ABP 3-2013
Page 1
A NEMA Low Voltage Distribution Equipment Section Document
ABP 3-2013
Molded Case Circuit Breaker Systems Testing with
Conductors
Published by
National Electrical Manufacturers Association
1300 North 17th Street, Suite 900
Rosslyn, Virginia 22209
www.nema.org
© 2014 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.
ABP 3-2013
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NOTICE AND DISCLAIMER
The information in this publication was considered technically sound by a consensus among persons
engaged in its development at the time it was approved. Consensus does not necessarily mean there was
unanimous agreement among every person participating in the development process.
The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which
the document herein is one, are developed through a voluntary 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. Although NEMA administers the process and establishes rules to promote
fairness in the development of consensus, it does not write the documents, nor does it 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, 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 particular purpose(s) or need(s). NEMA does
not undertake to guarantee the performance of any individual manufacturer’s 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 circumstance. 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.
© 2014 National Electrical Manufacturers Association
ABP 3-2013
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FOREWORD
This 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 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:
Senior Technical Director, Operations
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 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.
Eaton Corporation
General Electric
Siemens Industry, Inc.
Schneider Electric USA
Wichita Falls, TX
Pittsburgh, PA
Plainville, CT
Norcross, GA
Palatine, IL
© 2014 National Electrical Manufacturers Association
ABP 3-2013
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A NEMA Low Voltage Distribution Equipment Section Document
Molded case circuit breakers (MCCB) are designed to protect rated conductors and in particular, insulated wire.
For this reason, it is necessary to test circuit breakers with wire to demonstrate that protection.
Underwriters Laboratories (UL) 489 test procedures require testing the MCCB including the rated conductors
as a system to ensure system protection against overload and short circuit. UL listing requirements for
conductors do not include overload or short circuit testing nor does the National Electrical Code® (NEC®)
require that a conductor be tested by itself to establish these values. Conductors on the line and load sides of
the circuit breaker are tested on overload and short circuit during the MCCB systems test. The MCCB is tested
in an enclosure. A typical test set-up for MCCB systems testing would include the following: 4 feet of rated
conductors connecting the test station terminals to each pole of the line side of the circuit breaker and 10
inches of rated conductors connected to each pole of the load side of the circuit breaker. These conductors are
connected together to create a short. The rated conductors on the line and load sides of the MCCB are sized
in accordance with NEC Table 310.15(B)(16) and 240.4(D) and are based on the continuous current rating of
the circuit breaker. (An exception to this requirement is that, per 7.1.7.15 and Table 7.1.11.3.1.1 of UL 489, 277
V, 347 V, 480 V, and 600 V circuit breakers, rated 15A or less, may be tested with 12 AWG conductors.)
MCCB tests per the UL 489 Standard are divided into two categories: standard tests and high short circuit
interrupting tests.
The standard tests as defined in UL 489, Table 7.1.1.2 include overload and thermal tests, endurance followed
by low level short circuit interrupting tests, and standard low level short circuit interrupting tests. Each test
sequence begins with calibration tests and is concluded with a dielectric voltage withstand test. For the
calibration tests, each pole of a multi-pole circuit breaker is connected with a minimum of 4 feet of rated copper
conductor. The conductor size (AWG) is based on the temperature rating of the conductor as indicated by the
marking on the circuit breaker, i.e. 60°C or 75°C.
High short circuit interrupting tests are conducted per UL 489, Table 7.1.11.1. These high short circuit
interrupting tests are conducted at the assigned short circuit interrupting ratings as defined by the circuit
breaker manufacturer. The short circuit interrupting rating marking on UL listed MCCBs is a statement of the
maximum available fault current at the point of application of the MCCB, expressed in amperes RMS
symmetrical, which, by test with rated conductors, the circuit breaker can interrupt. The interrupting ratings are
fully applicable in systems where the analysis of available short circuit current is based on the impedance of
components in the distribution system. As mentioned earlier, it is only during these short-circuit interrupting
tests that rated conductors are subjected to such high currents and resulting temperature rise with (a) the
conductor insulation visually checked for damage, (b) the conductors checked for pullout at the terminals, and
(c) the molded case of the circuit breaker checked for damage from conductor whiplash. Other general
performance checks and tests conducted after the short-circuit tests include verification of no arcing to ground,
overcurrent trip-out tests, and a dielectric withstand test. These general performance tests confirm the circuit
breaker's ability to remain self-protecting and provide safe isolation.
Even though the UL 489 Standard and high interrupting rating tests provide verification through testing of the
ability of a MCCB to protect conductors, the issue of the use of conductors masking the real ability of a MCCB
to interrupt the fault current for which it is labeled persists. The NEC recognizes the suitability of this test circuit
by stating the following in NEC Section 110.10, Circuit Impedance, Short Circuit Current Ratings, and Other
Characteristics:
"The overcurrent protective devices, the total impedance, the equipment short-circuit ratings,
and other characteristics of the circuit to be protected shall be so selected and coordinated as
to permit the circuit protective devices used to clear a fault to do so without the extensive
damage to the electrical equipment of the circuit. This fault shall be assumed to be either
between two or more of the circuit conductors, or between any circuit conductor and the
equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in
accordance with their listing shall be considered to meet the requirement of this section."
Circuit breakers tested in accordance with the UL 489 Standard clearly meet this NEC requirement by
practical and consistent laboratory testing.
© 2014 National Electrical Manufacturers Association
ABP 3-2013
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To further understand the importance of total impedance in the system, not just in the conductor, consider that
a typical MCCB test circuit prior to a fault condition has only the static impedance of the connected devices
and the conductor, measured in milliohms. When a circuit breaker operates on overcurrent, an arc is drawn
between the contacts, and the resulting fault current is limited due to dynamic impedance. Dynamic
impedance is defined as the arc impedance introduced into a circuit by the opening of the circuit breaker
contacts during current interruption. When we consider the effect of wire impedance in the circuit, it is
important to consider it as part of the total system impedance. However, the wire impedance is small
compared to the dynamic impedance of the circuit breaker as it interrupts a fault. Many circuit breaker
designs, particularly the more recent circuit breaker designs, inject this dynamic impedance into the circuit
early during interruption to limit the fault current passing through the circuit breaker. The dynamic impedance
of the arc during the interrupting cycle of a MCCB plays a dramatic part in reducing the let through fault
current to downstream devices.
Figure 1 provides a graphic representation of the dynamic characteristics of arc impedance in one phase of a
high interrupting rating circuit breaker during short circuit interruption. The arc impedance increases rapidly
after contact separation, limits the fault current and the circuit breaker interrupts the circuit within 1/4 of a cycle.
The dynamic arc impedance makes the total impedance of the connected conductor or wire insignificant by
comparison. This dynamic impedance phenomenon occurs to some degree in every MCCB. Hence, all circuit
breakers limit current to some extent at their high-short current rating, UL defines the requirements for labeling
circuit breakers as current limiting.
The previous statement that the dynamic impedance is large compared to the wire impedance is true for 100 A
and larger circuit breakers. But, the higher impedance of the smaller conductors connected to circuit breakers
rated less than 100 A needs to be considered. UL 489 addresses this by requiring that circuit breakers rated
less than 100 A be subjected to an additional “bus bar conditions” test. In this test, 4 feet of 1 AWG wire is
connected to 10 inches of rated wire which in turn is connected to the circuit breaker. The lower impedance of
the 1 AWG wire insures that a small circuit breaker will adequately protect the wire connected to its load
terminals if the line side of the circuit breaker is connected to a bus bar in a panelboard.
ARC IMPEDANCE
Figure 1 Dynamic impedance of a 15 A high interrupting MCCB tested at 480 V/65 kA
© 2014 National Electrical Manufacturers Association
ABP 3-2013
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The Informational Note in Section 240.4 that was added in the 2011 NEC can be misleading if not properly
applied. As has previously been mentioned, UL 489 requires that circuit breakers equipped with lugs be
tested with rated wire. This testing demonstrates that the circuit breaker can protect the rated wire from
thermal damage; that the rated wire will be retained in the lug and that whiplash will not damage the circuit
breaker. It is unnecessary to use ICEA P-32-382 in cases where the circuit breaker has been tested with
rated wire, i.e. when equipped with lugs.
The established tests for determining the interrupting rating of a circuit breaker are a practical representation
of field installations of the products. The validity of standardized rating tests has been confirmed by accepted
field performance. When applied in accordance with their listed ratings as determined by standardized test
procedures, there is no evidence of problems with applications. Similar short circuit tests with rated
conductors for fusible disconnects and circuit breakers are done for all assembled products including
panelboards, switchboards, motor control centers, and combination starters. The field safety record shows
that when tested in accordance with the UL 489 Standard, the circuit breakers ability to protect conductors is
confirmed.
© 2014 National Electrical Manufacturers Association