Equipment used for decades at California's troubled San Onofre nuclear power plant might have temporarily severed the plant's emergency power supply during an earthquake.
Operator Southern California Edison said Tuesday it's disabled the equipment and is investigating.
The company reported the issue to federal regulators as "an unanalyzed condition that significantly degraded plant safety."
Edison says other backup systems were in place.
At issue are vibration sensors in use since 1981 on emergency diesel generators. The generators start if the plant loses its outside power supply -- a possibility during an earthquake.
But engineers at the idled plant between Los Angeles and San Diego found the sensors might incorrectly shut down the generators during an earthquake.
The plant needs power to control heat in the twin reactors.