Utility officials say one of several redundant security systems used to monitor the grounds of Southern California's San Onofre nuclear plant stopped working for 45 minutes, triggering a declaration of the lowest level of emergency.
Gil Alexander, a spokesman for the plant's operator, Southern California Edison, said an "unusual event" was declared at 6:12 a.m.
Although the security system was restored within 45 minutes, the Orange County plant remained on unusual event status until 9:50 a.m.
Alexander said the failure was not related to nuclear operations and no part of the plant's perimeter was left unguarded.
Edison did not release details of the incident or say what kind of equipment was involved.
Officials were still trying to determine the cause of the failure Saturday afternoon.
Alexander said the unusual event declaration required immediate notification of the Nuclear Regulatory Commission