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HomeNRC LibraryDocument CollectionsGeneral CommunicationsInformation Notices1993

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                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                               February 2, 1993


NRC INFORMATION NOTICE 93-09:  FAILURE OF UNDERVOLTAGE TRIP ATTACHMENT ON      
                               WESTINGHOUSE MODEL DB-50 REACTOR TRIP BREAKER   


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 the failure of an undervoltage trip attachment
on a reactor trip breaker at the Salem Nuclear Generating Station, Unit 1. 
The failure occurred during surveillance testing.  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

On July 25, 1991, Public Service Electric and Gas Company, the licensee for
Salem, Unit 1, was conducting the monthly surveillance tests of the
undervoltage trip attachment and the shunt trip attachment associated with
reactor trip circuit breakers.  Licensee personnel successfully completed the
shunt trip attachment surveillance tests.  However, when the undervoltage trip
attachment trip test was attempted on the "A" reactor trip breaker
(Westinghouse model DB-50), the circuit breaker required approximately 
16 cycles (0.267 seconds) to open.  For the function to be considered
operable, technical specifications require that the breaker open in 10 cycles 
(0.167 seconds) or less.  A second attempt to open the breaker using the
undervoltage attachment was unsuccessful; the breaker remained closed.

Discussion

On July 26, 1991, a Westinghouse representative and licensee personnel
performed various bench tests on the "A" reactor trip breaker.  The tests
indicated that the breaker had been properly maintained and that the breaker
trip bar was functioning normally.  However, when the undervoltage trip
attachment was tested, it again failed to open the breaker.  The undervoltage
trip attachment was not striking the circuit breaker trip bar with sufficient
force to lift the bar.  When the undervoltage trip attachment was lubricated
and retested, the undervoltage trip attachment armature successfully lifted
the circuit breaker trip bar, opening the breaker within the required 
10 cycles.  The licensee re-lubricated and tested the undervoltage trip


9301260286.

                                                            IN 93-09
                                                            February 2, 1993
                                                            Page 2 of 3


attachment with 300 grams of weight added to the trip bar as required by the
monthly surveillance test procedure.  The test was again performed
satisfactorily.  The trip attachment also lifted the trip bar with an added
460 grams, the amount of additional weight required by the licensee semi-
annual surveillance test procedure.

Initially, the cause of the failure appeared to have been inadequate
lubrication; however, the undervoltage trip attachment was returned to
Westinghouse for further evaluation.  Westinghouse later concluded that the
root cause of the failure of the device was abnormal frictional forces which
resulted primarily from the unpolished outside latch contact surfaces of the
undervoltage trip attachment.  Westinghouse representatives stated that these
areas had not been properly finished during the manufacturing process.  The
outside contact surface of the latch was "bright" on only one edge rather than
on the entire surface.  The "bright" edge also appeared to be burred or
serrated.

Westinghouse representatives also found excessive wear and grooving on the
undervoltage trip attachment latch spring at a location where the latch and
latch spring were making contact.  Contact between the latch and the latch
spring occurred because the latch spring was somewhat deformed and did not lie
flat as designed.  The latch spring deformation, grooving, and wear in
combination with the improperly finished latch contact surfaces created
additional internal frictional forces such that the undervoltage trip
attachment armature was unable to apply sufficient force to lift the breaker
trip bar.  

During the evaluation, Westinghouse personnel reassembled and cycled the
undervoltage trip attachment 250 times.  The undervoltage trip attachment was
then tested, lubricated, and retested.  Each time the undervoltage trip
attachment was tested, it provided sufficient force to operate the breaker
trip bar.  However, the force applied by the device appeared to decrease at
250 cycles but increase at 500 cycles.  The force then appeared to decrease
somewhat after 750 cycles, although this may be attributed to testing
inaccuracies.  While the testing was not considered conclusive, the
performance of the undervoltage trip attachment apparently degrades and then
improves to some plateau.  Westinghouse concluded that this change results
from, first, increasing friction as the latch cuts into the latch spring; and
later, decreasing friction as the parts wear and as pressure from the latch
spring decreases as it permanently deforms.

The undervoltage trip attachments equipped with improperly finished latches
were provided to various utilities by the Replacement Component Services (RCS)
Department of the Westinghouse Nuclear Services Division (NSD).  These
undervoltage trip attachments were supplied either individually or as part of
remanufactured or overhauled Westinghouse Model DB-50 circuit breakers with
installed undervoltage trip attachments.  Westinghouse determined that all the
improperly finished undervoltage trip attachment parts were from one lot of
parts that was made exclusively for RCS by a subcontractor.  The same
subcontractor made a second lot of undervoltage trip attachment parts; this
second lot contained no improperly finished parts.  Westinghouse .

                                                            IN 93-09
                                                            February 2, 1993
                                                            Page 3 of 3


representatives stated that none of the defective parts have been used in
undervoltage trip attachments or DB-50 circuit breakers available on the open
market as commercial grade items.  Westinghouse informed the NRC that it has
notified all customers of record who have purchased undervoltage trip
attachments or Model DB-50 circuit breakers with parts from the affected lot. 

Westinghouse personnel recommended that the Salem licensee visually inspect,
for proper finish, the undervoltage trip attachment latches on all other Model
DB-50 circuit breakers at the Salem facility.  They also recommended that the
licensee develop criteria for replacing the undervoltage trip attachment when
the difference in the force generated before and after lubrication exceeds a
predetermined value.

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.


                                           ORIGINAL SIGNED BY


                                        Brian K. Grimes, Director
                                        Division of Operating Reactor Support
                                        Office of Nuclear Reactor Regulation

Technical contacts:  N. Fields, NRR
                     (301) 504-1173

                     S. Alexander, NRR
                     (301) 504-2995

Attachment:  List of Recently Issued NRC Information Notices
.
```

*Page Last Reviewed/Updated Wednesday, February 16, 2011*

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