1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA ABP 7-2015 Engineering SeriesRatings: Is It Practical?A NEMA Low Voltage Distribution Equipment Section Document NEMA ABP 7-2015 Engineering Series Ratings: Is It Practical? Published by: National Electrical Manufacturers As
2、sociation 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2015 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 Arti
3、stic Works, and the International and Pan American copyright conventions. 2015 National Electrical Manufacturers Association 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
4、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 consen
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8、n published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide
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10、r 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
11、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
12、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. NEMA ABP 7-2015 Page i 2015 National Electrical Manufacturers Association Fore
13、word This is an update to the NEMA white paper originally published in 2005. 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
14、final NEMA approval and resulted in a number of substantive changes in this publication. This publication will 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 ev
15、olving technology. Proposed or recommended revisions should be submitted to: Vice President, Technical Services 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 t
16、he 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
17、 members: ABB Control, Inc.Wichita Falls, TX Eaton CorporationPittsburgh, PA General ElectricPlainville, CT Siemens Industry, Inc.Norcross, GA Schneider Electric USAPalatine, IL NEMA ABP 7-2015 Page ii 2015 National Electrical Manufacturers Association CONTENTS Foreword i 1 Introduction . 1 2 Existi
18、ng Installations . 1 3 Attempted Corrections with Series Ratings 2 4 Selection of Certified Rating Method 2 5 Analytical Method . 3 6 Downstream Circuit Breakers . 4 7 Tested Series Ratings 4 8 Are Analytical Methods Effective? 4 9 Low-voltage Power Circuit Breakers 5 10 Degrees of Protection . 6 11
19、 Summary 6 12 Bibliography 7 Appendix A 8 Appendix B 10 Figures Figure 1 Typical Installation with Increased Available Fault Current 2 Figure 2 System of Figure 1 with Added Series Protective Devices 3 NEMA ABP 7-2015 Page 1 2015 National Electrical Manufacturers Association 1 Introduction The 2005
20、National Electrical Code (NEC) introduced a change regarding series ratings for circuit breakers that deserves careful review before applying an engineered series rating. The additional section 240.86(A) reads: (A) Selected under Engineering Supervision in Existing Installations. The series rated co
21、mbination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install
22、, inspect, maintain and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end-use equipment. Notice that this new paragraph deals exclusively with existing installations where an increase in the available fault current (
23、due to factors such as increases in transformer size, lowering of transformer impedances, and changes in utility distribution systems) puts the existing circuit overcurrent equipment at peril in regard to interrupting fault currents and provides for a licensed professional engineer to determine the
24、series rating in these installations. In the 2008 NEC, a sentence was added to section 240.86(A) that reads: For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line
25、 side fully rated, current-limiting device. Several issues must be understood related to the new paragraph (A): a) Why is this provision for only existing installations? b) Is an added series rating a solution for the situation for which this is intended? c) What information does a licensed professi
26、onal engineer need, and what information does he/she have, that will permit him/her to determine an effective series rating? d) Are there any downstream circuit breakers that are known to be passive, or any suspected to not be passive? NOTEA circuit breaker is considered to be “passive” if its conta
27、cts remain closed during the interruption of the fault, such that it provides no additional arcing impedance to aid in the fault interruption. This additional impedance is often called “dynamic impedance.” 2 Existing Installations The basis for permitting engineered series ratings only on existing i
28、nstallations is to address conditions where available fault current has been increased after installation due to an upgrade in the electrical distribution system. With the modification, the existing equipment and overcurrent protective devices may be under-rated for the higher fault current. Under t
29、he NEC revision, an overcurrent protective device (OCPD) having an interrupting rating at least as high as the increased fault current can be added to protect the installed system if a licensed professional engineer determines that a series rating exists using this device and the installed OCPDs. Ut
30、ilities and other service operators occasionally revise their systems to provide for increased energy demand for power quality and other reasons. When these system changes result in a higher available fault current that exceeds the rating of installed equipment, a serious hazard exists to the facili
31、ty and to those working with or near the under-rated electrical equipment. Options for correcting this situation include replacing the equipment with properly rated equipment, replacing OCPDs with higher-rated NEMA ABP 7-2015 Page 2 2015 National Electrical Manufacturers Association OCPDs, or applyi
32、ng higher-impedance transformers or reactors and similar equipment modifications to reduce the available fault current. Some of these options can be complex and costly, so it is reasonable that a simple and low-cost solution would be desired. However, even with simple improvements, the true cost cou
33、ld be high if the improvements do not provide the expected protection and give a false sense of security. 3 Attempted Corrections with Series Ratings This definition of a series rating appeared in a 1994 article by the National Electrical Manufacturers Association (NEMA): Series rating: A short-circ
34、uit interrupting rating assigned to a combination of two or more overcurrent protective devices which are connected in series and in which the rating of the downstream device(s) in the combination is less than the series rating. 1 From the definition, we can see that the series rating will be higher
35、 than the rating of the downstream circuit breaker(s). By locating the higher-rated circuit breaker or fuse electrically ahead or upstream of the lower-rated circuit breaker, the higher-rated device will protect itself and the lower-rated downstream circuit breaker if the series is properly selected
36、. So, in the case of installed equipment that becomes under-rated, adding a fully rated fuse or circuit breaker electrically ahead of it might be considered a method of protecting it. The challenge is how to select the OCPD to be added to be sure that it will provide the required protection. Lets lo
37、ok at two methods, the Selection of Certified Rating Method and the Analytical Method, as possible ways to select this device. 4 Selection of Certified Rating Method Lets examine the simple three-tier system of Figure 1, which is typical of a number of commercial installations. Then, consider adding
38、 series devices as in Figure 2. Figure 1 Typical Installation with Increased Available Fault Current 12 00 AIR =4 2 kA40 k A 7 8 kA A va ila bl e27 k A 3 9 kA A va ila bl e13 .9 k A 1 6. 2 kAA va ila bl eIR =1 4 kA40 0 AIR =3 0 kAMain Feeder NEMA ABP 7-2015 Page 3 2015 National Electrical Manufactur
39、ers Association Figure 2 System of Figure 1 with Added Series Protective Devices Suppose we added a rated OCPD at location A (device A) on the supply side of the main. We would have to determine that the added device A will protect both the 1200 A main circuit breaker and the equipment in which it i
40、s installed. This step, if successful, would be an improvement in protecting the main and equipment, but it does not necessarily help us in protecting the feeders or the downstream system protected by the feeders. The first step is to determine whether a tested series rating already exists between d
41、evice A and the installed 1200 A main circuit breaker. If the rating is marked on the equipment, we know that by installing device A, the 1200 A main circuit breaker and the equipment in which it is installed will be protected. If a component recognized series rating is found to exist, but it does n
42、ot appear on the equipment label, we know only that the overcurrent protective devices will perform acceptably together under special test conditions, but we do not know whether they will perform acceptably in the end-use equipment, such as the switchgear or switchboard. The integrity of the enclosu
43、re, busing, and insulators in the end-use equipment must be considered as is required for application of tested series ratings. 5 Analytical Method If a tested series rating is not available, we might attempt an analytical method. From an analytical viewpoint, the performance of two OCPDs combined t
44、ogether for a series rating should be considered acceptable if all of the following criteria are met by the upstream OCPD: a) It reduces the let-through current to a value below the interrupting rating of the downstream circuit breaker. b) It clears the circuit at a time before the contacts of the d
45、ownstream circuit breaker begin to open. c) Items (1) and (2) are true for all current levels from the rating of the downstream circuit breaker through the series rating of the combination (not just at the maximum current level of the system). d) It has an interrupting rating at or above the enginee
46、red series rating. The question an engineer will have to answer is how to know whether the first three criteria are met. ABC1200 AIR =42 kA40 kA 78 kA Avai lable27 kA 39 kA Avai lable400 AIR =30 kA13.9 kA 16.2 kA A vailableIR =14 kAMain Feeder NEMA ABP 7-2015 Page 4 2015 National Electrical Manufact
47、urers Association While these criteria identify the ability of the upstream OCPD to protect the downstream circuit breaker, they do not determine whether they will perform acceptably in the end-use equipment, such as the switchgear or switchboard. One way to predict whether the equipment is also pro
48、tected is to estimate the let-through current and duration under which the equipment would have been tested. If these values are lower with the added device in the circuit, the equipment is likely protected. But how would an engineer determine the conditions under which the equipment would have been
49、 tested? 6 Downstream Circuit Breakers The Analytical Method above relates to protecting the 1200 A main circuit breaker and its end-use equipment by adding another device at location A. It has not addressed protection of the equipment on the load side of the main circuit breaker including the feeders and branches. Looking at Figure 2 again, we see that available fault current has increased for both the feeder and branch locations and exceeds the interrupting ratings of circuit breakers in these positions. A fault on the load terminals of the 400 A feeder wil
