United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 73-01: Faulty Overcurrent Trip Delay Device in Circuit Breakers for Engineered Safety Systems



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                                                                IEB 73-01

                                 UNITED STATES
                           ATOMIC ENERGY COMMISSION
                     DIRECTORATE OF REGULATORY OPERATIONS
                                   REGION V
                               2111 BANCROFT WAY
                          BERKELEY, CALIFORNIA 94704

                                       February 23, 1973


Gentlemen:

The attached Directorate of Regulatory Operations Bulletin No. 73-1, "Faulty
Overcurrent Trip Delay Device in Circuit Breakers for Engineered Safety
Systems," is sent to you to provide you with information we received from the
Wisconsin Electric Power Company and the Rochester Gas and Electric
Corporation concerning the Point Beach and Ginna pressurized water reactor
facilities.   This information may relate to the design, fabrication and
performance of certain equipment at your facilities.  Action requested on your
part is identified in Section B of the Bulletin.

                                             Sincerely,


                                             R. W. Smith
                                             Director

Enclosure:
As stated
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                                                    February 23, 1973
                                                    Directorate of Regulatory
                                                    Operations Bulletin 73-1


FAULTY OVERCURRENT TRIP DELAY DEVICE IN CIRCUIT BREAKERS FOR
ENGINEERED SAFETY SYSTEMS

We have received information from two licensees relating to the failure of
circuit breakers in engineered safety systems.  These circuit breakers,
Westinghouse Models D3-25, DB-50 and DB-75, are used extensively at both PWR
and BWR facilities.  These failures, which would negate operation of
engineered safety system components, were attributed to faulty overcurrent
trip delay devices.  Westinghouse has formulated an on-site testing program to
determine if the time delay devices are faulty and is developing a new end cap
for the dashpot used in the time delay devices.  Similar make and model
circuit breakers, when used for scram purposes, do not require the overcurrent
trip feature and, therefore, are not included in this problem.  Pertinent
details relating to this problem are contained in Section A below.  Action 
requested by this Bulletin is contained in Section B.

a.  Description of Circumstances

Plant A

During the performance of the routine monthly surveillance test of the Safety
Injection System, the pump motor tripped as it approached operating speed.  A
second attempt to start the pump produced the same result.  The cause of the
pump trip was attributed to malfunction of an overcurrent trip device.  This
device, which has an adjustable time delay, tripped out the pump motor on
starting current because of leakage in the time delay dashpot.  Subsequent
examination of the dashpot disclosed a crack in the end cap of the air
chamber.  The crack permitted air to leak into the chamber at an excessive
rate, reducing the time delay from approximately 45 seconds to 5 seconds. 
Testing of similar model breakers equipped with the overcurrent trip delay
feature revealed four additional units with low trip delay times caused by
cracks in the end caps.  Approximately 25 circuit breakers of the type
described are utilized to energize components of engineered safety feature
systems at this facility.

Plant B

A 480 volt AC bus circuit breaker tripped open when the auxiliary building
supply and the exhaust fans were started simultaneously.  The cause of the
failure was attributed to a crack in the end cap of the time delay dashpot in
the overcurrent trip device.  The trip delay unit and the circuit breaker in
question are similar to the units which failed at Plant A.
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The cracks found in the end caps of the time delay dashpots are considered a
common mode of failure.

 B.  Action Requested of the Licensee

It is requested that you determine whether safety related (Class IE) circuit
breakers of the described make and models utilizing overcurrent trip time
delay features are installed at your facilities and provide this office with
your findings.

If the subject breakers are installed, please include in your response a
description and the results of your program to determine if you have any
faulty overcurrent trip time delay devices installed and the scheduled
completion date of your corrective actions.  For facilities currently under
construction please inform us of your plans to assure that the appropriate
modification has been incorporated in all safety related circuit breakers of
the type described that have been or will be installed at your facility.  This
information should be provided to this office, in writing, within forty-five
days of your receipt of this bulletin.

Page Last Reviewed/Updated Tuesday, July 23, 2013