Information Notice No. 96-24: Preconditioning of Molded-Case Circuit Breakers Before Surveillance Testing
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
April 25, 1996
NRC INFORMATION NOTICE 96-24: PRECONDITIONING OF MOLDED-CASE CIRCUIT BREAKERS
BEFORE SURVEILLANCE TESTING
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the detrimental effect that preconditioning of
molded-case circuit breakers could have on the diagnostic validity of
surveillance tests. It is expected that recipients will review this
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 an inspection at the Diablo Canyon Nuclear Power Plant (IR 50-275,
323/94-27 dated December 21, 1994), NRC inspectors noted that the licensee
preconditioned circuit breakers before performing surveillance testing.
Before the overcurrent trip testing, Diablo Canyon staff exercised the circuit
breakers by manually opening and closing them three times. The plant staff
did not record the as-found condition of the breakers in terms of unexercised
trip currents and times.
During another inspection at the South Texas Project (IR 50-498, 499/94-35
dated January 19, 1995), NRC inspectors noted that the licensee exercised
similar circuit breakers manually at least five times before testing the
overcurrent trip unit.
A molded-case circuit breaker is one of the most widely used protective
devices for electrical circuits at nuclear power plants. Its proper operation
is essential for the protection and safe operation of plant electrical
equipment. Many circuit breakers are continuously energized for long periods.
Often they are in service for years and are rarely, if ever, called upon to
perform a protective function. Nevertheless, when needed, they must rapidly
isolate a faulted or overloaded circuit to prevent equipment damage.
Therefore, for the safe operation of the electrical distribution system
equipment of a nuclear power plant, it is important to periodically verify
their continued reliability.
9604220229. IN 96-24
April 25, 1996
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Periodic inspection and testing of circuit breakers in their as-found
condition is an appropriate way of demonstrating the functional operability of
the breaker and of detecting any degradation. However, the practice of
preconditioning before testing (e.g., lubricating pivot points and manually
cycling the breaker) defeats the purpose of the periodic test. Such
preconditioning does not confirm continued operability between tests nor does
it provide information on the condition of the circuit breaker for trending
purposes. Testing some circuit breakers in the as-found condition can provide
useful data on which to base decisions on surveillance intervals and the
ability of the untested circuit breakers to perform their intended function.
Since only a fraction of the circuit breakers are tested each refueling outage
to justify the operability of the remaining circuit breakers, preconditioning
before testing does not provide the expected assurance of the operability of
remaining breakers which are not tested. By preconditioning circuit breakers,
useful information may be lost because the breaker may not have been capable
of performing its intended function without preconditioning.
To correct this situation, Diablo Canyon Power Plant plans to revise the
electrical maintenance procedure before the next surveillance test has to be
performed. The revised procedures will state that electrical testing should
be performed before any intentional manual exercising of the breakers, and to
ensure that electrical testing on breakers with removable trip units is
performed before removing the trip unit.
Some plants have older technical specification bases that cite National
Electrical Manufacturers Association (NEMA) Standard AB 2-1980. This version
of the standard specifies mechanical cycling of circuit breakers prior to
electrical testing. In later versions of this standard, this procedure was
deleted. There appears to be no valid technical need for mechanical cycling
prior to electrical surveillance testing.
The NRC has issued other generic communications on molded-case circuit breaker
applications and testing: NRC Information Notice (IN) 92-51, and its
Supplement 1, "Misapplication and Inadequate Testing of Molded-Case Circuit
Breakers;" IN 93-26, "Grease Solidification Causes Molded Case Circuit Breaker
Failure to Close;" and IN 93-64, "Periodic Testing and Preventive Maintenance
of Molded-Case Circuit Breakers."
. IN 96-24
April 25, 1996
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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.
Brian K. Grimes, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Sikhindra Mitra, NRR
Steve Alexander, NRR
Christopher Myers, RIV
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