Bulletin 79-11: Faulty Overcurrent Trip Device in Circuit Breakers For Engineered Safety Systems
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 May 22, 1979 IE Bulletin No. 79-11 FAULTY OVERCURRENT TRIP DEVICE IN CIRCUIT BREAKERS FOR ENGINEERED SAFETY SYSTEMS Discussion: We have received information from Westinghouse and an NRC licensee relating to the potential failure of a circuit breaker in an engineered safety system of a nuclear power plant. This circuit breaker had a defect in one of its three time delay dashpots which resulted in a reduced time delay for overcurrent protection. The defect was a small hairline crack in the end cap of the dashpot. Further investigation by this licensee disclosed that 7 out of 17 spare dashpot end caps and 2 non-engineered safety feature breakers also had similar defects. The circuit breaker is a Westinghouse type DB-75. Westinghouse type DB-50 breakers also use the same type of dashpot and end cap. DB-50 and -75 breakers are used extensively in PWR's, and some BWR's may also have the same breakers. Similar make and model circuit breakers, when used for scram purposes, do not require the overcurrent trip feature and thus are not of concern. The end cap crack defect, if severe enough, could result in premature tripping of the circuit breaker because of insufficient time delay in overcurrent protection; i.e., the motor starting (inrush) current could cause the breaker to trip inadvertently and thus prevent the motor start. The defects reported by the licensee in April 1979, occurred in the replacement end caps which were provided to solve the problem described in IE Bulletin 73-1. The subject of Bulletin 73-1 was end caps made of a black phenolic material. As a result of that Bulletin, the black end caps were replaced with a new type made of fibre-filled polyester material called "navy-gray". Prior to the April 1979 report, there have been no reports of suspect "navy-gray" end caps either from scheduled testing or unusual behavior in service. The manufacturer of the "navy-gray" end caps believes the crack defects may be linked to a raw material batch problem. That is, the molding ingredient materials used may have neared the end of their shelf life before use. It is not believed the end caps, after fabrication, have a significant shelf life limit, due to the low residual stress and low crack propagation probabilities. . IE Bulletin No. 79-11 May 22, 1979 Page 2 of 3 Description of Event: The following information was obtained from the Licensee Event Report dated April 12, 1979 and a subsequent meeting with Westinghouse, the NRC staff and the licensee. During the 1979 surveillance tests, and a review of the previous refueling surveillance test results on a Westinghouse type DB-75 breaker (used as a 480v ESF bus supply breaker) a drift in the overcurrent trip time from the manufacturer's design minimum value of 6 seconds to 5.50 and 5.12 seconds was observed. The 1979 test results showed a deviation and inconsistency in the delayed trip timings among three consecutive tests. The overcurrent devices were removed from each of the three phases and a visual inspection indicated a hairline crack in the end cap of one of the devices. That cap was replaced (without checking for a possible crack in the replacement cap), and the breaker was again tested. This test also showed deviations and inconsistency in the delayed trip timings. The subsequent inspection of the devices revealed a hairline crack in the end cap which had just been installed. This prompted an inspection of the in-stock spare caps. Seven out of 17 caps were found to have similar cracks. Action Required of all Holders of an Operating License or Construction Permit: 1. Determine whether circuit breakers of the above described manufacture; and type with overcurrent trip devices are in safety-related Class IE service or in spares at your facilities. 2. If the subject breakers are in service in safety-related systems: within 30 days, review the existing test data for all overcurrent trip device calibrations since plant startup or since replacement caps were installed and tested in response to Bulletin 73-1, whichever is most recent. Determine if any delay times are: (1) outside of the acceptance band; (2) marginally acceptable - on the low side of the acceptance band; or (3) if any significant change in delay time performance has been observed. These breakers should be retested and end caps replaced as necessary to assure no loss of safety function. 3. Inspect all end caps in spares for cracks using at least a 3x magnifying glass. Caps having visible flaws should be discarded, or prevented from use in Class IE applications. 4. Review test procedures and test schedules for all safety-related circuit breakers to assure that all such breakers are tested at least each refueling outage to confirm overcurrent time delay protection. . IE Bulletin No. 79-11 May 22, 1979 Page 3 of 3 For facilities with an operating license, a written report of the above actions, including the date(s) when they will be completed shall be submitted within 45 days of receipt of this Bulletin. For facilities with a construction permit, a written report of the above actions, including the date(s) when they will be completed shall be submitted within 60 days of receipt of this Bulletin. Approved by GAO, B180225 (R0072), clearance expires 7/31/80. Approval was given under a blanket clearance specifically for identified generic problems.
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