NEMA ABP 1-2016 Selective Coordination of Low-Voltage Circuit Breakers.pdf

1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA ABP 1-2016Selective Coordination of Low-Voltage Circuit BreakersA NEMA Low-Voltage Distribution Equipment Section Document ABP 1-2016 Selective Coordination of Low-Voltage Circuit Breakers Published by: National Electrical M

2、anufacturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2016 National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Lit

3、erary 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. Consens

4、us 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. Th

5、is 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 inde

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8、 makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making this docum

9、ent 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 app

10、ropriate, 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 in

11、formation 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 c

12、ompliance 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 1-2016 2016 National Electrical Manufacturers Association i CONTENTS Page Foreword . III 1 INTRODUCTION 1 1.1 Pu

13、rpose 1 1.2 Scope 1 1.3 Definition of Selective Coordination 1 1.4 Coordination . 2 1.5 Examples 2 2 NEC SELECTIVE COORDINATION REQUIREMENTS 5 2.1 Requirements . 5 2.2 Challenges Meeting the Requirements 7 2.2.1 Local Jurisdiction Interpretation and Enforcement 7 2.2.2 Overriding Requirements . 8 3

14、CIRCUIT BREAKER TRIP RESPONSE FUNCTIONS 10 3.1 Fixed Thermal-Magnetic Type Circuit Breaker . 11 3.2 Adjustable Thermal-Magnetic Type Circuit Breaker . 12 3.3 Adjustable Electronic Type Circuit Breaker 14 3.4 Short-Time Withstand Current Rating 14 3.5 Instantaneous Override Function . 17 4 APPLICATIO

15、N INFORMATION FROM MANUFACTURERS . 18 4.1 Application of Time-Current Curves . 18 4.2 Limitation of Time-Current Curves 18 4.2.1 Overload Region 18 4.2.2 Instantaneous or Short-Circuit Region . 19 4.3 Short-Circuit Selective Coordination Tables . 22 4.4 Coordinating Ground-Fault Protection of Equipm

16、ent . 24 5 DESIGN GUIDELINES . 28 5.1 Simplify the One-Line Diagram . 32 5.1.1 Divide Larger Loads into Smaller Loads 32 5.1.2 Reduce the Number of Levels of Protective Devices 32 5.2 Reduce the Available Fault Current 33 5.2.1 Increase the Impedance of the System . 33 5.2.2 Use Step-Down or Isolati

17、on Transformers . 33 5.2.3 Take Advantage of the Added Arc Impedance of Circuit Breaker Combinations 37 5.3 Review Device Selection 37 5.3.1 Increase the Withstand Capabilities of the Upstream Line-Side OCPDs 37 5.3.2 Change the Type of Circuit Breaker 38 5.3.3 Select Current Limiting-Type Molded Ca

18、se Circuit Breaker 38 5.4 Special Equipment Application Requirements . 38 5.4.1 Generator Protection 38 5.4.2 Automatic Transfer Switches . 38 5.4.3 Busway . 40 5.4.4 Arc Flash Energy . 40 5.4.5 Zone Selective Interlocking 42 5.4.6 Bus Differential . 44 5.5 Field Adjustment . 45 5.6 Lifetime Selecti

19、ve Coordination 45 6 SUMMARY . 45 7 REFERENCES . 46 8 INFORMATIVE ANNEX . 47 ABP 1-2016 2016 National Electrical Manufacturers Association ii FIGURES Page Figure 1 System is selectively coordinatedfault at branch level OCPD 3 Figure 2 System is selectively coordinatedfault at feeder level OCPD . 3 F

20、igure 3 System is not selectively coordinatedfault at branch level OCPD 4 Figure 4 System is not selectively coordinatedfault at branch level OCPD 4 Figure 5 Code interpretation example . 8 Figure 6 Excerpt from NFPA 110-2013 Figure B.1(b) . 9 Figure 7 Typical TCCs for a low-voltage circuit breaker

21、. 11 Figure 8 Typical TCC for a fixed magnetic pickup action 12 Figure 9 Typical TCC for an adjustable magnetic pickup action . 13 Figure 10 Typical adjustable settings for circuit breakers . 14 Figure 11 Typical TCC characteristics for LVPCBs vs. molded-case/insulated-case circuit breakers . 16 Fig

22、ure 12 Typical adjustable versus fixed instantaneous pickup settings 17 Figure 13 Typical TCCs of two OCPDs . 19 Figure 14 Effect of current limiting on circuit breaker performance 20 Figure 15 Peak let-through current of a circuit breaker . 21 Figure 16 Circuit breaker settings (set so circuit brea

23、ker TCC does not overlap fuse TCC) 21 Figure 17 Circuit breaker settings (set to ensure coordination to full available bolted fault current) 22 Figure 18 The mandated limits for low-voltage ground-fault protection 25 Figure 19 Circuit breakers selectively coordinated with 1200 A ground fault . 26 Fi

24、gure 20 Circuit breaker and fuses with 1200 A ground fault NOT selectively coordinated 27 Figure 21 20 A single-pole lighting circuit breaker under 240 A ground fault . 28 Figure 22 Three levels of selective coordination required 33 Figure 23 Simplified scheme with two levels of selective coordinati

25、on . 33 Figure 24 Coordination requirements for transformer primary and secondary devices 34 Figure 25 Distribution of incoming phase currents into a delta primary winding . 35 Figure 26 Delta wye transformer windings with balanced 3-phase fault . 35 Figure 27 Delta wye transformer windings with sin

26、gle phase-to-phase fault 35 Figure 28 Single phase to ground . 36 Figure 29 Coordination requirements for transformer primary and secondary devices based on unknown high available primary fault current, known kVA, and known transformer impedance (the effect of conductor impedance is ignored) . 37 Fi

27、gure 30 Three circuit breakers achieving coordination using nested short-time bands . 42 Figure 31 A simplified ZSI communication scheme for three circuit breakers 43 Figure 32 The three circuit breakers of figure 31 at their backup protection settings and at the faster “in-zone” protection settings

28、 . 43 Figure 33 The effect of bus differential on a bus protected by zone-interlocked circuit breakers . 44 TABLES Page Table 1 Typical ratings of LVPCBs vs. molded-case/insulated-case circuit breakers 15 Table 2 Typical selective coordination table . 23 ABP 1-2016 2016 National Electrical Manufactu

29、rers Association iii FOREWORD This NEMA white paper 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

30、 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 led to a number of substantive changes in this publication. This publication wil

31、l be reviewed periodically 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: Senior Technical Director, Oper

32、ations National Electrical Manufacturers Association 1300 North 17th Street, Suite 900 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

33、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 included the following members: ABB, Inc.Memphis, TN Eaton CorporationPittsburgh, PA General ElectricPlainville, CT Siemen

34、s Industry, Inc.Norcross, GA Schneider Electric USAAndover, MA ABP 1-2016 2016 National Electrical Manufacturers Association 1 1 Introduction 1.1 Purpose The purpose of this paper is to provide guidance to engineers regarding the National Electrical Code (NEC) requirements for selective coordination

35、. This paper specifically addresses how to comply with these requirements for low-voltage circuit breakers (LVCBs). LVCBs include molded-case circuit breakers (MCCBs) listed to UL 489 and low-voltage power circuit breakers (LVPCBs) listed to UL 1066. (The MCCB category also includes insulated case c

36、ircuit breakers ICCBs.) 1.2 Scope This paper provides information on the following topics: 1) Description of the key functions of the overcurrent protective devices (OCPDs) used in low-voltage applications for meeting selective coordination requirements per the latest edition of the NEC 18 2) Discus

37、sion of selective 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

38、 with applicable authorities having jurisdiction (AHJ) 5) An overview of considerations for designing selectively coordinated systems 6) Background information illustrating why overlapping of time current characteristic curves does not equate to a loss of selective coordination 1.3 Definition of Sel

39、ective 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 18 definitions related to selective coordination are as follows: Selective coordination: “Localization of an overcurr

40、ent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full

41、range of overcurrent protective device opening times associated with those overcurrents.” Overcurrent: “Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short-circuit, or ground fault.” Overload: “Operation of equipment in excess of

42、 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.” Ground fault: “An unintentional, electrically conductin

43、g connection between an ungrounded conductor of an electrical circuit and the normally non-current carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.” Other relevant definitions: Short circuit current: “An overcurrent resulting from a fault of negligible imped

44、ance between live conductors having a difference in potential under normal operating conditions.” 13 With selective coordination, only the 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

45、ideally each OCPD shall be selectively coordinated with other upstream protective devices in the system. ABP 1-2016 2016 National Electrical Manufacturers Association 2 1.4 Coordination The use of the terms “selective coordination” and “coordination” in the 2012 edition of NFPA 99 and the 2014 editi

46、on of the NEC 19 has created a clear distinction between two levels of coordination that need to be understood by the engineer. The 2014 edition of the NEC changed the requirements around coordination for health care facilities. Article 517, “Health Care Facilities,” introduced a new section titled

47、“coordination.” This new language sought to ensure the curves do not overlap for times greater than or equal to 0.1 seconds. The language in the NEC stated that “overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a faults duration

48、extends beyond 0.1 seconds.” The informational note that was added provides further clarification of the level of current. This language in the code gives the engineer flexibility to establish coordination for selected times or currents to achieve an optimized solution. The engineer may need to reac

49、h a compromise between competing objectives of maximum protection, maximum service continuity, and/or budgetary constraints. With this in mind, the following definition is offered for the term “coordination”: Coordination: Localization of an overcurrent condition to minimize outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings, ensuring separation of their time current curves beyond a specified time period without regard to fault current magnitude