NEMA ABP 1 Selective Coordination

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

Selecti elective ve Coor Coordin dination ation

Published by National National Electrical Manufacturers Ass ociation 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.

NOTICE AND DISCLAIMER 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 seller’s 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.

NOTICE AND DISCLAIMER 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 seller’s 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

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Foreword ............................ .......................................... ............................. ............................. ............................ ............................. ............................. ......................... ......................... ............................. ............................. ................ 4 Introduction ............................. ........................................... ............................ ............................. ............................. ............................. ............................. .......................... ........................ .......................... .......................... ............ 5 1.1 Purpose ............................. ............................................ ............................. ............................ ............................. ............................. ............................. .......................... ........................ ........................... ..................... ....... 5 1.2 Scope ............................ .......................................... ............................ ............................. ............................. ............................. ............................. ........................... ........................... ........................... ....................... .......... 5 1.3 Definition of Selective Coordination ............................. ........................................... ............................ ............................ ............................. ............................. ............................ ................... ..... 5 National Electrical Code (NEC) NEC) [1] Selective Coordination Requirements ............................. ........................................... ............................ ........................... ............. 8 2.1 Requirements ........................... .......................................... ............................. ............................ ............................. ............................. ............................. ............................ .......................... ......................... ............ 8 2.2 Challenges Meeting the Requirements........................ Requirements...................................... ............................ ............................ ............................. ............................. ............................ ................. ... 10 2.2.1 Local Jurisdiction Interpretation and Enforcement ........................... ......................................... ........................... ........................... ............................ ................. ... 10 2.2.2 Overriding Requirements ............................ .......................................... ............................ ............................ ............................. ............................. ............................ ........................ .......... 10 Circuit Breaker Trip Response Functions....................... Functions..................................... ............................ ............................ ............................ ............................. ............................. ........................ .......... 11 3.1 Fixed Thermal-Magnetic Type Circuit Breaker............................. Breaker........................................... ............................ ............................. ............................. ............................ ................ 12 3.2 Adjustable Thermal-Magnetic Type Circuit Breaker.............................. Breaker............................................ ............................ ............................ ............................ ..................... ....... 14 3.3 Adjustable Electronic Type Circuit Breaker.................................. Breaker................................................ ............................. ............................. ............................ ............................ ................ 15 3.4 Short Time Withstand Current Rating ............................ .......................................... ............................ ............................. ............................. ............................ ............................ ................ 15 3.5 Instantaneous Override Function....................... Function...................................... ............................. ............................ ............................ ............................ ............................. ........................... ............ 18 Application Information from Manufacturers....................... 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........................... Region.......................................... ............................. ............................ ............................ ............................ .................... ...... 20 4.3 Short Circuit Selective Coordination Tables............................ Tables........................................... ............................. ............................ ............................ ............................ .................... ...... 23 4.4 Coordinating Ground-Fault Protection of Equipment ............................ .......................................... ............................ ............................ ............................ ..................... ....... 24 Design Guidelines ............................ .......................................... ............................. ............................. ............................. ............................. ............................ ........................... .......................... .......................... ............. 29 5.1 Simplify the One-line Diagram ........................... .......................................... ............................. ............................ ............................ ............................ ............................. ........................... ............ 33 5.1.1 Divide Larger Loads into Smaller Loads............................. 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........................... Switches.......................................... ............................. ............................ ............................ ............................ ............................. .................... ..... 39 5.4.3 Busway............................ Busway........................................... ............................. ............................ ............................. ............................. ............................. ........................... .......................... ........................ .......... 40 5.4.4 Arc Flash Energy.................................. Energy................................................. ............................. ............................. ............................. ............................ ............................. ......................... ............... ..... 41 5.4.5 Zone Selective Interlocking ........................... ......................................... ............................ ............................. ............................. ............................ ............................ ...................... ........ 41 5.5 Field Adjustment ............................ .......................................... ............................. ............................. ............................ ............................. ............................. ........................... .......................... ................... ...... 45 5.6 Lifetime Selective Coordination ............................ .......................................... ............................ ............................ ............................. ............................. ............................ ........................ .......... 45 Summary..................................... Summary....................... ............................. ............................. ............................. ............................. ............................. ............................. ........................ ........................ ............................ ................... ..... 46

© Copyright 2010 by the National Electrical Manufacturers Association.

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ABP 1-2010

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 Corporation—Pittsburgh, PA General Electric—Plainville, CT Siemens Industry, Inc.—Norcross, GA Schneider Electric USA—Palatine, IL


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ABP 1-2010

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 foll owing 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:

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Selective Coordin ation. “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.”

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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.”

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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:

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

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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). © Copyright 2010 by the National Electrical Manufacturers Association.

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ABP 1-2010 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 circuits—they are not affected, since only the 20 A OCPD closest to the fault operates to clear the fault. Power Distribution Equipment OCPD Not affected

800A

OCPD Opens 400A

Unnecessary power loss Fault

P-2

200A

P-1 20A

Selectively Coordinated For the full range of overcurrents possible at P-1, only t he 20A OCPD opens.

Figure 1.1 System Is Selectively Coor din ated—Fault at Branc h 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. Power Distribution Equipment OCPD Not aff ected

800A

OCPD Opens 400A

Unnecessary p ower loss Fault

P-2

200A

P-1 20A

Selectively Coordinated For the full range of overcurrents possi ble at P-2, only t he 200A OCPD opens.

Figure 1.2 System Is Selectiv ely Coor dinated—Fault at Feeder Level OCPD


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ABP 1-2010 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 Coor din ated—Fault at Branc h 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 level s that may not necessarily cause all of the OCPDs to open.)


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ABP 1-2010

Figure 1.4 System Is Not Selectively Coor din ated—Fault at Branc h 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.

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Nation al Electr ical Code (NEC) [1] Selective Coordi nation 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:

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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) Arti cle 517—Health Care Facilities 517.26 Application of Other Articles—“ The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.”


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ABP 1-2010 2) Arti cle 620—Elevators , Dumbwaiters , Escalators , Moving Walks, Wheelch air 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) Arti cle 700—Emergenc y 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) Arti cle 701—Legally 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) Arti cle 708—Criti cal 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 p ercent 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 © Copyright 2010 by the National Electrical Manufacturers Association.

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ABP 1-2010 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 overcurrents—overloads, short circuits, and ground faults. 2.2

Challenges Meetin g the Requirements

2.2.1

Local Jurisdic tion 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 AHJ’s 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 utili ty 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 o f 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

Overridi ng 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 © Copyright 2010 by the National Electrical Manufacturers Association.

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ABP 1-2010 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.

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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 sel ective 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 Fi gure 3.0. Note—For 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 components—the “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 an d trip unlatching mechanisms to operate. The “arcing time” is the time taken for the arc to be extingui shed 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.


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ABP 1-2010

In the Overload region—the 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 typicall y fairly long—seconds to hours.

In the Instantaneous or Short Circuit region—the overcurrent device responds instantaneously. Total Clearing times are typically very fast—less 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 ope rates directly from the load current or indirectly from current flowing through a current transformer. 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 thermalmagnetic 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 Circui t 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 © Copyright 2010 by the National Electrical Manufacturers Association.

12

ABP 1-2010 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. Manufacturer’s TCCs may demonstrate less tolerance for a particular device based on the device’s 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. © Copyright 2010 by the National Electrical Manufacturers Association.

13

ABP 1-2010 3.2

Adju stabl e Thermal-Magnetic Type Circui t 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 ele ctrical system.

Figure 3.2 Typical Time-Current Curve for an Adjustable Magnetic Pickup Ac tion 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. © Copyright 2010 by the National Electrical Manufacturers Association.

14

ABP 1-2010 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 sel ectively coordinate. 3.3

Adju stable Electron ic Type Circui t 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 requirements—some 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.

Long-time Pickup Long-time Delay T i m e

Short-time Pickup Short-time Delay (I2T IN)

I2T OUT

Adjustable Instantaneous Fixed Instantaneous

Multiples of Rated Current Figure 3.3 Typical Adjustable Settings for Circuit Breakers

3.4

Short Time Withst and Current Rating

Low-Voltage Circuit Breakers fall into two basic classifications of design—Low-Voltage Power Circuit Breakers (LVPCBs) and Molded Case Circuit Breakers (MCCBs). One of the most important application features that


15

ABP 1-2010 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 Rated ShortTime Withstand Current Duty Cycle Rated ShortCircuit Current (at Rated Maximum Voltage) Rated ShortCircuit Current Duty Cycle Short Circuit Test Power Factor

Frame Sizes: 600 A to 5000 A, ratings by combination of sensors and trip units Carry fault current For two 0.5 sec. periods

Frame Sizes: 15 to 6000 A

Not specified

200 kA max.

7.5 kA to 200 kA

O – (15 sec.) – CO

O – (2 to 60 min.) – CO

15% (X/R ratio 6.6) unfused LVPCBs 20% (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


16

ABP 1-2010 Figure 3.4 shows some typical Ti me-Current Curve characteristics for these various circuit breaker types. 1000

100

Molded Case CB with adjustable instantaneous trip

Adjustable Low Voltage Power CB without instantaneous trip

10 s d n o c e s

1

Adjustable Low Voltage Power CB or large MCCB with instantaneous trip

0.10

0.01

Fast instantaneous clearing time typical of MCCB

Curve without instantaneous typical of LVPCB without instantaneous or instantaneous override

Three cycle instantaneous clearing time typical of large MCCB or LVPCB 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,


17

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 what’s 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 l ower 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.


18

ABP 1-2010

Circuit Breaker Instantaneous Region 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 t he electronic logic trip unit to cause the circuit breaker to open without delay at very large fault levels. (IEEE 1015-2006 “Blue Book”) [8]

Figure 3.5 Typical Adjustable Settings for Circuit Breakers

4

Appli cation 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 uni ts 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 band—not 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.


19

ABP 1-2010

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

Lim itatio n of Time-Curr ent 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 devi ces. 4.2.2

Instantaneou s or Short Circui t 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 effect of the load side circuit breaker. Hence, in the instantaneous region, circuit breakers may be more or less © Copyright 2010 by the National Electrical Manufacturers Association.

20

ABP 1-2010 selective than traditional TCCs indicate. This is based on how the line side circuit breaker’s 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. The effect of current limitation on the line side circuit breaker’s trip performance may be illustrated by Figure 4.21.

90,000 80,000 70,000 60,000 50,000 s 40,000 e r 30,000 e p 20,000 m 10,000 A 0 -10,000 -20,000 -30,000 -40,000 -50,000 -60,000 -70,000 -80,000

Trip

Fault between line & load side devices Line side trip set at 36kA peak (25kA RMS)

Electronic Trip CB Peak let-through ~32kA

Fault above CL CB Current Limiting CB

-

Seconds

0.0083

0.0167

Fault below CL CB

Figur e 4.2-1 Effect of Current Limiting on Circuit Breaker Performance

In this Figure 4.2-1, the larger sine wave represents a prospective fault current or the fault magnitude possible at the load side circuit breaker’s line side terminals. The smaller half cycle sine wave represents the current limiting effect of the load side circuit breaker’s 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. Peak let-through currents may be provided by manufacturers in the form of peak let-through plots for various 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.


21

ABP 1-2010 100

√2 × RMS ) A k ( h g u o r h t t e L k a e P I

33kA Peak

50kA RMS = 33kA Peak let-through 33kA = 23kA Peak at PF = 1

10 10

Prospectiv e RMS Fault I (kA)

100

Figure 4.2-2 Peak Let-Through Currents of Circuit Br eaker 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 device’s 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 fuse’s time-current curve as drawn on a typical TCC showing a 0.01 second minimum response time.

1000 800A CB 100

10 200A J TD

1

0.10

S D N O C E S

0.01100

1K 10K Amperes RMS

100K

Figure 4.2-3 Circui t Breaker Settings (Set so Circ uit B reaker TCC does not Overlap Fus e TCC) However, when the fuse’s 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 availabl e bolted fault current.


22

ABP 1-2010

1000 800A CB 100

10200A J TD

1 S D

N O C E 0.10 S

0.01100

1K 10K Amperes RMS

100K

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 Circui t Selective Coord inatio n 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.


23

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 what’s 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 manufacturer’s tables to determine if improvements in the levels of selective coordination may be gained over the level of selectivity indicated by using traditional timecurrent 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© Copyright 2010 by the National Electrical Manufacturers Association.

24

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 1 recognizes ground faults as the most common type of electrical fault ; 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, i ncorrect 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."


25

ABP 1-2010 1000.00

1200A NEC maximum nominal pickup 100.00

10.00 s d n o c e S

1.00

UL 1053, maximum 2 second clear at 150% nominal setting NEC 3000A, maximum 1 second clear

0.10

0.01 100

1,000 10,000 Amp er es

100,000

Figure 4.1-1 The Mandated Lim its f or Lo w-Voltage OCPD Because of standard requirements, the shape of the ground-fault function’s 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 device’s 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.


26

ABP 1-2010 1000

250A Electronic CB 100A TM CB

100

10

s d n o c e S

1

0.10

0.01 10

100

1K

10K

100K

Amperes Figure 4.1-2 Circuit Br eaker Selectively Coordi nated with 1200A Ground Fault 2

The sloped portion of the ground-fault curve is called an I t 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.


27

ABP 1-2010 1000

100A TM CB 200A TD J fuse

100

200A J fuse 1200A GF

10

s d1 n o c e S

0.10

0.01 10

100

1K

10K

100K

Amperes 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 l arge 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 2 protective device. It is commonly believed that most faults are ground faults , and that most faults occur at end2

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.


28

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 hig hest pickup settings.

1000

20A 1P Branch CB

100

240A GF 1200A GF 10

s d n o c e S

1

0.10

0.01 10

100

1K

10K

100K

Amperes 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 a t 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

Desig n Guidelin es

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 © Copyright 2010 by the National Electrical Manufacturers Association.

29

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 rework 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 power—conduct a Short Circuit Study of the system 4) Select OCPDs that provide selective coordination—use Time-Current Curves and Selective Coordination Tables from manufacturers 5) Optimize the design—consider 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 Authorit y Having Jurisdictio n’s 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 building’s 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. It’s 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.


30

ABP 1-2010 3) Determine the available fault cur rents 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 coordi nation 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/or via Short Circuit Selective Coordination Tables, provided by the manufacturers of the devices. b) Use Time-Current Curves and Shor t Circui t Selective Coordi natio n Tables Time-Current Curves may be solely used when the fault currents are relatively low and the trip response time of the devices are relatively long (hundreds of milliseconds or longer). In this case, on a single plot, overlay the time-current curves for both the downstream and the upstream devices. On the fault current axis, locate the value of the available fault current at the downstream device. At this fault current value, determine if the upstream device can be set to remain closed, either via adjustable pickup or time-delay © Copyright 2010 by the National Electrical Manufacturers Association.

31

ABP 1-2010 settings, while allowing the downstream device to open. If these two devices are selectively coordinated, there will typically not be an y overlap in their time-current curve plots. The available fault current at the downstream device may be sufficiently high that when plotted with the time-current curves of the upstream device, it may not be clear from looking at the time-current curve plots if these two devices will selectively coordinate. There may be partial overlap of the curves in some regions. When there is overlap in the time-current curve plots of two devices, Selective Coordination Tables should be used to determine if coordination is still possible at the available short circuit fault current levels. Most manufacturers of Overcurrent Protective Devices (OCPD) provide these tables along with detailed instructions for using them. In general with these tables, the first step is to find the downstream device, and then locate the first upstream device that will coordinate at a fault current value that is equal to or greater than the available fault current at the downstream device. Most tables will typically show a maximum value up to which the combination of these two devices will coordinate. c) Use available “Software Tools” to speed up the analyses Depending on the types of OCPDs selected, and the level of complexity of the system being designed, it should be evident that this iterative design effort may become time consuming, error prone, and produce somewhat subjective results. In recent years as the National Electrical Code (NEC) [1] requirements for selective coordination have become more widespread, design engineers have recognized the need to take advantage of various new “tools” to simplify and improve the efficiency of the design efforts. As a result, third-party software companies now make available the time-current curves and the selective coordination tables in various electronic media, such that the time current data plots and lookup table results can be manipulated via software. To guide the calculation of the available fault currents in a Short Circuit Study, third-party “calculator” type software is now available. These “calculators” allow the designer to enter just some key, basic information that describes the configuration of the system, and then the software, with built-in formulas, does the required numerical operations and provides the desired calculated results. Electronic tools such as these serve to simplify and speed up the potentially time-consuming portions of the design process, freeing up the design engineer to focus their skills on much more important non-arithmetic considerations that typically need much more attention and deliberation. It can be anticipated that third-party software companies will continue to refine and enhance these “tools” as the NEC [1] requirements become even more widespread. d) Mixing of Overcurrent Protective Devices The design engineer may select overcurrent protective devices that may seem well suited for satisfying the requirements of the short circuit study, but may not be the best choice for selective coordination. As a result, the system may have a mixture of OCPDs of different types—circuit breakers or fuses. In addition, the design may have devices of the same type, but from different manufacturers. This scenario is particularly likely when an existing facility is being remodeled or expanded. Systems with a mix of fuses and circuit breakers present additional selectivity challenges. The mixing of multiple brands of circuit breakers may cause limitations in calculating the selectivity levels needed for the system. Fuse manufacturers provide time-current curves for their fuses. When used according to manufacturer recommendations, selectivity assessment for faults whose magnitudes are below where the upstream fuse crosses the 0.01 second axis on the Time-Current Curves (TCC) may be assessed via time-current curves alone. However, when fault currents are below the 0.01 second crossing for both fuses, then the fuses may be operating in their current and energy limiting range and hence the selectivity tables or recommended ratios provided by the manufacturer should be used. However, at the time of this writing, fuse selectivity tables and ratios apply only to one manufacturer at a time. Guidelines on how to assess selectivity across fuses made by different manufacturers do not exist at this time. Hence, any system implemented in this manner should use replacement fuses of the same type, brand, and size as the fuse being replaced. Similarly, when fuses are used downstream of circuit breakers, selectivity may be ascertained via the timecurrent curves alone as long as the available fault current does not exceed the instantaneous pickup of the © Copyright 2010 by the National Electrical Manufacturers Association.

32

ABP 1-2010 upstream circuit breaker shown on the time-current curves. When fault currents exceed the upstream circuit breaker’s pickup, selectivity may still be possible. However, more thorough analysis may be required. In some cases, analysis may be performed via the peak let-through current method described earlier in this document. At the time of this writing, manufacturers do not provide selectivity tables for circuit breakers used upstream of fuses, so there currently is not an easily verifiable way to analyze selectivity of instantaneous trip circuit breakers used on the line side of fuses. When that situation is encountered, the circuit breaker manufacturer should be contacted for additional i nformation. When fuses are on the line side of circuit breakers, similar complexities arise. The let-through energy of a circuit breaker may or may not be enough to melt an upstream fuse. However, neither manufacturers of circuit breakers nor fuses commonly provide sufficient information to allow the required analysis to be performed by system design professionals. When selectivity of fuses on the line side above circuit breaker combinations must be analyzed beyond where the fuse crosses the 0.01second axis of the time-current curve, the circuit breaker manufacturer should be consulted. Another situation that limits the availability of selectivity tools is the mix of multiple brands of circuit breakers. Selectivity in the short and long time range may be verified by the time-current curves. However, in the instantaneous range, selectivity tables are recommended, and at this time, like fuses, no cross brand selectivity tables are provided by any of the manufacturers. 5) Optimize the desig n Depending on how complex the electrical system is, the design engineer will likely find that there are points in the analysis that the selection of devices may present possible contradictions for decisions in system protection, selective coordination, or even personnel safety. The designer may be faced with iterative design changes by making tradeoffs in the types of devices to satisfy National Electrical Code (NEC) [1] requirements and customer specifications, in addition and maintenance considerations. Therefore, the next key step in this process is to gather all the resulting data from the various anal ysis of the short circuit studies, the selective coordination analysis, and any other relevant systems data, and then attempt to optimize the design of the overall system. While there are obviously no foolproof steps to ensure that every possible system configuration can be optimized, there are some basic guidelines that, when followed, will typically result in a system design that takes into consideration many of the key design, implementation, and maintenance concerns. The optimization process will involve an iterative look at some possible ways to streamline the final results— with approaches such as simplifying the original design, reducing the fault currents, reviewing the selection of Overcurrent Protective Devices (OCPD), and a focus on any special equipment application requirements. 5.1

Simpli fy the One-line Diagram

5.1.1

Divide Larger Loads into Smaller Loads

Where possible, split up larger loads into smaller loads such that the resulting fault currents will be lower. The lower fault currents may result in smaller protective devices and conducting cables, etc., thereby making selective coordination simpler. The tradeoff may be things such as more space, possibly higher (or lower) total costs for the smaller load devices, and more involved wiring for control schemes. 5.1.2

Reduce the Number of Levels of Protecti ve Devices

The fewer the number of levels of OCPDs, the simpler selective coordination becomes. Figures 5.1-1 and 5.1-2 demonstrate this approach.


33

ABP 1-2010 Panel-1

Panel-1

Panel-2

Pan el-2

Panel-3 Panel -3

Panel-3

Figu re 5.1-1 3 Levels of Selective Coordination Required

Figu re 5.1-2 Simplified Scheme with 2 Levels of Selective Coordination

In Figure 5.1-1, Panel-2 and Panel-3 are supplied, in series, with power from Panel-1, resulting in 3 levels of devices. The same number of panels may be re-configured as shown in Figure 5.1-2 into a 2-level scheme, thereby simplifying the selective coordination levels. 5.2

Reduce the Availabl e Fault Current

5.2.1

Increase the Impedance of the System

Conductors used in distributing power from sources to loads add resistive and inductive impedance to the system. Increased impedance will lower available fault current at the end of the conductors. In some cases, using longer conductors or dividing circuits into multiple smaller circuits will reduce fault current at different points in the circuit. Lower fault current may allow for easier selectivity. Longer conductors may have a slight effect on voltage drop and may increase the cost and complexity of the installation. However, the increased cost may be offset by the ability to use lower cost protective devices or a simpler distribution topology. The conductors that connect the various electrical equipment and devices in the system inherently add impedance. The more impedance, the lower the available fault currents will be, the simpler selective coordination will be. Equipment may be relocated, for example, such that longer cable runs are required. The tradeoff for longer cables will be their added costs, but these costs may be offset by the smaller and less costly Overcurrent Protective Devices (OCPD) that may now be used to handle the reduced fault currents. Note that whenever sizing cable conductors, in addition to selective coordination requirements, voltage drop and National Electrical Code (NEC) [1] and/or local code derating factors must also be considered. 5.2.2

Utilize Step-Down or Isolation Transfor mers

Transformers are used within commercial and industrial low-voltage power systems to reduce utilization voltage from the 480 or 600 volts, typically used for large machinery, to 120/208 or 240/120 volts used for smaller machinery, lighting, and general-purpose branch circuits. Transformers have a significant effect on selectivity in multiple ways: 1) They create a separately derived system where the transformer’s impedance becomes the main determinant of maximum fault current. 2) They have a winding-turns-ratio and a voltage ratio that determines the secondary voltage from primary voltage, and the primary current for a gi ven value of secondary current. 3) They have winding configurations (typically Delta-Wye) that cause secondary single-phase faults to be smaller multiple phase faults on the transformer’s primary conductors. © Copyright 2010 by the National Electrical Manufacturers Association.

34

ABP 1-2010

In addition, the primary main feeding the transformer and secondary main fed from the transformer do not need to be selective with each other. The NEC [1] specifically provides an exception for this device combination and consideration of the effect on system reliability shows that the effect on delivery of power to loads is the same regardless of which OCPD opens, or if both open. Both, however, need to be selective with the OCPD above and below them. See Figure 5.2.2-1 for illustration of selectivity needs around a transformer circuit. The transformer ratio may be calculated by dividing the rated primary voltage by the secondary primary voltage. For a 480 V to 208 V transformer, the turns-ratio is 2.31. This means that any balanced three-phase current on the secondary side conductors is reflected on the primary side conductors 2.31 times smaller. For example, as 10,000 A three-phase balanced fault on the secondary side would be fed by a 10,000/2.3 TO 1 (4,329 A) fault current on the primary side. Transformers also have a “winding-turns-ratio” defined as the ratio of wire turns in the primary winding to the number of turns in the secondary winding (N1/N2). This ratio determines the voltage and current ratio for a single-phase transformer and the winding current and winding voltage ratio in a three-phase transformer. The typical distribution transformer has a Delta primary winding. In a Delta winding, the current flowing through each winding is not the same as the current flowing into the winding from the phase conductors. The current from the phase conductors will divide into the winding by relationships determined by the three windings working together.

Should be selective with each other Should be selective with each other Need not be selective with each other

Should be selective with each other Should be selective with each other

Figu re 5.2.2-1 Selectivity Requirements f or Transformer Primary and Secondary Devices The division of currents is seen in Figure 5.2.2-2. The √3/3 factor affects how fault currents on the secondary circuit of the transformer are seen by Overcurrent Protective Devices (OCPDs) on the primary side of the transformer.


35

ABP 1-2010 A balanced three-phase fault is only affected by the transformer ratio as seen in Figure 5.2.2-3. However, single phase-to-phase and phase-to-ground faults are also affected by the Delta-Wye transformer winding configuration. Figure 5.2.2-4 shows the effect of the winding configuration on a single-phase fault and Figure 5.2.2-5 shows the effect on a ground fault on the secondary transformer circuit.

I phase 1.0A

√ 3/3 =0.58A

I winding

√ 3/3 =0.58A

I winding

Delta Winding

Delta Current Phasors

Figu re 5.2.2-2 Distribution of Incoming Phase Currents into a Delta Primary Winding

1.0 A

1.0/2.31 A N1

1.0/4 A 1.0/2.31 A

1.0/4 A

N2

3-phase Fault

1.0/4 A 1.0/2.31 A 1.0 A For 480-208 V transformer V ratio = 480/208V = 2.31 Turns Ratio = N1/N2 = 4

1.0 A

Figu re 5.2.2-3 Delta Wye Transformer Windings with Balanced 3-Phase Fault

3

0.5/2.31 A

2

= 0.87 A

0.5/2.31 A

0.5/2.31 A

0.5/2.31 A

1-phase Fault

1.0/2.31 A

3

3 • 3 = 3 2 0.87• 0.58 = 0.5

2

A

Figure 5.2.2-4 Delta Wye Transformer Windings with Single Phase-to-Phase Fault


36

ABP 1-2010

1.0 A

1.0 A/4

0A

1.0 A/4

1-phase to ground Fault

1.0 A/4 Primary current on two phases equal to secondary current divided by turns ratio Figu re 5.2.2-5 Single Phase to Ground

In addition to the effects of the winding configuration, transformers affect available fault current on their secondary side with their inherent impedance. Each transformer has characteristic impedance that may be provided by the manufacturer’s product information or may be read on the transformer’s nameplate. This impedance acts as a choke, limiting the maximum possible secondary fault current. To a lesser degree, the current available at the transformer’s primary terminals affects the fault current available at the secondary side of the transformer. Short circuit calculations can determine the precise fault current available based on both of these factors as well as the effect of conductor impedance in the circuit. However, dividing the secondary fullload-current by the transformer impedance and multiplying the result by 100 can conservatively estimate secondary fault current. The following formulas can be used to estimate fault current at the transformer’s secondary side if conductor impedance and source fault current is ignored. Maximum secondary fault current = 100 × FLA / Z% where Z% = percentage transformer impedance


37

ABP 1-2010 As an example, let’s look at the estimated requirements for a 75 kVA transformer with 4% Z. FLA = 75000 VA / 208 V / √3 = 208 A Maximum secondary fault current = 100 × FLA / Z% = 20,800 A / 4 = 5,200 A Ignoring conductor impedances and fault current available from the source system, Figure 5.2.2-6 shows the selectivity requirements at the various circuit breakers around a 75kVA transformer.

2,253A 3 2,253A 1 1,300A 1

φ φ - φ φ - G 480V Delta 75kVA 4%z 208V Wye

2,253A 3 2,253A 1 1,300A 1

φ φ - φ φ - G

5,200A 3 4,528A 1 5,200A 1

φ φ - φ φ - G

Figu re 5.2.2-6 Selectivity Requirements fo r Transformer Primary and Secondary Devices Based on Unknown High Avail abl e Primar y Fault Cur ren t, Kn ow n kVA, and Kno wn Transf or mer Impedan ce. The Effec t o f Conductor Impedance is Ignored Understanding how transformer ratios, windings, and impedance affect selectivity needs is required to optimally assess selectivity needs in a transformer circuit. Careful selection of transformer size and impedance can help to provide selectivity solutions or avert selectivity problems. In some cases using 1:1 transformers to “choke” down fault current can facilitate selectivity where high source fault current would have otherwise made selectivity difficult. For example, in the system illustrated in Figure 5.2.2-6, if the 75 kVA transformer was a 1:1, 480 V isolation transformer used in a system with 65 kA available, selectivity requirements could be simplified. Branch circuit breakers in the panel fed by the transformer would have fault currents in the 2,253-1,300 A range rather than a 65,000 A range to deal with. 5.2.3

Take Advant age of the Add ed Arc Impedance of Load Side and Line Circui t Breaker Combinations

The current limiting and peak let-through current effects between line side and load side Overcurrent Protective Devices (OCPDs) is described in some detail in section 4.2.2, Instantaneous or Short Circuit Region. In summary, most circuit breaker manufacturers provide selective coordination tables or other tools that show the levels of coordination between specific combinations of line side and load side circuit breakers. Design engineers should review this information to see if selectivity at higher current levels may be achieved.


38

ABP 1-2010 5.3

Review Device Selection

5.3.1

Increase the Withst and Capabilities of the Upstream Lin e Side Overcurrent Protective Devices

The withstand capabilities of the upstream line side circuit breaker should be selected to be as high as practical, such that it can remain closed while a downstream circuit breaker opens. This can be accomplished by selecting a larger frame Molded Case Circuit Breaker (MCCB) that has a higher Instantaneous Override trip level. By increasing the frame size of the upstream circuit breaker, from say a 1,200 A to a 2,000 A MCCB, most selective coordination tables will show an increase in the level of fault current that a downstream circuit breaker will coordinate. 5.3.2

Change the Type of Circui t Breaker

Selective Coordination Tables will show that by changing from a Molded Case Circuit Breaker to either an Insulated Case Circuit Breaker (ICCB) or a Low-Voltage Power Circuit Breaker (LVPCB) that the level of selective coordination with a downstream device will typically increase. This increase in the level of selective coordination is achieved by the inherent higher withstand capability ratings required in the design requirements of ICCBs and LVPCBs. The tradeoff may be higher costs and larger space for the ICCB or LVPCB, but these costs may be offset, for example, by the option to use the ICCB or LVPCB to remain closed while supplying power to reduced number of levels of OCPDs, as shown in Figure 5.1-2. 5.3.3

Select Current Limi ting Type Molded Case Circui t Breaker

Where possible, Current Limiting type Molded Case Circuit Breakers may be used for branch devices. Current Limiting devices respond very rapidly and significantly limit the let-through current, and in doing so coordinate better with upstream devices. 5.4

Special Equipment Application Requirements

5.4.1

Generator Protectio n

Generator manufacturers often supply their generators with circuit breakers on the output. These are typically thermal-magnetic or simple electronic types with only adjustable long time and instantaneous (LI) trip functions. Consideration must be given to whether the generator circuit breaker will coordinate with downstream circuit breakers. If the prospective fault current the generator can produce is below the maximum instantaneous trip point of the circuit breaker, then coordination can be achieved, regardless of the type or brand of downstream Overcurrent Protective Devices (OCPDs). If the prospective fault current that the generator can produce is above the maximum instantaneous trip point of the circuit breaker, then the let-through current of the downstream circuit breaker must be considered using the Selective Coordination Tables provided by the manufacturer. This means that the downstream OCPDs will need to be circuit breakers from the same manufacturer as the generator. Consideration must also be given to the protection of the generator, and to the careful selection of the protection settings of any generator protective devices supplied by the generator manufacturer. For example, in applications where the ratings required of the circuit breaker to protect the generator are such that it will not allow total selective coordination to be achieved with downstream OCPDs, it may not be necessary to change to a different generator breaker. In this case, an allowance from the Authorities Having Jurisdiction (AHJ) will be necessary to favor protection of the generator over total selective coordination of the system. There are applications where multiple generators are in place, and connecting and running multiple generators in parallel, while ensuring that short circuit protection and selective coordination, may become a significant challenge. In such an instance involving a complex system, an external scheme involving differential relaying may be necessary to provide adequate generator protection and selective coordination. 5.4.2

Auto matic Transfer Switches

Electrical equipment and components not designed to interrupt high values of fault current are usually rated in terms of “withstand” current. Withstand is typically defined as a high value of current that can be carried by the equipment or components for a specific amount of time, or long enough for a specific Overcurrent Protective Device (OCPD) upstream to clear the fault. Short circuit withstand ratings are commonly applied to distribution © Copyright 2010 by the National Electrical Manufacturers Association.

39

ABP 1-2010 equipment (such as switchboards and panels), busway, transfer switches, molded case switches, non-fused safety switches, and others. The application of transfer switches is a major consideration in the design of emergency, legally required standby, and optional standby systems. Transfer Switch short circuit ratings must be appropriate for the available fault current, the upstream overcurrent protective device, and the overcurrent device settings. Emergency and legally required standby systems require selective coordination. If coordination is accomplished using short time delays with circuit breakers, the transfer switches require a suitable short time rating as well. Not all transfer switches are rated for use with short time delays. Transfer switches are required to have withstand ratings of either a specific device rating or an “any-circuit breaker” rating. A transfer switch may have both types of ratings. In addition, a transfer switch may have a short time rating. These ratings must be evaluated based on the pairing of the transfer switch with the immediate upstream OCPDs in order to achieve proper coordination. The withstand ratings are marked for maximum available fault currents at maximum system voltages. The markings will specify the type of protection associated with each withstand rating. If the rating is accomplished using a specific circuit breaker, the manufacturer, type, and maximum ampere rating is included. For withstand ratings requiring the use of specific fuses, the rating includes the fuse class and its maximum ampere rating. The transfer switch may be marked as being suitable for use with any UL 489 [5] listed circuit breaker with an instantaneous trip response. The marking may also include short time current rating(s) that include maximum time duration(s). If the transfer switch does not have integral overcurrent protection, then it must be rated for the available fault current and the overcurrent protection being provided for both the normal and alternate source feeders. The following checks are involved i n assessing the switch ratings and switch protection: 1) Determine the available fault current and system voltage at the normal and alternate line terminals of the switch. 2) Select a switch that is rated for a fault current greater than, or equal to, that available fault current. 3) Check that the switch is rated for use with the type and maximum current rating for a circuit breaker of a particular manufacturer as selected or the class and maximum current rating of a selected fuse. 4) If a transfer switch has integral overcurrent protection, its time-current characteristic needs to be reviewed since it becomes an integral part of the selective coordination design. 5) If a circuit breaker is to utilize a short time delay, the transfer switch must include a short time rating that is suitable for use with the required short time current setting. The circuit breaker short time delay must be set to less than the transfer switch rated delay, and the circuit breaker instantaneous trip current setting must be less than the transfer switch short time current rating. If the transfer switch short time current rating is higher than the available fault current, the instantaneous setting is not required to protect the switch. Selective coordination can be achieved in a number of ways. Some approaches will require the transfer switch to be rated for use with short time delay functions. Other approaches such as locating the transfer switches close to the served loads may result in lower available fault currents and simpler coordination between the OCPDs. Applying more switches with lower continuous current ratings rather than a few large switches can simplify the coordination, and improve overall reliability. 5.4.3

Busway

Manufacturers may state busway withstand in terms of current and time or in terms of a specific type of protective device such as size and class of fuse. When protecting busway with circuit breakers with long delays or oversized frames due to selectivity considerations, it is important to verify that the bus is adequately protected and will operate within its claimed withstand ratings. In some cases, it may be required to select a larger bus size, different construction material, or different type of busway to ensure that its ratings are suitable for the protective device ahead of the busway. Alternatively, OCPD selection may need to be changed to provide suitable protection to the busway. © Copyright 2010 by the National Electrical Manufacturers Association.

40

ABP 1-2010 5.4.4

Arc Flash Energy

Selective Coordination requires upstream Overcurrent Protective Devices (OCPD) to wait longer to enable downstream protective devices to clear the fault. As protective devices permit fault currents to flow with intentional delay before the fault current is cleared, the amount of potential arc flash energy increases the longer the fault current is allowed to arc. As a result, a conflicting situation is put in place—Selective Coordination involves having OCPDs to remain closed during fault conditions, while Arc Flash Energy reduction requires these same devices to open as quickly as possible. The OCPDs are required to operate quickly enough for all levels of available fault current to adequately protect conductors and electrical equipment. However, while the level of arc energy that results in a selectively coordinated system may be fully within acceptable levels for equipment protection from damage, this level of arc energy is often very dangerous to personnel that may be working near that electrical equipment. NFPA 70E Standard for Electrical Safety in the Workplace [11] recognizes that there are circumstances that allow working on live, energized electrical equipment. As a result, recommendations and requirements are defined for the levels of protective systems and apparatus that must be in place when personnel are working near live energized electrical equipment. Strict guidelines are outlined in NFPA 70E for the various levels of Personnel Protective Equipment (PPE) that must be worn by personnel in certain hazardous situations. IEEE 1584 Guide for Performing Arc-Flash Hazard Calculations [12] provides guidelines for conducting a formal arc flash study of the devices in an electrical system. A comparative study of an electrical system that requires selective coordination versus one that does not require it will typically show that the selectively coordinated systems require higher levels of PPE for personnel working on or near that energized equipment. This comes from the longer times that upstream devices are required to remain closed, while downstream devices open, in order to meet selective coordination requirements. As a result, the design engineer should attempt to find a balance with satisfying selective coordination requirements without implementing devices that will cause excessive arc flash energy hazards to personnel working near energized electrical equipment. There are some types of overcurrent protective devices that can respond extremely quickly to a fault current. Their fast response of typically less than 1 half-cycle occurs at very high short current levels and limits the current and the corresponding potential arc flash energies to relatively low levels. On the other hand, for fault currents that are below the fast response, current limiting levels of these devices, their operating time is appreciably slower, and may actually result in longer arcing time and arcing energies. So while fast-acting, current limiting devices may seem an obvious design preference, the design engineer must examine the actual available fault currents, and select the appropriate type of OCPDs that optimizes the tradeoff between reduced arcing energies versus the fast response needed for downstream devices in a selectively coordinated system. Therefore, in addition to the short circuit fault current study for conductor and equipment protection and the selective coordination study to minimize unnecessary power outages, the design engineer should also conduct an Arc Flash study to identify the recommended PPE and other personnel safety considerations, and then select devices that most optimally meet these various requirements. 5.4.5

Zone Selective Interloc kin g

Selectivity between circuit breakers in the short time range and for ground faults is achieved with a combination of nested pickup settings and time delays bands. Figure 5.4.5-1 demonstrates three circuit breakers achieving selectivity using nested short time bands. Nested time bands achieve selectivity at a cost to arc flash hazard and equipment protection. The closer that circuit breakers are to the power source, their selected response time will be slower, and their pick-up levels will be less sensitive. These slower response times and/or less sensitive pick-up settings will have a negative effect on arc flash and equipment protection. Two circuit breaker control schemes are possible to improve protection— Zone Selective Interlocking (ZSI) and Bus Differential Protection, typically called by its ANSI designation of 87B protection. ZSI is the more commonly applied scheme for improving protection in low-voltage systems as most advanced electronic trips for Low-Voltage Power Circuit Breakers, Insulated Case Circuit Breakers, and many molded case Circuit breakers will provide a ZSI option. © Copyright 2010 by the National Electrical Manufacturers Association.

41

ABP 1-2010 1000

Fdr 100

Tie Main

10 s d n o c e S1

0.10

0.01100

1K

10K

100K

Amperes Figu re 5.4.5-1 Three Circuit Breakers Ach ieving Selectivity Using Nested Short Time Bands The basic premise of ZSI is that a pair of circuit breakers establishes sufficient communication between the downstream device and the upstream device such that the upstream device is aware when the downstream device has sensed a fault that exceeds its short time threshold and is timing towards a trip. This information allows the upstream device to change its time delay setting to a slower time delay to allow the faster downstream circuit breaker to fulfill its protection role. The upstream device operating at its slower setting provides suitable back-up protection in case the faster circuit breaker does not operate properly or does not clear the fault. ZSI allows each circuit breaker receiving the ZSI signal to operate faster for faults within its respective zone of protection than it does when it is acting in a back-up role to downstream devices. Figure 5.4.5-2 is a simplified ZSI communication scheme for three circuit breakers.


42

ABP 1-2010

Main Trip

Tie Trip

TCC shift caused by ZSI signal from CB below

ZSI signal to upstream CB

Fdr Trip

Figu re 5.4.5-2 A Sim pl if ied ZSI Comm un ic ati on Sch eme f or Three Circu it Br eaker s Different manufacturers may provide different ways to achieve this function. In most cases, however, the net result is very similar regardless of manufacturer. Figure 5.4.5-3 shows the same three circuit breakers of Figure 5.4.5-1 at their back-up protection settings and at the faster “in zone” protection settings.


43

ABP 1-2010

1000

T I M E I N

100

10 s d n Main o c Tie e S1 Fdr

S E C O N D S

Main In zone In Zone Tie In Zone Fdr

0.10

0.01100

1K

10K

Amperes

100K

1M

Figu re 5.4.5-3 The Three Circuit B reakers of Figure 5.4.5-1 at their Back-up Prot ection Settin gs and at the Faster “ in zone” Protection Settings This Figure 5.4.5-3 illustrates how the tie and main are able to provide faster protection for faults in their zone of protection without any sacrifice in selectivity. Selectivity is not enhanced by short time and ground fault ZSI, protection is. Protection is improved by allowing the same selectivity achievable via nested time delays to be maintained while protection in the upper layers of the system is accelerated. There are some limitations to ZSI applications in complex systems with multiple sources and tie circuit breakers. Manufacturers have different ZSI interconnection schemes and methods that may approach the complexities of multiple source systems differently. ZSI allows improvement in the speed of protection. However, it does not improve the sensitivity of protective settings in upstream devices. In applications where it is deemed important to provide sensitive fault detection in an upstream bus, differential protection may provide an al ternative way to control one or more circuit breakers. Bus Differential protection consists of a system that measures all the current into a zone and out of a zone. The sum of entering currents minus the sum of exiting currents should always equal zero. A non-zero quantity is indicative of current flowing outside of the expected circuit. Bus differential protection measures sources for a bus and compares against loads, if the total of one does not equal the total of the other there is a fault within the zone, i.e., there is a bus fault. Bus differential protection is not often applied in low-voltage systems because of the complexity of the scheme and the cost of implementation. The most common implementation requires a dedicated protective relay and dedicated current transformers used only for the bus differential relay. Figure 5.4.5-4 shows the effect of bus differential on a bus protected by zone-interlocked circuit breakers.


44

ABP 1-2010

1000

100

10 s d n o c e S1

Main In zone In Zone Tie Fdr 87B (Differential)

0.10

0.01100

1K

10K

100K

1M

Amperes Figu re 5.4.5-4 The Effect of Bus Differential on a Bus Protected by Zone-Interlocked Circuit Br eakers Bus differential protection should be able to provide sensitive and fast protection for a specific bus with no negative effect on the selectivity of other circuit breakers. In some cases, 87B protection may be the only way to provide selectivity in complex multiple sources systems without significant sacrifice in protection speed. A detailed discussion of differential protection is beyond the scope of this publication. 5.5

Field Adju stment

All the efforts that may go into designing a Selectively Coordinated electrical system will quickly be wasted if the Overcurrent Protective Devices (OCPD) are not properly set per the recommended settings from the coordination study. For example, in the interest of erring on the side of highest fault current protection, most manufacturers will, as standard practice, set their protective devices at the lowest, minimum pickup and trip time settings when they are shipped from their manufacturing factories. These minimum pickup and trip time settings are usually not in line with those recommended by the design engineer’s study. There is usually an appreciable time lag between when the design engineer developed the settings for the protective devices and when those devices arrive at the facility for installation and startup commissioning. It is therefore critical that the settings that were developed by the design engineer be documented and properly communicated to the personnel that perform the installation and startup of the electrical system. Setting of the devices as specified should be verified. 5.6

Lif etime Selectiv e Coordi nation

If circuit breaker selective coordination tables or fuse ratio tables were used, to maintain the selective coordination throughout the life of the system, Overcurrent Protective Devices (OCPD) of that type and from that same manufacturer will always need to be used in that system. Selective coordination may be affected by changes in the system. Protective device audits and tests should be performed to ensure that changes to the system haven’t affected either selective coordination or arc flash safety. Verify that the proper rating, brand, and type of each fuse are installed for each phase. Confirm that all circuit breaker and protective relay settings match the anal ysis studies. © Copyright 2010 by the National Electrical Manufacturers Association.

45

ABP 1-2010 Selective coordination requires very thorough upfront analysis and design, the selection of protective devices that considers the concerns of the local Authority Having Jurisdiction, the various requirements of equipment protection, and the impact of device selection on the arc flash energies that personnel may be exposed to. These analyses must be done by qualified personnel. Once completed, the results of the system design must be communicated to the various personnel that will install and maintain the electrical system per the specifications of the design engineer. For an electrical distribution system to be selectively coordinated, both an initial design phase and ongoing monitoring of the system’s changes must be conducted by the appropriate qualified personnel. It is recommended that the settings of circuit breaker electronic trip units and ground-fault relays be sealed, and that fusible switches be marked with the specific manufacturer and type of fuse to be i nstalled in them.

6

Summary

In recent years, the 2005 and 2008 National Electrical Code (NEC) [1] requirements for total selective coordination have changed the way engineers go about designing electrical distribution systems. Design engineers must now look at the electrical system from a more comprehensive perspective. They must now communicate even more with interested parties such as the local Authorities Having Jurisdiction, equipment manufacturers, and with the system installation and maintenance personnel. The design engineer must be prepared to optimize an initial design by taking into consideration various results from different analyses—short circuit analysis, selective coordination study, arc flash energy analysis—and review any other equipment or facility data that may have a bearing on the final installation of the electrical system. This design process is usually iterative in order to yield optimal results for addressing conflicts that typically arise. Manufacturers of OCPDs are actively seeking ways to expand the capabilities of their devices to meet the continually increasing demands for safety, flexibility, and ease-of-use in the design, installation, and application of their devices in electrical power distribution systems. They will continue to investigate and research innovative ways and technologies to meet these demands, and users can expect that while significant improvements have been made over recent years, continuous improvement in devi ces and analytical techniques will be ongoing.


46

NEMA ABP 1 Selective Coordination

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