The D.C. Public Service Commission has begun investigating the July 30 Capitol Hill blackout and several other power-related incidents to determine "whether Pepco is providing adequate and safe electrical service to District customers," the regulatory agency announced yesterday.

The investigation, ordered by PSC Chairman Patricia Worthy, could result in a recommendation for the utility to build a costly backup system that would result in higher rates for consumers, or for tighter preventive maintenance procedures, according to Howard C. Davenport, the PSC's general counsel. Davenport said he couldn't estimate how much it would cost to build such a system, which could include construction of additional circuits.

An hour-long power outage the morning of July 30 plunged Capitol Hill and parts of downtown into darkness, forcing federal and District workers out of buildings, stalling commuters in Metrorail stations and snarling traffic. The blackout, the largest single loss of energy in the central city in memory, was part of a recent rash of electric explosions and outages, including incidents at Sixth and D streets NW, and at 20th Street and Pennsylvania Avenue NW, in which explosions displaced manhole covers and disrupted traffic.

"Incidents disrupting important governmental and congressional activities require thorough investigation," Worthy said.

The three-member PSC regulates utility rates in the District and has authority to demand information from utilities. Its office of engineering will carry out the investigation.

Nancy Moses, a Pepco spokesman, said the utility would "cooperate fully" with the investigation, but said she did not know what Davenport meant by a "backup system."

Moses said Pepco had been informed by the PSC of an "informal investigation," but did not receive notification of the formal inquiry until after the commission issued press releases announcing the action.

Moses said the company expected the investigation to confirm Pepco's "long history" of reliable service. She said the Capitol Hill outage was "an unavoidable incident" involving a faulty underground cable.

The Capitol Hill power failure, which forced 17,000 federal workers out of 18 darkened buildings, coincided with the manhole explosion at Sixth and D NW, which sent 50-pound metal covers flying.

At the time of the blackout, Pepco officials said that it was caused by a small hole in the insulation around a huge feeder cable at the Benning Road power plant. The fault in the cable caused a power surge in three 69,000-volt feeder cables that resulted in a shutdown of power in the three. The power then was shifted automatically to a fourth cable, which overloaded and shut down, causing the blackout.

Davenport said there was no apparent relationship between the blackout and the manhole cover incidents. He said the PSC's decision to order an investigation was unusual and was necessary because of the scope of the power outage.

"The commission has deemed it appropriate to have its PSC staff examine what went wrong and how it was that three circuits went down and one circuit was overloaded," Davenport said.

He said the investigation would determine what the additional costs of a backup system would be.

The outage and manhole explosions July 30 came just four days after an underground explosion near 20th and Pennsylvania blew manholes 10 feet into the air and forced the evacuation of a two-square-block area around the site.

Electrical explosions at the Washington Hilton Hotel July 27 and 28 were not related to the manhole explosions and the Capitol Hill outage and are not part of the inquiry.