United States Nuclear Regulatory Commission - Protecting People and the Environment


ACCESSION #: 9702100321



                                                  GE Nuclear Energy



                                   General Electric Company

                                   175 Curtner Ave., San Jose, CA 95125



February 4, 1997

96-32NRC.DOC

MFN 002-97



Document Control Desk

United States Nuclear Regulatory Commission

Washington, DC 20555



Subject:  Possible Failure of GE Model AMH 4.76-250 Circuit Breakers to

          Latch Closed



This letter provides information concerning the possible failure of GE

Model AMH 4.76-250 circuit breakers to latch closed due to an inability

to maintain the gap between the manual trip rod and the trip paddle.

These devices were originally manufactured by GE and supplied to

licensees for balance-of-plant applications.  Since the specific

applications and associated safety functions of potentially affected

circuit breakers are not known to GE-NE, we have transferred information

pursuant to 10CFR Part 21.21 (b) to those licensees known to be affected.

However, since we cannot assure ourselves that all end-users have been

notified, we are also providing the information to the NRC for

appropriate action.



GE Model AMH 4.76-250 horizontal drawout circuit breakers were

manufactured between 1967 and 1975 by the GE Switchgear Department in

Philadelphia, PA and Burlington, IA, and are currently supported by GE

Electrical Distribution and Control (GE-ED&C), Philadelphia Operation.



In late July 1996, a licensee reported failures of AMH 4.76-250 circuit

breakers to latch closed.  A failure investigation was completed in

September 1996.  This was considered to have been an isolated incident

until similar failures were reported by another licensee in mid-November

1996.



The results of the failure investigation indicated that a weak trip shaft

reset spring can result in the inability to maintain the gap between the

manual trip rod and the trip paddle.  The weak spring allows the paddle

to strike the buffer pad on the breaker frame with sufficient force to

deform the paddle, thus reducing the gap.  Loss of the gap can prevent

the trip latch from remaining in the reset position during a closing

operation, causing the breaker not to latch closed.  Licensee reports and

testing by GE indicate that the failure to latch may not repeat

regularly.  Even in circuit breakers where the gap has been reduced to

nearly zero, failures are intermittent.



If the condition remains undetected, the continued bending and

readjustment of the paddle will result in failure of the paddle and/or

its support.  This could result in loss of the ability to manually trip

the breaker.



Licensees known to have GE Model AMH 4.76-250 circuit breakers have been

advised that any circuit breaker that will not maintain the trip shaft to

paddle gap should be evaluated for this possible failure mode.



GE-ED&C Philadelphia Operation has prepared a modification kit to address

the failure.  This modification includes a redesign of the trip paddle

and its associated support, as well as the spring discharge link.  In

testing, this redesign has eliminated bending of the manual trip paddle

in circuit breakers with a weak trip shaft reset spring.



GE-ED&C Philadelphia Operation intends to issue a Service Advise Letter

(SAL) to describe the failure mode and aid in troubleshooting suspected

circuit breakers.  The SAL is scheduled to be issued by April 30, 1997.



If you have any questions, please call me at (408) 925-1019.



Sincerely,



Michael A. Smith, Manager

Safety Evaluations Project



cc:  G. C. Cwalina (NRC-NRR/DISP/PSIB)

     K. R. Naidu (NRC-NRR/DISP/PSIB)

     H. J. Neems (GE-NE)

     G. W. Sanders (GE-NE)

     N. C. Shirley (GE-NE)

     J. A. Steininger (GE-NE)

     PRC File



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