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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 January 30, 1992 NRC INFORMATION NOTICE 92-09: OVERLOADING AND SUBSEQUENT LOCK OUT OF ELECTRICAL BUSES DURING ACCIDENT CONDITIONS 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 particular scenario identified by a licensee whereby electrical buses could be overloaded during accident conditions, deenergized, and prevented from being reenergized. 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 November 21, 1991, the Power Authority for the State of New York, the licensee for Indian Point Nuclear Generating Unit 3, reported that the vital buses could become overloaded during a loss-of-coolant-accident (LOCA) as a result of emergency operating procedures directing operators to restore non- essential loads to the buses if offsite power is not lost. The procedure would result in both emergency and non-emergency loads being powered concurrently from the same bus. The licensee's recent calculations reveal that performing this procedure could have overloaded buses. The resulting overcurrent condition could lock out a bus (i.e., deenergize the bus and prevent it from being reenergized from any source including the associated emergency diesel generator). Discussion A LOCA is generally analyzed as the most limiting condition in power demand for onsite electrical systems. When non-essential loads are added to the power system under this most demanding condition, electrical buses and breakers could be overloaded. The electrical protection system recognizes such an overloaded condition as an electrical fault on a bus and locks out the bus. The dc-powered breaker controls retain the faulted condition, and the emergency diesel generator will not be able to energize the bus until protective relays are manually reset. The scenario could result in disabling redundant trains of safety-related equipment. 9201240235 . IN 92-09 January 30, 1992 Page 2 of 2 This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact 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 contact: Thomas Koshy (301) 504-1176 Attachment: List of Recently Issued NRC Information Notices .
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