United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-43: Supplement 1: Mechanical Interference with Thermal Trip Function in GE Molded-Case Circuit Breakers

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               March 13, 1991


Information Notice No. 90-43, SUPPLEMENT 1:  MECHANICAL INTERFERENCE 
                                                 WITH THERMAL TRIP FUNCTION 
                                                 IN GE MOLDED-CASE CIRCUIT 
                                                 BREAKERS


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This supplement is intended to alert addressees to new information received 
from the General Electric Co. (GE) regarding the problem identified in NRC 
Information Notice (IN) 90-43, "Mechanical Interference With Thermal Trip 
Function in GE Molded-Case Circuit Breakers."  The new information could 
potentially expand the scope of the problem and limits the conclusiveness of 
the previously identified screening test.  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 supplement do not constitute NRC requirements; 
therefore, no specific action or written response is required.

Background:

IN 90-43 addressed a manufacturing deviation in GE types TED and THED, 
3-pole, molded-case circuit breakers that has caused failures of the thermal 
overcurrent trip function on pole C of those breakers when they are equipped 
with undervoltage release devices (UVRs).  The manufacturer, GE Electrical 
Distribution and Control (ED&C), determined that the cause of the failures 
was improper installation (misorientation) of the calibration screw spring 
clips on the thermal trip elements of the breakers at the factory.  The 
misoriented spring clips caused mechanical interference between C-pole 
thermal trip elements and installed UVRs.  UVR testing did not detect this 
condition because misoriented spring clips interfere only with thermal 
overcurrent trip operation and not with the operation of the UVR.  In 
addition, overcurrent trip testing was not being done after UVR 
installation.

Description of Circumstances:

Since the issuance of IN 90-43, GE ED&C instituted overcurrent trip testing 
of breakers following installation of internal accessory devices (including 
UVRs) at its accessory installation facility.  Subsequently, GE Nuclear 
Energy (GENE) informed the NRC that successful testing of the thermal 
overcurrent trip function of a UVR-equipped breaker with misoriented spring 
clips does not guarantee 


9103060369 
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                                                    IN 90-43, Supplement 1
                                                    March 13, 1991
                                                    Page 2 of 3     


that the breaker will not fail subsequently on its C-pole.  GENE also 
reported that the population of breakers that could have the misoriented 
spring clips was much greater than previously believed.

Discussion:

IN 90-43 discussed ED&C examination and testing of UVR-equipped, types TED 
and THED breakers that had failed pre-installation testing at the Oyster 
Creek Nuclear Generating Station.  This testing of specimens from a 1989 
GENE shipment to Oyster Creek implicated the misoriented spring clips.  
Subsequently, Oyster Creek returned the rest of the shipment to ED&C for 
testing and repair.  ED&C determined that the shipment was part of a large 
population of breakers believed to contain misoriented spring clips.  After 
testing all the breakers, ED&C repaired the failed breakers and returned to 
Oyster Creek those that passed the trip test without repairing them (i.e., 
without repositioning the spring clips).  However, in November 1990, GENE 
informed the NRC that when Oyster Creek retested the unrepaired breakers, 
some failed the same tests that they had reportedly passed at ED&C.

ED&C postulated that the unrepaired breakers (i.e., those that had passed 
the ED&C testing) failed at Oyster Creek because of tight clearances and 
minor shifts in component positions during handling.  This combination may 
have created an interference condition that was not present previously.  
Nevertheless, these test failures on breakers that previously passed 
indicate that as long as the breakers still contain misoriented spring 
clips, they are prone to failure on the C-pole if UVRs are installed.  
Therefore, passing a thermal overcurrent trip test is no longer considered 
conclusive for UVR-equipped breakers of the affected types with respect to 
the absence of a potential interference condition.

As noted in IN 90-43, GE ED&C has stated that the problem is limited because 
the misoriented spring clips can affect only the thermal overcurrent trip 
function on the C-pole of UVR-equipped, 3-pole, types TED and THED breakers.  
The thermal trips on poles A and B and the instantaneous magnetic trips on 
all three poles remain unaffected.  Although GE concluded that misoriented 
spring clips could only prevent a UVR-equipped breaker from tripping on a 
low-level overload solely on phase C, the combined current from multiple 
single phase loads connected to phase C (particularly if in excess of the 
design load factor) is one condition that could conceivably produce such a 
single phase overload.  

The NRC recently learned that Oyster Creek has taken effective corrective 
action by sending all the UVR-equipped, types TED and THED, molded-case 
circuit breakers in question back to the manufacturer, GE ED&C, where they 
were all checked for misoriented spring clips and repaired as necessary.

.

                                                    IN 90-43, Supplement 1
                                                    March 13, 1991
                                                    Page 3 of 3     


This information notice supplement requires no specific action or written 
response.  If you have any questions about the information in this 
supplement, please contact the technical contact listed below or the 
appropriate NRR project manager. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contact:  S. Alexander, NRR
                    (301) 492-0995

Attachment:  List of Recently Issued NRC Information Notices
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