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| Entered date | Event description | |
|---|---|---|
| ENS 41168 | 2 November 2004 20:22:00 | On November 2, 2004, at 11:05 PST, with Diablo Canyon Unit 2 in Mode 6 (Refueling), Emergency Diesel Generator (EDG) 2-1 auto started on an unplanned actuation signal from a valid 4160 volt Bus G undervoltage signal. All equipment responded as designed. After the EDG started, the auxiliary feeder breaker opened, and loads were automatically sequenced onto the EDG. At the time of the event, test equipment was being connected in preparation for an instrumented manual test start of EDG 2-1 prior to maintenance. On November 2, 2004, at 1708 PST, operators transferred Bus G to auxiliary power and shutdown DG 2-1. Prior to the event, Bus G was being supplied by auxiliary power, with startup power cleared for planned maintenance. Bus G was being prepared to be cleared for maintenance, therefore, required equipment was in-service on the other buses. Buses F and H were unaffected by this event and remain operable on auxiliary power, with EDG 2-2 (Bus H) and 2-3 (Bus F) operable. The following decay heat removal trains are powered from Bus G: Residual Heat Removal Pump 2-1 remained in standby, Component Cooling Water Pump 2-2 started on the transfer to EDG, and Auxiliary Saltwater Pump 2-2 re-started. Unit 2 is in day 9 of a refueling outage, with the reactor head removed, the refueling cavity filled, and the upper internals installed with rods latched. Unit 1 was unaffected and continues to operate in Mode 1 (Power Operation) at 100 percent power. The cause of the Bus G undervoltage signal is being investigated. The licensee stated that no undervoltage was seen on any other equipment but the undervoltage relays on Bus G did sense an undervoltage. Residual Heat Removal was unaffected by the event. Bus G has been returned to its pre-event configuration and is considered operable. The other EDGs were fully operable at the time of the event and there were no significant LCOs at the time. All systems functioned as required. The licensee has notified the NRC Resident Inspector |