Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 9, 1992
NRC INFORMATION NOTICE 92-51: MISAPPLICATION AND INADEQUATE TESTING
OF MOLDED-CASE CIRCUIT BREAKERS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems that were recently observed concerning
the application and testing of molded-case circuit breakers (MCCBs). It is
expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar
problems. However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
During research and inspection activities, the NRC staff has found that
licensees, when determining the MCCB parameters for motor loads, occasionally
underestimate or neglect to consider the inrush transient (starting) current
(ITC) occurring during the first few cycles after a motor is started. Often
only the locked rotor current (LRC) is considered in selecting the appropriate
MCCB.
If no special starting methodology is used to specifically limit the ITC, the
magnitude of the ITC can be as much as approximately three times the LRC.
Also, depending on the electrical characteristics of the power supply and the
impedance of the motor when started, the ITC can increase to as much as
approximately six times the value of the LRC. A current of this magnitude is
greater than the maximum instantaneous trip setting for circuit protection
recommended by the National Electric Code (NEC). Therefore, the NEC
recommended level of protection may be insufficient to prevent an unwanted
trip. Proper application of MCCBs depends strongly upon the use of accurate
design assumptions.
Another application problem involves the use of MCCBs equipped with both
thermal overload trip elements and instantaneous magnetic trip elements. A
newly purchased replacement Westinghouse Type HFB3125 ambient compensating
MCCB tripped when a hydrogen skimmer fan motor at the Catawba Nuclear Station
9207020233
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was started during testing after installation. Duke Power Company personnel
did not expect the MCCB to trip since the peak value of the ITC for the fan
motor was thought to be well below the instantaneous trip band of that MCCB
(NRC Inspection Report 99900404/90-01). A similar Westinghouse MCCB used
as a turbine room sump pump breaker tripped in a similar manner at the
Donald C. Cook Nuclear Power Plant. (NRC Inspection Report 99900404/90-21).
Westinghouse determined that most of its thermal-magnetic MCCBs with the
nonadjustable instantaneous magnetic trip function could trip inadvertently.
Westinghouse attributed this behavior to interaction between the thermal
overload trip function and the instantaneous magnetic trip function under
overload conditions. Under nonfault conditions, these functions may interact
when the circuit current causes the thermal trip element to deflect until it
presses on the MCCB's tripper bar at the same time that the instantaneous
magnetic armature is vibrating against the tripper bar. This interaction
causes the MCCB to trip.
Westinghouse notified nuclear licensees of this problem and recommended that
they test these MCCBs in their intended circuits before releasing them for
plant operation. Westinghouse also stated this recommendation on certificates
of conformance provided with its dedicated MCCBs sold as safety-related items.
Potential interactions between thermal overload and instantaneous trip
functions can be avoided by using a special class of MCCBs that are designed
specifically for motor loads. These MCCBs include only instantaneous magnetic
trip functions for protecting against faults or short circuits. The trip
setpoints are normally adjustable and more accurate than those in thermal-
magnetic MCCBs. These MCCBs, commonly referred to by the Westinghouse term
"motor circuit protectors," are intended for use in combination with motor
contactors or starters. Thermal overload relays contained in the contactors
or starters provide the coordinated protection against overloads and fault
currents which are below the magnitude necessary to actuate an instantaneous
trip of the MCCB.
Instances of premature tripping have occurred with MCCBs manufactured by ITE-
Siemens Company. The Carolina Power and Light Company, licensee for the
Shearon Harris Nuclear Power Plant, found that some of the 480 volt MCCBs
manufactured by ITE-Siemens, and purchased as commercial-grade, tripped
prematurely when tested (NRC Inspection Report 99901177/90-01). The licensee
had purchased these MCCBs as replacements for some older 600 volt ITE-Siemens
MCCBs. The ITE-Siemens Company tested the 480 volt MCCBs and found that these
trips were initiated by the MCCBs' instantaneous magnetic trip function, when
induction motor loads were started. The manufacturer tested the older 600
volt MCCBs and found no premature trips. Rather, the older 600 volt MCCBs
tripped at currents significantly above the levels at which an instantaneous
trip should have occurred.
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The licensee's testing of the 480 volt MCCBs was judicious. ITE-Siemens
stated that the instantaneous trip setpoints of commercial grade MCCBs with
nonadjustable magnetic trips are not normally verified at the factory.
However, ITE-Siemens and possibly other manufacturers of MCCBs will, upon
request, verify that the instantaneous magnetic trip points of their
commercial grade MCCBs with nonadjustable magnetic trips supplied to nuclear
utilities fall within the appropriate design band. Nevertheless, field
testing of other MCCBs may be needed to verify that their magnetic trip points
occur within the design band (but not below the band) because other
manufacturers may not routinely provide this verification for commercial grade
MCCBs. For this type of MCCB, most manufacturers check only that the MCCB
will trip instantaneously, without determining the current level at which the
trip occurs.
Discussion
An MCCB can render safety-related equipment inoperable if it trips
prematurely. Premature trips can result if the incorrect MCCB is selected
(and/or incorrectly adjusted) as a result of inadequate analysis of the load
circuit including dynamic analysis of expected transients when determining
ratings, settings and coordination requirements. Premature trips can also
occur with properly applied and set MCCBs that are operating out of tolerance
when incorrect bench testing methods and/or inadequate post-installation
testing fail to detect the out of tolerance performance.
Acceptable testing methods and specifications are provided in some MCCB
manufacturers' technical publications. Industry and professional
organizations also provide guidance on MCCB testing methods, including generic
acceptance criteria, in documents such as the National Electrical
Manufacturers Association (NEMA) Standard AB4-1991, "Guidelines for the
Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in
Commercial and Industrial Applications." It should be noted that the
application of generic field test tolerances to the instantaneous trip band on
manufacturers' time-current characteristic curves may not always ensure that
the MCCBs meet plant-specific breaker coordination, circuit protection or
technical specification requirements.
Testing of properly applied and set MCCBs in accordance with industry
recommended practices should provide reasonable assurance that the MCCBs'
instantaneous trip performance is acceptable for safety-related applications.
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This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Satish K. Aggarwal, RES
(301) 492-3829
Stephen D. Alexander, NRR
(301) 504-2995
Nick Fields, NRR
(301) 504-1173
Attachment: List of Recently Issued NRC Information Notices
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