Part 21 Report - 1997-110
ACCESSION #: 9702100321
GE Nuclear Energy
General Electric Company
175 Curtner Ave., San Jose, CA 95125
February 4, 1997
Document Control Desk
United States Nuclear Regulatory Commission
Washington, DC 20555
Subject: Possible Failure of GE Model AMH 4.76-250 Circuit Breakers to
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
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
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
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.
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)
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