The troubled San Onofre nuclear power plant operated for three decades with equipment that might have temporarily cut off the plant's emergency power supply in the event of an earthquake, government filings revealed Tuesday.

The disclosure by Southern California Edison about a possible backup power problem comes amid a probe into excessive wear on tubing that has sidelined the seaside plant for nearly four months.

The company disabled the equipment, a vibration sensor, and reported to federal regulators that the problem was being analyzed as a threat to plant safety. Other back-up systems were in place during that time.

"Engineers are continuing to analyze the condition and have not reached a final conclusion if the sensor would actually cause a shutdown during an earthquake," a company statement said.

A steady supply of electricity is a critical issue at nuclear plants, which need power to control heat in the reactors. A tsunami destroyed backup generators at the Fukushima Dai-ichi nuclear power plant _ setting off the world's worst nuclear disaster since Chernobyl.

At issue at San Onofre is a vibration sensor in use since 1981 on emergency diesel generators, which start if the plant's outside power is cut _ a possibility during an earthquake.

Engineers found the sensor _ designed to protect components inside the generators during operation _ might incorrectly stop them during an earthquake or an aftershock.

According to records filed with the Nuclear Regulatory Commission, Edison engineers are looking into whether "high vibration ... could interrupt the onsite electrical generation" during a temblor.

If the generators fail, the plant can use battery power for up to four hours to operate steam generators to cool the twin reactors.

The plant between Los Angeles and San Diego has been dark since January while investigators try to determine why tubing that carries radioactive water in relatively new steam generators eroded at an unusual rate, in some cases rapidly.

Gradual wear is common in such tubing, but the rate of erosion in some tubes at San Onofre alarmed officials since the equipment is relatively new. The company has said 1,300 tubes will be taken out of service, although the number is well within the margin to allow the generators to keep operating.

Edison initially targeted a June restart for at least one of the twin reactors, but that appears increasingly unlikely as investigators continue to review the widespread problem.