the earth electrode subsystem.
HIGHER FREQUENCY GROUND -- The interconnected metallic network (equipotential plane) intended to
serve as a common reference for currents and voltages at frequencies above 30 kHz and in some cases above
300 kHz. Pulse and digital signals with rise and fall times of less than 1 microsecond are classified as higher
frequency signals.
INTERFACE -- Any electrical connection (encompassing power transfer, signaling, or control functions)
between two or more equipments or systems.
ISOKERAUNIC (or isoceraunic) -- Showing equal frequency of thunderstorms.
ISOLATION -- Physical and electrical arrangement of the parts of an equipment, system, or facility to
prevent uncontrolled electrical contact within or between the parts.
LIGHTNING PROTECTION SUBSYSTEM -- A complete subsystem consisting of air terminals, interconnecting
conductors, ground terminals, arresters and other connectors or fitting required to assure a lightning discharge
will be safely conducted to earth.
LOWER FREQUENCY GROUND -- A dedicated, single-point network intended to serve as a reference for
voltages and currents, whether signal, control or power, from dc to 30 kHz and some cases to 300 kHz. Pulse
and digital signals with rise and fall times greater than 1 microsecond are considered to be lower frequency
signals.
MAGNETIC FIELD -- A vector field produced by a continuous flow of charge.
MULTIPOINT GROUND -- More than one path to ground.
NATIONAL ELECTRICAL CODE (NEC) -- A standard governing the use of electrical wire, cable, and fixtures
installed in buildings. It is sponsored by the National Fire Protection Association (NFPA-70) under the auspices
of the American National Standards Institute (ANSI-CI).
NEAR FIELD -- The region of the field immediately surrounding an antenna where the inductive and
capacitive fields predominate. In this region the angular distribution of the field varies with distance from the
antenna.
NEUTRAL -- The ac power system conductor which is intentionally grounded on the supply side of the first
service disconnecting) means.
It is the low potential (white) side of a single phase ac circuit or the low
potential fourth wire of a three-phase wye distribution system. The neutral (grounded conductor) provides a
current return path for ac power currents whereas the grounding (or green) conductor does not, except during
fault conditions.
PENETRATION -- The passage through a partition or wall of an equipment or enclosure by a wire, cable, or
other conductive object.
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PLANE WAVE -- An electromagnetic wave which predominates in the far field region of an antenna, and with
a wavefront which is essentially in a flat plane. In free space, the characteristic impedance of a plane wave is
377 ohms.
RACK -- A vertical frame on which one or more units of equipment are mounted.
RADIATION -- The emission and propagation of electromagnetic energy through space.
RADIATION RESISTANCE -- The resistance which, if inserted in place of an antenna, would consume the
same amount of power that is radiated by the antenna.
RADIO FREQUENCY INTERFERENCE (RFI) -- RFI is manmade or natural, intentional or unintentional
electromagnetic propagation which results in unintentional and undesirable responses from or perform ante
degradation or malfunction of, electronic equipment.
REFLECTING LOSS -- The portion of the transition loss, expressed in dB, that is due to the reflection of
power at a barrier or shield. Reflection loss is determined by the magnitude of the wave impedance inside the
barrier relative to the wave impedance in the propagation medium outside the barrier.
RF-TIGHT
-- Offering a high degree of electromagnetic shielding effectiveness.
SHIELD -- A housing, screen, or cover which substantially reduces the coupling of electric and magnetic fields
into or out of circuits or prevents the accidental contact of objects or persons with parts or components
operating at hazardous voltage levels.
SHIELDING EFFECTIVENESS -- A measure of the reduction or attenuation in the electromagnetic field
strength at a point in space caused by the insertion of a shield between the source and that point.
SIGNAL REFERENCE SUBSYSTEM -- A conductive sheet or cable network/mesh providing an equipotential
reference for C-E equipments to minimize interference and noise.
SIGNAL RETURN -- A current-carrying path between a load and the signal source. It is the low side of the
closed loop energy transfer circuit between a source-load pair.
STRUCTURE -- Any fixed or transportable building, shelter, tower, or mast that is intended to house
electrical or electronic equipment or otherwise support or function as an integral element of an electronics
complex.
SUPPORTING STRUCTURES, ELECTRICAL -- Normally nonelectrified conductive structural elements near
to energized electrical conductors such that a reasonable possibility exists of accidental contact with the
energized conductor.
Examples are conduit and associated fittings, junction and switch boxes, cable trays,
electrical/electronic equipment racks, electrical wiring cabinets, and metallic cable sheaths.
TRANSDUCER -- A device which converts the energy of one transmission system into the energy of another
transmission system.
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THUNDERSTORM DAY -- A local calendar day on which thunder is heard.
UNDESIRED SIGNAL -- Any signal which tends to produce degradation in the operation of equipments or
systems.
WAVE IMPEDANCE -- The ratio of the electric field strength to the magnetic field strength at the point of
observation.
ZONE OF PROTECTION -- (also known as CONE OF PROTECTION) That space that is below and adjacent to
a lightning protection subsystem that is substantially immune to direct lightning discharges.
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MIL-HDBK-419A
APPENDIX B
SUPPLEMENTAL BIBLIOGRAPHY
PART I.
SUBJECT CROSS REFERENCE.
BONDING -- 13, 15, 28, 29, 30, 32, 39, 45, 54, 60, 61, 76, 91, 96, 113, 120, 129, 159, 161, 199, 203, 207, 208.
CATHODIC PROTECTION -- 10, 43, 51, 55, 94, 108, 136, 144, 168, 169, 176.
CORROSION -- 8, 53, 56, 63, 75, 124, 143, 144, 164, 168, 169, 214.
EARTH ELECTRODE SUBSYSTEMS -- 11, 24, 25, 31, 33, 44, 51, 52, 63, 64, 65, 66, 70, 71, 85, 86, 87, 89, 101,
107, 120, 131, 137, 138, 145, 148, 170, 187, 190, 196, 205, 212, 213.
EMP -- 37, 47, 57, 58, 67, 68, 73, 74, 114, 128, 139, 140, 141, 150, 156, 163, 186, 202.
EQUIPMENT SHIELDING -- 3, 6, 7, 9, 14, 35, 36, 40, 41, 46, 49, 78, 83, 84, 90, 97, 99, 103, 109, 112, 115, 116,
122, 127, 133, 134, 135, 146, 151, 152, 153, 158, 159, 172, 173, 174 175, 180, 184, 194, 198, 204, 211.
FACILITY SHIELDING -- 9, 15, 82, 110, 115, 158, 183, 184, 203.
LIGHTNING PROTECTION -- 2, 5, 12, 16, 20, 21, 22, 23, 24, 26, 31, 69, 79, 80, 102, 120, 130, 147, 149, 154,
166, 179, 182, 185, 197, 200, 201, 209.
PERSONNEL SAFETY -- 20, 42, 44, 50, 66, 77, 80, 81, 95, 104, 117, 118, 119, 120, 121, 125, 142, 171, 191,
193.
POWER SYSTEMS GROUNDING -- 1, 4, 17, 18, 19, 34, 38, 62, 72, 92, 93, 95, 98, 100, 105, 106, 111, 117, 120,
123, 126, 132, 155, 160, 162, 165, 167, 177, 178, 181, 188, 195, 210.
SIGNAL GROUNDING -- 27, 42, 48, 49, 59, 72, 88, 109, 120, 159, 189, 192, 198, 203, 204, 206.
PART II. LISTINGS.
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MIL-HDBK-419A
3.
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32. Boyer, O.A. and Korges, E., “Connector Performance by Types,” AIEE Trans, Vol 75, Pt III, October 1956,
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82. Good, T. M.,
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B-7
MIL-HDBK-419A
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APPENDIX C
TABLE OF CONTENTS FOR VOLUME II
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
Page
1.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1
1.2
EARTH ELECTRODE SUBSYSTEM . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
Determination of Site Parameters . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
1.2.1
1.2.1.1
Soil
1.2.1.2
Geological
Effects
1.2.1.3
Physical
Features
1.2.1.4
Local
1.2.2
Design
Resistivity
.
.
.
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.
.
Climate
.
Procedure
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1-5
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1-5
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1-5
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1-6
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.
1.2.2.1
Selection of Electrode Configuration . . . . . . . . . . . . . . . . . . . . . .
1-6
1.2.2.2
Calculation of Earth Resistance. . . . . . . . . . . . . . . . . . . . . . . . .
1-9
1.2.2.3
Alternate Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.3
Design
1.2.4
Installation
1.3
Guidelines
1.3.2
Integral
Protection
Air
Terminals
1.3.2.1.2
1.3.2.2
of
Size
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System
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1-22
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1-23
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1-23
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1-24
.
.
.
.
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.
.
1-24
1-33
Conductors
1.3.2.2.2
Down
Fasteners
.
.
Conductors
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Grounding Conductors . . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
1-24
.
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1-23
.
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.
.
Location
Materials
.
1-9
1-14
.
Roof
1.3.3
.
.
Protection
and
1.3.2.2.1
1.3.2.3
.
LIGHTNING PROTECTION FOR STRUCTURES . . . . . . . . . . . . . . . . . . . .
Principles
1.3.2.1.1
.
Practices
1.3.1
1.3.2.1
.
.
.
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.
1-33
.
.
1-37
.
Separately Mounted Protection Systems . . . . . . . . . . . . . . . . . . . . . .
Type
.
.
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.
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.
.
1-40
1.3.3.1
Mast
1.3.3.2
Overhead Ground Wire Type . . . . . . . . . . . . . . . . . . . . . . . . . .
1-41
1.3.3.3
Waveguide Installation and Grounding . . . . . . . . . . . . . . . . . . . . . .
1-42
1.3.3.4
Cable Installation and Grounding . . . . . . . . . . . . . . . . . . . . . . . .
1-49
1.3.3.5
Lightning-Generated Transient Surge Protection . . . . . . . . . . . . . . . . .
1-49
1.3.3.5.1
Transient Source and Equipment Damage . . . . . . . . . . . . . . . . . . . .
1-49
1.3.3.5.2
Minimizing Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-50
1.3.3.5.3
Susceptible Components . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-50
1.3.3.5.4
Frequency of Transient Occurrence . . . . . . . . . . . . . . . . . . . . . .
1-51
1.3.3.5.5
Transient Definition, AC Service Conductors . . . . . . . . . . . . . . . . . .
1-51
1.3.3.5.6
Methods for Transient Protection on AC Service Conductors . . . . . . . . . . .
1-56
1.3.3.5.7
Use of Ferrous Metal Conduit. . . . . . . . . . . . . . . . . . . . . . . . .
1-56
1.3.3.5.8
Use of Overhead Guard Wires. . . . . . . . . . . . . . . . . . . . . . . . .
1-56
C-1
.
1-39
1-40
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
Protection of Underground Cables . . . . . . . . . . . . . . . . . . . . .
1-57
1.3.3.5.10
Buried Guard Wire
1-57
1.3.3.5.11
Secondary AC Surge Arrester. . . . . . . . . . . . . . . . . . . . . . . . .
1-59
1.3.3.5.12
Surge Arrester Installation . . . . . . . . . . . . . . . . . . . . . . . .
1-59
1.3.3.5.13
Operating Characteristics of Surge Arresters . . . . . . . . . . . . . . . . . .
1-60
1.3.3.5.14
Desirable Operating Characteristics for Transient Suppressors . . . . . . . . . .
1-67
1.3.3.5.15
Characteristics of Different Types of Surge Arresters . . . . . . . . . . . . . .
1-67
1.3.3.5.16
Transient Protection for Externally Exposed Equipment Lines . . . . . . . . . . .
1-73
1.3.3.5.17
Frequency of Transient Occurrence . . . . . . . . . . . . . . . . . . . . . .
1-73
1.3.3.5.18
Amplitudes and Waveforms of Occurring Transients . . . . . . . . . . . . . . .
1-73
1.3.3.5.19
Equipment
.
1-74
1.3.3.5.20
Protection Methods Against Transients. . . . . . . . . . . . . . . . . . . . .
1-74
1.3.3.5.21
Enclosing Cable Runs in Ferrous Metal Conduit . . . . . . . . . . . . . . . . .
1-74
1.3.3.5.22
Transient Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-75
1.3.3.5.9
Withstand
Levels
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1.3.3.5.23
Types of Available Transient Suppression. . . . . . . . . . . . . . . . . . . .
1-77
1.3.3.5.24
Operating Characteristics of Transient Suppressors . . . . . . . . . . . . . . .
1-77
1.3.3.5.25
Transient Suppressor Packaging Design. . . . . . . . . . . . . . . . . . . . .
1-78
1.3.3.5.26
Coaxial Cable Shield Connection Through an Entrance Plate . . . . . . . . . . .
1-78
1.3.3.5.27
Grounding
1-78
1.3.3.5.28
Transient Suppression for RF Coaxial Lines . . . . . . . . . . . . . . . . . . .
1-79
of
Unused
Wires
.
.
.
.
.
.
.
.
.
.
.
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.
.
.
.
.
.
Equipment-Level Transient Suppression . . . . . . . . . . . . . . . . . . . .
1-79
1.3.3.6
Lightning Generated Transient Protection Evaluation . . . . . . . . . . . . . . .
1-79
1.3.3.7
Transient
.
1-80
1-80
1.3.3.5.29
Protection
1.3.3.7.1
Protection
1.3.3.7.2
Transient
.
.
.
.
1-83
1.3.3.7.5
Minimizing Transient Damage . . . . . . . . . . . . . . . . . . . .
1-83
1.3.3.7.6
AC
1-84
1.3.3.7.7
Power Supply Transient Suppression . . . . . . . . . . . . . . . . . . . . .
1-89
1.3.3.7.8
Landline
1-89
1.3.3.8
Corrosion
1.3.3.9
Joints
1.4
.
Transient
.
.
.
.
.
Protection
.
.
.
Suppression
Control
.
Physical
Input
.
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.
Determination of Need for Transient Protection. . . . . . . . . . . . . . . . .
.
.
.
Determination of Equipment Damage (Withstand) Levels . . . . . . . . . . . . .
.
.
.
1.3.3.7.4
.
.
.
1.3.3.7.3
.
.
.
1-81
.
.
.
1-82
.
.
.
.
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.
.
.
.
.
Power
.
.
.
.
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.
.
.
.
Definition
.
.
.
1.3.3.10
Requirement
.
.
.
.
1-98
.
.
.
.
1-99
.
1-99
.
FAULT PROTECTIVE SUBSYSTEM . . . . . . . . . . . . . . . . . . . . . . . . .
1-99
1.4.1
Purpose
.
1-99
1.4.2
Equipment Fault Protection Subsystem Composition . . . . . . . . . . . . . . . . .
1-100
.
.
.
.
.
.
.
.
.
.
C-2
.
.
.
.
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.
.
.
.
.
.
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.4.3
Configuration of the Equipment Fault Protection Subsystem . . . . . . . . . . . . .
1-100
1.4.4
Pipes and Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-102
1.4.5
Electrical Supporting Structures . . . . . . . . . . . . . . . . . . . . . . . . .
1-102
1.4.5.1
Metal Conduit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
1.4.5.2
Cable Trays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
1.4.5.3
Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
1.4.5.4
Cable Armor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
Rotating Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
1.4.6
Power Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-104
1.4.7
Standby AC Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-104
1.4.8
Equipment Fault Protection Subsystems for Transportable Equipment . . . . . . . . .
1-104
1.4.9
MIL-STD-188-124A and NEC Compliance Evaluation . . . . . . . . . . . . . . . . .
1-105
1.4.5.5
1.4.9.1
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-105
1.4.9.2
MIL-STD-188-124A and NEC Compliance Inspection . . . . . . . . . . . . . . . .
1-105
Correction of Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-111
SIGNAL REFERENCE SUBSYSTEM FOR NEW FACILITIES . . . . . . . . . . . . . . .
1-113
Higher Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-113
Multipoint Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-118
1.4.9.3
1.5
1.5.1
1.5.1.1
1.5.1.1.1
Types of Equipotential Planes . . . . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.1
Copper Grid Embedded in Concrete . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.2
Equipotential Plane Under Floor Tile or Carpet . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.3
Overhead Equipotential Plane. . . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.4
Raised (Computer) Flooring.. . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.4.1
Bolted-Grid (Stringer) or Rigid Grid System. . . . . . . . . . . . . . . . .
1-125
1.5.1.1.1.4.2
Drop-In or Removable Grid System . . . . . . . . . . . . . . . . . . . .
1-131
1.5.1.1.1.4.3
1.5.1.1.1.5
1.5.1.1.1.6
1.5.2
1.6
Free-Standing, Pedestal-Only or Stringerless System . . . . . . . . . . . .
1-131
Ground Risers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-131
Equipment Cabinet Grounding . . . . . . . . . . . . . . . . . . . . . . .
1-131
Lower Frequency Signal Reference Network . . . . . . . . . . . . . . . . . . . .
1-131
GROUNDING PHILOSOPHY FOR EQUIPMENTS PROCESSING NATIONAL SECURITY
RELATED INFORMATION (RED/BLACK EQUIPMENTS) . . . . . . . . . . . . . . . .
1-134
BONDING PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-140
1.7.1
Application Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-140
1.7.2
Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-142
1.7.3
Bond Protection Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-143
Jumper Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-147
Typical Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-148
1.7
1.7.3.1
1.7.4
C-3
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
Cable Trays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
Tubing and Conduit . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-148
Other Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SHIELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-149
Establishing Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selection of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-154
1.7.4.1
1.7.4.2
1.7.4.3
1.8
1.8.1
1.8.2
1.8.3
1.8.4
1.9
1.9.1
1.9.2
1.10
Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMMON-MODE NOISE AND INSTRUMENTATION . . . . . . . . . . . . . . . . . .
EMP Bonding Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-173
MILITARY MOBILE FACILITIES . . . . . . . . . . . . . . . . . . . . . . . . . .
General Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.1
1.11.1.1.1.1
1.11.1.1.1.2
1.11.1.1.1.3
1.11.1.1.1.4
1.11.1.1.1.5
1.11.1.1.2
1.11.1.1.2.1
1.11.1.1.2.2
1.11.1.1.2.3
1.11.1.1.3
1.11.1.1.3.1
1.11.1.1.3.2
1.11.1.1.4
1.11.1.2
1.11.1.3
1.11.1.3.1
1-162
1-164
1-172
1.10.4
1.11.1.1.5
1-160
Earth Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EMP Shield Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.3
1.11.l.1
1-159
1-171
1.10.2
1.11.1
1-154
Design Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instrumentation Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .
EMP PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.1
1.11
1-149
1-172
1-172
1-173
1-176
1-177
1-177
1-177
Earth Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General Earth Electrode Subsystem Requirements . . . . . . . . . . . . . .
Earth Electrode Subsystem Types . . . . . . . . . . . . . . . . . . . . . .
1-177
Soil Resistance . . . . . . . . , . . . . . . . . . . . . . . . . . . . . .
Ground Rod Resistance . . . . . . . . . . . . . . . . . . . . . . . . . .
Ground Resistance Shells. . . . . . . . . . . . . . . . . . . . . . . . . .
1-178
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
1-178
1-177
1-177
1-178
1-178
1-178
Single-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
DC Power System (2-Wire) . . . . . . . . . . . . . . . . . . . . . . . . .
1-179
Lightning/EMP Protection Subsystem . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . .
Air Terminals . . . . . . . . . . .
1-179
Terminal Protection Devices . . . . . . . . . . . . . . . . . . . . . . . .
Signal Reference Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .
1-179
1-179
1-182
1-182
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .
Measuring Ground Resistance in Tactical Environments . . . . . . . . . . . . . .
Reducing Ground Resistance in Tactical Environments . . . . . . . . . . . . . . .
1-182
. . . . . . .
1-182
Existing Facilities . . . . . . . . . . . . . . . . . . . . .
C-4
1-182
1-182
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.11.1.3.2
Multiple Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.1.3.3
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.1.3.4
Chemical Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
Detailed Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
1-185
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.2.1.1
Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.1.2
Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2
1.11.2.1
Stand-Alone Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
Low Resistance Grounds . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2.1.1
Existing Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2.1.2
Earth Electrode Subsystem, Single Ground Rod . . . . . . . . . . . . . . .
1-187
1.11.2.2.2.1.3
Earth Electrode Subsystem, Multiple Ground Rods . . . . . . . . . . . . . .
1-187
Stand-Alone Shelter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.2.2
1.11.2 .2.2.1
1.11.2.3
1.11.2.3.1
Interconnection of Subsystems . . . . . . . . . . . . . . . . . . . . . . . .
1-187
Collocated Military Mobile Equipments . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.2.4.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.2.4.2
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.3.2
1.11.2.4
Collocated Shelters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.1
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.2
Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.3
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.4
Lightning/EMP Protection . . . . . . . . . . . . . . . . . . . . . . . . .
1-189
1.11.2.5.2.5
1.11.2.5
Collocated Shelters Greater than 8 Meters Apart . . . . . . . . . . . . . . .
1-189
1.11.2.5.2.5.1
Ground Resistance Difference of Less than 150 Ohms . . . . . . . . . . . .
1-189
1.11.2.5.2.5.2
Ground Resistance Difference of Greater than 150 Ohms . . . . . . . . . . .
1-189
1.11.2.5.2.5.3
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.1
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.2
Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.3
Safety/Equipment Ground (Greenwire) . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.4
Lightning/EMP Protection . . . . . . . . . . . . . . . . . . . . . . .
1-190
Fixed Prefabricated Shelters. . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.6
1.11.2.6.1
C-5
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.11.2.6.2
1.12
1.12.1
1.12.2
1.12.3
1.13
1.13.1
1.13.2
1.13.3
1.13.4
1.13.5
Electrical Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-191
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INSPECTION AND TEST PROCEDURES FOR A NEW FACILITY . . . . . . . . . . . .
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-191
Lightning Protection Network . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Reference and Fault Protection Subsystems. . . . . . . . . . . . . . . . . .
1-191
1-193
1-193
1-194
1-194
Bonds and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facility Checkout Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-195
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
Part II Lightning Protection Network . . . . . . . . . . . . . . . . . . . . . .
Part III Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . .
Part IV Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-196
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-200
Part I
1.14
1-191
1-196
1-197
1-198
1-199
CHAPTER 2 - EXISTING FACILITIES
2.1
2.2
2.2.1
2.2.2
2.2.2.1
2.2.2.2
2.2.2.2.1
2.2.2.2.2
2.2.2.2.3
2.2.2.2.4
2.2.2.2.5
2.2.2.2.6
2.2.2.2.7
2.2.2.3
2.2.2.3.1
2.2.2.3.2
2.2.2.3.3
2.2.2.4
. .
2-1
. .
2-1
. .
2-4
. .
2-4
Survey Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inspection Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
Bonds and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-4
.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
UPGRADING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facility Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION
.
.
2-5
2-5
2-10
. .
2-13
. .
2-16
. .
2-17
. .
2-17
Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bond Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-19
Ground System Noise Current. . . . . . . . . . . . . . . . . . . . . . . . .
Differential Noise Voltage . . . . . . . . . . . . . . . . . . . . . . . . . .
Survey Form. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-20
. . . . . . . . . . . . . . . . . . . . . .
Safety Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Grounding Practices . . . . . . . . . . . . . . . . . . . . . . . .
Ground System Noise Survey . . . . . . . . . . . . . . . . . . . . . . .
Lightning Protection Network
C-6
2-19
2-19
2-22
2-24
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 2 - EXISTING FACILITIES
Paragraph
Page
2.2.3
Guidelines
2.2.4
Expansion of Existing Facilities . . . . . . . . . . . . . . . . . . . . . . . . . .
for
Upgrading
2-35
2.2.5
Expansion of Existing Facilities for Higher-Frequency Grounds . . . . . . . . . . . .
2-35
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2-33
2-36
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Maintenance Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2.3.2
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2-36
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2-36
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Records
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Schedules
and
.
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2.3.1
2.3
MAINTENANCE
.
.
.
2.3.2.1
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . .
.
2-39
2.3.2.2
Lightning Protection Subsystem . . . . . . . . . . . . . . . . , . . . , . . . .
2-41
2.3.2.3
Bonding
.
2-42
2.3.2.4
Fault Protection Subsystem (Safety Ground) . . . . . . . . . . . . . . . . . . .
2-43
2.3.2.5
Signal Reference Subsystem (Signal Grounding) . . . . . . . . . . . . . . . . . .
2-44
2.3.2.6
Shielding
2-45
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2.3.3
Facility Maintenance Report . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-46
2.3.4
Performance Evaluation Program . . . . . . . . . . . . . . . . . . . . . . . . .
2-54
2.4
GROUNDING CONSIDERATIONS FOR CLASSIFIED INFORMATION PROCESSORS
(RED/BLACK EQUIPMENTS) INSTALLED PRIOR TO THIS HANDBOOK . . . . . . . . .
2.4.1
Introduction
.
.
.
2.4.2
Existing
Facilities
.
.
.
2.4.3
Protection
Grounds
2.4.4
Signal
Reference
2.4.5
Signal
Filter
2.4.6
Grounding
.
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Subsystem
Ground
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Precautions
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2-59
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2-59
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2-59
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2-59
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2-60
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2-61
.
2-61
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
3.1
INTRODUCTION
3.2
GROUNDING PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1
3.2.1.1
Signal
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Grounds
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3-1
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3-1
3-1
Lower Frequency Equipment.. . . . . . . . . . . . . . . . . . . . . . . . .
3-2
3.2.1.1.1
Signal Ground Network Configuration . . . . . . . . . . . . . . . . . . . . .
3-2
3.2.1.1.2
Signal Ground Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-2
3.2.1.1.3
Color
3-3
3.2.1.1.4
Cabinet Bus Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-3
3.2.1.1.5
Isolation
3-4
3.2.1.1.6
Signal
.
3-4
3.2.1.1.7
Signal Grounding . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
Higher Frequency Equipment . . . . . . . . . . . . . . . . . . . . . . . . . .
3-12
3.2.1.2
Code
.
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Interfacing
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C-7
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MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
Page
Paragraph
. . . . . . . . . . . . . . . . . . . .
3-13
Cable Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Equipments Containing Both Lower and Higher Frequency Circuits. . . . . . . . . .
3-13
3.2.2
Fault Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-14
3.2.3
Cabinet Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-16
3.3
BONDING PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-18
3.4
SHIELDING GUIDELINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parts Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-25
3.4.1
3.4.2
Layout and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-25
3.4.3
3-27
3.4.3.1
Equipment Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3.2
Penetrations and Apertures . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-28
3.5.1
COMMON-MODE NOISE CONTROL AND INSTRUMENTATION GROUNDING . . . . . . .
Common-Mode Noise Control. . . . . . . . . . . . . . . . . . . . . . . . . . .
3-34
3.5.2
Instrumentation Grounding. . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-34
Analog Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounded Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-35
Ungrounded Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-38
3-38
3.5.2.2
Digital Data Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-40
3.5.2.3
Recording Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-40
3.5.2.3.1
Magnetic Tape Recorders . . . . . . . . . . . . . . . . . . . . . . . . . .
3-40
3.5.2.3.2
Strip Chart Recorders . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-Y Plotters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-42
EMP CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EQUIPMENT INSPECTION AND TEST PROCEDURES . . . . . . . . . . . . . . . . .
3-42
3-43
3.7.1
Lower Frequency Equipments. . . . . . . . . . . . . . . . . . . . . . . . . . .
3-43
3.7.2
Higher Frequency Equipments . . . . . . . . . . . . . . . . . . . . . . . . . .
Hybrid Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-47
3.7.3
3.7.4
Installed Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-47
3.7.5
. . . . . . . . . . . .
. . . . . . . . . . . .
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. . . . . . . . . . . .
3-47
3.2.1.2.1
3.2.1.2.2
3.2.1.3
3.5
3.5.2.1
3.5.2.1.1
3.5.2.1.2
3.5.2.1.3
3.5.2.3.3
3.6
3.7
3.7.6
Signal Interfaces . . . . . . . . .
Fault Protection Subsystem . . . . . . . . . . . . . . .
Bonding . . . . . . . . . . . . . . . . . . . . . . . .
3-14
3-25
3-27
3-34
3-35
3-42
3-47
3-49
3-49
3.7.7
Shielding . . . . . . . . . . . . . . . . . . . . . . . .
3.7.8
Instrumentation System . . . . . . . . . . . . . . . . .
3.7.9
EMP Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-50
3.7.10
Other Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-50
3.7.11
Inspection Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-50
3.8
C-8
3-50
3-54
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 4 - NOTES
Paragraph
4.1
Page
SUBJECT TERM (KEY WORD) LISTING . . . . . . . . . . . . . . . . . . . . . . .
C-9/C-l0
4-1
MIL-HDBK-419A
APPENDIX D
NOTE: This appendix is a subjective index of material contained in both volumes of MIL-HDBK-419A. The
Roman numeral preceding the page number identifies the volume of interest.
INDEX A
Absorption loss, shield, I: 8-5, 8-8, 8-9, 8-27; II: 1-160, 1-162
equations for, I: 8-6
nomograph for, II: 1-161
AC resistance, I: 5-5
Air terminals, I: 3-13; II: 1-24, 1-27 to 1-33, 1-41, 1-179
height, II: 1-27, 1-28
location, II: 1-24 to 1-33
materials, II: 1-24
also see cone of protection
Amplifiers, grounding of, I: 6-19; II: 3-35 to 3-40
Analog devices, grounding of, II: 3-35
Antenna effects, I: 3-18, 6-14, 6-15
of groundwires, I: 6-16
and EMP pickup, I: 10-9
and lightning induced surges, I: 3-17
Apertures, shield, I: 8-32, 8-41, 10-11, 10-12, 10-15; II: 3-28
equations for, I: 8-34
control of leakage through, I: 8-42; II: 3-28
Arctic grounding, I: 2-66
electrode resistance, I: 2-71
improve grounding, I: 2-70
installation and measurements, I: 2-71
soil resistivity, I: 2-66
Armored cable, I: 8-60; II: 1-103, 1-171, 1-173
grounding of, II: 1-103
relative shielding effectiveness of, I: 8-60; II: 1-171
Arrester, surge, I: 3-25, 10-17 to 10-19; II: 1-59 to 1-70
Attractive area, I: 3-10
definition of, I: 3-10
how to determine, I: 3-11
also see effective height and cone of protection
D-1
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX B
Balancing, use of, I: 6-23; II: 3-4, 3-7
amplifiers, I: 6-21
signal lines, I: 6-24; II: 3-4, 3-7
also see noise minimization
Body resistance, human, I: 9-1
Bolting, I: 7-14; II: 1-140, 2-10
also see bond, electrical
Bond (and bonding), electrical, I: 7-1 to 7-36; II: 1-78, 1-109, 1-140, 1-195, 1-199, 3-18
area, I: 7-8
assembly, I: 7-10 to 7-17
completion of, I: 7-29; II: 1-140
connectors of, II: 1-140, 3-20, 3-27
contaminants, I: 7-7
definition, I: 7-1
direct, I: 7-4
earth electrode system, in, II: 1-22, 2-5
equipment, II: 3-18
guidelines for, I: 7-36; II: 1-140, 3-18
indirect, I: 7-16
also see bond strap
inspection of, II: 1-195, 2-5, 2-13, 3-49
lightning protection system, in, II: 1-79, 2-13
protection of, I: 7-29; II: 1-143 to 1-146
purposes of, I: 7-1
resistance, I: 7-3, 7-6; II: 1-194, 1-195, 2-10, 2-17, 2-19
shields in, I: 8-33, 8-41; II: 1-162, 3-25
structured, II: 1-140, 2-10
techniques, I: 7-10; II: 1-140
comparison of, I: 7-16
testing of, II: 2-19
torque, I: 7-7; II: 1-141
table of, II: 1-141
washer, use of, I: 7-15
workmanship, I: 7-34
D-2
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX B
Bond Protection Code, II: 1-143 to 1-147
Bond strap (or jumper), I: 7-21; II: 1-147 to 1-153, 2-10, 3-18
frequency, effects of, I: 7-19
guidelines for use of, II: l-148 to l-153, 2-10, 3-18
ratio, recommended, I: 7-21
Braided straps, I: 7-21
Brazing, I: 7-11; II: 1-140, 1-142
British Standard Code of Practice, I: 3-13
Buried metals, see incidental electrodes
Bus bar, use of, II: 3-3
D-3
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX C
Cabinets, grounding of, II: 1-131, 3-16
Cable routing, II: 1-171
interference control, for, II: 1-171
Cable shields, I: 8-59 to 8-63; II: 1-104, 3-2, 3-4, 3-35, 3-38, 3-49
bonding of, II: 3-8
braid, I: 8-59
conduit as, I: 8-60
grounding of, I: 8-61; II: 1-104, 3-2, 3-35, 3-38, 3-49
installation practices, I: 8-61
Cable trays, II: 1-103, 1-148
Calcium chloride, I: 2-60
also see chemical enhancement
Capacitance coupling, I: 3-21, 6-11
Capacitance, stray, I: 7-23
Cathodic protection, I: 2-63
Chemical enhancement, I: 2-60; II: 1-14, 1-185
Classified Information Processors (RED/BLACK Equipments), II: 1-134, 2-58
Climate, effects of, I: 2-7; II: 1-5, 1-143, 1-144
on bonds, II: 1-143, 1-144
on earth electrode subsystem design, II: 1-5
on soil resistivity, I: 2-5
Common-mode noise, I: 6-17 to 6-23
Common-mode rejection ratio, I: 6-21
Component damage, I: 10-15 to 10-17
D-4
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX C
Compton electrons, I: 10-l to 10-3
Concrete enclosed electrodes, I: 2-62
Conductive coupling, I: 6-5, 6-19
Conductor length criteria, ground, II: 1-57
Conductor parameters, I: 3-17, 5-1
ac resistance, I: 5-5
dc resistance, I: 5-1
proximity effects, I: 5-10
reactance, I: 5-7
also see inductance and skin effect
Conductor routing, see cable routing
Conductor selection, grounding, I: 5-l to 5-19; II: 1-107
I-beams, I: 5-15
rectangular bars, I: 5-13
stranded cables, I: 5-13
tubular (pipes), I: 5-13
Conduit, I: 8-60; II: 1-56, 1-74, 1-75, 1-103, 1-149, 1-159
as a shield, I: 8-60; II: 1-159
grounding of, II: 1-56, 1-75, 1-103
Cone of protection, I: 3-11; II: 1-27 to l-33
definition of, I: 3-13
example of, I: 3-13; II: 1-28
means of determining, II: 1-28, 1-30 to 1-33
Connectors, I: 8-59; II: 1-173, 3-13, 3-20, 3-49
bonding of, II: 1-173, 3-13, 3-20, 3-49
shields, I: 8-59
Contaminants, bond, I: 7-7; II: 1-142
removal of, I: 7-25; II: 1-142
D-5
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX C
Convenience outlets, II: 1-104, 3-14, 3-49
grounding of, II: 1-104, 3-14
inspection of, II: 3-49
Copper sulfate, I: 2-60
also see chemical enhancement
Corrosion, I: 7-29 to 7-35; II: 1-98, 1-99, 1-145, 1-146
in bonds, I: 7-30
protection against, I: 7-34; II: 1-98, 1-99, 1-145, 1-146
theory, I: 7-30
also see dissimilar metals
Counterpoise, II: 1-15, 1-19
Coupling, I: 6-1
capacitive, I: 6-11
conductive, I: 6-5
far-field, I: 6-14 to 6-17
free-space, I: 6-6
inductive, I: 6-8
near-field, I: 6-6
radiated, I: 6-14
D-6
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX D
Remountable enclosures, I: 8-66
Digital data systems, grounding of, II: 3-40
Discrepancy report, major, II: 2-38
Dissimilar metals, I: 7-31; II: 1-143, 1-145, 1-146
Down conductor, lightning, I: 3-17; II: 1-34, 1-37 to 1-39
location, II: 1-37, 1-39
routing, I: 3-17; II: 1-37, 1-39
size, II: 1-34
Drawings, requirements for, II: 2-4, 3-43
D-7
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX E
Earth electrode, I: 1-2, 2-1, 2-15; II: l-2 to l-22, 1-193, 1-196, 2-5, 2-39, 2-40
current handling capacity, I: 2-57; II: 1-6
design, II: 1-2, l-6 to 1-14
effective size of, I: 2-58
encasement, I: 2-62
enhancement, I: 2-59
functions of, I: 2-1; II: 1-6
heating, I: 2-57
impulse impedance, I: 2-32; II: 1-6
inspection of, II: 1-193, 2-5
installation practices, II: 1-22, 1-193
measurement, I: 2-35
resistance, I: 2-17; II: 1-9, 1-193
subsystem, I: 1-2; II: 1-193, 1-196
types of, I: 2-15
Earth resistance testing, I: 2-23, 2-35, 2-46; II: 1-9, 1-193, 2-5
fall-of-potential method, I: 2-35
large electrode system for, I: 2-44
three-point method of, I: 2-46
Effective height, I: 3-11
also see cone of protection
Electric dipole, I: 6-15
Electric shock, I: 9-1
Electrical equipment, grounding of, II: 1-104, 1-133
Electrical noise in communication systems, I: 1-4
Electrical noise reduction, I: 1-2
Electrical supporting structures, grounding of, II: 102 to 104
Electrochemical series, I: 7-31
D-8
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX E
Electromagnetic interference (EMI), I: 1-4, 8-74, 8-77; II: 1-113
Electromagnetic survey, I: 8-76; II: 1-154, 2-17
EMP (Electromagnetic Pulse), I: 10-l to 10-25; II: l-172 to l-177, 1-187, 1-190, 3-50
comparison with lightning, I: 10-5
current in long lines, I: 10-6 to 10-9
description, I: 10-1 to 10-5
equipment susceptibility to, I: 10-22
high-altitude EMP (HEMP), I: 10-5 to 10-25
protection, I: 10-13 to 10-25; II: 1-172 to 1-177, 1-187, 1-190, 3-50
Enclosures, electrical, II: 1-103, 3-27 to 3-33
Enclosures, shielded, I: 8-63
Epoxy, conductive, I: 7-16
Equipment grounding, II: 3-1 to 3-19
cabinet, of, II: 3-16, 3-46
fault protection, for, II: 3-14
inspection of, II: 3-43
signal network, II: 3-2
Equipment protection, I: 1-2
Equipment susceptibility, I: 10-15; II: 1-50
Equipotential plane, I: 5-26, 5-27; II: 1-120 to 1-133
Existing facilities, II: 2-1 to 2-54
expansion or modification of, II: 2-35
survey of, II: 2-4 to 2-32
upgrading, guidelines for, II: 2-33
D-9
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX F
Facility ground system, II: 1-113, l-l18 to 1-123, 1-199
combined elements, II: 1-121
description of, II: 1-113
structural steel as used in, II: 1-118, 1-120
Facility maintenance report, II: 2-46
Facility survey, II: 2-4, 2-24
Fall-of-potential method, I: 2-35 to 2-46; II: 1-182, 2-6
theory of, I: 2-35
Far-field coupling, I: 6-14 to 6-17
Fasteners, II: 1-39, 1-40, 1-147
Fault protection, I: 1-3, 2-2, 4-1
Faults, electrical, I: 2-2, 4-1; II: 1-6, 3-14
cause of, I: 4-1
protective measures against, I: 4-1; II: 1-6, 3-14
Feeder ground plate, II: 3-47
Field, high impedance, I: 8-15
low impedance, I: 8-10
plane wave, I: 8-13
Filters, I: 6-25, 10-18; II:3-26
Forms, II: 1-195 to 1-200, 2-24 to 2-32, 2-38, 2-46 to 2-53, 3-50 to 3-54
equipment inspection, II: 3-50 to 3-54
facility checkout, II: 1-195 to 1-200
facility maintenance report, II: 2-46 to 2-53
facility survey, II: 2-24 to 2-32
major discrepancy report, II: 2-38
Four-probe method, I: 2-15; II: 1-2 to 1-5
also see resistivity, soil
Frost line, II: 1-6
D-10
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX G
Galvanic series, I: 7-31
Gaskets, RF, I: 8-45; II: 1-162, 3-20, 3-22, 3-27, 3-49
Geological factors, II: 1-5
Glass, conductive, I: 8-52; II: 1-164
Ground fault interrupter (GFI), I: 4-2
Ground, floating, I: 5-15
Ground grid (or mesh), I: 2-15, 2-27, 2-33, 2-55, 5-27; II: 1-8
Ground, multipoint, I: 5-24 to 5-28; II: 1-120
Ground network configuration, I: 5-18
Ground network isolation, I: 5-28; II: 3-4
Ground rods, I: 2-15, 2-23, 2-27, 2-33, 2-48; II: 1-8 to 1-22, 1-178
arrays of, I: 2-27; II: 1-12
parallel, I: 2-23; II: 1-12
placement of, II: 1-14 to 1-19
resistance, equations for, I: 2-17; II: 1-9, 1-178
resistance, nomograph of, II: 1-11
selection of, II: 1-9 to 1-15
sizes of, I: 2-15
spacing of, I: 2-15
step voltage of, I: 2-48
Ground, single-point, I: 5-19 to 5-24; II: 3-43
Ground system, I: 1-2
Grounding, electrical power system, I: 1-3; II: 1-179, 2-16
single-phase, I: 4-4; II: 1-179
three-phase, I: 4-4; II: 1-178
D-11
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX G
Grounding safety, I: 1-5
Grounding, signal, I: 5-1 to 5-32; II: 1-113 to 1-133, 1-185 to 1-188, 2-17, 2-31, 3-1, 3-43
equipment in, II: 3-1
facilities, in, II: 1-113
network configurations, I: 5-18 to 5-31; II: 1-113, 1-186, 1-187, 2-17, 2-31
purposes of, I: 5-1
Guards (down conductor), II: 1-41, 2-13
Guidelines for
bonding, I: 7-36; II: 1-148 to 1-151, 1-173, 1-188, 1-195
earth electrode subsystem design, I: 1-14 to 1-22
earth electrode subsystem installation, II: 1-22
EMP protection, I: 10-13 to 10-25; II: 1-173, 1-188, 3-42
equipment inspections, II: 3-43
facility inspections, II: 1-195
facility upgrading, II: 2-1, 2-33 to 2-37
lightning protection, II: 1-23 to 1-46
personnel safety, I: 9-2
shielding, I: 8-54; II: 1-159
INDEX H
Hemispherical electrodes, I: 2-8 to 2-16
HEMP (High-Altitude EMP), I: 10-5 to 10-25
protection against, I: 10-13 to 1-25
Higher frequency grounding, I: 5-30, 5-31; II: 1-113 to 1-132, 1-194, 3-12, 3-47
equipment, in, II: 3-12, 3-47
facilities, in, I: 5-31; II: 1-113
network configurations, I: 5-30
Honeycomb, see waveguide-below-cutoff
Horizontal earth electrodes, I: 2-15, 2-23, 2-24; II: 1-8
Hybrid equipments, II: 3-47
D-12
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX I
Incidental electrodes, I: 2-53, 2-55; II: 1-8, 1-15
Inductance, I: 5-7, 7-17, 7-19 to 7-25
Inductive coupling, I: 6-8 to 6-10
Inspection procedures, II: l-193 to l-200, 2-l, 2-39 to 2-53, 3-43
equipment, II: 3-43
existing facilities, II: 2-1
maintenance, II: 2-39 to 2-53
new facilities, II: 1-193 to 1-200
Instrumentation, grounding of, II: 1-172, 3-34, 3-49
Instrumentation, test, II: 2-19 to 2-23
Interfacing, signal, II: 3-4,3-13
Interference coupling, I: 6-1
Interference reduction, see electromagnetic interference
Isolation, ground network, I: 5-28; II: 3-4
Isokeraunic, I: 3-4 to 3-11
INDEX J
Jumper, see bond strap
D-13
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX L
Labels, ground network, II: 3-3, 3-47
Laser hazards, I: 9-5
Layered earth, I: 2-32 to 2-36
Let-go current, I: 9-2
Lightning, I: 1-2, 2-1, 3-1 to 3-27; II: 1-23 to 1-43, 1-49, 1-197, 2-13, 2-41
cloud to cloud, I: 3-1
cloud to ground, I: 3-1, 3-3
cone protection, I: 3-11
description of, I: 3-1, 3-13 to 3-15
effects of, I: 3-13 to 3-25
flash parameters, I: 3-13
net work inspection procedures, II: 1-197, 2-13, 2-41
personnel hazards, I: 2-5, 2-47, 3-25
protective measures, I: 3-15, 3-25; II: 1-23 to 1-43, 1-49
strike prediction, I: 3-4 to 3-11
triggered, I: 3-4
Lightning discharge, I: 2-1
Lightning protection code, I: 3-13, 3-27
Lightning protection subsystem, I: 1-2
Lightning rods, see air terminals
Lower frequency grounding, I: 5-29; II: 3-2
equipment in, II: 3-2
facilities in, I: 5-29
network configuration, I: 5-29
D-14
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX M
Magnesium sulfate, I: 2-60
also see chemical enhancement
Maintenance, II: 2-36 to 2-58
procedures, II: 2-36, 2-39 to 2-47
records, II: 2-36
report form, II: 2-46 to 2-53
schedules, II: 2-36
Master Labeled Protection System, I: 3-27, 7-14; II: 2-13
Masts (lightning) protective, II: 1-23, 1-40 to 1-43
Metal framework, earth electrode, I: 2-16
MIL-C-5541, II: 1-145, 1-146
MIL-E-45782B, I: 8-63
MIL-STD-285, I: 8-73
MIL-STD-462, I: 8-73
MIL-STD-1377, I: 8-73
MIL-STD-10727, II: 1-145, 1-146
Mobile facilities, grounding of, II: 1-177 to 1-190
D-15
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX N
National Electric Code (NEC), I: 2-2, 2-5, 2-75, 3-21; II: 1-103, 1-104, 1-105, 2-10, 2-13, 3-14, 3-47
Near-field coupling, I: 6-6
Noise, I: 1-2, 2-6, 6-3, 6-7, 6-17 to 6-25
circuit, I: 6-3, 6-7
common-mode, I: 6-17 to 6-23
minimization, I: 6-23 to 6-25
also see electromagnetic interference
Noise reduction, I: 2-2
Noise survey, II: 2-17
Nomograph
bolts, torque on, I: 7-15
ground rod resistance, of, II: 1-11
shield absorption loss, of, II: 1-161
shield electric field reflection loss, of, II: 1-166
shield magnetic field reflection loss, of, II: 1-165
shield plane wave reflection loss, of, II: 1-167
skin effect, for, I: 5-8
Nuclear EMP effects, I: 10-1 to 10-25
INDEX O
Oppositely induced fields, I: 8-2
Overhead ground wire, II: 1-41 to 1-43
D-16
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX P
Perception current, I: 9-1
Personnel protection, I: 2-1, 2-5, 3-27, 7-1
Personnel safety, I: 1-2
Pilot streamer, I: 3-3
Pipes, utility, grounding of, I: 2-15; II: 1-102
Plates electrodes, I: 2-15, 2-23; II: 1-8
Protection, equipment, I: 1-2
Protective coatings, I: 7-34; II: 1-140, 1-145, 1-146
bonds, for, I: 7-30; II: 1-145, 1-146
bond washers, for, II: 1-140
Proximity effect, I: 5-10
D-17
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX R
Radio frequency (RF) radiation hazards, I: 9-5
Reactance, I: 5-7
Reaction current, I: 9-2
Recording devices, grounding of, II: 3-40, 3-42
Rectangular conductor, I: 5-13
Reflection loss of electromagnetic shield, I: 8-6; II: 1-161, 1-165 to 1-168
electric field, for, I: 8-13; II: 1-166
equations for, I: 8-6
magnetic field, for, I: 8-11; II: 1-165
plane wave, for, I: 8-15; II: 1-161, 1-167
theory of, I: 8-1
Reinforcing steel as shield, properties of, I: 8-56, II: 1-154, 1-156
Re-reflection correction factor, I: 8-19
Resistance requirements, I: 2-5
Resistive coupling, see conductive coupling
Resistivity mapping, soil, II: 1-4
Resistivity, soil, I: 2-5; II: 1-2 to 1-5,
measurement of, I: 2-8; II: 1-2 to 1-5
ranges, I: 2-7
temperature, as a function of, I: 2-8
RF radiation hazards, I: 9-5
Rivets (as bonds), I: 7-15
Roof conductor, lightning, I: 3-26; II: 1-24, 1-33 to 1-37
location, II: 1-24, 1-33 to 1-37
routing, II: 1-33 to 1-37
size, I: 1-24
D-18
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX S
Sacrificial anodes, I: 2-63
also see cathodic protection
Safety grounding, I: 1-2, 1-5, 4-1; II: 2-13
Salting methods (for electrode enhancement), I: 2-63; II: 1-185
Saltpeter, I: 2-60
also see chemical enhancement
Screen room, see shielded enclosures
Selection criteria, I: 2-1, 3-1, 7-1, 8-1; II: 1-6 to 1-9
bonds, for, I: 7-1
earth electrode subsystem, for, I: 2-1; II: 1-6 to 1-9
lightning protection, for, I: 3-1
shielding, for, I: 8-1
Semiconductor surge arresters, I: 10-18
also see arresters, surge
Shielded enclosures, I: 8-63 to 8-72
custom built, I: 8-70
double walled, I: 8-71
modular, I: 8-66
Shielding angle, see cone of protection
Shielding effectiveness (SE), I: 8-4, 8-19, 8-31, 8-59; II: 1-155 to 1-160, 1-168
building materials, of, I: 8-59; II: 1-155 to 1-160, 1-168
definition of, I: :8-4
equations for, I: 8-31
layered shields, of, I: 8-31
single thickness shields, of, I: 8-4
tables of, I: 8-6 to 8-54
also see absorption loss, reflection loss, and shields
D-19
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX S
Shielding, electromagnetic, I: 8-1
functions of, I: 8-1
theory of, I: 8-2
Shielding requirement, I: 8-14
Shields, I: 8-31; II: 1-154 to 1-165, 2-19, 3-25, 3-27, 3-35, 3-42, 3-53
components, II: 3-25
configuration of, I: 8-63; II: 1-162
design of, I: 8-74; II: 1-159
discontinuous, see apertures
equipment, guidelines for, II: 3-25
grounding of, I: 8-70; II: 1-162, 3-26, 3-35
inspection of, II: 2-19, 3-53
magnetic, I: 8-20, 8-41; II: 1-165, 3-42
material selections for, I: 8-53; II: 1-160, 1-162, 3-42
metal foils as, I: 8-71
personnel protection, I: 8-74; II: 1-159
seams in, I: 8-42; II: 1-162, 3-27, 3-49
testing of, I:8-72
thin film I: 8-31; II: 1-162
Shields, perforated, I: 8-33, 8-52; II: 1-162, 3-30
honeycomb, I: 8-52; II: 1-162, 3-30
screens, I: 8-33, 8-52; II: 3-30
Shock hazards, electric, I: 9-1 to 9-3
effects on human body, I: 9-1
prevention of, I: 9-3
Signal grounding terminals, II: 3-1, 3-44, 3-46
Signal reference, I: 1-3; II: 3-1, 3-35
Silver solder (for bonding), I: 7-14; II: 1-140
Site selection, II: 1-2 to 1-6
D-20
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX S
Site survey, I: 8-74; II: 1-2 to 1-6, 2-17
Skin effect, I: 5-3, 5-5, 5-8
formulas for, I: 5-5
nomograph of, I: 5-8
Sodium chloride, I: 2-60
also see chemical enhancement
Soft solder (for bonding), I: 7-14; II: 2-10
Soil enhancement, see chemical enhancement
Soil resistivity, I: 2-7; II: 1-2, 1-6
Solvents, use of, I: 7-26
Spark gaps, I: 10-17
also see arresters, surge
Standby generators, II: 1-104
Static electricity, I: 5-19, 9-3, 9-4
Step voltage, I: 2-49
Stepped-leader, I: 3-1
Stray current, I: 2-2, 6-5; II: 2-16, 2-17
Structural steel, I: 5-15; II: 1-39, 1-140, 1-153, 1-154
bonding of, II: 1-140, 1-153, 1-154
ground conductors, as, I: 5-15; II: 1-39
Structures, multiple, II: 1-15, 1-17, 1-18
Stuffing tube, I: 10-24
Surface hardness, see bonding, electrical
Surface preparation, I: 7-25
Surface transfer impedance, I: 8-59
D-21
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX T
Terminal protection devices, I: 10-17 to 10-19
Test procedures, I: 8-72; II: l-2 to 1-4, 2-5, 2-16, 2-17, 2-19, 3-43 to 3-46
bond resistance, II: 2-19,3-47
earth electrode resistance, II: 2-5
ground system noise, II: 2-20
network isolation, II: 2-19, 3-4, 3-41, 3-44, 3-45
shields, I: 8-72
soil resistivity, II: 1-2 to 1-4
stray current, II: 2-17, 2-20
Three-point method, I: 2-46
Thunderstorm day, see iokeraunic
Transducer grounding, II: 1-172, 3-35
Tubular conductor, I: 5-13
TT-C-490, II: 1-145, 1-146
Twisted wires, use of, I: 6-24; II: 1-171, 3-38, 3-40
INDEX U
Underground cables, protection of, II: 1-45, 1-57
Upgrading proceedings for facilities, II: 2-1, 2-33
D-22
MIL-HDBK-419A
APPENDIX D (Continued)
INDEX V
Varistors, I: 10-18
also see arrester, surge
Ventilation ports, shielding, of, I: 8-53; II: 1-162
Vertical structures, I: 10-9
INDEX W
Water retention, I: 2-60; II: 1-6
Water system as earth electrodes, I: 2-16; II: 1-182
Waveguide-below-cutoff, I: 8-50; II: 1-162, 1-164
Waveguide penetration, facility shield of, I: 10-19 to 10-25
Welding, I: 7-10; II: 1-22, 1-140, 3-18
Well casings, I: 2-16
Wells, grounding, II: 1-20, 1-22
Workmanship, I: 7-34
INDEX X
X-rays, I: 9-5
D-23
MIL-HDBK-419A
Custodians:
Army - SC
Preparing Activity:
Air Force -90
Navy - EC
Air Force - 90
Review Activities:
Army-SC, CR, AR, AC
Other Interest:
Navy - EC, NC, NV, OM
DNA - DS
Air Force -02, 04, 11, 14, 15, 17, 50, 90
OST (M-35)
DMSSO-SD
DCA - DC
Activity, Assignee
NSA - NS
Air Force -90
Joint Tactical C 3 Agency -JT
(Project SLHC-4191)
ECAC
MIL-HDBK-419A
29 DECEMBER 1987
SUPERSEDING
MIL-HDBK-419
21 JANUARY 1982
MILITARY HANDBOOK
GROUNDING, BONDING, AND SHIELDING
FOR
ELECTRONIC EQUIPMENTS AND FACILITIES
VOLUME II OF 2 VOLUMES
APPLICATIONS
AMSC N/A
EMCS/SLHC/TCTS
DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited
DEPARTMENT OF DEFENSE
WASHINGTON DC 20301
MIL-HDBK-419A
GROUNDING, BONDING, AND SHIELDING FOR ELECTRONIC EQUIPMENTS AND FACILITIES
1.
established
2.
This standardization handbook was developed by the Department of Defense in accordance with
procedure.
This publication was approved on 29 December 1987 for printing and inclusion in the military
standardization handbook series. Vertical lines and asterisks are not used in this revision to identify changes
with respect to the previous issue due to the extensiveness of the changes.
3.
This document provides basic and application information on grounding, bonding, and shielding
practices recommended for electronic equipment.
It will provide valuable information and guidance to
personnel concerned with the preparation of specifications and the procurement of electrical and electronic
equipment for the Defense Communications System. The handbook is not intended to be referenced in purchase
specifications except for informational purposes, nor shall it supersede any specification requirements.
4.
Every effort has been made to reflect the latest information on the interrelation of considerations
of electrochemistry, metallurgy, electromagnetic, and atmospheric physics.
It is the intent to review this
handbook periodically to insure its completeness and currency. Users of this document are encouraged to report
any errors discovered and any recommendations for changes or inclusions to: Commander, 1842 EEG/EEITE,
Scott AFB IL 62225-6348.
5.
Copies of Federal and Military Standards, Specifications and associated documents (including this
handbook) listed in the Department of Defense Index of Specifications and Standards (DODISS) should be
obtained from the DOD Single Stock Point:
Tabor Avenue, Philadelphia PA 19120.
Commanding Officer, Naval Publications and Forms Center, 5801
Single copies may be obtained on an emergency basis by calling
(AUTOVON) 442-3321 or Area Code (215)-697-3321.
obtained from the sponsor.
Copies of industry association documents should be
Copies of all other listed documents should be obtained from the contracting
activity or as directed by the contracting officer.
MIL-HDBK-419A
PREFACE
This volume is one of a two-volume series which sets forth the grounding, bonding, and shielding applications for
communications electronics (C-E) equipments and facilities.
Grounding, bonding, and shielding are complex
subjects about which in the past there has existed a good deal of misunderstanding. The subjects themselves are
interrelated and involve considerations of a wide range of topics from electrochemistry and metallurgy to
electromagnetic field theory and atmospheric physics. These two volumes reduce these varied considerations
into a usable set of principles and practices which can be used by all concerned with, and responsible for, the
safety and effective operation of complex C-E systems. Where possible, the principles are reduced to specific
steps. Because of the large number of interrelated factors, specific steps cannot be set forth for every possible
situation.
However, once the requirements and constraints of a given situation are defined, the appropriate
steps for solution of the problem can be formulated utilizing the principles set forth.
Both volumes (Volume I, Basic Theory and Volume II, Applications) implement the Grounding, Bonding, and
Shielding requirements of MIL-STD-188-124A which is mandatory for use within the Department of Defense.
The purpose of this standard is to ensure the optimum performance of ground-based telecommunications
equipment by reducing noise and providing adequate protection against power system faults and lightning
strikes.
This handbook emphasizes the necessity for including considerations of grounding, bonding, and shielding in all
phases of design, construction, operation, and maintenance of electronic equipment and facilities. Volume I,
Basic Theory, develops the principles of personnel protection, fault protection, lightning protection,
interference reduction, and EMP protection for C-E facilities.
In addition, the basic theories of earth
connections, signal grounding, electromagnetic shielding, and electrical bonding are presented. The subjects are
not covered independently, rather they are considered from the standpoint of how they influence the design of
the earth electrode subsystem of a facility, the selection of ground reference networks for equipments and
structures, shielding requirements, facility and equipment bonding practices, etc. Volume I also provides the
basic background of theory and principles that explain the technical basis for the recoin m ended practices and
procedures, illustrates the necessity for care and thoroughness in implementation of grounding, bonding, and
shielding; and provides supplemental information to assist in the solution of those problems and situations not
specifically addressed.
In Volume II, Applications, the principles and theories, including RED/BLACK protection, are reduced to the
practical steps and procedures which are to be followed in structural and facility development, electronic
engineering, and in equipment development.
These applications should assure personnel, equipment and
structural safety, minimize electromagnetic interference (EMI) problems in the final operating system; and
minimize susceptibility to and generation of undesirable emanations. The emphasis in Volume II goes beyond
development to assembly and construction, to installation and checkout, and to maintenance for long term use.
Four appendices are provided as common elements in both volumes. Appendix A is a glossary of selected words
and terms as they are used herein. If not defined in the glossary, usage is in accordance with Federal Standard
1037, Glossary of Telecommunication Terms.
Appendix B is a supplemental bibliography containing selected
references intended to supply the user with additional material. Appendix C contains the table of contents for
the other volume. Appendix D contains the index for the two-volume set.
MIL-HDBK-419A
TABLE OF CONTENTS
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.1
INTRODUCTION.
. . . . . . . . . . . . . . . .
1-1
1.2
EARTH ELECTRODE SUBSYSTEM . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Determination of Site Parameters. . . . . . . . . . . . . . . . . . . . . . . . .
1-2
1.2.1.1
Soil Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
1.2.1.2
Geological Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-5
1.2.1.3
Physical Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-5
1.2.1
Local Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-5
Design Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-6
1.2.2.1
Selection of Electrode Configuration . . . . . . . . . . . . . . . . . . . . . .
1-6
1.2.2.2
Calculation of Earth Resistance. . . . . . . . . . . . . . . . . . . . . . . . .
1-9
1.2.2.3
Alternate Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-9
1.2.3
Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-14
1.2.4
Installation Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-22
1.2.1.4
1.2.2
LIGHTNING PROTECTION FOR STRUCTURES . . . . . . . . . . . . . . . . . . . .
1-23
1.3.1
Principles of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-23
1.3.2
Integral Protection System . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-23
Air Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-24
1.3
1.3.2.1
1.3.2.1.1
Size and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-24
Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-24
Grounding Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-33
1.3.2.2.1
Roof Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-33
1.3.2.2.2
Down Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-37
Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-39
Separately Mounted Protection Systems . . . . . . . . . . . . . . . . . . . . . .
1-40
1.3.2.1.2
1.3.2.2
1.3.2.3
1.3.3
1.3.3.1
Mast Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-40
1.3.3.2
Overhead Ground Wire Type . . . . . . . . . . . . . . . . . . . . . . . . . .
1-41
1.3.3.3
Waveguide Installation and Grounding . . . . . . . . . . . . . . . . . . . . . .
1-42
1.3.3.4
Cable Installation and Grounding . . . . . . . . . . . . . . . . . . . . . . . .
1-49
1.3.3.5
Lightning-Generated Transient Surge Protection . . . . . . . . . . . . . . . . .
1-49
Transient Source and Equipment Damage. . . . . . . . . . . . . . . . . . . .
1-49
1.3.3.5.2
Minimizing Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-50
1.3.3.5.3
Susceptible Components . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-50
1.3.3.5.4
Frequency of Transient Occurrence . . . . . . . . . . . . . . . . . . . . . .
1-51
1.3.3.5.5
Transient Definition, AC Service Conductors . . . . . . . . . . . . . . . . . .
1-51
1.3.3.5.1
1.3.3.5.6
Methods for Transient Protection on AC Service Conductors . . . . . . . . . . .
1-56
1.3.3.5.7
Use of Ferrous Metal Conduit . . . . . . . . . . . . . . . . . . . . . . . . .
1-56
1.3.3.5.8
Use of Overhead Guard Wires . . . . . . . . . . . . . . . . . . . . . . . . .
1-56
i
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.3.3.5.9
Protection of Underground Cables. . . . . . . . . . . . . . . . . . . . . . .
1-57
1.3.3.5.10
Buried Guard Wire
1-57
1.3.3.5.11
Secondary AC Surge Arrester . . . . . . . . . . . . . . . . . . . . . . . . .
1-59
1.3.3.5.12
Surge Arrester Installation . . . . . . . . . . . . . . . . . . . . . . . .
1-59
1.3.3.5.13
Operating Characteristics of Surge Arresters . . . . . . . . . . . . . . . . . .
1-60
1.3.3.5.14
Desirable Operating Characteristics for Transient Suppressors . . . . . . . . . .
1-67
1.3.3.5.15
Characteristics of Different Types of Surge Arresters . . . . . . . . . . . . . .
1-67
1.3.3.5.16
Transient Protection for Externally Exposed Equipment Lines . . . . . . . . . . .
1-73
1.3.3.5.17
Frequency of Transient Occurrence
1-73
1.3.3.5.18
Amplitudes and Waveforms of Occurring Transients . . . . . . . . . . . . . . .
1-73
1.3.3.5.19
Equipment Withstand Levels
1-74
1.3.3.5.20
Protection Methods Against Transients . . . . . . . . . . . . . . . . . . . . .
1-74
1.3.3.5.21
Enclosing Cable Runs in Ferrous Metal Conduit . . . . . . . . . . . . . . . . .
1-74
1.3.3.5.22
Transient Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-75
1.3.3.5.23
Types of Available Transient Suppression. . . . . . . . . . . . . . . . . . . .
1-77
1.3.3.5.24
Operating Characteristics of Transient Suppressors . . . . . . . . . . . . . . .
1-77
1.3.3.5.25
Transient Suppressor Packaging Design . . . . . . . . . . . . . . . . . . . . .
1-78
1.3.3.5.26
Coaxial Cable Shield Connection Through an Entrance Plate . . . . . . . . . . .
1-78
1.3.3.5.27
Grounding
.
1-78
1.3.3.5.28
Transient Suppression for RF Coaxial Lines
1-79
of
Unused
Wires
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Equipment-Level Transient Suppression . . . . . . . . . . . . . . . . . . . .
1-79
1.3.3.6
Lightning Generated Transient Protection Evaluation . . . . . . . . . . . . . . .
1-79
1.3.3.7
Transient Protection
1-80
1.3.3.5.29
1.3.3.7.1
Protection Requirement
1-80
1.3.3.7.2
Transient Definition
1-81
1.3.3.7.3
Determination of Equipment Damage (Withstand) Levels . . . . . . . . . . . . .
1-82
1.3.3.7.4
Determination of Need for Transient Protection . . . . . . . . . . . . . . . . .
1-83
1.3.3.7.5
Minimizing Transient Damage
1-83
1.3.3.7.6
AC Power Input
1-84
1.3.3.7.7
Power Supply Transient Suppression
1-89
1.3.3.7.8
Landline Transient Suppression
1-89
1-98
1.3.3.8
Corrosion Control
1.3.3.9
Joints
1-99
Physical Protection
1-99
1.3.3.10
FAULT PROTECTIVE SUBSYSTEM . . . . . . . . . . . . . . . . . . . . . . . . .
1-99
1.4.1
Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-99
1.4.2
Equipment Fault Protection Subsystem Composition . . . . . . . . . . . . . . . . .
1-100
1.4
ii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Paragraph
Page
1.4.3
Configuration of the Equipment Fault Protection Subsystem . . . . . . . . . . . . .
1-100
1.4.4
Pipes and Tubes
1-102
1.4.5
Electrical Supporting Structures . . . . . . . . . . . . . . . . . . . . . . . . .
1-102
1.4.5.1
Metal Conduit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
1.4.5.2
Cable
1-103
1.4.5.3
Enclosures . . . . . . . . . . . . . . . . . . . . . . . .
1.4.5.4
Cable Armor
1-103
1.4.5.5
Rotating Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-103
Trays
. . . . . . . . . .
1-103
1.4.6
Power Distribution Systems.. . . . . . . . . . . . . . . . . . . . . . . . . .
1-104
1.4.7
Standby AC Generators
1-104
1.4.8
Equipment Fault Protection Subsystems for Transportable Equipment . . . . . . . . .
1-104
1.4.9
MIL-STD-188-124A and NEC Compliance Evaluation . . . . . . . . . . . . . . . . .
1-105
1.4.9.1
Measurements
1.4.9.2
MIL-STD-188-124A and NEC Compliance Inspection . . . . . . . . . . . . . . . .
1-105
1.4.9.3
Correction of Deficiencies
1-111
1.5
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1-105
SIGNAL REFERENCE SUBSYSTEM FOR NEW FACILITIES . . . . . . . . . . . . . . .
1-113
Higher Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-113
Multipoint Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-118
Types of Equipotential Planes . . . . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1
1.5.1.1
1.5.1.1.1
1.5.1 .1.1.1
Copper Grid Embedded in Concrete . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.2
Equipotential Plane Under Floor Tile or Carpet . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.3
Overhead Equipotential Plane. . . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.4
Raised (Computer) Flooring . . . . . . . . . . . . . . . . . . . . . . . .
1-120
1.5.1.1.1.4.1
Bolted-Grid (Stringer) or Rigid Grid System. . . . . . . . . . . . . . . . .
1-125
1.5.1.1.1.4.2
Drop-In or Removable Grid System . . . . . . . . . . . . . . . . . . . . .
1-131
1.5.1.1.1.4.3
Free-Standing, Pedestal-Only or Stringerless System . . . . . . . . . . . .
1-131
1.5.1.1.1.5
Ground Risers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-131
1.5.1.1.1.6
Equipment Cabinet Grounding . . . . . . . . . . . . . . . . . . . . . . .
1-131
Lower Frequency Signal Reference Network . . . . . . . . . . . . . . . . . . . .
1-131
1.5.2
1.6
1.7
GROUNDING PHILOSOPHY FOR EQUIPMENTS PROCESSING NATIONAL SECURITY
RELATED INFORMATION (RED/BLACK EQUIPMENTS) . . . . . . . . . . . . . . . .
1-134
BONDING PRACTICES
1-140
1.7.1
Application Guidelines
1-140
1.7.2
Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-142
1.7.3
Bond Protection Code
1-143
Jumper Fasteners
1-147
1.7.3.1
1.7.4
Typical Bonds
1-148
iii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
.
.
1-148
.
.
1-149
Other Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-149
SHIELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-154
1.7.4.1
Cable Trays . . . .
1.7.4.2
Tubing and Conduit . . . . . . . . . .
1.7.4.3
1.8
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1.8.1
Establishing Requirements
1-154
1.8.2
Design
1-159
1.83
Selection of Materials
1.8.4
Construction Guidelines
1.9
Guidelines
1-160
1-162
COMMON-MODE NOISE AND INSTRUMENTATION . . . . . . . . . . . . . . . . . .
1-164
1.9.1
Design Practices
1-171
1.9.2
Instrumentation Considerations. . . . . . . . . . . . . . . . . . . . . . . . . .
1-172
EMP PROTECTION
1-172
1.10.1
Earth Connection
1-172
1.10.2
EMP Shield Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-173
1.10.3
EMP Bonding Practices
1-173
1.10.4
Construction Guidelines
1-176
1.10
1.11
1-177
MILITARY MOBILE FACILITIES
General Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
1-177
Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Earth Ground
1-177
1.11.1.1.1.1
General Earth Electrode Subsystem Requirements . . . . . . . . . . . . . . .
1-177
1.11.1.1.1.2
Earth Electrode Subsystem Types . . . . . . . . . . . . . . . . . . . . . .
1-177
1.11.1.1.1.3
Soil Resistance
1-178
1.11.1.1.1.4
Ground Rod Resistance . . . . . . . . . . . . . . . . . . . . . . . . . .
1-178
1.11.1.1.1.5
Ground Resistance She's . . . . . . . . . . . . . . . . . . . . . . . . . .
1-178
l.11.1
1.11.l.l
1.11.1.1.1
1.11.l.l.2
1-177
1-178
Power Ground
1.11.1.1.2.1
Three-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
1-178
1.11.1.1.2.2
Single-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
1-179
1.11.1.1.2.3
DC Power System (2-Wire)
1-179
1.11.1.1.3
Lightning/EMP Protection Subsystem . . . . . . . . . . . . . . . . . . . . .
1-179
1.11.1.1.3.1
Air Terminals
1-179
1.11.1.1.3.2
Terminal Protection Devices . . . . . . . . . . . . . . . . . . . . . . . .
1-182
1.11.1.1.4
Signal Reference Subsystem. . . . . . . . . . . . . . . . . . . . . . . . .
1-182
1.11.1.1.5
Fault Protection Subsystem.. . . . . . . . . . . . . . . . . . . . . . . .
1-182
1.11.1.2
1.11.1.3
1.11.1.3.1
Measuring Ground Resistance in Tactical Environments . . . . . . . . . . . . . .
1-182
Reducing Ground Resistance in Tactical Environments . . . . . . . . . . . . . . .
1-182
1-182
Existing Facilities
iv
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
Multiple Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.1.3.3
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.1.3.4
Chemical Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
Detailed Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
1-185
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-185
1.11.2.1.1
Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.1.2
Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
Stand-Alone Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
Low Resistance Grounds . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2.1.1
Existing Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-186
1.11.2.2.2.1.2
Earth Electrode Subsystem, Single Ground Rod . . . . . . . . . . . . . . .
1-187
1.11.2.2.2.1.3
Earth Electrode Subsystem, Multiple Ground Rods . . . . . . . . . . . . . .
1-187
Stand-Alone Shelter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.1.3.2
1.11.2
1.11.2.1
1.11.2.2
1.11.2.2.2.1
1.11.2.3
1.11.2.3.1
Interconnection of Subsystems . . . . . . . . . . . . . . . . . . . . . . . .
1-187
Collocated Military Mobile Equipments . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.2.4.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-187
1.11.2.4.2
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.3.2
1.11.2.4
Collocated Shelters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.1
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2
1.11.2.5
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.1
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.2
Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.3
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
1-188
1.11.2.5.2.4
Lightning/EMP Protection . . . . . . . . . . . . . . . . . . . . . . . . .
1-189
1.11.2.5.2.5
Collocated Shelters Greater than 8 Meters Apart . . . . . . . . . . . . . . .
1-189
1.11.2.5.2.5.1
Ground Resistance Difference of Less than 150 Ohms . . . . . . . . . . . .
1-189
1.11.2.5.2.5.2
Ground Resistance Difference of Greater than 150 Ohms . . . . . . . . . . .
1-189
1.11.2.5.2.5.3
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.1
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.2
Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.3
Safety/Equipment Ground (Green Wire) . . . . . . . . . . . . . . . . .
1-190
1.11.2.5.2.5.3.4
Lightning/EMP Protection . . . . . . . . . . . . . . . . . . . . . . .
1-190
Fixed Prefabricated Shelters.. . . . . . . . . . . . . . . . . . . . . . . . .
1-190
1.11.2.6
1.11.2.6.1
General
Description
1-190
v
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1- NEW FACILITIES DESIGN CRITERIA
Page
Paragraph
1.11.2.6.2
1-190
Electrical Connection
1.12
FENCES. . .
1-191
1.12.1
Introduction .
1-191
1.12.2
Grounding . .
1-191
1.12.3
1.13
Installation . .
INSPECTION AND TEST PROCEDURES FOR A NEW FACILITY . . . . . . . . . . . .
1-191
1-193
1.13.1
Earth Electrode Subsystem
1-193
1.13.2
Lightning Protection Network
1-194
1.13.3
Signal Reference and Fault Protection Subsystems. . . . . . . . . . . . . . . . . .
1-194
1.13.4
Bonds and Bonding
1-195
1.13.5
Facility Checkout Form
1-196
Part I
1-196
1.14
Earth Electrode Subsystem
Part II Lightning Protection Network . . . . . . . . . . . . . . . . . . . . . .
1-197
Part III Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . .
1-198
Part IV Bonding
1-199
1-200
REFERENCES
CHAPTER 2 - EXISTING FACILITIES
2.1
INTRODUCTION
2-1
2.2
UPGRADING
2-1
2.2.1
Drawings
2-4
2.2.2
Facility Survey
2-4
2.2.2.1
Survey Steps
2-4
2.2.2.2
Inspection Procedure
2-5
2-5
2.2.2.2.1
Earth Electrode Subsystem
2.2.2.2.2
Bonds and Bonding
2-10
2.2.2.2.3
Lightning Protection Network . . . . . . . . . . . . . . . . . . . . . . . .
2-13
2.2.2.2.4
Safety Grounding
2-16
2.2.2.2.5
Signal Grounding Practices
2-17
2.2.2.2.6
Ground System Noise Survey . . . . . . . . . . . . . . . . . . . . . . . . .
2-17
2.2.2.2.7
Shielding
2-19
2.2.2.3
2-19
Test Procedures
2.2.2.3.1
Bond Resistance
2-19
2.2.2.3.2
Ground System Noise Current . . . . . . . . . . . . . . . . . . . . . . . . .
2-20
2.2.2.3.3
Differential Noise Voltage
2-22
2.2.2.4
2-24
Survey Form
vi
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 2- EXISTING FACILITIES
Page
Paragraph
2.2.3
Guidelines for Upgrading
2-33
2.2.4
Expansion of Existing Facilities
2-35
2.2.5
Expansion of Existing Facilities for Higher-Frequency Grounds . . . . . . . . . . . .
2-35
2.3
2-36
MAINTENANCE
2.3.1
Schedules and Records
2-36
2.3.2
Maintenance Procedures
2-36
2.3.2.1
Earth Electrode Subsystem
2-39
2.3.2.2
Lightning Protection Subsystem. . . . . . . . . . . . . . . . . . . . . . . . .
2-41
2.3.2.3
Bonding
2-42
2.3.2.4
Fault Protection Subsystem (Safety Ground) . . . . . . . . . . . . . . . . . . .
2-43
2.3.2.5
Signal Reference Subsystem (Signal Grounding) . . . . . . . . . . . . . . . . . .
2-44
2.3.2.6
Shielding
2-45
2.3.3
Facility Maintenance Report
2-46
2.3.4
Performance Evaluation Program . . . . . . . . . . . . . . . . . . . . . . . . .
2-54
2.4
GROUNDING CONSIDERATIONS FOR CLASSIFIED INFORMATION PROCESSORS
(RED/BLACK EQUIPMENTS) INSTALLED PRIOR TO THIS HANDBOOK . . . . . . . . .
2-59
2.4.1
Introduction
2-59
2.4.2
Existing Facilities
2-59
2.4.3
Protection Grounds
2-59
2.4.4
Signal Reference Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-60
2.4.5
Signal Filter Ground
2-61
2.4.6
Grounding
Precautions
.
.
.
.
.
. . . . . . . . . . . . . . . . . . . . . . . . .
2-61
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
3.1
INTRODUCTION
. . . . . . . . . . . . . . . . . . . . . . . . .
3-1
3.2
GROUNDING PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-1
Signal Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-1
3.2.1
3.2.1.1
.
.
.
.
.
.
.
.
Lower Frequency Equipment . . . . . . . . . . . . . . . . . . . . . . . . . .
3-2
3.2.1.1.1
Signal Ground Network Configuration
3-2
3.2.1.1.2
Signal Ground Terminals . . . . . .
3-2
3.2.1.1.3
Color Code . . . . . . . . . . . .
3-3
3.2.1.1.4
Cabinet Bus Bar . . . . . . . . .
3-3
3.2.1.1.5
Isolation . . . . . . . . . . . . .
3-4
3.2.1.1.6
Signal Interfacing . . . . . . . . .
3-4
3.2.1.1.7
Signal Grounding . . . . . . . . .
3-6
Higher Frequency Equipment . . . . .
3-12
3.2.1.2
vii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
Page
Paragraph
3.2.1.2.1
Signal Interfaces
3-13
3.2.1.2.2
Cable Connectors
3-13
3.2.1.3
Equipments Containing Both Lower and Higher Frequency Circuits .
3-14
3.2.2
Fault Protection
3-14
3.2.3
Cabinet Grounding
3-16
3.3
3.4
BONDING PRACTICES
3-18
SHIELDING GUIDELINES . . . . . . . . . . . . . . . . . . . .
3-25
3.4.1
Parts Selection
3-25
3.4.2
Layout and Construction . . . . . . . . . . . . . . . . . . . .
3-25
3.4.3
Equipment Enclosures
3-27
3.4.3.1
Seams
3-27
3.4.3.2
Penetrations and Apertures . . . . . . . . . . . . . . . . . .
3-28
3.5.1
Common-Mode Noise Control . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-34
3-34
3.5.2
Instrumentation Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-34
3.5
3.5.2.1
COMMON-MODE NOISE CONTROL AND INSTRUMENTATION GROUNDING . . . . . . .
3-35
Analog Systems
3.5.2.1.1
Grounded Transducers . .
3-35
3.5.2.1.2
Ungrounded Transducers .
3-38
3.5.2.1.3
Amplifiers . . . . . . .
3-38
3.5.2.2
Digital Data Systems . . .
3-40
3.5.2.3
Recording Devices . . . .
3-40
3.5.2.3.1
Magnetic Tape Recorders
3-40
3.5.2.3.2
Strip Chart Recorders . .
3-42
3.5.2.3.3
X-Y Plotters . . . . . .
3-42
3.6
EMP CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-42
3.7
EQUIPMENT INSPECTION AND TEST PROCEDURES . . . . . . . . . . . . . . . . .
3-43
3.7.1
Lower Frequency Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-43
3.7.2
Higher Frequency Equipments . . . . . . . . . . . . . . . . . . . . . . . . . .
3-47
3.7.3
Hybrid Equipments
3-47
3.7.4
Installed Equipments
3-47
3.7.5
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-47
3.7.6
Bonding
3-49
3.7.7
Shielding
3-49
3.7.8
Instrumentation System . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-50
3.7.9
EMP Design
3-50
3.7.10
Other Observations
3-50
3.7.11
Inspection Form
3-50
3.8
3-54
REFERENCES
viii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 4 - NOTES
Page
Paragraph
4.1
SUBJECT TERM (KEY WORD) LISTING . . . . . . . . . . . . . . . . . . . . . . .
4-1
APPENDICES
A
GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A-1
B
SUPPLEMENTAL BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . .
B-1
BI
SUBJECT CROSS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . .
B-1
BII
LISTINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B-2
C
TABLE OF CONTENTS FOR VOLUME I . . . . . . . . . . . . . . . . . . . . . . .
C-1
D
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D-1
ix
MIL-HDBK-419A
LIST OF FIGURES
Page
Figure
1-1
Measurement of Soil Resistivity. . . . . . . . . . . . . . . . . . . . . . . . . . .
1-3
1-2
Resistivity Determination of a Small Site . . . . . . . . . . . . . . . . . . . . . .
1-4
1-3
Minimum Earth Electrode Subsystem Configuration for Rectangular Shaped Facility . . . .
1-10
1-4
Nomograph for Determining the Resistance to Earth of a Single Ground Rod . . . . . . .
1-11
1-5
Effective Resistance of Ground Rods When Arranged in a Straight Line
or a Large Circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-12
1-6
Graph of Multiple-Rod Resistance Ratio . . . . . . . . . . . . . . . . . . . . . . .
1-13
1-7
Electrode Configuration for Irregular Shaped Facility . . . . . . . . . . . . . . . . .
1-16
1-8
Electrode Configuration for Adjacent Structures . . . . . . . . . . . . . . . . . . .
1-17
1-9
Electrode Configuration for Closely Spaced Structures . . . . . . . . . . . . . . . . .
1-17
1-10
Grounding System for Typical Radar Installation . . . . . . . . . . . . . . . . . . .
1-18
1-11
Details of Ground Rod/Earth Electrode Subsystem Installation . . . . . . . . . . . . .
1-19
1-12
Concrete Grounding Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-20
1-13
Typical Grounding Well Installation . . . . . . . . . . . . . . . . . . . . . . . . .
1-20
1-14
Connections to Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . .
1-21
1-15
Grounding Practices for Lightning Protection . . . . . . . . . . . . . . . . . . . . .
1-25
1-16
Location of Air Terminals for Common Roof Types . . . . . . . . . . . . . . . . . .
1-26
1-17
Location of Air Terminals on Gently Sloping Roofs . . . . . . . . . . . . . . . . . .
1-27
1-18
Air Terminal Placement on Flat-Roofed Structures . . . . . . . . . . . . . . . . . .
1-29
1-19
Graphical Method for Determining Need for Additional Air Terminals . . . . . . . . . .
1-30
1-20
Field Expedient Technique for Determining the Protection of Prominent Dormers . . . . .
1-31
1-21
Illustration of Method for Determining the Protection of Flat Surfaces
as Provided by Air Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-32
1-22
Criteria for Dead End Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-37
1-23
Recommended Construction Practices for Integral Lightning Protection Systems . . . . .
1-38
1-24
The Protected Zone Provided by Two Vertical Masts . . . . . . . . . . . . . . . . . .
1-40
1-25
Overhead Ground Wire Lightning Protection System . . . . . . . . . . . . . . . . . .
1-41
1-26
Waveguide Entry Plate Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-43
1-27
Grounding Detail for Elliptical Waveguide (Front View) . . . . . . . . . . . . . . . . .
1-44
1-28A
Grounding Detail for Elliptical Waveguide (Side View) . . . . . . . . . . . . . . . . .
1-45
1-28B
Heat Shrink Grounding . . . . . . . . . . . . . . . . . . . . . . . . . .
1-46
1-29
Ground Strap Detail for Elliptical Waveguide . . . . . . . . . . . . . . . . . . . . .
1-46
1-30
Strap Cutting Detail for Elliptical Waveguide . . . . . . . . . . . . . . . . . . . . .
1-46
1-31
Typical Communication Cable Entry Installation . . . . . . . . . . . . . . . . . . .
1-47
1-32
Ground Strap Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-48
1-33
Grounding
.
1-48
1-34
Mean Number of Thunderstorm Days per Year for the United States . . . . . . . . . . .
1-53
1-35
Lightning Protection for Underground Service. . . . . . . . . . . . . . . . . . . . .
1-58
1-36
Secondary AC Surge Arrester Installation, Grounded Service . . . . . . . . . . . . . .
1-61
1-37
Secondary AC Surge Arrester Installation, Ungrounded Service . . . . . . . . . . . . .
1-62
Steps
for
Cables
.
.
.
.
.
.
x
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
1-38
Typical Operating Curve for Two Series of Gas-filled Spark Gap Arresters
With Nonlinear Series Resistor. . . . . . . . . . . . . . . . . . . . . . . . . . .
1-69
1-39
Typical Arrester Operating Curves, ZNR and SAS . . . . . . . . . . . . . . . . . . .
1-72
1-40
Typical Transient Suppressor Installation, Facility and Equipment Level . . . . . . . . .
1-76
1-41
Typical Configuration for Protection of Equipment from Conducted Powerline Surges
and Transients (Neutral Grounded). . . . . . . . . . . . . . . . . . . . . . . . . .
1-42
Typical Configuration for Protection of Equipment from Conducted Powerline Surges
and Transients (Ungrounded). . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-43
1-86
1-87
Typical Configuration for Protection of Equipment from Conducted
Landline Transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-92
1-44
Transient Suppression for Coaxial Lines (DC to 3 MHz). . . . . . . . . . . . . . . . .
1-95
1-45
Transient Suppression for Twinaxial Lines (DC to 3 MHz) . . . . . . . . . . . . . . . .
1-96
1-46
Typical Equipment Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . .
1-101
1-47
Method for Determining the Existence of Improper Neutral Ground Connections . . . . . .
1-112
1-48
(Deleted)
1-49
Typical Equipotential Ground Plane for Multi-Deck Building . . . . . . . . . . . . . .
1-115
1-50
Typical Building Floor Plan (Top View) . . . . . . . . . . . . . . . . . . . . . . . .
1-116
1-51
Typical Multi-Deck Building Plan (Side View) . . . . . . . . . . . . . . . . . . . . .
1-117
1-52
Elements of the Facility Ground System (With Grid) . . . . . . . . . . . . . . . . . .
1-121
1-53
Typical Equipotential Ground Plane for New Construction Higher or Hybrid
Frequencies Facilities Installation . . . . . . . . . . . . . . . . . . . . . . . . . .
1-122
1-54
Ground Connector for Equipotential Plane in Concrete . . . . . . . . . . . . . . . . .
1-123
1-55
Typical Ground Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-123
1-56
Example of Cable to Bar Ground Connectors . . . . . . . . . . . . . . . . . . . . .
1-124
1-57
(Deleted)
1-58
Rigid Grid Floor System Details. . . . . . . . . . . . . . . . . . . . . . . . . . .
1-127
1-59
Example of Rigid Grid to Pedestal Bolted Connection . . . . . . . . . . . . . . . . .
1-128
1-60
Example of Rigid-Grid to Pedestal Clamped Connection . . . . . . . . . . . . . . . .
1-129
1-61
Example of Unacceptable Grid to Pedestal Bonding . . . . . . . . . . . . . . . . . .
1-130
1-62
Example of Drop-In Grid Floor Construction . . . . . . . . . . . . . . . . . . . . .
1-131
1-63
Example of Pedestal Only Floor Construction . . . . . . . . . . . . . . . . . . . . .
1-132
1-64
Typical Equipment Cabinet Grounding Detail . . . . . . . . . . . . . . . . . . . . .
1-133
1-65
Typical RED/BLACK Signal Reference Subsystem . . . . . . . . . . . . . . . . . . .
1-135
1-66
Typical RED Signal, Shield Ground, Bus Distribution System . . . . . . . . . . . . . .
1-136
1-67
Typical Intermediate Distribution Frame (Shield Ground Bus in Distribution Frames) . . . .
1-137
1-68
Typical Intermediate Distribution Frame (Data Concentrator Frame Installation) . . . . .
1-138
1-69
Facility Power and AC Ground Distribution . . . . . . . . . . . . . . . . . . . . . .
1-139
1-70
Order of Assembly for Bolted Connection . . . . . . . . . . . . . . . . . . . . . .
1-142
1-71
Bonding of Cable Trays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-148
xi
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
5-12
Single-Point Signal Ground (for Lower Frequencies) . . . . . . . . . . . . . . . . . .
5-20
5-13
Single-Point Ground Bus System Using Separate Risers (Lower Frequency) . . . . . . . .
5-21
5-14
Single-Point Ground Bus System Using a Common Bus . . . . . . . . . . . . . . . . .
5-22
5-15
Use of Single-Point Ground Configuration to Minimize Effect of Facility Ground
Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-23
5-16
Multipoint Ground Configuration . . . . . . . . . . . . . . . . . . . . . . . . . .
5-24
5-17
Use of Structural Steel in Multiple-Point Grounding . . . . . . . . . . . . . . . . . .
5-25
5-18
Recommended Signal Coupling Practice for Lower Frequency Equipment . . . . . . . . .
5-29
5-19
Ground Network Used as Signal Return (Practice Not Generally Recommended) . . . . . .
5-30
6-1
Idealized Energy Transfer Loop . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-2
6-2
Energy Transfer Loop With Noise Sources in Ground System. . . . . . . . . . . . . . .
6-3
6-3
Equivalent Circuit of Non-Ideal Energy Transfer Loop . . . . . . . . . . . . . . . . .
6-3
6-4
Practical Combinations of Source-Load Pairs . . . . . . . . . . . . . . . . . . . . .
6-4
6-5
Coupling Between Circuits Caused by Common Return Path Impedance. . . . . . . . . .
6-7
6-6
Conductive Coupling of Extraneous Noise into Equipment Interconnecting Cables
. . . . .
6-7
6-7
Magnetic Field Surrounding a Current-Carrying Conductor . . . . . . . . . . . . . . .
6-8
6-8
Illustration of Inductive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . .
6-9
6-9
Illustration of Capacitive Coupling . . . . . . . . . . . . . . . . . . . . . . . . .
6-11
6-10
Equivalent Circuit of Network in Figure 6-9 . . . . . . . . . . . . . . . . . . . . .
6-13
6-11
Characteristic Voltage Transfer Curve for Capacitive Coupling . . . . . . . . . . . . .
6-15
6-12
Electric Field Patterns in the Vicinity of a Radiating Dipole . . . . . . . . . . . . . .
6-16
6-13
Illustration of Conductively-Coupled Corn men-Mode Noise . . . . . . . . . . . . . . .
6-18
6-14
Common-Mode Noise in Unbalanced Systems . . . . . . . . . . . . . . . . . . . . .
6-20
6-15
Common-Mode Noise in Balanced Systems . . . . . . . . . . . . . . . . . . . . . .
6-22
7-1
Effects of Poor Bonding on the Performance of a Power Line Filter . . . . . . . . . . .
7-2
7-2
Current Flow Through Direct Bonds . . . . . . . . . . . . . . . . . . . . . . . . .
7-5
7-3
Nature of Contact Between Bond Members. . . . . . . . . . . . . . . . . . . . . .
7-6
7-4
Resistance of a Test Bond as a Function of Fastener Torque . . . . . . . . . . . . . .
7-9
7-5
Typical Exothermic Connections . . . . . . . . . . . . . . . . . . . . . . . . . .
7-12
7-6
Typical Bond Configuration Which Can be Implemented With the Exothermic Process. . . .
7-13
7-7
Nomograph for Torque on Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-15
7-8
Bonding Path Established by Rivets . . . . . . . . . . . . . . . . . . . . . . . . .
7-17
7-9
An Improperly Riveted Seam . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-17
7-10
Inductive Reactance of Wire and Strap Bond Jumpers . . . . . . . . . . . . . . . . .
7-11
Relative Inductive Reactance Versus Length-to-Width Ratio of Flat Straps . . . . . . . .
7-22
7-23
7-12
Frequency Variation of the Impedance of Simple Conductors . . . . . . . . . . . . . .
7-24
7-13
Equivalent Circuit for Bonding Strap . . . . . . . . . . . . . . . . . . . . . . . . .
7-24
xii
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
2-6
Typical Bonding Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-7
Typical Bonding Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-12
2-8
Severely Damaged Down Conductor . . . . . . . . . . . . . . . . . . . . . . . . .
2-14
2-9
Method for Determining the Existence of Improper Neutral Ground Connections . . . . . .
2-15
2-10
Measurement of Stray Current Level in Safety Ground Conductor . . . . . . . . . . . .
2-16
2-11
Typical Bond Resistance and Stray Current Measurement Locations
2-11
in an Electronic Facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-18
2-12
Bond Resistance Measurement Technique . . . . . . . . . . . . . . . . . . . . . . .
2-21
2-13
Test Setup for Stray Current Measurements . . . . . . . . . . . . . . . . . . . . .
2-22
2-14
Oscilloscope Connections for Measuring Voltage Levels on Ground Systems . . . . . . . .
2-23
2-15
Example of Equipotential or Multipoint Grounding . . . . . . . . . . . . . . . . . . .
2-37
2-16
Major Discrepancy Report Form . . . . . . . . . . . . . . . . . . . . . . . . . .
2-38
2-17
Typical Multiple Area Ground Distribution . . . . . . . . . . . . . . . . . . . . . .
2-62
2-18
Typical Signal, Shield Ground, Bus Distribution System for Single-Point Ground . . . . . .
2-63
2-19
Crypto Equipment Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-64
2-20
Typical Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-65
3-1
Grounding in Lower Frequency Equipment . . . . . . . . . . . . . . . . . . . . . .
3-3
3-2
Lower Frequency Signal Ground Bus Bar Installation in Rack or Cabinet . . . . . . . . .
3-5
3-3
Use of Balanced Lines to Avoid Ground Loops . . . . . . . . . . . . . . . . . . . . .
3-7
3-4
Effect of an Unbalanced Cable on the Single-Point Ground . . . . . . . . . . . . . . .
3-8
3-5
Effect of Arbitrarily Grounding the Source End of Unbalanced Equipment
Interconnecting Cables . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-9
3-6
Method of Grounding the Individual Shields on Long Lower Frequency Shield Cables . . . .
3-10
3-7
Grounding of Overall Cable Shields to Connectors . . . . . . . . . . . . . . . . . . .
3-11
3-8
Grounding of Overall Cable Shields to Penetrated Walls . . . . . . . . . . . . . . . .
3-12
3-9
Establishment of Shield Continuity Between Higher Frequency Equipments . . . . . . . .
3-13
3-10
Grounding Practices in Equipments Containing Both Higher Frequency and
Lower Frequency Circuits . . . . . . . . . . . . . . . . . . . . . . . . .
3-15
3-11
Typical Equipment Cabinet Grounding Detail . . . . . . . . . . . . . . . . . . . . .
3-17
3-12
Acceptable and Unacceptable Uses of Bonding Jumpers . . . , . . . . . . . . . . . .
3-19
3-13
Bonding of Subassemblies to Equipment Chassis . . . . . . . . . . . . . . . . . . . .
3-21
3-14
Bonding of Equipment to Mounting Surface . . . . . . . . . . . . . . . . . . . . . .
3-21
3-15
Typical Method of Bonding Equipment Flanges to Frame or Rack . . . . . . . . . . . .
3-22
3-16
Bonding of Rack-Mounted Equipments Employing Dagger Pins. . . . . . , . . . . . . .
3-22
3-17
Recommended Practices for Effective Bonding in Cabinets . . . . . . . . . . . . . . .
3-23
3-18
Method of Bonding Across Hinges . . . . . . . . . . . . . . . . . . . . . . . . . .
3-24
3-19
Bonding of Connector to Mounting Surface . . . . . . . . . . . . . . . . . . . . . .
3-24
3-20
Method of Making Permanent Seam Using a Gasket . . . . . . . . . . . . . . . . . .
3-29
xiii
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Page
Figure
3-21
Mounting of Gasket on Hinged Side of Equipment Doors and Panels . . . . . . . . . . .
3-29
3-22
Illustration of Proper and Improper Shield Penetration . . . . . . . . . . . . . . . . .
3-31
3-23
Use of Cylindrical Waveguide - Below-Cutoff for Control Shaft Shield Penetration . . . .
3-32
3-24
Method of Mounting Wire Screen Over a Large Aperture . . . . . . . . . . . . . . . .
3-33
3-25
Acceptable Methods of Shielding Panel - Mounted Meters . . . . . . . . . . . . . . . .
3-33
3-26
Grounding Practices for Single-Ended Amplifiers . . . . . . . . . . . . . . . . . . .
3-36
3-27
Grounding Practices for Differential Amplifiers . . . . . . . . . . . . . . . . . . . .
3-36
3-28
Method of Grounding Bridge Transducers . . . . . . . . . . . . . . . . . . . . . . .
3-37
3-29
Use of Isolated Differential Amplifier With Balanced Bridge Transducer . . . . . . . . .
3-37
3-30
Recommended Grounding Practices for Floating Traducers . . . . . . . . . . . . . .
3-39
3-31
Grounding for Single Channel Strip Chart Recorder . . . . . . . . . . . . . . . . . .
3-41
3-32
Resistive Isolation of Data Channels . . . . . . . . . . . . . . . . . . . . . . . . .
3-41
3-33
Single Ground Terminal Isolation Resistance Test for an Individual Equipment . . . . . . .
3-45
3-34
Signal Ground Terminal Isolation Resistance Test for an Equipment Assembly . . . . . . .
3-46
3-35
Measurement of Connector Bonding Resistance . . . . . . . . . . . . . . . . . . . .
3-48
xiv
MIL-HDBK-419A
LIST OF TABLES
Table
Page
1-1
Relative Advantages and Disadvantages of the Principal Types of Earth Electrodes . . . .
1-2
Minimum Requirements for Roof and Down Conductors on Structures Not Greater than
75 Feet (23 Meters) in Height . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-8
1-34
1-3
Minimum Requirements for Roof and Down Conductors on Structures Greater than
75 Feet (23 Meters) in Height . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-34
1-4
Solid Copper Wire - Weight, Breaking Strength, DC Resistance . . . . . . . . . . . . .
1-35
1-5
Frequency of Transient Occurrences . . . . . . . . . . . . . . . . . . . . . . . . .
1-51
1-6
Parameter for Direct Lightning Strike Current . . . . . . . . . . . . . . . . . . . .
1-54
1-7
Peak Currents from Direct Lightning Strikes . . . . . . . . . . . . . . . . . . . . .
1-55
1-8
Transient Surge Amplitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-55
1-9
Transient Occurrence, High-Incident Lightning Areas . . . . . . . . . . . . . . . . .
1-63
1-10
Transient Occurrence, Low-Incident Lightning Areas . . . . . . . . . . . . . . . . . .
1-63
1-11
Generalized Characteristics for Surge Arresters by Type . . . . . . . . . . . . . . . .
1-65
1-12
Typical Maximum Clamp Voltage for Gap Arresters . . . . . . . . . . . . . . . . . .
1-68
1-13
ZNR Type Devices (Molded Case Type) Typical Characteristics . . . . . . . . . . . . .
1-70
1-14
High Energy ZNR Surge Arrester Typical Characteristics . . . . . . . . . . . . . . . .
1-70
1-15
Test Results for Parallel-Connected ZNR . . . . . . . . . . . . . . . . . . . . . .
1-71
1-16
Transients Projected to Occur on Externally Exposed Line in High-Lightning
Incident Area Over 10-Year Period . . . . . . . . . . . . . . . . . . . . . . . . .
1-73
1-17
Transient Surges, Line-to-Ground . . . . . . . . . . . . . . . . . . . . . . . . . .
1-81
1-18
Transient Surges, Line-to-Line . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-82
1-19
Transient Surges Projected to Occur in 10-Year Period on Externally-Exposed
1-20
Landlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-83
Grounding Electrode Conductor Size . . . . . . . . . . . . . . . . . . . . . . . . .
1-107
1-21
Equipment Grounding Conductor Size Requirement . . . . . . . . . . . . . . . . . .
1-110
1-22
Size of Equipment Ground Cables . . . . . . . . . . . . . . . . . . . . . . . . . .
1-119
1-23
1-24
Minimum Torque Requirements for Bolted Bonds . . . . . . . . . . . . . . . . . . .
Compatible Groups of Common Metals . . . . . . . . . . . . . . . . . . . . . . . .
1-141
1-25
Bond Protection Requirements
1-144
1-26
Protective Finishes for Bond Members . . . . . . . . . . . . . . . . . . . . . . . .
1-145
1-27
Metal Connections for Aluminum and Copper Jumpers . . . . . . . . . . . . . . . . .
1-147
1-28
Attenuation Correction Factors for Reinforcing Steel . . . . . . . . . . . . . . . . .
1-157
1-29
Relative Conductivity and Relative Permeability of Common Metals. . . . . . . . . . .
1-163
1-30
Soil Resisitivity
1-179
3-1
Frequency Properties of Standard Sizes of Honeycomb . . . . . . . . . . . . . . . . .
(Ohm-m)
xv/xvi
1-143
3-30
MIL-HDBK-419A
CHAPTER 1
NEW FACILITIES DESIGN CRITERIA
1.1 INTRODUCTION.
This chapter presents the design, installation practices, test and acceptance procedures associated with the
incorporation of effective grounding, bonding, and shielding for a new facility.* The major elements of the
facility covered are the (1) earth electrode subsystem, (2) fault protection subsystem, (3) lightning protection
subsystem, (4) signal reference subsystem, (5) bonding, and (6) shielding.
Design and construction steps for
these six elements are contained in the following sections:
1.2
Earth Electrode Subsystem
1.3
Lightning Protection for Structures
1.4
Fault Protection Subsystem
1.5
Signal Reference Subsystem for New Facilities
1.6
Grounding, Philosophy for Equipments Processing National Security Related Information
1.7
Bonding Practices
1.8
Shielding
1.9
Common-Mode Noise and Instrumentation
1.10 EMP Protection
1.11 Military Mobile Facilities
1.12 Fences
1.13 Inspection and Test Procedures for a New Facility
Secure transmission facility requirements are covered in Section 1.6. Supplemental measures which are needed
to be incorporated in a facility to help reduce common-mode and instrumentation noise problems are presented
in Section 1.9, Common-Mode Noise and Instrumentation. The special construction practices recommended to
reduce facility vulnerability to the electromagnetic pulse (EMP) threat are contained in Section 1.10, EMP
Protection.
Tactical grounding requirements are presented in Section 1.11, Military Mobile Facilities.
Inspection and Test Procedures for a New Facility, provided in Section 1.13, should be utilized in verifying that
recommended practices and procedures are properly implemented and to help establish a perform ante baseline
against which future measurements can be compared.
To obtain optimum performance of electronic equipment and personnel safety while providing adequate
protection against power system faults, EMP, and lightning strikes, thorough consideration must be given to the
grounding system for the building; to the bonds needed and the method of their implementation and to the
shielding needed throughout the building for personnel safety and equipment interference control. For a new
facility, the requirements in each of these areas are defined and appropriate design steps set forth to assure
that the necessary measures are incorporated into the final structure and equipment installation.
*A new facility is considered to be one of new construction or an existing one that will undergo major
renovation or major equipment reconfiguration.
requirements
The project engineer shall determine the grounding
whenever minor equipment reconfigurations are accomplished in existing facilities. Refer to
Chapter 2 for additional information.
1-1
MIL-HDBK-419A
1.2 EARTH ELECTRODE SUBSYSTEM. The earth electrode subsystem establishes the electrical connection
between the facility and earth.
This connection is necessary for lightning protection, useful in power fault
protection, and and in the minimization of noise. The system should be tailored to reflect the characteristics
of the site and the requirements of the facility.*
It must be properly installed and steps must be taken to
assure that it continues to provide a low resistance connection throughout the life of the structure. To achieve
these objectives, first determine the electrical and physical properties of the site, design an earth electrode
subsystem appropriate for the site, install the subsystem in accordance with the recommended procedures, and
finally, measure the earth resistance of the subsystem to verify that it meets the recoin mended goals or design
specifications.
1.2.1 Determination of Site Parameters (Site Survey).
Before beginning the design, conduct a survey of the
site where the earth electrode subsystem is to be installed. Through this survey, determine the resistivity of
the soil, identify significant geological features, gather information on architectural and landscape features
which may influence the design of the subsystem, and review local climate effects. (If possible, conduct this
survey in advance of the final site selection in order to avoid particularly troublesome e locations.)
1.2.1.1 Soil Resistivity.
As the first step of the site survey, measure the resistivity of the soil at several
points over the area of the planned facility. For even the smallest facility, the effective facility area in so far
as the electrode subsystem is concerned is assumed to be at least 15 meters by 15 meters (50 feet by 50 feet).
For larger facilities, the facility areas are assumed to extend at least 6 meters (20 feet) beyond the basic
building or structural outline, i.e., the ground floor plan, substation grid, tower footing, transformer housing,
etc. It is necessary that the soil resistivity be known over the area encircled or covered by the earth electrode
subsystem.
a.
A single soil resistivity measurement is made using the four-probe method (see Volume I, Section
2.4) in the following manner:
(1) At a location near the center of the site, insert the four short probes supplied with the earth
resistance test set into the soil in a straight line as illustrated in Figure 1-1. A convenient probe spacing of 6
to 9 meters (20 to 30 feet) is recommended as a start. If probes are not supplied with the test set or if they
have been lost or misplaced, four metal (steel, copper, or aluminum) rods, 1/4 to 3/8 inch in diameter and 12 to
18 inches in length, may be used.
Drill and tap No. 6-32, 8-32, or 10-24 screws, according to rod size and
securely fasten the test set leads to the rods. Clamps may also be used for connecting the leads to the probes.
*The relationship between the performance of an electronic system and the resistance of the earth ground is
unclear.
The value of 10 ohms earth electrode resistance recommended in Section 1.2.2.la represents a
carefully considered compromise between overall fault and lightning protection requirements and the estimated
relative cost of achieving the resistance in typical situations.
In locations characterized by high soil
resistivities, to achieve 10 ohms could be very expensive. In such locations, examine all elements of the site,
consider the requirements of the planned facility, and then choose the best compromise based on soil conditions,
relative costs, etc.
1-2
MIL-HDBK-419A
(2) Following the manufacturer's instruction, obtain a resistance reading, R, with the test set.
(3) Convert the probe spacing, A, to centimeters. (See Page xvi for metric conversion factors.)
(4) Compute resistivity from
= 6.28AR
EXAMPLE:
(in ohm-cm)
Assume that a resistance of 2 ohms is measured with probe spacings of 20 feet.
Convert 20 feet to centimeters: 20 ft x 30.5 cm/ft = 610 cm
Calculate resistivity
= 6.28 x 610 x 2 = 7662 ohm -cm
A = ELECTRODE SPACING
B = DEPTH OF PENETRATION < A/20
D = DEPTH AT WHICH RESISTIVITY IS DETERMINED = A
Figure 1-1. Measurement of Soil Resistivity
1-3
(l-1)
MIL-HDBK-419A
b.
test area.
The reading obtained indicates the average resistivity of the soil in the immediate vicinity of the
A resistivity profile of the site requires that the above procedure be repeated at many sample
locations over the region being mapped.
For small sites up to 2500 square feet (232 square meters), make at
least one measurement at the center of the site and at each of the four corners of a 50-foot square as shown in
Figure 1-2.
Drive a stake or marker at the locations shown.
Position the potential and current probes in a
straight line with the stake or marker centered between the probes.
location and calculate the resistivity as in step a-4 above.
resistivity for the soil at the site.
Make a resistance measurement at each
Take the average of the five readings as the
If possible, soil measurements should be made during average/normal
weather conditions. Measurements should never be made immediately after a rain or storm.
c.
For larger sites, make measurements every 100 to 150 feet (30 to 45 meters) over the site area.
Include in the site area the locations of support elements such as transformer banks, towers, engine-generator
buildings, etc. Choose a sufficient number of test points to give an indication of the relative uniformity of the
soil composition throughout the area. Be particularly alert for the presence of localized areas of very high or
very low resistivity soils.
NOTE: NOT DRAWN TO SCALE.
Figure 1-2. Resistivity Determination of a Small Site
1-4
MIL-HDBK-419A
1.2.1.2 Geological Effects.
a.
Identify the significant geological features of the site. Specifically, attempt to establish:
(1) the distribution of major soil types (see Volume l, Section 2.3.2 ) to include the locations of
sand and gravel deposits,
(2) major rock formations,
(3) the presence of water sources to include underground streams, and
(4) the depth of the water table.
Utilize test borings, on site inspections, studies of local maps, and interviews with local construction
companies, well drillers, and other local personnel to obtain the desired information.
b.
Evaluate the information provided by these sources for indications of particularly troublesome (or
particularly helpful) characteristics that may influence the design or installation of the earth electrode
subsystem of the facility.
1.2.1.3 Physical Features.
Locate and identify those other physical features that will influence the general
placement of the earth electrode subsystem, the location of test and access points, physical protection
requirements, and the cost of materials and installation. For example, indicate on the general site plan:
a.
the planned physical layout of the building or structure,
b.
locations of paved roads and parking lots,
c.
drainage, both natural and man-made, and
d.
the location of buried metal objects such as pipes and tanks.
1.2.1.4 Local Climate.
a.
Review local climatic conditions and determine the annual amount and seasonal distribution of
rainfall, the relative incidence of lightning, and the depth of freezing (frost line) typical of the area. Obtain
the rainfall and frost line information from the local weather service; project the relative lightning incidence
from the isokeraunic maps given in Volume I, Section 3.4, Figures 3-2 and 3-3.
b.
Record the data and make it a part of the facility files for the site. Immediately, however, use this
information to aid in the design of the earth electrode subsystem for the facility to be constructed at the site.
1-5
MIL-HDBK-419A
1.2.2 Design Procedure.
1.2.2.1 Selection of Electrode Configuration.
Determine what type of earth electrode subsystem is most
appropriate for the facility (complex, building, structure, transformer bank, substation, etc). The directed
configuration is a ring ground outlined in paragraph 5.1.1.1.3 of MIL-STD-188-124A. If this configuration
cannot be employed, alternate configurations meeting these requirements are described in Section 1.2.2.3 of
this volume.
a.
Establish the primary functional requirements to be met by the earth electrode subsystem. For
example.
Lightning. For a facility located in an area of high lightning incidence or a high degree of exposure
to lightning, or both, (see Volume 1, Section 3.4) the earth electrode subsystem must safely dissipate the
lightning energy without melting conductors or overheating the soil (see Volume I, Section 2.8.2.2). Also, the
subsystem must minimize step voltages in areas where personnel are present.
Impulse Properties and RF Impedance Characteristics. If the antenna counterpoise must serve as an
earth electrode subsystem, it must have low rf impedance properties.
Mobile facilities or temporary transportable facilities will generally not justify the
Mobility.
installation of an extensive fixed electrode subsystem. For such facilities, install only a basic system capable
of providing the minimum acceptable lightning and personnel fault protection (see Section 1.11).
Resistance.
At fixed C-E facilities, the earth electrode subsystem should exhibit a resistance to
earth of 10 ohms or less. If 10 ohms is not economically feasible by the ring ground, alternate methods should
be considered.
Paragraph 5.1.1.1.3.2 of MIL-STD-188-124A refers.
Resistance measurements using the
fall-of-potential method shall be accomplished in 3-month increments for 12 months following installation.
Measurements shall be conducted in 21-month intervals after the first year.
b.
Evaluate local conditions.
Soil resistivity. Is soil resistivity low (< 5000 ohm-cm), average (5000 to 20,000 ohm -cm), or high
(> 20,000 ohm-cm)? The higher the soil resistivity, the more complex (and expensive) will be the electrode
subsystem necessary to achieve 10 ohms resistance.
Moisture content. Is the water table near the surface or far below grade, and is it subject to large
seasonal variations? Design the earth electrode subsystem so that it makes and maintains contact with soil that
stays damp or moist year round if at all possible. Penetration of the permanent water table is highly desirable.
Frost line. How deeply does the frost line extend, even during coldest periods? The resistivity of
soil rises greatly (see Volume I, Section 2.3.3) as the soil temperature drops below 32° F. Thus for maximum
stability of electrode resistance, the subsystem should penetrate far enough into the soil so that contact is
always maintained with unfrozen soil.
The earthing techniques described in this chapter are not directly
applicable to permafrost. In permafrost, fault protection must be provided through the use of metallic returns
accompanying the power conductors to insure the existence of a return path to the transformer or generator.
1-6
MIL-HDBK-419A
Personnel protection in permafrost requires an even greater emphasis on the bonding of all metal objects
subject to human contact and to the power system neutral and is described in Volume I, Chapter 2. Because of
the high resistance of permafrost, stray earth currents can be expected to be minimal with consequently
reduced concern with inter-facility power frequency noise problems (see Volume I, Section 2.1.3). In the event
that earth-current related noise problems exist, the common-mode rejection techniques described in Volume I,
Section 6.4 should be applied.
Rock Formations.
Are major rock formations near the surface and are they large enough to
influence the design and layout of the earth electrode subsystem?
In regions of shallow bedrock, vertical
ground rods may not be usable and horizontal grids, wires, or plates must be used. Large rock outcropping or
subsurface boulders may force the alternate routing of conductors or the placement of rods. There is no need
to incur the expense of drilling holes in rock to insert rods or lay wires because the resistivity of rock is so high
that generally the rods or wires would be ineffective.
Architectural
layout.
Design the earth electrode subsystem so that it will not be materially
influenced by the weather shielding effects of parapets and overhangs.
Lightning down conductor placement
and routing will frequently be influenced by architectural considerations. Design the earth electrode subsystem
to accommodate such considerations by providing convenient connection points near the down conductors.
Route the interconnecting cable of the earth electrode subsystem near down conductors to avoid long
extensions between the down conductor and the effective grounding point.
Configure the earth electrode
subsystem such that convenient connections are possible between the earth electrode subsystem and grounding
conductors of the power and signal ground systems inside the facility.
Landscape features.
Preferably locate ground subsystem conductors under sodded areas or those
otherwise covered with vegetation.
Locate conductors to take maximum advantage of the wetting effects of
runoff or drainage water from the roof, parking lots, etc.
Try to avoid placing major portions of this earth
electrode subsystem under extensive paved areas such as roads and parking lots.
c.
Considering the relative advantages and disadvantages given in Table 1-1, choose a basic type of
electrode most appropriate for meeting the functional requirements of the facility at the site under
construction.
d.
Estimate the relative costs to meet the objectives with the different types of configurations.
Include the cost of materials, installation costs, and relative maintenance and upgrading costs.
1-7
MIL-HDBK-419A
Table 1-1
Relative Advantages and Disadvantages of the Principal Types
of Earth Electrodes
Type
Ring Ground
Disadvantages
Advantages
Straightforward design. Easy to install
Not useful where large rock
(particularly around an existing facility).
formations are near surface.
Hardware readily available. Can be
extended to reach water table.
Horizontal Bare
Wires (Radials)
Can achieve low resistance where rock
Subject to resistance fluctuations
formations prevent use of vertical rods.
with soil drying.
Low impulse impedance. Good rf
counterpoise when laid in star pattern.
Horizontal Grid
Minimum surface potential gradient.
Subject to resistance fluctuations
(Bare Wire)
Straight for ward installation if done
with soil drying if vertical rods not
before construction. Can achieve low
used.
resistance contact in areas where rock
formations prevent use of vertical rods.
Can be combined with vertical rods to
stabilize resistance fluctuations.
Vertical Rods
Straightforward design. Easiest to
High impulse impedance. Not useful
install (particularly around an existing
where large rock formations are near
facility). Hardware readily available.
surface. Step voltage on earth
Can be extended to reach water table.
surface can be excessive under high
fault currents or during direct
lightning strike.
Plates
Can achieve low resistance contact in
Most difficult to install.
limited area.
Incidental
Can exhibit very low resistance.
Little or no control over future
Electrodes
alterations. Must be employed with
(Utility pipes,
other made electrodes.
building
foundations,
buried tanks)
1-8
MIL-HDBK-419A
1.2.2.2 Calculation of Earth Resistance.
Once the most appropriate configuration is chosen for the facility, calculate the resistance to earth for the
configuration.
If the calculated resistance meets the design goal (or requirement), complete the design to
include all necessary interconnections.
To illustrate this design procedure, assume that a 100 ft x 160 ft
rectangular configuration like that shown in Figure 1-3 is initially chosen.
Further, assume that the soil
resistivity measurements made during the site survey showed an average resistivity of 10,000 ohm-cm for the
area. In addition, the site survey indicated that all rock formations are at depths greater than 10 feet; the
water table never drops more than 5 feet below grade; and the frost line extends only to 1 foot below grade.
Therefore, 10-foot ground rods are initially selected for evaluation.
(The minimum rod diameter required in
MIL-STD-188-124A, para 5.1.1.1.4 is 3/4 inch.)
a.
Determine the resistance of one of the ground rods from Figure 1-4. First, place a straight edge
between the point marked 3/4 on line “d” and the point marked 10 on line
straightedge crosses.
Indicate on line “q” where the
Next, place the straightedge between the point just marked on “q” and the
10,000 ohm-cm point on the vertical line labeled “Resistivity.”
Read the resistance as 32 ohms at the point
where the straightedge crosses the vertical line labeled “Resistance.”
b.
Assume an initial spacing of 20 feet or twice the rod length (see Volume I, Section 2.6.2) between
rods. Figure 1-3 shows that 26 rods are required to encircle the structure.
Use Figure 1-5 to determine the
relative lowering of the resistance of one rod that is produced by 26 rods in parallel. (The answer is about 5.5
percent.) Thus the resistance of the 26 rods in 10,000 ohm-cm soil is
R = 32 x 0.055 = 1.76 ohms.
Figure 1-5 primarily applies to ground rods laid out in a straight line or around the perimeter of a site whose
dimensions are large with respect to the rod spacing.
If the rods are distributed in a grid pattern, as will
frequently be done for substations, use Figure 1-6 to estimate the net resistance. In many instances, the
answers provided by Figures 1-5 and 1-6 will agree. For this example, the resistance multiplier given by Figure
1-6 for 26 rods over an estimated area of 16,000 square feet (100' x 160') is 0.056 for a net resistance of
1.9 ohms.
1.2.2.3 Alternate
Configurations.
Nonideal sites will frequently be encountered. For example, large rock
formations may be present which prevent the uniform placement of ground rods around the site; bed rock may
be relatively near the surface; the water level may drop to several feet below grade; the soil resistivity may be
very high; or architectural and landscape requirements may preclude locating ground rods at particular points.
In such cases, modify the electrode configuration to conform to the constraints while achieving the desired
resistance. Typical suggested alternatives are:
1-9
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Figure 1-3.
1-10
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Figure 1-4. Nomograph for Determining the Resistance to Earth
of a Single Ground Rod (l-1)
a.
Change number of ground rods. The above example shows that fewer rods could be used and still
meet the 10-ohm goal. Thus, if rock outcropping were present at certain points around the perimeter, it would
be permissible to omit some of the rods. Since 10 ohms (the net effective resistance desired) is 31 percent of
32 ohms (the resistance of one 10-foot rod in soil of 10,000 ohm-cm), Figure 1-5 shows that as few as 3 rods
would be acceptable. On the other hand, if the soil resistivity is very high more rods will be necessary.
1-11
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Figure 1-5. Effective Resistance of Ground Rods When
Arranged in a Straight Line or a Large Circle
1-12
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Figure l-6. Graph of Multiple-Rod Resistance Ratio (l-2)
1-13
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b.
Use longer ground rods.
Rods longer than 10 feet (can be realized by assembling 10-foot sections)
may be used in high resistivity soil in place of a larger number of 10-foot rods. Where the ground water table is
greater than 10 feet below the surface at any season of the year or where the frost line is greater than 10 feet,
use the longer rods to maintain contact with the permanently moist, unfrozen soil. Use Figure 1-4 to estimate
the length needed, given the soil resistivity.
c.
Use horizontal wires or grids instead of vertical rods. Where bedrock or other obstacles prevent the
effective use of vertical rods, horizontal wires, grids, or radials should be used. (See Volume I, Section 2.6.1.2
for design data and equations.)
d.
Lower the soil resistivity through chemical enhancement (salting). Where the above alternatives are
not possible or are not cost effective, chemical enhancement is frequently the only choice left. Consult
Volume I, Section 2.9 before deciding what to do in this regard.
1.2.3 Design Guidelines.
a.
At each facility supplied by electric power, at least one ground rod should be installed near the
service disconnecting means and bonded to the earth electrode subsystem. If the transformer is located on the
site, a bare 1/0 AWG wire or cable should interconnect the ground rod at the transformer with the earth
electrode subsystem at the first service disconnect for lightning protection purposes.
b.
For lightning protection purposes, all facilities large or small or located in areas of low or high
lightning incidence will require an earth electrode subsystem, described in the previous section. Facilities
having structural extensions or equipment protrusions (such as antenna elements or towers) extending above the
surrounding terrain should have a continuous earth electrode subsystem enclosing each facility or should have
individual earth electrode subsystems connected together. See paragraph 5.1.1.3.8.1 of MIL-STD-188-124A.
c.
Most installations will require many interconnected ground rods. The configuration shown in Figure
1-3 is adequate for most facilities.
(The number of ground rods actually required at a given location will be
determined by the resistivity of the soil and the configuration of the installation.) Three-meter (ten-foot)
ground rods installed at 20-foot intervals around the perimeter of the structure provide good utilization of the
effective radius of the rod while providing several points of contact with the earth. If longer rods are required
to reach the water level, to make contact with lower resistivity soils, or to penetrate below the frost line,
greater spacings may be employed. The nominal spacing between rods should be between one and two times the
length of the rod however, it is necessary for a ground rod to be placed near each lightning down conductor, so
spacings should be limited to not more than 50 feet in order to conform to lightning protection requirements
(see Section 1.3.2.2.2).
d.
The rods and interconnecting cable comprising the earth electrode subsystem should be positioned
0.6 to 1.8 meters (2 to 6 feet) outside the drip line of the building or structure to insure that rain, snow, and
other precipitation wets the earth around the rods.
e.
For facilities which do not conform to a rectangular or square configuration, lay out the rod field to
generally follow the perimeter of the structure as illustrated in Figure 1-7.
1-14
MIL-HDBK-419A
f.
Where two or more structures or facilities are located in the same general area (less than 200 feet)
and are electrically interconnected with signal, control, and monitor circuits, either provide a common earth
electrode subsystem, or interconnect the separate earth electrode subsystems with two buried bare cables. A
common example of an installation where two separate structures are involved is a radar or communications
site where the equipment shelter is adjacent to the antenna tower. Signal cables (both coaxial and waveguide),
control cables, and power lines typically run between the tower and the shelter. The tower, being taller than
the shelter, is more susceptible to lightning strikes.
To minimize voltage differentials between the two
structures, the facilities should effectively share a common earth electrode subsystem. Separate structures
spaced closer than 6 meters (20 feet) should have a common earth electrode subsystem installed that encircles
both facilities as shown by Figure 1-8. Figure 1-9 shows the recoin mended arrangement when separations equal
to or greater than 6 meters (20 feet) but less than 60 meters (200 feet) are encountered.
One of the
interconnecting buried bare cables may also serve as a guard for buried signal or power cables. A typical site
installation involving three structures separated less than 200 feet is illustrated in Figure 1-10. Structures or
facilities having no interconnecting cables and separated by a distance greater than 60 meters (200 feet)
generally do not require their earth electrode subsystems be interconnected.
There may be a number of incidental, buried, metallic structures in the vicinity of the earth
g.
electrode subsystem. These structures should be connected to the subsystem to reduce the danger of potential
differences during lightning or fault protection; their connection will also reduce the resistance to the earth of
the electrode subsystem. Such additions to the earth electrode subsystem should include the rebar in concrete
footings, and buried tanks and pipes.
h.
To minimize resistance variations caused by surface drying of the soil and by the freezing of the soil
during winter and to minimize the possibility of mechanical damage to ground rods, connections, and
interconnecting cables, the tops of ground rods should be at least 0.3 meters (1.0 foot) below grade level. Bury
the bare 1/0 AWG interconnecting cable at least 0.45 meters (1.5 feet) below grade level. The recommended
practices are illustrated in Figure 1-11.
i.
If the subsystem is installed after foundations are poured, cables are installed, utility pipes installed,
etc., make proper provisions for performing the needed interconnections between the water system, lightning
down conductors, structural steel, buried lines and cables, and the electrodes.
Access to the earth electrode subsystem should be provided through the installation of one or more
j.
grounding wells at each site. Two acceptable types of grounding wells are illustrated in Figures 1-12 and 1-13.
Either clay pipe or poured concrete may be used.
Removable access covers must be provided. In very large
structures, particularly those in which grounding grids are installed underneath, the grounding well or wells may
be located inside the building in an accessible location.
More than one grounding well may be necessary
depending upon the size of the facility, the extent of the electrode subsystem, and the degree of accessibility to
the electrodes deemed desirable. Locate at least one of the ground wells in an area with access to open soil so
that resistance checks of the earth electrode subsystem can be made once the building is in use. The top view
of a representative ground rod installation shown in Figure 1-14 illustrates the required connections to the
signal reference subsystem, the lightning protection subsystem, and the facility ground network.
1-15
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Figure 1-7.
1-16
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Figure 1-8. Electrode Configuration for Adjacent Structures
Figure 1-9. Electrode Configuration for Closely Spaced Structures
1-17
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Figure 1-10. Grounding System for Typical Radar Installation
1-18
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Figure 1-11. Details of Ground Rod/Earth Electrode Subsystem Installation
1-19
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Figure l-12. Concrete Grounding Well
Figure 1-13. Typical Grounding Well Installation
1-20
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Figure 1-14.
1-21
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1.2.4 Installation Practices.
a.
Schedule the installation of the earth electrode subsystem so that any needed excavation, such as
hole and trench digging, can be performed while other excavating, clearing, and earth moving operations
associated with construction of the facility are in progress. If the subsystem is installed prior to completion of
other earth moving operations, take the precautions necessary to assure that the components are not damaged
or broken.
b.
Take special care to ensure that all metallic lines, such as water lines, sewer lines (if metal),
armored cable, etc., are carefully bonded to the earth electrode subsystem. Bonding jumpers of 1/0 AWG, or
larger, bare copper wire are to be used for this purpose.
c.
Before covering the earth electrode subsystem with backfill dirt or otherwise rendering it
inaccessible, make visual checks of all joints and connections to check mechanical integrity, to verify the
absence of voids or other indications of poor bonding, and to see that all required interconnections are made.
d.
All bonds in concealed locations must be brazed or welded. Any bonds between dissimilar metals,
such as between a copper wire and cast iron or steel pipe, must be thoroughly sealed against moisture to
minimize corrosion. Bolted clamp connections are to be made only in manholes or in grounding wells and are to
be readily accessible for verification of integrity.
e.
Drive rods only into undisturbed earth or into thoroughly tamped or compacted filled areas. Rods
and cables should be placed in the backfill around foundations only after the soil has been compacted or has had
adequate time to settle. Do not drive or lay rods in gravel beds which have been installed for drainage purposes
unless the rods extend through such beds far enough to provide at least 1.8 to 2.4 meters (6 to 8 feet) of contact
with the undisturbed earth underneath. Do not lay horizontal cables in such beds under any circumstances.
f.
Rods may be driven either by hand sledging, slide hammer, or with the use of power drivers. Use
driving nuts to prevent damage to the driven end, particularly, if two or more sections are to be joined. Deep
driven rods or those driven into hard or rocky soil generally require the use of power drivers with special driving
collars to prevent damage to the rod.
Attach the interconnecting cable to the rods by brazing, welding, or clamping. Use bolted, clampedg.
type connections only if the tops of the rods are accessible through grounding wells and a periodic maintenance
program is established to verify the integrity of the connection on a regular basis.
h.
As rods are installed, make a one-time resistance check of each rod once it reaches its intended
depth. After fulfilling the requirement of paragraph 5.1.1.1.7 of MIL STD 188-124A, resistance measurements
shall be conducted on the earth electrode subsystem (as a system) at 3-month intervals for 12 months after
installation and every 21 months thereafter. Use the measurement procedure outlined in Section 2.2.2.2.l.f. In
this way a continuous check is made of the electrode design. If the measured resistance of the rods is less than
the calculated resistance, the use of fewer rods may be acceptable as long as the minimum number required for
terminating lightning down conductors is installed. On the other hand, if the measured resistance of the rods is
greater than calculated, additional rods or longer rods should be installed during the construction stage rather
than waiting until the facility is completed to add additional rods.
1-22
MIL-HDBK-419A
1.3 LIGHTNING PROTECTION FOR STRUCTURES.
1.3.1 Principles of Protection.
A structure, for lightning protection purposes, is defined as a building mast, tower, or similar self-supporting
object other than power lines, power stations, and substations. To provide minimum protection for structures
against direct lightning strikes, four requirements must be fulfilled:
a.
an air terminal must be provided to intentionally attract the leader stroke,
b.
a path must be established that connects this terminal to earth with such a low impedance that the
discharge follows it in preference to any other,
c.
a low resistance connection must be made with the earth electrode subsystem, and
d.
a low impedance interface must be established between the earth electrode subsystem and earth.
These conditions are met when a lightning discharge is permitted to enter or leave the earth while passing
through only conducting parts of a structure.
The conditions can be satisfied by one of two methods, each
having specific applications. These methods are:
a.
the installation of an integral protection system consisting of air terminals interconnected with roof
and down conductors to form the shortest practicable distance to ground, or
b.
the installation of a separately mounted protection system of one of two types:
(1) a mast type consisting of a metal pole which acts as both air terminal and down conductor (a
nonconductive pole may be used if provided with metal air terminals and down conductors connected to an earth
ground), or
(2) two or more poles supporting overhead guard wires connected to an earth electrode subsystem
with down leads.
1.3.2 Integral Protection System.
When designing and installing an integral system of protection, perform the
following steps:
a.
Erect air terminals on the points of highest elevation and on other exposed areas to intercept the
stroke before it has an opportunity to damage the structure or equipments or components mounted thereon.
The terminal points must be placed high enough above the structure to eliminate the danger of fire from the
arc.
b.
Install roof and down conductors so that they offer the least possible impedance to the passage of
stroke currents between the air terminals and the earth.
The most direct path is the best. The radius of
conductor bends shall not be less than 8 inches nor shall the angle of such bends be less than 90 degrees.
Additional information may be found in para 3-12.5 of NFPA 78.
1-23
MIL-HDBK-419A
c.
Distribute ground connections symmetrically about the circumference of the structure rather than
grouping to one side.
d.
Interconnect all metal objects close to the discharge path to prevent side flashes. (Representative
interconnections are shown in Figure 1-15.)
e.
Make certain that the mechanical construction of the air terminal system is strong and that the
materials used offer high resistance to corrosion.
1.3.2.1 Air Terminals.
Air terminals (lightning rods) must intercept, or divert to themselves, any lightning
stroke that might otherwise strike the building or structure being protected.
Antennas and their associated
transmission lines/supporting structures shall be protected by air terminals meeting the requirements of
1.3.2.1.l.a rather than be dependent upon transient protection/suppression devices described in 1.3.3.5.22.
1.3.2.1.1 Size and Materials. To keep from exploding, igniting, or otherwise being destroyed, air terminals
should be made of copper, aluminum, brass, or bronze. The minimum sizes are 1.27 cm (1/2 inch) in diameter
for solid copper, brass, or bronze rods and 1.6 cm (5/8 inch) in diameter for solid aluminum rods.
a.
Air terminals must extend at least 25.4 cm (10 inches) directly above the object being protected and
be of sufficient height so as to provide a 1:1 zone of protection for adjacent objects (antennas and associated
support/control towers, etc).
Rather than choosing the shortest terminal which will provide this minimum
height, all parts of the structure must be checked graphically or analytically in the manner described in the
next section to determine if the zone of protection provided by the terminal is adequate.
Where taller
terminals are required to provide complete protection, adequate support and bracing as specified by ANSI-C5.1
(2.1.15) must be provided.
b.
Where air terminals are mounted on or very near (less than 1.5 meters (5 feet)) to vents or stacks
which emit potentially explosive or ignitable dusts, vapors, or gases, provide additional clearance.
(1) Over hooded vents emitting explosive substances under natural draft, the air terminals should
extend at least 1.5 meters (5 feet) above the opening.
(2) Above open stacks emitting explosive substances under forced drafts, air terminals should
extend at least 4.5 meters (15 feet) above the opening.
1.3.2.1.2
a.
Location.
Locate air terminals along the ridges of gable, gambrel, and hip roofs in the manner illustrated in
Figure 1-16.
b.
Place them on the corners and along the edges of gently sloping roofs as shown in Figure 1-17.
Gently sloping roofs are defined as (1) having a span of 40 feet or less with a rise-to-run ratio, i.e., pitch, of
one-eighth or less or (2) having a span greater than 40 feet and a rise-to-run ratio of one-quarter or less.
1-24
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Figure 1-15. Grounding Practices for Lightning Protection
1-25
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Figure 1-16.
1-26
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Figure 1-17. Location of Air Terminals on Gently Sloping Roofs
c.
On flat roofs position the air terminals around the perimeter in the manner shown in Figure 1-18.
Provide additional air terminals placed at 50-foot intervals over the interior of flat and gently sloping roofs
which exceed 50 feet in width.
d.
Terminals are to be provided within 2 feet of corners, the end of ridges, or edges of main roofs.
e.
Terminals less than 24 inches in height are to be spaced 20 feet or less. Terminals 24 inches or
taller may be placed at intervals not exceeding 25 feet.
f.
terminals.
Ensure that no part of the structure extends outside the cone of protection established by the air
Determine the cone of protection by preparing a simple scaled profile drawing of the structure and
then superimposing a 45-degree (a 1:1 cone of protection) triangle on the profile. The apex of the triangle
should coincide with the tip of the air terminal whose protected zone is being verified, as illustrated in Figure
1-19. Alternatively for existing structures, the field expedient method illustrated in Figure 1-20 showing a 2:1
cone of protection can be used to determine the coverage of prominent projections. This method is particularly
useful for small structures.
To determine if all parts of a flat roofed structure such as vents, pipes, cabling, or raised extensions are
protected, use the method illustrated in Figure 1-21 to calculate the zone protected by two vertical terminals.
This method can also be used to determine the coverage provided by vertical masts or horizontal wires. In
Figure 1-21 point P represents the point of discrimination. That is, the point of departure of the final stepped
leader of the downward traveling stroke (see Volume I, Section 3.2).
1-27
To determine if the air terminals are
MIL-HDBK-419A
actually the nearest objects to point P, use P as a center and swing an arc of radius X through the tips of the
terminals.
Let the value of this radius X be 100 feet, since 100 feet represents the shortest length usually
associated with a stepped leader (see Volume I, Section 3.2).
Because of the large differences between the
height of typical terminals and the striking distance X, graphical determination of the protected zone will
usually be awkward.
For greater accuracy, calculate the critical distances through the use of the following
equation:
(l-2)
which is valid for S
2X. In this equation, G is the minimum height between the terminals that is completely
protected; H is the height of the terminals, S is the spacing between terminals, and X is the radius of the arc.
Sample calculation.
To illustrate the application of this method, suppose it is necessary to determine the
minimum spacing between 3-foot air terminals that will guarantee that all parts of a flat roof remain in the
protected zone. In other words, what value of S corresponds to G = 0 in Equation 1-2? To perform the
calculation, first set G = 0:
Rearranging to be
and squaring both sides produces
Eliminating X2 and changing signs on both sides of the equation yields
or
Substituting H = 3 feet and X = 100 feet in this last equation shows that S must equal 48.6 feet or less to
guarantee that all parts of the roof remain within the protected zone.
1-28
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Figure 1-18.
1-29
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Figure 1-19.
1-30
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Figure 1-20.
1-31
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Figure 1-21.
1-32
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1.3.2.2 Groundi ng Conductors.
Provide each air terminal with a two-way path to earth through the
installation of roof and down conductors conforming to Table 1-2 for structures not greater than 75 feet in
height and conforming to Table 1-3 for structures greater than 75 feet in height. An exception is that air
terminals located on prominent dormers extending less than 16 feet from the main structure need have only one
connecting path from the terminal to the main down conductor as shown in Figure 1-22. Additional information
on copper wires is contained in Table 1-4.
1.3.2.2.1 Roof Conductors.
a.
Roof conductors should be routed along ridges of gable, gambrel, and hip roofs, and around the
perimeter of flat and gently sloping roofs.
Roof grounding conductors routed throughout decks, flat surfaces, and flat roofs should be
,
interconnected to form closed loops to insure that all air terminals have at least two paths to earth.
b.
c.
Ridge conductors may drop from a higher to a lower roof level without installing an extra down lead
at the point of intersection of the two roof levels if there are not more than two air terminals on the lower roof
level.
d.
On roofs that exceed 50 feet in width, additional conductors are to be provided to interconnect the
air terminals required to protect large flat areas (see Figure 1-18). One additional conductor for each 50 feet
in width is necessary. For example, on roofs 50 to 100 feet wide, add one additional run; on roofs 100 to 150
feet wide, add two additional runs; etc.
These additional runs must be interconnected together and to the
perimeter conductor at 150-foot intervals with cross conductors as illustrated in Figure 1-18.
1-33
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Table 1-2
Minimum Requirements for Roof and Down Conductors on
Structures Not Greater than 75 Feet (23 Meters) in Height (l-3)
Material
Type of Conductor
Copper
Aluminum
Cable
Strand Size
Weight per 1000 feet*
Area*
DC Resistance
14 AWG
187-1/2 pounds
59,500 Cir roils
0.176 ohms/1000 ft
12 AWG
95 pounds
98,500 Cir roils
0.176 ohms/1000 ft
Solid Strip
Thickness
Width
DC Resistance
14 AWG
1 inch**
0.176 ohms/1000 ft
12 AWG
1 inch**
0.176 ohms/1000 ft
Solid Rod
Weight Per 1000 feet
DC Resistance
186-1/2 pounds
0.176 ohms/1000 ft
95 pounds
0.176 ohms/1000 ft
Tubular Rod
Weight per 1000 feet
Wall Thickness
DC Resistance
187-1/2 pounds
0.032 inch
0.176 ohms/1000 ft
95 pounds
0.064 inch
0.176 ohms/1000 ft
* Acceptable substitutes are No. 2 AWG copper cables and 1/0 AWG aluminum cables.
**This is the minimum width for a strip void of perforations.
If perforated, the width shall be increased
equal to the diameter of the perforations.
Table 1-3
Minimum Requirements for Roof and Down Conductors on
Structures Greater than 75 Feet (23 Meters) in Height (1-3)
Material
Copper
Aluminum
Minimum
Weight
Weight Per
DC Resistance
Wire Strand Size*
Per Foot
1000 Feet
Per 1000 Feet
AWG
14
12
Ounces
6
3
Pounds
375
190
Ohms
0.088
0.088
*Equivalent standard AWG cable: Copper - 2/0, Aluminum - 4/0
1-34
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Table 1-4.
1-35
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Table 1-4.
1-36
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SEE
NOTE 2
NOTES:
1 – DEAD ENDS ARE NOT ACCEPTABLE ON MAIN RIDGES OR ON RIDGES OF
DORMERS OR SIDE WINGS AS HIGH OR HIGHER THAN THE MAIN RIDGE,
2- TOTAL CONDUCTOR LENGTH NOT TO EXCEED 16 FEET.
Figure 1-22. Criteria for Dead End Coverage
e.
Maintain a horizontal or downward course with roof conductors.
Provide "U" or "V" (up and down)
pockets with a down conductor from the base of the pocket (see Figure 1-23(a)) to ground or to a convenient
lead of the main down conductor.
f.
Route conductors through or around obstructions which lie in a horizontal plane with the conductor
(Figure 1-23(b) and (c)). Bends in the conductor should not include an angle of less than 90 degrees and should
maintain a radius of 8 inches or greeter (Figure 1-23(d)). In particular, re-entrant loops should be avoided (1-5).
When routing around obstructions, wide gradual bends are preferred.
Other recommended practices are
illustrated in Figures 1-23(e) thru (h).
g.
Securely attach the conductors directly to the ridge roll or roof with UL-approved fasteners every 3
h.
Conductors may be coursed through air up to 0.9 meters (3 feet) without support.
feet.
With an
acceptable support such as a 1.9 cm (3/4-inch) copper-clad ground rod or its equivalent, securely fastened at
each end, a conductor may be coursed up to 1.8 meters (6 feet) through air.
1.3.2.2.2 Down Conductors.
a.
Course down conductors over the extreme outer portions of the structure and separate them as far
apart as possible. Preferred locations are at diagonally opposite corners on square or rectangular structures and
symmetrically distributed around cylindrical structures.
b.
Locate down conductors as close as practical to air terminals and to the most convenient places for
attaching the conductors to the earth electrode subsystem of the structure.
equally and symmetrically spaced about the perimeter of the structure.
1-37
The down conductors should be
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Figure 1-23. Recommended Construction Practices for Integral Lightning Protection Systems
1-38
MIL-HDBK-419A
c.
At least two down conductors are required on all structures except on slender objects like flag poles,
antenna masts (not substantial towers), light poles, and the like.
d.
Provide one additional down conductor for each additional 30 meters (100 feet) or fraction thereof
on structures having a perimeter exceeding 75 meters (250 feet). On structures having flat or gently sloping
roofs and on irregular-shaped structures, the number of down conductors should be such that the length of the
average roof conductor joining them does not exceed 30 meters (100 feet). On structures higher than 18 meters
(60 feet) where down conductors are required, install at least one additional down conductor for each 18 meters
(60 feet) of height or fraction thereof; however, the spacing between down conductors need not be less than 15
meters (50 feet).
e.
Down conductors are to be provided or located appropriately to avoid dead ends in excess of 4.8
meters (16 feet) in length. See Figure 1-22, Note 1.
f.
Maintain down conductors in a downward course with routing around or through any obstruction
which may lie in the path. Sharp bends or turns are to be avoided with necessary turns limited to not less than
90 degrees and not less than 20 cm (8 inches) in radius.
Where large re-entrant loops (i.e., those with greater than 90-degree turns) cannot be avoided, e.g.,
g.
around cornices or over parapets, the conductor should be routed to ensure that the open side of the loop is
greater than one-eighth the length of the remaining sides of the loop.
It is advised, however, to course the
conductor through holes or troughs through the obstacles and avoid the loop completely (as shown in
Figure 1-23(e)) whenever possible.
h.
On structures with overhangs such as antenna towers with extended platforms or buildings utilizing
cantilevered
construction, run the down conductors vertically through the interior of the structure (l-5).
Internally routed conductors must be enclosed in nonmetallic, noncombustible ducts.
i.
Substantial metal structural elements of buildings may be substituted for regular lightning
conductors where, inherently or by suitable electrical bonding, they are electrically continuous from the air
terminal to the earth electrode connection.
The structural elements must have a conducting cross-sectional
area, including that in joints, at least twice that of the lightning conductor that would otherwise be used. There
need be no difference whether such conductors are on the interior or exterior of the structure when used for
down conductors. Steel frame buildings encased in bricks or other masonry products must have external air
terminals and roof conductors installed and bonded directly to the structural members to keep the lightning
discharge from having to penetrate the masonry shell to reach the frame members.
1.3.2.3 Fasteners.
a.
Securely attach air terminals and roof and down conductors to the building or other object upon
which they are placed.
b.
Fasteners (including nails, screws, or other means by which they are attached) should be substantial
in construction, not subject to breakage, and should be of the same material as the conductor or of a material
that will preclude serious tendency towards electrolytic corrosion in the presence of moisture because of
contact between the different metals. (For further information on corrosion, see Volume I, Section 7.8. )
1-39
MIL-HDBK-419A
c.
Keep all hardware, component parts, and joints that are not welded or brazed and that require
inspection for maintenance and repair readily accessible.
d.
Any special fixtures required for access should be permanently attached to prevent loss. However,
appropriate locks or other devices essential to safety, security, and physical protection of the hardware or of
the area in which it is located may be used.
1.3.3 Separately Mounted Protection Systems.
1.3.3.1 Mast Type.
a.
No part of the structure being protected should extend outside the protected zone as calculated by
the procedure illustrated by Figure 1-19 (a conservative estimate for two masts can be made with the aid of
Figure 1-24).
b.
Where it is impractical to provide a common mast to provide protection for an entire structure,
additional masts should be provided.
c.
If the pole is made of a nonconducting material, provide an air terminal extending not less than 0.6
meters (2 feet) nor more than 0.9 meters (3 feet) above the top of the poIe.
d.
Connect the base of the mast (if metal) or the down conductors to the earth electrode subsystem of
the protected structure with at least a No. 6 AWG copper conductor or equivalent.
Figure 1-24. The Protected Zone Provided by Two Vertical Masts
1-40
MIL-HDBK-419A
1.3.3.2 Overhead Ground Wire Type.
a.
If the poles are of a nonconducting material, an air terminal shall be securely mounted on the top of
each pole, extending not less than 0.45 meters (1.5 feet) above the top of the pole. Down conductors are run
down the side of the pole or the guy wire may be employed as the conductor as shown in Figure 1-25. If the guy
wire is used, it shall meet the requirements of paragraph 1.3.2.2 and both this wire and the overhead ground
wire are dead-ended at the pole.
separate cable.
The overhead ground wire and the guy wire shall be interconnected with a
Down conductors and guy wires used as down conductors are to be connected to the earth
electrode subsystem of the structure being protected.
Guy wires not located near existing earth electrode
subsystems shall be grounded either to their respective ground anchor (by use of an interconnecting cable) or to
a separate ground rod.
b.
The height of the poles should be sufficient to provide a clearance of not less than 1.8 meters (6
feet) between the overhead ground wire and the highest projection on the building. When the overhead ground
wire system is used to protect stacks or vents which emit explosive dusts, vapors, or gases under forced draft,
the cable is installed so that it has a clearance of at least 4.5 meters (15 feet) above the object receiving
protection.
c.
With either the mast type or the overhead ground wire type of system, the pole is placed at a
distance from the structure that is at least one-third the height of the structure, but in no instance less than 1.8
meters (6 feet). Figure 1-25 refers.
AIR TERMINAL
AIR TERMINAL
Figure 1-25. Overhead Ground Wire Lightning Protection System
1-41
MIL-HDBK-419A
1.3.3.3
Waveguide
Installation
and
Grounding.
Waveguide
between the antenna and the associated
transmit/receive equipment should be grounded in the following manner.
a.
Each waveguide shall be bonded to the down conductor of the air terminal at the top near the
antenna and also at the bottom near the vertical to horizontal transition point. The waveguide shall al SO b e
bonded to the antenna tower at the same points as well as at an intermediate point if the tower exceeds
60 meters (200 feet).
b.
All waveguide support structures shall be bonded to the tower.
The waveguides and supporting
structure shall be bonded together at the waveguide entry plate and connected to the earth electrode
subsystem.
c.
All waveguides, conduit or piping entering a building shall be bonded to the waveguide entry plate,
then to the earth electrode subsystem (see Figures 1-26 thru 1-31). For waveguide penetrations of a shielded
enclosure or entry plate see Volume 1, Section 10.4.2.4.
d.
Rigid waveguides within 1.8 meters (6 feet) of each other should be bonded together through the
entry plate or by means of a crimp type lug fastened under the waveguide flange bolts and No. 6 AWG wire.
The bond shall be extended to the bus at the waveguide entry point and connected to the earth electrode
subsystem.
e.
Determine location of ground strap position as shown in Figure 1-28A and remove waveguide jacket.
The ground strap is made from a piece of waveguide as detailed in Figures 1-29 and 1-30. Clean mating
surfaces (waveguide and strap) with solvent or cleaning fluid.
f.
Wrap the strap with No. 14 AWG copper wire (for 8 GHz waveguide as shown Figure I-28A). For
4 GHz waveguide, use No. 10 AWG solid copper wire.
Use adjustable stainless steel clamps as required to
secure the strap. Tighten screw until the clamp grips firmly. Excessive tightening could damage the waveguide
and impair the electrical characteristics. Weatherproof with Scotch Guard or equivalent and tape.
An alternate method of securing the strap to the waveguide is to use wrap-around heat shrink to
g.
cover the bond and to maintain weatherproofing. Solder one end of a solid copper wire (#10 for 4 GHz and #14
for 8 GHz waveguide) to one end of corrugated portion of the ground strap. Align the corrugated section of the
ground strap with the exposed section of the waveguide (see Figure 1-28 B). Tightly wrap the wire around the
ground strap and waveguide and solder the end of the wire to the ground strap for securing purposes. Apply the
wrap-around heat shrink around the waveguide and heat according to the manufacturer’s instructions.
h.
Remove all sharp and rough edges on ground strap.
i.
An alternate method for grounding waveguide is also shown on Figure 1-26.
1-42
MIL-HDBK-419A
NOTE: To satisfy HEMP requirements, peripherally bond waveguide to waveguide entry plate.
Figure 1-26. Waveguide Entry Plate Detail
1-43
MIL-HDBK-419A
Figure 1-27.
1-44
MIL-HDBK-419A
Figure 1-28A.
1-45
MIL-HDBK-419A
Figure 1-28B. Heat Shrink Grounding
Figure 1-29. Ground Strap Detail for Elliptical Waveguide
Figure 1-30. Strap Cutting Detail for Elliptical Waveguide
1-46
MIL-HDBK-419A
Figure 1-31. Typical Communication Cable Entry Installation
1-47
MIL-HDBK-419A
Figure 1-32. Ground Strap Detail
Figure 1-33. Grounding Steps for Cables
1-48
MIL-HDBK-419A
1.3.3.4 Cable Installation and Grounding.
Cables which enter a facility shall be installed generally using
Figure 1-31 as a guideline. The final design shall rest with the designer; however, the following steps apply in
general. (Figures 1-32 and 1-33)
a.
Remove outer cable jacket very carefully so as not to damage the cable shield (see Figure 1-33,
step 1).
b.
Preform ground strap to fit cable diameter and secure the first hose clamp as outlined in the next
step (see Figure 1-33, step 2)
c.
Fold back ground strap (about 3.2 cm (1-1/41”) long) over hose clamp and cable for a snug fit. Secure
second hose clamp around the folded strips of the ground strap described in the next step (see Figure 1-33,
step 3).
d.
For small diameter cable use a No. 6 AWG 7-strand copper wire with a lug connector on the other
end. Secure the stranded cable using the same method as for the strap.
e.
After attaching all ground straps, tape (weatherproof) the exposed area.
1.3.3.5 Lightning-Generated Transient Surge Protection. Electrical and electronic equipment at various
facilities has been severely damaged by lightning-generated transients. The transients occur on externally
exposed lines that directly interface equipment.
etc, that are exposed to weather elements.
Externally exposed lines are outside lines, buried, overhead,
The lines include incoming ac service conductors, and equipment
signal, status, control, grounding conductors and intrafacility ac and dc powerlines.
This section identifies
transient source and damage, waveforms and amplitudes of projected transients on different types of lines,
frequency of transient occurrence, and effective methods to implement to preclude equipment damage and
operational upset when transients occur.
1.3.3.5.1 Transient Source and Equipment Damage.
a.
Electrical and electronic equipment comprising an operating system is susceptible to damage from
lightning-generated transient surges via two primary sources as follows:
(1)
Transient surges coupled to equipment from incoming commercial ac power conductors.
(2) Transient surges coupled to equipment by connected facility control, status, power, ground,
data and signal lines that originate or terminate at equipment located externally to the building or structure
housing the equipment of interest.
b.
Damage resulting from lightning-generated transients occurs in many forms. Entire equipment
chassis have been exploded and burned, and wall-mounted equipments have been blown off the wall by largemagnitude transient energy. However, two forms of damage are most prevalent and are listed below:
(1) Sudden catastrophic component failure at the time of transient occurrence.
(2) Shortened operating lifetime of components resulting from over-stress at time of transient
occurrence.
1-49
MIL-HDBK-419A
1.3.3.5.2 Minimizing Damage.
a.
Damage can be minimized, and in most instances eliminated, by properly using the generally field-
proven protection methods detailed in this section.
In order to be cost effective and to provide effective
protection, allocation of protection must be divided into three general categories which are:
(1) Transient suppression (metal conduit and guard wires) for outside lines that interface
equipment to be protected.
(2) Installation of transient suppression devices on both ends of exterior lines immediately after
equipment building penetration or at exterior equipment termination, and on incoming ac service entrance lines
at the facility main service disconnect means.
On shielded facilities, transient suppression devices (TSD's)
should be installed in an entry vault or inside the main service disconnect box.
(3) Including transient suppression as an integral part of protected equipment at the exterior lineequipment interfaces.
b.
If realistic transient protection is to be designed, frequency of transient occurrence, amplitudes and
waveforms of transients, and the withstand level of protected equipment must be defined. The withstand level
is the short-duration voltage and current surge levels that equipment can withstand without overstressing or
immediate destruction of components occurring, and without equipment operational upset occurring.
The
information required for effective protection is provided in this section. The most susceptible components are
identified together with typical withstand levels. Frequency of transient occurrence is also provided. Because
of the large physical size of incoming ac service conductors, less impedance (resistance and inductance) is
presented to transient surge current flow.
As a result, amplitude and waveforms of transients appearing at ac
inputs are quite different from those appearing at control, status, data, signal , and in-system powerline inputs.
Therefore, protection for incoming ac power service conductors is discussed separately from that for other
externally exposed lines.
1.3.3.5.3 Susceptible Components.
Integrated circuits, discrete transistors and diodes, capacitors, and
miniature relays, transformers, and switches used in the design of solid-state equipment are very susceptible to
damage from lightning-generated transient surges.
Other components are not immune to damage but are
susceptible to a much lesser degree. Standards do not exist for specifying the withstand level against lightningtransients for most equipment and components.
Therefore, accurate information must be obtained from
manufacturers, laboratory testing performed or conservative engineering estimates made.
level limits for some common types of equipment and components are:
a.
Integrated circuits: 1.5 times normal rated junction and Vcc voltage.
b.
Discrete transistors: 2 times normal rated junction voltage.
c.
Diodes: 1.5 times peak inverse voltage.
d.
Miniature relays, transformers, and switches: 3 times rated voltage.
1-50
Typical withstand
MIL-HDBK-419A
e.
Capacitors: 1.5 times dc working voltage unless transient dielectric punch-through voltage known.
f.
DC power supplies with step-down transformer and diode bridge:
1.5 times diode peak inverse
voltage (PIV) rating times the transformer secondary to primary voltage ratio.
g.
Small motors, small transformers and light machinery: 10 times normal operating voltage.
h.
Large motors, large transformers and heavy machinery: 20 times normal operating voltage.
1.3.3.5.4 Frequency of Transient Occurrence.
Precise calculation of the number of lightning-generated
transients that will occur at a specific location n a specified time interval is not possible. However, enough
observations have been made to permit statistical evaluation of the number of lightning flashes that are likely
to occur in an area with a known average number of thunderstorm days per year. Some flashes may not produce
any transients while others will produce several transients. The available data, after considerable averaging
and rounding, is provided in Table 1-5. The table lists a typical number of transients that might be expected to
occur from lightning strikes at facilities located in high-and low-incident lightning areas.
When used in
conjunction with Figure 1-34, the table will permit calculation of the number of lightning surges that will occur
anywhere in the United States in a 10-year period. Decrease 1750 by 10% for each 10 decrease in the number
of thunderstorm days per year.
Table 1-5. Frequency of Transient Occurrences
Number of Lightning Surges
In 10 Years at One Facility
High Incident Area
(100 Thunderstorm
Days Per Year)
Low Incident Area
(10 Thunderstorm
Days Per Year)
1750
175
1.3.3.5.5 Transient Definition, AC Service Conductors.
Prediction of the exact amplitude, waveforms, and
number of transients that will occur at a particular facility over a specific time interval is not possible.
However, current amplitudes generated by many direct lightning strikes have been measured, and the
waveforms for the current have been measured and recorded. Also, sufficient data has been recorded to permit
statistical calculation of waveforms and amplitudes that are likely to occur. This data is provided in subsequent
paragraphs. Frequency of occurrence is provided in paragraph 1.3.3.5.4.
a.
Transient amplitudes from direct strikes.
Measured current amplitudes resulting from direct
lightning strikes have varied from 1,000 amperes to 250,000 amperes.
Results of several thousand measure-
ments have been reduced and are provided in Table 1-6. As shown in Table 1-6, typical peak current is 10 to 20
kiloamperes. Table 1-7 tabulates the peak current amplitudes measured for 2721 flashes. The median peak
value for the peak currents was approximately 15 kiloamperes. This is in agreement with the typical values
1-51
MIL-HDBK-419A
provided in Table 1-6, and there is agreement among authoritative sources that the peak current for a large
percentage of strikes is in the 10 to 30 kiloampere range.
Note that in Table 1-7, 1818 of the 2721 current
amplitudes or 66.8% were in the range of 1 to 20 kiloamperes.
Also note that only 14% were greater than
40,000 amperes, and it follows directly that 86% of the peak amplitudes were 40 kiloamperes or less. Only 45
of the 2721 measured amplitudes, or 1.65%, were above the 100-kiloampere level. Also, it is emphasized that
the peak current amplitudes noted in the foregoing resulted from direct strikes to metal towers for primary
transmission lines.
b.
Induced transient amplitude.
After installation of appropriate transient suppression, induced
transients will still occur as a result of close proximity, high-intensity strikes, and some transient energy will be
coupled through the service transformer onto the incoming ac service lines. The amplitude of those coupled and
induced transients will be reduced a minimum of 50% of direct strike amplitudes due to earth resistance,
attenuation of electromagnetic fields due to propagation through air, and coupling losses imposed by the service
transformer winding.
Therefore, 86% of the transient current surges appearing at a facility main service
disconnect means will be 20 kiloamperes or less, and the greatest percentage, 68%, of the surges will be in the
500 ampere to 10,000 ampere range. Only 1% of the surges will be above 50 kiloamperes, and only 0.25% will
be above 75 kiloamperes.
Table 1-8 provides a tabulation of transient amplitudes and the percentage of
transients on incoming ac lines that will as a maximum be of the amplitude listed.
c.
Transient waveforms, ac lines.
Waveshapes for transients will vary depending on the proximity of
the strike, intensity of the strike, and length and inductance of the incoming ac service lines. Table 1-6 lists
the typical time to peak current as 1.5 to 2 microseconds and 40 to 50 microseconds as the typical time from
the start of the pulse until the current decays to 50% of peak value.
Thus, a typical waveform for current
surges generated by a direct strike is 2-by-40 microseconds. Transients measured at main service disconnects
(amplitudes in excess of 3,000 volts) have had rise times of 1 to 2 microseconds and decay times of 20 to 40
microseconds. However, the inductance of some incoming ac service lines will slow down the rise time slightly.
Most manufacturers of secondary ac surge arresters use either 8-by-20 or 10-by-20 microsecond current
waveforms for testing and specification purposes, primarily because the waveform is relatively easy to generate
while a 2-by-40 microsecond waveform is quite difficult to generate. The 8-by-20 or 10-by-20 microsecond
waveforms are considered suitable for testing.
However, the user of the arrester should be aware of the
following:
(1) Transients with rise times faster than 8 microseconds may appear across the arrester
terminals resulting in a higher sparkover or turn-on voltage for the arrester than specified.
(2)
Transients with decay times up to 40 microseconds may appear across the arrester terminals
which will require the arrester to dissipate considerably more transient energy than would be required for a
20 microsecond decay time.
1-52
MIL-HDBK-419A
Figure 1-34.
1-53
MIL-HDBK-419A
Table 1-6
Parameter for Direct Lightning Strike Current
Parameter
Minimum
Typical
Maximum
1
2 to 4
26
3
40 to 60
100
1
10 to 20
250
< 0.5
1.5 to 2
30
<1
20
210
10
40 to 50
250
50
150
500
30
150
1600
Number of return
strokes per flash
Time between strokes
(ms)
Peak current per
return stroke (kA)
Time to peak current
(ps)
Rate of rise (kA/µs)
Time to half-value (µs)
Duration of continuing
current (ins)
Peak continuing current
(amperes)
1-54
MIL-HDBK-419A
Table 1-7. Peak Currents from Direct Lightning Strikes
No. of Flashes with
Range of current,
Peak Current in
No. at or
Percentage at or
(amperes)
Range
above Level
above Level
2,721
2,154
1,543
903
607
380
240
160
99
77
56
45
34
25
16
9
7
4
4
3
3
2
2
1
100
79.2
56.7
33.2
22.3
14.0
8.82
5.88
3.64
2.83
2.06
1.65
1.25
0.918
0.588
0.331
0.257
0.137
0.147
0.110
0.110
0.073
0.073
0.037
567
611
640
296
227
140
80
61
22
21
11
11
9
9
7
2
3
0
1
0
1
0
1
1
1,000 5,000
5,001 - 1 0 , 0 0 0
10,001 - 20,000
20,001 - 30,000
30,001 - 40,000
40,001 - 50,000
50,001 - 60,000
60,001 - 7 0 , 0 0 0
70,001 - 80,000
80,001 - 90,000
90,001 - 100,000
100,001 - 110,000
110,001 - 120,000
120,001 - 130,000
130,001 - 140,000
140,001 - 150,000
150,001 - 160,000
160,001 - 170,000
170,001 - 180,000
180,001 - 190,000
190,001 - 200,000
200,001 - 210,000
212,000
218,000
2,721
Table 1-8. Transient Surge Amplitudes
Transient Surge
Percentage of Transients
Amplitude (Amperes)
at Listed Amplitude
500
2,501
5,001
10,001
20,001
30,001
40,001
50,001
75,001
to
to
to
to
to
to
to
to
to
2,500
5,000
10,000
20,000
30,000
40,000
50,000
75,000
100,000
21%
23%
24%
19%
8%
3%
1%
0.9%
0.1%
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MIL-HDBK-419A
1.3.3.5.6
Methods for Transient Protection on AC Service Conductors.
Proper use of the following provides
effective protection against lightning generated transients on incoming ac powerlines.
a.
Completely enclosing buried lines in ferrous metal, electrically continuous, watertight conduit.
b.
Use of overhead guard wires to protect overhead lines.
c.
Installation of a secondary ac surge arrester at the facility main service disconnect means.
d.
Including surge suppressors as in integral part of equipment at ac power inputs and rectifier outputs
of low-level (5 to 48 volt) power supplies, when a power supply operates from commercial ac power and supplies
operating power for solid-state equipment.
e.
Installation of suitable surge arresters on the primary and secondary of the service transformer.
f.
Installation of powerline filters shall be in accordance with NACSIM 5203.
1.3.3.5.7 Use of Ferrous Metal Conduit. Since transients are induced on buried lines by electromagnetic waves
created by lightning current flow, all buried incoming ac service lines should be completely enclosed in ferrous
metal, watertight conduit. To be effective, the conduit must be electrically continuous and effectively bonded
to the building entry plate and grounded to earth ground at each end. No. 2 AWG bare copper stranded cable is
suitable for the earth ground connection, and exotherrnic welds provide effective bonding in earth. Approved
pressure connectors are suitable for use above ground. The conduit should extend from the service transformer
secondary to the facility main service disconnect means.
This use of metal conduit will eliminate low-level
induced transients, and will attenuate otherwise high-amplitude induced transients by 90% minimum. Although
the conduit provides effective protection against induced transients, it does not provide protection against
transients that enter the service conductors directly from the secondary of the service transformer.
1.3.3.5.8 Use of Overhead Guard Wires. Since enclosing overhead incoming ac service lines in metal conduit is
not feasible, experimentation has proved that the use of an overhead guard wire provides an effective level of
protection for overhead service conductors against direct lightning strikes. This guard wire also provides a low
level of protection against transients induced on lines by close proximity strikes as well as nearby cloud to cloud
discharges. The guard wire must be located above and parallel to the service conductors. To be effective, the
height of the guard wire must be that required to form a 1:1 cone of protection for the service conductors (see
Volume I, Section 3.5.2), and the guard wire must extend from the secondary of the service transformer for the
facility to the facility service entrance fitting. Also, at each end the guard wire must extend to, and be bonded
to, an effective earth ground or to the earth electrode subsystem of the facility.
When the distance between
terminating facilities exceeds 250 feet, the guard wire shall also be bonded to a ground rod meeting the
requirements of
MIL-STD-188-124A,
paragraph
5.1.1.1.4.
Also
refer to
MIL-STD-188-124A,
paragraph 5.1.1.3.10.2 regarding the type and size requirements of the guard wire. Since the guard wire and the
earth electrode subsystem are comprised of different metals, exothermic welding is recommended.
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MIL-HDBK-419A
1.3.3.5.9 Protection of Underground Cables.
a.
Protect against direct lightning strikes to buried cable by installing a guard wire above the cables or
cable duct. A 1/0 AWG bare copper cable laid directly over the protected cables as shown in Figure 1-35(a) is
At least 25.4 cm (10 inches) should be maintained between the protected cables and the guard
recoin mended.
wire.
b.
For a relatively narrow spread of the cables, 0.9 meters (3 feet) or less, or for a duct less than 0.9
meters (3 feet) wide, only one guard wire cable is necessary. For wider cable spreads or wider ducts, at least
two 1/0 AWG cables should be provided as illustrated in Figure 1-35(b). (Since the guard wire and protected
cables are embedded in the earth, the applicable cone of protection is not known.)
1.3.3.5.10 Buried Guard Wire. Experimental use of a buried guard wire embedded in soil above and parallel to
buried cable runs not enclosed in metal conduit has provided effective attenuation of lightning-induced
transients.
Use of the guard wire is recommended for protection of buried equipment lines not enclosed in
metal conduit. Bare 1/0 AWG copper wire has provided the most effective protection during experimental use.
To be effective, the guard wire must be embedded in the soil a minimum of 25 cm (10 inches) above and parallel
to the protected cable run or duct. When the width of the cable run or duct does not exceed 0.9 meters (3 feet),
one guard wire, centered over the cable run or duct, provides adequate protection. When the cable run or duct
is more than 0.9 meters (3 feet) wide, two guard wires should be installed. The guard wires should be spaced at
least 30 cm (12 inches) apart and be not less than 30 cm (12 inches) nor more than 45 cm (18 inches) inside the
outermost wires or the edges of the duct.
To be effective, the guard wires must be bonded to the earth
electrode subsystem at each terminating facility.
Exothermic welds provide the most effective bonding. The
requirement and need for underground guard wires shall be determined by the project and civil engineer and
shall be determined on a case and location basis dependent upon the priority of the circuit and the degree of
lightning
anticipated.
1-57
MIL-HDBK-419A
(a) CABLE SPREAD LESS THAN 3 FEET
(b) CABLE SPREAD 3 FEET OR GREATER
Figure 1-35. Lightning Protection for Underground Cables
1-58
MIL-HDBK-419A
1.3.3.5.11 Secondary AC Surge Arrester. Installation of a properly selected secondary ac surge arrester at the
facility main service disconnect means provides the best method for ensuring that high energy transients are
not coupled to equipment by ac distribution lines within the facility.
The surge arrester installed must have
certain characteristics to ensure adequate protection.
a.
Characteristics.
( 1 ) B e c a p a b l e o f safely dissipating transients of amplitudes and waveforms expected at the
facility for a predetermined period of time. Selection of an arrester that will provide protection for a period of
ten years is recoin mended.
(2)
Have a turn-on time fast enough to ensure that transient energy will not cause damage before
the surge arrester turns on and clamps.
(3)
Maintain a low enough discharge (clamp) voltage while dissipating transient current to prevent
damage to protected equipment.
(4)
(5)
b.
Have a reverse standoff voltage high enough to ensure nonconduction during normal operation.
Be capable of complete extinguishing after firing on an energized line.
Additional requirements.
In addition to the above, the surge arrester must be properly installed to
ensure optimum operation. The input to each phase arrester contained in the surge arrester should be fused to
provide protection against overload of, or damage to, the ac supply in the event an arrester should short. Also,
Indicator lights and an audible alarm that go off when a fuse opens should be provided on the front of the surge
arrester enclosure as a maintenance aid.
1.3.3.5.12 Surge Arrester Installation.
optimum operation.
Proper installation of the surge arrester is of vital importance for
A surge arrester with excellent operating characteristics cannot function properly if
correct installation procedures are not used. The most important installation criteria are provided below and
applies to surge arrester phase input connections and the ground connection.
All surge arresters should be
installed in accordance with the manufacturer's recommendations.
a.
Installation criteria.
(1) If possible, install arresters inside the first service disconnect box to keep interconnecting lead
lengths as short as feasible.
(2) Use interconnecting wire of sufficient size to limit resistance and inductance in the transient
path to ground through the surge arrester.
(3) Interconnecting wiring should be routed as straight and direct as possible with no sharp bends,
and the least number of bends possible.
(4) Do not include loops in the wiring.
1-59
MIL-HDBK-419A
(5)
b.
Must be grounded by the shortest low impedance path available.
Surge arrester input connections.
Installation of surge arresters is shown for grounded and
ungrounded service in Figures 1-36 and 1-37 respectively. For best possible protection, the line supply side of
the main service disconnect means should be connected to the phase input(s) of the surge arrester. However,
when necessary to facilitate removal of ac power for surge arrester maintenance, it is permissible to connect
the surge arrester to the load side of the main service disconnect means.
In order to prevent introducing
excessive inductance and resistance in the transient path to the surge arrester, No. 4 AWG (minimum) insulated
stranded copper wire of the minimum feasible length must be used to make the interconnection(s) unless
otherwise recommended and guaranteed by the manufacturer.
contain loops or sharp bends.
Also, the interconnecting wiring must not
Otherwise, the response time of the surge arrester will be delayed and a higher
clamp voltage than that of the surge arrester will be impressed across the protected equipment, thus increasing
the possibility of damage.
In the event a very fast transient should occur, it is quite likely that the surge
arrester would never turn on, and all of the transient energy would be dissipated by supposedly protected
equipment.
c.
Surge arrester ground connection.
When the surge arrester is not properly grounded, its response
time will be delayed and a higher clamp voltage than that of the surge arrester will be impressed across the
equipment being protected.
This can also be expected if the earth ground connection for the surge arrester
contains loops or sharp bends or is not properly bonded to the earth electrode subsystem. To overcome this
problem, stranded copper wire specified in accordance with Article 280 of the NEC must be used to make the
ground connection unless other specifications are provided by the [manufacturer of the surge arresters.
Figure 1-36 shows the surge arresters installed to ensure the [nest direct route to ground thereby minimizing
the lead inductance(s) and ensure the firing of the surge arresters. For best results exothermic welds should be
UL –approved pressure connectors are suitable for
used for bonding to the earth electrode subsystem.
above-ground bonds.
1.3.3.5.13 Operating Characteristics of Surge Arresters. Operating characteristics of different types of surge
arresters are discussed in the following subparagraphs.
Guidelines for selection of an adequate surge arrester
are also provided.
a.
Transient dissipation capability. Selection of a surge arrester that will provide adequate protection
against worst case transients is recoin mended.
Waveforms are defined in Section 1.3.3.5.5. The worst case
waveform is 2-by-40 microseconds. The number and amplitude of transients that can be expected to occur can
be determined by referring to Tables 1-5 and 1-8.
(1) In a high-lightning incident area (average of 100 thunderstorm days per year), 1750 transients
are expected to occur in a 10-year period. Referring to Table 1-8, it can be determined that transient
amplitudes and occurrence may be as listed in Table 1-9.
(2) In a low-lightning incident area (average of 10 thunderstorm days per year), only 175 transients
are expected to occur in a 10-year period. Transient occurrence and amplitudes may be as listed in Table 1-10.
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MIL-HDBK-419A
Figure 1-36.
1-61
MIL-HIIBK-419A
Figure 1-37. Secondary AC Surge Arrester Installation, Ungrounded Service
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MIL-HDBK-419A
Table 1-9. Transient Occurrences, High-Incident Lightning Areas
Transient
No. of Transients
Amplitude (Amperes)
in 10-year Period
500
to
2,500
368
2,501
to
5,000
402
5,001
to
10,000
420
10,001
to
20,000
333
20,001
to
30,000
140
30,001
to
40,000
52
40,001
to
50,000
17
50,001
to
75,000
16
75,001
to 100,000
2
Table 1-10. Transient Occurrences, Low-Incident Lightning Areas
No. of Transients
Transient
Amplitude (Amperes)
in 10-year Period
500
to
2,500
37
2,501
to
5,000
40
5,001
to
10,000
42
10,001
to
20,000
33
20,001
to
30,000
14
30,001
to
40,000
5
40,001
to
50,000
1.75
50,001
to
75,000
1.5
75,001
to
100,000
0.175
(3) Transient amplitudes are less at small electronic facilities. Recorded data substantiates that
large electronic facilities tend to attract higher intensity strikes than small electronic facilities. The transient
amplitudes listed in Sections 1.3.3.5.13a(l) and a(2) are for large electronic facilities, and the amplitudes should
be decreased by 50% for small electronic facilities. Large electronic facilities are defined as requiring more
than 100 amperes per phase for norm al operation. The transient amplitudes of Tables 1-9 and 1-10 should be
decreased by 50% when relating to a small facility.
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MIL-HDBK-419A
b.
Turn-on time.
Turn-on time (response time) is the time required for an arrester to turn on and
clamp a transient after turn-on voltage is impressed across device terminals. All basic suppressor devices used
in manufacture of surge arresters are voltage dependent for ionization, breakdown, and other phenomena
associated with breakdown. Therefore, a low turn-on voltage enhances a faster turn-on time. Turn-on time
requirements for a surge arrester must be directly related to the withstand level for equipment and components
being protected.
For instance, if only heavy duty electrical equipment, such as motors, contractors, and
switches are being protected, relatively slow turn-on of 1 to 5 microseconds can be tolerated. However, if
solid-state electronic equipment, or a combination of electrical and electronic solid-state equipment is being
protected, turn-on time becomes much more critical.
desirable.
criteria.
In general, the most rapid response time available is
However, cost and current dissipation capability normally place constraints on such selection
Four types of arresters are currently manufactured as noted below. Additional data for each type is
provided in 1.3.3.5.15.
c.
(1)
Gas-filled spark gap with series-connected nonlinear resistance.
(2)
Zinc oxide nonlinear resistor (ZNR) or metal oxide varistor (MOV).
(3)
Solid-state.
(4)
Hybrid of above components (development stage).
Important turn-on time characteristics.
surge arresters are listed in Table 1-11.
Generalized characteristics for the three basic types of
Turn-on time of 50 nanoseconds is sufficiently fast to protect all
except very critical components that would directly receive transient energy prior to turn-on and clamp of the
surge arrester.
Solid-state units may be used for protection of very critical equipment components, and the
gas-filled spark gap type will provide adequate protection for heavy duty electrical equipment such as motors,
contractors and switches.
However, arresters with slow turn-on time and high turn-on voltage should not be
used to protect electronic equipment that has low-voltage, fast turn-on transient suppression devices or circuits
included as an integral part of the equipment. Otherwise, the transient suppression in the equipment will turn
on and attempt to dissipate transient energy before the surge arrester installed at the main service disconnect
means turns on. In most cases, this will rapidly destroy equipment-level transient suppression. The impedance
and inductance of power distribution panels and power distribution wiring within the facility will tend to slow
down transient rise time and also dissipate some transient energy both before and after the surge arrester turns
on. The resistance and inductance works in conjunction with the surge arrester at the main service disconnect
means to provide additional protection. However, the true degree of protection thus provided varies widely due
to varying transient waveforms, and size and length of distribution wiring within the facility. In summary, the
most important characteristics for turn–on time are:
(1) Turn-on time must be rapid enough to preclude damage to equipment resulting from overvoltage before the surge arrester turns on and clamps the incoming transient.
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MIL-HDBK-419A
Table 1-11. Generalized Characteristics for Surge Arresters by Type
Type
Gas-filled
spark gap
MOV or ZNR
Turn-on Time
Current Capacity
Firing/Clamp Voltage
cost
5-250 nanoseconds
Extreme duty to 150,000
High -350 to 5500
Moderate -$25
volts (firing)
to $750
for 10 kV/µs rise
amperes lifetime: 2500
time
surges at 10,000 amperes
50 nanoseconds
Varies - can be equivalent
Moderate -300 to
Moderate -$50
or less, any rise
to spark-gap type
3000 volts (clamp)
to $1,000
10 nanoseconds
Varies - Generally 50 to
L OW -275 to 750
High -$100 to
or less, any rise
100 amperes except for
volts (clamp)
$25,000
time
costly units
time
Solid State
( 2 ) T u r n - o n v o l t a g e a n d t i m e for the surge arrester must be compatible with the same
characteristics of transient suppressors/circuits included as an integral part of protected equipment. Otherwise
equipment-level transient suppressors/circuits will attempt to dissipate the transient before the surge arrester
turns on.
When this occurs, the equipment level transient suppression will likely be destroyed resulting in
damage or operational upset of protected equipment.
d.
Discharge (clamp) voltage. The clamp voltage, sometimes referred to as the discharge voltage, for
a surge arrester is the voltage that appears across the arrester input terminals and the ground terminal while
conducting a transient surge current to ground.
The clamp voltage waveform occurring across the surge
arrester installed at the main service disconnect means appears across the protected equipment after losses
imposed by inductance and resistance of power distribution lines and panels.
(1) In general, a surge arrester with the lowest clamp voltage possible is desirable. An all-solidstate arrester provides the lowest clamping voltage available (Table 1-11).
However, as with turn-on time,
other factors such as current dissipation capability and cost normally place constraints on simply installing a
surge arrester at the main service disconnect means with the lowest clamping voltage available.
(2) In new facilities calling out the latest design equipment, transient surge suppression generally
is included as an integral part of the equipment ac input. Higher clamping voltages can therefore be tolerated
at the main service disconnect means.
When good engineering design practices are used, equipment level
suppressors will have a slightly lower turn-on voltage threshold and a slightly faster turn-on time than the surge
arrester at the main service disconnect means.
This permits the equipment-level suppressors to maintain a
lower clamping level to provide maximum equipment protection.
equipment level suppressor(s) will turn on first.
1-65
Therefore, when a transient occurs, the
MIL-HDBK-419A
(3) This circuit operation may generate the requirement for a properly sized (2-microhenry
minimum) inductor to be installed in series with applicable ac conductors.
If its need has been ascertained, it
must be installed between the surge arrester and the integral equipment-level transient suppressor. It may also
be designed as an integral part of the surge arrester or the equipment-level transient suppressor.
(4) The equipment-level suppressor will immediately start toward its clamp voltage as transient
current is conducted. Because of resistance and inductance in the power distribution lines and panels, the surge
arrester will turn on very soon (nanoseconds) after the equipment-level suppressor(s), and will dissipate most of
the remaining transient energy.
After the surge arrester turns on, the equipment level suppressor(s) are
required to dissipate only the transient energy resulting from the clamp voltage of the surge arrester.
Thus, the surge arrester dissipates most of the transient surge, and the equipment-level
(5)
suppressor(s) provide equipment protection against fast rise time transients and reduce the surge arrester clamp
voltage to levels that can be safely tolerated by protected equipment. In summary, the clamp voltage for the
surge arrester must be low enough while dissipating a high-energy transient to provide adequate equipment
protection taking into consideration:
(a)
Protection provided by transient suppression that is an integral part of the facility
(b)
Impedance (resistance and inductance) of power distribution lines and panels within the
equipment.
facility.
e.
Reverse standoff voltage.
Reverse standoff voltage is specified in various ways by surge arrester
manufacturers such as maximum allowable voltage, voltage rating, and reverse standoff voltage.
For usage
herein, reverse standoff voltage is defined as the maximum voltage that can be applied across the surge
arrester and still permit the surge arrester to remain in an off state (current leakage through arrester to ground
100 microampere or less). Good engineering practice dictates that the surge arrester remains off during
normal operation.
(1) Design of effective lightning transient protection requires that the surge arrester turn on very
rapidly at the lowest voltage possible when a transient occurs.
In addition, it is desirable that a low clamp
voltage be maintained across the surge arrester while conducting surge current to ground. Turn-on voltage and
associated turn-on time as well as clamp voltage are proportional to reverse standoff voltage.
That is, an
arrester with a low reverse standoff voltage has a lower turn-on voltage (and thus a faster turn-on time) and a
lower clamp voltage than an arrester with a higher reverse standoff voltage. Therefore, it is important that the
surge arrester has the lowest possible reverse standoff voltage.
(2)
For effective protection, the reverse standoff voltage should be between 200 to 300 percent of
nominal line-to-ground voltage of the appropriate ac service lines for a spark gap type surge arrester that is to
be installed line to ground. The reverse standoff voltage should also be between 200 to 300 percent of nominal
line-to-line voltage of appropriate ac service lines for a spark gap type surge arrester that is to be installed
line to line.
The reverse standoff voltage for MOV and ZNR type arresters should be 175 ± 25 percent of the
nominal line-to-ground or line-to-line voltages of the appropriate ac service lines.
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MIL-HDBK-419A
1.3.3.5.14 Desirable Operating Characteristics for Transient Suppressors. The transient suppressor characteristics listed below are required for effective protection at the facility level:
a.
Turn-on (response) time: 50 nanoseconds or less.
b.
Standoff voltage and leakage current:
To ensure that the suppressor remains off except during
transient occurrence, the standoff voltage should be between 200 to 300 percent above the nominal line voltage
for spark gap type suppressors and approximately 175 ± 25 percent for MOV and ZNR type suppressors.
Leakage current should not exceed 100 microampere at standoff voltage.
c.
Polarity: Bipolar or unipolar, depending on line voltage.
d.
Turn-on voltage:
125 percent of standoff voltage maximum at one milliampere for MOV and ZNR
type suppressors. Also, 125 percent of the standoff voltage for gas-filled spark gap suppressors.
e.
Clamp voltage:
(Also known as discharge voltage) should not exceed 200 percent of the turn-on
voltage for transients 100 amperes peak or 225 percent of the turn-on voltage for transients 1000 amperes
peak.
f.
Operating life: Capable of dissipating number and amplitude of transients projected to occur over a
10-year period. See Section 1.3.3.5.17.
Self-restoring capability: Essential that suppressor automatically restores to off state when applied
g.
voltage drops below turn-on voltage.
1.3.3.5.15 Characteristics of Different Types of Surge Arresters.
Various types of surge arresters are
presently available for purchase as off-the-shelf items from a multitude of manufacturers. Most have desirable
characteristics, and also have undesirable characteristics.
Some types have the capability of dissipating
tremendous amounts of current, but turn on relatively slowly (150 to 200 nanoseconds) after turn-on voltage
appears across device terminals.
Another type turns on more rapidly (50 nanoseconds or less) but will not
dissipate as much current as the slower devices, unless many devices are connected in parallel which is not
totally desirable. Solid-state arresters are available which have very fast turn-on times but most of them are
limited in current dissipation capability except for expensive units that range in cost from $7,500 to $25,000.
Several hybrid units are currently under development that consist of a solid-state suppressor for dissipation of
low-energy transients, and a separate suppressor section for dissipation of high-energy transients. The two
suppressor sections are normally separated by a choke in series with the protected phase line. The three most
important characteristics of an ac surge arrester are the capability to dissipate the required levels of surge
current, maintain a low discharge (clamp) voltage while dissipating the transient current, and a fast response
time. The fast response time is important to prevent the appearance of high level transient energy (overshoot
voltage) across protected equipment for an intolerable length of time before the arrester turns on and clamps.
Various types of suppressors are discussed below together with typical operating characteristics.
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MIL-HDBK-419A
a.
Gas-filled spark gap with series-connected silicon carbide block. The gas-filled spark gap arrester is
capable of conducting very high currents.
Some units have an extreme duty discharge capacity of 150,000
amperes peak for one transient with a 10-by-20 microsecond waveform.
Minimum life of such units is
dissipation of 2500 surges of 10,000 amperes peak surge current with a 10-by-20 microsecond waveform.
Impulse sparkover (turn-on) voltage is 1400 volts peak for a transient with a 10 kV/ µs waveform for two types
of arresters.
Some typical discharge (clamp) voltages are listed in Table 1-12 for 10-by-20 microsecond
waveforms of the transient amplitudes listed:
Table 1-12. Typical Maximum Clamp Voltage for Spark Gap Arresters
Peak Surge
Maximum
Amplitude
Clamp Voltage
10,000 Amperes
2,000 volts
40,000 Amperes
3,000 volts
150,000 Amperes
5,500 volts
(1) Follow current. The typical discharge (arc) voltage across a spark gap is 20 to 30 volts while it
is in full conduction.
Because of the low arc voltage, the voltage and current available from the ac power
supply would maintain the spark gap in an on state after a transient was dissipated until the first zero crossing
of the power supply or until a supply line fuse opened, a line burned open, the spark gap burned open, or the
service transformer burned open.
For this reason, a silicon carbide block (nonlinear resistor) is connected in
series with a spark gap to ground to ensure that the spark gap extinguishes on the first zero crossing of the
connected line, and, more importantly, to limit follow current through the spark gap after a transient is
dissipated until the first zero crossing of the powerline (8.3 milliseconds maximum). The silicon carbide block is
a nonlinear resistance, and resistance decreases as applied voltage increases. Thus, the resistance is relatively
high at powerline voltages to limit follow current, but decreases to a fraction of an ohm when high-level
transient voltage is applied.
However, the resistance remains high enough to generate a relatively high clamp
voltage when discharging high-amplitude transient currents.
(2) Sparkover (turn-on) voltage.
Sparkover time for the spark gap arrester is directly related to
transient risetime since a finite amount of time is required for the spark gap to ionize and transition from the
off mode through the glow region and into the arc mode of operation. Also, ionization time is to some extent
related to the risetime of the transient. Transition time from off to arc mode of operation is typically 150 to
200 nanoseconds after sparkover voltage appears across arrester terminals.
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MIL-HDBK-419A
Figure 1-38. Typical Operating Curve for Two Series of Gas-Filled Spark Gap
Arresters with Nonlinear Series Resistor
(3) Summary.
In summary, the gas-filled spark gap is capable of discharging high-amplitude
transients, but has a relatively slow response time and a relatively high discharge voltage.
Follow current
(10 to 80 amperes typical) occurs, but normally presents no significant problems. Figure 1-38 depicts typical
operating curves for two series of gas-filled spark gap arresters with a series-connected silicon carbide resistor.
b.
ZNR and MOV type arresters. The ZNR type arresters have several desirable characteristics. Other
types of MOV arresters are currently under development that have voltage-current characteristics similar to
the ZNR type. The ZNR type arresters have a relatively fast turn-on time (50 nanoseconds or less), low turn-on
voltage, relatively low clamping voltage, and various levels of current dissipation capability since the ZNR
types are available in different energy level packages.
Table 1-13 lists related characteristics for ZNR
available in one type of energy level package, and Table 1-14 lists related characteristics for a high-energy
level package.
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MIL-HDBK-419A
Table 1-13. ZNR Type Devices (Molded Case Type) Typical Characteristics
Range of Available Devices
Parameter
DC Breakdown Voltage at
20 mm Disc
25 mm Disc
32 mm Disc
200 to 910 volts
200 to 910 volts
200 to 910 volts
525 to 2800 Volts
590 to 3200 Volts
640 to 3800 Volts
2.5 to 5 kA
5 to 10 kA
10 to 20 kA
1 Milliampere
Maximum Clamping Voltage
at Maximum Surge Current
Maximum Surge Current
(8 x 20 Microsecond Waveform)
Life
Depends on Surge Current and Waveform*
*Maximum surge current (8 x 20 microseconds) can be applied twice without incurring damage or over
stressing the devices.
Table 1-14. High Energy ZNR Surge Arrester Typical Characteristics
Size:
Three 80 m m Discs in Parallel
Powerline Voltage:
250 V AC Maximum
DC Breakdown Voltage at
560 Volts
1 Milliampere:
Maximum Clamping Voltage:
Current
Clamping Voltage
(10 x 20 Microseconds)
1-70
10 kA
1300 volts
40 kA
1600 Volts
150 kA
2450 Volts
MIL-HDBK-419A
Table 1-15. Test Results for Parallel-Connected ZNR
Number of
Clamp Voltage (Peak)
Surge Amplitude
Surges Applied
2000
250A @ 1000V
300V
2500
400A @ 1600V
315V
225
20,000A @ 8.75kV
500V
25
40,000A @ 16.8kV
650V
50,000A @ 20kV
700V
8
(1) Current dissipation.
Testing has established that connection of the devices listed in
Table 1-13 in parallel for line-to-ground or line-to-line protection is feasible. Use of the ZNR in parallel
provides increased current dissipation capability and a lower maximum clamping voltage than a single, highenergy ZNR can provide. Five of the devices were connected in parallel and surged as listed in Table 1-15. The
clamp voltages listed in Table 1-15 occurred. Current division was very good.
(2)
Turn-on.
Although the ZNR devices used in ZNR-type arresters are not solid-state junction-
type devices, the arrester acts very much like junction-type devices.
That is, when breakdown voltage is
reached, transition from off to on occurs very rapidly as shown in Figure l-39b which is a typical operating
curve for a ZNR.
Since the devices used in ZNR-type surge arresters are essentially nonlinear resistors,
resistance decreases rapidly as applied voltage across the device increases above breakdown voltage.
Therefore,
current flow through this type of arrester increases rapidly after breakdown as shown in
Figure l-39b. Primarily because of resistance and capacitance of the ZNR, the clamp voltage slightly lags the
transient current waveform.
The ZNR-type arrester automatically restores to the off state when applied
voltage falls below turn-on voltage. Therefore, no follow current occurs during the turn off phase.
c.
Solid-state type arresters.
So many different types of solid-state arresters are currently
manufactured that it is difficult to generally evaluate them. In general, solid-state arresters manufactured by
connecting silicon avalanche diode suppressors (SAS) in series to attain the desired current handling capability
have truly fast response times of 1 to 10 nanoseconds.
However, this type of arrester is generally limited to
handling approximately 500 amperes surge current (waveform 8-by-20 to 8-by-40 microseconds). Figure 1-39a
is a typical operating curve for a solid-state suppressor.
This type of arrester also has a low clamp voltage
(normally 160% of breakdown voltage , maximum) compared to other types of arresters.
Other solid-state
arresters are a combination of silicon avalanche diodes or rectifier diodes connected in a bridge network
followed by a second stage consisting primarily of a silicon-controlled rectifier (SCR) with a varying-value
current-limiting resistor in series with the SCR.
This type arrester has a slow response time, sometimes
approaching 1 microsecond, because of the slow turn-on time for the SCR. Also, the clamping voltage can be
1-71
MIL-HDBK-419A
high depending on the value of the SCR current-limiting resistor.
Because of the proliferation of solid-state
arresters available, it is strongly recommended that complete laboratory demonstration testing be required
prior to implementation of the solid-state arresters.
a.
TYPICAL OPERATING CURVE FOR SILICON AVALANCHE SUPPRESSOR
b.
TYPICAL OPERATING CURVE FOR ZNR SUPPRESSOR
Figure 1-39. Typical Arrester Operating Curves, ZNR and SAS
d.
Hybrid type arresters.
Hybrid type arresters are currently in development that consist of a
combination of gas-filled spark gaps and ZNR or MOV, and two-stage arresters consisting of a solid-state stage
for dissipation of low-energy-content transients and a separate stage for dissipation of high-energy transients
consisting of gas-filled spark gaps and ZNR or MOV. The two stages are separated by a very low dc resistance
choke so that the low-energy dissipation stage fires first to achieve fast response time. When sufficient voltage
develops across the choke, the high energy dissipation stage turns on and dissipates the high level transient
energy.
Insufficient data currently exists to support analyzing the hybrid type arresters. A hybrid should
emerge that effectively utilizes the best characteristics of available devices, (e. g., the fast turn-on and low
clamping voltage characteristics of silicon avalanche diode suppressors and the high current dissipation
capability of ZNR or MOV and gas-filled spark gaps).
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MIL-HDBK-419A
1.3.3.5.16 Transient Protection for Externally Exposed Equipment Lines.
In order to effectively protect
equipment against damage from lightning generated transients on externally exposed (outside) equipment lines,
the following must have some definition which is provided in subsequent paragraphs.
a.
Frequency of Transient occurrence.
b.
Amplitude and Waveform of Occurring Transients.
c.
Equipment Withstand Levels.
d.
Protection Methods Against Transients.
Frequency of Transient Occurrence.
1.3.3.5.17
There is no existing method for precise calculation of the
number of lightning generated transients that will occur at a specific location in a given period of time.
However, by using the best available data listed in Section 1.3.3.5.4, projections are that 1750 transients will
occur in a 10-year period at a facility located in a high-lightning incident area with an average of 100
thunderstorm days per year, and only 175 transients will occur in a 10-year period at a facility in a low-incident
lightning area with an average of 10 thunderstorm days per year.
Note that the number of transients is
decreased by one order of magnitude for the low-lightning incident area. Therefore, by using Figure 1-34 to
determine the average number of thunderstorm days per year in a specific location, and decreasing 1750 by 10%
for each 10 decrease in the average number of thunderstorm days per year, the number of transients projected
to occur at any location in the United States can be determined.
Table 1-16. Transients Projected to Occur on Externally Exposed Line in
High-Lightning Incident Area Over 10-Year Period
No. of Transients
Percentage
Peak Voltage (Volts)
2
0.1
750 to 1,000
750 to 1,000
15
0.9
500
to
749
500 to
749
400
to
499
400
to
499
300
to
399
Peak Current (Amperes)
18
1
53
3
300
to
399
140
8
200
to
299
200
to
299
332
19
100
to
to
24
50
to
199
99
100
420
50
to
199
99
403
23
25
to
49
25
to
49
367
21
5
to
24
5
to
24
Note: The source impedance for design purposes is assumed to be 1 ohm.
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MIL-HDBK-419A
1.3.3.5.18 Amplitudes and Waveforms of Occurring Transients.
Transients occurring on landlines have been
defined as 10-by-1000 microsecond, 1000-volt peak pulses where 10 microseconds is the time from the start of
the transient to peak voltage, and 1000 microseconds is the time from the start of the transient until the
amplitude exponentially decays to 50% of peak value. Source impedance cannot be precisely defined but for
design purposes is assumed to be 1 ohm. Therefore, for design purposes, a typical worse case lightning-induced
transient can be defined as 10-by- 1000 microseconds, 1000 volts peak with a peak surge current of 1,000
amperes.
Using Table 1-8, the 1750 transient pulses defined in Section 1.3.3.5.17 and the worst case transient
pulse defined above, the number of transients of varying amplitude would be as listed in Table 1-16 over a
10-year period for an externally exposed line in a high-incident lightning area (average of 100 thunderstorm
days per year).
1.3.3.5.19 Equipment Withstand Levels.
Equipment withstand levels were generally defined in Section
1.3.3.5.3. Nothing of substance can be added. However, manufacturers generally do not specify equipment or
component withstand levels against lightning generated transient surges.
It is imperative that the withstand
level be analyzed and determined for each item of equipment to be protected. The withstand level should be
10% below both the damage threshold level and operational upset level for the equipment. The damage
threshold level is defined as the level where immediate component destruction occurs or the repeated
application energy level that decreases useful operating lifetime of equipment components, whichever is lower.
The operational upset level is defined as the transient voltage that causes an intolerable change in equipment
operation.
It is imperative that an accurate withstand level be established. Otherwise, designed transient
suppression may not be effective, or conversely, costly transient protection may be designed when not required.
Methods listed below are effective, when properly
1.3.3.5.20 Protection Methods Against Transients.
implemented, in providing equipment protection against lightning generated transients appearing on externallyexposed equipment signal, status, control and ac and dc intrafacility lines.
Subsequent paragraphs delineate
proper implementation techniques for the listed methods.
a.
Completely enclosing buried lines end-to-end in ferrous metal, watertight conduit.
b.
Installation of buried guard wire above buried cable runs not in metal conduit.
c.
Connecting transient suppressors line-to-ground on both ends of externally exposed equipment lines
as soon as feasible after building penetration or at point of termination at exterior equipment.
d.
Including transient suppressors or transient suppression circuits as an integral part of protected
equipment at all external line-equipment interfaces.
e.
Peripherally bonding the shields of rf coaxial lines to building entry plates by use of bulkhead
connector plates.
1.3.3.5.21
Enclosing Cable Runs in Ferrous Metal Conduit.
Transients are induced on external lines by
electromagnetic waves created by lightning current flow, and by cloud-to-cIoud lightning discharges. Therefore, completely enclosing buried external cable runs in ferrous metal, watertight, electrically continuous
conduit provides an effective protection level against lightning-generated transients.
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MIL-HDBK-419A
Cost considerations.
a.
When a buried cable run is 90 meters (300 feet) or less in length, it is
economically feasible to enclose the cable run end-to-end in metal conduit.
When the cable run exceeds 90
meters (300 feet), it is normally more economically feasible to provide transient suppression at building
penetration and equipment level than to install the conduit. However, use of metal conduit provides effective
protection against induced transients, regardless of the length of the cable run. The conduit must extend from
building penetration to building penetration, or building penetration to exterior equipment termination.
b.
Grounding of conduit. To be effective, the conduit must be electrically continuous and effectively
bonded to earth ground at each end. If building entry plates are available the conduit should be peripherally
welded.
NO. 2 AWG bare copper stranded cable is suitable for the earth ground connection, and exothermic
welds provide effective bonding underground. Approved pressure connectors are suitable for use above ground.
For runs over 90 meters (300 feet), the conduit should be connected to earth ground at each end and every 30
meters (100 feet).
The structural steel of antenna towers may be used to effectively ground the conduit
provided the total bond resistance from the conduit to the earth electrode System is 5 milliohms or less.
c.
Transient suppression for lines in metal conduit. Only one level of transient suppression is required
for exterior line/equipment interfaces to provide effective protection against induced transients conducted by
lines in metal conduit. The one level of suppression may be located at building penetration or designed as an
integral part of the applicable equipment.
The one level of suppression may consist of a single suppressor
connected line to ground, or two resistors connected in series with the external line input and a silicon
avalanche diode connected between the junction of the two resistors and earth ground or equipment case
ground, depending on location of the transient suppression.
d.
Amplitude of transients on external lines enclosed in metal conduit.
The number of lightning
generated transients occurring on external cables will not change as a result of enclosing cable runs in metal
conduit.
However, the voltage and current amplitudes will decrease a minimum of 90%. Therefore, Table 1-16
can be used to determine the number and amplitude (voltage and current) of transients that are projected to
occur on externally exposed lines, enclosed in metal conduit, in high-lightning incident areas.
1.3.3.5.22 Transient Suppression.
In order to provide effective equipment protection against lightning
generated transients, externally exposed lines must have transient suppression installed on each end where the
line directly interfaces electrical/electronic equipment.
This requirement applies in all cases when the
withstand level of the interfaced equipment is below the transient levels projected to occur at the
line/equipment interface. As previously noted, transient amplitudes projected to occur on lines enclosed endto-end in electrically continuous, ferrous metal conduit are only 10% of the transient amplitudes projected to
occur on lines not enclosed in metal conduit (Table 1-16). Primarily because of insertion losses and impedance
mismatch, transient suppression is not currently available that is satisfactory for installation on externally
exposed rf coaxial lines at building penetration when the lines carry signals above 3 MHz in frequency.
Therefore, all protection for these line/equipment interfaces must be designed as an integral part of the
equipment.
The most effective design for equipment protection is provided by installing a high energy level
transient suppressor at building penetration (on all lines that carry signals 3 MHz or less in frequency)
connected line to earth ground, and including low-energy suppression as a part of integral equipment design.
Figure 1-40 depicts typical transient suppression at the facility and equipment level for both coaxial cables and
single wires or pairs. Suppressors installed at building penetration should be located in the junction box that
first terminates the externally exposed lines after building penetration.
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DEMARCATION
J-BOX (PART OF
SHELTER/BUILDING)
NOTE :
a.
TYPICAL TRANSIENT PROTECTION CONFIGURATION
NOTE :
b.
SAS MAY BE UNIPOLAR OR
BIPOLAR DEPENDING ON
LINE VOLTAGE
TRANSIENT PROTECTION FOR
SHIELD REQUIRED ONLY WHEN
SHIELD IS NOT GROUNDED AT
EQUIPMENT
TRANSIENT PROTECTION FOR EXTERNALLY-EXPOSED
COAXIAL CABLES
Figure 1-40. Typical Transient Suppressor Installation, Facility and Equipment Level
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a.
Grounding for transient suppression. In order for the transient suppression to operate properly and
provide optimum equipment protection, the ground side of the transient suppressor must be connected as
directly as possible to ground. The ground for the high-energy level suppressor must be connected directly to
the nearest J-Box wall.
b.
Suppressor installation.
Suppressors can be installed between applicable terminal boards and the
ground with short direct connections. Maintaining short lead lengths is important to prevent inductance of long
lead lengths from delaying turn-on and response of the transient suppressors.
1.3.3.5.23 Types of Available Transient Suppressors. Three different types of suppressors are available to
provide transient protection as listed below. Operating characteristics for each type are provided in subsequent
paragraphs, followed by desirable operating characteristics.
a.
Zinc oxide nonlinear resistor (ZNR) or metal oxide varistor (MOV).
b.
Silicon avalanche diode suppressor (SAS).
c.
Gas-filled spark gap.
1.3.3.5.24
a.
Operating Characteristics of Transient Suppressors.
Characteristics of ZNR-type suppressors.
(1)
Response time: 50 nanoseconds or less, any risetime.
(2)
Clamping voltage: 225% of breakdown voltage maximum for surge currents projected.
(3)
Breakdown voltage: 22 V dc to 1800 V dc at 1 milliampere.
(4)
Standoff voltage: 14 V dc to 1599 V dc.
(5)
Surge current dissipation: 500 to 2000 amperes, 8-by-20 microsecond waveform.
Lifetime:
(6)
protection, projected.
b.
Variable, depends on amplitude of surge current, satisfactory for 10-year
Characteristics of SAS-type suppressors.
(1) Response time: 1 nanosecond or less, any risetime.
(2) Clamping voltage: 165% of breakdown voltage maximum at rated peak pulse current.
(3) Breakdown voltage: 6.8 V dc to 200 V dc at 1 milliampere.
(4) Standoff voltage: 5.5 V dc to 200 V dc.
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(5) Surge current dissipation: Peak pulse current ratings from 139 amperes for 6.8 V dc suppressor
to 5.5 amperes for 200 V suppressor for 10-by-1000 microsecond waveforms.
(6)
Lifetime:
Not presently defined.
Requires current-limiting resistor in series with protected
line to provide required surge current dissipation at facility level.
c.
Characteristics of gas-filled spark gap suppressors.
(1) Response time: 3 to 5 microseconds for 10-by-1000 microsecond waveforms.
(2) Clamping voltage: Arc voltage is 20 volts typical.
(3) Breakdown voltage: 300 to 500 volts typical.
(4) Standoff voltage: 75 V dc to 1000 V dc.
(5) Surge current dissipation: 5,000 amperes for 10-by-50 microsecond waveform.
(6) Lifetime:
Varies depending on surge current amplitude, 50 surges of 500 amperes peak current
with 10-by-1000 microsecond waveform typical.
1.3.3.5.25 Transient Suppressor Packaging Design.
twisted shielded pairs is not critical.
Packaging of transient suppressors for standard wires and
Leads should be as short as feasible to enable short, direct connections
without bends. Transient suppressors for coaxial and twinaxial lines should be contained in a metal and epoxy
package with appropriate connectors on each end, one male, and one female, to permit inline installation at the
connector panel in the demarcation junction box. Two suppressors must be included in all twinaxial protector
packages.
1.3.3.5.26 Coaxial Cable Shield Connection Through an Entrance Plate. Effective transient protection can be
provided by peripherally bonding each rf coaxial cable to a metal bulkhead connector which in turn is
peripherally bonded to the building entry plate and grounded to the earth electrode subsystem. This scheme will
route transient currents from cable shields to earth ground instead of through terminating equipment to ground.
Also, transient surge currents will be shunted to ground before transient energy is cross-coupled to other
equipment lines in the facility.
constructed of steel.
The entry plate should be a minimum of 0.64 cm (1/4-inch) thick, and
The entry plate must contain the required number of appropriate coaxial feedthrough
connectors to terminate all applicable incoming lines. The connectors must also provide a path to ground for
connected cable shields. If external and internal coaxial cables are of a different physical size, the changeover
in connector size should be accomplished by the feedthrough connectors of the entry plate. The entry plate
should be connected to the earth electrode subsystem with a 1/0 AWG (minimum) insulated copper cable. The
cable should be bonded to the entry plate and the earth electrode subsystem with exothermic welds.
1.3.3.5.27 Grounding of Unused Wires.
A11 unused wires/pairs of Communication cable runs should be
—
connected to ground at each end. This action will reduce transients on the unused lines which otherwise could be
coupled to in-service lines of the cable.
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1.3.3.5.28 Transient Suppression for RF Coaxial Lines.
At the present time, effective transient suppressors
for connection from line-to-ground at building penetration for externally exposed rf coaxial lines carrying
signals above the 3 MHz range are still in the development stage, primarily because of insertion losses. The
best method for protecting the lines at present is end-to-end enclosure in ferrous metal conduit, and providing
transient suppression as an integral part of using equipment.
1.3.3.5.29 Equipment-level Transient Suppression.
Equipment-level
transient protection is discussed in
paragraph 1.3.3.7 of this chapter. In general, effective protection is provided by low-value resistors in series
with external line inputs, and silicon avalanche diode suppressors connected line-to-ground. Suppressors are
currently available as special order items that are suitable for connection line-to-ground on rf lines carrying
signals up to 500 MHz. The suppressors consist of a spark gap, a silicon avalanche diode suppressor in parallel
with an rf choke, or a combination ZNR and rf choke.
1.3.3.6 Lightning Generated Transient Protection Evaluation.
This portion of the procedure is performed to
determine whether effective and adequate transient suppression is provided for protection against damage from
lightning-generated transients. The procedure consists of a detailed review of facility drawings and a detailed
visual inspection.
a.
Facility drawings.
Review facility drawings required to determine the following. Sketch items of
interest to aid in subsequent visual examination.
(1) Are lightning protectors installed on the primary and secondary of commercial ac service
transformer(s)?
(2)
Are buried, incoming ac power service lines enclosed in watertight, ferrous metal conduit
connected to earth ground at the service transformer and to the earth electrode subsystem at the facility end?
Is No. 2 AWG (minimum) bare, stranded copper wire used for earth ground connections?
(3) Are overhead incoming ac power service lines protected by an overhead guard wire from the
service transformer to the facility service entrance? Is the guard wire connected to earth ground at each end?
Does the guard wire provide a 1:1 cone of protection for the incoming service lines?
(4) Is an ac surge arrester installed at the facility main service disconnect means (each main
disconnect if more than one)? Note manufacturer and part number on sketch.
(5) Are the external landlines and lines which terminate at exterior equipment (including rf
coaxial lines that connect to facility equipment) enclosed in watertight, ferrous metal conduit if the cable runs
are 90 meters (300 feet) or less in length? Is the conduit connected to the applicable earth electrode subsystem
at each end?
(6)
Do buried landlines (more than 90 meters (300 feet) in length and not enclosed in ferrous
conduit) have a guard wire installed end-to-end in the cable trench?
Is the guard wire connected to the earth
electrode subsystem at each end?
(7) Are all rf coaxial cables grounded to the metal bulkhead connector plate at building
penetration?
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(8) Are transient suppressors or transient suppression circuits installed line-to-ground on each end
of all exterior lines not enclosed in ferrous metal conduit (except rf lines carrying signals above 3 MHz) at first
termination after building penetration?
b.
Inspection.
A survey form in Section 2.2.2.4, Part II, is provided for guidance in accomplishing a
thorough visual inspection. Detailed written notes fully describing all noted deficiencies should be made.
c.
Corrective action. Specific corrective action to accomplish in response to each noted deficiency is
difficult to detail. For instance, cable runs less than 90 meters (300 feet) in length are not normally enclosed
end-to-end in electrically continuous, watertight, ferrous metal conduit. Intensity and incidence of lightning in
the immediate area, together with economic feasibility and operational requirements, are normally the
overriding factors in determining whether the installation of metal conduit is justified and feasible. In most
cases, for the example cited, installation of transient suppression circuits on each end of externally exposed
equipment lines is the most feasible solution.
However, installation of transient suppression directly at the
line-equipment interface may also be warranted, depending on equipment susceptibility and lightning incidence.
Consider each deficiency individually.
Refer to Sections 1.3.3.5 and 1.3.3.7 as required, and correct
deficiencies in the most feasible manner. Some typical and required corrective actions are listed below:
(1) If a secondary ac surge arrester is not installed at the facility, and there is any history of
lightning incidence in the area, install a surge arrester on the line or load side of the main service disconnect
means. Refer to Section 1.3.3.5 to determine that the surge arrester selected will be adequate and effective.
(2) If the surge arrester and transient suppressor does not have a low-impedance, effective path to
Neither the arrester nor
earth ground, take whatever action is necessary to provide effective grounding.
suppressor will provide effective transient protection if an effective ground is not available.
(3) If no transient suppressors are installed on externally exposed equipment lines not enclosed
end-to-end in metal conduit, and the lines interface susceptible equipment, as a minimum install transient
suppressors on each end of each line that interfaces susceptible equipment.
Refer to Sections 1.3.3.5 and
1.3.3.7 as required.
1.3.3.7 Transient Protection.
1.3.3.7.1
Protection
Requirement.
Individual items of electrical and electronic equipment that directly
interface any externally exposed equipment lines, including commercial ac , may require transient protection
that is designed as an integral part of the equipment. Whether or not protection is required is dependent on the
damage susceptibility of the equipment of interest, the level of transient suppression provided on externally
exposed lines at building penetration or external equipment termination and the level of transient energy that is
projected to be conducted to the equipment.
For use herein, externally exposed lines are defined as lines
exposed to outside weather elements and environmental conditions.
grade surface , or be buried in earth.
The lines may run overhead, run along
Included are ac power input lines and signal, control, status, and
intrafacility powerlines. The lines are commonly referred to as landlines. Transient protection is not required
in equipment when an interfaced landline is fiber optic in lieu of a metallic line. In order to provide effective
transient protection, the damage (withstand) level for the equipment must be determined, and the amplitude
and number of transients that will be conducted to the equipment must be known. This information is provided
in this section. Three areas of equipment circuitry normally require transient protection, and are listed below:
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a.
The ac power input.
b.
Where other externally exposed lines interface with the equipment.
c.
Rectifier outputs of 5 to 48 V dc power supplies that operate from commercial ac power and supply
operating power for solid-state equipment.
1.3.3.7.2
Transient Definition.
The waveform and amplitude of transients that may appear on commercial ac
input lines and other landlines connected to equipment are provided in this paragraph.
a.
AC powerline transients. The number and amplitude of lightning generated transients projected to
occur on ac power inputs to equipment over a 10-year period are listed in Tables 1-17 and 1-18. The waveform
for the transients is 8-by-40 microseconds where 8 microseconds is the risetime from zero to peak amplitude,
and 40 microseconds is the time from the start of the transient until exponential decay to 50% of peak value.
The transients listed are based on the data in Section 1.3.3.5. The transients listed in the two tables represent
clamp voltages that will appear across equipment by the facility secondary ac arrester installed at the main
service disconnect means (see Section 1.3.3.5) when discharging transient surges.
Voltages and currents
actually appearing across protected equipment will necessarily be related to the amounts and type of equipment
operating from power supplied by the main service disconnect means.
Table 1-17. Transient Surges, Line-to-Ground, Expected to
Appear Across Equipment by Secondary AC
Surge Suppressor Over a 10-Year Period
Surge Current Amplitude
Number of Surges
(8-by-40 µs)
1.5 kV, 100 A
1,500
2 kV, 200 A
700
2.5 kV, 300 A
375
3 kV, 500 A
50
3.5 kV, 1 kA
5
4 kV, 1.5 kA
2
4.5 kV, 2 kA
1
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Table 1-18. Transient Surges, Line-to-Line, Expected to
Appear Across Equipment by Secondary
AC Surge Suppressor Over a 10-Year Period
(Ungrounded Service Only)
Surge Current Amplitude
Number of Surges
(8-by-40 µs)
1,000
500 V, 50 A
b.
750 V, 100 A
100
1 kV, 200 A
50
1.5 kV,300A
10
Landline transients.
The number and amplitude of transients projected to be conducted to each
landline equipment interface are listed in Table 1-19.
The waveform for the transients is 10-by-1000
microseconds where 10 microseconds is the risetime from zero to peak amplitude for the transient, and 1,000
microseconds is the time from the start of the transient until exponential decay to 50’% of peak amplitude. The
information presented in Table 1-19 is based on data contained in Section 1.3.3.5. Since an equipment designer
will not normally know whether external lines will be enclosed in ferrous metal conduit, different transient
amplitudes are not provided in Table 1-19 for external lines enclosed in metal conduit.
1.3.3.7.3 Determination of Equipment Damage (Withstand) Levels.
Manufacturers do not normally specify
withstand levels for components. Therefore, an analysis should be performed to determine the withstand level
for each item of equipment that directly interfaces any externally exposed lines including ac input lines.
Transients that are projected to be conducted to equipment are provided in Tables 1-17, 1-18, and 1-19. The
analysis should be based either on results of laboratory tests or engineering analysis. Also the analysis must
include all equipment circuitry that will be exposed to transients. Three factors determine the withstand level
for the equipment as follows:
a.
Component destruction level.
The component destruction level is the transient energy level that
either causes immediate component destruction or degrades component operation to a point so that useful
operation cannot be achieved. This energy level is not usually specified or controlled by the manufacturer.
b.
Shortened component operating life. Useful component operating life can be appreciably shortened
by repeated overstressing of components. The overstressing occurs as a result of repeated application of some
level of transient energy. This energy level may be difficult in some cases to determine, but is certainly
meaningful when designing protection against transients.
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Table 1-19. Transient Surges Projected to Occur in 10-Year
Period on Externally-Exposed Landlines
Peak Amplitude
Number of Surges
(Voltage and Current)
100 V, 50 A
1,000
500
c.
500 V, 100 A
50
750 V, 375 A
5
1,000 V, 1,000 A
Operational upset level.
The operational upset level is the transient energy level that causes a
change in the equipment operating state. Since a change in the equipment operating state will normally create
an intolerable change in associated system operation, transient protection must ensure that transient energy
levels appearing across protected equipment do not cause operational upset.
To establish the equipment withstand level, compare the transient energy levels that cause immediate
component destruction, component overstressing, or equipment operational upset. Select the lower of the three
transient energy levels, and establish the withstand level at 10% below the lowest transient energy level.
1.3.3.7.4 Determination of Need for Transient Protection. Power supplies (5 to 48 V) operating from ac inputs
and supplying operating power for solid-state equipment always require internal transient protection. Other
equipment that directly interfaces externally exposed lines, including commercial ac inputs, may or may not
require transient protection designed as an integral part of the equipment.
To determine whether transient
protection is required, compare the equipment withstand level with the transients of Table 1-17, 1-18, or 1-19,
as applicable.
If the equipment withstand level is above the transient amplitudes provided in the tables,
equipment-level transient protection is not required.
When the transient amplitudes are above the equipment
withstand level, equipment-level transient protection is required , either at the ac input, other externallyexposed line-equipment interfaces, or both.
1.3.3.7.5 Minimizing Transient Damage.
When equipment requires protection against lightning generated
transient damage, transient suppression design must ensure that transients are attenuated to the equipment
withstand level prior to entering any equipment component.
Therefore, the transient suppression must be
effective at the external line-equipment interface.
a.
New equipment.
(1) AC inputs.
The most feasible method for providing transient suppression is to design the
suppression as an integral part of the equipment.
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(2) Other external line interfaces (dc to 3 MHz).
The most effective method for providing
transient suppression is to design low-energy level transient suppression as an integral part of the equipment
and specify that high-energy level transient suppression, of a design provided by the manufacturer, be installed
on applicable lines in cable demarcation junction boxes at building penetration or exterior equipment
termination. Total transient suppression may be designed as an integral part of the equipment but caution must
be exercised to ensure that a separate, dedicated path to earth ground be provided for the high-energy level
dissipation section of the transient suppression.
(3) External line interfaces (above 3 MHz).
All transient suppression must be designed as an
integral part of the applicable equipment. This is necessary because effective suppression devices/circuits are
not currently available for in–line installation on rf lines above 3 MHz, primarily because of high insertion
losses. If useable, effective high-energy level suppression becomes available in the future, the most effective
transient protection can be realized by installing high-energy level suppression on applicable lines at a metal
bulkhead connector plate at building penetration and including low-energy transient suppression as a part of the
equipment.
b.
Existing equipment.
The most effective transient protection can be provided as described in a(l),
(2), and (3) above. When room is not available in the existing equipment to add required transient suppression
components, the components can be installed in a small enclosure affixed to the chassis or cabinet rack for all
except rf lines that carry rf signals above 3 MHz.
1.3.3.7.6 AC Power Input.
The clamp voltage, appearing across protected equipment by the secondary ac
surge arrester installed at the facility main service disconnect means, when dissipating a transient surge, may
be higher than the withstand level for the equipment.
Therefore, effective transient suppression must be
designed as an integral part of the equipment.
a.
Transient suppression design. To provide effective protection, equal suppression must be installed
line-to-ground on each service conductor input and the neutral input.
For floating (ungrounded) line-to-line
power inputs, line-to-ground suppression must be installed and line-to-line suppression is optional. Suppressors
installed at the equipment power input should have a slightly lower turn-on voltage and a slightly faster
response time than suppressors of the secondary ac surge arrester at the main service disconnect means. This
permits the suppressors integral to the equipment to clamp short-duration overshoot voltage that occurs before
the secondary ac surge arrester can turn on and clamp in response to a transient. Also, with a lower turn-on
voltage, the suppressors at the equipment will nave a lower clamp voltage for a given transient surge than the
secondary arrester and thus provides optimum equipment protection.
However, with the specified character-
istics, the surge suppressors at the equipment will tend to dissipate the occurring transient before the secondary
arrester turns on.
with the input line.
Therefore, it is imperative to have an inductor or a minimum 10 foot cable added in series
If the inductor is properly chosen, the secondary surge arrester may then turn on very
rapidly after the equipment suppressor(s) turn on because of the voltage increase across the inductor. The
voltage increase is caused by current drain through the equipment suppressors to ground. Figure 1-41 depicts a
typical suppression circuit for use at the equipment level on ac inputs with a neutral. Figure 1-42 depicts a
typical suppression circuit for use on ungrounded (line-to-line) inputs.
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b.
Components.
(1) Inductor L1.
The inductor Ll, shown in Figures 1-41 and 1-42, is necessary to provide a
voltage increase to cause the secondary ac surge arrester at the main service disconnect means to turn on very
rapidly when suppressor RV1 turns on and conducts transient current to ground. The inductor must be capable
of safely passing normal operating voltages and current, and current resulting from 130% overvoltage for a
period of 50 milliseconds. Also, the inductor must:
(a) Have a very low dc resistance.
(b) Present a high impedance to transient surges.
(c) Present a very low impedance to 60 Hz line voltage.
(d) Be capable of safely passing the transient current listed in Table 1-17.
(2) Suppressor RV1.
Figure 1-41 shows RV1 as a metal oxide varistor (MOV) because the zinc
oxide nonlinear resistor type of MOV is especially well suited for this particular application.
Other types of
MOV are constantly being upgraded and are now possibly suitable for use. Other devices are also suitable for
use, and, in some cases will be required.
Silicon avalanche diodes are effective for use in protecting very
susceptible equipment. Data for different type suppressor are provided in Section 1.3.3.5. Use of a gas-filled
spark gap for use at the location of RV1 is not recommended for two reasons.
(a) Available gas-filled spark gaps with the required current handling capability have a
relatively high sparkover (turn-on) voltage and relatively slow turn-on times. Therefore, if spark gaps are used
for transient suppression at ac inputs, additional suppression including inductors, MOV and/or silicon avalanche
diode suppressors must be added to provide required protection.
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a.
TYPICAL TRANSIENT SUPPRESSION
FOR HOT AC INPUT TO EQUIPMENT
b.
TYPICAL TRANSIENT SUPPRESSION
FOR NEUTRAL AC INPUT TO EQUIPMENT
Figure 1-41. Typical Configuration for Protection of Equipment from
Conducted Powerline Surges and Transients (Neutral Grounded)
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Figure 1-42. Typical Configuration for Protection of Equipment from
Conducted Powerline Surges and Transients (Ungrounded)
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(b) Arc voltage for spark gaps is a nominal 20 to 30 volts. Therefore, when the transient
occurs causing the spark gap to turn on, normal line voltage is interrupted which will usually cause operational
upset of the affected equipment. Also, since the arc voltage is only 20 volts and is across a 120-volt supply, the
spark gap will likely remain in the arc mode of operation and draw current until the supply voltage waveform
crosses zero or until the supply circuit breaker opens. It is likely that the spark gap will be destroyed before
the supply circuit breaker opens. Either condition is very undesirable.
These two components form an LC network to filter out high
(3) Inductor L2 and capacitor Cl.
frequency components of transient surges and are required only for equipment susceptible to high frequency,
very short duration (less than 1 nanosecond) transient pulses that might pass across RV1.
c.
Transient suppression grounding.
When at all feasible, transient suppressor grounds should be
directly bonded to case ground. When the direct bond is not feasible, the suppressor grounds must be connected
as short and direct as possible to case ground, and the case must have a low bond resistance to earth ground.
Otherwise, the suppressors cannot operate properly.
d.
Functional
characteristics.
Functional characteristics for transient suppression at the ac input-
equipment interface must be as follows for effective transient suppression.
(1)
Voltage characteristics. The operating (reverse standoff) voltage must be between 200 to 300
percent of the normal line voltage for gas-filled spark gap suppressors.
For MOV, ZNR, and SAS type
suppressors, the reverse standoff voltage should be 175 ± 25 percent of the normal line voltage. Turn-on
voltage, discharge (clamp) voltage and the a amplitude and time duration of any overshoot voltage must be
sufficiently low to preclude equipment damage or operational upset.
Leakage current. Leakage current for each suppression component at reverse standoff voltage
(2)
must not exceed 100 microamperes.
(3) Self-restoring capability.
The surge suppressors must automatical ly restore to an off state
when transient voltage falls below turn-on voltage for the suppressor.
(4) Operating lifetime. Equipment transient suppression must be capable of safely dissipating the
number and amplitude of surges specified in Table 1-17 or 1-18 as applicable. Clamp voltage shall not change
more than l0 percent over the operating lifetime.
(5) In-1ine devices.
Only inductors designed to have low dc resistance shall be used as in-line
devices for suppression of conducted powerline transient.
In-line inductors shall safely pass equipment
operation voltages and line current with 130 percent overvoltage conditions for a period of 50 milliseconds.
Suppression components should be housed in a separate, shielded, compartmentalized
enclosure as an integral part of equipment design. Bulkhead-mounted, feedthrough capacitors should be used as
necessary to prevent high-frequency transient energy from coupling to equipment circuits. Suppression
e.
Housing.
components should be directly bonded to equipment case ground when at all feasible. Suppressor Connections to
ground must be short, straight, and direct.
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MIL-HDBK-419A
1.3.3.7.7 Power Supply Transient Suppression. Power supplies (5 to 48 V dc) that operate from commercial ac
power inputs and furnish operating voltage to solid-state equipment must have a transient suppressor installed
between the rectifier output and case ground. This protection (in addition to the service disconnect arrester
and powerline suppression at equipment entrances) is required because of the adverse electromagnetic
environmental operating conditions for much military equipment.
provide the best protection for this particular application.
A silicon avalanche diode suppressor will
The silicon avalanche diode suppressor is
recommended because of the very fast response time of the device, since the primary purpose is to clamp very
fast risetime and very short duration transients. In addition, the silicon avalanche diode suppressor provides the
lowest clamping voltage available.
Thus, when this device is used, the clamped output of the transient
suppression at the ac input-equipment interface will be clamped to a lower level by the avalanche diode at the
rectifier.
This, in turn, provides optimum protection for solid-state voltage regulators and other solid-state
components receiving operating voltage from the power supply. Operating characteristics for the suppressor
installed at the rectifier output must be as follows if the suppressor is to provide the desired function:
a.
Operating (reverse standoff) voltage.
Reverse standoff voltage must be 5 percent above maximum
rectifier output voltage.
b.
Leakage current.
Leakage current to ground should not exceed 100 microamperes at standoff
voltage.
c.
Turn-on voltage. Turn-on voltage must be as near standoff voltage as possible using state-of-the-
art suppressors, and shall not exceed 125 percent of reverse standoff voltage.
d.
Discharge (clamp) voltage. Clamp voltage must be the lowest possible value that can be obtained
using state-of-the-art suppressors not to exceed 160 percent of turn-on voltage.
e.
Overshoot voltage.
operational upset.
Overshoot voltage must be sufficiently low to preclude equipment damage or
Time duration of overshoot voltage shall be limited to the shortest possible time not
exceeding 2 nanoseconds.
f.
Self-restoring
capability.
Transient suppressors installed in power supplies must automatically
restore to an off state when line transient falls below rated turn-on voltage for the suppressor.
Operating lifetime. The transient suppressors must safely dissipate 1000 surges with an amplitude
g.
of 200 volts above rectifier output voltage and a waveform of 8-by-40 microseconds. Eight microseconds
defines the time from the start of the transient to peak voltage, and 40 microseconds is the time from the start
of the transient until the transient exponentially decays to 50 percent of peak value.
1.3.3.7.8 Landline Transient Suppression.
When the equipment withstand level is below the transient energy
level projected to occur at direct landline-equipment interfaces, transient suppression must be provided by
Generally, all direct landline-equipment interfaces will require transient suppression.
equipment design.
However, when the landlines are totally enclosed end-to-end in ferrous metal conduit, a much lesser degree of
suppression is required than when the landlines are direct earth-buried or overhead cable runs. A t the time of
new equipment design, when provisions for transient protection must be included, the manufacturer may not
know whether externally exposed landlines will be totally enclosed in ferrous metal conduit. When the
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MIL-HDBK-419A
manufacturer is not conclusively certain that external landlines will be enclosed in metal conduit, designed
transient protection must ensure that the equipment will be adequately protected against the transient levels of
Table 1-19. Subsequent paragraphs provide design guidelines for transient suppression for all types of landlines.
Coaxial and twinaxial lines are treated separately.
Also, externally-exposed landlines that carry signals of
3 MHz to 400 MHz are treated separately.
a.
Control, status, intrafacility power, and audio landlines.
Control, status, intrafacility power, and
audio lines, other than coaxial or twinaxial lines, are most effectively protected by transient suppression
designed as an integral part of the equipment, and specified transient suppression installed at building
penetration or exterior equipment termination. Effective design is shown in Figure 1-43.
(1) Suppression design and component selection.
Transient suppression will effectively protect
equipment only when proper components are selected so that the components operate in conjunction to provide
the desired function. This is necessary so that the clamped output of the suppression components/circuits can
provide optimum equipment protection. Actual suppression components are shown in Figure 1-43 as GT1, RV1,
RV2, and TS1. The suppression component at the equipment entrance should be chosen so that it has a lower
turn-on and clamping voltage than the suppression component at the facility entrance. Therefore, resistor R1
must provide a voltage to turn on the suppression component at the facility entrance and limit current flow
through the suppressor at equipment entrance. Otherwise, the suppression component at the facility entrance
may not turn on when a transient occurs. The component will not normally turn on when a transient of less than
400 volts peak amplitude occurs and the component is a gas-filled spark gap (GT1). However, when a transient
of greater amplitude occurs, the suppression component at the facility entrance must turn on. Otherwise, the
suppression component at the equipment entrance will attempt to dissipate the entire transient to ground. As a
result, the suppression component at the equipment entrance will attain a higher clamp voltage as it dissipates
additional transient current.
The higher clamp voltage is reflected across protected equipment. In addition,
the suppression component is likely to fail.
(a) Gas-filled spark gap GT1.
A gas-filled spark gap is suitable for use as a transient
suppressor at the building/facility entrance in some cases. The device has a relatively high sparkover (turn-on)
voltage and a relatively slow turn-on time when compared with a metal oxide varistor (MOV) or silicon
avalanche diode suppressor (SAS).
For typical lightning-induced transients on landlines, turn-on voltage is a
nominal 500 volts with an associated turn-on time of 5 microseconds. These characteristics are satisfactory as
long as the value of resistor R1 is 10 ohms or more, and the peak pulse current rating for the suppression
component at the equipment entrance is not exceeded.
When R1 is 10 ohms, a peak current of 50 amperes is
required to provide a voltage of 500 volts across R1 which is the nominal turn-on voltage for GT1. Since GT1
turns on after a nominal 5 microseconds, the peak pulse current rating for most MOV and SAS devices will not
be exceeded. After the spark gap turns on, arc voltage across the device is a nominal 20 volts. This may be
sufficiently below the normal line voltage to create operational upset of the protected equipment, which in
some cases cannot be tolerated. If normal line voltage is greater than 20 volts, difficulty may be encountered
in turning off the device, depending on available current.
The arc mode of operation may be sustained by
current greater than 1 ampere for some devices. When the value of R1 is less than 10 ohms, an MOV or other
equivalent suppressor must be used at the facility entrance because a spark gap will not turn on before the
suppressor at the equipment entrance is damaged by overcurrent, particularly when the suppressor at equipment
entrance is an SAS.
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MIL-HDBK-419A
(b) Metal oxide varistor (MOV) RVl, RV2.
As shown in Figure 1-43, MOVs can be used in
Various configurations to provide effective transient suppression.
Turn-on time for the MOV is less than 50
nanoseconds, and turn–on voltage ranges from 22 to 1800 volts. Clamp voltage is not as low as for SAS devices
and turn-on time is not as fast. The turn-on time for SAS devices is typically less than 10 nanoseconds, and less
than 1 nanosecond in some configurations. The configuration shown in Figure 1-43c is especially effective for
protecting highly susceptible equipment.
The configurations shown by Figures 1-43a and l-43b provide
adequate protection when the protected equipment can safely withstand the rated clamping voltage for the
MOV at the equipment entrance.
An MOV with a 20 mm element diameter will normally provide required
protection at the facility entrance, and a 10 m m element diameter MOV will normally provide required
protection at the equipment entrance.
To enable desirable functioning, the turn-on voltage of the MO V
suppressor at the facility entrance should exceed that of the MOV at the equipment entrance by approximately
10%. This is desirable to permit the MOV at the equipment entrance to turn on and dissipate low-amplitude
transients while reflecting a low clamp voltage to protected equipment.
When a high-amplitude transient
occurs, the voltage increase across Rl will cause the MOV at the facility entrance to turn on. When the MOV
at the facility entrance turns on, it dissipates most of the remaining transient energy, thereby eliminating or
greatly reducing the energy to the 110 V at the equipment entrance. Thus, the MOV at the equipment entrance
W Ill
conduct only a small amount of current and maintain a low clamp voltage that will appear across the
protected equipment. The MOV operating characteristics are similar to those for a pair of back-to-back zener
diodes. Therefore, the device responds the same to a negative or positive transient voltage.
(c) Silicon avalanche diode suppressor (SAS) TS1.
The SAS device has the fastest turn-on
time of any of the three suppressor devices shown in Figure 1-43.
Turn-on time is typically less than
10 nanoseconds and can be less than 1 nanosecond in some configurations depending on lead length and the path
to ground for the device. Turn-on voltage ranges from 6.8 volts to 200 volts. Devices may be connected in
series to obtain higher turn-on voltages and to improve power handling capability, For example, two devices
connected in series can dissipate approximately 1.8 times the power dissipated by a single device. The clamping
voltage for the device is also lower than for MOV devices. The maximum clamping voltage for the SAS devices
is approximately 1.6 times the turn-on voltage at peak pulse current.
Peak pulse current ranges from 139
amperes for a 6.8-volt device to 5.5 amperes for a 200-volt device over a period of 1 millisecond. Devices
recommended for use at the equipment entrance have a peak pulse power dissipation rating of 1500 watts over a
period of 1 millisecond.
Devices are available in both unipolar and bipolar configurations. Operation of a
unipolar device is very similar to that of a zener diode, and operation of a bipolar device is very similar to that
of a pair of back–to-back zener diodes.
For the most effective protection, unipolar devices should be used on
lines that carry unipolar voltage provided the ac noise level on the applicable line is less than 0.5 volt. Use
bipolar devices on lines that carry bipolar (at) voltage and on lines with an ac noise level greater than 0.5 volt.
Select SAS devices based on the reverse standoff voltage rating. The reverse standoff voltage must be greater
than maximum line operating voltage, and should exceed normal line voltage by 20% when possible.
(d) Resistor R1.
The function of resistor R1 is to provide current limiting for the
suppression device at the equipment entrance and to provide a turn-on voltage for the suppressor at the facility
entrance.
Empirical evidence has shown that the power rating for the resistor should be 5 watts. The
resistance value should be as high as equipment operation will permit. Typical values are 10 to 50 ohms. Values
as low as 2 ohms have been successfully used. However, when the value is less than 10 ohms, the suppressor at
the facility entrance must be an MOV or equivalent type suppressor.
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MIL-HDBK-419A
AT FACILITY
ENTRANCE
AT EQUIPMENT
ENTRANCE
RI
5W
R2
IW
LANDLINE
EQUIPMENT
CIRCUIT
a. CONFIGURATION NO. 1
R2
lW
RI
5W
LANDLINE
EOUIPMENT
CIRCUIT
b. CONFIGURATION NO. 2
R2
IW
RI
5W
LANDLINE
c. CONFIGURATION NO. 3
Figure 1-43. Typical Configuration for Protection of Equipment from
Conducted Landline Transients
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EQUIPMENT
CIRCUIT
MIL-HDBK-419A
(e)
Resistor R2 attenuates current flow to protected
Resistor R2 and capacitor C1.
equipment resulting from clamp voltage of the transient suppressor at the equipment entrance.
The resistor
also speeds up, and in some cases, generates turn-on of the transient suppressor at the equipment entrance. In
addition j the resistor limits current drain from protected equipment when a transient with polarity opposite
that of tile equipment power supply occurs.
A power rating of 1 watt is sufficient for the resistor. The
resistance value should be as high as can be tolerated by applicable equipment, taking into consideration the
value of resistor R1 and the impedance of the associated landline. The purpose of capacitor C1 is to filter out
some high-frequency transient components, and the value of C1 should be selected accordingly. In some cases,
equipment operating characteristics and line length may preclude the use of resistor R2 and eapacitor C1.
(2)
Grounding for suppresion components/circuits.
The high-energy transient suppressors, shown
at the facility entrance in Figure 1-43 must be grounded to earth ground by means of the shortest path. This
will minimize the large voltage spikes, caused by L di/dt effects when high-amplitude transient currents flow
through the high-energy transient suppressor onto the ground, which in turn may damage protected equipment
or the low-energy transient suppressors at the equipment entrance.
(a) Grounding of transient suppressor at facility entrance. The high-energy transient
suppressors installed at the facility entrance should be located in a junction box or the main (first) service
disconnect where incoming lines are first terminated.
The most effective ground for the suppressors can be
provided by a ground bus bar located in the first service disconnect or the junction box. The transient
protection devices (TPD's) must be bonded to the TPD box and grounded by the shortest means. It is important
that the ground wire has no sharp turns or bends, and is as short as feasible. The ground bus bar should be
located to permit short, direct connection of suppressors between landline terminations and earth ground.
(b) Grounding of transient suppressor at equipment entrance. The low-energy transient
suppressor at the equipment entrance should be directly bonded to the equipment case when possible. The
ground side of the suppressor at the equipment entrance must be connected with a short, straight, direct
connection to equipment case to be effective. Connection of the suppressor to equipment case references both
the suppressor and equipment circuits to the same ground potential, thus providing optimum equipment
protection.
(3)
Packaging design.
Transient suppression components/circuits included as an integral part of
equipment design should be enclosed in a shielded, compartmentalized section of the equipment.
necessary to preclude cross-coupling of transient energy to other equipment circuits.
This is
The suppression
components must be located so that transients are attenuated prior to entering any equipment component
susceptible to damage, including EMI filters.
Packaging design for transient suppression specified for
installation at facility entrance is not critical. However, the design should provide for short, direct connection
of transient suppressors between the line termination and ground.
b.
Coaxial and twinaxial lines (dc to 3 MHz). The same transients are projected to occur on externally
exposed coaxial and twinaxial lines as on the control and status lines discussed in paragraph 1.3.3.7.8a. In
general, the same transient protection described in paragraph 1.3.3.7.8a will provide effective transient
protection for equipment that directly interfaces the coaxial and twinaxial lines. That is, the most effective
transient protection is provided by installing a high-energy transient suppressor and resistor at the facility
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MIL-HDBK-419A
entrance or exterior equipment termination, with low-energy transient suppression included as an integral part
of tile equipment as shown in Figure 1-43. However, in many cases, end equipment connected to coaxial lines
cannot tolerate added capacitance imposed by capacitor C 1 . Also, in most cases, the added resistance of
resistor R2 cannot be tolerated. Because most end equipment connected to coaxial and twinaxial lines has a
relatively low withstand level, the configuration shown in Figure 1-43c, without resistor R 2 and capacitor C1 ,
should be used for transient suppression.
The silicon avalanche diode suppressor TS 1 should always be bipolar.
The configuration shown by Figure 1-43c should be used for protection of equipment that directly interfaces
externally exposed twinaxial lines. In most cases, it is necessary to use a bipolar SAS since the twinaxial lines
normally conduct both dc and low–level audio signals. Specific design criteria is provided in paragraphs (1) and
(2) below.
Facility entrance suppression. The high-energy transient suppression specified for location at
(1)
facility entrance or exterior equipment termination should be designed for in-line installation on applicable
lines.
The lines should be terminated at a metal connector plate located in a junction box at the facility
entrance or exterior equipment termination. Transient suppression components should be enclosed in a sealed,
metal enclosure with appropriate connectors to facilitate in-line installation. The ground side of suppressor(s)
in tile sealed package must be connected as directly as possible with No. 12 AWG copper wire (minimum) to a
ground point located on the exterior of the sealed package to facilitate connection to a ground bus or tie point
in the junction box. The package for a twinaxial line must include two suppression circuits, one for each of the
two center conductors. Also, when a coaxial cable shield is not directly grounded at interfaced equipment, the
enclosure for In-1ine Installation must also contain two transient suppression circuits, one for the cable center
conductor and one for the cable shield. Circuit configurations for each type of line are depicted in Figures 1-44
and 1-45.
Primarily because of the grounding configuration , MOV or equivalent devices should be used at
facility entrance.
(2)
Equipment entrance suppression.
Equipment entrance suppression is shown in Figure 1-44 for
coaxial line-equipment interfaces. The transient suppression should be enclosed in shielded, compartmentalized
areas to prevent cross-coupling of transient energy to other equipment circuitry. The transient suppression
must be located so that transients are attenuated prior to entering any susceptible equipment components,
Including EMI filters.
Because of the normally low withstand levels for end equipment, only bipolar avalanche
diode suppressors should be used at equipment entrance.
However, MOV suppressors may be used when the
protected equipment can safely withstand tile clamp voltages that will appear across protected equipment. For
the most effective protection, the ground side of transient suppressors should be bonded directly to equipment
case. When direct bonding is not possible, short, direct connections to equipment case must be used.
c.
Transient suppression for lines in metal conduit. When externally exposed lines are enclosed end-to-
end in ferrous metal conduit, the amplitude of transients
projected to be conducted to equipment will be
attenuated a minimum of 90%. The number of transients that occur will not change. Therefore, the number of
transients listed in Table 1-19 will still occur, but amplitudes will be only 10% of the amplitudes listed in
Table 1-19. When the equipment manufacturer is absolutely certain that all externally exposed equipment lines
will be enclosed in ferrous metal conduit, total transient suppression should be designed as an integral part of
the equipment.
The total transient suppression should consist of a 5-watt resistor in series with the landline
input, and an MOV or SAS connected line-to-ground on the equipment side of the 5-watt resistor.
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MIL-HDBK-419A
Figure 1-44. Transient Suppression for Coaxial Lines (DC To 3 MHz)
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MIL-HDBK-419A
Figure 1-45. Transient Suppression for Twinaxial Lines (DC To 3 MHz)
d.
Functional
characteristics.
For effective transient suppression, the suppression components must
have certain minimum operating or functional characteristics.
These characteristics are defined in
paragraphs 1.3.3.7.8d(l) and (2) for high-and low-energy transient suppressors, respectively.
(1)
High-energy transient suppression characteristics.
(a)
Reverse standoff voltage.
Reverse standoff voltage for spark gap type suppressors
should be between 200 and 300 percent of the nominal operating line voltage.
For MOV, ZNR, and SAS type
suppressors, the reverse standoff voltage should be 175 + 25 percent of the nominal line voltage.
Leakage current.
(b)
reverse standoff voltage.
I.eakage current to ground should not exceed 100 microamperes at
Turn-on voltage.
Turn-on voltage should not exceed 125 percent of reverse standoff
(c)
voltage.
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MIL-HDBK-419A
(d) Overshoot voltage. Overshoot voltage should be the lowest voltage that can be obtained,
for the shortest time possible, using the best state-of-the-art suppressors available.
(e)
Clamp (discharge) voltage.
Clamp voltage of the transient suppressors should be as low
as possible and not more than 225 percent of turn-on voltage when discharging a transient with 1000 amperes
peak amplitude.
(f)
Operating life. The transient suppressor must be capable of discharging the number of
transients listed in Table 1-19 with peak amplitudes that are 90% of those listed in Table 1-19. Clamp voltage
must not change more than 10 percent over the operating lifetime.
(g) Self-restoring capability. The transient suppressor must automatically restore to the off
state when the transient voltage level falls below turn-on voltage.
(2) Low-energy transient suppressor characteristics.
(a)
Reverse standoff voltage.
The reverse standoff voltage rating of the transient
suppressor should be between 200 to 300 percent above the nominal line voltage for spark gap type suppressors.
For MOV, ZNR, and SAS type suppressors, the reverse standoff voltage should be 175 ± 25 percent of the
nominal line voltage.
(D)
Turn-on voltage. Turn-on voltage of the suppression component at the equipment must
be as close to reverse standoff voltage as possible using state-of-the-art devices, and shall not exceed 125
percent of reverse standoff voltage.
(c) Overshoot voltage. Overshoot voltage must be the lowest value that can be obtained, for
the shortest time possible, using state-of-the-art suppressors. Overshoot voltage shall be low enough to
preclude equipment damage or operational upset. The requirement will apply for transients with rise times as
fast as 5,000/µs.
(d)
Leakage current.
Leakage current to ground should not exceed 100 microampere at
reverse standoff voltage.
(e)
Clamp voltage.
Clamp voltage must remain below the equipment withstand level while
dissipating transient currents with peak amplitude that are 10 percent of those listed in Table 1-19. The clamp
voltage must not change more than 10 percent over the operating lifetime.
(f)
Operating life.
The transient suppressor must be capable of safely dissipating the
number of transients listed in Table 1-19, with current amplitudes that are 10 percent of those listed in
Table 1-19.
e.
RF coaxial lines (above 3 MHz).
At the present time, there is some difficulty encountered in
providing effective transient suppression for lines that conduct signals above 3 MHz in frequency, and especially
above 10 MHz. Most suppression devices that provide low-level clamping of transients have enough capacitance
to create high insertion losses when installed line to ground on the conductor. Packaging of the devices for inline installation without causing high insertion losses is also difficult and expensive. Gas-filled spark gaps have
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MIL-HDBK-419A
been successfully packaged for in-line installation on critical rf lines, but unit cost is excessive. Also, gas-filled
spark gaps do not always provide satisfactory protection because of high sparkover (turn-on) voltage, slow
turn-on time, and low arcing voltage.
Therefore, the best alternative at present is to include transient
suppression design as an integral part of new equipment.
Transient suppression design. Potential sources of effective transient suppression are gas(1)
filled spark gaps, MOV in series with rf chokes, and surge-rated, low capacitance silicon avalanche diodes
paralleled with selected rf chokes.
A11 of the suppression devices and components are for line-to-ground
connection at the line-equipment interface.
(2)
Transient suppression grounding. The total transient suppression is included as an integral part
of the equipments, and may have to dissipate the transient currents listed in Table 1-19. However, in most
cases, these lines will be enclosed in ferrous metal conduit, and the amplitude of occurring transients will
therefore be only 10% of the values listed in Table 1-19.
In either case, the transient suppression should be
grounded directly to equipment case ground using the shortest and most direct method possible. The equipment
case must, in turn, be effectively connected to the earth grounding system via the equipment rack and the
equipment grounding conductor, when applicable.
(3) Packaging design. The transient suppression should be located in a shielded, compartmentalized section of the equipment and located so that conducted transients are attenuated prior to entering any
susceptible circuit component.
1.3.3.8 Corrosion Control.
a.
The materials of which lightning protection subsystems are made must be highly corrosion resistant.
Junctions or contact between dissimilar metals must be avoided; where such unions are unavoidable, moisture
must be permanently excluded from the contacting surfaces.
b.
Where any part of a copper protective system is exposed to the direct action of chimney or other
corrosive gases, the exposed copper elements are to be protected by a continuous hot dip coating of lead. The
coating should extend at least 0.6 meters (2 feet) below the top of the chimney or past the vent or flue opening.
c.
Where aluminum down conductors are used, do not permit them to come in contact with the soil.
(1) Connections between aluminum down conductors and copper ground electrode risers are not to
be made lower than one foot above grade level; use UL-approved bimetallic connectors for these connections.
(2)
Aluminum parts, including fasteners and anchors, should be protected from direct contact with
concrete or mortar wherever such concrete or mortar is wet or damp or may become intermittently wet or
damp.
(3)
Aluminum parts also must be protected from contact with alkaline-based paints.
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MIL-HDBK-419A
d.
Aluminum parts are not to be used on copper roofing materials and must not contact other copper
surfaces such as gutters, flashings, and trim.
Similarly, do not use copper lightning protection materials on
aluminum structures or on structures using aluminum roofing materials or aluminum siding. Avoid contact
between copper conductors, terminals, and fasteners and aluminum gutters, windows, and trim.
e.
In aluminum lightning-protection systems, copper, copper-covered, or copper-alloy fixtures and
fittings must not be used for connectors.
Where aluminum must connect to copper, only UL-approved
bimetallic connectors are to be used.
1.3.3.9 Joints.
a.
Welded or brazed bonds are preferred over all other types; in particular, junctions in inaccessible
locations should be welded or brazed whenever practical.
b.
Never use soldered connections for bonding any part of the lightning protection system.
c.
Bolted or clamp-type connections should employ only UL-approved connectors.
d.
Where bolted connections to flat surfaces are necessary, the surface contact area should be 3 square
inches (19.5 square cm) or greater.
1.3.3.10 Physical Protection.
a.
Protect all elements of the lightning protection system from damage and physical abuse by routing
conductors to take advantage of any protection offered by structural features.
Install appropriate guards or
covers preferably made of wood or noncombustible synthetic material.
b.
Where conductive conduit is used, bond the conduit to the enclosed lightning conductor at each end
of each isolated section of the conduit. (Standard conduit grounding lugs are acceptable. )
The use of ferrous conduit to enclose lightning conductors should be avoided because it increases the
c.
impedance of the lightning conductor.
1.4 FAULT PROTECTION SUBSYSTEM.
1.4.1 Purpose.
In Volume I, the equipment fault protection subsystem was described as a network which
ensures that personnel are protected from shock hazard and equipment is protected from damage or destruction
resulting from faults that may develop in the electrical system. To accomplish this, ground connections must
be adequate for both normal and fault currents. The fault protection subsystem includes the green wire and all
exposed noncurrent-carrying metal parts of fixed equipment such as raceways and other enclosures which are
likely to be energized under power fault conditions.
Any conductor used for grounding purposes shall not
penetrate any designated rf barrier, screen room, shielded enclosure etc., but shall rather be bonded to a welded
stud on the barrier. In general, the equipment fault protection subsystem will conform to the requirements
established in MIL-STD-188-124A.
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MIL-HDBK-419A
1.4.2 Equipment Fault Protection Subsystem Composition.
One of the major shortcomings in grounding
systems is the interconnection and reversal of ac neutral and protective wires of the ac power distribution at
various power distributions panels and at equipment througout a facility. These installation errors result in
additional electrical noise and ac currents in the ground system. The equipment fault protection subsystem
should generally follow a configuration from a central or main ground point which, ideally, should be at the
primary power station transformer ground point: or, it should be bonded directly to the earth electrode
subsystem at thle communications building, if a protective wire is not available to the main ground point. The
configuration consists of a central main or trunk lead from the power source with protective conductors to the
various intermediate power panels and equipment.
The protective wire is carried along with the phase and
neutral wires from the main ground point to the main circuit breaker pannel, from there to intermediate circuit
breaker panels to the equipment panels, and finally to the equipment.
1.4.3 Configuration of the Equipment Fault Protection Subsystem.
a.
The equipment fault protection subsystem consists primarily of the grounding conductors of the
interior ac power distribution system.
The grounding conductors are green insulated or bare wires running in
the same conduit or duct with the neutral and phase conductors. revered grounding conductors are preferred to
reduce EMI.
Figure 1-46 illustrates a typical equipment fault protection subsystem.
Key points to be noted
are:
(1) The conduit is grounded to the power panel at each end, but it is not used in lieu of a grounding
conductor which continues through the conduit to the protective bus bar.
(2) The ac neutral lead can be grounded at the first service disconnect means. In this case, the ac
neutral also serves as a protective conductor back to the source. For best results, the ac neutral {grounded) and
green (grounding) wire should be grounded at the service transformer and the first service disconnect means
through the five-wire distribution system.
b.
To protect personnel from exposure to hazardous voltages, all exposed metallic elements of
electrical and electronic equipment shall be connected to ground. In the event of inadvertent contact between
the "hot" lead and chassis, frame, or cabinet through human error, insulation failure, or component failure, a
good, direct, known fault current path will be established to quickly remove the hazard. The neutral lead shall
be grounded for fault protection preferably at the distribution transformer and if the transformer is outside the
building, at one additional point outside the building. It should never be grounded on the load side of the first
service disconnect means.
c.
Grounding conductors and ground connections for transformers, switchgears, motors etc., shall
comply with the requirements of the NEC Articles 250-92 and 250-95.
Metal boxes, fittings, and noncurrent-carrying metal parts of other fixed equipment do not require
d.
additional protection if metallically connected to the grounded cable armor or bonded to the grounded members
of the building.
MIL-STD-188-124A provides that the path to ground for circuits, equipment, and conductor
enclosures be permanent and continuous.
The path must have (1) the capacity to conduct safely any fault
current likely to be imposed upon it, and (2) sufficiently low impedance to limit voltage to ground and to aid the
operation of circuit protective devices.
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MIL-HDBK-419A
Figure 1-46.
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MIL-HDBK-419A
e.
Article 250-91 of the NEC describes
the types and materials used for equipment grounding
conductors. Types include solid and stranded (insulated or bare) wire or other shapes, such as metallic tubes,
pipes, and conduit.
cable
The grounding conductor types permitted by the NEC also include various metal ducts,
trays, and raceways however these types shall not be used in lieu of the equipment grounding conductors.
The NEC also permits/allows certain types of armored cable sheath be used as grounding conductors.
f.
Experience with military C -E facilities has proven that a low-noise, low-impedance equipment fault
protection subsystem can be maintained over a prolonged period of time if separately designed and installed
ground conductors are provided. Therefore, a separate equipment fault protection conductor shall be included
with the ac power distribution if not provided in the power cable.
installed in the
S ame
A grounding (green) wire should be used and
conduit as the other ac wires. When ferrous ducts or conduits are used to protect or shield
the neutral and phase conductors, the lowest impedance will result when this grounding conductor is installed in
tile same duct or conduit.
The impedance can be further decreased if the grounding conductor is wrapped
around the other conductors and bonded to tile duct or conduit at both ends.
In a correctly installed power
distribution system, there should be no power current on the grounding conductor, except during a fault
condition.
It should be noted that there are two types of faults causing overcurrent devices to operate. The
first is an overload condition in equipment.
In this case, fault current is on the neutral and phase leads. The
second fault is where a phase or hot lead is inadvertently grounded. The faint current in this case is on the base
lead and the grounding conductor. In both cases, the overcurrent protective device, usually a circuit breaker, is
Due to the fault currents that can flow either on the phase leads, neutral, or
opened in tile phase or hot lead.
grounding conductor, i t is recommended that a 2-inch separation be maintained between power runs and signal
runs when neither is in conduit.
1.4.4 Pipes and Tubes.
a.
All metallic pipes and tubes (including conduit) and their supports should be electrically continuous
and are to be bonded to the facility ground system at least at one point. If any run of metal pipes or tubes
exceeds 3 meters (10 feet) in length, it should be bonded to the facility ground system at each end. Also, longer
runs should be bonded to the facility ground system at intervals of approximately 45 meters (150 feet).
b.
At indoor locations, these bonds may be made with clamps which provide continuous pressure. Pipes
installed out of doors should be bonded to the facility ground system at entry point or wherever feasible by
welding or brazing. Compatible stainless steel straps may be used with stainless steel pipe. In the event that a
direct bond cannot be made, zinc-plated hose clamps or stranded, bare copper, untinned bond straps may be
used. All bonds should be adequately protected against corrosion in humid or corrosive environments.
c.
Joints in metal pipes and tubes should have a dc resistance no greater than 5 milliohms. In the case
of threaded joints, the threads should be cleaned and firmly tightened (200 ft-lbs for hard wall conduit) and
protected against corrosion. Leaded or caulked joints, flared and other compression fittings, and 0-ring fittings
should all be measured to verify that the joints have a dc resistance no greater than 5 milliohms.
1.4.5 Electrical Supporting Structures.
Electrical supporting structures such as conduit, cable trays or
raceways, wiring system enclosures , and metallic power cable sheaths should be electrically continuous and are
to be bonded to the facility ground system at multiple points,
prevent these structures from rising to a hazardous potential.
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In the event of a fault, this continuity will
MIL-HDBK-419A
1.4.5.1 Metal Conduit.
a.
All metal conduit is to be grounded, regardless of whether it is used for enclosing power cables or
for signal and control cables.
All joints between sections of conduit and between conduit, fittings and boxes should be made
b.
electrically continuous when they are installed.
c.
All pipe and locknut threads should be thoroughly cleaned before they are engaged and then
tightened flrmly. For additiona1 information see Sections 7.6 and 7.7 of Volume I.
d.
Gouging locknuts must positively penetrate all paint or other nonconductive finishes.
e.
Any joints not inherently continuous should be bonded with jumpers of No. 12 AWG or larger copper
These jumpers should be welded or brazed in place or attached with clamps, split bolts, grounding
wire.
bushings, or screws and lockwashers.
f.
Protect the bonds against weather, corrosion, and mechanical damage.
g.
Firmly tighten the screws on the cover plates of pull boxes, junction boxes, and outlet boxes.
h.
All conduit brackets and hangers should be securely bonded to both the conduit and to the structural
member to which they are attached.
Bond conduit runs, to include the individual sections, couplings, line
fittings, pull boxes, junction boxes, outlet boxes, etc., to the facility ground system at intervals not exceeding
15 meters (50 feet). The resistance to each connection should not exceed 5 milliohms.
1.4.5.2 Cable Trays.
Make all cable tray systems electrically continuous by bonding together each individual
section as described in Section 1.7.4.1.
support.
Bond each support bracket or hanger to the cable trays which they
Connect the cable tray assemblies to the facility ground system with copper cables or equivalent
conforming to the 2000 cmil per foot criterion.
Make the connections within two feet of each end and at
intervals not exceeding 15 meters (50 feet) along the run. Where metal covers are used, they should be securely
bolted in place.
1.4.5.3 Enclosures.
Ground all enclosures of electrical and electronic wiring and distribution equipment in
accordance with MIL-STD-188-124A.
1.4.5.4 Cable Armor. The armor on electrical power cables should be bonded to the facility ground system at
each end if the cables are 3 meters (10 feet) or longer.
Provide supplemental connections at intervals not
exceeding 15 meters (50 feet). The resistance of each connection should not exceed 5 milliohms.
1.4.5.5 Rotating Machinery,
The frames of motors, generators, and other types of electrical rotating
machinery are to be connected to the facility ground system in accordance with the NEC requirements (1-6).
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1.4.6 Power Distribution Systems.
The neutral of an aC power distribution system is to be grounded to the earth electrode subsystem at
a.
the first service disconnect as well as to the ground terminal at the secondary distribution transformer. For
separate facilities served by a common distribution system, each disconnecting means should be grounded to the
earth electrode subsystem common to the facility. This connection may. be lifted if noise or hum problems are
encountered. At separate facilities having a common earth electrode subsystem, each service disconnecting
means should be grounded to the closest point on the earth electrode subsystem.
b.
All distribution neutrals are to be isolated from equipment and structural elements except for the
connection at the first service disconnect.
Connect the ground terminals of convenience outlets to the facility ground system with the green
c.
wire specified by the NEC. Do not use wire mold or plug mold distribution strips which depend upon serrated or
toothed fingers for grounding.
Effectively ground the ground terminals on such strips with auxiliary grounding
conductors equivalent to the green wire requirements of Table 250-95 of the NEC.
d.
For a dc power system, ground one leg with a single connection to the earth electrode subsystem.
The size of the grounding conductor should conform to the requirements of the NEC. Whether grounded at the
source or at the load, provide a dedicated current return conductor from the load to the source to assure that
the dc load current in the facility ground system or the lower frequency signal ground network is minimized.
1.4.7 Standby AC Generators.
The frames and housing of ac generators should be grounded as prescribed by
the NEC. Ground the neutral to the facility main ground plate or to the earth electrode subsystem, whichever
IS
closest,
When generators are connected in parallel, interconnect the neutrals and ground them to the
facility’s earth electrode subsystem with a common grounding conductor.
1.4.8 Equipment Fault Protection Subsystems for Transportable Equipment.
a.
To protect personnel from exposure to hazardous voltages, all exposed metal elements of equipment
and supporting’ structures shall be interconnected by a green wire from the ac power distribution system and
referenced back to the power source.
The grounding requirements of a transportable facility installed in the
field and operating from transportable engine generators is relatively simple. The primary requirement is to
ensure that (1) all vans, vehicles, traders, and engine generator units are interconnected through a protective
(green wire) network, and (2) the power neutral is grounded from a common bus that is connected to an earth
electrode at tile generator. Where parts are movablc or subject to vibration, metal straps may be used in lieu
of the green wire.
b.
When transportable facilities are powered from a commercial base ac source or are integrated into a
permanent installation with nontransportable facilities, personnel protection requirements become more
complex. When part of a fixed installation, the transportable system shall be integrated into the facility ground
system by extending the earth electrode subsystem to provide connections for the transportable facility. All
metallic components of the facility shall be
interconnected through the equipment fault protection subsystem
and bonded to the earth electrode subsystem at the main power panel, or back to the primary power source
through the ground conductor of the power distribution cable.
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1.4.9 MIL-STD-188-124A and NEC Commpliance Evaluation.
1.4.9.1 Measurements. This portion of the survey is performed to determine if the facility complies with the
requirements of the MIL-STD-188-124A as applicable to military installations in regard to grounding, bonding,
and shielding.
Powerline and equipment grounding conductor current measurements, not related to the NEC,
are made at the time of the survey for convenience.
provided.
Survey form contained in Section 2.2.2.4 Part IV is
Guidance in making the current measurement is provided in Sections 2.3.2.4 and 2.3.3 Part IV.
Prepare sketches, as appropriate, that may aid in explaining the results of the survey or illustrating the
installation. Attach the sketches to the survey data.
a.
The verification required by some steps may involve more than an inspection. For example, in
verifying that all neutral conductors are color-coded white or natural gray, if a green colored wire is found
connected to the neutral bus it will be necessary to trace out the conductor to determine its proper function. If
the verification is not accomplished at the time of the inspection, the discrepancy should be recorded on the
data sheet and noted as a potential violation.
b.
During the inspection, it may be desirable to correct a deficiency at that time (e.g., cleaning a bond
area of paint). In such cases, record the discrepancy on the data sheet and note the corrective action taken.
1.4.9.2 MIL-STD-188-124A and N E C Compliance Inspection.
Sections a(l), a(9), d(9), and d(15) are not MIL-STD-188-124A or NEC requirements but are for
Note:
information.
a.
Service entrance.
Perform the following to determine that wiring at service entrances is in
accordance with MIL-STD-188-124A requirements.
(1) Determine if the input to the facility, from the power company, is single phase or three phase,
If three phase is delta or wye, and if one of the service conductors is identified (grounded).
The identified
conductor will be the neutral.
Verify that each run of cable, conduit, etc., contains all phases and the identified conductor
(2)
and that each identified conductor is grounded at or in the vicinity of and ahead of the service disconnecting
means. For example, if the source is a transformer whose secondary is a 3-phase, 4-wire wye with the neutral
grounded and the power is routed to the service disconnect switch through 3 conduits, each conduit must
contain all 3 phases and neutral, and the neutral must be grounded in the vicinity of the service disconnect
switch.
(3) For each building or service supplied by a single source, verify that the identified conductor is
routed to each service disconnect switch and that the conductor is connected to the earth electrode at each
building with a grounding electrode conductor that meets the requirements of Table 1-20.
(4) If the grounding electrode conductor is routed through a metallic enclosure (conduit, etc.),
verify that the enclosure is bonded to the conductor at both ends.
Note: It may not be possible to verify this connection at the electrode end as it may be inaccessible.
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MIL-HDBK-419A
(5)
Verify that the color of the identified conductor is white or natural gray. If larger than a
No. 6 AWG and of a different color (not green), it should be reidentified white or natural gray with paint or
tape, or by other means such as tags or labels.
(6)
Verify that the equipment grounding conductors ("safety" or "green" wire) are green or green
with one or more stripes, or if larger than No. 6 AWG and of a color other than green, not white, has been
reidentified with green tape, paint, or other means.
(7)
Verify that all metal noncurrent carrying service equipment is effectively bonded by one of
tile methods specified below and that all non-conductive coating in the bonding path has been removed:
(a)
Bonding jumpers connected by pressure connector, clamps, or other means.
(b)
Threaded couplings and threaded bosses on enclosures with joints that are tight when
rigid conduit is involved.
(c)
Threaded coupling used for metallic tubing and rigid conduit is tight.
(d) Bonding jumpers are used around knockouts that are punched or otherwise formed so as
to impair the electrical connection.
(e) Bonding-type locknuts and bushings on other devices.
Verify that all covers for wireways, junction and pullboxes, surface raceways, etc., are
(8)
installed and secured.
(9) Using a clamp-on ammeter, measure the current in each phase conductor and the identified
service conductors. Also measure the current in the grounding electrode conductor. Record the current levels
and wire sizes on Part IV of the survey form in Chapter 2.
b.
Separately derived power sources.
For premises derived sources, (a premises wiring system with
power derived from an on-site generator, transformer or converter windings that have no direct electrical
connection to supply conductors originating in another system) perform the following for each source.
(1)
Verify that the neutral conductor is grounded where the following exists:
(a)
AC circuits of less than 50 volts (peak to peak) where:
1.
Supplied to transformers if the transformer supply system exceeds 150 volts (peak
2.
—
Supplied by transformers if the transformer supply system is ungrounded.
3.
—
Installed as overhead conductors outside of buildings.
to peak) to ground.
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Table 1-20. Grounding Electrode Conductor Size
Size of Largest Service-Entrance
Minimum Size of Grounding
Conductor or Equivalent for
Electrode Conductor
Parallel Conductors (AWG)
(AWG)
Copper
Aluminum or
Aluminum or
Copper-Clad
Copper-Clad
Aluminum
Copper
Aluminum l
2 or smaller
0 or smaller
8
6
l or 0
2/0 or 3/0
6
4
2/0 or 3/0
4/0 or 250 MCM
4
2
Over 3/0 thru
Over 250 MCM
2
0
0
3/0
2/0
4/0
3/0
250 MCM
350 MCM
thru 500 MCM
Over 500 MCM
Over 350 MCM
thru 600 MCM
Over 600 MCM
thru 900 MCM
Over 900 MCM
thru 1100 MCM
Over 1100 MCM
thru 1750 MCM
Over 1750 MCM
NOTE: Where the service conductors or the equivalent size of parallel conductors exceed 1100 MCM, the size
of the grounding electrode conductor shall not be less than 12-1/2 percent of the area of the service
conductor(s).
l
Aluminum or copper-clad aluminum grounding conductors shall not be used where in direct contact with
masonry or the earth or where subject to corrosive conditions.
Where used outside, aluminum or copper-clad
aluminum grounding conductors shall not be installed within 45 cm (18 inches) of the earth.
(b)
The source can be grounded such that the maximum voltage to ground on the ungrounded
conductors does not exceed 150 volts.
(c) Where the source is nominally rated 480/277-volt, 3-phase, 4-wire wye in which the
midpoint of one phase is used as a circuit conductor.
(d)
Where the source is nominally rated 240/120-volts, 3-phase, 4-wire wye in which the
midpoint of one phase is used as a circuit conductor.
(e) Where a grounded service conductor is uninsulated.
(2) Where a source is grounded, verify that the installation complies with 1.4.9.2b(l). Also see
Article 250-26 of the NEC.
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MIL-HDBK-419A
c.
Power transfer and bypass switches. If the facility contains power transfer and/or bypass switches,
perform the following for each switch:
(1)
Verify that an identified conductor, if used, is brought into the switch from each power source.
(2)
Verify that the identified conductors are not grounded within the switch.
(3)
Verify that the identified conductors are white or natural gray. If larger than No. 6 AWG and
of another color (not green), it should be reidentified white or natural gray with paint or tape or by other means
such as tags or labels.
(4)
Verify that all raceways, conduits, enclosures, etc., are adequately grounded.
(5)
Verify the phase, identified and grounding conductors brought into the switch from each source
are routed together.
(6)
d.
Verify that output phase, identified and grounding conductors are routed together.
Power panels. For power panels, excluding service entrance, verify the conditions listed below. In
some instances, steps 11 through 14 may be more readily accomplished by working back from the equipment or
load end.
(1)
Verify that the phase, identified and equipment grounding conductors are routed into the panel
together through the same conduit, raceway, cable, etc.
(2)
Verify that the identified conductor is connected to the neutral bus.
(3)
Verify that the neutral bus is not grounded.
(4)
Verify that all wires connected to the neutral bus are white or natural gray or if larger than a
No. 6 AWG and of a different color, not green, have been reidentified with white or natural gray paint or tape
or by other means such as tags or labels.
(5)
Verify that no green, white or natural gray wires are used as phase conductors, or if white (but
not green), have been reidentified with paint, tape, tags, or labels.
(6) If an equipment grounding conductor is a separate conductor brought into the panel, verify
that it is bare, or if insulated, that it is green, or green with one or more stripes, or if larger than a No. 6 AWG
and of another color, not white, it has been reidentified with paint, tape, or tags.
(7) If the equipment grounding conductor is an insulated or bare wire, verify that it is connected
either to the ground bus or if the bus does not exist, that it is connected to the frame of the panel with
UL-approved
connectors.
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MIL-HDBK-419A
(8) The equipment grounding conductor is a separate conductor, must be run in the same conduit
as the feeder, and should be wrapped around the insulated conductors.
(9) Using a clamp-on ammeter, measure the current in each input phase, the identified conductor,
and the equipment grounding conductor. Record the data on Part IV of the survey form in Chapter 2.
(10) Verify that bonds in the ac or dc power systems are not dependent upon solder for their
electrical and mechanical connections. Solder may be used only to supplement mechanical connections to lower
the overall impedance in RF/signal circuits or subsystems.
(11) Verify that all related phase and equipment grounding conductors (“safety grounds”) to all
circuits supplied by the panel are routed through the same conduit, raceway, cable, etc.
(12) Verify that all separate equipment grounding conductors leaving the panel are green or green
If larger than No. 6 AWG and of another color, not white, they shall
with one or more yellow stripes or bare.
be reidentified with paint, tape, tags, or other means at each end and at all places where the conductor is
accessible. Bare conductors should not be utilized where EMI or TEMPEST must be considered.
(13) Verify that the equipment grounding conductor for each circuit is at least as large as that
given in Table 1-21 based upon the size of the overcurrent device protecting the circuit phase conductors.
(14) Verify that all bonding connections are made through surfaces that have been cleaned of
insulating finishes or by some method, i.e., gouging locknuts fully tightened, that inherently accomplishes the
same result.
(15) Using a clamp-on ammeter, measure the current in each equipment grounding conductor
leaving the panel. Record the current on Part IV of the survey form in Chapter 2.
e.
Wireways, raceways, cable trays.
For all wireways, raceways, cable trays, etc., verify the
following.
(1) All covers, where applicable, are in place and properly secured.
(2) All sections are electrically connected, and any insulating finishes in the bonding path have
been removed.
(3) If the wireway, raceway, cable tray, etc., contains neutral or equipment grounding conductors
that have been reidentified, verify that reidentification is accomplished at various intervals throughout their
length.
f.
Equipment.
For all equipment, verify the items listed below. In some instances, verification may
require that the equipment be shut down.
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MIL-HDBK-419A
Table 1-21. Equipment Grounding Conductor Size Requirement
Size
Rating or Setting of Automatic
Overcurrent Device of Equipment
Circuit.
Aluminum or
Copper-Clad
Amperes
Copper
Aluminum
Wire No.
Wire No.
15
14
12
20
12
10
30
10
8
40
10
8
60
10
8
100
8
6
200
6
4
400
3
1
600
1
2/0
800
0
3/0
1000
2/0
4/0
1200
3/0
250 MCM
1600
4/0
350 MCM
2000
250 MCM
400 MCM
(1) Where the equipment grounding conductor is a separate conductor, verify that the conductor is
routed through the same conduit, raceway, etc., as the phase and neutral conductors.
(2)
Verify that the equipment grounding path back to the power panel is continuous and that any
insulating finishes in the grounding path have been removed.
(3)
Verify that the size of the equipment grounding conductor is at least as large as that listed in
Table 1-21 for the overcurrent device serving the equipment.
(4)
Verify that the neutral is not connected to the chassis or frame of the equipment. This may be
verified visually or with an ohmmeter.
Isolation of neutral conductor.
g.
perform the following test:
(1)
When it is possible to deenergize a facility, or a portion thereof,
With the electrical power removed disconnect the facility neutral from ground or in the case
of a portion of the facility (e.g., a power panel) the incoming neutral. See Figure 1-47.
1-110
MIL-HDBK-419A
(2)
panel frame.
Measure the resistance between the neutral bus and the equipment grounding conductor or
A low value of resistance (< 10 ohms) indicates that the neutral may be grounded at some place
other than at the first service disconnect.
Grounding of the neutral at places other than at the first service
disconnect violates the MIL-STD-188-124A and the NEC and will result in power current flow through the
equipment ground network.
1.4.9.3 Correction of Deficiencies. The results of the survey should be thoroughly reviewed to determine the
overall impact of correcting the deficiencies. complete and strict compliance with the requirements of
MIL-STD-188-124A and the NEC is required. However, some corrections could be expensive and not result in
any improvement in the operation of the facility. Some types of deficiencies should be corrected. Presented
below is a listing of violations that could be encountered and possible corrective actions.
a.
Undersized equipment grounding conductor, replace with proper size conductor.
b.
Equipment grounding conductor and/or neutral conductor not routed with phase conductors. Reroute
the grounding and/or neutral conductor to be in the same raceway as the phase conductors.
c.
Equipment is not grounded by means of an equipment grounding conductor meeting the requirements
of 1.4.9.2d(7) or 1.4.9.2d(8) but is grounded by means of its installation (e.g., mounted in a rack). Ground the
equipment by means of a separate green or identified conductor routed with the phase and neutral conductors.
d.
Neutral bus in power panel (other than service entrance) is grounded to the panel frame. Remove
the grounding connection.
e.
A green wire connected to the neutral bus is found to be connected to an equipment chassis and is
supposed to be the equipment conductor. Disconnect the conductor from the neutral bus and reconnect it to the
ground bus or panel frame with UL-approved connectors.
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MIL-HDBK-419A
Figure 1-47. Method for Determining the Existence
of Improper Neutral Ground Connections
1-112
MIL-HDBK-419A
f.
Bond is obtained through a painted surface.
Disassemble, remove paint and reassemble. Protect
with waterproof paint if exposed to moisture.
A black wire, not reidentified, is found to be used as an equipment grounding conductor. If it is
g.
larger than a No. 6 AWG, it may be reidentified with green paint or tape or by other means at each end and
wherever accessible.
If smaller than No. 6, it should be replaced to comply with the NEC. However, an
acceptable substitute would be to reidentify it with green paint or tape if replacement is impractical or
expensive.
h.
Grounding conductor is routed through conduit and the conduit is not grounded. Ground the conduit
at both ends by means of a grounding bushing or clamp, a jumper wire, and a split-bolt connector. The jumper
wire is to be the same size as the grounding conductor.
i.
conductor.
Service neutral is not grounded but equipment enclosure is grounded by means of a grounding
Ground the neutral by connecting it to the grounding conductor/bus in the first service disconnect.
Conductor should be replaced.
Conductor insulation is damaged and conductor is exposed.
j.
Alternate correction is to cover the damaged area with insulating tape until the insulation of the repaired area
is equal to the insulation of the conductor.
k.
Power panel is grounded by a soldered connection.
Provide supplemental grounding by means of a
bolted grounding connector.
1.
Ground bus is not grounded and equipment grounding conductors terminate at equipment frame, not
at receptacles. Connect ground bus to panel frame by means of UL-approved connectors.
m.
Raceway contains neutral and grounding conductors of different systems (e.g., commercial and
regulated power) and conductors are not distinguishable.
paint, tape or tags.
together.
Distinguish conductors from each other by means of
Alternately, tie the phase, neutral and equipment grounding conductors of each system
1.5 SIGNAL REFERENCE SUBSYSTEM FOR NEW FACILITIES.
1.5.1 Higher Frequency Network. The higher frequency network is a conductive sheet, grid, or cable network
mesh providing multiple low resistance paths between any two points within the structure and between any
point in the structure
and the earth electrode subsystem.
It consists of three primary components:
(1) equipotential plane, (2) equipment ground conductors, and (3) structural steel elements and electrical
supporting structures, (see Figures 1–49, 1-50, and 1-51) connected to the earth electrode subsystem. The
grounding (green) wire sha1l not be considered a substitute for this subsystem.
The optimum interconnecting
cable and mesh spacing of the equipotential plane should be 1/8 of a wavelength with regard to the highest
frequency of concern. In practice this may not be feasible and the interconnecting cable and mesh spacing
should therefore be as short and small as practical.
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1-114
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Figure 1-49.
1-115
MIL-HDBK-419A
Figure 1-50.
1-116
MIL-HDBK-419A
Figure 1-51.
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MIL-HDBK-419A
a.
In steel frame buildings, make all structural members of the building (e.g., building columns, wall
frames, roof trusses, etc.) electrically continuous by bonding each joint and interconnection with a welded,
brazed, soldered, or high–compression bolted connection.
Where direct bonds of these types are not possible,
bridge the joint with a l/0 AWG stranded copper cable both ends of which are brazed, welded, or bolted in
place. This does not include rebars.
b.
cables.
Connect the bonded structural steel network to the earth electrode subsystem with 1/0 AWG copper
The distance between adjacent connections from the building structure to the earth electrode
subsystem should not exceed 15 meters (50 feet).
c.
Where steel frame construction is not used, install a supplemental network consisting of large
copper cables conforming to Table 1-22.
d.
Equipment cabinets, electrical supporting structures , and utility pipes are to be connected to this
structural steel or copper cable grid (equipotential plane) with #6 AWG copper wire. This interconnecting wire
should be as short as feasible, preferably not over 24 inches to minimize high frequency reactance. (Electrical
supporting structures include all the conduit, raceways, switch and breaker panels, and other hardware (not
energized) commonly associated with the communication electronic facility. )
1.5.1.1 Multipoint Ground System, The multipoint ground system requires the existence of an equipotential
ground plane for the system. Such an equipotential plane exists in a building with a metal floor or ceiling grid
electrically bonded together, or in a building with a concrete floor with a ground grid embedded in it, connected
to the facility ground.
Equipment
cabinets
are then
connected
to
the
equipotential
plane.
Chassis are
connected to the equipment cabinets and all components, signal return leads, etc., are connected to the chassis.
The equipotential plane is then terminated to the earth electrode subsystem to insure personnel safety and a
low impedance path for lower frequency signals.
At higher frequencies, the large conducting surface, embedded in the floor or the metallic raised floor under
the equipments to be grounded, presents a much lower characteristic impedance than a signal wire, even if both
were improperly terminated.
This is true because the characteristic impedance (Z 0) is a function of L/C. As
capacity to earth increases, Z 0 decreases. Normally, the capacity of a metallic sheet to earth is higher than
that of wire. If the size of the sheet is increased and allowed to encompass more area, the capacitance
increases.
Also, the unit length inductance decreases with width, which further decreases Z 0.
If the dimensions
of a metallic sheet increase extensively (as in the case of a conducting subfloor), the characteristic impedance
approaches a very low value. In this case, even if improperly terminated, the impedance would be quite low
throughout a large portion of the spectrum.
all equipments bonded to it.
a.
This, in turn, would establish an equipotential reference plane for
With this reference plane bonded to earth, the following advantages are obtained:
Any "noisy" cable or conductor connected to the receptor through or along such a ground plane will
have its fiels contained between the conductor and the ground plane.
The noise field can be "shorted out" by
filters and bond straps because the distance between these "transmission line" conductors is very small.
Shorting out the noise field has the desirable effect of keeping noise current from flowing over the receptor
case and along any antenna input cables.
1-118
MIL-HDBK-419A
Table 1-22
Size of Equipment Ground Cables
Cable Size
Maximum Path Length
(AWG)
(FT)
750 MCM
375
600 MCM
300
500 MCM
250
350 MCM
175
300 MCM
150
250 MCM
125
4/0
105
3/0
84
2/0
66
1/0
53
1
41
2
33
4
21
6
13
8
8
Busbar
(IN.)
4 x 1/4
636
4 X l/8
318
3 x 1/4
476
3 X 1/8
238
2 x 1/4
318
2 X 1/8
159
2 X 1/16
79
1 x 1/4
159
1 X 1/8
79
1 X 1/16
39
1-119
MIL-HDBK-419A
b.
Filters at the interface terminals of equipment can operate more effectively when both terminals of
their equivalent "transmission line" are available.
A S in a, above, a large conducting surface makes it possible
to contain the field carried by the offending conductor, in such a way that it can be more easily prevented from
traveling further.
c.
A large conducting surface will also provide isolation between any rooftop antennas and from cable
runs below it.
1.5.1.1.1 Types of Equipotential Planes. Conducting media that can be utilized for the equipotential plane are
(a) a copper grid embedded in the concrete floor or raiscd metal floor such as computer floor, (b) a subfloor of
aluminum, copper, phospher bronze screen or sheet metal laid underneath the floor tile or carpet, or (c) a
ceiling grid above tile equipment. The grid openings should not be larger than 1/20 wavelength at the highest
frequency of concern up to four inches. A S a design objective (DO) the grid openings should not be larger than
four inches. The following equipotential planes may be utilized on new facilities or those facilities undergoing a
major rehab, or upgrading of communications electronics equipments.
1.5.1.1.1.1 Copper Grid Embedded in Concrete.
Since a large solid conducting surface is not economically
feasible for some installations, a ground reference plane, made up of a copper grid, or copperclad construction
mesh with 4 inch openings may be embedded in the concrete with ground risers installed to the surface of the
concrete as shown in Figure 1-54. The mesh is commercially available in AWG wire sizes Nos. 6, 8, 10, and 12.
It is normally furnished in 3.7m (12 foot) rolls, but can be obtained in various widths up to 5.5m (18 feet). See
Figure 1-52.
Where sections of mesh are joined together, there should be a one foot overlap and bonded
together every two feet by welding, brazing, or manufactured connectors that are connected to the grid and
give grounding access at the floor surface. See Figures 1-53, 1-54, 1-55, and 1-56. Normally, if the grid is
embedded in a concrete floor, the latter method provides the easiest grounding source. The equipotential plane
shall be welded to the main structural steel of the building at multiple locations.
Where frame buildings are
utilized the plane is connected to the earth electrode subsystem at multiple locations using 1/0 AWG copper
conductors. If metal floor systems are used (metal floors with concrete poured over the floor) then the floor
system itself can be used as the equipotential plane. In fact, this would be the preferred method of establishing
the plane.
1.5.1.1.1.2 Equipotential Plane Under Floor Tile or Carpet.
An equipotential plane can be realized by
installing a metal sheet or roll of either aluminum, copper, or phospher bronze under the floor tile or carpet.
This sheet may be either thin gauge solid metal or window screen type material bonded to the floor with mastic
and tile or carpet installed on top of it. In existing facilities where equipments are already installed, the plane
need not be installed under tile equipment cabinets, but must be bonded to the cabinets on all four sides. The
plane shall be bonded to the main structural steel members of the building at multiple locations. The structural
steel shall in turn be bonded to the earth electrode subsystem.
1.5.1.1.1.3 Overhead Equipotential Plane. Where it is not practicable to install a plane on the floor around the
equipment, it is possible to install an overhead equipotential plane in or on the ceiling of the equipment room.
This can be accomplished by installing either thin metal sheets or screen either above or on the ceiling. Care
must be taken to keep bonding straps from the equipment to the plane as short as possible. Generally phospher
bronze screen is used in this application because it is light, durable, and easy to work.
The plane must be
connected to the building steel which in turn is bonded to the earth electrode subsystem.
1-120
MIL-HDBK-419A
Figure 1-52.
1-121
MIL-HDBK-4l9A
1.5.1.1.1.4 Raised (Computer) Flooring. Raised floors are used to structurally support equipment cabinets and
provide a space between the original facility floor and raised floor plates for cabling, air plenum or air
conditioning ducting, piping, drains, etc. Raised floors provide an esthetic room appearance. Three general
types of floor systems manufactured are: (a) the bolted-grid (stringer) or the rigid grid system, (b) the drop-in grid
or removable grid type, and (c) the free–standing, stringerless or pedestal-only type. Only type (a) is acceptable
as an equipotential plane.
Figure 1-53. Typical Equipotential Ground Plane for New Construction
nigher or hybrid Frequencies Facilities Installation
1-122
MIL-HDBK-419A
FOR FLAT BAR TO GROUND CABLE IN CONCRETE
USE:
These Ground Connectors are attached to a
ground cable (Equipotential plane) in the floor,
and after the concrete is poured, a 2" X 2" flat
plate is left exposed, flush with the finished sur–
face.
The connectors may be located at predetermined locations, or if set at intervals throughout
the floor or wall base, a ground pad is always
r e a d i l y a v a i l a b l e f o r- g r o u n d i n g e l e c t r i c a l e q u i p –
ment.
Contact surface of fitting is 2" square and
comes with either two or four 1/8" D&T holes which
This equican be used. for connection purposes.
potential plane is bonded to the earth electrode
subsystem at numerous points.
Figure 1-54. Ground Connector for Equipotential Plane in Concrete
Figure 1-55. Typical Ground Connectors
1-123
MIL-HDBK-419A
1 Bolt
1 Piece Design
FOR TWO PARALLEL
CABLES TO FLAT BAR
FOR CABLE TO
FLAT BAR
2 Bolts
FOR CABLE TO
FLAT BAR
FOR TWO PARALLEL
CABLES TO FLAT BAR
NOTE: Bolts furnished will fasten
connector to plate up to ¼ "
thick. Longer bolts will be
furnished when necessary if
plate thickness is specified.
Figure 1-56. Examples of Cable to Bar Ground Connectors
1-124
MIL-HDBK-419A
1.5.1.1.1.4.1 Bolted-Grid (Stringer) or Rigid Grid System Raised Floors. Shown in Figures 1-58, 1-59, and 1-60
arc bolted-grid floor systems.
The systems are similar to the drop-in grid except the grids, when properly
installed, are securely bolted or clamped in place. The drop-in panels must be metal or wood with metal plate
on both sides with a selected floor covering. They should be no larger than 24" x 24". Although the panels may
not make a good low resistance contact with the stringers, the high distributed capacity makes the floor appear
to be an electrically continuous sheet at rf frequencies. The equipment cabinets shall be connected to the floor
stringers by bonding straps which must be kept as short as possible. This will provide a low impedance path to
earth at the lower frequencies. Materials used for stringers and pedestal heads are steel and aluminum. Raised
flooring to be used for equipotential planes should be purchased to conform to the requirements of
MIL-F-29046 (TD).
In general, the grounding aspects of raised flooring have been excellent. Problem areas that designers should be
aware of are:
a.
Installation
practices.
Inadequate bonded joints between pedestal heads and stringers have
sometimes resulted from poor installation practices primarily due to:
(1) pedestal heads heavily oxidized and dirty when bolted,
(2)
use of poor bolting hardware (speed nuts, sheet metal screws), or
(3)
bolting hardware not installed or not properly tightened.
Clipnuts shall not be used in place of standard nuts since they generally will deform and therefore produce loose
joints.
The installer may receive aluminum pedestal heads from the manufacturing plant that are heavily oxidized.
The joint surface requires minor abrasion and perhaps a light coating of a joint protective compound. The joint
compound should be particularly considered for non-carpeted floors where moisture, cleaning compounds and
wax would settle, degrading the joint (sometimes severely) in a several year period.
The use of improper bolting hardware has caused unreliable joints.
All bolted bonds must meet the
requirements of Table 1-23. Sheet metal screws have on occasion been employed to support the stringer to the
pedestal, Clipnuts employed in lieu of standard nuts will deform and in turn produce loose joints, and shall not
therefore be used.
This "clip" nut called the grip lock nut is shown in Figure 1-61. MIL-F-29046 has been
modified to specifically prohibit the use of such type of hardware.
It is recommended that the installation crew be briefed and the floor tested, before the floor panels are
installed.
b.
Floor system checkout.
Composite bonds between cabinet chassis and the raised floor shall not
exceed a specified resistance value, usually 1 milliohm. Typically, a pedestal head to stringer resistance will
read about 40 micro-ohms and should not exceed 100 micro-ohms.
For
additional
information
see
MIL-F-29046,
c.
Resistance
measurement
equipment.
The instrument
recommended
to
obtain
resistance
measurements of 100 micro-ohms for these measurements is a modified Shall cross Model 670A Milliohmmeter
or equal.
1-125
MIL-HDBK-419A
THIS PAGE INTENTIONALLY LEFT BLANK
1-126
MIL-HDBK-419A
Figure 1-58.
1-127
MIL-HDBK-4l9A
Figure 1-59. Example of Rigid-Grid to Pedestal Bolted Connection
1-128
MIL-HDBK-419A
1-129
MIL-HDBK-419A
Figure 1-61. Example of Unacceptable Grid-to-Pedestal Bonding
d.
Connections from the equipment racks and the earth
C o n n e c t i o n s t o t h e raised f l o o r s y s t e m .
electrode subsystem to the floor are important. Clamps, if used, should be installed on the upper pedestal
assembly to avoid the relatively high resistance between the lower assembly (that has the base) and the upper
column. The stringer to pedestal fastener hardware can often be changed to allow bolting a bonding cable
terminal directly to the pedestal head.
It is feasible to obtain additional grid locking hardware and use it to
bolt the bonding cable terminal to the floor grid.
Another means of terminating a bonding cable is to drill a
hole and bolt it to a non-heavy weight bearing stringer.
e.
Corrosion control.
In extremely humid environments where corrosion is common, the use of
corrosion prevention compounds is recommended.
Bolted joints can be covered with a non-corrosive silicone-
rubber compound that will protect the joint for the life of the installation. An ice cube rubbed on the siliconerubber will smooth it.
f.
Carpeting.
Carpeting selected as a floor-covering, should be of a low static or static-free type to
prevent possible static discharge or component failure. See MIL-F-29046 for additional information.
Maintenance measurements. To determine degradation of the floor, resistance measurements and
g.
method should be documented and available so that repeat measurements can be made if ground reference
subsystem problems are suspected or periodic checks for degradation made.
h.
Reliability.
The MIL-F-29046 specification provides information for raised floor procurements.
The guidelines in this Handbook will provide a ground reference subsystem that is well-designed, properly
installed and lasts for the life of the electronic system installation.
1-130
MIL-HDBK-419A
1.5.1.1.1.4.2 Drop-In or Removable Grid System. The Drop-In Grid System is shown in Figure 1-62. The grids
or stringers are retained by engaging pins or depressions in the pedestal head. The stringers supply support and
when newly installed provide comparatively low resistance contact to the pedestal head. Equipment cabinets
resting on the floor panels provide increased contact pressure in certain areas, Severe corrosion and unreliable
electrical contact have resulted due to dirt, moisture and floor cleaning/waxing compounds filtering through
crevices.
This floor system is also considered unsuitable for a reference plane. Floor panels resting on the
pedestals and grids are commonly 24” x 24” although they may be purchased in 30” x 30” dimension.
1.5.1.1.1.4.3
Free-Standing,
Pedestal-Only or Stringerless System.
The pedestal-only system is shown in
Figure 1-63. The pedestal base is glued or “shot” in place to form the basic understructure. The pedestal heads
are leveled and the floor panel is installed.
The conductivity between distant pedestals is variable and
unreliable, making it unsuitable for a ground reference.
1.5.1.1.1.5. Ground Risers. The type of ground riser to be used depends on the type of equipotential plane to
be installed and whether the subject building will be new construction, a major modification to an existing
building in which new equipments will be installed, or an existing building in which only the ground system will
be upgraded while the equipments remain in place. The latter case is discussed in Section 2.2.3.
1.5.1.1.1.6 Equipment Cabinet Grounding. Each individual unit or piece of equipment should either be bonded
to its rack or cabinet (see Figure 1-64), or have its case or chassis bonded to the nearest point on the
equipotential plane. Racks and cabinets should also be bonded to the nearest point of the plane.
1.5.2 Lower Frequency Signal Reference Network. Lower frequency signal reference subsystems are not to be
installed in communications-electronics facilities.
Figure 1-62. Example of Drop-In Grid Floor Construction
1-131
MIL-HDBK-419A
Figure 1-63.
1-132
MIL-HDBK-419A
Figure 1-64.
1-133
MIL-HDBK-419A
1.6 GROUNDING PHILOSOPHY FOR EQUIPMENTS PROCESSING NATIONAL SECURITY RELATED
INFORMATION (RED/BLACK EQUIPMENTS). Grounding of equipment, conduit, and frames for safety
protection in areas processing National Security related information (RED data) is no different than any other
facility. Typically a RED and a BLACK signal ground is established by a direct connection totally within a
controlled space to an equipotential ground plane and earth electrode subsystem (see Figure 1-65). Cable
shields from the RED equipment to the RED side of the crypto are grounded at least at both ends. Cable
shields from the BLACK side of the crypto equipment through the BLACK IDF to the BLACK equipment are
normally grounded at both ends.
For unbalanced signaling, signal ground is usually established by a direct
connection from an isolated signal ground bus in the RED distribution frame to an equipotential ground plane
and in turn to the earth electrode subsystem (see Figures 1-66,
1-67, and 1-68.) Figure 1-69 provides
information on the facility power and ac ground distribution system. BLACK signal ground is used to provide a
signal ground reference in the BLACK distribution frame and for signal line filters. For all aspects other than
grounding, bonding, and shielding of secure installations, refer to NACSIM 5203. Refer to Section 2.4 of this
Volume for the grounding of existing RED/BLACK equipments and systems.
Note: An equipotential ground plane is considered as earth for the signal reference subsystem of communication systems, regardless of elevation from physical earth.
1-134
MIL-HDBK-419A
Figure 1-65.
1-135
MIL-HDBK-419A
Figure 1-66. Typical Red Signal, Shield Ground, Bus Distribution System
1-136
MIL-HDBK-419A
Figure 1-67. Typical Intermediate Distribution Frame (Shield Ground Bus in Distribution Frames)
1-137
MIL-HDBK-419A
Figure 1-68. Typical Intermediate Distribution Frame (Data Concentrator Frame Installation)
1-138
MIL-HDBK-419A
Figure 1-69.
1-139
MIL-HDBK-419A
High quality bonds between conducting elements throughout the facility are
1.7 BONDING PRACTICES.
essential ingredients to the effective functioning of all grounding and shielding networks.
It is thus of
paramount importance that thorough consideration be given to bonds and bonding throughout design and
construction of a facility.
A bonding resistance of 1 milliohm indicates a high quality junction has been
achieved. See Volume I, Chapter 7, for additional information.
1.7.1 Application Guidelines.
a.
Utilize welding (exothermic) whenever possible for permanently joined bonds. The welds must be
adequate to support the mechanical load demands on the bonded members, and the following minimum
requirements must also be met:
(1) On members whose maximum dimension is 5 cm (2 inches) or less, the weld must extend
completely acres the side or surface of largest dimensions.
(2) On members whose largest dimension is greater than 5 cm (2 inches) but less than 30 cm
(12 inches), one weld of at least 5 cm (2 inches) in length must be provided.
(3) On members whose largest dimension is greater than 30 cm (12 inches), two or more welds,
each not less than 5 cm (2 inches) in length, are to be provided at uniform spacings across the surface of largest
dimension. The maximum spacing between successive welds must not exceed 30 cm (12 inches).
(4) At butt joints, use complete penetration welds on all members whose thickness is 0.63 cm
(1/4 inch) or less. Where the thickness of the members is greater than 0.63 cm (1/4 inch), the depth of the weld
must not be less than 0.63 cm (1/4 inch).
(5) Fillet welds are to have an effective size equal to the thickness of the members or as specified
by Construction Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings (1-7).
(6) At lap joints between members whose thickness is less than 0.63 cm (1/4-inch), double fillet
welds must be provided.
b.
Use brazing (or silver soldering) for permanently bonding copper and brass.
c.
Do not use soldered connections in the fault protection subsystem, the lightning protection
subsystem or the earth electrode subsystem.
d.
All structural bolted connections must conform to the torque requirements of Table 1-23.
e.
The proper order of assembly for bolted bonds is illustrated in Figure 1-70.
Position load
distribution washers directly underneath the bolt head or under the nut next to the primary member.
Lockwashers may be placed between the nut and any load distribution washers. Toothed lockwashers should not
be placed between the primary bonded members but may be used as shown in Figure 1-70 for interior locations
that are not exposed to moisture and where electrochemically compatible metals for the washer and bond
members are utilized.
1-140
MIL-HDBK-419A
Table 1-23
Minimum Torque Requirements for Bolted Bonds
Bolt Size
#8
#l0
1/4”
5/16”
Min. Torque
Tension
(in -lbs)
(lbs)
Bond Area
(in. 2)
32
18
625
0.416
36
20
685
0.456
24
23
705
0.470
32
32
940
0.626
20
80
1840
1.225
28
100
2200
1.470
18
140
2540
1.690
20
150
2620
1.750
16
250
3740
2.430
24
275
3950
2.640
7/16”
14
400
5110
3.400
20
425
5120
3.420
1/2”
13
550
6110
4.070
20
575
6140
4.090
5/8”
11
920
7350
4.900
3/4”
10
1400
9300
6.200
7/8”
1"
9
1950
11100
7.400
8
2580
12900
8.600
3/8”
f.
Threads/Inch
Once the mating surfaces have been cleaned of all nonconductive material, join the bond members
together as soon as possible. If delays beyond two hours are necessary in corrosive environments, the cleaned
surfaces must be protected with an appropriate coating which, of course, must be removed before completing
the bond.
Alligator clips and other spring loaded clamps are to be employed only as temporary bonds. Use
g.
them primarily to insure that personnel are not inadvertently exposed to hazardous voltages when performing
repair work on equipment or on facility wiring.
1-141
MIL-HDBK-419A
Figure 1-70. Order of Assembly for Bolted Connection
1.7.2 Surface
a.
Preparation.
Welding generally requires only the removal of foreign material which might prevent a homogeneous
weld from being established.
b.
Before performing exothermic welding, dirt and other debris must be wiped or brushed away from the
weld area and water must be dried off before positioning the molds.
c.
Surfaces to be brazed or soldered are to be cleaned of all foreign matter and metallic films that would
prevent adhesion of the filler metal to the primary members, and appropriate fluxes are to be applied. After
the bond has been completed, remove any excess flux or neutralizers to prevent future corrosion.
d.
The mating surfaces of bolted and other compression type bonds require careful cleaning in accordance
with the discussion in Volume I, Section 7.6. The basic requirements are:
(1) All nonconductive material must be removed.
Such materials include paints and other organic
finishes; anodize films; oxide and sulfide films; and oil, grease and other petroleum products.
(2) All corrosive agents must be removed. Such agents include water, acids, strong alkalies, and any
other materials which provide conductive electrolytic paths.
(3) All solid matter which would interfere with the establishment of a low resistance path across the
bond interface or which forms a wedge or barrier to keep the bond area open to the entrance of corrosive
materials or agents must be removed. Such solid materials include dust, dirt, sand, metal filings, and corrosion
by-products.
1-142
MIL-HDBK-419A
1.7.3 Bond Protection Code.
For bonds of high reliability, corrosion must be prevented by (1) avoiding the
pairing of dissimilar metals and (2) preventing the entrance of moisture or other electrolytes into the bond area.
Metals to be indirect contact should fall as close together in the galvanic series (see Volume I, Section 7.8.l.2)
as possible.
Compatible groupings of the common metals are given in Table 1-24. The corrosive action
between metals of different groups will be greatest when the metallic union is openly exposed to salt spray,
rain, or other liquids.
The less exposed the bond, the less the rate of corrosion.
The relative degrees of
exposure may be defined as follows (1-8):
Exposed:
Open, unprotected exposure to weather.
Sheltered:
Limited protection from direct action of weather. Locations in louvered housings, sheds,
and vehicles offer sheltered exposure.
Housed:
Located in weatherproof buildings.
When bonds under these different exposure conditions must be made between different groups, they should be
protected as indicated by Table 1-25. Condition A means that the couple must have a protective finish applied
after metal-to-metal contact has been established so that no liquid film can bridge the two elements of the
couple.
Condition B means that the two metals may be joined with bare metal exposed at junction surfaces.
The remainder of the bond must be given an appropriate protective finish. Condition C indicates that the
combination cannot be used except under very unusual circumstances where short life expectancy can be
tolerated or when the equipment is normally stored and exposed for only short intervals. Protective coatings
for bonds subjected to weather exposure, corrosive fumes, or excessive dust are mandatory. Consult Table 1-26
for assistance in choosing a method for protecting the bond members against corrosion.
Table 1-24
Compatible Groups of Common Metals
Group
I
Metals
Magnesium
II
Aluminum, aluminum alloys, zinc, cadmium
III
Carbon steel, iron, lead, tin, lead-tin solder
IV
Nickel, chromium, stainless steel
V
Copper, silver, gold, platinum, titanium
1-143
MIL-HDBK-419A
Table 1-25
Bond Protection Requirements
Condition
Anode
of Exposure
I
II
Exposed
A
A
Sheltered
A
A
Housed
A
A
Exposed
C
A
B
Sheltered
A
B
B
Housed
A
B
B
III
Iv
II
III
Exposed
C
A
B
B
Sheltered
A
A
B
B
Housed
A
B
B
B
Exposed
C
C
C
A
Sheltered
A
A
A
B
Housed
A
A
B
B
1-144
Cathode
IV
V
MIL-HDBK-419A
Table 1-26
Protective Finishes for Bond Members
Finishing Requirements
Type Bond
1.
Between Similar Metals
a.
Clad and corrosion resistant aluminum
Clean and deoxidize
(6061)
b.
Non-corrosion resistant aluminum
Chemically treat per MIL-C-5541 (l-9) using colored
inspectable coating on both members of joint (Alodine
600, Iridite 14).
c.
Steel (alloy and carbon)
If entire part is finished, plate with tin, MIL-T-10727
(1-10), Type I or II. If only faying surface is finished,
plate with tin using brush plating method.
d.
Corrosion-resistance steel (18-8
Clean per TT-C-490 (1-11) Method I (abrasive) or
stainless steel)
Method VI (phosphoric
acid etch) for machined
surfaces.
e.
Copper and copper alloys
If entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
2.
Dissimilar Metals
a.
Corrosion resistant aluminum mated
Clean and deoxidize
with the following metals:
(1) Non-corrosion resistant aluminums
Chemically treat per MIL-C-5541, colored inspectable
coating (Alodine 600, Iridite 14).
(2) Steel (alloy and carbon)
If entire part is finished, plate with tin, MIL-T-10727,
Type I or 11.
If only faying surface is finished, plate
with tin using brush plating method.
(3) Copper and copper alloys
If entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
1-145
MIL-HDBK-419A
Table l-26 (Continued)
Protective Finishes for Bond Members
Finishing Requirements
Type Bond
(4) Corrosion-resistant
Clean per TT-C-490, Method I (abrasive) or Method VI
(phosphoric acid etch).
b.
Non-corrosion resistant aluminum mated
Chemically treat per MIL-C-5541, colored inspectable
with the following metals:
coating (Alodine 600, Iridite 14).
(1) Steel (alloy and carbon)
If entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
(2) Copper and copper alloy
[f entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
(3) Corrosion resistant steel
Clean per TT-C-490, Method I (abrasive) or Method VI
(Phosphoric acid etch).
c.
Steel (alloy and carbon) mated with the
If entire part is finished, plate with tin, MIL-T-10727,
following metals:
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
(1) Copper and copper alloys
If entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
(2) Corrosion resistant steel
Clean per TT-C-490, Method I (abrasive) or Method VI
(phosphoric acid etch).
d.
Copper and copper alloys
If entire part is finished, plate with tin, MIL-T-10727,
Type I or II.
If only faying surface is finished, plate
with tin using brush plating method.
(1) Corrosion resistant steel
Clean per TT-C-490, Method I (abrasive) or Method VI
(phosphoric acid etch).
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MIL-HDBK-419A
1.7.3.1 Jumper Fasteners.
Acceptable fastener materials for bonding aluminum and copper jumpers to
structures are indicated in Table 1-27. The arrangement of the metals is in the order of decreasing galvanic
activity. The screws, nuts, and washers to be used in making the connections as indicated are:
Type I - Cadmium or zinc plated steel, or aluminum
Type II - Passivated stainless steel
Where either type of securing hardware is indicated, Type II is preferred from a corrosion standpoint.
Table 1-27
Metal Connections for Aluminum and Copper Jumpers
Connection
Metal Structure
Connection For
Screw
For Tinned
Screw
(Outer Finish Metal)
Aluminum Jumper
Type
Copper Jumper
Type
Magnesium and Magnesium
Direct or Magnesium
alloys
washer
Type I
Aluminum or
Type I
Magnesium
washer
Zinc, Cadmium, Aluminum
Direct
Type I
Aluminum washer
Type I
Direct
Type I
Direct
Type I
Direct
Type I
Direct
Type I
and Aluminum alloys
Steel (except stainless
steel)
Tin, Lead, and Tin-lead
or II
solders
Copper and Copper
Tinned or Cadmium
Type I
alloys
plated washer
or II
Nickel and Nickel
Tinned or Cadmium
Type I
alloys
plated washer
or II
Stainless Steel
Tinned or Cadmium
Type I
plated washer
or II
Silver, Gold and
Tinned or Cadmium
Type I
precious metals
plated washer
or II
1-147
Direct
Type I
or II
Direct
Type I
or II
Direct
Type I
or II
Direct
Type 1
or II
MIL-HDBK-419A
1.7.4 Typical Bonds.
1.7.4.1 Cable Trays.
Utilize cable trays as part of the overall system bonding scheme. Bond each section of
each tray in the manner shown in Figure 1-71 to the following section to provide a continuous path. The trays
should also be connected to equipment housings by wide, flexible, solid bond straps as illustrated in Figure 1-72.
Figure 1-71. Bonding of Cable Trays
Figure 1-72. Bonding of Equipment Cabinets to Cable Tray
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1.7.4.2 Tubing and Conduit. Long spans of conduit should be properly bonded to the structure at both ends and
at several intermediate points.
Ordinary clamps cannot be used to bond flexible conduit since the required
pressure on a comparatively small surface area may be sufficiently high to compress or collapse the conduit.
Instead of ordinary clamps, use a flared, split sleeve (Figure 1-73) fitted around the flexible conduit.
This
sleeve distributes the high pressure of the bonding clamp over a large area, thereby exerting low pressure on the
conduit.
Figure 1-74 illustrates a method for bonding to rigid conduit. With either type of clamp, the conduit
or tubing should be cleansed of paint and foreign material over the entire surface covered by the clamps. A l l
insulating finishes should be removed from the contact area before assembly, and anodized screws, nuts, and
washers should not be used to attach contacting parts.
1.7.4.3 Other Examples.
Figures 1-75 through 1-80 illustrate recommended bonding methods appropriate for
most facilities.
Figure 1-73. Bonding to Flexible Cable and Conduit
Figure 1-74. Bonding to Rigid Conduit
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MIL-HDBK-419A
Figure 1-75. Connection of Bonding Jumpers to Flat Surface
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MIL-HDBK-419A
Figure 1-76. Bolted Bond Between Flat Bars
Figure 1-77. Bracket Installation (Rivet or Weld)
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Figure 1-78. Use of Bonding Straps for Structural Steel Interconnections
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Figure 1-79. Direct Bonding of Structural Elements
Figure 1-80. Connection of Earth Electrode Riser to Structural Column
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MIL-HDBK-419A
1.8 SHIELDING. The shielding provided in a given facility should be adequate to provide the needed equipment
and personnel protection; however, it need not go beyond what is required for that particular facility. To
determine the shielding required at a facility, the electromagnetic environment at the planned location should
first be surveyed;
then this threat environment should be compared with the response properties or
susceptibilities of the equipment to be located in that environment. If a need for shielding is indicated, then it
should be provided either as a part of the facility or the equipment shielding should be upgraded. The final
decision will be based on a trade off between the known (or estimated) shielding requirements and the relative
cost to provide this shielding.
Conducted as well as radiated susceptibility and emission requirements of C-E
equipments should meet the specifications of MIL-STD-461.
1.8.1 Establishing Requirements.
a.
Tailor the shielding of the facility according to the needs of the equipments or systems to be located
there by
(1) Conducting an electromagnetic survey at the facility location (see Volume I, Section 8.12.2)
(The performance of these surveys requires specialized instrumentation, careful equipment calibration
procedures, and calibrated antennas. Have this survey performed by an experienced team.),
(2)
examining the history of performance of the similar equipments at other sites with comparable
electromagnetic environments, and
(3)
b.
considering the measured EMI characteristics of the equipments (if available).
If measured susceptibility data (the incident field levels which cause equipment interference) are
available, determine the amount of additional shielding necessary by subtracting the equipment susceptibility
level (in dB above a microvolt per meter, dB µ V/m) from the field strength (as measured in dB µ V/m) of the
incident signals. If the measured signal strength is greater than the susceptible level, arrange to provide the
extra shielding necessary either as part of the structure or building or require that the equipment’s shielding be
upgraded (see Section 3.4).
If susceptibility data is not available, make a best estimate of the amount of
required shielding from the historical performance of the equipments (or similar types) at other sites.
c.
Before deciding what type or how much supplemental shielding material is necessary, estimate the
amount of shielding inherently provided by conventional building materials and techniques. For example:
(1) Use Figures 1-81 and 1-82 to estimate the shielding provided by normal construction
techniques (steel skeleton with brick or concrete block exterior with standard wood, gypsum board, or concrete
block interior walls).
(2) Reinforced concrete offers additional shielding because of the presence of the rebar.
Estimate the shielding effectiveness of single course rebar to low frequency magnetic fields from the curves
shown in Figure 1-83.
(Use Table 1-28 to obtain attenuation correction factors to apply to Figure 1-83 for
other size rebar and other spacings.)
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MIL-HDBK-419A
Figure 1-81. Measured Electromagnetic Shielding Effectiveness
of a Typical Building at 6 Feet Inside Outer Wall (1-12)
Figure 1-82. Measured Electromagnetic Shielding Effectiveness
of a Typical Building at 45 Feet Inside Outer Wall (1-12)
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MIL-HDBK-419A
Figure 1-83.
1-156
MIL-HDBK-419A
Table 1-28
Attenuation Correction Factors for Reinforcing Steel (1-14)
No. of Courses
Correction Factor
Bar Diameter
Bar Spacing
(in.)
(in.)
2.257
12
Single
+5
1.692
14
Single
0
1.000
18
Single
-6
2.257
20
Double
+8.5
1.692
14
Double
+13
1.000
16
Double
+5
(dB)
(3) Use Figure 1-84 to determine the relative attenuation of rebar (and other wire mesh or grid)
to higher frequency electric fields and plane waves. To use this figure, first calculate the ratio of the wire (or
bar) diameter, d, to the wire spacing, S. Then determine the ratio of S to the wavelength,
f, of interest
at the frequency,
in meters = 3 x 108 divided by f in hertz). For example, determine the shielding effectiveness
at 100 MHz of a 1“ x 2“ grid made of No. 10 AWG (0.1” diameter) wire.
Calculation Steps
depending upon the polarization of the incident wave.
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MIL-HDBK-419A
(d) The Shielding Effectiveness (SE) (depending upon the polarization of the field) from
Figure 1-84 is either
or
(e) Use the lowest SE (25 dB) for design purposes.
d.
If these calculations or estimates indicate a need for additional shielding, incorporate the shielding
into the design of the structure, and schedule its installation at a time in the construction phase when it can be
done most economically.
-
Figure 1-84. Shielding Effectiveness of a Grid as a Function of Wire
Diameter, Wire Spacing, and Wavelength (1-15)
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MIL-HDBK-419A
1.8.2 Design Guidelines.
a.
Design the shielding to conform to the needs of the system. Consider the relative ease of shielding
an individual equipment rather than shielding a room or the entire structure.
b.
Assure that the shielding provided is sufficient to meet system needs (both known and predicted) but
do not excessively over design.
c.
Use the inherent shielding properties of the structure to maximum advantage. Employ the small
amount of shielding (typically 10-20 dB) offered by reinforced concrete.
However, do not expect common
building materials such as brick, concrete, wood, fiberglass, or plastic to provide any significant shielding to
electromagnetic signals (1-16).
d.
Locate most sensitive and most critical equipments as close to the core of the structure as
operational requirements will permit.
e.
To minimize the attenuation requirements on shields, predetermine the location of likely sources of
interference such as power substations, engine-generators, and RF transmitters; maximize the separation
between such sources and potentially susceptible equipments or systems.
f.
Where a choice exists as to exterior skin materials for the shelter or structure (e.g., fiberglass
versus sheet steel or aluminum) choose metals to take advantage of their improved shielding properties. (In
order to utilize metal sidings as effective shields, seams must be electrically continuous.)
Insure that shield continuity is maintained at points of entry of signal cables, power conductors,
g.
utility lines, and ground conductors.
h.
Make sure that windows, doors, and ventilation ports are shielded along with the walls. Use well
bonded screen wire for windows, use metal doors, and apply honeycomb ducts or appropriate screening over
ventilation ports.
i.
Equip all power lines supplying shielded areas with power line filters.
Use steel conduit in preference to aluminum conduit to take advantage of the improved magnetic
j.
shielding properties of steel.
k.
Use enclosed metal wiring ducts or raceways in preference to open mesh or unenclosed types.
l.
If the only purpose of the shield is to establish a personnel barrier to prevent inadvertent contact
with dc and power frequency hazardous voltages, consider the use of nonconductive shields which may be less
expensive.
(If metal shields are used to provide shock protection, they must be well grounded to the power
safety ground - the green wire network.)
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MIL-HDBK-419A
1.8.3 Selection of Materials. The selection of a shielding material can be done either by (1) choosing a possible
metal of a given thickness and then determining if the shielding effectiveness is equal to or greater than the
field attenuation desired, or (2) starting with desired attenuation, determining what thickness of metal sheet or
what type of screen is required. Either approach is acceptable.
a.
As the first step in the selection of a shield type and material, ascertain the nature of the field by
determining whether it is an electric field, magnetic field, or a plane wave. (The distance between the source
and the shield relative to signal wavelength gives an indication of the impedance characteristics of the incident
Note the source may be either the actual signal generator such as a transmitter or it may be the
field.
current-carrying conductor such as a power line or signal cable.)
(1) First compute the wavelength,
For f in hertz,
or the incident signal or signals from
will be in meters.
(2) If source location is known, measure or estimate r. Then calculate
r is distance from source in meters
Value of r must be greater than
D is length of antenna in meters
(3) If
is less than unity, the incident field will either be a high impedance electric field or it
will be a low impedance magnetic field. To determine which one, try to establish what type of source produced
the field. (An electric field source is characterized by a high source impedance and relatively low currents.
Examples are high voltage dc power supplies; static discharges; short monopole antennas; etc. A magnetic field
source is generally characterized as a low impedance, high current source. Typical magnetic sources are loop
antennas and power lines.)
(4) If
b.
is unity or greater, assume the incident field is a plane wave.
Next, use Figure 1-85 to obtain the absorption loss of the material selected for a plane wave. To
use this nomograph, draw a straight line between a point on the right hand vertical scale that corresponds to the
particular metal involved and the correct point on the thickness scale (center scale on the nomography). Mark
where the straight line crosses the unlabeled pivot line. Next place a straight edge between the marked point
on the pivot line and the frequency of interest (1eft most vertical scale).
Read the absorption loss off the
compressed scale just to the left of the thickness scale. (The determination of the absorption loss of a 14 mil
sheet of stainless steel at 1 kHz is illustrated on the figure. First, line 1 is drawn between stainless steel on the
right hand scale and 14 mils on the thickness scale. Then line 2 is drawn between 1 kHz on the left hand scale
and the crossover point. The indicated absorption loss is 3 dB.)
If the specific metal of interest is not indicated on the right hand scale, obtain both the relative conductivity,
g r, and the relative permeability, µ r, from Table 1-29. Multiply g r times µ r; use the product as the right hand
location for line 1 and complete the determination. (Given the frequency and the desired absorption loss, this
nomograph can be used to determine the thickness and/or the type of metal needed.)
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MIL-HDBK-419A
Figure 1-85. Shield Absorption Loss Nomograph (1-17)
1-161
MIL-HDBK-419A
c.
The total shielding effectiveness is the sum of the absorption loss and the reflection loss. Use
Figure 1-86 to determine the reflection loss of various metals to magnetic fields; use the nomograph of
Figure 1-87 to determine the reflection loss of electric fields. The procedures for using these nomography are
similar to that described previously for determining absorption loss. Note that the right hand scale is based on
the ratio of relative conductivity to relative permeability instead of the product of the two as used on the
absorption loss nomograph.
Determine the reflection loss for plane waves with the use of Figure 1-88. Simply lay a straightedge between
the metal of interest (or the correct g r/ µr ratio) on the right hand scale and the frequency of interest on the
left hand scale; read the reflection loss of the scale in between.
Thin shields with low values of absorption loss can experience re-reflections which may cause the estimates of
shielding effectiveness to be in error, If the absorption loss is less than 10 dB, see Volume I, Section 8.3.3 for
ways to account for the effect of re-reflections.
d.
Consider the use of thin metal foils for shielding high frequency (broadcast frequencies and above)
plane and electric fields. Use Figures 1-89 and 1-90 to estimate the amount of shielding that can be achieved
with copper and aluminum.
1.8.4 Construction Guidelines.
a.
Securely ground all metal shields.
b.
All seams and joints must be well bonded.
must provide a high degree
Welded seams are highly desirable in enclosures which
80 dB) of RF shielding or are intended for EMP protection. Where welding is
impractical, solder or knitted wire gaskets should be used to supplement the mechanical fasteners (see
Volume I, Section 8.5.2). Figures 1-91 and 1-92 show two recommended techniques for constructing seams in
shields.
c.
Limit openings (windows, doors, ventilation ports) and penetrations (signal lines, power lines,
utilities) to the lowest possible number and restrict their dimensions to a minimum.
(1) If holes through the shield are necessary, see Volume I, Section 8.4.3 to determine the
optimum size and spacing.
(2) Use honeycomb (see Volume I, Section 8.5.3.1) for the shielding of ventilation ports wherever
possible. Where forced ventilation is used through ports shielded with either honeycomb or wire mesh, predict
the pressure drop with the aid of Figure 1-93. (A larger blower will generally be necessary to provide the same
volume of air through a shielded port than would be required through an unshielded port.)
d.
Peripherally bond metallic utility lines to the shield at the point of entrance. Nonmetallic lines
entering through waveguide-below-cutoff (see Volume I, Section 8.5.3.1) ducts or tubes may also be used for
water, gas, compressed air, etc.
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MIL-HDBK-419A
Table 1-29
Relative Conductivity and Relative Permeability of Common Metals
Metal
Relative
Relative
Conductivity
Permeability
( gr)
( µr)
Initial
Alfenol
.011
3,450
Beryllium
.377
1
Brass
.442
1
Cadmium
.230
1
Chromax
.017
Chromium
.663
1
Cobalt
.177
70
Constantan
.039
Copper
Gold
Comments
Maximum
116,000
66% Cu, 34% Zn
15% Cr, 35% Ni, 50% Fe
250
55% Cu, 45% Ni
1.000
1
.707
1
Commercial annealed
HyMu80
.030
20,000
100,000
80% Ni, 20% Fe
Iron, pure
.178
25,000
350,000
Annealed
Iron, Swedish
.172
250
5,500
Iron, cast
.057
100
600
Kovar A
.006
Lead
.079
1
Magnesium
.387
1
Manganin
.039
84% Cu, 12% Mn, 4% Ni
Monel Metal
.041
67% Ni, 30% Cu, 1.4% Fe,
29% Ni, 17% Co,
0.3% Mn, 53.7% Fe
1% Mn
Mumetal
.034 - .069
20,000
100,000
110
600
71-78% Ni, 4.3-6% Cu,
0-2% Cr, bal. Fe
Nickel
.250
Nickel-silver
.062
Palladium
.160
1
Permalloy
.038
2,500
25,000
Permendure
.066
800
4,500
50% Co, 1-2% V, bal. Fe
Platinum
.164
1
Rhodium
.338
1
Rhometal
.019
1,000
5,000
36% Ni, 64% Fe
64% Cu, 18% Zn, 18% Ni
1-163
45% Ni, 55% Fe
MIL-HDBK-419A
Table 1-29 (Continued)
Relative Conductivity and Relative Permeability of Common Metals
Metal
Relative
Relative
Conductivity
Permeability
Comments
( µr )
Initial
Sendust
.022
-
.029
30,000
Maximum
120,000
10% Si, 5% Al, 85% Fe
(cast)
Silver
Steel
.078
1.064
1
-
50
.133
Steel, manganese
.025
Steel, silicon
.034
Steel, stainless
.019
Supermalloy
Tin
.029
100,000
.151
1
Titanium
.036
1
Tungsten
.315
1
Zinc
.287
1
100
0.4%-0.5% C, bal. Fe
13% Mn, 1% C, 86% Fe
500
7,000
4% Si, 96% Fe (hot rolled)
0.1% C, 18% Cr, 8% Ni,
73.9% Fe
e.
1,000,000
79% Ni, 5% Mo, 16% Fe
Cover all openings required for visual access with wire screen or conductive glass (see Volume I,
Section 8.5.3.2). Insure that the screen or glass is carefully bonded to the enclosure around the perimeter of the
opening.
f.
Doors should be metal with solid, uniform contact around the edges. Wire mesh gaskets or finger
stock should be provided.
For large shielded enclosures where high traffic volume is expected, consider the use of waveguideg.
below-cutoff hallways.
1.9 COMMON-MODE NOISE AND INSTRUMENTATION. There are several steps which can be taken during
the design and construction stages of a facility to minimize subsequent common-mode noise problems in
instrumentation, equipment, and systems.
The recommended steps should be recognized as being appropriate
for interference control in general and not limited strictly to common-mode noise.
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Figure 1-86. Nomograph for Determining Magnetic Field Reflection Loss (1-17)
1-165
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Figure 1-87. Nomograph for Determining Electric Field Reflection Loss (1-17)
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Figure l-88. Nomograph for Determining Plane Wave Reflection Loss (l-l7)
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MIL-HDBK-419A
Figure 1-89. Shielding Effectiveness of Aluminum
Foil Shielded Room (1-18)
Figure 1-90. Shielding Effectiveness of Copper Foil Shielded Room (l-l8)
1-168
MIL-HDBK-419A
Figure 1-91. Formation of Permanent Overlap Seam
Figure 1-92. Good Corner Seam Design
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Figure 1-93.
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MIL-HDBK-419A
1.9.1 Design Practices.
a.
Sensitive data and instrumentation facilities should be located as far as possible from high voltage
(66 kV and above) transmission lines.
b.
The routing of data and signal lines should be perpendicular to main power lines wherever possible.
Where parallel runs cannot be avoided, maximum separation must be maintained. In many instances, routing of
the data and signal cables in ferrous conduit may be necessary.
c.
possible.
Distribution feeders to the facility should be routed perpendicularly to high voltage power lines, if
In any event, long parallel runs between distribution feeders and the main power line should be
avoided.
d.
Where overhead distribution lines are necessary, pre-assembled aerial cable should be used in
preference to open wires.
Since the conductors of pre-assembled aerial cable are twisted, the associated
magnetic field is greatly reduced.
e.
All internal distribution power conductors near sensitive test and measurement facilities and
carrying more than 5 amperes should be twisted.
A suggested rate of twist is one complete twist for each
length equal to approximately 25 times the diameter of the insulated power conductor.
f.
Metallic enclosures should be used for power conductors wherever possible to take advantage of the
shielding they offer. In order of preference, the types of enclosures recommended are:
(1) Conduit.
From the standpoint of noise reduction, rigid steel conduit is the most effective
enclosure for power conductors and should be used wherever practical.
Electrical metallic tubing (EMT) and
rigid aluminum or copper conduit provide effective electrostatic shielding, but their magnetic shielding
properties are at least an order-of-magnitude poorer than rigid steel conduit.
( 2 ) C a b l e armor.
Armored cable is sometimes used in lieu of conduit and individual insulated
conductors. The armor provides an effective electrostatic shield but is not as effective as rigid steel conduit
for magnetic shielding. Steel armor is preferable to aluminum or bronze.
(3) Flexible conduit. Because of its construction, standard construction grade flexible conduit is a
poorer electrostatic shield than either of the above and provides considerably less magnetic shielding than rigid
steel conduit. It is recommended that the use of flexible conduit be restricted to short lengths and only where
required to absorb vibration or to permit position adjustment of the equipment or device served.
(4) Wireway or cable tray. Wireways, which are rectangular sheet metal duct-like enclosures, and
cable trays are not nearly as effective for electrostatic or magnetic shields as rigid steel conduit. Unless the
wireway or cable tray is made of a ferrous metal and all discontinuities are carefully bonded, its use for the
shielding of power conductors should be limited.
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Electrical power equipment such as transformers, line voltage regulators, motors, generators, and
g.
switching devices should be separated as far as possible from data system equipment and conductors. The
architectural arrangement of the facility should allow for the maximum distance between these devices and the
data systems.
This requirement also applies to heating, ventilating, and air conditioning equipment which
utilize electric motors and high amperage switching devices.
The maximum distance will be limited by the
voltage drop which can be tolerated in feeders to the system equipment.
h.
Use squirrel cage induction motors, which do not utilize slip rings or commutators, wherever
possible.
i.
Where necessary to specify motors with commutators, specify those properly designed to minimize
arcing. Arcing at the commutator or slip rings can be decreased by careful mechanical design such as requiring
adequately sized shafts and bearings which maintain concentricity to minimize brush bounce and vibration.
1.9.2 Instrumentation Considerations. Where transducers and associated processing devices are to be installed
as an integral part of a facility, the instrumentation system must be designed and installed such that it does not
compromise the single-point signal ground networks used by other lower frequency systems.
In particular,
where the systems interface, care must be utilized to assure that the grounding integrity of each is maintained.
Derive the ac power for the test equipment from the same branch circuit supplying the equipment or system
being measured. If this practice raises system reliability problems, low amperage breakers or fuses should be
provided for the test equipment outlets. If the outlets for test equipment cannot be connected to the branch
circuits feeding the primary equipment, then the test equipment branch circuit should be restricted only to test
equipment use. In particular, rotating machinery, industrial machines, appliances, vending and office machines,
and any other non-EMI protected equipment should not be connected to that branch circuit.
1.10 EMP PROTECTION.
EMP protective measures are based on intercepting the incident energy and
dissipating it or reflecting it away from the threatened device, equipment, or facility. These measures are
implemented by providing adequate metal shielding around the facility (or the equipment inside); by installing
fast response surge arresters on power, signal, and control lines; by terminating the shields and arresters in an
earthing connection offering a low impulse impedance; by carefully controlling the points of penetration of
collectors; and, finally, by paying particular attention to all bonds throughout the protective system.
1.10.1 Earth Connection.
A radial, or star, configuration is preferred to other types of earth electrode
subsystems because of its lower impulse impedance (see Volume I, Section 2.6.3). Where ENIP protection is to
be provided in addition to conventional signal and safety protection, supplemental radials may be added to the
conventional system.
One low-impulse impedance radial should be placed at each location where there are over voltage arresters or
protectors on incoming external lines or conductors.
An example of such a location is the point where
commercial power lines enter the first stepdown transformer. Another location is at the point where external
conductors enter the shelter itself and where protectors or arresters are located.
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MIL-HDBK-419A
Water pipes or conduit should be connected to the earth electrode subsystem to prevent ground currents from
entering the structure.
Further, at the first service disconnect the ac neutral should be grounded at only one
point (to EES) to prevent the possibility of damage to transformers from circulating currents. This does not
negate the NEC requirement to ground the neutral at the transformer.
1.10.2 EMP Shield Applications.
a.
Whenever feasible, shielding of the overall building should be done in preference to room or area
protection. Individual room or area shields should only be utilized to provide additional protection of critical
equipment when normal protective methods will not reduce EMP to an acceptable level, or where, in
retrofitting an existing structure, the cost of protecting the entire building is excessive.
b.
Commercial enclosures may be used for small rooms and bolted construction is acceptable. For
large room construction, however, continuously welded steel is preferred.
c.
Electrical wiring and components should be protected from EMP fields by a shield such as ferrous
conduit, RF shielded raceway, or cable armor, that completely surrounds the items to be protected. Electronic
components may be shielded with sheet metal housings.
d.
All metallic penetrations of the facility shielding should enter at a common location as illustrated in
Figure 1-94.
All shielded cables, conduits, and pipes should be bonded to an entry plate as shown in
Figure 1-95. This plate should be large enough so that no penetrations will occur within 1 foot of the nearest
edge.
The entrance plate should be continuously welded, around its perimeter, to the building shield. The
conduit should be of steel with threaded or welded couplings. Conduit runs should be as short as practical with
joints held to a minimum. Transient protection for cables entering a building at points away from the building
entry plate is provided by following procedures outlined in Section 1.3.3.5.26.
1.10.3 EMP Bonding Practices.
a.
Homogeneous welds should be used whenever possible because they offer the best protection against
penetration of the EMP signal.
b.
When bolts are used as fasteners, the body of the bolt should not be welded or brazed. The nut and
washers should be located inside the shield region where they will not be exposed to the incident field. Nuts
should be checked for tightness periodically during EMP hardness assurance test cycles.
c.
Pipes, conduit, and connector shells should be welded or brazed to the shield completely around their
perimeter at the point of penetration of the shielded region. Conductors used for grounding purposes shall not
penetrate any metallic barrier designated as an EMP shield, i.e., shielded enclosure, EMP vault etc., but shall
rather be bonded to a welded stud on the barrier.
d.
Indirect bonding jumpers and straps should be as wide as practical and as short as possible to
minimize the inductance of the path for the EMP-induced current.
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MIL-HDBK-419A
Figure 1-94. Typical Single-Point Entry for Exterior Penetrations (Top View)
1-174
MIL-HDBK-419A
Figure 1-95.
1-175
MIL-HDBK-419A
1.10.4 Construction Guidelines (1-14).
The following is a list of additional construction practices which have
proven effective in reducing problems of EMP interference and/or instrumental damage:
a.
Isolate power by using internal motor-generator sources and installing lightning arresters on lines.
b.
Put all external wires in continuous, properly grounded ferrous conduit.
c.
Screen over air-conditioning ducts, where they enter shielded areas, must be peripherally bonded to
the shield.
d.
Interconnect the steel reinforcing bars in concrete into the shielding and grounding systems for the
structure.
e.
Use lightning arresters on power station transformers.
f.
Provide all surge arresters with shortest possible leads.
g.
lengths.
Ground cable outer shields and insure that the shields are continuous and closed throughout their
h.
Bury power and signal cables in ferrous conduit as deeply as is economically feasible (greater than
0.9 meter (3 feet)) to reduce current surges and to slope wave fronts induced on the cables.
i.
Install transient protective devices (TPD’s) on all antennas and other electric lines exposed to the
external environment.
Educate personnel in proper protection practices; for example, extension cords connected to outside
j.
plugs should not be brought into shielded areas.
k.
Bond together and ground all nonelectrical conductors such as elevator cables, metal airducts, and
storage cabinets.
l.
All conduit penetrations must be peripherally bonded to the shield.
m.
Avoid use of nonconducting lubricants when fastening conduit pipes together.
n.
Ensure that a low resistance circumferential electrical weld exists wherever conduit penetrates the
EMP barrier.
o.
When closed, the cover should provide peripheral contact with the box.
Either use adequate surge protection on oil-filled transformers and other high voltage gear tc
p.
prevent explosions or use only dry transformers inside the shielded enclosure.
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q.
Provide adequate surge protection for emergency power equipment.
r.
Do not rely on fuses or circuit breakers for EMP protection.
s.
Provide automatically closing doors in preference to manually closed doors.
t.
Put single-phase protection on each phase of 3-phase power systems as well as on the neutral.
u.
Use passive low pass L-C radio interference filters on signal, control, and telephone lines.
v.
Since electromagnetic fields in the corners of a shielded structure are usually higher than in other
parts of the structure, when convenient do not locate known sensitive equipments in corners.
1.11 MILITARY MOBILE FACILITIES.
1.11.1 General Tactical Grounding Requirements.
1.11.1.1 Facility Ground System. The facility ground system connects any metallic element of the associated
subsystems to earth by way of an earth-electrode configuration. It establishes a reference potential common to
any equipment or subsystem, and makes the ground potential available throughout the system. This section
describes the four subsystems that comprise the facility ground system and should be addressed during the
design and installation of any electrical and electronic equipment, subsystem, and system. Although, it is not
possible to have a fixed set of rules governing the grounding of all conceivable electrical or electronic
equipment or system configurations, the guidelines presented here should be adapted to the requirements of a
particular tactical installation.
1.11.1.1.1 Earth Ground. A good, basic earth ground or earth electrode subsystem is the fundamental network
for establishing a ground point for the three remaining ground subsystems; lightning/EMP, signal reference, and
fault protection. An ideal earth electrode subsystem will provide a common potential reference point anywhere
in the system to eliminate undesirable voltages and currents.
1.11.1.1.1.1 General Earth Electrode Subsystem Requirements. An earth electrode subsystem is a network of
electrically interconnected rods, plates, mats, or grids installed with a system to establish a low-resistance
contact with earth.
As a design objective, the dc resistance to earth of the earth electrode subsystem should
not exceed 10 ohms.
1.11.1.1.1.2 Earth Electrode Subsystem Types.
There are earth electrode subsystems for the following two
types of facilities:
a.
Fixed Site Facilities.
Descriptions of earth electrode subsystems installed in fixed facility or
semi-permanent buildings or installations are contained in Sections 1.2 and 1.4.
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b.
Tactical Facilities.
Tactical earth electrode subsystems are connected to existing buried low
resistance facilities, if available, or to driven ground rods or ground-rod configurations. Total resistance to
earth is affected by the type of soil, contact resistance between ground rod and soil, and by the connector
resistance.
1.11.1.1.1.3 Soil Resistance.
Resistivity of the soil into which the earth electrode subsystem is buried
constitutes a basic constraint on achieving low ground resistance.
Soil resistivity, measured in ohm-m, is
defined as the electrical resistance of a cube of homogeneous material (soil). Table 1-30 contains sample
resistivity values of various soil types.
ground resistance.
Soil temperature and moisture content are other variables affecting
Ground resistance increases inversely as the soil temperature with only slight changes in
ground resistance occurring above 32 degrees Fahrenheit. Soil resistance also varies widely as a function of
moisture content.
Additional information on soil resistance is contained in Sections 1.2.2 and 1.2.3 of this
Volume, and Section 2.3 of Volume I.
1.11.1.1.1.4 Ground Rod Resistance.
Ground rod resistance is primarily a function of the depth the rod is
driven into the earth and the soil resistivity. Theoretically, the resistance (R) of a ground rod driven vertically
into uniform soil is:
R =
where
is the resistivity of the soil,
(l-3)
and d are the rods length and diameter, respectively. Figure 1-96
illustrates the measured effect of rod length on total ground resistance.
1.11.1.1.1.5 Ground Resistance Shells.
Associated with a driven ground rod injected with current are
imaginary ground resistance shells. The concentric shells of resistance outward from the rod are a function of
the earth’s resistance to flow of current.
The shell having the smallest cross-sectional area closest to the
ground rod will exhibit the largest incremental resistance.
Approximately 90 percent of the shells of total
resistance to the earth occur, on the average, within a radius of two rod lengths from the electrode.
1.11.1.1.2 Power Ground. Power supplied to tactically-deployed equipments and systems may be derived from
three sources; transportable ac power generators, commercially-available ac power, and battery supplied dc
power. The ac neutrals are always floated on the load side of the first service disconnect within the shelter.
The 5-wire system described in 1.11.1.1.2.1 is preferred for new systems. If multiple shelters are serviced from
a single power source (transformer or generator), or if hum is encountered, the neutral conductor should be
grounded at the source only.
1.11.1.1.2.l Three-Phase Power Distribution System.
Transportable power generators presently used with
military mobile equipments are 3-phase, 4-wire, 120/240 V ac wye distribution systems. Ground points of a
3-phase wye system are illustrated in Figure 1-97.
Five-wire ac power grounding requires that the neutral
(white) or grounded conductor be connected to an earth ground at the source (generator or transformer) and
again at the supply side of the first service disconnect/power entry panel (PEP). The grounding (green)
conductor should also be connected to earth ground at the first service disconnect and at the transformer.
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Table 1-30. Soil Resistivity (ohm-m)
Resistivity
Type of Soil
Minimum
Average
Maximum
3 x 104
5 x 104
5 x 105
6 x 102
2.5 x 103
7 x 103
Clay, shale, gumbo, loam
3 x 102
4 x 103
2 x 104
Same as above with varying
1 03
1.5 x 104
10 5
5 x 104
10 5
10 6
Sandy, dry, flat, typical
coastal areas
Fills, ashes, cinders, brine,
waste
proportion of sand and gravel
Gravel sandstones with little
clay, loam, or granite
1.11.1.1.2.2 Single-Phase Power Distribution System.
Commercially-supplied ac power is single-phase,
110/220V. Power neutral of these systems is first grounded at the transformer secondary and also at the first
service disconnect. Figure 1-98 illustrates the ground connections of a single-phase power system.
1.11.1.1.2.3 DC Power System (2-Wire).
A 2-wire dc power generator is grounded by connecting either the
positive or negative conductor to ground at one point only, preferably at the source. The neutral (or grounded)
conductor should not be grounded at the Power Entry Panel (PEP). Figure 1-99 shows the ground connections
for the 28 V dc power system. The 3-wire dc power system requires that the neutral wire (white) or grounded
conductor be connected to the earth ground at the source (generator or transformer) only.
1.11.1.1.3 Lightning/EMP Protection Subsystem.
lightning/EMP ground subsystems.
Low resistance earth grounds are important for
For these subsystems, low resistances to earth are necessary to reduce the
possibility of arcing generated by potential differences between the earth and nearby equipments or shelters
and to reduce step potentials and voltage gradients in the vicinity where a lightning discharge enters the earth.
See Sections 2.8.1.3 and 3.6.3.4 of Volume I for additional information. The need for lightning protection in a
tactical environment is determined by the frequency and intensity of lightning activity in the area and by the
type of structures needing protection. The lightning protection subsystem down conductor should be connected
to the earth electrode subsystem at a point removed from the signal reference and fault protection subsystem.
Two general ways of protecting against lightning damage are air terminals and surge protection devices.
1.11.1.1.3.1 Air Terminals. To protect a shelter from damage caused by a lightning stroke, an air terminal of
adequate mechanical strength, length, and electrical conductivity to withstand the stroke must be provided to
intercept the discharge before it penetrates the structure.
between the air terminal and earth electrode subsystem.
A low-impedance path (cable) must be established
The resistance of the earth electrode subsystem
should be less than 10 ohms. Detailed construction of air terminal systems are given in Section 1.3.2.1.
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Figure 1-96.
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Figure 1-97.
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1.11.1.1.3.2 Terminal Protection Devices.
Terminal Protection Devices (TPDs) and electromagnetic shields
provide additional means of lightning and/or EMP protection (Section 1.3.3.5.11). The TPDs are fast-response
protection devices installed on exposed circuits such as power lines, signal, and control cables. Lead lengths of
each TPD shall be kept to an absolute minimum.
They are installed on lines for the purpose of shunting
extraneous pulses to ground and are typically installed on signal and power lines at their point-of-entry into a
shelter’s signal entry panel.
Examples of commonly used terminal protection devices are carbon blocks, gas-
filled spark gaps, zener diodes, and EMI power and signal line filters. Surge arresters used to protect a system
against lightning may serve to protect it against certain types of EMP given the response time of the arrester is
properly designed (see Volume I, Section 10.4.2.3).
1.11.1.1.4 Signal Reference Subsystem.
Grounding techniques used in the signal reference subsystem are a
function of operating frequencies. Lower frequency circuits (30 kHz and below) shall be single point grounded.
Higher frequency circuits (above 30 kHz) shall employ an equipotential plane which may, in the case of a
metallic van or shelter, be the skin of the housing.
All equipment cases or cabinets must be bonded to the
equipotential planes by the shortest and most direct route. in fixed site facilities an equipotential plane will be
installed in accordance with Section 1.5.1. If a combination of both higher and lower frequencies circuits exist,
use the higher frequency signal ground technique.
circuits.
All digital circuits are considered higher frequency signal
Signal reference subsystems, in a multishelter configuration shall be bonded to a common earth
electrode subsystem at one point only to minimize inter-shelter interference on signal cabling.
1.11.1.1.5 Fault Protection Subsystem.
The fault protection subsystem (grounding/green conductor) shall be
designed to carry current only in the event of equipment or system faults.
The fault protection subsystem
includes equipment racks, cabinets, conduit, junction boxes, raceways, ductwork, pipes and other normally non–
current carrying metal elements.
For shelterized equipments, the fault protection subsystem is connected to
the earth electrode subsystem via the power entrance panel.
Care should be taken to ensure the fault
protection subsystem and the signal reference subsystem are not connected to the earth electrode subsystem at
the same point.
1.11.1.2 Measuring Ground Resistance in Tactical Environments.
The resistance to earth of the earth
electrode subsystem shall be measured by the fall of potential technique (see MIL-STD-188-124A para 5.1.1.1.7
or Section 2.7.2 of Volume I). If the tactical situation does not permit this method to be used, the three-point
or triangulation method is an adequate substitute. Section 2.7.3 of Volume I refers.
1.11.1.3 Reducing Ground Resistance in Tactical Environments. Three basic methods should be considered for
grounding tactical equipments and systems; (1) utilization of earth electrode subsystems of existing/permanent
facilities, (2) utilization of recently eonfigured earth electrode, ground rod/ground rod configurations, and (3)
utilization of antenna counterpoises (including radial grounds). These methods and means of reducing related
resistance are described below.
1.11.1.3.1 Existing Facilities.
Wherever feasible, installation of earth electrode subsystems should take
advantage of the low-resistance properties of existing facilities such as water pipes, water well casings,
plumblng, and other metals embedded in and in contact with the earth. Resistance of the facilities should be
measured prior to use to determine if the 10 ohm or less resistance criteria is met. Use of existing facilities as
grounding means is especially desirable in permafrost situations.
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NOTE: LIFT WHEN SINGLE TRANSFORMER SUPPLIES POWER TO MORE THAN ONE BUILDING OR
BECAUSE OF OBJECTIONABLE CURRENT, NOISE OR INTERFERENCE.
Figure 1-98. Grounding of Single-Phase, 3-Wire 110/220V Power System
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Figure 1-99.
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1.11.1.3.2 Multiple Electrode System.
The resistance of a single vertically driven ground rod may also be
reduced if additional ground rods are connected in parallel with the given ground rod using a 1/0 AWG bare
copper cable to interconnect the rods. It is however important to note that total system resistance is sensitive
to electrode spacing. Electromagnetic interaction between multiple (74) ground rods that are spaced too closely
prevents the resistance of the total earth electrode subsystem connected in parallel from being l/M times the
resistance of a single rod.
If the electrodes in a multiple electrode system are spaced at 1.5 to 2 times the
length of a rod, the interactive influence is minimized and total resistance of the system will approach the
ideal.
1.11.l.3.3 Earth Electrode Subsystem.
The earth electrode subsystem should, soil and tactical conditions
permitting, consist of properly spaced ground rods interconnected in parallel by a bare 1/0 AWG copper cable.
The interconnecting cable for tactical situations should be clamped to the ground rods to facilitate installation
and transportability.
Earth electrode subsystems shown in Figures 1-9 and 1-100 may be installed around the
perimeter of temporary enclosures housing several stand-alone equipments such as portable single subscriber
terminals, telephone, or small switchboards. These earth electrode subsystems should extend 0.6 to 1.8 meters
(2 to 6 feet) beyond the dripline of the enclosure to ensure that any form of precipitation wets the soil around
the system.
Earth electrode subsystems in radial or star configurations may be employed but are less suitable
for (a) grounding equipments operating at rf such as radar or microwave systems, or (b) providing low impedance
grounds between interfacing shelters required to lessen interference or voltage surges caused by lightning
discharges.
Means of calculating ground resistance of the entire earth electrode subsystem are described in
Section 2.6 of Volume I while measurements of these systems are described in Section 2.7.
1.11.1.3.4 Chemical Treatment. The resistance of driven ground rods may be reduced by chemically treating
the soil around the rod and the interconnecting cable/wire.
Addition of ion-producing chemicals such as
magnesium sulphate (epsom salts), sodium chloride (table salt), and potassium nitrate (saltpeter) as well as
bentonite to the soil adjacent to an electrode has the net effect of increasing the apparent cross-sectional area
of the electrode and minimizing the current density of the soil.
Use of magnesium sulphate or bentonite is
recommended because of their low corrosive effect on metal and high electrical conductivity.
A circular
trench approximately 0.3 meters (1 foot) deep and 0.9 meters (3 feet) in diameter should be dug around the
electrode at a radius of 0.45 meters (1.5 feet) from the center of the electrode. The trench is filled with the
saline solution and covered with earth.
In order to provide the best distribution of the treating material with
the least corrosive effect the solution should not actually touch the electrode.
Additional information is
provided in Section 2.9 of Volume I.
1.11.2 Detailed Tactical Grounding Requirements.
1.11.2.1 Introduction. It is important that serious consideration be given to grounding implementation. Proper
grounding can have a significant impact on the ability to maintain communications under adverse conditions.
This section contains grounding requirements for tactical deployments of mobile equipment.
Grounding
methods set forth are based not only on implementation considerations but also on complying with specific
measured resistance requirements. The tactical deployments of mobile equipments are considered to be of four
types; i.e., stand-alone equipment, stand-alone shelter, collocated equipments and collocated shelters.
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1.11.2.1.1 Training.
Installers and operators of communications equipment should be formally trained in the
installation and maintenance concepts of grounding systems.
This training should include instructions in the
various types of grounding techniques and configurations, such as those listed:
Typical Training Requirements
Grounding Techniques -
Resistance Measuring Methods
Reduction in Ground Resistance
Initial Establishment of Grounding Systems as a Function of Terrain
Grounding Configurations - Earth Electrode Subsystem (Single Ground Rod)
Earth Electrode Subsystem (Multiple Ground Rods)
Equipotential Plane
Antenna Counterpoises
Radial Single-Point Ground Networks
Ground Systems on a Nodal Basis
1.11.2.1.2 Testing.
Ground resistance measurements should be made upon installation of a ground system and
at periodic intervals should the system
remain in place for any length of time, or at any time extraneous noise
occurs in the system. An earth resistance measurement set should be authorized to each unit to perform these
resistance measurements. The earth resistance goal of 10 ohms or less should be obtained for stand-alone and
collocated equipment. Where collocated equipment systems are separated by greater than 8 meters (26.5 feet),
their difference in resistance-to-ground measurements in tactical situations may be higher. If noise or other
undesirable effects are produced as a result of these higher ground resistance differences, the earth electrode
subsystems of each facility should be interconnected using two bare 1/0 AWG copper cables or chemical
treatment for soil enhancement should be applied to the subsystem having the higher resistance.
1.11.2.2 Stand-Alone Equipment.
1.11.2.2.1 General Description. The stand-alone equipments of the military mobile system are generally selfcontained transportable field equipment. These equipments interface with other equipment over WF-16 wire or
coaxial cables.
Stand-alone equipments generally are totally self-contained with integral power supplies and
grounding system.
The primary emphasis of low resistance grounds for stand-alone equipments is to assure
personnel safety and lightning protection.
Lightning protection is needed to protect operating personnel from
the effects of lightning that may impinge upon interfacing cable or from direct strike on the shelter.
1.11.2.2.2 Grounding Procedure. Means of providing lightning and safety protection on stand-alone equipments
include low resistance grounds and installation of surge arresters on interfacing cabling.
1.11.2.2.2.1 Low Resistance Grounds. Obtaining and maintaining low resistance grounds are the responsibility
of user personnel.
To provide adequate lightning protection the resistance to ground should be less than
10 ohms. Realizable grounding alternatives for stand-alone equipments are:
1.11.2.2.2.1.1 Existing Facilities.
If available, operating personnel should connect an interconnecting ground
cable to an existing low resistance facility as specified in 1.11.1.3.1.
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1.11.2.2.2.1.2 Earth Electrode Subsystem, Single Ground Rod.
A single driven ground rod connected to the
equipment via an interconnecting cable may be used, if the design resistance value is achievable. The rod
should generally be between 1.8 and 3.0 meters (6 and 10 feet) long. Selection of the required type of ground
rod should be based upon the expected soil conditions at the tactical site location. The resistance between the
rod and earth should be measured in accordance with Section 2.7.2 of Volume I. Where measured resistance is
not low enough, a saline solution (see 1.11.1.3.4) should be added to the soil adjacent to the rod to reduce ground
resistance.
1.11.2.2.2.1.3 Earth Electrode Subsystem, Multiple Ground Rods.
Where soil resistance cannot be reduced by
chemical means additional electrodes may be connected in parallel with the given ground rod.
The
interconnecting cable should be 1/0 AWG bare copper cable, and the ground rods should be spaced 1.5 to 2 rodlength's apart to minimize overlapping shells (see 1.11.1.1.1.5).
1.11.2.3 Stand-Alone Shelter.
1.11.2.3.1 General Description.
A stand-alone shelter is comprised of equipment housed in a mobile metallic
shelter and typically, is not situated close enough to other equipments to merit construction of a common
extensive earth electrode subsystem between its interfacing systems. Power supplied to the shelter may come
from a power generator or a commercial source. Interfacing with the shelter may be through the power cable.
The need for grounding stand-alone shelters is to provide a ground for (a) the fault protection subsystem, (b) to
"bleed off" static charges or EMI from interfacing signal cables, (c) the signal reference subsystem, and (d) the
(signal reference subsystem), lightning protection subsystem.
1.11.2.3.2 Interconnection of Subsystems. The signal reference and fault protection subsystems are connected
to the earth electrode subsystem because of the following reasons: (1) the skin of the shelter generally serves
as the equipotential plane for the signal reference subsystem, (2) the electronic equipment systems are
connected directly to the skin of the shelter by the shortest route possible, and (3) the fault protection
subsystem is connected to the grounding bus in the power entrance panel, and in turn, to the earth electrode
subsystem.
Since the power entry panel is bonded to the skin of the shelter, no loops are formed, and
everything within the shelter will remain at the same potential in the event of power faults, EMP, lightning or
EMI.
If the lightning activity in the deployment area warrants additional shelter protection, air terminals
should be installed atop the shelter as per 1.11.1.1.3.1.
If the installation is long-term, lightning protection
shall be mandatory.
1.11.2.4 Collocated Military Mobile Equipments.
1.11.2.4.1 General Description.
Collocated mobile equipments are equipments operating individually but
housed together within a single transportable enclosure, e.g., tarpaulin.
rack mounted and may be situated on the ground.
Typically, these equipments are not
Metallic shelter enclosures are not considered here
(reference 1.11.2.5). Intra-enclosure communication links may exist among equipments, but normally links are
established between an equipment and an external system. Basic operational characteristics of collocated
equipments are similar to stand-alone equipments. Grounding requirements are primarily for personnel safety
from lightning and power faults.
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1.11.2.4.2 Grounding
Procedure. Each stand-alone equipment is deployed with at least one ground rod. If the
—
total number of equipments within the enclosure are small enough and can be positioned such that the ground
rod for each can be used without compromising grounding integrity, then existing low resistance facilities or a
single driven ground rod per equipment may be used (reference 1.11.2.2.2.1) to ground collocated equipments.
Where large numbers of equipments are housed within an enclosure for which the individual grounding procedure
is not reasonable, a simple earth electrode subsystem should be deployed around the enclosure. The size of the
ground system and the number of attached rods needed to achieve the required ground resistance should be
determined according to 1.11.1.3.3.
In deployment areas requiring additional lightning\EMP protection
measures described in 1.11.1.1.2 shall be incorporated.
1.11.2.5
1.11.2.5.1
Collocated Shelters.
General Description.
Collocated shelters are transportable metallic shelters that share common
signal and/ or power cables and arc classified in two general categories; those located within 8 meters
(26.5 feet) of one another and those located greater than 8 meters (26.5 feet) from one another (see
Figure 1-100). Collocated shelter configurations are typical of an equipment system that must be housed in
multiple shelters.
Grounding requirements for collocated shelters are required to provide personnel and
equipment protection from the effects of lightning power faults and to provide a reference for signal grounds.
Particular consideration must be given to collocated shelters receiving power from the same power source or
communicating over inter-shelter signal cables.
The need to establish an all encompassing shelter grounding
system for collocated shelters situated more than eight meters apart should be a function of ground resistance
measurements taken at each shelter site. The ground system of each shelter should be interconnected as shown
in Figure 1-100 using two bare 1/0 AWG copper cables. If noise or other undesirable effects are produced as a
result of these higher ground resistance differences, the system having the higher resistance can be reduced by
use of chemical treatment or enhancement described in 1.11.1.3.4 or Section 2.9 of Volume I.
1.11.2.5.2 Grounding Procedure.
1.11.2.5.2.1 Power Ground.
Shelters powered by a single, common power source should have all grounded
conductors (neutrals) grounded to one point at the generator.
Where several power generators are connected in
parallel, the power neutrals of the generators should be interconnected and grounded at a single point. For
collocated shelters not sharing a common source but supplied by individual power sources, all neutrals may be
bonded together and grounded at one point.
The equipment shelters should share a common earth electrode
subsystem if they are located within 8 meters (26.5 feet) of each other.
1.11.2.5.2.2 Signal Ground. Collocated shelters less than 8 meters (26.5 feet) with interfacing communications
cables should have the signal grounding conductors bonded to a common earth electrode subsystem (see
1.11.1.3.3). This provides all interfacing shelters with a common signal reference plane.
1.11.2.5.2.3 Fault Protection Subsystem.
Intra-shelter green wire equipment grounding conductors should be
grounded at the power entrance panel of each shelter via the earth electrode subsystem.
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1.11.2.5.2.4
Lightning/EMP
Protection.
Electrical surge arresters should be used on all power and signal
cables at the shelter points-of-entry to provide protection from lightning and EMP induced currents on
inter-sllelter cabling. Lightning protection should also be provided in accordance with Section 1.3.
1.11.2.5.2.5 Collocated Shelters Greater than 8 Meters Apart.
Collocated shelters located greater than
8 meters (26.5 feet) apart refers to equipment systems consisting of multiple shelters (which has interfacing
shelters located as much as 250 feet apart) as opposed to an equipment system in which all elements are housed
in a single shelter. These shelters may therefore also be considered as stand-alone shelters. Where deployment
requires shelters to be located morc than 8 meters (26.5 feet) apart, grounding should be accomplished in
accordance with Figure 1-100 and as follows:
1.11.2.5.2.5.1 Ground Resistance Difference of Less Than 150 Ohms. If ground resistance measurements of all
shelters differ by less than 150 ohms, ground each shelter as a stand-alone shelter (reference 1.11.2.3).
1.11.2.5.2.5.2 Ground Resistance Difference of Greater Than 150 Ohms. If differences in ground resistance
measurements are 150 ohms or more, take corrective action (reference 1.11.1.3) to reduce resistance, and
ground each shelter as a stand-alone shelter (reference 1.11.2.3).
Figure 1-100. Connecting Ground Subsystems for Collocated Shelters Greater than 20 Feet Apart
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1.11.2.5.2.5.3 Earth Electrode Subsystem.
The earth electrode subsystem of each shelter shall be inter-
connected by two bare copper cables (see Figure 1-100). Tie all shelter earth electrode subsystems together to
form a common earth electrode subsystem (reference Table 1-22 for appropriate size cable to be used).
1.11.2.5.2.5.3.1 Power Ground. All grounded (neutral) conductors of each shelter should be grounded to the
earth electrode subsystem via the power entrance panel.
1.11.2.5.2.5.3.2
Signal
Ground.
The signal reference subsystem (skin or shelter) should be grounded at the
earth electrode subsystem at a single point.
1.11.2.5.2.5.3.3 Safety/Equipment Ground (Green Wire). The equipment grounding conductors of each shelter
are connected to the ground bus at the power entrance panel which in turn is grounded to the earth electrode
subsystem.
1.11.2.5.2.5.3.4 Lightning/EMP Protection.
Reference 1.11.2.5.2.4 for this requirement.
If additional
lightning protection is required, each air terminal with its associated down conductor should be grounded to a
ground rod of the earth electrode subsystem.
This requirement may necessitate the installation of additional
ground rods.
1.11.2.6 Fixed Prefabricated Shelters.
1.11.2.6.1 General Description.
Fixed prefabricated shelters are generally designed having the major
components prefabricated and then assembled on-site into a fixed shelter which can be considered as a fixed
facility.
As such, it will have its own earth electrode subsystem (ring ground) meeting the requirements of
Section 1.2.
It should also have a lightning protection subsystem meeting the requirements of 1.11.1.1.3.1
whenever the shelter is located outside the cone of protection of a higher grounded tower. The shell of metallic
prefabricated shelters should be constructed to be electrically continuous and grounded to the earth electrode
subsystem to bleed off static charges and reduce the effects of interference to C–E equipments and circuits. If
metallic and electrically continuous, the skin of a fixed prefabricated shelter may serve as the equipotential
plane.
If the skin is not metallic or electrically continuous, a separate equipotential plane meeting the
requirements of Section 1.5 will be required.
1.11.2.6.2 Electrical Connection.
If the skin of the shelter is metallic, and electrically continuous, it shall be
bonded to the grounding (green) cable of the fault protection subsystem at the first service disconnect or the
power entrance panel which, in turn, is grounded to the earth electrode subsystem by the shortest route. Both
the grounded (neutral) and grounding (green) wires are bonded together inside the first service disconnect, and
grounded to the earth electrode subsystem. The grounded wire may, however, be lifted from ground if hum or
noise problems are encountered when one power source supplies power to two or more shelters.
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1.12 FENCES.
1.12.1 Introduction.
Perimeter or security fences designed as part of a facility’s ground system shall be
constructed of galvanized steel chain-link fencing; vinyl-coating shall not be used. The supporting post and top
rails or wire shall be electrically conductive materials.
A #6 AWG (minimum) copper conductor, called a
reinforcing wire shall be woven through the entire length of the fence.
reinforcing wire shall be grounded periodically to ground rods.
Both the fence post and the fence
A suitable interval for the ground rods is
l00 feet for small sites and 500 feet for large sites. Installation will be shown on Figure 1-101.
1.12.2
Grounding.
Fences should be grounded on each side of every gate and, where crossed by high tension
lines, at points 150 feet on each side of high tension crossing. If a fence consists of wooden post and horizontal
metal strands only, down conductors should be run the full height of the fence post and securely fastened to
each wire so as to be electrically continuous. The connection to the ground may be made at the post, if the
post is metal and is electrically continuous with the fence.
1.12.3 Installation. Installation of fence grounds shall be accomplished as follows:
a.
Thread a bare conductor, #6 AWG or larger, through individual links of the fence. This must be
accomplished by threading two or three links at a time and pulling the conductor through until all slack has been
removed; then repeat the process.
The copper wire shall be continuous between posts. Spl ices, if required,
shall be made at fence post bonds.
b.
Prepare bonding surface in accordance with Section 1.7.2.
c.
Bond reinforcing wires at each post with exothermic welds or by brazing.
d.
Bond ground straps to fence posts with exothermic welds or by brazing. Bonds to ground rods shall
be exothermic welds or clamped and brazed.
e.
Seal all bonding surfaces in accordance with Section 1.7.3.
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Figure 1-101. Method of Grounding a Fence
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1.13 INSPECTION AND TEST PROCEDURE FOR A NEW FACILITY. The grounding, bonding, and shielding
practices and procedures recommended in this chapter should be implemented a S integral elements of the
facility during the construction of the building or structure. To ensure that the implementation is accomplished
in a timely manner, the construction efforts should be carefully monitored from the onset of excavation through
completion of the facility. Prior to acceptance of the facility, complete the Facility Checkout Form provided
in Section 1.13.6. The following guidelines are provided to aid in the inspection and checkout of the facility.
1.13.1 Earth Electrode Subsystem.
a.
observed.
Observe installation procedures.
Specifically see that the recommendations of Section 1.2.4 are
Verify that ground rods conforming to the sizes specified in MIL-STD-188-124A are used. If the
ground rods are driven in place, see that driving collars or nuts are used to prevent damage to the rods. Watch
for bent and broken or bulged couplings between sections. Seriously weakened or damaged couplings should be
replaced before driven below grade.
b.
Spot check the resistance of rods as they are driven. Use the fall-of-potential method described in
Section 2.2.2.2.1 to determine the resistance of a rod when it reaches the design or specified depth. With the
aid of Figures 1-4, 1-5, and 1-6, project the net resistance of the total number of ground rods. This projection
should indicate if the planned electrode subsystem will achieve 10 ohms (or less) resistance. As additional rods
are driven, continue to spot check the resistance of individual rods by measuring the resistance at each
successive fourth or fifth rod. This procedure will permit a decision to be made on the necessity for adjusting
the electrode configuration (either adding to or subtracting from) to achieve the required resistance (see
Section 1.2.2.3).
c.
See that cable interconnecting the rods is of a correct size (1/0 AWG). Inspect all connections
between cable sections and all interconnections between cable and ground rods. All connections to be buried
and subsequently made inaccessible must be welded or brazed. Restrict the use of clamps or bolted connections
to locations which will remain accessible.
d.
Check to see that provisions are made for interconnecting the earth electrode subsystem with metal
utility lines, buried tanks, and other underground metals.
e.
Verify that risers or cables of appropriate size are installed for lightning down conductor, signal
ground, and power system ground connections (see Section 1.3.2.2). Ensure that risers used for lightning down
conductors are not used as part of the signal reference or fault protection subsystems.
f.
Once the complete minimum system is installed, measure the resistance to earth for the system
using the fall-of-potential method described in Section 2.2.2.2.1.
If the resistance is greater than 10 ohms,
alternate methods described in Section 2.9 of Volume I for reducing the resistance-to-earth shall be considered.
g.
Insure that all changes or modifications are properly indicated on the facility drawings.
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1.13.2 Lightning Protection Network.
a.
Determine the cone of protection established by the air terminals (or by the mast or overhead
ground wire, if a separately installed system is provided).
structure (be sure to include all views).
Locate air terminals on a scaled drawing of the
Using the procedures of Section 1.3.2.1, determine if all parts of the
facility are adequately included within the cones of protection established by the air terminals. In deficient
areas, determine what additional measures, if any, need to be taken (Sections 1.3.2.1 and 1.3.2.2 should be
consulted for guidance).
b.
Inspect air terminals for type of materials (Section 1.3.2.1.1), for correct height (Section 1.3.2.1.2),
and proper placement (Section 1.3.2.1).
c.
Inspect roof conductors for proper size and correct choice of materials (Tables 1-2 and 1-3), proper
routing (Section 1.3.2.2.1), and for proper use of fasteners (Section 1.3.2.3).
d.
Inspect down conductors for proper size and appropriate choice of material (Tables 1-2 and 1-3).
Verify that the routing of down conductors conforms to the recommendations of Section 1.3.2.2.2.
Where
structural steel members are used as down conductors, the recommendations of Section 1.3.2.2.2.1 should be
observed. Inspect fasteners and hardware for accessibility, strength, and corrosion resistance as recommended
by Sections 1.3.2.3 and 1.3.3.8.
e.
Verify that adequate guards are provided (Section 1.3.3.10).
1.13.3 Signal Reference and Fault Protection Subsystems.
a.
b.
elements.
Inspect to verify that equipotential planes exist in conformance to Section 1.5.1.1.
In steel frame buildings, verify that the equipotential plane is bonded to the main structural steel
In wooden or masonry buildings inspect to assure that multiple downleads are bonded to the plane.
Insure the red and black signal grounds are bonded to the equipotential plane as outlined in Section 1.6.
c.
Verify that the structural steel elements are bonded at the joints to produce a low resistance
(< 1 milliohm) joint. Review Section 1.5.1 for recommended fastening procedures. Welded joints conforming to
Section 1.7.1 are preferred. Mechanically fastened joints should be carefully cleaned, bolts adequately torqued
(see Table 1-5), and proper bond protection supplied.
Visually inspect cleaning procedures, perform spot checks
torque measurements, and visually verify that paints and sealants are applied as needed. Perform spot check
measurements of bond resistance at structural joints using the double balanced bridge technique described in
Section 2.2.2.3.1. Where bond resistances greater than 1 milliohm are encountered, require that bond surfaces
be recleaned, bolts retorqued, or supplemental jumpers provided as needed to achieve 1 milliohm.
d.
In non-steel frame or masonry buildings, inspect the installation of the supplemental grounding
network for conformance to the recommendations of Section 1.5.1. In particular, verify that the grounding
cables provide the required 2,000 circular roils per running foot of conductors.
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e.
Verify that the ground risers are bonded to the equipotential plane as specified in Section 1.5.1.1 and
that the bond resistance does not exceed 1 milliohm.
Inspect to assure that the ground risers are located to
provide the shortest possible lengths to the equipotential plane.
f.
Verify that at least two electrical paths exist between the equipotential plane and the earth
electrode subsystem. Preferably the plane should be bonded to the building main structural steel (or downleads
in wooden buildings) at least every 3 meters (10 feet). Measure the resistance between selected points on the
plane and the earth electrode subsystem to verify that the total resistance does not exceed 5 milliohms. If the
resistance does exceed 5 milliohms check all joints for proper bonding and down hauls for proper sizes. See that
all deficient conductors are replaced and that all poor bonds are redone.
Inspect all conduit metallic pipes and tubes for continuity and bonding as recommended in
g.
Section 1.4.4.
h.
Verify that all electrical supporting structures and cable ways are interconnected and bonded as
recommended in Section 1.4.5.
i.
Inspect the grounding of the electrical distribution system for conformance with Section 1.4.6.
1.13.4 Bonds and Bonding.
a.
In addition to the inspection of structural joints, generally inspect all bonds for proper cleaning,
correct fastening or assembly, and for adequate corrosion protection.
Be particularly alert for conformance
with the recommendations of Sections 1.7.1 and 1.7.3.
b.
Perform resistance checks on selected bonds. Use the double balanced bridge method described in
Section 2.2.2.3.1.
All bonds should exhibit a resistance of 1 milliohm or less; those which do not must be
redone.
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PART I - EARTH ELECTRODE SUBSYSTEM
1.13.5 Facility Checkout Form.
Facility
Date
Location
Inspector
A.
Soil Resistivity ---------------------------------------
(ohm-cm).
(Obtain from site survey (see Section 1.2.1.1) or from the measured resistance of a rod or group of rods
(see Section 1.13.1). Use Figures 1-4, 1-5, and 1-6 to obtain a