ABP 1-2010 Selective Coordination Final

A NEMA Low-Voltage Distribution Equipment Section Document ABP 1-2010

Selective Coordination

Published by National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 www.nema.org Copyright 2010 by the National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions.

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The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document.

NEMA standards and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document.

NEMA disclaims and makes no guaranty or warranty, express or implied, as to the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safety-related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement.

ABP 1-2010

CONTENTS

Foreword ......................................................................................................................................................................... 4 1 Introduction ..................................................................................................................................................................... 5

1.1 Purpose ................................................................................................................................................................. 5 1.2 Scope .................................................................................................................................................................... 5 1.3 Definition of Selective Coordination ....................................................................................................................... 5

2 National Electrical Code (NEC) [1] Selective Coordination Requirements ...................................................................... 8 2.1 Requirements ........................................................................................................................................................ 8 2.2 Challenges Meeting the Requirements................................................................................................................ 10

2.2.1 Local Jurisdiction Interpretation and Enforcement ..................................................................................... 10 2.2.2 Overriding Requirements ........................................................................................................................... 10

3 Circuit Breaker Trip Response Functions...................................................................................................................... 11 3.1 Fixed Thermal-Magnetic Type Circuit Breaker..................................................................................................... 12 3.2 Adjustable Thermal-Magnetic Type Circuit Breaker............................................................................................. 14 3.3 Adjustable Electronic Type Circuit Breaker.......................................................................................................... 15 3.4 Short Time Withstand Current Rating .................................................................................................................. 15 3.5 Instantaneous Override Function......................................................................................................................... 18

4 Application Information from Manufacturers.................................................................................................................. 19 4.1 Application of Time-Current Curves..................................................................................................................... 19

4.1.1 Overload Region ........................................................................................................................................ 19 4.2 Limitation of Time-Current Curves ....................................................................................................................... 20

4.2.1 Overload Region ........................................................................................................................................ 20 4.2.2 Instantaneous or Short Circuit Region........................................................................................................ 20

4.3 Short Circuit Selective Coordination Tables......................................................................................................... 23 4.4 Coordinating Ground-Fault Protection of Equipment ........................................................................................... 24

5 Design Guidelines ......................................................................................................................................................... 29 5.1 Simplify the One-line Diagram ............................................................................................................................. 33

5.1.1 Divide Larger Loads into Smaller Loads..................................................................................................... 33 5.1.2 Reduce the Number of Levels of Protective Devices ................................................................................. 33

5.2 Reduce the Available Fault Current ..................................................................................................................... 34 5.2.1 Increase the Impedance of the System...................................................................................................... 34 5.2.2 Utilize Step-Down or Isolation Transformers .............................................................................................. 34 5.2.3 Take Advantage of the Added Arc Impedance of Load Side and Line Circuit Breaker Combinations........ 38

5.3 Review Device Selection ..................................................................................................................................... 39 5.3.1 Increase the Withstand Capabilities of the Upstream Line Side Overcurrent Protective Devices .............. 39 5.3.2 Change the Type of Circuit Breaker ........................................................................................................... 39 5.3.3 Select Current Limiting Type Molded Case Circuit Breaker ....................................................................... 39

5.4 Special Equipment Application Requirements ..................................................................................................... 39 5.4.1 Generator Protection .................................................................................................................................. 39 5.4.2 Automatic Transfer Switches...................................................................................................................... 39 5.4.3 Busway....................................................................................................................................................... 40 5.4.4 Arc Flash Energy........................................................................................................................................ 41 5.4.5 Zone Selective Interlocking ........................................................................................................................ 41

5.5 Field Adjustment .................................................................................................................................................. 45 5.6 Lifetime Selective Coordination ........................................................................................................................... 45

6 Summary....................................................................................................................................................................... 46

Copyright 2010 by the National Electrical Manufacturers Association. 3

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Foreword

This is a new NEMA White Paper. It was developed in response to the requirements in the National Electrical Code for selective coordination in order to assist engineers in designing selectively coordinated power systems using low-voltage circuit breakers. To ensure that a meaningful publication was being developed, draft copies were sent to a number of groups within NEMA having an interest in this topic. Their resulting comments and suggestions provided vital input prior to final NEMA approval and resulted in a number of substantive changes in this publication. This publication will be periodically reviewed by the Molded Case Circuit Breaker Product Group of the Low-Voltage Distribution Equipment Section of NEMA for any revisions necessary to keep it up to date with advancing technology. Proposed or recommended revisions should be submitted to: Vice President, Technical Services National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 This White Paper was developed by the Molded Case Circuit Breaker Product Group of the Low-Voltage Distribution Equipment Section of NEMA. Approval of this White Paper does not necessarily imply that all members of the Product Group voted for its approval or participated in its development. At the time it was approved, the Molded Case Circuit Breaker Product Group had the following members: ABB Control, Inc.Wichita Falls, TX Eaton CorporationPittsburgh, PA General ElectricPlainville, CT Siemens Industry, Inc.Norcross, GA Schneider Electric USAPalatine, IL


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1 Introduction

1.1 Purpose To provide guidance to engineers regarding the 2008 National Electrical Code (NEC) [1] requirements for Selective Coordination in articles 620, 700, 701, and 708. This paper specifically addresses how to comply with these requirements for low-voltage Overcurrent Protective Devices (OCPD).

1.2 Scope This paper provides information on the following topics:

1) Description of the key functions of the OCPDs used in low-voltage applications for meeting Selective Coordination requirements per the latest version of the NEC [1].

2) Discussion of selectivity coordination application information provided by manufacturers and implications for system design.

3) The importance of including both phase currents as well as ground-fault currents for Selective Coordination.

4) The role of the system design engineer and the necessary interaction with applicable Authorities Having Jurisdiction (AHJ).

5) An overview of considerations for designing selectively coordinated systems.

1.3 Definition of Selective Coordination The goal of Selective Coordination is to isolate the faulted circuit while maintaining power to the balance of the electrical distribution system.

NEC Article 100 [1] definitions related to selective coordination are as follows:

Selective Coordination. Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.

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.

Other relevant definitions from The Authoritative Dictionary of IEEE Standard Terms, IEEE 100 include:

Short Circuit Current. An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.

Ground Fault. An insulation fault between a conductor and ground or frame. With Selective Coordination, only the Overcurrent Protective Device (OCPD) nearest to the fault should open to clear the fault. This overcurrent fault condition may be caused by an overload, a short circuit, or a ground fault, and ideally each OCPD shall be selectively coordinated with other upstream protective devices in the system.

The concept of selective coordination is probably best understood via graphical presentations.

Example 1 A system that Is Selectively Coordinated

Figure 1.1 shows a typical electrical system with multiple levels of branch and feeder Overcurrent Protective Devices (OCPD).


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In Figure 1.1, for a fault below the 20 A OCPD in panel P-1, only the 20 A OCPD should open. Electrical power continues to be available in all other circuitsthey are not affected, since only the 20 A OCPD closest to the fault operates to clear the fault.

P-120A

200A

800A

400A

P-2

Selectively CoordinatedFor the full range of overcurrents possible at P-1, only the 20A OCPD opens.

Power Distribution Equipment

OCPD Not affected

OCPD Opens

Unnecessary powerloss

Fault

P-120A

200A

800A

400A

P-2

Selectively CoordinatedFor the full range of overcurrents possible at P-1, only the 20A OCPD opens.


OCPD Not affected

OCPD Opens


Fault

Figure 1.1

System Is Selectively CoordinatedFault at Branch Level OCPD

In Figure 1.2, the same system is shown, except with the fault now located between panels P-1 and P-2. Since this system is selectively coordinated, only the 200 A OCPD in panel P-2 operates to clear the fault.

P-120A

200A

800A

400A

P-2

Selectively CoordinatedFor the full range of overcurrents possible at P-2,

only the 200A OCPD opens.


OCPD Not affected

OCPD Opens


Fault

P-120A

200A

800A

400A

P-2

Selectively CoordinatedFor the full range of overcurrents possible at P-2,

only the 200A OCPD opens.


OCPD Not affected

OCPD Opens


Fault

Figure 1.2

System Is Selectively CoordinatedFault at Feeder Level OCPD


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Example 2 A system that Is Not Selectively Coordinated

Figure 1.3 shows the same scenario as in Figure 1.1, except in this case, the system is NOT selectively coordinated.

Figure 1.3

System Is Not Selectively CoordinatedFault at Branch Level OCPD

In the scenario of Figure 1.3, where the system is NOT selectively coordinated, an overload or fault downstream of the 20 A OCPD in panel P-1 causes both the 200 A and the 20 A OCPD to open. If this system was selectively coordinated, only the 20 A OCPD should open.

If the fault current were a short circuit condition such that the currents were great enough to cause the 800 A circuit breaker to open, the scenario would be as shown in Figure 1.4. The 800 A, the 400 A, the 200 A, and the 20 A OCPDs may ALL open instead of just the 20 A OCPD, since the system is NOT selectively coordinated.

(Note that the opening of all of these OCPDs in this scenario is theoretical. In practice, impedances in the circuit may typically limit the current to levels that may not necessarily cause all of the OCPDs to open.)


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Figure 1.4

System Is Not Selectively CoordinatedFault at Branch Level OCPD

The purpose of selective coordination is to isolate the faulted circuit, regardless of the type of fault, while maintaining power to the balance of the electrical distribution system. For short circuit selectivity, each pair of Overcurrent Protective Devices (OCPD) should ideally be selective up to the maximum fault current available at the load terminals of the downstream device. This level of current defines the maximum fault current of concern for selective coordination. The devices must also be selective for all lower fault currents.

2 National Electrical Code (NEC) [1] Selective Coordination Requirements

2.1 Requirements The National Electrical Code Section 240.12 [1] defines Electrical System Coordination as follows:

Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted.

(1) Coordinated short-circuit protection (2) Overload indication based on monitoring systems or devices.

Selective Coordination first became a requirement in the 1993 edition of the National Electrical Code (NEC) [2]. In the 1993 NEC edition [2], Article 620 for Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and Stairway Chair Lifts was the first to add requirements for Selective Coordination.

In the 2005 NEC [3], these requirements were expanded to include the following additional types of systems:

Emergency Systems in Section 700.27 Legally Required Standby Systems in Section 701.18 Health Care Facilities in Section 517.17

In the 2008 NEC [1], these requirements for selective coordination were further expanded into the new Article 708 for Critical Operations Power Systems (COPS) in Section 708.54.

In the 2008 edition of the NEC [1], the following articles require selective coordination:

1) Article 517Health Care Facilities

517.26 Application of Other ArticlesThe essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.


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2) Article 620Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and Stairway Chair Lifts

620.27 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.

3) Article 700Emergency Systems

700.27 Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

Exception: Selective coordination shall not be required in (1) or (2):

(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exist(s) on the transformer secondary,

(2) Between overcurrent protective devices of the same size (ampere rating) in series.

4) Article 701Legally Required Standby Systems

701.18 Coordination. 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 in (1) or (2):

(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exist(s) on the transformer secondary,

(2) Between overcurrent protective devices of the same size (ampere rating) in series.

5) Article 708Critical Operations Power Systems

708.54 Coordination. Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

In addition, Section 517.17 states:

(C) Selectivity. Ground-fault protection for operation of the service and feeder disconnecting means shall be fully selective such that the feeder device, but not the service device, shall open on ground faults on the load side of the feeder device. A six-cycle minimum separation between the service and feeder ground-fault tripping bands shall be provided. Operating time of the disconnecting devices shall be considered in selecting the time spread between these two bands to achieve 100 percent selectivity.

Additionally, Section 708.52 (B) states:

Feeders. Where ground-fault protection is provided for operation of the service disconnecting means of feeder disconnecting means as specified by 230.95 or 215.10, an additional step of ground-fault protection shall be provided in all next level feeder disconnecting means downstream toward the load.

Additionally, Section 708.52 (D) states:

Selectivity. Ground fault protection for the operation of the service and feeder disconnecting means shall be fully selective such that the feeder device, but not the service device, shall open on ground faults on the load side of the feeder device. A six-cycle minimum separation between service and feeder ground-fault tripping bands shall be provided. Operating time of the disconnecting devices shall be considered in selecting the time spread between these bands to achieve 100 percent selectivity.

In each of the National Electrical Code (NEC) [1] sections above, the spirit of the NEC [1] requirement is that the Overcurrent Protective Devices (OCPD) in these types of electrical distribution systems are coordinated such that their operation does not cause unnecessary power loss whenever a fault occurs. Whenever a fault does


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occur, only the OCPD closest to the fault should respond, and allow power to remain in all other unaffected parts of the electrical system. The OCPDs should be selectively coordinated to respond to all types of overcurrentsoverloads, short circuits, and ground faults.

2.2 Challenges Meeting the Requirements 2.2.1 Local Jurisdiction Interpretation and Enforcement Authorities Having Jurisdiction (AHJs) do not often have the expertise to analyze or interpret short circuit and selective coordination studies. Furthermore, there is significant controversy on the exact intent and interpretation of some of the NEC [1] passages referencing selectivity. At the time of this writing, there is considerable variation regarding interpretation of the requirements, enforcement practices, and enforcement rigor.

While the local AHJ does not have to be expert at how electrical systems are designed to meet these selective coordination requirements, they do have to understand what the NEC [1] requirements mandate. More importantly, the AHJ must understand how to interpret documentation that has been provided by engineers or contractors and must determine how to enforce the requirements.

Below are some examples where the AHJs interpretation of NEC [1] requirements illustrate this challenge.

The NEC [1] requires selective coordination for all supply-side overcurrent protective devices in circuits such as legally mandated emergency, life safety, and critical operation power system types of loads. Examples of these types of loads are lights, pumps, and fans that would play critical life safety roles during fires, natural disasters, building collapses, loss of utility power, and other similar catastrophic situations.

Authorities Having Jurisdiction (AHJ) must determine which portions of the electrical systems are covered by the various NEC [1] clauses and then must determine what to enforce and how to enforce it. In recent years, electrical system designers are being reminded to seek input from their local AHJ early in the design process, relative to interpretations of NEC [1] requirements for their local city or municipalities. It is important that the designer understand how the applicable AHJ will interpret and enforce the NEC [1] with respect to the subject system.

For example, when considering selective coordination for all supply-side overcurrent protective devices, some jurisdictions may interpret the meaning of this phrase differently. For some AHJ, where the focus is specifically on the wording all supply-side selective coordination may be interpreted to be required for both the normal and alternate power sources. Some other AHJ may choose to focus on the placement of the requirement being in the Emergency, Legally Required Standby and Critical Operations Power Systems articles of the National Electrical Code (NEC) [1], and interpret the requirement to be applicable only for the alternate, emergency power source circuits, as implied by the scopes of the articles where the selectivity requirements have been added. At the time of this writing, there are different interpretations of these NEC [1] Sections within the construction industry.

In another example of the interpretation challenge, when a new installation is being added to an existing facility, shall all the OCPDs in the existing facility be made to selectively coordinate with those OCPDs in the new installation? Again, depending on any number of different factors, different Authorities Having Jurisdiction (AHJs) may make different decisions as to how to interpret and enforce the NEC [1] requirements in a case such as this.

While the NEC [1] requirements may be drafted in reasonably clear text, the practical interpretation and enforcement are sometimes a subjective matter, and may be controversial. This may be best handled by early communications between the local AHJ and electrical system design engineers, such that all the parties involved can air positions and come to agreements that satisfy NEC [1] requirements and user needs.

2.2.2 Overriding Requirements Some jurisdictions may have overriding requirements like the Agency for Health Care Administration (AHCA) in Florida and the Office of Statewide Health Planning and Development (OSHPD) in California, or may have amended the NEC [1] requirements previously mentioned.

Statewide agencies may regulate specific types of occupancies such as hospitals and may enforce specific requirements that are different from the NEC for those occupancies. Within those states, the state agency will


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override NEC [1] requirements that may pertain to other occupancies not covered by the state agency. Sometimes cities, counties, and other governmental organizations may also have specific requirements that amend the NEC or use sections of the NEC from older editions of the NEC. Again, electrical system designers are urged to understand the NEC [1] requirements as applicable to the occupancy they are designing for and the governmental agencies that have jurisdiction over those specific occupancies.

3 Circuit Breaker Trip Response Functions

There are various methods to obtain Selective Coordination between OCPDs. Generally, selectivity is achieved by adjusting the line side or source device to be less sensitive and slower than the load side device. This is particularly true in the overload region of the various trip curves. In NEC [1] articles 700 and 701 there are exceptions where two or more devices in series need not be selective. The intent is that when two or more devices are feeding the exact same circuit with no loads connected in between, then they need not be selective with each other. However, they do need to be selective with other devices above and below. The exceptions are as follows:

1. Two protective devices of the same continuous ampere rating directly connected in series.

2. The feeder breaker on the primary side of a transformer and the main breaker on the secondary side of a transformer.

For both of these exceptions, it would not matter which OCPD would open, or if they both opened, since the protected circuit would be disconnected in either case.

The response of OCPDs to fault currents is typically shown via Time-Current Curves (TCCs).

An example is shown in Figure 3.0. The TCCs of OCPDs can generally be broken into two separate regions to better understand the two separate time response characteristics of these devices. These regions are called the Overload region and the Instantaneous or Short Circuit region, as shown in Figure 3.0.

NoteFor countries that use International Electrotechnical Commission (IEC) standards, there are somewhat different terminologies that are used in discussing TCCs. The IEC/TR 61912-2 [4] document uses the terminology Fault-current zones to describe the high current areas of TCCs. The different terminologies, either Fault-Current zones or the Instantaneous or Short Circuit regions are both intended to describe that area of the TCCs where currents are above an Overload condition.

The TCC in Figure 3.0 also shows the tolerance bands for the time it takes the device to operate. The TCC shows the maximum tolerance of this time, called the Total Clearing time.

The Total Clearing time for an OCPD has two main componentsthe operating time and an arcing time. The operating time includes all of the sequence of events that occur within the device from the point in time when the device senses that an overcurrent condition has occurred, until current arcing begins. In fuses, this operating time includes the time for events such as sensing and melting elements to respond. In circuit breakers, it includes the time for sensing components and trip unlatching mechanisms to operate.

The arcing time is the time taken for the arc to be extinguished and the current is reduced to zero.

A simple thermal magnetic circuit breaker consists of two key tripping mechanisms. The curved inverse time portion known as the Overload region is generally controlled by a bimetallic strip that flexes with heat caused by current flowing through the strip or by heat caused by a nearby resistive element that has current flowing through it.


ABP 1-2010

In the Instantaneous or Short Circuit regionthe overcurrent device responds instantaneously. Total Clearing times are typically very fastless than 30 milliseconds (ms) for molded case CB and 60 ms for Low-Voltage Power CB. This flat portion of the trip curves referred is generally controlled by a magnetic unlatching mechanism that operates directly from the load current or indirectly from current flowing through a current transformer.

In the Overload regionthe overcurrent device has an inverse-time operating response, meaning that the response time for the device to open decreases as the fault current level increases. Total Clearing times are typically fairly longseconds to hours.

Figure 3.0 Typical Time-Current Curves for an Overcurrent Protective Device

The overall Time-Current Curve (TCC) is the combination of these two protective elements. The transition may be vertical as shown in Figure 3.0, which indicates a relatively simple transition from the slow bimetallic mechanism operation to the faster magnetic operation, or it may be more sloped showing a more complex interaction between the two mechanisms.

In the example shown in Figure 3.0, for a fault current of say 3,000 A, the time-current curves show that this circuit breaker rated at 70 A will trip instantaneously, in a time that is less than 30 ms. For another circuit breaker, rated at say 1,000 A, this same 3,000 A fault will likely cause that larger circuit breaker to trip in the overload region, in tens of seconds or longer, depending on the design and user settings.

Selecting Overcurrent Protective Devices (OCPD) that provide selectivity for faults in their respective overload ranges may be accomplished by providing overload functions that are increasingly less sensitive and slower as the circuit goes from branch to main. For any specific fault current, if the load side device operates in its instantaneous region and the line side device operates in its overload region, selectivity is easily achieved. However, when a fault is in the range where the instantaneous responses of multiple series devices overlap then selectivity may be harder to achieve.

Therefore, a key to optimized selective coordination is the instantaneous response of the circuit breakers that are being considered in the design of the electrical system. There are a number of different types of instantaneous functions associated with circuit breakers, and their similarities and differences.

For circuit breakers, the tripping function is accomplished by designs that operate on thermal-magnetic principles, or on designs that operate using electronic circuits. In either of these trip designs, whether thermal-magnetic or electronic, various adjustable or fixed setting options are often possible. Their differences and how it relates to selective coordination is key to understanding how selectivity may be achieved.

3.1 Fixed Thermal-Magnetic Type Circuit Breaker The response time in the instantaneous region of a particular family of circuit breaker is typically drawn at a constant value in the range of 16 to 30 milliseconds (ms). Once the fault current exceeds the trip threshold, called the pickup level of the device, the magnetic fields from this current are sufficient to unlatch the device


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from its closed state. The only factor in the operation after this point is the time it takes for the contacts to physically open and for the electrical arc to be extinguished. This complete action typically takes place within one cycle of the electrical current for smaller devices and possibly two cycles for larger devices, without any intentional mechanical or electronic delay on the part of the device.

Figure 3.1

Typical Time-Current Curve for a Fixed Magnetic Pickup Action

In Figure 3.1, the Magnetic Pickup level of the device is fixed by design to operate once the current exceeds approximately 1,000 A. The device will trip with no intentional delay, in approximately 1-cycle (17 ms).

There are various tolerances associated with the dimensional and material properties of the components used in the design of the device. The result of these variations in the design materials causes a tolerance in the response levels of both the pickup current and also the exact trip time. The total tolerance is represented by the band shown around the nominal current and time on the Time-Current Curves (TCC).

Standards such as UL 489 [5] specify the maximum tolerance (such as -20% to +30%) allowed for an adjustable instantaneous setting marked on the circuit breaker. Manufacturers TCCs may demonstrate less tolerance for a particular device based on the devices actual performance. In the case of Low-Voltage Circuit Breakers, the TCCs provided by a manufacturer reflect applicable clearing time tolerances that are demonstrated by the corresponding circuit breaker.

For selective coordination applications, the designer of the electrical system must therefore select Overcurrent Protective Devices (OCPD) in such a manner that the OCPDs coordinate at the calculated fault currents, whether the fault current is in the overload or instantaneous range of the various devices. Typically, line side devices are selected such that the instantaneous trip level of the device can be set higher than the available fault current at the load side devices terminals. Conversely, a load side branch device is usually selected such that it will respond instantaneously to faults above the normal expected currents required to sustain the load.


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3.2 Adjustable Thermal-Magnetic Type Circuit Breaker Circuit breakers with adjustable instantaneous trips are available from most manufacturers over a wide range of circuit breaker sizes and types. An adjustable instantaneous trip offers system designers greater flexibility by allowing selection of an optimized instantaneous protection function that allows normal load fluctuations while tripping for higher abnormal currents. A simple example of this option with three settings is shown in Figure 3.2.

In this example, the electrical system designer has the flexibility to select the instantaneous pickup setting to be at current level Low amperes as in Figure 3.1, or adjust it higher to levels Medium or High amperes based on the needs of the electrical system.

Figure 3.2

Typical Time-Current Curve for an Adjustable Magnetic Pickup Action

Traditionally, when performing a selective coordination study, the goal is to achieve selective trip coordination by adjusting trip bands on the various devices to achieve a separation of the tolerance bands to the point where there is white space or a visible space between them. There have been various opinions and recommendations for how much white space is adequate to ensure selective trip coordination, especially in the area of medium- and high-voltage circuit breakers where an external sensing and tripping device is employed. When the trip curves for the external relays were drawn, an allowance for the reaction and clearing time of the circuit breaker was necessary. The achievement of white space was considered good design practice and carried over into all trip curve coordination.

Low-Voltage Circuit Breaker Time-Current Curves (TCC) represent the operation of the circuit breaker as a complete system. Per applicable UL standards [5], Low-Voltage Circuit Breakers and their respective trip systems are tested and listed as a system. A Low-Voltage Circuit Breaker TCC includes sensing time, signal processing time, mechanical operation time, and arc extinguishing time, plus all the associated tolerances. Hence modern circuit breaker manufacturers do not generally require that additional tolerance or clearing time be allocated between Low-Voltage Circuit Breaker curves in a composite TCC. If two circuit breakers are operating at similar temperatures, it can be expected that they will be selective for a given fault current even if the respective TCC are close enough together that white space is not evident in the composite TCC.


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Today, modern Low-Voltage Circuit Breakers with integral trip units operate at higher speeds than in the past, even to the point where some molded case circuit breakers are current limiting. Modern trip units also employ many techniques to improve their performance and accuracy; even the standard thermal-magnetic trip units are better today than in the past. Electronic trip units employ high-speed microprocessors to achieve the highest levels of accuracy, repeatability, and reliability. The time-current curves for modern circuit breakers now accurately reflect not only the trip unit reaction times but also the total clearing time, including all tolerance allowances. What this means is that it is no longer necessary to allow white space between Low-Voltage Circuit Breaker trip curve bands to ensure selective coordination. Even if the outer edges of the bands touch, the included clearing times and tolerances ensure that the two devices will selectively coordinate.

3.3 Adjustable Electronic Type Circuit Breaker Electronic trip units are characterized by their adjustability (Figure 3.3), their accuracy, and their repeatability. This repeatability that is inherent with electronic design allows less variability in the point at which the device will pickup during a fault condition. As a result, circuit breakers with electronic trip units typically have much narrower tolerance bands as compared to other designs of Overcurrent Protective Devices (OCPD).

There are presently no unique industry standards for the pickup tolerances for circuit breakers with electronic trip units. While these devices comply with tolerance requirements of the present UL 489 [5] for molded-case circuit breakers, for example, most circuit breaker manufacturers publish time-current curves with tolerances that are considerably narrower than the UL 489 requirementssome typically shown in the range of 10% to 15% tolerances.

Most electronic circuit breaker designs have simple switches on the devices that provide for several adjustable selections of the pickup setting for instantaneous response, as shown in Figure 3.3. These adjustable electronic circuit breakers therefore provide the electrical system designer with two key advantages. First, they provide maximum flexibility in adjusting the desired level of pickup current, and second, they inherently have the narrowest tolerances for coordinating the response of multiple OCPDs.

T i me

Multiples of Rated Current

Long-time Pickup

Long-time Delay

Short-time Pickup

I2Short-time Delay ( T IN)

Adjustable Instantaneous I2 T OUT

Fixed Instantaneous

Figure 3.3

Typical Adjustable Settings for Circuit Breakers

3.4 Short Time Withstand Current Rating Low-Voltage Circuit Breakers fall into two basic classifications of designLow-Voltage Power Circuit Breakers (LVPCBs) and Molded Case Circuit Breakers (MCCBs). One of the most important application features that

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distinguish a LVPCB from a MCCB is the ability of the LVPCB to withstand very high overcurrent levels without tripping.

There is a special type of Molded Case Circuit Breaker called an Insulated Case Circuit Breaker (ICCB). These circuit breakers have many of the Low-Voltage Power Circuit Breaker (LVPCB) characteristics, including short time current duty cycles and stored energy mechanisms.

The main difference is that Insulated Case Circuit Breakers, like Molded Case Circuit Breakers, are tested in accordance with UL 489 [5]. Table 3.4 shows just some of the key differences in the ratings between Power Circuit Breakers (UL 1066) [6] and Molded Case / Insulated Circuit Breakers (UL 489) [5].

Required Ratings

UL 1066 [6] (LVPCBs)

UL 489 [5] (MCCBs & ICCBs)

Rated (Maximum)

Voltage

254 V, 508 V, or 635 V (unfused), or 600 V (if integrally fused)

120, 120/240, 240, 277, 347 V,

480Y/277, 480, 600Y/347, or 600 V

Rated Frequency DC, 50 Hz, or 60 Hz DC, 50 Hz, or 60 Hz

Rated Continuous

Current

Frame Sizes: 600 A to 5000 A,

ratings by combination of

sensors and trip units

Frame Sizes: 15 to 6000 A

Rated Short- Time Withstand Current

Duty Cycle

Carry fault current For two 0.5 sec.

periods Not specified

Rated Short- Circuit Current

(at Rated Maximum Voltage)

200 kA max. 7.5 kA to 200 kA

Rated Short- Circuit Current

Duty Cycle O (15 sec.) CO O (2 to 60 min.) CO

Short Circuit Test Power Factor

15% (X/R ratio 6.6) unfused LVPCBs20% (X/R ratio 4.9) fused LVPCBs [4.9 is the X/R as stated in ANSI/IEEE

C37.50 which is referenced in UL 1066 [6] for LVPCBs]

20% (X/R ratio 4.9)

Table 3.4

Typical Ratings of Low-Voltage Power Circuit Breakers vs. Molded Case / Insulated Case Circuit Breakers


ABP 1-2010

Figure 3.4 shows some typical Time-Current Curve characteristics for these various circuit breaker types.

Figure 3.4

Typical Time-Current Curve Characteristics for Low-Voltage Power Circuit Breakers vs. Molded Case / Insulated Case Circuit Breakers

The Short Time Withstand Current Rating of a LVPCB is the level of rms symmetrical current that a circuit breaker can carry in the closed position for a specified period of time. This term is typically used in association with LVPCBs, and not with MCCBs. Some MCCB manufacturers may publish a Short Time Withstand Current Rating where they exist.

The Short Time Withstand Current rating represents the mechanical and thermal ability of the circuit breaker to withstand an overcurrent for the given amount of time. This specific rating is published by the manufacturer.

Rated short-time withstand current:

The maximum root-mean-square (rms) total current that a circuit breaker can carry momentarily without electrical, thermal, or mechanical damage or permanent deformation. The current shall be the rms value,

seco

nds

Fast instantaneous clearing time typical of MCCB

Three cycle instantaneous clearing time typical of large MCCB or LVPCB

Curve without instantaneous typical of LVPCB without instantaneous or instantaneous override

0.01

0.10

1

10

100

1000

Adjustable Low Voltage Power CB without instantaneous trip

Molded Case CB with adjustable instantaneous trip

Adjustable Low Voltage Power CB or large MCCB with instantaneous trip


ABP 1-2010

including the dc component, at the major peak of the maximum cycle as determined from the envelope of the current wave during a given test time interval. (adapted from IEEE Std. C37.100-1992) [7]

LVPCBs are typically used in electrical distribution systems to feed a switchboard, a motor control center, or other electrical panelboards. A number of circuit breakers in these power distribution centers may then be used to feed a variety of separate loads. To coordinate the tripping characteristics of the LVPCB with other downstream circuit breakers, it is very desirable to have the mechanical characteristics of the circuit breaker so that its "withstand current" rating is as high as possible. Short Time Withstand ratings allow the circuit breaker to intentionally delay up to 30 cycles (0.5 seconds) before tripping, depending on the manufacturer and design. The result is to enable the LVPCB to remain closed, allowing selective coordination with downstream circuit breakers to open and clear a fault.

3.5 Instantaneous Override Function The typical range of instantaneous pickup adjustment for circuit breakers is from around 1.5 up to 12 (or higher) times the continuous ampere rating of the circuit breaker, depending on the manufacturer and design. In the example of the 70 A circuit breaker in Figure 3.0, this circuit breaker could be adjusted to trip instantaneously at the 1.5x setting (105 A), or as high as the 12x setting (840 A).

In addition to being able to adjust the range of instantaneous pickup settings from a low value to a high value, some circuit breaker manufacturers also have electronic designs that allow the instantaneous function to be turned OFF. When a circuit breaker with an electronic trip unit is specified without an instantaneous pickup function, it typically contains whats called an instantaneous override function, as shown in Figure 3.5.

The instantaneous override function is also set to pickup and trip the circuit breaker instantaneously, but its pickup level is permanently set at a much higher level than the typical maximum instantaneous settings of 12 times the continuous ampere rating of the circuit breaker (Figure 3.5). The pickup level of the instantaneous override is typically set relatively close to the Short Time Withstand rating of the circuit breaker, depending on the manufacturer and design. As a result, the instantaneous override pickup setting of the 70 amp circuit breaker of Figure 3.0 may be as high the Short Time Withstand capability of the circuit breaker, of say 10,000 A.

This ability of a circuit breaker to remain closed at relatively high fault currents is a key benefit in being able to selectively coordinate Overcurrent Protective Devices (OCPD). In this example, a 70 A circuit breaker with an instantaneous override set at 10,000 A will coordinate (stay closed) with a downstream overcurrent protective device that is set to trip instantaneously at fault currents levels that are lower than 10,000 A.

Therefore, one of the key ways for maximizing selective coordination is to apply an upstream circuit breaker using an electronic trip unit without the adjustable instantaneous trip function. These circuit breakers do not have an adjustable instantaneous characteristic; the built-in instantaneous override feature will instantaneously trip the circuit breaker when the current level exceeds the published Short Time Withstand values, but will allow this circuit breaker to remain closed at lower fault current levels.


ABP 1-2010

Circuit Breaker Instantaneous Region

Figure 3.5

Typical Adjustable Settings for Circuit Breakers

4 Application Information from Manufacturers

4.1 Application of Time-Current Curves 4.1.1 Overload Region The correct method for determining selective coordination and the protection of equipment is via a coordination study. This method provides a thorough analysis of the requirements, and results in documented evidence that the coordination and protection requirements have been adequately achieved.

The selective coordination study involves a time-current coordination study by comparing the timing characteristics of the various protective devices being considered with each other. In addition, the study also looks at the potential damage characteristics of equipment being protected. For electronic or thermal-magnetic circuit breakers, the appropriate settings for the circuit breaker trip units are developed in the coordination study.

The short circuit currents available at different points in the system must also be understood. To ensure an optimal analysis, a coordination study is typically performed in conjunction with a Short Circuit Study. This study evaluates the short circuit currents that may available in the system and allow the designer to see, at the same time, the impact of these short circuit currents on the selection of devices to meet both selective coordination and protection requirements.

When discussing selective coordination, Time-Current Curves (TCCs) for Overcurrent Protective Devices (OCPD) (circuit breakers and fuses) are properly displayed as a bandnot a single line. Note that because of the time difference between minimum response time and total clearing time, a band must always be shown around that curve. Without this band, a user may accidentally create a selective coordination error resulting from hidden curve overlap.

Instantaneous Pick-Up The nominal value of current at which an adjustable circuit breaker is set to trip instantaneously. (IEEE 1015-2006 Blue Book) [8]

Instantaneous Trip A qualifying term indicating that no delay is purposely introduced in the tripping action of the circuit breaker. (IEEE 1015-2006 Blue Book) [8]

Instantaneous Override The override trip is an independent instantaneous trip set near the circuit-breaker withstand level that overrides the electronic logic trip unit to cause the circuit breaker to open without delay at very large fault levels. (IEEE 1015-2006 Blue Book) [8]


ABP 1-2010

effect of the load side circuit breaker. Hence, in the instantaneous region, circuit breakers may be more or less

Figure 4.1 Typical Time-Current Curves of Two Overcurrent Protective Devices

The time-current trip curves provide a quick and easy way to identify if selective coordination exists between Overcurrent Protective Devices (OCPD). By overlaying the trip curves of two circuit breakers onto one graphical plot, the designer can determine whether selective coordination exists. If the trip curves of two circuit breakers intersect, the area of intersection indicates conditions under which both circuit breakers may trip. If these two circuit breakers were used in an electrical system, the overlap of trip curves could result in both circuit breakers tripping, causing unnecessary power loss to some portions of the electrical distribution system. On the other hand, if the trip curves of two circuit breakers do not touch, the circuit breakers are said to be coordinated.

4.2 Limitation of Time-Current Curves 4.2.1 Overload Region The Time-Current Curves are broken into two separate regions called the Overload region and the Fault Current or Short Circuit region, as shown in Figure 3.0.

In the overload region as shown in Figure 4.1, the curves of two devices in series are typically separated by time, and the trip response time involved is relatively long (seconds or minutes or even hours).

Therefore, in the overload region where fault currents are relatively low, and the response time of OCPDs is typically not much faster than around one second or so, selective coordination is relatively easy to accomplish between most devices. In this region, the Time-Current Curves of the various OCPDs are typically an adequate tool for determining selective coordination of devices.

4.2.2 Instantaneous or Short Circuit Region Traditional interpretation of time-current curves in the instantaneous region is the same as the interpretation in the overload region. An overlap of the curves indicates potential lack of selectivity and, a lack of overlap indicates probable selectivity. However, Time-Current Curve (TCC) analysis alone ignores the current limiting


ABP 1-2010

The effect of current limitation on the line side circuit breakers trip performance may be illustrated by Figure 4.2-

selective than traditional TCCs indicate. This is based on how the line side circuit breakers instantaneous trip function reacts to a fault current flowing through both devices as altered by the typically smaller load side circuit breaker or fuse. The line side circuit breaker will react to the peak let-through current allowed to flow by the smaller, or faster, OCPD for a given prospective fault current.

1.

CurrentLimiting CB

ElectronicTrip CB

Trip

Fault below CL CB

Fault above CL CB

-80,000-70,000-60,000-50,000-40,000-30,000-20,000-10,000

010,00020,00030,00040,00050,00060,00070,00080,00090,000

- 0.0083 0.0167Seconds

Am

pere

s

Line side trip set at36kA peak (25kA RMS)

Fault between line &load side devices

Peak let-through~32kA

CurrentLimiting CB

ElectronicTrip CB

Trip

Fault below CL CB

Fault above CL CB

CurrentLimiting CB

ElectronicTrip CB

Trip

Fault below CL CB

Fault above CL CB

-80,000-70,000-60,000-50,000-40,000-30,000-20,000-10,000

010,00020,00030,00040,00050,00060,00070,00080,00090,000

- 0.0083 0.0167Seconds

Am

pere

s

Line side trip set at36kA peak (25kA RMS)

Fault between line &load side devices

Peak let-through~32kA

Figure 4.2-1

Effect of Current Limit Breaker Performance

this Figure 4.2-1, the larger sine wave represents a prospective fault current or the fault magnitude possible at

Peak let-through currents may be provided by manufacturers in the form of peak let-through plots for various

ing on Circuit

Inthe load side circuit breakers line side terminals. The smaller half cycle sine wave represents the current limiting effect of the load side circuit breakers current limitation on the larger prospective fault current. The dashed line is the instantaneous trip setting, in instantaneous or peak amperes, of the line side circuit breaker. As may be seen from this diagram, even though the prospective fault current could have had a peak ampere value over 80 kA, the current limiting effect of the load side device limited the peak current to approximately 32 kA ensuring selectivity with the line side device set at 36 kA.

circuit breakers or fuses. Values for peak let-through current at a specific prospective fault current may be selected from these graphs. If a line side circuit breaker trip is set above the peak allowed to flow through by the downstream device then the pair should be selective for the defined prospective fault current and below. In the example shown in Figure 4.2-2, the current limiting circuit breaker allows a peak let-through current of 33 kA for a prospective fault of 50 kA rms. As long as the line side circuit breaker is set above 23 kA rms, selectivity up to 50 kA is possible.


ABP 1-2010

10

100

10 100Prospective RMS Fault I (kA)I P

eak

Let-

thro

ugh

(kA

)

2 RMS

33kA Peak

50kA RMS = 33kA Peak let-through

33kA = 23kAPeak at PF = 1

10

100

10 100Prospective RMS Fault I (kA)I P

eak

Let-

thro

ugh

(kA

)

2 RMS

33kA Peak

50kA RMS = 33kA Peak let-through

33kA = 23kAPeak at PF = 1

Figure 4.2-2 Peak Let-Through Currents of Circuit Breaker

Understanding how the current limiting behavior of a current limiting fuse or circuit breaker is sensed by a line side device that operates based on instantaneous peak currents can also prevent setting circuit breakers too low when the downstream devices curve is drawn only down to the 0.01 axis on the Log-Log Time-Current Curve (TCC).

Figure 4.2-3 shows a circuit breaker set high enough to not overlap with the fuses time-current curve as drawn on a typical TCC showing a 0.01 second minimum response time.

100 1K 10K 100K0.01

0.10

1

10

100

1000

Amperes RMS

SE

CO

ND

S

800A CB

200A J TD

800A CB

200A J TD

100 1K 10K 100K0.01

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1000

Amperes RMS

SE

CO

ND

S

800A CB

200A J TD

800A CB

200A J TD

100 1K 10K 100K0.01

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1

10

100

1000

Amperes RMS

SE

CO

ND

S

800A CB

200A J TD

800A CB

200A J TD

Figure 4.2-3 Circuit Breaker Settings (Set so Circuit Breaker TCC does not Overlap Fuse TCC)

However, when the fuses peak let-through current is taken into consideration, the circuit breaker must be set as shown in Figure 4.2-4 to ensure selectivity up to the full available bolted fault current.


ABP 1-2010

100 1K 10K 100K0.01

0.10

1

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1000

Amperes RMS

SE

CO

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200A J TD

800A CB

200A J TD

800A CB

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1000

Amperes RMS

SE

CO

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200A J TD

800A CB

200A J TD

800A CB

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100

1000

Amperes RMS

SE

CO

ND

S

200A J TD

800A CB

200A J TD

800A CB

Figure 4.2-4 Circuit Breaker Settings (Set to Ensure Selectivity to Full Available Bolted Fault Current)

Circuit breaker manufacturers have developed additional analytical methods and advanced proprietary electronic trip algorithms that allow selectivity of multiple current limiting circuit breakers in series and also allow electronic trips to be set at lower, more sensitive settings than the above described peak let-through based method. Testing performed by the manufacturers under a variety of fault conditions should confirm the validity of the methods used. Description of these methods is beyond the scope of this document.

Manufacturers will provide short circuit selectivity tables or other tools that document the instantaneous selectivity that may be achieved with their devices based on the peak let-through current or energy of the load side devices and how the line side devices respond to that let-through current or energy.

4.3 Short Circuit Selective Coordination Tables The time-current curves of Overcurrent Protective Devices (OCPD) are typically developed by conducting current interruption tests at various levels of overload and short circuit current. The time taken for the device to completely interrupt the current flow is then measured and plotted to generate the time-current curves. These tests are done on individual devices, and the corresponding time-current curves plotted.

In the case of selective coordination, the idea is to see how two of these devices perform, not as individual devices, but instead, how they perform when connected in series with the same fault current flowing. At current levels in the overload region, time-current curves for the individual devices may be overlaid on each other to visually see if selective coordination is achievable. At higher short circuit current levels, the time-current curves alone may not show as complete a picture as possible. The time-current curves alone do not include the impact of the added impedance of the downstream circuit breaker if it begins to open faster than the upstream circuit breaker, and the resulting higher coordination levels.

At these high fault current levels, if the time-current curves do not indicate that the two circuit breakers in question are coordinated, then selective coordination performance should be determined by looking at the additional information provided by the manufacturer of the OCPDs. Most manufacturers provide additional selective coordination information that is summarized in the form of tables such as Table 4.3, [9] and show the interrupting capabilities of the two devices when connected in series.


ABP 1-2010

Table 4.3

Typical Selective Coordination Table These Selective Coordination Tables typically show the downstream circuit breaker data on one axis and the upstream circuit breaker on the second axis. The numbers that fill in the matrix between these two axes represent the levels of coordination between the upstream and downstream devices. The tables are also intended to be a quick and visually easy-to-use way to determine selective coordination, without design engineers needing to perform complex, error prone calculations. To further the easy-to-use approach, there are software companies that have set-up programs that automate the navigation through the tables, to speed up and simplify the interpretation of the information in these tables.

In some application cases, this increased level of coordination between whats determined by time-current curves alone, versus the use of Selective Coordination Tables, may make an appreciable difference in criteria such as the physical size, costs, and availability in the selection of these devices. Most manufacturers of Overcurrent Protective Devices (OCPD) publish both time-current curves and Selective Coordination Tables. The electrical system designer should consult the manufacturers tables to determine if improvements in the levels of selective coordination may be gained over the level of selectivity indicated by using traditional time-current curves analysis.

4.4 Coordinating Ground-Fault Protection of Equipment Dedicated equipment ground-fault protection such as ground-fault relays or the integral ground-fault function in circuit breaker trips, or switches, is often applied in low-voltage systems. Equipment ground-fault protection is not intended to provide protection against shock or electrocution. Devices that provide Equipment Ground-Fault protection must meet the applicable Sections of the National Electrical Code (NEC) [1], UL 489 Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures [5], and UL 1053 Ground-Fault Sensing and Relaying Equipment [10]. This discussion applies to equipment ground-fault protection, not Ground-Fault Circuit Interrupters (GFCI) personnel protective devices.

The National Electrical Code (NEC) [1] article 230.95 requires equipment ground-fault protection to be provided on solidly grounded wye electric services of more than 150 volts to ground but not exceeding 600 volts phase-


ABP 1-2010

to-phase for each service disconnect rated 1000A or more. Exceptions are made for legally mandated emergency and standby systems as well as systems where a disorderly shutdown may present more risk to human life than a fire caused by an arcing ground fault. Because of the National Electrical Code (NEC) [1] requirement and the desire to protect against low magnitude arcing faults in 480Y/277V systems, ground-fault protection is common in systems rated 1000 A or larger.

The mandate and need for ground-fault protection arises out of the potential for an arcing ground-fault current to be low relative to the settings of the phase protection devices. Prior to ground-fault protection mandates being added to the NEC [2] in 1972, a large number of building and electrical system fires were attributed to arcing ground faults that persisted long enough to seriously damage equipment or start building fires. The industry recognizes ground faults as the most common type of electrical fault1; hence in systems that require higher reliability, it is common to include more than one level of ground-fault protection. Many systems, and hospitals meeting the additional ground-fault selectivity requirements of Section 517.17, will have two or more levels of ground-fault protection in series. The intent of the second level of ground-fault protection is to increase system reliability by preventing the service entrance main Overcurrent Protective Device (OCPD) from opening from a ground-fault below a second-level feeder OCPD. However, as the following text will describe, incorrect selection of downstream OCPDs, complicated by multiple levels of ground-fault protection, may decrease system reliability.

Multiple standards define performance for ground-fault protective devices. The NEC [1] defines maximum pickup to be 1200 A and the maximum clearing time at 3000 A to be 1 second. UL 1053 [10] defines maximum clearing time at 150% of nominal pickup setting as 2 seconds. Figure 4.1-1 shows the various mandated limits along with a typical ground-fault protective device curve at maximum pickup allowed for any size of low-voltage OCPD.

1 J.R. Dunki-Jacobs, F.J. Shields with Conrad St. Pierre Industrial Power System Grounding Design Handbook, self published, 2007:

- Pg. 175: "Non-bolted faults generally are intermittent rather than continuous faults, and occur mostly as ground faults for the reason that, among electrical faults, ground faults statistically prevail."

- Pg. 189: "Statistically, ground faults make up around 95% of all faults. Of these, in industrial systems, a large portion may be initiated as arcing-ground faults in low-voltage systems.

- Pg. 336: "As more than 90 percent of all faults in electrical systems in industry involve ground, an effective ground-fault detection and protective system merits prime consideration."

- Pg. 444: "Statistics tacit (sic) indicate that about 95% of all short circuits in industrial plants are line-to-ground faults, of which most are of the arcing fault variety."


ABP 1-2010

0.01

0.10

1.00

10.00

100.00

100 1,000 10,000 100,000Amperes

Sec

onds

1000.001200A NECmaximumnominal pickup

UL 1053, maximum 2 second clear at 150% nominal setting

NEC 3000A, maximum 1 second clear

0.01

0.10

1.00

10.00

100.00

100 1,000 10,000 100,000Amperes

Sec

onds

1000.001200A NECmaximumnominal pickup

UL 1053, maximum 2 second clear at 150% nominal setting

NEC 3000A, maximum 1 second clear

Figure 4.1-1 The Mandated Limits for Low-Voltage OCPD

Because of standard requirements, the shape of the ground-fault functions protective curve is more limited than the shape of phase protection devices. Phase protection devices response must be shaped to allow normal transient currents associated with motor starting and transformer inrush to flow; hence, the downstream phase protection devices response may not be slower or less sensitive than the ground-fault protection in an upstream Overcurrent Protective Device (OCPD).

Ground-fault protective devices are able to use various sensing mechanisms or calculations to discern a ground-fault current separate from balanced phase current even if the phase current includes a phase-to-phase fault component. However, phase protection devices cannot separate a ground fault from a phase fault. A ground fault with enough fault current can operate phase protection. However, a phase fault should not operate properly functioning ground-fault protection

This requires that for complete system selectivity, that phase protection devices and ground-fault protection devices be coordinated with each other, as shown in Figure 4.1-2.


ABP 1-2010

10 100 1K 10K 100K0.01

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TIME

IN S

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100A TM CB

250A Electronic CB

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IN S

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nds

100A TM CB

250A Electronic CB

Figure 4.1-2

Circuit Breaker Selectively Coordinated with 1200A Ground Fault The sloped portion of the ground-fault curve is called an I2t slope and is a user selectable response typically provided by circuit breaker and Ground-Fault (GF) relay manufacturers. In this figure, a 100 A thermal magnetic lighting type branch circuit breaker and a 250 A molded case circuit breaker are shown to be barely selective with the maximum National Electrical Code [1] allowed 1200 A ground-fault setting. The difference in sensitivity to fault types between ground-fault relays and normal overcurrent protection provides additional selectivity complexity within systems that include both phase and ground-fault protection.

Figure 4.1-3 demonstrates a 100 A thermal magnetic circuit breaker that is not selectively coordinated with the 1200 A Ground-Fault (GF) function and two 200 A class-J fuses. One of the fuses shown in Figure 4.1-3 is selectively coordinated, and the other is not.


ABP 1-2010

10 100 1K 10K 100K0.01

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TIME

IN S

EC

ON

DS

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1200A GF

200A TD J fuse

100A TM CB

200A J fuse

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DS

Amperes

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nds

1200A GF

200A TD J fuse

100A TM CB

200A J fuse

Figure 4.1-3

Circuit Breaker and Fuses with 1200 A Ground Fault NOT Selectively Coordinated These two figures demonstrate that phase protection devices connected downstream of equipment ground-fault protection must be carefully selected with respect to size, type and individual response characteristics to obtain selectivity. However in all cases the downstream device may need to be significantly smaller than the device that incorporates the ground-fault protection.

Due to the limits on ground-fault response and the shape of typical fuses and circuit breaker phase protection downstream of an Overcurrent Protective Device (OCPD) equipped with equipment ground-fault protection, downstream phase protectors may need to be relatively small. In the case of circuit breakers the downstream circuit breakers adjustment flexibility may allow for devices as large as 250 A, potentially more depending on the degree of curve shaping flexibility in the downstream device. Fuses may need to be under 100 A to be selective with ground-fault functions as high as 1200 A if they are of the time delayed type, normal non-time delay type fuses may be larger.

In systems with multiple levels of ground-fault protection, feeders with Ground-Fault Protection may be impossible to make selective with branch circuit breakers as small as 20 A 1 pole. Any fault in a single-phase circuit protected by a one-pole OCPD will be sensed as a ground fault by an upstream three-phase ground-fault protective device. It is commonly believed that most faults are ground faults2, and that most faults occur at end-

2 Per the references: - In actual practice, unbalanced faults are much more common, especially line-to-ground in grounded systems. Per the IEEE Color

Book Series - Orange Book pg. 175

- Ground faults comprise the majority of all faults that occur in industrial and commercial power systems. Per the IEEE Color Book Series - Buff Book pg. 4

- Operating records show that the majority of the electrical circuit faults originate as phase-to-ground failures. Per the IEEE Color Book Series - Red Book pg. 187

- Most electric-circuit faults occur as phase-to-ground breakdowns. Protection Fundamentals for Low-Voltage Electrical Distribution Systems in Commercial Buildings, IEEE JH 2112-1, 1974, pg. 113.


ABP 1-2010

use equipment and circuits. Hence, ground and phase protection selectivity with branch circuit OCPDs is very important in systems where selectivity is deemed important for system reliability. Figure 4.1-4 shows a system, as may be found in a hospital application with two levels of ground fault, as required by National Electrical Code (NEC) [1] article 517.17, set at the highest pickup settings.

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20A 1P Branch CB

240A GF

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1200A GF

20A 1P Branch CB

240A GF

Figure 4.1-4

20A 1-Pole Lighting Circuit Breaker under 240A Ground Fault The feeder with Ground-Fault (GF) protection in this figure is a 400 A circuit breaker with ground-fault set at 240 A nominal pickup. This GF function is barely selective with a 20A 1-pole lighting type circuit breaker. A lower setting of the ground-fault circuit breaker would be impossible to coordinate with a small branch circuit single pole circuit breaker.

In power delivery systems where continuity of power is important, GF protection, circuit size, Overcurrent Protective Device (OCPD) type, as well as device settings must be selected carefully to optimize selectivity. Ground-fault protection in small feeder circuit breakers may reduce system reliability by causing a lack of selectivity between feeder trips and branch circuit breakers.

5 Design Guidelines

In order to properly design a selectively coordinated system, the design Professional Engineer must recognize and understand how the various technical, business, and personnel issues of such a system are interrelated. The Overcurrent Protective Devices (OCPD) and associated control and monitoring equipment must all have technical (electrical, mechanical, thermal, etc.) capacities that are equal to or greater than the system that they are being applied to. The choice of these components drives short- and long-term costs, overall system reliability, and maintenance considerations, and impacts the lives of the personnel that must install and maintain these devices.

As a result, the design of selectively coordinated systems must consider more than just the alignment of equipment selection with National Electrical Code (NEC) [1] requirements and/or technical customer specifications. As design Professional Engineers have worked over recent years to implement these systems, comments from feedback exchanges indicate that there are general approaches that typically yield successful


ABP 1-2010

results when designing selectively coordinated systems. Feedback has also indicated that the earlier in the design process that the various selective coordination requirements are considered, the smoother the entire process will be. For example, getting preliminary data about things such as the available fault currents from the utility and/or generators, estimates of cable lengths, and OCPDs typically results in designs that minimize re-work and time-consuming revisions.

Successfully designed systems typically follow some fairly straightforward guidelines:

1) Understand the overall requirements and objectives of the electrical system

2) Understand how the local Authority Having Jurisdiction interprets the NEC [1] with respect to the proposed system

3) Determine the available fault currents at each device, from all sources of powerconduct a Short Circuit Study of the system

4) Select OCPDs that provide selective coordinationuse Time-Current Curves and Selective Coordination Tables from manufacturers

5) Optimize the designconsider special application requirements, and make iterative changes to simplify the impact of the design on initial installation, ongoing maintenance, and the safety of operating personnel

1) Understand the overall electrical system

Prior to designing a system, the design engineer must understand the overall requirements and objectives of the electrical system, particularly in the area of selective coordination. The understanding of these requirements should be documented. The requirements for selective coordination often go hand-in-hand with systems that involve standby generators and automatic transfer switches (ATS).

a) System drawings and documentation should indicate the nature of the application that the ATS scheme is to be used for. These applications may be for situations that involve emergency, life safety, critical care, elevators or similar people movers, legally required standby, critical operations power systems, some other local NEC [1] requirement.

b) System documentation should clearly identify where requirements need to meet selective coordination per NEC [1] Sections 620.62, 700.27, 701.18, or 708.54, as required.

In some cases, preliminary drawings may not clearly identify what areas of the electrical system require selective coordination. If selective coordination requirements are not clearly and fully addressed early in the design phase of a project, equipment manufacturers that may bid on the project for example, can make erroneous assumptions that may later impact the physical size, performance capabilities, costs, availability, etc., of equipment being provided.

2) Understand the local Authority Having Jurisdictions interpretations

The design engineer should develop a sound understanding of how the local jurisdiction interprets the selective coordination NEC [1] requirements with respect to the various areas of the proposed electrical system. Early discussions should be conducted with the appropriate Authorities Having Jurisdiction (AHJ) to clarify how any areas of potential ambiguity will be interpreted.

In an example such as the addition of a new building wing to an existing hospital, a number of opportunities for confusion may arise. Exactly what portions of the existing buildings equipment shall coordinate with the equipment of the new wing, and whether the devices connected to both the emergency generator and the normal utility shall be selectively coordinated, may have different answers depending on the exact nature of the application. Its best to proactively surface these issues in order to discuss and determine the answers as early as possible in the design phase, so that the upcoming selection of the appropriate equipment can be made without unnecessary redesign.


ABP 1-2010

3) Determine the available fault currents

In order to determine if devices are selectively coordinated, the system designer must know which devices will stay closed and which devices will open when fault currents flow through those devices. In order to know which devices will open or stay closed, a study must be conducted to determine the prospective fault currents that are available at each device in the electrical system. To make this determination, some basic information is needed as follows:

a) A One-Line diagram of the electrical system

b) Voltage at various points in the system

c) Short Circuit Fault Currents at various points in the system

A simple electrical system may have a single power source, and their analysis will usually be simple and straightforward. Other, more complicated systems may have multiple power sources, requiring more involved analysis. In either case, whether simple or complicated, the analysis for determining the available fault currents will follow the same general approach.

The approach for determining available fault currents in a complex scheme is similar to that of the single source scheme, except that a number of factors must be considered. The system design engineer must make sure that other factors such as fault current contribution from motors will require adjustments for changes in X/R ratios, and the effect of power loss due to various cable-length impedances are all accounted for and included in the analysis. In the more complicated schemes, there are a number of different components in the electrical system that may impact the available fault current at each Overcurrent Protective Device (OCPD). The analysis of how these various components impact the system design must be done by qualified system design personnel.

In schemes involving both a normal utility power source and an alternate emergency generator power source, the design engineer must work with the local Authority Having Jurisdiction (AHJ) to establish if both the utility and the generator power source, or just the generator power source, are to be considered in the analysis of the available fault currents. In general, generators will typically have much lower available fault currents than the normal utility source, making selective coordination somewhat simpler. There are, however, applications such as large data centers and hospitals where the available fault currents from the generator power source may be quite high.

Early discussions between the design engineer and the local AHJ will typically surface the appropriate approach to take in addressing these options, during the actual system design phase.

4) Select overcurrent protect devices for selective coordination

At this point in the design of the system, the design engineer may preliminarily select OCPDs that satisfy the requirements for being able to appropriately interrupt fault currents and provide protection to limit damage. The next step in the design process is to examine whether these preliminarily selected devices will also selectively coordinate. For this discussion, we determined that selective coordination of all devices connected on both the normal utility and the emergency generator power sources are required by the local AHJ.

a) Start at the smallest device and work from the bottom up

To begin the analysis, start with the smallest device that is the farthest downstream point of the utility system. Using the fault current available to this device from the Short Circuit study, examine if this downstream device will coordinate with the device that is immediately upstream from it. This examination may be done looking at both Time-Current Curves and/o

Documents

ABP 1-2010 Selective Coordination Final