Malfunctions in two of the three main feeder lines at a Pepco substation caused power failures at six government buildings in the Foggy Bottom area yesterday afternoon.

Shortly before 3 p.m. yesterday, power failed at the Old Naval Hospital, the Federal Reserve Board Annex, the Potomac Sewage Pumping Station, the Office of Personnel Management and the departments of State and Interior.

"It just went off," said Lucy Young, a worker in the Office of Personnel Management. "The lights, the computers, phones, elevators, everything just went black."

Pepco spokeswoman Nancy Moses said it initially appeared that two of the three 69,000-volt feeder lines at the utility's F Street substation failed.

The first feeder failed yesterday morning when a private contractor dug into a cable in Virginia. At 2 p.m., the second feeder dropped out of service for reasons not yet known.

With the entire load of electricity being carried by the single remaining line, Pepco shut down service to some of the substation's customers in order to avoid a more widespread failure.

Many government employes in the affected buildings were sent home for the day. Some workers at the Interior Department were trapped in elevators but were evacuated by 4 p.m. By that time, most of the power service had been restored, Pepco said.