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 . 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 .ENDEND
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Page Last Reviewed/Updated Tuesday, March 09, 2021