Eaton Bussmann series Selecting Protective Devices han Owner's Manual

Selecting Protective Devices

SPD

Electrical protection handbook

Based on the 2017 NEC ®

Selecting protective devices

Introduction

Welcome to Eaton’s Bussmann series Selecting Protective

Devices (SPD) handbook. This reference document is based on the 2017 National Electrical Code (NEC

®

) and is a comprehensive guide to electrical overcurrent protection and electrical design considerations. The information within this resource is presented on numerous applications as well as code and standard requirements for a variety of electrical equipment and distribution systems.

For this edition, considerable change was made to the overall structure and organization to improve readability, and to organize the discussions around the important topics of safety and protection.

This edition is comprised of major sections containing content that can be easily located by three methods.

•

Table of contents with new or expanded content highlighted in red.

•

Index arranged alphabetically by topic with corresponding page numbers

•

NEC index to located information associated with specific code references.

For more technical resources and general product information, visit

Eaton.com/bussmannseries

.

George Ockuly

This 2018 Edition of the Bussmann series Selecting Protective Devices

(SPD) handbook is dedicated to George Ockuly for his service to the electrical industry.

Ockuly joined Bussmann in 1972, where he held various positions of increasing importance, including district sales engineer, regional sales manager, director of North American sales, and finally vicepresident of sales. His responsibilities included worldwide codes and standards activities, technical literature development, application engineering, product certification and testing, training, and product safety. Mr. Ockuly retired after 28 years of service.

Outside of Bussmann,

Ockuly held positions on NEMA’s Standards

Policy Committee, the International and

Regional Standards

Committee, the fuse section, and served as vice chairman of the

Codes and Standards

Committee.

He was a member of the NFPA Board of Directors, a member of the

National Electrical Code Panels 10 and 11, and was a principal on NFPA

70B Recommended Practices for Electrical Equipment Maintenance.

He served as vice-president of the U.S. National Committee of the

International Electrotechnical Commission (USNC/IEC), and technical advisor for UNSC, IEC C32B Low Voltage Fuses. He is a senior member of the Institute of Electrical and Electronics Engineers (IEEE), a member of the Electronic Industries Alliance, and an associate member of the

International Association of Electrical Inspectors (IAEI). He is a recipient of the NEMA Kite and Key award which recognizes individuals who have advanced the interests of the electrical industry through active and sustained involvement in the affairs of the association.

About the front cover

Electricians installing a Bussmann series Quik-Spec™ Coordination

Panelboard (QSCP) which uses the Compact Circuit Protector and UL ®

Class CF CUBEFuse™. The QSCP makes it easy to achieve selective coordination by using published upstream fuse and circuit breaker tables. Available in flush- or surface-mount NEMA 1 enclosures or a

NEMA 3R enclosure, the QSCP increases worker electrical safety by featuring dead front protection and finger-safe fuses.

This handbook is intended to clearly present product data and technical information that will help the end user with design applications. Eaton reserves the right, without notice, to change design or construction of any products and to discontinue or limit their distribution. Eaton also reserves the right to change or update, without notice, any technical information contained in this handbook.

Once a product has been selected, it should be tested by the user in all possible applications. Further, Eaton takes no responsibility for errors or omissions contained in this handbook, or for misapplication of any Bussmann series product.

Extensive product and application information is available online at: Eaton.com/ bussmannseries

National Electrical Code ® is a trademark of the National Fire Protection Association,

Inc., Batterymarch Park, Quincy, Massachusetts, for a triennial electrical publication. The term, National Electrical Code, as used herein means the triennial publication constituting the National Electrical Code and is used with permission of the National Fire Protection Association, Inc.


Benefits of the modern current-limiting fuse

Provides a flexible and worry-free solution

•

With interrupting ratings up to 300 kA, fuses can be installed in almost any system without fear of misapplication

•

With straight voltage ratings, fuses can be installed in any system independent of its grounding as opposed to slash voltage rated devices that can only be installed on a solidly grounded Wye system

•

The fuse’s interrupting rating is typically at least equal to, or in many cases greater than, the available fault current at the line terminals

•

The fuse’s high interrupting rating provides flexibility should system changes, such as utility transformers or equipment relocation, increase fault current levels

Saves time and money

•

Fuses eliminate the need for expensive, time-consuming fault current studies when using 300 kA interrupting rated Low-

Peak fuses

•

Current-limiting fuses make achieving selective coordination easy and simple by maintaining a minimum amp ratio between upstream and downstream fuses. Using published ratio tables eliminates the need for selective coordination studies and ensures the affected circuits are isolated and prevents unnecessary power loss to upstream portions of the electrical system.

•

Because fuses are an enclosed, non-venting design, they eliminate the need for additional system guards or barriers to protect from venting

•

Fuses reduce the need for OCPD maintenance as they require no additional maintenance or servicing beyond periodically checking conductors and terminations

1

Increases electrical safety

•

Finger-safe protection is provided in the latest fuse technology for fuse holders and blocks, switches, and power distribution fuse blocks, including the patented Bussmann™ series

Low-Peak™ CUBEFuse™ and revolutionary Compact Circuit

Protector (CCP) disconnect switch

•

With interrupting ratings up to 300 kA, available fault currents exceeding these high values due to system changes is virtually eliminated

•

UL Class branch circuit fuses have physical rejection features that help ensure the same voltage and equal to or greater interrupting ratings are retained throughout the system’s life

•

Arc flash hazards can be greatly reduced when fuses operate in their current-limiting range

Reduces risk and improved reliability

•

Fuse rejection features reduce the potential to install an overcurrent protective device (OCPD) with different performance characteristics and lower interrupting ratings that can compromise the protection level

•

Fuses do not vent during a fault, safely containing and extinguishing the arcing inside the fuse body. On some

OCPDs venting is inherent, possibly causing damage to other system components

•

Factory-calibrated replacement fuses ensure the same protection level throughout the system’s life and eliminate the possible need to test and recalibrate an OCPD after a fault

•

The fuse’s enclosed, sand-filled design operates on proven thermal principles that eliminate the risk something may not be properly adjusted or operate correctly under short-circuit conditions as is the case with some mechanical OCPDs

•

The enclosed, fixed design eliminates the need to adjust and change device settings in the field, thus reducing confusion and risk of misapplication

Helps achieve high equipment short-circuit current ratings

(SCCR)

•

•

Fuses have high interrupting ratings (up to 300 kA) and will not be the limiting factor in a panel SCCR

Current limitation drastically reduces the peak let-through current to protect downstream components and help raise branch SCCRs

Reduces downtime and improved protection

•

Specifying Type 2 “No Damage” (versus Type 1) protection with properly sized current-limiting fuses helps eliminate the need to replace components after a fault

•

Current limitation helps reduce the extreme, destructive thermal and mechanical forces associated with short-circuit events

Facilitates code compliance

•

Compliance with NEC 110.9 is easily achieved with high interrupting ratings up to 300 kA

•

Compliance with NEC 110.10 for protecting equipment and components from extensive damage from short-circuits is easy with current-limiting fuses

•

Compliance with OSHA 1910.334(b)2 is met by eliminating the invitation for an operator to reset the OCPD after a fault without first determining its cause. Resetting circuit breakers or replacing fuses without investigating and fixing the cause is prohibited by federal law.

Eaton.com/bussmannseries 1-1


1-2

Table of Contents

Section Topic

1 Benefits of the modern, current-limiting fuse

2 Electrical safety

3 Fuseology and breaker basics

3.1 Fuseology

3.2 Breaker basics

4 Power system analysis

4.1 Fault current calculations

4.2 Selective coordination

4.3 Arc flash

5 Maintenance

5.1 Overview

5.2 Maintenance frequency and procedures

5.3 MCCB maintenance example

5.4 Circuit beaker testing considerations

5.5 OCPD servicing and maintenance

5.6 Testing knifeblade fuses

5.7 After an OCPD opens

5.8 Calibration decal on equipment

6 Electrical safe work practices

6.1 Overview

6.2 The electrical safety program

6.3 Shock hazard

6.4 Arc flash hazard

6.5 Maintenance

7 Equipment application/protection

6-13

7-1

Fuse sizing for building electrical systems up to 600 V 7-1

7.1 Appliances 7-4

7.2 Ballasts 7-4

6-1

6-1

6-2

6-5

5-3

5-3

5-3

6-1

Page

1-1

2-1

3-1

3-1

3-54

4-1

4-1

4-11

4-30

5-1

5-1

5-1

5-2

5-2

5-3

7.3 Batteries/battery charging

7.4 Busway

7.5 Capacitors

7.6 Circuit breakers

7.7 Conductors

7.8 Electric heat

7.9 Elevators

7.10 Generator protection

7.11 Ground fault protection

7.12 Industrial control panels

7.13 Industrial machinery

7.14 Motor/motor circuit protection

7.15 Panelboards and other fusible equipment

7.16 Solenoids

7.17 Switchboards

7.18 Transfer switches

7.19 Transformers

7.20 Uninterruptible Power Supplies (UPS)

7.21 Variable frequency drive and power

electronic device protection

7.22 Welders

7-159

7-162

7-48

7-55

7-76

7-76

7-139

7-143

7-144

7-147

7-152

7-158

7-25

7-44

7-44

7-48

7-5

7-5

7-7

7-7


Section Topic

8 Special applications

8.1 Data centers

Page

8-1

8-1

8.2 HVAC systems

8.3 Photovoltaic power generation

8.4 Fuse applications in hazardous locations

9 Appendix

9.1 Electrical formulas

8-10

8-12

8-20

9-1

9.2 Glossary of common electrical terms

9.3 Selective coordination inspection form

9-1

9-2

9-4

9.4 Interrupting rating and short-circuit

current rating inspection form

9.5 Content index related to the 2017 NEC

9.6 Content index related to the 2018 NFPA 70E

9-5

9-8

9-8

9.7 Content index related to the OSHA CFR 1910 9-8

9.8 Content index by subject 9-8


2 Electrical safety

The safety implications for electrical system design, installation, inspection, testing, maintenance, trouble-shooting and repair are significant.

In most cases, applicable enforceable codes and standards provide requirements that are the minimum for safety. In order to provide electrical systems and equipment well suited for the owner’s environment and needs, designers and installers must go beyond these minimum requirements to provide systems and equipment that are efficient and adequate for the present and to easily accommodate future changes. There may also be additional safety features that can be incorporated beyond the minimum required by Codes and standards.

In some cases, this publication on Selecting Protective Devices (SPD) for overcurrent protection applications presents solutions that merely meet the minimum Code requirements. For other cases, there are recommended solutions that provide superior safety, reliability and practicality.

Safety in regards to electrical equipment and systems has evolved to mean more than just protecting people and property against shock and fire hazards due to equipment failures. It includes considerations for electrical systems that deliver electrical power to loads vital for life safety and public safety, such as emergency systems and critical power operations systems. In these cases, the code and standard requirements focus on electrical system reliability and power continuity to the loads which are vital for life and public safety.

Another safety prospective is electrical safety related work practices.

Federal regulations mandated by OSHA require owners to provide a safe workplace. This includes workers who must work on or near electrical equipment and systems. The hazards of electrical shock, arc flash and arc blast can be eliminated or mitigated by good design practices, proper installation and maintenance procedures.

Selecting and using overcurrent protective devices (OCPDs) can have a profound impact on the level of safety an electrical system provides. To that end, this handbook covers many subjects associated with selecting overcurrent protective devices:

• Fuseology and breaker basics cover how overcurrent devices work, their varieties, ratings and operating characteristics that make them suitable for various applications.

• Power system analysis, covered in Section 4 examines fault currents, selective coordination and arc flash that directly related to electrical system safety and reliability. Specifically covered is this section are the National Electrical Code (NEC ® ) requirements related to these subjects.

• The impact maintenance, or the lack there of, has on ensuring overcurrent protective device operation and performance over time is covered in Section 5.

• Electrical safe work practices focuses on NFPA 70E and OSHA requirements, and how to ensure worker safety.

• Equipment application and protection deals with applying OCPDs for various applications, and why some are better suited for use with regards to operation, reliability, electrical safety and reducing or eliminating equipment damage.

Our section on special applications focuses on those considerations unique to protecting data centers, HVAC and photovoltaic systems, and fuses used in hazardous locations.

All the sections described above can stand on their own, but they also interrelate. Taken all together, they will provide a comprehensive understanding about selecting protective devices for reliability, code, standards and regulatory compliance, and, most importantly, safety for people, plant and equipment.

Fault currents release tremendous amounts of destructive energy and magnetic force. Selecting the correct overcurrent protective device can help ensure they do not result in a short-circuit event like the one shown above.

2


The power of space

The revolutionary

CUBEFuse™

Bussmann

™

series Low-Peak

delivers the smallest footprint compared to any Class J or RK fuse solution — requiring up to 70% less space when combined with its unique fuse holder or UL

Compact Circuit Protector.

®

98 Listed

™

compared to any Class J or RK fuse solution — requiring up to 70% less space when combined

Freeing up space is powerful. And the CUBEFuse does just that, while packing up to a 300 kA interrupting rating and enabling higher panel

SCCR. Plus, it features plug-in capability for easier installation.

CUBEFuse.com

The evolution continues. 2018.


3 Fuseology and breaker basics

Contents

3.1 Fuseology

3.2 Breaker basics

Section page

1

54

Sidebars

Sidebars in this handbook contain additional information or present related subject material

Look for these in the yellow boxes Section page

• Friemel’s Laws of Overcurrent Protection

• Factory calibrated replacements

• Single-phasing…does it have issues with fuses?

• The Bussmann series Low-Peak fuse system

• The NEC and “Fuse only ratings”

• Test conditions for a 300 kA interrupting rated fuse

• Rules for medium voltage current-limiting fuses

• R-Rated medium voltage fuses and motor circuits

11

14

• Selective coordination 30

• Exceptions in the code for applying supplemental OCPDs 39

• 10 Reasons why supplemental protectors are not allowed to protect branch circuits 41

44

45

3

4

8

9

3.1 Fuseology

Contents

3.1.1 Overcurrent protective device basics

3.1.1.2 Friemel’s Laws of Overcurrent Protection

Section page

2

2

3.1.2 How fuses work

3.1.2.1 Overcurrent protection, overloads and short-circuits

3.1.3 Construction

3.1.3.1 Non-time delay (fast-acting) fuses

3

3

4

5

3.1.3.2 Dual-element, time-delay fuses

3.1.4 Ratings

3.1.4.1 Volts

3.1.4.2 Amps

3.1.4.3 Interrupting rating

3.1.5 Performance characteristics

3.1.5.1 Current limitation/fuse current let-through curves

3.1.5.2 Current let-through curves

3.1.5.3 The OCPD’s role in electrical safety

3.1.5.4 Time-current characteristic curve (TCC)

3.1.5.5 Selective coordination

3.1.6 Fuse types and classes

3.1.6.1 Low voltage branch circuit fuses

3.1.6.2 Supplemental/application limited OCPDs

3.1.6.3 Medium voltage fuses

3.1.6.4 High speed fuses

3.1.6.5 Photovoltaic fuses

3.1.7 Fuseology summary —

the power of the modern, current-limiting fuse 53

29

30

31

31

39

44

48

51

11

16

16

20

26

9

10

6

9

First published in the 1920s, the Bussmann Fuseology handbook on fuses has promoted electrical safety by advancing the understanding of overcurrent protection.

3


Section 3 — Fuseology and breaker basics

3.1.1 Overcurrent protective device basics

Fuseology is the study of the fuse’s fundamental operating principles.

These include the ratings and operating characteristics that make the fuse an efficient overcurrent protective device (OCPD) as well as its construction that creates its unique leadership role in circuit protection.

In the simplest terms, a fuse is an overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of overcurrent through it.

A fuse is comprised of all the parts that form a unit that can perform these functions. It may or may not be the complete device necessary to connect it into an electrical circuit.

Electrical distribution systems can be simple or complicated.

Regardless, they cannot be absolutely fail-safe and are subject to destructive overcurrent events such as overloads, ground faults or short-circuits. Harsh environments, general deterioration, damage

(whether accidental or from natural causes), excessive electrical system expansion or overloading are common factors leading to overcurrent events. Reliable OCPDs like the fuse shown in Figure 3.1.1.a prevent or minimize costly damage to transformers, conductors, motors and many other components and loads that make up a complete power distribution system. Reliable circuit protection is also essential to electrical safety for personnel as well as avoiding severe monetary losses from power blackouts or prolonged facility downtime.

3.1.1.2 Friemel’s Laws of Overcurrent Protection and the

NEC

Law 1 — Interrupting rating

• OCPDs shall be applied with an interrupting rating equal to or greater than the maximum available fault current

• Code compliance: 110.9

Law 2 — Component protection

• OCPDs shall be selected and installed to clear a fault without extensive damage to electrical equipment and components

• Code compliance: 110.10

Law 3 — Selective coordination

• A properly engineered and installed electrical system will restrict outages to ONLY the nearest upstream OCPD for the full range of overcurrents and associated opening times, leaving the remainder of the system undisturbed and preserving service continuity

• Code compliance: 620.62, 645.27, 695.3(C)(3), 700.32, 701.27, 708.54

Friemel’s Law

1. Interrupting rating

2. Component protection

3. Selective

Coordination

Code section

110.9

110.10 and numerous sections involving SCCR

620.62, 645.27, 695.3,

700.32, 701.27, 708.54

How it’s achieved

Current limiting fuses, especially Low-Peak

Current limiting fuses, especially Low-Peak and high SCCR control panel products (CCP, PDFB, etc.)

Current limiting fuses, especially Low-Peak,

QSCP, Power Module,

CCPLP and engineering services

Figure 3.1.1.2.a

Friemel’s Laws of Overcurrent Protection in table form.

3-2

Figure 3.1.1.a

A regular and x-ray view of the Bussmann™ series Low-Peak™ LPN-RK dual-element fuse showing the overload and short-circuit links.

The fuse is a reliable and simple OCPD made in a variety of configurations that are fundamentally comprised of a “fusible” link or links encapsulated in a tube or housing that are connected to terminals.

The link’s electrical resistance is so low that it acts as a conductor until it encounters current levels above its amp rating. Then it melts and opens the circuit to protect conductors, components and loads.

Fuses for electrical distribution systems typically have three unique performance characteristics that address Friemel’s Laws of Overcurrent

Protection (see sidebar “Friemel’s Laws of Overcurrent Protection” for details):

• High interrupting rating to safely open very high fault currents without rupturing.

• Current limitation to “limit” fault currents to low values for optimum component and equipment protection, and help equipment achieve high short-circuit current ratings (SCCRs).

• Electrical system selective coordination for the full range of overcurrent events to help prevent needless “blackouts” caused by upstream OCPDs cascading open when applied with the correct amp rating ratios.



3.1.2 How fuses work

As an overcurrent protective device, the fuse acts as “electricity’s safety valve” by providing a weak link in the circuit path that, when properly applied, will melt and open the circuit to minimize or eliminate any damage that can be caused by an excessive flow of current. There are many fuse varieties and constructions, each developed to address an application’s need for a particular kind of overcurrent protection.

3.1.2.1 Overcurrent protection

An overcurrent is either an overload or a fault/short-circuit . The overload current is an excessive current flow relative to normal operating current, but still confined to the normal circuit paths provided by the conductors, components and loads. A fault (often referred to as a shortcircuit) flows outside the normal circuit paths.

Overloads

Overloads are most often up to 6 times the normal current level. They are usually caused by harmless, temporary in-rush currents that occur when motors start up or transformers are energized. Such overloads, or transients, are normal occurrences, and their brief duration is not harmful to circuit components as the associated temperature rise is minimal with no harmful affect. It’s important that OCPDs are properly sized and have the appropriate operating characteristics so they do not react to these temporary overloads or cause “nuisance openings.”

Persistent, non-temporary overloads can result from defective motors

(worn bearings) or when too many loads are on a single circuit and must not be permitted to last long enough to damage electrical system components such as conductors. This damage may eventually lead to severe fault events if the overload is not interrupted.

Due to the overload’s inherent low magnitude nature, removing them within seconds or even minutes will generally prevent thermal damage.

Faults

Faults, also referred to as short-circuits, differ from overloads as they can be hundreds to thousands of times greater than the normal operating current. A high level short-circuit may be up to 30 kA or 200 kA, and must be interrupted as quickly as possible to minimize the damage that can include:

• High magnetic forces that warp and distort busbars and associated bracing beyond repair

• Severe insulation damage

• Melting or vaporizing conductors

• Vaporizing metal, including buswork in electrical equipment

• Ionized gases

• Arcing fires

• Explosions

Note: “fault current” is a general term that’s used in this publication and includes ground fault, arcing fault and short-circuit currents.

Friemel’s Laws of Overcurrent Protection

Paul Friemel was known in the electrical industry as the Professor of Overcurrent Protection from the mid-1960s until his passing in

2015. As a licensed professional engineer, he presented seminars on electrical overcurrent protection for more than 40 years.

Among his many accolades, he was awarded the Outstanding

Educator Award by the IEEE as a Life Senior Member in 2004 and recognized as an Outstanding Professional Engineer by the St.

Louis Society of Professional Engineers in 2010. He served on the

St. Louis Electrical Code Review Committee for St. Louis County for over two decades, actively participated in the International

Association of Electrical Inspectors, and was a guest lecturer at

Washington University and the University of Missouri. He was a long standing member of the Electrical Board of Missouri and

Illinois where he served several terms on the board of directors.

Friemel taught the three

C

’s of overcurrent protection which are now known as Friemel’s Laws of Overcurrent Protection:

1. Interrupting rating (

C apacity)

2.

C omponent Protection

3. Selective

C oordination

An understanding of these three key electrical overcurrent protection principles will lead to a safe, reliable and code compliant electrical system.

3



3.1.3 Construction

The fuse is a highly efficient OCPD with a simple design based upon basic principles of physics to interrupt and limit overcurrent events.

Insight into their construction helps in understanding their application.

As shown in Figure 3.1.3.a, fuses have four parts common to most designs: case/housings (tube or cartridge), terminals (end blades or ferrules), fuse link (element), and arc-quenching filler. There are different fuse types that provide the operating characteristics required to address differing circuit protection needs.

End blades Tube/cartridge Fuse link

Arc-quenching filler

Figure 3.1.3.a

A dual-element, time-delay Low-Peak LPS-RK fuse showing the four common construction characteristics.

A fuse’s construction typically offers these benefits:

Physical rejection — Fuses have rejection features based on physical size or by a construction characteristic. Generally, a fuse of one class and case size cannot be installed in another fuse class and case size mounting. This ensures that the replacement fuse being installed will have the same voltage and interrupting ratings. A mild exception is that Class R fuses can be installed in Class H(K) fuse mountings for a protection upgrade, but, lower performing Class H(K) fuses cannot be installed in Class R fuse holders or blocks.

The Class J fuse is another example. Its size rejection prevents installing any other fuse type and virtually eliminates installing the wrong fuse type having different, potentially lower performance characteristics.

Unless a user replaces the holder, block or switch, it’s very difficult to install the wrong replacement fuse.

Enclosed, non-venting design — Fuses do not vent when they interrupt fault currents. All arcing is contained and extinguished inside the fuse body. This reduces the risk of metal vapors causing unnecessary damage to other components inside an enclosure. As part of their design, some mechanical OCPDs will vent when they interrupt fault currents. In addition, using fuses reduces cost by eliminating the need for guards or barriers to protect from the venting.

Enclosed, fixed, thermal design — Modern current-limiting fuses are constructed with an enclosed case, tube or body and have no moving parts when they open from an overcurrent. By operating on thermal energy principles of physics, the fuse improves electrical system reliability by not relying on springs, levers or latches that require periodic maintenance to ensure continued proper operation.

Factory calibrated replacements

There is no worry that a fuse may seize or not operate as intended as it’s factory calibrated with no need for field adjustment. This minimizes possible misapplication by eliminating the need to adjust or change device settings in the field. Engineers and specifiers can be certain the required overcurrent protection level is met and retained.

When fuses are replaced, system integrity is maintained by ensuring the same protection for many years to come.

Using thermal or electronic OCPDs in electrical systems to protect against overloads, such as motor starters, is beneficial as they can easily be reset by an operator or user (after the overload cause has been corrected) so that production can quickly resume. On the other hand, if a fault occurs, a qualified electrician must investigate and remedy the cause prior to resetting the device or replacing the fuse. If an unqualified person is allowed to simply reset a device, a safety hazard could occur if the fault is still on the line.

Fuses help in complying with federal law and other safety standards by eliminating the invitation for an operator to “reset” a device after a fault without investigating or remedying the cause. OSHA 1910.334(b)2 does not allow this practice and similar requirements are found in NFPA 70E Section 130. Fuses help prevent this from happening as a qualified person is much more likely to be involved in replacing the fuse. In addition, many maintenance personnel in industrial facilities prefer fuses for the simple reason that the troubleshooter is more likely to investigate the cause for the fuse opening rather than simply replacing the fuse.

3-4 Eaton.com/bussmannseries


3.1.3.1 Non-time-delay fuse

Depending upon the fuse’s amp rating, the “single-element” non-time delay fuse (often called a fast-acting fuse) may have one or more links.

They are electrically connected to the terminals (end blades or ferrules)

(see Figure 3.1.3.1.a) and enclosed in a case/housing (tube or cartridge) that contains an arc-quenching filler material that surrounds the link.

Many Bussmann series Limitron™ fuses are “single-element” fuses.

Under normal operation, when the fuse is applied at or near its amp rating, it simply functions as a conductor. If an overload occurs and persists for more than a short time interval, as illustrated in Figure

3.1.3.1.b, the link’s temperature eventually reaches a level that causes a restricted link segment (neck) to melt. As a result, a gap is formed and an electric arc established. As the arc causes the link to “burn back,” the gap becomes progressively larger. The electric arc’s resistance eventually reaches such a high level that it cannot be sustained and is extinguished with the help of the filler material’s arc-quenching properties (see Figure 3.1.3.1.c). The fuse will have then completely cut off all current flow in the circuit.

Present day single-element fuse designs respond very quickly to overcurrents with excellent fault current component protection.

However, temporary, harmless overloads (in-rush currents associated with inductive loads such as motors, transformers and solenoids) may cause nuisance openings unless these fuses are oversized. Therefore, they are best used in circuits not subject to heavy inrush currents.

Whereas overload normally falls between 1.35 and 6 times normal current, fault currents are quite high and the fuse may be subjected to fault currents of 30 kA or higher. The fuse’s current-limiting response to such high currents is extremely fast as its restricted link sections will simultaneously melt within a matter of two or three-thousandths of a second.

The multiple arcs’ high total resistance, together with the arc-quenching filler material, results in rapid arc suppression and clearing the fault (see

Figures 3.1.3.1.d and Figure 3.1.3.1.e). Fault current is cut off in less than a quarter-cycle, long before it can reach its full value (fuse operating in its current-limiting range).

Figure 3.1.3.1.a

Cutaway view of typical single-element fuse.

Figure 3.1.3.1.b

Under sustained overload, a section of the link melts and an arc is established.

Figure 3.1.3.1.c The “open” single-element fuse after opening a circuit overload.

Figure 3.1.3.1.d

When subjected to a fault current, several sections of the fuse link melt almost instantly.

Figure 3.1.3.1.e

The “open” single-element fuse after opening a shorted circuit.

Bussmann series UL Listed branch circuit fuses play a major role in industrial or commercial facilities by providing reliable, maximum protection to power systems. Their physical size or rejection features prevent replacing a fuse with one from another fuse class. This helps ensure the correct replacement fuse is always installed and the voltage and interrupting ratings remain the same. Shown are the case sizes for each fuse class relative to the size of a US quarter (left edge of image).

3

FRN-R — Class RK5, 250 V, 200 kA IR up to 600 A

LPN-RK — Class RK1, 250 V, 300 kA IR up to 600 A

LPJ — Class J, 600 V, 300 kA IR up to 600 A

LP-CC, FRQ-R, KTK-R — Class CC,

600 V, 200 kA IR up to 30 A

FRS-R — Class RK5, 600 V, 200 kA IR up to 600 A

LPS-RK — Class RK1, 600 V, 300 kA IR up to 600 A

JJN — Class T, 300 V, 200 kA IR up to 1200 A

JJS — Class T, 600 V, 200 kA IR up to 800 A



3.1.3.2 Dual-element, time-delay fuse

There are many advantages to using “dual-element,” time-delay fuses that feature an overload link and a short-circuit element connected in series — hence, the “dual-element” designation. Unlike single-element fuses, Bussmann series dual-element, time-delay fuses can be sized closer to the load to provide high performance for both short-circuit and overload protection.

The overload element provides the intentional “time-delay” that permits temporary overloads to harmlessly pass. This is the reason these fuses can be sized much closer to the load than non-time delay fuses that must be oversized to pass inrush currents and not produce nuisance openings.

The short-circuit element is there to handle fault currents, and when the fuse is operating in its current-limiting range, it’s not possible for the full available fault current to flow through the fuse — it’s a matter of physics. The small restricted link sections in the short-circuit element quickly vaporize with the filler material assisting in forcing the current to zero; and so it’s able to “limit” the fault current.

Anatomy of a dual-element, time-delay fuse

Arc-quenching filler

The overload element is held in tension to the shortcircuit element by the trigger spring until the fusing alloy fractures

Insulated end caps help prevent accidental contact with live blades

The trigger spring pulls the overload element away from the short-circuit element to open the fuse

Figure 3.1.3.2.c

Overload element operation.

Operation under persistent overload conditions as shown in Figure

3.1.3.2.c causes the trigger spring to fracture the calibrated fusing alloy and releases the “connector.” The insets show the overload element before and after it opens. The coiled spring pushes the connector from the short-circuit element and the circuit is interrupted.

Figure 3.1.3.2.a Typical Class R Low-Peak fuse.

The Low-Peak LPS-RK-100SP, 100 A, 600 V, Class RK1, dual-element fuse has excellent time-delay to withstand high inrush currents along with excellent current limitation and a 300 kA interrupting rating. Figure

3.1.3.2.a shows the fuse’s internal construction. The real fuse has a nontransparent tube and arc-quenching material that completely surrounds the element and fills the tube’s internal space.

Overload element

Short-circuit element with “neck” restrictions that arc and “burn back” under fault conditions.

Figure 3.1.3.2.b

“Dual-element” construction.

The true dual-element fuse has separate and distinct overload and shortcircuit elements connected in series as shown in Figure 3.1.3.2.b.

Figure 3.1.3.2.d

Short-circuit element operation under fault conditions.

For operation under fault conditions, the short-circuit element is designed with minimum metal in the restricted portions to greatly enhance the fuse’s current limitation and minimize the short-circuit current let-through. Fault current causes the short-circuit element’s restricted portions to quickly vaporize and commence arcing as shown in Figure 3.1.3.2.d. The arcs burn back the element, resulting in longer arcs that reduce the current with the arc-quenching filler helping to extinguish the arcs and force the current to zero.

Figure 3.1.3.2.e

Arc-quenching filler material helps suppress the arcing by melting and forming folgurite.

As a result of short-circuit operation, the special small granular, arcquenching material plays an important part in the interruption process as it assists in quenching the arcs by absorbing their thermal energy and melting to form an insulating barrier material called folgurite as shown in

Figure 3.1.3.2.e.



Advantages of dual-element over single element fuses

Bussmann series dual-element, time-delay fuses have six distinct advantages over single-element, non-time delay fuses:

1. Motor overload and short-circuit protection

When Bussmann series dual-element, time-delay fuses protect circuits with high inrush currents, such as motors, transformers and other inductive components, the Bussmann series Low-Peak and Fusetron™ dual-element, time-delay fuses can be sized close to full-load amps to maximize overcurrent protection. Sized properly, they will hold until normal, temporary overloads subside. For example, a 200 volt threephase 10 Hp motor with a 1.15 service factor has a 32.2 A full-load current rating (see Figure 3.1.3.2.f).

200 V

3-phase

M

10 Hp / 32.2 FLA

Low-Peak or Fusetron dual-element fuse

Figure 3.1.3.2.f

Motor circuit with a dual-element, time-delay fuse.

A 40 A, dual-element, time-delay fuse will protect the 32.2 A motor, compared to a much larger, 100 A, non-time delay, single-element fuse that would be necessary to withstand the temporary inrush current. If a harmful, sustained 200% overload occurred in the motor circuit, the

100 A, non-time delay, single-element fuse would never open and the motor would be damaged because it only provides ground fault and short-circuit protection. Additionally, the non-time delay fused circuit would require separate motor overload protection per the NEC. In contrast, the 40 A time-delay dual-element fuse provides the same ground fault and short-circuit protection, plus overload protection

(eliminating the code requirement for separate motor overload protection) (see Figure 3.1.3.2.g).

2. Permit using smaller and less costly switches

Bussmann series dual-element, time-delay fuses permit using smaller, space saving and less costly switches because a properly sized higher amp rated single-element fuse would make it necessary to use larger switches as the switch rating must be equal to or larger than the fuse’s amp rating. As a result, a larger switch may cost two or three times more than necessary rather than using a dual-element Bussmann series

Low-Peak or Fusetron fuse. (Note: should a larger switch already be installed for single-element fuses, smaller, properly sized dual-element fuses can also be installed for motor overload or back-up protection using fuse reducers. These permit installing a smaller case size fuse into a larger case size mounting.)

3. Better short-circuit component protection (current limitation)

Bussmann series dual-element, time-delay fuses provide better component protection than non-time delay, fast-acting fuses that must be oversized for circuits with in-rush or temporary overloads. Oversized non-time delay fuses respond slower to faults than smaller, time-delay fuses because the current will build up to a higher level before the fuse opens, thus the oversized fuse’s current limitation is less than a fuse with an amp rating that’s closer to the circuit’s normal full-load current.

4. Simplify/improve selective coordination for blackout prevention

The larger an upstream fuse is relative to a downstream fuse (feeder to branch), the less likely an overcurrent in the downstream circuit to cause both fuses to open (lack of selective coordination). To be selectively coordinated, Bussmann series Low-Peak fuses require only a 2:1 amp rating ratio. Contrast this to a fast-acting, non-time delay fuse that would require at least a 3:1 amp rating ratio between a large upstream, lineside Low-Peak time-delay fuse and the downstream, loadside

Bussmann series Limitron fuse.

As shown in Figure 3.1.3.2.h, closely sized Bussmann series Low-Peak dual-element fuses in the branch circuit for motor overload protection provides a large difference in the amp ratings (3.75:1 ratio) between the feeder and branch fuses, compared to the single-element, nontime delay Limitron fuse (1.67:1 ratio) with the 90 A Limitron fuse not conforming to the 3:1 published ratio needed for selective coordination.

3

Fuse and switch sizing for 10 Hp motor (200 V, 3 Ø, 32.2 FLA)

Fuse type Max fuse (A) Required switch (A)

Dual-element, time-delay

Fusetron FRS-R or FRN-R

40* 60

Single element non-time delay

Limitron

100 † 100

* Per NEC 430.32

† Per NEC 430.52

In normal installations, Bussmann series dual-element fuses sized for motor-running, overload protection, provide better fault protection plus a high degree of back up protection against motor burnout from overload or single-phasing should other overload protective devices fail (see sidebar “Single-phasing…are fuses an issue?” on page 3-8). If thermal overloads, relays or contacts should fail to operate, the properly sized dual-element fuse will act independently to provide “back-up” protection for the motor.

When secondary single-phasing occurs, current in the remaining phases increases from 173% to 200% of the motor’s rated full-load current.

When primary single-phasing occurs, unbalanced voltages occurring in the motor circuit also cause excessive current. Dual-element fuses sized for motor overload protection can help protect against overload damage caused by single-phasing.

200 V

3-phase

Fusetron dual element 40 A fuse

60 A switch

M

10 Hp / 32.2 FLA

208 V

3-phase

Selective coordination

150 A: 40 A = 3.75:1 adequate

(minimum ratio need only be 2:1)

Low-Peak dual element 150 A fuse

Low-Peak dual element 40 A fuse

No selective coordination

150 A:90 A = 1.67:1 inadequate

(minimum ratio must be at least 3:1)

M 10 Hp / 32.2 FLA

M 10 Hp / 32.2 FLA

208 V

3-phase

Limitron non-time delay 90 A fuse

Low-Peak or Limitron dual element 150 A fuse

Figure 3.1.3.2.h

Using Low-Peak fuses permits closer fuse sizing to load for better protection and using smaller, less costly switches while retaining selective coordination to help prevent blackouts.

200 V

3-phase

Limitron non-time delay 100 A fuse

100 A switch

M

10 Hp / 32.2 FLA

Figure 3.1.3.2.g

Closer fuse sizing to load can result in using smaller, less costly switches.



Single-phasing…are fuses an issue?

Single-phasing conditions on three-phase motor circuits can create unbalanced voltage and/or overcurrent conditions that, if allowed to persist, will damage motors. In modern motor circuits, properly applied fuses and overload protective devices provide a high degree of single-phasing protection. Major considerations of singlephasing include the following.

• Single-phasing cannot be eliminated, there are numerous causes including:

•

The utility loses one phase

•

Overheated conductor termination

•

Disconnect does not “make” one pole

•

Controller contact burns open

• Prior to 1971, single-phasing plagued three-phase motors installed per the NEC because overload protection was only required on two phases. The 1971 NEC remedied this problem by adding the requirement for three-phase-motor circuits to have motor overload protection on all three phases. This provided protection for the worst condition seen when a utility loses a phase on the transformer primary.

• Three properly sized (to the actual motor running current) motor overload protective devices, now required in NEC 430.37, provide sufficient protection

• Most electronic overloads, soft-start controllers, and drives have options to sense voltage imbalance to provide single-phasing protection

• Although circuit breakers do not cause single-phasing, unless one pole’s contact does not “make,” they do not provide singlephasing protection

• Fuses provide excellent short-circuit, current limitation to protect motor circuit starters and conductors, including Type 2

“No-Damage” protection when properly sized

• Fusible motor control centers benefit from the fast clearing time of a current-limiting fuse that also helps to reduce incident energy levels, mitigate arc flash hazards and protect workers

5. Better motor protection in elevated ambient temperatures

Before selecting a fuse or any OCPD, the application’s ambient temperature should be known so the proper amp rating can be determined through what’s called “derating.” Like all fuses, the dual-element fuse should be derated based on increased ambient temperatures. The fuse derating curves closely parallel motor derating curves in elevated ambient temperatures. Figure 3.1.3.2.i illustrates the affect ambient temperature has on Bussmann series Fusetron and Low-

Peak dual-element fuse operating characteristics. This unique feature allows for optimum motor protection, even in high temperatures. For derating affects of single-element or non-time delay fuses, see Figure

3.1.3.2.j.

150

140

130

120

110

100

90

80

70

60

50

Affect on opening time

Affect on carrying capacity rating

40

30

-76 -40 -4 32 68 104 140 176 212

(-60) (-40) (-20) (0) (20) (40) (60) (80) (100)

Ambient °F (°C)

Figure 3.1.3.2.i

Ambient temperature dual-element fuse derating curve.

120

110

100

90

-76 -40 -4 32 68 104 140 176 212

(-60) (-40) (-20) (0) (20) (40) (60) (80) (100)

Ambient °F (°C)

Figure 3.1.3.2.j Ambient temperature single-element derating curve.

6. Provide Type 2 “No Damage” motor starter protection when properly sized

Fuses help reduce downtime when Type 2 “No Damage” (versus Type

1) protection is specified with properly sized fuses. Type 2 protection ensures that no damage, within specified limits, occurs to the contactor or overload relay. With Type 2 protection, light contact welding is allowed, but must be easily separable allowing equipment to be placed back into service without having to replace or re-calibrate any components. A current-limiting device is necessary to achieve Type 2, often requiring Class CC, CF, J, or RK1 fuses. In this scenario, when the branch-circuit fuse protects the motor circuit, the starter does not need replacing and downtime is reduced or eliminated.



The Bussmann series Low-Peak fuse system

Feeder for MCC

KRP-C-_SP

Branch for large motor

KRP-C-_SP

Feeder for

MLO lighting panels

LPJ-_SP

LPS-RK-_SP

M

Reduced voltage starter for large motor

LP-CC

Quik-Spec Coordination Panelboard with Low-Peak CUBEFuse

M

Branch for resistance load

KRP-C-_SP

LPJ-_SPI LPS-RK-_SP

LP1

LPJ-_SP LPS-RK-_SP

Resistance load

Specifying the Low-Peak fuse family throughout a building results in:

• Built-in fuse size and class rejection for greater safety

• Selective coordination with a minimum 2:1 amp ratio

• Maximum current-limiting protection for distribution equipment

• Type 2 “No Damage” motor starter protection when properly sized

• Reduced inventory

• Up to 300 kA interrupting ratings

• Arc flash hazard mitigation

3.1.4 Ratings

All fuses have three basic ratings:

• Voltage (AC, DC or both)

• Amp

• Interrupting

Understanding these three ratings, their significance and how they apply to circuit protection is crucial to specifying the correct, and in many cases, optimal circuit protection.

3.1.4.1 Voltage rating

One aspect of proper OCPD application requires the OCPD’s voltage rating be equal to or greater than the system voltage. When an OCPD is applied beyond its voltage rating, there may not be any initial indications that anything is wrong, but when it attempts to interrupt an overcurrent, adverse consequences can result and it may self-destruct in an unsafe manner. There are two OCPD voltage rating types: straight voltage rated and slash voltage rated.

Straight rated devices

A straight voltage rated OCPD can be installed in any electrical system regardless of the grounding system.

All fuses are straight voltage rated and their proper application is straightforward (i.e., 600 V, 480 V, 240 V). These OCPDs have been evaluated for proper performance with full phase-to-phase voltage used during testing, listing and marking.

The fuse’s voltage rating is its ability to open under an overcurrent condition while suppressing the internal arcing that occurs after the link melts and an arc is produced. If a fuse is applied with a voltage rating lower than the circuit voltage, arc suppression will be impaired, and, under some conditions, it may not safely clear the overcurrent.

The fuse’s voltage rating must be at least equal to or greater than the circuit voltage. For example, a 600 V rated fuse can be used in a 208 V circuit, but a 250 V rated fuse cannot be used in a 480 V circuit.

Most low voltage power distribution fuses have 250 V or 600 V ratings

(other ratings include 125 V, 300 V, and 480 V). Bussmann series

Low-Peak LPJ (Class J) fuses are rated at 600 V and can be used on any 600 V or less system, whether it’s solidly grounded, ungrounded, impedance grounded or corner grounded Delta.

A straight rated OCPD (whether a fuse or circuit breaker) that protects a single pole can be used to protect single-phase, line-to-neutral loads when supplied from a three-phase, solidly grounded circuit. For example, a 300 V rated fuse can be used to protect single-phase, lineto-neutral loads when supplied from a three-phase, solidly grounded,

480/277 V circuit, where the single-phase, line-to-neutral voltage is 277

V. This is allowed in this application because a 300 V fuse will not have to interrupt a voltage greater than its 300 V rating.

Slash rated devices

Slash voltage rated OCPDs have limitations imposed upon them that straight rated voltage OCPDs do not. Multiple-pole, mechanical OCPDs with a slash voltage rating, such as circuit breakers, self-protected starters and manual motor controllers, are limited in their application and require extra evaluation for use.

The slash rating can be broken down into its higher and lower numbers and are understood as follows:

• The lower rating number pertains to overcurrents at line-to-ground voltages, intended to be cleared by one pole of the device.

• The higher rating number pertains to overcurrents at line-to-line voltages, intended to be cleared by two or three poles of the device.

3



The proper slash rated circuit breaker application is such that:

• The line-to-ground voltage does not exceed the device’s lower voltage rating

• The line-to-line voltage (between any two conductors) does not exceed the device’s higher voltage rating. (Reference NEC Section

240.85.)

Understanding the higher and lower ratings is important as slash rated device misapplication can result in it being applied outside its voltage rating with dire consequences should the device be called upon to interrupt overcurrents.

Slash voltage rated circuit breakers are not intended to open line-to-line

(phase-to-phase) voltages across only one pole. Where it is possible for line-to-line voltage to appear across only one pole, a straight rated

OCPD must be used. For example, a 480 V circuit breaker may have to open an overcurrent at 480 V with only one pole, such as might occur when Phase A goes to ground on a 480 V, B-phase, corner grounded

Delta system.

Slash voltage rated circuit breakers can only be used on solidly grounded power distribution systems. The proper application of molded case circuit breakers on three-phase systems, other than solidly grounded Wye, particularly on corner grounded Delta systems, must consider the circuit breakers’ individual pole-interrupting capability. (Ref.

NEC Section 240.85).

Slash rated devices cannot be used on the following systems (Ref. NEC

Section 430.83(E)):

• Impedance-grounded

• Ungrounded Wye systems

• Ungrounded Delta systems

• Corner-grounded Delta systems

Other slash rated devices have these same limitations. They include, but are not limited to:

• Manual motor controllers — UL 508

• Self-protected Type E combination starters — UL 508

• Supplementary protectors — UL 1077. These look like small circuit breakers and are sometimes referred to as a mini-breaker. However, these devices are not rated for branch circuit protection and cannot be used where branch circuit protection is required.

Product standards require slash voltage rated devices to be marked with their rating such as 480 Y/277 V. If a machine or equipment panel utilizes a slash voltage rated device, it’s recommended that the equipment nameplate or label designate the slash voltage rating as the equipment voltage rating. UL 508A industrial control panels require the electrical panel voltage marking to be slash-rated if one or more devices in the panel are slash voltage rated.

3.1.4.2 Amp rating

In general, the OCPD amp rating indicates the amount of current that can flow through the device without causing it to open. Standard amp ratings for fuses and inverse time circuit breakers are shown in the

Figure 3.1.4.2.a below (Reference NEC Section 240.6).

Understanding this NEC table is important. NEC Section 240.6 is referenced whenever the requirements specify “... the next standard overcurrent device size shall be used...” The next standard OCPD size is not based on a manufacturer’s literature, but always obtained from NEC

240.6.

Fuse only ratings

1 3 6

Fuse and circuit breaker ratings

15 20 25

40

80

150

300

600

1600

5000

45

90

175

350

700

2000

6000

50

100

200

400

800

2500

—

10

30

60

110

225

450

1000

3000

—

601

35

70

125

250

500

1200

4000

—

Figure 3.1.4.2.a NEC Table 240.6, standard amp ratings. Also see sidebar on The NEC and “Fuses only ratings.”

In selecting the fuse’s amp rating, consideration must be given to the load type and code requirements. The fuse amp rating normally should not exceed the circuit’s conductor current carrying capacity that’s determined by ampacity adjustment factors covering how and where it’s routed or other NEC related adjustment areas. For the most part, if a conductor’s current carrying capacity is 20 A, a 20 A fuse is the largest that should be used to protect it.

There are specific circumstances in which the OCPD amp rating is permitted to be greater than the circuit’s current carrying capacity, with motor circuits a common exception. Dual element time-delay fuses are generally permitted to be sized up to 175% and non-time delay fuses up to 300% of the motor’s full-load amps. As a rule, the fuse amp rating and switch combination should be selected at 125% of the continuous motor load current (this usually corresponds to the circuit capacity, which is also selected at 125% of the load current). There are exceptions, such as when the fuse-switch combination is approved for continuous operation at 100% of its rating.

Figure 3.1.4.2.b Fires can result if the correct OCPD amp rating is not applied.

The photograph in Figure 3.1.4.2.b vividly illustrates the impact overcurrents have on electrical components when the OCPD’s amp rating is not sized to the component’s rating.



The NEC and “Fuse only ratings”

As part of the 1978 NEC, the “fuse only ratings” shown in Table

3.1.4.3.b were added because public inputs focused on protecting motors and the desire to provide the smallest fuse amp rating possible for effective short-circuit protection. There were two inputs accepted.

One public input addressed fuses rated less than 15 amps with the submitter noting in the substantiation that these fuses are often required on motor branch circuits to provide short-circuit and ground-fault protection. The substantiation for these fuse ratings came from test results showing fuses rated 1, 3, 6 and 10 amps provided the intended protection in motor branch circuits for motors with full load currents less than 3.75 amps (3.75 x

400% = 15). These ratings are also commonly shown on control manufacturers’ overload relay tables. Overload relay elements for very small motors, with small full load motor currents, have such a high resistance that a bolted fault at the controller load terminals produces a less than 15 amp fault current, regardless of the available current at the line terminals. An overcurrent protective device rated or set for 15 amps is unable to provide the shortcircuit or ground fault protection required by Section 110.10 in such circuits.

The other public input added the 601 A Class L fuse for motor protection as the 601 A fuse size was not listed in this table, and the next standard size up that would be permitted would be a 700

A fuse. When the NEC called for the next standard OCPD size to be permitted, and when the calculated amp rating is greater than

500 A, only a 700 A Class L fuse would have been permitted for the installation.

Before the 1978 NEC, the 1975 NEC cycle placed the 601 A fuse as an exception to Section 430-52 (the requirements for rating or setting for individual motor circuits). This exception is still a part of the NEC as Exception “d” to this requirement and states,

“The rating of a fuse of 601—6000 ampere classification shall be permitted to be increased but shall in no case exceed 300 percent of the full-load current.”

The public input pointed out in the substantiation that “since the intent of Table 430-52 and Section 430-52 is to encourage closer short-circuit protection, it seems prudent to encourage availability and use of 601 amp fuses in combination with motor controllers that can accept a Class L fuse.

The submitter recognized that inverse time circuit breakers are not subjected to the same limitation that fuses are when related to the fuse mounting means. For this reason, a distinction between 600 and 601 amps in circuit breakers has no purpose, and thus simply adding 601 A to the list for all OCPDs was not supported by the code panel.

3.1.4.3 Interrupting rating

An OCPD must be able to safely interrupt destructive fault current energy. If a fault current exceeds a level beyond the OCPD’s capability, it may rupture, causing damage and posing a safety hazard. The rating that defines OCPD’s capacity to maintain its integrity when reacting to fault currents is its interrupting rating. It’s important when applying a fuse or circuit breaker, to use one that can safely interrupt the largest potential fault currents. Most modern, current-limiting fuses have a 200 kA or

300 kA interrupting rating and can be used in just about any system without fear of misapplication. NEC 110.9 requires equipment intended to break current at fault levels to have an interrupting rating sufficient for the available fault current at point of application.

The fuse interrupting rating is not dependent on a particular voltage when applied within its rating. For example, a 600 Vac rated LPJ fuse has a UL Listed 300 kA interrupting rating for any voltage up to 600 Vac.

Whether for the initial installation or system updates, a fusible system can maintain a sufficient interrupting rating throughout its life. There is little need for additional fault current calculation or worry that a fuse will be misapplied due to an improper interrupting rating. Nor is a shortcircuit study needed when applying Bussmann series Low-Peak fuses for selective coordination, so meeting NEC 110.9 requirements is easy.

Additionally, high interrupting ratings help equipment achieve a high short-circuit current rating that may be limited by the installed OCPD’s low interrupting rating. Finally, fuses provide peace of mind as the interrupting rating is always at least equal to or, in many cases, greater than the available fault current at the line terminals.

When applying a fuse or circuit breaker, as shown in Figure 3.1.4.3.a, the chosen OCPD must be able to safely interrupt the largest available fault currents at its line terminals (Ref. NEC Section 110.9).

Fuse must have an interrupting rating of at least 50 kA

Available fault current = 50 kA

Circuit breaker must have the capability to interrupt at least 50 kA

Figure 3.1.4.3.a The interrupting rating of the fuse or circuit breaker must be greater than the calculated maximum available fault current at its line terminals.

As with other ratings, applying an OCPD in an AC or DC system matters because interrupting ratings for alternating current (AC) will generally be different from direct current (DC), with AC interrupting ratings being higher in general. The primary difference between AC and DC interrupting ratings is alternating current has a zero voltage potential that happens 60 times a second (60 Hz) when its sine wave passes through zero. It’s when the voltage potential is at zero that arc suppression is easiest to achieve. On the other hand, there aren’t any “zero voltages” in DC, so the arc that’s generated never experiences “zero volts” and, as such, is more difficult to suppress.

Products must be rated for the application within which they are placed.

The fuse’s simplicity limits areas of misapplication with this regard, as most modern current-limiting fuses have an AC interrupting rating of

200 kA or 300 kA. For example, Bussmann series Low-Peak* fuses are

UL Listed at 300 kA IR at 600 Vac, allowing them to be safely applied on any 600 V or less system and still provide a 300 kA interrupting rating.

* Does not include LP-CC Class CC fuses which are 200 kA.

Table 3.1.4.3.b on the following page illustrates the highest AC and DC interrupting ratings available for Bussmann series low voltage branch circuit fuses by fuse class.


3


Bussmann series fuse voltage and interrupting ratings

Bussmann series product UL Class Catalog symbol Amps

CC LP-CC Up to 30

Low-Peak

Fast-acting

CUBEFuse

CF*

J

L

RK1

CF*

TCF

LPJ

KRP-C

LPN-RK

LPS-RK

FCF

Up to 100

Up to 600

601 to 6000

Up to 600

Up to 600

Up to 100

FRN-R Up to 600

Fusetron RK5

Limitron

CC

J

L

RK1

T

FRS-R

FNQ-R

KTK-R

JKS

KLU

KTU

KTN-R

KTS-R

JJN

JJS

Up to 600

Up to 30

Up to 30

Up to 600

601 to 4000

601 to 6000

Up to 600

Up to 600

Up to 1200

Up to 800

General purpose

G

H(K)

SC

NON

NOS

Up to 60

250

600

600

600

600

600

600

600

250

600

300

600

600 (1/2-20 A)

480 (25-60 A)

Up to 600

Vac

600

600

600

600

250

600

600

250

Up to 600

* UL Class CF fuses have UL Class J electrical performance.

Table 3.1.4.3.b

Interrupting ratings of Bussmann series UL branch circuit fuses.

50

—

Interrupting rating examples

Figure 3.1.4.3.c shows four different scenarios involving an OCPD with an interrupting rating of 10 kA and varying levels of fault currents that they will be called upon to interrupt. This illustrates the importance of knowing the available fault current and the advantage of applying a fuse with 100 kA, 200 kA or higher interrupting rating.

In the first three scenarios, the circuit current conditions are within the OCPD’s safe operating capabilities. However, the fourth instance involves an OCPD misapplication. A fault on the device’s loadside resulted in a 50 kA fault current that’s well above the OCPD’s interrupting rating. This resulted in a violent rupture and possible damage to equipment or injury to personnel. Using high interrupting rated fuses

(typically rated at 200 kA or 300 kA) would prevent this potentially dangerous situation.

The examples on the next page are from fault current tests. They demonstrate the destructive power associated with fault currents.

Circuit with overcurrent protective device

• Current rating = 100 A

• interrupting rating = 10 kA

OCPD

100 amps

80

AMMETER

200

10,000

X

LOAD

Circuit condition

Normal

Applicatio n and action of protective device

Proper

Overload current greater than device’s am p rating

Proper, safe interruption of overload current

Fault curren t within device interruptin g rating

Proper, safe interruption of fault current

50,000

X Fault curren t exceeds device interrupting rating

Improper: explosion or rupture could result

Figure 3.1.4.3.c

Interrupting ratings are important for protecting against short-circuits.


20

20

—

—

—

—

100

—

20

100

—

10

IR DC (kA)

20

100

100

100

100

100

50

20


Misapplied circuit breaker

Figure 3.1.4.3.d is a series of images depicting a test conducted on a

480 V circuit breaker with a 14 kA interrupting rating and a test circuit capable of delivering fault current of 50 kA at 480 V. The dramatic results are shown below. This video is available through the QR code below.

Misapplied general purpose fuses

Figure 3.1.4.3.e is a series of images depicting the same test circuit as the previous test with a pair of 600 V, general purpose fuses having a 10 kA interrupting rating. Notice in this test (as well as the circuit breaker test), the large destructive force that was released. Misapplying OCPDs in this manner is a serious safety hazard as shrapnel and molten metal could strike electricians or maintenance personnel, or anyone who happens to be nearby. This video is available through the QR code below.

1 2 1 2

3

3

Figure 3.1.4.3.d

A circuit breaker severely misapplied beyond its interrupting rating.

4 3

Figure 3.1.4.3.e

A fuse severely misapplied beyond its interrupting rating.

4



Properly applied Low-Peak fuses

Figure 3.1.4.3.f is a series of images depicting the same test circuit as the previous two tests (50 kA available at 480 V) only this time the test was performed with modern, Bussmann series Low-Peak currentlimiting fuses with a 300 kA interrupting rating. Notice that the fault was contained and cleared without violence. This video is available through the QR code below.

1

3

2

Figure 3.1.4.3.f

A fuse applied within its interrupting rating.

As depicted in Figure 3.1.4.3.g, it becomes necessary to determine the available fault currents at each OCPD location. The fault currents in an electrical system can be easily calculated if sufficient information is known. The advantage of high 200 kA or 300 kA fuse interrupting ratings are that they can be used to eliminate the need for fault current calculations — 200 kA and 300 kA will exceed available fault currents for virtually all power distribution system.

Test conditions for a 300 kA interrupting rated fuse

The NEC defines interrupting rating as the highest current at rated voltage that an overcurrent protective device can safely interrupt under standard test conditions. The phrase “under standard test conditions” considers the importance of understanding how the overcurrent protective device is tested in order to ensure it is properly applied.

The UL 248 Standard defines the branch circuit fuse test configuration to establish the necessary performance requirements for interrupting ratings. The process to achieve a UL Listed 300 kA interrupting rating is:

• To confirm that the interrupting capacity is not less than the interrupting rating, the test circuit is established without any additional conductor lengths in the test circuit configuration.

The fuse is efficient at interrupting very high fault currents and does not require any help from additional impedances in the configuration.

• The test circuit is calibrated to have at least 300 kA fault current at the rated fuse voltage. During the test circuit calibration, a busbar is used in place of the fuse to verify the 300 kA fault current level.

• The busbar is then replaced with a fuse and the test conducted.

If the fuse passes the test, the fuse can be marked with a

300 kA interrupting rating.

This test procedure ensures the fuse has an interrupting rating equal to or greater than the fault current available at its line terminals for both three-phase bolted faults and for one or more phase-to-ground faults. Per UL/CSA/ANCE 248 Fuse Standards, fuses are tested and evaluated as single-pole devices. Although most electrical systems are designed with OCPDs having adequate three-phase interrupting ratings, the single/individual pole interrupting capabilities are easily overlooked. Because the fuse interrupting rating is all encompassing, there is no need for concern about single-pole interrupting capabilities.

300 kA Low-Peak fuses

Bussmann series Low-Peak fuses (excluding Class CC) are the only fuses tested and Listed by UL to 300 kA IR. This high interrupting rating is capable of safely interrupting virtually any available fault current level to be encountered in a 600 V or less system. The

300 kA IR provides assurance that when a properly sized Low-

Peak fuse is installed, the system is covered for any worst case overcurrent event.



Electrical system short-circuit current levels and appropriate protection

Knowing the available fault current levels throughout an electrical system helps determine the necessary OCPD interrupting ratings.

Figure 3.1.4.3.g is a one-line diagram showing available fault current levels at different points in an electrical system.

Resistor keeps first fault current low:

5 amps or so.

A

277 V 277 V 480 V

C

277 V B

480 V

Service panel

X

75 kA

A

B

C

Branch panel

First fault to steel conduit

75 kA

25 kA

X

X

X

30 kA

X

15 kA

Figure 3.1.4.3.i

The grounding resistor’s impedance keeps the ground fault current low.

Figure 3.1.4.3.j illustrates the system should a second fault occur before the first fault can be addressed. The fault is essentially line-to-line with the conductors and ground path impedance, and the fuse must interrupt this second fault. Since a fuse’s interrupting rating is the same as its single-pole interrupting capacity, fuses with 200 kA or 300 kA interrupting rating can be applied without further analysis for single-pole interrupting capabilities.

Single-pole must interrupt fault current: fuses’s marked IR is its single-pole IR.

A simple solution

Service panel

Branch panel

3

Figure 3.1.4.3.g

Determining the fault current at each OCPD is necessary to ensure proper device application.

Fuses can be applied on single-phase or three-phase circuits without any concern for single-pole interrupting capabilities. There is no need to perform special calculations due to the grounding system utilized. All

Bussmann series Low-Peak LPJ, KRP-C, LPS-RK and LPN-RK fuses have

UL Listed 300 kA single-pole interrupting ratings.

This is a simple solution to ensure adequate interrupting ratings for present and future systems regardless of the grounding scheme.

Figure 3.1.4.3.h illustrates the fusible, high impedance grounded system allowing the fuse performance during fault conditions to be reviewed.

Resistor

Service panel

Branch panel

A

277 V 277 V 480 V

C

277 V B

480 V

A

B

C

First fault to steel conduit

A

277 V 277 V 480 V

C

277 V B

480 V

A

B

C

Figure 3.1.4.3.h

Fusible high impedance grounded system.

Figure 3.1.4.3.i illustrates what occurs during the first system fault which is limited by the grounding resistor’s impedance that keeps the ground fault current in the low range of 5 amps. Here the fuse does not open as expected and designed.

High fault current value because ground resistor

is no longer in path

Second fault to enclosure

Figure 3.1.4.3.j

The fault is essentially line-to-line with the conductors and ground path impedance.

As fault current levels increase with the continued growth in electrical power generation, so too has the need for OCPDs with higher interrupting ratings.

Utilities desire to obtain greater efficiencies, lower energy costs and improved voltage regulation by installing lower impedance transformers with larger kVA ratings that produce higher available fault currents. To meet this challenge, OCPDs that only interrupt moderate fault current levels are being replaced with fuses able to interrupt fault currents up to 300 kA.

Utilities are increasing available fault current through installing lower impedance and larger kVA transformers.



3-16

3.1.5 Performance characteristics

3.1.5.1 Current limitation

Current limitation is a function of how quickly the fuse can react to a fault and clear it before the fault current can build up to destructive magnitudes.

NEC 240.2 defines a current-limiting device as:

“ Current-Limiting Overcurrent Protective Device: A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.”

Fuses that limit the let-through current to downstream components provide protection from thermal and magnetic forces while providing numerous benefits.

• Current-limiting fuses enhance workplace safety by reducing the incident energy personnel are exposed to under fault conditions. Arc flash hazards may be greatly reduced when compared to using a noncurrent limiting OCPD, especially at 600 amps and below.

• Fuses can protect components and equipment from extreme thermal and magnetic forces by clearing the fault current within the first half or quarter cycle. Conductors, busway, motor starters, switches and other devices can be well protected with current-limiting fuses. In addition, fuses, when properly sized, help comply with NEC 110.10 by protecting equipment and components from extensive damage.

• Fuse current limitation helps equipment achieve a high SCCR. Branch circuit components inside industrial control panels and machines,

HVAC control panels, and other UL 508A Listed equipment can be protected by current-limiting fuses in the feeder circuit. The low peak let-through can help raise the overall equipment short-circuit current rating — even with lower rated devices in the branch circuit — so long as the peak let-through current is less than the branch components’

SCCR and the branch circuit overcurrent protective device’s interrupting rating is sufficient.

• Current limitation is a key part of selective coordination and currentlimiting fuses save the engineer or designer time and money by reducing the need for a selective coordination study. As long as a minimum amp ratio is maintained between upstream and downstream fuses (2:1 for Bussmann series Low-Peak fuses), selective coordination is achieved and unnecessary power loss to upstream circuits is prevented for any fault current up to the interrupting rating of the fuse. See “Bussmann series fuse selectivity ratios” (Table 3.1.5.5.b on page 3-30).

Figure 3.1.5.1.a shows the minimum Low-Peak fuse amp rating ratios required for “selective coordination” (discrimination) between upstream and downstream fuses.

X

LPS-RK-400SP

LPS-RK-200SP

KRP-C-800SP

2:1 (or more)

2:1 (or more)

Figure 3.1.5.1.a

Low-Peak fuses only need a 2:1 amp ratio for selective coordination.

It’s a simple matter to selectively coordinate modern design fuses. By maintaining a minimum fuse amp rating ratio between an upstream and downstream fuse, selective coordination is achieved. Minimum selectivity ratios for Bussmann series fuses can be found on page 3-30.

Most electrical distribution systems today can deliver very high fault currents, some in excess of 200 kA. Many circuit components have relatively low (a few thousand amps) short-circuit current withstand capabilities.

If the components can’t withstand these high fault currents, they can be easily damaged or destroyed. The fuse’s current-limiting ability allows components with low SCCRs to be specified despite high available fault currents.

Protecting electrical system components from fault currents is critical when selecting OCPDs. The engineer or specifier should consider the electrical circuit components’ SCCR, such as wire, bus or motor starters, and whether proper circuit protection will improve reliability and reduce the possibility of injury. Electrical systems can be destroyed if

OCPDs do not limit the fault current to within the system’s component

SCCRs. Merely matching the component amp rating with the protective device amp rating does not ensure component protection under fault conditions.

Current limitation is illustrated in Figures 3.1.5.1.b and 3.1.5.1.c. When not protected by a current-limiting OCPD, the prospective available short-circuit current is shown in Figure 3.1.5.1.b. A non-current-limiting

OCPD permits fault current to build up to its full value and lets through an immense amount of destructive current, heat and magnetic energy before opening. Some OCPDs will permit short-circuit currents to flow for more than 1 cycle.

100 kA peak

Areas within waveform loops represent destructive energy impressed upon circuit components

Shortcircuit initiation

Normal load current

Time

Non-current limiting OCPD opens short-circuit in about 1 cycle

Figure 3.1.5.1.b

Fault current can become great in the first full cycle.

The waveform in Figure 3.1.5.1.c demonstrates this same short-circuit when the fuse operates within its current-limiting range. A currentlimiting fuse has such a fast response speed that it cuts off the current long before it can build up to its full peak value.

100 kA peak

10 kA peak

Normal load current

Shortcircuit initiation

Time

Current limiting fuse opens and clears short-circuit in less than 1/2 cycle

Figure 3.1.5.1.c

Cutting off the fault current in the first 1/4 cycle greatly reduces its magnitude.

The shaded area under the curve represents energy being dissipated in the circuit with both magnetic forces and thermal energy being directly proportional to the square of the current, making it important to limit the short-circuit current to as small a value as possible. The maximum magnetic forces vary as the square of the “peak” current and thermal energy varies as the square of the “RMS” current.

In the first major fault current cycle loop depicted by the waveform in Figure 3.1.5.1.b, a non-current limiting OCPD would let through approximately 100 times as much destructive energy as the currentlimiting fuse would — (100 kA ÷ 10 kA) 2 = 100.



The current-limiting fuse in Figure 3.1.5.1.c opens and clears the fault within the first 1/2 cycle and limits the let-through energy to a fraction of the system’s available short-circuit current.

This performance is an important consideration when selecting OCPDs.

Most fuses are current-limiting and greatly reduce a fault current’s destructive peak and duration to protect downstream equipment, and can reduce the bracing needs for bus structures and minimize the need for components to have high SCCRs (withstand ratings).

Current-limiting fuses reduce the magnetic forces on downstream equipment during fault events that, if not limited, can reach levels of 30 kA or higher (even above 200 kA) in the first half cycle (0.008 seconds, at 60 Hz). The immense heat that can be produced in circuit components can cause conductor insulation damage or violent component explosions. At the same time, huge magnetic forces developed between conductors can crack insulators and distort or destroy bracing structures with the maximum mechanical force exerted being proportional to the square of the instantaneous peak current (I their full potential.

P

2 ).

Thus, it is important that an OCPD limit fault currents before they reach

Cable whip test

The “cable whip test” is a current limitation demonstration that visually illustrates the difference between current-limiting and non-current limiting OCPDs. The following tests (A and B) were conducted with the same parameters/configuration:

• Test voltage: 480 volts

• Conductor: 90 ft. of 2/0 AWG cable placed on the test lab floor

• The short-circuit current during a calibration test: asymmetrical with an approximately 26 kA RMS Sym. component

Test A was conducted without current limitation and Test B utilized a

200 A current-limiting fuse.

Tests videos are available through the QR codes. Figure 3.1.5.1.d is a photograph of the test configuration that was performed without an upstream current-limiting OCPD. Figure 3.1.5.1.e illustrates how Test A was conducted with the resulting peak current that flowed in one cycle.

480 V

480 V

1 cycle opening, non-current limiting

90 feet 2/0 AWG total

26 kA

X

Test results:

• Ip let-through = 48,100 A

• Clearing time = 0.0167 sec.

Figure 3.1.5.1.e

Test A with a non-current limiting OCPD.

Test B results and video are available in Figure 3.1.5.1.f. This test was conducted with an upstream current-limiting device with dramatically different results from Test A.

200 A current limiting

Class RK1 fuse

90 feet 2/0 AWG total

26 kA would flow without current limitation

X

Test results:

• Ip let-through = 10,200 A

• Clearing time = 0.004 sec.

Figure 3.1.5.1.f

Test B with a 200 A current-limiting OCPD.

Reviewing the Test A and Test B cable whip results demonstrate a significant reduction in let-through current by the 200 A currentlimiting fuse versus the one-cycle non-current limiting OCPD. Since the mechanical forces exerted on electrical equipment is directly proportional to the instantaneous peak current squared (I

P

2 through by over 95% as shown in this equation:

) let-through, the current-limiting fuse reduced the maximum mechanical force let-

(10,200 ÷ 48,100) 2 ≈ 1/22

3

Figure 3.1.5.1.d

Test A: One cycle interrupting time — non-current limiting OCPD.



Test A and B short-circuit current waveforms

The equivalent Test A and Test B waveforms are illustrated in Figure

3.1.5.1.g and Figure 3.1.5.1.h.

48,100 A peak

Test A

Short-circuit initiation

Simulated normal load current*

48,100 A peak

Current trace

Fault cleared in

1 cycle (0.0167 sec.)

* Norman load current did not flow prior to short-circuit initiation.

Figure 3.1.5.1.g

Test A fault current waveform.

Figure 3.1.5.1.g illustrates the current trace of Test A depicting normal current flow until the fault occurs, and then the fault current that flows for one cycle, achieving a peak let-through of 48,100 A.

How to use current limitation charts

Analysis of current-limiting fuse let-through graphs

The degree of current limitation of a given size and type of fuse depends, in general, upon the available fault current that can be delivered by the electrical system. Current-limitation of fuses is best described in the form of a let-through curve (see Figure 3.1.5.1.i) that, when applied from a practical point of view, is useful to determine the let-through currents when a fuse opens.

Fuse let-through curves are plotted from actual test data. The test circuit that establishes line A—B corresponds to a short circuit power factor of 15% that is associated with an X/R ratio of 6.6. The fuse curves represent the cutoff value of the prospective available fault current under the given circuit conditions. Each type or class of fuse has its own family of let-through curves.

The let-through data has been generated by actual short- circuit tests of current-limiting fuses. It is important to understand how the curves are generated, and what circuit parameters affect the let-through curve data. Typically, there are three circuit parameters that can affect fuse letthrough performance for a given available fault current. These are:

1. Short-circuit power factor

2. Short-circuit closing angle

3. Applied voltage

Current-limiting fuse let-through curves are generated under worst case conditions, based on these three variable parameters. The benefit to the user is a conservative resultant let-through current (both I p

I

RMS

and

). Under actual field conditions, changing any one or a combination of these will result in lower let-through currents. This provides for an additional degree of reliability when applying fuses for equipment protection.

Test B

Short-circuit initiation

Simulated normal load current*

Test A current trace

Test B current trace

Fault cleared in less than 1/4 cycle (0.004 sec)

* Norman load current did not flow prior to short-circuit initiation.

Figure 3.1.5.1.h

Test B fault current waveform.

Figure 3.1.5.1.h illustrates the current trace of Test B showing normal current until the fault occurs, and then the short-circuit current being cleared in less than 1/4 cycle by the Bussmann series current-limiting

Low-Peak LPS-RK-200SP fuse that limited the instantaneous peak current to only 10.2 kA.

For simplicity, this section does not provide the measurement parameter to assess the thermal energy let-through for these tests. However, the recording instrumentation documented that Test B let-through current was 1/123 the thermal energy compared to Test A.



Current-limiting fuse analysis

400,000

300,000

200,000

100,000

80,000

60,000

30,000

20,000

10,000

8000

6000

4000

3000

A

2000

1000

B

C

Current

800 A

D t m t c t a

C I p

available fault current = 198 kA

A I

RMS

available fault current = 86 kA

D I p

peak fuse let-through current = 49 kA

B I

RMS

fuse let-through current = 21 kA t m t a

Time

= fuse melt time

= fuse arc time t c

= fuse clearing time

3

B

A

Prospective fault current - RMS Sym. amps

Figure 3.1.5.1.i

Current-limiting fuse let-through curve for a Bussmann series 800 A KRP-C-800SP Class L Low-Peak, time-delay fuse.

Prior to using the fuse let-through curves, it must be determined what let-through data is pertinent to equipment SCCRs (withstand ratings).

Equipment SCCR can be described as how much fault current the equipment can handle, and for how long. Based on standards presently available, the most important data that can be obtained from the fuse let-through curves and their physical effects are:

• Peak let-through current (mechanical forces)

• Apparent prospective RMS symmetrical let-through current (heating effect)

• Clearing time: less than 1/2 cycle when fuse is in its current-limiting range (beyond where fuse curve intersects A—B line).

This typical example in Figure 3.1.5.1.j shows the available fault current in an 800 A circuit and an 800 A Low-Peak current-limiting time-delay fuse.

Available

86 kA

RMS Sym.

KRP-C800SP fuse Fault

X

Figure 3.1.5.1j

800 A Low-Peak current-limiting time-delay fuse and associated let-through data

Fuse current limitation let-through curves

Using the example given, one can determine the pertinent let-through data for the KRP-C-800SP amp Low-Peak fuse. The let-through curve pertaining to the 800 A Low-Peak fuse is illustrated in Figure 3.1.5.1.i.

A. Determine the peak let-through current.

Step 1. Enter the chart on the prospective short-circuit current scale at 86 kA and proceed vertically until the 800 A fuse curve is intersected.

Step 2. Follow horizontally until the Instantaneous peak let-through current scale is intersected.

Step 3. Read the peak let-through current as 49 kA (if a fuse had not been used, the peak current would have been 198 kA).

B. Determine the apparent prospective RMS Sym. let-through current.

Step 1. Enter the curve on the prospective short-circuit current scale at 86 kA and proceed vertically until the 800 A fuse curve is intersected.

Step 2. Follow horizontally until line A—B is intersected.

Step 3. Proceed down to the prospective short-circuit current.

Step 4. Read the prospective fault current — RMS Sym. amps as

21 kA. (The RMS Sym. let-through current would be 86 kA if there were no fuse in the circuit.)



3.1.5.2 Current let-through curves

Class CF fuses

TCF_ and TCF_RN current-limiting, dual-element, time-delay fuse RMS let-through currents (kA)

Prosp. short C.C.

1000

3000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

80,000

100,000

150,000

200,000

250,000

300,000

1

1

1

1

1

1

1

1

1

15

I

RMS

1

2

2

2

2

2

1

1

2

3

3

3

3

3

2

2

2

2

2

2

2

2

2

1

1

1

Fuse size (amps)

30 60

I

RMS

1

I

RMS

1

2

2

2

3

3

3

2

3

3

4

5

5

5

5

3

4

4

4

4

4

3

3

4

2

2

3

100

I

RMS

1

5

6

7

7

8

5

5

5

FCF_RN current-limiting, non-time delay fuse RMS let-through currents (kA)


1000

3000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

80,000

100,000

150,000

200,000

250,000

300,000

1

1

1

1

1

1

1

1

1

15

I

RMS

1

2

2

2

2

2

1

2

2

3

3

2

3

3

2

2

2

2

2

2

1

1

2

1

1

1


30 60

I

RMS

1

I

RMS

1

1

2

2

3

3

3

2

3

3

5

5

4

4

5

3

3

4

4

4

4

3

4

4

2

2

3

100

I

RMS

1

7

8

6

6

8

4

5

5

100,000

10,000

1000

A

100

Prospective Short-Circuit Current (RMS Sym. Amps)

B


100 A

60 A

30 A

15 A

TCF CUBEFuse FCF CUBEFuse



Class J fuses

LPJ-SP current-limiting, dual-element, time-delay fuse RMS let-through currents (kA)


1000

3000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

80,000

100,000

150,000

200,000

250,000

300,000

1

1

1

1

1

1

1

1

1

15

I

RMS

1

1

2

2

1

2

1

1

1

1

2

2

1

1

1

1

1

1

30

I

RMS

1

1

3

3

2

3

2

2

2

5

5

4

4

3

3

4

2

3

3

2

2

2

1

2

2


60

I

RMS

1

1

100

I

RMS

1

2

200

I

RMS

1

2

2

2

3

3

4

4

3

3

3

4

4

4

5

6

6

4

5

5

7

7

6

6

4

5

5

10

11

9

9

6

7

8

9

9

10

7

8

8

5

6

7

400

I

RMS

1

3

14

16

17

18

11

12

12

12

12

13

10

10

11

5

8

9

600

I

RMS

1

3

19

21

23

24

14

15

17

JKS current-limiting, non-time delay fuse RMS let-through currents (kA)

Prosp. short

C.C.

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

2

2

2

2

2

2

1

1

2

30

I

RMS

1

1

2

2

2

2

3

3

3

3

3

3

3

2

2

3

60

I

RMS

1

2

3

4

4

5

5

5

6

6

6

7

4

5

5

3

4

4


100

I

RMS

2

3

200

I

RMS

3

4

3

3

3

4

5

6

7

7

8

6

6

7

9

10

8

9

9

10

11

11

9

9

10

7

8

9

400

I

RMS

4

6

12

13

13

14

16

15

16

17

13

13

14

600

I

RMS

5

9

10

11

12

17

18

18

22

24

100,000

10,000

1,000

A

100

400,000

100,000

10,000

A

1,000

B


B

AMP RA

600

400

200

100

60

30

600A

400A

200A

100A

60A

30A

15A

3




Class RK1 fuses — 250 V and 600 V

250 V LPN-RK-SP current-limiting, dual-element, time-delay fuse RMS let-through currents (kA)

Prosp. short

C.C.

1000

2000

3000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

250,000

300,000

2

2

2

2

2

2

1

1

1

1

1

30

IRMS

1

1

1

3

3

2

3

2

2

2

3

3

3

3

3

3

2

3

2

2

3

60

IRMS

1

1

1

5

6

4

5

4

4

4

5

5

5

4

4

4

7

7

6

6

4

4

4

3

3

2

3

3


100

IRMS

200

IRMS

1

2

2

1

2

3

6

6

6

5

5

3

4

5

8

7

8

7

7

7

9

11

11

12

11

11

12

9

10

10

8

8

5

7

9

400

IRMS

1

2

3

15

16

17

18

12

13

13

14

16

16

12

13

13

11

11

5

9

12

600

IRMS

1

2

3

19

20

21

22

16

17

17

600 V LPS-RK-SP current-limiting, dual-element, time-delay fuse RMS let-through currents (kA)

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

250,000

300,000

Prosp. short

C.C.

1000

2000

3000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

2

2

2

2

2

2

3

3

3

3

2

2

2

1

2

2

30

IRMS

1

1

1

1

1

4

4

4

3

4

4

6

6

5

5

3

3

3

2

3

3

60

IRMS

1

1

1

2

2

5

5

6

5

5

5

7

7

6

7

4

4

4

3

3

4


100

IRMS

200

IRMS

2

2

1

2

3

3

3

1

2

4

6

6

6

5

5

6

8

8

9

7

7

8

10

11

12

12

13

13

14

11

12

13

15

16

17

18

10

10

10

8

9

9

400

IRMS

1

2

3

5

7

16

17

17

15

15

16

19

21

22

23

13

13

14

11

12

12

600

IRMS

1

2

3

5

10

400,000

100,000

10,000

A

1,000

B


B

AMP RA

600

400

200

100

60

30



400,000


B

Class RK5 fuses — 250 V and 600 V

250 V FRN-R current-limiting, dual-element, time-delay fuse

RMS let-through currents (kA)

Prosp. short

C.C.

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

6

6

6

7

8

5

5

6

4

4

4

4

5

60

IRMS

2

3

3

3

3

3

4

4

3

3

3

2

2

2

2

2

30

IRMS

1

2

2

7

8

8

6

6

5

6

7


100

IRMS

200

IRMS

3

4

5

5

7

8

11

12

13

8

9

10

10

11

10

11

8

9

9

13

14

14

16

18

19

20

21

17

18

19

24

26

12

13

14

15

15

400

IRMS

5

10

11

24

25

26

21

22

23

29

32

16

17

18

19

20

600

IRMS

5

10

15

600 V FRS-R current-limiting, dual-element, time-delay fuse

RMS let-through currents (kA)

Prosp. short

C.C.

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

4

4

4

3

3

3

3

3

60

IRMS

1

2

2

2

2

4

5

6

3

3

3

2

2

2

2

2

30

IRMS

1

1

1

2

2

3

3

4

7

7

7

6

6

5

5

6


100 200

IRMS

3

IRMS

4

4

4

5

5

5

6

7

7

8

9

9

13

14

16

11

12

12

9

9

8

8

10

17

17

17

13

13

14

14

15

400

IRMS

5

9

10

11

12

18

21

23

23

23

24

18

18

19

20

22

600

IRMS

5

10

14

15

17

25

27

32

100,000

10,000

A

1,000


400,000

B

100,000

10,000

A

1,000

Amp Rating

600A

400A

200A

100A

60A

30A

15A

Amp Rating

600

400

200

100

60

30

3




Class T fuses — 300 V and 600 V

Prosp. short

C.C.

500

1000

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

150,000

200,000

300 V JJN current-limiting, non-time delay fuse RMS let-through currents (kA)

1

1

1

1

1

1

1

1

1

1

1

1

1

1

15

I

RMS

1

1

1

2

2

2

2

2

2

2

2

2

1

2

1

1

1

1

60

I

RMS

1

2

3

3

1

1

1

1

2

2

1

1

1

1

1

1

1

1

30

I

RMS

1

2

2

2

4

4

4

3

3

3

3

3

3

2

3

2

2

2

2

1

1


100 200 400

I

RMS

1

I

RMS

1

I

RMS

1

1

2

2

3

4

4

1

3

3

3

3

4

4

4

4

5

5

6

7

7

6

7

8

8

5

6

5

5

6

6

7

8

9

9

9

9

10

10

11

11

8

8

7

7

6

6

1

5

600

I

RMS

1

12

13

15

11

12

13

14

10

10

11

11

15

15

7

9

1

5

800 1200

I

RMS

1

I

RMS

1

1

5

9

10

13

15

16

17

11

12

13

13

17

18

16

17

19

19

22

23

600 V JJS current-limiting, non-time delay fuse RMS let-through currents (kA)

Prosp. short

C.C.

15

I

RMS

500 1

1000 1

5000 1

10,000 1

15,000 1

20,000 1

25,000 1

30,000 1

35,000 1

40,000 1

50,000 1

60,000 1

70,000 1

80,000 1

90,000 1

100,000 2

150,000 2

200,000 2

2

2

2

1

1

1

1

1

1

1

1

30

I

RMS

1

2

3

3

2

2

2

2

2

3

2

2

2

1

2

1

1

2

60

I

RMS

1

3

4

4

3

3

3


100 200 400 600 800

I

RMS

1

I

RMS

1

I

RMS

1

I

RMS

1

I

RMS

1

2

3

1

2

3

1

3

3

4

4

1

4

6

7

7

1

5

8

10

10

9

11

1

5

12

4

4

4

3

3

3

5

5

5

5

6

6

7

8

9

9

10

10

11

12

13

13

14

16

15

17

18

13

14

15

5

6

6

4

4

4

7

8

9

7

7

7

12

14

16

11

11

12

19

22

24

17

17

18

22

25

28

19

20

21

B

400,000

300,000

200,000

100,000

80,000

60,000

40,000

30,000

20,000

10,000

8000

6000

4000

3000

2000

1000

800

600

400

200

600 800 1000

6000 8000

60,000 80,000 100,000


60

30

15

AMP RATING

1200

800

600

400

200

100

B

400,000

300,000

200,000

100,000

80,000

60,000

40,000

30,000

20,000

10,000

8000

6000

4000

3000

2000

1000

800

600

400

200

60,000 80,000 100,000


800

600

400

200

100

60

30

15



Class CC fuse

LP-CC current-limiting, time-delay fuse RMS let-through currents (kA)

Prosp. short

C.C.

1000

3000

5000

10,000

20,000

30,000

40,000

50,000

60,000

80,000

100,000

200,000

290

315

340

350

390

420

525

1-1/4

I

RMS

100

140

165

210

260

525

610

650

735

785

830

1100

2-8/10

I

RMS

135

210

255

340

435

800

870

915

1050

1130

1210

1600


15

I

RMS

240

350

20

I

RMS

305

440

420

540

680

570

700

870

1030

1150

1215

1300

1500

1600

2000

1300

1390

1520

1650

1780

2000

2520

25

I

RMS

380

575

690

870

1090

1520

1700

1820

1980

2180

2400

3050

30

I

RMS

435

580

710

1000

1305

A

B


20

15

2-8/10

1-1/4

3

Class L fuses

KRP-C-SP current-limiting, time-delay fuse RMS let-through currents (kA)

Prosp. short 601

C.C.

I

RMS

5000 5

800

I

RMS

5

I

RMS

5

I

RMS

5


1200 1600 2000 2500 3000 4000 5000 6000

I

RMS

5

I

RMS

5

I

RMS

5

I

RMS

5

I

RMS

5

I

RMS

5

10,000 8

15,000 9

20,000 10

25,000 11

10

12

13

14

10

15

17

19

10

15

20

22

10

15

20

25

10

15

20

25

10

15

20

25

10

15

20

25

10

15

20

25

10

15

20

25

30,000 11

35,000 12

40,000 13

50,000 14

14

15

16

17

20

21

22

23

24

25

26

28

27

29

30

32

30

35

35

37

30

35

40

50

30

35

40

50

30

35

40

50

30

35

40

50

60,000 15

70,000 15

80,000 16

90,000 17

100,000 17

150,000 20

200,000 22

250,000 24

300,000 25

22

25

27

29

31

18

19

20

21

30

34

37

40

43

25

26

27

29

36

41

45

49

52

30

32

33

34

41

47

51

55

59

34

36

38

39

47

54

59

64

68

40

42

44

45

58

67

73

79

84

49

52

54

56

70

80

87

94

100

60

62

65

67

60

70

76

79

81

93

100

104

102 114

110 123

117 140

60

70

80

90

1000,000

100,000

10,000

1000

A


B

6000 A

5000 A

4000 A

3000 A

2500 A

2000 A

1200 A

800 A

601 A



3.1.5.3 The OCPD’s role in electrical safety

OCPD selection and performance play a significant role in electrical safety. Extensive tests and analysis by industry have shown that the energy released during an arcing fault is related to two OCPD characteristics:

1. The time it takes the OCPD to open

2. The amount of fault current the OCPD lets through

For instance, the faster OCPD clears the fault, the lower the energy released. If the OCPD can also limit the current, thereby reducing the actual fault current magnitude that flows through the arc, the lower the energy released. The lower the energy released, the better for both worker safety and equipment protection.

Simple method for arc flash hazard analysis per 2018

NFPA 70B

The following is an example of identifying the arc flash hazard per

130.5(E) for the arc flash boundary (AFB) and 130.5(G) using the incident energy analysis method.

Various information about the system may be needed to complete this analysis, but two values are absolutely necessary:

1. The available 3 Ø bolted fault current

2. The fuse type/amp rating

Consider the one-line diagram in Figure 3.1.5.3.a and then follow the examples that take the steps needed to conduct an arc flash hazard analysis.

LPS-RK-600SP

600 A, Class RK1 fuses

600 V, 3Ø

MLO panel

42 kA available bolted fault current

• Incident energy 0.25 cal/cm 2

• 6” AFB

@18”

Example 1: Arc flash hazard analysis using Bussmann series current-limiting fuses (notes referenced appear on page 3-28)

The following is a simple method when using certain Bussmann series fuses; this method is based on actual data from arcing fault tests (and resulting simplified formulas shown in NFPA 70E Annex D.4.6 and

2002 IEEE 1584) with Bussmann series current-limiting fuses. Using this simple method, the first thing that must be done is to determine the incident energy exposure level . We have simplified this process when using LPS-RK, LPJ, TCF, LP-CC or KRP-C Low-Peak fuses, or JJN or JJS Limitron fuses and FCF fuses. In some cases the results are conservative; see Note 6.

In this example, the lineside OCPD in Figure 3.1.5.2.a is an LPS-RK-

600SP, Low-Peak current-limiting fuse. Simply take the available 3 Ø bolted fault current at the panel — in this case 42 kA — and locate it on the vertical column in Table 4. Then proceed directly to the right to the

401-600 A fuse column and identify the I.E. (incident energy) and AFB.

With 42 kA of 3 Ø bolted available fault current, the table shows that when relying on the LPS-RK-600SP Low-Peak fuse to interrupt an arcing fault, the incident energy is 0.25 cal/cm 2 . Notice the variables required are the available 3 Ø bolted fault current and the Low-Peak currentlimiting fuse amp rating. See Notes 7 and 8.

The next step in this simplified arc flash hazard analysis is to determine the AFB. With an incident energy of 0.25 cal/cm 2 and using the same table, the AFB is approximately 6 inches, which is found next to the incident energy value previously located. See Note 6. This AFB distance means that anytime work is to be performed inside of this distance, including voltage testing to verify that the panel is de-energized, the worker must be equipped with the appropriate PPE.

The last step in the arc flash hazard analysis is to determine the appropriate PPE for the task. To select the proper PPE, utilize the incident energy exposure values and the requirements from NFPA 70E.

NFPA 70E Table 130.5(G) has requirements for the PPE based upon the incident energy exposure level. NFPA 70E Annex H is a resource for guidance in selecting PPE; specifically Tables H.2 and H.3.

When selecting PPE for a given application or task, keep in mind that these requirements from NFPA 70E are minimum requirements. Having additional PPE, above what is required, can further assist in minimizing the effects of an arc flash incident. Another thing to keep in mind is that

PPE available on the market today does not protect a person from the pressures, shrapnel and toxic gases that can result from an arc-blast, which are referred to as “physical trauma” in NFPA 70E. Existing PPE is only tested to minimize the potential for burns from the arc flash. See

Notes 1 and 2.

Figure 3.1.5.3.a: One-line diagram used in arc flash hazard analysis.

The following information utilizes the simplified fuse formulas based upon IEEE 1584-2002 Guide for Arc Flash Hazard Analysis and shown in

2018 NFPA 70E Annex D.4.6 and shows the steps necessary to conduct an arc flash hazard analysis when using Low-Peak fuses and Table

3.1.5.3.b: arc flash incident energy.

1. Determine the available bolted fault current on the lineside terminals of the equipment that will be worked on.

2. Identify the amp rating of the upstream Low-Peak fuse that’s protecting the panel where work is to be performed.

3. Consult Table 3.1.5.3.b to determine the Incident Energy Exposure

(I.E.) level.

4. Determine the AFB that will require personal protective equipment

(PPE) based upon the incident energy. This is simplified by using the column for AFB in Table 3.1.5.3.b.

5. Identify the minimum requirements for PPE when work is to be performed inside of the AFB by consulting the requirements found in

NFPA 70E Table 130.5(G).



Arc flash incident energy levels based upon 1-600 A Low-Peak LPS-RK and 601-2000 A Low-Peak KRP-C fuses

Incident Energy (I.E.) values expressed in cal/cm 2 , Arc Flash Boundary (AFB) expressed in inches.

96

98

100

102

88

90

92

94

74

76

78

80

82

84

86

104

106

66

68

70

72

58

60

62

64

44

46

48

50

52

54

56

36

38

40

42

28

30

32

34

20

22

24

26

12

14

16

18

Bolted fault current

(kA)

1

6

8

10

4

5

2

3

1-100 A

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

I.E.

2.39

0.25

0.25

0.25

0.25

0.25

0.25

0.25

101-200 A 201-400 A 401-600 A 601-800 A 801-1200 A 1201-1600 A 1601-2000 A

AFB I.E.

AFB I.E.

AFB I.E.

AFB I.E.

AFB I.E.

AFB I.E.

AFB I.E.

AFB

29 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6

6

6

6

5.20

0.93

025

0.25

6 0.25

6 0.25

6 0.25

49 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

15 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6

6

6 20.60

>120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6 1.54

21 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6

0.75

0.69

0.63

13

12

12

>100

36.85

12.82

>120

>120

90

>100

>100

75.44

>120

>120

>120

>100

>100

>100

>120

>120

>120

>100

>100

>100

>120

>120

>120

>100

>100

>100

>120

>120

>120

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

6 0.25

6 0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6 0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6 0.25

6 0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

0.57

0.51

0.45

0.39

0.33

0.27

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

11

10

9

8

7

7

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6.71

0.60

0.59

0.48

0.38

0.28

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

0.25

0.25

6 0.25

6 0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

0.25

0.25

6 0.25

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

58 49.66

>120 73.59

>120 >100 >120 >100 >120

11 23.87

>120 39.87

>120 >100 >120 >100 >120

11

10

1.94

1.82

25

24

11.14

10.76

82

80

24.95

24.57

>120

>120

>100

>100

>120

>120

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

8

7

1.70

1.58

1.46

1.34

1.22

1.10

0.98

0.86

0.74

0.62

0.50

0.38

6 0.25

6 0.25

6 0.25

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

Table 3.1.5.3.b

Arc flash incident energy levels. See next page for applicable notes.

23 10.37

22

21

19

18

17

16

14

13

11

10

8

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

9.98

8.88

7.52

6.28

5.16

4.15

3.25

2.47

1.80

1.25

0.81

0.49

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

6 0.39

6 0.39

0.39

0.39

0.39

6

6

6 0.39

6 0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

78

76

70

63

55 22.71

>120 28.67

>120

48 22.34

>120 28.41

>120

42 21.69

>120 28.15

>120

35 18.58

116 27.90

>120

29 15.49

24 12.39

18

14

10

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

24.20

23.83

23.45

23.08

9.29

6.19

3.09

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.92

2.80

8 2.67

8 2.54

2.42

2.29

2.17

2.04

1.91

1.79

1.66

1.54

1.41

1.28

1.16

1.03

0.90

0.78

0.65

0.53

0.40

>120

>120

>120

>120

102

88

72

55

>100

>100

29.18

28.92

27.64

27.38

27.13

26.87

>120

>120

>120

>120

>120

>120

>120

>120

29 19.61

28 18.57

27 17.54

26 16.50

25 15.47

24 14.43

22 13.39

16

15

13

12

21 12.36

20 11.32

19 10.29

18 9.25

8.22

7.18

6.15

5.11

10

9

4.08

3.04

34 26.61

>120

33 26.36

>120

33 26.10

>120

33 25.84

>120

33 25.59

>120

33 25.33

>120

33 25.07

>120

33 24.81

>120

33 24.56

>120

33 24.30

>120

33 24.04

>120

33 23.75

>120

32 22.71

>120

31 21.68

>120

30 20.64

>120

66

61

55

48

88

83

77

72

120

116

111

107

102

97

93

41

34

3



Table notes for arc flash hazard analysis in Table 3.1.5.3.b

1. This information is not to be used as a recommendation to work on energized equipment. This information is to help assist in determining the PPE to help safeguard a worker from the burns that can be sustained from an arc flash incident. This information does not take into account the effects of pressure, shrapnel, molten metal spray or the toxic vapor resulting from an arc-fault. This information does not address the maintenance conditions of the overcurrent protective device.

2. This data is based upon the simplified fuse formulas in NFPA 70E

Annex D.4.6 and 2002 IEEE 1584 guide for arc flash hazard analysis.

3. PPE must be utilized any time work is to be performed on equipment that is not placed in an electrically safe work condition.

Voltage testing, while completing the lockout/tagout procedure

(putting the equipment in an electrically safe work condition), is considered as working on energized parts per OSHA 1910.333(b).

4. The data is based on 32 mm (1-1/4”) electrode spacing, 600 V 3 Ø ungrounded system, and 20” x 20” x 20” box. The incident energy is based on a working distance of 18 inches, and the AFB is based on 1.2 cal/cm 2 (threshold for a second-degree “just curable” burn).

5. The data is based upon tests that were conducted at various fault currents for each Bussmann series Low-Peak KRP-C and LPS-RK fuse indicated in the charts. These tests were used to develop the formulas as shown in NFPA 70E Annex D.4.6 and 2002 IEEE 1584.

Actual results from incidents could be different for a number of reasons, including:

• System voltage

• Short-circuit power factor

• Distance from the arc

• Arc gap

• Enclosure size

• Fuse manufacturer

• Fuse class

• Orientation of the worker

• Grounding scheme

• Electrode orientation

100 A LPS-RK fuses were the smallest fuses tested. Data for the fuses smaller than that is based upon the 100 A data. Arc flash values for actual 30 and 60 A fuses would be considerably less than

100 A fuses. However, it does not matter since the values for the

100 A fuses are already so low.

6. The fuse incident energy values were chosen not to go below 0.25 cal/cm 2 even though many actual values were below

0.25 cal/cm 2 . This was chosen to keep from encouraging work on energized equipment without PPE because of a low AFB.

7. Table 3.1.5.3.b can also be used for LPJ, TCF, FCF, JJS and LP-CC fuses to determine the incident energy available and AFB.

8. These values from fuse tests take into account the translation from available three-phase bolted fault current to the arcing fault current.

9. To determine the AFB and incident energy for applications with other fuses, use the basic equations in 2002 IEEE 1584 or NFPA 70E

Annex D.4.

10. Where the arcing current is less than the current-limiting range of the fuse when calculated per NFPA 70E Annex D.4.6 and 2002 IEEE

1584, the value for incident energy is given as >100 cal/cm 2 . For the incident energy and arc flash boundary in these cases, use 2002

IEEE 1584 basic equation methods with the fuse time-current curve.



1

0.8

0.6

0.4

0.3

Point F

0.2

0.1

0.08

0.06

0.04

0.03

0.02

Point E 100

80

60

40

30

20

Point G

10

8

6

4

3

Point D

2

3.1.5.4 Time-current characteristic curve (TCC)

Time-current characteristic curves (TCC) are graphical representations of the OCPD’s operation under different overcurrent conditions as plotted by amps and time (see Figure 3.1.5.4.a). TCCs also provide a visual means for comparing OCPD operation and whether they will selectively coordinate, or not (see Selective coordination on page 3-30).

Figure 3.1.5.4.a illustrates the TCCs for 400 A and 100 A dual-element, time-delay fuses in series as depicted in the one-line diagram. The graph’s horizontal axis represents the RMS symmetrical current in amps. The vertical axis represents the time in seconds. Each fuse is represented by a band comprised of the minimum melt characteristic

(solid blue line showing the lower operating characteristics) and the total clear characteristics (hash red line showing the higher operating characteristics). The area between these two lines represents the fuse’s tolerance band under specific test conditions that, for a given overcurrent, a specific fuse, under the same circumstances, will open at a time within the its time-current tolerance band.

600

100 A 400 A

400

300

200

400 A

Point C

Point B

100 A

X Available fault current = 1 kA

Fuses have an inverse time-current characteristic meaning the greater the overcurrent, the faster they open and interrupt. For example, the

100 A fuse in Figure 3.1.5.4.a subjected to a 200 A overcurrent shows by its TCC that it will open and clear in approximately 200 seconds, and for a 2000 A overcurrent, its TCC shows it will open and clear in approximately 0.15 second.

In some cases, assessing coordination between two or more fuses is possible by comparing their TCCs. This method is limited to only the overcurrent range up to the point at which the upstream fuse crosses

0.01 second. For example: assume there is a 1 kA RMS symmetrical overcurrent on the 100 A fuse’s loadside. To determine the time it would take this overcurrent to open both the 100 A and 400 A fuses:

• Find 1 kA on the horizontal axis (Point A)

• Follow the dotted line vertically to the intersection of the 100 A fuse’s total clear curve (red line at Point B) and the 400 A fuse’s minimum melt curve (blue line at Point C).

• Then, horizontally from both intersection points, follow the dotted lines to Points D and E.

At 1.75 seconds, Point D represents the maximum time the 100 A fuse will take to open the 1 kA overcurrent. At 90 seconds, Point E represents the minimum time at which the 400 A fuse could open this overcurrent. These two fuses are coordinated for a 1 kA overcurrent.

For overcurrents up to approximately 11 kA (Point H), it can be determined that these two fuses selectively coordinate as there isn’t any curve overlap and the current is less than where the 400 A upstream fuse crosses 0.01 second. When overcurrents exceed 11 kA, selective coordination cannot be determined by using TCCs and fuse selectivity ratio tables must be used. Using the Bussmann series fuse selectivity ratio table makes it simple to determine whether fuses selectively coordinate or not and eliminates the need for plotting and comparing fuse TCCs (see the Bussmann series fuse selectivity ratios

Table 3.1.5.5.b on page 3-30).

3

Technical assistance is available to all customers. Application support is available Monday-Friday, 7:00 a.m. - 5:00 p.m. Central Time. Toll-free phone: 855-287-7626 (855-BUSSMANN), email: [email protected].

0.01

Point A 1000 A

Minimum melt

Current in amps

Point H

Total clearing

Figure 3.1.5.4.a

Minimum melt an d total clearing curves for 100 A and 400 A dual-element, time-delay fuses.



3.1.5.5 Selective coordination

While important, selecting OCPDs based solely on their ability to carry system load current and interrupting the maximum fault current at their respective application points is not enough. As the demand for power system reliability increases, the OCPD’s performance in the system becomes more and more critical as its function should, ideally, limit a power system outage to only that portion of the circuit which is faulted.

The selected OCPD types and ratings (or settings) determine whether they are selectively coordinated upstream and downstream in a system so that only the nearest upstream OCPD will open for the full range of overcurrents and opening times, and leave the remainder of the system undisturbed to preserve service continuity.

X

KRP-C-800SP

LPS-RK-400SP

LPS-RK-200SP

2:1 (or more)

2:1 (or more)

Figure 3.1.5.5.a

Low-Peak fuses applied with a simple 2:1 amp rating ratio achieves selective coordination.

Figure 3.1.5.5.b shows the minimum amp rating ratios for Bussmann series Low-Peak fuses required to provide “selective coordination”

(discrimination) between upstream and downstream fuses.

It’s a simple matter to selectively coordinate a system using published fuse amp rating ratio tables and maintaining the minimum ratios between an upstream and downstream fuse. This will ensure selective coordination is achieved for all fault currents up to the fuse’s interrupting rating. These selectivity ratios are for all overcurrent levels up to the fuse interrupting or 200 kA, whichever is lower.

For an in-depth examination of this subject, see selective coordination in

Section 4.

Coordination, Selective (selective coordination)

Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.

Selective coordination is mandatory per the NEC for the circuit paths of some vital loads on specific systems including:

•

•

•

•

•

•

Elevator circuits — 620.62

Critical operations data systems — 645.27

Campus style fire pumps — 695.3

Emergency systems — 700.32

Legally required standby systems — 701.27

Critical operations power systems — 708.54

Bussmann series fuse selectivity ratios

Circuit

Amp rating range

601-

6000 A

Fuse type

Timedelay

Trade name

(fuse class)

Low-Peak

(L)

Bussmann fuse symbol

KRP-C_SP

601 to

6000 A

601 to

4000 A

0 to 600 A

0 to 600 A

0 to 100 A

0 to 600 A

601 to

6000 A

0 to 600 A

0 to 1200 A

0 to 600 A

0 to 60 A

Timedelay

Timedelay

Dualelement

Dualelement

Dualelement

Dualelement

Fastacting

Fastacting

Fastacting

Fastacting

Timedelay

Low-Peak

(L)

Limitron

(L)

Low-Peak

(RK1)

Low-Peak

(J)

CUBEFuse

(CF 2 )

Fusetron

(RK5)

Limitron

(L)

Limitron

(RK1)

Limitron

(T)

Limitron

(J)

SC

(G)

KRP-C-SP

KLU

LPN-RK-SP

LPS-RK-SP

LPJ-SP

TCF

FRN-R

FRS-R

KTU

KTN-R

KTS-R

JJN

JJS

JKS

SC

2:1

2:1

—

—

—

—

2:1

—

—

—

—

601-

4000 A

Timedelay

Limitron

(L)

1-100 A

Time-delay

CUBEFuse

(CF 2 ) (J)

Downstream / loadside fuse

0-600 A

601-

6000 A


Low-Peak Low-Peak

(RK1)

Fusetron

(RK5)

Fastacting

Limitron

(L)

0-600 A

Fastacting

Limitron

(RK1)

0-

1200 A

Fastacting

Limitron

(T)

0-600 A

Fastacting

Limitron

(J)

0-

60 A

Timedelay

SC

(G)

KLU

2.5:1

2:1

—

—

—

—

2.5:1

—

—

—

—

TCF

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

LPJ-SP

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

LPN-RK-SP

LPS-RK-SP

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

FRN-R

FRS-R

4:1

4:1

8:1

8:1

8:1

2:1

6:1

8:1

8:1

8:1

4:1

KTU

2:1

2:1

—

—

—

—

2:1

—

—

—

—

KTN-R

KTS-R

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

JJN

JJS

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

JKS

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

SC

2:1

2:1

4:1

4:1

4:1

1.5:1

2:1

4:1

4:1

4:1

2:1

0-30 A

(CC)

LP-CC

FNQ-R

KTK-R

2:1

2:1

2:1

2:1

2:1

2:1

2:1

—

—

—

—

General notes: Ratios given in this table apply to only Bussmann fuses. When fuses are within the same case size, consult Bussmann.

1. Where applicable, ratios are valid for indicating and non-indicating versions of the same fuse. At some values of fault current, specified ratios may be lowered to

permit closer fuse sizing. Consult Bussmann.

2. Time-delay Class CF TCF CUBEFuse OCPDs are 1 to 100 A Class J performance; dimensions and construction are a unique, finger-safe design.

Table 3.1.5.5.b

This selectivity ratio table identifies the fuse amp rating ratios that ensure selective coordination.



3.1.6 Fuse types and classes

3.1.6.1 Low voltage, branch circuit fuses

The NEC defines the branch circuit OCPD as capable of providing protection for service, feeder and branch circuits and equipment over the full range of overcurrents between its rated current and its interrupting rating. They’re also the only OCPDs the NEC permits to be installed in a building’s electrical system. The definition found in Article

100 is as follows:

“Overcurrent Protective Device, Branch Circuit. A device capable of providing protection for service, feeder, and branch-circuits and equipment over the full-range of overcurrents between its rated current and its interrupting rating. Such devices are provided with interrupting ratings appropriate for the intended use but no less than 5,000 amperes.”

Per this definition, branch circuit OCPDs are suitable to protect branch and feeder circuits and service conductors at any point in the electrical system, and must be capable of protecting against the full range of overcurrents, including overloads and faults. In addition, the OCPD must have an interrupting rating sufficient for the application per NEC 110.9.

Branch circuit OCPDs meet common, minimum standard requirements for spacing and operating time-current characteristics defined by UL.

Figure 3.1.6.1.a illustrates acceptable OCPDs that can be used for branch circuit protection.

This is inherent in all current-limiting fuse classes. Each fuse class must meet:

• Maximum let-through limits (I p

and I 2 t) during fault conditions

• Minimum voltage ratings

• Minimum 200 kA interrupting ratings for Class CC, CF, J, L, R and T

• Physical rejection of

•

Different fuse classes and case sizes*

•

Non current-limiting fuses (see Figures 3.1.6.1.b and 3.1.6.1.c)

* Branch circuit fuse blocks and holders are made to hold a fuse class case size that corresponds to a particular amp range. This prevents fuses from the same class with a larger case size from being installed and helps prevent overfusing.

There are instances where it is desirable to install a class fuse with a smaller case size than the block or holder. For these situations, it is permitted to use fuse reducers.

These product standards ensure branch circuit fuses provide specific, minimum circuit protection when current-limiting fuses and equipment are used. A given fuse class will ensure the voltage and interrupting rating, and degree of current limitation for the electrical system’s life.

For example, by using Class J fuses and equipment, only Class J fuses can be installed. This ensures the voltage rating is always 600 V (whether the system is 120, 208, 480, or 575 V), the interrupting rating is at least

200 kA, and the fault current protection provided by its current-limiting, let-through characteristics. If the fuse needs replacing, only a Class J fuse can be installed.

3

Device type

UL 248 branch circuit fuses

Acceptable devices

Class CC

Class CF

Class G

Class H(K)

Class J

Class L

Class RK1

Class RK5

Class T

Bussmann series fuses

LP-CC, FNQ-R, KTK-R

TCF, FCF

SC

NON, NOS

LPJ-, JKS, DFJ

KRP-C, KLU, KTU

LPN-RK, LPS-RK, KTN-R,

KTS-R

FRN-R, FRS-R

JJN, JJS

UL 489 circuit breakers

UL 1066 circuit breakers

Molded case CBs

Insulated case CBs

Low voltage power CBs

—

Figure 3.1.6.1.a

Acceptable OCPDs for branch circuit protection.

The UL 248 fuse standards cover distinct low-voltage (600 volts or less) fuse classes. Of these, modern current-limiting fuse Classes CC, CF, G,

J, L, R and T are the most important. The branch circuit current-limiting fuses’ rejection feature helps ensure electrical system safety over its life because it prevents installing other fuse types or larger case sizes. Thus, fuses that cannot provide a comparable minimum protection level for critical ratings and performance cannot be inadvertently installed.

Figure 3.1.6.1.b

L ow-Peak Class J fuses achieve rejection by their unique physical size that is unlike other UL class fuses.

Rejection ferrule will also fit

Class H or K5 mountings

Rejection blade will also fit

Class H mountings

Figure 3.1.6.1.c

Class R fuse rejection clips (restriction on the ferrule or notch on the blade) that will only accept Class R fuses.



Bussmann series branch circuit power distribution fuses

Class CC

Time-delay, Low-Peak LP-CC

• 600 Vac, 1/2 to 30 A current-limiting 200 kA IR AC

• UL Std. 248-4 Class CC, Guide JDDZ, File E4273,

1/2-2.8 A (300 Vdc 20 kA IR), 3-15 A (150 Vdc 20 kA IR), 20-30 A (300 Vdc 20 kA IR), CSA Class

1422-02, CSA File #53787

The Bussmann series Low-Peak LP-CC was developed specifically for a growing need in the industry to have a compact, space saving branch circuit fuse for motor circuits

(see data sheet no. 1023).

Time-delay, Limitron FNQ-R

• 600 Vac, 1/4 to 30 A, current-limiting 200 kA IR AC

• UL Std. 248-4 Class CC, Guide JDDZ, UL File

E4273, CSA Class 1422-01, CSA File 53787

Ideal for control transformer protection, the FNQ-R can be sized to meet requirements of NEC 430.72 and UL 508. Its small size and branch circuit rating allow it to be used for motor branch circuit and short-circuit protection required by

NEC 430.52 (see data sheet no. 1014)

Non-time delay (fast-acting), Limitron KTK-R

• 600 Vac, 1/10 to 30 A, current-limiting 200 kA IR AC

• UL Std. 248-4 Class CC, Guide JDDZ, UL File

E4273, CSA Class 1422-02 CSA File 53787

KTK-R fuses are small, high performance, fast-acting, singleelement fuses for protecting branch circuits, motor control circuits, lighting ballasts and street lighting fixtures (see data sheet no. 1015).

A

B

Class CF

The UL Class CF CUBEFuse provides the same electrical performance as UL Class J fuses. Available in time-delay or fast-acting versions, the

CUBEFuse is the world’s first finger-safe fuse with the smallest installed footprint of any power class fuse and meets IEC 60529 requirements for IP20 fingersafe protection.

The CUBEFuse mounts in 35 mm DIN-Rail and panel mountable amp rating rejecting holders (30, 60 and 100 A) that will not accept a fuse rating greater than the holder’s. Additionally, the CUBEFuse can be mounted in the UL 98 Compact Circuit Protector available in 1-, 2- and

3-pole factory configured units in the same 30, 60 and 100 amp rating rejection increments.

Time-delay, Low-Peak CUBEFuse TCF

• 600 Vac, 1 to 100 A, dual-element, current-limiting

300 kA IR AC

• UL Listed Class CF, Std. 248-8 Class J performance, Guide JDDZ, File E4273, 300 kA IR

AC, (300 Vdc – 100, kA IR), CSA Class 1422-02,

CSA File 53787, 200 kA IR AC, (300 Vdc, 100 kA IR)

The Low-Peak TCF CUBEFuse provides Type 2 “No

Damage” motor starter protection when sized properly. Available with optional open fuse indication

(6 to 100 A) (see data sheet no. 9000).

Non-time delay (fast-acting), CUBEFuse FCF

• 600 Vac/dc, 1 to 100 A, current-limiting 300 kA (up to 60 A) 200 kA (70 to 100 A) IR, 50 kA IR DC

• UL Listed Class CF, Guide JDDZ, File E4273, CSA

Class 1422-02, CSA File 53787, 200 kA IR AC

The fast-acting FCF CUBEFuse is a non-indicating fuse specifically designed to meet the needs of UPS and critical power applications (see data sheet no. 2147)

Dimensions — in (mm)

Amp range

Up to 30

A

1.5 (38)

B

0.41 (10)


Amp range

1-15

17-1/2

20

25-30

35-40

45-50

60

70

80-90

100

A

1.88 (48)

2.13 (54)

3.01 (76)

B

0.75 (19)

1.0 (25)

C

1.0 (25)

1.13 (29)

1.26 (32)

D

0.23 (6)

0.27 (7)

0.31 (8)

0.36 (9)

0.44 (11)

0.49 (12)

0.57 (14)

E

0.63 (16)

0.58 (15)


Quik-Spec Coordination Panelboard (QSCP)

The Bussmann series Quik-Spec™ Coordination Panelboard uses the

CUBEFuse for its branch circuit OCPD. The QSCP makes selective coordination easy in an all-fused system utilizing the fuse selectivity ratios table.

Features

• Addresses NEC selective coordination requirements

• Flexible configurations — up to 400 A, 600 Vac or less

• Same size footprint as traditional circuit breaker panelboards

• Finger-safe construction for greater safety

• Saves time

• Easy-to-spec

• Amp rating rejection branch switches help prevent overfusing

• Built-in spare fuse storage

(See data sheet no. 1160 and application note no. 3148.)

Class G

Non-time delay/fast-acting (up to 6 A) and time-delay

(7 to 60 A) general purpose SC

• 600 Vac (1/2 to 20 A), 480 Vac (25 to 60 A),

170 Vdc 1/2 to 20 A), 300 Vdc (25-60 A), current-limiting 100 kA IR AC, 10 kA IR DC

• UL Std. 248-5 Class G, Guide JDDZ, File

E4273 0-20 A (170 Vdc 10 kA IR), 25-30 A (300

Vdc 10 kA IR), 35-60 A (300 Vdc 10 kA IR),

CSA Class 1422-01, CSA File 53787

A high performance general-purpose branch circuit fuse for lighting, appliance and motor branch circuits. SC fuse lengths vary with amp rating from 1-5/16 to 2-1/4 inches to serve as a rejection feature and help prevent oversizing


A

13/32 (10)


Fuse amp rating

1/2 to 15

20

25-30

35-60

A

1-5/16 (33)

1-13/32 (36)

1-5/8 (41)

2-1/4 (57)


3



Class J

Time-delay Low-Peak LPJ-SP

• 600 Vac, 1 to 600 A, dual-element, currentlimiting, 300 kA IR AC

• UL Std. 248-8 Class J, Guide JDDZ, File E4273,

300 kA IR AC, 1 to 600 A

(300 Vdc, 100 kA IR), CSA Class 1422-02, CSA

File 53787

Space saving LPJ fuses have the time-delay

“advantage” that permits them to pass temporary, harmless overloads while offering back-up overload,and short-circuit protection. Ideal for

IEC starters, they provide Type 2 “No Damage” protection when properly sized (see data sheet no.

1006, up to 60 A, and no. 1007, 70 to 600 A).

Non-time delay (fast-acting) Limitron JKS

• 600 Vac, 1 to 600 A, current-limiting, 200 kA IR

AC



JKS Limitron fuses are essentially the same as RK1 Limitron fuses, but smaller in physical size. JKS fuses are single-element units with no intentional time-delay and are thus best applied in circuits free of the temporary overloads from motors and transformers. The smaller dimensions of Class J fuses prevent their replacement with conventional fuses (see data sheet no. 1026, up to

60 A, and no. 1027, 70 to 600 A).

High speed drive fuse DFJ

• 600 Vac, 450 Vdc (15 to 600 A), 1 to 600 A, current-limiting 200 kA IR AC, 100 kA DC



The DFJ high speed fuse offers the advantage of meeting NEC and UL branch circuit protection requirements. Designed specifically for protecting drives, soft starters, solid state relays and other power electronics, the DFJ is capable of limiting fault currents like a semiconductor fuse. The DFJ fits into all standard Class J fuse holders and blocks (see data sheet no. 1048).

A

E B

A

C

D I F B

1 to 60A


Amp range

1-30

A

2.25

(57)

B

0.81

(21)

35-60

70-100

110-

200

225-

400

450-

600

2.38

(60)

4.63

(118)

5.75

(146)

7.12

(181)

8.0

(203)

1.06

(27)

1.13

(29)

1.63

(41)

2.11

(54)

2.6

(66)

C

—

—

3.63

(92)

4.38

(111)

5.25

(133)

6.0

(152)

3-34

D

—

—


2.63

(67)

3.0

(76)

3.26

(83)

3.31

(84)

H

70 to 600A

E

F

—

—

E

0.5

(13)

0.63

(16)

1.0

(25)

1.38

(35)

1.87

(48)

2.12

(54)

0.75

(29)

1.13

(29)

1.62

(41)

2.0

(51)

G

—

H

—

G

I

—

—

0.13

(3)

0.19

(5)

0.25

(6)

0.53

(14)

— —

0.41

(10)

0.38

(10)

0.56

(14)

0.72

(18)

0.28

(7)

0.28

(7)

0.4

(10)

0.53

(14)

Innovative Bussmann series products, like the patented Class J power distribution fuse block above, combine fuse block and power distribution block into one assembly to reduce component count and speed equipment assembly.

Class L

Time-delay, Low-Peak KRP-C

• 600 Vac, 601 to 6000 A, current-limiting, 300 kA IR AC

• UL Std. 248-10 Class L, Guide JFHR, File

E56412, 300 kA IR AC, 601-2000 and 3000 A

(300 Vdc 100 kA IR), CSA Class 1422-02, CSA

File 53787

The KRP-C is an all purpose fuse for both overload and short-circuit protection of high capacity systems. Its minimum time-delay of four seconds at five times amp rating permits sizing closer to loads. The use of downstream 1/10 to 600

A Low-Peak dual-element time-delay fuses and upstream 601 to 6000 A KRP-C Low-Peak fuses is recommended for easy selective coordination and blackout protection with a simple 2:1 amp rating ratio. Low-Peak fuses can also reduce bus bracing and provide excellent overall protection of circuits and loads (see data sheet no. 1008, 601 to 2000

A, and no. 1008, 2001 to 6000 A).


Non-time delay (fast-acting), Limitron KTU

• 600 Vac, 601 to 6000 A current-limiting,

200 kA IR AC

• UL Std. 248-10 Class L, Guide JDDZ, File


The KTU is a single-element non-time delay fuse that’s very fast-acting with a high degree of current limitation to provide excellent component protection. In motor circuits, the KTU is sized at approximately 300% of motor full-load amps (see data sheet no. 1010).

Time-delay, Limitron KLU

• 600 Vac, 601 to 4000 A, current-limiting,

200 kA IR AC

• UL Std. 248-10 Class L, Guide JDDZ, File


The KTU has a minimum five second delay at

500% of rated current. The KTU is not as currentlimiting as KRP-C or KTU fuses (see data sheet no. 1013).

3

All slots and holes

J1

J3

I

J4

J2

J1

J3

J4

J2

A

A

C1

601 to 800 A

C2

801 to

2001 to

3000 A

3500 to

4000 A

4500 to

6000 A

D

G

F

B


Amp range

601-800

801-1200

1350-1600

1800-2000

2001-2500

3000

3500-4000

4500-5000

6000

A

8.63 (219)

10.75 (273)

B

2.4 (61)

3.0 (76)

3.5 (89)

4.8 (122)

5.0 (127)

5.75 (146)

6.25 (159)

7.13 (181)

C1 C2 D

6.75 (172) 5.75 (146) 3.75 (95)

F G

2.00 (51) 0.38 (10)

2.38 (60) 0.44 (11)

2.75 (70)

3.50 (89)

0.5 (13)

0.75 (19) 4.00 (102)

4.75 (121)

5.25 (133)

5.75 (146)

1.0 (25)

I

0.63 (16)

J1

—

—

—

—

J2

—

—

—

—

1.75 (45) 1.38 (35)

J3

—

—

—

J4

—

—

—

0.88 (22) 0.81 (21)

1.63 (41) 0.88 (22)



Class RK1

Time-delay, Low-Peak LPN-RK and LPS-RK

• LPN-RK (250 Vac) and LPS-RK (600 Vac), up to

600 A, current-limiting, dual-element, 300 kA IR AC,

100 kA IR DC

• UL Std. 248-12 Class RK1, Guide JDDZ, File E4273,

CSA Class 1422-02, CSA File 53787 — LPN-RK

0-60 A (125 Vdc, 50 kA IR), 70-600 A (250 Vdc,

50 kA IR), LPS-RK 0-600 A (300 Vdc, 50 kA IR)

Low-Peak RK1 fuses provide a very high degree of fault current limitation of Limitron fuse plus the overload protection of the Fusetron fuse in all types of circuits and loads.

They can be closely sized to motor full load amps for reliable backup protection. Close sizing to loads permits using smaller and more economical switches (and fuses), better selective coordination, and a greater degree of current limitation for component protection. RK1 Low-

Peak fuses are rejection type but also fit non-rejection type fuse holders to easily replace lower-rated Class H(K) fuses in existing installations

(see LPN-RK data sheet no. 1001, up to 60 A, and no. 1002, 70 to

600 A, and LPS-RK data sheet no. 1003, up to 60 A, and no. 1004, 70 to

600 A).

Non-time delay (fast-acting), Limitron KTN-R and KTS-R

• KTN-R (250 Vac) and KTS-R (600 Vac), up to 600 A, current-limiting, 200 kA IR AC

• UL Std. 248-12 Class RK1, Guide JDDZ, File


KTN-R and KTS-R single-element, fast-acting fuses have no intentional time-delay and provide a high degree of fault current limitation (component protection). They are well suited for circuits and loads without the in-rush currents. RK1 Limitron fuses are rejection type but also fit non-rejection type fuse holders to easily replace lower-rated Class H(K) fuses in existing installations (see KTN-R data sheet no.

1043, and KTS-R data sheet no. 1044).

Class RK5 — Time-delay, Fusetron FRN-R and FRS-R

• FRN-R (250 Vac) and FRS-R (600 Vac), up to 600 A, current-limiting, dual-element, 200 kA IR AC,

20 kA IR DC

• UL Std. 248-12 Class RK5, Guide JDDZ, File

E4273, CSA Class 1422-02, CSA File 53787 —

FRN-R and FRS-R, up to 600 A, 200 kA IR AC,

FRN-R 125 Vdc, 20 kA IR DC (up to 60 A and

110 to 200 A), 250 Vdc, 20 kA IR DC (225 to

600 A), FRS-R 300 Vdc, 20 kA IR DC (up to 30

A and 65 to 600 A), 250 Vdc, 20 kA IR DC (35 to 60 A)

FRN-R and FRS-R RK5 time-delay fuses provide excellent overload protection for loads with inrush current like motors, transformers and solenoids. Fusetron fuses are not as fast-acting on shortcircuits as Low-Peak fuses, and do not give as high a degree of component short-circuit protection.

Like the Low-Peak fuse, Fusetron fuses can be sized closer to loads to permit using smaller size and less costly switches. RK5 Fusetron fuses are rejection type but also fit non-rejection type fuse holders to easily replace lower-rated Class H(K) fuses in existing installations (see

FRN-R data sheet no. 1017, up to 60 A, and no. 1018, 70 to 600 A, and

FRS-R data sheet no. 1019, up to 60 A, and no. 1020, 70 to 600 A).

Basic dimensions are same as Class H(K), general purpose (NON and

NOS) fuses. Note : relating to dimensional compatibility these fuses can replace existing Class H, RK1 and RK5 fuses.

A A

B

Up to 60 A 70 to 600 A


250 V fuses

Amp range

Up to 30

35-60

A

2 (51)

3 (76)

B

0.56 (14)

0.81 (21)

600 V fuses

A

5.0 (127)

5.5 (140)

RK5 FRN-R, FRS-R, — RK1 KTN-R, KTS-R

70-100 5.88 (149) 1.06 (27)

110-200

225-400

7.13 (181)

8.63 (219)

1.56 (40)

2.38 (61)

2.88 (73)

7.88 (200)

9.63 (245)

11.63 (295)

13.38 (340) 450-600 10.38 (264)

RK1 LPN-RK, LPS-RK

70-100 5.88 (149)

110-200 7.13 (181)

1.16 (30)

1.66 (42)

7.88 (200)

9.63 (245)

225-400

450-600

8.63 (219)

10.38 (264)

2.38 (61)

2.88 (73)

11.63 (295)

13.38 (340)

B

0.81 (21)

1.06 (27)

1.34 (34)

1.84 (47)

2.59 (66)

3.13 (80)

1.16 (30)

1.66 (42)

2.38 (61)

2.88 (73)

B

Bussmann series modular Class R fuse blocks feature a snap-together construction to create the required number of poles. Either DIN-Rail or panel mount, these blocks are available with optional covers that provide a lockout/tagout provision for enhanced electrical safety. Also available in power distribution fuse block versions.

Class H(K)

General purpose, NON and NOS

• NON (250 Vac) and NOS (600 Vac) up to 600 A, non-current-limiting, 50 kA IR AC up to 60 A) and 10 kA (65 to 600 A)

• UL Std. 248-9 Class K5, Std. 248-9, UL Std.

248-6 Class H, Guide JDDZ, File E4273, CSA

Class 1421-01, CSA File 53787 (NON 65-600 A)

10 kA IR AC, (NOS 70-600 A) 10 kA IR AC.

NON and NOS Class H(K) general purpose fuses are not considered current-limiting fuses, do not incorporate intentional time-delay and are used in circuits with low available fault currents. We recommend upgrading to Class R fuses that can be installed without the need to change fuse blocks or holders (see NON/NOS data sheet no.

1030).


Class T

Non-time delay/fast-acting, Limitron JJN and JJS

• JJN (300 Vac up to 1200 A) and JJS (600 Vac up to 800 A), current-limiting, 200 kA IR AC

• UL Std. 248-15 Class T, Guide JDDZ, File E4273,

JJN 15-600 A (160 Vdc, 20 kA IR), JJN 601-1200 A

(170 Vdc 100 kA IR), CSA Class 1422-02,

CSA File 53787

JJN and JJS fuses are the space-saving counterparts to KTN-R/KTS-R Limitron fuses.

At one-third the size, they are well suited for applications where space is very restricted. These single-element fuses are extremely fast-acting and provide a high degree of current limitation on short-circuits for excellent component protection.

These fuses will give only short-circuit protection and must be oversized for circuits with inrush currents common to motors, transformers and other inductive components (see JJN data sheet no. 1025, and JJS data sheet no. 1029).

A A

A

C

D

B B 1.0 (25)

JJN up to 60 A

JJS up to 30 A

JJS 35 to 60 A


Amp range A

300 V JJN

Up to 30 0.88 (22)

35-60

70-100

110-200

225-400

0.88 (22)

2.16 (55)

2.44 (62)

2.75 (70)

450-600

601-800

801-1200

600 V JJS

Up to 30

35-60

70-100

110-200

225-400

450-600

601-800

3.06 (78)

3.38 (86)

4.00 (102)

1.50 (38)

1.56 (40)

2.95 (75)

3.25 (83)

3.63 (92)

3.98 (101)

4.33 (110)

B

0.41 (10)

0.56 (14)

0.75 (19)

0.88 (22)

1.00 (25)

1.25 (32)

1.75 (45)

2.00 (51)

0.56 (14)

0.81 (21)

0.75 (19)

0.88 (22)

1.00 (25)

1.25 (32)

1.75 (45)

JJN 70 to 1200 A

JJS 70 to 800 A

C

—

—

1.56 (40)

1.69 (43)

1.84 (47)

2.03 (52)

2.22 (56)

2.53 (64)

—

—

2.36 (60)

2.50 (64)

2.72 (69)

2.96 (75)

3.17 (81)

D

—

—

0.84 (21)

0.84 (21)

0.86 (22)

0.88 (22)

0.89 (23)

1.08 (27)

—

—

1.64 (42)

1.66 (42)

1.73 (44)

1.78 (45)

1.88 (48)

B


3



Bussmann series branch circuit fuse selection chart (600 V or Less)

Amp rating Catalog

Circuit Load (A) Fuse type symbol

Conventional dimensions—Class RK1, RK5 (0-600 A), L (601-6000 A)

All type loads (optimum overcurrent protection)

Up to 600

Low-Peak (dualelement, timedelay)

LPN-RK-SP

LPS-RK-SP

KRP-C-SP

Voltage rating (AC)

250 V

600 V

600 V

Fuse class

RK1††

L

Interrupting rating (kA)

300

300

Remarks

All-purpose fuses. Unequaled for combined short-circuit and overload protection.

Motors, welder, transformers, capacitor banks(circuits with heavy inrush currents)


Power electronic applications such as drives and SSRs

Non-motor loads

(circuits with no heavy inrush currents)

Fusetron (dualelement, timedelay)

Low-Peak (dualelement, timedelay)

Drive fuse (high speed Class J)

Limitron (fastacting)

FRN-R

FRS-R

KLU

250 V

600 V

600 V

RK5††

L

Non-motor loads

(circuits with no heavy inrush currents).

Limitron fuses

Up to 600


KTN-R

KTS-R

250 V

600 V

RK1†† 200 particularly suited for circuit breaker 601 to 6000 KTU 600 V L 200 A fast-acting, high performance fuse.

protection. Up to 600 A

Reduced dimensions for installation in restricted space—CUBEFuse Class CF (0-100 A), Class J (0-600 A), T (0-1200 A), CC (0-30 A), G (0-60 A)


Up to 600

Up to 100

Up to 600

Up to 600

Up to 600

CUBEFuse

(finger-safe, dual-element, time-delay)

TCF

LPJ

DFJ

JKS

JJN

600 V

600 V

600 V

600 V

300 V

(CF)

J***

J

J

J

T

200

200

300

300

200

200

200

Moderate degree of current limitation. Time-delay passes in-rush currents.

All-purpose fuse. Time- delay passes in-rush currents.

Same short-circuit protection as Low-

Peak fuses but must be sized larger for circuits with inrush currents; i.e., up to 300%.

Finger-safe, all-purpose fuses.

Unequaled for combined shortcircuit and overload protection.

(Specification grade product)

All-purpose fuses. Unequaled for combined short-circuit and overload protection. (Specification grade product)

Where branch circuit protection is needed with high speed fuse characteristics.

Very similar to KTS-R Limitron, but smaller.

The space saver (1/3 the size of

KTN-R).

Up to 800

Limitron fastacting

JJS 600 V T 200

The space saver (1/3 the size of

KTS-R).

Motor loads (circuits with heavy inrush currents)

Non-motor loads

(circuits with no heavy inrush currents)

Control transformer circuits and lighting ballasts; etc

Up to 30

Low-Peak

(time-delay)


Limitron (timedelay)

LP-CC

KTK-R

FNQ-R

600 V

600 V

600 V

CC

CC

CC

200

200

200

Very compact (13/32” x 1-1/2”); rejection feature. Excellent for motor circuit protection.

Very compact (13/32” x 1-1/2”); rejection feature. Excellent for outdoor highway lighting.

Very compact (13/32” x 1-1/2”); rejection feature. Excellent for control transformer protection.

General purpose; i.e., lighting panelboards

Miscellaneous

Plug fuses can be used for branch circuits and small component protection.

Up to 60

Up to 600

Up to 30

Up to 12

General purpose (1/2-6

A fast-acting,

7-60 A timedelay)

General purpose

Type S (dualelement, timedelay)

Type T (dualelement, timedelay)

Type W (fastacting)

SC

NON

NOS

S

T

W

600 (0-20 A)

480 V (25-

60 A)

250 V

600 V

125 V

125 V

125 V

G

H or K5†

S

**

**

100

10

10

10

10

Current limiting; 13/32” dia. x varying lengths per amp rating.

Forerunners of the modern cartridge fuse.

Base threads of Type S differ with amp ratings (size rejecting). T and W are Edison base. T and S Type fuses recommended for motor circuits. W not recommended for circuits with motor loads.

** UL Listed as Edison base plug fuse.

† Some amp ratings are available as UL Class K5 with a 50 kA interrupting rating.

†† RK1 and RK5 fuses fit standard switches, equipped for non-rejection fuses (K1, K5 and H) fuse blocks and holders; however, the rejection feature of Class R switches and fuse blocks designed specifically for rejection type fuses (RK1 and RK5) prevents the insertion of the non-rejection fuses (K1, K5, and H).

*** Class J performance, special finger-safe dimensions.

**** For many of these fuse types, there are indicating and non-indicating versions, each with different catalog numbers.

Table 3.1.6.1.d

Branch circuit fuse selection chart.



3.1.6.2 Supplemental/application limited OCPDs

The supplemental or application limited OCPDs in Figure 3.1.6.2.a are not branch circuit rated (cannot be installed in a building’s electrical system) and serve specific functions within a circuit. Two application limited OCPD examples include motor circuit protectors and supplemental protective devices.

Application limited OCPDs cannot be used in place of branch circuit

OCPDs, however a branch circuit rated OCPD can be used in lieu of an application limited OCPD.

Understanding the differences between these devices is important to ensure their proper application. Not using a branch circuit OCPD where required could result in potentially serious electrical safety hazards to people or damage to property. In addition, NEC violations could be tagged by the authority having jurisdiction (AHJ), resulting in project delays and unplanned delays and costs.

UL 248-14 supplemental fuses

UL 1077 supplemental protectors

(mini circuit breakers)

Figure 3.1.6.2.a

Supplemental OCPDs cannot be used for branch circuit protection.

Supplemental OCPDs are not general use devices and must be evaluated for appropriate application in every instance where they are used. Supplemental OCPDs are extremely application oriented, and prior to application, the differences and limitations of these devices must be investigated and found acceptable.

Bussmann series supplemental fuses

The following pages contain examples of non-time delay/fast-acting and time-delay fuses with their specifications that must be considered before a correct selection can be determined for a particular application.

Of particular note with these fuses are the following:

• Agency information is not applicable to all ratings

• Specific fuse amp ratings may have different voltage ratings and corresponding interrupting ratings

• Construction may vary, depending on amp rating, and impact the available mounting means (see FNA, page 3-43)

• Some are pin-indicating to provide a visual notification means or activating a microswitch for remote monitoring systems

• Their labels are color coded to indicate maximum voltage rating that can coincide within an amp range inside a fuse family

All these factors must be reviewed to be sure the appropriate supplemental fuse is specified and meets the application’s requirements for:

• Operation (time-delay or non-time delay/fast-acting)

• Voltage rating (by fuse amp rating)

• Amp rating

• Interrupting rating at applied amp rating and system voltage

• Special needs (pin-indication)

Exceptions in the Code for applying supplemental OCPDs

There are exceptions that do allow using a supplemental, application specific OCPD in a branch circuit, but defined NEC conditions must be met.

(1) Permitted for specific branch circuit applications under limited conditions per the specific reference in the NEC: These OCPDs have some limitation(s) and are not true branch circuit devices, but may be permitted if qualified for the use in question. Examples include:

• High speed fuses that are not branch circuit OCPDs, but can be used for fault current protection on motor circuits utilizing power electronic devices by 430.52(C)(5).

• Motor Circuit Protectors (MCPs) are recognized devices

(not listed) and can be used for fault current protection of motor branch circuits, if applied in combination with a listed combination starter for which the MCP has been tested and found acceptable [per 430.52(C)(3)].

• Self-protected starters listed only for motor branch circuit protection; they cannot be used on other branch circuit types or for main or feeder protection.

• When considering supplemental, application specific OCPDs, special attention must be paid to the circuit type, NEC requirements and the device’s product listing or recognition.

(2) Supplemental overcurrent protective devices: These devices have limited applications and must always be in compliance with

240.10.

240.10 Supplementary Overcurrent Protection. Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment...it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection.

3



Applying supplemental OCPDs in branch circuits

The NEC defines a supplemental OCPD as “A device intended to provide limited overcurrent protection for specific applications and utilization equipment such as luminaires (lighting fixtures) and appliances. This limited protection is in addition to the protection provided in the branch circuit by the required branch-circuit overcurrent protective device.”

Supplemental OCPDs can:

• Only be used for additional protection when installed on the branch circuit overcurrent device’s loadside

• Not be applied where branch circuit OCPDs are required

• Be properly used in appliance applications and for additional

(supplemental) protection where branch circuit overcurrent protection is already provided. In appliance applications, the supplemental device inside the appliance provides protection for internal circuits and supplements the branch circuit OCPD’s protection.

Using supplemental OCPDs is permitted by 240.10 for lighting and appliances shown in Figure 3.1.6.2.b. The supplemental protection is in addition to that provided by the branch circuit OCPD protecting the branch circuit located in the lighting panel in Figure 3.1.6.2.b.

Luminaires

X

LPJ-200SP

Distribution panel

TCF20RN

Faulted ballast

X

Luminaires

12 AWG wire

Branch circuit protective device

Supplemental protection

(KTK-R-3 fuses)

LPJ-200SP

Distribution panel

TCF20RN

Faulted ballast

12 AWG wire

Branch circuit protective device

Supplemental protection

(GLR-3 fuses)

Branch lighting panel

Figure 3.1.6.2.b

Supplemental OCPDs may be used per 240.10, but will not be considered as protecting a branch circuit.

Branch circuit OCPDs are permitted for supplemental protection and can replace a supplemental OCPD (see Figure 3.1.6.2.c). Rather than using a supplemental OCPD for supplemental luminaire protection, a branch circuit OCPD is used. The fact that a branch circuit OCPD (a KTK-R-3 fuse) is used where a supplemental device is permitted does not turn the circuit between the lighting panel and the fixture from a branch circuit into a feeder circuit. In the case of Figure 3.1.6.2.c, the branch circuit starts on the 20 A fuse’s loadside in the lighting panel.

Branch lighting panel

Figure 3.1.6.2.c

Branch circuit OCPDs can be used for supplemental protection.

Another difference and limitation is that supplemental OCPDs may have creepage and clearance spacing that are considerably less than a branch circuit OCPD. Two such creepage and clearance spacing differences include:

• A supplemental protector, recognized to UL 1077, has 3/8 inch through air spacing requirements between terminals and 1/2 inch over surface at 480 V.

• A branch circuit rated UL 489 molded case circuit breaker with

1 inch through air and 2 inches over surface has spacing requirements between terminals at 480 V.

Further, branch circuit OCPDs have standard overload characteristics to protect branch and feeder circuits, and service entrance conductors.

Supplemental OCPDs do not have standard overload (time-current) characteristics and may differ considerably from standard branch circuit overload characteristics. Also, supplemental OCPDs have interrupting ratings that range from 32 A to 100 kA. When supplemental OCPDs are considered for proper use, it’s important to be sure the interrupting rating equals or exceeds the available fault current, and that the device has the proper voltage rating for the installation (including compliance with slash voltage rating requirements, if applicable).


10 Reasons why supplemental protectors are not allowed to protect branch circuits

1. Supplemental protectors are not intended to be used, nor are they evaluated for branch circuit protection in UL 1077.

2. Compared to branch circuit OCPDs, supplemental protectors have drastically reduced spacings, and often depend upon a separate, upstream branch circuit OCPD.

3. Supplemental protectors do not have standard calibration limits or overload characteristic performance levels, and cannot ensure proper protection for branch circuits.

4. Multi-pole supplemental protectors used in three-phase systems are not evaluated for protection against all overcurrent types nor tested to protect circuits from all fault types (e.g., line-to-ground faults on B-phase grounded systems).

5. Most supplemental protectors are fault current tested with an upstream branch circuit OCPD and rely upon this device for proper performance.

6. Supplemental protectors do not require testing for closing into a fault.

7. Supplemental protector re-calibration (for supplemental protection by circuit breakers) is not required and depends upon the manufacturer’s preference. There is no performance assurance following a fault or resetting the device. The product standard does not require supplemental devices to be re-calibrated and operational after interrupting a fault.

8. Considerable damage to a supplemental OCPDs is allowed following short-circuit testing.

9. Supplemental protectors are not intended for use as a disconnecting means.

10. Supplemental protectors are not evaluated for fault current performance such as energy let-through limits or protecting test circuit conductors.


3



BAF non-time delay (fast-acting) 13/32” x 1-1/2”

Fast-acting, supplemental fuse. Green color code (250

Vac max) (see data sheet no. 2011).

For superior protection, Eaton recommends upgrading to Bussmann series Low-Peak Class CC fuses (see data sheet no. 1023).

Ratings

Fuse amp range

1/4 to 1

1-1/2 to 2-1/2

3

4 to 10

IR at system voltage

250 Vac

35 A

100 A

100 A

200 A

12 to 15

20 to 30

Agency information

750 A

200 A

125 Vac

10 kA

10 kA

10 kA

10 kA

10 kA

10 kA


UL CSA

—

—

X

X

X

—

X

X

X

X

X

—

• UL Listed, Std. 248-14, 250 Vac (3 to 15 A) Guide JDYX,

File E19180, CSA Certified, 250 Vac (1/4 to 15 A) Class 1422-01, File

53787

BBS non-time delay (fast-acting) 13/32” x 1-3/8”

Fast-acting supplemental fuse. Color codes black (600 Vac max 1/10 to 6

A), green (250 Vac max 7 to 10 A), and purple (48 Vac max 12 to 30 A) (see data sheet no. 2010).


Ratings

Fuse amp range

1/10 to 6

7 to 10

12 to 30*


600 Vac 250 Vac

10 kA —

—

—

10 kA

—

48 Vac

—

—

—


UL

X

X

—

CSA

X

X

—

* For interrupting rating, contact factory.


• UL Listed, Std. 248-14 (1/10-6 A@600 Vac, 7-10 A@250 Vac), Guide

JDYX, File E19180, CSA Certified, C22.2 No. 248.14 (1/10-6 A @

600 Vac, 7-10 A @ 250 Vac), Class 1422-01, File 53787, CE

KLM non-time delay (fast-acting) 13/32” x 1-1/2”

Fast-acting supplemental fuse. Color code black

(600 Vac/dc max) (see data sheet no. 2020).


For protecting PV systems, use PVM 10x38mm PV fuses (see data sheet no. 10121).

Ratings

Fuse amp range

1/10 to 30

IR at rated voltage

600 Vac

100 kA

Electrical characteristics

600 Vdc

50 kA


UL

X

CSA

X

% of amp rating

110%

135%

Opening time

4 hours minimum

AC opens within 1 hour


• UL Listed, Std. 248-14, Guide JDYX, File E19180, CSA Certified, C22.2

No. 248. 14, Class 1422-01, File 53787, RoHS compliant, CE

KTK non-time delay (fast-acting) 13/32” x 1-1/2”

Fast-acting supplemental fuse. Black color code

(600 Vac max) (see data sheet no. 1011).

For superior protection, Eaton recommends upgrading to Bussmann series Low-Peak Class

CC fuses (see data sheet no. 1023).

Ratings

Fuse amp range

1/10 to 30



600 Vac

100 kA


UL

X

CSA

X


No. 248.14, Class 1422-01, File 53787, HRC-MISC, RoHS compliant,

CE

Bussmann series modular fuse blocks easily dovetail together to created the desired number of poles. Installation flexibility is provided with either DIN-Rail or panel mounting. They are available with optional covers to provide IP20 finger-safe protection and feature a builtin lockout/tagout provision for added electrical safety.



MIC non-time delay (fast-acting) 13/32” x 1-1/2” pin-indicating

Fast-acting, pin-indicating supplemental fuse. Green color code (250 Vac max 1 to 15 A), grey (32 Vac max 20 to 30 A)


Ratings

Fuse amp range

1

2 to 3

5 to 10

15

20 to 30

IR at voltage rating

250 Vac

35 A

100 A

200 A

750 A

—

Electrical characteristics

32 Vac

—

—

—

—

10 kA


UL

X

X

X

X

—

% of fuse rating

110%

135%


Opening time

Indefinitely

1 hour max

• UL Listed, Std. 248-14, 1-15 A, Guide JDYX, File E19180, CE

CSA

—

—

—

—

—

FNA time-delay 13/32” x 1-1/2” pin-indicating

Pin-indicating time-delay supplemental fuse. Color coded green (250 Vac max 1/10 to 6 A), blue

(125 Vac max 6-1/4 to 15

A) and grey (32 Vac max

20 to 30 A) (see data sheet no. 2029).

Ratings Dual-tube construction 12 A and up

Fuse amp range

1/10 to 8/10

1 to 6

6-1/4 to 15

20 to 30


250 Vac 125 Vac 32 Vac

35 A

200 A

—

—

10 kA

10 kA

10 kA

—

—

—

—

1 kA


UL CSA

X

X

X

—

X

X

X

—


• UL Listed, 1/10 to 8/10 A @ 125/250 Vac, 1-15 A @ 125 Vac,

Guide JDYX, File E19180, CSA Certified, 1/10 to 10 A @ 125 Vac,

Class 1422-01, File 53787, CE

FNM time-delay 13/32” x 1-1/2”

Time-delay supplemental fuse. Color code green (250 Vac max) (see data sheet no. 2028).

For superior protection, Eaton recommends upgrading to Bussmann series Low-Peak Class

CC fuses (see data sheet no. 1023).

Ratings

Fuse amp range

1/10 to 1

1-1/8 to 3-1/2

4 to 10

12 to 30

IR at system voltage Agency information

250 Vac 125 Vac UL CSA

35 A

100 A

200 A

10 kA

10 kA

10 kA

10 kA

—

X

X

X

X

X

X

X

X


• UL Listed, Std. 248-14, Guide JDYX; File E19180, CSA Certified,

Class 1422-01, File 53787, RoHS compliant, CE

FNQ time-delay 13/32” x 1-1/2”

Time-delay supplemental fuse. Color code orange (500 Vac max) (see data sheet no. 1012).

For superior protection, Eaton recommends upgrading to Bussmann series Limitron FNQ-R

Class CC fuses (see data sheet no. 1014).

Ratings

Fuse amp range

1/10 to 30


500 Vac

10 kA


UL CSA

X X



No. 248.14, Class 1422-01, File 53787, HRC-MISC, RoHS compliant,

CE

3

Bussmann series finger-safe CH modular fuse holders for supplemental, PV and

Class CC fuses feature a snap-together construction to create the required number of poles. These DIN-Rail mount holders are available with optional open fuse indication and PLC remote fuse monitoring for faster troubleshooting, and accessories like comb busbars for easy ganging.



3.1.6.3 Medium voltage fuses

Medium voltage fuses generally have ratings that range from 2.5 kV to

38 kV and are designated under one of three ANSI/IEEE C37.40 defined classifications:

•

General Purpose Current-Limiting: A fuse capable of interrupting all currents from the rated interrupting current down to the current that causes the fusible element to melt in one hour

• Back-up Current-Limiting: A fuse capable of interrupting all currents from the maximum rated interrupting current down to the rated minimum interrupting current

• Expulsion: A vented fuse in which the expulsion effect of gasses

(produced by the arc and housing, either alone or aided by a spring) extinguish the arc

The general purpose and back-up current-limiting fuses are constructed in a sealed, non-venting design that, when the element melts from a current within the fuse’s interrupting rating, produces arc voltages exceeding the system voltage which, in turn, forces the current to zero.

The arc voltages are produced by a series of high resistance arcs within the fuse’s element to create a fuse that typically interrupts high fault currents within the first 1/2 cycle.

The expulsion fuse, in contrast, depends on the interruption process being initiated by a single arc that acts as a catalyst to create and cause a de-ionizing gas to escape from its housing.

The arc is then elongated, either by the gases’ force or a spring so that, at some point, the arc elongates sufficiently enough to prevent a restrike after the AC current cycle passes through zero volts and may take many cycles to clear.

Application

Many rules for applying expulsion and current-limiting fuses are the same, with some additional rules applied to current-limiting fuses because they operate much faster on high fault currents. The three basic factors to consider when applying any medium voltage fuse are:

• Voltage rating

• Continuous current carrying capacity

• Interrupting rating

Voltage rating

As a rule, medium voltage fuses should be applied on systems as close to their voltage rating as possible (unlike low voltage fuses that can be applied on a system at or below their rating). This is particularly important with current-limiting fuses that function by creating multiple high resistance arcs that will drive up the fuse’s peak arcing voltage.

The arcing voltage should never exceed the system basic insulation level

(BIL) and create a safety hazard.

Continuous current carrying capacity

Continuous current values shown on the fuse label represent the continuous current the fuse can carry without exceeding the temperature rise specified in ANSI C37.46. An application that exposes the fuse to a current slightly above its continuous rating, but below its minimum interrupting rating, may cause damage to the fuse from excessive heat. This is the main reason motor circuit protection uses overload relays in series with back-up current-limiting fuses.

Interrupting rating

As with all fuses, medium voltage fuses need to have an interrupting rating equal to or greater than the available fault current.

Rules for medium voltage current-limiting fuses

To ensure proper current-limiting fuse application, it’s important to apply the following:

• As stated earlier, current-limiting fuses produce arc voltages that exceed the system voltage. Care must be taken to ensure the peak voltages do not exceed the system’s basic insulation level

(BIL). If the fuse voltage rating is not permitted to exceed system voltage by 140%, there should not be a problem. This does not mean that a higher rated fuse cannot be used, but points out that one must ensure the system’s BIL will handle the peak arc voltage produced.

• As with the expulsion fuse, current-limiting fuses must be properly coordinated with other system OCPDs. For this to happen, the rules for applying an expulsion fuse must be used at all currents that cause the fuse to interrupt in 0.01 second or greater.

When other current-limiting OCPDs are on the system, it becomes necessary to use I 2 t (the thermal energy required to melt a specific fuse element at rated current under test condition, expressed

“current squared times seconds” or as A 2 s “amps squared times seconds” values for coordination at currents causing the fuse to interrupt in less than 0.01 second. These may be supplied as minimum and maximum values, or minimum melting and total clearing I 2 t curves. In either case, apply the following:

• The fuse’s minimum melting I 2 t should be greater than the downstream current-limiting device’s total clearing I 2 t.

• The fuse’s total clearing I 2 t should be less than the upstream current-limiting device’s minimum melting I 2 t.

Applying R-Rated fuses

The current-limiting fuse should be selected so that the overload relay’s curve crosses the fuse’s minimum melting curve at a current greater than 110% of the motor’s locked rotor current.

A preliminary choice is obtained through the following formula:

6.6 x Full Load Current ÷ 100 = R rating of fuse

This value is rounded up to the next R-Rated fuse size.

Example:

A 2300 V motor has a 100 amp full load current rating and 600 amps locked rotor current. The preliminary choice is:

6.6 x 100 ÷ 100 = 6.6

When rounded up to the next standard R-Rated size, it becomes a 9R fuse, but this must be checked against the appropriate time-current characteristics curves, shown in Figure 3.1.6.3.a.



1000

100

Overload relay

6R 9R 12R

JCK-9R

10

1

125% motor

FLA

0.1

0.01

110% locked rotor current

M

Thermal overload relay

Contactor

Motor

FLA 100 A

Locked rotor current 600 A

Current in amps

Figure 3.1.6.3.a

Special care must be taken to ensure the fuse amp rating is compatible with the motor’s overload relay.

The overload relay in this example has the time-current characteristic shown in Figure 3.1.6.3.a. To ensure the proper fuse is selected, one must plot 110% of the locked rotor current and the range (6R, 9R, 12R) of R-Rated fuses on the same graph as the overload relay.

The selected fuse should be the smallest whose minimum melting characteristic crosses the overload relay at a current greater than 110% of the motor’s locked rotor current. In this example, it would be a

2400 V 9R fuse determined in the given formula. This agrees with the quick selection choice. Depending on the type of installation and starter being used, a JCK-9R, JCK-A-9R, or 2BCLS-9R would be the correct choice.

Additional rules

When choosing an expulsion fuse, it’s important that it be properly coordinated with other upstream and downstream OCPDs. To accomplish this, one must consider the devices’ melting and clearing characteristics. Two curves, the minimum melting and the total clearing curve, provide this information. To ensure proper coordination, the following should apply:

• The total clearing curve for any downstream protective device must be below a curve representing 75% of the applied fuse’s minimum melting curve

• The total clearing curve for the applied fuse must lie below a curve representing 75% of the minimum melting curve for any upstream protective device

R-Rated medium voltage fuses and motor circuits

R-Rated medium voltage fuses are back-up current-limiting fuses used in junction with medium voltage motors and controllers.

These fuses are only for short-circuit protection and do not protect themselves or other components during extended overloads. They offer a high level of fault current interruption in a self-contained, non-venting package that can be mounted indoors or in an enclosure. All Bussmann series R-Rated fuses come with open fuse indication, with some available with a Hookeye option for use with a hookstick for non-loadbreak isolation.

R-Rated fuses do not have “amp ratings,” but rather an R-rating when they meet the following requirements:

• The fuse will safely interrupt any current between its minimum and maximum interrupting rating

• The fuse will melt in a range of 15 to 35 seconds at a value of

100 times the “R” number (ANSI C37.46).

Application

Medium voltage motors are efficiently protected by overload relays applied in conjunction with back-up current-limiting fuses that open the circuit under high fault conditions. The overload relay is chosen to interrupt currents below the fuse’s minimum interrupting rating.

Since multiple devices are used to provide motor protection, it is very important that they be properly coordinated. The motor starter manufacturer typically designates the proper fuse R-rating, overload relay and contactor.

3

The Eaton Bussmann Division’s Paul P. Gubany Center for High Power

Technology is the electrical industry’s most comprehensive facility for testing and certifying device and equipment SCCRs. Capable of performing electrical tests up to 600 V three-phase, 300 kA fault current for meeting ANCE, ANSI, CE, CSA, ETL, IEC and UL testing requirements.



R-Rated for motor circuits

• 2.4 kV: 2CLS, 2ACLS, JCK,

JCK-A, 2HCLS, 2BCLS —

25 to 450 A

• 4.8 kV: 5CLS, JCL, 5LCLS,

JCL-A, 5ACLS, 5HCLS,

5BCLS, 5MCLS —

30 to 800 A

• 7.2 kV: 8CLS, 7CLS, 8ACLS,

7ACLS, 7BCLS —

70 to 800 A

• 50 kA IR Sym.

• 80 kA IR Asym.

R-Rated medium voltage fuses are back-up current-limiting fuses used in conjunction with medium voltage motors and motor controllers to provide short-circuit protection.

Current-limiting fuses may be designated as R-Rated if they meet the following requirements:

• The fuse will safely interrupt all currents between its minimum and maximum interrupting ratings.

• The fuse will melt in a range of 15 to 35 seconds at a value of 100 times the “R” number (ANSI C37.46).

Bussmann series R-Rated fuses offer a high level of fault current interruption in a self-contained, non-venting package that can be mounted indoors or in an enclosure.

Available styles include standard, AMPGARD™ Hookeye, hermetically sealed and bolt-on with open fuse indication standard on all fuses (see data sheet no. 6001).

E-Rated for transformer and feeder protection

• 2.75 kV: 2CLE, JCX —

1 to 450 A (see data sheet no. 10350)

• 5.5 kV: AHLE, BHLE, HCL, 5CLE, 5HLE,

JCY, MV055F —


• 8.3 kV: 8CLE, 8HLE, 8AHLE, 8BHLE, 8HCL

— 10 to 350 A (see data sheet no. 10352)

• 15.5 kV: 15CLE, 15HLE, 15LHLE, MV155F,

15BHLE, 15HCL —


• 5.5 to 38 kV (DIN dimensioned for switchgear):

55GDMSJ, 55GFMSJ, 155GQQSJ, 175GDMSJ,

175GFMSJ, 175GXMSJ, 175GXQSJ, 258GDQSJ,

258GXQSJ, 258GXZSJ, 38GFZSJ —


• 25 to 65 kA IR catalog number dependent

E-Rated medium voltage fuses are general purpose current-limiting fuses. The E-rating defines the fuse’s melting time-current characteristic and the ratings are used to allow electrical interchangeability among different manufacturers. A general purpose E-Rated fuse must meet these requirements:

• The current responsive element shall melt in 300 seconds at an RMS current within the range of 200% to 240% of the fuse’s continuous current rating (ANSI C37.46).

• The current responsive element above 100 amps shall melt in 600 seconds at an RMS current within the range of 220% to 264% of the fuse’s continuous current rating (ANSI C37.46).

Bussmann series E-Rated fuses provide primary protection for transformers, feeders and branch circuits. They are non-venting fuses which must be mounted indoors or in an enclosure. Their current-limiting ability reduces the fault current (I 2 t) that the system components must withstand.


E-Rated for potential and small transformers

• 2.475 kV: 2NCLPT —

0.25 to 5 A

• 3.6 kV: 3.6ABCNA,

3.6ABWNA, 3.6CAV —

2 to 10 A

• 5.5 kV: JCW, 5CLPT, 5NCLPT,

5.5ABWNA, 5.5AMWNA,

5.5CAV, 5.5CAVH —

0.5 to 15 A

• 7.2 kV: 7.2ABWNA, 7.2ABCNA,

7.2AMWNA, 7.2CAV —

0.5 to 10 A

• 8.3 kV: 8CLPT, CLPT, 8NCLPT —

0.5 to 10 A

• 12 kV: 12ABCNA, 12CAV —

2 to 3.15 A

• 15.5 kV: 15CLPT, 15NCLPT, 15.5CAV, 15.5CAVH —

0.5 to 10 A

• 17.5 kV: 17.5ABGNA, 17.5CAV —

2 to 10 A

• 24 kV: 24ABGNA, 24CAV —

2 to 4 A

• 25.5 kV: 25CLPT —

0.5 to 1 A

• 36 kV: 36ABGNA, 36CAV —

2 to 4 A

• 38 kV: 38CAV, 38CAVH, 38CLPT —

0.5 to 4 A

• 25 to 80 kA IR Sym., catalog number dependent.

Low amp, E-Rated medium voltage fuses are general purpose currentlimiting fuses defined by their melting time-current characteristic that permits their electrical interchangeability with fuses having the same

E-rating. To be E-Rated, the fuse responsive element shall melt in 300 seconds at a RMS current within the range of 200% to 240% of the fuse’s continuous current rating. (For fuses rated 100E or less)(ANSI

C37.46).

Bussmann series low amp, E-Rated fuses provide primary protection for potential, small service and control transformers. These fuses offer a high level of fault current interruption in a self-contained, non-venting package that can be mounted indoors or in an enclosure (see data sheet no. 6002).


3



3.1.6.4 High speed fuses

High speed fuses are often called “semiconductor fuses” and sometimes feature a diode symbol on their label. This does not mean there’s a diode in the fuse’s construction. The symbol is there to indicate the fuse is primarily used to protect solid state devices.

In general terms, high speed fuses are not full range fuses. They are applied for short-circuit protection that requires a very “high speed” response to faults. As such, their proper specification and selection requires greater attention to application details and correction factors too numerous to cover in this section. More information on this subject is available in the Bussmann series high speed fuse application guide, publication no. 10507.

Protecting solid-state power equipment often differs significantly from electrical equipment and requires the unique operating characteristics only offered by high speed fuses. Because power diodes and silicon controlled rectifiers (SCRs) cannot withstand heavy fault current, they require ultra-fast current-limiting fuses. The circuits in which these fuses are applied have certain requirements that are generally more stringent than those for typical 60 cycle AC power distribution systems.

The diodes or SCRs in solid-state power equipment have relatively low fault current withstand capabilities. Their thin silicon material has a very low transient thermal capacity that makes them highly susceptible to damage from the heat produced by low, moderate and high faults that can occur in a very short time. Thus, it’s essential to restrict fault energy with a high speed fuse.

NEC 430.52(C)(5) recognizes using high speed fuses for motor protection

There are several criteria used to judge high speed fuse performance.

Among these are the current-limiting/short-circuit capability and the ability to quickly interrupt DC currents found in rectifiers and drives.

From a design standpoint, I 2 t is most often used to evaluate the current-limiting/short-circuit performance. I 2 t (RMS amps-squared seconds) indicates the heating affect associated with a current pulse. A semiconductor’s data sheet often specifies its maximum I 2 t withstand capability. If the selected fuse has an I 2 t let-through less than the semiconductor’s I 2 t withstand rating, it is protected.

High speed fuses are often applied where DC interrupting capabilities are essential, with some high speed fuses specifically designed and rigorously tested to have excellent DC operating characteristics.

Specialized knowledge about circuit types is essential for proper application. Included in Figures 3.1.6.4.a through 3.1.6.4.h are common circuits protected by high speed fuses.

Typical circuits

I

2

I

2

I

3

I

3

I

2

I

2

Figure 3.1.6.4.c

Single-phase, full-wave, bridge.

Figure 3.1.6.4.d

Three-phase, half-wave.

Figure 3.1.6.4.e

Three-phase, full-wave.

I

1

I

1

I

1

Load

Load

Load

I

1

I

1

Load

Load

Figure 3.1.6.4.a

Single-phase, half-wave.

I

2

I

1

Load

Figure 3.1.6.4.f

Six-phase, single wave.

I

2 I

1

Load

Figure 3.1.6.4.b

Single-phase, full-wave, center-tap.

Figure 3.1.6.4.g

Single-phase, anti-parallel, AC control.



I

2

I

1

Figure 3.1.6.4.h

Three-phase, anti-parallel, AC control.

Not all systems with diodes or SCRs have the fuse provide full protection and they are applied to achieve varying design objectives:

1. Prevent device rupture — The fuse merely needs to interrupt current before the diode or SCR ruptures.

2. Isolate failed device — Used typically where only three or more diodes or SCRs are used per conduction path. An individual fuse is not intended to protect an individual device, but rather its purpose is to isolate the diode or SCR after it shorts out and permit the overall circuit to continue operating. At this level, the fuse must be able to protect the diodes or SCRs that are splitting the fault current in another leg, as illustrated in Figure 3.1.6.4.i.

Isolation

A

Load

Normal, conducting

CHSF — UL compact high speed fuse

• 50 to 400 A

• 500 Vac/dc

• 200 kA IR

• Operating class aR, UL

Recognized, Guide JFHR2,

File E56412, CSA Component

Acceptance, Class 1422-30, File

53787, IEC aR (self certified),

RoHS compliant, CE

Bussmann series compact high speed fuses feature spacesaving case sizes for protecting semiconductor devices while providing superior current cycling performance to help withstand demanding applications.

The CHSF fuse requires up to 48% less enclosure space to help reduce the overall assembly size. Its innovative design allows for a significantly smaller package without compromising heat rise performance, preventing extensive equipment redesign.

With a bolt-on design the CHSF provides design flexibility for installation in fuse blocks or direct mounting on busbars and meets JASO D622 spec for thermal shock, humidity, and vibration (see data sheet no.

10414).

3

B

C

Shorted

+

Normal, blocking

_

Fuse on shorted diode in leg B should be able to open and clear before any damage is done to the diodes in leg A

Figure 3.1.6.4.i

High speed fuses can be applied to simply isolate faulted circuit paths and allow the remainder of the device to operate.

3. Protect the device (faults) — In this case, the fuse is applied to protect the diode or SCR against faults external to the SCR or diode.

Typically, the fuse is selected to give a much lower let-through current than that required for design objectives 1 and 2 shown above.

DFJ — UL Class J full range high speed fuse

• 1 to 600 A

• 600 Vac/450 Vdc

• IR

•

200 kA RMS Sym.

• 100 kA DC

• Operating class aR, UL Listed, Std

248-8, Class J, Guide JDDZ, File

E4273, CSA Certified, C22-2 No

248.8, Class 1422-02, File 53787

UL Class J high speed, full range current-limiting fuses provide maximum protection for AC and DC drives and controllers.

The UL Class J DFJ fuse has the lowest I 2 t of any branch circuit fuse to protect power semiconductor devices that utilize diodes, GTOs, SCRs and SSRs.

The DFJ fuse combines the performance of high speed fuses in a full range Class J branch circuit package, allowing the use of readily available Class J fuse blocks, holders and switches. The DFJ uses standard Class J holders and blocks (see data sheet no. 1048).



British style (BS88)

CT, ET, EET, FE, FEE, FM, FMM,

LCT, LET, LMT, LMMT, MT, MMT

• 6 to 900 A

• 240 to 690 V

• 200 kA IR AC

• UL Recognized

• Designed and tested to BS88:

Part 4 and IEC 60269: Part 4

A wide range of British style semiconductor fuses that use innovative arc-quenching techniques and high grade materials to provide:

• Minimal energy let-through (I 2 t)

• Excellent DC performance

• Good in-rush withstand profile

Found in equipment manufactured in the United Kingdom or British

Commonwealth countries. North American manufacturers have begun to specify BS88 fuses — particularly in UPS applications at 240 volts or less — to take advantage of their size, performance and cost benefits.

See Bussmann series full line product catalog 1007 for complete product offering.

Ferrule

FWA, FWX, FWH, FWC, FWP,

FWK, FWJ, FWL, FWS

• 1 to 100 A

• 150 to 2000 V

• 200 kA IR AC

• UL Recognized

• Designed and tested to IEC

60269:Part 4

Bussmann series full line of ferrule style (cylindrical and clip-mounted) high-speed fuses are designed and tested to meet standards and requirements in various locations around the world. Their unique design and construction provide:

• Superior cycling

• Low energy let-through (I2t)

Ferrule high-speed fuses provide an excellent solution for small UPS, small AC drives and other low power applications where space is at a premium.


Square body

170M

• 10 to 7500 A

• 690 to 1300 V

• 200 kA IR AC

• UL Recognized

• Designed and tested to IEC

60269: Part 4

Complete range of square body high-speed fuses and accessories suited for high power applications requiring a compact design with superior performance. End fitting options include:

• DIN 43 653

• DIN 43 620

• Flush end (Metric/US)

• French style

• US style


North American

FWA, FWH, FWJ, FWP, FWX,

KAC, KBC

• 1 to 4000 A

• 130 to 1000 V

• 200 kA IR AC

• UL Recognized

North American The Bussmann series high speed fuses have a complete range of blade and flush-end styles, and accessories designed to provide:

• Low energy let-through (I 2 t)

• Low watts loss

• Superior cycling capability

• Low arc voltage

• Excellent DC performance

While there are currently no published standards for these fuses in medium power applications, the industry has standardized on mounting centers that accept Bussmann series fuses.




3.1.6.5 Photovoltaic (PV) fuses

Unlike a grid connected AC system, the available fault current within a PV system is limited, requiring the OCPD to operate effectively on higher DC voltage and low fault current. For this reason, PV-specific fuses were developed.

The International Electrotechnical Commissions and Underwriters

Laboratories recognize that PV system protection is different and is reflected in IEC 60269-6 (gPV) and UL 248-19 with PV OCPDs for protecting strings and arrays specifically designed to meet these standards:

• Fully tested to the requirements of IEC 60269-6 and exceeding the requirements of operating at 1.45 x In (1.45 times the nominal current).

• Meet the requirements of UL 248-19 that are very similar to the IEC standards, except they operate at 1.35 x In (1.35 times the nominal current).

These IEC and UL ratings do not reflect a continuous service rating. The assigned service rating should be reduced as ambient temperatures increase. To ensure PV fuse longevity, they should not be subjected to a continuous current of more than 80% of the assigned IEC and UL ratings.

Photovoltaic system trends

As with any electrical system, primary goals in a photovoltaic system are efficiency and economies. To do this requires:

• Using smaller, less costly conductors and system components

• Simplifying system design (e.g., in-line holders that combine fuse and holder in one assembly)

• Operating at higher system voltages

Additionally, circuit protection comes into play on the balance of system components from the DC (direct current) side (arrays, combiner boxes, recombiner boxes and inverters - where high speed fuses are needed) to the AC (alternating current) side and, ultimately, connecting to a building’s electrical system or the grid.

For more information on selecting and sizing PV fuses, see the solar circuit protection application guide, publication no. 10191.

PVM 600 Vdc 13/32” x 1-1/2” midget

A range of UL 248-19 fast-acting 600 Vdc 13/32” x 1-1/2”

(10x38mm) midget fuses specifically designed to protect solar power systems in extreme ambient temperature, high cycling and low level fault current conditions (reverse current, multi-array fault) (see data sheet no. 2153).

PVS-R 600 Vac/dc Class RK5

A range of UL 248-19 fast-acting 600 Vdc Class RK5 fuses specifically designed to protect photovoltaic power systems in extreme ambient temperature, high cycling and low level fault current conditions (reverse current, multi-array fault) (see data sheet no. 4203)

Ratings

• Volts

•

600 Vac to UL 248-12 and 600 Vdc to UL 248-19

• Amps 20-400 A

• IR

•

200 kA RMS Sym. AC

•

20 kA DC (20-60 A)

•

10 kA DC (70-400 A)

Agency information

• UL Listed, Std. 248-12, Class RK5, Guide JFGA, File E335324.

Photovoltaic to UL 248-19, CSA Component Certified C22.2

PV 1000 Vdc 10x38mm PV fuses

A range 10x38mm, 1000 Vdc PV fuses for the protection and isolation of photovoltaic strings that are specifically designed for use in PV systems with extreme ambient temperature, high cycling and low fault current conditions (reverse current, multi-array fault) string arrays. For application flexibility, the

PV 10x38mm fuse is available as a cylindrical fuse as well as bolt-on, single and dual PCB tab and in-line crimp terminal versions (see data sheet no. 10121).

Ratings

• Volts 1000 Vdc

• Amps 1-20 A

• IR 50 kA

Terminals and conductors

• Crimp connection for single, 75°C/90°C Cu Stranded 12-8 AWG PV wire

Agency information

• UL Listed to 248-19

RoHS Compliant

† , Guide JFGA, File E335324, IEC 60269-6 (gPV),

CSA File 53787, Class 1422-30 (1-15 A), 20 A Pending, CCC (1-20 A),

† Except crimp terminal version that is UL Recognized to UL 248-19, Guide

JFGA2, File E335324.

Ratings

• Volts 600 Vdc

• Amps 4-30 A

• IR 50 kA DC (4-30 A)

Agency information

• UL Listed 248-19, Guide JFGA, File E335324, CSA Component

Certified C22.2, RoHS compliant

3



HPV 1000 Vdc in-line PV fuse assembly

A single-pole, non-serviceable photovoltaic in-line fuse holder and fuse assembly in an IP67 dust tight, submersible insulating boot for use in photovoltaic wire harnesses (see data sheet no. 2157).

Ratings

• Volts 1000 Vdc

• Amps 1-20 A

• IR 33 kA

Agency information

• UL Listed to 4248-1 and 4248-19, File E 348242, CSA Component

Acceptance, Class 6225 30, File # 47235, IP67 submersible, RoHS compliant, CE

PV 1500 Vdc 10x85mm

A range of 10x85mm PV fuses specifically designed for protecting and isolating photovoltaic strings. These fuses are capable of interrupting low overcurrents associated with faulted PV systems (reverse current, multi-array fault). Also available with crimp terminals (see data sheet no.

10658).

Ratings

• Volts 1500 Vdc

• Amps 2.25-30 A

• IR 30 kA, 1 ms

Agency information

• UL 248-19, IEC 60269, RoHS compliant

PV 1000/1100 Vdc 14x51mm

A range of 14x51mm PV fuses specifically designed for protecting and isolating photovoltaic strings. These fuses are capable of interrupting low overcurrents associated with faulted PV systems (reverse current, multi-array fault) (see data sheet no. 720132).

Ratings

• Volts

•

1000 Vdc (25 and 32 A)

•

1100 Vdc (15 and 20 A)

• Amps 15-32 A

• IR 10 kA

Agency information

• UL Listed, Guide JFGA, File E335324. Photovoltaic to, UL 248-19,

IEC 60269-6 gPV, CSA Pending, CCC Pending, RoHS compliant

PV15M-4A-CT 1500 Vdc in-line crimp terminal fuse

Bussmann series PV15M-4A-CT is a 1500 Vdc in-line photovoltaic fuse with crimp terminals for use in wire harnesses and other applications where an in-line PV fuse is desirable. This in-line fuse can be electrically insulated with customer-supplied overmolding or approved heat-shrink


Ratings

• Volts 1500 Vdc

• Amps 4 A

• IR 15 kA DC

• Time constant 1~3 ms

Agency information

• UL Recognized, 248-19, File E484317 Vol. 1 Sec. 1, CSA pending,

RoHS compliant

NH 1000 Vdc blade and bolt-on

A range of 1000 Vdc NH size PV fuses specifically designed for protecting and isolating array combiners/re-combiners, disconnects and inverters (see data sheet no. 720133).

Ratings

• Volts 1000 Vdc

• Amps 32-400 A

• IR 50 kA

Agency information

• UL Listed, Guide JFGA, File E335324, Photovoltaic to UL 248-19, IEC

60269-6 gPV, CSA Class 1422-30, File 53787 (32-160 A), UL Listed,

IEC gPV, CSA, CCC Pending, RoHS compliant

PV XL 1000 Vdc and 1500 Vdc

A range of XL size PV fuses specifically designed for protecting and isolating photovoltaic array combiners and disconnects. These fuses are capable of interrupting low overcurrents associated with faulted

PV systems (reverse current, multi-array fault). Optional microswitches available for use in monitoring systems (see data sheet no. 10201).

Ratings

• Volts

• 1000 Vdc (60-630 A)

•

1500 Vdc (50-400 A)

• Amps

•

63-630 A (1000 Vdc)

• 50-400 A (1500 Vdc)

• IR

•

50 kA (1000 Vdc 63-160 A 01XL and 350-630 A 3L)

•

30 kA (1000 Vdc 200 A 1XL, 160-355 A 2XL)

• 30 kA (1500 Vdc 50-160 A 01XL, 100-200 A 1XL, 125-250 A 2XL,

250-400 A 3L)

Agency information

• UL 248-19, Guide JFGA, File E335324, IEC 60269-6 gPV,

CSA Class 1422-30, File 53787, RoHS compliant



3.1.7 Fuseology summary — the power of the modern, current-limiting fuse

Current limitation Factory calibrated replacements

• Enhances workplace safety by reducing the incident energy and arc flash hazards personnel may be exposed to under fault conditions

• Protects components and equipment from extreme thermal and mechanical forces associated with a fault event

• Helps compliance with NEC 110.10 by protecting equipment and components from extensive fault current damage when properly sized

• Helps achieve high equipment short-circuit current ratings by drastically reducing peak let-through current

• Type 2 “No Damage” (versus Type 1) protection with properly sized fuses reduces downtime and improves system protection

Highest interrupting ratings up to 300 kA

• Improved system reliability throughout the system’s life by ensuring the same protection level is installed after a fault

• Maintained system integrity as replacement fuses utilize the latest design and performance improvements

• Helps comply with OSHA 1910.334(b)2 by eliminating the invitation for an operator to reset a device after a fault occurs without first determining its cause (resetting circuit breakers or replacing fuses in a circuit without investigating and fixing the cause is prohibited by federal law)

Enclosed non-venting design

• Provides flexibility for use in any system with available fault currents up to 300 kA without fear of misapplication

• Provides a no-worry solution in the case system changes (utility transformer or equipment relocation) result in higher available fault current

• Saves time by eliminating the need for fault current studies when using 300 kA IR Low-Peak fuses

• Improves compliance with NEC 110.9 and removes the fear of misapplication

• Helps achieve high equipment short-circuit current ratings as the fuse will not be the limiting factor

• Peace of mind as a fuse’s interrupting rating is always at least equal to or, in many cases, greater than the available fault current at the line terminals

Straight voltage rating

• Fuses do not vent, so they eliminate the venting that’s inherent on some mechanical OCPDs that could cause unnecessary damage to other system components

• Reduces cost by eliminating the need for additional system guards or barriers to protect from venting

Enclosed, fixed, thermal design

• Improved fault protection integrity by not relying on springs, levers and latches to open the circuit

• Reduced operating costs by eliminating the need for device maintenance or calibration

• Minimizes possible misapplication and confusion by eliminating the need to adjust and change device settings in the field

Finger-safe designs

• Straight voltage rated fuses provide flexibility in any system regardless of its grounding (a slash voltage rated OCPD is limited to installation in ONLY a solidly grounded Wye system)

Physical rejection

• Increase system and personnel safety by utilizing the latest in fingersafe fuse technology covering blocks, holders, disconnect switches and power distribution fuse blocks

• Of special note is the finger-safe Bussmann series CUBEFuse that’s used with the patented UL 98 Listed Bussmann series Compact

Circuit Protector (CCP) switch — together they provide a finger-safe fused switch that’s horsepower rated and can be used on branch circuits

• Provides a safe and reliable system by ensuring the same class fuse with the same voltage and interrupting rating is installed

• Reduces the risk that the wrong fuse type with a higher short-circuit energy let-through and lower interrupting ratings can be installed and compromise the protection level

3



3.2 Breaker basics

Contents

3.2.1 Introduction

3.2.2 Ratings

3.2.3 Circuit breaker types and classifications

3.2.4 How circuit breakers work

3.2.5 Circuit breaker time-current characteristic curves

3.2.1 Introduction

Section page

54

54

55

56

58

The NEC defines a circuit breaker as “A device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.” There are two basic circuit breaker classifications and three types used for low voltage circuit protection. The two basic classes are:

1. Low Voltage Power Circuit Breaker (LVPCB) which comply with these standards:

•

ANSI Std. C37.6 — Preferred Ratings

•

ASNI Std. C37.17 — Trip Devices for LVPCB

•

ANSI Std. C37.50 — Test Procedures

•

IEEE Std. C37.13 — LVPCB used in Enclosures

•

UL 1066 - LVPCB

2. Molded Case Circuit Breakers (MCCBs) that comply with these standards:

•

UL 489 — MCCB

•

UL 489 — Molded Case Switches (MCS)

The classifications themselves lend their names to the first two of the three circuit breakers types. The third circuit breaker type is derived from the molded case class and known as an insulated case circuit breaker.

The three circuit breaker types are:

1. Low Voltage Power Circuit Breakers (LVPCBs)

2. Molded Case Circuit Breakers (MCCBs)

3. Insulated Case Circuit Breakers

(ICCBs)

Molded case circuit breaker current carrying parts, mechanisms and trip devices are completely contained within a molded case of insulating material. MCCBs are available in small and medium frame sizes with various interrupting ratings for each frame size. Some larger MCCBs are available in drawout design and used primarily in panelboards and switchboards where they are mostly fixed mounted.

MCCB

LVPCB

ICCB

Low voltage power circuit breakers are used primarily in drawout switchgear. LVPCBs have replaceable contacts and are designed to be field maintainable. The term power circuit breaker also applies to medium voltage (1 - 72.5 kV) or high voltage (over 72.5 kV) breakers.

Preferred low voltage power circuit breaker ratings are as indicated in ANSI IEEE C37.16. Standards for low voltage AC power circuit breakers used in enclosures are as indicated in ANSI/IEEE C37.13.

Test procedures for low voltage power circuit breakers used in enclosures are as indicated in ANSI/IEEE C37.50-1973. Application recommendations are discussed in ANSI/IEEE C37.16-1980 and application factors in ANSI/IEEE C37.20. LVPCBs are generally UL Listed and can be UL labeled.

ICCBs are rated and tested according to the UL 489 Standard. However, they utilize design characteristics from both LVPCB and MCCB breaker classes. They are of large frame size with short time capabilities and

3-54 Eaton.com/bussmannseries utilize stored energy operating mechanisms. ICCBs are designed and tested to the UL 489 Standard, and used primarily in fixed mounted switchboards, but are also available in drawout configurations. They are generally considered not field maintainable, but they do have several maintenance operations that can be field performed.

3.2.2 Ratings

3.2.2.1 Voltage ratings

Voltage is an extremely important rating for overcurrent protective devices (OCPDs). The proper OCPD application according to its voltage rating requires that the device voltage rating be equal to or greater than the system’s voltage. For example, a 600 V rated circuit breaker can be used in a 208 V circuit, but a 250 V rated circuit breaker cannot be used in a 480 V circuit. When a circuit breaker is applied beyond its voltage rating, there may not be any initial indicators anything is wrong. Adverse consequences typically result when it is applied outside of its voltage rating and it attempts to interrupt an overcurrent, at which point it may self-destruct in an unsafe manner.

There are two circuit breaker voltage rating types:

• Straight voltage

• Slash voltage

Straight voltage rated breakers

Proper straight rated circuit breaker application is relatively straightforward. These devices are marked with a straight voltage rating

(e.g., 240 V, 480 V, 600 V) and are evaluated for proper operation with full phase-to-phase voltage used for testing, listing and marking.

Slash voltage rated breakers

Some circuit breakers have a slash voltage rating. The slash rating can be broken down into the higher and lower of two numbers included in its rating. They are understood as:

• The lower rating pertains to overcurrent protection at line-to-ground voltages, intended to be cleared by one pole of the device

• The higher rating pertains to overcurrent protection at line-to-line voltages, intended to be cleared by two or three poles of the device

The proper slash-rated circuit breaker application is such that the lineto-ground voltage cannot exceed the device’s lower voltage rating, and the voltage between any two conductors (line-to-line) cannot exceed the devices higher voltage rating. (Reference NEC Section 240.85) Slash voltage rated circuit breakers are not intended to open phase-to-phase voltages across only one pole. Where it’s possible for phase-to-phase voltage to appear across only one pole, a straight rated breaker must be used. For example, a 480 V circuit breaker may have to open an overcurrent at 480 V with only one pole, such as might occur when

Phase A goes to ground on a 480 V, B-phase, corner grounded delta system.

The slash rated breaker misapplication can cause it to be applied outside its voltage rating with dire ramifications should the device be called upon to interrupt fault currents.

Slash voltage rated circuit breakers can only be utilized on solidly grounded Wye systems.

Slash voltage rated circuit breakers cannot be used on the following systems (use only straight voltage rated devices on these systems):

• Impedance-grounded

• Ungrounded Wye

• Ungrounded Delta

• Corner-grounded Delta

Similarly, slash rated equipment also have limitations. For example, a slash voltage rated motor controller is restricted to only solidly grounded systems and not appropriate for corner grounded delta, impedance grounded or ungrounded systems. (NEC Section 430.83(E))


Devices that may be slash-rated include, but are not limited to:

• Molded case circuit breakers — UL 489

• Manual motor controllers — UL 508

• Self-protected Type E combination starters — UL 508

• Supplemental protectors — UL 1077 (these look like and are sometimes referred to as mini-breakers, but they are not circuit breakers; they are not rated for branch circuit protection and can’t be a substitute where branch circuit protection is required.)

If devices have a slash voltage rating limitation, product standards require them to be marked with the rating such as 480Y/277 V. If a machine or equipment electrical panel utilizes a slash voltage rated device, it’s recommended that the equipment nameplate (or label) designate this slash voltage rating as the voltage rating. UL 508A industrial control panels requires the electrical panel voltage marking to be slash rated if one or more devices in the panel are slash voltage rated.

A straight rated fuse or circuit breaker protecting a single pole can be used to protect single-phase line-to-neutral loads when supplied from a three-phase, solidly grounded circuit. For example, a 300 V rated fuse can protect single-phase line-to-neutral loads when supplied from threephase, solidly grounded, 480/277 V circuits, where the single-phase lineto-neutral voltage is 277 V. This is permitted because in this application, a 300 V fuse will not have to interrupt a voltage greater than its 300 V rating.

3.2.2.2 Amp ratings

Every circuit breaker has a specific amp rating. Standard amp ratings for inverse time circuit breakers are shown in the table below (NEC Section

240.6).

Circuit breaker ratings (amps)

15

40

80

20

45

90

150

300

600

1600

5000

175

350

700

2000

6000

25

50

100

200

400

800

2500

—

30

60

110

225

450

1000

3000

—

35

70

125

250

500

1200

4000

—

Understanding and using this NEC table is important. NEC Section

240.6 is leveraged whenever requirements specify “...the next standard overcurrent device size shall be used....” The next standard overcurrent device size is not based on manufacturers’ literature; it is always obtained from NEC 240.6.

In selecting a circuit breaker amp rating, consideration must be given to the load type and code requirements. The circuit breaker amp rating normally should not exceed the circuit’s current carrying capacity. The conductor current carrying capacity is that current rating arrived upon after ampacity adjustment factors are applied to a given conductor, based on how and where it’s routed or other NEC related ampacity adjustment factors. For example, if a conductor’s current carrying capacity is 20 A, a 20 A circuit breaker is the largest that should be used to protect this conductor.

3.2.2.3 Interrupting ratings

A circuit breaker interrupting rating establishes the maximum fault current value at specified voltage which the device has been evaluated to safely interrupt under standard test conditions. Circuit breaker interrupting ratings vary based upon applied voltage. Interrupting ratings, if other than 5 kA, are required to be marked on circuit breakers

(240.83(C)).

3

3.2.3 Circuit breakers types and classifications

Description

Select trip short-time rating

Operator type

Closing speed

Mounting

Interrupting rating

Current limiting

Relative cost

Available frame sizes

Maintenance

Enclosure types

Series ratings

Enclosed rating

Standards

LVPCB (Type Magnum DS ® and Series NRX ® ) ICCB (Type Magnum SB and Series NRX) MCCB (QUICKLAG ® /Series C ® /Series G ® )

Selective trip over full range of fault currents up to interrupting rating (high short-time ratings)

Selective trip over partial range of fault currents within the interrupting rating

(medium short-time ratings). Typically up to 35 kA

Selective trip over a smaller range of fault currents within the interrupting rating (low short-time ratings).

Typically 10-13 times the frame size

Types of operators: mechanically operated and electrically operated two-step stored energy

Types of operators: mechanically operated and electrically operated twostep stored energy

Types of operators: mechanically operated over-center toggle or motor operator

5-cycle closing for electrically operated devices

Available in drawout construction permitting racking to a distinct “test position” and removal for maintenance

5-cycle closing for electrically operated devices

Available in drawout construction permitting racking to a distinct “test position” and removal for maintenance

Greater than 5-cycle closing for electrically operated devices

Typically fixed-mounted but large frame sizes may be available in drawout construction

Interrupting duty at 635 Vac: 42-130 kA and current limiting with or without fuses up to 200 kA

Special current limiting types available with or without fuses up to 200 kA

Higher

Small number of frame sizes available.

Typical 800-6000 A

Extensive maintenance possible on all frame sizes

Used in enclosures, MCCs, switchboards and switchgear

Not available in series ratings

100% continuous current rated in its enclosure

ANSI/IEEE C37 UL 1066

Interrupting duty at 508 Vac: 42-130 kA

Special current limiting types available without fuses up to 150 kA

Medium

Small number of frame sizes available.

Typical 800-6000 A

Limited maintenance possible on larger frame sizes

Used in enclosures, MCCs and switchboards

Not available in series ratings

80% continuous-current rated, unless specifically stated to be rated 100% in an enclosure

NEMA AB1/AB3 UL 489 or UL 1066

Interrupting duty at 480 Vac: 22-100 kA without fuses and up to 200 kA with integral fuses or for currentlimiting type

Current limiting available with and without fuses up to 200 kA

Low

Large number of frame sizes available. Typical 100-2500 A

Very limited maintenance possible on larger frame sizes

Used in enclosures, panelboards, switchboards, MCCs and control panels

Available in series ratings

80% continuous-current rated, unless specifically stated to be rated 100% in an enclosure

NEMA AB1/AB3 UL 489



3.2.3.1 Molded case circuit breakers (MCCB)

As a class, molded case circuit breakers are tested and rated according to UL 489. Their current carrying parts, mechanisms, and trip devices are completely contained within a molded case of insulating material.

MCCBs are available in small and medium frame sizes with various interrupting ratings for each frame size.

Current-limiting molded case circuit breakers are also available. They are characterized by fast interruption short-circuit trip elements.

Molded case circuit breakers are designed and tested to the UL

489 Standard. Some of the larger molded case circuit breakers are available in drawout design. They are used primarily in panelboards and switchboards where they are mostly fixed mounted.

3.2.3.2 Insulated case circuit breaker (ICCB)

Insulated case circuit breakers are also rated and tested according to the UL 489 Standard. However they utilize characteristics of design from both classes. They are of large frame size, have short time capabilities and utilize stored energy operating mechanisms.

Insulated case power circuit breakers, designed and tested to the UL

489 Standard, are used primarily in fixed mounted switchboards but are also available in drawout configuration. They are generally considered not field maintainable but there are several maintenance operations that can be performed in the field.

3.2.3.3 Power circuit breaker (PCB)

Low voltage power circuit breakers are used primarily in draw-out switchgear; they have replaceable contacts, and are designed to be maintained in the field.

The term power circuit breaker also applies to medium voltage

(1 kV to 72.5 kV) or high voltage (over 72.5 kV) breakers.

3.2.4 How circuit breakers work

Bimetal element

Loadside

(downstream)

1

Trip bar

Latch

2

3

Magnetic element

Spring-loaded contacts

Arc chute

Lineside

(upstream)

Figure 3.2.4.a

Circuit breaker overload operation

Figures 3.2.4.b and 3.2.4.c illustrate thermal circuit breaker operation by a bimetal element sensing a persistent overload. The bimetal element senses an overload condition by the unequal expansion rates of its material. In some circuit breakers, the overload sensing function is performed by electronic means. Regardless, the unlatching and interruption process is the same. Figure 3.2.4.b illustrates that as the overload persists, the bimetal sensing element bends. If the overload persists for too long, the force exerted by the bimetal sensor on the trip bar becomes sufficient to unlatch the circuit breaker. Figure

3.2.4.c shows that once a circuit breaker is unlatched, it’s on its way to opening. The spring-loaded contacts separate and the overload is cleared. There can be some arcing as the contacts open, but the arcing is not as prominent as when interrupting fault current.

All circuit breakers are mechanical OCPDs that share three operating functions:

1. Thermal, magnetic or electronic current sensing

2. Mechanical unlatching (opening) mechanism

3. Current/voltage interruption means, whether mechanical contact parting, arc chute or both

The circuit breaker’s operating physics are significantly different than a fuse’s. First, the circuit breaker senses the overcurrent. If the overcurrent persists for too long, the sensing means causes or signals the contact mechanism to unlatch. The unlatching function causes the contacts to start parting. As the contacts start to part, the current is “stretched” through the air causing arcing between the contacts to commence. The further the contacts separate, the longer the arc becomes, which aids in interrupting the overcurrent. However, in most cases, especially for fault current, the contacts alone are not sufficient to interrupt. The arcing is “thrown” to the arc chute which further stretches and cools the arc so that interruption can be made. Figure

3.2.4.a shows a simplified model with the three operating components

(1, bimetal and magnetic elements, 2, latch and 3, spring-loaded contacts and arc chutes) for a thermal magnetic circuit breaker, which is the most commonly used circuit breaker. It should also be noted that there are various contact mechanism designs that can significantly affect the interruption process.

Bimetal element

Loadside

(downstream)

Figure 3.2.4.b

Trip bar

Latch

Lineside

(upstream)


Bimetal element

Loadside

(downstream)

Lineside

(upstream)


Figure 3.2.4.c

Circuit breaker fault (short-circuit) operation

Figures 3.2.4.d, 3.2.4.e and 3.2.4.f illustrate circuit breaker operation during a fault current. The magnetic element (often referred to as the instantaneous trip that operates without any intentional delay) senses the higher level overcurrent condition. In some circuit breakers, the

“instantaneous trip” is performed by electronic means. In either case, the unlatching and interruption process is the same as illustrated in

Figures 3.2.4.e and 3.2.4.f. Figure 3.2.4.d illustrates the high current due to a short-circuit causing the trip bar to be pulled toward the magnetic element. If the fault current is high enough, the resulting magnetic force causes the trip bar to exert enough force to unlatch the circuit breaker.

Figure 3.2.4.e shows that once unlatched, the contacts start to part.

It is important to understand that once a circuit breaker unlatches it will open, but interrupting the current does not commence until the contacts start to part. As the contacts part, the current continues to flow through the air (arcing current) between the stationary and the movable contacts. At some point the arc is “thrown” to the arc chute that further “stretches” and cools the arc. The speed at which the contacts open depends on the circuit breaker design. The total current interruption time for circuit breaker “instantaneous tripping” depends on the specific design and condition of the breaker’s mechanisms. Smaller amp rated circuit breakers may open and clear in as little as 1/2 cycle or less. Larger amp rated circuit breakers may clear in a range typically between 1 to 3 cycles, depending on the design. Circuit breakers that are listed and marked as current-limiting can interrupt in 1/2 cycle or less when the fault current is in the circuit breaker’s current-limiting range. With the assistance from the arc chute, as well as the alternating current crossing zero voltage (60 or 50 times a second) and the contacts traveling a sufficient distance, the fault current is interrupted (see Figure

3.2.4.f). As energy is released in the contact interruption path and via the arc chutes during the current interruption process, circuit breakers are designed with specific interrupting ratings at specific voltage ratings.

For instance, a circuit breaker may have a 14 kA IR at 480 Vac and 25 kA

IR at 240 Vac.

Loadside

(downstream)

Figure 3.2.4.e

Loadside

(downstream)

Figure 3.2.4f

Magnetic element

Trip bar


Magnetic element


Arc chute

Lineside

(upstream)

3

Magnetic element

Lineside

(upstream)

Loadside

(downstream)

Figure 3.2.4.d

Latch

Lineside

(upstream)



3.2.5 Circuit breaker time-current characteristic curves

When using molded case circuit breakers, there are three basic curve considerations to understand:

1. Overload region

2. Instantaneous region with unlatching

3. Interrupting rating

Overload region - overloads can typically be tolerated by circuit components for relatively longer times than faults with OCPD opening times ranging from seconds to minutes. As can be seen in Figure

3.2.4.g, the overload region has a tolerance band (between minimum unlatching and maximum interrupting time), which means the breaker should open within that area for a particular overcurrent.

Instantaneous region — here the circuit breaker will open as quickly as possible. The instantaneous trip (IT) setting indicates the full load rating multiple at which the circuit breaker starts to operate in its instantaneous region. Circuit breakers with instantaneous trips have either (1) fixed settings or (2) adjustable settings. The instantaneous region in Figure 3.2.4.g, (and for this example), is shown to be adjustable from 5x to 10x the breaker amp rating. When the breaker senses an overcurrent in the instantaneous region, it unlatches the contacts and permits them to commence the parting process.

The unlatching time is represented by the curve labeled “average unlatching times for instantaneous tripping” (this is the instantaneous trip curve continuing below 0.01 second). This is important when evaluating lineside to loadside breaker coordination. The manufacturer of the circuit breaker depicted in Figure 3.2.4.g also published unlatching times for various currents (table in upper right). Unlatching starts the contacts parting and the overcurrent is not cleared until the breaker’s contacts are mechanically separated and the arc is extinguished

(depicted in Figure 3.2.4.f as the maximum interrupting time).

Consequently, the time from unlatching to interruption, is indicated by the overload region between the minimum unlatching time and the maximum interrupting time curves. This time range affects the ability circuit breakers with instantaneous trips to selectively coordinate when the overcurrent magnitude is in the instantaneous trip range.

Two instantaneous trip settings for a 400 A breaker are shown in

Figure 3.2.4.g (solid blue and red dashed lines). The solid blue line instantaneous trip region represents an IT = 5x, or five times 400 A =

2000 A. At this setting, the circuit breaker will trip instantaneously on currents of approximately 2000 A or more. The ± 25% band represents the area in which it’s uncertain whether the overload trip or the instantaneous trip will operate to clear the overcurrent. The dashed red line instantaneous trip region represents the same 400 A breaker with an IT = 10x, or 10 times 400 A = 4000 A. At this setting the overload trip will operate up to approximately 4000 A (±10%). Overcurrents greater than 4000 A (±10%) would be sensed by the instantaneous setting.

The ± 25% and ±10% band mentioned in this paragraph represents a tolerance that can vary by circuit breaker manufacturer and type.

Many lower amp rated circuit breakers (100 A and 150 A frame CBs) have non-adjustable or fixed instantaneous trip settings. For larger molded case, insulated case and power breakers the instantaneous trip setting can usually be adjusted by an external dial.

The circuit breaker’s IT is typically set at its lowest setting when shipped from the factory. Note that most published circuit breaker time-current curves show the vertical time axis from 0.01 second up to about 100 or 1000 seconds. The published curves do not normally provide the instantaneous unlatching characteristic. However, if a circuit breaker has an instantaneous trip, it has unlatching times usually less than 0.01 second.

Some circuit breakers have short time-delay trip settings (STD). The short time-delay trip option can be used in conjunction with (1) an instantaneous trip settings or (2) without instantaneous trip settings.

Typically, MCCBs and ICCBs with short time-delay settings have an instantaneous trip override. This means at some fault current level, the instantaneous trip operates to protect the circuit breaker. LVPCBs can be specified with a short time-delay setting which does not inherently incorporate an instantaneous trip override.


0.01

0.008

0.006

0.004

0.003

0.002

1000

800

600

400

300

200

100

80

60

40

30

20

1

0.8

0.6

0.4

0.3

0.2

4

3

10

8

6

2

0.1

0.08

0.06

0.04

0.03

0.02

0.001

Min unlatching time

Overload region

Adjustable instantaneous trip set at 5 times

IT = 5x

(± 10% band)

Max IR time

Average unlatching times for tripping breakers magnetically

Current in

RMS amps

Time in seconds

5000

10,000

15,000

20,000

25,000

0.0045

0.0029

0.0024

0.0020

0.0017

Interrupting ratings

Volts A RMS Sym.

240 V

480 V

600 V

42,000

30,000

22,000

Adjustable magnetic instantaneous trip set at 10 times

IT = 10x

(± 10% band)

Instantaneous region

Average unatching time for instantaneous tripping

Current in amps

Max interrupting time

IR at 480 V

Figure 3.2.4.g: Typical circuit breaker time-current characteristic curve.

Interrupting rating - the interrupting rating is represented in Figure

3.2.4.g by a vertical line at the lower right end of the curve. The circuit beaker’s interrupting rating varies based on the voltage level (see the table in Figure 3.2.4.g that lists the interrupting ratings for this specific circuit breaker). For coordination purposes, the vertical line is often drawn at the fault current level in lieu of the interrupting rating (if the interrupting rating is greater than the available fault current). However, if the fault current is above the interrupting rating, a misapplication and

NEC 110.9 violation is evident. In Figure 3.2.4.g, the circuit breaker interrupting rating at 480 V is 30,000 amps.


Two instantaneous trip circuit breakers

Figure 3.2.4.h illustrates a 90 A circuit breaker and an upstream 400 A circuit breaker having an instantaneous trip setting of 5x (5 times 400

A = 2000 A). The minimum instantaneous trip current for the 400 A circuit breaker could be as low as 2000 A times 0.75 = 1500 A (± 25% band). If a fault above 1500 A occurs on the 90 A breaker’s loadside, both breakers could open. The 90 A breaker may unlatch before the

400 A breaker. However, before the 90 A breaker can part its contacts and clear the fault current, the 400 A breaker could have unlatched and started the irreversible contact parting process.

Assume a 4000 A fault exists on the 90 A circuit breaker’s loadside, the sequence of events would be:

1. The 90 A breaker will unlatch (Point A) and free the breaker mechanism to start the contact parting process.

2. The 400 A breaker will unlatch (Point B) and it, too, would begin the contact parting process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible.

3. At Point C, the 90 A breaker will have completely interrupted the fault current.

4. At Point D, the 400 A breaker will also have opened, which unnecessarily disrupts power to all other loads.

100

80

60

40

30

20

10

8

6

4

3

2

0.8

1

0.6

0.4

0.3

0.2

0.1

0.08

0.06

0.04

0.03

0.02

1000

800

600

400

300

200

90 A circuit breaker

400 A circuit breaker

I.T. = 5X

400 A

90 A

X 4000 A

•

•

D

C

0.01

0.008

0.006

0.004

0.003

0.002

A

•

•

B

0.001

80 100

1500A

4000A

30 kA IR

Current in amps 14 kA IR

Figure 3.2.4.h These two specific circuit breakers with the settings as stated are coordinated for any overcurrent up to approximately 1500

A. However, this is a non-selective system where fault currents above

1500 A* cause a blackout to all the loads fed by the 400 A breaker. As mentioned previously, this is typical for molded case circuit breakers due to the instantaneous trip and band of operation on medium to high fault conditions. Additionally, this can affect other, larger upstream circuit breakers, depending upon the upstream circuit breaker’s size and instantaneous setting, and the fault current magnitude.

* Circuit breaker manufacturers provide coordination tables that show specific circuit breakers types and amp ratings coordinating to fault values greater than the crossing point where two circuit breaker time-current characteristic curves intersect.

3


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SCCR requirements?

Finding out is easy. Eaton’s handbook Code

changes affecting Short-Circuit Current Ratings

based on the 2017 NEC will help you:

Know why SCCR is important.

Understand hazards associated with insufficient SCCR.

Know what specific NEC chapters, articles, parts and sections are involved.

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Eaton.com/nec2017sccr

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NEC is a registered trademark of the National Fire Protection Association.


4 Power system analysis

Contents

4.1 Fault current calculations

4.2 Selective coordination

4.3 Arc Flash

Section page

1

11

30

4.1 Fault current calculations

Contents

4.1.1 Overview

4.1.2 Code requirements

4.1.3 One- line diagram

4.1.4 Procedures and methods

4.1.5 Point-to-point calculation method

4.1.6 Equipment evaluations

4.1.7 Tables

4.1.1 Overview

Section page

1

1

1

3

4

8

8

The fault current calculation is the most basic calculation performed on a power distribution system and vital for the proper electrical equipment application. There are several NEC sections with requirements directly pertaining to the proper electrical product application and available fault current. Safe and reliable electrical equipment application, including

OCPDs, relies on such power systems analysis study information obtained from fault current and selective coordination studies.

4.1.2 Code requirements

Knowing available fault current throughout the power distribution system is important for proper product application. The NEC recognizes the importance of fault currents in many areas within its requirements, including these important topics and sections:

Available fault current markings

• 110.24 — Service Entrance Equipment

• 409.22(B) — Industrial Control Panels

• 440.10(B) — Air Conditioning & Refrigeration Equipment

• 620.51(D)(2) — Elevator Control Panels

• 670.5(2) — Industrial Machinery

Applying solutions within their ratings

• 110.9 — Interrupting Rating

• 110.10 — Component Protection

• 110.24 — Available Fault Current

• 240.4 — Conductor Protection

• 250.122 — Equipment Grounding Conductor Protection

• 409.22(A) — Industrial Control Panels

• 440.10(A) — Air Conditioning & Refrigeration Equipment

• 620.16(B) — Elevator Control Panels

• 670.5(1) — Industrial Machinery

Marking Short-Circuit Current Ratings (SCCR)

• 230.82(3) — Meter Disconnect

• 409.110(4) — Industrial Control Panels

• 430.8 — Motor Controllers

• 430.98 — Motor Control Centers

• 440.4(B) — Air Conditioning & Refrigeration Equipment

• 620.16(A) — Elevator Control Panel

• 670.3(A)(4) — Industrial Machinery

• 700.5(E) — Transfer Equipment for Emergency Systems

• 701.5(D) — Transfer Equipment for Legally Required and Standby

Systems

• 702.5 — Transfer Equipment for Optional Standby Systems

• 708.24(E) — Transfer Equipment for Critical Operations Power

Systems

Selective coordination

• 620.62 — Selective Coordination for Elevator Circuits

• 645.27 — Critical Operations Data Systems

• 695.3(C) — Multi-building Campus-Style Complexes

• 700.32 — Emergency Systems

• 701.27 — Legally Required Standby Systems

• 708.54 — Critical Operations Power Systems

4.1.3 One-line diagram

The one-line diagram, often referred to as a single-line, plays an important role in many aspects of power distribution system design, maintenance and construction. A one-line diagram graphically represents the power distribution system. Developing this diagram is the first step in making fault current, selective coordination and incident energy studies. This diagram should show all fault current sources and significant circuit elements. Significant circuit element reactance and resistance values should be included in the diagram. The one-line diagram should be updated any time the power distribution system changes. Changes must be reviewed with attention paid to the impact upon the studies that are based on this diagram’s contents.

4.1.3.1 Fault current contributors

Fault current sources in a power distribution system include:

Utility

Utilities provide power through a transformer or series of transformers depending upon where in the distribution system the facility obtains its power. Most rural locations have a transformer dedicated to a facility or multiple facilities. In some urban areas, for reliability sake, power is derived from utility secondary networks where utility transformers are operated in parallel. Available fault currents on these secondary network systems are very high, in a range greater than 100 kA and upwards of

200 kA.

The fault current that’s typically provided from the utility is an infinite bus calculation based upon the supply transformer’s kVA size and minimum impedance. For applications on a secondary network, consulting the utility is the only way to obtain the available fault current for any given installation.

Generators

On-site generation for backup power must be a consideration for the power distribution system equipment. In most cases, the local generation will not provide fault currents greater than what can be seen from a utility. When large systems have multiple generators installed in parallel, such as hospitals or other similar applications, it’s conceivable that available fault currents are greater than that available from utility sources.

4


4-2

Section 4 — Power system analysis

The available fault current will depend upon the kVA of the generator and sub-transient reactance. NEC Section 445.11 specifies the information to be included on the generator’s nameplate consisting of:

• Sub-transient, transient, synchronous, and zero sequence reactances

• Power rating category

• Insulation system class

• Indication if the generator is protected against overload by inherent design, overcurrent protective relay, circuit breaker or fuse

• Maximum fault current for inverter-based generators, in lieu of the synchronous, sub-transient, and transient reactances

The sub-transient reactance is an impedance value used in determining generator fault contribution during the first cycle after a fault occurs.

In approximately 0.1 second, the reactance increases to the transient reactance which is typically used to determine the fault current contribution after several cycles. In approximately 1/2 to 2 seconds, the generator’s reactance increases to the synchronous reactance, which is the value that determines current flow after a steady-state condition is reached by the system, should fault currents be permitted to flow this long.

Motors

Voltages collapse during a fault, and when this happens to operating motors, their rotors will continue to turn and convert this rotating motion from a load to a fault current source. It’s not practical to consider the contribution of each small motor in a system. IEEE rules of thumb deal with small motors by combining motors ≤ 50 Hp at each bus to which they are attached, and modeling them as one motor with an assumed sub-transient reactance, with 25% the typical assumed value.

The basic equation to determine motor contribution I sc

is:

I

SC

motor = (Motor FLA x 100) ÷ %Xd’

Motor sub-transient reactances range from 15% to 25%, with 25% being the more popular value used.

The motor’s sub-transient reactance is an impedance value used to determine the motor’s fault current contribution during the first cycle after a fault occurs. In approximately 0.1 second, the reactance increases to the transient reactance, which is typically used to determine the fault current contribution after several cycles. In approximately 1/2 to 2 seconds, the motor’s reactance increases to the synchronous reactance, which is the value that determines current flow after a steady-state condition is reached by the system, should fault currents be permitted to flow this long.

Alternate power sources

Alternative energy sources are becoming more and more common in power distribution systems. In addition to an inverter that’s collecting energy from solar or wind power, batteries are also fault current contributors and should be considered when appropriate.

4.1.3.2 Fault current reducers (impedances)

Impedance components considered in fault calculations, and shown on one-line diagrams, include:

Conductors

Unlike rotating machinery and transformers, conductors have a resistance and reactance mix to add to the power distribution system.

Impedance values can be obtained from Table 9 of the NEC. As an example, a 500 kcmil copper conductor in a metallic raceway has impedance values as:

• Resistance = 0.029 ohms per 1000 ft.

• Reactance = 0.048 ohms per 1000 ft.

This is typically expressed in a rectangular format as:

Z

Conductor

= 0.029 + j0.048 ohms per 1000 ft.*

* “j” is a 90 degree operator signifying a vector at a 90 degree angle. Each impedance is comprised of real and reactive components. Real components are a magnitude at a 0 angle and inductive reactive components are a magnitude at a 90 degree angle.

This conductor’s impedance is represented graphically in Figure 4.1.3.2.a.

R = 0.029 Ohms/1000 ft.

Figure 4.1.3.2.a

Conductor length is important when determining the impedance of any branch, feeder or service circuit. The impedance for 200 ft. of the 500 kcmil conductor referenced above is calculated as:

Z

(200 ft.)

Z

(200 ft.)

= 0.0006 + j0.0096 Ohms

Impedance values will be different depending upon the conductor size and material (copper or aluminum) as well as what raceway in which it is installed and its length. One-line diagrams must have enough information to determine the correct conductor ampacity as well as determining the correct impedance for calculations.

Transformers

Transformers add considerable impedance to a power distribution system. The transformer nameplate will include its %Z value, which is based on the transformer’s secondary.

When actual information is not available, the rule of thumb for typical transformer impedance ranges is shown in the following table.

kVA 3-phase

112.5

150

225

300

500

750

1000

1500

2000

2500

Ohms/1000

Z = 0.0561

Ø = 58.86°

6.0

7.0

7.0

8.0

9.0

X/R

3.0

3.5

4.0

4.5

5.0

ft.

X = j0.048 Ohms/1000 ft.

Range of %Z

1.6 — 2 Min — 6.2

1.5 — 2 Min — 6.4

2.0 — 2 Min — 6.6

2.0 — 4.5 Min — 6.0

2.1 — 4.5 Min — 6.1

3.2 — 5.75 — 6.75 — 6.8

3.2 — 5.75 — 6.75 — 8.0

3.5 — 5.75 — 6.75 — 6.8

3.5 — 5.75 — 6.75 — 6.8

3.5 — 5.75 — 6.75 — 6.8

Table notes:

1. Underlined values are from ANSI C57.12.10-1977[1], ANSI C57.12.22-1980 [2] and NEMA 210-1976 [10].

2. Network transformers with three-position switches have 5.0%Z for 500-750 kVA. See ANSI C57.12-40-1982 [3].

3. Three-phase banks with three single-phase transformers may have values as low as 1.2%.

The infinite bus calculation for the maximum fault current that can be possibly seen on the transformer’s secondary is calculated with the following equation:

I

Infinite bus

= FLA x 100

% Z



A transformer manufacturer determines impedance through what’s called a short-circuit test conducted as follows:

• The transformer secondary is short-circuited

• Voltage on the primary is increased until full load current flows in the secondary

• The applied voltage divided by the rated primary voltage (times 100) is the transformer’s impedance

Example: For a 480 volt rated primary, if 9.6 volts cause secondary full load current to flow through the shorted secondary, the transformer impedance is 9.6 ÷ 480 = 0.02 = 2%Z.

Busway

Busway presents a flexible method to distribute power in a facility.

Busway originated in Detroit’s automotive industry during late 1920’s in a response to a need for overhead wiring systems that would simplify electric motor-driven machine connections and permit a convenient arrangement for these production line machines. Busway has grown in popularity for many applications beyond manufacturing.

Busway presents a very low impedance to the distribution system, making it a very efficient power distribution means with attractive low voltage drop qualities. Busway also presents a low impedance during fault events.

The following table presents typical busway impedance values for use in voltage drop and fault calculations.

1600

2000

2500

3000

4000

5000

Typical busway parameters, line-to-neutral, in m Ω /100 ft, 25°C

Aluminum Copper

Current rating (amps) R X R X

600

800

2.982

2.00

1.28

0.80

2.33

1.63

1.57

1.25

1000

1200

1350

1.60

1.29

1.03

0.64

0.55

0.44

1.27

0.97

0.86

0.92

0.69

0.63

0.89

0.70

0.57

0.46

0.34

—

0.38

0.32

0.26

0.21

0.16

—

0.72

0.58

0.41

0.37

0.28

0.20

0.55

0.46

0.32

0.29

0.21

0.16

Reactors

There are various reasons that reactors are used in a power distribution system. One reason is to limit fault current. Current-limiting reactors, connected in series, are primarily used to reduce fault currents and to match the impedance of parallel feeders. For example, to reduce the available fault and arcing current at the equipment, low voltage motor control centers can be supplied with three single-phase reactors that limit available fault current.

Reactors are also used in grounding neutrals of generators directly connected to the distribution system bus to limit the line-to-ground fault to somewhat less than the three-phase fault at the generator terminals.

If the reactor is so sized, in all probability, the system will remain effectively grounded.

4.1.4 Procedures and methods

To determine the fault current at any point in the system, first secure an up to date one-line diagram. The one-line diagram must include all major fault current sources and impedances to fault currents. Next, an impedance diagram is created that includes all major power system components represented as impedances.

The impedance tables in Section 4.1.7 include three-phase and singlephase transformers, cables and busway. Use these tables if information from the manufacturer is not readily available.

Fault current calculations are performed without current-limiting devices in the system. To determine the maximum “available” fault current, calculations are made as though these devices are replaced with copper bars. This is necessary to project how the system and the currentlimiting devices will perform.

Also, multiple current-limiting devices do not operate in series to

“compound” a current-limiting effect. The downstream, loadside fuse will operate alone under a fault condition if properly coordinated.

The application of the point-to-point method permits determining available fault currents with a reasonable degree of accuracy at various points for either three-phase or single-phase electrical distribution systems. This method can assume unlimited primary fault current

(infinite bus) or it can be used with limited available primary fault current.

4.1.4.1 Maximum and minimum fault currents

Fault current calculations should be performed at all critical points in the system including:

•

•

•

•

•

•

•

Service entrance equipment

Transfer switches

Panelboards

Load centers

Motor control centers

Disconnects

Motor starters

Normally, fault studies involve calculating a bolted three-phase fault condition. This can be characterized as all three phases “bolted” together to create a zero impedance connection. This establishes a

“worst case” (highest current) condition that results in maximum threephase thermal and mechanical stress in the system.

From this calculation, other fault condition types can be approximated.

This “worst case” condition should be used for interrupting rating, component protection, “Table” method for determining PPE per NFPA

70E and selective coordination.

Arc flash hazard analysis calculations should consider both maximum and minimum fault current calculations. Incident energy depends upon current and time. For lower arcing current values, clearing times could be longer than those for higher arcing current values, which could result in higher incident energy values. Therefore, an arc flash analysis must consider both spectrum of available fault current for calculating arcing currents, which are then compared with OCPD TCC curves to determine clearing times.

There are several variables in a distribution system affecting calculated bolted three-phase fault currents. Variable values applicable for the specific application analysis must be selected. The point-to-point method presented in this section includes several adjustment factors given in notes and footnotes that can be applied, and that will affect results.

Some of the parameters that must be considered include utility source fault current, motor contribution, transformer percent impedance tolerance and voltage variance.

In most situations, the utility source(s) or on-site energy sources (such as generators) are the major fault current contributors. The point-topoint method includes steps and examples that assume an infinite available fault current from the utility source. Generally, this is a good assumption for highest, worst case conditions since the property owner has no control over the utility’s system and future utility changes. In many cases, a large increase in the utility available fault current does not increase the building system’s fault current a great deal on the secondary of the service transformer. However, there are cases where the actual utility medium voltage available fault current provides a more accurate fault current assessment (minimum bolted fault current conditions) that may be needed to assess arc flash hazards.

When motors are in the system, motor fault current contribution is also a very important factor to include in any fault current analysis. When a fault occurs, motor contribution adds to the fault current magnitude, with running motors contributing four to six times their normal full load current. Series rated combinations can’t be used in specific situations due to motor fault current contributions (see the section on Series

Ratings in this book).


4


For short time duration capacitor discharge currents, certain IEEE

(Institute of Electrical and Electronic Engineers) publications detail how to calculate these currents if they are substantial.

4.1.5 Point-to-point calculation method

The application of the point-to-point method permits determining available fault currents with a reasonable degree of accuracy at various points for either 3 Ø or 1 Ø electrical distribution systems.

4.1.5.1 Basic pint-to-point calculation

The following are the basic steps to employ in the point-to-point method of calculating fault current.

Step 1: Determine the transformer full load amps (FLA) from either the nameplate, the following formulas or Table 4.1.7.1.a:

3 Ø Transformer I

FLA

= kVA x 1000

E

L-L

x √ 3

1 Ø Transformer I

FLA

= kVA x 1000

E

L-L

Step 2: Find the transformer multiplier (see Notes 1 and 2).

Multiplier =

100

%Z transformer

Note 5 . On a single-phase center-tapped transformer, the L-N fault current is higher than the L-L fault current at the secondary terminals.

The fault current available (I) for this case in Step 4 should be adjusted at the transformer terminals as follows: At L-N center tapped transformer terminals, I

L-N

= 1.5 x I

L-L

at transformer terminals.

Depending upon wire size, at some distance from the terminals, the L-N fault current is lower than the L-L fault current. The 1.5 multiplier is an approximation and will theoretically vary from 1.33 to 1.67. These figures are based on a change in turns ratio between primary and secondary, infinite source available, zero feet from transformer terminals and 1.2 x

%X and 1.5 x %R for L-N versus L-L resistance and reactance values.

Begin L-N calculations at the transformer’s secondary terminals, then proceed point-to-point.

Step 5: Calculate “M” (multiplier) or take from Table 4.1.7.2.

M = 1

1+f

Step 6: Calculate the available short-circuit symmetrical RMS fault current at the point of fault. Add motor contribution, if applicable.

I

SC RMS Sym.

= I

SC

x M

Note 1 . Get %Z from nameplate or Table 4.1.7.1.a. Transformer impedance (Z) is used to determine what the fault current will be on the transformer secondary.

Note 2 . 25 kVA and larger UL 1561 listed transformers have a ± 10% impedance tolerance. Fault current levels can be affected by this tolerance. Therefore, for high end ,worst case, multiply %Z by 0.9. For low end of worst case, multiply %Z by 1.1. Transformers constructed to ANSI standards have a ± 7.5% impedance tolerance (two-winding construction).

Step 3: Determine by formula or Table 4.1.7.1.a the transformer letthrough fault current. See Notes 3 and 4.

I sc

= Transformer FLA x Multiplier

Step 6A: Significant motor fault current contribution may be added at all fault locations throughout the system. A practical motor fault current contribution estimate is to multiply the total motor current in amps by 4.

Values of 4 to 6 are commonly accepted.

4.1.5.2 Point-to-point calculation when available primary fault current is known

The following procedure can be used to calculate the fault current level at a downstream transformer’s secondary in a system when the fault current level at the transformer primary is known.

Main transformer

X

I sc

primary

X

I sc

secondary

Note 3 . Utility voltages may vary ± 10% for power and ± 5.8% for 120 volt lighting services. Therefore, for highest fault current conditions, multiply values as calculated in Step 3 by 1.1 or 1.058 respectively. To find the lower end worst case, multiply results in Step 3 by 0.9 or 0.942 respectively.

Note 4 . Motor fault current contribution, if significant, may be added at all fault locations throughout the system. A practical motor fault current contribution estimate is to multiply the total motor current in amps by 4.

Values of 4 to 6 are commonly accepted.

Step 4: Calculate the “f” factor.

3 Ø Faults f =

√ 3 x L x I

3Ø

C x n x E

L-L

HV utility connection

X

I sc

primary

Figure 4.1.5.2.a

X

I sc

secondary

Step A: Calculate the “f” factor (I

SC

primary known)

3 Ø Transformer (I

SC

primary and I

SC

secondary are 3 Ø fault values); f = I

SC

primary x V

Primary

100,000 x kVA

x √ 3 x %Z transformer

1 Ø Transformer (I

SC secondary is L-L );

primary and I

SC

secondary are 1 Ø fault values: I

SC f = I

SC

primary x V primary

100,000 x kVA

x %Z transformer

4-4

For the next two equations, see note 5

1 Ø Line-line faults f =

2 x L x I

L-L

C x n x E

L-L

1 Ø Line-neutral faults f = 2 x L x I

L-N

C x n x E

L-N

Where:

L = Conductor length to the fault in feet

C = Constant from Table 4.1.7.6.a of “C” values for conductors and Table

4.1.7.7.a of “C” values for busway n = Number of conductors per phase (adjust C value for parallel runs)

I = Available fault current in amps at circuit’s beginning

E = Circuit voltage


Step B: Calculate “M” (multiplier)

M = 1

1+f

Step C: Calculate the fault current at the transformer secondary (see

Note under Step 3 of “Basic point-to-point calculation procedure”)

I

SC secondary

=

V

V primary x M x I

SC primary secondary


4.1.5.3 Point-to-point calculation

Example 1

Infinite bus Note: The following “Step” numbers pertain to the steps described in “4.1.5.1. Point-to-point calculation basic.”

I

1500 kVA transformer,

480 V, 3Ø,

3.5%Z, 3.45%X, 0.56%R, fl

= 1804 A

25’ 500 kcmil Cu conductor,

3 single conductors,

6 per phase, magnetic conducit

X

1

Fault X1

Step 1: I fL

= 1500 x 1000 = 1804 A

√ 3

Step 2: Multiplier =

100

3.5 x 0.9

†

= 31.746

Step 3: I

I

SC

SC

I

SC

= 1804 A x 31.746 = 57,279 A

motor contribution* = 4 x 1804 A = 7217 A

total = 57,279 A + 7217 A = 64,496 A

X

2

2000 A switch,

KRP-C-2000SP fuse

Fault X2

Step 4:

√

400 A switch,

LPS-RK-400SP fuse


3 single conductors, magnetic conducit

M

X

3

Step 5: M =

1

1+ 0.0388

= 0.9626

Step 6: I

I

SC

SC

I

SC

= 57,279 A x 0.9626 = 55,137 A

motor contribution* = 4 x 1804 A = 7217 A

total =55,137 A + 7217 A = 62,354 A

* See Note 4 on page 4-4. Assumes 100% motor load. If 50% of this load is from motors, I

SC

motor contribution = 4 x 1804

A x 0.5 = 3608 A.

† See Note 2 on page 4-4.

Fault X3

Step 4:

√ 3 x 50 x 55,137 = 0.4484

Step 5: M =

1

1+ 0.4483

= 0.6904

Step 6: I

SC

= 55,137 A x 0.6904 = 38,067 A

I

I

SC

SC

motor contribution = 4 x 1804 A = 7217 A

total = 38,067 A + 7217 A = 45,284 A

4

Motor contribution*

Example 2

Infinite bus

1000 kVA transformer,

480 V, 3Ø,

I

3.5%Z, fl

= 1203 A



4 per phase,

PVC conducit

X

1

X

2

1600 A switch,

KRP-C-1500SP fuse

Note: The following Step numbers pertain to the steps described in “4.1.5.1 Point-to-point calculation basic.”

Fault X1

Step 1:

480 x √ 3

Fault X3

Step 4: √ 3 x 20 x 36,761 = 0.1161

2 x 11,424 x 480

Step 2:

3.5 x 0.9

= 31.746

Step 3: I

SC

= 1202.8 A x 31.746 = 38,184 A

Step 5: M=

1

1+ 0.1161

= 0.8960

Step 6: I

SC

= 36,761 A x 0.8960 = 32,937 A

Fault X2

Step 4:

√ 3 x 30 x 38,184

26,706 x 4 x 480

= 0.0387

Step 5: M =

1

1+ 0.0387

= 0.9627

Step 6: I

SC

= 38,184 A x 0.9627 = 36,761 A

Fault X4

Step A:

32,937 x 480 x √

100,000 x 225

Step B: M =

1

1+ 1.3144

= 0.4321

Step C: I

SC

400 A switch,

LPS-RK-350SP fuse

20’ 2/0 Cu conductor,


2 per phase,

PVC conducit

225 kVA transformer

208 V, 3Ø,

1.2%Z

X

3

X

4



4.1.5.4. Single-phase system fault currents

Fault current calculations on a single-phase center tapped transformer system require a slightly different procedure than 3 Ø faults on 3 Ø systems.

Primary available fault current

It is necessary that the proper impedance be used to represent the primary system. For 3 Ø fault calculations, a single primary conductor impedance is used from the source to the transformer connection. This is compensated for in the 3 Ø fault current formula by multiplying the single conductor or single-phase impedance by 1.73 ( √ 3).

However, for single-phase faults, a primary conductor impedance is considered from the source to the transformer, and back to the source.

This is compensated for in the calculations by multiplying the 3 Ø primary source impedance by two.

Center-tapped transformer impedance

The center-tapped transformer impedance must be adjusted for the halfwinding (generally line-to-neutral) fault condition.

Primary Primary

Secondary Secondary

Short-circuit Short-circuit

Figure 4.1.5.4.a

L

1

N L

2

Figure 4.1.5.4.b

Figure 4.1.5.4.b illustrates that during line-to-neutral faults, the full primary winding is involved, but only the half-winding on the secondary is involved. Therefore, the actual transformer reactance and resistance of the half-winding condition is different than the actual transformer reactance and resistance of the full winding condition. Thus, adjustment to the %X and %R must be made when considering line-to-neutral faults. The adjustment multipliers generally used for this condition are:

• 1.5 times full winding % R on full winding basis

• 1.2 times full winding % X on full winding basis

Note: %R and %X multipliers given in “Impedance Data for Single-

Phase Transformers” Table may be used. However, calculations must be adjusted to indicate transformer kVA ÷ 2.

Cable and two-pole switch impedances

The cable and two-pole switch impedance on the system must be considered “both-ways” since the current flows to the fault and then returns to the source. E.g., if a line-to-line fault occurs 50 feet from a transformer, then 100 feet of cable impedance must be included in the calculation. (Figure 4.1.5.5.c.)

Primary Secondary

N

L

1

Short-circuit

L

2

50 Feet

Figure 4.1.5.4.c

The following calculations illustrate 1 Ø fault calculations on a singlephase transformer system. Both line-to-line and line-to-neutral faults are considered.

Note in these examples:

• The multiplier is 2 for some electrical components to account for the single-phase fault current flow

4-6

• The half-winding transformer %X and %R multipliers for the line-toneutral fault situation along with impedance and reactance data



Single-phase system fault current calculation

75 kVA transformer,

120/240 V, 1Ø,

1.40%Z, 31.22%X, 0.68%R,

I fl

= 312.5 A

Infinite bus

X

1

Example 1 — line-to-line

Note: The following Step numbers pertain to the steps described in “4.1.5.1. Point-to-point calculation basic.”

Fault X1

Step 1:

240

Step 2: Multiplier =

100

1.4 x 0.9*

= 79.37

Step 3: I

SC

L-L =312.5 A x 79.37 = 24,802 A

Fault X2

Step 4:

22,185 x 1 x 240

Step 5: M =

1

1+ 0.2329

= 0.8111

Step 6: I

SC

L-L = 24,802 A x 0.8111 = 20,116 A

Fault X3

Step 4:

4774 x 1 x 240

Step 5: M =

1

1+ 1.7557

= 0 .3629

Step 6: I

SC

= 20,116 A x 0.3629 = 7300 A

* In addition, 25 kVA and larger UL 1561 listed transformers have a ±10% impedance tolerance that can affect fault current.

Therefore, for high end worst case, multiply %Z by 0.9. For low end of worst case, multiply %Z by 1.1. Transformers constructed to ANSI standards have a ±7.5% impedance tolerance (two-winding construction).

4



400 A switch,

LPS-RK-400SP fuse

50’ 3 AWG Cu conductor,


X

2

X

3

Example 2 — line-to-neutral

Note: The following Step numbers pertain to the steps described in “4.1.5.1. Point-to-point calculation basic.”

Fault X 1

Step 1:

240

Step 2:

1.4 x 0.9

= 79.37

Step 3*: I

SC

I

SC

L-L = 312.5 A x 79.37 = 24,802 A

L-N = 24,802 x 1.5 = 37,202 A

Fault X2

Step 4:

22,185 x 1 x 120

Step 5: M =

1

1+ 0.6987

= 0.5887

Step 6*: I

SC

L-L = 37,202 A x 0.5887 = 21,900 A

Fault X3

Step 4:

4774 x 1 x 120

Step 5: M =

1

1 + 3.8323

= 0.2073

Step 6*: I

SC

= 21,900 A x 0.2073 = 4540 A

* The L-N fault current is higher than the L-L fault current at the single phase center-tapped transformer’s secondary terminals. The available fault current (I) for this case in Step 4 should be adjusted at the transformer terminals as follows:

At L-N center-tapped transformer terminals, I

L-N

= 1.5 X I

** Assumes same size neutral and line conductors.

L-L

at transformer terminals.


4-8


4.1.6 Equipment evaluations

The first step to properly applying electrical solutions, as well as complying with Code requirements, is the fault current study. Once the fault current levels are determined, equipment has to be evaluated for proper application including:

• OCPD interrupting ratings

• System selective coordination

• Component protection (SCCR)

• Incident energy analysis

See the various sections in this handbook for further information on these topics.

Low voltage fuses have their interrupting rating expressed as symmetrical component of fault current. They are given an RMS symmetrical interrupting rating at a specific power factor. This means that the fuse can also interrupt the asymmetrical current associated with this rating. Thus, only the symmetrical component of fault current need be considered to determine the necessary low voltage fuse interrupting rating.

The NEC includes requirements for marking and/or documenting, available fault current for various locations throughout the power distribution system. NEC 110.24 requires field marking service equipment (other than dwelling units and certain industrial facilities) with the maximum available fault current. Additionally, other requirements include either marking the available fault current on the equipment, or documenting the available fault current covering industrial control panels, HVAC equipment, elevator control panels and industrial machinery.

In addition to OCPD interrupting ratings and equipment SCCR ratings, the available fault current is used for other purposes, including determining selective coordination and arc flash boundary, along with the proper arc rated PPE per NFPA 70E. Whether determined by the incident energy method or arc flash PPE category method (70E 130.5), the available fault current is required.

4.1.7 Tables

4.1.7.1 Fault currents available from transformers

Table 4.1.7.1 includes values based on actual field nameplate data or from utility transformer worst case impedance.

Voltage and phase kVA

25

37.5

50

75

100

167

45

75

112.5

150

2000

2500

75

112.5

150

225

225

300

500

750

1000

1500

300

500

750

1000

1500

2000

2500

4.00

4.00

1.00

1.00

1.20

1.20

1.12

1.11

1.24

3.50

3.50

3.50

% impedance †

(nameplate)

1.50

1.50

1.50

1.50

1.60

1.60

1.00

1.00

1.11

1.07

1.20

1.30

3.50

3.50

3.50

4.00

4.00

5552

6940

90

135

181

271

625

833

1388

2082

2776

4164

Full load amps

104

156

208

313

417

696

125

208

312

416

361

602

903

1204

1806

2408

3011

61,960

83,357

124,364

66,091

88,121

132,181

154,211

192,764

10,035

15,053

16,726

25,088

Fault current amps ††

12,175

18,018

23,706

34,639

42,472

66,644

13,879

23,132

31,259

43,237

33,451

51,463

28,672

38,230

57,345

66,902

83,628

Available Fault Current Calculator

FC 2 is an online or downloadable application

(for both Apple and Android devices) that utilizes the point-to-point method for calculating and documenting available fault current levels in single- and three-phase systems.

It’s capable of producing equipment labels in English,

Spanish or French for local language needs. Scan the QR code or visit the Bussmann division website at

Eaton.com/bussmannseries.

Table 4.1.7.1.a

* Single-phase values are L-N at transformer terminals. These figures are based on change in turns ratio between primary and secondary, 100,000 kVA primary, zero feet from transformer terminals, 1.2 %X and 1.5 %R multipliers for L-N vs.

L-L reactance and resistance values, and transformer X/R ratio = 3.

** Three-phase fault currents based on “infinite” primary.

† 25 kVA or greater UL Listed transformers have a ± 10% impedance tolerance.

Fault current shown in Table 4.1.7.1.a reflect -10% condition. Transformers constructed to ANSI standards have a ± 7.5% impedance tolerance (twowinding construction)

†† System voltage fluctuations will affect the available fault current. For example, a 10% increase in system voltage will result in a 10% greater available fault current than as shown in Table 4.1.7.1.a.



4.1.7.2 “M” multiplier

The “M” multiplier is used in the point-to-point calculations. The basic equation used to derive the values in this table is:

The “f” value is based upon a calculation as described in Step 4 of the point-to-point method for calculating fault currents.

0.06

0.07

0.08

0.09

0.10

0.15

f

0.01

0.02

0.03

0.04

0.05

0.20

0.25

0.30

0.35

0.40

0.94

0.93

0.93

0.92

0.91

0.87

M

0.99

0.98

0.97

0.96

0.95

0.83

0.80

0.77

0.74

0.71

1.00

1.20

1.50

1.75

2.00

2.50

f

0.50

0.60

0.70

0.80

0.90

3.00

3.50

4.00

5.00

6.00

0.50

0.45

0.40

0.36

0.33

0.29

M

0.67

0.63

0.59

0.55

0.53

0.25

0.22

0.20

0.17

0.14

20.00

30.00

40.00

50.00

60.00

70.00

f

7.00

8.00

9.00

10.00

15.00

80.00

90.00

100.00

—

—

Table 4.1.7.2 “M” multiplier

4.1.7.3 Transformer single-phase impedance data

Table 4.1.7.3 is reprinted from IEEE Std. 242-1986 (R1991), IEEE

Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. Copyright 1986 by the Institute of

Electrical and Electronics Engineers, Inc. with the permission of the

IEEE Standards Department.

25 kVA and greater UL Listed transformers have a ± 10% tolerance on their impedance nameplate.

0.05

0.03

0.02

0.02

0.02

0.01

M

0.13

0.11

0.10

0.09

0.06

0.01

0.01

0.01

—

— kVA 1 Ø

25.0

37.5

50.0

75.0

100.0

167.0

250.0

333.0

500.0

Suggested

X/R ratio for calculation

1.1

1.4

1.6

1.8

2.0

2.5

3.6

4.7

5.5

Normal range of percent impedance*

1.2–6.0

1.2–6.5

1.2–6.4

1.2–6.6

1.3–5.7

1.4–6.1

1.9–6.8

2.4–6.0

2.2–5.4

Impedance multipliers — line-neutral faults** for %X

0.6

0.6

for %R

0.75

0.75

0.6

0.6

0.6

1.0

0.75

0.75

0.75

0.75

1.0

1.0

1.0

0.75

0.75

0.75

Table 4.1.7.3

* National standards do not specify %Z for single-phase transformers. Consult manufacturer for values to use in calculation.

** Based on winding rated current (one-half nameplate kVA divided by secondary line-to-line voltage).

This table has been reprinted from IEEE Std. 242-1986 (R1991), IEEE

Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems, Copyright 1986 by the Institute of

Electrical and Electronics Engineers, Inc. with the permission of the

IEEE Standards Department.

4.1.7.4. Impedance data for single-phase and three-phase transformers supplement

The data included in Table 4.1.7.4 provides actual transformer nameplate ratings taken from field installations. 25 kVA and greater UL Listed transformers have a ± 10% tolerance on their nameplate impedance.

1 Ø

10

15

—

—

—

—

333

—

500 kVA

3 Ø

—

—

75

150

225

300

—

500

—

%Z

1.2

1.3

1.11

1.07

1.12

1.11

1.9

1.24

2.1

Suggested X/R ratio for calculation

1.1

1.1

1.5

1.5

1.5

1.5

4.7

1.5

5.5

Table 4.1.7.4

4.1.7.5 Various fault current types as a percent of threephase bolted faults (typical)

This table provides some general information on various fault current types as a percentage of three-phase bolted fault currents. These are general rules of thumb that should not replace actual calculations that can be provided by software applications.

Fault type

Three-phase bolted

Line-to-line bolted

Line-to-ground bolted

Line-to-neutral bolted

Three-phase arcing

Line-to-line arcing

Line-to-ground arcing minimum

Percentage

100%

87%

25-125%* (Use 100% near transformer, 50% otherwise)

25-125% (Use 100% near transformer, 50% otherwise)

89% maximum

74% maximum

38% minimum

Table 4.1.7.5

* Typically much lower, but can actually exceed the three-phase bolted fault if it is near the transformer terminals. Will normally be between 25% to 125% of three phase bolted fault value.

4



4.1.7.6 “C” values for conductors

Table 4.1.7.6 data is used as part of the point-to-point fault current calculation when determining the “f” factor as part of Step 4.

AWG or kcmil

Copper

14

12

1

1/0

3

2

10

8

6

4

2/0

3/0

4/0

250

300

350

400

500

600

750

1000

Aluminum

14

400

500

600

750

1000

1/0

2/0

3/0

4/0

250

300

350

2

1

4

3

12

10

8

6

600 V

10,755

12,844

15,082

16,483

18,177

19,704

20,566

22,185

22,965

24,137

25,278

389

617

981

1557

2425

3806

4774

5907

7293

8925

5777

7187

8826

10,741

12,122

13,910

15,484

16,671

18,756

20,093

21,766

23,478

237

376

599

951

1481

2346

2952

3713

4645

Steel conduit

5 kV

Three single conductors

15 kV

Non-magnetic conduit

600 V 5 kV 15 kV

—

—

—

1551

2406

3751

4674

5736

7029

8544

10,062

11,804

13,606

14,925

16,293

17,385

18,235

19,172

20,567

21,387

22,539

—

—

—

950

1476

2333

2928

3670

4575

5670

6968

8467

10,167

11,460

13,009

14,280

15,355

16,828

18,428

19,685

21,235

—

—

—

—

2389

3696

4577

5574

6759

7973

9390

11,022

12,543

13,644

14,769

15,678

16,366

17,492

17,962

18,889

19,923

—

—

—

—

1472

2319

2904

3626

4498

5493

6733

8163

9700

10,849

12,193

13,288

14,188

15,657

16,484

17,686

19,006

389

617

982

1559

2430

3826

4811

6044

7493

9317

11,424

13,923

16,673

18,594

20,868

22,737

24,297

26,706

28,033

29,735

31,491

237

376

599

952

1482

2350

2961

3730

4678

5838

7301

9110

11,174

12,862

14,923

16,813

18,506

21,391

23,451

25,976

28,779

10,878

13,048

15,351

17,121

18,975

20,526

21,786

23,277

25,204

26,453

28,083

—

—

—

1555

2418

3789

4745

5926

7307

9034

—

—

—

951

1479

2342

2945

3702

4632

5766

7153

8851

10,749

12,343

14,183

15,858

17,321

19,503

21,718

23,702

26,109

10,319

12,360

14,347

15,866

17,409

18,672

19,731

21,330

22,097

23,408

24,887

—

—

—

—

2407

3753

4679

5809

7109

8590

5646

6986

8627

10,387

11,847

13,492

14,955

16,234

18,315

19,635

21,437

23,482

—

—

—

—

1476

2333

2929

3673

4580

600 V

389

617

982

1559

2431

3830

4820

5989

7454

9210

11,245

13,656

16,392

18,311

20,617

22,646

24,253

26,980

28,752

31,051

33,864

5852

7327

9077

11,185

12,797

14,917

16,795

18,462

21,395

23,633

26,432

29,865

237

376

599

952

1482

2351

2963

3734

4686

—

—

—

1557

2425

3812

4785

5930

7365

9086

11,045

13,333

15,890

17,851

20,052

21,914

23,372

25,449

27,975

30,024

32,689

5820

7271

8981

11,022

12,636

14,698

16,490

18,064

20,607

23,196

25,790

29,049

—

—

—

951

1480

2347

2955

3719

4664

Three-conductor cable

Steel conduit

5 kV 15 kV

—

—

—

—

2415

3779

4726

5828

7189

8708

10,500

12,613

14,813

16,466

18,319

19,821

21,042

23,126

24,897

26,933

29,320

5717

7109

8751

10,642

12,115

13,973

15,541

16,921

19,314

21,349

23,750

26,608

—

—

—

—

1478

2339

2941

3693

4618

Non-magnetic conduit

600 V 5 kV 15 kV

389

617

982

1560

2433

3838

4833

6087

7579

9473

11,703

14,410

17,483

19,779

22,525

24,904

26,916

30,096

32,154

34,605

37,197

5876

7373

9243

11,409

13,236

15,495

17,635

19,588

23,018

25,708

29,036

32,938

237

376

599

952

1482

2353

2966

3740

4699

—

—

—

1558

2428

3823

4803

6023

7507

9373

11,529

14,119

17,020

19,352

21,938

24,126

26,044

28,712

31,258

33,315

35,749

5852

7329

9164

11,277

13,106

15,300

17,352

19,244

22,381

25,244

28,262

31,920

—

—

—

952

1481

2350

2959

3725

4682

11,053

13,462

16,013

18,001

20,163

21,982

23,518

25,916

27,766

29,735

31,959

—

—

—

—

2421

3798

4762

5958

7364

9053

5771

7202

8977

10,969

12,661

14,659

16,501

18,154

20,978

23,295

25,976

29,135

—

—

—

—

1479

2344

2949

3709

4646

Table 4.1.7.6.a “C” values for conductors.

Note: These values are equal to one over the impedance per foot, and based upon resistance and reactance values found in IEEE Std. 241-

1990 (Gray Book), IEEE Recommended Practice for Electric Power Systems in Commercial Buildings & IEEE Std. 242-1986 (Buff Book), IEEE

Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. Where resistance and reactance values differ or are not available, the Buff Book values have been used. The values for reactance in determining the C Value at 5 kV & 15 kV are from the Gray

Book only (Values for 14-10 AWG at 5 kV and 14-8 AWG at 15 kV are not available and values for 3 AWG have been approximated).



4.1.7.7 “C” values for busway

Table 4.1.7.7 data is used as part of the point-to-point fault current calculation when determining the “f” factor as part of Step 4.

Without coordination With coordination

Ampacity

225

400

600

Busway

Plug-in Feeder High impedance

Copper Aluminum Copper Aluminum Copper

28700

38900

41000

800

1000

46100

69400

1200 94300

1350 119000

1600 129900

23000

34700

38300

57500

89300

97100

104200

120500

18700

23900

36500

49300

62900

76900

90100

101000

12000

21300

31300

44100

56200

69900

84000

90900

—

—

—

—

15600

16100

17500

19200

2000 142900

2500 143800

3000 144900

4000 —

135100

156300

175400

—

134200

180500

204100

277800

125000

166700

188700

256400

20400

21700

23800

—

Table 4.1.7.7.a “C” values for busway.

Note: These values are based on a survey of industry and equal to one over the impedance per foot for busway impedance. Busway manufacture information should be consulted for specific applications.

4.2 Selective coordination

Contents

4.2.1 Overview

4.2.2 Coordination analysis

4.2.3 Selective coordination tools

4.2.4 Fuse selective coordination

4.2.5 Fuse selectivity ratio tables

4.2.6 Circuit breaker selective coordination

4.2.7 Fuse and circuit breakers selective coordination

4.2.8 Code requirements

4.2.9 Selective coordination design guide

4.2.10 Coordination

4.2.11 Summary

4.2.1 Overview

Section page

11

12

12

13

15

16

22

24

26

30

30

Selective coordination is critical for electrical distribution system reliability. A reliable system is not only important for life safety, it’s important from a business perspective as nothing will stop all activity, paralyze production, inconvenience and disconcert people more than a major power failure.

Selectively coordinated overcurrent protective devices address localizing faulted conditions on the power distribution system and is quite often a reliability design goal. In addition, the NEC mandates selectively coordinated OCPDs for circuits that supply power to vital loads in specific building system applications.

A properly engineered and installed system that’s selectively coordinated will allow only the nearest upstream OCPD to open for the full range of overcurrents (both overloads and all fault types), leaving the remainder of the system undisturbed and preserving continuity of service. Figure 4.2.1.a illustrates the difference between a selectively coordinated system and one that is not. Isolating the circuit’s faulted portion is important for overall system reliability.

Opens

Unnecessary power loss

X

Fault

Opens

Not affected

X

Fault

Figure 4.2.1.a

Selective coordination isolates the circuit’s faulted portion and only the faulted portion. The OCPD closest to the fault is the only device to open to limit the impact on the balance of the system. To achieve selective coordination, the selection and installation of OCPDs and their ratings or settings are important. This must be addressed in any project’s design phase. Once switchboards, distribution panels, motor control centers, lighting panelboards and OCPDs are selected and installed, retroactively “fixing” a system that does not selectively coordinated can be expensive.

The following sections explain how to evaluate whether OCPDs provide selective coordination for the full range of overcurrents.

4


4-12


4.2.2 Coordination analysis

Point E 100

80

60

Point G

10

8

6

4

3

Point D

2

40

30

20

1

0.8

0.6

0.4

0.3

Point F

0.2

0.1

0.08

0.06

0.04

0.03

0.02

600

400

300

200

100 A 400 A

Point C

400 A

100 A

X Available fault current = 1 kA

Fuses have an inverse time-current characteristic. This means the greater the overcurrent, the faster they interrupt. Look at the 100 A fuse curve: for a 200 A overcurrent, the fuse will clear the overcurrent in approximately 200 seconds, and for an 2000 A overcurrent, the fuse will clear the overcurrent in approximately 0.15 second.

In some cases, it’s possible to assess coordination between two or more fuses through comparing their time-current curves. This method is limited to only the overcurrent range up to the point at which the upstream fuse crosses 0.01 second. For example: assume an 1000

A RMS symmetrical overcurrent on the loadside of the 100 A fuse.

To determine the time it would take this overcurrent to open the two fuses:

• Find 1000 A on the horizontal axis (Point A)

• Follow the dotted line vertically to the intersection of the 100 A fuse’s total clear curve (Point B) and the 400 A fuse’s minimum melt curve

(Point C).

• Then, horizontally from both intersection points, follow the dotted lines to Points D and E.

Point B

0.01

Point A 1000 A

Current in amps

Point H

Minimum melt Total clearing

Figure 4.2.2.a

Figure 4.2.2.a illustrates the time-current characteristic curves for a 400

A and 100 A time-delay, dual-element fuses in series, as depicted in the one-line diagram. The graph’s horizontal axis represents the RMS symmetrical current in amps. The vertical axis represents the time, in seconds. Each fuse is represented by a tolerance band which is the space between the minimum melt characteristic (solid line) and the total clear characteristics (hash line). This band represents the fuse’s tolerance under specific test conditions. For a given overcurrent, a specific fuse, under the same circumstances, will open at a time within the fuse’s time-current tolerance band.

At 1.75 seconds, Point D represents the maximum time the 100 A fuse will take to open the 1000 A overcurrent. At 90 seconds, Point

E represents the minimum time at which the 400 A fuse would open this overcurrent. These two fuses are coordinated for the 1000 A overcurrent.

For overcurrents up to approximately 11,000 A (Point H), since no curve overlapping exist and the current is less than where the upstream fuse crosses 0.01 second, it can be determined that the two fuses are selectively coordinated. The 100 A fuse will open before the 400

A fuse can melt for all currents up to approximately 11,000 A. When currents exceed 11,000 A, selective coordination cannot be determined by the time-current curves. For currents in this curve region, the fuse selectivity ratio tables must be used. Eaton’s Bussmann division publishes fuse selectivity ratios that make it simple to assess whether fuses selectively coordinate. Using the selectivity ratios makes plotting fuse TCC curves unnecessary.

4.2.3 Selective coordination tools

There are many resources available to the qualified person who must specify the correct OCPDs to achieve selective coordination. Systems that leverage the fuse, circuit breaker or both technologies can achieve selective coordination for the full range of overcurrents, but attention to proper procedures and resources is important.

The following tools are available to assist in determining selective coordination:

4.2.3.1 Systems analysis software applications

Computer programs allow the designer to select OCPD time-current curves published by manufacturers and overlay, on one graph, multiple curves of selected OCPDs in a particular circuit. These curves provide the relationship of devices and show how each will respond in relation to each other for any given current value. The qualified individual must review the plotted curves in relation to the available fault current to determine whether or not selective coordination is achieved.

4.2.3.2 Manually creating a TCC curve

When computer software programs are not available, it’s possible for the qualified individual to manually create TCC curves and analyze one or more OCPDs in the system. Time-current curve overlays (from manufacturer published data) can be hand traced onto log-log paper.

The qualified individual must review the plotted curves in relation to the available fault current to determine whether or not selective coordination is achieved.



4.2.4 Fuse selective coordination

Selective coordination for fused systems is simple when the fuses are specified per the selective coordination amp ratio tables. When selectively coordinating a fused system, the available fault current is only used to determine proper interrupting and short-circuit current ratings — as fuse tables only stipulate upstream/downstream amp ratios that apply equally to all available overcurrents. Because fuses have such a high interrupting capability, calculating available fault currents throughout the distribution system for selective coordination purposes is often not necessary. The fuse amp ratio table established and discussed in Section 4.2.5 is an important tool to understand before conducting a fused system selective coordination analysis.

4.2.4.1 Fuse example 1

The following fused system illustrates how simple it is to achieve selective coordination. Review the fused system one-line diagram in

Figure 4.2.4.1.a. All fuses shown are Low-Peak fuses. The selectivity ratio table provides the minimum amp rating ratio that must exist between a lineside and loadside fuse in order to achieve selective coordination. If the entire electrical system maintains these minimum fuse amp rating ratios for each circuit path, the entire electrical system will be selectively coordinated for all overcurrents (note there is no need to plot time-current curves). It’s important to understand that the fuse amp ratio tables apply only to that manufacturer’s specific fuses.

Point B: LPJ-400SP fuse downstream of KRP-C-1200SP fuse

Use the same steps as in the previous paragraph. The amp rating ratio for the two fuses in this circuit path is 1200:400, or a 3:1 ratio.

The selectivity amp ratio table shows that the amp ratio must be maintained at 2:1, or more, to achieve selective coordination for these specific fuses. Since these fuses have a 3:1 ratio, and a minimum 2:1 is all that’s needed, these two fuses are selectively coordinated for any overcurrent up to 200 kA. The result is this entire circuit path is selectively coordinated for all overcurrents up to 200 kA as shown in

Figure 4.2.4.1.b.

KRP-C-1200SP

Low-Peak fuse

LPJ-400SP

Low-Peak fuse

LPJ-100SP

Low-Peak fuse

C

KRP-C-1200SP

Low-Peak fuse

X

Only this fuse opens

4

B

LPJ-400SP

Low-Peak fuse

A

LPJ-100SP

Low-Peak fuse

X

Any fault level

Figure 4.2.4.1.b

4.2.4.2 Fuse example 2

Figure 4.2.4.2.a is an example where the selected fuses do not initially meet the minimums in the selectivity amp ratio table. This system does not selectively coordinate. This example will work through resolving this situation without changing fuse sizes which would impact equipment and conductor sizes.

Figure 4.2.4.1.a

Always begin closest to the load and check device pairs for selectivity.

A review of the system shown in Figure 4.2.4.1.a includes the following steps:

Point A: LPJ-100SP fuse downstream of LPJ-400SP fuse

In this circuit path, the amp rating ratio for these fuses is 400:100, or a 4:1 ratio. Checking the selectivity amp ratio table, the intersection of lineside LPJ (left column) to loadside LPJ (top row), yields a minimum

2:1 ratio. This indicates there is selective coordination for these two fuses for any overcurrent up to 200 kA. For any overcurrent on the LPJ-

100SP fuse’s loadside, only the LPJ-100SP fuse opens. The LPJ-400SP fuse remains in operation as well as all other fuses in the system.

KRP-C-800SP

Low-Peak fuse

2:1

FRS-R-400

Fusetron fuse

2:1

FRS-R-200

Fusetron fuse

Selectivity amp ratios

Lineside fuses

KRP-C-SP

FRS-R

LPS-RK-SP

KRP-C-SP

2:1

—

—

Loadside fuses

FRS-R

4:1

2:1

8:1

LPS-RK-SP

2:1

1.5:1

2:1

X

Overloads or faults of any level up to 200 kA

Figure 4.2.4.2.a

To address the lack of selective coordination, each upstream/ downstream fuse pair must be evaluated for using alternative fuses.

Figure 4.2.4.2.b shows an analysis for obtaining selective coordination by specifying other fuse types while keeping each fuse’s amp rating the same. The original non-selectively coordinated system is one-line diagram “A” in Figure 4.2.4.2.b.



The following steps are necessary to evaluate this system. Always start at the fuse furthest from the source and work back towards the source as follows:

FRS-R-200 fuse downstream of FRS-R-400 fuse

The FRS-R-200 fuse selectively coordinates with the FRS-R-400 fuse since they have a 2:1 amp ratio and the selectivity amp ratio table for

FRS-R to FRS-R fuses is a 2:1 minimum.

FRS-R-400 fuse downstream of KRP-C-800SP fuse

As applied, these two fuses have a 2:1 amp rating ratio. The selectivity amp ratio table requires at least a 4:1 ratio for selectivity. To address this, the Bussmann series Fusetron FRS-R-400 fuse can be substituted with another that fits the same block and has similar, if not the same, overload characteristics. The LPS-RK-400SP fuses can be applied because they can be sized for loads in the same manner as the FRS-R-

400 fuses, while offering better current-limiting performance for a better ratio with the upstream KRP-C-800SP fuse. This simple substitution ensures a minimum 2:1 ratio can be maintained for selective coordination. This is shown as “B” in Figure 4.2.4.2.b. Because the FRS-

R-400 was substituted with a different fuse type that’s already been evaluated with the downstream FRS-R-200 fuse, it necessary to start the evaluation again from the most downstream point in the system to ensure selectivity with the downstream fuse was not lost.

FRS-R-200 fuse downstream of LPS-RK-400SP fuse

Because the FRS-R-400 fuse was changed to the LPS-RK-400SP fuse, evaluating the downstream FRS-R-200 fuse is necessary to ensure selectivity is maintained. Upon review, the FRS-R-200 and LPS-RK-

400SP fuses do not meet the minimum selectivity 8:1 ratio for these fuses. The FRS-R-200 must be substituted to maintain the existing

2:1 amp rating ratio needed for selective coordination. The FRS-R-200 fuse is changed to an LPS-RK-200SP fuse to maintain the 2:1 ratio and achieve selective coordination (the minimum ratio is 2:1 for pairs of

Low-Peak fuses). This is shown as “C” in Figure 4.2.4.2.b and the entire system is now selectively coordinated.

LPS-RK-400SP fuse downstream of KRP-C-800SP fuse

The LPS-RK-400SP fuse selectively coordinates with the KRP-C-

800SP fuses since is has a 2:1 ratio and the selectivity amp ratio table minimum is 2:1 between LPS-RK-SP and KRP-C-SP fuses.

A B C

KRP-C-800SP

Low-Peak fuse

4:1

FRS-R-400

Fusetron fuse

2:1

FRS-R-200

Fusetron fuse

KRP-C-800SP

Low-Peak fuse

2:1

LPS-RK-400SP

Low-Peak fuse

8:1

FRS-R-200

Fusetron fuse

KRP-C-800SP

Low-Peak fuse

2:1

LPS-RK-400SP

Low-Peak fuse

2:1

LPS-RK-200SP

Low-Peak fuse

X

Overloads or faults of any level up to 200 kA

X X

Achieve selective coordination, typically with lower arc flash and better component protection

Figure 4.2.4.2.b

4.2.4.3. Fusible solutions

Performing a fused system selective coordination analysis is relatively simple. However, there are many fuse types with their associated ratios.

For 600 V or less electrical systems, the following Low-Peak fuses are recommended for applications from 1/10 to 6000 A, (note the LPN-RK-

SP is rated 250 V or less, but all other Low-Peak fuses are rated 600

V or less and can be used on any system up to 600 V). All Low-Peak fuses have 2:1 selectivity amp ratio with any other Low-Peak fuse in the system.

Fusible panelboards

The Bussmann series Quik-Spec™ Coordination Panelboard (QSCP) provides a fusible branch panelboard solution, making it simple and cost effective to selectively coordinate lighting and other branch circuits with upstream Bussmann series fuses.

This QSCP is available in Main Lug

Only (MLO), as well as fused or nonfused main disconnect configurations with a choice of 18, 30 and 42 branch positions in NEMA 1 or 3R enclosures to easily meet branch or service panel installation needs. This fused panelboard uses the Bussmann series finger-safe CUBEFuse (1 to 100 A,

UL Listed, Class CF, current-limiting, time-delay or fast-acting versions) for the branch circuit OCPDs as an integral part of the innovative, UL 98, horsepower rated Compact Circuit

Protector Base (CCPB) disconnect switch that’s available in 1-, 2- and

3-poles.

Figure 4.2.4.3.a

The fused main disconnect options are either 100 through 400 A

Class J Bussmann series Low-Peak LPJ-SP fuses ,or 30, 60 or 100 A

CUBEFuse. The panel is rated 600 Vac/125 Vdc and provides high SCCRs up to 200 kA. The footprint is the same size as traditional circuit breaker panelboards: 20” W x 5-3/4” D x 33” to 69” H (height depends on configuration and number of branch circuit positions).

Two key features of this panelboard is the fuse/CCPB disconnect switch interlock that prevents fuse removal in the ON position along with an amp rating rejection feature that coincides with standard branch circuit amp ratings to help prevent overfusing.

Figure 4.2.4.3.b

The Low-Peak CUBEFuse and Low-Peak LPJ-SP fuses are easy to selectively coordinate with each other and any other Low-Peak fuses used in upstream power distribution panelboards and switchboards.

Merely maintain at least a 2:1 fuse amp rating ratio between upstream and downstream Low-Peak fuses and selective coordination is ensured up to 200 kA.

For more information on the QSCP solution visit Eaton.com/ bussmannseries for data sheet no. 1160, application notes and more.

4.2.4.4. Summary — fuse selective coordination

With modern current-limiting fuses, selective coordination can be achieved by adhering to published selectivity amp rating ratios. There is no need to plot time-current curves nor calculate the available fault currents (for systems up to 200 kA). Simply maintain the minimum amp rating ratios provided in the selectivity amp ratio table and the system will be selectively coordinated. Not only is this simple method quick and easy, but selectivity is retained regardless if the available fault current increases due to a transformer change or for any other reason. To maintain a selectively coordinated system throughout its life, an electrician should always replace an opened fuse with one from the same manufacturer matching the type and amp rating (i.e., same catalog number). The ratios shown in the selectivity amp ratio table are only valid for the Bussmann series fuses shown. Do not mix Bussmann series fuses with another manufacturer’s fuses.



If a design does not initially provide selective coordination, investigate other Bussmann series fuse types that may have different selectivity ratios. If another fuse type is evaluated, the application sizing guidelines for that fuse should also be reviewed. If selective coordination still cannot be achieved, then a design change may be necessary.

4.2.5 Fuse selectivity ratio tables

For 600 V or less systems that leverage the fuse for overcurrent protection, they can use published selectivity ratio tables. The published ratios apply for all overcurrent conditions (overloads and faults). Using the fuse selectivity ratio method is easy and quick. Knowledge of fault currents for fuse selective coordination applications (provided the fuse interrupting ratings are not exceeded) is not required to properly select and apply fuses when using these ratio tables.

These selectivity ratios in Figure 4.2.5.a are for all overcurrent levels up to the fuse interrupting rating, or 200 kA whichever is lower.

Maintaining the specified current rating ratio for each given fuse as per the table below ensures the downstream fuse will always open before the upstream fuse for all currents up to the fuse’s interrupting rating.

Nor is there a need to calculate the available fault current throughout the system, provided they are always less than the fuses’ interrupting ratings, typically 200 kA.

Notice the Low-Peak fuses (highlighted in yellow — LPJ-SP, LPN-RK-

SP, LPS-RK-SP, and KRP-C-SP) as well as the CUBEFuse (TCF) — only require a 2:1 amp rating ratio to achieve selective coordination and simplifies the design process.

Amp rating range

601 to

6000 A

601 to

4000 A

0 to 600 A

0 to 600 A

0 to 100 A

0 to 600 A

601 to

6000 A

0 to 600 A

0 to 1200 A

0 to 600 A

0 to 60 A

Dualelement

Fastacting

Fastacting

Fastacting

Fastacting

Timedelay

Timedelay

Timedelay

Dualelement

Dualelement

Dualelement

Circuit

Fuse type

Trade name

(fuse class)

601-

6000 A

Timedelay

Low-Peak

(L)

Bussmann fuse symbol

KRP-C-SP

Low-Peak

(L)

Limitron

(L)

Low-Peak

(RK1)

Low-Peak

(J)

CUBEFuse

(CF 2 )

Fusetron

(RK5)

Limitron

(L)

Limitron

(RK1)

Limitron

(T)

Limitron

(J)

SC

(G)

KRP-C-SP

KLU

LPN-RK-SP

LPS-RK-SP

LPJ-SP

TCF

FRN-R

FRS-R

KTU

KTN-R

KTS-R

JJN

JJS

JKS

SC

2:1

2:1

—

—

—

—

2:1

—

—

—

—

601-

4000 A

Timedelay

Limitron

(L)

1-100 A

Time-delay

CUBEFuse

(CF 2 ) (J)

Downstream / loadside fuse

0-600 A

601-

6000 A


Low-Peak Low-Peak

(RK1)

Fusetron

(RK5)

Fastacting

Limitron

(L)

0-600 A

Fastacting

Limitron

(RK1)

0-1200

A

Fastacting

Limitron

(T)

0-600 A

Fastacting

Limitron

(J)

0-60

A

Timedelay

SC

(G)

0-30 A

(CC)

KLU TCF LPJ-SP

LPN-RK-SP

LPS-RK-SP

FRN-R

FRS-R

KTU

KTN-R

KTS-R

JJN

JJS

JKS SC

LP-CC

FNQ-R

KTK-R

2.5:1

2:1

—

—

—

—

2.5:1

—

—

—

—

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

2:1

2:1

2:1

2:1

2:1

1.5:1

3:1

3:1

3:1

3:1

3:1

4:1

4:1

8:1

8:1

8:1

2:1

6:1

8:1

8:1

8:1

4:1

2:1

2:1

—

—

—

—

2:1

—

—

—

—

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

2:1

2:1

3:1

3:1

3:1

1.5:1

2:1

3:1

3:1

3:1

2:1

2:1

2:1

4:1

4:1

4:1

1.5:1

2:1

4:1

4:1

4:1

2:1

2:1

2:1

2:1

2:1

2:1

2:1

2:1

—

—

—

—

Figure 4.2.5.a

This fuse selectivity ratio table identifies the fuse amp rating ratios that ensure selective coordination.

General notes: Ratios given in this table apply to only Bussmann series fuses. When fuses are within the same case size, consult factory.

1. Where applicable, ratios are valid for indicating and non-indicating versions of the same fuse. At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Consult factory.

2. Time-delay Class CF TCF or TCF_RN CUBEFuse are 1 to 100 A Class J performance; dimensions and construction are a unique, finger-safe design.

4

How fuse ratio tables are used

The fuse selective coordination ratio tables make life for the designer easy by taking the burden off of the shoulders of the qualified individual.

To read the fuse selectivity table shown in this section, one must understand its content.

The top rows contain loadside fuse information and the left columns contain lineside fuse information. The intersection for any given row and column establishes that fuse pair’s amp rating ratio. These selectivity ratios are for all overcurrents up to the fuse interrupting ratings or 200 kA, whichever is lower. Selective coordination is ensured for the fuse pairs selected per this table.

The design professional should specify fuses that adhere to the selectivity table shown. The design professional does not need to plot time-current curves or perform a fault current analysis — provided the available fault current is less than 200 kA or less than the interrupting rating of the fuses. All that’s necessary is to make sure the fuse types and amp rating ratios for the mains, feeders and branch circuits meet or exceed the applicable selectivity amp ratios. If the ratios are not satisfied, then the designer should investigate another fuse type or design change.

The role of the installer is to install the proper fuse type and amp rating with a full understanding that the manufacture and model/catalog numbers are important to ensure selectivity is maintained. It is not possible to mix manufacturers as the tables are based on the specific manufacturer’s product.



How fuse ratio tables are made

The fuse selectivity ratio tables result from testing fuse pairs whose performance is dictated by the basic physics of current limitation. The principle requirements for fuse selective coordination is that the total clearing energy of the loadside fuse is less than the melting energy of lineside fuse.

Figure 4.2.5.b demonstrates the basic fuse selective coordination principles when the available fault current is greater than the current value where the upstream fuse curve crosses 0.01 second.

Lineside KRP-C-1200SP

1200 A fuse melting

T m

T c

Available fault current

Available fault current

Loadside

X

LPS-RK-600SP

600 A fuse clearing

The 1200 A fuse melting energy must be greater than the 600 A fuse clearing energy

Figure 4.2.5.b

T

T m c

T c

= melting time

= clearing time

For the high fault current levels that both fuses see, the downstream fuse clearing time, as it relates to the upstream fuse, is critical in determining if the downstream fuse will open and the upstream fuse will not. Current limitation principles can help describe the physics behind the selective coordination ratio tables.

The available fault current that could flow is depicted by the dotted line of Figure 4.2.5.b. The current-limiting nature of the current-limiting fuse closest to the fault is such that it must clear the fault before the upstream fuse that sees the same current level does. Note that T m

in c

is the fuse’s total clearing Figure 4.2.5.b is the fuse’s melting time and T time. The area under the current curves over a time period indicates the energy let-through. The amount of thermal energy delivered is directly proportional to the square of the current multiplied by clearing time (I 2 t). The amount of energy being released in the circuit while the fuse element is melting (or vaporizing) is called the melting energy, and energy released during the entire interrupting process (melting plus arcing) is called total clearing. To achieve a selectively coordinated system the downstream fuse’s T c upstream fuse’s T m

and clearing I 2 t must be less than the

and melting I 2 t.

The fuse selectivity ratio tables result from the physics just discussed and have been tested in a lab for the fuse pairs in the tables. Adhering to fuse selectivity ratios makes it easy to design and install fusible systems that are selectively coordinated.

4.2.6 Circuit breaker selective coordination

The first step in the circuit breaker selective coordination process is calculating available fault currents. The next step is plotting and overlaying the TCC curves for the circuit breaker pairs being evaluated.

When it comes to selective coordination, circuit breaker performance depends upon the available fault current that passes through each device. Circuit breaker pairs must be analyzed to determine if the downstream device will open and the upstream device remain closed for all currents up to the fault current that both devices will see.

Before attempting a selective coordination analysis for a circuit breaker system, it’s important to review circuit breaker basics (as covered in breaker basics in Section 3.2), is another important section to review and understand as circuit breaker selective coordination principles are reinforced when one understands how circuit breakers work - especially in relation to how they respond to overcurrents.

The ability to achieve selective coordination with circuit breakers depends upon the amount of fault current and the selected circuit breaker types. The upstream circuit breaker’s ability to hold its contacts closed long enough to let the downstream device clear the fault current is the basic premise. This performance depends upon the amount of current flowing through both circuit breakers, and the relationship of their trip curves at that fault current level.

Circuit breakers are not like fuses. Applying circuit breakers to achieve selective coordination is not, and cannot be, based upon an amp ratio of their handle ratings. These devices are current dependent and react to the fault current that passes through them. The flexibility that some circuit breakers provide through adjustable pickup values and delays requires that TCC curves be plotted to ensure selectivity in their overload regions.

4.2.6.1. Circuit breaker example 1

Figure 4.2.6.1.a shows three circuit breakers in series. This figure is a

TCC curve plot of a 30 A BAB thermal magnetic molded case circuit breaker downstream from a thermal magnetic F Frame 150 A FD circuit breaker which, in turn, is downstream from an N Frame 800 A ND molded case circuit breaker with an electronic trip unit. Circuit breaker pairs are reviewed for selectivity beginning with the circuit breaker that’s furthest downstream from the source.

BAB 30 A downstream from FD 150 A

Per the TCC curve in Figure 4.2.6.1.a, the BAB 30 downstream from the

FD 200 selectively coordinates to 750 A, which is the leading edge of the FD 150 A circuit breaker’s instantaneous pickup. Based on the TCC curves, the selectivity threshold is 750 A. Any current greater than 750

A passing through this circuit breaker pair will open both. Leveraging the circuit breaker-to-circuit breaker Table 4.2.6.5.a, this same circuit breaker pair will selectively coordinate up to 1500 A. Based on the tested pair information provided as part of the circuit breaker selective coordination tables, the selectivity threshold is 1500 A. When these two devices see currents greater than 1500 A, they will both open. These devices do not selectively coordinate when the available fault current exceeds 1500 A.

FD 150 A downstream from ND 800 A

Per the TCC curve in Figure 4.2.6.1.a, the FD 150 A downstream from the ND 800 A selectively coordinates up to 11,900 A, which is the leading edge of the ND 800 A circuit breaker’s instantaneous pickup.

Based on the TCC curves, any current greater than 11,900 A passing through this pair of circuit breakers will open both. Leveraging the circuit breaker-to-circuit breaker tables, this same circuit breaker pair will selectively coordinate up to 30 kA. This means that, based on the tested pair information provided as part of the circuit breaker selective coordination tables, the selectivity threshold is 30 kA. When these two devices see currents greater than 30 kA, they will both open. These devices do not selectively coordinate when the available fault current exceeds 30 kA.


1000

100

30 A

150 A

CURRENT IN AMPERES

800 A


To avoid system changes, the BAB 30 A can be replaced with a TCF 30

A CUBEFuse. Based on the circuit breaker-to-fuse selective coordination amp rating Table 4.2.7.c, selective coordination is achieved with the upstream FD 150 A circuit breaker for all currents up to 35 kA. No additional upstream changes are needed to the circuit breakers for selective coordination up to 8500 A.

4.2.6.3 Circuit breaker example 3

1200 A MCCB

I.T. = 6X = 7200 A

10

400 A MCCB

I.T. = 10X = 4000 A

1

0.10

0.01

1 10 100 1K 10K

Ref. Voltage: 480V Current in Amps x 1

X

Fault > 7200 A

100 A MCCB

I.T. = non-adjustable

100K

Figure 4.2.6.3.a

shows a one-line diagram with three molded case circuit breakers in series: 1200 A main, 400 A feeder and 100 A branch circuit. The other circuit breakers on the one-line diagram supply other circuits and loads. The fault current path from the power source is depicted by the red arrow/line. For the coordination analysis, faults on both the branch and feeder circuit must be analyzed.

4

Figure 4.2.6.1.a Simply plotting time current characteristic curves in a circuit breaker system is not enough to ensure selective coordination.

The fault current must be known and compared to the upstream circuit breaker’s instantaneous pick-up. For values beyond this point, the circuit breaker-to-circuit breaker tables must be consulted. In the case of the

30 A and 150 A breakers, the level of selective coordination is increased from 750 A to 1500 A by using the tested values. Beyond this point, both devices will open; in which case the 150 A breaker rating would need to increase to accommodate higher fault currents.

4.2.6.2 Circuit breaker example 2

For the same three circuit breakers in series discussed in example 1,

4.2.6.1.a, an evaluation can be made when the available fault current that all three would see is 8500 A. In this case, the BAB 30 A must selectively coordinate with the FD 150 circuit breaker for all currents up to 8500 A. Based on the trip curves shown in Figure 4.2.6.1.a, 8500 A is greater than the instantaneous pickup of both the BAB 30 and the FD

150 circuit breakers, but less than the ND 800 A. Based on information provided by the TCC curves, it would appear both the BAB 30 A and FD

150 A circuit breakers would not selectively coordinate at this high fault current level. Leveraging the circuit breaker-to-circuit breaker curves, the

BAB 30 A would definitely not selectively coordinate with the FD 150, as they will both open for all currents greater than 1500 A.

To fix this, the FD 150 A circuit breaker could be increased to an LD 300

A circuit breaker, but system changes would have to occur as an LD

300 A would not be able to provide the same conductor protection level as the FD 150 A. Conductors would have to increase in size to ensure

NEC requirements for protection are still satisfied. Should the LD 300

A be chosen, the feeder circuit conductors would have to increase from what’s adequate for 150 A to what’s needed for 300 A.



The following discussion will analyze the TCC curves shown in Figure

4.2.6.3.b.

TCC curve analysis for branch circuit fault

For a branch circuit fault current less than 3600 A on the 100 A circuit breaker’s loadside, the 400 A and 1200 A circuit breakers will keep their contacts closed, as the instantaneous pickup values are greater than the 3600 A, providing the 100 A branch circuit breaker sufficient time to open and clear the fault without causing the upstream devices to open.

If the fault current flowing is greater than 3600 A but less than 6500

A, both the 100 A branch and the 400 A feeder circuit breakers may unnecessarily open, as the fault current exceeds the instantaneous trip pickup values for both. The upstream 1200 A feeder breaker would not open, as the fault current flowing is less than its instantaneous pickup value. The outage to the distribution system in this case would cause unnecessary power loss to loads.

If the branch circuit fault current is greater than 6500 A, it’s possible that the 100 A, 400 A and then the 1200 A main circuit breakers could all unnecessarily open, causing an extensive outage to the facility.

TCC curve analysis for feeder circuit fault

For any feeder fault less than 6500 A on the 400 A circuit breaker’s loadside, the 400 A and 1200 A circuit breakers will selectively coordinate, as the fault current is less than the 1200 A main circuit breaker’s instantaneous pickup. For feeder faults greater than 6500 A, the 1200 A circuit breaker is not selectively coordinated with the 400

A feeder circuit breaker, and they may both open as the fault current exceeds their instantaneous pickup.

TCC curve analysis conclusions

When only TCC curves are used, the available fault current (as compared to the circuit breakers’ TCC curves) must be compared to the instantaneous pickup values of each circuit breaker in series. The circuit breakers’ performance depends upon how much current passes through them.

When the available fault current exceeds the instantaneous pickup values of two or more circuit breakers, circuit breaker selective coordination tables should be leveraged to determine selectivity.

4.2.6.4 Circuit breaker solutions

Circuit breaker selective coordination is dependent upon the upstream circuit breakers’ ability to keep their contacts closed long enough for downstream circuit breakers to open, clear and isolate the system’s faulted portion. Because of this fundamental principle, circuit breakers that have higher instantaneous pickup values and longer short-time delay values should be used closer to the utility source (see Section

3.2 for more information). Circuit breakers equipped with electronic trip units offer more flexibility when it comes to meeting selective coordination and protection needs. Microprocessor based trip units offer the ability to add intentional delays to provide downstream devices the necessary time to clear and isolate faulted portions of the circuit.

The following will review the various circuit breaker families and discuss their location in the circuit.

Figure 4.2.6.3.b



4.2.6.5 Circuit breaker selective coordination tables

600 V or less systems that leverage circuit breakers can also take advantage of published selective coordination tables. These tables include circuit breaker pairs that selectively coordinate up to the fault current values shown.

These circuit breakers will NOT selectively coordinate for fault current levels above these values.

These tables are leveraged when the maximum available fault current exceeds the instantaneous pickup of both circuit breakers being evaluated. The table below is for select Eaton circuit breakers from

Eaton publication number IA01200002E.

60

70

80

90

100

Upstream breaker

Breaker family

Type trip unit

Digitrip RMS trip unit

OPTIM trip unit

Minimum trip (plug/trip)

EG

T/M

F

T/M

F

T/M

F

T/M

F F

ETU ETU

F J

ETU T/M

J

T/M

J

T/M

—

—

—

—

—

—

—

—

310+

—

310+

—

310+

—

—

—

—

—

—

—

125 A 100 A 150 A 225 A 15 A 60 A 100 A 70 A 150 A 250 A

20

30

40

50

Maximum trip (frame)

Pow-R-Line: main

Downstream breaker

Pow-R-Line: branch

Pow-R-Line: sub-feed

Pow-R-Line panelboard/switchboard

125 A 100 A 200 A 225 A

3E

3E

—

Main Branch Sub-feed

BR, BAB, HQP and QC (10 kA at 240 Vac) single-, two- and three-pole

15 — 1a, 3a, 4, Swbd — 1.2

1.0

1.0

1.5

1.5

2.2

2.2

80 A

1a, 2a, 3a, 3E

3a, 4, Swbd

1a, 2a, 3E

0.6

0.6

160 A 225 A 125 A 225 A 250 A

1.2

1.2

2.3

2.3

1.0

1.0

3a

4, Swbd

3a

2.1

2.1

4.0

3.4

—

—

—

—

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1.2

1.2

0.8

0.8

1.0

1.0

—

1.5

1.5

1.5

2.2

2.2

2.2

0.6

0.6

—

1.2

1.2

1.2

2.3

2.3

2.3

0.7

—

—

2.1

1.5

1.5

3.4

3.4

2.5

1a

1a

1a

1a

1a

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

1a, 3a, 4, Swbd —

0.8

—

—

—

—

—

—

—

—

—

1.5

1.5

—

—

—

2.2

2.2

2.2

2.2

1.8

—

—

—

—

—

1.2

1.2

—

—

—

2.3

2.3

2.3

2.3

2.3

—

—

—

—

—

1.5

1.5

—

—

—

2.5

2.5

2.5

2.3

2.3

Table 4.2.6.5.a

This table pertains only to circuit breakers manufactured by Eaton. Values in the shaded area are kA. E.g., 1.5 = 1500 amps.

How circuit breaker selective coordination tables are made

Circuit breaker manufacturers publish circuit breaker-to-circuit breaker coordination tables based on testing. In addition to these coordination tables, Eaton also publishes circuit breaker-to-fuse coordination tables to take advantage of the downstream current-limiting Bussmann series fuses.

The tables illustrate the performance of two circuit breakers in series and the maximum fault current values up to which the circuit breakers selectively coordinate. The tables assume all circuit breaker settings are on maximum. Using the tables does not preclude the plotting of circuitbreaker trip curves to ensure selective coordination in the overload and short-time regions.

The testing is typically conducted by using a test configuration as established in UL 489 for testing series rated pairs. The same conductor lengths are used in the test configuration as the fault current is increased through each circuit breaker pairs until both trip. That point at which both circuit breakers trip is the threshold current level, where selectivity is no longer achievable.

How circuit breaker selective coordination tables are used

To read the circuit breaker-to-circuit breaker selectivity table shown above, one must understand the content. The top rows show upstream

(lineside) circuit breaker information and the left columns show the downstream (loadside) circuit breaker information. Their intersection within the table for any given row and column establishes current value beyond which both circuit breakers will open. The installer just needs to install the proper circuit breakers and, where applicable, program the proper settings. It’s still necessary to plot time-current curves as the table only applies when the fault current is in both circuit breakers’ instantaneous region. It’s also necessary to perform a fault current study as the values within the table are the fault currents beyond which selective coordination is not achieved.

Selectively coordinating circuit breakers first requires calculating the maximum fault current. Next, the circuit breakers’ TCC curves are plotted and evaluated with respect to the calculated fault current.

When the fault current that both devices will see exceeds both circuit breakers’ instantaneous pickup, the circuit breaker-to-circuit breaker selective coordination tables are used to determine if selectivity is achieved at the higher current level. If selective coordination is not achieved, the current level must be reduced or the upstream circuit breaker must be increased in size. A simple a remedy to this situation is to replace the downstream circuit breaker with a current-limiting

Bussmann series fuse, such as the TCF CUBEFuse in a Bussmann series Quick-Spec Coordination Panelboard (QSCP)(see section 4.2.4.3).

4



How circuit breaker-to-fuse selective coordination tables are made

The circuit breaker-to-fuse selective coordination tables are result from testing pairs upstream (lineside) circuit breakers and downstream

(loadside) fuses, the performance of which is dictated by the current limiting fuses’ basic physics. The basic principle leveraged for fuse to circuit breaker selectivity is similar to that discussed in 4.2.5 covering fuse selectivity ratio tables. The downstream (loadside) fuse’s total clearing energy must be less than what is required to open the upstream (lineside) circuit breaker.

The downstream fuse’s fast-acting nature is such that the fault current is removed so quickly that the upstream circuit breaker is not able to respond, thus keeping its contacts closed through much higher fault current levels than what’s achievable for circuit breaker-to-circuit breaker pairs as discussed in Section 4.2.6.5. Those values presented within this circuit breaker-to-fuse table are often the upstream circuit breaker’s interrupting ratings.

This circuit breaker-to-fuse selectivity table is the result of the physics and lab testing for the fuse and circuit breaker pairs shown. The test circuit configuration is the same circuit configuration used for the circuit breaker-to-circuit breaker tables discussed in 4.2.6.5. The testing is conducted such that the upstream circuit breaker settings, where applicable, were set at maximum.

How circuit breaker-to-fuse selective coordination tables are used

To read the circuit breaker-to-fuse selectivity table shown in Section

4.2.7.1, one must understand the content. The table illustrates that for each upstream (lineside) circuit breaker there are downstream (loadside)

CUBEFuse amp ratings that selectively coordinate up to a maximum fault current, which happens to be the listed circuit breakers’ interrupting ratings. As an example, an F Frame FD 150 A circuit breaker upstream of a 70 A or less CUBEFuse will selectively coordinate for all fault currents up to and including 65 kA. This assumes that the upstream circuit breaker settings, if applicable, are set to maximum.

Molded case circuit breakers (MCCB)

Molded case circuit breakers are offered in ratings from 15 A through

2500 A, in either a simple thermal magnetic construction or as microprocessor-driven electronic trip units. In this family, the thermal magnetic MCCB is the lowest cost solution.

All molded case circuit breakers have a built-in instantaneous override to protect it from high fault currents. One of the most difficult and stressful things for a circuit breaker to do during a fault is to keep its contacts closed. In addition to the instantaneous override, some MCCBs are equipped with an adjustable instantaneous trip.

These devices are typically found downstream in the power distribution system, closer to the load and further away from the utility. The circuit breakers in Figure 4.2.6.5.a could be found at any point in the power distribution system, with their application driven by the needs of the loads served, as well as the performance for selectivity. Selecting the device frame ratings and amp ratings shown in Figure 4.2.6.5.a for any given installation are typically based upon the load, available fault currents, the circuit-breaker selective coordination tables and

TCC curves. The larger the MCCB frame, the higher the instantaneous current pickup values are to address the higher fault current values to which they will selectively coordinate with downstream devices.

Figure 4.2.6.5.a

Within these frame ratings, Eaton manufactures special circuit breakers to provide a cost-effective thermal magnetic solution to help meet selective coordination needs by lowering the long time pickup to protect smaller conductors, and increase the instantaneous pickup values to provide selective coordination with higher fault current levels. The L and

N frame circuit breakers were selected providing the LHH and NHH circuit breakers for these applications. These devices are available for applications from 125 A to 400 A, with instantaneous pickup values as high as 93 times their handle rating, depending upon the device’s continuous current rating. For circuit breaker selective coordination purposes, the larger the window between continuous current rating and instantaneous pickup values, the better for MCCB applications.

For smaller amp rated MCCBs, typical instantaneous pickup values are anywhere from 10 to 12 times the continuous current rating. Larger

MCCBs can have higher multipliers for instantaneous pickup values in the range of 20 time their continuous current rating or higher.

MCCB TCC curve characteristics

•

Continuous current : Long time pickup values ranging from 15 A through 2500 A

•

Instantaneous pickup : Pickup values from 10 times as a minimum to very high maximums dependent upon the selected device. The TCC curves for individual MCCBs should be consulted.

• Short time delays : Standard thermal magnetic solutions do not provide the ability to offer short time delay settings. Electronic trip units, when equipped on MCCBs, have the ability to offer delay from

2.5 to 18 cycles for currents from 2 to 8 times or even 12 times the

MCCB’s continuous current rating.

Insulated case circuit breakers (ICCB)

The insulated case circuit breaker is a bridge between an MCCB and the Power Circuit Breaker (PCB), offering higher instantaneous pickup values and longer short time delay capabilities in a more economical package than a power circuit breaker. ICCBs can be found much closer to the service entrance, if not the service main, depending upon the application and selective coordination needs.

Molded case circuit breakers and insulated case circuit breakers typically comply with UL 489 (the standard for molded case circuit breakers) or UL 1066 (the UL standard for low-voltage AC and DC power circuit breakers). These circuit breakers employ a two-step stored energy mechanism and are found in draw-out construction enclosures, such as switchboards and Motor Control Centers (MCCs). They’re available from

800 A to 6000 A with interrupting ratings from 35 kA to 150 kA.



The largest difference between ICCBs, and PCBs and MCCBs is their interrupting and withstand capabilities. All ICCBs will be equipped with electronic trip units that provide a high degree of flexibility with regard to selective coordination and equipment protection.

• Short time delays : Delays from 2.5 to 30 cycles for currents from

2 times the continuous current rating of the PCB up to the PCB’s interrupting rating.

Figure 4.2.6.5.b

ICCB TCC curve characteristics

• Continuous current : Long time pickup values ranging from 800 A through 6000 A

• Instantaneous pickup : Pickup values from 2 to 12 times the continuous current rating. The TCC curves for individual ICCBs should be consulted.

• Short time delays : The flexibility of short time delay offered will depend upon the electronic trip unit solutions selected. ICCBs have the ability to offer delay from 2.5 to 30 cycles for currents from 2 to

10 times the continuous current rating of the MCCB.

Power circuit breakers (PCB)

The power circuit breaker is a full featured versatile device that offers a world of flexibility to meet a variety of power distribution system needs.

These devices are equipped to provide delay up to 30 cycles for fault currents up to their interrupting ratings. All PCBs are equipped with electronic trip units and are listed to UL 1066 and ANSI/IEEE C37.

PCBs are typically found close to service entrance equipment, and in equipment such as switchboards, switchgear and MCCs. The enclosures that usually hold PCBs are of “drawout” construction. They are available from 800 A to 6000 A with interrupting ratings from 42 kA to 200 kA.

PCB TCC curve characteristics

• Continuous current : Long time pickup values ranging from 800 A through 6000 A

• Instantaneous pickup : Pickup values from 2 times the continuous current rating to the circuit breaker’s interrupting rating.

Figure 4.2.6.5.c

Figure 4.2.6.5.c illustrates a 30 A molded case circuit breaker fed by a

200 A LVPCB and 800 A LVPCB. The 200 A and 800 A circuit breakers have short-time delay (STD) settings that provide selective coordination.

The 200 A circuit breaker has its STD set at 6 cycles and the 800 A circuit breaker has its STD set at 20 cycles.

4.2.6.6 Summary — circuit breaker selective coordination

It’s possible to design electrical systems that selectively coordinate with circuit breakers. Applying circuit breakers requires analysis and specifying the proper circuit breaker types and settings. It’s necessary to calculate the available fault currents at each circuit breaker’s point of application, plot trip curves and/or review selective coordination tables to evaluate how devices will respond in relation to each other when compared to the maximum available fault currents.

The nature of applying circuit breakers to achieve selective coordination is using devices that can keep their contacts closed so that downstream devices can open and clear faults. This intentional delay will increase incident energy in these applications, and steps should be taken to mitigate these effects through employing arc energy reduction technologies. Section 4.3 of this document should be consulted.


4


4.2.7 Fuse and circuit breaker selective coordination

For downstream fuses and upstream circuit breakers, it’s not a simple matter to determine if a fuse and circuit breaker will be selectively coordinated when tested pair tables, as shown in Figure 4.2.7.c, are not available. Even if the TCC curves plotted for a downstream fuse and an upstream circuit breaker show that the curves don’t cross, selective coordination may not be possible for high fault currents that exceed a specific current value. Testing can be conducted to determine the selectivity level achieved between these two OCPD types. The

Bussmann Division’s Paul P. Gubany Center for High Power Technology is available to perform this testing. Look under Bussmann series Services at Eaton.com/bussmannseries.

Figure 4.2.7.a shows an example of a 400 A circuit breaker with a downstream 100 A fuse. Coordination is shown in the TCC curve up to about 3 kA, where the circuit breaker trip curve crosses the 0.01 second axis. Coordination cannot be ensured above this value without testing or further analysis because the fuse may not clear the fault prior to the upstream circuit breaker unlatching (the current axis is 10x).

If a fuse is upstream and a circuit breaker downstream, at some point the fuse’s TCC curve will cross the circuit breaker’s TCC curve. The general rule is that for fault currents at that crossover point and higher, the upstream fuse is not coordinated with the downstream circuit breaker. Figure 4.2.7.b shows a 400 A fuse upstream from a 100 A circuit breaker. Coordination is not possible above approximately 5 kA as shown in the overlapping time-current curves (the current axis is 10x).

Figure 4.2.7.b

Figure 4.2.7.a



4.2.7.1 Circuit breaker and fuse selective coordination

In addition to the circuit breaker-to-circuit breaker selective coordination tables, Eaton provides circuit breaker-to-fuse selectivity tables leveraging the fuse’s fast current-limiting operation when placed downstream of a circuit breaker*.

240 Vac Eaton thermal magnetic circuit breaker “quick pick” selective coordination with CUBEFuse amp ratings*

Eaton lineside circuit breakers

Breaker frame Breaker family

G Frame

E Frame

GHB

EGB

F Frame

EHD

FD

FD

FD

J Frame

K Frame

JD

JD

KD

HKD

Min. amp rating Max amp rating

100

125

100

100

125

100

100

150

225

100

150

225

70

150

200

400

70

150

400

400

Max circuit fault current (kA) Loadside TCF or FCF CUBEFuse (amps)**

65

25

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50, 60

18

65

65

65

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50, 60, 70

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

65

65

65

100

15, 20, 25, 30

15, 20, 25, 30, 35, 40, 50, 60, 70

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

* For circuit breakers with an adjustable instantaneous trip, selective coordination is based upon instantaneous trip set at maximum.

** TCF (time-delay) and FCF (fast-acting) fuses can be used on any 600 Vac or less system. The CUBEFuse has a 300 kA interrupting rating at 600 Vac or less.




G Frame

E Frame

GHB

EGB

F Frame

EHD

FD

FD

FD

J Frame

K Frame

JD

JD

KD

HKD

Min. amp rating

100

125

100

100

150

225

70

150

200

400

Max amp rating

100

125

100

100

150

225

70

150

400

400


14 †

18

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50, 60

14

35

35

35

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50, 60, 70

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

35

35

35

65

15, 20, 25, 30

15, 20, 25, 30, 35, 40, 50, 60, 70

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100



† 480/277 Vac




F Frame

FD

FD

FD

J Frame

K Frame

JD

JD

KD

L Frame

HKD

LD

HLD

Min. amp rating

100

150

225

70

150

200

400

300

300

Max amp rating

100

150

225

70

150

400

400

600

600


18

18

15, 20, 25, 30, 35, 40, 50

15, 20, 25, 30, 35, 40, 50, 60, 70

18

18

18

25

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

15, 20, 25, 30

15, 20, 25, 30, 35, 40, 50, 60, 70

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

35

25

35

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100

15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100



Figure 4.2.7.c Circuit breaker-to-fuse selective coordination tables for 240, 480 and 600 V systems.

4



4.2.8 Code requirements

For building electrical systems, the topic of OCPD selective coordination can be segmented into two areas:

• A desirable design consideration

• An NEC requirement

In most cases, selective coordination is a desirable design consideration and not an NEC requirement. It’s in the building owner’s or tenants’ best interest to have selectively coordinated overcurrent protection to avoid unnecessary blackouts. Selective coordination should be evaluated in the context of the reliability desired for the power system to deliver power to loads.

Because reliability is a key safety component, selective coordination is mandatory per the NEC for specific applications pertaining to life safety or national security.

The NEC has mandatory selective coordination requirements for these systems:

• Emergency Systems — Article 700: 700.32

• Legally Required Standby Systems — Article 701: 701.27

• Critical Operations Power Systems — Article 708: 708.54

Additionally, selective coordination is required in elevator circuits

(620.62), in certain fire pump applications (695.3(C)(3), critical operations data systems (645.27) and for certain emergency system wiring schemes (700.10(B)(5)(b) which are not discussed in depth in this section.

Notice these requirements are not in NEC Chapters 1 through 4,

(such as Articles 210 Branch Circuits, 215 Feeders, or 240 Overcurrent

Protection)) that generally pertain to all premise electrical installations.

Instead, these requirements are in Chapters 5 through 7, which are under special occupancies and special conditions, respectively.

The NEC gives these systems special attention because they have unique requirements. Articles 700, 701, and 708 cover circuits and systems intended to deliver reliable power for loads that are vital to life safety, public safety or national security. Reliability for these systems must be greater than normal systems covered in Chapters 1 through 4.

Articles 700, 701, 708 and 517 are unique and have more restrictive minimum requirements (versus the general requirements for normal systems) so that these systems provide more reliable power to vital loads, with selective coordination being one that supports higher reliability.

A few of the more restrictive minimum requirements in Article 700 are:

• Periodic testing, maintenance and record retention

• Alternate power sources

• Separate wiring from emergency source to emergency loads

(separate from all other wiring

• Special fire protection for wiring

• Locating wiring to avoid outage due to physical damage during fires, floods, vandalism, etc.

• Automatic transfer switches (ATS) with sophisticated sensors, monitors and controls

• Separate ATSs and load segmenting (emergency, legally required standby and optional standby) with sophisticated load shedding, if required

The reason these articles for special systems exist is that the electrical industry, the standard making bodies, the technical code panel members and Homeland Security believe special rules are needed to ensure minimum requirements for reliable power to designated vital loads.

To better understand why we have more restrictive requirements, it’s important to understanding the loads being served by these special systems (vital loads that pertain to life safety, public safety and national security). For instance, the Informational Note found in 700.2 defines emergency systems as:

“Emergency systems are generally installed in places of assembly where artificial illumination is required for safe exiting and for panic control in buildings subject to occupancy by large numbers of persons, such as hotels, theaters, sports arenas, healthcare facilities and similar institutions. Emergency systems may also provide power for such functions as ventilation where essential to maintain life, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and similar functions.”

The NEC sections defining selective coordination, and those requiring the OCPDs in the circuit paths supplying these vital loads to be selectively coordinated, are as follows:

4.2.8.1 Article 100 definitions

NEC Article 100 contains only those definitions essential to the application of this Code. In general, only those terms used in two or more articles are defined in Article 100, where other definitions can be found in the specific Articles to which they apply. Because selective coordination is used in multiple NEC articles, its definition is included here.

“Coordination, selective (selective coordination)

Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents (CMP-10).”

This definition establishes some very important criteria including these key points:

• OCPD Selection.

The proper selection, installation and setting of the OCPDs in a power distribution system is critical to the devices’ performance in practice. Once the fault current and selective coordination studies are completed, the OCPDs must be specified to achieve selective coordination at the designated fault current, and must also be the devices installed in the application. Some circuit breakers may require field adjustments to ensure selective coordination as dictated by the study.

•

Current levels.

The definition makes it clear that OCPDs must selectively coordinate for the full range of overcurrents, including overloads and maximum fault currents. As the definition states, the goal is to restrict outages to the circuit or equipment affected which can only be achieved by ensuring the downstream device clears overcurrents before the upstream devices for all overcurrent values up to and including the maximum fault current that the devices could see in the system.



4.2.8.2 Article 620 elevators

Article 620 covers electrical equipment installation and wiring used in connection with elevators, dumbwaiters, escalators, moving walks, platform lifts and stairway chairlifts. Quite often this equipment is used for safety related purposes, and selective coordination is important to their operation during emergencies. Section 620.62 was introduced in the 1990 NEC and it addresses the selective coordination requirements for these installations.

“620.62 Selective Coordination

Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.

Selective coordination shall be selected by a licensed professional engineer or other qualified person engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain and operate the system.”

An important thing to highlight with regard to selective coordination is the NEC repeatedly establishes the need to ensure a professional engineer or a qualified individual is engaged in the OCPDs’ selection for selective coordination. This requirement was established in the 2014

NEC.

4.2.8.3. Article 645 information technology equipment

Article 645 covers equipment, power-supply wiring, equipment interconnecting wiring, and grounding of information technology equipment and systems in an information technology equipment room.

Reliability is critical for this infrastructure and an important safety consideration. Section 645.27 provides the selective coordination requirements for this equipment and was introduced in the 2014 NEC.


Critical operations data system(s) overcurrent protective devices shall be selectively coordinated with all supply-side overcurrent protective devices.”

This requirement applies to all OCPDs on the supply-side of the critical operations data system’s OCPDs.

4.2.8.4. Article 695 fire pumps

Article 695 covers the installation of electric power sources and interconnecting circuits (as well as switching and control equipment) dedicated to fire pump drivers. Fire pumps are critical for safety during emergencies and this equipment’s reliability drives the selective coordination requirements found in Section 695.3 and was introduced in the 2011 NEC.

“695.3 Power Source(s) for Electric Motor - Driven Fire

Pumps

(C) Multi-building Campus-Style Complexes

If the sources in 695.3(A) are not practicable and the installation is part of a multi-building campus-style complex, feeder sources shall be permitted if approved by the authority having jurisdiction and installed in accordance within (C)(1) and

(C)(3) or (C)(2) and (C)(3).

(3) Selective Coordination

The overcurrent protective device(s) in each disconnecting means shall be selectively coordinated with any other supplyside overcurrent protective device(s).”

4.2.8.5. Article 700 emergency systems

Article 700 applies to the electrical safety of emergency system installation, operation, and maintenance. This consist of circuits and equipment intended to supply, distribute and control electricity for illumination, power (or both) to required facilities when the normal electrical supply is interrupted. Requirements for selective coordination can be found in Section 700.32 that was introduced in the 2005 NEC,

(which at that time was Section 700.27). Adding exceptions began as part of the 2008 NEC, recognizing that two OCPDs in series, such that when opening either device results in losing the same load, do not have to selectively coordinate. The requirement that selective coordination be addressed by a licensed professional engineer or other qualified person was introduced as part of the 2014 NEC.


Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.

Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.

Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.”

4.2.8.6. Article 701 legally required standby systems

Article 701 applies to the electrical safety of the legally required standby system installation, operation and maintenance. This consist of circuits and equipment intended to supply, distribute and control electricity to required facilities for illumination or power (or both) when the normal electrical supply is interrupted. Requirements for selective coordination can be found in Section 701.27 and introduced in the 2005 NEC. Adding exceptions began as part of the 2008 NEC, recognizing that two OCPDs in series, such that when opening either device results in losing the same load, do not have to selectively coordinate. The requirement that selective coordination be addressed by a licensed professional engineer or a qualified person was introduced in the 20014 NEC.


Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.


Exception: Selective coordination shall not be required between two overcurrent protective devices located in series if no loads are connected in parallel with the downstream device.”

4



4.2.8.7 Article 708 critical operations power systems

Article 708 was introduced in the 2008 NEC. This article applies to the installation, operation, monitoring, control and maintenance of those portions of the premise’s wiring system intended to supply, distribute, and control electricity to designated critical operations areas (DCOA) in the event of disruption to elements of the normal system.

A critical operations power system is one designated as such by municipal, state, federal or other codes by any government agency having jurisdiction, or by facility engineering documentation establishing the necessity for such a system. These systems include, but are not limited to designated critical operations areas including:

• Power systems

• HVAC

• Fire alarm

• Security

• Communications and signaling

The informational note number 1 to the scope of Article 708 provides useful information for understanding what critical operations power systems are. It states the following:

“Critical operations power systems are generally installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health or safety; and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority.”

As a result of such events as 9/11, and hurricane Katrina and Irma,

Homeland Security requested NFPA to develop requirements for electrical systems that are vital to the public. Article 708 (COPS) includes selective coordination as a requirement; and is a minimum requirement for electrical systems that are important for national security and public safety. Selective coordination requirements can be found as part of 708.54 which was introduced in the 2008 NEC as part of the new Article 708. Adding exceptions began in the 2008

NEC recognizing that two OCPDs in series, such that when opening either device results in losing the same load, do not have to selectively coordinate. The requirement that selective coordination be addressed by a licensed professional engineer or a qualified person was introduced in the 2014 NEC.


Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.


Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.”

4.2.9 Selective coordination design guide

Achieving a selectively coordinated power distribution system in the most cost effective manner is the goal of every power systems engineer. The power systems engineer must leverage all available tools to achieve this goal. The engineer must understand overcurrent protective device principles and functionality, and take into consideration some basic power system design principles to achieve the most cost effective and efficient life safety system possible. Sometimes the installation cost must also consider the cost of time and resources for the design itself.

The information provided here is meant to serve as a guide for consulting engineers when designing systems where selective coordination is desired or required. These basic principles can be applied by the consulting engineer for any part of the power distribution system to provide customers a well-engineered, selectively coordinated system.

4.2.9.1 Leveraging the fuse’s power

When applying the fuse, maintaining an amp ratio from one fuse to another ensures selective coordination without regard to fault current levels. The value of this to the engineer is speed in achieving an effective design without calculating available fault currents nor plotting TCC curves. A reliable solution is also achieved for the power distribution system’s life — even if available fault current changes. New technologies have reduced the fusible equipment’s footprint, and the fuse itself provides an economical, logical choice — especially when selective coordination is a key design goal or Code requirement.

4.2.9.2 OCPDs that have to selectively coordinate

The Code text for the selective coordination requirements in 700.32 is carefully worded; stating that all emergency OCPDs shall selectively coordinate with all supply-side overcurrent devices. This helps ensure that these vital loads are not unnecessarily disrupted, whether fed from the normal source or the alternate source. Wording for 701.27 legally required standby systems and 708.54 critical operations power systems is similar, except for the system type nomenclature.

Figures 4.2.9.2.a and 4.2.9.2.b illustrate that all emergency overcurrent protective devices must selectively coordinate through to the alternate power source. Additionally, the emergency OCPDs on the transfer switch’s loadside must selectively coordinate with the OCPDs in the normal circuit path.

Normal source

Alternate source

N E

ATS

Panel

Figure 4.2.9.2.a



Normal source

Alternate source

N E

ATS

OCPDs that do not have to selectively coordinate

Sections 700.32, 701.27 and 708.54 have exceptions for selective coordination that’s shown in Figure 4.2.9.2.d. The exception does not reduce life safety because there aren’t any additional portions of the electrical system would be unnecessarily shut down. The red OCPDs in both circuits don’t have to selectively coordinated with each other.

Two OCPDs in series

Transformer primary and secondary

600 A OCPD

Panel

Series circuit

400 A OCPD

X

No other devices in parallel

X

Figure 4.2.9.2.b

However, based on wording, there is a difference on the minimum requirement for the OCPDs in the normal source path located on the transfer switch’s lineside.

The example in 4.2.9.2.c covers each OCPD in the system with a review of the selective coordination requirement.

Normal source

Alternate source

6 4

5

N

3

E

ATS

Figure 4.2.9.2.d

4.2.9.3. Worst case fault current

To assess whether the OCPDs selectively coordinated in the circuit path for these vital loads, it’s important to consider the maximum available fault current, whether from the normal or alternate source (see

Figure 4.2.9.3.a). This is required per 700.4(A) Capacity and Rating: “…

The emergency system equipment shall be suitable for the maximum available fault current at its terminals.” Generally, the normal source can deliver more fault current than the emergency generator/alternate source. If the alternate source can deliver more fault current, then it must be used for determining compliance with NEC 110.9 (interrupting rating) and 110.10 (short-circuit current rating).

Normal source

Alternate source

4

2

1 N E

ATS

Emergency system OCPDs

Normal system OCPDs supplying emergency system OCPDs

Normal system OCPDs

Figure 4.2.9.2.c

Practical requirement application example:

•

•

OCPD 1 Must selectively coordinate with OCPD 2, 3, 4, 5 and 6

OCPD 2 Must selectively coordinate with OCPD 3, 4, 5 and 6

• OCPD 3 Must selectively coordinate with OCPD 4

• OCPD 5 Does not have to selectively coordinate with OCPD 6 because OCPD 5 is not an emergency system OCPD.

Whether or not OCPD 5 selectively coordinates with OCPD 6 is a design decision. Although having OCPD 5 NOT selectively coordinate with OCPD 6 is permitted, the best practice is to have them selectively coordinated.

X

Panel

Figure 4.2.9.3.a

To ensure selective coordination, OCPDs must selectively coordinate for the full range of possible overcurrents. This includes overloads, faults, ground faults, arcing faults and bolted faults. Whether the OCPDs are current-limiting fuses or circuit breakers, they must be selectively coordinated for the maximum available fault current at each point of application in the system.



4.2.9.4 Ground fault protection relays

If a circuit path includes a ground fault protection relay (GFPR), then the selective coordination analysis should include the GFPR. One approach is to first perform the fuse or circuit breaker selective coordination analysis (as described in the previous sections and accounting for all overcurrent types). Then perform a separate analysis for how the fuse or circuit breaker and GFPR coordinate with each other for ground faults.

4.2.9.5 Panels in series

When designing a selectively coordinated system, it’s important to minimize the number protective device “series levels.” Figure 4.2.9.5.a illustrates a 400 A panelboard feeding a 200 A panelboard which, in turn, feeds a 175 A panelboard and then shows the same equipment arranged differently so that the 400 A panelboard now feeds both the

200 A and 175 A panelboard. By eliminating panelboard levels, the engineer effort needed to selectively coordinate the OCPDs is reduced.

400 A main

400 A panelboard

200 A feeder

200 A panelboard

175 A feeder

400 A main

200 A feeder

200 A panelboard

400 A panelboard

175 A feeder

175 A panelboard

175 A panelboard

Figure 4.2.9.5.a

4.2.9.6 Fused branch and/or feeder panels

Achieving selective coordination relies heavily on how quickly a downstream OCPD can clear a fault, ensuring the upstream OCPDs can remain closed. Having a downstream device that can clear very quickly helps ensure the upstream OCPDs are the smallest devices possible for selective coordination. Current-limiting OCPDs will operate faster at high current, where the fuse operates fastest in its current-limiting region.

Thus, applying a fuse close to the load allows for upstream OCPDs to be smaller and more effective.

By using selective coordination tables that have fuses as the downstream OCPD, engineers can achieve selective coordination to higher fault current levels with either upstream fuses or breakers.

4.2.9.7 480/277 V lighting loads

It’s difficult to use single- and double-pole circuit breakers for feeding lighting loads from 277 V circuits, because the available fault current is typically higher than what’s allowed for selectively coordinating the breakers per the selective coordination tables.

The Bussmann series Quik-Spec Coordination Panelboard (QSCP) is an ideal solution for these applications because a simple 2:1 amp ratio can be maintained in a footprint that’s the same as a circuit breaker solution.

When using circuit breakers for supplying lighting loads, it’s better to utilize smaller, kVA step-down transformers from the 480 V to

208 Y/120 V. This will result in low fault current levels at 208 Y/120 V and make it possible for the secondary main and branch breakers to selectively coordinate. Although the transformer’s primary circuit breaker and secondary main circuit breaker need not selectively coordinate, the secondary branch devices must selectively coordinate with the transformer’s primary circuit breaker. The maximum breaker size that can be placed on the transformer primary is determined by the NEC and the smallest breaker that can be placed on the transformer’s primary is determined by inrush current levels and selective coordination needs.

The total cost of ownership and project cost must be considered when adding transformers just to achieve selective coordination. This increased cost and complexity can be avoided by installing fusible panelboards.

4.2.9.8 Automatic transfer switch withstand ratings

Automatic transfer switches (ATS) manufactured in accordance with UL

1008 have short-circuit withstand ratings of either 1.5 or 3 cycles.

Therefore, the upstream OCPD feeding the ATS must have an instantaneous element that clears a through-fault, downstream of the

ATS, in less than the 1.5 or 3 cycle rating — or be a fuse. Because the fuse is not impacted as much by the available fault current, protecting the ATS is not a big concern, but should always be evaluated. Several

ATS equipment manufacturers, including Eaton, have 30-cycle withstand ratings on their higher amp rated ATSs that’s available upon request.

The following table illustrates the instantaneous trip requirements for protective devices feeding UL Listed ATSs.

ATS withstand

1.5 cycles

3 cycles

30 cycles

Required feeder protective device instantaneous clearing time

≤ 1.5 cycles

≤ 3.0 cycles

≤ 30 cycles

480 V panel

Device #9 LPJ-150SP

150 A Class J fuse

480 V elevator module

See note

1

2

3

1 ATSs with a 1.5-cycle withstand rating are typically rated 400 A or less and used in applications with a maximum available fault current of 10 kA.

2 ATSs with a 3-cycle withstand rating are typically rated greater than 400 A and used in applications with a maximum available fault current exceeding 10 kA.

3 ATSs with a 30-cycle withstand rating are typically used when there is a requirement for selective coordination. The upstream circuit breaker’s instantaneous trip function can be disabled, as long as the available fault current is leas than the 30-cycle withstand (short-time) rating of the ATS and circuit breaker.

Where the application requires selective coordination, it may be necessary to disable the instantaneous trip function on the power circuit breaker that’s upstream from the ATS so that selective coordination with downstream devices is maintained. In this case, it’s important to ensure the ATS has a 30-cycle withstand rating that’s high enough for the system’s available fault current.

4.2.9.9 Elevator disconnects

As defined in NEC 620.62, the selective coordination requirements for elevator circuits state that “…where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply-side overcurrent protective devices.”

It’s common practice to use fused elevator modules as the shunt-trip disconnecting means (circuit breakers may also be used). The following examples illustrate applying both equipment options.

Example 1: The one-line diagram in Figure 4.2.9.9.a illustrates a typical fused elevator module feeding two elevators.

Device #10

LPJ-60SP

60 A Class J fuse

Device #11

LPJ-60SP

60 A Class J fuse

Elevator #1

30 Hp

Figure 4.2.9.9.a

Elevator #2

30 Hp

The fused elevator disconnects (Devices #10 and #11) selectively coordinate with the upstream feeder fuse (Device #9) provided a 2:1 ratio is maintained. There is no need to plot time current characteristic curves or know the available fault current.



Example 2: The one-line diagram in Figure 4.2.9.9.c illustrates a typical application of shunt-trip circuit breakers feeding two elevators.

Device #13

FD 80 A

480 V panel

Device #12 LG 250 A

480 V elevator module

Device #14

FD 80 A

4.2.9.10 Main lug only (MLO) and through-feed lug (TFL) panels

When designing a selectively coordinated system, it’s important to minimize the number OCPD “levels” that need coordinating. It’s common practice to “daisy chain” panels (feed one sub-panel from another sub-panel). As shown in Figure 4.2.9.10.a, Devices #4 and #5 must selectively coordinate with Devices #2 and #3 and all downstream devices must selectively coordinate with Device #1.

Device #1

Panel A

Elevator #3

30 Hp

Elevator #4

30 Hp

Figure 4.2.9.9.c

In the one-line diagram above, the circuit breaker elevator disconnects

(Device #13 and #14) selectively coordinate with the upstream feeder circuit breaker (Device #12) per the time-current curve shown in Figure

4.2.9.9.d, provided Device #12’s available fault current is below the instantaneous pickup setting, or that Device #12 and Device #13 or #14 have been tested to show they selectively coordinate for the available fault current levels.

1000

100

Panel B

Device #2

Device #3

Device #4

Device #5

Panel C

Figure 4.2.9.10.a

In lieu of sub-feed circuit breakers, using main lug only (MLO) panels with through-feed lugs (TFL) reduces the selective coordination requirement to just the branch circuit devices in Panels A, B and C, and the main device in Panel A. See Figure 4.2.9.10.b.

4

Device #1

10 Panel A

Panel B

Panel C

1

0.10

0.01

0.5 1 10 100 1K

Ref. Voltage: 480V Current in Amps x 10

10K

Figure 4.2.9.9.d

When only one elevator is present in an electrical system (or when multiple elevators are fed from separate sources), NEC 620.62 does not require selective coordination. Therefore, fused elevator modules or selectively coordinated circuit breakers are not required. However selective coordination may be required if the elevator is fed from an emergency or legally required standby source, as defined in NEC 700.32 and 701.27.

Figure 4.2.9.10.b

The conductor size for the feed through panels (Panels B and C in this example) must be the same size as those in through feed panel

A. In addition, panelboards B and C must have a main lug and bus rating equal to Device #1. This will ensure that all cables and buses are protected by the upstream breaker, Device #1.

4.2.9.11 Generator breaker selection

Most fuse and circuit breaker manufacturers have performed testing to develop comprehensive selective coordination tables. However, no cross-manufacturer selective coordination testing has been performed.

This becomes an issue when the entire electrical distribution system is comprised of one manufacturer’s equipment and the generator protective device from another manufacturer. To avoid this, it’s suggested that all protective devices be from the same manufacturer, including the generator protective device(s).

4.2.9.12 Series rated systems

The premise behind a series rated combination is that both the upstream and the downstream circuit breakers interrupt in the event of a fault. Since the objective of selective coordination is to localize the overcurrent event to just the affected equipment, series rated systems are not allowed where selective coordination is required. System designs must use fully rated equipment to meet selective coordination.



4.2.9.13 Zone selective interlock (ZSI)

A circuit breaker zone selectively interlocked system’s goal is to reduce incident energy. ZSI is not a tool for selective coordination. Two circuit breakers that are zone selectively interlocked must first selectively coordinate for all fault currents as required by the NEC, or by the design goals. ZSI is put in place to ensure the upstream circuit breaker trips without an intentional delay should a fault occur between the two connected devices. ZSI does NOT provide selective coordination.

4.2.10 Coordination

2017 NEC introduced in Article 517 the concept of “coordination” for healthcare systems instead of “selective coordination”. The change made in the NEC aligned with a change made in NFPA 99, “Healthcare

Facilities Code,” which has purview over healthcare facility performance.

The NFPA 99 technical panel reduced the selectivity level to address the circuit breaker’s increased incident energy due to the fact that the circuit breaker’s instantaneous pickup will require increasing to achieve selective coordination.

517.31(G) requires that the OCPDs serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.

“517.31 Essential Electrical Systems for Hospitals

(G) Coordination — Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.

Exception No. 1 : Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary.

Exception No. 2 : Between overcurrent protective devices of the same size (ampere rating) in series.

Informational Note : The terms coordination and coordinated as used in this section do not cover the full range of overcurrent conditions.”

The minimum performance level required in 517.31(G) is less restrictive.

The “coordination” of OCPDs disregards fault current that the OCPDs see during a fault to determine selectivity. The 517.31(G) informational note establishes the fact that fault currents are not considered.

To ensure a higher level of reliability for vital loads, many system designs will continue incorporating selective coordination in essential electrical systems. Also, there are some healthcare facilities or parts of facilities, such as administrative buildings, which are required to comply with the NEC and its requirements for:

• Elevators (620.62)

• Critical operations data systems (645.27)

• Emergency systems (700.32)

• Legally required standby systems (701.27)

• Critical operations power systems (708.54).

4.2.11 Summary

Selective coordination is a desirable design goal, and for some installations a Code requirement. The NEC provides the minimum requirements for special systems that are essential for life safety, public safety and national security. For any system where reliability is of utmost importance, selective coordination should be used to increase the reliability of delivering power to vital loads. Selective coordination can be achieved with fuses, circuit breaker or fuse/circuit breaker solutions, but attention to details and being designed by a qualified individual is essential.

4.3 Arc flash

Contents

4.3.1 Overview

4.3.2 Arcing current

4.3.3 Incident energy

4.3.4 Fuse equations

4.3.5 Circuit breaker example

4.3.6 Fuse example

4.3.1 Overview

Section page

30

31

32

32

35

36

Understanding what incident energy levels are at various locations in an electrical distribution system is important for electrical safety. This information can also be input into a reliability review that indicates the extent of damage to expect should an arc flash event occur. An important arc flash analysis output is guidance on the requisite PPE when justified, energized work is conducted. The correct PPE selection and other safe work practices are covered in Section 6.

The steps necessary to conduct an arc flash study starts with these steps:

Step 1: Collect system information.

Information accuracy is important for any analysis. Whether it be an existing installation or new construction, the power distribution system must be properly evaluated and documented. The foundation of power systems analysis studies is collected information accuracy and the conservative nature of any assumptions made. New and existing systems present challenges unique to each, with existing installations, that have not been maintained, posing the biggest challenge.

Existing facilities may need to receive a walk down of the facility to update or verify one-line diagrams. New facilities under construction may go through study phases; the first of which is based upon many conservative assumptions until as-built drawings are obtained after the construction is complete.

Step 2: Operation modes.

Once the system configuration is understood, one-line diagrams are updated and accurate, a review must be made to determine the various power distribution system configurations that will impact the available fault currents.

Examples of the varying configurations include:

• Number of utility feeders that are, or could be, in and out of service

• Unit substations that can be supplied by one or two primary feeders

• MCCs with more than one feeder having the ability to energize one or two feeders

• Large motors that may or may not be running during fault conditions

• Generators running in parallel with the utility supply ,or in standby.

Step 3: Fault current study.

The fault current study is critical to the incident energy calculation (Ref. 4.1). This fault study must include both maximum and minimum expected fault currents at each major piece of electrical distribution equipment. When performing incident energy calculations, minimum fault currents (when no motors are connected as an example) could result in higher incident energy values due to longer clearing times at lower fault currents.



Step 4: Coordination study.

The coordination study is critical because arcing fault current clearing times are determined by comparing the arcing currents with the TCC curves of the OCPDs in the system. The

TCC curve is selected as part of the selective coordination study.

Note: Steps 5, 6 and 7 reference IEEE 1584-2002 and system limitations covering:

• 0.208 kV to 15 kV, three-phase

• 50 Hz to 60 Hz

• 700 A to 106,000 A available short-circuit current

• 13 mm to 152 mm conductor gaps

Step 5: Calculating arcing currents.

Arcing currents are calculated based on the equations of IEEE 1584-2002, “IEEE Guide for Performing

Arc-Flash Hazard Calculations”. The total arcing current at each piece of equipment must be determined, as well as the arcing current level that passes through the upstream OCPD. The total arcing current is used to calculated the incident energy. The arcing current that will pass through the upstream OCPD is used to determine that upstream device’s clearing time.

Step 6: Calculating incident energy.

Equations available from

IEEE 1584-2002 are used with the assembled information. Multiple calculations are made for the various identified power system configurations to determine which configuration provides the highest calculated incident energy. The calculated incident energy will depend upon voltage, equipment type and working distances.

Step 7: Determine flash-protection boundary.

The flash-protection boundary is determined through iterative calculations based on the same equations used to calculate incident energy. The iterations are designed to determine the distance from the arcing source at which the onset of a second degree burn could occur. Most programs include the flash-protection boundary based on an incident energy of 5.0 J/cm 2 (1.2 cal/cm 2 ). To convert from J/cm 2 to cal/cm 2 divide J/cm 2 by 4.184.

4.3.2 Arcing current

Arcing current is a fault current flowing through an electrical arc plasma.

This is commonly referred to as arc fault current and/or arc current.

Once the three-phase arcing current is calculated, the operating time for the OCPDs at that arcing current can be determined for incident energy calculations.

The equations for the arcing current are based on actual test data. Lab testing was conducted under specific conditions with critical data points recorded. Equation derivations where made to mathematically replicate what was witnessed in the lab. The following includes the equations for calculating arcing current.

4.3.2.1 System voltage under 1000 V

This is the equation to use for applications where system voltage is less than 1000 V: l g

I a

= K + (0.662) (l g

(l g

) (I bf

) (I bf

) + (0.0966) (V) + (0.000526) (G) + (0.5588) (V)

) -0.00304 (G) (l

G

) (I bf

)

Where: l g

= The log10

I a

= Arcing current (kA)

K = -0.153 for open configurations, or -0.097 for enclosed equipment

I bf

= Bolted fault current for three-phase faults (RMS Sym.) (kA)

V = System voltage (kV)

G = The gap between conductors, in millimeters, see Table 4.3.2.1.a

System voltage (kV)

0.208 to 1

>1 to 5

>5 to 15

Equipment type

Open air

Switchgear

MCCs and panels

Cable

Open air

Switchgear

Cable

Open air

Switchgear

Cable

“G” typical conductor gap (mm)

10-40

32

25

13

102

13-102

13

13-153

153

13

Distance “x” factor (mm)

2.000

1.473

1.641

2.000

2.000

0.973

2.000

2.000

0.973

2.000

Table 4.3.2.1.a

4.3.2.2 System voltage 1000 V to 15 kV

This is the equation to use for applications where system voltage is

1000 V to 15 kV: l g

I a

= 0.00402 + 0.983 (l g

I bf

)

There aren’t any box configuration specifications when applying the high voltage equations to determine arcing current.

To arrive at I used: a

, the log10 must be addressed and the following equation

I a

= 10l g

I a

IEEE 1584-2002 also advises that two arcing current values are to be calculated in preparation for the calculating two incident energy values based on both of these arcing currents’ clearing time. The second arcing current calculation is 85% of the original calculation’s arcing current.

The reason for the second arcing current calculation is due to the OCPD trip curve characteristics. Depending where on the curve the arcing current lies, small changes in arcing current can cause a large increase in clearing time which will have a very big impact on incident energy.

4



4.3.3 Incident energy

Once the arcing current has been determined and the coordination study or TCC curve is obtained, the input information for the incident energy equations is available for calculating incident energy.

Incident energy is defined by IEEE 1584-2002 as: “The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is measured in joules per centimeter squared (J/cm 2 ).” To convert from J/cm 2 to cal/cm 2 one must divide J/cm 2 by 4.184.

Calculating incident energy is a three-step process. The first is to calculate the log

10

of incident energy normalized for a 0.2 second arc clearing time at a 610 mm working distance. The next step is to convert the Log 10 to an incident energy value. The final step is an equation that considers the arcing current’s actual clearing time, working distance and other variables.

4.3.3.1 Step 1: Normalized equation

l g

E n

= K

1

+ K

2

+ 1.081 l g

I a

+ 0.0011 G

Where:

E n

= Incident energy in Joules/cm 2 normalized for time and distance

K

1

= -0.792 for open configurations, or -0.555 for enclosed equipment

K

2

= 0 for ungrounded and high resistance grounded systems, or -0.113 for grounded systems

G = The gap between conductors which is dependent upon the equipment type being applied x = A distance value dependent upon the equipment type

The parameter “x” accounts for the gaps between buses in the panel. The equipment’s construction plays a role in the incident energy amount that can be expected. Typical gaps to be used for the “x” value in the equation can be obtained from table 4.3.2.1.a.

4.3.4 Fuse equations

Equations have been developed to calculate incident energy when the arcing currents are in the specific fuse’s current limiting region. These formulae were developed based upon testing at 600 V and a 455 mm distance using one manufacturer’s fuses.

The common variables included in these equations are:

• I bf

is bolted fault current for three-phase faults (kA RMS Sym.)

• E is incident energy (cal/cm 2 )

The equations for each fuse class and size can only be used in the current ranges specified for each. Incident energy for currents outside the specified ranges must be calculated as per 4.3.3.

Class L and RK1 fuse incident energy equations

4.3.3.2 Step 2: Converting from log

10

Use the following equation to convert from the LOG base and provide an energy value used in the next step.

E n

= 10l g

E n

Where l g

E n

is the calculated value from Section 4.3.3.1.

4.3.3.3 Step 3: Calculating incident energy

The final step in the process is to actually calculate the incident energy value based on more detailed parameters specific to the application, using this equation:

E = C f

E n

( t ) ( 610

0.2 D x x ) cal/cm

2

Where:

E = Incident energy (cal/cm 2 )

C f

= A constant, 1.0 for voltage above 1 kV and 1.5 for voltage at or below 1000 volts

E n

= Normalized incident energy calculated as part of 4.3.3.2

t = Arcing time in seconds determined from the upstream OCPD’s

TCC curve

D = Working distance from the arcing point to the person, in mm

The parameter “D” is the working distance from the arcing point to the person measured in millimeters. Typical working distances are shown in the following table. Working distance accounts for the distance between the worker and the arc source.

Equipment class

15 kV switchgear

5 kV switchgear

Low voltage switchgear

Low voltage MCCs and panelboards

Cable

Other

Table 4.3.3.3a


“D” (mm)

910

910

610

455

455

To be determined in field

Bolted fault current range (kA) Incident energy cal/cm 2

Class L 1601-2000 A

22.6 ≤ I bf

≤ 65.9

65.9 < I bf

≤ 106

Class L 1201-1600 A

- 0.1284 I

- 0.5177 I bf bf

+ 32.262

+ 57.917

15.7 ≤ I bf

≤ 31.8

31.8 < I bf

< 44.1

44.1 ≤ I bf

≤ 65.9

65.9 < I bf

≤ 106

Class L 801-1200 A

- 0.1863 I bf

+ 27.926

-1.5504 I bf

+ 71.303

2.9398

- 0.0631 I bf

+ 7.0878

- 0.1928 I bf

+ 14.226

0.0143 I bf

2 - 1.3919 I bf

+ 34.045

0.3896

15.7 ≤ I bf

≤ 22.6

22.6 < I bf

≤ 44.1

44.1 < I bf

≤ 106

Class L 601-800 A

15.7 ≤ I bf

≤ 44.1

44.1 < I bf

≤ 106

Class RK1 401-600 A

8.5 ≤ I bf

≤ 14

14 < I bf

≤ 15.7

15.7 < I bf

≤ 22.6

22.6 < I bf

≤ 106

Class RK1 201-400 A

- 0.0601 I

0.25

- 3.0545 I bf bf

+ 2.8992

0.599

- 0.0507 I bf

+ 43.364

+ 1.3964

0.25

3.16 ≤ I bf

≤ 5.04

5.04 < I bf

≤ 22.6

22.6 < I bf

≤ 106

Class RK1 101 A—200 A

1.16 ≤ I bf

≤ 1.60

1.60 < I bf

≤ 3.16

3.16 < I bf

≤ 106

Class RK1 up to 100 A

0.65 ≤ I bf

≤ 1.16

1.16 < I bf

≤ 1.40

1.40 < I bf

≤ 106

- 19.053 I bf

+ 96.808

- 0.0302 I bf

+ 0.9321

0.25

- 18.409 I bf

- 4.2628 I bf

+ 36.355

+ 13.721

0.25

- 11.176 I bf

- 1.4583 I

+ 13.565

bf

+ 2.2917

0.25


4.3.4.1. Arc flash incident energy table

The incident energy table in this section can be used to determine incident energy let-through for current-limiting fuses. The data points within this table are based upon the equations above and the following assumptions:

• Electrode spacing: 32 mm (1-1/4”)

• Enclosure: 20” x 20” x 20” box

• 600 V, 3 Ø ungrounded system

• Working distance: 18”

• Arc flash boundary is based on 1.2 cal/cm 2 second-degree “just curable” burn

, the threshold for a

The data is based upon tests conducted at various fault currents for each Bussmann series Low-Peak KRP-C-SP and LPS-RK-SP fuse in the table. These tests were used to develop the formulas as shown in NFPA

70E Annex D.7.6 and 2002 IEEE 1584. Actual results from incidents could be different for the following reasons:

• System voltage

• Short-circuit power factor

• Distance from the arc

• Arc gap

• Enclosure size

• Fuse manufacturer

• Fuse class

• Orientation of the worker

• Grounding scheme

• Electrode orientation

100 A LPS-RK-SP fuses were the smallest fuses tested. Data for smaller fuses is based upon the 100 A data. Arc flash values for actual 30 and

60 A fuses would be considerably less than 100 A fuses. However, it does not matter since the values for the 100 A fuses are already so low.

The fuse incident energy values were chosen not to go below

0.25 cal/cm 2 ,even though many actual values were below

0.25 cal/cm 2 . This was chosen to keep from encouraging work on energized equipment without PPE because of a low AFB.

This arc flash incident energy table can also be used for LPJ-SP, TCF,

FCF, JJS and LP-CC fuses to determine the available incident energy and AFB.

These values from fuse tests take into account the translation from available 3-phase bolted fault current to the arcing fault current.

To determine the AFB and incident energy for applications with other fuses, use the basic equations in 2002 IEEE 1584 or NFPA 70E Annex D.7.

Where the arcing current is less than the fuse’s current-limiting range when calculated per NFPA 70E Annex D.7.6, and 2002 IEEE 1584, the value for incident energy is given as >100 cal/cm 2 . For the incident energy and arc flash boundary in these cases, use 2002 IEEE 1584 basic equation methods with the fuse time-current curve.

The steps necessary to conduct an arc flash hazard analysis using this table are:

1. Determine the available bolted fault current on the lineside terminals of the equipment that will be worked upon.

2. Identify the amp rating of the upstream Low-Peak fuse that’s protecting the equipment where work is to be performed.

3. Consult the table to determine the incident energy exposure and the arc flash boundary (AFB).

4. Identify the minimum requirements for PPE when work is to be performed inside the AFB by consulting the requirements found in

NFPA 70E.

4



76 0.25

78 0.25

80 0.25

82 0.25

84 0.25

86 0.25

88 0.25

90 0.25

92 0.25

94 0.25

96 0.25

98 0.25

100 0.25

102 0.25

104 0.25

106 0.25

46 0.25

48 0.25

50 0.25

52 0.25

54 0.25

56 0.25

58 0.25

60 0.25

62 0.25

64 0.25

66 0.25

68 0.25

70 0.25

72 0.25

74 0.25

Bolted fault current

(kA)

1-100 A

IE

1 2.39

AFB

101-200 A

IE AFB

201-400 A

IE AFB

401-600 A

IE AFB

601-800 A

IE AFB

801-1200 A

IE AFB

1201-1600 A

IE AFB

1601-2000 A

IE AFB

29 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

2 0.25

3 0.25

6

6

5.20

0.93

49

15

>100

>100

>120

>120

>100

>100

>120

>120

>100

>100

>120

>120

>100

>100

>120

>120

>100

>100

>120

>120

>100

>100

>120

>120

4 0.25

5 0.25

6 0.25

8 0.25

10 0.25

12 0.25

14 0.25

6

6

6

6

6

6

6

025

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6 20.60

>120 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6 1.54

21 >100 >120 >100 >120 >100 >120 >100 >120 >100 >120

6

0.75

0.69

0.63

0.57

0.51

13

12

12

11

10

>100

36.85

12.82

6.71

0.60

>120

>120

90

58

11

>100

>100

75.44

49.66

23.87

>120

>120

>120

>120

>120

>100

>100

>100

73.59

39.87

>120

>120

>120

>120

>120

>100

>100

>100

>100

>100

>120

>120

>120

>120

>120

>100

>100

>100

>100

>100

>120

>120

>120

>120

>120

16 0.25

18 0.25

20 0.25

22 0.25

24 0.25

26 0.25

28 0.25

30 0.25

32 0.25

34 0.25

36 0.25

38 0.25

40 0.25

42 0.25

44 0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.45

0.39

0.33

0.27

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

9

8

7

7

6

6

6

6

6

6

6

6

6

6

6

0.59

0.48

0.38

0.28

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

11

10

8

7

6

6

6

6

6

6

6

6

6

6

6

1.94

1.82

1.70

1.58

1.46

1.34

1.22

1.10

0.98

0.86

0.74

0.62

0.50

0.38

0.25

25 11.14

24 10.76

23 10.37

22 9.98

21

19

18

17

16

14

13

11

10

8

6

8.88

7.52

6.28

5.16

4.15

3.25

2.47

1.80

1.25

0.81

0.49

82 24.95

>120 >100 >120

80 24.57

>120 >100 >120

78 24.20

>120 >100 >120

76 23.83

>120 >100 >120

70 23.45

>120 29.18

>120

63 23.08

>120 28.92

>120

55 22.71

>120 28.67

>120

48 22.34

>120 28.41

>120

42 21.69

>120 28.15

>120

35 18.58

116 27.90

>120

29 15.49

24 12.39

18

14

10

9.29

6.19

3.09

102

88

72

55

34

27.64

27.38

27.13

26.87

26.61

>120

>120

>120

>120

>120

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6 0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6 0.25

6 0.25

6

6

6

6

6

6

6 0.25

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6 0.25

6 0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

6 0.39

6 0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

0.39

8

8

8 2.92

8 2.80

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.93

2.67

2.54

2.42

2.29

2.17

2.04

1.91

1.79

1.66

1.54

1.41

1.28

1.16

1.03

0.90

0.78

0.65

0.53

0.40

28 18.57

27 17.54

26 16.50

25 15.47

24 14.43

22 13.39

21 12.36

20 11.32

13

12

10

9

19 10.29

18 9.25

16

15

8.22

7.18

6.15

5.11

4.08

3.04

33 26.36

>120

33 26.10

>120

33 25.84

>120

33 25.59

>120

33 25.33

>120

33 25.07

>120

33 24.81

>120

33 24.56

>120

33 24.30

>120

33 24.04

>120

33 23.75

>120

32 22.71

>120

31 21.68

>120

30 20.64

>120

29 19.61

120

55

48

41

34

77

72

66

61

116

111

107

102

97

93

88

83

Table 4.3.4.1.a Arc flash incident energy table*.

* Bussmann series 1-600 A Low-Peak LPS-RK-SP fuses and 601-2000 A Low-Peak KRP-C-SP fuses, Incident Energy (IE) values expressed in cal/cm 2

(AFB) expressed in inches.

, Arc Flash Boundary



4.3.5 Circuit breaker example

An electrical panel is supplied from a 1200 A circuit breaker with the TCC curve shown in Figure 4.3.5.a. The available fault current at the downstream electrical panel is 51,907 A. The system is a solidly grounded system and the enclosure to be labeled is a 208 V panelboard.

Determine the incident energy label for this service entrance equipment.

Incident energy calculation

Step 1: The variables to use in the equation from section 4.3.3.1 include:

I a

= 11.76 kA and 13.84 kA. Two incident energy values will be calculated

K

1

= -0.555 for enclosed equipment (this is panelboard application)

K

2

= -0.113 as this example has a solidly grounded system

G = 25 as the gap determined as per Table 4.3.2.1.a for panelboards

The equation of 4.3.3.1 is solved as follows for each value of I a

:

For I a

= 13.84 kA: l g

E n

= (-0.555) + (-0.113) + 1.081 (l g

13.84) + (0.0011) (25) l g

E n

= 0.5931

For I a

= 11.76 kA: l g

E n

= (-0.555) + (-0.113) + 1.081 (l g

11.76) + (0.0011) (25) l g

E n

= 0.5166

Step 2: The previous values of l g from log10 base as:

E n

for both values of I a

are converted

For I a

= 13.84 kA:

E n

= 10 0.5931

E n

= 3.918

For I a

= 11.76 kA:

E n

= 10 0.5166

E n

= 3.2856

Step 3: This calculation is done in two parts for both arcing current values. The information that must first be determined includes the

OCPD’s clearing time based upon each arcing current value. The TCC curve in Figure 4.3.5.a is used to determine the clearing times.

The clearing times for each arcing current value is taken by following the X-axis in Figure 4.3.5.a up to where the circuit breaker curve’s top band is intersected and then following it left on the Y-axis to determine the time in seconds. This is an approximation process with more precise results obtainable using a software application.

4

Figure 4.3.5.a

The arcing current is the first value to calculate. The variables to use in the equation of 4.3.2.1 include:

K = -0.097 as this is a panelboard

I bf

= 51.97 kA as defined in the problem statement

V = 0.208 V

G = 25 mm as obtained from Table 4.3.2.1.a

l g

I a

= -0.097 + (0.662) (l g

(0.5588) (0.208) (l g

51.97) + (0.0966) (0.208) + (0.000526) (25) +

51.97) - 0.00304 (25) (l g

51.97) l g

I a

= 1.1411

I a

= 10 1.1411

I a

= 13.84 kA

85 percent of the calculated arcing current provides the second arcing current value to be considered when calculating incident energy.

I a

(85%) = 0.85 x 13.84 = 11.76 kA

Arcing current (kA)

13.84

11.76

E n

3.918

3.2856

t (seconds)

0.6

0.85

Table 4.3.5.b

The following equation for incident energy from 4.3.3.3 is used for the arcing currents and clearing times shown in the table above:

E = C f

E n

( t ) ( 610 x

0.2 D x

)

Where:

C f

= 1.5 (this equipment’s voltage is below 1 kV)

E n

= 3.918 and 3.2856 (from Table 4.3.5.b) t = 0.6 and 0.85 (from Table 4.3.5.b)

D = 455 mm (from Table 4.3.3.3.a, this equipment is a panelboard) x = 1.641 (from Table 4.3.2.1.a, this equipment is a panelboard)

For the 11.76 kA arcing current:

E = 1.5 x 3.2856 x

( 0.85

0.2

) x ( 610 1.641

455 1.641

)

E = 33.89 cal/cm 2



For the 13.84 kA arcing current:

E = 1.5 x 3.918 x

( 0.6

0.2

) x ( 610 1.641

455 1.641

)

E = 28.5237 cal/cm 2

The higher incident energy is calculated from the lower arcing fault current. For this application the label incident energy should be

33.89 cal/cm 2 .

4.3.6 Fuse example

The following is a simple method when using certain Bussmann series fuses and based on actual data from arcing fault tests (and resulting simplified formulas shown in 2018 NFPA 70E Annex D.4.6 and IEEE

1584-2002) with Bussmann series current-limiting fuses. Using this simple method, the first thing is to determine the incident energy exposure. This process is simplified when using Low-Peak LPS-RK-SP,

LPJ-SP, TCF, LP-CC or KRP-C-SP fuses or JJS Limitron and Class CF FCF fuses. In some cases the results are conservative.

LPS-RK-600SP

600 A, Class RK1 fuses

42 kA available bolted fault current

The last step in the arc flash hazard analysis is to determine the appropriate PPE. To select the proper PPE, utilize the incident energy exposure values and the requirements from NFPA 70E. NFPA 70E

130.7(C)(1) through (C)(16) for the PPE level based upon the incident energy. The 2018 NFPA 70E Annex H is a resource for guidance in selecting PPE, specifically Tables H.3(a) and (b).

When selecting PPE, keep in mind that the requirements from NFPA

70E are minimum requirements. Having additional PPE, above what is required, can further assist in minimizing an arc flash incident’s affects.

Another thing to keep in mind is that PPE available on the market today does not protect a person from the pressures, shrapnel and toxic gases that can result from an arc-blast (referred to as “physical trauma” in

NFPA 70E). Existing PPE is only tested to minimize the potential for burns from the arc flash.

This information is not to be used as a recommendation to work on energized equipment. This information is to help assist in determining the PPE to help safeguard a worker from the burns that can be sustained from an arc flash incident. This information does not take into account the effects of pressure, shrapnel, molten metal spray or the toxic vapor resulting from an arc-fault. This information does not address the maintenance conditions of the overcurrent protective device.

This data is based upon the simplified fuse formulas in NFPA 70E Annex

D.4.6 and 2002 IEEE 1584 guide for arc flash hazard analysis.

For a more complete presentation on this subject, see Section 3.1.5.3

“The OCPD’s role in electrical safety.”

600 V, 3Ø

MLO panel

• Incident energy 0.25 cal/cm

• 6” AFB

2 @18”

Figure 4.3.6.a

In this example, the lineside OCPD in Figure 4.3.6.a is a Low-Peak

LPS-RK-600SP current-limiting fuse. Simply take the available 3 Ø bolted fault current at the panel, in this case 42,000 A, and locate it on the vertical column in the arc flash incident energy Table 4.3.4.1.a. Then proceed directly to the right to the 401-600 A fuse column and identify the IE (Incident Energy) and AFB (Arc Flash Boundary).

With 42 kA of 3 Ø bolted available fault current, Table 4.3.4.1.a shows that when relying on the Low-Peak LPS-RK-600SP fuse to interrupt an arcing fault, the incident energy is 0.25 cal/cm current-limiting fuse’s amp rating (see 4.3.4.1.).

2 . Notice the variables required are the available 3 Ø bolted fault current and the Low-Peak

The next step in this simplified arc flash hazard analysis is to determine the AFB. With an incident energy of 0.25 cal/cm 2 and using the same table, the AFB is approximately 6 inches, which is found next to the incident energy value previously located (see 4.3.4.1). This AFB distance means that anytime work is performed inside this distance, including voltage testing to verify that the panel is de-energized, the worker must wear the appropriate PPE.



5 Maintenance

Contents

5.1 Overview

5.2 Maintenance frequency and procedures

5.3 MCCB maintenance example

5.4 Circuit breaker testing considerations

5.5 OCPD servicing and maintenance

5.6 Testing knifeblade fuses

5.7 After an OCPD opens

5.8 Calibration decal on equipment

Section page

1

1

2

2

3

3

3

3

5.1 Overview

Overcurrent protection is similar to auto insurance. When a person buys auto insurance, they hope they never have to submit an accident claim. Should they have a major accident, they’re grateful the insurance company financially protects them. The insurance for electrical systems and equipment are overcurrent protective devices (OCPDs), which are intended to protect against overload and short-circuit (fault) conditions that may arise. People install OCPDs hoping there will never be an overcurrent condition requiring them to open — especially due to a fault.

NFPA 70E 225.3 has a very specific requirement pertaining to fault currents and circuits breakers. It reads:

“225.3 Circuit Breaker Testing After Electrical Faults.

Circuit breakers that interrupt faults approaching their interrupting ratings shall be inspected and tested in accordance with the manufacturer’s instructions.”

Should an overcurrent event occur, the OCPD needs to operate as originally specified. If the OCPD does not operate as fast as it should, or fails to operate, the OCPD investment is nullified and property damage, lost business time/production, and possible harm to property and people can occur. In addition, arc flash mitigation for electrical safe work practices typically relies on overcurrent protective devices operating as intended.

Reliability may be the most important criteria for OCPD type evaluation and selection. What good is an OCPD that may not function, or not function properly, when needed? For a particular circuit, an overcurrent event that must be cleared by the OCPD can occur from the day it’s installed, to decades later, or never. Whether a 1000 A or a 20 A circuit, the OCPD reliability is important for fire safety, life safety, and worker safety.

In the design stage, OCPD maintenance should be considered and recommendations made to the owner on the necessary periodic inspections, testing, maintenance and documentation over the electrical system’s life-cycle.

Modern current-limiting fuses are inherently reliable in terms of overcurrent interruption over the product’s life, with life cycle maintenance requirements a primary consideration when deciding whether to use fuses.

The NEC is predominantly an installation standard and has few OCPD maintenance requirements. However, the NEC recognizes that proper installation alone is not adequate for safety; maintenance during the system’s life is also necessary.

NEC 90.1(B) Adequacy — This contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentially free from hazard, but not necessarily efficient, convenient, or adequate for good service or electrical system’s future expansion.

NFPA 70E-2018 Standard for Electrical Safety in the Workplace does have OCPD maintenance requirements. A few important requirements are:

130.5(B) Estimate of Likelihood and Severity — The estimate of the likelihood of occurrence of injury or damage to health and the potential severity of injury or damage to health shall take into consideration the following:

(1) The design of the electrical equipment, including its overcurrent protective device and its operating time.

(2) The electrical equipment operating condition and condition of maintenance.

IN No. 1 — Improper or inadequate maintenance can result in increased fault clearing time of the overcurrent protective device, thus increasing the incident energy. Where equipment is not properly installed or maintained, PPE selection based on incident energy analysis or the PPE category method might not provide adequate protection from arc flash hazards.

205.4 General Maintenance Requirements — Overcurrent protective devices shall be maintained in accordance with the manufacturers’ instructions or industry consensus standards.

Maintenance, tests, and inspections shall be documented.

210.5 Protective Devices — Protective devices shall be maintained to adequately withstand or interrupt available fault current..

IN — Improper or inadequate maintenance can result in increased opening time of the overcurrent protective device, thus increasing the incident energy.

5.2 Maintenance frequency and procedures

Important OCPD decision factors include reliability, maintenance frequency and procedures, and cost (including downtime) required to retain the original specified protection level.

The best sources for OCPD maintenance frequency, necessary tests and specific methods include OCPD manufacturer’s instructions, NFPA

70B-2016 Recommended Practice for Electrical Equipment Maintenance, and ANSI/NETA MTS-2015, Standard for Maintenance Testing

Specifications for Electrical Power Equipment and Systems.

NFPA 70B provides maintenance frequency guidelines, as well as guidelines for setting up an electrical preventative maintenance (EPM) program, including sample forms and requirements for electrical system maintenance. ANSI/NETA MTS-2015 is more prescriptive about what maintenance and testing is required for electrical power system devices and equipment. Visual, mechanical, and electrical inspections and tests are specified by equipment type, as well as what results are acceptable.

This standard includes guidelines for maintenance frequency required for electrical system power equipment in Appendix B, Frequency of

Maintenance Tests.

5


Section 5 — Maintenance

5-2

2016 NFPA 70B OCPD Frequency of Maintenance

The complete NFPA 70B text has more comprehensive practices and annex information than shown here. This is merely a representation of what’s provided by NFPA 70B. NFPA 70B stresses that for specific situations the frequency of maintenance is dependent upon many variables such as environmental conditions and operating conditions.

11.4 Frequency of Tests — Most routine testing can best be performed concurrently with routine preventive maintenance, because a single outage will serve both procedures. For that reason, the testing frequency generally coincides with maintenance frequency. The optimum cycle depends on the use to which the equipment is put, and the equipment’s operating and environmental conditions. In general, this frequency can range from six (6) months to three (3) years, depending on conditions and equipment use.

The difficulty in scheduling an outage for testing and maintenance should never be a factor in performing these two vital procedures.

Equipment for which an outage is difficult to obtain is usually the most vital equipment in the electrical system’s operation.

Consequently, this “equipment failure” would most likely create the most problems relative to continued system operation. In addition to routine testing, tests should be performed any time equipment has been subjected to conditions that could possibly cause it to be unable to continue performing its design function properly.

Below are considerations for low-voltage fuses.

Annex L Maintenance Intervals (partial extract)

Item/equipment

Fuses, 1000 V or less

Task/function

Fuse terminals and fuseclips

Interval Reference

Visual inspection/clean 3 years

Clip contact pressure 3 years

3-5 years

18.1.2

18.1.3

18.1.3

Fuses

Cleaning of contact surfaces

Visual inspection for discoloration and damage

3 years 18.1.3

18.1.2 Inspection — Fuse terminals and fuseclips should be examined for discoloration caused by heat from poor contact or corrosion. Early detection of overheating is possible through the use of infrared examination. If evidence of overheating exists, the cause should be determined.

18.1.3 Cleaning and Servicing — The power source to fuse holders should be disconnected before servicing. All fuse holder connections should be tightened. All connections to specifications should be torqued where available. Fuseclips should be checked to ascertain that they exert sufficient pressure to maintain good contact. Clips making poor contact should be replaced or clip clamps used. Contact surfaces of fuse terminals and clips that have become corroded or oxidized should be cleaned. Silverplated surfaces should not be abraded. Contact surfaces should be wiped with a noncorrosive cleaning agent. Fuses showing signs of deterioration, such as discolored or damaged casings or loose terminals, should be replaced.

5.3 MCCB maintenance example

This example uses ANSI/NETA MTS-2011 Appendix B: Frequency of Maintenance Tests tables. Assume a molded case circuit breaker

(MCCB) in average condition. The condition has been established by prior maintenance, retaining records, and trending the data. Since workers rely on this circuit breaker for arc flash protection, “high reliability” is desired. Referencing the Maintenance Frequency Matrix

(shown in reprinted ANSI/NETA MTS table) using average condition and high reliability, results in a 0.50 multiplier. Then referencing the

Inspections and Tests, Frequency in Months table for a molded case circuit breaker (left Section column denoted as 7.6.1.1) the following are prescribed frequencies for inspections and tests: visual every one (1) month; visual and mechanical every 12 months; and visual, mechanical and electrical every 36 months. Then, the 0.50 multiplier is applied to these with these results:

• Visual — every two weeks

• Visual and mechanical — every six months

• Visual, mechanical and electrical — every 18 months

ANSI/NETA MTS-2011 in Section 7.6.1.1 Circuit Breaker

Air, Insulated-Case/Molded Case provides lists of prescribed inspections and test procedures including references to tables in the standard with acceptable test values. Item five of nine in the visual and mechanical inspection list is “Operate the circuit breaker to insure smooth operation,” which is a procedure to exercise the circuit breaker. There is a list of thirteen electrical tests procedures which are reprinted below.

ANSI/NETA-2011 Section 7.6.1.1

2. Electrical Tests

1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section

7.6.1.1.1.

2. Perform insulation-resistance tests for one minute on each pole, phase-to-phase and phase-to-ground with the circuit breaker closed, and across each open pole. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.

3 Perform a contact/pole-resistance test.

4. (Optional) Perform insulation-resistance tests on all control wiring with respect to ground. The applied potential shall be 500 volts

DC for 300-volt rated cable and 1000 volts DC for 600-volt rated cable. Test duration shall be one minute. For units with solid-state components, follow manufacturer’s recommendation.

5. Determine long-time pickup and delay by primary current injection.

6. Determine short-time pickup and delay by primary current injection.

7. Determine ground-fault pickup delay by primary current injection.

8. Determine instantaneous pickup current by primary injection.

9. (Optional) Test functions of the trip unit by means of secondary injection.

10. Perform minimum pickup voltage test on shunt trip and close coils in accordance with Table 100.20.

11. Verify correct operation of auxiliary features such as trip and pickup indicators, zone interlocking, electrical close and trip operation, tripfree, ant pump function, and trip unit battery condition.

12. Reset all trip logs and indicators.

13. Verify operation of charging mechanism.

Copyright © 2011 International Electrical Testing Association. Reprinted with permission, Courtesy of International Electrical Testing Association.

5.4 Circuit breaker testing considerations

Maintenance files should be kept, and record the “as found” condition/ performance and the “as left” condition/performance, as well as key trending data. Over time, these maintenance files and trending data will often indicate when a circuit breaker’s declining and needs replacement.

Low voltage power circuit breakers are designed to be reconditioned, if necessary. Molded case and insulated case circuit breakers are not intended to be internally examined and maintained/reconditioned.

Instead a replacement device should be supplied if they are shown to be out of calibration or inoperative. Circuit breaker testing, maintenance and reconditioning should be performed by persons qualified for this work. Circuit breaker testing equipment is available in the market as well as companies that specialize in testing and maintenance, and reconditioning circuit breakers and other components.



5.7 After an OCPD opens

Bussmann™ series clip clamps can be used to help restore fuse contact weakened fuseclips.

5.5 OCPD servicing and maintenance

NFPA 70B-2016 has guidelines for testing fuses:

21.18.1

— Fuses can be tested with a continuity tester to verify that the fuse is not open. Resistance readings can be taken using a sensitive 4 wire instrument such as a Kelvin bridge or microohmmeter. Fuse resistance values should be compared against values recommended by the manufacturer.

21.18.2

— Where manufacturer’s data is not readily available, resistance deviations of more than 15 percent for identical fuses in the same circuit should be investigated.

Normally on low voltage systems, a simple fuse continuity test is sufficient. Low resistance denotes a fuse is good and extremely high resistance indicates a fuse is open and needs replacing. For some applications, such as high speed fuses used in large power electronic applications and medium voltage fuse applications, maintenance contractors performing periodic shut down maintenance often will check the fuses’ resistance. This requires using sensitive resistance measurement instruments such as a Kelvin bridge or micro-ohmmeter.

5.6 Testing knifeblade fuses

Other important considerations for selecting the OCPD type is servicing and trouble shooting. This is an area where misinformation exists and often, a lack of proper safe work practices.

When an OCPD device opens due to a fault, OSHA and NFPA 70E do not permit circuit breakers to be re-closed or fuses to be replaced until it is safe to do so.

2018 NFPA 70E 130.6(M) and OSHA 1910.334(b)(2)* Re-closing

Circuits After Protective Device Operation

After a circuit is de-energized by the automatic operation of a circuit protective device, the circuit shall not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual reclosing of circuit breakers or re-energizing circuits through replaced fuses shall be prohibited. When it is determined from the design of the circuit and the overcurrent devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, examination of the circuit or connected equipment shall not be required before the circuit is re-energized.

* Shown is wording from 2018 NFPA 70E. The OSHA wording is different, but has the same meaning.

This is an important safety practice. If an OCPD opened under a fault condition, damage may have resulted. If the fault’s cause is not identified and rectified, re-energizing the circuit again into the fault condition might result in an even more severe fault event than the first.

What constitutes “can be safely energized?” First, ensuring the fault’s cause has been properly repaired, but that is not sufficient. When fault current flows through the distribution system to the fault’s location, damage to circuit components carrying the fault can occur. Inspect and test the circuit to ensure that the fault current did not damage circuit components that now are or soon could be the source of another fault.

If all the components check out as in good condition, the circuit may still not be safely re-energize. The OCPD(s) must be verified as safe to re-energize. New fuses of the proper type and amp rating must be inserted and the circuit re-energized by closing the disconnect.

A continuity test across any knifeblade fuse should be taken ONLY across the fuse blades.

Do NOT test a knifeblade fuse with meter probes to the fuse caps.

5

Contrary to popular belief, fuse manufacturers do not generally design their knifeblade fuses to have electrically energized fuse caps during normal fuse operation. The caps’ electrical inclusion into the circuit results from the coincidental mechanical contact between the fuse cap and terminal extending through it. In most knifeblade fuse brands, this mechanical contact is not guaranteed; therefore, electrical contact is not guaranteed. Thus, a resistance reading or voltage measurement taken across the fuse caps does not indicate whether or not the fuse is open.

In an effort to promote safer work environments, Bussmann series

Fusetron (Class RK5) and Low-Peak (Class RK1) knifeblade fuses feature insulated end caps to reduce the possibility of accidental contact with a live part (see Figure 5.6.a). With these fuses, the informed electrician knows that the end caps are isolated. With older style knifeblade fuses, with non-insulated end caps, the electrician using the caps for testing doesn’t really know if the fuse is energized or not.

Figure 5.6.a

Always test for voltage across the blades, not the caps

5.8 Calibration decal on equipment

A best practice after conducting periodic maintenance, or maintenance after fault interruption on OCPDs is to apply a decal on the equipment’s outside. The decal is color coded and can be an aid for hazard identification and risk assessment for electrical safety. NFPA 70B

Recommended Practice for Electrical Equipment Maintenance makes this recommendation in 11.27 Test or Calibration Decal System.

See Figures 5.8.a to 5.8.c for a decal system example. This maintenance decal system complies with NFPA 70B 11.27. After the technician performs inspections and tests, and, if necessary, remedial measures, one of three color coded decals is affixed to the equipment.

The decal and test records can communicate the OCPD’s condition of maintenance. This is especially important for arc flash risk assessment.

For instance, NFPA 70E 130.5 Arc Flash Risk Assessment requires the OCPD’s design, opening time and condition of maintenance be considered. When an OCPD is not properly maintained and an arcing fault occurs, the resulting arc flash incident energy may be much greater than calculated because the OCPD is not clearing the arcing current in the time indicated by its published data. The possible result: a worker is wearing PPE with a certain arc rating based on an incident energy calculation, but the arc flash incident energy is actually much greater than calculated.


Section 5 — Maintenance

TESTED

Project No.:__________________

Test Date:__________________

Figure 5.8.a

White decal communicates the OCPD is electrically and mechanically acceptable, and it should perform to the manufacturer’s original specification.

LIMITED SERVICE

Project No.:__________________

Test Date:__________________

Figure 5.8.b

Yellow decal communicates the OCPD may have minor deficiencies, but is electrically and mechanically acceptable. A trip indicator (indicating whether the overcurrent interrupted was an overload or fault) that does not function properly is an example of such a minor deficiency.

DEFECTIVE

Project No.:__________________

Test Date:__________________

Kerry Heid, Magna Electric Corporation, summarized the NETA 2008 survey results in Summer 2011 NETA World article, Survey Says!

Maintenance is Critical as Part of Your Electrical Safety Program.

“A 2008 survey was conducted by NETA on the performance of electrical protective devices. A national survey of field performance on approximately 340,000 protective devices was reviewed, and the results of those findings are quite alarming.”

“Based on the results of the survey, approximately 23 percent of the circuit breakers tested had an issue affecting the protective device operation. This data closely correlates with failure data presented in IEEE Std. 493-2007, Table 5-1 in the fair (18.1%) to poor (32.8%) maintenance quality category. With percentages in these ranges, approximately one in four of the devices in the field will not operate as indicated on the time current curves. The impact to personnel in the field is that in most cases incident energies will significantly increase due to the defective equipment.”

“Another alarming statistic was the fact that on average 10.5 percent of the devices did not function at all when tested. This means that when overload or short-circuit current is applied to the device, it was found to be inoperable. If a fault were to occur, it would severely impact personnel safety when working on or near that particular piece of equipment. Of the units with issues affecting performance, 42.8 percent were mechanical issues, and

26.7 percent had issues related to electrical diagnostic testing.

Lubrication issues were the predominant mechanical failure at 51.4 percent.”

5-4

Figure 5.8.c

Red decal communicates the OCPD has not passed one or more inspections, or tests and it is not suitable to be in service. Example deficiencies are failure to trip on calibration test or unacceptable high values during a contact resistance test.

The International Electrical Testing Association (NETA) has numerous articles and papers on this subject. The NETA contractors conduct maintenance on electrical equipment including OCPDs. For several years, the NETA has been surveying its members regarding equipment maintenance and associated reliability.

Pertinent quotes from NETA Maintenance Testing Research on Electrical

Power System Equipment Performance, by Kerry Heid, Magna Electric

Corporation and Ron Widup, Shermco Industries, 2012 PowerTest

Conference:

“Surveys on electrical power distribution equipment and systems indicate a high number of equipment failures that directly impact personnel safety and intensify equipment damage.”

“The 2008 paper focused on data obtained during electrical maintenance testing activities, indicating that 23% of service-aged electrical power systems equipment did not operate within the initial design parameters and 10.5% had serious issues affecting equipment performance and safety. These are alarming results when one considers that even a few milliseconds of operating lag in a power system protection scheme can mean drastic increases in energy levels released during a fault.”

“In general terms of reliability, fuses were ranked as having the lowest failure rate with molded case breakers showing the highest level of failure rate amongst the survey participants. The survey indicated the following ranking for failures: molded-case or insulated-case breakers, low voltage air circuit breakers, mediumvoltage relay breaker combo, and fuses.”

“A few respondents indicate that a high amount of failures occur with breakers, particularly molded case breakers during acceptance testing. This is due to a number of factors such as operation, adjustment and settings issues typically found with circuit breakers.”



6 Electrical safe work practices 6.2 The electrical safety program

Contents

6.1 Overview

6.2 The electrical safety program

6.3 Shock hazard

6.3.1 Shock risk assessment

6.3.2 Shock risk control solutions

6.3.3 Ground-fault circuit interrupters

6.4 Arc flash hazard

6.4.1 Arc flash risk assessment

6.4.2 Incident energy analysis method

6.4.3 Arc flash PPE category method

6.4.4 Equipment labeling

6.4.5 Personal Protective Equipment (PPE)

6.4.6 Arc flash risk control solutions

6.5 Maintenance

6.5.1 OCPD maintenance

6.5.2 Risk control solutions

6.1 Overview

There is a great deal of activity in the electrical industry concerning electrical safety. A culture change is underway and the electrical worker is at its center. Effectively implementing electrical safe work practices mandates that owners, employers and employees work together because safety is a shared responsibility. Electrical hazards include shock, arc flash and arc blast. Significant knowledge has been gained through testing and analysis concerning arc flash hazards. The

2018 edition of NFPA 70E, the “Standard for Electrical Safety in the

Workplace,” is the foremost consensus standard on electrical safety.

References to NFPA 70E are to the 2018 edition.

This standard was formed in 1976 when NFPA received a formal request from the Occupational Safety and Health Administration (OSHA). The request recognized the need for a consensus document to address electrical safe work practices that was fully compliant with the NEC. The committee on Electrical Safety Requirements for Employee Workplaces,

NFPA 70E, was needed for a number of reasons, including:

• The NEC is an installation standard. OSHA addresses employee safety in the workplace through both installation requirements and safe work practices

• Most NEC sections do not relate directly to task-based worker safety

• Safety related work and maintenance practices are generally not covered, or not adequately covered, in the NEC

• A national consensus standard on electrical safety for workers did not exist, but was needed — an easy to understand document that addresses worker electrical safety

The first 1978 edition of NFPA 70E was published in 1979. In most cases, OSHA regulations can be viewed as the “Shall” and NFPA

70E as the “How.” In most cases, OSHA requirements are written in performance style while NFPA 70E is always written as prescriptive text.

All OSHA requirements for electrical safe work practices are included in

NFPA 70E. Compliance with 70E translates directly to compliance with

OSHA, with NFPA 70E going beyond minimum OSHA compliance.

Section page

1

1

2

3

10

10

8

9

7

7

3

4

5

13

13

14

An employer must implement and document an overall electrical safety program that directs activity appropriate to the risk associated with electrical hazards. The electrical safety program, when a part of the employer’s overall occupational health and safety management system, is an important tool that’s helping change the industry’s safety culture.

NFPA 70E, through the requirements of 110.1, provides a detailed outline for this electrical safety plan covering:

• Inspection (110.1(B))

• Condition of maintenance (110.1(C))

• Awareness and self-discipline (110.1(D))

• Electrical safety program

• Principles (110.1(E))

•

Controls (110.1(F))

•

Procedures (110.1(G))

• Risk assessment procedure (110.1(H))

• Job Safety planning and job briefing (110.1(I))

• Incident investigations (110.1(J))

• Auditing (110.1(K))

Before work that involves exposure to electrical hazards is performed, a job safety plan must be completed. Before work is performed, all of the associated tasks with the job safety plan must be understood with an identification of shock and arc flash hazards. The tasks to be performed are reviewed to assess the risk involved with respect to the identified hazards. The next step is to implement risk control according to a hierarchy of risk control methods provided in section 110.1(H)(3). The hierarchy of risk control methods is comprised of six levels with the first being most effective and the last, least effective.

Hierarchy of Risk Control (110.1(H)(3) and information from Annex Table

F.3) covers:

1. Elimination — creation of an electrically safe work condition

2. Substitution — reduce energy by using 24 volt control circuits instead of line voltage at 120, 277, etc.

3. Engineering controls — guarding of exposed energized conductors or circuit parts, remote switching, racking, etc.

4. Awareness — warning signs, identifying potential hazards

5. Administrative controls — verification of proper maintenance and installation, alerting techniques, auditing requirements, and training requirements

6. PPE — appropriate PPE used for shock and arc flash hazards

The first three steps (Elimination, Substitution, and Engineering controls) are considered to be the most effective methods for reducing risk because they are typically applied at the source of possible injury, or damage to health, and are less likely to be affected by human error.

The last three steps (Awareness, Administrative controls and PPE) are considered to be the least effective methods for reducing risk because they are not applied at the source of possible injury, or damage to health, and are more likely to be affected by human error.

6


Section 6 —Electrical safe work practices

6-2

The goal of NFPA 70E is to eliminate the hazard. The OSHA general industry standard was modified to parallel 70E requirements. Section

1910.333(a)(1) requires live parts to be de-energized before an employee works on or near them except for two demonstrable reasons by the employer:

1. De-energizing introduces additional or increased hazards (such as cutting ventilation to a hazardous location) or

2. Infeasible due to equipment design or operational limitations (such as when voltage testing is required for diagnostics).

Similarly, NFPA 70E 130.2 requires energized electrical conductors and circuit parts to be put in an electrically safe work condition before an employee works within the Limited Approach Boundary (LAB) of those conductors or parts, or the employee interacts with equipment without exposed energized parts, but an increased likelihood of an arc flash hazard exist. Energized work is only permitted where it is justified in accordance with NFPA 70E 130.2(A). NFPA 70E, Article 100, defines an electrically safe work condition as: “A state in which an electrical conductor or circuit part has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to verify the absence of voltage, and if necessary, temporarily grounded for personnel protection.”

NFPA 70E 130.3(A)(2) requires work on electrical conductors or circuit parts not in an electrically safe work condition to be performed only by qualified persons. In some situations, an arc flash hazard may exist beyond the LAB.

NFPA 70E 130.2(A)(1) permits energized work if the employer can demonstrate energized work introduces additional or increased risk or per NFPA 130.2(A)(2) i.e., if the task to be performed is infeasible in a de-energized state due to equipment design or operational limitations.

Financial considerations are not an adequate reason to perform energized work. Not complying with these regulations and practices violates federal law in the form of OSHA regulations.

When energized work is justified per NFPA 70E 130.2(A)(1) or (A)(2), a shock hazard analysis, in accordance with NFPA 70E 130.4 and an arc flash risk assessment, which may include determining the arc flash

PPE if an arc flash hazard exists, in accordance with NFPA 70E 130.5, is required. When an energized electrical work permit is required, it must include items from NFPA 70E 130.2(B)(2). Some key items of the energized electrical work permit include:

• Determining the limited approach and restricted approach shock protection boundaries in accordance with NFPA 70E 130.4

• The arc flash boundary in accordance with NFPA 70E 130.5

• The necessary protective clothing and other Personal Protective

Equipment (PPE) in accordance with both NFPA 70E 130.4 and 130.5

Similarly, OSHA 1910.132(d)(2) requires the employer to verify that the required workplace hazard assessment has been performed through a written certification that states:

• The workplace evaluated

• The person certifying that the evaluation has been performed

• The date(s) of the hazard assessment

• Identifies the document as a certification of hazard assessment

Note : de-energized electrical parts are considered as energized until all steps of the lockout/tagout procedure are successfully completed per

OSHA 1910.333(b)(1). Similarly, all electrical conductors and circuit parts are not considered to be in an electrically safe work condition until all the requirements of Article 120 have been met per NFPA 70E.

Verifying the absence of voltage is a critical step in the implementing an electrically safe work condition. All required PPE must be worn by qualified persons when verifying the absence of voltage. This means that the equipment is considered energized until the verification is complete. Adequate PPE may also be required during load interruption and the visual inspection that verifies all disconnecting devices are open.

6.3 Shock hazard

NFPA 70E defines a shock hazard as “A source of possible injury or damage to health associated with current through the body caused by contact or approach to energized electrical conductors or circuit parts.”

At higher voltages, the worker doesn’t have to come in contact with energized conductors or circuit parts, close proximity could be enough.

Injury and damage to health resulting from shock is primarily dependent on:

• Current magnitude — amount of current flowing through the body, influenced both by the magnitude of the driving voltage and the total resistance of the path that current takes through your body

•

Current path — the path the current flows through the body with current flowing through the chest cavity is more likely to cause harm than current flowing through an extremity

• Time current flows — Length of time the body is in the circuit.

Current magnitude depends upon the circuit’s impedance and the impedance of those parts of the human body that come in contact with an energized conductor. The table below shows the human body’s resistance based upon various conditions.

Condition

Finger Touch

Hand holding wire

Finger-thumb grasp

Hand holding pliers

Palm touch

Hand around 1-1/2” pipe

Figure 6.3a

Resistance, (ohms)

40,000 to 1,000,000

15,000 to 50,000

10,000 to 30,000

5000 to 10,000

3000 to 8000

1000 to 3000

Wet

4000 to 15,000

3000 to 6000

2000 to 5000

1000 to 3000

1000 to 2000

500 to 1500

Two hands around

1-1/2” pipe

Hand immersed

Foot immersed

Human body, internal, excluding skin

500 to 1500

—

—

200 to 1000

250 to 750

200 to 500

100 to 300

This table was compiled from data developed by Kouwenhoven and Milnor.

The path current takes through the body is important as the chest cavity, which contains the heart and lungs, is the most severe path current can take (see Figure 6.3a).

Dry



The length of time that current flows through the body is dependent upon the OCPD’s clearing time or other protective device in the circuit.

Ground-fault circuit interrupters (GFCI) are designed to limit the time a person is exposed to ground fault currents. The UL category code for a GFCI circuit breaker is DKUY, the UL category code for a GFCI receptacle is KCXS, with both listed to the UL 943 standard titled

“Ground-Fault Circuit Interrupters.”

The physiological reaction to current passing through the body ranges from perception to muscular contractions, inability to let go, ventricular fibrillation, tissue burns to death. From research conducted in the

1960’s and 1970’s, the current levels in the table below provide the physiological reaction to current.

For AC fixed electrical equipment systems, the LAB is established at three feet, six inches (3 ft. 6 in.) for all equipment from 50 to 750 volts.

The RAB for AC fixed electrical equipment systems from 50 to 150 volts is to just avoid contact but for 151 to 750 volts is the restricted approach is one foot (1 ft.).

Arc flash boundary (AFB) dependent on fault level and duration, and must wear appropriate PPE

Shock approach boundaries (system voltage dependent)

Current level

0.5 — 3 mA

3 — 10 mA

10 — 40 mA

30 — 75 mA

100 — 200 mA

200 — 500 mA

1500 + mA

Physiological reaction

Tingling Sensation

Muscle contractions and pain

“Let-go” threshold

Respiratory paralysis

Ventricular fibrillation

Heart clamps tight

Tissue and organs start to burn

Restricted, qualified persons only.

Limited, qualified person (or unqualified only if accompanied by qualified person.

Figure 6.3.1.a

Figure 6.3.1.a is a graphic depiction of the two shock approach boundaries. For electrical hazard analysis and worker protection, it’s important to observe the shock approach boundaries together with the arc flash boundary.

6.3.2 Shock risk control solutions

6.3.1 Shock risk assessment

The shock risk assessment per NFPA 70E 130.4 requires the identifying the shock hazard, estimating the likelihood of injury or damage to health, including the potential severity and determining additional protective measures required, including PPE. A shock hazard analysis consists of the following:

• Determine if work is being conducted on exposed energized conductors or equipment greater than 50 volts. Where lower voltages exist, the system capacity and overcurrent protection must be evaluated to determine if an increased exposure exists.

• Identify and understand the shock hazards

• Estimate the likelihood of injury or damage to health and their potential severity

• Determine if additional protective measures are required, including the use of PPE

If additional protective measures are required, they must be selected and implemented according to the risk controls hierarchy in the following order: elimination, substitution, engineering controls, awareness, administrative controls and PPE.

When additional protective measures include PPE, the following must be determined:

• The voltage to which personnel will be exposed

• The boundary requirements

• The personal and other protective equipment required by this standard to protect against the shock hazard

Any equipment above 50 volts presents a potential shock hazard to the electrical worker when exposed energized conductors, parts, or components are present.

The LAB is the distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists. The RAB is a distance from an exposed energized electrical conductor or circuit part within which there is an increased likelihood of electric shock, due to electrical arc-over combined with inadvertent movement. The distances for both of these boundaries related to shock are found in Tables 130.4(D)(a) for

AC systems and 130.4(D)(b) for DC systems.

The risk assessment procedure must require implementing preventive and protective risk control methods.

The most effective methods to reduce risk include:

• Elimination

• Substitution

• Engineering controls

The least effective methods to reduce risk include:

• Awareness

• Administrative controls

• PPE

This section will focus on the most effective methods to reduce the risk of shock.

6.3.2.1 Finger-safe solutions

Finger-safe products help reduce the chance that shock can occur. When all components in a panel are finger-safe or covered, a worker has a much lower chance of coming in contact with energized parts.

Shown below is the finger-safe CUBEFuse (a very current-limiting protective device). Also shown are SAMI™ fuse covers, Safety J™ fuse holders for LPJ fuses, CH fuse holders, new modular fuse blocks with optional, finger-safe covers, a variety of Bussmann series fused disconnect switches (with fuse and terminal shrouds). All these devices can reduce the chance that a worker, tool or other conductive item will come in contact with a live part.

6

CUBEFuse

SAMI fuse covers

Safety J fuse holders

CH fuse holders

Fuse blocks with covers

Fused disconnects



6.3.2.2 Barriers

Equipment that employ barriers designed to prevent the electrical worker from coming in contact with energized conductors or parts decrease the likelihood of contact. Barriers come in many different forms; from those that engulf lugs and exposed conductors to those that “barrier off” portions of the enclosure. The risk analysis must consider if these barriers must be removed in order to conduct justified energized work.

Electrical distribution equipment, such as panelboards, employ deadfront barriers that permit the portions of the equipment to which access is needed (for example switch handles) to protrude through the deadfront barrier while keeping energized parts unexposed. The QSCP panelboard below is an example of such a fused solution; it enables the

UL 98 switch to be turned ON and OFF and fuse replacement without exposing any energized parts.

The 2017 NEC introduced a new requirement for service entrance panelboards to have a lineside barrier to protect workers from exposed, energized lugs and conductors from where the lineside conductors are landed on the main switch. (NEC Section 408.3(A)(2)) This requirement does not apply to those service entrance panels that are leveraging the

6-disconnect rule as permitted in 408.36 as part of the exceptions.

Figure 6.3.2.2.a


Figure 6.3.2.2.b

6.3.3 Ground-fault circuit-interrupters

GFCI protection is the last line of defense when it comes to shock protection. A ground fault device operates off the basic principal of differential current; that current that flows out to the load through the line conductor has to come back from the load over the neutral conductor. The conductors involved are the expected current paths.

A ground fault device employs two key components that work together to determine if ground fault current is flowing. The system is comprised of sensing equipment and relaying equipment. The sensing equipment is in the form of a current transformer that can be placed at various locations within the circuit.

Figure 6.3.3.a


6.4 Arc flash hazard

NFPA 70E defines an arc flash hazard as “a source of possible injury or damage to health associated with the release of energy caused by an electric arc.” If an arcing fault occurs, the tremendous energy released in a fraction of a second can result in tremendous heat, clothing ignition, severe burns or death.

The likelihood of an arc flash is increased in some cases when personnel interact with energized electrical equipment. Table 130.5(C) provides examples of tasks that increase the likelihood of an arc flash incident. When conductors or circuit parts are exposed, and when personnel are interacting with the equipment, taking measurements or other similar justified energized work, an arc flash is possible.

An arc flash incident is not likely to occur under normal operating conditions when enclosed energized equipment has been properly installed and maintained.

Where work is performed inside the LAB and arc flash boundary (AFB), shock and arc flash hazards exist.

The arc temperature can reach approximately 35,000°F, or about four times as hot as the sun’s surface. These temperatures can cause serious or fatal burns and/or ignite flammable clothing.

The arc flash severity depends upon the amount of incident energy available at the location where work will be performed. Incident energy, per NFPA 70E, “is the amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is typically expressed in calories per square centimeter (cal/cm 2 ).” The incident energy amount at any given location depends upon the amount of arcing fault current magnitude and the time that the arcing current is permitted to flow. Section 4.3 is a resource to on how to calculate arcing current and incident energy.

To demonstrate the relationship between arcing current and time, with regard to incident energy, testing was conducted on the same electrical circuit set-up with different OCPDs. An ad hoc electrical safety working group, within the IEEE Petroleum and Chemical Industry Committee, conducted these tests to investigate arc fault hazards. These and other tests are detailed in Staged Tests Increase Awareness of Arc-Fault

Hazards in Electrical Equipment, IEEE Petroleum and Chemical Industry

Conference Record, September, 1997, pp. 313-322. One finding in this

IEEE paper is that current-limiting OCPDs reduce damage and arc-fault energy (provided the fault current is within the current-limiting range).

To better assess the benefit of limiting the current of an arcing fault, it is important to note some key human injury thresholds.

Injury

Just curable burn

Incurable burn

Eardrum rupture

Lung damage

Threshold

80°C / 175°F (0.1 sec)

96°C / 205°F (0.1 sec)

720 lbs/ft 2

1728 — 2160 lbs/ft 2

OSHA required ear protection 85 db (for sustained time period)

Note: a 3 dB increase is equivalent to doubling the sound level

Figure 6.4.a

Figure 6.4.a is a model of an arc fault and the physical consequences that can occur. The unique aspect of an arcing fault involves the fault current flowing through the air between conductors, or the air between a conductor(s) and a grounded part. The arc has an associated arc voltage because there is arc impedance.

The concentration of fault current and arc voltage at one point results in releasing tremendous energy that takes several forms. The high arc temperature vaporizes the conductors and results in an explosive change in state (vaporized, copper expands to 67,000 times its original volume, e.g., one cubic inch of copper expands to 38.8 cubic feet !).

With the expansive vaporizing of conductive metal, a line-to-line or lineto-ground arcing fault can escalate into a three-phase arcing fault in less than a thousandth of a second. This event can be so rapid that the human system can’t react quickly enough to take corrective measures. If an arcing fault occurs while a worker is in close proximity, the worker’s survivability depends mostly upon (1) system design aspects (such as

OCPD characteristics) and (2) precautions the worker has taken prior to the event (such as wearing PPE that’s appropriate for the hazard).

Test results were recorded by sensors on mannequins for comparison to these parameters.

All three tests shown in Figures 6.4.b through 6.4.d were conducted on the same electrical circuit that included:

• Available bolted three-phase, fault current of 22,600 symmetrical RMS amps

• Voltage: 480 V, three-phase

• An arcing fault was initiated in a Size 1 combination motor controller enclosure with the door open, as if an electrician were working on a “live unit” or before it was placed in an electrically safe work condition.

Test 4 and Test 3 were identical except for the OCPD protecting the circuit. In Test 4, a 640 A OCPD protecting the circuit interrupts the fault current in 6 cycles. In Test 3, a KRP-C-601SP, 601 amp, current-limiting

Class L fuses protecting the circuit cleared the fault current in less than

1/2 cycle and limited the current. The arcing fault was initiated on the lineside of the motor branch circuit device in both Test 4 and Test 3, making the fault on the feeder circuit but within the controller enclosure.

In Test 1, the arcing fault was initiated on the loadside of the branch circuit overcurrent protective devices, LPS-RK-30SP, 30 A current-limiting

Class RK1 fuses. These fuses limited the fault current to a much lower value and cleared the fault in approximately 1/4 cycle or less.

Figures 6.4.b through 6.4.d are results recorded from the various sensors on the mannequin closest to the arcing fault. T1 and T2 recorded the temperature on the bare hand and neck respectively. The hand with T1 sensor was very close to the arcing fault. T3 recorded the temperature on the chest under the shirt. P1 recorded the pressure on the chest. And the sound level was measured at the ear. Some results

“pegged the meter” meaning the actual values were unable to be recorded because the actual level exceeded the sensor/recorder setting range. These values are shown as “>” indicating that the actual value is greater, but it’s unknown how much greater than the actual value attained.

6



Figure 6.4.b Test 4

Figure 6.4.b are Test 4 results, the staged test protected by an OCPD that interrupted the fault current in six cycles (0.1 second). This is not a current-limiting OCPD. It should be noted that there was an additional, unexpected fault in the wireway and the blast caused the cover to hit the mannequin’s head. An analysis shows this test’s incident energy was 5.8 cal/cm 2 at an 18 inch working distance, with a resulting 47 inch arc flash boundary per IEEE 1584-2002 (basic equations).

Figure 6.4.c Test 3

Figure 6.4.c are Test 3 results, the staged test protected by KRP-C-

601SP Low-Peak™ current-limiting Class L fuses . These fuses operated in their current-limiting range clearing the fault in less than a 1/2 cycle

(0.008 second). An analysis showed this test’s calculated incident energy was 1.58 cal/cm 2 at an 18 inch working distance with a 21 inch arc flash boundary per IEEE 1584-2002 (simplified fuse equations).



Conclusions that can be drawn from these tests include:

(1) Arcing faults can release tremendous amounts of energy in many forms in a very short time period.

(2) The OCPD’s characteristics have a significant impact on the results.

• A 601 amp, current-limiting OCPD reduced (limited) the current peak and clearing time. The clearing time was 1/2 cycle or less.

• The measured Test 1 results are significantly less than those in

Test 4, and even those in Test 3. The reason is Test 1 utilized a much smaller (30 amp), current-limiting OCPD.

• Test 3 and Test 1 both show the benefits of using current-limiting

OCPDs. Test 1 results demonstrate that the greater the currentlimitation, the more the arcing fault energy may be reduced.

• Both Test 3 and Test 1 utilized very current-limiting fuses, but the lower amp rated fuses limit the fault current more than the larger amp rated fuses (Note, the fault current must be in the OCPD’s current-limiting range to receive the benefit of the lower current let-through). See the oscillographs below depicting Test 4, Test 3 and Test 1.

Test 4: Non-current limiting OCPD (6 cycles)

Test 3: current limiting

KRP-C-601SP

Reduced fault current via current limitation

Test 1: current limiting

LPS-RK-30SP

Figure 6.4.d Test 1

Figure 6.4.d are Test 1 results, The staged test protected by LPS-RK-

30SP, Low-Peak current-limiting Class RK1 fuses ). These fuses were in their current-limiting range and cleared the fault in approximately 1/4 cycle (0.004 second). An analysis showed this test’s calculated incident energy was less than 0.25 cal/cm 2 at an 18 inch working distance with a less than 6 inch arc flash boundary per IEEE 1584-2002 (simplified fuse equations).

6.4.1 Arc flash risk assessment

In all justified energized work, an arc flash risk assessment is an important part. This includes the process for determining if equipment has been placed into an electrically safe work condition. As per NFPA

70E, the following aspects of an arc flash risk assessment must be established:

• Estimate of likelihood and severity (130.5(B))

• Determine additional protective measures (130.5(C))

• Documentation of the risk assessment (130.5(D))

• Establish an arc flash boundary (130.5(E))

• Determine arc flash PPE (130.5(F))

• Incident energy analysis method (130.5(G))

• Equipment labeling (130.5(H))

6.4.2 Incident energy analysis method

One method for determining the proper PPE to wear, as well as the arc flash boundary, is the incident energy analysis method of 130.5(G).

There are several incident energy calculation methods within IEEE

1584-2002 commonly used for 15 kV or less. The IEEE 1584-2002 basic equation method has these steps:

• Complete a fault current study that calculates the available fault currents throughout the power distribution system (see Section 4.1)

• Calculate arcing currents at each piece of electrical distribution equipment based upon IEEE 1584-2002 (see Section 4.3)

6



• Document a selective coordination study establishing OCPD trip curves. For existing systems, visually verify the settings of all OCPDs in the system. For new systems, the OCPD settings or proper selection must be verified to have been implemented in the final design (see Section 4.2).

• For each location within the power distribution system where justified energized work may be performed, the incident energy must be calculated for the task working distance (based upon IEEE 1584-2002 equations) along with the available arcing currents and their respective clearing times for the protecting overcurrent protective device (see section 4.3)

• The arc flash boundary (AFB) must be determined based upon the distance where the incident energy is limited to 1.2 cal/cm 2, (130.5(E))

• Repeat calculating the incident energy and AFB for 85% of the available arcing current. In the next step, use the higher incident energy resulting from the 100% and 85% arcing fault current calculations.

• The correct arc-rated clothing/PPE is selected from Table 130.5(G)

NFPA 70E Annex D provides information on calculating both the arc flash boundary and incident energy.

IEEE 1584-2002 is the foremost industry consensus standard for calculations on 15 kV or less systems for faults in a box or in the open.

This guide has the basic calculation methods; simplified fuse method, and simplified circuit breaker method.

It is important to note that current-limiting OCPDs (when the fault current is in their current-limiting range) can reduce the required AFB and the required arc-rated PPE as compared to non-current limiting

OCPDs. In addition, once the arcing current is in the current-limiting range of a specific fuse type/amp rating, the incident energy is very low (typically 0.25 cal/cm 2 ) and remains at this low level even for higher available arcing fault currents.

There are various resources and tools available in the industry to aid in performing the IEEE 1584-2002 calculations. Section 4.3 includes a table method derived using the IEEE 1584-2002 simplified methods for fuses.

6.4.3 Arc flash PPE category method

The second method offered in 130.5(F) is the arc flash PPE category method in accordance with 130.7(C)(15) which is titled “Arc Flash PPE

Category Method.” 130.7(C)(15) includes references to two tables:

Table 130.7(C)(15)(a), Arc-Flash PPE Categories for Alternating Current

(AC) Systems

Table 130.7(C)(15)(b), Arc-Flash PPE Categories for Direct Current (DC)

Systems

These tables offer the PPE category as well as arc-flash boundary based upon the equipment and various parameters. The arc flash PPE category method can be used for many situations provided all table parameters are met. The conditions of use (as to when Tables 130.7(C)(15)(a) and

130.7(C)(15)(b) are permitted to be used) are a part of Section 130.7(C)

(15)(a) and 130.7(C)(15)(b). If all the use conditions are not satisfied, the tables cannot be used and an incident energy method must be used.

Table conditions of use

Using the NFPA 70E tables has its limitations. The following conditions that must be satisfied in order to use this table to perform the PPE category method:

• Limited to AC systems for Table 130.7(C)(15)(a) and DC systems for

Table 130.7(C)(15)(b)

• Limited to equipment types and voltage ratings listed in table

• Parameters under the specific equipment type being evaluated

•

Available fault current at equipment installation cannot exceed the parameter maximum value in the table

•

The clearing time for the OCPD type at the given value of maximum available, bolted short-circuit current in the table cannot exceed the parameter maximum fault clearing time value the in table

•

The working distance cannot be less than the parameter value in the table

If all conditions are met, the PPE category and arc flash boundary can be used in conjunction with Table 130.7(C)(15)(c) to select PPE. The PPE categories are 1, 2, 3, and 4.

Table 130.7(C)(15)(a) has notes at the end that are important to its application.

The first note acknowledges the fast acting nature of current-limiting fuses and current-limiting circuit breakers. For equipment rated 600 volts and less that are protected by an upstream current-limiting OCPD sized at 200 A or less, the arc flash PPE category is permitted to be reduced by one realizing that the lowest PPE category is 1.

An informational note provides typical OCPD type fault clearing times.

They are:

Device

Current-limiting fuses when the fault current is within the current-limiting range

Molded case circuit breakers rated less than 1000 volts with an instantaneous integral trip

Insulated case circuit breakers rated less than 1000 volts with an instantaneous integral trip or relay operated trip.

Relay operated circuit breakers rated 1 kV to 35 kV when the relay operates in the instantaneous range (i.e., “no intentional delay”)

Low-voltage power and insulated case circuit breakers with a short time fault clearing delay for motor inrush

Voltage power and insulated case circuit breakers with a short time fault clearing delay without instantaneous trip

Cycles

0.5

1.5

3.0

5.0

20

30

In addition, NFPA 70E 130.5(H) requires that the calculations and data used for the information on the equipment label be reviewed for accuracy at intervals not to exceed 5 years. Where this review results in a change that renders the equipment label inaccurate, the label must be updated with the correct information.



6.4.4 Equipment labeling

Labeling is an important method for raising the awareness of hazards.

Electrical distribution equipment labeling requirements can be found in the NEC as well as NFPA 70E.

NEC 110.16, “Arc-Flash Hazard Warning,” includes labeling requirements for two important areas. 110.16(A) includes the general requirements that specify a label for electrical equipment, such as switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures, and motor control centers, in other than dwelling units.

This label’s application is limited to equipment that’s likely to require examination, adjustment, servicing, or maintenance while energized.

The required label can be field or factory installed because it’s very general and designed to warn qualified persons of potential electric arc flash hazards. The marking must also meet the requirements found in section 110.21(B), and must be located on the equipment such that it is clearly visible to the electrical worker. Figure 6.4.4.a is an example of this label.

The nominal system voltage is obtained from the equipment nameplate information as shown in Figure 6.4.4.b. The available fault current is that which is at the service OCPD. The clearing time required in (3) is the clearing time of the available fault current required to be shown as part of (2). This exercise must leverage the OCPD’s TCC curve and the current value to determine the clearing time.

This label information correlates with the PPE category method described in 6.4.3 and is enough to establish a PPE category when performing energized work.

The exception to 110.16(B) permits the application of a label as outlined in NFPA 70E. It can be marked with any of the approved methods in

NFPA 70E section 130.5(H). The required information includes:

1. Nominal system voltage

2. Arc flash boundary

3. At least one of following:

• Available incident energy and the corresponding working distance, or the arc flash PPE category in Table 130.7(C)(15)(a) or Table

130.7(C)(15)(b) for the equipment, but not both

• Minimum clothing arc rating

• Site-specific PPE level

Additional information is often included on the label, such as the values determined by the shock approach boundaries.

Figure 6.4.4.c is an example of a label required by NFPA 70E 130.5(H).

Figure 6.4.4.a Label complying with NEC 110.16(A)

NEC section 110.16(B) now requires service equipment rated at 1200 amps or more to be labeled in a manner that provides information help determination the arc rated PPE. This requirement applies to:

• That equipment in other than dwelling units

A dwelling unit is defined in the NEC Article 100 as “a single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation.” 110.16(B) requirements would apply to any structure other than a single family dwelling unit, as it’s the only structure that is specified as a dwelling unit. Most single family dwelling units would not meet the next requirement.

• Service equipment rated 1200 amps or larger

This stipulation pertains to the equipment and not the OCPDs within.

The information to determine if the service equipment is impacted by this requirement is found on the manufacturer’s equipment label / nameplate as shown in Figure 7.3.4.b.

Figure 6.4.4.c

The last paragraph of 130.5(H) requires the calculation method and data to support this information shall be documented. For instance, in both the incident energy method and arc flash PPE category method, the available fault current must be determined in the analysis process. The fault current calculating method and the results must be documented and retained. This information may be required for a future OSHA inspection/investigation. Also, if future system changes occur, this documentation will assist in determining whether the arc flash hazard results changed.

6

Figure 6.4.4.b

As with 110.16(A), the label must meet the requirements of 110.21(B).

This label must also contain the following:

(1) Nominal system voltage

(2) Available fault current at the service OCPDs

(3) The service equipment OCPDs’ clearing time based on the available fault current at the service equipment

(4) The date the label was applied



6.4.5 Personal Protective Equipment (PPE)

Employees must wear appropriate protective equipment and be trained in its use for the possible electrical hazards they may face. PPE is the last line of defense for the electrical worker. PPE examples could include a hard hat, face shield, neck protection, hearing protection, arc rated (AR) clothing, arc flash suit, insulated rubber gloves with leather protectors, and insulated leather footwear. Selecting the required arcrated PPE will be driven by either the PPE category method or the incident energy analysis method previously covered in 6.4.2 and 6.4.3.

As stated previously, the common working distance used for most low voltage incident energy measurement research and testing is 18 inches from the arcing fault source. The closer the worker is to the arcing fault, the higher the incident energy and arc blast energy. This means that when the arc flash hazard analysis results in relatively high incident energy at 18 inches from the arcing fault source, the incident energy and arc blast energy at the arcing fault’s point can be considerably greater. Put another way, even if the body has sufficient PPE for an 18 inch working distance, severe injury can still result for any part of the body closer than 18 inches to the arc source. The incident energy is approximately inversely proportional to the distance squared. That is, if the incident energy is 8 cal/cm 2 at 18 inches working distance, the incident energy at 9 inches Is approximately 32 cal/cm 2 and at 4.5 inches approximately 128 cal/cm 2 .

6.4.6 Arc flash risk control solutions

Preventative and protective risk control methods should be implemented to help reduce electrical worker risk. Section 110.1(H)(3) of NFPA 70E provides a control method hierarchy with elimination, substitution and engineering controls at the top of the list. The following solutions can be used to help reduce the risk for arc flash hazards.

6.4.6.1 Local disconnects

Adding a local disconnect to end use equipment where most maintenance and troubleshooting will occur provides the electrical worker multiple benefits. Having a disconnect close to the load to be serviced can increase the likelihood that work will be performed in an electrically safe state. Figure 6.4.6.1.a illustrates HVAC equipment with local disconnects.

When a local disconnect is provided for specific equipment, the ability to apply current-limiting fuses is in place to provide that equipment the most effective means possible to reduce incident energy. Fuses working in their current-limiting range can be sized closer to the load and provide an effective means to ensure any justified energized work is done so with the lowest possible incident energy.

The installing horsepower-rated disconnects (with a permanently installed lockout/tagout provision) within sight and within 50 feet of every motor or driven machine can help increase electrical safety. Doing this fosters safer work practices and can be used for an emergency disconnect if there is an incident as well as promoting electrically safe work habits.

When selecting fusible OCPDs, the fuse holder or switch type is very important for determining proper application. The most economical solution is often a standard UL 4248 Listed fuse holder, but it does not offer a disconnecting means for the fuses, which is required, in most cases, per NEC 240.40. A disconnecting means can be ahead of the fuse holder or a fused UL 98 disconnect or UL 508 disconnect can be selected. The UL 98 fused disconnect offers the widest application range, since it can be used in all locations whether as the motor branch circuit disconnect, controller disconnect, or within sight motor disconnect. UL 508 disconnects can only be used in motor branch circuits on the motor branch circuit OCPD’s loadside.

The Compact Circuit Protector (CCP) is the smallest, most economical

UL 98 Listed fusible disconnect switch available. There are two CCP types: one for Class CC and UL supplemental fuses that’s available as a

30 A disconnect, accepting fuses up to 30 A, the other for the UL Class

CF CUBEFuse that’s available in ratings up to 400 A.

Figure 6.4.6.1.a

6.4.6.2 Lockout/tagout

The use of lockout/tagout methods is vital for establishing an electrically safe work condition as mandated in Article 120 of NFPA 70E. The general requirement in this Article is that each employer establish, document, and implement a lockout/tagout program.

The solutions shown in Figures 6.4.6.2.a through 6.4.6.2.c include those that would meet the NEC requirements and have permanent provisions to receive a lock to meet the work practices required by 70E.

Figure 6.4.6.2.a

Figure 6.4.6.2.b

Figure 6.4.6.2.c



6.4.6.3 Maintenance

Maintenance plays an important role in any electrical distribution system’s life. NFPA 70E uses the term “condition of maintenance” throughout the document and it refers to the electrical equipment’s state with regard to the manufacturers’ instructions and recommendations, and applicable industry codes, standards, and recommended practices. The proper operation of OCPDs, switches, transfer switches, GFCIs and other safety related solutions depends upon a rigorous and vigilant maintenance program.

The condition of maintenance is critical for OCPDs being relied upon to mitigate incident energy if an arcing fault incident occurs when a worker is working on equipment not in an electrically safe work condition. If the OCPD does not operate as intended, or does not operate when an incident occurs, the actual incident energy impressed on the worker may be much greater. NFPA 70E 205.4 requires OCPDs to be maintained and the inspections, tests and maintenance to be documented.

If the electrical system is an existing fusible system, consideration should be given to upgrading the installed fuse base to Bussmann series Low-Peak fuses. If the installed fuses are not the most current-limiting type fuses, upgrading to those in the Low-Peak family may reduce the hazards associated with arc flash. Visit Eaton.com/ bussmannseries to review the Low-Peak Fuse Upgrade Program.

6.4.6.4 Ratings

The proper electrical equipment and component application within their ratings is very important for safety. OCPDs must have interrupting ratings and electrical distribution equipment must have the short-circuit current ratings equal to or greater than the available fault current at their lineside terminals. An OCPD subjected to interrupting a fault current beyond its interrupting rating can violently rupture. Electrical distribution equipment applied beyond its SCCR can also result in an unintended rapid disassembly. Consideration for interrupting ratings and

SCCR should be for the system’s life. System changes, such as when transformers are replaced or systems are upgraded and the available short-circuit currents increase, must initiate equipment evaluations.

Modern fuses have interrupting ratings of 200 kA and 300 kA, which virtually eliminates this hazard contributor.

6.4.6.5 Current limiting OCPDs

Current limitation can play an important role in reducing incident energy.

Current-limiting fuses are the industry’s fastest devices when the arcing current is in their current-limiting region. Current limitation is discussed in depth as part of Sections 3.1 and 3.2 of this handbook.

Systems should leverage the most current-limiting OCPDs possible.

There are many OCPD choices in the market and many are not marked as “current-limiting” and can’t be considered current-limiting. For those that are marked current-limiting, there are different current-limitation degrees to consider. The brand to use for 600 V and less, electrical distribution applications and general equipment circuit protection are the

Bussmann series Low-Peak fuses. The Low-Peak fuse family is the most current-limiting fuse family type for general and motor circuit protection.

If the actual, maximum full load current on an existing main, feeder or branch circuit is significantly below its designed circuit amp rating, replace existing fuses with lower amp rated Low-Peak fuses, e.g., an industrial found that many of the 800 amp feeders to their MCCs were lightly loaded; for better arc flash protection they installed 400 and 600 amp current-limiting fuses and switches in the feeders.

Figure 6.4.6.5.a

6.4.6.6 Arcflash Reduction Maintenance System (ARMS)

The ARMS is a device that places the circuit into a maintenance mode to reduce the system’s incident energy for the time period that work is being performed. This arc reduction method is the fastest solution in the circuit breaker offering for incident energy reduction.

Figure 6.4.6.6.a

6.4.6.7 Zone Selective Interlocking (ZSI)

Zone selective interlocking provides incident energy reduction when the arcing fault occurs within the protection zone as shown in Figure

6.4.6.7.a. ZSI equipped circuit breakers work in pairs consisting of an upstream and a downstream circuit breaker. These devices are connected together via a control circuit to provide the upstream device with the ability to know if the downstream device sees a detected fault.

This connection is usually referred to as a restraining signal, as when the downstream device sees a fault, the restraining signal sent to the upstream device prevents it from tripping instantaneously and instructs it to trip per its programmed settings for selective coordination. The result is the downstream device will clear the fault per its programmed trip settings for selective coordination.

Should the fault occur between the two devices, the upstream device would not receive the restraining signal and would trip without an intentional time-delay. Circuit breakers equipped with this feature reduce the damage at the fault’s point if it occurs at a location within the zone of protection.

It is important to understand the zone of protection provided as a fault on the upstream device’s lineside or the downstream device’s loadside would not be met with incident energy reduction for the electrical worker.

6



Zone 1

Zone 2

Main breaker “A”

Zone 3

Figure 6.4.6.7.a

Digitrip 3000 “A”

Feeder breaker “B”

Digitrip 3000 “B”

Interlocking wire

Downstream breaker “C”

Digitrip 3000 “C”

6.4.6.9 Remote operation

Placing electrical workers outside the arc flash boundary (while performing such tasks as racking in or out a circuit breaker) places them at a safe distance from the equipment. There are many methods currently on the market that make remote operation a reality for the electrical installation. These include but are not limited to:

• Communications: OCPDs and other solutions exist that have the ability to communicate electronically with computers. These communication methods make it possible to open switches or circuit breakers remotely from a computer in an office or from a touch screen display outside the electrical room.

• Remote operation accessories: Solutions exist that can be placed on electrical equipment to mechanically operate handles via a remote location through a long umbilical cord. This method places the worker outside the arc flash boundary while circuit operations are made.

LOAD

Figure 6.4.6.9.a

6.4.6.10 Barriers

Doing more with the doors closed is a significant trend in equipment development. There are more solutions on the market than ever before that enable racking electrical components in and out with the protective doors closed. Switchgear, switchboards, motor control centers and even circuit breaker panelboards are offered with this functionality.

Figure 6.4.6.7.b

6.4.6.8 Active arc flash mitigation systems

Active arc flash mitigation systems employ technologies beyond just looking at current flow to detect when an arcing event has occurred within a specific piece of electrical equipment. The arc detection process will typically involve:

• An arc is detected by light sensors and lineside CTs

• The arc flash relay sends a signal to trigger an arc quenching device

• The arc quenching device will create a lower-impedance arc event across all 3 phases which are contained inside a sacrificial pressure vessel

• The arcing fault is quickly extinguished inside the switchgear

This process occurs in approximately 53 milliseconds reducing the time and incident energy.

Again, it’s important to understand the limitations to these solution types as they only work on the electrical equipment within which they are installed. And it is vital to note some technologies do not work when the enclosure doors are open and justified energized work is performed.

Figure 6.4.6.10.a

Figure 6.4.6.10.b



6.5 Maintenance

Electrical distribution system maintenance can take many forms, all of which are important for system safety over its lifetime. Electrical equipment maintenance should be performed based on manufacturer requirements. Maintenance must also be performed to ensure all equipment is working together correctly as a system.

The OCPD reliability can directly impact arc flash hazards. Poorly maintained OCPDs may result in higher arc flash hazards. Devices, such as GFCIs, if not tested per manufacturers’ instructions may not provide the required protection when needed. Smoke detectors and similar equipment, too, have life limitations beyond which they must be replaced. Manufacturer instructions are important to keep on file and read with regard to recommended maintenance practices.

NFPA 70E 130.5(B) reads in part:

The estimate of the likelihood of occurrence of injury or damage to health and the potential severity of injury or damage to health must take into consideration:

1. The design of the electrical equipment, including its overcurrent protective device and its operating time

2. The electrical equipment operating condition and condition of maintenance.

130.5 has two Informational Notes (IN) concerning the importance of overcurrent protective device maintenance:

IN No. 1 : Improper or inadequate maintenance can result in increased opening time of the overcurrent protective device, thus increasing the incident energy.

IN No. 2 : For additional direction for performing maintenance on overcurrent protective devices see Chapter 2, Safety-Related

Maintenance Requirements.

The 130.5 requirement to take into consideration the maintenance condition of OCPDs is very relevant to arc flash hazards. Their reliability can directly impact the incident energy. Poorly maintained OCPDs may take longer to clear or not clear at all, resulting in higher arc flash incident energies.

Panel

800 A OCPD

6 cycle opening for arcing current

22.6 kA Sym available fault current

480 V, 3Ø

Incident energy analysis and AFB analysis for panel:

• Incident energy 5.8

cal/cm2 @ 18”

• Arc flash boundary 47 ”

Figure 6.5.a

Figure 6.5.a illustrates an arc flash hazard analysis calculation assuming the OCPD has been maintained and operates as specified by the manufacturer’s performance data. In Figure 6.5.b illustrates that the actual arc flash event can be significantly higher if the lack of maintenance causes the OCPD clearing time to be greater than its specified performance data. Calculations are per IEEE 1584-2002.

800 A OCPD

30 cycle opening for arcing current

22.6 kA Sym available fault current

480 V, 3Ø

Panel

Actual incident energy and AFB due to poor maintenance:

• Incident energy 29 cal/cm2 @ 18”

• Arc flash boundary 125 ”

Figure 6.5.b

6.5.1 OCPD maintenance

If lack of maintenance causes the OCPD to clear in 30 cycles, the actual arc flash hazard will be much greater than the calculated arc flash hazard.

The OCPD is a critical component in the power system that’s necessary to reduce incident energy for the electrical worker. NFPA 70E includes references to OCPD maintenance that recognizes their significance in the system.

The following are the NFPA 70E references for OCPD maintenance:

205.4: Requires OCPDs to be maintained per manufacturers’ instructions or industry consensus standards. “Maintenance, tests, and inspections shall be documented.”

210.5: Requires OCPDs to be maintained to safely withstand or be able to interrupt the available fault current. Informational Note makes mention that improper or lack of maintenance can increase arc flash incident energy.

225.1: Requires fuse body and fuse mounting means to be maintained. Mountings for current-limiting fuses cannot be altered to allow for insertion of non-current-limiting fuses.

225.2: Requires molded cases circuit breaker cases and handles to be maintained properly.

225.3: Requires circuit breakers that interrupt faults approaching their interrupting rating be inspected and tested in accordance with the manufacturer’s instructions.

OSHA 1910.334(b)(2) Use of Equipment

Re-closing circuits after protective device operation — after a circuit is de-energized by a circuit protective device— the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual re-closing of circuit breakers or reenergizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the circuit design and the

OCPDs involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.

A key phrase in the regulation is “circuit can be safely energized.” When complying with NFPA 70E 225.3 it’s impractical, if not impossible, to determine the fault level interrupted by a circuit breaker.

6



Sources for guidance in setting up maintenance programs, determining the maintenance frequency and providing prescriptive procedures include:

1. Equipment manufacturer’s maintenance manuals

2. NFPA 70B Recommended Practice for Electrical Equipment

Maintenance

3. ANSI/NETA MTS-2011, Standard for Maintenance Testing

Specifications for Electrical Distribution Equipment and Systems. This standard includes guidelines for the required maintenance frequency for electrical system power equipment in Appendix B, Frequency of

Maintenance Test, as well as prescriptive inspections and tests in the standard.

Maintenance calibration decal systems (Figure 6.5.1.a) can assist in evaluating the OCPD maintenance condition required in NFPA 70E

130.5.

TESTED

Project No.:__________________

Test Date:__________________

Figure 6.5.1.a

The internal parts of current-limiting fuses do not require maintenance for arc flash protection considerations. However, it’s important to periodically check fuse bodies and mountings.

In addition, periodically check conductor terminations for signs of overheating, poor connections and/or insufficient conductor ampacity.

Infrared thermographic scans are one method for monitoring these conditions. Records on maintenance tests and conditions should be retained and trended.

The NEMA document pertinent to the subject of maintenance include

“Guidelines for Inspection and Preventive Maintenance of Molded-Case

Circuit Breakers Used in Commercial and Industrial Applications” is the industry reference for molded case circuit breaker maintenance.

6.5.2 Risk control solutions

A maintenance program that is formalized and comprehensive can go a long way towards reducing risk for those electrical professionals working on the power distribution system.

Using OCPDs that are reliable and do not require maintenance to ensure performance per the original specifications is a wise choice.

Modern fuses are reliable and retain their ability to react quickly under fault conditions. When a fuse is replaced, a new, factory-calibrated replacement is put back into service and the circuit retains reliable protection with performance equal to the original specifications. When an arc fault or overcurrent occurs, the OCPD must be able to operate as intended.


7 Equipment application/protection

Contents Section page

Introduction, fuse sizing for building electrical systems up to 600 V 1

7.1 Appliances 4

7.2 Ballasts

7.3 Batteries/battery charging

4

5

7.4 Busway

7.5 Capacitors

7.6 Circuit breakers

7.7 Conductors

7.8 Electric heat

7.9 Elevators

7.10 Generator protection

7.11 Ground fault protection of equipment

7.12 Industrial control panels

25

44

44

48

48

55

5

7

7

7.13 Industrial machinery

7.14 Motor/motor circuit protection

7.15 Panelboards and other fusible equipment

7.16 Solenoids

7.17 Switchboards

7.18 Transfer switches

7.19 Transformers

7.20 Uninterruptible Power Supply (UPS)

7.21 Variable frequency drive and power electronic device protection

7.22 Welders

144

147

152

158

76

76

139

143

159

162

Introduction

All electrical distribution system components work together to deliver or utilize electric power, and they must be applied within their listing and protected according to the minimum National Electrical Code (NEC) requirements.

The fundamental principles behind proper equipment protection lies in the magnitude and time duration that current flows in a power distribution system. The overcurrent protective device’s (OCPD) role is twofold:

1. Control the amount of time that current is permitted to flow when its magnitude is outside the equipment’s safe operating limits.

2. Limit the magnitude of high-level, destructive currents that can inflict damage to system components due to magnetic forces and thermal energy.

The fuse’s current-limiting nature is a powerful feature that can limit the amount of damage caused to the distribution system when faults occur.

This section provides guidance to ensure maximum electrical system protection.

This section pertains to the two roles OCPDs perform when called upon to protect specific applications.

This section starts with covering fuse sizing and conductor ampacity for building electrical systems that are up to 600 V. This provides a context for the applications covered in this section as they are almost always connected to a building’s electrical system for their power.


7


Section 7 — Equipment application/protection

7-2

Fuse sizing for 600 V building electrical systems

General guidelines are given for selecting fuse amp ratings for most circuits. For specific applications warranting other fuse sizing, the load characteristics and appropriate NEC sections should be considered. The selections shown here are not, in all cases, the maximum or minimum amp ratings permitted by the NEC. Demand factors as permitted by the

NEC are not included. Study the pertinent NEC sections noted by “()” and reference pertinent footnotes.

1

Recommended fuses:

• Low-Peak KRP-C Class L, time-delay , 601 to 6000 A

• Low-Peak 250 V LPN-RK/600 V LPS-RK Class RK1,

dual element, time-delay, up to 600 A

• Fusetron 250 V FRN-R/600 V FRS-R Class RK5,

dual element, time-delay, up to 600 A

10 8

Fuse

> 600 A

Motor starter overload relay

2

Feeder with no motor load

3

6a 6b

M

Large motor

Feeder with all motor load

4

7

Feeder with combo motor/ no motor load

5 5 5

Cont.

loads, lighting/ heating

Noncont.

loads

M M M

7 7 7

M M M

9

Power factor correction capacitor

Dual-element, time-delay fuses

5 5

Class CF (up to 400 A), and J, RK1 and RK5 ( up to 600 A)

For fuses above 600 A, use Class L time-delay fuses with ratings from

601-6000 A. While these fuses are not dual-element construction, the

Bussmann series KRP-C-SP is a time-delay fuse.

1. Main service.

Size fuse according to method in 4 below.

2. Feeder circuit with no motor loads.

(215.3) The fuse size must be at least 125% of the continuous load† plus 100% of the noncontinuous load. Do not size larger than the conductor’s ampacity*.

3. Feeder circuit with all motor loads.

(430.62) Size the fuse at

150% to 175% of the largest motor’s full load current** plus the full-load current** of all other motors ∆ .

4. Feeder circuit with mixed loads ∆ .

(430.63) Size fuse at sum of: a. 150% to 175%†† of the largest motor’s full-load current**, plus b. 100% of all other motors’ full-load current**, plus c. 125% of the continuous, non-motor load†, plus d. 100% of the non-continuous, non-motor load

5. Branch circuit with no motor load.

(210.20) The fuse size must be at least 125% of the continuous load† plus 100% of the noncontinuous load. Do not size larger than the conductor’s ampacity*.


Continuous loads

11

12

6. Motor branch circuit with overload relays.

Where overload relays are sized per 430.32 for motor running overload protection, there are various alternatives:

6a. Motor branch circuit short-circuit and ground fault protection. (430.52) (most common).

Size the fuse between

150 to 175%†† of the full load current.** Provides branch circuit short-circuit and ground fault protection only.

6b. Motor branch circuit short-circuit and ground fault protection

(430.52) as well as backup overload protection.

Size FRN-R and FRS-R Class RK5 dual-element, time-delay fuses at 125% and LPN-RK-SP and LPS-RK-SP Class RK1 dual-element, timedelay fuses at 130% of motor full-load current or next higher size. This results in closer fuse sizing and provides some backup running overload protection. In addition, it provides motor branch circuit short-circuit and ground fault protection. Sizing in this manner may result in better motor protection if the overload relays are not properly sized or calibrated.

7. Motor branch circuit with only fuse protection.

Where the fuse is the only motor protection, the following FRS-R and FRN-R, Class

RK5, fuses provide motor running overload protection (430.32) and short-circuit protection (430.52):

• Motor 1.15 service factor or 40°C rise. Size the fuse at 110% to

125% of the motor full-load current on the name plate [430.6(a)

(2)].

• Motor less than 1.15 service factor or over 40°C rise. Size fuse at 100% to 115% of motor full-load current on the name plate

[430.6(a)(2)].

8. Large motor branch circuit.

Fuse larger than 600 A. [436.52(c) and 430.52(c)(1) Exceptions 2(d)]. For large motors, size Low-Peak

KRP-C-SP time-delay fuse at 175% to 300% of the motor full-load current**, depending on the starting method; i.e., part-winding starting, reduced voltage starting, etc.

9. Power factor correction capacitors.

[460.8(b)]. Size dual-element fuses as low as practical, typically 150% to 175% of capacitor rated current.

10. Transformer primary fuse (without secondary fuse protection).

[450.3(b)] When transformer primary current is equal to or greater than 9 amps, the dual-element, time-delay fuse should be sized at

125% of transformer primary current or the next size larger if 125% does not correspond to a standard fuse size. Note: Secondary conductors must be protected from overcurrent damage per Article

240.

11. Transformer primary fuse (with secondary fuse protection).

[450.3(b)] May be sized at 250% of transformer primary current if the secondary is fused per 12 below.

12. The secondary fuse is sized at no more than 125% of secondary full-load current.

[450.3(b)] Note: Secondary conductors must be protected at their ampacities per Article 240.


Non-time delay and all Class CC fuses

(FCF, JKS, KTN-R, KTS-R, JJN, JJS, LP-CC, KTK-R and FNQ-R)

1. Main service.

Size fuse according to method in 4.

2. Feeder circuit with no motor loads.

(215.3) The fuse size must be at least 125% of the continuous load† plus 100% of the noncontinuous load. Do not size larger than the conductor’s ampacity.*


(430.62) Size the fuse at

300% of the largest motor’s full-load current** plus the full-load current** of all other motors.

4. Feeder circuit with mixed loads.

(430.62) Size fuse at sum of: a. 300% of the full-load current** of the largest motor, plus b. 100% of the full-load current** of all other motors, plus c. 125% of the continuous, non-motor load†, plus d. 100% of the non-continuous, non-motor load

5. Branch circuit with no motor loads.

(210.20) The fuse size must be at least 125% of the continuous load† plus 100% of the noncontinuous load. Do not size larger than the conductor’s ampacity.*

6a. Motor branch circuit with overload relays.

(430.52) Size the fuse at 300% of the full load current**. Provides branch circuit short-circuit and ground fault protection only. Other means must be utilized to provide motor overload protection (see 430.32). (If

300% is not a standard fuse amp rating, 430.52(C)(1) Exception

1 permits the next standard fuse amp rating. If the motor cannot start with this size fuse, 430.52(C)(1) Exception 2 permits increasing the fuse size up to 400% provided the fuse rating does not exceed 600 A.)

6b. Motor branch circuit short-circuit and ground fault protection (430.52) as well as backup overload protection.

Not applicable for non-time-delay fuses; use FRN-R and FRS-R,

Class RK5, dual-element time-delay fuses or LPN-RK-SP and

LPS-RK-SP Class RK1, dual-element, time-delay fuses (see 6b under dual-element time-delay fuse selection). Non-time-delay fuses cannot be sized close enough to provide motor running backup overload protection. If sized for motor overload backup protection, non-time-delay fuses would open due to motor starting current.

7. Motor branch circuit with only fuse protection.

Not applicable for non-time-delay fuses; use FRN-R and FRS-R, Class RK5, dualelement time-delay fuses (see 7 under dual-element time-delay fuse selection). Non-time-delay fuses cannot be sized close enough to provide motor running overload protection. If sized for motor overload protection, non-time-delay fuses would open due to motor starting current.

8. Power factor correction capacitors.

[460.8(B)] Size non-time-delay fuses as low as practical, typically 250% to 300% of capacitor rated current.

Conductor ampacity selection

1. Feeder circuit and main circuit with mixed loads.

(430.24) conductor ampacity at least sum of: a. 100%†† of the full-load current** of the largest motor, plus a. 100% of the full-load current** of all other motors, plus c. 125% of the continuous, non-motor load†, plus d. 100% of the non-continuous, non-motor load

2. Feeder circuit with no motor load.

[215.2(a)(1)] Conductor ampacity at least 125% of the continuous load plus 100% of the non-continuous load.


(430.24) Conductor ampacity at least 125% of the largest motor full-load amps plus 100% of all other motors’ full-load amps.

4. Feeder circuit with mixed loads.

(430.24) Size according to method 1 above.

5. Branch circuit with no motor load.

[210.19(a)(10] Conductor ampacity at least 125% of the continuous load plus 100% of the non-continuous load.

6, 7 and 8. Motor branch circuits.

(430.22) Conductor ampacity at least 125% of the motor full-load current.

9. Capacitor connected to motor branch circuit.

(460.8) Conductor ampacity at least 135% of capacitor rated current, and at least 1/3 the motor circuit conductors’ ampacity.

10, 11. Conductor ampacity minimum 125% of transformer full-load current.

12. Conductor ampacity per 1 above.

† 100% of the continuous load can be used rather than 125% when the switch and fuse are listed for 100% continuous operation as an assembly (e.g.,

215.3 Exc 1). Some bolted pressure switches and high pressure contact switches 400 A to 6000 A with Class J and L fuses in specified assemblies are listed for 100% continuous operation.

* Where conductor ampacity does not correspond to a standard fuse amp rating, the next higher amp rating fuse is permitted when 800 A or less

[(240.4(B)]. Above 800 A the conductor ampacity must be equal or greater than the fuse amp rating [(240.4(C)]. However, per 240.91(B), when above

800 A for supervised industrial installations, the conductor ampacity is permitted to be 95% of the fuse amp rating as long as the equipment is listed for that size conductor and the conductor is protected within its time vs. current limits [240.4 Informational Note].

∆ In many motor feeder applications dual-element fuses can be sized at ampacity of feeder conductors.

• Available short-circuit current and the clearing time of the overcurrent device must be considered so that the conductor’s ICEA (P32.382) withstand rating is not exceeded.

** On general motor applications, motor full load amps for calculating conductor ampacity and for calculating fuse amp ratings for motor branch circuit shortcircuit and ground fault protection (430.52) are selected from NEC Tables

430.247 through 430.250 per 430.6(A)(1). However, the motor nameplate current rating is used for sizing motor overload protection (430.32) per

430.6(A)(2).

†† 430.52(C)(1) allows a maximum of 175% for time-delay fuses, for all but wound rotor and DC motors. A range of 150% to 175% was used for these guidelines, even though 430.52(C)(1) allows a maximum of 175% for timedelay fuses as stated above. The reason for showing this range is to highlight the possibility for application selection. In some situations, there may be a difference in the switch amp rating or fuse block amp rating in selecting

150% versus 175%. Using 175% is permitted and is suggested for heavy starting current or longer starting time applications.

Further note: the NEC permits larger sizing via two exceptions. 430.52(C)

(1) Exception 1 permits the next standard size if 175% does not correspond with a standard fuse amp rating. If the motor cannot start with this size fuse,

430.52(C)(1) Exception 2 permits increasing a time-delay fuse size up to

225%.

(Note that while a time-delay fuse may not exceed 225% when using

Exception 2, using a time-delay fuse could exceed 225% when applying

Exception 1. For example, assume a motor with a FLA of 1.0 amp. 430.52(C)

(1) would allow a 1.75 amp fuse. Exception 1 would allow a 3 amp timedelay fuse per 240.6(A). Exception 2 limits the time-delay fuse to 2.25 amps as a maximum, but Exception 2 is not utilized or needed if Exception 1 is adequate.)

*** The conductor ampacity may have to be greater due to using adjustment or correction factors per 210.(19)(A)(1) and 215.2(A)(1).


7


7.1 Appliances

The NEC defines an appliance as “utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions such as clothes washing, air-conditioning, food mixing, deep frying, and so forth.” Appliance branch circuits must be protected in accordance with NEC 240.5. If an OCPD amp rating is marked on an appliance, the branch circuit OCPD amp rating cannot exceed that rating marked as per 422.11(A). Section 430.6(A)(1) exception No. 3 addresses situations where the appliance is marked with both a horsepower rating and an amp rating.

For branch circuits which supply a single non-motor operated appliance rated more than 13.3 A, the fuse rating must not exceed 150% of the appliance rating [422.11(E)(3)].

Electric resistance element heating appliances rated more than 48 A must have the heating elements subdivided such that each subdivision does not exceed 48 A and each subdivision must be protected by a branch circuit listed fuse not to exceed 60 A. These fuses must be factory installed by the manufacturer, be accessible, and be suitable for branch circuit protection [422.11(F)(1)].

Fixed appliances are considered protected when supplied from a 15,

20, 25, or 30 A branch circuit. Fixed cooking appliances are permitted to be protected by 40 or 50 A branch circuits (210.23(C)). Household appliances with surface heating elements that have a maximum rating greater than 60 A must be divided into two or more circuits, each of which is protected by a fuse of no greater than 50 A [422.11(B)].

For “equipment not fastened in place,” NEC 210.23(A)(1) requires any one cord-and-plug-connected utilization equipment to not exceed 80 percent of the branch circuit’s amp rating.

7-4

7.2 Ballasts

NEC 410.130(E) requires integral thermal protection for ballasts, except where used for egress lighting. Testing agencies list ballasts for general use in lighting fixtures that pass specific thermal and short-circuit tests. The ballast must have a thermal protector to sense certain over temperature conditions and must also be able to withstand a 200 A fault current when tested with a 20 A fuse. See the Figure 7.2.a for a typical ballast test.

Most electrical systems today will deliver a fault current greater than

200 A to a row of fixtures (see Figure 7.2.b). In order to comply with the last sentence in NEC 110.10 (equipment short-circuit current rating), it’s necessary to have ballasts applied in accordance with their listing and therefore the fixtures must be specified to incorporate individual ballast fusing within the fixture and external to the ballast.

Fusing each fixture also provides for isolating any faulted ballast and reducing costly and dangerous blackouts. When a ballast does fail, only the fuse protecting that individual fixture opens - the remaining fixtures continue in normal operation. Without individual ballast protection, a faulted ballast could cause the branch circuit OCPD to open and shut off all the lights. With individual fusing, the maintenance electrician can troubleshoot the problem much more quickly because only one fixture is

“out.” And this troubleshooting can be performed as part of a scheduled maintenance procedure. Adding external fuses avoids an “emergency” situation where employees are left in the dark.

Note: Refer to the fixture manufacturer for recommended fuse size.

Bussmann series in-line fuses and holder are made specifically for light fixtures.

Ballasts fuse selection

Location Type

Fluorescent

Notes

Consult fixture manufacturer for size and type

Indoor

Outdoor

All other (mercury, sodium, etc.)

Mercury, sodium, etc.

Figure 7.2.a UL 935 short-circuit test for ballast protectors.

277 V lighting panel

20 feet, 10 AWG THW wire

2000 A available

Fuse opens

X

Fixture Faulted ballast Ballasts

Row of lighting fixtures

Figure 7.2.b

Individual ballast fusing.

Consult fixture manufacturer for size and type

Consult fixture manufacturer for size and type.

200 A

0.9 - 1.0 PF

Thermal protector

20 A fuse

Fault

X

Ballast

Ballast winding

Fuse recommendation Fuse holder recommendation

GLR, GMF, GRF

GLQ, GMQ

BAF, KTK, FNM, FNQ

HLR

HLQ

HPF, HPS

KTK-R, FNQ-R, LP-CC

BBS

SC up to 15 A

HPS-RR, HPF-RR

HPS-L, HPF-L

HPF-EE, HPS-EE

SC 20 A

SC 25-30 A

BAF, KTK, FNM, FNQ

KTK-R, FNQ-R, LP-CC

HPF-JJ, HPS-JJ

HPF-FF, HPS-FF

HEB, HEX, HPC-D

HEY, HEZ



7.3 Batteries/battery charging

Batteries can be a part of an overall system to supply electrical power.

The components that comprise this system include an inverter, a battery management system and batteries. Just as with any power distribution system, all these components must be protected from the ill effects of current under abnormal or fault conditions.

The NEC defines the battery system as an“Interconnected battery subsystem consisting of one or more storage batteries and battery chargers, and can include inverters, converters, and associated electrical equipment.”

The current a battery provides is “direct current” (DC) and not a sinusoidal alternating current (AC). The battery’s DC current supplied at a specific DC voltage is much different than an AC current as there are no “zero crossings” with DC. Overcurrent protective devices leverage the AC system’s zero crossings to interrupt the flow of current. The overcurrent protective devices used in DC systems must be listed for these systems and/or they require testing to ensure they can safely interrupt DC current at specified DC voltages. In most cases, especially in higher current applications, high speed, electric vehicle

(EV) or photovoltaic (PV) fuses are selected. Bussmann series product application engineers can assist in evaluating AC-rated fuses for battery applications. Otherwise, only those devices listed for DC systems should be used in DC systems.

Fuses and circuit breakers provide short-circuit and overload protection for batteries, conductors and the other distribution equipment within the system. Not all battery systems are equal and each should be understood from a system perspective. The battery ancillary equipment, as well as the battery itself, can be simple or complex, depending upon the function that they serve.

As with all power distribution systems, there are three competing objectives when designing the battery system protection scheme: a. Minimize the risk of equipment damage during electrically faulted conditions.

b. Limit the battery system’s number and duration of service interruptions.

c. Cost.

Protective devices serve to minimize the damage to the battery. High currents of sufficient duration may cause damage to cable insulation, conductor material, battery posts, battery intercell connectors, battery post seals and battery cell covers.

Batteries are a part of many different system types that provide energy storage and ride-through energy. The following NEC Articles include requirements pertaining to batteries and battery charging systems based on their application.

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Article 480, Storage Batteries

Article 706, Energy Storage Systems

Article 500, Hazardous (Classified) Locations, Classes I, II, and III,

Divisions 1 and 2

Article 503, Class III Locations

Article 505, Zone 0, 1, and 2 Locations

Article 511, Commercial Garages, Repair and Storage

Article 513, Aircraft Hangars

Article 517, Health Care Facilities

Article 551, Recreational Vehicles and Recreational Vehicle Parks

Article 552, Park Trailers

Article 600, Electric Signs and Outline Lighting

Article 625, Electric Vehicle Charging System

Article 640, Audio Signal Processing, Amplification, and

Reproduction Equipment

Article 645, Informational Technology Equipment

Article 646, Modular Data Centers

Article 680, Swimming Pools, Fountains, and Similar Installations

Article 690, Solar Photovoltaic (PV) Systems

Article 691, Large-Scale Photovoltaic (PV) Electric Power

Production Facility

Article 692, Fuel Cell Systems

Article 694, Wind Electric Systems

Article 695, Fire Pumps

Article 700, Emergency Systems

Article 701, legally Required Standby Systems

Article 705, Interconnected Electric Power Production Sources

Article 706, Energy Storage Systems

Article 708, Critical Operations Power Systems (COPS)

Article 712, Direct Current Microgrids

Article 760, Fire Alarm Systems

7.4 Busway

NEMA standards require that busways have a symmetrical short-circuit withstand rating at least as great as the average available symmetrical fault current. NEMA established a minimum three-cycle short-circuit rating, tested as three cycles of peak current (I

P

).

“Busways may be used on circuits having available fault currents greater than the three cycle rating of the busway rating when properly coordinated with current-limiting devices (NEMA Publication no. BU1).”

If a busway is listed or labeled for a maximum short-circuit current with a specific overcurrent device, it cannot be used where greater fault currents are available. If a busway is listed or labeled for a maximum short-circuit current without a specific overcurrent device (i.e., for three cycles), current-limiting fuses can be used to reduce the fault current to within the busway’s withstand rating.

Refer to Figure 7.4.a for an analysis of the short-circuit rating requirements for the 800 A plug-in bus.

Available

70 kA RMS Sym.

KRP-C-800SP

Low-Peak time-delay fuse

Fault

X

800 A switch

Bracing required?

800 A plug-in bus

Figure 7.4.a

Analysis of the short-circuit rating requirements.

Determining a busway’s short-circuit current rating

The 800 A plug-in bus could be subjected to a 70 kA fault current on its lineside; however, the 800 A Low-Peak Class L KRP-C-800SP timedelay fuse would limit this available current. When protected by a KRP-

C-800SP fuses, the 800 A bus need only be braced for 19 kA RMS

Sym. This is derived by using the KRP-C-SP fuse let-through chart (see current-limiting fuse let-through data in Section 3). Table 7.4.c on the following page illustrates the minimum required bracing to be 20 kA

RMS Sym. when protected by KRP-C-800SP fuses with 80 kA available fault current.

7



This would allow a standard 22 kA RMS Sym. (three-cycle) rated bus to be specified, whereas, if a non-current limiting OCPD were specified, the bracing requirements would be 70 kA for three cycles.

“Current limiting fuses generally reduce bus bracing requirements to allow a standard short-circuit rated busway to be specified.”

The busway short-circuit, short time rating has a mechanical limit.

Exceeding this limit invites physical damage from the high magnetic forces associated with a fault’s peak current. The mechanical limit typically applies for high faults near and below the busway short-circuit current rating. Allowable durations of short-circuit current longer than the three-cycles at 60 Hz (0.05 second) required at the maximum shortcircuit rating are obtained from a constant I 2 t “mechanical damage limit” curve.

Typically, for currents below one-half of the short-circuit current rating, where mechanical stresses are reduced to one-quarter of the maximum rating, the mechanical strength becomes less important than the thermal capability. The lower limit duration at one-half the busway rating is determined by the busway thermal (I 2 t) capabilities.

The following example shows busway fault current protection with current-limiting fuses (see Figure 7.4.b). This study looks at the development of the busway mechanical withstand curves and the fuses’ time-current curves (a busway used as a branch circuit must be protected against overcurrent in accordance with NEC 210.20).

In this example, the 800 A plug-in busway has a 70 kA short-circuit current rating for three cycles.

A plot of the busway mechanical limit characteristic on log-log paper passes through the short-circuit rating at (70 kA, 0.05 second) and is a constant I 2

70 kA).

t down to 32.5 kA (one-half the short-circuit current rating of

Assume the available fault current at the busway is equal to its 70 kA rating. The OCPD is assumed to have the proper interrupting rating.

Shown is the system’s plot using Low-Peak Class L and Class RK1 fuses. Current limitation by the KRP-C800SP will offer the busway shortcircuit protection, as it lets-through current is 19 kA in less than 1/2 cycle.

Note: The busway is protected by the fast response in the high fault current region. Protection is achieved, as is selective coordination, with the downstream LPS-RK-400SP fuse.

100

80

60

40

30

20

1,000

800

600

400

300

200

LPS-RK-400SP

KRP-C-800SP

Busway amps

100

225

400

600

601

800

1200

1600

2000

3000

4000

Fuse amps*

100

Available fault current — amps RMS Sym.

25 kA 50 kA 75 kA 100 kA 200 kA

3400 4200 4800 5200 6500

225

400

6000

9200

7000

11,000

8000

13,000

9000

14,000

12,000

17,000

600 12,000 15,000 17,000 19,000 24,000

601 11,000 14,500 17,000 18,000 24,000

800 14,200 17,500 20,000 23,000 29,000

1200 16,000 22,500 26,000 28,000 39,000

1600 22,500 28,500 33,000 36,000 46,000

2000 25,000 32,000 37,000 40,000 52,000

3000 25,000 43,000 50,000 58,000 73,000

4000 25,000 48,000 58,000 68,000 94,000

* Fuses are:

- 100 to 600 A Low-Peak dual-element, current-limiting fuses; LPS-RK-SP (Class

RK1) or LPJ-SP (Class J)

- 800 to 4000 A Low-Peak time-delay, current-limiting fuses; KRP-C-SP (Class L)

Table 7.4.c

Minimum required bracing for bus structures up to 600 V.

UL Standard 891 details fault current durations for busway within switchboards for a minimum of three cycles, unless the main OCPD clears the short in less than three cycles.

10

8

6

4

3

2

KRP-C-800SP

0.4

0.3

0.2

1

0.8

0.6

LPS-RK-400SP

0.1

0.08

0.06

0.04

0.03

0.02

Busway mechanical capability

0.01

Current in amps

65,000 A fault

7-6

Figure 7.4.b

Busway fault current protection with current-limiting fuses.



7.5 Capacitors

The reason for fusing capacitors is short-circuit protection. A capacitor fails when the dielectric is no longer able to withstand the applied voltage and it “shorts out.” Proper fusing is intended to remove the shorted capacitor from the circuit, prevent it from rupturing and protect the conductors from damage due to fault current. However, proper fusing must also be sized such that the capacitor can operate normally; that is the fuse should not open under the normal, steady state current, or the inrush current when voltage is applied. For example, when a capacitor’s circuit is energized, the capacitor can draw a very high inrush current for a very brief time. Therefore, a capacitor fuse must have the characteristics to not open during the initial current inrush. Also, the capacitor’s steady state current is directly proportional to the applied voltage; when the voltage increases the capacitor current increases.

Capacitors are rated in reactive kilovolt-amperes (kilovars or kVAr) or kilovolt-amperes capacitive (kVAc). Both ratings are synonymous.

The current corresponding to the kVAr rating of a 3-phase capacitor (ic) is computed using this formula: ic = kVAr x 1000 √ 3 x V

1 kVAr = 1000 VA (reactive)

In a typical capacitor bank configuration there will be a fuse in line with an overload relay and a capacitor. A fuse must be provided on each ungrounded conductor (no protection is required for a capacitor connected on the loadside of a motor running overcurrent device). The fuse rating must be as low as practical [460.8(B)].

Generally, size dual element, current-limiting fuses from 150% to 175% of the capacitor’s rated current and size non-time delay, fast-acting, current-limiting fuses from 250% to 300% of the capacitor’s rated current.

Conductor ampacity must be at least 135% of the capacitor rated current [460.8(A)]. The conductor ampacity for a capacitor connected to a motor circuit must be at least 1/3 the motor circuit conductor ampacity [460.8(A)].

Fuse sizing per NEC 460

Protected by

Time-delay fuses

Non time-delay fuses

Sizing

150% to 175% of Full Load Current (FLC)

250% to 300% of Full Load Current (FLC)

Fuse/volt recommendation

Up to 250 V: LPN-RK-SP, FRN-R

Up to 600 V: LPS-RK-SP, FRS-R, LPJ-SP, LP-CC, FNQ-R, TCF

Up to 250 V: KTN-R

Up to 300 V: JJN

Up to 600 V: KTS-R, JKS, KTK-R, JJS, FCF-RN

On loadside of motor running overcurrent device

Protection recommended as shown, but not required —

7.6 Circuit breakers

Contents

7.6.1 Overview

7.6.2 Tested series rated combinations

7.6.3 Series rated systems — new installations

7.6.4 Series rated systems — existing installations

7.6.5 Labeling requirements

7.6.6 Motor contribution and limitations

Section page

7

8

9

10

8

9

7.6.7 Examples

7.6.8 Selective coordination

7.6.9 Component protection

7.6.10 Recommended solution

7.6.11 Example of practical series rated combination application 12

7.6.12 Series combination ratings tables 13

10

11

11

11

7.6.1 Overview

A circuit breaker should not be applied where the available short-circuit current at its lineside terminals exceeds its interrupting rating. This is an NEC 110.9 requirement. However, 240.86 contains allowances for protecting downstream circuit breakers with fuses or circuit breakers where the available short-circuit current exceeds the downstream circuit breaker’s interrupting rating.

The terms for this protection is “series rated combination,” “series rating” or “series combination rating”. Properly applying series ratings has many technical limitations and additional NEC requirements that must be met. Series rated combinations allowed per 240.86 should be used sparingly. The series rating requirements are different for new installations versus existing installations.

Starting on page 7-13 are tables for commercially available fuse/ circuit breaker series rated combinations published by panelboard and switchboard manufacturers. These tables, along with a compliance check list for evaluating a series rated combination for a specific installation can be viewed or downloaded from Eaton.com/bussmannseries.

Systems can be either fully rated or series rated. As far as interrupting ratings are concerned, fully rated systems are recommended with the benefit that they can be used everywhere.

Fully rated system

A fully rated system is one that has all the individual OCPD individual interrupting ratings equal to or greater than the available short-circuit current at their line terminals per 110.9. Fully rated systems can be protected by all fuses, all circuit breakers, or fuse/circuit breaker combinations. The interrupting rating is required by 240.60(C) to be marked on the branch circuit fuse (unless its interrupting rating is 10 kA). The interrupting rating is required by 240.83(C) to be marked on the branch circuit circuit breaker (unless its interrupting rating is 5 kA).

In this section, “individual” or “stand-alone” interrupting rating is used to denote the fuses’s or circuit breaker’s interrupting rating. It’s the

“individual” or “stand-alone” interrupting rating that is marked on a fuse or circuit breaker (see Figure 7.6.a). A major advantage with modern current-limiting fuses is that they have 200 kA or 300 kA interrupting ratings that can protect against virtually any fault current level.

X

LPJ-200SP fuse

300,000 A IR Up to I sc

= 300,000 A available fault current

X

Up to I sc


LPJ-20SP fuse

300,000 A IR

Figure 7.6.1.a Current-limiting fuses with up to 300 kA IR can protect against virtually any fault.


7

7-8


Series rated system

A series rated combination is a specific circuit breaker/circuit breaker combination or fuse/circuit breaker combination that can be applied at available short-circuit current levels above the loadside (protected) circuit breaker’s interrupting rating, but not above the lineside (protecting) device’s interrupting rating.

A series rated combination can consist of:

• Fuses protecting circuit breakers

• Circuit breakers protecting circuit breakers

With the stipulations that a fuse cannot protect a fuse, and a circuit breaker cannot protect a fuse.

Figure 7.6.1.b illustrates a fuse/circuit breaker series rated combination.

While there are common series rated requirements for new and existing installations, each has additional, specific requirements. The following addresses the common and specific requirements for new and existing installations.

Series combination IR 200,000 A

X

LPJ-400SP fuse

300,000 A IR

Up to I sc


X

Up to I sc

= 300,000 A available fault current 20 A xyz circuit breaker

CB Company

10,000 A IR

Figure 7.6.1.b A fuse/circuit breaker combination can achieve a high IR.

Fully rated vs. series rated systems

Fully rated systems are the preferred choice for many reasons. If fully rated fuses are used and the proper choices are made, the systems will not have any limitations as described in the previous paragraphs. In addition, if a fully rated system uses modern current-limiting fuses with

200 kA or higher interrupting ratings, the system will likely remain fully rated over its life — even if changes or additions occur that increase the available short-circuit current.

Series rated combinations should be used sparingly. The most suitable application for series rated combinations is for branch circuit, lighting panel circuit breaker protection. Lighting panels typically do not have significant motor loads, so the motor contribution limitation [240.86(C)] is not an issue for series rated combinations. However, series rated combinations used for power panel or main/feeder applications can pose a problem upon initial installation or if future loads change.

A recommendation is to use fully rated fuses for all lighting panelboards, power panelboards, distribution panelboards, motor control centers, motor branch circuits, emergency circuits, elevator circuits and switchboards.

Most series rated combinations cannot be selectively coordinated.

This is a major limitation that most building owners or tenants do not want. Unnecessary blackouts in an electrical system is unacceptable in today’s business environment, technology driven healthcare systems, or emergency circuits. Consider the consequences if there is a disaster to a portion of the building; it is important for safety egress to have as much of the electrical system in service as possible.

7.6.2 Tested series rated combinations

The series combination has to be evaluated and found suitable for a specific manufacturer’s panelboard, loadcenter, switchboard or other equipment. Section 240.86(B) requires that when a series rating is used, the switchboard, panelboard, loadcenter, or other equipment be marked by the manufacturer for use with the specific series rated combinations, indicating they’ve been investigated for such use with those specific series rated combinations. For instance, the series rated combination shown in Figure 7.6.1.b is tested and marked for use in a particular manufacturer’s panelboard as shown in Figure 7.6.2.a. Notice in these two figures that the loadside circuit breaker has an individual marked

10 kA interrupting rating. However, with the series rated combination testing and marking, it may be possible to use it where there is 200 kA available fault current. Also, note that this rating applies to:

1. A specific manufacturer’s type and size circuit breaker.

2. When used in a specific manufacturer’s type panelboard, switchboard or other equipment.

3. When protected on the lineside by a specific OCPD maximum amp rating.

4. The panelboard is factory marked with the necessary series combination rating specifics.

The lineside (protecting) fuse or circuit breaker can be installed in the same panelboard or a separate enclosure.

Panel manufacturer’s label

NRTL listing of series combination rating of

200,000 amps when CB Co.

XYZ Circuit Breaker protected by maximum

400 A Class J fuse

Figure 7.6.2.a NEC 240.86(B) labeling.

Because there is often not enough room in the equipment to show all of the legitimate series rated combinations, UL 67 (panelboards) allows for a bulletin to be referenced and supplied with the panelboard. These bulletins typically provide all the acceptable series rated combinations for that panelboard.

Bussmann has researched the major manufacturers’ application literature and published the tables starting on page 7-13. These tables show, by manufacturer, the various series rated fuses and breakers combinations that are acceptable by panelboard and switchboard type.

Note that more combinations may be available for loadcenters and metercenters; refer to the equipment manufacturer’s literature.

Although series rated combinations save a little on the initial equipment costs, there are many issues involved with designing and utilizing series rated combinations. If series rated combinations are considered for use, there are other NEC requirements that must be met. Since series rated combinations are evaluated by laboratory testing under specific conditions, these other requirements are extremely important in order to make sure a series rated combination is, in fact, applied per its testing, listing and marking [110.3(B)].

7.6.3 Series rated systems — new installations

The industry has devised a method for a Nationally Recognized

Testing Laboratory (NRTL) to test a manufacturer’s specific type and size circuit breaker combination beyond its marked interrupting rating when protected by specific type lineside fuses or circuit breakers of a maximum amp rating. An NRTL does not list the fuse/circuit breaker combination by itself as a series rated combination.

For new installations, the series rated combinations must be tested and marked on specific panelboards and switchboards [240.86(B)]. While testing determines the series combination rating, that rating is not marked on circuit breakers or fuses.



As will be shown in this section, the manufacturer of the panelboard, loadcenter, switchboard or other equipment in which the protected circuit breaker is installed must mark the equipment with the tested series rated combination details. In a later section, field labeling per NEC

110.22 and motor contribution limitation requirements are discussed.

A series rated combination compliance checklist for new installations is available in the Inspection Checklist section of this publication.

Labeling requirements:

• Factory label: the switchboard, panelboard or other equipment is required to be tested, listed and factory marked for use with series rated combination to be utilized per 240.86(B).

• Field label: installer (electrical contractor) to affix labels on the equipment enclosures, which note the series combination rating and call out the specific replacement OCPDs to be utilized. If the upstream OCPD protecting the downstream circuit breaker is in a different enclosure, then both enclosures need to have field-installed labels affixed.

MDP1

Field installed label

CAUTION

Series rated combination with panel LDP1 rated 200,000 amps.

Replace with only

LPJ-200SP Class J fuse.


CAUTION

Series rated combination with LPJ-200SP Class J fuses in MDP1 rated 200,000 amps.

Replace with only CB Co.

XYZ circuit breaker.

LDP1

Figure 7.6.3.a New installation labeling per 240.86(B) and 110.22(C).

7.6.4 Series rated systems — existing installations

2. If the existing system used series ratings with Class R fuses, analyze whether a specific Bussmann series Class RK1, J or T fuse may provide the protection at the higher short-circuit current level. The series ratings for panelboards that use lineside Class R fuses have been determined with special, commercially unavailable* Class RK5 umbrella fuses. Actual, commercially available Bussmann series Class

RK1, J or T fuses will have current-limiting let-through characteristics considerably better than the Class RK5 umbrella fuse limits.

* Commercially unavailable umbrella fuses are only sold to electrical equipment manufacturers in order to perform equipment short-circuit testing.

3. Supervise lineside current-limiting fuse short-circuit testing to verify that protection is provided to circuit breakers that are identical to the existing, installed circuit breakers.

4. Perform an analysis to determine if current-limiting fuses installed on the existing circuit breakers, lineside provides adequate protection for the circuit breakers. For instance, if the existing equipment is passive during the interruption period, such as with low voltage power circuit breakers (approximately 3-cycle opening time), then the lineside fuse short-circuit let-through current (up, over and down method) must be less than the circuit breaker’s interrupting rating.

7.6.5 Labeling requirements

Field labeling: for engineered series ratings (see Figure 7.6.5.a), affix labels on the equipment enclosures, which note engineered series rating, the series combination rating and call out the specific replacement OCPDs. If the upstream OCPD protecting the downstream circuit breaker is in a different enclosure, then both enclosures need to have field installed labels affixed. 240.86(A) and 110.22(B).

MDP1


CAUTION

Engineered

Series rated combination system with panel LDP1 rated 100,000 amps.

Replace with LPJ-200SP

Class J fuse only.

For existing installations, NEC 240.86(A) permits licensed professional engineers to select series rated combinations by other means than testing by an NRTL.

When buildings undergo improvements, or when new transformers are installed, the new available fault currents can exceed the existing circuit breakers’ interrupting ratings. This is a serious safety hazard and does not comply with NEC 110.9. In the past, an owner in this situation faced the possibility of replacing the existing circuit breaker panel with a new circuit breaker or fusible panel with OCPDs having sufficient interrupting ratings for the new available fault currents. This could be very expensive and disruptive.

Now, for existing systems, a licensed professional engineer can determine if a lineside fuse or circuit breaker upgrade can constitute a sufficient series rated combination with existing loadside breakers. This option may represent a significant cost savings versus replacing the existing gear.

The professional engineer must be qualified by primarily working in the design or maintenance of electrical installations. Documents on the selection must be stamped and available to all necessary parties. The series rated combination must also be labeled in the field, including upstream OCPD identification.

There may be several analysis options for a licensed professional engineer to comply with 110.9 where existing circuit breakers have inadequate interrupting ratings. In some cases, a suitable method may not be feasible.

Methods

1. Check to see if a new fused disconnect can be installed ahead of the existing circuit breakers utilizing a tested series rated combination.

Even though the existing system may not take advantage of series ratings, the panel manufacturer may have a table or booklet that provides all the possible tested fuse-circuit breaker series ratings combinations.


CAUTION

Engineered

Series rated combination system with LPJ-200SP Class

J fuses in MDP1 rated

100,000 amps. Replace with

XXX circuit breakers only.

LDP1

Figure 7.6.5.a Existing installation labeling per 240.86(A) and 110.22(B).


7


7.6.6 Motor contribution and limitation

Where motors are connected between the lineside (protecting) device and the loadside (protected) circuit breaker, 240.86(C) has a critical limitation on series rated combination use. This section requires that a series rated combination must not be used where the motor full load current sum exceeds 1% of the loadside (protected) circuit breaker’s individual interrupting rating (see Figure 7.6.f). The reason is that when a fault occurs, running motors momentarily contribute current to the short-circuit (usually about four to six times their full load rating). This added motor contribution may result in short-circuit current that exceeds what the loadside (protected) circuit breaker was tested to handle per the series rated combination testing. Motor contribution is critical for initial installations, but in addition, future system changes can negate the series combination rating.


Total calculated load: 1000 amps

System motor load (MCC1 and MCC2): 500 amps

Series rated combination

Main

Feeder

MCC1 MCC2

This does not comply with NEC 240.86(C)

Motor contribution

CB standalone IR = 10 kA X

M M

Motor FLA > 100 A (1% IR)

Figure 7.6.6.a Motor contribution considerations.

Test set-up series rated

Fuse or circuit breaker

UL 489 testing circuit for series combination ratings does not include fault current contribution from system motors, which in particular applications can contribute fault current on the loadside of the series combination protective device.

Motor contribution in practical apllications not part of series rating testing and listing.

M M

When a fault occurs in actual systems, motors contribute approximately 4 to 6 times their full load amps.

M M M M

Series combination

IR = 22 kA

Figure 7.6.6.b UL 489 testing configuration.

This is one of the major reasons that series rated combinations are generally recommended only for lighting panel applications. Lighting panels typically do not have significant motor loads, so the motor contribution between the feeder overcurrent device and lighting panel branch circuit breakers is not an issue upon initial installation, or in the future. However, series rated combinations used for power panel or main/feeder applications can often pose a problem upon initial installation or if future loads change.

7.6.7 Examples

Example 1

The implications of 240.86(C) are contained in Figure 7.6.7.a. With an installation having a 1000 A total load, 50% motor load (500 A motor load), the motor contribution could be an issue in selecting a series rated combination. If a main/feeder series rating were to be considered, the feeder circuit breaker must have at least a 50 kA individual or standalone interrupting rating per 240.86(C) (1% of 50,000 = 500). If the protected circuit breaker needs at least a 50 kA individual interrupting rating, it negates the reason that series rated combinations are utilized for most applications.

This circuit breaker is the protected circuit breaker in a series rated combination.

What is the minimum individual interrupting rating required for this circuit breaker?

Figure 7.6.7.a Example 1 diagram.

Example 2

Use Table 7.6.7.b to evaluate the “protected” (loadside) circuit beaker in a series rated combination for meeting the motor contribution limits in 240.86(C). In Figure 7.6.7.a, the connected motors could contribute current where the feeder circuit breaker (“protected” device of the

“series combination”) would have to interrupt, but that the main circuit breaker (“protecting” device of the “series combination”) would not have to interrupt is represented by normal full load current of 500 A. In column A of the table below, the 500 A full load motor current exceeds

420 A. Therefore, a series rating with a “protected” circuit breaker having a standalone interrupting rating of 42 kA AIR is insufficient to meet 240.86(C). A series combination that uses a “protected” circuit breaker with a standalone interrupting rating of at least 50 kA would be required to meet 240.86(C).

Note: Do not confuse the “protected” circuit breaker’s standalone interrupting rating with the series combination rating. The series combination rating is the rating for both devices working together to interrupt fault currents. The series combination rating is much greater than the “protected” circuit breaker’s standalone interrupting rating.

Motor full load amps must not exceed this value, If using series combination with“protected” circuit breaker having standalone interrupting rating in Column B

(A)

75 A

100 A

140 A

180 A

200 A

220 A

250 A

300 A

350 A

420 A

500 A

650 A

“Protected” circuit breaker standalone interrupting rated in series combination

(B)*

7500 AIR

10,000 AIR

14,000 AIR

18,000 AIR

20,000 AIR

22,000 AIR

25,000 AIR

30,000 AIR

35,000 AIR

42,000 AIR

50,000 AIR

65,000 AIR

* Some possible circuit breaker interrupting ratings per UL 489, Table 8.1.

Table 7.6.7.b Example 2 motor contribution table.

7-10


Example 3

Assessing the series combination rating for motor contribution limits in the following system (see.


All circuit breakers in PDP1 are series rated with LPJ-600SP fuses.

This series combination IR is 100 kA.

LPJ-600SP

PDP1 load schedule

Circuit Load

1 25 Hp air handler

2

3

4

25 Hp air handler

75 A static

100 A compressor

5

6

7

8

9

10

100 A compressor

10 Hp pump

75 A static

Spare

Spare

Spare


Power distribution panel PDP1

- MLO 600 A

- All circuit breakers have standalone 22 kA IR

Figure 7.6.7.c

Example 3 diagram.

Step 1: Determine total motor load

Quantity Component

2

2

100 A compressors

25 Hp motors @ 34 A each

Amps

200

68

1 10 Hp pump @14 A 14

Total motor load connected between series rated devices 282

Step 2: Is the series rated combination shown acceptable?

No. The series combination shown has a 100 kA series combination rating, which is sufficient for the 37 kA available short-circuit current at

PDP1. The 600 amp LPJ-600SP fuses have a 300 kA interrupting rating, which is sufficient for the 58 kA available short-circuit current at the main switchboard. However, the series combination “protected” circuit breakers, which are located in PDP1, have a standalone or individual 22 kA interrupting rating. The motor load connected between the protecting and protected devices in the series rated combination cannot exceed

1% of the protected circuit breaker’s standalone interrupting rating. The motor load is 282 A, which exceeds 1% of 22 kA (220 A), making this series rated combination non-compliant with 240.86(C).

Then consider the building’s uncertain future. Many buildings, such as office buildings, manufacturing facilities, institutional buildings and commercial spaces, by their nature, incur future changes. A properly designed and initially installed series combination rating could be compromised should building loads change to a larger percentage of motor loads.

As just illustrated, it’s not enough to just check the available shortcircuit current against the series combination rating. 240.86(C) also requires the designer, contractor and AHJ to investigate the protected circuit breaker’s individual or standalone interrupting rating in the series combination. This is necessary for series rated combinations used in new installations as well as existing series rated combinations, when refurbishing or upgrading existing systems.

7.6.8 Selective coordination

In most applications, series rated combinations cannot be selectively coordinated. In order to protect the loadside circuit breaker, the lineside (protecting) device must open in conjunction with the loadside

(protected) circuit breaker. This means the entire panel can lose power because the device feeding the panel must open under short-circuit conditions.

When applying series rated combinations, it’s difficult to meet the selective coordination requirements for:

• Elevator circuits (620.62)

• Critical operations data systems (645.27)

• Emergency systems (700.28)

• Legally required standby systems (701.27)

• Critical operations power systems (708.54)

Applying series rated combinations reduces overall emergency circuit reliability caused by the inherent lack of selective coordination (see

Figure 7.6.8.a).

Not affected

Opens

Unnecessary power loss

7.6.9 Component protection

7.6.10 Recommended solution

X

Fault

Series combinations

Figure 7.6.8.a Selective coordination and series ratings.

Using series rated combinations does not assure circuit component protection. The series rating only pertains to the OCPDs. Specifically, it means a lower interrupting rating loadside circuit breaker can be used in an application with higher available fault currents. In practical applications, the other circuit components (conductors, busway, contactors, etc.) should be independently assessed for protection under the worst case short-circuit conditions.

If a series rated combination is to be used, the designer and contractor should select the tested and marked lineside protection that will assure reliable performance over the electrical system’s lifetime. If the lineside (protecting) OCPD does not function as intended, due to lack of maintenance or loss of calibration, the original protection level is lost.

Fuses are recommended when using series rated combinations.

Modern current-limiting fuses are the most reliable OCPDs available.

While periodic fuse maintenance is not required, it’s recommended that disconnects, and all conductor and fuse terminations be periodically inspected and maintained. Regardless, whether it’s the first day of service or years later, modern current-limiting fuses will respond to protect the circuit components as originally designed.

7



If and when fuses are called upon to open on an overcurrent, installing the same type and amp rated fuses provides the circuit with new, factory-calibrated fused protection. The original design integrity can be maintained throughout the electrical system’s life. With fuses there is typically no worry about replacing an incorrect one in per the series rating. Modern current-limiting fuses have mountings that only accept the same fuse class and case size. All the testing, listing and marking of series rated combinations that utilize fuses as the lineside (protecting) device are tested with the maximum fuse amp rating that fits into the fuse mounting. For instance, all the series ratings with lineside fuses are at the maximum amp ratings for 30, 60, 100, 200, 400, 600 amp standard fuse mounting.

Per UL/CSA/ANCE 248 fuse standards, the lineside fuses used in testing for series rated combinations are special “umbrella” fuses that intentionally exceed the maximum short-circuit current let-through values for specific fuse classes and amp ratings. These special

“umbrella” fuses add an extra safety factor that ensures the shortcircuit current let-through energy represents the worst case for all the commercially available fuses for that fuse class and amp rating. As mentioned previously, it is an umbrella fuse with the highest amp rating that fits in a standard fuse mounting. In addition, the commercially available fuses undergo periodic follow up testing witnessed by the listing agency to verify that the products continue to have short-circuit let-through values under the umbrella limits.

7.6.11 Example of practical series rated combination application

MDP1

X

1


Lighting panel: all CBs 30 A

208Y /120 V

X

2


LDP1

Every major panelboard manufacturer has a suitable fuse/circuit breaker series rated solution. The example that follows uses Eaton equipment

(the table on page 7-13). The selected Eaton panelboard is a type PRL1A with BA single-pole, 20 A, circuit breakers (which have a 10 kA individual interrupting rating) protected by 200 amp Bussmann series LPJ-

200SP fuses (which have a 300 kA interrupting rating). From the table, this series combination rating is 200 kA. That means if all the other requirements are met, the BA circuit breakers in this type panelboard can be applied in a system having an available short-circuit current up to

200 kA at the point where the panelboard is installed. The requirements that must be met are:

1. The series combination rating must be equal to or greater than the available short-circuit current at the circuit breaker location( X2).

Remember, the loadside circuit breaker in a series rated combination can be applied beyond its individual interrupting rating (a BA circuit breaker in this case has a 10 kA individual interrupting rating).

2. In this example, the series rated combination interrupting rating is 200 kA and there is 25 kA available short-circuit current. The protecting overcurrent protective device’s interrupting rating must have an individual interrupting rating equal to or greater than the available short-circuit current at its point of application (X1). In this example, the

200 amp LPJ-200SP fuses have a 300 kA individual interrupting rating and there is 45 kA available short-circuit current.

3. The loadside (protected) circuit breaker’s individual interrupting rating must meet the minimum required in 240.86(C) due to motor contribution. In this case, it’s a lighting panel application and there aren’t any motor loads on the LPJ-200SP fuses’ loadside.

4. Selective coordination in this application is not required per the NEC as this is not a healthcare application, an elevator circuit, nor a part of an emergency legally required standby, or critical operations power system circuit. However, the owner and designer should consider the consequences of not having selective coordination. If selective coordination were deemed necessary, another approach would have to be taken.

5. The panelboard must be marked by the manufacturer and provide sufficient details about the tested series combination rating.

The installer must field install a label on the panelboard and the distribution panelboard providing specific details for the installed series combination rating, the devices and their respective locations.

These are critical to verify the proper ratings for the initial installation and during the system’s life.

See the following series rating tables.

Figure 7.6.11.a Series rated combination application.

In Figure 7.6.11.a, the 208Y/120 V, 200 A lighting panel LDP1 has 25 kA available short-circuit current. The distribution panel MDP1 has 45 kA available. The lighting panel has all single-pole, 20 A circuit breakers.

The typical standard 20 A lighting panel circuit breaker has a 10 kA interrupting rating, which is insufficient for the 25 kA available fault current. The options are:

1. Use a higher interrupting rated circuit breaker for the lighting panel, which may cost more and require more space

2. Use a series rated combination. The series rated combination option can be investigated by looking at the fuse/circuit breaker tables by panelboard manufacturer that are in this handbook’s appendix.



7.6.12 Series combination ratings tables

Eaton series ratings

Switchboards: PRL-C / PRL-i

Panelboards: PRL 5P, PRL 4, PRL 3A and Pow-R-Command Panelboards

Max system volts SCIR* (kA) Lineside fuse

100

LPN-RK

JJN, LPJ

120/240 Vac

200

LPN-RK

JJN, LPJ

JJN, LPJ

LPN-RK

240 Vac

480/277 Vac

480 Vac

600 Vac

100

200

65

100

200

100

200

100

200

JJN, LPJ

JJN

KRP-C

KRP-C

LPN-RK

LPN-RK

JJN, LPJ

JJN, LPJ

JJS, LPJ

JJS, ;PJ

LPS-RK

LPJ

JJS

LPS-RK

JJS, LPJ

LPS-RK

JJS, LPJ

KRP-C

KRP-C

LPS-RK

JJS, LPJ

KRP-C

LSP-RK

JJS, LPJ

400

200

100

200

600

600

100

400

100

200

1200

800

100

200

400

600

200

400

1200

400

600

400

600

4000

4000

100

Max fuse amps

200

400

100

200

400

200

200

200

Loadside

Circuit breaker

GB, GHB

BA, BAB, HQP, QBHW, QPHW

BA, BAB, HQP, QBHW, QPHW, GB, GHB


GB, GHB

GHB

GB, CA

BAB_H, QBHW_H, HQP_H, QPHW_H

CA, CAH, HCA

EHD, FD

FDB, ED, JDB, JD, DK, KDB, KD

GHB

BAB_H, QBHW_H, HQP_H, QPHW_H, CAH, HCA, GB

GHB

BAB_H, QBHW_H, HQP_H, QPHW_H, CAH, HCA

GHB

GB

GHBS

GHBS

GHB

EHD, FD, HFD, FDC

GHB, EHD, FD, HFD, FDC, JD, HJD, JDC

GHB

GHB

EHD

EHD, FD, HFD, FDC

MC, HMC, NC, HNC

MC, HMC

FD, HFD, FDC

JD, HJD, JDC

KD, HKD, KDC

LC

FD, HFD, FDC

JD, HJD, JDC

LC

LC

KD, HKD, KDC, LC

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

Amps Poles

All 1, 2

All 1, 2

1, 2

1, 2

1, 2

1, 2, 3

2, 3

2, 3

2, 3

1, 2, 3

2, 3

1, 2, 3

2, 3

2, 3

2, 3

1, 2, 3

2, 3

1, 2, 3

2, 3

1, 2

1, 2

1, 2, 3

2, 3

2, 3

1, 2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

* Series combination rating

Table notes:

1. The data in these tables was compiled from information in Eaton, Series Rating Information Manual, catalog reference number 1C96944Ho1 Rev. E, pages 18-24, and Eaton, Consulting Application Catalog 12th Edition, pages F1-11 - F1-12. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.

2. The lineside fused switch may be in a separate enclosure or in the same enclosure as the loadside circuit breaker. A lineside fused switch may be integral or remote.

3. Max fuse current rating denotes the largest amp fuse that may be used for that series rated combination. A lower amp fuse may be substituted for the listed fuse.

7




Panelboards: PRL 1A, PRL 2A, PRL 1A-LX, PRL 2A-LX

Max system volts

120/240 Vac

SCIR* (kA) Lineside fuse Max fuse amps

100

LPN-RK

JJN, LPJ

200

400

200

LPN-RK

JJN, LPJ

JJN, LPJ

100

200

400

240 Vac

480/277 Vac

100

200

65

100

200

LPN-RK

JJN, LPJ

JJN

KRP-C

KRP-C

LPN-RK

LPN-RK

JJN, LPJ

JJN, LPJ

JJS, LPJ

JJS. LPJ

LPS-RK

LPJ

JJS

LPS-RK

JJS, LPJ

200

400

600

6000

6000

100

200

200

400

200

100

200

600

600

100

400

Load sde

Circuit breaker

GB, GHB


BA, BAB, HQP, QBHW, QPHW, GB, GHB


GB, GHB

GHB

GB, CA

BAB_H, QBHW_H, HQP_H, QPWH_H

CA, CAH, HCA

EHD, FD

FDB, ED, JDB, JD, DK, KDB, KD

GHB

BAB_H, QBHW_H, HQP_H, QPHW_H, CAH_H, HCA, GB

GB, GBH

BAB_H, HQP_H, QBHW_H, QPHW_H, CA, CAH, HCA

GHB

GB

GHBS

GHBS

GHB

EHD, FD, HFD, FDC

GHB, EHD, FD, HFD, FDC, JD, HJD, JDC

GHB

GHB

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

Amps Poles

All 1, 2

All 1, 2

1, 2

1, 2

1, 2

1, 2, 3

2, 3

2, 3

2, 3

1, 2, 3

2, 3

1, 2, 3

2, 3

2, 3

2, 3

1, 2, 3

1, 2

1, 2

1, 2

1, 2, 3

2, 3

2, 3

1, 2, 3

1, 2, 3

* Series combination rating.

Table notes:

1. The HQP and QPHW are not listed for use in the PRL1A-LX panel.

2. PRL 1A and PRL 1A-LX are for use at 240 V maximum

3. Branch breakers for maximum 120/240 V systems include BAB, HQP, QBHW and QPHW.

4. Branch breakers for maximum 240 V systems include BAB_H, HQP_H, QBHW_H and QPHW_H.

5. PRL 2Aand PRL 2A-LX, branch breakers include GHB, GHBS and GB.

6. PRL 1A-LX and PRL 2A-LX main and sub-feed breakers include ED, FD, HFD, FDC.

7. PRL 1A and PRL 2A main and sub-feed breakers include CA, CAH, HCA, ED, FD, HFD, FDC, JD, HJD, JDC, KD, HKD and KDC.

8. The data in these tables was compiled from information in Eaton, Series Rating Information Manual, catalog reference number 1C96944Ho1 Rev. E, pages 18-24, and

Eaton, Consulting Application Catalog 12th Edition, pages F1-11 - F1-12. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.






Triple series rating - switchboards: PRL-C and PRL-i

Panelboard types: PRL 5P, PRL 4, PRL 3A, PRL 2A, PRL 2A-LX, PRL 1A, PRL 1A-LX and Pow-R-Command panels

Max system Volts SCIR* (kA) Lineside fuse

120/240 Vac

240 Vac

100

100

KRP-C (6000 A max)

KRP-C (6000 A max)

Tenant main type

DK, KDB, KD

JD, JDB

FD

FD, FDB

EHD

DK, KDB, KD

JD, JDB

FD

FD, FDB

EHD

Branch type

Circuit breaker

GB, GHB

GB, GHB

GB, GHB

HQP

BA, BAB

BA, BAB, HQP

GHB

GB, EHD

CA, CAH, HCA

FD, FDB

JD, JDB

GHB

GB

GHB

GB

BAB_H, QBHW_H, HQP_H, QPHW_H

BAB_H, HQP_H

All

All

All

All

All

All

All

All

All

All

All

Amps Poles

All 1, 2

All 1, 2

All 1, 2

15-70 1, 2

All 1, 2

All 1, 2

1, 2, 3

2, 3

2, 3

2, 3

2, 3

1, 2, 3

2, 3

1, 2, 3

2, 3

2,3

2, 3


Table notes:

1. The data in these tables was compiled from information in Eaton, Series Rating Information Manual, catalog reference number 1C96944Ho1 Rev. E, pages 18-24, and Eaton, Consulting Application Catalog 12th Edition, pages F1-11 - F1-12. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.



7



General Electric series ratings

Spectra Series (see notes on page 7-20)

Max system volts SCIR* (kA)

240 Vac

277 Vac

42

100

200

100

65

Lineside fuse

LPJ, JJN

KRP-C

JJN, LPJ

JJN

KRP-C

LPN-RK

JJN, LPJ

KRP-C

LPS-RK

JJS, LPJ

LPJ

LPS-RK

400

100 JJS, LPJ

480 Vac

600 Vac

200

200

JJS

KRP-C

KRP-C

KRP-C


** Includes all sensor/rating plug or setting values within stated frame size.


600

800

1200

2000

2000

2500

2000

2500

600

600

100

200

800

1200

2000

2500

200

Max fuse amps

600

2000

400

600

400

600

2000

100

200

400

TED

THED

SED, SEH, SEL

TEY

SED, SEH, SEL

TED, THED

TED, THED6

TEY

SED, SEH, SEL

TED

TED, THED6

SFH, SFL

SGH, SGL

TEY

SED, SEH, SEL

SKH, SKL

THJK

SKH, SKL

SGH, SGL

Loadside

Circuit breaker

TJD

TJD

TQD

THHQB

TQD

TQD

TJD

SFH

TJD

THJK

TEB, TED

SFH, SFL

SED, SEH, SEL

TEB

TEB, TED

TJD

SFH, SFL

SED, SEH, SEL

SGD, SGH, SGL

TED

THED

TEY

SED, SEH, SEL

TEY

TED

TPV, THPV

TPV, THPV

TPV, THPV

TPV, THPV

15-50

15-30

15-150

15-100

15-150

15-150

15-100

15-100

15-150

15-50

15-100

70-250

125-600

15-100

15-150 2, 3

300-1200 2, 3

125-600 2, 3

300-1200 2, 3

125-600 2, 3

800 A

FRAME**

3

2500 A

FRAME**

800 A

FRAME**

2500 A

FRAME**

3

3

3

2, 3

2, 3

2, 3

1

2, 3

2, 3

2, 3

2, 3

1

1

2, 3

1

2, 3

2, 3

15-100

15-100

250-400

70-250

15-150

125-600

15-50

15-30

15-100

15-150

15-100

15-50

Amps

250-400

250-400

125-225

40-100

100-225

125-225

250-400

70-250

250-400

250-600

15-100

70-250

15-150

1

1

1

2, 3

1

1

1, 2

2, 3

2, 3

2, 3

2, 3

2, 3

Poles

2, 3

2, 3

2, 3

3

2

3

2, 3

2, 3

2, 3

2, 3

1, 2, 3

2, 3

2, 3



AL / AQ Panelboard (see notes on page 7-20)


240 Vac

42

65

100

200

Lineside fuse

JJN

JJN, LPJ

KRP-C

JJN

JJN, LPJ, LPN-RL

KRP-C

LPN-RK

JJN

JJN, LPJ

JJN

KRP-C

LPN-RK

JJN, LPJ

KRP-C

Max fuse amps

600

600

2000

600

600

3000

200

200

400

600

800

1200

2000

200

400

600

2000

Loadside

Circuit breaker

THQL-GF

THQL

TJD

TJD

THHQL

THHQL

TFJ

TFJ

THQL

THQP

THQL

TQD

THHQL, THHQB

TFJ

TQD

TQD

TJD

TFJ

SFH

TJD

THQL

TFJ

SFH, SFL

SED, SEH, SEL

THQL

TFJ

TJD

SFH, SFL

SED, SEH, SEL

SGD, SGH, SGL

70-225

100-225

125-225

250-400

70-225

70-250

250-400

15-100**

70-200

70-250

15-150

15-100**

Amps

15-30

15-100**

250-400

250-400

15-70

15-125

70-225

70-225

15-100**

15-50

15-100**

125-225

40-100

70-225

250-400

70-250

15-150

125-600

2, 3

1, 2

2, 3

2, 3

2, 3

1, 2

2, 3

2

3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

Poles

1

1, 2, 3

2, 3

2, 3

1

2

2, 3

2, 3

1, 2, 3

1, 2

1, 2, 3

2, 3

3


** THQL 1-pole rating is 70 A maximum. Maximum system voltage is 120/240 Vac., THQL 2 pole 110-125 A ratings are also series rated on 120/240 Vac maximum services.

7




ALC / AQC Panelboard (see notes on page 7-20)


240 Vac

42

65

100

200

Lineside fuse

JJN

JJN

JJN, LPJ

KRP-C

LPN-RK

JJN, LPJ

Max fuse amps

600

600

JJN, LPJ, LPN-RL 600

KRP-C

LPN-RK

JJN

3000

200

200

400

600

1200

200

400

Loadside

Circuit breaker

THQL-GF

THQL

THHQL

THHQL

TFJ

TFJ

THQL

THQP

THQL

TQD

THHQL, THHQB

TFJ

TQD

TQD

TFJ

SFH

THQL

TFJ

SFH, SFL

SED, SEH, SEL

THQL

TFJ

SFH, SFL

SED, SEH, SEL

125-225

70-225

70-250

15-100**

70-200

70-250

15-150

15-100**

70-225

70-250

15-150

Amps

15-30

15-100**

15-70

15-125

70-225

70-225

15-100**

15-50

15-100**

125-225

40-100

70-225

100-225

2, 3

1, 2

2, 3

2, 3

2, 3

3

2, 3

2, 3

1, 2

2, 3

2, 3

Poles

1

1, 2, 3

1

2

2, 3

2, 3

1, 2, 3

1, 2

1, 2, 3

2, 3

3

2, 3

2

600


** THQL 1-pole rating is 70 A maximum. Maximum system voltage is 120/240 Vac. THQL 2-pole 110-125 A ratings are also series rated on 120/240 Vac maximum services.




AE / AD Panelboard (see notes on page 7-20)


277 Vac

480 Vac

100

65

100

Lineside fuse

LPS-RK

JJS, LPJ

LPJ

LPS-RK

JJS, LPJ

JJS

KRP-C


AEC Panelboard (see notes on page 7-20)

Max system volts SCIR* (kA) Lineside fuse

600

600

100

200

Max fuse amps

100

200

400

400

600

800

1200

2000

Loadside

Circuit breaker

TED

THED

TEY

SED, SEH, SEL

TEY

TED

TED

THED

SED, SEH, SEL

TEY

SED, SEH, SEL

TED, THED

TED, THED6

TEY

SED, SEH, SEL

TED

TED, THED6

TFJ

TJJ

SFH, SFL

SGH, SGL

TEY

SED, SEH, SEL

SKH, SKL

TJL

SKH, SKL

SGH, SGL

277 Vac

480 Vac

100

65

100

LPS-RK

JJS, LPJ

LPJ

LPS-RK

JJS, LPJ

400

600

600

100

200

Max fuse amps

100

200

400

600

Loadside

Circuit breaker

TED

TEY

SED, SEH, SEL

TEY

TED

TED

SED, SEH, SEL

TEY

SED, SEH, SEL

TED

TED

TEY

SED, SEH, SEL

TED

TED

TFJ

SFH, SFL

SGH, SGL

TEY

SED, SEH, SEL



Amps

15-50

15-100

15-150

15-100

15-50

15-50

15-150

15-100

15-150

15-150

15-100

15-100

15-150

15-50

15-100

70-225

70-250

125-600

15-100

15-150

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

1

2, 3

2, 3

2, 3

1

1

1

2, 3

1

1

2, 3

2, 3

Poles

1

1

1

2, 3

1

15-100

15-150

15-50

15-100

70-225

125-400

70-250

125-600

15-100

15-150

300-1200

125-400

300-1200

125-600

Amps

15-50

15-30

15-100

15-150

15-100

15-50

15-50

15-30

15-150

15-100

15-150

15-150

15-100

2, 3

2, 3

2, 3

2, 3

1

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

Poles

1

1

2, 3

1

1

1

2, 3

1

7-19

7



The following circuit breakers may be substituted for the circuit breakers shown in the series rating tabulations. Devices with MicroVersaTrip Plus and PM trip units may also be substituted, provided the short-circuit rating is equal to or greater than series connected rating. Reference. GE publication DET-008A.

SGD

SGH

SGL

SKH

SKL

TPV

THPV

TQD

TFJ

SFH

SFL

TJJ

THJK

Circuit breaker

THQL

THHQL

THQL-GF

TED

SED

SEH

SEL

Substitute circuit breakers

THQB, THQC, THQE, THHQL, THHQB, THHQC

THHQB, THHQC

THQB-GF, THQC-GF

THED

SEH, SEL, SEP

SEL, SEP

SEP

THQD

TFK, THFK

SFL, SFP

SFP

TJK, THJK, TJ4V, THJ4V, THJ9V, TJH

THJ4V, THJ9V, TJH, TJL

SGH, SGL, SGP

SGL, SGP

SGP

SKL, SKP

SKP

SS, SH, TP, TCV, THP, THC, THCV

SH, THP, THC, THCV

Table notes:

1. The data in these tables was compiled from information in GE Electrical

Distribution and Control publication, catalog reference number GEP-11OOP and

GE Electrical Distribution and Control publication - UL Component Recognized

Series Ratings, publication reference number DET-008A. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information.

The information contained in the tables may change without notice due to equipment design modifications.





Siemens series ratings

Switchboards SB1, SB2, SB3

Panelboard S1


120/240 Vac

240 Vac

SCIR* Lineside fuse

LPJ, LPN-R

65

100

100

200

JJN

KRP-C

JJN

LPJ, LPN-RK

JJN

KRP-C

LPN-RK

JJN

LPJ

LPJ, LPN-RK

JJN

KRP-C

Max fuse amps

600

1200

6000

200

600

600

1200

6000

200

400

600

600

1200

6000

Loadside

Circuit breaker

QPH, BQH, BLH

QP, BQ, BL

Amps

15-70

15-125

15-100

15-70

15-125

15-100

Poles

1 (120 V)

2

3

1 (120 V)

2

3

HQP, HBQ, HBL, QPH, BQH, BLH 15-100

QPF, BQF, BLF, QE, BE, BLE, QEH, BLEH, BLHF, QPHF, BQHF 15-30

QEH, BLEH, QE, QPHF, BLHF, BLE, QPF, BLF

QT

15-60

15-50

15-70

QPH, BQH, BLH, HQP, HBQ, HBL

ED4, HED4

ED4, ED6, HED4, HED6

15-125

15-100

15-100

15-125

FD6-A, FXD6-A

JD6-A, JXD6-A, JXD2-A, SJD6-A

LD6-A

SLD6-A

LXD6-A

ED4, HED4


FD6-A, FXD6-A


LD6-A

SLD6-A

LXD6-A

ED4, HED4


FD6-A, FXD6-A


LD6-A

SLD6-A

LXD6-A

SMD6

SND6

PD6, PXD6, SPD6

RD6, RXD6

3

1 (120 V)

2

1 (120 V), 2

1 (120 V)

2

70-250

200-400

200-600

300-600

450-600

15-100

15-125

70-250

200-400

200-600

300-600

450-600

15-100

15-125

3

1 (120 V)

2, 3

2, 3

2, 3

2, 3

3

2, 3

1 (120 V)

2, 3

2, 3

2, 3

2, 3

3

70-250

200-400

200-600

300-600

450-600

500-800

500-1200 3

1200-1600 3

1600-2000 3

3

2, 3

3

2, 3

1 (120 V)

2, 3

2, 3

2, 3

2, 3

QJH2, QJ2H, QJ2

QJ2

QJH2, QJ2H

HFD6, HFXD6

HFD6, HFXD6

HFD6, HFXD6

MD6, MXD6, HMD6, HMXD6

ND6, NXD6, HND6, HNXD6

125-200

125-225

125-225

2, 3

2, 3

2, 3

70-250

70-250

70-250

500-800

2, 3

2, 3

2, 3

2, 3

500-1200 2, 3


Table notes:

1. The data in these tables was compiled from information in Siemens SpeedFax 2000 Electrical Products publication, catalog reference number GNPC-01000. The

Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.




7


Siemens series ratings

Switchboards SB1, SB2, SB3

Panelboards 82, SE, S3, S4, 85


480 Vac

SCIR*

50

100

480/277 Vac 200

Lineside fuse

LPJ

LPJ

JJS, LPJ

LPJ, LPS-RK

JJS, LPJ, LPS-RK

JJS

JJS, KRP-C

KRP-C

LPS-RK

JJS, LPJ

Max fuse amps

400

400

600

600

600

800

1200

6000

100

200

Loadside

Circuit breaker

ED4

ED4

FD6-A, FXD6-A

HFD6, HFXD6

5-50

70-250

70-250

JD6-A, JXD6-A, HJD6-A, HJXD6-A 200-400 2,3 LD6-A, HLD6-A 200-600

LXD6-A, HLXD6-A

HFD6, HFXD6

JD6-A, JXD6-A, HJD6-A, HJXD6-A

LD6-A, HLD6-A

LXD6-A, HLXD6-A

HFD6, HFXD6

MD6, MXD6, HMD6, HMXD6

ND6, NXD6, HND6, HNXD6

BQD, CQD

BQD**, CQD**

BQD, CQD

BQD**, CQD**

Amps

60-100

15-100

450-600

70-250

200-400

200-600

450-600

70-250

500-800

500-1200

15-100

20-30

15-100

20-30

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

Poles

1 (277 V)

2, 3

1 (277 V)

2, 3

2, 3

2, 3

2, 3

2, 3

1 (277 V)

2, 3

1 (277 V)

2, 3


** BQD and CQD circuit breakers are series rated from 15-100 A for Series 7A, S2 and S3 panelboard applications only.

Table notes:

1. The data in these tables was compiled from information in Siemens SpeedFax 2000 Electrical Products publication, catalog reference number GNPC-01000. The

Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.





Square D series ratings

I-Line Switchboard/Panelboard


240 Vac

480 Vac

SCIR*

100

200

100

200

KRP-C

KRP-C

KRP-C

LPS-RK

JJS

JJS

LPJ

KRP-C

KRP-C

KRP-C

KRP-C

LPS-RK

JJS

JJS

Lineside fuse

LPN-RK

JJS

JJS

LPJ

KRP-C

KRP-C

KRP-C

LPN-RK

JJS

JJS

LPJ

LPJ

LPJ

KRP-C

KRP-C

KRP-C

600

800

1200

1600

2000

600

400

800

800

1200

2000

600

600

800

1200

2000

600

600

800

600

Max fuse amps

600

600

800

600

800

400

600

800

1200

2000

Loadside

Circuit breaker

FH, KA, KH, LA, LH, MA, MH, MX

FA

FH, KA, KH, LA, LH, MA, MH, MX

FA, FH, KA, KH, LA, LH, MA, MH, MX

KA

FH, LA, LH

KH, MA, MH, MX

FH, FC, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

FA

FH, FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

FA, FH, FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

FH, LA, LH

FC, KH, KC, LC, LX, MA, MH, MX

NA, NC, NX

FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA

FA, FH

FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA

FA, FH, FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA

KA

KH, LA, LH

MA

FC, KC, LC, LX, MH, MX, NA

FC, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

FA, FH

FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

FA, FH

FC, KA, KH, KC, LA, LH, LC, LX, MA, MH, MX, NA, NC, NX

LA, LH

FC, KC, LC, LX, MA, MH, MX

NA, NC, NX

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

All

Amps Poles

All 2, 3

All 2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3

2, 3


Table notes:

1. The data in these tables was compiled from information in Square D, Series Rating Data Bulletin No. 2700DB9901 and Square D Digest 171. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.



7



Square D series ratings


NQOD Panelboards

SCIR*

(kA) Lineside fuse Max fuse amps

Loadside

Circuit breaker Amps Poles

240 Vac 200

JJS, LPJ

JJN

200

400

QO, QOB

QO, QOB (AS)

QO, QOB (GF I)

QO, QOB

QO, QOB (AS)

QO, QOB (GF I)

All

All

All

All

All

All

1, 2, 3

1, 2, 3

1, 2, 3

1, 2, 3

1, 2, 3

1, 2, 3

Note for NQOD panelboards: 1P for use at 120 V only

NEHB Panelboards

480Y/277 Vac 100 JJS, LPJ

Note for NEHB Panelboards: 1P for use at 277 V only

NF panelboard

480Y/277 Vac

100

200

JJS, LPJ

JJS, LPJ

Note for NF Panelboards: 1P for use at 277 V only

SF switchboards with I-Line or NQOD distribution

120/240 Vac 42 JJS

240 Vac 42 JJS

480 Vac

50

65

JJS

JJS

200

400

200

400

800

800

800

EH, EHB

EDB, EGB, EJB

QO-VH, QOB-VH

QO-VH, QOB-VH, FA, Q4

Q2-H

FA, FH

KA, KH, LA, LH

All

All

All

All

All

All

All

1, 2, 3

1, 2, 3

1 (120 V)

2, 3

2

2, 3


Table notes:

1. The data in these tables was compiled from information in Square D, Series Rating Data Bulletin No. 2700DB9901 and Square D Digest 171. The Bussmann Division assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.





7.7 Conductors

Contents

7.7.1 NEC Article 100 definitions

7.7.2 General

7.7.3 Conductor ampacity tables

7.7.4 Determining load amps

7.7.5 Conductor selection

7.7.6 Conductor termination considerations

7.7.7 Conductor and termination considerations

7.7.8 Ampacity adjustments

7.7.9 Voltage drop considerations

7.7.10 Overcurrent protective device selection

7.7.11 Tap conductors and OCPDs

7.7.12 Small conductors

7.7.13 Flexible cords

7.7.14 Exceptions for listed surge protective devices

7.7.15 Battery conductors

38

38

7.7.16 Branch circuits - lighting and/or appliance load (no motor load) 39

7.7.17 Feeder circuits (no motor load) 39

7.7.18 Service conductors

7.7.19 Wire and cable short-circuit protection

7.7.20 Cable limiters

7.7.21 Transformer secondary conductors

7.7.22 Motor circuit conductor protection

39

39

42

43

43

Section page

25

25

26

26

31

33

37

37

38

38

27

28

29

7.7.1 NEC Article 100 definitions

“Ampacity: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Continuous load: A load where the maximum current is expected to continue for 3 hours or more.

Informational Note: A current in excess of rating may be accommodated by certain equipment and conductors for a given set of conditions. Therefore, the rules for overcurrent protection are specific for particular situations.

Neutral conductor: The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.

Overcurrent: Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short-circuit, or ground fault.

Overload: Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short-circuit or ground fault, is not an overload.”

7.7.2 General

The basic conductor selection and protection process includes the following:

1. Calculate the load current: The load amps for the branch and feeder circuits must be calculated taking into consideration continuous and non-continuous loads.

a. Find calculated load for continuous amps i. Apply 1.25 multiplying factor ii. Apply 1.00 multiplying factor for fully rated conductors b. Find calculated load for non-continuous amps i. Apply 1.00 multiplying factor in all applications c. Find total calculated load i. Add calculated continuous and non-continuous loads

2. Select the conductor: Select the conductor that has an ampacity to handle the load calculated as per 210.19 or 215.2. The conductor size will either be based upon the continuous plus non-continuous loads or be based upon the conditions of use, after adjustment and correction factors.

a. Continuous plus non-continuous (No adjustments and correction factors) i. Use rated ampacity of the conductor based on table

310.15(B)(16)

1. Ambient temperature is greater than 77°F and less than 87°F

2. No more than three current carrying conductors in a raceway, cable or Earth (direct buried) b. Conditions of use (adjustment and correction factors) i. Adjustment factor - more than three current carrying conductors in raceway, cable, or Earth (direct buried)

1. Obtain multiplier from Table 310.15(B)(3)(a) adjustment factors ii. Correction factor — ambient temperature less than 78°F and greater than 86°F

1. Obtain multiplier from Table 310.15(B)(2)(a) ambient temperature correction factors

2. Add 60°F to ambient temperature if installed in conduit on a roof in direct sunlight less than 7/8” from the rooftop iii. Total Adjustment — number of conductors + ambient

1. The conductor‘s new ampacity is:

MF

NoC

x MF

Temp

x Rated ampacity = Adjusted ampacity

3. Termination consideration: Depending upon the size of conductor landed on a specific termination with a temperature rating, the maximum amps the termination is rated for is shown in Table 7.7.6.a.

a. If the adjusted ampacity of the conductor is less than that shown in the table of Figure 7.7.6.a, use the adjusted ampacity b. If the adjusted ampacity of the conductor is greater than that shown in the table of Figure 7.7.6.a, use the value shown in the table of Figure 7.7.6.a.

4. Voltage drop considerations: The voltage at the load must be adequate for the application. If it isn’t, make necessary adjustments in conductor size.

5. OCPD selection: The overcurrent protective device selected must protect the conductor from overcurrents. This may require resizing the conductor so it may be protected by the OCPD.

7



Branch and feeder conductors must be sized appropriately for the load.

Section 210.19 of the NEC, “Conductors — Minimum Ampacity and

Size”, requires as part of 210.19(A)(1) branch circuit conductors to have an ampacity not less than the maximum load that it will serve. A similar requirement is found for feeders as part of Section 215.2, “Minimum

Rating and Size”. It is 215.2(A)(1) that requires feeder conductors to have an ampacity that is not less than that which is required to supply the calculated load.

Conductors that are of an appropriate ampacity that can carry the calculated load must then be afforded protection from overcurrents as per 240.4,“Protection of Conductors”. Per the definition of overcurrent found in Article 100 of the NEC, for a conductor an overcurrent is any current in excess of the ampacity of the conductor. Per Article 100 an overcurrent may result from overloads, short-circuits, or ground faults.

NEC 110.10 requires that conductors must also be protected from the effects of short-circuit current damage. For these high fault currents, no device is faster than the current-limiting fuse which removes the most stress and heat from the distribution system in the safest most economical way possible.

The proper selection and application of the conductor is as much a part of its adequate protection as is the overcurrent protective device selection.

7.7.3 Conductor ampacity tables

The NEC includes six tables that provide allowable ampacity of conductors. These tables provide the rated amps for the conductors which is the amount of current that each conductor can carry indefinitely and continuously.

Just as in any rating of a product, there are stipulations on how they achieved their rating. These stipulations are also included within the table. As an example, let’s review the most commonly used table providing allowable ampacities in the NEC, Table 310.15(B)(16). This table provides the rated ampacity for conductors that meet the following qualifications:

• Insulated conductors

• Rated up to and including 2000 V

• Insulation rating of 60°C through 90°C (140°F through 194°F)

• When not more than three current-carrying conductors are in raceway, cable, or earth (directly buried),

• When in an ambient temperature of 30°C (86°F)

Addressing each qualification:

• Insulated conductors: this table addresses conductors that are insulated. Insulation is the outer coating on the copper conductors.

• Rated up to and including 2000 volts: All products must be applied within their rating. The ampacities in this table apply to conductors with a voltage rating of 2000 volts and less. Each conductor is listed and tested and labeled with a voltage rating.

•

Insulation rating of 60°C through 90°C (140°F through 194°F):

A conductor will be tested and listed at a specific insulation rating.

A conductor will be marked with the temperature at which it was evaluated. Conductors will have a dry or wet conductor rating. For example, THHN in Table 7.7.5.a has a 90°C rating dry only.

• Not more than three current-carrying conductors: The rated ampacity listed in this table is achieved under specific conditions one of which is having not more than three current-carrying conductors in a raceway. If a raceway has more than three current carrying conductors the ampacity must be adjusted. For that adjustment, reference 310.15(B)(3)(a). See 310.15(B)(5) to determine if any neutral conductors present are current carrying.

• Ambient of 30°C (86°F): The rated ampacity in this table is achieved under specific conditions one of which is having an ambient temperature of 30°C (86°F). For those conductors routed in areas in temperatures that exceed or are less than 30°C (86°F), adjustment factors must be used to adjust the ampacity in this table. For this adjustment activity, reference 310.15(B)(2).

As an example, a 1/0 copper conductor of insulation type THHW can carry 170 amps indefinitely when not more than three current-carrying conductors are in a raceway, cable, or direct buried in earth and when the ambient temperature is 30°C (86°F). This information is obtained from the 90°C column of Table 310.15(B)(16).

The other tables in Section 310.15 address the ampacity rating of other conductor configurations and include:

• Table 310.15(B)(17) (formerly Table 310.17) allowable ampacities

•

Single, insulated conductors

•

Rated up to and Including 2000 V

•

In free air

•

Based on ambient temperature of 30°C (86°F)


•

Insulated conductors

•


•

Insulation rating of 150°C through 250°C (302°F through 482°F).

•

Not more than three current-carrying conductors in raceway or cable

•

Based on ambient air temperature of 40°C (104°F)


•

Single, insulated conductors

•


•

Insulation rating of 150°C through 250°C (302°F through 482°F), in free air

•


• Table 310.15(B)(20) (formerly Table 310.20) ampacities

•

Not more than three single insulated conductors

•

Rated up to and including 2000 V

•

Supported on a messenger

•


• Table 310.15(B)(21) (formerly Table 310.21) ampacities

•

Bare or covered conductors

•

In free air

•

Based on 40°C (104°F) ambient, 80°C (176°F), total conductor temperature, 610 mm/sec (2 ft/sec) wind velocity

7.7.4 Determining load amps

The first step in the process of selecting a conductor is understanding the current it will be called upon to carry. This section will address branch circuits and feeder circuits.

Branch circuits

NEC Article 100 defines a branch circuit as the circuit conductors between the final overcurrent protective device protecting the circuit and the outlet(s). Refer to Figure 7.7.4.a for various examples of branch circuit and feeder circuits.



Feeder circuit

Branch circuit

Heater

Figure 7.7.4.a Feeder and branch circuits.

Section 210.19 addresses conductor minimum ampacity and size. This section tells us that the branch circuit conductor must have an ampacity not less than the maximum load that is served by the branch circuit. To determine this maximum load consideration must be given to whether a load is continuous or not. The NEC defines a continuous load as that load where the maximum current is expected to continue for three hours or more. In determining the amp load for the branch circuit the following equation is leveraged:

Calculated branch circuit amps = I

NC

+ (1.25 x I

C

)

Where:

I

NC

= Non-continuous current

I

C

= Continuous current

Once the load amps that the circuit must supply indefinitely is determined, conductor selection can proceed.

Example: A branch circuit supplies 20 heaters that have four fluorescent bulbs (F54T5HO) per fixture. Manufacturer information tells us that each heater draws 0.84 amps at 277 volts. This is a warehouse and these heaters will be on 24 hours per day and 7 days a week. What is the minimum ampacity for this branch circuit?

The total amps for this circuit is calculated as:

Heater amps = 20 x 0.84 A = 16.8 A

The heater load must be treated as a continuous load when sizing the branch circuit. The branch circuit minimum ampacity is calculated as:

Branch circuit amps = 1.25 x 16.8 A = 21 A

The branch circuit must be sized per a minimum ampacity of 21 A and be able to continuously serve a load of 16.8 A indefinitely.

Feeder circuits

NEC Article 100 defines a feeder as all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

Refer to Figure 7.7.4.a for various examples of branch circuit and feeder circuits.

Section 215.2 addresses minimum conductor rating and size.

This section tells us that the branch circuit conductor must have an ampacity not less than that required to serve the load as calculated per specific Parts of Article 220.

• Part III, Feeder and Service Load Calculations

• Part IV, Optional Feeder and Service Load Calculations

• Part V, Farm Load Calculations

• To determine the maximum load, 215.2 requires consideration be given to whether a load is continuous or not. The NEC defines a continuous load as that load where the maximum current is expected to continue for three hours or more. In determining the amp load for the feeder circuit the following equation is leveraged:

Calculated feeder amps = I

NC

+ (1.25 x I

C

)

Where:

I

NC

= Non-continuous current

I

C

= Continuous current

Once the amps that the circuit must supply indefinitely is determined, conductor selection can proceed.

Example: A feeder circuit supplies a 200 A panel. The panel was sized to be able to supply the load calculated as per Parts III, IV, and V of Article

220. The loads on this panel are heating loads for a warehouse and will be on 24 hours per day and 7 days a week. What is the minimum ampacity for this feeder circuit?

The total amps for this circuit is given by the panel size for this application. The panel size of 200 A was determined based upon the loads being treated as continuous loads and multiplying those load amps by 1.25. The engineer has added additional room for growth and so the

200 A panel is oversized from the standard load calculations of Article

220. The feeder circuit for this application is a 200 A feeder. It must be capable of supplying 200 A continuously and indefinitely.

7.7.5 Conductor selection

Proper selection of a conductor for an application must take into consideration where and how the conductor will be routed to serve the load. Table 310.104(A) provides information to help understand the construction of conductors and their application provisions. If for example a conductor will be in a wet or damp location, this table can provide guidance on which conductors are rated for that application.

Chapter 3 of the NEC includes detailed information important for proper selection and application of various types of conductors for many different applications.

Example: A branch circuit includes 20 heaters that have 4 heating elements per fixture. Per manufacturer information, each heater draws

0.84 A at 277 V. This is a warehouse and the heaters will be on 24 hours per day and 7 days a week. The ampacity of this branch circuit is calculated to be 21 A which is 1.25 x (20 x 0.84 A). The majority of this circuit will be routed in a raceway in a part of the building that is 115°F.

What conductor type would fit this application?

Based on Table 310.104(A), an insulation with 90°C for dry and damp locations is selected. Heat-resistant thermoplastic type THHN conductor is selected for this application. This conductor has an insulation temperature of 90°C. Based on manufacturer information this conductor is rated for 600 V. These conductors have multiple ratings. Depending upon the product application, allowable temperatures are shown in Table

7.7.5.a.

7

Insulation Applications/max. temperatures

THHN or

T90 Nylon

Dry locations not to exceed 90°C

THWN-2

THWN

TWN75

Wet or dry locations not to exceed 90°C or locations not to exceed 75°C when exposed to oil

Wet locations not to exceed 75°C or dry locations not to exceed 90°C or locations not to exceed 75°C when exposed to oil

Wet locations not to exceed 75°C

MTW

AWM

Wet locations or when exposed to oil at temperatures not to exceed 60°C or dry locations not to exceed

90°C (with ampacity limited to that for 75°C conductor temperature per NFPA 79)

Dry locations not to exceed 105°C only when rated and used as appliance wiring material

Table 7.7.5.a



Example: A feeder circuit supplies a 200 A panel that was sized as per the load calculations in Parts III, IV, and V of Article 220. The loads on this panel are heater loads for a warehouse and will be on 24 hours per day and 7 days a week. It was determined that the ampacity of this feeder is 200 A. The feeder will be routed in a raceway with other conductors such that there are nine current carrying conductors in the raceway. The temperature in the portion of the building where the majority of the length of this feeder circuit is routed is 115°F. What conductor type would fit this application?

Based on Table 310.104(A), select an insulation with 90°C for dry and damp locations. Heat-resistant thermoplastic type THHN conductor is selected. This conductor has an insulation temperature of 90°C.

Manufacturer information states it is rated for 600 V. The manufacturer information notes that there are multiple options for this conductor.

Depending upon the product application, allowable temperatures are shown in Table 7.7.5.a.

7.7.6. Conductor termination considerations

Both ends of a conductor whether in a branch circuit or feeder circuit will be terminated on a terminal. The temperature rating of the terminal will be a limiting factor as to the amount of current that the conductor is permitted to carry. Figure 7.7.6.a is a table developed from 310.15(B)

(16) that illustrates the maximum current permitted for each termination with the specified temperature rating and conductor size. This is a limiting factor in the determination of the ampacity of the circuit that has nothing to do with the capability of the conductor but rather the limitations of the electrical equipment.

Section 110.14 is that area of the NEC that speaks to electrical connections. This section of the NEC reminds us to be aware of dissimilar metals and other details important to ensure proper termination of conductors. Section 110.14(C), “Temperature Limitations”, requires that the temperature rating associated with the ampacity of a conductor must be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than those specified for terminations must be permitted to be used for ampacity adjustment, correction, or both. It is also permitted to land a conductor with a higher temperature rated insulation but it is not permitted to use the ampacity at that higher temperature rated insulating ampacity rating. This is true even when adjusting or correcting the ampacity of this higher insulation rated conductor. Any adjustment or correction factors that arrive upon an ampacity that is greater than that which is established by the lowest temperature rated termination cannot be used. Section

310.15(B) requires that a conductor ampacity adjustment can be taken from the ampacity based on the insulation rating and can be used as the ampacity of the conductor only if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).

It is also not permissible to replace a lug/terminal on equipment with higher temperature rated terminals that have not been tested with those lugs at the higher temperature ratings.

Section 110.14(C)(1) provides termination temperature ratings when the equipment is not available for inspection to determine the actual rating of the termination. Figure 7.7.6.a simplifies the requirements of 110.14(C)

(1) in table format.

Unless the equipment is listed and marked otherwise, conductor ampacities must be limited by the termination temperature ratings.

Section 110.14(C)(1) addresses two types of equipment:

1. Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 through 1 AWG conductors a. Conductors rated at 60°C (140°F) can be used with ampacity determined based on 60°C (140°F) b. Conductors with higher temperature ratings can be used, provided the ampacity of such conductors is determined based on the 60°C

(140°F) ampacity of the conductor size used c. Conductors with higher temperature rating can be used at their higher temperature ampacity as long as the product is listed as such d. Conductors for motors marked with design letters B, C, or D , that have an insulation rating of 75°C (167°F) or higher can be used as long as their ampacity doesn’t exceed the 75°C (167°F) ampacity

2. Termination provisions of equipment for circuits rated over 100 amps, or marked for conductors larger than 1 AWG a. Conductors must be rated for 75°C (167°F) and the ampacity at that temperature rating can be used b. Conductors with higher temperature ratings can be used, provided the ampacity of such conductors is determined based on the 75°C

(167°F) ampacity of the conductor size used c. Conductors with higher temperature rating can be used at their higher temperature ampacity as long as the product is listed as such

Example: A THWN-2 4 AWG conductor is terminated on a circuit breaker which has a lug rated for 75°C at one end and an industrial control panel at the other. What is the ampacity of this conductor based on Table 310.15(B)(16)?

As per Table 310.104(A), a THWN-2 conductor is rated for 90°C. Section

110.14 requires that the ampacity be limited to the temperature rating that aligns with the temperature rating of the terminals upon which this conductor is landed. Both ends of the this conductor have to be considered. One end of the conductor is terminated on a circuit breaker which, based on the information provided, is rated for 75°C. The other end of the conductor is terminated on an industrial control panel for which no information is available. Based on 110.14(C)(1) and because this conductor is not larger than a 1 AWG, the assumed temperature rating of the termination is 60°C.

Based on this information the 60°C column of Table 7.7.6.a or the 60°C column of NEC Table 310.15(B)(16) is used to determine the maximum current that can be used for a 4 AWG conductor which is 70 A.



300

350

400

500

600

700

750

800

1

1/0

2/0

3/0

4/0

250

Size AWG/ kcmil

18

16

14

12

4

3

2

10

8

6

900

1000

1250

1500

1750

2000

60°C

(140°F)

—

Copper

—

15*

20*

30*

40*

55*

70*

85*

95*

110*

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

75°C

(167°F)

—

—

20

25

35

50

65

85

100

115

130

150*

175*

200*

230*

255*

285*

310*

335*

380*

420*

460*

475*

490*

520*

545*

590*

625*

650*

665*

—

—

—

—

—

—

—

—

85*

—

—

—

—

—

60°C

(140°F)

75°C

(167°F)

Aluminum or copper-clad aluminum

— —

—

—

15*

—

—

20

25*

35*

40*

55*

65*

75*

65

75

90

30

40

50

—

—

—

—

—

—

425*

445*

485*

520*

545*

560*

230*

250*

270*

310*

340*

375*

385*

395*

100

120*

135*

155*

180*

205*

* Default minimum value when equipment terminal rating is unknown.

Table 7.7.6.a

Maximum equipment termination current.

7.7.7 Conductor and termination considerations

Figure 7.7.7.a

This disconnect’s middle, lineside conductor became loose and created an excessive thermal condition that damaged the terminal, along with the middle and the adjacent conductors.

A fuse, as well as a circuit breaker, is part of a system where there are electrical, mechanical and thermal considerations. All three are interrelated. If there is too much electrical current for the circuit, the components will overheat. If a conductor termination is not properly torqued, it can be a “hot spot” and contribute excess heat that’s detrimental to the termination, conductor insulation and even the

OCPD. If the conductor size is too small for the circuit load, or for the fuse/termination or circuit breaker/termination rating, the undersized conductor will create excess heat, which can damage the device.

Figure 7.7.7.b

Both the proper conductor size and termination method are critical!

Many so called OCPD “nuisance” openings or failures can be traced to improper termination methods or conductor sizing as the root cause.

Poorly made or improper electrical connections can result in fire or other damage to property, and can cause injury and death. If there are loose terminal connections, then:

• The conductor overheats and the conductor insulation may break down that can lead to a fault; typically line-to-ground. If conductors with different potentials are touching, the insulation of both may deteriorate, and a phase-to-neutral or phase-to-phase fault occurs.

• Arcing can occur between the conductor and lug. Since a poor connection is not an overload or a short-circuit, the OCPD does not operate.

• The excessive heat at the conductor’s termination increases the temperature beyond the fuseclip material’s thermal rating and can result in the fuseclip losing its spring tension and creating a hot spot at the interface between the fuse and clip. See Figure 7.7.7.c.

• The excessive thermal conditions described above may cause a device’s (block, switch, fuse, circuit breaker, etc.) insulating system to deteriorate and result in a mechanical and/or electrical breakdown. For instance, the excessive thermal condition of a conductor termination at a circuit breaker can degrade the insulating case material or fuse block material may carbonize.

7



Normally, a fuse is mounted in a fuseclip or bolted to a metal surface.

It’s important that the fuse-to-clip/metal surfaces are clean and mechanically tight to minimize electrical resistance. If not, this interface will be a high resistance connection, which can lead to a hot spot.

With a fuseclip application, the temperature rise from a poor fuse/ clip connection can cause even further clip tension deterioration causing the hot spot condition getting worse.

Figure 7.7.7.c The fuse clip on the right has excellent tension that provides a good mechanical and electrical interface (low resistance) between the fuse and clip. The clip on the left experienced excessive heat caused by an improper conductor termination or undersized conductor and lost it tension as a result. Consequently, the mechanical and electrical interface between the fuse and clip was inadequate which further accelerated the unfavorable thermal condition.

Loose terminal and connection causes

Below are some possible loose terminal connection causes for various termination methods that can result in excessive OCPD/termination/ conductor heating:

• The copper or aluminum conductor’s gauge and type must be within the connector’s specifications.

Terminals are rated to accept a specific conductor type(s) and size(s). A conductor that’s too large or small for the connector will result in a poor connection. Additionally, it must be verified that the terminal is suitable for aluminum, copper or both. Usually the termination means is rated for acceptable conductor type(s) and size range, with these ratings marked on the device

(block, switch, circuit breaker, etc.) or specified in the device’s data sheet.

•

The connector is not torqued to the manufacturer’s specification.

Conductors expand and contract with temperature changes caused by load fluctuations. If the connections are not torqued correctly, loose connections may result after a number of expansion/contraction cycles. For a mechanical screw, nut, bolt or box lug type connection, follow the manufacturer’s specified torque that’s typically marked on the device. For a specific connector, the specified torque may be different for different wire sizes.

• The conductor is not properly crimped. A poor crimp connection could be between a conductor and a ring terminal, quickconnect terminal or an in-line device.

If using a crimp/compression connection, use the manufacturer’s recommended crimp tool with the designated die location and number of crimps.

• The quick-connect terminal is not seated properly.

If the malefemale connections are not fully seated, a hot spot may result.

• The quick-connect terminal is being used beyond its amp rating. Quick-connects typically have limited continuous current ratings that must not be exceeded. Typical possible maximum quickconnect ratings are 16 or 20 A (some are less) and are also based on a specified conductor size. If a quick-connect is used beyond its amp rating, excessive temperature will result which can degrade its tension properties, leading to ever increasing temperatures until the connection fails.

• The conductor is not properly soldered to a solder terminal.

Again, if there isn’t a good connection between the two, a hot spot will result.

• The terminal is rated to only accept one conductor, but multiple conductors are being used.

Again, the product specifications must be checked to see if the terminal is rated for dual conductors.

If the product is not marked suitable for dual conductors, then only one conductor can be used for the terminal. Inserting too many conductors will cause a poor connection, which can result in overheating at the connector.

• The terminal is not rated for a finely stranded conductor. The common electrical connectors and terminals for electrical equipment are rated to accept conductors with the number of stands not exceeding Class B and Class C stranding. If conductors with finer stranding are used, the connectors and terminals must be suitable and identified for the specific stranded conductor class(es). See

NEC 110.14 for the requirement and NEC Chapter 9, Table 10 for the number of strands for Class B and Class C conductors.

Properly torque terminations

Proper conductor installation and maintenance practices are to properly torque the termination during initial installation and then to periodically conduct visual and thermal inspections (such as infrared scan).

When installing a conductor into a termination, applying the device manufacturer’s specified torque for the type and size conductor is critical.

The specified torque value ensures the proper force is being applied on the conductor in the termination with a resulting low contact resistance.

Applying a torque value below the manufacturer’s specification can result in a higher resistance at the conductor termination that may result in excessive heat and causes damage to the conductor and device. Applying a torque value exceeding the specification can result in damaging the termination device and/or the conductor.

The conductor termination torque values are part of the testing and listing procedures when a manufacturer’s device is evaluated for compliance to product standards by a nationally recognized testing laboratory. NEC 110.3(B) requires installing the equipment to the torque values that were used in the listing or product labeling. Therefore, when installing conductors it’s important to use a calibrated torque tool and torque to the device manufacturer’s specification. A device’s conductor termination specified torque values typically are on the device label as shown in Figure 7.7.7.d. However, these specifications may be in the instructions or data sheet.

18-10 AWG (1-2.5 mm 2 ) Single & Dual 20 in-lb (2.26 N•m)

8-6 AWG (3.15-4 mm 2 ) Single & Dual 35 in-lb (3.95 N•m)

4 AWG (5 mm 2 ) Single 35 in-lb (3.95 N•m)

75°C, CU ONLY, SAME AWG & TYPE FOR DUAL

Figure 7.7.7.d

Terminal torque specifications are generally marked on the device’s label or may be found in its instructions or data sheet.

If all connections were properly torqued, many electrical device failures would not occur. The installer needs to ensure a proper conductor termination by using a calibrated torque measurement tool and the device manufacturer’s specified torque value for the conductor type and gauge. Through recent surveys conducted at various electrical industry events, it was found that approximately 75% of the terminations made without a torque measuring tool do not come within plus or minus 20% of the manufacturer’s specifications. These findings reinforce the point that there is a need to ensuring proper termination methods by using installation tools that measure torque.

Some maintenance practices involve periodically checking the conductor termination by checking the torque or just retightening. This should not be done and is affirmed in the NEC Informative Annex I: “Because it is normal for some relaxation to occur in service, checking torque values sometime after installation is not a reliable means of determining the values of torque applied at installation.”



Often it’s assumed that terminations inevitably become loose after a system’s extended cycling or just through time in service. After all, every metal has the physical property that it will experience a certain amount of relaxation, and it’s perceived that this relaxation is a cause of concern. However, manufacturers have taken these physical properties into account through their product design and testing. If equipment, conductors and terminations are used in applications for which they are designed and listed, and the terminations are made with the proper torque value during installation, then the connection will remain within its required values. All this requires that conductor termination devices used are suitable for their application. By contrast, many common conductor termination devices are not suitable for applications with vibrations, such as at a generator’s terminals.

When a loose conductor termination has resulted in thermal damage to the conductor at and near the termination, remove the conductor from the terminal. First, determine if the terminal is suitable for continued use. Stripped threads make terminals unsuitable for use and terminal discoloration may indicate it isn’t suitable for further use, either. If the terminal is deemed suitable for further use, cut the conductor’s damaged portion and reinstall it using a calibrated torque tool set to the proper value.

Improper overcurrent protection can be a root cause of conductor termination damage. Conductors can become loose under screws or lugs if they have carried excessive amounts of short-circuit current. High fault current can result in high mechanical forces causing conductor movement which degrades the contact points between the conductors and terminal devices. In addition, the excessive heat generated by the fault current contributes to the problem. Since the conductor is deformed during termination, the portion at the termination that’s damaged by fault current needs to be cut off and the conductor properly re-terminated so that new and correct contact points are created.

Unfortunately, terminated conductor damage due to fault current may not manifest itself as a problem until long after the fault’s cause has been corrected. Properly applied current-limiting fuses can prevent terminated conductor damage caused by fault currents.

7.7.8 Ampacity adjustments

Each conductor used must be reviewed to ensure the ampacity of the conductor can serve the connected load and is protected at its ampacity by an overcurrent protective device. Section 310.15 addresses ampacities for conductors and reminds us that conductors must not be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. (310.15(A)(3))

Once a conductor has been selected to be used in an application, the correct size must be determined and will be dependent upon the ampacity taking into consideration the environment in which it will be applied. The ampacity of the conductor must be greater than the current it must carry to serve the load.

There are requirements in the NEC for adjusting and determining the conductor’s ampacity, including:

1. Ampacity limitations due to terminations

2. Ambient temperature

3. Number of conductors in a raceway or cable

There are code requirements that affect the size of conductor to be used (e.g., motor circuits or continuous loads) but only items 2 and

3 shown above are used to regulate ampacity to keep the conductor within its maximum temperature rating.

NEC 310.10 contains the basic rule that, in effect, requires an ampacity adjustment for ambient temperature and number of conductors in a raceway or cable.

Temperature considerations for a conductor that impact its ampacity include:

1. Ambient temperature

2. Heat generated internally in the conductor as a result of current flow

3. The rate at which generated heat dissipates from the conductor, and

4. Adjacent current-carrying conductors

Items (1) and (4) are the two that require attention and for which guidance is provided by NEC Section 310.15.

7.7.8.1. Ambient temperature adjustment

All ampacity tables must use some ambient temperature as a basis and

NEC Table 310.15(B)(16) uses 30°C (86°F). At the bottom of this table, direction is provided to Section 310.15(B)(2) for ampacity correction factors where the ambient temperature is other than 30°C (86°F).

Use of these factors is fairly straightforward. As per the parent text of 310.15(B), the temperature correction and adjustment factors are permitted to be applied to the ampacity for the temperature rating of the conductor as long as the corrected and adjusted ampacity is not greater than the ampacity for the termination’s temperature rating in accordance with 110.14(C).

The temperature adjustment factors are found in Table 7.7.8.1.a (NEC

310.15(B)(2)(a)) and 7.7.8.1.b (NEC 310.15(B)(2)(b)). The first table aligns with ampacity tables 310.15(B)(16) and 310.15(B)(B)(17). The second table aligns with ampacity tables 310.15(B)(18), 310.15(B)(19), and

310.15(B)(20).

56-60

61-65

66-70

71-75

76-80

81-85

26-30

31-35

36-40

41-45

46-50

51-55

For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below.

Ambient

Temperature

(°C)

10 or less

Temperature rating of conductor

60°C

1.29

75°C

1.20

90°C

1.15

Ambient temperature

(°F)

50 or less

11-15

16-20

21-25

1.22

1.15

1.08

1.15

1.11

1.05

1.12

1.08

1.04

51-59

60-68

69-77

1.00

0.91

0.82

0.71

0.58

0.41

1.00

0.94

0.88

0.82

0.75

0.67

1.00

0.96

0.91

0.87

0.82

0.76

78-86

87-95

96-104

105-113

114-122

123-131

-

-

-

-

-

-

0.58

0.47

0.33

-

-

-

0.71

0.65

0.58

0.50

0.41

0.29

132-140

141-149

150-158

159-167

168-176

177-185

Table 7.7.8.1.a

NEC Table 310.15(B)(2)(a) ambient temperature correction factors based on 30°C (86°F).

7



For ambient temperatures other than 40°C (104°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below.

Ambient Temperature rating of conductor Ambient

76-80

81-90

91-100

101-110

111-120

121-130

131-140

141-160

161-180

181-200

201-225

36-40

41-45

46-50

51-55

56-60

61-65

66-70

71-75 temperature

(°C)

10 or less

11-15

16-20

21-25

26-30

31-35

60°C

1.58

1.50

1.41

1.32

1.22

1.12

1.00

0.87

0.71

0.50

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

75°C

1.36

1.31

1.25

1.20

1.13

1.07

1.00

0.93

0.85

0.76

0.65

0.53

0.38

-

-

-

-

-

-

-

-

-

-

-

-

90°C

1.26

1.22

1.18

1.14

1.10

1.05

1.00

0.95

0.89

0.84

0.77

0.71

0.63

0.55

0.45

-

-

-

-

-

-

-

-

-

-

150°C

1.13

1.11

1.09

1.07

1.04

1.02

1.00

0.98

0.95

0.93

0.90

0.88

0.85

0.83

0.80

0.74

0.67

0.60

0.52

0.43

0.30

-

-

-

-

200°C

1.09

1.08

1.06

1.05

1.03

1.02

1.00

0.98

0.97

0.95

0.94

0.92

0.90

.088

0.87

0.83

0.79

0.75

0.71

0.66

0.61

0.50

0.35

-

-

250°C

1.07

1.06

1.05

1.04

1.02

1.01

1.00

0.99

0.98

0.96

0.95

0.94

0.93

0.91

0.90

0.87

0.85

0.82

0.79

0.76

0.72

0.65

0.58

0.49

0.35

temperature

(°F)

50 or less

51-59

60-68

69-77

78-86

87-95

96-104

105-113

114-122

123-131

132-140

141-149

150-158

159-167

168-176

177-194

195-212

213-230

231-248

249-266

267-284

285-320

321-356

357-392

393-437

Figure 7.7.8.1.b NEC Table 310.15(B)(2)(b) ambient temperature correction factors based on 40°C (104°F).

Example: What is the ampacity of a 1/0 AWG, aluminum, type THHN conductor when the ambient temperature is 100°F?

The process for adjustment is:

1. THHN is an insulated conductor rated 600 V and is not in free air.

Table 310.15(B)(16) for the rated ampacity of the conductor is used.

2. Table 310.104(A) shows that the THHN conductor has an insulation rating of 90°C.

3. Because this is a 90°C insulated aluminum conductor, adjustments are made for temperature based on the 90°C column of Table

310.15(B)(16). The starting ampacity is 135 amps.

4. Table 310.15(B)(2)(a) is next consulted to determine the ampacity adjustment factor for a conductor applied at 100°F. This table shows for a temperature range of 96 to 104°F the multiplier to be used for a

90°C temperature rated conductor is 0.91.

5. Finally enough information is obtained to determine the ampacity of this 1/0 AWG conductor in an ambient of 100°F with this calculation:

Adjusted ampacity = 0.91 x 135 A = 122.85 A

7.7.8.2 Number of conductors adjustment

As noted above, the rated ampacity tables 310.15(B)(16) and 310.15(B)

(18) are applicable when not more than three current carrying conductors are in the raceway. For those installations where there are more than three current carrying conductors ampacity adjustment must be addressed through application of multiplying factors shown in Table

310.15(B)(3)(a).

Number of current-carrying conductors*

4-6

7-9

10-20

21-30

31-40

41 and greater

50

45

40

35

% values in NEC ampacity tables 310.16 to

310.19 as adjusted for ambient temperature if necessary

80

70

* Number of conductors is the total number of conductors in the raceway or cable, including spare conductors. The count must be adjusted in accordance with 310.15(B)(5) and (^). The count must not include conductors that are connected to electrical components that cannot be simultaneously energized.

Table 7.7.8.2.a

NEC Table 310.15(B)(3)(a) adjustment factors for more than three current-carrying conductors.

Example: What is the ampacity of eight 12 AWG copper THHN conductors installed in one conduit?

The process for adjustment is:

1. THHN is an insulated conductor rated 600 V and is not in free air.

Table 310.15(B)(16) is the applicable table for the rated ampacity of the conductor.

2. Table 310.104(A) shows that the THHN conductor has an insulation rating of 90°C.

3. Because this is a 90°C insulated copper conductor, adjustments for temperature can be made based on the 90°C column of Table

310.15(B)(16). The starting ampacity is 30 A.

Note: This No. 12 copper THHN conductor in Table 310.15(B)(16) has two stars next to it which per the foot note references 240.4(D) for conductor overcurrent protection limitations. Section 240.4(D) requires that the OCPD protecting a 12 AWG aluminum and copper-clad aluminum conductor not exceed 15 A unless specifically permitted in 240.4(E) or (G). The requirements of 240.4(D) do not impact the ampacity determination of a conductor, it only impacts the sizing of the overcurrent protective device for that conductor. For this ampacity adjustment, it is permitted to use the 90°C column but the adjusted ampacity cannot be more than what 240.4(D) requires unless the application falls under the provisions of 240.4(E) or 240.4(G).

4. Table 310.15(B)(3)(a) is next consulted to determine the adjustment factor for an application with eight current-carrying conductors in a single raceway. Per this table for 7 to 9 current-carrying conductors in the same raceway the adjustment factor is 0.70.

5. Finally enough information is available to determine the ampacity of this 12 AWG copper THHN conductor in a raceway that has a total of eight current-carrying conductors with this calculation: Adjusted ampacity = 0.70 x 30 A = 21 A

The adjusted ampacity is 21 A. The requirement of Section 240.4(D)(5) is such that, as long as this application does not fall under the provisions of 240.4(E) or 240.4(G), this circuit must be protected by a maximum of a 20 A OCPD. If the demands of the load is such that a 20 A OCPD would be too small, a larger size conductor must be used.

7.7.8.3 Ambient temperature and number of conductors adjustment

For those applications where a conductor will be routed in an ambient outside of its rating and in a raceway with more conductors than with which it was rated, both adjustment calculations are made and their impact is cumulative.

Example: What is the ampacity of four 1/0 THW copper conductors when the ambient temperature is expected to reach 110°F?



The process for adjustment is:

1. THW is an insulated conductor rated 600 V and is not in free air. Table

310.15(B)(16) is used for the rated ampacity of the conductor.

2. Table 310.104(A) shows that the THW conductor is available with insulation ratings of both 75°C and 90°C. Table 310.15(B)(16) has THW under the 75°C column only.

3. Because this is a 75°C insulated copper conductor, adjustments for temperature are made based on the 75°C column only of Table

310.15(B)(16) and not the 90°C column. Starting ampacity is 150 A for both ambient and number of conductor adjustment factors.

4. Table 310.15(B)(2)(a) is next consulted to determine the ampacity adjustment factor for a conductor applied at 110°F. This table shows for a temperature range of 105 to 113°F the multiplier to be used for a

75°C temperature rated conductor is 0.82.

5. Table 310.15(B)(3)(a) is next consulted to determine the adjustment factor for an application with four current-carrying conductors in a single raceway. Per this table for 4 to 6 current-carrying conductors in the same raceway the adjustment factor is 0.80.

6. Finally enough information is obtained to determine the ampacity of this 1/0 AWG THW conductor in a raceway that has four currentcarrying conductors and in an ambient of 110°F using this calculation:

Adjusted ampacity = 0.80 x 0.82 x 150 A = 98.4 A

7.7.9 Voltage drop considerations

A power distribution system should be capable of providing power to all equipment within their published voltage limits under all normal operating conditions. In addition, voltage considerations should also include motor voltage drop during starting and restrictions placed on the user by the utility company to prevent disturbances to their system when starting large motors.

The National Electrical Code has many Informational Notes throughout the document in various areas that speak to recommended voltage drop maximums. There are a few specific requirements that place required do not exceed percentage voltage drop values for an application.

Branch circuits 210.19(A): “Branch Circuits Not More Than 600

Volts”. Informational Note No. 4 recommends branch circuits be sized to prevent a voltage drop exceeding 3% at the farthest outlet and a maximum of 5% voltage drop on both feeders and branch circuits.

Feeder circuits 215.2(A)(1): Informational Note No. 2 recommends feeders be sized to prevent a voltage drop exceeding 3% at the farthest outlet and a maximum of 5% voltage drop on both feeders and branch circuits.

Ampacity calculations 310.15(A)(1): Section 310.15 focuses on ampacities for conductors rated 0-2000 volts. Informational note #1 makes the user of the NEC aware that the conductor ampacities and adjustments addressed by Section 310.15 do not take into consideration voltage drop. The user is referenced to Sections 210.19(A) for branch circuits and 215.2(A) for feeder circuits when it comes to voltage drop considerations.

Ampacity calculations 310.60(C) Tables: Conductors Rated 2001 to 35,000 Volts. Informational Note No. 2 makes the user of the NEC aware that the conductor ampacities and adjustments provided by

Section 310.60, which is titled “Conductors Rated 2001 to 35,000 Volts”, do not take into consideration voltage drop. The user is referenced to

Sections 210.19(A) for branch circuits and 215.2(A) for feeder circuits when it comes to voltage drop considerations.

Phase converters 455.6(A): The informational note for this section recommends a maximum voltage drop of 3% for the single-phase conductors from the source of supply to the phase converter. This maximum value is established to help ensure proper starting and operation of the motor loads.

Storage batteries 480.4(B): “Intercell and Intertier Conductors and

Connections”. The Informational note for this section recommends a maximum voltage drop of 3% when supplying the maximum anticipated load and a maximum of 5% to the furthest point of connection. This informational note also advises that these values of voltage maximums may not be appropriate for all batter applications. Reference is made to IEEE 1375-2003, “Guide for the Protection of Stationary Battery

Systems” for more information.

Agricultural buildings 547.9(C): “Service Disconnecting Means and

Overcurrent Protection at the Distribution Point”. The informational note for this section recommends a maximum voltage drop of 2% and connecting loads line-to-neutral to help reduce neutral-to-earth voltages in livestock facilities.

Recreational Vehicles and Recreational vehicle parks 551.72(D):

“Neutral Conductors”. The informational note to this section tells the user of the NEC that circuit lengths will typically be very long in recreational vehicle parks. It reminds one of the fact that the ampacity information provided as part of Article 310 does not take into consideration voltage drop noting that selection based on Article 310 alone may be inadequate for the installation. Informational note #2 provides insight to what voltage drop is and what current should be used when calculating voltage drop. The suggested current to be used here is based on the calculated load with appropriate demand factors as per Section 551.73(A).

Electric welders 630.31: “Ampacity of Supply Conductors”. The informational note for this section of the NEC simply advises that the voltage drop should be limited to a value that is satisfactory for the performance of the welder. The informational note reads “The ampacity of the supply conductors for resistance welders necessary to limit the voltage drop to a value permissible for the satisfactory performance of the welder is usually greater than that required to prevent overheating.”

Sensitive electronic equipment 647.4(D): “Voltage Drop”. This section of the NEC specifies a not to exceed voltage drop percentage for branch circuits of 1.5% and a not to exceed combined voltage drop of feeder and branch-circuit conductors of 2.5%. Special consideration is given for cord-connected equipment in an effort to limit voltage to

1.5% where portable cords may be used as a means of connecting equipment. These requirements, found in 647.4(D)(2) require that the voltage drop on branch circuits supplying receptacles are not to exceed

1%. Additional information is provided with regard to the load current that should be used when calculating voltage drop. This section sets the minimum current to be 50% of the branch-circuit rating. In addition, the maximum voltage drop for the feeder and branch circuit combined cannot exceed 2.0%.

Code reference is 647.6(B), Grounding Conductors Required.

Informational Note No. 1 to this section reminds the user of the NEC that when the current carrying conductors are increased in size for voltage drop considerations, Section 250.122 should be consulted to make the appropriate adjustments to the equipment grounding conductor.

Solar photovoltaic (PV) systems 690.45

“Size of Equipment

Grounding Conductors”. This section of the NEC tells us that for PV systems, when the current carrying conductors are increased in size to account for voltage drop, the requirements of Section 250.122 are not required to be met with regard to the equipment grounding conductor sizing.

Fire pumps, Article 695.6(B): “Conductor Size”. This section provides guidance on sizing the conductors supplying the fire pump motor and reminds the user of the NEC that the voltage drop requirements of

Section 695.7 must also be met.

“Voltage Drop”. requirements for fire pumps in 695.7 focuses more on not exceeding a percent voltage drop for a normal starting condition of the fire pump motor. 695.7(A) titled “Starting” notes that the voltage at the fire pump controller line terminals must not drop more than 15% below the normal voltage when the motor is starting under normal conditions.

7



Energy storage systems 706.31(B): “Intercell and Intertier Conductors and Connections”. The informational note to this section recommends a maximum voltage drop of 3% calculated based on maximum anticipated load. In addition a recommendation is made of not exceeding 5% for total voltage drop to the furthest point of connection. This informational note also reminds the user of the NEC that these voltage drop maximums may not be appropriate for all battery applications. Reference is made to IEEE 1375-2003, Guide for the Protection of Stationary

Battery Systems, for further guidance for overcurrent protection and associated cable sizing.

Conductor impedance, Chapter 9 Table 9: It’s Table 9, “Alternating-

Current Resistance and Reactance for 600-Volt Cables, 3-phase, 60

Hz, 75°C (167°F) — Three Single Conductors in Conduit” of the NEC where guidance on the impedance values is found to use in voltage drop calculations. Note 2 to this table tells us that leveraging the effective impedance Z in voltage drop calculations provides a decent approximation for line-to-neutral voltage drop.

When circuit changes are made to reduce voltage drop, the impact to the available fault current must also be reviewed. Reducing voltage drop is primarily achieved through increasing the size of the current carrying conductors. As conductors sizes get larger conductor impedances get smaller. Lower impedance results in higher fault currents. The effects of increasing fault currents must not be overlooked and include SCCR and selective coordination for circuit breaker applications.

Single-phase calculations

The calculation of voltage drop is a simple basic equation based upon the circuit in Figure 7.7.9.a.

V

Source

V conductor 1 — Line

V conductor 2 — Neutral

Current supplied via Conductor 1 (line) must return via Conductor 2 (neutral) — both conductor lengths must be accounted for when determining voltage drop

V

Load

Figure 7.7.9.a

Line and neutral conductors must both be considered for voltage drop calculations.

Kirchoff’s Voltage Law tells us that the algebraic sum of all voltages in a loop must equal zero. The following equation is constructed:

0 = V source

- V

Conductor 1

- V

Conductor 2

- V

Load

V

Load

= V source

- V

Conductor 1

- V

Conductor 2

The voltage across the conductor is determined by multiplying the load current by the impedance. As per Note 2 of Table 9, the effective impedance Z of the conductor can be used as the conductor impedance.

One can obtain the current used for these calculations a few different ways depending upon the application and the anticipated application:

1. Ampacity of the conductor: This assumption will provide the worst case voltage drop for the circuit as the highest load current would be used resulting in the most voltage drop across the conductor.

2. 80% of the conductor’s ampacity: This assumption is more close to those applications that leverage the sizing of conductors based on

1.25 times the calculated continuous load currents.

3. 50% of the conductor’s ampacity: This assumption is used in

Article 647 for sensitive electronic loads.

4. Maximum anticipated load: This recommendation is used in

Articles 706 for Energy Storage Systems and Article 551 for

Recreational Vehicles and Recreational Parks.

5. Actual load: The actual load on the circuit in question can be used.

The basic equation for calculating voltage drop across a conductor for a two wire DC circuit, a two wire AC circuit, or a three wire AC singlephase circuit is as follows:

Voltage drop = 2 x L x Z

Conductor

x I

Load

÷ 1000

Where:

L = The one way length of the conductor. The factor of 2 in the numerator takes into consideration the entire circuit length including the return path.

Z

Conductor

= Conductor impedance in Ω /1000 ft. This information is from

NEC Chapter 9 Table 9

I

Load

= The current selected at which the voltage drop must be determined for the application. The value here may vary depending upon application.

Percent voltage drop is calculated as:

% Voltage drop = V

Load

- V

Source

÷ V

Source

Example 1: For a 120 V circuit, what is the voltage drop of a 12 AWG conductor that supplies a 15 A load located 100 feet from the power supply?

VD = 2 x L x Z

Conductor

x I

Load

÷ 1000

= 2 x 100 ft. x 1.7 ( Ω ÷100 ft) x 15 A) ÷ 1000 = 5.1 volts

The percent voltage drop assuming a source of 120 V is calculated as:

% Voltage drop = V

Load

- V

Source

÷ V

Source

= 5.1 V ÷ 120 V x 100% = 4.25%

The voltage at the load assuming a source voltage of 120 V is equal to

120 V - 5.1 V or 114.9 V.

Example 2: For a 240 V circuit, what is the voltage drop of a 6 AWG conductor that supplies a 45 A load located 120 ft. from the power supply?

VD = 2 x L x Z

Conductor

x I

Load

÷ 1000

= 2 x 120 ft. x 0.29 ( Ω ÷1000 ft.) x 45 A ÷ 1000 =3.132 V

The percent voltage drop assuming a source of 240 V is calculated as:

% Voltage drop = V

Load

- V

Source

÷ V

Source

= 3.132 V ÷ 240 V x 100% = 1.3%

The voltage at the load assuming a source voltage of 240 V is equal to

240 V - 3.132 V or 236.9 V.

Three-phase calculations

Three-phase systems have to be treated differently than single-phase systems.



Copper conductors — ratings and volt loss†

2

1

0

6

4

3

3

2

1

10

8

6

4

0

00

000

0000

250

300

350

400

Wire size

T, TW

(60°C)

Steel conduit

14 20*

Ampacity/type

RH,

THWN,

RHW,

THW

(75°C)

RHH,

THHN,

XHHW

(90°C)

20* 25*

12

10

8

00

000

0000

250

300

350

400

500

600

25*

30

40

55

70

85

95

110

125

145

165

195

215

240

260

280

320

335

30

40

55

70

85

95

110

125

145

165

195

215

240

260

280

25*

35*

50

65

85

100

115

130

150

175

200

230

255

285

310

335

380

420

35*

50

65

85

100

115

130

150

175

200

230

255

285

310

335

30*

40*

55

75

95

110

130

150

170

195

225

260

290

320

350

380

430

475

750

1000

400

455

475

545

535

615

34

26

36

31

68

62

78

72

84

78

88

82

42

36

Non-magnetic conduit (lead covered cables or installation in fiber or other non-magnetic conduit, etc.)

14 20* 20* 25* 6140 5369 4876 4355 3830 3301 6200

12 25* 25* 30* 3464 3464 3158 2827 2491 2153 4000

40*

55

75

95

110

130

150

170

195

225

260

290

320

350

380

Direct current

6140

3860

2420

1528

982

616

490

388

308

244

193

153

122

103

86

73

64

52

43

2420

1528

982

616

470

388

308

244

193

153

122

103

86

73

64

Three-phase (60 cycle, lagging power factor)

Volt loss

Single-phase(60 cycle, lagging power factor)

100%

5369

3464

2078

1350

848

536

433

346

277

207

173

136

109

93

77

67

60

50

43

2078

1350

848

536

433

329

259

207

173

133

107

90

76

65

57

90%

4887

3169

1918

1264

812

528

434

354

292

228

196

162

136

123

108

98

91

81

75

1908

1255

802

519

425

330

268

220

188

151

127

112

99

89

81

80%

4371

2841

1728

1148

745

491

407

336

280

223

194

163

140

128

115

106

99

90

84

1714

1134

731

479

395

310

255

212

183

150

128

114

103

94

87

70%

3848

2508

1532

1026

673

450

376

312

264

213

188

160

139

129

117

109

103

94

89

1516

1010

657

435

361

286

238

199

174

145

125

113

104

95

89

60%

3322

2172

1334

900

597

405

341

286

245

200

178

154

136

128

117

109

104

96

92

1316

882

579

388

324

259

219

185

163

138

121

110

102

94

89

100%

6200

4000

2400

1560

980

620

500

400

320

240

200

158

126

108

90

78

70

58

50

2400

1560

980

620

500

380

300

240

200

154

124

104

88

76

66

90%

5643

3659

2214

1460

937

610

501

409

337

263

227

187

157

142

125

113

105

94

86

79

72

5630

3647

2203

1449

926

599

490

381

310

254

217

175

147

129

114

103

94

80%

5047

3281

1995

1326

860

568

470

388

324

258

224

188

162

148

133

122

114

104

97

91

84

5029

3264

1980

1310

845

553

456

358

295

244

211

173

148

132

119

108

100

70%

4444

2897

1769

1184

777

519

434

361

305

246

217

184

161

149

135

126

118

109

103

97

90

4422

2877

1751

1166

758

502

417

330

275

230

201

167

145

131

120

110

103

60%

140

128

118

109

103

375

300

253

214

188

159

3812

2486

1520

1019

669

448

148

135

126

120

111

106

102

95

331

283

232

206

178

157

3836

2508

1540

1040

690

468

394

500

600

750

1000

320

335

400

455

380

420

475

545

430

475

535

615

52

43

34

26

46

39

32

25

71

65

58

51

77

72

65

59

80

76

70

63

82

77

72

66

54

46

38

30

82

75

67

59

90

83

76

68

93

87

80

73

94

90

83

77

* The overcurrent protection for conductor types marked with an (*) must not exceed 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper; or 15 A for 12

AWG and 25 A for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

† Figures are L-L for both single-phase and three-phase. Three-phase figures are average for the three-phases.

7



Aluminum conductors — ratings and volt loss†

Ampacity/type

Volt loss

Three-phase (60 cycle, lagging power factor) Single-phase(60 cycle, lagging power factor)

350

400

500

600

750

1000

2

1

0

00

000

0000

250

300

Wire size

T, TW

(60°C)

Steel conduit

12 20*

10 25

8

6

4

3

30

40

55

65

2

1

0

00

000

0000

75

85

100

115

130

150

RH,

THWN,

RHW,

THW

(75°C)

90

100

120

135

155

180

20*

30*

40

50

65

75

RHH,

THHN,

XHHW

(90°C)

100

115

135

150

175

205

25*

35*

45

60

75

85

Direct current

638

506

402

318

259

200

6360

4000

2520

1616

1016

796

100%

554

433

346

277

225

173

5542

3464

2251

1402

883

692

90%

541

432

353

290

241

194

5039

3165

2075

1310

840

668

80%

502

405

334

277

234

191

4504

2836

1868

1188

769

615

70%

458

373

310

260

221

184

3963

2502

1656

1061

692

557

60%

411

338

284

241

207

174

3419

2165

1441

930

613

497

100%

250

300

350

400

500

600

750

170

190

210

225

260

285

320

205

230

250

270

310

340

385

230

255

280

305

350

385

435

169

141

121

106

85

71

56

148

124

109

95

77

65

53

173

150

135

122

106

95

84

173

152

139

127

112

102

92

168

150

138

127

113

105

96

161

145

134

125

113

106

98

172

144

126

110

90

76

62

1000 375 445 500 42 43 73 82 87 89 50

Non-magnetic conduit (lead covered cables or installation in fiber or other non-magnetic conduit, etc.)

12

10

8

20*

25

30

20*

30*

40

25*

35*

45

6360

4000

2520

5542

3464

2251

5029

3155

2065

4490

2823

1855

3946

2486

1640

3400

2147

1423

6400

4000

2600

6

4

3

40

55

65

50

65

75

60

75

85

1616

1016

796

1402

883

692

1301

831

659

1175

756

603

1045

677

543

912

596

480

1620

1020

800

640

500

400

320

260

200

6400

4000

2600

1620

1020

800

210

225

260

285

320

375

75

85

100

115

130

150

170

190

250

270

310

340

385

445

90

100

120

135

155

180

205

230

280

305

350

385

435

500

100

115

135

150

175

205

230

255

121

106

85

71

56

42

638

506

402

318

252

200

169

141

105

93

74

62

50

39

554

433

346

277

225

173

147

122

125

114

96

85

73

63

532

424

344

281

234

186

163

141

125

116

100

90

79

70

490

394

322

266

223

181

160

140

123

114

100

91

82

73

443

360

296

247

209

171

153

136

118

111

98

91

82

75

394

323

268

225

193

160

145

130

122

108

86

72

58

46

640

500

400

320

260

200

170

142

90%

625

499

407

335

279

224

5819

3654

2396

1513

970

771

200

174

156

141

122

110

97

85

615

490

398

325

270

215

188

163

5807

3643

2385

1502

959

760

144

132

111

98

85

73

80%

5201

3275

2158

1372

888

710

580

468

386

320

270

221

200

176

160

146

129

118

107

95

5184

3260

2142

1357

873

696

566

455

372

307

258

209

185

162

145

134

115

104

92

81

70%

529

431

358

301

256

212

4577

2889

1912

1225

799

644

194

173

159

146

131

121

111

100

60%

475

391

328

278

239

201

3948

2500

1663

1074

708

574

186

168

155

144

130

122

114

103

512

415

342

285

241

198

177

157

4557

2871

1894

1206

782

627

142

132

115

106

94

85

456

373

310

260

223

185

167

150

3926

2480

1643

1053

668

555

137

128

114

105

95

86

* The overcurrent protection for conductor types marked with an (*) must not exceed 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper; or 15 A for 12

AWG and 25 A for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

† Figures are L-L for both single-phase and three-phase. Three-phase figures are average for the three-phases.



7.7.10 Overcurrent protective device selection

Once a conductor has been selected considering ampacity as well as considerations for voltage drop, selecting the OCPD can proceed. The basic premise to selecting the overcurrent protective device is that the amp rating of the OCPD must not be greater than the ampacity of the conductor. The conductor’s ampacity must also include any adjustments due to temperature or number of current carrying conductors in a raceway.

Section 240.4(B) provides an allowance to go to the next larger fuse size when the ampacity of the conductor does not correspond to a standard size fuse. This is only applicable when its rating does not exceed 800 A and the conductor is not part of a multi-outlet branch circuit supplying receptacles for cord and plug connected portable loads.

Standard fuse sizes per NEC 240.6 are: 1, 3, 6, 10, 15, 20, 25, 30, 35,

40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,

350, 400, 450, 500, 600, 601, 700, 800, 1000, 1200, 1600, 2000, 2500,

3000, 4000, 5000 and 6000 A.

Note: The small fuse amp ratings of 1, 3, 6 and 10 were added to provide more effective short-circuit and ground-fault protection for motor circuits, in accordance with 430.40 and 430.52 and listing agency requirements for protecting the overload relays in controllers for very small motors.

For fuse amp ratings over 800 A, per 240.4(C), the ampacity of the conductor must be equal to or greater than the rating of the fuse as required in 240.6. For supervised industrial installations, see 240.91.

7.7.11 Tap conductors and OCPDs

Fuses must be installed at the point where the conductor receives its supply, i.e., at the beginning or lineside of a branch circuit or feeder,

NEC 240.21.

• (B)(1) OCPDs are not required at the conductor supply if a feeder tap conductor is not over ten feet long; is enclosed in raceway; does not extend beyond the switchboard, panelboard or control device which it supplies; and has an ampacity not less than the combined computed loads supplied, and not less than the rating of the equipment containing an overcurrent device(s) supplied, unless the tap conductors are terminated in a fuse not exceeding the tap conductor ampacity. For field installed taps, the ampacity of the tap conductor must be at least 10% of the overcurrent device protecting the feeder conductors [240.21(B)(1)].

• (B)(2) OCPDs are not required at the conductor supply if a feeder tap conductor is not over 25 feet long; is suitably protected from physical damage by being enclosed in an approved raceway or other approved means; has an ampacity not less than 1/3 that of the device protecting the feeder conductors and terminate in an overcurrent device sized not more than the tap conductor ampacity [240.21(B)(2)].

• (B)(3) OCPDs are not required at the conductor supply if a transformer feeder tap has primary conductors at least 1/3 the ampacity of the overcurrent device protecting the feeder, and secondary conductors are at least 1/3 the ampacity of the overcurrent device protecting the feeder, when multiplied by the transformer turns ratio. The total length of one primary plus one secondary conductor (excluding any portion of the primary conductor that is protected at its ampacity) is not over

25 feet in length; the secondary conductors terminate in an OCPD rated at the ampacity of the tap conductors; and if the primary and secondary conductors are suitably protected from physical damage

[240.21(B)(3)].

• (B)(4) OCPDs are not required at the conductor supply if a feeder tap is not over 25 feet long horizontally and not over 100 feet long total length in high bay manufacturing buildings where only qualified persons will service such a system. Also, the ampacity of the tap conductors is not less than 1/3 of the fuse rating from which they are supplied. The size of the tap conductors must be at least 6 AWG copper or 4 AWG aluminum. They may not penetrate walls, floors, or ceilings, and the taps are made no less than 30 feet from the floor.

The tap conductors terminate in an OCPD that limit the load to the tap conductors’ ampacity. They are physically protected by being enclosed in an approved raceway or other approved means and contain no splices.

[240.21(B)(4)].

50 A rated conductor

150 A feeder fuse

100 A fuse

480 V 240 V


300 A feeder fuse

2:1

Transformer


25 feet or less


100 A fuse

480 V 480 V


1:1

Transformer


Figure 7.11.a

Note: Smaller conductors tapped to larger conductors can be a serious hazard. If not adequately protected against short-circuit conditions (as required in NEC 110.10 and 240.1(FPN)), these unprotected conductors can vaporize or incur severe insulation damage. Molten metal and ionized gas created by a vaporized conductor can envelop other conductors (such as bare bus), causing equipment burndown. Adequate short-circuit protection is recommended for all conductors. When a tap is made to a switchboard bus for an adjacent panel, such as an emergency panel, the use of Bussmann series cable limiters is recommended for protection of the tapped conductor. These current-limiting cable limiters are available in sizes designed for short-circuit protection of conductors from 12 AWG to 1000 kcmil. Bussmann series cable limiters are available in a variety of terminations to make application to bus structures or conductors relatively simple.

• (B)(5) OCPDs are not required at the supply for an outside tap of unlimited length where all of the following are met:

•

The conductors are outdoors except at the point of load termination.

•

The conductors are protected from physical damage in an approved manner.

•

The conductors terminate in an OCPD that limits the load to the ampacity of the conductors.

•

The fuses are a part of or immediately adjacent to the disconnecting means.

•

The disconnecting means is readily accessible and is installed outside or inside nearest the point of entrance or where installed inside per 230.6 nearest the point of conductor entrance [240.21(B)

(5)]. See the Figure7.11.b.

500 kcmil

150 kVA

3-phase

208/120 V secondary

1 AWG

(rated 130 A)

Fuse or CB

125 A max

Figure 7.11.b

• (C)(1) OCPDs are not required on the secondary of a single-phase two-wire or three-phase, three-wire, Delta-Delta transformer to provide conductor protection where all of the following are met:

•

The transformer is protected in accordance with 450.3.

7



•

The overcurrent protective device on the primary of the transformer does not exceed the ampacity of the secondary conductor multiplied by the secondary to primary voltage ratio. [240.21(C)

(1)]. Selecting the next higher standard size overcurrent protective device is NOT allowed.

• (C)(2) OCPDs are not required on the secondary of a transformer to provide conductor protection where all of the following are met:

•

The secondary conductors are not over 10 feet long.

• The secondary conductors’ ampacity is not less than the combined computed loads.

• The secondary conductor ampacity is not less than the rating of the device they supply or the rating of the overcurrent device at their termination. Selecting the next higher standard size overcurrent protective device is NOT allowed.

•

The secondary conductors do not extend beyond the enclosure(s) of the equipment they supply and they are enclosed in a raceway.

•

For field installations where the secondary conductors leave the enclosure or vault from where they receive their supply, the secondary conductor ampacity is not less than 1/10 of the rating of the OCPD protecting the transformer’s primary multiplied by the turns ratio. [240.21(C)(2)].

• (C)(3) Transformer secondary conductors do not require fuses at the transformer terminals when all of the following conditions are met.

•

Must be an industrial location.

• The conditions of maintenance and supervision in a given industrial location ensure that only qualified personnel service the system.

• Secondary conductors must not be more than 25 feet long.

•

Secondary conductor ampacity must be at least equal to the secondary full-load current of transformer and sum of terminating, grouped, overcurrent devices. Selecting the next higher standard size overcurrent protective device is NOT allowed.

•

Secondary conductors must be protected from physical damage in an approved raceway or other approved means. [240.21(C)(3)].

Note: Switchboard and panelboard protection (408.36) and transformer protection (450.3) must still be observed.

• (C)(4) Outside conductors that are tapped to a feeder or connected to the secondary terminals of a transformer do not require OCPD protection when all of the following are met:

•

The conductors are protected from physical damage in an approved means.

•

The conductors terminate in an OCPD, no larger than the ampacity of the conductors.

•

The conductors are outside, except for point of load termination.

•

The overcurrent device is near or a part of the disconnecting means.

•

The disconnecting means is readily accessible outdoors or, if indoors, nearest the point of the entrance of the conductors or where installed inside per 230.6 nearest the point of conductor entrance [240.21(C)(4)].

7.7.12 Small conductors

NEC 240.4(D) determines protection of small conductors. The overcurrent protective device is required to not exceed the following, unless specifically permitted by 240.4(E) for tap conductors or 240.4(G) for specific conductor applications:

• 18 AWG Copper — 7 A or less provided continuous loads do not exceed 5.6 A and overcurrent protection is provided by one of the following:

•

Class CC, J, or T fuses

•

Branch circuit-rated fuses or circuit breakers listed and marked for use with 18 AWG copper wire

• 16 AWG copper — 10 A or less provided continuous loads do not exceed 8 amps and overcurrent protection is provided by one of the following:

•

Class CC, J, or T fuses

•

Branch circuit-rated fuses or circuit breakers listed and marked for use with 16 AWG copper wire

• 14 AWG copper or 12 AWG aluminum and copper-clad aluminum —

15 A or less

•

12 AWG copper - 20 A or less

•

10 AWG aluminum and copper-clad aluminum - 25 A or less

• 10 AWG copper - 30 A or less

It’s important to note that 310.106 (and Table 310.106(A)) lists the minimum size conductor as 14 AWG. 16 and 18 AWG conductors can only be used provided they are permitted elsewhere in the Code. In addition to allowances for small motors per 430.22(G) 16 and 18 AWG conductors are permitted for power circuits in industrial machinery per

NFPA 79 and UL 508A. However, there are strict limitations on the overcurrent protection. See NFPA 79 for more information.

7.7.13 Flexible cords

Per NEC 240.5 flexible cords and extension cords must have overcurrent protection rated at their ampacities. Supplementary fuse protection is an acceptable method of protection. For 18 AWG fixture wire of 50 feet or more, a 6 A fuse would provide the necessary protection. For 16

AWG fixture wire of 100 feet or more, an 8 A fuse would provide the necessary protection.

For 18 AWG extension cords, a 10 A fuse would provide the necessary protection for a cord where only two conductors are carrying current, and a 7 amp fuse would provide the necessary protection for a cord where only three conductors are carrying current.

7.7.14 Exception for listed surge protective devices

Exceptions to 240.21(B)(1)(1)b and

240.21(C)(2)(1)b permits sizing of tap conductors for listed surge protective devices and other listed non-energy consuming devices to be based on the manufacturer’s instructions.

This surge protective device is prewired with specific conductors that are shown in the device’s instructions. Surge protective devices are non-energy consuming devices that do not have a calculated load as referenced by

240.21(B)(1)(1)b and 240.21(C)(2)(1)b.

For surge protective devices visit Eaton.

com/bussmannseries.

7.7.15 Battery conductors

Conductors connected to storage battery systems must be protected in accordance with their ampacity per 240.4. For non-hazardous environments the location of the overcurrent protective device must be as close as practicable to the storage battery terminals in accordance with 240.21(H). The installation of overcurrent protective devices on battery systems in hazardous locations is permitted. However, the additional requirements for hazardous locations must be followed.



7.7.16 Branch circuits — lighting and/or appliance load

(no motor load)

The branch circuit rating must be classified in accordance with the rating of the overcurrent protective device. Classifications for those branch circuits other than individual loads must be: 15, 20, 30, 40 and 50 A

(210.3).

Branch circuit conductors must have an ampacity of the rating of the branch circuit and not less than the load to be served (210.19).

The minimum size branch circuit conductor that can be used is 14 AWG

(210.19). For exceptions to minimum conductor size, see 210.19.

Branch circuit conductors and equipment must be protected by a fuse with an amp rating which conforms to 210.20. Branch circuit conductor ampacity must be at least the larger of two calculations (a) or (b): a. The sum of non-continuous load plus 125% of the continuous load.

b. Maximum load to be served after applying adjustment or correction factors (as calculated per Article 220).

An example calculation is shown in NEC Information Annex D, Example

D3(a). The fuse size must not be greater than the conductor ampacity

(for exceptions, see 210.20). Branch circuits rated 15, 20, 30, 40 and 50

A with two or more outlets (other than receptacle circuits of 210.11(C)(1) and (C)(2) must be fused at their rating and the branch circuit conductor sized according to Table 210.24 (see 210.24).

7.7.17 Feeder circuits (no motor load)

The feeder fuse amp rating and feeder conductor ampacity minimum size must be at least the larger of two calculations (a) or (b): (a) the sum of non-continuous load plus 125% of the continuous load or (b) maximum load to be served after applying adjustment or correction factors (as calculated per Article 220). An example calculation is shown in NEC Information Annex D, Example D3(a). The feeder conductor can be protected by an OCPD the next higher standard overcurrent device rated (above the ampacity of the conductors being protected) as long as the OCPD size is rated 800 A or less. If the OCPD size is greater than

800 A, the conductor must be protected by an OCPD not greater than the conductor ampacity (for exceptions, see 240.3). Motor loads must be computed in accordance with Article 430; Conductors for motor branch and feeder circuits in Section 7.14.5.

7.7.18 Service conductors

Each ungrounded service entrance conductor must have a fuse in series with an amp rating not higher than the ampacity of the conductor (for exceptions, see 230.90(A) which permits the use of 240.4(B) or (C) and

240.6. The service fuses must be part of the service disconnecting means or be located immediately adjacent thereto (230.91).

Service disconnecting means can consist of one to six switches for each service (230.71) or for each set of service entrance conductors permitted in 230.2. When more than one switch is used, the switches must be grouped together (230.71).

Service equipment must have adequate short-circuit ratings for the available fault current.

110.24 requires the maximum available fault current and date of calculation to be field marked on the service equipment. This is to ensure that the overcurrent protective devices have sufficient interrupting rating and that the service equipment short-circuit current rating are equal to or exceed the available short-circuit current. If electrical installation modifications are made the maximum available fault current should be recalculated and new field marking for the service equipment. It should be verified that the service equipment short-circuit current rating and overcurrent protective devices’ interrupting ratings are adequate for the new available fault current.

110.24 is not required for dwelling units or certain industrial installations.

7.7.19 Wire and cable short-circuit protection

The circuit shown originates at a distribution panel where 40 kA RMS

Sym. fault current is available. To determine the proper fuse, first establish the short-circuit withstand data for the 10 AWG THW copper cable shown in Figure 7.7.19.a

40 kA

RMS Sym.

available

Distribution panel

Low-Peak LPS-RK-30SP dual-element fuse

Fault

X

To load

10 AWG THW copper

Figure 7.7.19.a.

The following table shows the short-circuit withstand of copper cable with 75°C thermoplastic insulation based on Insulated Cable Engineers

Association (ICEA) formulas.

The short-circuit withstand of the 10 AWG THW copper conductor is

4300 A for one cycle (0.0167 seconds). This conductor’s short-circuit protection requires selecting an OCPD that will limit the 40 kA RMS

Sym. available to a value less than 4300 A, and clear the fault in one cycle or less.

The Low-Peak LPS-RK30SP dual-element fuse let-through graph in

Section 3 shows that it will let-through an apparent prospective RMS current that’s less than 1800 A, when 40 kA is available (and would clear the fault in less than 1/2 cycle).

Fault currents for insulated cables

Increases in power distribution system kVA capacity have resulted in the potential for extremely high magnitude fault currents that can seriously damage conductor insulation from the induced, high conductor temperatures. As a guide in preventing such serious damage, maximum allowable short-circuit temperatures, which begin to damage conductor insulation, have been established for various insulation types as follows:

• Paper, rubber and varnished cloth, 200°C

• Thermoplastic, 150°C

The graph in Figure 7.7.19.c shows the currents which, after flowing for the times indicated, will produce these maximum temperatures for each conductor size. The system short-circuit capacity, conductor cross-sectional area and OCPD opening time should be such that these maximum allowable fault currents are not exceeded.

Using the formula shown on the ICEA protection table will allow calculating conductor withstand ratings. It may be advantageous to calculate withstand ratings below one cycle, when the opening time of the current-limiting device is known; see table below. See Bussmann series current-limiting fuse let-through data to obtain LPS-RK data in

Section 3.

6

4

14*

12*

10

8

Copper wire size, 75°C thermoplastic

18*

16*

Maximum fault current withstand in amps

1/8 cycle*

1/4 cycle*

1/2 cycle*

1 cycle

2 cycles

3 cycles

1850 1300 900 700

3000 2100 1500 1100

500

700

400

600

4800 3400 2400 1700 1200 1000

7600 5400 3800 2700 1900 1550

12,000 8500 6020 4300 3000 2450

19,200 13,500 9600 6800 4800 3900

30,400 21,500 16,200 10,800 7600 6200

48,400 34,200 24,200 17,100 12,100 9900

* Extrapolated data.

Table 7.7.19.b Copper, 75° thermoplastic insulated cable damage at 60 Hz.

7



100,000

80,000

60,000

40,000

30,000

20,000

Allowable fault currents for thermoplastic insulated copper conductors*

10,000

8,000

6,000

4,000

3,000

2,000

1,000

800

600

400

300

200

100

1 Cycle - 0.0167 second

2 Cycles - 0.0033 second






100 Cycles - 1.6667 seconds

Conductor size

Curves based on formula:

[

I

A

]

2

t = 0.0297 log

[

T

2

+234

T

1

+234

]

Where:

I = short-circuit current - Amps

A = Conductor area - Circular mils t = Time of short-circuit - seconds

T

1

= Maximum operating temperature - 75°C

T

2

= Maximum short-circuit temperature - 150°C

* © 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA).

Permission has been given by ICEA to reprint this chart.

Figure 7.7.19.c Short-circuit current withstand graph for copper cables with thermoplastic insulation.

Protecting equipment grounding conductors

Safety issues arise when the equipment grounding conductor analysis

(EGC) is discussed. NEC Table 250.122 offers minimum sizing for equipment grounding conductors.

Equipment grounding conductors are much more difficult to protect than phase conductors because the OCPD is most often several sizes larger than the equipment grounding conductor’s ampacity.

The equipment grounding conductor protection problem was recognized decades ago when Eustace Soares, wrote his famous book “Grounding

Electrical Distribution Systems for Safety.” In his book he states that the “validity” rating corresponds to the amount of energy required to cause the copper to become loose under a lug after the conductor has had a chance to cool down. This validity rating is based upon raising the copper temperature from 75°C to 250°C.

In addition to this and the ICEA charts, a third method promoted by

Onderdonk allows for calculating the energy necessary to cause the conductor to melt (75°C to 1083°C).

Table 7.7.19.d summarizes the values associated with various size copper conductors.

The word “Minimum” in the heading of NEC Table 250.122 means just that - the values are minimums - they may need increasing due to the available fault current and the OCPD’s current-limiting, or non-currentlimiting ability. 250.4(A)(5) and 250.4(B)(4) require grounding conductors sized adequately for the fault current that could be let-through.

This means that based on the available fault current and the OCPD characteristics (let-through current), the grounding conductor may have to be sized larger than the minimum in Table 250.122.

Good engineering practice requires calculating available fault currents

(three-phase and phase-to-ground values) wherever equipment grounding conductors are used. OCPD manufacturers’ literature must be consulted. Let-through energy for these devices should be compared with the equipment grounding conductor’s short-circuit current withstand. Wherever let-through current exceeds the “minimum” equipment grounding conductor withstand, the equipment grounding conductor size must be increased until the withstand is not exceeded.



Conductor size

18

16

14

12

10

8

6

1

1/0

2/0

4

3

2

500

600

700

750

800

900

1000

3/0

4/0

250

300

350

400

7101

8285

9468

11,835

14,202

16,569

1981

2500

3150

3972

5009

5918

17,753

18,936

21,303

23,670

5 Sec. rating (amps)

ICEA P32-382 insulation damage

150°C

38

61

97

155

246

391

621

988

1246

1571

Soares 1 amp/30cm validity

250°C

55

88

137

218

346

550

875

1391

1754

2212

2790

3520

4437

5593

7053

8333

10,000

11,667

13,333

16,667

20,000

23,333

25,000

26,667

30,000

33,333

Table 7.7.19.d Comparison of equipment grounding conductor fault current withstands.

5144

6490

8180

10,313

13,005

15,365

18,438

21,511

24,584

30,730

36,876

43,022

46,095

49,168

55,314

61,460

Onderdonk melting point

1083°C

99

158

253

401

638

1015

1613

2565

3234

4078

252.0

343.0

448.0

700.0

1008.0

1372.0

19.6

31.2

49.6

78.9

125.0

175.0

1576.0

1793.0

2269.0

2801.0

I 2 t rating x10 6 (amps squared seconds)

ICEA P32-382 insulation damage

Soares 1 amp/30cm validity

150°C

0.007

250°C

0.015

0.019

0.047

0.120

0.039

0.094

0.238

0.303

0.764

1.93

4.88

7.76

12.3

0.599

1.51

3.83

9.67

15.4

24.5

500.0

680.0

889.0

1389.0

2000.0

2722.0

38.9

61.9

98.4

156.0

248.0

347.0

3125.0

3556.0

4500.0

5555.0

132.0

210.0

331.0

532.0

845.0

1180.0

1700.0

2314.0

3022.0

4721.0

6799.0

9254.0

10,623.0

12,087.0

15,298.0

18,867.0

Onderdonk melting point

1083°C

0.049

0.124

0.320

0.804

2.03

5.15

13.0

32.9

52.3

83.1

Take the example in Figure 7.17.19.e. The EGC to be protected can withstand 4300 A for 1 cycle. A current-limiting fuse will limit the current to within the EGC’s withstand rating. An LPS-RK60SP will limit the line-to-ground current to approximately 3300 A and provide sufficient protection.

Grounded service neutral

Compliance

50,000 A RMS

Grounding electrode

Metal service equipment enclosure

60 A current limiting fuses with

1/4 cycle opening time under fault conditions

10 AWG copper equipment grounding conductor

3Ø load

Non-metallic raceway

Metal enclosure

Conforms to 110.10, Table 250.122, and 250.4(A)(5) or 250.4(B)(4)

Figure 7.7.19.e

Tap conductor sizing by the engineering method

The NEC has additional sizing latitude for feeder tap conductors used in supervised industrial installations. Tap conductors are now considered protected under short-circuit current conditions by using an engineering method to select the conductor size based on the proper feeder OCPD characteristics. This allowance can only be used in supervised industrial installations.

Per 240.2, three conditions must be met to be qualified as a supervised industrial installation:

• The maintenance crew must be qualified and under engineering supervision.

• The premises wiring system load (based on industrial process(es) and manufacturing activities) must be 2500 kVA or greater as calculated in accordance with Article 220.

• There must be at least one service at 277/480 or 480 volts or higher.

The physics formulas shown in Table 240.92(B) are the same as in the

ICEA protection table, and can be used to find the maximum shortcircuit current and time for proper conductor protection under shortcircuit conditions.

7



Table 240.92(B) tap conductor short-circuit current ratings.

Tap conductors are considered to be protected under short-circuit conditions when their short-circuit temperature limit is not exceeded.

Conductor heating under short-circuit conditions is determined by the short-circuit formula for:

(1) Copper conductors (I 2 /A 2 )t = 0.0297 log

10

[(T2 + 234)/(T1 + 234)]

(2) Aluminum conductors (I 2 /A 2 )t = 0.0125 log

10

[(T2 + 228)/(T1 + 228)]

Where:

I = Short-circuit current in amps

A = Conductor area in circular mils t = Time of short-circuit in seconds (for times ≤ to 10 seconds)

T1 = Initial conductor temperature in °C (conductor insulation rating)

T2 = Final conductor temperature in °C (threshold for insulation damage, see Table 7.7.19.e)

Conductor Insulation

Paper, rubber, varnished cloth

Copper

Thermoplastic

Cross-linked polyethylene

Ethylene propylene rubber

Aluminum

Paper, rubber, varnished cloth

Thermoplastic

Cross-linked polyethylene

Ethylene propylene rubber

T2 (°C)

200

150

250

250

200

150

250

250

Table 7.7.19.e

The change in 240.92(B) allows supervised industrial installations increased flexibility for feeder tap conductor where protection can be proven by physics formulas, in lieu of the simple ratios in 240.21(B)(2),

(B)(3) and (B)(4). Thus, feeder tap conductor sizing can be accomplished using formulas for selecting OCPDs based on conductor insulation thermal damage levels and the OCPD’s let-through energy under short-circuit conditions. Previous tap conductor sizing did not take into consideration any fault current or OCPD current-limiting characteristics, only the amp rating ratios that may result in overly conservative tap conductor sizing.

16 and 18 AWG conductors for industrial machinery power circuits

Typically 14 AWG or larger conductors are required for power circuits.

However, per 430.22(G), 240.4(D), NFPA 79 (12.6.1.1 and 12.6.1.2) and

UL 508A (66.5.4 Exception and Table 66.1A) 16 and 18 AWG conductors are permitted for motor and non-motor circuits under specified conditions. Using 16 and 18 AWG conductors reduces wiring costs in industrial machinery. The Table below illustrates where Class CC, J and

T fuses can be used to protect 16 and 18 AWG conductors in power circuits per NFPA 79 and UL 508A.

Sizing Table 7.7.19.f for LP-CC (Class CC) and LPJ (Class J time-delay) fuse protection for 16 and 18 AWG conductors in industrial machinery power circuits per 430.22(G), 240.4(D) NFPA 79 and UL 508A

16 and 18 AWG conductors are easily damaged by fault currents.

Many OCPDs are unable to protect these small conductors. However, the Small Wire Working Group of the NFPA 79 technical committee performed tests and evaluated criteria to demonstrate that Class CC or

J fuses are among those able to provide protection. Other branch-circuit rated fuses or circuit breakers can only be used if marked for protecting

16 and 18 AWG conductors.

UL issued a special service investigation to look into using 16 and

18 AWG conductors for power branch circuits in industrial machinery applications (file number E4273) to verify the test results. The analysis, test program and results can be viewed in an IEEE paper presented at the 2002 IEEE Industrial and Commercial Power Systems Technical

Conference, titled “An Investigation of the Use of 16 AWG and 18 AWG

Conductors for Branch Circuits in Industrial Machinery Built to NFPA 79

2002.”


Conductor size (AWG)

Max load amps

8

16

18

8

5.5

5.6

5

3.5

Load type

Nonmotor

Motor

Motor

Nonmotor

Motor

Motor

Max LP-CC amps

10

300% of motor FLA or next standard size*


7



Max LPJ or

TCF amps

10



7



Motor overload relay trip class

—

Class 10

Class 20

—

Class 10

Class 20

* Standard fuse sizes are 1, 3, 6, 10, 15, 20, 25 and 30. Where the motor’s starting current opens the fuse, the maximum setting can be increased, but not exceed 400% for LP-CC or 225% for LPJ.

Table 7.7.19.f

7.7.20 Cable limiters

Cable limiters are distinguished from fuses by their application for providing only short-circuit protection. They are not designed to provide overload protection. Typically, cable limiters are selected based on conductor size. They are available in a wide range to accommodate the many copper or aluminum conductor sizes and termination methods.

There are two broad cable limiter categories:

• 600 V or less rated for large commercial, institutional and industrial applications

• 250 V or less rated for residential and light commercial applications

In institutional, commercial and industrial systems, cable limiters are used at both ends of each cable on three or more cables-per-phase applications, and located between the transformer and switchboard as shown in Figure 7.7.20.a. For residential systems, the limiters are generally applied on single cables as shown in Figure 7.7.20.b.


Open limiter

Service disconnect

X

Open limiter

A faulted cable is opened and isolated by the limiters on each end to permit the other cables to remain in service and delivering power

Figure 7.7.20.a Multi cable-per-phase commercial/industrial service entrance

#1

#2

#3

#4

Residences

X

Open limiter

The faulted cable is isolated, permitting the other cables to continue supplying power without interruption

Figure 7.7.20.b

Single cable-per-phase basis for residential applications have limiters applied at the lateral feeder source end.

Cable limiters may be located on the supply side of the service disconnecting means, and provide these advantages:

• Isolating one or more faulted cables so that only the affected cable(s) are isolated by the cable limiters opening at each end (assuming three or more cables per phase, with cable limiters on each end).

• Isolating a faulted cable permits more convenient repair service scheduling.

• Equipment burndown hazards caused by a fault on the main OCPD’s lineside is greatly reduced. Typically, without cable limiters, a fault between the transformer and service switchboard is given little or no protection.

• Their current-limiting ability can minimize arc flash hazards by reducing the arc flash current’s magnitude and exposure duration. There are many different limiters available for cables from 12 AWG to 1000 kcmil and many different termination types. Below are the available

Bussmann series 600 V and 250 V cable limiters.

600 V limiters

Catalog no.

Tubular terminals*

Cable size

(AWG/kcmil) Catalog no.

KCY

KCZ

4

3

KCF

KCH

KCA

KCB

KCC

KCD

2

1

1/0

2/0

KCJ

KCM

KCV

KCR

KCE 3/0 KCS

Tubular terminal and offset bolt-type terminal*

KQV

KQT

12

10

KDD

KDE

KFZ

KIG

8

6

KDF

KDH

Cable size

(AWG/kcmil)

4/0

250

350

500

600

750

1000

2/0

3/0

4/0

250

KDY

KDA

KDB

KDC

4

2

1

1/0

KDJ

KDM

KDU

KDR

350

500

600

750

Compression connector rod terminal and tubular terminal*

KEX 4/0 KQO 350

KFH-A 250 KDT 500

Center bolt-type terminal and offset bolt-type terminal**

KPF 4/0 KDP 500

KFT

KEW

250

350

KFM 750

* Copper conductors only.

** Copper or aluminum conductors.

250 V limiters

Catalog no.

UHA

UHJ-M

UHJ-T

UHJ-W

Cu cable size

(AWG/kcmil)

3/0

3/0

350

600

7.7.21 Transformer secondary conductors

Al cable size

(AWG/kcmil)

4/0

250

500

800

Secondary conductors need to be protected from damage by the proper overcurrent protective device. Although 240.4(F) provides an exception for conductors supplied by a single-phase transformer with a two-wire secondary, or a three-phase delta-delta transformer with a three-wire, single voltage secondary, it is recommended that these conductors be protected by fuses on the secondary sized at the secondary conductor ampacity. Due to energizing the transformer’s primary windings inrush current, overcurrent protective devices on the primary may not be able to be sized low enough to meet the requirements of 450.3 and provide protection to the secondary conductors.

7.7.22 Motor circuit conductor protection

Motors and motor circuits have unique operating characteristics and circuit components, and therefore must be dealt with differently than other load types. Generally, two levels of overcurrent protection are required for motor branch circuits:

• Overload protection: Motor running overload protection is intended to protect the system components and motor from damaging overload currents.

• Short-circuit protection (includes ground fault protection): Shortcircuit protection is intended to protect the motor circuit components such as the conductors, switches, controllers, overload relays, etc., against fault currents. This level of protection is commonly referred to as motor branch circuit protection.

Frequently, due to inherent limitations in various OCPD types for motor applications, two or more separate protective devices are used to provide overload and short-circuit protection. An exception is the dualelement fuse. For most motor applications, the benefits of dual-element fuse characteristic allow sizing the Fusetron™ Class RK5 fuses to provide both protection functions for motor circuits.

7



7.8 Electric heat

Resistance type electric space heating equipment rated more than 48 A must have its heating elements subdivided. Each subdivided load must not exceed 48 A, and the fuse for each load should not exceed 60 A

[424.22(B)]. If a subdivided load is less than 48 A, the fuse rating must be 125% of that load.

Exception: Boilers employing resistance type immersion electric heating elements in an ASME rated and stamped vessel may be subdivided into circuits not exceeding 120 A, and protected by a fuse at not more than

150 A [424.22(B) and 424.72(A)]. If a subdivided load is less than 120 A, the fuse rating must be 125% of that load.

Fuse sizing per NEC 424

Electric resistance heating appliances using elements rated more than 48 A must have the heating elements subdivided such that each subdivision does not exceed 48 A, and each subdivision must be protected by a branch circuit listed fuse not to exceed 60 A. These fuses must be factory installed by the manufacturer, be accessible and be suitable for branch circuit protection [422.11(F)(1)].

Heating type

Space heating

Boilers (ASME rated and stamped vessel)

Sizing

125% or next size larger but in no case larger than

60 amps for each subdivided load

125% or next size larger but in no case larger than

150 amps for each subdivided load

Fuse/volt recommendation

Up to 250 V: LPN-RK-SP, FRN-R, NON

Up to 300 V: JJN

Up to 480 V; SC 25 to SC 60

Up to 600 V: LPS-RK-SP, FRS-R, NOS, JJS, LPJ_SP, LP-CC, FNQ-R,

JKS, KTK-R, TCF, SC 1/2 to SC 20, FCF_RN

7.9 Elevators

Contents

7.9.1 Overview

7.9.2 NEC 620.62 requirements

7.9.3 2017 NEC and SCCR marking requirements

7.9.1 Overview

Section page

44

45

46

Where the elevator shaft and/or equipment room has a sprinkler fire suppression system installed, the elevator Code (ANSI/ASME A17.1) requires main line power to the elevator be removed prior to the application of water. This is typically accomplished via a shunt trip device. NEC 620.51(B) allows this requirement to be implemented at the elevator disconnecting means.

There are options available to achieve this depending upon the customer’s needs. The simplest option is to use a shunt trip circuit breaker in either the feeder supplying the elevator or the elevator disconnect. For elevators with battery lowering systems, an additional contact is required per NEC 620.91(C) to disable the battery lowering system when the elevator disconnect is manually operated for maintenance. NFPA 72 requires the control circuit between the Fire

Alarm System and the shunt trip be monitored for integrity. In addition, the shunt-trip voltage must also be monitored by the Fire Alarm

System. Loss of voltage to the control circuit for the disconnecting means must cause a supervisory signal to be indicated at the control unit and required remote annunciation. In some cases, it may require a means to test the shunt trip operation or have one contact operate the shunt trip of more than one elevator. Per the 2017 NEC, if the elevator is designated as an emergency system load, the disconnect must be protected by a surge protective device (SPD). All of these options and special wiring can be challenging depending upon the location of the shunt trip circuit breaker as well as who has responsibility for providing these additional options. Because of this, several manufacturers offer an all-in-one shunt trip elevator disconnect switch that includes all the prewired accessories needed to comply with the various Code sections.

The Bussmann series Power Module™ is a simple solution available to engineering consultants, contractors and inspectors to help comply with all of these requirements in one enclosure.

The Power Module contains a shunt trip molded case switch with trailing fuses together with the components necessary to comply with the fire alarm system requirements and shunt trip control power all in one package. This greatly simplifies the specification for engineering consultants. This also greatly simplifies installation for contractors because it only requires connecting the appropriate wires. This simplifies things for inspectors, too, because everything is in one place with the same wiring every time. The Power Module utilizes Low-Peak LPJ-

SP fuses to protect the elevator branch circuit from the damaging effects of fault currents as well as helping to provide an easy selective coordination method when supplied with upstream Low-Peak fuses with at least a 2:1 amp rating ratio. More information about the Bussmann series Power Module can be found at Eaton.com/bussmannseries.

Typical elevator installations

The architect has a choice of either traction or hydraulic elevators.

Traction elevators are typically faster and more energy efficient than hydraulic elevators, and are often used for high-rise buildings. Hydraulic elevators are typically more cost effective and used for buildings up to 5-6 stories. Traction elevators are typically installed in a “bank of elevators” where fused switches, or circuit breakers in a panelboard are located in the machine room serving the bank of elevators. Each fused switch or circuit breaker in the panelboard is used as an elevator disconnect. Hydraulic elevator installations are typically supplied from the main switchboard and have an elevator fused switch or circuit breaker in the machine room that serves as the elevator disconnect.

As mentioned, traditional installations of a traction or hydraulic elevator include a separate machine room. The vast majority of equipment serving the elevator is located within this room (i.e., elevator control panel). Also located in this room might be, but not limited to: exhaust fan, cooling unit (depending on local requirements and/or requirements set forth by the individual elevator supplier), lighting, voice and/or data drop serving the elevator cab emergency phone, elevator control panel primary fused disconnect, elevator feeder shunt-trip circuit (i.e., shunttrip circuit breaker), elevator cab lighting fused switch, and convenience receptacle(s).

Machine room less (MRL) elevator installations incorporate the elevator control panel and often the primary disconnecting means in a convenient package that is located within the elevator shaft. However, space is frequently limited within the shaft and as such many ancillary components (shunt trip circuit breakers, fused disconnects) may need to be located outside of the elevator shaft. It is important for design engineers to communicate this with the architect and owner during the design phase, as it will impact space needs elsewhere in the facility to accommodate electrical equipment.



A traditional elevator installation (which includes a machine room) requires access inside the elevator shaft. Convenience receptacles and lighting for maintenance purposes are required at the base of the pit and, in some instances (elevator manufacturer dependent), at the top of the elevator shaft. It is important for the design engineer to coordinate with the architect to determine the elevator manufacturer basis of design to determine if and when additional power and lighting is required.

It is of great importance for the design engineer to communicate with their local authority having jurisdiction (AHJ) to determine fire alarm requirements for the respective elevator installation. While machine roomless designs have been commercially available for many years, their use may still be unknown to some AHJs. Design practices that are acceptable in some jurisdictions may not be in others. As always, the best advice in all instances — do your homework, communicate often, and document decisions made. Years may lapse between the design and installation stages of a project. Good documentation is of key importance to recalling what decisions were made and why.

7.9.2 NEC 620.62 requirements

NEC 620.62, “Selective Coordination,” requires that where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means must be selectively coordinated with any other supply side overcurrent protective devices.

A design engineer must specify and the contractor must install main, feeder, sub-feeder and branch circuit protective devices that are selectively coordinated for all overload and fault current values.

7.9.2.1 Example 1

To better understand how to assess selectively coordinated overcurrent protective devices in an electrical system refer to this publication’s section on selective coordination. Figure 7.9.2.1.a offers a brief elevator system coordination assessment using fuses in Power Module elevator disconnects with upstream fuses in the feeders and main.

The fuse selectivity ratio guide is all that’s needed to ensure selectivity for any fault current in this system. To assure selective coordination, the

Low-Peak fuses selected require only a minimum a 2:1 amp rating ratio.

In this example, there is a 4:1 ratio between the 1600 A main fuse and the first level 400 A feeder fuse, and a 2:1 ratio between the first level

400 A feeder fuse and the 200 A second level feeder fuse. There is a 2:1 ratio between the second level 200 A feeder fuse and the 100 A branch circuit fuse. Since a minimum of a 2:1 ratio is satisfied at all levels for this system, selective coordination is achieved and 620.62 is met.

Because the ratio tables were utilized, drawing fuse time-current curves was not necessary to assess selective coordination. The fuse timecurrent curves for this example are shown as Figure 7.9.2.1.b.

0.8

1

0.6

0.4

0.3

0.2

1,000

800

600

400

300

200

10

8

6

4

3

2

100

80

60

40

30

20

KRP-C-1600SP

LPS-RK-400SP

LPS-RK-200SP

LPJ-100SP

4:1

Utility transformer

1600 A main KRP-C-1600SP fuse

400 A feeder LPS-RK-400SP

0.1

0.08

0.06

0.04

0.03

0.02

2:1

0.01

7

200 A feeder LPS-RK-200SP

Current in amps

2:1

Figure 7.9.2.1.b

A selectively coordinated system using fuses.

100 A branch LPJ-100SP

M M M

EL

1

EL

2

EL

3

Figure 7.9.2.1.a

Coordination assessment using fuses.

Using the one-line diagram of Figure 7.9.2.1.a, perform a coordination study to determine if the system complies with the 620.62 selective coordination requirements. EL-1, EL-2, and EL-3 are elevator motors.

Section 4.2 offers a more in-depth discussion on how to analyze systems to determine if selective coordination can be achieved.

In this example, LPJ-100SP fuses are used for the branch protection,

LPS-RK-200SP and LPS-RK-400SP fuses for the feeder protection, and a

KRP-C-1600SP fuse for the main protection.

Power Module switch

The Bussmann series Power

Module™ switch (PS) and Power

Module Panel (PMP) come factory configured and only require electrical connection for installation

Power Module panel



Elevator control panels

Traditionally, the electrical system design engineer has given little attention to the elevator control panel. In fact, an entire elevator system for a project is most often designed and specified by the architect who has little understanding of the electrical distribution system. Because of this, there is often a communications gap between the electrical system designer, whose design typically stops at the required elevator disconnecting means, and the architect who designs the elevator system. To address this issue, new requirements have been added to the 2017 NEC that deal with the proper installation of elevator control panels.

These requirements were added to the 2017 NEC due to the concern that elevator control panels can be installed in locations where the available fault current can be high and the inspector may not be able to clearly determine the SCCR of the elevator control panel unless it is marked on the equipment. In addition, The 2017 NEC now requires the elevator control panel marked SCCR value must be adequate for the available fault current to ensure a safe installation. In order to aid enforcement, the 2017 NEC additionally requires marking the available fault current at the elevator control panel. Below is a summary of these new 2017 NEC sections:

• 620.16(A) Manufacturers must mark their elevator control panels with an equipment short-circuit current rating (SCCR). The SCCR can be based on an assembly product standard listing and labeling, or an approved analysis method, such as UL 508A, Supplement SB.

• 620.16(B) If the elevator control panel SCCR is not equal to or greater than the available fault current, the elevator control panel must not be installed.

• 620.51(D)(2) An elevator control panel must now be field marked with the maximum available fault current along with the date the calculation was made. Further, if there is a change to the available fault current, then this field marking must be revised.

Complying with these new requirements

In order to comply with these requirements a chain of events must occur.

1. The electrical system designer calculates the maximum available fault current at the elevator control panel.

2. The electrical system designer needs to communicate this information to the person responsible for specifying the elevator control panel, which is most often the architect.

3. The party responsible for procuring the elevator control panel must state the minimum acceptable SCCR or the maximum available fault current where the controller will be installed.

4. The elevator control panel manufacturer must determine the required elevator control panel SCCR as stated in the elevator control panel specification and provide an elevator control panel SCCR that is equal to or greater than the available fault current that’s indicated in the design documents where there are multiple elevator control panels at different locations.

With these new NEC requirements, there is now the potential that an elevator control panel SCCR may be required that’s higher than what elevator manufacturers have historically furnished. As such, elevator controller manufacturers should make the effort to rethink their designs to avoid costly system design changes. For instance, a typical elevator control panel may traditionally have SCCR ratings from 5 to 10 kA.

However, it is likely that for many elevator control panels, this may not be high enough for the available fault current where they will eventually be installed. Some elevator control panel manufacturers believe this is an issue for the electrical system designer and installer to address and remedy by adding impedance to the system for lowering the available fault current. This, however, can result in a dramatic increase in cost, space availability (lack thereof), and reduced efficiency. For instance, one method to reduce the available fault current is to add an isolation transformer ahead of the elevator control panel.


Many new buildings are incorporating an elevator design that does not include an equipment room (sometimes called MRL elevators).

All equipment in a MRL design is installed within the elevator shaft or exterior compartment near the elevator shaft, including the elevator control panel and the elevator disconnecting means. In these installations, it is best to design the elevator control panel so the elevator control panel SCCR is adequate for the available fault current.

This is something that can be easily accomplished if the elevator control panel manufacturer uses components in combination with OCPDs that are tested and listed with high combination SCCRs. In many cases, the

OCPDs that are suitable for this level of protection are current-limiting fuses rather than a traditional circuit breaker. In this case, it may be best if the elevator control panel manufacturer does not include the elevator disconnecting means (often offered as a circuit breaker) but instead have the installer provide a separate fused disconnect to comply with NEC

620.51(A) and also achieve a high SCCR for the elevator control panel when fuses are provided as specified by the elevator control panel and marked on the elevator control panel nameplate.

7.9.3 2017 NEC and SCCR marking requirements

These are to be found in NEC Chapter 6 Special Equipment, Article 620

Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts, and

Stairway Chairlifts and pertain to:

• Part II. Conductors, 620.16 Short-Circuit Current Rating

• Part VI. Disconnecting Means and Control

•

620.51 Disconnecting Means

• 620.51(D)(2) Available Short-Circuit Current Field Marking

Significance of the additions

These new requirements (see Figure 7.9.c for marking examples) make it easier for inspectors to verify elevator control panels have sufficient equipment SCCR for the available short-circuit current.

• 620.16(A) - manufacturers must mark their elevator control panels with an equipment short-circuit current rating (SCCR). The SCCR can be based on an assembly product standard listing and labeling, or an approved analysis method, such as UL 508A, Supplement SB.

• 620.51(D) - An elevator control panel must now be field marked with the maximum available short-circuit current along with the date the calculation was made.

• 620.16(B) - if the SCCR of an elevator control panel is not equal to or greater than the available short-circuit current, the elevator control panel must not be installed.

• With the marked SCCR and marked available short-circuit current on the elevator control panel, an inspector can easily check for compliance.

The person responsible for the specifications or for ordering the elevator controls needs to communicate the minimum elevator control panel

SCCR level that is required for a specific installation to the equipment supplier. This requires knowing the available short-circuit current.

Elevator control suppliers should require customers to provide the minimum SCCR level as necessary in their specification data.

If there is a change to the available short-circuit current, then 620.51(D)

(2) requires revising the field marking. Then the equipment SCCR must be verified that it is not less than this revised available short-circuit current.

Changes to the electrical distribution system can increase the available short-circuit current when a larger kVA or lower percent impedance transformer is installed, or premise wiring changes.


Sample Label

Model:

Job #:

Input:

SCE-AC-0 X

XXXXXXXXXX

XXXV rms,

Output: Motor:

Brake:

3

XXX hp,

XXXX Vdc

Date:

XXX

08/25/2016

Ø, 50/60Hz, XXXXA rms

XXX Amps DC

Serial #:

XX.X

Hz,

8001424

XXXXX FLA

Suitable for use on a circuit capable of delivering not more

10,000 600

CONVIENT À CIRCUIT POUVANT DÉBITER UN COURANT DE DÉFAUT

10,000

UNE TENSION MAXIMALE DE V.

ETL Recognized

3084676

CONFORMS TO

ANSI/ASME STD A17.5

CERTIFIED TO

CAN/CSA STD B44.1

EN 12016 Compliant

46-03-0129 R1 Motion Control Engineering A Nidec Kinetek Company www.nidec-MCE.com MADE IN USA

Project Name:

Fault Name:

System:

Office Building A

Elevator Tower B

Three-Phase

Avail. Fault Current L-L-L

(Amps) :

9,055

Voltage L-L (Volts) : 480

Calculation Performed On: Sep 2, 2016 @ 10:20am

Calculation performed via Eaton's Bussmann Series Available Fault Current Calculator v1.5

Figure 7.9.c

Elevator control panel has a manufacturer’s nameplate

SCCR of 10 kA and is field marked with available short-circuit current

9055 A. Photo courtesy of MCE.

What to look for:

• The elevator control panel’s manufacturer nameplate is marked with its SCCR.

• The control panel is field marked with the available short-circuit current and documentation supporting the calculation is available.

• If the control panel’s SCCR is equal to or greater than the marked available short-circuit current.


7


7.10 Generator protection

If a short-circuit is applied directly to a synchronous generator’s output terminals, it will initially produce an extremely high current, then gradually decay to a steady-state value. This change is represented by a varying reactive impedance. Three specific reactances are used for short-circuit fault currents:

• Subtransient reactance X d

” , which is used to determine the fault current during the first 1 to 5 cycles

• Transient reactance X d

’ , which is used to determine the fault current during the next 5 to 200 cycles

• Synchronous reactance X d state fault current

, which is used to determine the steady-

The subtransient reactance X d

” will range from a minimum of approximately 9% for a two-pole, wound-rotor machine to approximately

32% for a low-speed, salient-pole, hydro-generator. The initial symmetrical fault current can be as much as 12 times full load current.

Generator short-circuit current is determined using this equation:

I

SC

= Voltage ÷ X d

’

NEC 445.11 requires that a generator be provided with a nameplate with the following information:

• Manufacturer’s name

• Rated frequency

• Number of phases if AC

• Rating in kilowatts or kilovolt-amperes

• Power factor

• Normal volts and amps corresponding to the rating

• Rated ambient temperature

• Rated temperature rise

Nameplates or manufacturer’s instructions must provide the following information for all stationary generators and portable generators rated more than 15 kW:

1. Subtransient, transient, synchronous, and zero sequence reactances

2. Power rating category

3. Insulation system class

4. Indication if the generator is protected against overload by inherent design, an overcurrent protective relay, circuit breaker, or fuse

5. Maximum short-circuit current for inverter-based generators, in lieu of the synchronous, subtransient and transient reactances

Depending on the generator type, the zero sequence impedance may be less than the subtransient reactance, and the ground fault current substantially higher than the three-phase short-circuit current. For example, a 2500 kVA, 480/277 V, four-pole, 2/3 pitch standby generator has: a 0.1411 per unit subtransient reactance X zero sequence X o d

” and a 0.033 per unit

reactance. The ground fault current is approximately a third larger than the three-phase fault current. The ground fault current can be reduced to the three-phase level by simply adding a small reactance between the generator’s neutral and ground while still being considered solidly grounded.

The electric power system analysis must be performed based on the worst case operating conditions. Typically this is when all sources are paralleled.

If the system can operate with both the utility and generator supply in parallel, then the equipment must be rated for the combined fault current plus motor contribution. If the generator and utility will not be paralleled, then both cases will need to be looked at independently and the worst case used for selecting the equipment ratings.

Generator protection will vary and depend on the size of the generator, system type and the generator’s importance. Generator sizes are defined as: small, 1000 kVA maximum up to 600 V (500 kVA maximum when above 600 V); medium over 1000 kVA to 12,500 kVA maximum regardless of voltage; large, from 12,500-50,000 kVA. The simplest is a single generator system used to feed emergency and/or standby loads. In this case, the generator is the only source available when it’s operating and it must keep operating until the normal source is restored.

7.11 Ground fault protection of equipment

Contents

7.11.1 Overview

7.11.2 Ground-fault protection of equipment (GFPE)

7.11.3 NEC requirements

7.11.4 Selective coordination

7.11.5 GFPE considerations

7.11.6 Design options

7.11.1 Overview

Section page

48

49

49

51

52

54

This section covers the NEC requirements for ground fault protection of equipment (GFPE), how to comply with fusible equipment, options to design systems without GFPE and selective coordination considerations for circuits with GFPE.

The NEC defines Ground Fault Protection of Equipment (GFPE) as “a system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.”

The pertinent NEC requirements for ground fault protection of equipment (GFPE) are located in 230.95, 210.17, 215.10, 240.13, 517.17,

695.6(G), 700.31, 701.26, and 708.52. A few key requirements are:

• GFPE is required on 1000 A or greater service disconnects for

480/277 V, solidly grounded Wye systems

• If a GFPE is located on the service or feeder of a healthcare or COPS facility, then GFPE must be on the next level of feeders, per 517.17(B) and 708.52(B) respectively

• GFPE is not required for the alternate source of emergency systems

(700.31) and legally required standby systems per 701.26

• GFPE is not allowed on the circuit paths for fire pumps per 695.6(G)

• For healthcare essential electrical systems, additional levels of GFPEs cannot be located on the loadside of certain transfer switches, per

517.17(B)

GFPE is only required in a few, certain applications. If the use of GFPE is not desired, in some cases, there may be design options in which GFPE is not required, such as impedance grounded systems.



7.11.2 Ground-fault protection of equipment (GFPE)

A ground fault protection relay in conjunction with a sensor are used with a fusible switch to provide the addition of GFPE. The switch needs to be listed as suitable with the GFPR. When the ground fault current magnitude and time reach the GFPR’s pick-up setting (amp and timedelay setting), the control scheme signals the circuit disconnect to open.

GFPRs only monitor and respond to ground fault currents.

Fuses and circuit breakers respond to any overcurrent condition type: overloads and short-circuit currents, including ground faults. Per the

NEC, for most premise circuits, the service, feeder, and branch circuit

OCPDs (fuses or circuit breakers) provide protection for all overcurrent types, including ground faults. However, for some very high amp circuits, the NEC requires the addition of GFPE, which is intended to provide equipment protection from lower magnitude ground fault currents.

GFPE typically provides only equipment protection from low magnitude ground faults. GFPE and disconnecting means typically are too slow for higher magnitude ground faults. Equipment protection against higher magnitude ground faults depends on the conventional OCPD response speed (fuses or circuit breakers).

Providing ground fault protection of equipment with a GFPR requires a sensor, monitor, shunt trip and circuit disconnecting means. A fusible switch with shunt trip capability can be equipped with GFPR. Figure

7.11.2.a shows a bolted pressure switch with GFPR.

Circuit breakers with shunt trip capability also can be equipped in a similar manner. Some electronic trip circuit breakers have the GFPE option built into the electronic trip unit..

GFPE limitations

People protection. These devices are not intended to protect people from electric shock hazards. Ground fault circuit interrupters (GFCIs) perform this function and are required for certain branch circuits, and are intended to protect people from line-to-ground shock hazards.

Ground fault prevention.

GFPE and OCPDs can minimize the damage to equipment caused by ground faults, but they do not protect against a ground fault event. There are other methods recommended to prevent the ground fault, such as insulated bus or barriers.

Other than zero sequence currents.

The GFPE cannot provide protection from faults of any magnitude that stay within the intended current path such as 3-phase, phase-phase, or phase-neutral faults.

Detecting these faults is typically left to the fuse or circuit breaker.

Non-Current limitation.

A GFPE in itself will not limit the line-to-ground or phase-to-phase short-circuit current. Therefore, it’s recommended that current-limiting OCPDs be used in conjunction with a GFPE. Using current-limiting fuses offers:

• Some degree of arcing and low magnitude ground fault protection by the GFPR operating the switch

• Limiting current for high magnitude ground faults and short-circuits with current-limiting fuses provides component protection for the switchgear

GFPE characteristics and settings

GFPEs typically have adjustable trip settings and various shaped timecurrent characteristic curves. Manufacturers typically ship GFPEs with these set to the minimum which leads to many nuisance trips. To ensure best possible circuit coordination, these should be field adjusted to the desired setting.

The trip setting generally consists of an amp setpoint and a time setpoint, both selected from a range. Understanding a GFPE’s characteristics is important in assessing the equipment’s protection and coordination levels. Too often a GFPE on a service is adjusted to a low amp and instantaneous trip setting. With this setting, a ground fault on a

20 A branch circuit may cause a GFPE to unnecessarily open the service disconnect. If the GFPE is set properly, a fault on a 20 A branch circuit would be interrupted by the 20 A fuse or circuit breaker.

Figure 7.11.2.a

Fusible bolted pressure switch equipped with ground fault protection relay and sensor. Courtesy of Boltswitch, Inc.

NEC 230.95 has a maximum 1200 A current limit for GFPE characteristics and an operational limit of 1 second at 3000 A. GFPEs are available with various time-current shaped characteristics; some with a step function and others with an inverse time function as shown in Figure 7.11.4.c. A GFPE’s time-current characteristic curve shape, various amp and time-delay setpoints permit selecting time-current characteristics that provide the needed equipment protection level along with the desired coordination level.

7.11.3 NEC requirements

The pertinent NEC sections with requirements for ground fault protection of equipment (GFPE) include:

230.95.

Ground-fault protection of equipment must be provided for solidly grounded Wye electric services of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each service disconnect rated 1000 amperes or more.

480Y/277 V, solidly grounded Wye only connected service disconnects,

1000 A and larger must have GFPE in addition to conventional overcurrent protection. However, a GFPE is not required on a service disconnect for a continuous process where its opening will increase hazards (240.13). All Delta connected or impedance grounded services are not required to have GFPE. The maximum setting for the GFPE can be set to pick up ground faults at a 1200 A maximum and actuate the main switch or circuit breaker to disconnect all phase conductors. A

GFPE with a deliberate time-delay characteristic up to 1 second, may be specified for currents greater than or equal to 3000 A. (The use of such a GFPE greatly enhances system coordination and minimizes power outages - see Figure 7.11.4.c).

215.10. “Each feeder disconnect rated 1000 A or more and installed on solidly grounded wye electrical systems of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase, shall be provided with ground-fault protection of equipment in accordance with the provisions of 230.95.” There are two exceptions. Exception No. 2: GFPE is not required on feeder equipment when GFPE is provided on the feeder’s supply side and on the transformer’s loadside supplying the feeder (except for certain healthcare and COPS facilities, Article 517 and

708). Note: 210.13 and 215.10 are only valid if there isn’t any upstream

GFPE.

7



Violation

Compliance

4160 V service

4160 V service

215.10.

Exception No. 2

480Y/277 V Feeder w/o GFPR

> 1000 A

480Y/277 V Feeder with GFPR

> 1000 A

GFPR

Compliance

480Y/277 V

> 1000 A

Feeder of any rating no GFPR required

(except per Article 517 and 708)

210.13.

“Each branch-circuit disconnect rated 1000 A or more and installed on solidly grounded wye electrical systems of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase, shall be provided with equipment ground-fault protection in accordance with the provisions of 230.95.” There are two exceptions which are the same as those for 215.10.

Additional required levels

517.17(B).

If GFPE is provided on the service or feeder as specified in

230.95 or 215.10 of a healthcare facility, GFPE must be provided in all next level feeder disconnecting means downstream toward the load.

This is done so that selective coordination can minimize the outage should a ground fault occur downstream. Merely providing coordinated

GFPE does not prevent a main service blackout caused by feeder or branch circuit ground faults. The phase OCPDs must also be selectively coordinated. Selective coordination by definition requires that the phase

OCPDs working in conjunction with the GFPE be selectively coordinated for all values of current, including medium and high magnitude ground fault currents. This is because the conventional phase OCPDs may operate at these higher levels.

708.52(B).

If GFPE is provided on the service or feeder as specified in

230.95 or 215.10 of a COPS, GFPE must be provided in all next level feeder disconnecting means downstream toward the load. For COPS, the separation between GFPE time bands for any feeder and main GFPE must be at least six cycles in order to achieve coordination between these two GFPEs. If the requirements of 230.95, 240.13, or 215.10 do not require a GFPE, and no GFPE is utilized on the main service disconnect or feeder disconnect, then no GFPE is required on the next level downstream. See Figure 7.11.2.a.

Load/application specific requirements

426.28.

Ground-fault protection of equipment must be provided for fixed outdoor electric deicing and snow-melting equipment.

427.22.

GFPE for electric heat tracing and heating panels.

100

80

60

40

30

20

1000

800

600

400

300

Feeder GFPR

200

10

8

6

4

3

2

1

0.8

0.6

0.4

0.3

0.2

0.1

0.08

0.06

0.04

0.03

0.02

Feeder GFPR

800 A, 2 cycles

0.01

Main GFPR

480Y/277 V

3Ø/4W

SWBD

GFPR

Main GFPR

1200 A, 12 cycles

Min. 6 cycle separation

GFPR

Current in amps

Figure 7.11.2.a

Main and feeder GFPR curves

240.13.

Ground-fault protection of equipment must be provided in accordance with the provisions of 230.95 for solidly grounded Wye electrical systems of more than 150 volts to ground but not exceeding

1000 volts phase-to-phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more.


Eaton fusible vacuum starter in a motor control center protected by

Bussmann series dual-element, time-delay, current-limiting Fusetron

FRN-R fuses.


Where GFPE is not required

There are many branch, feeder and services, and others that do not require or permit GFPE. The general rules for where GFPE is not required are determined based on the boundaries of the NEC requirements including:

• Service, feeder or branch disconnects less than 1000 A

• 208Y/120 V, 3 Ø, services or feeders do not require GFPE

• Single-phase services or feeders including 240/120 V

• Resistance or impedance grounded systems, such as 480 V, high resistance grounded Wye systems.

• High or medium voltage services or feeders; greater than 1000 V.

(See NEC 240.13 and 215.10 for feeder requirements.)

• Services or feeders on Delta systems (grounded or ungrounded) such as 480 V, 3 Ø, 3 W Delta, or 240 V, 3 Ø, 4 W Delta with midpoint tap

• Services with six or less disconnects (Section 230.71) where each disconnect is less than 1000 A. A 4000 A service could be split into five 800 A switches

230.95.

This section’s GFPE provisions must not apply to a service disconnect for a continuous industrial process where a non-orderly shutdown will introduce additional or increased hazards.

240.13.

The provisions of 240.13 must not apply to the disconnecting means for:

• Continuous industrial processes where a non-orderly shutdown will introduce additional or increased hazards

• Installations where GFPE is provided by other requirements for services or feeders

• Fire pumps

517.17(B).

For healthcare essential electrical systems, additional levels of GFPEs can’t be on the Loadside of certain transfer switches

695.6(G).

Ground-fault protection of equipment must not be installed in any fire pump power circuit

700.31.

Ground-Fault Protection of Equipment. The alternate source for emergency systems must not be required to provide groundfault protection of equipment with automatic disconnecting means.

Ground-fault indication at the emergency source must be provided in accordance with 700.6(D) if ground-fault protection of equipment with automatic disconnecting means is not provided.

701.26.

The alternate source for legally required standby systems must not be required to provide ground-fault protection of equipment with automatic disconnecting means. Ground-fault indication at the legally required standby source must be provided in accordance with 701.6(D) if ground-fault protection of equipment with automatic disconnecting means is not provided.

708.52.

Critical operations (including multiple occupancy buildings) with critical operation areas. Where GFPE is provided for operation of the service disconnecting means or feeder disconnecting means as specified by 230.95 or 215.10, an additional step of ground-fault protection must be provided in all next-level feeder disconnecting means downstream toward the load. Such protection must consist of overcurrent devices and current transformers or other equivalent protective equipment that causes the feeder disconnecting means to open. When GFPE is first installed, each level must be tested to ensure that GFPE is operational. GFPE for operation of the service and feeder disconnecting means must be fully selective such that the feeder device, but not the service device, must open on ground faults on the loadside of the feeder device. Separation of GFPE time-current characteristics must conform to the manufacturer’s recommendations and must consider all required tolerances and disconnect operating time to achieve 100 percent selectivity.

.4

.3

.2

1

.8

.6

10

8

6

4

3

2

.1

.08

.06

.04

.03

.02

.01

7.11.4 Selective coordination

GFPEs should be included in a selective coordination analysis. If a particular GFPE use causes a lack of selective coordination, there may be other GFPE options available, or there may be alternate design options.

The following content on ground fault protection provides more information on the requirements and considerations for applying GFPEs.

Analysis of GFPE curves and overcurrent device curves

To a fuse or circuit breaker, ground fault current is sensed just as any other current. If the ground fault current is high enough, the fuse or circuit breaker responds before the GFPE (depending on the GFPE setting, OCPD characteristics and response speed and ground fault current magnitude). Therefore, when analyzing ground fault protection, it’s necessary to study the GFPE and OCPD characteristics as a combination.

The GFPE and OCPD combination has a ground fault “effective curve.”

This is a composite of the GFPE and OCPD curves. When analyzing line-to-ground faults, the “effective” GFPE and OCPD curve must be examined (see Figure 7.11.b).

1000

800

600

400

300

200

MAIN GFPR

1000 A or larger switch

Currentlimiting fuse

480Y/277V

3Ø/4W

GFPG relayF

100

80

60

40

30

20

SWBD

KRP-C 1600SP

7

CURRENT IN AMPS

Figure 7.11.4.a

“Effective” time-current curve for line-to-ground fault with a 1600 A fuse and GFPE set at 1200 A.

Figure 7.11.4.a is the “effective” ground fault curve for a 1600 A fuse in combination with a GFPE set at 1200 A pickup and 12-cycle delay.

Figure 7.11.4.b is the “effective” ground fault curve for a 1600 A circuit breaker in combination with a GFPE set at 1200 A and 12-cycle delay.

Notice in Figures 7.11.4.a and 7.11.4.b that for ground fault current less than approximately 14,000 A, the GFPE sensor responds and signals the bolted pressure switch (Figure. 7.11.4.a) or circuit breaker (Figure 7.11.4.b) to open. For ground fault current greater than approximately 14,000 A in

Figure 7.11.4.a the fuses will respond faster than the GFPE, and in Figure

7.11.4.b the circuit breaker phase overcurrent sensors will respond faster than the GFPE. In Figure 7.11.4.a, the fuses become current-limiting above approximately 22,000 A regardless of whether the fault is a ground fault or other fault type.



MAIN GFPE

MAIN

GFPE

Figure 7.11.4.b “Effective” time-current curve for line-to-ground fault with 1600 A circuit breaker and GFPE set at 1200 A.

7.11.5 GFPE considerations

When GFPE is used in a system, selective coordination should include an analyzing the ground fault circuit paths.

As previously mentioned, GFPE only monitors and responds to ground fault currents. Branch circuit fuses and circuit breakers sense and respond to all overcurrent types. Therefore, when analyzing a circuit path for selective coordination, GFPEs should be included. For circuit paths with GFPEs, there are two components in a coordination analysis:

1. Analyze only the circuit paths, considering the fuses or circuit breakers for all overcurrent types. Previous sections in this publication cover this in depth.

2. Analyze the circuit paths for just ground faults. In this case, the GFPE characteristics and the fuse or circuit breaker characteristics must be considered together. Remember, fuses and circuit breakers monitor and respond to any overcurrent type, so they should also be factored in. The following are some important considerations for this analysis.

One step GFPE

When a ground fault occurs on a feeder or branch circuit, it’s highly desirable for the feeder or branch circuit OCPD to open and clear that fault before the main device opens, thus preventing an unnecessary system blackout. However, this is not always the case when GFPE is located on the main or when the OCPDs are not selectively coordinated.

To avoid unnecessary service disruptions or blackouts:

1. The main OCPD characteristics must be analyzed with relation to the feeder and branch circuit OCPDs.

2. The feeder and/or branch circuit OCPDs’ characteristics must be analyzed with relation to the main GFPE characteristics, and with the next lower GFPE level, if provided.

Selective coordination should be investigated for low and high magnitude ground faults. Generally on low magnitude ground faults, the feeder OCPD must be selective with the main GFPE. For high magnitude ground faults, It’s necessary also to consider selective coordination between the main phase OCPD and downstream phase

OCPDs.

Two step GFPE

Two step ground fault relaying includes ground fault relays on the main service and feeders.

In many instances, this procedure can provide a higher degree of ground fault coordination to prevent unnecessary service blackouts.

Yet it’s mistakenly believed by many that two step GFPE ensures total ground fault coordination.

For complete selective coordination of all ground faults, the conventional phase OCPDs must be selectively coordinated, as well as the GFPE.

The fact is that even with this two-step GFPE provision, ground fault coordination is not assured on many systems where the main fuses or circuit breakers are not selectively coordinated with the feeder fuses or circuit breakers. The analysis must also include the phase OCPDs since they also respond to all fault current types, including ground faults.

In many cases, two step relays do provide a higher degree of ground fault coordination. When properly selected, the main fuse can be selectively coordinated with the feeder fuses. Thus, on all feeder ground faults or short-circuits, the feeder fuse will always open before the main fuse.

When selectively coordinated main and feeder fuses are combined with selectively coordinated main and feeder GFPE, ground fault coordination between the main and feeder is predictable.

Figures 7.11.5.a and 7.11.5.b illustrate a selectively coordinated main and feeder for all ground fault, overload and short-circuits levels. Any fault on the feeder will not disrupt the main service.

This system offers full selective coordination for all ground fault or shortcircuit levels.

1. The feeder GFPE is set at a lower time band than the main GFPE, therefore they are coordinated.

2. The feeder fuses are selectively coordinated with the main fuses for all ground faults, short-circuits or overloads on the feeder’s loadside.

The feeder fuses would open and clear the fault before the main fuses open.

If downstream circuits must be selectively coordinated with the feeder

GFPE and OCPDs, the analysis needs to include the downstream

OCPDs.

For healthcare facilities (517.17) and Critical Operations Power Systems

(708.52), the main and feeders are required to be 100% selectively coordinated for all ground fault current magnitudes, including low, medium and high ground fault currents. The system shown in Figures

7.11.5.a and 7.11.5.b comply with 517.17 and 708.52.



KRP-C1200SP

Does not open

Main

GFPR

1200 A

18-cycle delay

LPS-RK200SP

Only feeder disrupted

Feeder

GFPR

100 A

6-cycle delay

X

Any level ground fault current

Figure 7.11.5.a

Mains and feeders selectively coordinated for all ground faults.

1,000

800

600

400

300

200

Main GFPR

100

80

60

40

30

20

LPS-RK-200SP

KRP-C1200SP

10

8

6

4

3

2

Feeder GFPR

1

0.8

0.6

0.4

0.3

0.2

0.1

0.08

0.06

0.04

0.03

0.02

0.01

Current in amps

Figure 7.11.5.b

Time-current curve comparisons for a selective coordinated fused system with GFPR.

Low magnitude ground faults on feeders — one step GFPE

For low magnitude feeder ground faults, the feeder OCPD can open and clear the circuit without disrupting the main service if the feeder OCPD curve lies to the left of the GFPE curve and they do not cross each other at any point.

In Figures 7.11.5.c and 7.11.5.d, the GFPE located on the main has an 18-cycle operating time-delay and 1200 A pickup. Its inverse-time characteristic with the maximum 1 second opening time at 3000 A improves selective coordination with downstream devices.

10

8

6

4

3

2

0.8

1

0.6

0.4

0.3

0.2

0.1

0.08

0.06

0.04

0.03

0.02

0.01

100

80

60

40

30

20

1000

800

600

400

300

200

LPS-RK-200SP

X

Current in amps

Figure 7.11.5.c

Selective coordination considerations for low magnitude feeder ground faults. Longer GFPR delay permits larger feeder fuse to coordinate with main relay.

Figure 7.11.5.c illustrates that an inverse-time main ground fault relay may permit a larger size feeder fuse to selectively coordinate with the ground fault relay. In this case, the inverse time ground fault relay is set at 1200 A and an 18-cycle delay. A 200 amp LPS-RK-200SP feeder fuse coordinates with this main ground fault relay. A 400 A JKS-400 non-time delay fast-acting feeder fuse coordinates with this same main GFPR

(figure not included).

High magnitude ground faults on feeders — one step GFPE

For higher magnitude ground faults, it’s generally necessary to consider the main OCPD characteristics as well as the GFPE. Conventional phase OCPDs (fuses or circuit breakers) operate the same way for high magnitude ground faults or high magnitude phase-to-phase shortcircuits. Therefore, when a high magnitude feeder ground fault occurs, the main OCPD must be considered in relation to the feeder OCPD. To achieve selective coordination and prevent a blackout for high magnitude ground faults, the feeder OCPD must be selective with the main OCPD.

Selective coordination considerations for high magnitude feeder ground faults involve analyzing the main and feeder OCPDs. In this case, the fuses are selectively coordinated so that an unnecessary blackout does not occur.

7



0.1

0.08

0.06

0.04

0.03

0.02

1

0.8

0.6

0.4

0.3

0.2

0.01

4

3

2

10

8

6

1000

800

600

400

300

200

100

80

60

40

30

20

KRP-C1200SP

LPS-RK-200SP

X

KRP-C1200SP

Current in amps

Figure 7.11.5.d

Selective coordination considerations for high magnitude feeder ground faults involves analysis of main and feeder

OCPDs. In this case the fuses are selectively coordinated so that an unnecessary blackout does not occur.

Figure 7.11.5.d illustrates that the 200 amp feeder LPS-RK-200SP fuse selectively coordinates with the inverse-time main GFPR for all ground fault levels. Also, for any overcurrent type, including low level and high level ground faults, the LPS-RK-200SP fuse selectively coordinates with the 1200 amp main KRP-C-1200SP fuses. Figure 7.11.5.a fuse time-current curves show coordination for the curve portions shown

(up to approximately 17,000 A). For currents greater than 17,000 A, using the Selectivity Ratio Guide presented in the section on selection coordination shows that LPS-RK-200SP fuses selectively coordinate with

KRP-C-1200SP fuses up to 200 kA for any overcurrent type, including ground fault currents.

7.11.6 Design options

GFPEs are only required in certain applications. If using GFPEs causes selective coordination issues, or isn’t desired, there are design options to resolve the issues, including:

• Use inverse-time ground fault relays and set the amp and time delay setpoints as high as practical.

• Utilize a 480 V high resistance grounded Wye system. This system type does not require GFPEs. This system also reduces the probability of a hazardous arcing-fault starting from line-to-ground faults and enhances worker safety. Loads requiring neutrals must be fed from downstream transformers, which can be 208Y/120 V solidly grounded

Wye systems or 480Y/277 V solidly grounded Wye systems with 800

A or less secondary feeder disconnects.

• Design 480Y/277 V solidly grounded Wye services using up to six 800

A or less disconnects (230.71).

• For circuits supplying loads having alternate sources, place the automatic transfer switches close to the loads. Use smaller transfer switches placed closer to the final panelboard or large branch circuit loads. This option requires more transfer switches and longer cable runs. However, it enhances power to vital load reliability.

• Some municipalities enforce selective coordination only between the

OCPD’s rated current setting and not through the GFPE ground fault current and time settings. Consult your AHJ for local requirements.



7.12 Industrial control panels

7.12.1 Assembly SCCR

Contents

7.12.1 Overview

7.12.2 Assembly SCCR

7.12.1 Overview

Section page

55

55

NEC Article 409 covers the installation requirements for industrial control panels. As indicated in the Informational Note in 409.1, UL 508A is the product safety standard for industrial control panels.

The NEC defines an industrial control panel in Article 100 as an assembly of two or more components consisting of one of the following:

• Power circuit components only

• Control circuit components only

• Combination of power and control circuit components

The components, and associated wiring and terminals are mounted on a subpanel or contained in an enclosure. Generally, industrial control panels do not include the controlled equipment.

Power circuit components, such as motor controllers, overload relays, fused disconnect switches, and circuit breakers carry main power to loads such as motors, lighting, heating, appliances and general use receptacles.

Control circuits, as defined in Article 100, carry the electric signals directing the performance of the controller, but do not carry the main power current. Common control circuit components include push buttons, pilot lights, selector switches, timers, switches, and control relays.

Per NEC 409.21, overcurrent protection is required to be provided ahead of the industrial control panel or by a single main OCPD within the panel. The rating of the OCPD supplying the industrial control panel is required to not be greater than the sum of the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device provided with the industrial control panel, plus 125 percent of the fullload current rating of all resistance heating loads, plus the sum of the full-load currents of all other motors and apparatus that could be in operation at the same time. 409.110 requires the industrial control panel to be marked with:

• Manufacturer name

• Voltage, number of phases, frequency and full-load current for each supply circuit

• Short-circuit current rating (SCCR)

The SCCR is based on assembly listing and labeling or an approved method such as UL 508A, Supplement SB. If the panel only contains control circuit components (i.e., no power circuit components), an SCCR marking is not required.

NEC 409.22 prohibits installing industrial control panels where the available fault current exceeds the SCCR as marked in accordance with

409.110. Additionally, where an industrial control panel is marked with an SCCR, the available fault current at the industrial control panel’s install location and the date the fault current calculation was performed is required to be documented and made available to those authorized to inspect the installation. Typically, where the available fault current exceeds 5000 A, the designer and installer need to advise the industrial control panel manufacturer of the available fault current so industrial control panels with adequate SCCR can be designed/manufactured.

This may require the use of current-limiting fuses to achieve an SCCR adequate for the installation.

See the Industrial Control Panel - SCCR section in this publication.

Although a main disconnect is not required to be installed in the industrial control panel’s feeder or branch circuits per NEC 409.30, if disconnecting means are used to supply motors, they must comply with

Part IX of Article 430.

Prior to the 2011 NEC there was no definition of SCCR (sometimes referred to as “withstand rating”), although it was referenced in several sections on the marking and proper application of various types of equipment. Because the term is referenced in multiple locations of the

Code, it was necessary to add a definition to Article 100 of the NEC.

Article 100 definition

“ Short-Circuit Current Rating.

The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.”

What’s an SCCR?

SCCR is the maximum short-circuit (fault) current a component or assembly can safely withstand when protected by a specific overcurrent protective device(s) or for a specified time. Adequate equipment SCCR is required per NEC 110.10.

AWG range

2-6

2-14

2-14

Max. Class J fuse amps

400

200

175

Resulting

SCCR (kA)

200

50

100

This finger-safe power distribution block, protected with Class J fuses, is rated for use on a circuit capable of delivering no more than the SCCR shown (kA RMS Sym. or DC amps 600 V maximum).

Figure 7.12.2.a

Figure 7.12.a is a power distribution block (PDB) that has a default

SCCR of 10 kA per UL 508A Table SB4.1. However, this PDB has been combination tested and UL Listed with higher SCCRs when in combination with specific types and maximum amp rating currentlimiting fuses. The label is marked with a 200 kA SCCR when protected by 400 A or less Class J fuses and the conductors on the lineside and loadside are in the range of 2 to 6 AWG.

“Short-circuit current rating” is not the same as “interrupting rating” and the two must not be confused. Interrupting rating is the highest current at rated voltage that a device is identified to interrupt under standard test conditions; it does not ensure protection of the circuit components or equipment. Adequate interrupting rating is required per NEC 110.9.

The fuse in Figure 7.12.2.b has a UL Listed interrupting rating of 300 kA

@ 600 Vac or less.

Figure 7.12.2.b Fuse with 300 kA IR at 600 Vac.

When analyzing assemblies for SCCR, the interrupting rating of overcurrent protective devices and the SCCRs of all other components affect the overall equipment/assembly SCCR. For instance, the SCCR for an industrial control panel typically cannot be greater than the lowest interrupting rating of any fuse or circuit breaker, or the lowest SCCR of all other components in the enclosure.

7



The importance of SCCR

SCCRs establish the level of fault current that a component or piece of equipment can safely withstand (based on a shock or fire hazard external to the enclosure). Without knowing the available fault current and SCCR, it is impossible to determine if components or equipment can be safely installed.

Specification and installation of new equipment with higher SCCRs, such as 200,000 amps, makes it easy to meet the requirements of the

NEC. In addition, when equipment is later moved within a facility or from plant to plant, equipment with the highest ratings can be moved without worrying about unsafe situations that might arise from placing the equipment in a new location where the available fault current is higher than the old location and now above the rating of the equipment.

Complying with the code

To assure proper application, the designer, installer and inspector must ensure that the marked SCCR of a component or equipment is greater than the calculated available fault current.

In order to ensure compliance, it’s necessary to:

1. Determine the available fault current at the point of installation of the component or equipment.

2. Ensure the component or equipment marked SCCR is equal to or greater than the available fault current.

Figure 7.12.c illustrates SCCR compliance from a system perspective.

Any installation where the equipment marked SCCR is less than the available fault current is a lack of compliance, a safety hazard, and violates NEC 110.10. In these cases, the equipment cannot be installed until the component or equipment SCCR is sufficient or the fault current is reduced to an acceptable level..

Figure 7.12.2.c

Using current-limiting fuses and assembly SCCR

Current-limiting fuses are an effective means used by equipment manufacturers to increase the equipment SCCR per methods in accordance with product standards such as UL 508A. Therefore, it may not be appropriate to utilize the let-through current of externally field installed fuses to reduce the available fault current level below an equipment inadequate SCCR. Use of manufacturer specific published let-through current of externally mounted fuses is not in compliance with these methods. UL 508A Supplement SB uses UL

248 documented peak let-through limits that apply for all manufacturers fuses and not the RMS let-through of a specific manufacturer.

7-56

Courtesy NJATC.


When marked properly per UL procedures, peak let-through limits of externally mounted fuses may be used to increase branch components

SCCR but they cannot increase the overcurrent protective device interrupting rating or combination motor controller SCCRs found inside the panel.

If the equipment has an SCCR less than the available fault current, and does not contain overcurrent protective devices with inadequate interrupting ratings or combination motor controllers with inadequate

SCCRs, the authority having jurisdiction (AHJ) may allow the use of fuse manufacturer published let-through RMS data. For installations where the fault current exceeds the marked equipment SCCR and interrupting rating of overcurrent devices or SCCR of combination motor controller in the equipment, other methods should be explored such as adding impedance to reduce the fault current or redesigning and evaluating the existing equipment SCCR based on field inspection by a nationally recognized testing laboratory (NRTL).

Independent of connected downstream equipment, upstream currentlimiting fuses will reduce the available fault current per manufacturer’s publish data and provide many benefits including:

• Potentially reducing arc flash hazards at downstream equipment

• Simple selective coordination with upstream current-limiting fuses in feeders or mains using published amp rating ratios

• Superior short-circuit protection for any Loadside-connected conductors and busway

• Maintain protection level by reducing maintenance beyond periodically checking conductors and terminations

Determining assembly SCCR: “Two Sweep” method and procedures

How to determine assembly SCCR

For components, the Short-Circuit Current Rating (SCCR) is typically determined by product testing. For assemblies, the SCCR can be determined through the equipment product listing standard or by an approved method. UL 508A is the standard for Safety for Industrial

Control Panels. UL 508A, Supplement SB, provides an analytical method to determine the SCCR of an industrial control panel. This method is based upon the “weakest link” approach. In other words, the assembly marked SCCR is limited to the lowest rated component SCCR or the lowest rated overcurrent protective device interrupting rating. Since testing is not required with this method, it is typically the preferred method to use in determining the assembly SCCR and approved by the

NEC.

There are two basic concepts that must be understood and identified before analyzing the assembly SCCR per UL 508A, Supplement SB. The first is power circuit versus control circuit. The second is branch circuit versus feeder circuit. The differences and importance of these concepts are detailed as follows:

1. A power circuit is defined as the conductors and components of branch and feeder circuits. A branch and feeder circuit carries main line power current to external loads such as motors, lighting, heating, appliances and general use receptacles. A control circuit is a circuit that carries the electric signals directing the performance of a controller, and which does not carry the main power current. Only devices in power circuits and overcurrent devices protecting control circuits or power supplies (see Note 1) affect the assembly SCCR.

Note 1: Supplement SB3.2.1: For control circuits tapped from the feeder circuit, the overcurrent protection for the common control circuit or for the primary of a control transformer or power supply must be a branch circuit protective devices having a short-circuit current rating not less than the overall panel short-circuit current rating. For control circuits tapped from the load-side of a motor branch circuit protective device, the overcurrent protection for the common control circuit or for the primary of a control transformer or power supply, the short-circuit current rating of the overcurrent protection must be included in the determination of the branch circuit short-circuit current rating.


Note 2: There are some devices that are applied in power circuits but do not affect the assembly SCCR. These include:

• Power transformers, reactors, current transformers, dry-type capacitors, resistors, and voltmeters (deletion of “varistors” is a proposed revision so that SPDs affect the SCCR, not currently in UL

508A).

• The “S” contactor of a Wye-Delta motor controller

• Enclosure air conditioners that are cord-and-attachment-plug connected

• Wiring ferrules

• Line filters used with power conversion equipment (proposed revision, not currently in UL 508A)

2. Per UL 508A: a branch circuit is defined as the conductors and components following the final branch circuit overcurrent protective device protecting a load. A feeder circuit is the conductors and circuitry on the supply side of the branch circuit overcurrent protective device(s).

Note: In some cases, current-limiting devices in the feeder circuit can be used to increase the SCCR of branch circuit components. In addition, larger spacings are required for components used in feeder circuits versus when used in branch circuits. This is especially important for power distribution and terminal blocks, if used in feeder circuits.

Using the “Two Sweep” method based on UL 508A

After all the power circuit components and overcurrent devices protecting control circuits and power supplies have been identified, the

“Two Sweep” method based on UL 508A can be used to determine the assembly Short-Circuit Current Rating (SCCR). The purpose of performing two sweeps in this method is to ensure that the overcurrent protective device interrupting rating (or SCCR for combination motor controllers) are never increased by an upstream overcurrent protective device. UL 508A strictly prohibits any overcurrent protective device interrupting rating (or SCCR for combination motor controllers) from being raised beyond the marked interrupting rating or SCCR by an upstream overcurrent protective device. Hence, series rating of overcurrent devices (combination of an under rated circuit breaker applied at higher fault currents when provided with an appropriately tested or selected upstream fuse or circuit breaker) is prohibited.

Series ratings, per NEC 240.86 are applicable for panelboards and switchboards, not industrial control panels, industrial machinery or motor control centers.

Sweep 1: The component protection sweep

The first sweep reviews all components in the branch, feeder, subfeeder and supply circuits, and determines the component with the lowest SCCR. It is important to note these components can have either standard or high fault SCCRs. Per UL 508A Supplement SB2.2 and

SB2.3, they are defined per below:

STANDARD FAULT SHORT-CIRCUIT CURRENT RATING — Short-circuit current rating of a component as specified in Table SB4.1 (default values).

HIGH FAULT SHORT-CIRCUIT CURRENT RATING — Marked shortcircuit current rating of a component that is greater than the standard fault short-circuit current rating.

Note: There are exceptions where the overcurrent device required to achieve a high fault SCCR may not be required per UL 508A Supplement

SB4.2.3, these include:

• When the specified branch circuit protection related to the high fault short-circuit current rating is a Class CC, G, J, L, RK1, RK5, or T fuse, a fuse of a different class is able to be used at the same high fault rating where the peak let-through current and I 2 t of the new fuse is not greater than that of the specified fuse. See Table SB4.2 for maximum let-through currents and I 2 t.

• The specified branch circuit protection is able to be provided in the field when the panel is marked in accordance with SB5.1.2. SB5.1.2 requires the industrial control panel to be marked with the required branch circuit protective device type and size required to be installed in the field.

• When the specified branch circuit protection related to the high fault short-circuit current rating is a listed circuit breaker marked “currentlimiting”, a different current-limiting circuit breaker is able to be used at the same high fault rating where the peak let-through current and

I 2 t of the new current-limiting circuit breaker is not greater than that of the specified circuit breaker. Refer to published let-through values for current-limiting circuit breakers provided by the manufacturer. Figure

SB4.1 is provided to assist in determining the peak let-through current and I 2 t from the manufacturers data sheets.

• When the specified branch circuit protection related to the high fault

SCCR is a non-current limiting overcurrent device, a current-limiting fuse according to Table SB4.2 can be used at the same high fault rating where the interrupting rating of the current-limiting fuse is equal to or greater than the specified overcurrent device, and where the rated current of the fuse is equal to or less than the specified overcurrent device.

Sweep 2: The overcurrent protection sweep

The second sweep reviews all overcurrent protective devices in the branch, feeder and supply circuits, and determines the lowest interrupting rating (or SCCR for combination motor controllers).

The lowest rating from Sweep 1 and Sweep 2 identifies the assembly

SCCR. Because this method determines the assembly SCCR, it may be referred to as the “FIND IT.”

Note: It is necessary to complete both sweeps and all steps to determine an assembly’s SCCR marking. If an assembly SCCR marking is inadequate, then see the “FIX IT” portion at the end of this section for suggestions on how to increase an assembly’s marked SCCR.

Procedures for the “Two Sweep” method

Each sweep of this method is broken down into steps:

• Sweep 1 has five steps

• Sweep 2 has three steps

The following shows the procedure for completing the steps of both sweeps.

Sweep 1: Verifying assembly component SCCRs

Step 1: Determine the component SCCR for each branch circuit:

• Identify all component SCCRs and any special conditions that exist to utilize the ratings by one of the following methods:

1. The SCCR based on the default ratings per UL 508A Table SB4.1

(see Table 7.12.e SCCR1 - default SCCR Ratings).

2. The SCCR marked on the component or instruction sheet provided with the component.

3. The SCCR based on testing with a specific overcurrent protective device and/or combination of components in accordance with product standards and documented by the manufacturer. Example: a motor controller may have a high fault SCCR of 100 kA with a 30

A Class J fuse, but only 5 kA with a 30 A non-current-limiting overcurrent protective device.

• Apply the lowest SCCR of any component used in a branch circuit as the SCCR for that branch circuit. Repeat this for each branch circuit in the assembly.

• Note the lowest branch circuit SCCR for every branch circuit in the assembly or panel.

7



Step 2: Determine the component SCCR for each feeder circuit

(includes supply, feeders and sub-feeders):

• Identify all component SCCRs and any special conditions that exist to utilize the ratings by one of the following methods:

1. The SCCR based on the default ratings per UL 508A Table SB4.1 (see

Table 7.12.e SCCR1 - Default SCCR Ratings).

2. The SCCR marked on the component or instruction sheet provided with the component.

3. The SCCR based on testing with a specific overcurrent protective device and/or combination of components in accordance with product standards and documented by the manufacturer. Example: a power distribution block may have a high fault SCCR of 100 kA with a 200

A Class J fuse, but only 10 kA with a 200 A non-current-limiting overcurrent protective device.

• Apply the lowest SCCR of any component used in the feeder circuit as the SCCR of the feeder circuit.

• Note the lowest feeder circuit SCCR.

Step 3: If using a power transformer (a transformer that supplies loads) in a feeder circuit, modify the transformer circuit SCCR, if possible, as follows:

• Determine if the SCCR of the downstream components or interrupting rating of overcurrent devices can be increased by applying the following procedure:

1. Determine the let-through short-circuit current based upon calculation using the formulas in SB4.3.1(a) or use of Tables of SB4.3 or SB4.4 provided the impedance of the transformer is at least 2.1% or higher

(see Table 7.12.d for Transformer1).

2. On the transformer secondary,identify the lowest component SCCR or overcurrent protective device interrupting rating.

3. If the lowest component SCCR or overcurrent protective device interrupting rating is greater than the let-through short-circuit current, apply the transformer’s primary overcurrent protective device interrupting rating to the entire transformer circuit. Otherwise apply the lowest downstream component SCCR or overcurrent protective device interrupting rating to the transformer circuit.

Single-phase transformer secondary available short-circuit currents (A)†

Min. transformer secondary voltage (V)

Trans. 120/

3

5 max kVA

1

10

15

25

37.5

50

75

120

400

1200

1990

3970

5960

240††

300

900

1490

2980

4470

9930 7450

14,890 11,170

208

230

690

1150

2290

3440

5730

8590

240

200

600

1000

1990

2980

4970

7450

277

180

520

860

1720

2580

4300

6450

347

140

420

690

1380

2060

3440

5150

19,850 14,890 11,450 9930 8600 6870

29,770 22,330 17,180 14,890 12,900 10,300

480

100

300

500

1000

1490

2490

3730

4970

7450

600

80

240

400

800

1200

1990

2980

3970

5960

† Z assumed to be 2.1%. All values are rounded up.

†† Short-circuit current shown is line-to-neutral. (1.5 times line-to-line.)

Table 7.12.2.d

Transformer1: Single-phase transformer secondary shortcircuit current (UL 508A SB4.3).

Three-phase transformer secondary available short-circuit currents (A)†

Trans. Min. transformer secondary voltage (V)

15

20

25

30 max kVA

5

10

45

75

100

208Y/

120††

830

1660

2480

3310

4140

4960

7440

12,400

208

670

1330

1990

2650

3310

3970

5950

9920

240

580

1150

1720

2300

2870

3440

5160

8600

16,530 13,220 11,460

480Y/

277††

360

720

1080

1440

1800

2150

3230

5370

7160

480

290

580

860

1150

1440

1720

2580

4300

5730

600Y/

347††

290

580

860

1150

1440

1720

2580

4300

5730

600

230

460

690

920

1150

1380

2070

3440

4590

† Z assumed to be 2.1%. All values are rounded up.

†† Short-circuit current shown is line-to-neutral (1.25 times line-to-line).

Table 7.12.2.e

Transformer2: Three-phase transformer secondary shortcircuit current (UL 508A SB4.4).

Step 4: If using a current-limiting overcurrent protective device in the feeder circuit, modify branch circuit component SCCRs other than the interrupting rating of branch circuit overcurrent protection devices such as fuses and circuit breakers or, the SCCR of Type A (fusible combination motor controllers), Type C (circuit breaker combination motor controllers), Type D combination motor controllers (instantaneous trip circuit breakers/motor circuit protectors [MCPs]) and Type E or F

(self-protected) combination motor controllers, if possible, as follows:

1. Determine the peak let-through value of the current-limiting overcurrent protective devices at a given prospective fault current.

a. If the overcurrent protective device is a current-limiting fuse, determine the peak let-through umbrella value dictated by the product standard for the fuse class and amp rating utilized at the level of fault current desired (50, 100, 200 kA). See Table 7.12.g for

SCCR2 - UL Umbrella Limits at Rated Voltage (based on UL 508A

Table SB4.2).

b. If the overcurrent protective device is a marked current-limiting circuit breaker, manufacturer’s let-through curves can be used to determine the peak let-through value at a given prospective fault current.

2. Ensure that the peak let-through value is less than any of the SCCRs determined for the branch circuit in Step 1.

3. If condition “2” above is met, apply a SCCR to branch circuit components fed by the feeder based upon the value of the prospective fault current used to determine the peak let-through value of the current-limiting overcurrent protective device.

Note 1: Per SB4.3.4, the specified circuit breaker marked “currentlimiting” or current-limiting Class of fuse supplied in the feeder circuit that limits the peak let-through current available in accordance with

SB4.3.2 and SB4.3.3 is able to be provided in the field when the panel is marked in accordance with SB5.1.3.

Note 2: Per SB5.1.3, an industrial control panel marked with a high fault short-circuit current rating and is not provided with the required feeder circuit protective device as specified in the SB4.3.4 must be marked with the type and size of feeder circuit protection required to be installed in the field. This marking must be included as part of the marking in SB5.1.1.

Step 5: Determine the assembly SCCR for Sweep 1

• Determine the Sweep 1 assembly SCCR by utilizing the lowest rated branch or feeder circuit component SCCR.



Sweep 2: Verify assembly SCCR based upon overcurrent protective device interrupting rating (or SCCR for combination motor controllers)

Step 1: Determine the interrupting ratings (or SCCR of combination motor controllers) of all the overcurrent protective devices used in feeder (includes supply, feeders and sub-feeders) and branch circuits, including those overcurrent protective devices protecting control circuits and power supplies.

Step 2: Identify the lowest overcurrent protective device interrupting rating or SCCR for combination motor controllers.

Step 3: Compare the lowest overcurrent protective device interrupting rating or SCCR with the component SCCRs from Sweep 1, Step 5. The lowest rating encountered is the assembly SCCR.

This SCCR is then marked on the assembly. If this SCCR is not sufficiently high enough, there are “FIX IT” solutions at the end of this section that can be investigated to achieve a higher SCCR.

Note 1: Per SB5.1.1 the nameplate rating of an industrial control panel must include: “Short-circuit current rating: ___kA rms symmetrical, ___V maximum” or the equivalent.

Note 2: Per UL 508A 49.5 if multiple sources of supply to the industrial control panel are present, each set of input terminals must have a shortcircuit current rating.

Note 3: Per UL 508A Table 52.1, the short-circuit current rating marking location must be visible without opening the enclosure’s door or cover.

Note 4: Per UL 508A 49.6 when an industrial control panel contains slash voltage rated components, such as 120/240 V, 480Y/277 V, or

600Y/347 V, the voltage rating of the industrial control panel must be the complete slash voltage rating, when intended for connection to the higher voltage, or not more than the lower voltage rating.

Component

Busbars

Circuit breaker (including GFCI type)

Current meters

Current shunt

Fuse holder

Industrial control equipment a. Auxiliary devices (overload relay) b. Switches (other than mercury tube type) c. Mercury tube switches rated:

Over 60 amps or over 250 volts

250 volts or less, 60 amps or less and over 2 kVA

250 volts or less and 2 kVA or less

Motor controller, rated in horsepower (kW)†† a. 0-50 (0-37.3) b. 51-200 (38-149) c. 201-400 (150-298) d. 401-600 (299-447) e. 601-900 (448-671) f. 901-1500 (672-1193)

Meter socket base

Miniature or miscellaneous fuse

Receptacle (GFCI type)

Receptacle (other than GFCI)

Supplementary protector

Switch unit

Terminal block or power distribution block

Multi-point interconnection power cable assembly

Cable assemblies and fittings for industrial control and signal distribution

Multi-wire (power distribution) lug

Default SCCR

(kA)

10

5

*

10

10

5

5

5

3.5

1

2

10

0.2

5

10

10

5**

10**

18**

20**

42**

85**

10

10†

10

10

* An SCCR is not required when connected via a current transformer or current shunt. A directly connected current meter must have a marked SCCR.

** Standard short-circuit current rating for motor controller rated within specified horsepower range.

† The use of a miniature fuse is limited to 125 V circuits.

†† Includes combination motor controllers, float and pressure operated motor controllers, power conversion equipment and solid state motor controllers.

Highest rated horsepower of motor controller.

Table 7.12.2.f

SCCR1 - Default SCCR Ratings (UL 508A Table SB4.1)

Umbrella fuse limits

UL / CSA / ANCE Fuse Standards set maximum I p

and I 2 t let-through limits for short-circuit current performance of current-limiting fuses. The limits vary by fuse class, amp rating and available short-circuit current. To receive a listing, a commercially available current-limiting fuse must be tested and evaluated under short-circuit current tests per the applicable standard and witnessed by a National Recognized Testing Laboratory

(NRTL). One evaluation criteria of the testing is that the fuse’s I let-through measured during the short-circuit tests cannot exceed the

Standard’s “umbrella limits” for I p

and I 2 p

and I 2 t let-through established for that fuse class, amp rating, and available short-circuit current*. See Table t

7.12.g SCCR2 - UL Umbrella Limits at Rated Voltage for the umbrella limits applicable to most of the current-limiting fuses.

* NOTE: These tests are performed at the fuse’s rated voltage, with only one fuse in the circuit and by controlled closing of the test circuit so that the fuse “starts to arc” between 60 and 90 degrees on the voltage waveform. These test conditions are the most severe for fuse interruption. In addition, current-limiting fuses are required to have periodic NRTL witnessed follow-up testing in the same manner. The fuses for NRTL witnessed follow-up testing are pulled from inventory.

Umbrella fuses

An umbrella fuse is a special fuse that is designed to have short-circuit current I p

and I 2 t let-through that are at least equal to or greater than the fuse standard limit. Umbrella fuses are not intended to be sold as commercially available fuses.

These devices fall under UL 248-16 Test Limiters. UL uses the term

“test limiters” interchangeably as umbrella fuses. UL 248-16 states:

“…test limiters are calibrated to specific limits of peak let-through current and clearing I 2 t at 250, 300, 480, or 600 Vac. Test limiters are non-renewable and current-limiting, with test current ratings up to

200,000 A. They are calibrated to maximum peak let-through current and clearing I 2 t limits for the fuses specified in this Standard and are used for withstand testing of equipment designed to accept those fuses.”

Umbrella fuses are used for test purposes in qualifying a combination

SCCR with a specific component. For instance, a controller manufacturer wants the controller to be marked with a 100,000 A SCCR at 600 V when protected by 60 A Class J fuses. The NRTL witnessed tests would be with 60 A Class J umbrella fuses in combination with the controller on a test circuit of 100,000 A at 600 V. If the results satisfy the UL 508 Industrial Control Standard evaluation criteria, the controller can be labeled with a 100,000 A, 600 V SCCR when protected by Class

J fuses 60 A (or less). Another use of umbrella fuses is for series rated fuse/circuit breaker panelboard and switchboard combinations per NEC

240.86. For more information on series ratings see the section on Series

Rating: Protecting Circuit Breakers. However, UL 508A Supplement SB4 does not permit series rated combinations for use in establishing the

SCCR for industrial control panels. Therefore, the interrupting rating of overcurrent devices cannot be raised by another upstream overcurrent device.

7



Fuse type

Class CC

Class G

Class RK1

Class RK5

Class T, 300 V*

225

250

300

350

400

450

500

600

90

100

110

125

150

175

200

700

800

1000

1200

50

60

70

80

30

35

40

45

15

20

25

6

10

1

3

Amp rating

15

20

30

15

400

600

30

60

100

200

400

600

60

100

200

20

30

60

30

I

Between threshold and 50 kA

2 t x 10 3

2

2

7

—

10

40

100

400

1200

3000

50

200

500

1600

5200

10000

—

3.5

—

15

—

40

—

150

—

550

—

1,000

—

1,500

—

3,500

I p

x 10 3 (kA)

3

22

—

29

—

37

—

50

3

6

—

6

10

14

18

33

45

11

20

22

32

50

65

—

5.0

—

7

—

9

—

13

—

100 kA

175

225

300

400

550

600

800

1,000

38

40

50

75

88

115

150

1,200

1,500

3,500

3,500

11

15

25

30

3.5

6

8.5

9

0.4

0.6

1

1.5

2

2.5

2.7

I 2 t x 10 3

7

3.8

2

3

1200

3000

50

200

500

1600

5000

10000

40

100

400

25

10

5

7

* When values at 50 kA and 200 kA are needed, the standard case size must be used.

Note: Values shown are UL umbrella limits.

Table 7.12.g

SCCR2 - UL umbrella limits at rated voltage (UL 508A Table SB4.2)

28

32

37

37

21

22

24

27

45

50

65

65

11.6

12

12

13

14

15

16

10

10.7

8

9

7.2

7.6

7

7

0.8

1.3

2

3

4

4.5

5.5

25

40

60

80

35

50

11

21

I p

x 10 3 (kA)

3

4

7.5

4

10.5

8.7

5

7

12

16

22

200 kA

I 2 t x 10 3

3

3

7

—

11

50

100

400

1600

4000

50

200

500

2000

6000

12000

—

—

150

—

550

—

1,000

—

1,500

—

4,000

3.5

—

15

—

40

I p

x 10 3 (kA)

4

5

12

—

32

50

75

100

50

70

14

26

12

16

20

30

—

—

35

—

46

—

65

—

80

—

20

9

—

12

—

15



Fuse type

Class CF (up to 400 A),

J and T 600 V*

Class L

250

300

350

400

450

500

600

700**

90

100

110

125

150

175

200

225

Amp rating

1

3

6

10

15

20

25

30

60

70

80

35

40

45

50

800**

800

1200

1600

2000

2500

3000

4000

5000

6000

Between threshold and 50 kA 100 kA

I 2 t x 10 3 I p

x 10 3 (kA) I 2 t x 10 3

0.8

1.2

2

— —

7 6

5

5.5

3

4

7

—

30

—

—

8

—

30

50

60

12

17

18

22

60

—

200

—

1000

—

12

—

16

—

25

—

450

600

800

1,100

1,500

2,000

2500

3,500**

75

80

100

150

175

225

300

350

2500

—

4000**

10,000

12,000

22,000

35,000

35

—

50**

80

80

100

110

— —

4000**

10,000

12,000

22,000

35,000

75,000

10,0000

15,0000

35,0000

35,0000

* When values at 50 kA and 200 kA are needed, the standard case size must be used.

** Value applies to Class T fuses. Values at 700 A are included per UL 248, but have not been added to UL 508A Supplement SB.

Note: Values shown are UL umbrella limits.

Table 7.12.g

SCCR2 - UL umbrella limits at rated voltage (UL 508A Table SB4.2)

I p

x 10 3 (kA)

1

1.5

2.3

3.3

4

5

6

7.5

12.5

14

14.5

15.5

17

18.5

20

22.5

7.5

8

8.5

9

10

11.5

12.5

55**

80

80

100

120

165

175

36

42

45

50**

24

26

29

30

220

—

200 kA

I 2 t x 10 3

—

7

—

30

—

80

—

300

—

1100

—

2500

—

4000**

10,000

15,000

30,000

40,000

75,000

100,000

150,000

350,000

500,000

I p

x 10 3 (kA)

—

45

—

70

—

75**

80

120

150

165

180

200

250

300

350

30

—

12

—

16

—

20

—

7



Example using the “Two Sweep” method: “FIND IT”

The following example illustrates the procedures outlined for the two sweep method to determine the assembly SCCR. The example is based on the industrial control panel shown in Figures 7.12.1.h and 7.12.1.i.

Figure 7.12.1.h shows the graphical representation of the industrial control panel while Figure 7.12.1.i is its the one-line diagram that details each power circuit component and their ratings.

This example illustrates how each sweep and their steps are performed and documented in the tables. After both sweeps and all steps have been completed, the result identifies the assembly SCCR or“FIND IT”.

Later, methods are outlined to increase the assembly SCCR or “FIX IT.”

Figure 7.12.1.h

Industrial control panel graphical representation.

7

8

9

10

4

5

6

Circuit number Device description

1 Molded case circuit breaker protecting an IEC contactor

2

Self-protected Type E combination motor controller protecting an IEC contactor (Type F combination motor controller, additional components may be required)

3

Instantaneous trip circuit breaker (MCP) protecting an IEC starter (Type D combination motor controller) special assembly conditions required)

Molded case circuit breaker protecting an IEC starter contactor (Type C combination motor controller)

Class CC fused switch protecting an IEC starter

Class CC fused switch protecting variable frequency drive and contactor

Molded case circuit breaker and GFCI receptacle

Molded case circuit breaker protecting power transformer

Power distribution block

Class J fused switch


Example using the “two sweep” method: “FIND IT”


Note: It is important to record the voltage ratings for all components and overcurrent protective devices. The assembly is marked based upon the lowest or most restrictive device voltage rating. If there are devices with slash voltage ratings (such as 480/277 V), these are more limiting than straight or full voltage ratings

(such as 480 V). Assemblies with 480/277 V devices are suitable for only 480/277 V solidly grounded Wye systems. These assemblies cannot be applied on 480 V ungrounded, resistance grounded or corner grounded systems. (See slash voltage rated devices in Section 3 for more information.)

Figure 7.12.1.i

Industrial control panel one-line diagram.

Circuit number

1

2

3

4

5

6

7

8

9

10

Type

Branch

Branch

Branch

Branch

Branch

Branch

Branch

Sub-feeder

Feeder

Supply

Device descriptions

- Molded case circuit breaker: IR = 14 kA @ 480/277 V

- IEC contactor: SCCR = 5 kA @ 600 V

- Self-protected Type E combination motor controller with lineside terminal kit: SCCR = 65 kA @ 480/277 V


- Instantaneous trip circuit breaker (MCP): unmarked IR

- IEC starter: SCCR = 5 kA @ 600 V

- Molded case circuit breaker: IR = 14 kA @ 480 V


- Bussmann series Class CC Compact Circuit Protector (CCP): SCCR = 200 kA @ 600 V

- Bussmann series LP-CC fuses: IR = 200 kA @ 600 V


- Bussmann series Class CC Compact Circuit Protector (CCP): SCCR = 200 kA @ 600 V

- Bussmann series LP-CC fuses: IR = 200 kA @ 600 V

- Variable frequency drive: SCCR = 5 kA @ 480 V


- Molded case circuit breaker: IR = 10 kA @ 120 V

- GFCI receptacle: unmarked SCCR

- Molded case circuit breaker: IR = 14 kA @ 480/277 V

- 3 kVA 480 V-120 V secondary power transformer (does not affect SCCR)

- Power distribution block: unmarked SCCR

- Bussmann series 100 A Class J fused switch: SCCR = 200 kA @ 600 V

- Bussmann series 100 A LPJ fuses: IR = 300 kA @ 600 V

7



“Two sweep” method: Sweep 1, Step 1 - branch circuit components

Sweep 1: Verifying component SCCRs

Step 1: Determine the lowest rated component in each branch circuit.

Note: Determine only component SCCRs. OCPD interrupting ratings and SCCRs are ignored in this step.

Branch circuit 1

• IEC contactor: SCCR = 5 kA @ 600 V

• High fault SCCR as Type C combination motor controller (with a circuit breaker) does not exist

• SCCR = 5 kA @ 600 V


• IEC Starter: SCCR = 5 kA @ 600 V

• High fault short-circuit current rating as Type D combination motor controller rating with MCP (only with same manufacturer) = 65 kA @

480 V

• SCCR = 65 kA @ 480 V



• High fault short-circuit current rating as Type F Combination motor controller rating with self-protected starter (only with same manufacturer) = 65 kA @ 480/277 V

• SCCR = 65 kA @ 480/277 V


• IEC starter: SCCR = 5 kA @ 600 V

• High fault short-circuit current rating as Type C combination motor controller rating with circuit breaker (only with same manufacturer) =

25 kA @ 480 V

• SCCR = 25 kA @ 480 V


• IEC starter: SCCR = 5 kA @ 600 V

• High fault short-circuit current rating with Class CC fuses = 100 kA @

600 V

• SCCR = 100 kA @ 600 V




• Variable Frequency Drive: SCCR = 5 kA @ 480 V


• High fault short-circuit current rating with Class CC fuses:

•

200 kA @ 600 V for variable frequency drive

•

100 kA @ 600 V for IEC contactor

• SCCR = 100 kA @ 600 V


• GFCI Receptacle: unmarked SCCR (2 kA per Table 7.12.e Default

SCCR Ratings)

• Higher combination rating with circuit breaker does not exist

• SCCR = 2 kA @ 120 V (does not affect panel voltage rating)

Results of Sweep 1, Step 1: SCCR = 2 kA @ 480/277 V

Summary

• Lowest Step 1 SCCR is 2 kA @ 480/277 V

Note: Red table cells denote limiting components and voltages for each step.

Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7

Sub-feeder circuit 8

Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR Volts

5 kA

65 kA

65 kA

25 kA

600 V

480/277 V

480 V

480 V

100 kA 600 V

100 kA 600 V

2 kA

—

—

—

—

—

—

—

Sweep 1-Step 2

(feeder)

SCCR Revisions

Sweep 1-Step

3 (trans)

SCCR Volts SCCR

Sweep 1 Results

Sweep 1-Step

4 (C-L OCPDs) Sweep 1-Step 5

SCCR SCCR Volts

Sweep 2-Steps 1 and

2 (overcurrent device)

IR/SCCR Volts

7



“Two sweep” method: Sweep 1, Step 2 - feeder circuit components

Sweep 1: Verifying component SCCRs

Step 2: Determine the component SCCR for each feeder, sub-feeder and supply circuit.

Sub-feeder circuit 8

• This is a transformer circuit and is covered by Sweep 1, Step 3

Feeder circuit 9

• Power distribution block (PDB): unmarked SCCR (10 kA per Table

7.12.e SCCR1 - Default SCCR Ratings)

• SCCR = 10 kA @ 600 V

Note: PDB must have proper spacings for feeder application per

UL 508A.

Supply circuit 10

• Bussmann series 100 A Class J fused switch: SCCR = 200 kA @ 600 V

• SCCR = 200 kA @ 600 V


Summary

• Lowest Step 2 SCCR is 10 kA @ 600 V.

• Lowest Step 1 or Step 2 SCCR is 2 kA @ 480/277 V.


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

—

—

—

Assessment

Sweep 1-Step 1

(branch)

SCCR

5 kA

Volts

600 V

Sweep 1-Step 2

(feeder)

SCCR Volts SCCR

— —

65 kA

65 kA

25 kA

480/277 V —

480 V

480 V

100 kA 600 V

—

—

—

100 kA

2 kA

600 V

—

—

—

—

—

—

—

—

—

SCCR Revisions

Sweep 1-Step

3 (trans)

Sweep 1 Results

Sweep 1-Step


SCCR SCCR Volts

—

—

—

—

10 kA

—

600 V

200 kA 600 V

“Two Sweep” method: Sweep 1, Step 3 - components/transformers



IR/SCCR Volts


Sweep 1: Verifying equipment component SCCRs

Step 3: Determine if power transformers in the feeder, sub-feeder or supply circuit can raise the interrupting rating of branch circuit overcurrent protective devices and branch circuit component SCCRs

(circuit breaker and GFCI receptacle):



• 3 kVA sub-feeder transformer with a 120 V secondary can be used to raise the secondary components SCCR since the transformer’s letthrough current is 1.2 kA. Refer to table transformer1.

• Since all 120 V secondary branch circuit overcurrent protective devices and components have an interrupting rating/SCCR (circuit breaker

IR = 10 kA) or SCCR (GFCI receptacle SCCR = 2 kA) of 1.2 kA or higher, the interrupting rating of the transformer primary overcurrent protective device (sub-feeder circuit 8) can be assigned to the entire branch circuit 7 (circuit breaker and GFCI receptacle).

• Revised branch circuit 7 SCCR = 14 kA


Summary

• Branch circuit 7 SCCR was raised to 14 kA

• Branch circuit 1 is still the limiting SCCR factor


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR Volts

5 kA

65 kA

65 kA

600 V

Sweep 1-Step 2

(feeder)

SCCR Revisions

Sweep 1-Step

3 (trans)

SCCR Volts SCCR

Sweep 1 Results

Sweep 1-Step


SCCR SCCR Volts

—

480/277 V —

480 V —

—

—

—

—

—

—

—

—

—

25 kA 480 V

100 kA 600 V

100 kA 600 V

2 kA —

—

—

—

—

—

—

—

—

10 kA

—

—

—

—

—

—

—

—

14 kA

—

600 V —

200 kA 600 V —



IR/SCCR Volts

7



“Two sweep” method: Sweep 1, Step 4 - current-limiting overcurrent devices


Step 4: Determine if current-limiting overcurrent protective devices (C-L

OCPDs) are used in the feeder, sub-feeder or supply circuit that can raise branch circuit component ratings (other than devices that provide branch circuit overcurrent protection or combination motor controllers).

100 A Class J fuses

Fault current

(kA)

Peak letthrough (kA)

50

100

200

12

14

20


The 100 A Class J fuse in supply circuit 10 is a current-limiting device.

Use Table 7.12.g SCCR2 - UL Umbrella Limits at Rated Voltage to identify the peak let-through values:

• Compare the peak let-through values with result of Step 1 and increase branch circuit component ratings where possible.

Note: Since the 100 A Class J fuse peak let-through of 20 kA at a fault current of 200 kA is less than the SCCR of Step 1 for branch circuits 2 through 6, the SCCR is raised to 200 kA (contactor or variable frequency drive SCCR only). The SCCR of components in feeder circuit 9, subfeeder circuit 8 or supply circuit 10 cannot be raised per UL 508A.


Summary

• Branch circuit 1 SCCR cannot be raised.

• Increased SCCR of branch circuits 2 through 6 to 200 kA (contactor or variable frequency drive SCCR only)

• Branch circuit 7 SCCR cannot be raised in this step because it was raised by Step 3

• Feeder circuit 9, sub-feeder circuit 8 or supply circuit 10 cannot be raised in this step (only branch circuit components can be raised)


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR Volts

5 kA

65 kA

600 V —

480/277 V —

Sweep 1-Step 2

(feeder)

Sweep 1-Step

3 (trans)

SCCR Volts SCCR

—

—

SCCR Revisions

—

—

—

200 kA

Sweep 1 Results

Sweep 1-Step


SCCR SCCR Volts

65 kA

25 kA

480 V

480 V

100 kA 600 V

100 kA 600 V

—

—

—

—

—

—

—

—

—

—

—

—

200 kA

200 kA

200 kA

200 kA

2 kA

—

—

—

—

—

—

—

10 kA

—

—

14 kA

—

600 V —

—

—

—

— — 200 kA 600 V — —



IR/SCCR Volts



Step 5: Determine the lowest branch or feeder circuit component SCCR based on all steps in Sweep 1 and retain for Sweep 2.

• Lowest SCCR resulted from branch circuit 1 in Step 1

• Branch circuit 2 limited voltage in Step 1

• Sweep 1 Lowest SCCR = 5 kA @ 480/277 V

Note: Sweep 2 must still be completed to determine SCCR marking.



Summary

• After completing all five steps in Sweep 1, the resulting SCCR remains at a low 5 kA @ 480/277 V due to the 5 kA rated contactor in Branch

Circuit 1 and the slash voltage rated contactor in Branch Circuit 2 (when protected by a slash voltage rated self-protected Type E combination motor controller).


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR Volts

5 kA 600 V

65 kA

Sweep 1-Step 2

(feeder)

SCCR

—

480/277 V —

—

—

SCCR Revisions

Sweep 1-Step

3 (trans)

Sweep 1 Results

Sweep 1-Step


Volts SCCR

—

—

SCCR

—

200 kA

SCCR

5 kA

200 kA

Volts

600 V

480/277 V

480 V 65 kA

25 kA

480 V

480 V

100 kA 600 V

100 kA 600 V

2 kA

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

14 kA

—

200 kA

200 kA

200 kA

200 kA

—

—

200 kA

200 kA

200 kA

200 kA

14 kA

—

480 V

600 V

600 V

—

—

—

—

—

—

10 kA 600 V —

200 kA 600 V —

—

—

10 kA

200 kA

600 V

600 V



IR/SCCR Volts

7



Sweep 2: Verifying assembly SCCR based upon overcurrent protective device interrupting rating (or SCCR for combination motor controllers)

Step 1: Determine overcurrent protective device interrupting rating or

SCCR:


• Molded case circuit breaker

• IR = 14 kA @ 480 V



• IR = 14 kA @ 480/277 V


• High fault short-circuit current rating as Self-protected Type E combination motor controller (with lineside terminal kit)

• SCCR = 65 kA @ 480/277 V

Note: Self-protected Type E combination motor controllers are not rated with an interrupting rating. So for this Step 1, its SCCR is used per UL

508A.


• Bussmann series LP-CC fuses

• IR = 200 kA @ 600 V


• MCP — High fault short-circuit current rating as Type D combination motor controller rating with IEC Starter (same manufacturer)

• SCCR = 65 kA @ 480 V

Note: Per UL 508A, in order to ensure proper application in industrial control panels, the MCP must be procedure described to verify use as part of a listed Type D combination motor controller and the corresponding SCCR.



• Bussmann series LP-CC fuses

• IR = 200 kA @ 600 V


Verifying assembly overcurrent protective device interrupting rating or SCCR of combination motor controller

Step 2: Identify the lowest overcurrent protective device interrupting rating or SCCR of combination motor controller.

Feeder circuit 9

• No overcurrent protective device in this circuit


• Molded case circuit breaker analyzed in Sweep 1, Step 3

• IR = 10 kA, but raised to 14 kA due to transformer and interrupting rating of sub-feeder circuit 8 molded case circuit breaker


• Bussmann series 100 A LPJ fuses

• IR = 300 kA @ 600 V



• IR = 14 kA @ 480/277 V

Results of Sweep 2, Steps 1 and 2: SCCR = 14 kA @ 480/277 V (Sweep 2, Step 2 only)

Summary

• The lowest overcurrent protective device interrupting rating or SCCR of combination motor controllers in this Step is 14 kA @ 480/277 V based upon the interrupting rating of branch circuits 1, 2, 4 and sub-feeder circuit 8


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR

5 kA

65 kA

65 kA

Volts

600 V

480/277 V —

480 V

Sweep 1-Step 2

(feeder)

—

—

—

—

—

SCCR Revisions

Sweep 1-Step

3 (trans)

SCCR Volts SCCR

—

—

—

Sweep 1-Step


SCCR

—

200 kA

200 kA

Sweep 1 Results

SCCR

5 kA

200 kA

200 kA

Volts

600 V

480/277 V 65 kA

480 V



IR/SCCR Volts

14 kA

65 kA

480/277 V

480/277 V

480 V

25 kA

100 kA

2 kA

—

—

480 V

600 V

100 kA 600 V

—

—

—

—

—

—

—

—

10 kA

—

—

—

—

—

—

—

—

14 kA

—

600 V —

200 kA

200 kA

200 kA

—

—

—

200 kA

200 kA

200 kA

14 kA

—

10 kA

480 V

600 V

600 V

—

—

600 V

14 kA

200 kA

200 kA

—

14 kA

—

480 V

600 V

600 V

—

480/277 V

—

— — 200 kA 600 V — — 200 kA 600 V 300 kA 600 V


7


Step 3: Determine final assembly SCCR based upon results of Sweep

1 (component SCCR) and Sweep 2 (overcurrent protective device interrupting rating or SCCR for combination motor controller).

• Sweep 1 lowest SCCR = 5 kA @ 480/277 V

• Sweep 2 lowest IR or SCCR = 14 kA @ 480/277 V

• Resulting assembly SCCR = 5 kA @ 480/277 (see figure below)

Plastics Processing M achine

Se ria l N um b er

C urren t

La rges t M oto r H .P.

M ax O C P D evice

Vo lta ge

S N 23 5 6Y U P 7 7

87 A m peres

25 H o rse p ow e r

1 00 A m p e re

48 0 /2 7 7 v olts

Ph ase & Freq ..

Short-Circuit

Current Rating

D ia gram N um b ers

3 ph a se , 4 w ire, 6 0 H z

5 ,0 0 0 A m p e re s R M S

CM 12.1 T HR U C M 12.5

Q u a lity M ac h in e T o o l

S o m ew h e re, U S A

Example of assembly SCCR label marking based on the “2 Sweep” method.

Note: The assembly would have to be marked with 5 kA SCCR and

480/277 V. Equipment with 480/277 V devices are suitable for only

480/277 V solidly grounded Wye systems and cannot be applied on 480

V ungrounded, resistance grounded or corner grounded systems. See the section on Slash Voltage Ratings for more information.)

Results of Sweep 2, Step 3: assembly SCCR = 5 kA, voltage = 480/277

Summary

• The lowest SCCR of both Sweep 1 and Sweep 2 is 5 kA @ 480/277 V

• The 5 kA SCCR is based on the contactor in branch circuit 1, analyzed in Sweep 1 - Step 1

• The 480/277 V slash voltage rating is from multiple components in Sweep 1 - Steps 1 and 5, and Sweep 2, Steps 1, 2 and 3

• The assembly SCCR is 5 kA @ 480/277 V


Branch circuit 1

Branch circuit 2

Branch circuit 3

Branch circuit 4

Branch circuit 5

Branch circuit 6

Branch circuit 7


Feeder circuit 9

Supply circuit 10

Assessment

Sweep 1-Step 1

(branch)

SCCR Volts

5 kA 600 V

65 kA

Sweep 1-Step 2

(feeder)

SCCR

—

480/277 V —

SCCR Revisions

Sweep 1-Step

3 (trans)

Volts SCCR

—

—

—

—

Sweep 1-Step

SCCR

—

200 kA

Sweep 1 Results


SCCR

5 kA

200 kA



Volts

600 V

480/277 V

IR/SCCR Volts

14 kA

65 kA

480/277 V

480/277 V

65 kA

25 kA

480 V

480 V

100 kA 600 V

100 kA 600 V

2 kA

—

—

—

—

—

—

—

—

—

—

—

—

—

10 kA

200 kA

—

—

—

—

—

—

—

—

—

—

14 kA

—

600 V —

600 V —

200 kA

200 kA

200 kA

200 kA

—

—

—

—

200 kA

200 kA

200 kA

200 kA

14 kA

—

10 kA

200 kA

480 V

480 V

600 V

600 V

—

—

600 V

600 V

65 kA

14 kA

200 kA

200 kA

—

14 kA

—

300 kA

480 V

480 V

600 V

600 V

—

480/277 V

—

600 V



Increasing assembly SCCR — “FIX IT”

What follows are methods to increase, or “FIX,” a low assembly

SCCR using the appropriate overcurrent protective devices with higher interrupting ratings and components with high fault SCCRs.

To increase the assembly SCCR, identify the “weak links” and determine alternatives to increase the SCCR. Industrial control panels are required to be marked with an SCCR. NEC 409.22 requires the industrial control panel SCCR to be not less than the available fault current. Many OEMs and end users are finding that SCCR ratings of 65 kA, 100 kA, or higher with full voltage ratings (480 V in lieu of 480/277

V) are often needed to ensure NEC compliance and provide flexibility for future changes to the system or when moving the assembly to another location. The process to “FIX” these “weak links” follows.

“Weak link” 1 — Branch circuit 1: SCCR = 5 kA and slash voltage rated devices

The first “weak link” from the “Two Sweep” example is the IEC contactor (5 kA SCCR) and the slash voltage rated circuit breaker

(480/277 V) from branch circuit 1. As shown in Figure 7.12.1.j, not only does the circuit breaker have a low interrupting rating (14 kA) and is slash voltage rated (480/277 V), but the other circuit components, such as the IEC contactor (5 kA), can additionally limit the SCCR since high fault SCCRs are not available.

The solution is to use a fully rated overcurrent protective device with a high interrupting rating and a high fault SCCR combination rating with the IEC contactor. In this example, the circuit breaker can be replaced with the Bussmann series Compact Circuit Protector (CCP) with Class

CC fuses rated 600 V and 200 kA. Since the Class CC CCP utilizes Class

CC fuses, and since the IEC contactor in this example had a high fault

SCCR of 100 kA with Class CC fuses, the SCCR is now 100 kA. An additional benefit of the CCP saves space when compared to typical lighting and industrial style circuit breakers.

“Weak link” 2 — Feeder circuit 9: SCCR = 10 kA

The next “weak link” is the unmarked power distribution block. The easy solution to this is to find a power distribution block that has a high

SCCR when protected by a specific overcurrent device upstream. Since the overcurrent device upstream is a Class J fuse, the solution would be to use a Bussmann series high fault SCCR power distribution block or terminal block. This is important to note, as many power distribution blocks and terminal blocks require a current-limiting fuse to achieve a high fault SCCR. In addition, since the power distribution block is in the feeder circuit, feeder circuit spacings are also required per UL 508A. The

Bussmann series PDB (open style) or PDBFS (enclosed style) Series of power distribution blocks are Listed to UL 1953 ensuring compliance with feeder circuit spacing requirements in UL 508A and are UL Listed with high fault SCCR with Class J fuses as shown in Figure 7.12.1.k.

Figure 7.12.1.k

High fault SCCR PDBs

Often the power distribution block is the “weak link” keeping assembly

SCCR low. Using high fault SCCR PDBs protected with Class J fuses can deliver a high fault SCCR. The table below shows the possible high fault SCCRs.

AWG wire range

2-6

2-14

Class J fuse max. amps (A)

400

200

175

Combination SCCR

(kA)

200

50

100

This power distribution block is rated for use on a circuit capable of delivering no more than the SCCR shown (kA RMS Sym. or DC amps

600 V maximum). For other SCCR options, see Bussmann series data sheet no 10536.

Figure 7.12.1.j The Bussmann series CCP with Class CC fuses can easily increase SCCR by replacing low IR and slash rated OCPDs.

7

Figure 7.12.1.l power distribution block and power distribution fuse block.

Bussmann series PDBFS’s SCCR is only 10 kA with a circuit breaker, unless otherwise indicated in Bussmann series data sheet no 10536.

Enhanced SCCR and component protection is easily achieved by using

Bussmann series power distribution fuse blocks that combine the power distribution block and fuse block into one unit. They feature SCCRs up to 200 kA, fuse amp ratings up to 400 A and are available in 1-, 2- and

3-pole versions for UL supplemental, Class CC, H(K), J and R fuses.



“Weak Link” 3 — Branch circuit 4: SCCR = 14 kA and sub-feeder circuit 8: SCCR = 14 kA and slash voltage rating

The next “weak link” is the 14 kA circuit breaker in branch circuit 4 and the 14 kA slash rated (480/277 V) circuit breaker in sub-feeder circuit 8.

There are two possible solutions for this, either increase the interrupting rating of both circuit breakers and change to a full or straight voltage rated circuit breaker in sub-feeder circuit 8 or change to the Bussmann series CCP (or modular fuse holder if a branch circuit disconnect is not required) as shown in “Weak Link 1.” An economical solution is to change to the Bussmann series CCP or modular fuse holder with Class

CC fuses. In branch circuit 4, this change increases the interrupting rating to 200 kA as well as the IEC starter’s SCCR to 100 kA (high fault

SCCR) through the use of Class CC fuses so that branch circuit 4 is now rated 100 kA. The change to sub-feeder circuit 8 not only increases the interrupting rating to 200 kA, but also improves the voltage rating from

480/277 V (limits the assembly) to 600 V (not limited). This remedy is shown in Figure 7.12.1.m.

“Weak Link” 5 — Branch circuit 2, 3 and 4: manufacturer limitation

Where fusible devices are used in motor circuits, high fault SCCR with motor circuit components from multiple manufacturers are available increasing an OEMs’ flexibility in sourcing components. This typically reduces costs and provides alternatives during extended product delivery situations. For instance, fuses protecting motor circuit components listed at 100 kA high fault SCCR generally are available from several motor circuit component manufacturers. In contrast, the self-protected Type E combination motor controller and contactor in branch circuit 2 requires the same manufacturer for each component to be selected if high fault SCCRs are desired. This remedy is shown in

Figure 7.12.1.o.

Figure 7.12.1.m

“Weak Link” 4 — Branch circuit 2: slash voltage rated components

The next “weak link” is the slash voltage rating in branch circuit 2.

While the self-protected Type E combination motor controller is compact in size and has a relatively high fault SCCR (65 kA), it typically comes with a slash voltage rating. The solution is to either add an overcurrent device with a high interrupting rating ahead of the self-protected Type

E combination motor controller (changing this device’s application to a manual motor controller) or change to the CCP or modular fuse holder with Class CC fuses and a magnetic starter. The most economical solution to achieve a high SCCR and full voltage rating is to change to the CCP or modular fuse holder with Class CC fuses and a magnetic starter. With this change the circuit is rated 100 kA @ 600 V. This remedy is shown in Figure 7.12.1.n.

Figure 7.12.1.o

Plastics Processing M achine

Se ria l N um b er

C urren t

La rges t M oto r H .P.

M ax O C P D evice

Vo lta ge

Ph ase & Freq ..

Short-Circuit

Current Rating

D ia gram N um b ers

S N 23 5 6Y U P 7 7

87 A m peres

25 H o rse p ow e r

1 00 A m p e re

600 Volts

3 ph a se , 4 w ire, 6 0 H z

100,0 0 0 A m p e re s R M S

CM 12.1 T HR U C M 12.5

Q u a lity M ac h in e T o o l

S o m ew h e re, U S A

Figure 7.12.1.p

“FIX IT” Summary

The Figure 7.12.1.p shows the solutions to the “weak links.” The panel now has a high assembly SCCR with a full voltage rating.

Figure 7.12.1.n



Increasing assembly SCCR: “FIX IT” — typical “weak links” and improving SCCR

The following table highlights the typical “weak links” in industrial control panels and provides Bussmann series product solutions, along with the added benefits that these solutions can provide for industrial control panels.

“Weak link”

UL 1077 supplemental protectors

Limiting factors:

- Some may have an interrupting rating of 5 kA to 10 kA. Default rating is 200 A if unmarked.

- Not permitted for feeder or branch circuit protection.

Increase the interrupting rating

“FIX IT”

Use Bussmann series current-limiting fuses and the CCP (Class CC or CUBEFuse) or modular fuse holder to achieve high fault SCCRs by replacing the low interrupting rated UL 1077 supplementary protector with modern current-limiting fuses with high IRs of up to 300 kA and UL 4248 modular fuse holders or UL 98 disconnects with SCCR of

200 kA.

UL 489 instantaneous trip circuit breaker (MCP)

Limiting factors:

- SCCR is dependent upon Type D combination motor controller high fault SCCR when used with a listed combination motor controller.

Default rating can be as low as 5 kA. Varies by manufacturer.

- Procedure described to verify proper application.

Increase the interrupting rating

Use Bussmann series current-limiting fuses and the CCP (Class CC or CUBEFuse) or modular fuse holder to achieve high fault SCCR. Modern currentlimiting fuses are available with high interrupting ratings of up to 300 kA and UL 4248 modular fuse holders or UL 98 disconnects are available with

SCCR of 200 kA.

Power distribution block in feeder circuit

Limiting factors:

- If the power distribution block is not marked

(or indicated in instruction sheets) with a high fault SCCR, the 10 kA default rating must be used.

- For feeder circuit applications, power distribution blocks must have feeder spacings per UL 508A.

- Power distribution blocks recognized to UL

1059 typically do not comply.

Use power distribution blocks with high SCCR

Bussmann series PDBs Listed to UL 1953 with high SCCRs up to 200 kA when protected by Class

J and CF fuses. By replacing a low rated PDB with a finger-safe PDBFS a panel can achieve the high fault SCCR and proper spacings needed for feeder circuit applications. Also consider using power distribution fuse blocks that simplify wiring, reduce components and deliver up to 200 kA SCCR with amp ratings up to 400 A.

UL 489 molded case circuit breakers with low interrupting ratings Increase the interrupting rating

Assembly limiting factor:

- Typically have interrupting ratings of 10 kA to

14 kA.

- Higher interrupting ratings are available at increased cost.

Self-protected Type E combination motor controller

Assembly limiting factor:

- Slash voltage rating (480/277 V) limits the application options for the assembly to only a solidly grounded Wye system.

- Line-to-ground interrupting capability is limited.

- SCCR at 600/347 V is typically limited.

- May require additional lineside adapter accessory to be used as a self-protected Type

E combination motor controller.

Use Bussmann series current-limiting fuses and the CCP (Class CC or CUBEFuse) or modular fuse holder to achieve high fault SCCR by replacing the low interrupting rated circuit breaker with modern current-limiting fuses which are available with high interrupting ratings of up to 300 kA. UL 4248 modular fuse holders or UL 98 disconnects are available with SCCR of 200 kA.

Use a device with a straight voltage rating

Use Bussmann series current-limiting fuses and the CCP (Class CC or CUBEFuse) or modular fuse holder with high fault SCCR and straight voltage rated motor controller to allow for installation on any type of system grounding.

7

Additional SCCR resources

FC

2

Available Fault Current Calculator

Bussmann series FC 2 Available Fault Current Calculator is the online and mobile app that makes it easy to calculate fault current levels and generates NEC 110.24 compliant labels and one-line diagrams. Knowing available fault current is vital for compliance with NEC and OSHA SCCR requirements. This free mobile (Apple and Android devices) can be downloaded at the app store, or use the web-based version.

The SCCR Protection Suite

The SCCR Protection Suite provides OEM designers an easy means to fix weak links. This online application provides access to a comprehensive product portfolio of circuit protection and wiring distribution/termination/switching devices

(and their component SCCRs) to meet SCCR needs up to

200 kA.

OSCAR™ SCCR online compliance software

Bussmann series OSCAR SCCR online compliance software easily guides subscribers through entering an electrical panel’s components to calculate the equipment SCCR, provide a one-line diagram and detailed documentation. This online application is a quick and accurate means for compliance with 2017 NEC and UL 508A Supplement SB equipment

SCCR marking requirements. Request a free 7-day trial.



7.13 Industrial machinery 7.14 Motor/motor circuit protection

Contents

Overview

7.14.1 Motor branch circuit devices

Section page

7.14.2 Supplemental OCPDs for use in motor control circuits

7.14.3 Branch circuit OCPDs and disconnects

76

78

82

83

7.14.4 Overload protection

7.14.5 Disconnecting means for motor circuits

7.14.6 Motor starter protection

7.14.7 Group motor protection

7.14.8 Motor control circuit protection

7.14.9 The myth of OCPD resettability

7.14.10 Voltage unbalance and single-phasing

86

89

110

111

112

133

134

Overview

NEC 670 covers the installation requirements for industrial machinery and NFPA 79 is the electrical standard for industrial machinery.

670.2 defines industrial machinery as a power driven machine (or group of machines), not portable by hand while working, which is used to process material. It can include associated equipment used to transfer material or tooling, to assemble/disassemble, to inspect or test, or to package. The associated electrical equipment is considered as part of the industrial machine.

670.3(A) requires the industrial machinery to be marked with the following:

• Voltage, number of phases, frequency and full-load current for each supply circuit

• Maximum amp rating of the short-circuit and ground-fault protective device

• Amp rating of the largest motor

• Short-circuit current rating (SCCR) based on the assembly’s listing and labeling or an approved method such as UL 508A, Supplement SB.

To determine the SCCR of an industrial machine control panel, follow the procedure for industrial control panel SCCR in this publication.

Unlike industrial control panels, 670.4(B) requires a disconnecting means. If overcurrent protection is included with the disconnecting means, it is required to be marked as such per 670.3(B). Overcurrent protection is required to be provided and sized in accordance with

670.4(C) ahead of the industrial control panel, or by a single main OCPD within the panel.

Similar to industrial control panels, the overcurrent protection is required to not be greater than the sum of the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device provided with the industrial control panel, plus 125 percent of the full-load current rating of all resistance heating loads, plus the sum of the full-load currents of all other motors and apparatus that could be in operation at the same time.

NEC 670.5 prohibits installing industrial machinery where the available fault current exceeds the SCCR as marked in accordance with 670.3(A).

It also requires industrial machinery to be marked in the field with the maximum available fault current at the point of installation. This marking is required to include the date the fault current calculation was performed and withstand the environment involved. Typically where fault currents exceed 5000 A, the designer and installer need to advise the industrial machinery’s manufacturer of the available fault current so industrial machinery with adequate SCCR can be designed/ manufactured. The use of current-limiting fuses may be required to achieve an SCCR adequate for the installation.

In addition, NEC 670.6 requires industrial machinery with safety interlock circuits to have surge protection installed.

Of all the branch circuits encountered in the electrical industry, motor branch circuits remain the most unique. Listed here are a few reasons why:

• The harsh demand of motor loads, such as inrush and locked rotor currents

• The desire for various functionality, such as remote push button and automatic control

• The multitude of potential device types used in motor circuits and associated permitted functions for different parts of the motor circuit

• Combination of higher probability to incur faults and many motor circuit components such as starters, overload heaters, and contactors that have low short-circuit current ratings (SCCRs) or may not be completely protected from damage under short-circuit conditions (See

Type 2 “No Damage” protection)

NEC

Section

430 parts

Figure 7.14.a

IX

IV

VII

III

IX

NEC 430 parts.

M

Motor branch circuit and controller disconnect

Motor branch circuit fault and ground fault protection

Motor controller

Motor overload protection

“At the motor” disconnect

(430.102(B))



In order to provide reliable motor branch circuit protection, a thorough understanding of the requirements for various functional motor branch circuits parts and their intended purpose is required. Motor branch circuits can be broken down into 4 and sometimes 5 major functional blocks as shown in Figure 7.14.a. (This figure is a subset of NEC 430.1 found at the beginning of Article 430.)

They include:

1. Motor branch-circuit and controller disconnect

2. Motor branch-circuit short-circuit and ground fault protection

3. Motor controller

4. Motor overload protection

5. Sometimes an additional motor disconnect, often referred to as the

“at the motor” or “in sight from motor” disconnect may be required if the motor branch-circuit and controller disconnect is not in sight of the motor and driven machinery location

Overcurrent protection for motor circuits can be broken into two parts:

1. Motor overload protection (430.32)

2. Motor branch circuit short-circuit and ground fault protection (430.52)

Motor overload protective devices provide protection from low-level, long time overcurrent conditions which generally cause motor or motor branch circuit components to overheat over a long period of time (10 seconds or longer). Motor branch circuit devices provide short-circuit and ground fault protection for motor branch circuits and the components of the circuit (i.e. motor starters, conductors, equipment grounding conductors, etc.). Proper OCPD selection is extremely important. If not properly protected from fault currents, motor circuit components can be extensively damaged. It’s possible for the component to violently rupture and emit conductive gases that present safety hazards and can lead to other faults.

The motor branch-circuit and controller disconnect and the “at the motor” disconnect provide a means to isolate the motor circuit or motor from the supply source supply for maintenance work (electrically safe work condition) and serves as an emergency disconnect. Motor controllers can be manual or automatic and serve as an ON/OFF function for the motor and, as the name implies, control the motor’s operation.

In addition to these functional blocks, there are various requirements for motor control circuit components and other specialized components.

This discussion will focus on the motor (power) branch circuit requirements and the devices corresponding thereto. Various devices are available on the market to provide these functions. Some devices perform only one of these functions and some combine multiple functions. Some devices, such as UL 508 disconnects (manual motor controllers) and Manual Motor Protectors (MMPs) have spacing requirements that are less than UL 98 disconnects or UL 489 molded case circuit breakers, and, therefore have limitations on their application.

Suitability for use of motor branch circuit devices

Two of this section’s main objectives are to provide an understanding of devices that can be used in motor branch circuits, and then understand how each device must be judged as suitable per the NEC for specific motor circuit functions. Device product listing or recognition is one means to judge suitability for use. However, these facts are often overlooked or ignored, and devices get applied in applications beyond their intended use and listing and posing a safety hazard. It’s important for designers and installers to recognize and understand the various

NEC motor circuit functions and requirements. Additionally, one needs to know how to read device labeling, markings, and instructions in order to determine the proper applications for devices based on this information and the NEC requirements. NEC 110.3(A) and (B) identify the proper examination, identification, installation and use of equipment.

The text of NEC 110.3(A) and (B) is partially reprinted here.

110.3 Examination, Identification, Installation, Use, and Listing

(Product Certification) of Equipment.

(A) Examination . In judging equipment, considerations such as the following must be evaluated:

(1) Suitability for installation and use in conformity with the provisions of this Code

Informational Note 1: Equipment may be new, reconditioned, refurbished or remanufactured.

Informational Note 2: Suitability of equipment use may be identified by a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application. Special conditions of use or other limitations and other pertinent information may be marked on the equipment, included in the product instructions, or included in the appropriate listing and labeling information. Suitability of equipment may be evidenced by listing or labeling.

(B) Installation and Use. Listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling.

(C) Listing.

Product testing, evaluation, and listing (product certification) must be performed by recognized qualified electrical testing laboratories and must be in accordance with applicable product standards recognized as achieving equivalent and effective safety for equipment installed to comply with this Code.

In addition, the specific application must comply with NEC 110.9 and

110.10. This means each OCPD must have an interrupting rating equal to or greater than the available fault current, and the SCCR for each component must be equal to or greater than the available fault current.

Table 7.14.b summarizes the suitability of some common devices for the five possible NEC motor branch circuit functions. The device suitability should be evidenced by its product listing mark and any instructions included in the listing or labeling. The NEC requirements for each function are found in Article 430 under their respective parts as shown in Figure 7.14.b. Remember, for specific applications all OCPD interrupting ratings (NEC 110.9) and all component SCCRs (NEC 110.10) must be equal to or greater than the available fault current at the point of installation.

7



UL 248 fuses and disconnect

UL 489 circuit breaker

Instantaneous trip circuit breaker

Self protected combination starter (Type E starter)

IEC manual motor controller

(manual motor protector)

Magnetic motor starter

Manual motor

Controller (UL

508 switch)

UL 1077 supplemental protector

Allowed uses per NFPA 79 and NEC

Motor branch circuit and controller disconnect

Motor branch circuit fault and ground fault protection

Motor controller

Motor overload protection

“At the motor” disconnect

(430.102(B))

Yes 1

Yes

Yes 2

Yes

Yes

Yes 8

Yes

Yes 3

Yes 5,6

Yes 5,6

No

No

Yes 2 Yes No

M

Figure 7.14.b

1. When used in conjunction with a UL 98 fusible switch.

2. Where used in conjunction with a UL 98 or UL 508 fusible switch. If UL 508 switch, see footnote 4.

3. Often cannot be sized close enough.

4. Must be located on the Loadside of motor branch short-circuit protective device, marked “Suitable as Motor Disconnect,” and be provided with a lockable handle.

7.14.1 Motor branch circuit devices

Yes 6,7

Yes 6,8

Yes 9

Yes 10

Yes

No

No

Yes 9

Yes 10

Yes 4

No

No

Yes

Yes

No

No

No

Yes 9

No

Yes 4

No

No

No

No

No

5. When used in conjunction with a motor starter as part of a listed and labeled combination motor controller.

6. Limited to single motor circuit applications.

7. Additional terminal kit often required.

8. If slash voltage rated, limited to solidly grounded Wye systems ONLY.

9. Additional contactor required for remote control.

10. Class 10 overload protection only.

Branch circuit fuses as listed to UL/CSA/ANCE 248

Standards

Branch circuit fuses cannot be replaced with fuses having a lower voltage rating, and current-limiting branch circuit fuses installed in rejection-style clips cannot be replaced with non-current-limiting fuses. UL branch circuit fuses are

Class CC, CF, G, H, J, K1, K5, L, RK1,

RK5 and T, and plug fuses. These fuses are listed for branch, feeder, and main protection and have interrupting ratings that range from 10 kA to 300 kA. In a motor circuit they provide branch circuit, short-circuit, and ground fault protection. In addition, enhanced overcurrent protection such as back-up overload and

Type 2 “No Damage” protection can be provided by selecting certain fuse types and ratings.

Allowed uses

• Motor branch short-circuit and ground fault protection

• Motor overload protection (some fuse types based upon their amount of time delay)

• Group motor protection as the short-circuit and ground fault protective device per NEC 430.53

• Motor branch circuit and “at-the-motor” disconnecting means when used in conjunction with a UL 98 fusible switch

• Motor controller when used in conjunction with a UL 98 fusible switch, UL 508 manual motor controller, or UL 1429 pullout switches

Identification

Fuses listed to UL/CSA/ANCE

248 will contain a marking near the agency symbol. This marking should read “Listed Fuse.”


Disconnect switches (fused and non-fused) listed to UL 98

Disconnect switches from 30 to 6000 A that may be used in mains, feeders and branch-circuits for service equipment, panelboards, switchboards, industrial control equipment and motor control centers. They may also be used as a motor controller (ON/OFF function) to meet NEC Article 430, Part VII, and may be used as both a motor branch-circuit disconnect (NEC 430.109) or “at-themotor” disconnect and a motor controller

(NEC 430.111).

Allowed uses

• Motor branch circuit and controller disconnect or “at-the-motor” disconnect

• Motor controller

Identification

Disconnect switches as listed to UL

98 will contain a marking near the agency symbol. This marking should read “Listed Misc. Sw” or “Open

Type Switch.”


Pullout switches listed to UL 1429

Fused and non-fused switches ranging from 30 to 400 A at 600 V or less. Pullout switches with horsepower ratings are suitable for branch-circuit and controller disconnect or “at-the-motor” disconnect to meet NEC 430.109, as motor controllers to meet NEC Article 430 Part VII (if rated 100

Hp or less. Per UL 1429, pullout switches are not permitted to be used as a motor controller for motors above 100 Hp), and in general use for panelboards, switchboards, etc. They may be used as both a motor branch circuit and controller disconnect or “at-the-motor” disconnect, and a motor controller to meet NEC 430.111. Pullout switches with only amp ratings (no Hp ratings) are suitable only for general use, not motor circuits. If they are marked “Motor Circuit Pullout Switch,” they may only be used in a motor circuit. When used with properly sized branch circuit fuses, pullout switches may be used for motor, motor circuit and group motor protection.

Allowed uses

• Motor branch circuit and controller disconnect or “at-the-motor” disconnecting means


Identification

Pullout switches listed to UL 1429 will contain a marking near the agency symbol. This marking should read “Listed Pullout Switch.”

Motor switches (manual motor controllers) listed to UL 508

UL 508 switches may be used as a motor controller (ON/OFF function) to meet NEC Article 430 Part VII. As motor controllers, they have creepage and clearance distances that are less than those required by UL 98.

As a result, they can’t be used as a motor branch circuit and controller disconnect to meet NEC 430.109.

If the device is listed as a “Manual

Motor Controller” and is additionally marked “Suitable As Motor

Disconnect,” it’s permitted to serve as an “at-the-motor” disconnect if it’s located between the final motor branch circuit short-circuit and ground fault protective device, and the motor. This listing is optional; verification will be required if it’s intended to be used for this purpose.

Allowed uses


• “At the Motor” disconnect if marked “Suitable as Motor Disconnect” and located between the final motor branch-circuit short-circuit and ground fault protective device and the motor.

Identification

Motor switches/manual motor controllers listed to UL 508 will contain a marking near the agency symbol. This marking should read “Listed Manual Motor Controller” or an abbreviation such as “Man. Mtr. Cntlr.”

Manual motor controllers listed for use as an “at-the-motor” disconnect means will be marked “Suitable as Motor Disconnect.”

Fuse holders listed to UL 4248

When used with a motor branch-circuit and controller disconnect, and properly sized branch-circuit fuses, fuse holders may provide main, feeder, branch circuit, motor circuit, and group motor protection.

They can’t be used alone as a motor branch circuit and controller disconnect or as an

“at-the-motor” disconnect to meet NEC

430.109, nor can they be used alone as a motor controller (ON/OFF function) to meet

NEC Article 430, Part VII.

Identification

Fuse holders listed to UL 4248 will contain a marking near the agency listing symbol. This marking should read “Listed Fuse

Holder.”

Thermal-magnetic (inverse time) circuit breakers listed to

UL 489

Circuit breakers listed to UL 489 are intended to provide branch, feeder and main protection, with interrupting ratings from 5 kA to 200 kA. Properly sized inverse time circuit breakers provide motor branch circuit shortcircuit and ground fault protection.

They may be used for group motor protection and are suitable for use as a motor branch circuit and controller disconnect or “at-themotor” disconnect per NEC 430.109, as a motor controller (ON/OFF function) per NEC Article 430 Part VII, and as both a motor branch circuit and controller disconnect, or “at-the-motor” disconnect and motor controller per NEC 430.111.

Allowed uses

• Motor branch circuit short-circuit and ground fault protection

• Motor overload protection

• Group motor protection as the short-circuit and ground fault protective device per NEC 430.53



Identification

Circuit breakers listed to UL 489 will contain a marking near the agency symbol. This marking should read “Listed Circuit Breaker” or an abbreviation such as “Cir. Bkr.”

7



Instantaneous trip circuit breakers (MCPs) recognized to

UL 489

Circuit breakers without overload (thermal) protection capability are known as

MCPs. They’re intended to provide only branch circuit short-circuit and ground fault protection for individual motor branch circuits. They may not be used to provide main, motor feeder, motor overload, general branch circuit or group motor protection. Because they’re only recognized and not listed, they can’t be used with any manufacturer’s control equipment. NEC 430.52 requires that

MCPs must only be used as part of a listed combination controller (typically from the same manufacturer). MCPs are only short-circuit tested in combination with a motor controller and overload device. As a result, they’re not labeled with an interrupting rating by themselves. Per NEC 430.109, they may be used as a motor branch circuit and controller disconnect or “at-the-motor” disconnect only as a part of a listed combination motor controller.

Allowed uses

• Motor branch circuit short-circuit and ground fault protection only when part of a listed combination motor controller

• Motor branch circuit and controller disconnect or “at-the-motor” disconnect only when part of a listed combination motor controller


Identification

Instantaneous trip circuit breakers recognized to UL 489 will contain a UL recognition or CSA component acceptance marking. This marking indicates that the product can’t be used “stand alone” and is limited to certain conditions of use.

Molded case switches listed to UL 489

Molded case switches can be used with fuses and are very similar to molded case thermal magnetic circuit breakers, except they don’t have any thermal overload protection, and may or may not be equipped with a “magnetic” instantaneous trip as a self-protect mechanism. They may be used in mains, feeders and branch circuits for service equipment, panelboards, switchboards, industrial control equipment, motor control centers and motor branch circuits.

They are suitable for use as a motor branch circuit and controller disconnect or “at-the-motor” disconnect per NEC 430.109. They may be used as a motor controller (ON/OFF function) to meet NEC Article 430 Part VII and as both a motor branch circuit and controller disconnect or “at-themotor” disconnect and motor controller to meet NEC 430.111.

Allowed uses



Identification

Molded case switches listed to UL

489 will contain a marking near the agency symbol. This marking should read “Listed Molded Case Switch.”

Self-protected combination starters (Type E) listed to UL 508

Self-protected combination starters could be referred to as “coordinated protected starters,”

“self-protected starters,” “self-protected combination controllers,” “Type E combination starters” or “Type E starters.” In some cases, self-protected combination starters can be marked and applied as either self-protected combination starters or manual motor controllers. However, the device ratings will typically be much more restrictive when applied as a self-protected combination starter. Selfprotected combination starters are intended to provide motor overload and motor branch circuit short-circuit and ground fault protection by combining a magnetic shortcircuit trip and adjustable motor overload in one package. A selfprotected combination starter is a listed combination starter suitable for use without additional motor branch circuit overcurrent protection, and is limited to single motor circuits. Type E starters have additional test requirements for low level short-circuit interrupting tests followed by endurance tests that are not required for other combination motor controllers. Self-protected starters can be either manual or electromechanical.

A slash voltage rated self-protected combination starter is limited to use only on solidly grounded Wye systems. They can’t be used on ungrounded, corner grounded or impedance grounded systems.

Creepage and clearance on the line terminals must be the same as UL

489 and UL 98 devices. Because of this, a self-protected combination starter that’s marked for use with a terminal kit must be installed with a terminal kit to ensure lineside terminal spacings are adequate. Additional accessories, such as lockable handles, may be needed in order for the device to be suitable for use. Self-protected combination starters are suitable for use as a motor branch circuit and controller disconnect or

“at-the-motor” disconnect per NEC 430.109, as a motor controller (ON/

OFF function) per NEC Article 430, Part VII, and as both a motor branch circuit disconnect or “at-the-motor” disconnect and motor controller per

NEC 430.111. These devices are only permitted on single motor branch circuits.

Allowed uses





Identification

Self-protected combination starters as listed to UL 508 will contain a marking near the agency symbol. For factory-assembled units, this marking should read “Listed Self-Protected Combination Motor

Controller.” If separate components are used, the manual self-protected combination starter must be marked “Self-Protected Combination Motor

Controller when used with (manufacturer and loadside component catalog number or “Motor Controllers Marked For Use With This

Component”). If not marked with manufacturer and catalog number, the other assembly components must be marked “Suitable For Use On

Loadside Of (manufacturer and catalog number) Manual Self-Protected

Combination Motor Controller.”

Additionally, self-protected combination starters which are limited in application to only solidly grounded Wye systems will be marked as slash voltage rated, e.g., 480Y/277 V or 600Y/347 V.

When marked as slash voltage rated, they can’t be used on ungrounded, corner grounded, or impedance grounded systems.



Type F combination starters listed to UL 508

An IEC contactor combined with a self-protected combination starter, may be referred to as a “Type

F” starter. However, this does not make it a “selfprotected” starter unless tested and listed as a

“Type E” starter. If listed as a Type F combination starter, the additional tests required for Type E starter status have not been performed.

Allowed uses



• Motor branch circuit and controller disconnect or

“at-the-motor” disconnect


Identification

Type F combination starters listed to UL 508 will contain a marking near the agency symbol. For factory assembled units, this marking should read “Combination Motor Controller.” If separate components are used, the manual self-protected combination starter must be marked “Combination Motor Controller when used with (manufacturer’s loadside component and catalog number or “Motor Controllers Marked

For Use With This Component”. If not marked with manufacturer and catalog number, the other assembly components must be marked

“Suitable For Use On Loadside Of (manufacturer and catalog number)

Manual Self-Protected Combination Motor Controller.”

Additionally, Type F combination starters which are limited in application to only solidly grounded Wye type systems will be slash voltage rated marked, e.g., 480Y/277

V or 600Y/347 V. When slash voltage rated marked, they can’t be used on ungrounded, corner grounded, or impedance grounded systems.

Manual motor controllers (manual motor protectors) listed to UL 508

Manual motor starters, sometimes called MMPs, are permitted to provide motor overload protection as required per NEC 430.32, and to provide motor control. MMPs are not listed nor permitted to provide motor branch circuit short-circuit and ground fault protection.

Their creepage and clearance distances are typically not as great as required in UL 489, and, therefore, they cannot be tested and listed as a circuit breaker. They require a motor branch circuit overcurrent protective device and a motor branch circuit and controller disconnect on the lineside for both single motor and group motor applications.

Some IEC manual motor protectors have been tested and listed for group motor applications. This allows several of them to be protected by a single motor branch circuit overcurrent protective device, such as an upstream fuse provided it doesn’t exceed the maximum size allowed per the device listing. In group motor applications, other limitations such as horsepower ratings and tap rule restrictions must also be investigated. Devices listed for use in group motor installations will be marked to indicate that they have undergone the appropriate testing, and are suitable for such use.

The slash voltage rating ( 480Y/277 V) limits their use to only solidly grounded Wye systems. Manual motor controllers may be used as a motor controller (ON/OFF function) to meet NEC Article 430 Part VII.

Unless otherwise marked, MMPs do not meet requirements for a motor branch circuit and controller disconnect or “at-the-motor” disconnect as required in NEC 430.109. If it is marked “Suitable as Motor Disconnect,” it’s permitted to serve as an “at-the-motor” disconnect if it is located between the final motor branch circuit short-circuit and ground fault protective device and the motor. If investigated for tap conductor protection in group motor installations, they can be additionally marked

“Suitable for Tap Conductor Protection in Group Installations.” These additional markings and listings are optional, so a device marking review will be required if it is intended to be used for this purpose.

Allowed uses


• Group motor applications only as the protected (downstream) device when it’s tested, listed and marked, and the upstream fuse

(protecting device) is sized within the maximum allowed per the device’s listing and other limitations, such as horsepower ratings and tap rules are met.


• “At-the-motor” disconnect if marked “Suitable as Motor Disconnect” and located between the final motor branch circuit short-circuit and ground fault protective device and the motor.

• Protecting tap conductors in group installations if marked “Suitable for

Tap Conductor Protection in Group Installations” and located on the loadside of the final motor branch circuit short-circuit and ground fault protective device.

Identification

Manual motor protectors listed to UL 508 will contain a marking near the agency symbol. This marking should read “Listed Manual Motor

Controller” or an abbreviation such as “Man. Mtr. Cntlr.”

Manual motor controllers listed for use within group motor applications, as the protected overload/controller device, will be marked for such use along with the required maximum upstream fuse size. Manual motor controllers additionally listed for use as an “at-the-motor” disconnect will be marked “Suitable as Motor Disconnect.” Manual motor controllers additionally listed for use to protect tap conductors in group installations will be marked

“Suitable for

Tap Conductor

Protection in Group

Installations.”

7

Integrated starters listed to UL 508

Integrated starters are a modular type motor starter.

They consist of an IEC manual motor controller and an IEC contactor. Various controller types, control units, communication modules and accessories are available. Users can select from different components to meet their specific application needs.

These starters may be factory assembled units or assembled in the field.

Application requirements are the same as manual motor controllers, including the need for motor branch circuit overcurrent protective device and a lineside disconnect suitable for motor branch circuits and motor controllers upstream.

In some cases, integrated starters may be additionally tested and listed as a self-protected Type E or Type F starter when the appropriate components and accessories are installed. When applied as a selfprotected Type E or Type F starter, the device ratings are usually limited compared to the device ratings when applied as a manual motor controller or motor starter.



Magnetic motor starters

Magnetic motor starters combine a magnetic contactor and overload relay. The magnetic starter’s overload relay is intended to provide single motor overload protection per NEC 430.32. The magnetic motor starter’s horsepower rated magnetic contactor is intended for use as a motor controller (ON/OFF function) to meet NEC Article 430 Part VII.

The horsepower rated magnetic contactor also allows for remote motor operation. Available in NEMA or IEC, magnetic motor starters must be protected by a separate motor branch circuit overcurrent protective device per NEC 430.52. A lineside disconnecting means suitable for a motor branch circuit NEC 430.109 must also be installed.

Allowed uses



Identification

Magnetic motor starters listed to UL 508 will contain a marking near the agency symbol. This marking should read “Listed

Industrial Control Equipment” or an abbreviation such as “Ind. Cont. Eq.”

7.14.2 Supplemental overcurrent protective devices for use in motor control circuits

Branch circuit vs. supplemental overcurrent protective devices (OCPDs)

Branch circuit OCPDs can be installed anywhere overcurrent protection is needed, from protecting motors and motor circuits, control circuits and group motor circuits, to protecting distribution and utilization equipment. Supplemental OCPDs can only be used where proper overcurrent protection is already being provided by a branch circuit

OCPD, by exception (430.72(A)), or if additional overcurrent protection is not required, but desired for increased overcurrent protection and isolating loads. Supplemental OCPDs can often be used to protect motor control circuits, but they can’t be used to protect motors or motor branch circuits. A very common misapplication is using a supplemental

OCPD, such as a UL Recognized 1077 supplemental overcurrent protective device, for motor branch circuit short-circuit and ground fault protection, and motor branch circuit and controller disconnect or

“at-the-motor” disconnect. Supplemental OCPD testing is inferior to the more stringent requirements of branch circuit devices, such as UL

Listed 489 circuit breakers. IT’S A SERIOUS MISAPPLICATION AND

SAFETY CONCERN to apply these devices beyond their ratings! Caution should be taken to ensure that the proper OCPD is being used for the application. Supplemental OCPDs are described below.

Most supplemental OCPDs have very low interrupting ratings, and caution must be taken to ensure the interrupting rating is equal to or greater than the available short-circuit current.

Supplemental fuses as listed or recognized to the UL/CSA/

ANCE 248-14 Standard

Supplemental fuses can have varying voltage and interrupting ratings within the same case size. Supplemental fuse examples are 13/32’’ x 1-1/2’’, 5x20 mm and 1/4’’ x 1-1/4’’ fuses. Interrupting ratings range from 35 A to 100 kA.

Supplemental protectors (“mini-breakers”) recognized to UL

1077

Supplemental protectors (often referred to as mini-circuit breakers) are not permitted for use as a branch circuit

OCPD. They are not permitted to provide motor circuit or group motor protection and can only be used for protecting an appliance or other electrical equipment where branch circuit overcurrent protection is already provided, or is not required. Creepage and clearance distances that are less than those required in UL 489, and therefore they can’t be listed as a circuit breaker or used as a motor branch circuit and controller disconnect or “at-the-motor” disconnect to meet the requirements of NEC 430.109. Interrupting ratings are typically very low and those devices that are short-circuit tested in series with a fuse must be applied with a branch circuit rated fuse on their lineside.

Identification

Supplemental protectors marked recognized to UL 1077 will contain a recognized symbol rather than a listed symbol.

Warning —

Supplemental protectors are NOT suitable for motor branch circuit protection

In numerous applications throughout the industry, supplemental protectors are being improperly used for motor branch circuit overcurrent protection and as motor branch circuit and controller disconnects or “at-the-motor” disconnects. These are

MISAPPLICATIONS and their prevalence has prompted numerous safety notices, articles and technical bulletins to alert users to the danger of this practice.

Why supplemental OCPDs are misapplied

Top reasons for misapplication include:

• Supplemental protectors look very similar to molded case circuit breakers leading to the assumption they provide the same protection

• Supplemental protectors are often labeled as circuit breakers or miniature circuit breakers (MCB) in literature

• Per IEC standards, many devices are rated as a circuit breaker and creates confusion between North American and IEC ratings that leads to misapplication

Avoiding misapplication

For correct OCPD application, suitable protection for the motor branch circuit needs to specified and installed. The simplest correction to any misapplication is to replace a misapplied supplemental OCPD with one that’s suitable for branch circuit protection.

How to confirm proper application

NEC 430.52 lists acceptable motor branch circuit short-circuit and ground fault protection devices. Time-delay and non-time delay (fastacting) branch circuit fuses used in conjunction with a disconnect are acceptable for reliable and safe protection.

Summary

Many reasons lead to misapplication, including mistaking supplemental protectors for North American circuit breakers. The key to properly identifying supplemental protectors is to look for the “recognized” symbol on its marking. If the device is marked “recognized,” more than likely it’s a supplemental protector and should be replaced with a branch circuit OCPD.



7.14.3 Branch circuit OCPDs and disconnects

Fuse solutions

When selecting fuses, the fuse holder or switch type is very important in determining proper application. While the most economical solutions are often standard UL 4248 Listed fuse holders, they don’t offer a fuse disconnecting means as required per NEC 240.40. A disconnecting means can be ahead of the fuse holder or a UL 98 or UL 508 fused disconnect switch can be selected. UL 98 fused disconnects offer the widest range of applications, while UL 508 disconnects are limited to only motor circuit applications with additional restrictions as noted in

Figure7.14.3.a.

The Bussmann series Compact Circuit Protector (CCP) is the smallest, most economical UL 98 Listed fusible disconnect switch available. There are two CCP types: one with Class CC fuses, the other with Class CF fuses. The Class CC CCP is full voltage rated and available up to 30 A.

The Class CF CCP (uses the UL Class CF CUBEFuse) is available in

DIN-Rail mount and bolt mount versions that are full voltage rated and available up to 100 A.

Red text indicates applications that are limited or restricted.

UL 98 Listed CCP with Class CC fuses or Class CF

CUBEFuse™

UL 4248 Listed fuse holder with

Class CC fuses or

CUBEFuse with holder

UL 4248 Listed holder with Class

CC fuses and

UL 508 Listed disconnect

(manual motor controller)

UL 508 Listed disconnect

(manual motor controller) with integral Class CC fuses

UL 98 Listed disconnect with


Class CC fuses

UL 98 Listed fusible disconnect with Class CC or

J fuses

Visual reference

Branch circuit overcurrent protection

Branch circuit disconnect

At the motor circuit disconnect

Feeder circuit overcurrent protection

Feeder circuit disconnect

Cost

Yes

Yes

Yes

Yes

Yes

$$-$$$

Yes

No

No

Yes

No

$-$$

Yes

No

Yes*†

N/A**

No

$$$

Yes

No

Yes*

N/A**

No

$$$

Yes

Yes

Yes

Yes

Yes

$$$$

Yes

Yes

Yes

Yes

Yes

$$$$$

* Manual motor controller must be additionally marked “Suitable as Motor Disconnect” and be installed on the loadside of the final branch circuit overcurrent protective device.

** Class CC fuse can provide feeder circuit overcurrent protection but UL 508 manual motor controller cannot be applied in a feeder circuit.

† The manual motor controller is the “at the motor” disconnect, not the fuse holder. Table 1 — CCP compared to fuse holders, disconnects with fuses and fused disconnects

Figure 7.14.3.a

CCP compared to fuse holder, disconnect with fuses, and fusible disconnect.

7



Fuse and circuit breaker solutions

To provide branch or feeder circuit overcurrent protection, the OCPD must be either a UL Listed 248 “Class” fuse or a UL Listed 489 circuit breaker. To provide a branch or feeder circuit disconnect, a UL 98 Listed fused disconnect switch or a UL Listed 489 circuit breaker must be selected. The CCP can replace low-rated circuit breakers or misapplied supplemental protectors in branch circuit applications, and provide a higher short-circuit current rating at a similar or lower cost. The CCP is a cost-effective solution similar in size to a supplemental protector or lighting-style circuit breaker, but has higher voltage and interrupting ratings with better short-circuit component protection. Compared to a similarly rated industrial circuit breaker, the CCP is one-third the size.

Figure 7.14.3.b shows the rating differences between the CCP and a supplemental protector, lighting circuit breaker (240 V and 480Y/277 V) and fully rated (600 V) industrial circuit breaker.


UL 98 Listed CCP with Class CC or

Class CF CUBEFuse

UL 1077 Recognized supplemental protector

UL 489 Listed circuit breaker



Visual representation

Branch or feeder circuit overcurrent protection

Branch or feeder circuit disconnect

Voltage rating (AC)

Interrupting rating

Overcurrent protection method

Cost

Yes

Yes

600 V

200 kA

Class CC fuse or

CUBEFuse

$$-$$$

No

No

Typically 277 V or less

Typically 5-10 kA

Yes

Yes

Typically 240 V or less Typically 480/277 V† or

Typically 10-14 kA

Yes

Yes

Typically 14-18 kA

* Cost increases as interrupting rating increases.

† Limits application to solidly grounded wye systems only, not permitted on ungrounded, resistance grounded or corner grounded systems.

Figure 7.14.3.b

CCP Compared to supplemental protector, lighting circuit breakers and fully rated industrial circuit breakers.

Yes

Yes

Typically 600 V or less

Varies 14-100 kA*

Thermal magnetic trip Thermal magnetic trip Thermal magnetic trip Thermal magnetic trip

$ $$ $$$ $$$$



Motor circuit solution comparisons

There are many options available for motor circuits with the CCP along with a magnetic starter providing a very compact, cost-effective solution. Figure 7.14.3.c is an application comparison for the CCP/ magnetic starter combination when compared to other viable solutions.


UL 98 Listed

CCP with Class

CC or Class CF

(CUBEFuse) fuses and magnetic starter


Class CC fuses or CUBEFuse with holder and magnetic starter

UL 508 Listed self-protected starter (SPS) and magnetic contactor

UL 4248 Listed

CCP with Class

CC or Class CF

(CUBEFuse) fuses and manual motor protector (MMP) and magnetic contactor

UL 489

Recognized motor circuit protector (MCP) and magnetic starter**

UL 489 Listed circuit breaker and magnetic starter

Visual representation

Branch circuit overcurrent protection

Motor circuit disconnect

Voltage rating (AC)

Yes

Yes

600 V

Yes

No

600 V

Yes†††

Yes

Typically 480/277

V† or 600/347 V†

Typically 30 kA or

65 kA††

Yes Yes** Yes

Yes*

Typically 480 V or

600 V

Typically 30 kA or

65 kA††

Yes** Yes

Typically 600 V Typically 600 V

Varies 14 kA to 100 kA***

Varies 14 kA to 100 kA***

SCCR

High SCCR with multiple manufacturers

Cost

Typically 100 kA

Yes

Typically 100 kA

Yes No No No
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