Charles Coleman, a spokesman for Southern California Edison, said Thursday that the plant went offline at 3:38 p.m. Pacific as it is supposed to do when there is an electrical disturbance.
Coleman says there is no danger to any residents or employees at the plant, but it is not currently generating power.
The larger outage is affecting 500 to 2,000 SoCal Edison customers in southern Orange County and Riverside County.
Orange County sheriff's spokesman John McDonald says deputies have been dispatched to major intersections to direct traffic because traffic signals are out.
There have also been reports of people stuck in elevators.