5 MB
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 I 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.
This standardization handbook was developed by the Department of Defense in accordance with
established procedure.
2.
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, electromagnetics, 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: Commanding Officer, Naval Publications and Forms Center, 5801
Tabor Avenue, Philadelphia PA 19120. Single copies may be obtained on an emergency basis by calling
(AUTOVON) 442-3321 or Area Code (215)-697-3321. Copies of industry association documents should be
obtained from the sponsor. 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 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 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
1.1
1.2
1.3
1.4
1.5
1.5.1
1.5.2
1.5.3
1.6
1.7
Page
GENERAL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APPLICATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCED DOCUMENTS. . . . . . . . . . . . . . . . . . . . . . . . . .
DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facility Ground System. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounding and Power Distribution Systems . . . . . . . . . . . . . . . . . . . . .
Electrical Noise in Communications Systems. . . . . . . . . . . . . . . . . . . .
BONDING, SHIELDING, AND GROUNDING RELATIONSHIP . . . . . . . . .
GROUNDING SAFETY PRACTICES. . . . . . . . . . . . . . . . . . . . . .
1-1
l-l
l-l
l-l
l-2
l-2
l-3
l-4
l-5
l-5
CHAPTER 2 - EARTHING AND EARTH ELECTRODE SUBSYSTEM
2.1
OBJECTIVES.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1
Lightning Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2
Fault Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3
Noise Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4
Summary of Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
RESISTANCE REQUIREMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2
Resistance to Earth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.1
National Electrical Code Requirements . . . . . . . . . . . . . . . . . . . . .
2.2.2.2
Department of Defense Communications Electronics Requirements . . . . . . . . .
2.2.3
Lightning Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3
SOIL RESISTIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2
Typical Resistivity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3
Environmental Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4
MEASUREMENT OF SOIL RESISTIVITY. . . . . . . . . . . . . . . . . . . . . . .
2.4.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2
Measurement Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2.1
One-Electrode Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2.2
Four-Terminal Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5
TYPES OF EARTH ELECTRODE SUBSYSTEMS. . . . . . . . . . . . . . . . . . . .
2.5.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2
Ground Rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.3
Buried Horizontal Conductors. . . . . . . . . . . . . . . . . . . . . . . . . . .
i
2-l
2-l
2-2
2-2
2-2
2-5
2-5
2-5
2-5
2-5
2-5
2-7
2-7
2-7
2-7
2-8
2-8
2-8
2-8
2-13
2-15
2-15
2-15
2-15
TABLE OF CONTENTS (Continued)
Paragraph
Page
Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.4
Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.5
Metal Frameworks of Buildings. . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.6
Water Pipes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.7
Incidental Metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.8
Well Casings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.9
RESISTANCE PROPERTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6
Simple Isolated Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1
Driven Rod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1.1
Other Commonly Used Electrodes . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1.2
Resistance of Multiple Electrodes . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.2
Two Vertical Rods in Parallel. . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.2.1
Square Array of Vertical Rods . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.2.2
2.6.2.3
Horizontal Grid (Mesh). . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Rods Connected by a Grid . . . . . . . . . . . . . . . . . . . . . . .
2.6.2.4
Transient Impedance of Electrodes . . . . . . . . . . . . . . . . . . . . . . . .
2.6.3
Effects of Nonhomogeneous (Layered) Earth. . . . . . . . . . . . . . . . . . . .
2.6.4
Hemispherical Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.4.1
Vertical Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.4.2
Grids.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.4.3
MEASUREMENT OF RESISTANCE-TO-EARTH OF ELECTRODES. . . . . . .
2.7
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.1
Fall-of-Potential Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2
Probe Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2.1
Extensive Electrode Subsystems.. . . . . . . . . . . . . . . . . . . . . . . .
2.7.2.2
2.7.2.3
Test Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three-Point (Triangulation) Method. . . . . . . . . . . . . . . . . . . . . . . .
2.7.3
2.8
OTHER CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1
Surface Voltages Above Earth Electrodes. . . . . . . . . . . . . . . . . . . . .
Step Voltage Safety Limit. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1.1
Step Voltages for Practical Electrodes. . . . . . . . . . . . . . . . . . . . . .
2.8.1.2
Flush Vertical Rod. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1.2.1
Buried Vertical Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1.2.2
Buried Horizontal Grid. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1.2.3
Minimizing Step Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.1.3
Heating of Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.2
Steady State Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.2.1
Transient Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.2.2
Minimum Electrode Size . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8.2.3
2-15
2-15
2-16
2-16
2-16
2-16
2-17
2-17
2-17
2-23
2-23
2-23
2-27
2-29
2-30
2-32
2-32
2-32
2-33
2-33
2-35
2-35
2-35
2-36
2-42
2-45
2-46
2-47
2-47
2-47
2-49
2-49
2-53
2-55
2-56
2-57
2-57
2-57
2-59
ii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
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2-59
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2-60
2-62
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2-63
2-63
2-65
2-65
2-66
2-66
2-66
2-70
2-71
2-71
2-71
2-71
2-75
THE PHENOMENON OF LIGHTNING. . . . . . . . . . . . . . . . . . . . . . . .
DEVELOPMENT OF A LIGHTNING FLASH. . . . . . . . . . . . . . . . . . . . .
INFLUENCE OF STRUCTURE HEIGHT. . . . . . . . . . . . . . . . . . . . . . . .
STRIKE LIKELIHOOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATTRACTIVE AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structures Less Than 100 Meters High . . . . . . . . . . . . . . . . . . . . . . .
Cone of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIGHTNING EFFECTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flash Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanical and Thermal Effects . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conductor Impedance Effects. . . . . . . . . . . . . . . . . . . . . . . . . .
Induced Voltage Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Capacitively-Coupled Voltage . . . . . . . . . . . . . . . . . . . . . . . . .
Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BASIC PROTECTION REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . .
DETERMINING THE NEED FOR PROTECTION . . . . . . . . . . . . . . . . . . . .
Strike Likelihood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-l
3-3
3-3
3-4
3-10
3-10
3-11
3-13
3-13
3-15
3-17
3-17
3-18
3-21
3-21
3-25
3-26
3-26
ELECTRODE ENHANCEMENT . . . . . . . . . . . . . .
2.9
2.9.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .
Water Retention . . . . . . . . . . . . . . . . . . . . . . .
2.9.2
2.9.3
Chemical Salting. . . . . . . . . . . . . . . . . . . . . .
2.9.4
Electrode Encasement . . . . . . . . . . . . . . . . . . . .
Salting Methods. . . . . . . . . . . . . . . . . . . . . . . .
2.9.5
CATHODIC PROTECTION . . . . . . . . . . . . . . . . .
2.10
2.10.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .
2.10.2
Protection Techniques. . . . . . . . . . . . . . . . . . . . .
2.10.3
Sacrifical Anodes . . . . . . . . . . . . . . . . . . . . . . .
Corrosive Atmospheres . . . . . . . . . . . . . . . . . . . .
2.10.4
GROUNDING IN ARCTIC REGIONS. . . . . . . . . . . . . . .
2.11
Soil Resistivity. . . . . . . . . . . . . . . . . . . . . . . .
2.11.1
Improving Electrical Grounding in Frozen Soils . . . . . . . . .
2.11.2
2.11.2.1
Electrode Resistance . . . . . . . . . . . . . . . . . . . .
2.11.2.2
Installation and Measurement Methods . . . . . . . . . .
2.11.2.2.1
Electrode Installation . . . . . . . . . . . . . . . . . . .
2.11.2.2.2
Backfill . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . .
2.12
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CHAPTER 3 - LIGHTNING PROTECTION SUBSYSTEM
3.1
3.2
3.3
3.4
3.5
3.5.1
3.5.2
3.6
3.6.1
3.6.2
3.6.3
3.6.3.1
3.6.3.2
3.6.3.3
3.6.3.4
3.7
3.8
3.8.1
iii
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
3.8.2
3.8.3
3.9
3.10
Page
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3-26
3-27
3-27
3-28
FAULT PROTECTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power System Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ground-Fault-Circuit-Interrupter (GFCI) . . . . . . . . . . . . . . . . . . . . . .
EARTH CONNECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AC POWER LINE GROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-l
4-l
4-3
4-3
4-3
4-5
4-6
Type of Construction . . . . . . . . . . . . . . . . . . . .
Criticalness to System Mission . . . . . . . . . . . . . . . .
APPLICABLE CODES. . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . .
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CHAPTER 4 -FAULT PROTECTION SUBSYSTEM
4.1
4.1.1
4.1.2
4.2
4.3
4.4
4.5
CHAPTER 5 - GROUNDING OF SIGNAL REFERENCE SUBSYSTEM
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1
CONDUCTOR CONSIDERATIONS. . . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Direct Current Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1
Alternating Current Impedance . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2
Skin Effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2.1
AC Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2.2
Reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2.3
Proximity Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2.4
Resistance Properties vs Impedance Properties . . . . . . . . . . . . . . . . . . .
5.2.3
Effects of Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4
Stranded Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.1
Rectangular Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.2
Tubular Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.3
Structural Steel Members . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.4
SIGNAL REFERENCE SUBSYSTEM NETWORK CONFIGURATIONS . . . . . . .
5.3
Floating Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1
Single-Point Ground (for Lower Frequencies) . . . . . . . . . . . . . . . . . . . .
5.3.2
Multipoint Ground (for Higher Frequencies).. . . . . . . . . . . . . . . . . . . .
5.3.3
Equipotential Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.1
Types of Equipotential Planes. . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.2
5.3.4
Floating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iv
5-1
5-l
5-l
5-l
5-3
5-5
5-7
5-10
5-10
5-12
5-13
5-13
5-13
5-15
5-15
5-15
5-18
5-24
5-26
5-27
5-28
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
5.4
5.4.1
5.4.2
5.4.3
5.5
SITE APPLICATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lower Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Higher Frequency Network. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-28
5-29
5-30
5-31
5-32
CHAPTER 6 - INTERFERENCE COUPLING AND REDUCTION
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
6.2
COUPLING MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conductive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1
Free-Space Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2
Near-Field Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2.1
Inductive Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2.2
Capacitive Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2.3
Far-Field Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2.4
6.3
COMMON-MODE NOISE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic Theory of Common-Mode Coupling. . . . . . . . . . . . . . . . . . . . . .
6.3.1
Differential Amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.2
MINIMIZATION TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4
6.4.1
Reduction of Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reference Plane Impedance Minimization . . . . . . . . . . . . . . . . . . . .
6.4.1.1
6.4.1.2
Spatial Separation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reduction of Circuit Loop Area. . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1.3
Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1.4
6.4.1.5
Balanced Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.2
Alternate Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5
FACILITY AND EQUIPMENT REQUIREMENTS. . . . . . . . . . . . . . . .
6.6
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-l
6-5
6-5
6-6
6-6
6-8
6-11
6-14
6-17
6-19
6-23
6-23
6-23
6-23
6-24
6-24
6-24
6-24
6-24
6-25
6-25
CHAPTER 7 - BONDING
7.1
7.2
7.3
7.4
7.4.1
7.4.1.1
7.4.1.2
7.4.1.3
DEFINITION OF BONDING. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PURPOSES OF BONDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RESISTANCE CRITERIA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DIRECT BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contact Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-l
7-l
7-3
7-4
7-6
7-7
7-7
7-7
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
Page
7.4.1.4
7.4.2
7.4.2.1
7.4.2.2
7.4.2.3
7.4.2.4
7.4.2.5
7.4.2.6
7.4.2.7
7.5
7.5.1
7.5.2
7.5.2.1
7.5.2.2
7.5.2.3
7.6
7.6.1
7.6.2
7.6.3
7.6.4
7.7
7.8
7.8.1
7.8.1.1
7.8.1.2
7.8.2
7.8.3
7.9
7.10
7.11
7-8
7-10
7-10
7-11
7-14
7-14
7-15
7-16
7-16
7-16
7-19
7-19
7-19
7-19
7-23
7-25
7-26
7-26
7-29
7-29
7-29
7-30
7-30
7-31
7-31
7-34
7-34
7-34
7-36
7-37
Bond Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Direct Bonding Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soft Solder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rivets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conductive Adhesive. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . .
INDIRECT BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Skin Effect.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bond Reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stray Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SURFACE PREPARATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Platings and Inorganic Finishes . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion By-Products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPLETION OF THE BOND. . . . . . . . . . . . . . . . . . . . . . . . . . .
BOND CORROSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical Basis of Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrochemical Series . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galvanic Series. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relative Area of Anodic Member . . . . . . . . . . . . . . . . . . . . . . . . .
Protective Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
WORKMANSHIP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SUMMARY OF GUIDELINES. . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER 8 - SHIELDING
8.1
8.2
8.2.1
8.2.2
8.2.3
FUNCTION OF AN ELECTROMAGNETIC SHIELD. . . . .
BASIC SHIELDING THEORY . . . . . . . . . . . . . . . . . .
Oppositely Induced Fields . . . . . . . . . . . . . . . . . .
Transmission Line Analogy . . . . . . . . . . . . . . . . . .
Nonuniform Shielding . . . . . . . . . . . . . . . . . . . .
vi
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8-l
8-2
8-2
8-2
8-4
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
8.3
SHIELDING EFFECTIVENESS OF CONTINUOUS SINGLE-THICKNESS SHIELDS.
8.3.1
Absorption Loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reflection Loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2
8.3.2.1
Low Impedance Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2.2
Plane Wave Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.2.3
High Impedance Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.3
Re-Reflection Correction Factor . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.4
Total Shielding Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.4.1
Measured Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.4.2
SHIELDING EFFECTIVENESS OF OTHER SHIELDS. . . . . . . . . . . . . . . . . .
8.4
8.4.1
Multiple Solid Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4.2
Coatings and Thin-Film Shields. . . . . . . . . . . . . . . . . . . . . . . . . .
8.4.3
Screens and Perforated Metal Shields . . . . . . . . . . . . . . . . . . . . . . .
8.5
SHIELD DISCONTINUITY EFFECTS (APERTURES). . . . . . . . . . . . . . . . . .
8.5.1
Seams Without Gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seams With Gaskets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5.2
Penetration Holes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5.3
8.5.3.1
Waveguide-Below-Cutoff . . . . . . . . . . . . . . . . . . . . . . . . . . .
Screen and Conducting Glass . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5.3.2
SELECTION OF SHIELDING MATERIALS. . . . . . . . . . . . . . . . . . . . . .
8.6
USE OF CONVENTIONAL BUILDING MATERIALS. . . . . . . . . . . . . . . . . .
8.7
Concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7.1
Reinforcing Steel (Rebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7.2
CABLE AND CONNECTOR SHIELDING . . . . . . . . . . . . . . . . . . . . . . .
8.8
Cable Shields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.8.1
Terminations and Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.8.2
SHIELDED ENCLOSURES (SCREEN ROOMS).. . . . . . . . . . . . . . . . . . . .
8.9
Demountable (Modular) Enclosures . . . . . . . . . . . . . . . . . . . . . . . .
8.9.1
Custom Built Rooms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.9.2
Foil Room Liners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.9.3
TESTING OF SHIELDS.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.10
Low Impedance Magnetic Field Testing Using Small Loops. . . . . . . . . . . . . .
8.10.1
8.10.2
Additional Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PERSONNEL PROTECTION SHIELDS. . . . . . . . . . . . . . . . . . . . . . . .
8.11
DETERMINATION OF SHIELDING REQUIREMENTS. . . . . . . . . . . . . . . . . .
8.12
Equipment Disturbances . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.12.1
Electromagnetic Environmental Survey . . . . . . . . . . . . . . . . . . . . . .
8.12.2
Equipment EMI Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.12.3
vii
8-4
8-5
8-6
8-10
8-13
8-15
8-19
8-19
8-27
8-27
8-31
8-31
8-32
8-33
8-41
8-42
8-45
8-45
8-50
8-52
8-53
8-56
8-56
8-56
8-59
8-59
8-63
8-63
8-66
8-70
8-71
8-72
8-73
8-74
8-74
8-74
8-76
8-76
8-77
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Page
Paragraph
8.13
8.13.1
8.13.2
8.13.3
8.13.4
8.14
SYSTEM DESIGN CONSIDERATIONS. . . . . . . . . . . . . . . . . . . . . . . .
Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-77
8-78
8-78
8-78
8-78
8-79
CHAPTER 9 - PERSONNEL PROTECTION
9.1
9.1.1
9.1.2
9.2
9.3
9.4
9.5
9.6
ELECTRIC SHOCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Levels of Electric Shock. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shock Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
STATIC ELECTRICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RADIO FREQUENCY (RF) RADIATION HAZARDS. . . . . . . . . . . . . . . . . .
LASER HAZARDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-RAY RADIATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9-l
9-l
9-3
9-3
9-5
9-5
9-6
9-6
CHAPTER 10 - NUCLEAR EMP EFFECTS
10.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2
EMP GENERATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.1
High-Altitude EMP (HEMP).. . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.1.1
Early-Time HEMP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.1.2
Late-Time HEMP (MHDEMP). . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.1.3
Intermediate-Time HEMP. . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.2
Surface-Burst EMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.3
Other EMP Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.4
Comparison With Lightning. . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3
HEMP INTERACTION WITH SYSTEMS.. . . . . . . . . . . . . . . . . . . . . . .
10.3.1
Current in Long Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.1.1
Long Overhead Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.1.2
Long Buried Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.1.3
Vertical Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.2
HEMP Interaction With Local Structure. . . . . . . . . . . . . . . . . . . . . .
10.3.2.1
Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.2.2
Penetrating Conductors. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apertures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.2.3
viii
10-l
10-l
10-l
10-l
10-3
10-3
10-3
10-4
10-5
10-5
10-6
10-6
10-7
10-9
10-9
10-9
10-10
10-11
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
Paragraph
PROTECTION AGAINST HEMP. . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4
HEMP Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.1
10.4.1.1
Shield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Penetrating Conductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.1.2
Apertures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.1.3
Allocation of Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2
Amount of Protection Needed . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2.1
Where Protection is Applied . . . . . . . . . . . . . . . . . . . .
10.4.2.2
Terminal Protection Devices. . . . . . . . . . . . . . . . . . . . . . .
10.4.2.3
Spark Gaps and Gas Tubes . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2.3.1
Metal-Oxide Varistors . . . . . . . .. . . . . . . . . . . . . . . . . . . .
10.4.2.3.2
10.4.2.3.3
Semiconductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2.3.4
Waveguide Penetration of Facility Shield. . . . . . . . . . . . . . . . . . . . .
10.4.2.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2.4.1
10.4.2.4.2
In-Line Waveguide Attachment. . . . . . . . . . . . . . . . . . . .
Sleeve and Bellows Attachment. . . . . . . . . . . . . . . . . . . . . .
10.4.2.4.2.1
10.4.2.4.2.2
Braided Wire Sleeve. . . . . . . . . . . . . . . . . . . . . . . . . . .
Stuffing Tube for Waveguide. . . . . . . . . . . . . . . . . . . . . . .
10.4.2.4.2.3
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5
10-13
10-13
10-13
10-13
10-15
10-15
10-15
10-17
10-17
10-17
10-18
10-18
10-18
10-19
10-19
10-21
10-21
10-23
10-24
10-25
CHAPTER 11 - NOTES
11.1
SUBJECT TERM (KEY WORD) LISTING . . . . . . . . . . . . . . . . . . . . . . .
11-1
APPENDICES
A
B
BI
BII
C
D
GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SUPPLEMENTAL BIBLIOGRAPHY. . . . . . . . . . . . . . . . . . . . . . . . .
SUBJECT CROSS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . .
LISTINGS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TABLE OF CONTENTS FOR VOLUME II. . . . . . . . . . . . . . . . . . . . . . .
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
A-l
B-l
B-l
B-2
C-l
D-l
MIL-HDBK-419A
LIST OF FIGURES
Figure
2-l
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-21
2-22
2-23
2-24
2-25
2-26
2-27
2-28
2-29
2-30
2-31
2-32
2-33
Page
Voltage Differentials Arising from Unequal Earth Electrode Resistances and Unequal
Stray Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Voltage Differentials Between Structures Resulting from Stray Ground Currents. . . .
Typical Variations in Soil Resistivity as a Function of Moisture, Temperature, and
Salt Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Flow From a Hemisphere in Uniform Earth. . . . . . . . . . . . . . . . . .
Idealized Method for Determining Soil Resistivity. . . . . . . . . . . . . . . . . . .
Effect of Rod Length Upon Resistance . . . . . . . . . . . . . . . . . . . . . . .
Effect of Rod Diameter Upon Resistance. . . . . . . . . . . . . . . . . . . . .
Earth Resistance to Shell Surrounding a Vertical Earth Electrode . . . . . . . . . . . .
Resistance of Buried Horizontal Conductors . . . . . . . . . . . . . . . . . . . . .
Resistance of Buried Circular Plates. . . . . . . . . . . . . . . . . . . . . . . . .
Ground Rods in Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ratio of the Actual Resistance of a Rod Array to the Ideal Resistance of N Rods
in Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transient Impedance of an Earth Electrode Subsystem as a Function of the Number
of Radial Wires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Distribution in Nonuniform Soil . . . . . . . . . . . . . . . . . . . . . . .
Fall-of-Potential Method for Measuring the Resistance of Earth Electrodes . . . .
Effect of Electrode Spacing on Voltage Measurement . . . . . . . . . . . . . . . . .
Resistance Variations as Function of Potential Probe Position in Fall-of-Potential
Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Earth Resistance Curves for a Large Electrode Subsystem . . . . . . . . . . . . . . .
Earth Resistance Curve Applicable to Large Earth Electrode Subsystems . . . . . . .
Intersection Curves for Figure 2-18 . . . . . . . . . . . . . . . . . . . . . . . . .
Triangulation Method of Measuring the Resistance of an Earth Electrode. . . . .
Variation of Surface Potential Produced by a Current Flowing Into an Isolated
Ground Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Potential Variation Along a Grid. . . . . . . . . . . . . . . . . . . . . . .
Effect of Chemical Treatment on Resistance of Ground Rods . . . . . . . . . . . . . .
Seasonal Resistance Variations of Treated and Untreated Ground Rods . . . . . . . . . .
Trench Method of Soil Treatment . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternate Method of Chemical Treatment of Ground Rod.. . . . . . . . . . . . . . .
Relative Depths of Unconsolidated Materials, Subarctic Alaska. . . . . . . . . . .
Typical Sections Through Ground Containing Permafrost. . . . . . . . . . . .
Illustration Showing Approximate Variations in Substructure . . . . . . . . . .
Installation of an Electrode During the Process of Backfilling . . . . . . . . . .
Apparent Resistivity for Two Soils at Various Moisture and Soil Contents . . . . .
Configuration of Nearly Horizontal Electrodes Placed in the Thawed Active Layer . .
X
2-3
2-4
2-9
2-11
2-14
2-18
2-18
2-20
2-24
2-25
2-26
2-28
2-31
2-34
2-37
2-38
2-41
2-44
2-45
2-47
2-48
2-52
2-54
2-61
2-61
2-64
2-64
2-67
2-68
2-69
2-72
2-73
2-73
MIL-HDBK-419A
LIST OF FIGURES (Continued)
2-34
2-35
2-36
Resistance-to-Ground Curves for an Electrode Driven Into Ice-Rich Silt . . . . . . . . .
Resistance-to-Ground Curves for an Electrode Surrounded by Backfill of Saturated Silt.
Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated
Salt-Soil Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated
Salt-Soil Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistance-to-Ground Curves for Electrodes Placed in Holes Modified by Spring
Changes. . . . . . . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2-73
2-74
3-l
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
Charge Distribution in a Thundercloud . . . . . . . . . . . . . . . . . . . . . . . .
Mean Number of Thunderstorm Days Per Year for the United States . . . . . . . .
Worldwide Isokeraunic Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Attractive Area of a Rectangular Structure . . . . . . . . . . . . . . . . . . . . .
Effective Height of a Structure . . . . . . . . . . . . . . . . . . . . . . . . . . .
Zones of Protection Established by a Vertical Mast and a Horizontal Wire.. . . . . . . .
Some Commonly Used Lightning Shielding Angles. . . . . . . . . . . . . . . . . . .
Illustration of Processes and Currents Which Occur During a Lightning Flash to Ground .
Inductive Coupling of Lightning Energy to Nearby Circuits . . . . . . . . . . . . . . .
Normalized Voltage Induced in a Single-Turn Loop by Lightning Currents . . . . . .
Capacitive Coupling of Lightning Energy. . . . . . . . . . . . . . . . . . . . . . .
Coupling of Lightning Energy Through an Interconnected Facility . . . . . . . .
Step-Voltage Hazards Caused by Lightning-Induced Voltage Gradients in the Earth.. . . .
3-2
3-5
3-6
3-12
3-12
3-14
3-14
3-15
3-19
3-20
3-22
3-23
3-24
4-1
4-2
4-3
4-4
Grounding for Fault Protection. . . . . . . . . . . . . . . . . . . . . . . .
Single-Phase 115/230 Volt AC Power Ground Connections . . . . . . . . . . . .
Three-Phase 120/208 Volt AC Power System Ground Connections . . . . . .
Connections for a Three-Phase “Zig-Zag” Grounding Transformer . . . . .
.
.
.
.
4-2
4-4
4-5
4-6
5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
5-11
Surface Resistance and Skin Depth for Common Metals . . . . . . . . . . . . .
Resistance Ratio of Isolated Round Wires . . . . . . . . . . . . . . . . . . . . . .
Nomograph for the Determination of Skin Effect Correction Factor . . . . . . .
Low Frequency Self Inductance Versus Length for 1/0 AWG Straight Copper Wire . . .
Self Inductance of Straight Round Wire at High Frequencies. . . . . . . . . . . . . .
Resistance Ratio of Rectangular Conductors . . . . . . . . . . . . . . . . . . . . .
Resistance Versus Length for Various Sizes of Copper Tubing . . . . . . . . . . . . . .
AC Resistance Versus Frequency for Copper Tubing. . . . . . . . . . . . . . . . . .
Resistance Ratio of Nonmagnetic Tubular Conductors . . . . . . . . . . . . . . . . .
Inductance Versus Frequency for Various Sizes of Copper Tubing. . . . . . . . . . . .
Floating Signal Ground.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-4
5-6
5-8
5-9
5-9
5-14
5-14
5-16
5-17
5-18
5-19
2-37
2-38
xi
.
.
.
.
.
.
.
.
2-74
2-74
2-74
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Figure
1-72
1-73
1-74
1-75
1-76
1-77
1-78
1-79
1-80
1-81
Page
1-85
1-86
1-87
1-88
1-89
1-90
1-91
1-92
1-93
1-94
1-95
1-96
1-97
1-98
1-99
1-100
1-101
Bonding of Equipment Cabinets to Cable Tray. . . . . . . . . . . . . . . . . . . . .
Bonding to Flexible Cable and Conduit . . . . . . . . . . . . . . . . . . . . . . . .
Bonding to Rigid Conduit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Connection of Bonding Jumpers to Flat Surface . . . . . . . . . . . . . . . . . . . .
Bolted Bond Between Flat Bars . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bracket Installation (Rivet or Weld). . . . . . . . . . . . . . . . . . . . . . . . .
Use of Bonding Straps for Structural Steel Interconnections. . . . . . . . . . . . . . .
Direct Bonding of Structural Elements.. . . . . . . . . . . . . . . . . . . . . . .
Connection of Earth Electrode Riser to Structural Column . . . . . . . . . . . . . . .
Measured Electromagnetic Shielding Effectiveness of a Typical Building at 6 Feet
Inside Outer Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measured Electromagnetic Shielding Effectiveness of a Typical Building at 45 Feet
Inside Outer Wall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of Rebars. . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of a Grid as a Function of Wire Diameter, Wire Spacing, and
Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shield Absorption Loss Nomograph . . . . . . . . . . . . . . . . . . . . . . . . .
Nomograph for Determining Magnetic Field Reflection Loss. . . . . . . . . . . . . .
Nomograph for Determining Electric Field Reflection Loss. . . . . . . . . . . . . . .
Nomograph for Determining Plane Wave Reflection Loss . . . . . . . . . . . . . . . .
Shielding Effectiveness of Aluminum Foil Shielded Room. . . . . . . . . . . . . . . .
Shielding Effectiveness of Copper Foil Shielded Room. . . . . . . . . . . . . . . . .
Formation of Permanent Overlap Seam. . . . . . . . . . . . . . . . . . . . . . .
Good Corner Seam Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pressure Drop Through Various Materials Used to Shield Ventilation Openings. . . .
Typical Single-Point Entry for Exterior Penetrations (Top View).. . . . . . . . . .
Entry Plate Showing Rigid Cable, Conduit, and Pipe Penetrations. . . . . . . . . .
Effect of Rod Length on Ground Resistance. . . . . . . . . . . . . . . . . . . . .
Grounding of 120/208V 3-Phase, 4-Wire Wye Power Distribution System. . . . . .
Grounding of Single-Phase, 3-Wire 110/220V Power System. . . . . . . . . . . . .
Grounding of 28 VDC 2-Wire DC Power System. . . . . . . . . . . . . . . . . . . .
Connecting Ground Subsystems for Collocated Shelters Greater than 20 Feet Apart . .
Method of Grounding a Fence. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-1
2-2
2-3
2-4
2-5
Transmitter Building. . . . . . . . . . . . . . . . .
Communication Center/Receiver Building Expansion. .
Earth Resistance Measurement at a Typical Facility. .
Resistance Measurement Work Sheet. . . . . . . . .
Sample of a Completed Resistance Measurement Work
1-82
1-83
1-84
xii
. . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
Sheet . . . . . . . . . . . . . .
1-148
1-149
1-149
1-150
1-151
1-151
1-152
1-153
1-153
1-155
1-155
1-156
1-158
1-161
1-165
1-166
1-167
1-168
1-168
1-169
1-169
1-170
1-174
1-175
1-180
1-181
1-183
1-184
1-189
1-192
2-2
2-3
2-7
2-8
2-9
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Figure
Page
7-14
7-15
7-16
7-17
7-18
7-19
True Equivalent Circuit of a Bonded System . . . . . . . . . . . . . . . . . . . . .
Measured Bonding Effectiveness of a 9-1/2 Inch Bonding Strap . . . . . . . . . . . . .
Measured Bonding Effectiveness of 2-3/8 Inch Bonding Strap . . . . . . . . . . . . . .
Basic Diagram of the Corrosion Process . . . . . . . . . . . . . . . . . . . . . . .
Anode-to-Cathode Size at Dissimilar Junctions . . . . . . . . . . . . . . . . . . . .
Techniques for Protecting Bonds Between Dissimilar Metals. . . . . . . . . . . . . .
7-24
7-27
7-28
7-30
7-35
7-35
8-1
8-2
8-3
8-4
8-5
8-6
8-7
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
8-16
Electromagnetic Transmission Through a Slot . . . . . . . . . . . . . . . . . . . .
Transmission Line Model of Shielding. . . . . . . . . . . . . . . . . . . . . . . .
Absorption Loss for One Millimeter Shields. . . . . . . . . . . . . . . . . . . . . .
Wave Impedance Versus Distance from Source . . . . . . . . . . . . . . . . . . . .
Reflection Loss for Iron, Copper, and Aluminum With a Low Impedance Source. . .
Universal Reflection Loss Curve for a Low Impedance Source. . . . . . . . . . .
Plane Wave Reflection Loss for Iron, Copper, and Aluminum
. . . . . . . . .
Universal Reflection Loss Curve for Plane Waves . . . . . . . . . . . . . . . . . . .
Universal Reflection Loss Curve for High Impedance Field . . . . . . . . . . .
Reflection Losses for Iron, Copper, and Aluminum With a High Impedance Source . .
Graph of Correction Term (C) for Copper in a Magnetic Field . . . . . . . . . . .
Absorption Loss and Multiple Reflection Correction Term When I’ = 1. . . . . . .
Theoretical Attenuation of Thin Copper Foil . . . . . . . . . . . . . . . . . . . . .
Theoretical Attenuation of Thin Iron Sheet . . . . . . . . . . . . . . . . . . . . . .
Measured Shielding Effectiveness of High Permeability Metals . . . . . . . . .
Measured Shielding Effectiveness of High Permeability Material as a Function
of Measurement Loop Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measured Shielding Effectiveness of Two Sheets of High Permeability Metal. . . .
Measured and Calculated Shielding Effectiveness of Copper Screens to Low
Impedance Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Size . . . .
Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Spacing. . . .
Slot Radiation (Leakage). . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness Degradation Caused by Surface Finishes on Aluminum. . . . .
Influence of Screw Spacing on Shielding Effectiveness . . . . . . . . . . . . . . . . .
Shielding Effectiveness of AMPB-65 Overlap as a Function of Screw Spacing Along
Two Rows, 1.5 Inches Apart. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of an AMPB-65 Joint as a Function of Overlap . . . . . . .
Typical Mounting Techniques for RF Gaskets . . . . . . . . . . . . . . . . . . . . .
Enlarged View of Knitted Wire Mesh. . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of Conductive Glass to High Impedance Waves. . . . . . .
Shielding Effectiveness of Conductive Glass to Plane Waves . . . . . . . . . . . . . .
8-3
8-4
8-9
8-10
8-12
8-13
8-14
8-15
8-16
8-17
8-22
8-22
8-26
8-26
8-29
8-17
8-18
8-19
8-20
8-21
8-22
8-23
8-24
8-25
8-26
8-27
8-28
8-29
xiii
8-29
8-32
8-37
8-40
8-40
8-43
8-44
8-46
8-46
8-47
8-49
8-50
8-54
8-55
MIL-HDBK-419A
LIST OF FIGURES (Continued)
Figure
8-30
8-31
8-32
8-33
8-34
8-35
8-36
8-37
8-38
8-39
8-40
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
10-14
10-15
Page
Light Transmission Versus Surface Resistance for Conductive Glass. . . . . . . . . . .
Shielding Effectiveness of Some Building Materials . . . . . . . . . . . . . . . . . .
Center Area Attenuation of Induced Voltage by 15 Foot High Single-Course
Reinforcing Steel Room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Transfer Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of Various Types of RF Cables as a Function of Frequency . . .
Connector for Shield Within a Shield.. . . . . . . . . . . . . . . . . . . . . . . .
RF-Shielded Connector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effectiveness of Circumferential Spring Fingers for Improving the Shielding of a
Connector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Use of Finger Stock for Door Bonding. . . . . . . . . . . . . . . . . . . . . . . .
Coaxial Loop Arrangement for Measuring Shield Effectiveness . . . . . . . . . . .
Coplanar Loop Arrangement for Measuring Shield Effectiveness. . . . . . . . . .
8-55
8-57
EMP From High Altitude Bursts . . . . . . . . . . . . . . . . . . . . . . . . . .
Schematic Representation of High-Altitude EMP Generation . . . . . . . . . . . . . .
Surface-Burst Geometry Showing Compton Electrons and Net Current Density,
. . . .
Short-Circuit Current Induced at the End of a Semi-Infinite Above-Ground Wire
By an Expodential Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Normalized Current Waveform for Various Valves of the Depth Parameter
(Expodential Pulse) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Short Circuit Current Induced at the Base of a Vertical Riser by a Vertically
Polarized Incident Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shield to Exclude Electromagnetic Fields.. . . . . . . . . . . . . . . . . . . . . .
Electromagnetic Penetration Through Small Apertures. . . . . . . . . . . . . . . . .
Shielding Integrity Near Interference - Carrying External Conductors. . . . . . . . . .
Magnetic Field Penetration of Apertures. . . . . . . . . . . . . . . . . . . . . . .
Exclusion of Waveguide Current From Interior of Facility . . . . . . . . . . . . . . .
Waveguide Feedthroughs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bellows With Slitted Sleeve Waveguide Attachment . . . . . . . . . . . . . . . . . .
Braided Wire Sleeve Clamped to Waveguide. . . . . . . . . . . . . . . . . . . . . .
Stuffing Tube for Waveguide . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-2
10-2
10-4
xiv
8-58
8-62
8-62
8-65
8-65
8-66
8-69
8-75
8-75
10-7
10-8
10-9
10-11
10-12
10-14
10-16
10-19
10-20
10-22
10-23
10-24
MIL-HDBK-419A
LIST OF TABLES
Table
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
Facility Ground System: Purposes, Requirements, and Design Factors. . . . . . . .
Approximate Soil Resistivity. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistivity Values of Earthing Medium. . . . . . . . . . . . . . . . . . . . . . . .
Resistance Distribution for Vertical Electrodes. . . . . . . . . . . . . . . . . . . .
Simple Isolated Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistance Accuracy Versus Probe
Spacing . . . . . . . . . . . . . . . . . . . .
Step Voltages for a Buried Vertical Ground Rod. . . . . . . . . . . . . . . . . . . .
Methods of Reducing Step Voltage Hazards. . . . . . . . . . . . . . . . . . . . . .
Effect of Moisture Content on Earth Resistivity. . . . . . . . . . . . . . . . . . . .
Effect of Temperature on Earth Resistivity.. . . . . . . . . . . . . . . . . . . . .
2-6
2-9
2-10
2-21
2-22
2-43
2-50
2-56
2-66
2-66
3-1
Range of Values for Lightning Parameters . . . . . . . . . . . . . . . . . . . . . .
3-16
5-1
5-2
5-3
5-4
5-5
5-6
Properties of Annealed Copper Wire. . . . . . . . . . . . . . . . . . . . . . . . .
Parameters of Conductor Materials . . . . . . . . . . . . . . . . . . . . . . . . .
DC Parameters of Some Standard Cables.. . . . . . . . . . . . . . . . . . . . . .
Sixty-Hertz Characteristics of Standard Cables. . . . . . . . . . . . . . . . . . . .
One-Megahertz Characteristics of Standard Cables. . . . . . . . . . . . . . . . . .
Impedance Comparisons Between #12 AWG and 1/0 AWG.. . . . . . . . . . . . . . .
5-2
5-3
5-11
5-11
5-12
5-12
7-1
7-2
7-3
7-4
7-5
7-6
7-7
DC Resistance of Direct Bonds Between Selected Metals . . . . . . . . . . . . . . . .
Ratings of Selected Bonding Techniques. . . . . . . . . . . . . . . . . . . . . . .
Calculated Inductance of a 6 Inch (15.2 cm) Rectangular Strap . . . . . . . . . . . . .
Calculated Inductance
of 0.05 Inch (1.27 mm) Thick Straps. . . . . . . . . . . . .
Calculated Inductance
of Standard Size Cable . . . . . . . . . . . . . . . . . . .
Standard Electromotive Series. . . . . . . . . . . . . . . . . . . . . . . . . . .
Galvanic Series of Common Metals and Alloys in Seawater. . . . . . . . . . . . . . .
7-8
7-18
7-20
7-20
7-21
7-32
7-33
8-1
8-2
8-3
8-4
8-5
8-6
8-7
8-8
8-9
8-10
Electrical Properties of Shielding Materials at 150 kHz. . . . . . . . . . . . . . . . .
Absorption Loss, A, of 1 mm Metal Sheet.. . . . . . . . . . . . . . . . . . . . . .
Coefficients for Magnetic Field Reflection Loss.. . . . . . . . . . . . . . . . . . .
Calculated Reflection Loss in dB of Metal Sheet, Both Faces . . . . . . . . . . . . . .
Coefficients for Evaluation of Re-Reflection Correction Term, C . . . . . . . . . .
Correction Term C in dB for Single Metal Sheet.. . . . . . . . . . . . . . . . . . .
Calculated Values of Shielding Effectiveness. . . . . . . . . . . . . . . . . . . . .
Measured Shielding Effectiveness in dB for Solid-Sheet Materials . . . . . . . . . . . .
Summary of Formulas for Shielding Effectiveness. . . . . . . . . . . . . . . . . . .
Magnetic Material Characteristics . . . . . . . . . . . . . . . . . . . . . . . . .
8-7
8-8
8-11
8-18
8-20
8-21
8-23
8-28
8-30
8-31
xv
MIL-HDBK-419A
LIST OF TABLES (Continued)
Table
8-11
8-17
8-18
8-19
Calculated Values of Copper Thin-Film Shielding Effectiveness in dB Against
Plane-Wave Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effectiveness of Non-Solid Materials Against Low Impedance and Plane-Waves . . . .
Effectiveness of Non-Solid Shielding Materials Against High Impedance Waves. . . .
Comparison of Measured and Calculated Values of Shielding Effectiveness for
No. 22, 15 mil Copper Screens . . . . . . . . . . . . . . . . . . . . . . . . . . .
Characteristics of Conductive Gasketing Materials . . . . . . . . . . . . . . . . . .
Shielding Effectiveness of Hexagonal Honeycomb Made of Steel With 1/8-Inch
Openings l/P-Inch Long . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of Cable Shields. . . . . . . . . . . . . . . . . . . . . . . . . . . .
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
8-12
8-13
8-14
8-15
8-16
xvi
8-33
8-38
8-39
8-41
8-48
8-51
8-60
8-64
8-67
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. Electrical noise reduction is discussed as it relates to the proper
installation of ground systems. 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.
l-l
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 . Electrical noise reduction. Electrical noise reduction is accomplished on communication circuits by
ensuring 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. Metallic pipe systems shall not be used as the sole earth
electrode subsystem. 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. To effectively protect a building,
mast, tower, or similar self-supporting objects from lightning damage, an air terminal (lightning rod) of
adequate mechanical strength and electrical conductivity to withstand the stroke impingement must be
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 overvoltages 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.
1-3
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 some ac 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.
The ability of an electrical shield to drain off induced electrical charges and to carry sufficient
b.
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 Communications-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.
1-5
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. Ample evidence (2-1)* exists to show
that such gradients are more than adequate to cause death. 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.
2-1
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. As shown in Chapter 6, such intersite currents can induce
common-mode noise 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).
2-2
Figure 2-1. Voltage Differentials Arising From Unequal Earth Electrode Resistances and Unequal Stray Currents
Figure 2-2. Voltage Differentials Between Structures Resulting From Stray Ground Currents
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.
2-5
Table 2-1
Facility Ground System: Purposes, Requirements, and Design Factors
Purpose
Requirements
Lightning Protection
Dissipate lightning energy in
earth.
Multiple connections to earth electrode
subsystem, high peak power transfer
capability, low impulse impedance to
minimize magnitude of transient potentials.
Lightning protection subsystem must
be sized to dissipate energy in
a lightning pulse (worst case) without
producing hazardous voltages or
damage to itself.
Fault Protection
Provide fault current path
to operate equipment breakers, blow fuses, etc.
Low resistance in the return path for
fault current, maintain voltage of equipment enclosures near earth potential.
Resistance should be low enough
to permit operation of facility
over-current devices when faults
occur.
Signal Reference
Reduce noise in signal circults, provide leakage path
for static charges, establish
voltage reference.
Establish reference potential for signal voltages, provide sink for static
charge.
Fault currents and lightning protection system currents normally
should not flow in the signal reference
network; earth connection should not
degrade signal quality.
Earth Electrode
Law resistance path to earth.
Provides link for lightning
protection, fault protection and
signal reference subsystems to earth.
Installed around periphery of
building or tower to be protected.
Subsystem
Design Factors
MIL-HDBK-419A
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.
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
ohms
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. Soil is typically rather nonhomogeneous; many types will be encountered at most
locations. 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
(2-3)
where
I = current entering the electrode and
= 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)
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
103
33
Moist Soil
102
104
330
Dry Soil
103
105
3300
Bed Rock
104
106
33000
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
Minimum
(ohm-cm)
Surface soils, loam, etc.
Clay
Sand and gravel
Surface limestone
Granites, basalts, etc.
Decomposed gneisses
Slates, etc.
Fresh Water Lakes
Tap Water
Sea Water
Pastoral, low hills, rich soil, typical
of Dallas, Texas; Lincoln, Nebraska
areas
Flat country, marshy, densely wooded
typical of Louisiana near Mississippi
River
Pastoral, medium hills and forestation,
typical of Maryland, Pennsylvania, New
York, exclusive of mountainous territory
and seacoasts
Rocky soil, steep hills, typical of New
England
Sandy, dry, flat, typical of coastal
country
City, industrial areas
2-10
Resistivity
Average
(ohm-cm)
Maximum
(ohm-cm)
MIL-HDBK-419A
Table 2-3 (Continued)
Resistivity Values of Earthing Medium (2-5), (2-6), (2-7)
Medium
Minimum
(ohm-cm)
Fills, ashes, cinders, brine, waste
Clay, shale, gumbo, loam
Same-with varying proportion of
sand and gravel
Gravel, sand stones with little clay
or loam, granite
Resistivity
Average
(ohm-cm)
6 x 102
3 x 102
2.5 x 103
4 x 103
103
1.5 x 104
5 x 104
105
Figure 2-4. Current Flow From a Hemisphere in Uniform Earth
Maximum
(ohm-cm)
7 x 103
2 x 104
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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:
the voltage drop between the electrode and a point infinitely distant,
the current entering the electrode,
earth resistivity, and
radius of hemisphere.
Rewriting Equation 2-7 as
(2-8)
shows that the resistivity can be determined by knowing r, E, and I.
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)
where
input current,
and
(2-10)
where
= output current.
But since
the total potential V is
v = v1 + 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.
AIR
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 lo-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
impractical. 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. Such a
system usually extends over the entire area. 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. A burial depth of 1.5 to 2.4 meters (5 to 8
feet) below grade should be maintained. This system is considered very expensive for the value produced and
generally not recommended.
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:
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
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
MlL-HDBK-419A
I
0
1
I
I
2
3
I
I
I
I
I
I
8
9
6
7
4
5
GROUND ROD DEPTH IN FEET
I
I
I
10
11
12
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
(2-20)
where
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,
Thus 92 percent of the resistance of a lo-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
Total Resistance
(%)
Approximate
Distance from Rod
(feet)
3/4-inch pipe,
driven 3-feet
deep
90
95
98
99
6
12
31
61
3/4-inch pipe,
driven 5-feet
deep
90
95
98
99
9
18
46
92
1-1/4-inch pipe,
driven 10-feet
90
95
98
99
18
35
88
176
2-1/2-inch pipe,
driven 20-feet
deep
90
95
98
99
25
69
173
345
Type of Rod Electrode
2-21
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
where s = spacing between rods.
2-23
MIL-HDBK-419A
Figure 2-9. Resistance of Buried Horizontal Conductors
2-24
Figure 2-10. Resistance of Buried Circular Plates
MIL-HDBK-419A
Figure 2-11. Ground Rods in Parallel
2-26
MIL-HDBK-419A
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 RN 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
(2-26)
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 (1/N) of the resistance that one of the rods would exhibit if isolated.
2-27
MIL-HDBK-419A
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,
= total length of conductors used,
= A = area covered by grid, and
= 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
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
= length of conductors in grid
= resistance of bed of rods, as found from Figure 2-12
= mutual resistance which accounts for interaction
of rods on grid
, or
(2-30)
(2-31)
where
= radius of grid wire,
= depth of grid, if buried, and
= length of rod, if the grid is near surface.
2-30
MIL-HDBK-419A
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. The lines radiating outward from the earth electrode indicate the flow of
current. 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
shin-cm):
ohm-cm) over granite
(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
be the effective length (2-3)
“effective length” of the ground rod. Letting
(2-34)
where
= physical length of rod,
= resistivity of upper layer,
= resistivity of subsoil, and
h = depth of upper layer.
the effective length of the rod is reduced to
When the subsoil has a higher
Note that if
resistivity than the top layer of soil
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
the resistivity of the subsoil.
homogeneous with resistivity equal to
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
a grid in the superficial layer is given by (2-3)
and a subsoil with resistivity p2, the resistance to earth of
(2-36)
2-33
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Figure 2-14. Current Distribution in Nonuniform Soil
2-34
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the earthing resistance is approximately
(2-37)
it is approximately
(2-38)
If, for example, the diameter, De, of the grid equals 500 meters, the resistivity, pl, of the superficial layer
of the subsoil equals 200 ohm-meters, and the length,
of
equals 10,000 ohm-meters, the resistivity,
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,
as shown in Figure 2-15(a). The drop in voltage between the earth
electrode and the potential electrode,
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 measurement.) Several resistance measurements are
taken by moving the potential probe,
from the position of the earth electrode, along a straight line to the
current probe,
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 recommended for
measurement of single rod or multi-rod earth electrode subsystems.
2-35
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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,
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,
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
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,
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 p is the average soil resistivity; the minus sign indicates that the current, I, flows into
and out from
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
is the voltage that is being measured and is:
(2-41)
when x = r
2-36
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POTENTIAL PROBE POSITIONS
Figure 2-15. Fall-of-Potential Method for Measuring the Resistance of Earth Electrodes
2-37
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(b) WIDELY SPACED ELECTRODES
Figure 2-16. Effect of Electrode Spacing on Voltage Measurement
2-38
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If the
is the radius of the hemisphere that is equivalent to the current probe
radius of the electrode under test, it is seen that when x = D -
and r is the equivalent
(2-42)
(2-43)
But the true value of resistance corresponds to
(2-44)
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)
(2-46)
2-39
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which can be solved as follows:
(2-47)
= 0.618D
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,
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 radius 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 location 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
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Figure 2-17. Resistance Variations as Function of Potential Probe Position in Fall-of-Potential Method (2-12)
2-41
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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
For an accuracy of 90 percent, the probe
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
should
the longest diagonal dimension
of the array. Thus for an accuracy of 90 percent, the location for
be
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. In addition, a connection to the electrical
center of the subsystem may not be possible. 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,
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
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Table 2-6
Resistance Accuracy Versus Probe
Spacing
Accuracy
(percent)
Probe Spacing
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 + 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
+ X) from the true center (D). This means that the distance,
from the point of actual connection (O) to the system to the location at which the correct resistance to earth
exists will be
(2-50)
where
= 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
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= 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
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To determine the true resistance of the earth electrode, X is allowed to assume convenient increments from
zero to
For each
the value of measured resistance corresponding to the resultant
(calculated with
Equation 2-50) is read from the curves of Figure 2-18 and plotted against X. For example, if X and
both
equal 305 M (1000 feet), considering only the right hand curve in Figure 2-18, the value of
is 240, and R is
0.08 ohms. Next let X be 244 m (800 feet). The corresponding value of
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
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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
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,
(2-51)
voltage drop from test electrode to electrode B, divided by
current into test electrode, X,
(2-52)
=
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.
Either alternating or direct current may be used as the source of test current. For the three-point
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 (10-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
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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
2-47
MIL-HDBK-419A
Figure 2-21. Triangulation Method of Measuring the Resistance of an Earth Electrode
2-48
MIL-HDBK-419A
where
= surface earth resistivity, (ohm - meters),
= 10 for a minimum value,
t = duration of shock (sec).
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
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When the step length is much less than the rod length, i.e., when
as
the step voltage can be approximated
(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
Ratio of Step Voltage
To Electrode Potential
Rod Length
(Ft)
0.75
0.68
0.61
0.53
0.48
5
10
20
50
100
2-50
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The step voltage near the lo-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 lo-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
(2-61)
which is 41 times higher than the safe step voltage derived above.
The resistance of the 10-foot by 1-inch (3.05 m x 2.54 cm) rod in 1000 ohm-cm soil is
(2-62)
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 1-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
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Figure 2-22. Variation of Surface Potential Produced by a Current
Flowing Into an Isolated Ground Rod
2-52
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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. Maximum 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:
If h = 3 f e e t , the maximum step voltage is approximately
instead of 41,400 volts, which was characteristic of the flush-driven
rod.
2-53
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2-54
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2.8.1.2.3 Buried Horizontal Grid. An expression for the resistance to earth for a buried grid was presented in
Section 2.6.2.3. Equations 2-27 and 2-28 are the sum of a resistance of a superficial plate
and a
resistance term representing the per unit diffusion resistance of the earth electrode material
A voltage
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
ohm-cm--and if the lightning current is 2 x
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
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
ohm-cm and an
MIL-HDBK-419A
= 322 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
Design Approach
Remarks
1 . Minimize resistance to earth of electrode
system.
Resistance to earth is directly proportional to soil
resistivity.
2 . Bury earth electrode to reduce maximum
gradient on surface of earth.
Connection to earth electrode must be insulated to
withstand 5 x
volts.
3 . Bury a grid beneath the earth, surrounding
the earth electrode.
Tends to equalize the surface potential over area of
grid.
4 . Erect barricade so that personnel cannot
enter area of danger.
Fence must be grounded.
2-56
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2.8.2 Beating of Electrodes. It is necessary to use enough material in an earth electrode to prevent excessive
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,
= electrode resistance to earth, and
= permissible temperature rise
For
= 60°C, the permissible steady state current is limited by
when
ohm-cm. Since the voltage at the earth electrode is equal to the product I R, the corresponding
voltage limits are
and
2.8.2.2 Transient Current. 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
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where
duration of the transient, in seconds,
temperature rise,
soil resistivity, ohm-cm, and
transient current density.
Letting
=
60°C, one has
The current density, i, at the surface of a short ground rod is approximately constant over the length of the rod
and is given by
where
d
I
=
rod diameter (cm),
=
rod length (cm), and
=
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
temperature rise is:
(2-71)
2-58
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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
If the earth is moist soil with a
of approximately
ohm-cm, the limit becomes
(2-74)
In granite with a
of approximately
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
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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 resistivity. Planting of appropriate ground covers, such as
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
of the earth electrode subsystem will keep soil salt in solution as conductive ions. Drainage water which is high
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
- epsom salts.
b.
Copper sulphate
C.
Calcium chloride
d.
Sodium chloride (NaCl) - common salt.
e.
Potassium nitrate
- blue vitriol.
- 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.
2-60
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Largereductions 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.
1600
1400
1200
1000
800
600
400
200
0
Figure 2-24. Effect of Chemical Treatment on Resistance of Ground Rods
130
120
110
100
90
80
70
60
50
40
30
20
0
Figure 2-25. Seasonal Resistance Variations of Treated and Untreated Ground Rods
2-61
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Chemical treatment is limited in its effectiveness, however. Consider, for example, a square array of
100 ground rods of length
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
then
The upper bound on the effectiveness of chemical enhancement can be illustrated by determining the 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, De, of the equivalent plate
would be 2030 feet (619 m), and its resistance to earth would be (2-3):
(2-76)
= 8 ohms
The most that chemical enhancement could reduce the resistance of this large array would be by a factor of
1.58.
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.
2-62
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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. This keeps the soil moist and thereby reduces the resistance of the
electrode to earth. 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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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. Insulation, however, is not an acceptable corrosion preventive
for earth electrodes. The second technique for reducing galvanic corrosion is avoiding the use of dissimilar
metals at a site. 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 sacrifical 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 Sacrifical 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 niobium 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 Atmospheres. 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. Seasonal changes in temperature and moisture will therefore extensively affect the soil
resistance. 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
Resistivity, ohm-cm
Moisture Content
% By Weight
Top Soil
0
1,000
2.5
5
10
15
20
30
250,000
165,000
53,000
17,000
12,000
6,400
Sandy Loam
x 104
1,000
x 10 6
150,000
43,000
18,500
10,500
6,300
4,200
Table 2-10. Effect of Temperature on Earth Resistivity *
Temperature
20
10
0
0
-5
-15
Resistivity
ohm-cm
68
50
32 (water)
32 (ice)
23
5
*For sandy loam, 15.2% moisture.
2-66
7,200
9,900
13,800
30,000
79,000
330,000
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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 O°C
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 a foot in the arctic
to depths in excess of 12 feet in the subarctic. The seasonal thaw zone remains unfrozen only during the short
summer months. During this period, it is possible to recognize terrain features which can be located in the
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. Consequently, it is easily seen how one area versus another might be more
suitable for good grounding. 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 layers, 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. The degree of improvement will
depend on the local existence and accessibility of conductive soils. 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
use 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. The resistance of ground, R, of a single vertical electrode of length
and radius a in cm, emplaced in homogeneous soil of resistivity
(ohm - cm) is found from:
in cm,
(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).
Handheld 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
Salt 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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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
see page.
Figure 2-32. Apparent Resistivity for Two Soils at Various Moisture and Salt Contents
Figure 2-33. Configuration of Nearly Horizontal
Electrodes Placed in the Thawed Active Layer
Figure 2-34. Resistance-to-Ground Curves for
an Electrode Driven into Ice-Rich Silt
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Figure 2-36. Resistance-to-Ground Curves for
an Electrode Surrounded by a Water-Saturated
Salt-Soil Backfill
Figure 2-35. Resistance-to-Ground Curves for an
Electrode Surrounded by a Backfill of Saturated
Silt
Figure 2-38. Resistance-to-Ground Curves for
3 Electrodes Placed in Holes Modified by Spring
Charges and Filled with a Salt-Water Solution.
Figure 2-37. Resistance-to-Ground Curves for an
Electrode Surrounded by a Water-Saturated
Salt-Soil Backfill
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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,
Batterymarch 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.
"
Bulletin of the Bureau of Standards, Vol 12,
2-8. F. Wenner, “A Method of Measuring Earth Resistivity,
pp 469-478, 1915-1916.
2-9. H. B. Dwight, ‘Calculation of Resistance to Ground,” Elec. Engr, Vo1 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. F. 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, JuIy/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
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 (3-1). 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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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 progressing 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 contacts 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
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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
activity. Local thunderstorm activity can be projected from isokeraunic maps similar to those shown in
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 determination 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,
can
be reasonably predicted from (3-5):
(3-1)
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3-5
Figure 3-3. Worldwide Isokeraunic Map (Sheet 1 of 4)
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3-7
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3-8
Figure 3-3. Worldwide Isokeraunic Map (Sheet 4 of 4)
MIL-HDBK-419A
where
is the number of thunderstorm days per year. Out of the total number of flashes per unit area, the
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, A, can be represented (3-8) as:
Thus in a given location the flash density,
year, is:
i.e., the number of discharges to earth per square kilometer per
(3-3)
for a specific location, first determine Ty 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 Ty, 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, ra, in meters of a structure is (3-5);
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,
Thus A, for a lo-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)
3-10
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,
and width, w.
The roof area is given by x 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
the sides contribute
The total attractive area is the sum of the roof area
the corners
the ends
and the sides
to produce a total of
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
times the attractive area,
density,
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, Ty is determined to be approximately 54 thunderstorm days per year. The flash density as given by
Equation 3-1 is 20.4 flashes/km /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 /year to earth
can be expected. Within the area of the structure (0.01 km2) 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
structures. 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. 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
MIL-HDBK-419A
Figure 3-4. Attractive Area of a Rectangular Structure
Figure 3-5. Effective Height of a Structure
3-12
MIL-HDBK-419A
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
be provided in
important areas while a 2:1 zone
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. To establish even 2:1 type coverage on large buildings,
exceptionally tall air terminals would be required. Experience, however, shows that extremely tall terminals
are not needed for effective protection. 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 PROVIDED BY A HORIZONTAL
AERIAL GROUND WIRE AT HEIGHT H.
Figure 3-6. Zones of Protection Established by a Vertical Mast and a Horizontal Wire
REFERENCE
D/H
RECOMMENDED FOR
AOA'
2/1
63
o
NFPA 78
ORDINARY CASES
BOB'
1/1
45
o
NFPA 78
BRITISH CODE
IMPORTANT CASES
ORDINARY STRUCTURES
COC'
0.58/1
30
o
BRITISH CODE
CRITICAL STRUCTURES
Figure 3-7. Some Commonly Used Lightning Shielding Angles
3-14
MIL-HDBK-419A
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
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 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.
3-15
MIL-HDBK-419A
Table 3-1
Range of Values for Lightning Parameters (3-5)
Parameter
Number of return strokes per flash
Duration of flash(s)
Time between strokes (ms)
Peak current per return stroke (kA)
Charge per flash (C)
Time to peak current
Rate of rise
Time to half-value
Duration of continuing current (ms)
Peak continuing current (A)
Charge in continuing current (C)
Minimum
1
0.03
3
1
1
<0.5
< l
10
50
30
3
Typical
Maximum
2 to 4
0.2
40 to 60
10 to 20
15 to 20
1.5 to 2
20
40 to 50
150
150
25
26
2
100
250
400
30
210
250
500
1600
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:
IZ (current x impedance) drop resulting from the lightning pulse traveling down power lines or signal
a.
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
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
will be
(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):
(3-8)
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-4194
The voltage, V, developed across an inductance is given by
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
which
corresponds to a di/dt of
amps/second. Thus the voltage developed by the discharge pulse through the
30-meter (100 foot) downconductor is
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). The voltage induced in the loop is dependent upon the
dimensions of the loop
its distance from the down conductor
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
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,
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
minus r1 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
MIL-HDBK-419A
Figure 3-9. Inductive Coupling of Lightning Energy to Nearby Circuits
3-19
MIL-HDBK-419A
Figure 3-10. Normalized Voltage Induced in a Single-Turn Loop by Lightning Currents
3-20
MIL-HDBK-419A
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
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,
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
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
MIL-HDBK-419A
If
is 25 ohms while
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
to approximately the magnitude of
(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
other 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. As 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
Figure 3-12. Coupling of Lightning Energy Through and Interconnected Facility
MIL-HDBK-419A
Figure 3-13. Step-Voltage Hazards Caused by Lightning-Induced Voltage Gradients in the Earth
3-24
MIL-HDBK-419A
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. In homogeneous soil, the current
rapidly leaves the electrode. 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. However, every building or structure does not require that a lightning protection system be
installed. 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 Systems 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, Batterymarch 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
because of objectionable current, noise or interference.
more than one building or
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,
of a conductor of uniform cross section is proportional to the length and inversely
proportional to the cross-sectional area, that is
where is the resistivity of the conductor material,
is the length of the conductor in the direction of current
flow, and A is the cross-sectional area of the conductor. Values of
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
No.
Diameter
mils
mm
4/0
460.0
11.7
211 600
107.2
0.049
0.161
3/0
409.6
10.4
167 800
85.0
0.062
0.203
2/0
364.8
9.3
133 100
67.4
0.078
0.256
1/0
324.9
8.3
105 500
53.4
0.098
0.322
1
289.3
7.3
83 690
42.4
0.124
0.407
2
257.6
6.5
66 370
33.6
0.156
0.512
4
204.3
5.2
41 740
21.1
0.248
0.814
6
162.0
4.1
26 250
13.3
0.395
1.296
8
128.5
3.3
16 510
8.4
0.628
2.060
10
101.9
2.6
10 380
5.3
0.999
3.278
12
80.8
2.1
6 530
3.3
1.588
5.210
14
64.1
1.6
4 107
2.1
2.525
8.284
16
50.8
1.3
2 583
1.3
4.016
13.176
18
40.3
1.0
1 624
0.8
6.385
20.948
20
31.9
0.8
1 022
0.5
10.150
33.300
Cross-Sectional Area
5-2
Resistance in Ohms
per km
per 1000 ft
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,
is defined as the depth at which
the current density is attenuated to
= 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
crosssectional 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).
where
is the resistivity of the material in ohm-cm, f is the frequency in hertz, and
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)
5-3
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
(5-3)
(5-4)
where P is the circumference of the conductor and Rs 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
is the relative permeability of the conductor, f is the frequency in hertz, and
in ohms for 1 cm of conductor.* In the case of copper wire, Equation 5-6 becomes
is the dc resistance
(5-7)
where
is the wire diameter in mils, or becomes
(5-8)
where
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
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
is a skin effect correction factor which may
be determined (for copper) from Figure 5-3. For
and d in centimeters, Equation 5-9 becomes
(5-10)
For materials other than copper,
can be obtained from Figure 5-3 by using
instead of the actual
frequency f, where
is the resistivity of the material and
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
(if surrounded by air) is
where all the dimensions are in inches. As the frequency increases, a limiting value of inductance,
approached:
is
(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
for a 1/0 AWG solid round copper conductor as a function of length, and
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 Held 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 is
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.
b . The advantage offered by a large cable, e.g., 1/0 AWG, will be somewhat more pronounced for
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.
to 115.8
for lengths of 30 meters
d . Since even 1/0 AWG cables exhibit impedances from 22.6
(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
Diameter
(mils)
Size
(AWG)
No. 1 2
DC Resistance
(Ohms/1000 ft)
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)
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
MIL-HDBK-419A
Table 5-5
One-Megahertz Characteristics of Standard Cables
Size
AWG
Length
(Ft)
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
Frequency
Length
60 H z
30 ft.
16.20
16.20
9.23
60 Hz
100 ft.
16.20
16.20
7.99
100 f t .
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
recommended 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
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
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
MIL-HDBK-419A
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.
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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
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Figure 5-9. Resistance Ratio of Nonmagnetic Tubular Conductors (5-3)
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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.
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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).
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Figure 5-13. Single-Point Ground Bus System Using Separate Risers (Lower Frequency)
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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,
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
Figure 5-15. Use of Single-Point Ground Configuration to Minimize Effect of Facility Ground Currents
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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
at the highest
frequency of concern (5-11).
Figure 5-16. Multipoint Ground Configuration
5-24
Figure 5-17. Use of Structural Steel in Multiple-Point Grounding
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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
is a function of L/C, hence as capacity
to earth increases,
decreases. The capacity of a metallic sheet or grid to earth is much 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
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 facility ground system. Equipment cabinets are then connected to the
equipotential plane. Chassis are connected to the equipment cabinets and all components, signal return leads,
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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;
a.
Any “noisy” cable or conductor connected to the receptor, i.e., receivers, modems, etc., through or
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.
b.
Filters at the interface terminals of equipment can operate more effectively when both terminals of
their equivalent “transmission line” are available. As 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 may also shield or isolate rooftop antennas from noisy cables below it.
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.
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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.
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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
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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.
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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 multipoint 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.
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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 760 639.
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, 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.
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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. Also 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 microvolts 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.
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An ideal signal system is a simple signal generator-load pair as shown in Figure 6-1. With no extraneous
voltages present within the loop, this simple pair is free of interference. 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
and
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:
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.
Connect the low side of the loop to the reference plane at either Node 1 or Node 2 but not at both.
Reduce the impedance,
of the path connecting the two noise sources.
Reduce the magnitudes of
and
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.
LOAD
Figure 6-1. Idealized Energy Transfer Loop
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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
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(a) ONE SOURCE DRlVlNG MULTIPLE LOADS
SOURCE
NO. 1
LOAD
NO. 1
SOURCE
NO. 2
(b) CASCADED PAIRS.
(c) SEPARATE PAIRS CONNECTED
TO COMMON GROUND PLANE.
Figure 6-4. Practical Combinations of Source-Load Pairs
6-4
LOAD
NO. 2
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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. Interference is broadly
classified by its coupling means; i.e., as either being conductive or free-space. 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.
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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
flowing in Circuit 1 of Figure 6-5 returns to its
source through signal reference impedance
producing a voltage drop
in the reference plane. The
impedance
is common to Circuit 2, hence
appears in Circuit 2 as a voltage in series with the desired
signal voltage source,
This undesired source produces an interference voltage,
across the load of
Circuit 2; similarly, the desired current,
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,
flowing in
the reference (or ground) plane.
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,
produces a voltage EN in the reference plane impedance,
This voltage produces a current in
the interconnecting loop which in turn develops a voltage across
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 6-6. 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
is the magnetic field strength and
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
where
(t) is the induced voltage and
magnetic flux is given by
is the total magnetic flux linking the susceptible circuit loop. This
where
is the magnetic flux density,
is the absolute permeability of the medium, and
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
webers,
(6-5)
where
and
are the distances from i(t) to the two sides of the loop which are parallel to i(t), and
is the
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
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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
induced in the circuit loop
(6-6)
In free space,
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
(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 both), the frequency, f, or the time rate of
change, di/dt, of the interfering sources, and increases with a decrease in the distance,
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 same induced voltage may be small compared to the circuit signal voltages and thus not
create any problems. For these reasons, low impedance circuits are usually more susceptible to inductive
coupling than are high impedance circuits.
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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,
exists between the conductors. The value of this capacitance is a function of the
geometry of the signal lines. For parallel wires, Cc is given by (6-2).
where rc 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 replacing rc with r1 and with
(see Figure 6-9).
The interference source voltage,
produces a current flow through the mutual capacitance, Cc, between the
two signal conductors and develops an induced voltage,
in the susceptible circuit. The equivalent circuit
for Figure 6-9 is given in Figure 6-10(a) where the parallel combination of
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
where
and
(6-15)
Substitution of Equations 6-14 and 6-15 into Equation 6-13 yields
(6-16)
where
which is generally true at lower frequencies, the equivalent circuit of Figure
6-10(b) is applicable and
At higher frequencies, the equivalent circuit of Figure 6-10(c) is applicable and
(6-18)
These equations illustrate the induced voltage,
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
the stray capacitance in the susceptible circuit,
and
voltage,
the values of the coupling capacitance,
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,
depends upon
the stray capacitance in the interference source circuit,
in Figure 6-9.
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If the impedance of the susceptible circuit is totally resistive, i.e.,if
Equation 6-16 as
the induced voltage is given by
(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
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
until
approaches the value
; i.e., the reactance of
and
in parallel. For larger values of
the ratio asymptotically approaches
The behavior
of this voltage ratio with frequency is illustrated in Figure 6-11. The ratio is zero at dc and asymptotically
approaches
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
Close to the conductor (near field),
the conductor, i.e., each term is proportional to either 1/r, 1/r2, or
2
3
the 1/r and 1/r components dominate and the electromagnetic energy oscillates between the space
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
is 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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LOG f
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-l). 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
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
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
is chosen instead of
Thus
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 common-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
Figure 6-13. Illustration of Conductively-Coupled Common-Mode Noise
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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,
represents some signal voltage from an unbalanced source, i.e., the output signal of some transducer or
measuring amplifier, and
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.
are the series resistances in the interconnecting cables between the source and amplifiers. The voltage source
Vc m with output resistance
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
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,
is small and can be neglected. With this approximation, it can be
shown that the output voltage of the amplifier is given by
where K is the voltage gain of the amplifier.
There are two signal contributions to the output signal
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
- By decreasing
decreases proportionally.
the common-mode noise voltage at the output terminals
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
are manipulated such that
(6-22)
the common-mode noise voltage at the amplifier output terminals can be made to vanish.
c.
- If Z1 is sufficiently large compared to
is sufficiently large
Increase
compared to
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
and output
r e s i s t a n c e connected to the two inputs of an amplifier. In this case, the center tap of the source is
connected to the ground reference terminal. As before, if it is assumed that
is small, it can be shown that
is given by
(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 (Vcm) and divided by the amplifier output due to
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
(6-24)
CMR=
6-21
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Figure 6-15. Common-Mode Noise in Balanced Systems
6-22
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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
CMR =
(6-25)
6.3.2 Differential Amplifier. A differential amplifier is designed to make
large compared to
and
large compared to
Since
are normally functions of frequency, it can be seen that the CMR
will also be a function of frequency. Typically
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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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 width (r2 - 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. In a balanced circuit, the two signal conductors are symmetrical with
respect to ground. At equivalent points on the two conductors the desired signal is opposite in polarity and
equal in amplitude relative to ground. 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
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
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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 the 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).
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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, whether 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 drop 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. Poor joints in lightning protection networks can be particularly
dangerous. 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
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,
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
is high relative to
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. In this way, resistive voltage drops are minimized which helps with noise
control. 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,
path through the conductors on either side of the bond is given by
of the
(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
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
of most electrical bonding surfaces will be several orders of magnitude greater. When two such surfaces are
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
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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 due 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
Brass-Brass
6
Aluminum-Aluminum
Brass-Aluminum
Brass-Steel
Aluminum-Steel
Steel-Steel
25
50
150
300
1500
Notes: Apparent Bond Area:
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
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
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 strength 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 arc, Heliarc 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 heat generated (in excess of
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
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
MIL-HDBK-419A
Figure 7-5. Typical Exothermic Connections
7-12
MIL-HDBK-419A
COURTESY OF ERIC0 PRODUCTS, INC., CLEVELAND, OHIO
Figure 7-6. Typical Bond Configurations Which Can Be Implemented With The Exothermic Process
7-13
MIL-HDBK-419A
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 flexibility
of the system. 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. A low-rated method
expertly performed, will work better than a high-rated poorly performed 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
Table 7-2
Ratings of Selected Bonding Techniques*
-Chemical-
Thermal
Solder
Low Resistance
Resistance
Electrical
Stability
Properties Voltage Drop
Current
Capacity
Pulloff Force
Mechanical Low Creep
Properties Strength
Solid Wire
Conductor Stranded Wire
Aluminum Wire
ApplicaBus Bars and
bility
Structures
Environmental
High Temperature
Low Temperature
Thermal Shock
Vibration
Corrosive
Atmosphere
Aging
ExoGas or thermic
Braze Arc Weld Weld
Mechanical
Spot
Weld
Conductive
Adhesive
Bolts
Crime
Rivets
Clamps
Wire
9
8
9
9
9
9
5
8
7
8
9
9
5
5
8
8
9
Screws
Twisted
Terminal Wires
9
10
10
10
10
3
9
9
10
9
10
10
10
10
10
10
10
10
7
5
8
9
9
9
5
10
10
10
9
1
9
9
9
8
9
9
8
8
9
6
10
10
10
10
10
10
10
10
10
10
10
7
1
5
4
9
9
9
9
9
9
9
9
8
8
3
3
5
10
8
9
2
2
4
10
9
5
10
9
2
10
8
1
10
9
8
10
10
9
2
10
9
9
10
10
8
4
9
9
9
10
2
4
8
8
8
9
9
6
9
7
10
7
3
9
8
6
9
10
9
7
10
10
10
10
10
10
10
10
10
9
9
9
10
10
10
10
10
10
10
10
Cost
Economy
Tooling
Process
7
6
4
5
3
5
4
5
4
5
Accessibility in
Assembly
Method Needs
Little Space
Ease of Repair
8
8
5
6
4
5
5
5
8
9
Source: Adapted from Reference (7-8)
5
4
6
8
8
9
9
9
9
9
1
10
5
7
2
1
7
8
2
2
3
7
9
8
10
10
10
10
9
9
Wire
Nuts
9
10
9
10
4
7
8
8
10
10
10
10
8
9
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
ohm-cm, (6.77 x
ohm-inches), a
copper conductor 2.5 cm, (1 inch) wide, 40 mils 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
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
(in.)
(in.)
0.5 (12.7 mm)
0.5
0.5
1.0 (25.4 mm)
1.0
1.0
2.0 (50.8 mm)
2.0
2.0
L
0.112
0.110
0.107
0.092
0.091
0.089
0.072
0.071
0.071
0.01 (0.25 m m )
0.05 (1.27 mm)
0.10 (2.54 mm)
0.01
0.05
0.10
0.01
0.05
0.10
Table 7-4
Calculated Inductance
of 0.05 Inch (1.27 mm) Thick Straps
Length
Width (in.)
0.5 (12.7 mm)
1.0 (25.4 mm)
2.0 (50.8 mm)
6 in. (15.2 cm)
0.110
0.091
0.071
7-20
12 in. (30.4 cm)
0.261
0.222
0.183
36 in. (91 cm)
0.984
0.866
0.745
MIL-HDBK-419A
Table 7-5
Calculated Inductance
of Standard Size Cable
Length
AWG No.
6 in.
12 in.
36 in.
4/0
1/0
2
4
6
10
14
0.098
0.108
0.115
0.122
0.129
0.144
0.158
0.238
0.259
0.273
0.287
0.301
0.329
0.358
0.914
0.977
1.020
1.063
1.105
1.189
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
strap decreases significantly as the length to width
ratio decreases. The curve shows that a strap with an
ratio of 5 to 1 has an inductive reactance that is 45 percent that of a thin wire (i.e., very high ratio of to
b); a 3 to 1 ratio decreases this percentage to 38 percent. 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
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.
represents the ac resistance of the strap;
is the inductance which may be calculated
with either Equation 7-2 or 7-3; and
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
term can be
ignored. Thus, not considering
the equation for the magnitude of impedance,
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
Figure 7-11. Relative Inductive Reactance versus Length-to-Width Ratio of Flat Straps (7-10)
7-23
MIL-HDBK-419A
Figure 7-14. True Equivalent Circuit of a Bonded System
7-24
MIL-HDBK-419A
and the
The inherent inductance of a bonded object, e.g., an equipment rack or cabinet, is represented by
capacitance between the bonded members, i.e., between the equipment and its reference plane, is represented
can again be ignored. Thus, the primary (i.e., the lowest)
by Cc. In most situations
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. Positive values of bonding effectiveness indicate a lowering of the induced
voltage. 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.
bonding;
at low frequencies where the reactance of the strap is low, bonding straps will provide effective
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 degreasing 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-419A
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
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. Removal down to a bright metal finish is
generally adequate. 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.
because
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. Corrosion is the deterioration of a substance (usually a metal) because of a reaction
with its environment. 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
environments. 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
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.
ENVIRONMENT
(ELECTROLYTE)
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. Metals high in the series react
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. The farther apart the metals are in the series, the
higher the voltage between them. 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 AC TIVE 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 is greater for a small electrode than for a larger one. The greater the current density of the current
leaving an anode, the greater is 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 the
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. Bonds
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 7-19. Techniques for Protecting Bonds Between Dissimilar Metals
7-35
MIL-HDBK-419A
7.10 SUMMARY OF GUIDELINES.
l
Bonds must be designed into the system.
Specific attention should be directed to the
interconnections not only in 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.
l
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.
l
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.
o
Bonding jumpers are only a substitute for direct bonds. If the jumpers are kept as short as possible,
have a low resistance and low
ratio, and are not higher in the electrochemical series than the
bonded members, they can be considered a reasonable substitute.
l
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.
o
Protection of the bond from moisture and other corrosive elements must be provided.
l
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, “Electromagnetic 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. “Electromagnetic Compatibility,” AFSC Design Handbook
2 March 1984.
1-4, Air Force Systems Command,
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 (1963).
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, e i t h e r a t t h e s o u r c e o r a t t h e r e c e p t o r . A n
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:
(8-2)
The variables
and
are the electric and magnetic field strengths without the shield present, and
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. According to the planewave theory developed by Schelkunoff
(8-1), an electromagnetic shield transmits EM 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. Consider an incident EM wave with a power of
impinging upon a
flat shield as in Figure 8-2. When the wave encounters the first surface of the shield, a portion
of the
8-2
MIL-HDBK-419A
incident power is reflected back toward the source; the remainder
penetrates the shield and begins to
propagate through the shield. The ratio of reflected power to incident power
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
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
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
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. Nonuniform shielding theory has been developed to deal with wave transmission
through defects. 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 nomographs is based upon the
separation of the shielding effectiveness into three additive terms: absorption loss, reflection loss, and a
correction term to account for re-reflections within the shield.
8-4
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:
where
= thickness of the shield,
= propagation constant of the shield,
= transmission coefficient,
and
= 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. In a particular
shielding application, the values of the constants
depend upon the conductivity
permeability
depend also upon the wave impedance of
and permittivity
of the shielding material. The values 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
and conductivity relative to copper (gr). Simplified approximate expressions
have been derived for the reflection and correction terms. The selection of the appropriate approximate
expression will depend upon whether the wave impedance is low (Zw <
magnetic field), medium
plane wave), or high
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 (8-2) to be given by:
MIL-HDBK-419A
where
131.4 if
is expressed in meters,
3.34 if
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
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 mils. Shield thicknesses are
commonly expressed in either millimeters (mm) or milli-inches (mils); these two units are related as follows:
1 mm = 39.37 mils 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; nomographs 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:
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)
Relative
Conductivity
Relative
Permeability
Metal
Absorption Loss
(dB)
Silver
Copper, annealed
Copper, hard-drawn
Gold
Aluminum
Magnesium
Zinc
Brass
Cadmium
Nickel
Phosphor-bronze
Iron
Tin
Steel, SAE 1045
Beryllium
Lead
Hypernick
Monel
Mu-metal
Permalloy
Steel, stainless
1.05
1.00
0.97
0.70
0.61
0.38
0.29
0.26
0.23
0.20
0.18
0.17
0.15
0.10
0.10
0.08
0.06
0.04
0.03
0.03
0.02
1
1
1
1
1
1
1
1
1
1
1
1,000
1
1,000
1
1
80,000
1
80,000
80,000
1,000
*With no saturation by incident field.
8-7
1 mm thick
1 mil thick
51.96
50.91
49.61
42.52
39.76
31.10
27.56
25.98
24.41
22.83
21.65
665.40
19.69
509.10
16.14
14.17
3484.00*
10.24
2488.00*
2488.00*
224.40
1.32
1.29
1.26
1.08
1.01
.79
.70
.66
.62
.58
.55
16.90
.50
12.90
.41
.36
88.50*
.26
63.20*
63.20*
5.70
MIL-HDBK-419A
In a manner analogous to the classical equations (8-1) describing reflections in transmission lines, the
shield reflection loss can be expressed as:
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)
Frequency
60.0 Hz
1.0 kHz
10.0 kHz
150.0 kHz
1.0 MHz
3.0 MHz
10.0 MHz
15.0 MHz
100.0 MHz
1.0 GHz
1.5 GHz
10.0 GHz
1,000
1,000
1,000
1,000
700
600
500
400
100
50
10
1
Relative Conductivity,
1
1
1
1
1
1
1
1
1
1
13
54
171
663
1,430
2,300
3,830
4,200
5,420
12,110
6,640
5,420
1
1
4
13
56
131
228
416
509
1,310
4,160
5,090
13,140
Iron - 0.17, Copper - 1.0, Aluminum - 0.61.
8-8
1
1
1
1
1
1
1
1
1
0.8
3.0
10.0
40.0
103.0
178.0
325.0
397.0
1,030.0
1
1
1
3,250.0
3,970.0
10,300.0
MIL-HDBK-419A
Figure 8-3. Absorption Loss for One Millimeter Shields
8-9
MIL-HDBK-419A
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
where r is the distance from the antenna and is the wavelength of the EM field. For such magnetic
(low 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),
conductivity of shield material relative to copper,
relative permeability of shield material,
and the constants
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
Coefficient
Units for Distance (r)
Meters
.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
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,
and
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
where
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 ur/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
322, if r is in meters
354, if r is in inches,
8-15
MIL-HDBK-419A
and
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
is
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
Table 8-4
Calculated Reflection Loss in dB of Metal Sheet, Both Faces (8-2), (8-3)
Reflection Loss*
Frequency
Iron
0.17)
Magnetic
Electric
Plane
Magnetic
Electric
Plane
Magnetic
Electric
Field
Field
Wave
Field
Field
Wave
Field
Field
r = 30.5 cm
r = 30.5 cm
Field
r = 30.5 cm
r = 30.5 cm
Field
r = 30.5 cm
r = 30.5 cm
241
113
22
279
150
0.9
204
101
34
242
138
1000
8.0
174
91
44
212
128
1000
10.0
139
79
56
177
117
54
1.0 MHz
700
28.0
116
72
64
152
108
62
60.0 Hz
1000
1.0 kHz
1000
10.0 kHz
150.0 kHz
- 0 . 9 **
15.0 MHz
400
42.0
83
63
76
117
96
74
115
100.0 MHz
100
57.0
64
61
84
92
88
82
90
1.5 GHz
10
59.0 +
59 +
59
76+
76 +
76
10.0 GHz
1
61.0 +
61 +
61
69+
68 +
68
* If Absorption Loss <15 dB, use Correction Factor C
** The Reflection Loss for iron is zero at 620 Hz, it is a negative quantity at 60 Hz, zero at 31.5 Hz, and becomes positive at lower frequencies.
* * * Plane-Wave Field not provided (normally close to that of copper),
+ At these frequencies and for given distance, the field becomes a plane wave.
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
(8-11)
where A is the shield’s absorption loss (see Equation 8-5) and
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,
is essentially unity.
Whenever the approximation
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. The entries under “SOURCE” designate the EM wave
impedance classification: 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
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
Table 8-5
MIL-HDBK-419A
Table 8-6
Correction Term C in dB for Single Metal Sheet (8-2)
Thickness*
(mils)
Frequency
60 Hz
100 Hz
Copper,
-22.2
-21.7
-19.2
-15.6
-13.0
-9.0
-4.0
-0.8
+0.3
1
5
10
20
30
50
100
200
300
-42.0
-28.0
-22.0
-16.0
-13.0
-9.0
-4.0
-0.6
+0.1
-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
1.0
0.9
0.8
0.4
0.06
0.00
-30.00
-16.00
-10.00
-5.00
-3.00
-0.60
+0.50
0.00
-40.00
-25.00
-20.00
-14.00
-11.00
-7.00
-3.00
+0.10
+0.60
-20.00
-7.00
-3.00
+0.20
+0.60
+0.10
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
-20.0
-7.0
-3.0
+0.1
+0.6
+0.1
0.0
-10.0
-0.6
+0.6
+0.1
0.0
near field of loop
1.00
0.90
0.50
0.08
0.06
0.00
-1.60
-0.60
+0.06
0.00
Iron,
1
5
10
20
30
50
100 kHz
plane waves and near field of electric dipole
Iron,
1
5
10
20
30
50
10 kHz
near field of loop
Copper,
1
5
10
20
30
50
100
200
300
1 kHz
-1.8
0.0
plane waves and near field of electric dipole
-17.00
-5.00
-1.30
+0.50
+0.40
0.00
-8.0
+0.2
+0.4
0.0
*l mil equals 0.0254 mm
8-21
-1.3
0.0
-3.0
+0.1
0.0
MIL-HDBK-419A
Figure 8-12. Absorption Loss and Multiple Reflection Correction Term when
8-22
= 1 (8-2)
MIL-HDRK-419A
Table 8-7
Calculated Values of Shielding Effectiveness (8-2)
Thickness
Frequency
(mils)
(Hz)
Source*
R
(dB)
Copper
A
(dB)
C
(dB)
SE=A+R+C
(dB)
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
1M
L
64.2
33.400
0.00
97.60
10
1M
D
152.0
33.400
0.00
185.40
10
1M
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
(mils)
Frequency
Source*
(Hz)
R
(dB)
Iron
C
(dB)
SE=A+R+C
(dB)
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
1
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
1
M
L
28.0
363.00
0.00
391.00
10
1
M
D
116.0
363.00
0.00
479.00
10
1
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
(mils)
Frequency
(Hz)
Source*
R
A
(dB)
Aluminum
(dB)
C
(dB)
SE=A+R+C
(dB)
10
1 M
62.0
26.0
0.00
88.0
10
1 M
150.0
26.0
0.00
176.0
10
1 M
10
15 M
79.0
100.0
0.00
179.0
10
15 M
115.0
100.0
0.00
215.0
10
15 M
10
100 M
82.0
260.0
0.00
342.0
10
100 M
90.0
260.0
0.00
350.0
10
100 M
*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 8-14. 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 1-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
Table 8-8
Measured Shielding Effectiveness in dB for Solid-Sheet Materials (8-3)
Type
Wave
Material
Thickness
(mils)
0.1 kHz
1 kHz
Nominal Effectiveness
10 kHz 85 k H z
200 kHz 2 MHz
5 MHz 10 MHz
Plane
Cu
2.5
109
106
114
Plane
A l
5.0
107
109
118
Plane
Stainless
Steel
18.0
97
95
99
Plane
Steel
4.5
105
99
101
Plane
AA- Conetic
Foil
3.5
97
130
Low
Impedance
Cu
Low
Impedance
A l
Low
Impedance
Steel
Low
Impedance
Cu-Clad Steel
Clad 2 Sides
125.0
63.0
31.0
4.5
8
22
58
4
11
29
97
59
34
125.0
63.0
31.0
5
1
1
18
16
10
50
35
24
78
63.0
31.0
25
4
40
28
80
59
31.0
94
107
120
55
92
120
MIL-HDBK-419A
Figure 8-16. Measured Shielding Effectiveness of High Permeability
Material as a Function of Measurement Loop Spacing (8-6)
8-29
Table 8-9
Summary of Formulas for Shielding Effectiveness
S E = A + R + C
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
(Co-Netic AA)
Outer Material
(Netic S 3-6)
Initial Permeability
20,000.00
300.0
Permeability at 0.02 tesla
80,OOO.OO
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.
the
Solid material shielding theory is applicable to thin-film shields. For shields much thinner than
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
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.
8-32
This unit is related to mils by
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)
0.0041 M i l s
Thickness
0.86 Mils
0.086 M i l s
0.049 Mils
1 GHz
1 MHz
1 GHz
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
Single
Reflection
Loss, R
109
79
109
79
109
79
109
79
Multiple
Reflection
Correction
Term, C
-47
-17
-26
-6
-21
0.6
-3.5
Shield
Effectiveness,
62
SE
62
83
88
90
108
Frequency
Absorption
Loss, A
78
171
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
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
acting as
many
stacked
MIL-HDBK-419A
The shielding effectiveness, in decibels, is expressed as follows (8-8):
(8-12)
where, as with solid shields, A, represents the absorption or attenuation term,
the multi-reflection correction term.
The
additional
terms
the reflection loss term, and
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:
=
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,
=
aperture reflection loss in dB,
where
(8-15)
=
correction factor for aperture reflections (negligible when
is greater than 10 dB)
(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
for rectangular apertures and radiated fields
(8-19)
=
jfd x 1.47 x
for circular apertures and radiated fields
(8-20)
f
=
frequency in MHz
r
=
distance from signal source to shield in inches
j
=
k
= 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),
(8-21)
8-34
MIL-HDBK-419A
where
a = area of each hole in square inches
n = number of holes per square inch
= 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)
K3 = correction factor for coupling between closely spaced shallow holes
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
The impedance ratio for the magnetic wave and rectangular apertures is given by
and the reflection term is
8-35
MIL-HDBK-419A
The multi-reflection correction term is
The number of openings correction factor is
The skin depth correction factor is
Finally, the hole-coupling correction factor is given by
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.
Figure 8-18. Measured and Calculated Shielding Effectiveness of
Copper Screens to Low Impedance Fields (8-8)
8-37
Table 8-12
Effectiveness of Non-Solid Shielding
Materials Against Low Impedance and Plane Waves (8-7)
Nominal Effectiveness
(dB)
Impinging
Wave
Low
Impedance
General
Mesh
(Screening)
Detail
Material
2 layers
1 inch
apart
Cu
(oxidized)
No. 22
Cu
No. 16
Bronze
No.
Perforated
Sheet
Plane
4
45 mil
dia.,
225 sq.
inch
Thickness
(mils)
0.1
kHz
2
10
85
1
10
kHz
kHz
MHz
MHz
6
18
31
43
43
17
21
18
Galvanized
Steel
10
9380 MHz
3040 MHz
Al
20
dia. = 13
Mesh
(Screening)
No. 22
1
kHz
Cu
dia. = 15
60
62
34
36
200 kHz
1 MHz
118
106
5 MHz
100
100 MHz
80
Table 8-13
Effectiveness of Non-Solid Shielding Materials Against High Impedance Waves (8-3)
General
FORM
Detailed
Material
Hexcell
1/4-inch cell,
1 inch thick
TV Shadow
Mask
(PhotoEtched)
9-mil holes,
28-mil centers
Lektromesh
40 count
25 count
40 count
25 count
Cu-Ni
1/8-inch dia,
3/16-inch
1/4-inch dia,
5/16-inch
centers
7/16-inch dia,
5/8-inch
centers
Steel
Steel
Perforated
Sheet
Mesh
No. 16
(Screening)
16
No.
No.
No.
No.
No.
x 16/sq in.
22
12
16
10
4
No. 2
Al
Thickness
(mils)
3
Nominal Effectiveness
(14 kHz to 1000 MHz)
(dB)
>90
>90
78
78
Cu
>2
0.2
0.2
0.4
0.5
36
49
57
56
0.4
0.2
0.2
0.2
1.7
0.5
0.5
0.4
0.27
>0.6
60
58
60
48
46
37
35
45
55
36
Cu
Bronze
Monel
Galvanized
Steel
20 (dia)
20 (dia)
18 (dia)
30 (dia)
65 (14 kHz to 60 MHz)
50
45 (14 kHz to 60 MHz)
40
35
28 (14 kHz to 40 MHz)
24
.26
12
50
50
Al
Al
Air-Flow Static
Pressure
(inches of water)
400 cu ft/min
200 cu ft/min
0.06
>90
>90
95% Cu
5%
100%
Open
Area
%
50
76
88
MIL-HDBK-419A
8-40
MIL-HDBK-419A
Table 8-14
Comparison of Measured and Calculated Values
of Shielding Effectiveness for No. 22, 15 Mil Copper Screens (8-8)
Test
Type
Magnetic
field
Plane
Wave
Electric
field
Calculated
Effectiveness
(MHZ)
Measured
Effectiveness
(dB)
0.085
1.000
10.000
31
43
43
29
46
49
0.200
1.000
5.000
100.000
118
106
100
80
124
110
95
70
0.014
65
**65
Frequency
(dB)
**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 SHIELD 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
enclosures. Combinations of frequency and slot lengths to the right of the 1/100 wavelength line would tend to
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:
Seams between two metal surfaces, with the surfaces in intimate contact (such as two sheets of
a.
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.
8-42
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 bonderrite, 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.
Mating surfaces should be bonded immediately after protective coatings are removed to avoid
oxidation. Refinishing after bonding is acceptable from the standpoint of shielding effectiveness.
f.
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.
Soldering may be used to fill the resulting seam, but should not be employed to provide bond
h . 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)
8-43
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. Bolts, screws, or various types of
clamp and slide fasteners have been used for this purpose. 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 (1-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 (10-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
screwed or bolted together. Figures 8-24 and 8-25 indicate the change in shielding effectiveness of an
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.
8-45
MIL-HDBK-419A
8-46
MIL-HDBK-419A
Figure 8-25. Shielding Effectiveness of an AMPB-65 Joint as a Function of Overlap (8-6)
8-47
MIL-HDBK-419A
Table 8-15
Characteristics of Conductive Gasketing Materials
Material
Chief Advantages
Chief Limitations
Compressed knit ted wire
Most resilient all-metal gasket (low
flange pressure required). Most
points of contact. Available in
variety of thicknesses and
resiliencies, and in combination
with neoprene and silicon.
Not available in sheet (Certain
intricate shapes difficult to make).
Must be 0.40 inch or thicker.
Brass or beryllium copper
with punctured holes
Best break-thru of corrosion
protection films.
Not truly resilient nor generally
reusable.
Oriented wires in rubber or
silicon
Combines fluid and rf seal. Can
be effective against corrosion
films if ends of wires are sharp.
May result in larger size gasket
for same effectiveness.
Aluminum screen impregnated
with neoprene
Combines fluid and conductive
seal. Thinnest gasket. Can be cut
to intricate shapes.
Very low resiliency (high flange
pressure required).
Soft Metals
Cheapest in small sizes.
Cold flows, low resiliency.
Metal over rubber
Takes advantage of the resiliency
of rubber.
Foil cracks or shifts position.
Generally low insertion loss yielding
poor rf properties.
Conductive rubber
Combines fluid and conductive
seal.
Practically no insertion loss, giving
very poor rf properties.
Contact Fingers
Best suited for sliding contact.
Easily damaged. Few points of
contacts.
8-48
Figure 8-26. Typical Mounting Techniques for RF Gaskets
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
The cutoff frequency for the TE mode of rectangular waveguide is
In these equations,
= 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.
8-50
MIL-HDBK-419A
At any frequency,
considerably less than cutoff (i.e.,
the attenuation,
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
The shielding
as honeycomb) with openings that operate on the waveguide-below-cutoff principle.
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 1/8-inch hexagonal openings 1/2-inch long.
Table 8-16
Shielding Effectiveness of Hexagonal Honeycomb Made of Steel
with 1/8-inch Openings 1/2-Inch Long (8-10)
Frequency
Shielding
Effectiveness
(MHz)
(dB)
0.1
50.0
100.0
500.0
2,200.0
45
51
57
56
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,
8-51
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 aperture; because of the space requirements involved.
8-52
MIL-HDDK-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, and its relative permeability,
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.
fields.
At low frequencies (LF), only magnetic materials can furnish appreciable shielding against magnetic
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
Figure 8-28. Shielding Effectiveness of Conductive Glass to High Impedance Waves (8-11)
MIL-HDBK-419A
SURFACE RESISTANCE (OHMS/SQ)
Figure 8-29. Shielding Effectiveness of Conductive Glass to Plane Waves (8-11)
SURFACE RESISTANCE (OHMS/SQUARE)
Figure 8-30. Light Transmission Versus Surface Resistance for Conductive Glass (8-7)
8-55
MIL-HDBK-419A
8.7 USE OF CONVENTIONAL BUILDING MATERIALS. Conventional building materials are not normally
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 preliminary 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 steel 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. There will be less shielding near the
edges of the room. 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.
8-56
MIL-HDBK-419A
Figure 8-31. Shielding Effectiveness of Some Building Materials (8-12)
8-57
Figure 8-32. Center Area Attenuation of Induced Voltage by 15 Foot High Single-Course Reinforcing Steel Room (8-13)
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. Because of cable
proximity in cable runs or elsewhere, 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
As with other shields, the EM wave shielding
effectiveness and (2) surface transfer impedance,
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,
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
in terms of these parameters are available (8-14). For uniform current distribution
along a cable shield, the resulting
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
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 (8-14). The relative
shielding effectiveness of single and double braided cables as a function of frequency is shown in Figure 8-34.
8-59
MIL-HDBK-419A
Table 8-17
Comparison of Cable Shields
Single +
Layer
Braid
Multiple +
Layered
Braid
Foil ++
Conduit ++
Flexible
Conduit
Good
Good
Exc.
Exc.
Good
Shield
Effectiveness*
(Radio Frequency)
Good
Exc.
Exc.
Exc.
Poor
Normal Coverage
60-95%
95-97%
100%
100%
90-97%
Fatigue Life
Good
Good
Fair
Poor
Fair
Tensile Strength
Exc.
Exc.
Poor
Exc.
Fair
Shield
Effectiveness*
(Audio Frequency)
*Poor < 20 dB; Fair, 20-40 dB; Good, 40-60 dB; Exc. > 60 dB.
+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 finks 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 if the cable. These
discontinuities usually result from poor splicing or 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 permeability 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.
8-60
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.
b . Where a high degree of shielding is needed, cables with multiple shields separated by insulation
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.
g . Grounding a number of conductor shields by means of a single wire to a connector ground pin should
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.
8-61
MIL-HDBK-419A
Figure 8-34. Shielding Effectiveness of Various Types of RF Cables as a Function of Frequency (8-15)
8-62
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 gained 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. Table 8-19 summarizes some of the more
significant features of twelve different types of screen rooms. 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 demountable enclosure which may be relocated quite simply when the need arises.
8-63
Table 8-18
Connector Application Summary
Connector
Series
Voltage
Rating
Characteristic
Impedance
Frequency
Range
Method of
Assembly
For RG-/U Cables
Disconnect Style
5, 6, 8, 9, 10, 11,
12, 13, 14, 17, 18
Screw-on type
500 V
peak
50 ohm
70 ohm
Up to
10 GHz
Manual
8, 9, 29, 55, 58,
58A, 59, 62, 116
Push-on type
1500 V
peak
50 ohm
Up to
7 GHz
Manual
C
8, 9, 10, 12, 14,
55, 58
Bayonet Lock type
1000 V
peak
50 ohm
UHF
8, 9, 10, 11, 12,
13, 44, 58, 62,
63, 65, 71
Screw-on type
500 V
LC
17, 18
Screw-on type
5000 V
peak (modified
to 10 kV)
50 ohm
BN
8, 9, 10, 17, 18
Screw-on type
5000 V
peak
50 ohm
BN
55, 58, 59, 62, 71
Screw-on type
250 V
peak
(nonconstant)
BNC
55, 58, 59, 62, 71
Bayonet lock type
250 V
peak
50 ohm
174
Screw-on & Push-on
types
-----
50 ohm
75 ohm
N
GR-874
Subminiature
(nonconstant)
Manual
Up to
200 MHz
Manual
-----
Manual
Manual
Up to
200 MHz
Manual
Up to
10 GHz
Manual &
Crimp-on
Crimp-on
MIL-HDBK-419A
Figure 8-36. RF-Shielded Connector
8-65
MIL-HDBK-419A
Figure 8-37. Effectiveness of Circumferential Spring Fingers
for Improving the Shielding of a Connector (8-18)
8.9.1 Demountable (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 the 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 the 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.
8-66
MIL-HDBK-419A
Table 8-19
Characteristics of Commercially Available Shielded Enclosures (8-13)
Frequency
(kHz)
Room Description
Magnetic
Shielding
Effectiveness
(dB)
MAX
MIN
15
61
56
Styrofoam core, sheet metal skin.
Braided gasket material on door.
200
15
150
97
90
87
96
54
63
Hollow core construction. Piano
hinge on &or with finger stock.
15
200
100
118
81
108
1000
100
80
Construction similar to above,
except no overhead door.
18
150
93
120
64
95
26 gauge steel with folded and soldered seams
between panels. Commercially available door
with double row of beryllium copper finger stock.
All power lines provided with filters.
14
280
58
75
34
58
Continuously soldered 20 gauge sheet metal with
1.25
zinc electroplate. Two commercial
doors with finger stock (2 rows). Power line
filtering installed. Room size = 20 x 20 x 8 feet.
14
100
70
65
90
Continuously inert-gas welded sheet steel, 12 gauge
with overlapping seams. Standard commercial
shielded room door with double row finger stock.
14
200
90
130
74
106
Copper screen cell type
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.
8-67
MIL-HDBK-419A
Table 8-19 (Continued)
Room Description
Frequency
Magnetic
Shielding
Effectiveness
(kHz)
(dB)
MAX
MIN
Similar to above in construction features.
15
100
111
99
50
81
Double shielding of 10 gauge continuously inert-gas
welded low carbon sheet steel, 2” spacing between
walls, pneumatic bladder, expanding panel sliding
doors (no gasket).
0.1
1
15
100
62
108
120
25
52
92
107
Room partitioned into three separate rooms; two
are 12’ x 12’ x 10’ and the third is 12’ x 12’ x 14’.
All seams continuously inert-gas welded 16 gauge
sheet steel. Doors have pneumatic bladder with
triple row of finger stock.
0.5
1.0
5.0
10
15
104
122
80
39
20
Room divided into three cells. Single shielding
1
15
100
10,000
sheet steel continuously inert-gas welded, with
pneumatic bladder, and expanding panel sliding
doors (EMI gasket for contact surface). Total
room size 30 x 70 x 12 feet.
8-68
115
114
140
119
73
104
114
61
MIL-HDBK-419A
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 demountable 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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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 Britain 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. This approach may
achieve the highest performance realizable at an economical price. 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
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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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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
nonconducting and function more as a simple physical barrier which prevents accidental contact. Metal
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.
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MIL-HDBK-419A
GENERATOR
Figure 8-39. Coaxial Loop Arrangement for Measuring Shield Effectiveness
Figure 8-40. Coplanar Loop Arrangement for Measuring Shield Effectiveness
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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. If the
performance of the specific equipment is not known, 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. High voltage transmission systems, in
particular, frequently generate noise through corona discharge and arcing across dirty connectors and
insulators. The frequency spectrum of this noise generally extends well into the HF region (3-30 MHz) 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
factors. The best indicator as to how much shielding is going to be required for a given piece of equipment or
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.
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MIL-HDBK-419A
8.13.1 Size. 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. This phenomenon is very important in choosing shields to protect against
EMP for instance. 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 Cost.
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 434 850.
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,
Electromagnetics Inc, Bethesda MD, 20 August 1963.
8-13. “EMP Protection for Emergency Operating Centers,"
Washington DC, July 1972.
Contract AF
30(602)-2691,
White
TR-61A, Defense Civil Preparedness Agency,
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.
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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(l), 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, Contemporary 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.
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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.* At voltages higher then 240 volts, the contact resistance of the skin becomes less
important. 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 (9-1) (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.
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MIL-HDBK-419A
Shock currents greater than the reaction current produce an increasingly severe muscular reaction. Above a
certain level, the shock victim becomes unable to release the conductor. 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 the 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)
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
Effects
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.
l.
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 attractive 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
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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.
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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 gastrointestinal 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
meters) and 420 nm. This decrease is caused by the strong
absorption at the lens of the eye. 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.
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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
appropriate target. In addition to being produced by
produced by high-voltage (> 15 kV) tubes used for other
radiation in equipments be identified and shielded so that
a sufficiently high velocity before colliding with an
specifically designed equipment, X-rays can also be
applications. It is important that all sources of X-ray
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 Program.”
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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 electrodynamic
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 are typical for this early-time portion of the HEMP (10-2).
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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
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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 MHDEMP 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
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.
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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.
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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. HEMP interaction with systems may be separated into long-line
effects and local effects. 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.
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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
polarization (dashed line) and vertical polarization (solid line) of the incident field. The curve
is the
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
The current in Figure 10-4 is normalized by
containing the characteristic impedance of the line, the peak field
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 ?oynting 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 (a).
For a
line 10 meters (33 feet) above soil having
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
For the
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
where d is the depth of burial. The current is normalized to the inductance per unit
depth parameter
length (L) of the cable, the magnitude of the incident exponential pulse (E o ), the decay time-constant
soil tine-constant
field strength
conductivity
the
and a directivity function (D). The induced current is proportional to the incident
the 3/2 power of the decay time-constant
For a horizontally polarized, vertically incident pulse having
long cable buried near the surface (d = 0) in soil of conductivity
10-7
and the inverse square root of the soil
= 50 kV/m and
= 250 ns, a
S/m will have about 2.8 kA induced in it.
MIL-HDBK-419A
Figure 10-5.
The Normalized Current Waveform for Various Values of the Depth
Parameter p (Exponential Pulse)
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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
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.
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Table 10-1
Shielding by Diffusion
Internal Voltage Induced in Loop *
Shield
Thickness
(mm)
Copper
Aluminum
Steel
5.8 x 107 S/m
3.7 x 107 S/m
6 x 106 S/m
= 200)
0.2
0.34 V
0.85 V
0.076 V
1.0
2.6 mV
6.4 mV
1.1 mV
5.0
21.0
51.0
15.0
*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
where C is the wire-to-shield capacitance and
C = 10 pF and
=
is the load impedance on the wire. (For nominal values of
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).
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Figure 10-7. Shield to Exclude Electromagnetic Fields
10.3.2.3 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
can induce charge on internal cables as illustrated in
Figure 10-8a. The external magnetic field
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
proportional to
and the voltage induced by the magnetic field is
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
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
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Figure 10-8. Electromagnetic Penetration Through Small Apertures
10-12
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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 joint 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.
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Figure 10-9. Shielding Integrity Near Interference-carrying External Conductors
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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. Zero stress certainly satisfies this
definition, but more practical values can be found. 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.
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Figure 10-10. Magnetic Field Penetration of Apertures
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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.
For example, suppose the facility is a node with a 100-pair cable
linking it to other parts of a network. 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, 1000 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 arc
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
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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. Gas tubes have small capacitances and
virtually no loss in the nonconducting states. 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 intermodulation
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 intermodulation 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.
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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 this 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
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Figure 10-12. Waveguide Feedthroughs
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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.
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Figure 10-13. Bellows with Slitted Sleeve Waveguide Attachment
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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). In addition, it is desirable that the
sleeve have an optical coverage of at least 85%. 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
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10.4.2.4.2.3 Stuffing Tube for Waveguide. In this method, illustrated in figure 10-15, the connection to the
waveguide is 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
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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., “How 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. Hamm, and E. F. Vance.
10-25/10-26
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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
11-1/11-2
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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 I2R 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
inch). A circular mil is equal to
or 78.54 percent of a square mil (1 square mil =
square inch). The
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. EMC is the result of an engineering planning process applied during the
life cycle of equipment. 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 disconnect(ing) 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 performance
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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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, 7 1 , 85, 86, 87, 8 9 , 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, 2 0 , 2 1 , 22, 23, 24, 2 6 , 3 1 , 6 9 ,7 9 , 80, 102, 120, 130, 147, 149, 154,
166, 179, 182, 185, 197, 200, 201, 209.
PERSONNEL SAFETY - - 2 0 , 42, 44, 50, 66, 7 7 , 8 0 , 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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3 . Adams, W. S., “Graphical Presentation of Electromagnetic Shielding Theory,” Proceedings of the Tenth
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17. Bellaschi, P. L., “Impulse and 60-Cycle Characteristics of Driven Grounds,” AIEE Trans., Vol 60,
March 1941, pp 123-128.
18. Bellaschi, P. L., Armington, R. E., and Snowden, A. E., “Impulse and 60-Cycle Characteristics of Driven
Grounds--II,” AIEE Trans., Vol 61, June 1942, pp 349-363.
19. Bellaschi, P. L. and Armington, R. E., “Impulse and 60-Cycle Characteristics of Driven Grounds--III, Effect
of Lead in Ground Installation,” AIEE Trans., Vol 62, June 1943, pp 334-344.
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32. Boyer, O. A. and Korges, E., “Connector Performance by Types," AIEE Trans, Vol 75, Pt III, October 1956,
pp 907-913.
33. Braine, M. R., “The Siting of Station Earths for Telecommunication Installations,” Point-to-Point
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Pt III, February 1955, pp 1580-1585.
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October 1954, p M-23.
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47. Clark, D. B. and Lassiter, H. A., “Nuclear Electromagnetic Pulse (NEMP) Protection for Communications
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61. Dixon, C. R. and Nelson, F. G., “The Effect of Elevated Temperature of Flash-Welded Aluminum Copper
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62. Dorey, F. M., “System Grounding in Industrial Plants,” Electrical Engineering, Vol 72, No. 2,
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82. Good, T. M., “A Method of Evaluating The Effectiveness of Radio-Frequency Gasket Materials,”
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107. Humpheries, J. D., “Earth-Electrode Systems for Large Electric Stations,” Proc. IEE, Vol 104, Pt A,
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117. Kaufmann, R. H., “Equipment Grounding:
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121. Kline, R. L. and Friauf, J. B., “Electric Shock, Its Causes and Its Prevention,” BuShips Journal, July 1954,
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123. Krida, H. and McCurry, E. T., “A New Grounding and Testing Device for Metal-Clad Switchgear,” AIEE
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124. Kuhn, R. J., “Grounding and Corrosion Protection on Underground Electric Power Cable Sheaths and Oil- or
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125. Lee, R. H., “Electrical Grounding: Safe or Hazardous?” Chemical Engineering, Vol 76, No 16, July 1969.
126. Loucks, W. W. and Lemire, W.A.R., “Transmission and Distribution Grounding in the Hydro-Electric Power
Commission of Ontario,” AIEE Trans, Vol 70, Pt II, 1951, pp 1493-1499.
127. Madden, Charles, “EMI Shielding with Electrically Conductive Pressure Sensitive Tapes,” Proceedings of
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pp 109-112.
130. McCann, G. D., Beck, E., and Finzi, L. A., “Lightning Protection For Rotation Machines,” AIEE Trans,
Vol 63, June 1944, pp 319-332.
131. McCrocklin, A. J., Jr and Wenlandt, C. W., “Determination of Resistance to Ground of Grounding Grids,”
AIEE Trans, Vol 71, Pt III, December 1952, pp 1062-1064.
132. McGreer, T. H., “Grounding of Power Station 4, 160-Volt Auxiliary Systems,” AIEE Trans, Vol 76, Pt III,
February 1958, pp 1459-1463.
133. Mendez, H. A., “Shielding Theory of Enclosures with Apertures,” AFOSR-TR-0100, Contract
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134. Merewether, David E., “Design of Shielded Cables Using Saturable Ferromagnetic Materials,” IEEE Trans
on Electromagnetic Compatibility, Vol EMC-12, No. 3, August 1970.
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B-16
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APPENDIX C
TABLE OF CONTENTS FOR VOLUME II
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
1.1
1.2
1.2.1
1.2.1.1
1.2.1.2
1.2.1.3
1.2.1.4
1.2.2
1.2.2.1
1.2.2.2
1.2.2.3
1.2.3
1.2.4
1.3
1.3.1
1.3.2
1.3.2.1
1.3.2.1.1
1.3.2.1.2
1.3.2.2
1.3.2.2.1
1.3.2.2.2
1.3.2.3
1.3.3
1.3.3.1
1.3.3.2
1.3.3.3
1.3.3.4
1.3.3.5
1.3.3.5.1
1.3.3.5.2
1.3.3.5.3
1.3.3.5.4
1.3.3.5.5
1.3.3.5.6
1.3.3.5.7
1.3.3.5.8
Page
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . .
EARTH ELECTRODE SUBSYSTEM . . . . . . . . . . . . . .
Determination of Site Parameters. . . . . . . . . . . . . .
Soil Resistivity . . . . . . . . . . . . . . . . . . . . .
Geological Effects . . . . . . . . . . . . . . . . . . .
Physical Features. . . . . . . . . . . . . . . . . . . .
Local Climate . . . . . . . . . . . . . . . . . . . . .
Design Procedure . . . . . . . . . . . . . . . . . . . . .
Selection of Electrode Configuration . . . . . . . . . . .
Calculation of Earth Resistance. . . . . . . . . . . . . .
Alternate Configurations. . . . . . . . . . . . . . . . .
Design Guidelines. . . . . . . . . . . . . . . . . . . . .
Installation Practices . . . . . . . . . . . . . . . . . . .
LIGHTNING PROTECTION FOR STRUCTURES.. . . . . . . .
Principles of Protection . . . . . . . . . . . . . . . . . .
Integral Protection System . . . . . . . . . . . . . . . . .
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1-1
1-2
1-2
1-2
1-5
1-5
1-5
1-6
1-6
1-9
1-9
1-14
1-22
1-23
1-23
1-23
1-24
1-24
1-24
1-33
1-33
1-37
1-39
1-40
1-40
1-41
1-42
1-49
1-49
1-49
1-50
1-50
1-51
1-51
1-56
1-56
Use of Overhead Guard Wires. . . . . . . . . . . . . . . . . . . . . . . . .
1-56
Air Terminals . . . . . . . . . . . . . . . . . . . . .
Size and Materials . . . . . . . . . . . . . . . . . .
Location. . . . . . . . . . . . . . . . . . . . . . .
Grounding Conductors . . . . . . . . . . . . . . . . . .
Roof Conductors . . . . . . . . . . . . . . . . . . .
Down Conductors . . . . . . . . . . . . . . . . . . .
Fasteners
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Separately Mounted Protection Systems . . . . . . . . . . .
Mast Type . . . . . . . . . . . . . . . . . . . . . . .
Overhead Ground Wire Type . . . . . . . . . . . . . . .
Waveguide Installation and Grounding . . . . . . . . . . .
Cable Installation and Grounding . . . . . . . . . . . . .
Lightning-Generated Transient Surge Protection . . . . . .
Transient Source and Equipment Damage. . . . . . . . .
Minimizing Damage . . . . . . . . . . . . . . . . . .
Susceptible Components . . . . . . . . . . . . . . . .
Frequency of Transient Occurrence . . . . . . . . . . .
Transient Definition, AC Service Conductors . . . . . . .
Methods for Transient Protection on AC Service Conductors
Use of Ferrous Metal Conduit. . . . . . . . . . . . . .
C-l
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MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
1.3.3.5.9
1.3.3.5.10
1.3.3.5.11
1.3.3.5.12
1.3.3.5.13
i.3.3.5.14
1.3.3.5.15
1.3.3.5.16
1.3.3.5.17
1.3.3.5.18
1.3.3.5.19
1.3.3.5.20
1.3.3.5.21
1.3.3.5.22
1.3.3.5.23
1.3.3.5.24
1.3.3.5.25
1.3.3.5.26
1.3.3.5.27
1.3.3.5.28
1.3.3.5.29
1.3.3.6
1.3.3.7
1.3.3.7.1
1.3.3.7.2
1.3.3.7.3
1.3.3.7.4
Protection of Underground Cables. . . . . . . . . . . . .
Buried Guard Wire . . . . . . . . . . . . . . . . . . .
Secondary AC Surge Arrester. . . . . . . . . . . . . . .
Surge Arrester Installation . . . . . . . . . . . . . . . .
Operating Characteristics of Surge Arresters . . . . . . . .
Desirable Operating Characteristics for Transient Suppressors
Characteristics of Different Types of Surge Arresters . . . .
Transient Protection for Externally Exposed Equipment Lines .
Frequency of Transient Occurrence . . . . . . . . . . . .
Amplitudes and Waveforms of Occurring Transients . . . . .
Equipment Withstand Levels . . . . . . . . . . . . . . .
Protection Methods Against Transients. . . . . . . . . . .
Enclosing Cable Runs in Ferrous Metal Conduit . . . . . . .
Transient Suppression . . . . . . . . . . . . . . . . . .
Types of Available Transient Suppression. . . . . . . . . .
Operating Characteristics of Transient Suppressors . . . . .
Transient Suppressor Packaging Design. . . . . . . . . .
Coaxial Cable Shield Connection Through an Entrance Plate
Grounding of Unused Wires . . . . . . . . . . . . . . .
Transient Suppression for RF Coaxial Lines. . . . . . . .
Equipment-Level Transient Suppression . . . . . . . . .
Lightning Generated Transient Protection Evaluation . . . .
Transient Protection . . . . . . . . . . . . . . . . . .
Protection Requirement . . . . . . . . . . . . . . . .
Transient Definition. . . . . . . . . . . . . . . . . .
Determination of Equipment Damage (Withstand) Levels . .
Determination of Need for Transient Protection. . . . . .
Minimizing Transient Damage . . . . . . . . .
1.3.3.7.5
1.3.3.7.6
AC Power Input. . . . . . . . . . . . . . . .
Power Supply Transient Suppression . . . . . . .
1.3.3.7.7
Landline Transient Suppression . . . . . . . . .
1.3.3.7.8
Corrosion
Control. . . . . . . . . . . . . . . .
1.3.3.8
Joints . . . . . . . . . . . . . . . . . . . . .
1.3.3.9
1.3.3.10
Physical Protection . . . . . . . . . . . . . . .
FAULT PROTECTIVE SUBSYSTEM . . . . . . . . .
1.4
1.4.1
Purpose . . . . . . . . . . . . . . . . . . . . .
1.4.2
Equipment Fault Protection Subsystem Composition . .
C-2
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. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
1-57
1-57
1-59
1-59
1-60
1-67
1-67
1-73
1-73
1-73
1-74
1-74
1-74
1-75
1-77
1-77
1-78
1-78
1-78
1-79
1-79
1-79
1-80
1-80
1-81
1-82
1-83
1-83
1-84
1-89
1-89
1-98
1-99
1-99
1-99
1-99
1-100
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
1.4.3
1.4.4
1.4.5
1.4.5.1
1.4.5.2
1.4.5.3
1.4.5.4
1.4.5.5
1.4.6
1.4.7
1.4.8
1.4.9
1.4.9.1
1.4.9.2
1.4.9.3
1.5
1.5.1
Page
Configuration of the Equipment Fault
Pipes and Tubes. . . . . . . . . .
Electrical Supporting Structures . .
Metal Conduit . . . . . . . . .
Cable Trays . . . . . . . . . .
Enclosures . . . . . . . . . . .
Cable Armor. . . . . . . . . .
Rotating Machinery . . . . . . .
Power Distribution Systems . . . .
Standby AC Generators . . . . . .
Protection Subsystem
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. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
Equipment Fault Protection Subsystems for Transportable Equipment . . . . . . . . .
MIL-STD-188-124A and NEC Compliance Evaluation. . . . . . . . . . . . . . . . .
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MIL-STD-188-124A and NEC Compliance Inspection. . . . . . . . . . . . . . . .
Correction of Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . .
SIGNAL REFERENCE SUBSYSTEM FOR NEW FACILITIES . . . . . . . . . . . . . . .
Higher Frequency Network . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1
Multipoint Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1
Types of Equipotential Planes. . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.1
Copper Grid Embedded in Concrete . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.2
Equipotential Plane Under Floor Tile or Carpet . . . . . . . . . . . . . . . .
1.5.1.1.1.3
Overhead Equipotential Plane. . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.4
Raised (Computer) Flooring . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.4.1
Bolted-Grid (Stringer) or Rigid Grid System. . . . . . . . . . . . . . . . .
1.5.1.1.1.4.2
Drop-In or Removable Grid System . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.4.3
Free-Standing, Pedestal-Only or Stringerless System . . . . . . . . . . . .
1.5.1.1.1.5
Ground Risers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5.1.1.1.6
Equipment Cabinet Grounding . . . . . . . . . . . . . . . . . . . . . . .
1.5.2
Lower Frequency Signal Reference Network . . . . . . . . . . . . . . . . . . . .
1.6
GROUNDING PHILOSOPHY FOR EQUIPMENTS PROCESSING NATIONAL SECURITY
RELATED INFORMATION (RED/BLACK EQUIPMENTS) . . . . . . . . . . . . . . . .
1.7
BONDING PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.1
Application Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.2
Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.3
Bond Protection Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.3.1
Jumper Fasteners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.4
Typical Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C-3
1-100
1-102
1-102
1-103
1-103
1-103
1-103
1-103
1-104
1-104
1-104
1-105
1-105
1-105
1-111
1-113
1-113
1-118
1-120
1-120
1-120
1-120
1-120
1-125
1-131
1-131
1-131
1-131
1-131
1-134
1-140
1-140
1-142
1-143
1-147
1-148
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
Page
1.7.4.1
Cable Trays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.4.2
Tubing and Conduit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7.4.3
Other Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SHIELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8
Establishing Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8.1
1.8.2
Design Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selection of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8.3
1.8.4
Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMMON-MODE NOISE AND INSTRUMENTATION. . . . . . . . . . . . . . . . . .
1.9
1.9.1
Design Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.9.2
Instrumentation Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10
EMP PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.1
Earth Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.2
EMP Shield Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.3
EMP Bonding Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10.4
Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11
MILITARY MOBILE FACILITIES . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1
General Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
Facility Ground System . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1
Earth Ground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.1
1.11.1.1.1.1
General Earth Electrode Subsystem Requirements . . . . . . . . . . . . . . .
Earth Electrode Subsystem Types . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.1.2
1.11.1.1.1.3
Soil Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.1.4
Ground Rod Resistance . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.1.5
Ground Resistance Shells. . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.2
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.2.1
Three-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
1.11.1.1.2.2
Single-Phase Power Distribution System . . . . . . . . . . . . . . . . . . .
1.11.1.1.2.3
DC Power System (2-Wire) . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.3
Lightning/EMP Protection Subsystem . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.3.1
Air Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.3.2
Terminal Protection Devices . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.4
Signal Reference Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.1.5
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.2
Measuring Ground Resistance in Tactical Environments . . . . . . . . . . . . . .
1.11.1.3
Reducing Ground Resistance in Tactical Environments . . . . . . . . . . . . . . .
1.11.1.3.1
Existing Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-148
1-149
1-149
1-154
1-154
1-159
1-160
1-162
1-164
1-171
1-172
1-172
1-172
1-173
1-173
1-176
1-177
1-177
1-177
1-177
1-177
1-177
1-178
1-178
1-178
1-178
1-178
1-179
1-179
1-179
1-179
1-182
1-182
1-182
1-182
1-182
1-182
C-4
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
Page
Multiple Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.3.2
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.3.3
Chemical Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.1.3.4
Detailed Tactical Grounding Requirements . . . . . . . . . . . . . . . . . . . . .
1.11.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.1
Training. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.1.1
Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.1.2
Stan&Alone Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.2
General Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.2.1
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.2.2
Low Resistance Grounds . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.2.2.1
Existing Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.2.2.1.1
Earth Electrode Subsystem, Single Ground Rod . . . . . . . . . . . . . . .
1.11.2.2.2.1.2
Earth Electrode Subsystem, Multiple Ground Rods . . . . . . . . . . . . . .
1.11.2.2.2.1.3
Stand-Alone Shelter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.3
General Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.3.1
Interconnection of Subsystems . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.3.2
Collocated Military Mobile Equipments . . . . . . . . . . . . . . . . . . . . .
1.11.2.4
General Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.4.1
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.4.2
Collocated Shelters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5
General Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.1
Grounding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.2
Power Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.2.1
Signal Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.2.2
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.2.3
Lightning/EMP Protection . . . . . . . . . . . . . . . . . . . . . . . . .
1.11.2.5.2.4
Collocated Shelters Greater than 8 Meters Apart . . . . . . . . . . . . . . .
1.11.2.5.2.5
Ground Resistance Difference of Less than 150 Ohms . . . . . . . . . . . .
1.11.2.5.2.5.1
1-185
1-185
1-185
1-185
1-185
1-186
1-186
1-186
1-186
1-186
1-186
1-186
1-187
1-187
1-187
1-187
1-187
1-187
1-187
1-188
1-188
1-188
1-188
1-188
1-188
1-188
1-189
1-189
1-189
Ground Resistance Difference of Greater than 150 Ohms .
1.11.2.5.2.5.2
Earth Electrode Subsystem . . . . . . . . . . . . . .
1.11.2.5.2.5.3
Power Ground . . . . . . . . . . . . . . . . . .
1.11.2.5.2.5.3.1
Signal Ground . . . . . . . . . . . . . . . . . .
1.11.2.5.2.5.3.2
Safety/Equipment Ground (Greenwire) . . . . . . . .
1.11.2.5.2.5.3.3
Lightning/EMP Protection . . . . . . . . . . . . .
1.11.2.5.2.5.3.4
Fixed Prefabricated Shelters . . . . . . . . . . . . . . . .
1.11.2.6
General Description. . . . . . . . . . . . . . . . . . .
1.11.2.6.1
C-5
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1-189
1-190
1-190
1-190
1-190
1-190
1-190
1-190
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 1 - NEW FACILITIES DESIGN CRITERIA
Paragraph
Page
1.11.2.6.2
Electrical Connection . . . . . . . . . . . . . . . . . . .
1.12
FENCES . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.12.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .
1.12.2
Grounding . . . . . . . . . . . . . . . . . . . . . . . . . .
1.12.3
Installation. . . . . . . . . . . . . . . . . . . . . . . . . .
1.13
INSPECTION AND TEST PROCEDURES FOR A NEW FACILITY . . .
1.13.1
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . .
1.13.2
Lightning Protection Network . . . . . . . . . . . . . . . . .
1.13.3
Signal Reference and Fault Protection Subsystems. . . . . . . . .
1.13.4
Bonds and Bonding . . . . . . . . . . . . . . . . . . . . . .
1.13.5
Facility Checkout Form . . . . . . . . . . . . . . . . . . . .
Part I
Earth Electrode Subsystem . . . . . . . . . . . . . . .
Part II Lightning Protection Network . . . . . . . . . . . . .
Part III Facility Ground System . . . . . . . . . . . . . . . .
Part IV Bonding . . . . . . . . . . . . . . . . . . . . . . .
1.14
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . .
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1-190
1-191
1-191
1-191
1-191
1-193
1-193
1-194
1-194
1-195
1-196
1-196
1-197
1-198
1-199
1-200
2.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
UPGRADING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1
Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2
Facility Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.1
Survey Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2
Inspection Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.1
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.2
Bonds and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.3
Lightning Protection Network . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.4
Safety Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.5
Signal Grounding Practices . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.6
Ground System Noise Survey . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.2.7
Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.3
Test Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.3.1
Bond Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.3.2
Ground System Noise Current. . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.3.3
Differential Noise Voltage. . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.4
Survey Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-1
2-1
2-4
2-4
2-4
2-5
2-5
2-10
2-13
2-16
2-17
2-17
2-19
2-19
2-19
2-20
2-22
2-24
CHAPTER 2 - EXISTING FACILITIES
C-6
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 2 - EXISTING FACILITIES
Paragraph
2.2.3
2.2.4
2.2.5
2.3
2.3.1
2.3.2
2.3.2.1
2.3.2.2
2.3.2.3
2.3.2.4
2.3.2.5
2.3.2.6
2.3.3
2.3.4
2.4
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
2.4.6
Page
Guidelines for Upgrading. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Expansion of Existing Facilities . . . . . . . . . . . . . . . . . . . . . . . . . .
Expansion of Existing Facilities for Higher-Frequency Grounds . . . . . . . . . . . .
MAINTENANCE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Schedules and Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maintenance Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Earth Electrode Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lightning Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .
Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fault Protection Subsystem (Safety Ground) . . . . . . . . . . . . . . . . . . .
Signal Reference Subsystem (Signal Grounding) . . . . . . . . . . . . . . . . . .
Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facility Maintenance Report . . . . . . . . . . . . . . . . . . . . . . . . . . .
Performance Evaluation Program . . . . . . . . . . . . . . . . . . . . . . . . .
GROUNDING CONSIDERATIONS FOR CLASSIFIED INFORMATION PROCESSORS
(RED/BLACK EQUIPMENTS) INSTALLED PRIOR TO THIS HANDBOOK . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Existing Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Protection Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Reference Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Filter Ground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounding Precautions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-33
2-35
2-35
2-36
2-36
2-36
2-39
2-41
2-42
2-43
2-44
2-45
2-46
2-54
2-59
2-59
2-59
2-59
2-60
2-61
2-61
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
3.1
3.2
3.2.1
3.2.1.1
3.2.1.1.1
3.2.1.1.2
3.2.1.1.3
3.2.1.1.4
3.2.1.1.5
3.2.1.1.6
3.2.1.1.7
3.2.1.2
INTRODUCTION . . . . . . . . . .
GROUNDINGPROCEDURES. . . .
Signal Grounds . . . . . . . . . .
Lower Frequency Equipment . . .
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Signal Ground Network Configuration
Signal Ground Terminals . . . . . .
Color Code. . . . . . . . . . . .
Cabinet Bus Bar . . . . . . . . .
Isolation. . . . . . . . . . . . .
Signal Interfacing . . . . . . . . .
Signal Grounding . . . . . . . . .
Higher Frequency Equipment . . . . .
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3-1
3-1
3-1
3-2
3-2
3-2
3-3
3-3
3-4
3-4
3-6
3-12
MIL-HDBK-419A
TABLE OF CONTENTS (Continued)
CHAPTER 3 - EQUIPMENT DESIGN CRITERIA
Paragraph
3.2.1.2.1
3.2.1.2.2
3.2.1.3
3.2.2
3.2.3
3.3
3.4
3.4.1
3.4.2
3.4.3
3.4.3.1
3.4.3.2
3.5
3.5.1
3.5.2
3.5.2.1
3.5.2.1.1
3.5.2.1.2
3.5.2.1.3
3.5.2.2
3.5.2.3
3.5.2.3.1
3.5.2.3.2
3.5.2.3.3
3.6
3.7
3.7.1
3.7.2
3.7.3
3.7.4
3.7.5
3.7.6
3.7.7
3.7.8
3.7.9
3.7.10
3.7.11
3.8
Page
Signal Interfaces . . . .
Cable Connectors . . . .
Equipments Containing Both
Fault Protection . . . . . .
Cabinet Grounding . . . . .
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Lower
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Higher Frequency
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Circuits.
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BONDING PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SHIELDING GUIDELINES. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parts Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Layout and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Equipment Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Penetrations and Apertures. . . . . . . . . . . . . . . . . . . . . . . . . .
COMMON-MODE NOISE CONTROL AND INSTRUMENTATION GROUNDING. . . .
Common-Mode Noise Control. . . . . . . . . . . . . . . . . . . . . . . . . .
Instrumentation Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analog Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounded Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ungrounded Transducers . . . . . . . . . . . . . . . . . . . . . . . . . .
Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Digital Data Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Recording Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Magnetic Tape Recorders . . . . . . . . . . . . . . . . . . . . . . . . .
Strip Chart Recorders . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-Y Plotters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EMP CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EQUIPMENT INSPECTION AND TEST PROCEDURES . . . . . . . . . . . . . . . .
Lower Frequency Equipments. . . . . . . . . . . . . . . . . . . . . . . . . .
Higher Frequency Equipments . . . . . . . . . . . . . . . . . . . . . . . . .
Hybrid Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Installed Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fault Protection Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .
Bonding . . . . . .
Shielding. . . . . . .
Instrumentation System
EMP Design . . . . .
Other Observations . .
Inspection Form . . .
REFERENCES . . . .
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C-8
3-13
3-13
3-14
3-14
3-16
3-18
3-25
3-25
3-25
3-27
3-27
3-28
3-34
3-34
3-34
3-35
3-35
3-38
3-38
3-40
3-40
3-40
3-42
3-42
3-42
3-43
3-43
3-47
3-47
3-47
3-47
3-49
3-49
3-50
3-50
3-50
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-10
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-l
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: 1-148 to 1-153, 2-10, 3-18
R/b 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: 10-1 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: 5-1 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 1-33
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
Demountable 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: 1-2 to 1-22, 1-193, 1-196, 2-5, 2-39, 2-40
current handling capacity, I: 2-57; II: 1-6
design, II: 1-2, 1-6 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: 10-1 to 10-25; II: 1-172 to 1-177, 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, 1-118 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: 1-193 to 1-200, 2-1, 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
network 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
lie-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
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: 1-2 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
Navy - EC
Air Force - 90
Other Interest:
DNA - DS
OST (M-35)
Activity, Assignee
Air Force - 90
(Project SLHC-4191)
Preparing Activity:
Air Force - 90
Review Activities:
Army - SC, CR, AR, AC
Navy-EC, NC, NV, OM
Air Force - 02, 04, 11, 14, 15, 17, 50, 90
DMSSO-SD
DCA - DC
NSA - NS
Joint Tactical
Agency -JT
ECAC
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