United States Nuclear Regulatory Commission - Protecting People and the Environment


ACCESSION #:  9702210229                                                  



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GE                                                      GE Nuclear Energy



                                   General Electric Company

                                   175 Curtner Ave., San Jose, CA 95125



February 17, 1997

96-34NRC.DOC

MFN 004-97



Document Control Desk

United States Nuclear Regulatory Commission 

Washington, DC 20555



Subject:  Possible Failure of Hex Head Set Screws in Older GE Type AK-15

          and AK-25 Circuit Breakers



This letter provides information concerning the possible failure of the

hex head set screw that attaches the buffer paddle to the trip shaft of

GE Type AK-15 and AK-25 circuit breakers manufactured prior to 1976. 

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 Nuclear Energy, 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 Type AK-15 and AK-25 circuit breakers were originally manufactured by

the GE Switchgear Department in Philadelphia, PA and Plainville, CT, and

are currently supported by GE Electrical Distribution and Control,

Burlington, IA.



Two licensees reported to GE in late 1996 that they had found broken hex

head set screws on GE Type AK-25 circuit breakers.  This set screw is

used to attach the buffer paddle to the trip shaft on older AK-15 and AK-

25 circuit breakers by fastening through a lock nut, the buffer paddle

and a square nut, and tightening against the trip shaft.  The screw is

torqued to 20 to 22 inch-pounds with the lock nut loose, and the lock nut

is then torqued to 20 inch-pounds.



In both of the reported failures, the set screw broke at the top of the

lock nut.  Because the lock nut was tight, the screw did not loosen and

circuit breaker operability was not adversely affected.  Based on testing

performed by the licensees, it appears that the breakage resulted from

hydrogen embrittlement due to improper heat treatment when the set screws

were plated.





The hex head set screw was used in this application on AK-15 and AK-25

circuit breakers and their variants (AKF and AKU) for many years, but the

design was changed to a slotted round head screw in 1975.  GE Nuclear

Energy (GE-NE) is not aware of any problems with the slotted round head

screw.



The presence of the hex head set screw is best determined by visually

inspecting the buffer paddle.  Since this may not be possible with the

circuit breaker installed in its cubicle, a screening process can be used

based on the date code on the circuit breaker nameplate.  Most circuit

breakers have a two-later date code stamped after the serial number on

the nameplate.  In some instances, the two-letter date code is stamped

elsewhere on the nameplate.  The affected date codes are:



Table omitted.



                                    2





In both of the reported cases, the hex head screw broke above the lock

nut, and the circuit breaker continued to operated normally.  However, it

is possible that the portion of the screw that breaks off could fall into

the circuit breaker mechanism and cause binding.  It is also conceivable

that the screw could break at another point, resulting in the buffer

paddle becoming loose on the trip shaft.  This could cause the mechanism

to bind and/or fail when called upon to trip.



Since these circuit breakers were supplied for balance-of-plant

applications, it is not possible for GE-NE to evaluate the safety

significance of potential failures to determine if a defect exists in the

context of 10CFR Part 21.



GE-NE recommends that the hex head set screw be replaced with the slotted

round head screw.  The GE part number for the slotted round head screw is

N37P14008B for non-safety related applications or QN37P14008B for safety-

related applications.



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)

     J.  M.  Austin (GE-NE)

     N.  C.  Shirley (GE-NE)

     J.  A.  Steininger (GE-NE)

     PRC File



                                    3





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