A hole the size of a quarter in insulation around a power cable set off the chain reaction Tuesday that plunged Capitol Hill and parts of downtown Washington into darkness from a massive power failure, Potomac Electric Power Co. officials said yesterday.

The company replaced eight feet of the 69,000-volt cable -- at a cost of about $4,000 -- to repair the "fault" that ultimately caused chaos in 18 federal buildings, portions of the Metrorail system and hundreds of homes around Capitol Hill. Power was restored in about one hour at most locations, after an incident that caused the largest single loss of energy in the downtown area that Pepco officials could recall.

The city has experienced a rash of electrical problems in the past week, including Tuesday's power failure, electrical equipment explosions inside the Washington Hilton Hotel and blasts that sent manhole covers flying at busy downtown intersections on two separate occasions.

But David W. Masters, vice president of Pepco's electric system, said that the incidents are unrelated. "There is nothing going on in the electrical system that is out of the ordinary," he said yesterday. The Pepco system, Masters said, is viable and well maintained.

According to Masters, Pepco's monitoring equipment "worked exactly as it's designed to work" during the power failure and prevented an even more extensive power shutdown.

The small hole in the insulation around a huge feeder cable at a Benning Road power plant allowed an electrical charge to escape into the ground. That created a power surge in three 69,000-volt feeder cables that carry power to a series of substations, including one on New Jersey Avenue at E Street NW that serves the Capitol Hill area.

The surge was read by monitoring equipment as a problem on the three lines, and it automatically shut them down. Their electrical power 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 Pepco customers.

According to Masters, Pepco has not determined the exact cause of the manhole blasts that occurred Tuesday shortly after the power failure, and last Thursday, when several covers went flying 10 feet into the air during an underground explosion and fire at Pennsylvania Avenue and 20th Street NW.

Masters said the company has a maintenance program to check its thousands of manholes for problems, and that such blasts occur infrequently. "It seems like a string of bad luck . . . because they've occurred back to back," he added. The blasts, he said, can be set off by tiny insulation problems, and occur in underground electrical systems throughout the country.

A spokesman for Boston Edison, for instance, said manhole blasts disrupted service in downtown Boston last summer. "The manholes are inspected on a regular basis," he said. "But one could test perfectly fine today, and you could have it happen tomorrow. There is no way to predict an explosion."

Tuesday's power failure shut down parts of Metro's automatic control system, and service was disrupted when Red Line trains were forced to run at sharply reduced speeds. Because of similar problems in the past, Metro has decided to purchase a backup generator, a spokesman said yesterday.