1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA ABP 6-2015What is the Purpose of a Molded Case Circuit Breaker?A NEMA Low Voltage Distribution Equipment Section Document ABP 6-2015What is the Purpose of a Molded Case Circuit Breaker?Published by:National Electrical Manufa
2、cturers Association1300 North 17thStreetSuite 900Rosslyn, Virginia 22209www.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
3、Artistic Works, and the International and Pan American copyright conventions. 2015 National Electrical Manufacturers AssociationNOTICE AND DISCLAIMERThe information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the documen
4、t 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.American National Standards Institute, Inc. (ANSI) standards and guideline publications, of which the document contained herein i
5、s 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
6、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 in
7、jury, 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
8、the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products o
9、r 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
10、 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
11、 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 insta
12、llations 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. 2015 National Electrical Manufactu
13、rers Association 2015 National Electrical Manufacturers AssociationABP 6-2015Page iForewordThis is an update to the NEMA white paper originally published in 1994. To ensure that a meaningful publication was being developed, draft copies were sent to a number of groups within NEMA having an interest
14、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 Distributi
15、on 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, OperationsNational Electrical Manufacturers Association1300 North 17thStreet, Suite 900Rosslyn, VA 22209This w
16、hite 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
17、 was approved, the Molded Case Circuit Breaker Product Group had the following members:ABB Inc.Wichita Falls, TXEaton CorporationPittsburgh, PAGeneral ElectricPlainville, CTSiemens Industry, Inc.Norcross, GASchneider Electric USAPalatine, IL 2015 National Electrical Manufacturers AssociationPage iiA
18、BP 6-2015 2015 National Electrical Manufacturers AssociationABP 6-2015Page 1 2015 National Electrical Manufacturers AssociationOver the past half century, advances in molded case circuit breaker (MCCB) technology have led to a product that is a key element in the electrical safety of residences, off
19、ice buildings, and sophisticated commercial and industrial systems. MCCBs have become so commonplace that their purpose is taken for granted and often misunderstood. MCCBs contribute to electrical safety in a variety of circumstances by providing protection for conductors in the applications mention
20、ed above. Under what conditions do they provide protection?There are several devices that physically resemble MCCBs but have different applications. The MCCB we will describe in this paper is an overcurrent protective device incorporating both inverse time overload (protects against thermal damage f
21、rom overload conditions) and instantaneous (protects against thermal and mechanical damage from high-magnitude faults) protection. Larger MCCBs may also be referred to as insulated case circuit breakers (ICCB). ICCBs are also MCCBs, but have some additional characteristics mentioned later in this pa
22、per. These devices are constructed to meet the requirements of UL 489, and are suitable to protect conductors as required by the National Electrical Code(NEC). Other devices of molded case construction not addressed in this paper are: molded case switches (UL 489). These devices have no inverse time
23、 overload or short-circuit protection. In some cases, they may have fixed instantaneous protection (for self-protection only). These devices are not suitable to protect conductors. motor circuit protectors (UL 489). These devices also do not have overload protection, but have an adjustable instantan
24、eous protection that is especially suited for use in a motor starter (along with a separate overload device provided with the contactor) or as part of a motor protection scheme. Motor circuit protectors are not designed to protect conductors from overloads. supplementary protectors (UL 1077). These
25、devices have both inverse time overload (thermal) and instantaneous protection, but are listed under UL 1077 instead of UL 489, and also cannot be used to protect branch circuit conductors as required in the NEC. They are defined by UL 1077 as “.intended for use as overcurrent, or over- or under-vol
26、tage protection within an appliance or other electrical equipment where branch circuit overcurrent protection is already provided, or is not required.“NEC defines a circuit breaker as a device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a pred
27、etermined overcurrent without damage to itself when properly applied within its rating.Simply stated, a circuit breaker must be able to be switched open and closed manually and to protect a conductor from overcurrent by opening automatically. This allows an MCCB to be used repeatedly without replace
28、ment.The three most common types of circuit breakers available for use today in low-voltage power distribution systems are low-voltage power circuit breakers, ICCBs, and MCCBs. The first two choices are large, very robust devices designed for high withstand applications that require the circuit-brea
29、ker contacts to stay closed for a brief period during a fault to facilitate critical coordination with downstream devices. These types are used only for a small percentage of circuit-protection applications. MCCBs comprise the majority of all other circuit breakers used in distribution and are the t
30、opic of this paper.The switching function of a circuit breaker is simple and readily understood. It provides a means of easily isolating a circuit or circuits protected by the circuit breaker from the rest of the electrical system, allowing for the safe modification or maintenance of the circuit, or
31、 for repair or replacement of appliances or equipment in the circuit. All circuit breakers are intended to switch loads. In addition, some are specifically rated for switching loads such as the lighting in commercial and in industrial buildings, on a regular basis.The protection function is not quit
32、e so easily understood. What is an MCCB intended to protect? MCCBs have specific requirements that they are expected to meet, per NEC and Underwriters Laboratories (UL). 2015 National Electrical Manufacturers AssociationPage 2ABP 6-2015Section 240.1 of NEC has the following Informational Note descri
33、bing overcurrent protection:Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in the conductors or conductor insulation.So, circuit breakers are intended to protect conductors (insu
34、lated wires) by opening automatically before damage is caused by excessive temperatures. This protection applies to the permanently installed wiring in homes, offices, and industrial buildings. In protecting these wires, overcurrent-protective devices reduce the risk of fire, personal injury, and da
35、mage to equipment and property.Circuit breakers listed by UL are subjected to a rigorous test program to assure the specifications for overcurrent protection are met. Initial samples are subjected to endurance, overload, and short circuit tests with a length of the wire to be protected in the test c
36、ircuit. Follow up tests on samples randomly chosen from production are run on a regular basis by UL, and production samples are continually checked to be sure ongoing production is properly calibrated.In summary, circuit breakers are intended to protect the wiring, permanently installed conductors,
37、and certain extension and power supply cords from high temperatures caused by currents in excess of the rating of the conductors. When the installation location of the MCCB is subjected to temperature ranges above/below 25 degrees Celsius, rerating may be necessary in order to ensure overcurrent pro
38、tection is achieved at the temperature extremes. At temperatures below 25 degrees Celsius, the actual trip thresholds may be higher than the marked ampere rating. At temperatures above 25 degrees Celsius, the actual trip threshold may be lower than the marked ampere rating. Always consult with manuf
39、acturers regarding rerating of MCCBs. While “conductor“ protection is the key element in electrical system safety, there are potentially dangerous conditions that do not involve overcurrent. Standard MCCBs do not provide protection against these conditions. The following are important examples: elec
40、tric shock. Standard overcurrent protective devices will not protect personnel from the effects of electric shock. Minute currents, 20 mA (0.02A) or less, can have fatal results if they pass through the human body. Obviously, an overcurrent protective device rated 15- or 20-ampere will take no notic
41、e of such small currents, however dangerous they may be. A special form of MCCB called a ground-fault circuit interrupter (GFCI) can be used to protect personnel against the effects of electric shock. These sophisticated electronic devices will “trip” when the current to ground is 6 mA or higher and
42、 detect currents as small as 4 mA, opening the circuit so quickly that the shock that does occur will be over before serious injury can occur to an otherwise healthy person. GFCI protection is required by NEC for areas such as bathrooms, garages, outdoor receptacles, crawl spaces, unfinished basemen
43、ts, kitchens, sinks, boathouses, and certain other locations where the likelihood of electrical shock is the greatest. Such devices are also available in receptacle and other forms. arc fault. An arc fault is an unintended arc created by current flowing through an unplanned path. Arcing creates high
44、 intensity heating at the point of the arc, resulting in burning particles that may easily ignite surrounding material, such as wood framing or insulation. The temperatures of these arcs can exceed 10,000 degrees Fahrenheit. An arc-fault circuit interrupter (AFCI) is a product that is designed to de
45、tect a wide range of arcing electrical faults to help reduce the electrical system from being an ignition source of a fire. Conventional overcurrent protective devices do not detect low-level, hazardous arcing currents that have the potential to initiate electrical fires. It is well known that elect
46、rical fires do exist and take many lives and damage or destroy significant amounts of property. Electrical fires can be a silent killer occurring in areas of the home that are hidden from view and early detection. The 2015 National Electrical Manufacturers AssociationABP 6-2015Page 3objective is to
47、protect the circuit in a manner that will reduce its chances of being a source of an electrical fire. AFCI protection is required by Section 210.12 of the 2014 NEC for all 15- and 20-ampere 120-volt branch circuits supplying outlets or devices installed in dwelling unit kitchens, family rooms, dinin
48、g rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, or similar rooms or areas. The AFCI is required to be a listed combination type arc-fault circuit interrupter. There are six permitted ways to meet this requirement. One is a list
49、ed circuit-breaker type combination AFCI. Receptacle type AFCIs are also permitted, subject to certain application restrictions in new construction (see NEC for details). ground-fault protection of equipment (GFPE). Ground faults are created when an ungrounded conductor comes into contact with a grounded metal object. As with arc faults, a standard circuit breaker may not be capable of detecting a low level ground fault, and such a fault could cause ignition of a flammable material or damage to the equipment.GFPE is designed to protect equipment from damaging line-
