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Information Notice No. 92-45: Incorrect Relay Used in Emergency Diesel Generator Output Breaker Control Circuitry
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 June 22, 1992 NRC INFORMATION NOTICE 92-45: INCORRECT RELAY USED IN EMERGENCY DIESEL GENERATOR OUTPUT BREAKER CONTROL CIRCUITRY Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a potential problem that may exist at their facilities resulting from the failure of a relay in the control circuitry for an emergency diesel generator (EDG) output breaker to perform its intended design function. 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 information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On April 3, 1992, during the weekly idle start test of an EDG at the St. Lucie Plant, Unit 1, of the Florida Power and Light Company (the licensee), the control room operator was unable to close the output circuit breaker after the EDG had started, warmed up, and achieved rated speed. The licensee analyzed the control circuitry and found that a relay was not functioning properly for an unknown reason. The licensee sent the relay to Westinghouse for failure analysis. This relay is intended to prevent the output circuit breaker from closing when the generator is operating at a frequency below 54 Hz. However, the licensee found independent of the specific cause of the failure which occurred on April 3, 1992, that the relay had not been properly selected and was not able to perform its intended function. When operating as actually designed, the relay would permit the circuit breaker to close at all frequencies up to 62.5Hz. The licensee had not noticed this discrepancy during previous testing because the EDG usually achieves 90 percent of full voltage (the circuitry also includes an under-voltage permissive relay function) at almost the same time as it achieves 90 percent of full frequency. The licensee determined that the relay had been incorrectly selected because of an ambiguity in the Westinghouse catalog description for this particular relay (Style 177C717G03, 60 Hz/120 V). Westinghouse has since corrected the catalog description. The licensee corrected the error in relay selection by replacing the relay with another relay that properly performs the under-frequency permissive function. 9206060071. IN 92-45 June 22, 1992 Page 2 of 2 Discussion The EDG control circuitry is designed to prevent the output circuit breaker from closing prematurely. This circuitry includes under- frequency and under-voltage permissive relays to ensure that the EDG attains both 90 percent of rated frequency and 90 percent of rated voltage before the output circuit breaker closes. Under emergency conditions (e.g., accident conditions with loss of offsite power), use of the incorrect relay could allow the output circuit breaker to close before the EDG reaches the necessary operating conditions possibly resulting in shutdown of the diesel engine and/or damage to the safety-related loads. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Walter P. Haass, NRR (301) 504-3219 E. Nick Fields, NRR (301) 504-1173 Attachment: List of Recently Issued NRC Information Notices .
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