A massive power failure yesterday plunged Capitol Hill buildings and parts of downtown Washington into darkness -- forcing 17,000 federal workers out of 18 blackened buildings, stalling commuters in Metrorail stations, snarling traffic and leaving the U.S. Senate to convene in a chamber eerily illuminated by a single light.

The U.S. Capitol went black at 8:56 a.m., along with an area covering 140 city blocks, when a "fault," or errant electrical charge, in a Potomac Electric Power Co. line set off a chain reaction that ultimately cut power at the New Jersey Avenue substation that serves customers on and around Capitol Hill.

In the Capitol, emergency generators kicked on almost immediately to provide what officials called "safety lighting." One such light allowed the Senate to convene at 9:30 a.m. -- with senators called to the chamber by the banging of a metal wastebasket rather than the ringing of electrically powered bells that normally signal the session's start. "We work in the dark most of the time anyway," quipped Senate Majority Leader Robert J. Dole (R-Kan.).

Other parts of the city were not so fortunate. Riders on the subway system were diverted from their destinations, with many ordered off trains and onto crowded buses for the trip downtown; prisoners awaiting trial in D.C. Superior Court sweated in stuffy basement cells, and the D.C. fire department raced to 10 buildings to rescue people trapped in stalled elevators.

Pepco officials said that they restored power shortly after 10 a.m., but that some buildings with circuit breakers that needed to be reset experienced longer blackouts. About 2,500 homes in Northeast had been affected. No injuries were reported.

Meanwhile, a second and possibly related incident set off fires and small explosions in underground power lines at two Northwest intersections, sending manhole covers flying from the streets -- an incident similar to one Thursday near Pennsylvania Avenue and 20th Street NW that forced closure of streets and the evacuation of nearby buildings.

The District has experienced a rash of electrical power problems in the past week, including the blast that sent manhole covers flying and two successive electrical equipment explosions at the Washington Hilton that forced the huge hotel to shut its doors Saturday. Pepco officials said yesterday that the incidents are not related.

They pointed out that the Hilton explosions occurred in a private building and did not involve the power company's equipment. The underground explosions are still under investigation, but Pepco officials said they believe that low-voltage cables sparked a fire and explosion in high-voltage feeder lines.

David W. Masters, vice president of electrical systems for Pepco, said that the company has regular inspection and maintenance programs for all of its high- and low-voltage equipment and that the recent problems are "coincidental."

"We got a little string of bad luck," he said.

A D.C. fire official yesterday likened the recent series of electrical problems to fires, which sometimes go in cycles. "We might go for a long time without any fires, and then get hit with a whole bunch right in a row," he said.

All over the city yesterday there were scenes spawned by the blackout. Computers at the D.C. Municipal Center crashed. Comptroller General Charles A. Bowsher, about to make a major announcement to an audience of division chiefs from around the nation, found himself in pitch-blackness at the General Accounting Office, where workers were later sent home for the day. At other buildings, federal employes were banished to plazas and sidewalks until power was restored. Telephone bells went mute all over Capitol Hill, and members of Congress enjoyed an impromptu "candlelight breakfast" in the Capitol with the Turkish ambassador.

"There was a sense of excitement in the halls, with everything pitch-black," said Ann Pincus, press aide to Sen. Charles McC. Mathias Jr. (R-Md.). "It was just like a terrible snowstorm when everyone gets stranded together."

Tourists and workbound commuters on the Metro system fared perhaps the worst when some underground stations were plunged into darkness and service on the Red and Yellow lines was disrupted. Metro officials said the power failure did not affect the electricity needed to run the trains but did shut down parts of the automatic control system, forcing the trains to run at greatly reduced speeds. Hour-long delays occurred as trains crept between the Rhode Island and Dupont Circle stations, which bounded the affected areas, according to one official.

Trains that were outside those points when power failed were not allowed to proceed farther, and commuters were told to get off and take buses, the official said. Power to lights, Farecard machines and escalators in three Red Line stations also failed, forcing the use of battery-powered backups. At Judiciary Square, the batteries ran down, cutting even the auxiliary power.

Associated Press reported that a train with 200 passengers halted 100 feet short of the Judiciary Square station and remained there for nearly an hour. When it finally arrived at the station, passengers picked their way to immobile escalators, their paths illuminated only by flashlights held by Metro police.

Metro officials said an apparently unrelated malfunction in a switch at L'Enfant Plaza, just before the Pepco failure, forced them to halt Yellow Line service as a precaution. Passengers were told to use the Blue Line, which provides a less direct route to most of the same stations.

Meanwhile, Red Line commuters forced off their trains gave bleak reports of the ride downtown. Myra Lyman, who boarded a train at Silver Spring, reported a 20-minute delay, then an announcement that the train would not be moving.

Lyman hitched a ride to Dupont Circle, where she picked up a bus. "There were a lot of older people around who couldn't get rides like me," she said. "And the Metro guys don't even say, 'Sorry for the inconvenience.' That's what's so upsetting."

The havoc, according to Pepco officials, can be laid to an errant electrical charge that ultimately caused the power shutdown. Officials said the problem began at a power plant in the 3400 block of Benning Road NE that sends power through huge, 69,000-volt feeder lines to a series of substations, including the one on New Jersey Avenue at E Street NW that serves the Capitol Hill area.

The errant electrical charge created a power surge that was incorrectly read by monitoring equipment as a problem on three of the four huge feeder lines serving the New Jersey Avenue substation, automatically shutting them down. Their electrical charge was shifted to the fourth line, overloading it and shutting it down, too. That killed power running from the substation to 28 smaller lines that serve the substation's customers.

Though officials were still investigating the cause of the explosions that blew manhole covers at Sixth and D streets Northwest and Fourth and H streets Northwest, they said it may have been related to the massive failure.

The manhole blasts caused traffic tie-ups at those intersections, while the massive power failure shut down traffic signals at intersections throughout much of the affected area, according to D.C. officials. Some of the lights were out until 2 p.m., according to Charles Stuart of the Bureau of Traffic Services.

While the Capitol, the Supreme Court and three Senate office buildings were plunged into darkness, security measures were turned on full force. "We simply closed the buildings to everyone except staff," said U.S. Capitol Police Capt. Robert Howe. "Security was never in jeopardy."

One Senate staff member said guards turned their flashlights on the staff badges of aides walking into the building, then double-checked by shining the lights on their faces. Volunteers from the American Association of Retired Persons, who were trying to deliver jars of Lifesavers to senators, had their jars hand-searched by guards. Normally, the jars would have been run through the X-ray machines at building doorways.

Emergency generators kept power going to essential operations in the Capitol -- safety lights and the four elevators reserved for senators and House members. "They have to move around," said Elliott Carroll of the Capitol Architect's office.

Senate staff members in the Hart Building, where the halls are illuminated by a sweeping skylight, were inconvenienced too. "The Coke machines were down," said intern Randy Ellis. "And the coffee machines were off, so a caffeine panic ensued."

Meanwhile, nervous law students taking the D.C. bar examination at nearby Georgetown University were further unnerved when the power failed in the test room. That part of the exam was rescheduled for Thursday.

"A few people acted like lawyers and got hot and bothered about it," said Katherine McManus, one of the test-takers. "But most people took it real well and were just resigned to it. What else could you do?"