Two Pepco transmission power lines failed in a 17 1/2-hour period, forcing the company to cut service yesterday to some downtown offices for an hour and to limit electricity use to other downtown customers.

The problems come at a time when scorching temperatures and high humidity have meant heavy demands for power. Since July 18, temperatures in Washington have reached the 90s every day except Aug. 1, when the high was 87.

Late yesterday, about nine buildings were still without electricity, but a Potomac Electric Power Co. spokesman said the company hoped to have repairs completed when most downtown offices open today. The cause of the failures, which decreased the output of power at a substation at 1616 L St. NW, was not known late yesterday.

While the power outage, from 11:20 a.m. to 12:30 p.m. yesterday, meant only mild inconveniences for most Pepco customers in a 16-block area, the potential for greater problems kept utility crews working and company clerks calling building managers and asking them to turn off unnecessary lights, business equipment and air conditioning.

In downtown Washington yesterday, workers left darkened buildings and ventured out into the midday sun as temperatures climbed into the 90s. Most returned to warmer offices where air conditioners were turned off or adjusted a few degrees higher.

The two failed power lines also meant longer waits for elevators, because some building elevators were shut down, longer lunches or early departures. Metro service was unaffected, but lights were out at some downtown intersections, where District police officers directed traffic.

Most workers were back in their offices or on their way home when scattered thunderstorms began about 2 p.m. After the rain, the area got a reprieve as temperatures dropped to 74 by 4:30 p.m.

The L Street substation distributes power to Pepco customers between 15th and 18th streets NW and H and M streets NW. Pepco was forced to disconnect lines for an hour to businesses in an area between 16th and 18th streets NW and H and K streets NW.

The first sign of a problem came Tuesday when one of the transmission power lines went out at 6 p.m. The major problem came when the second line failed at 11:15 a.m. yesterday as a crew was repairing the first. Both lines run from a generating station to the L Street substation, where electricity is broken down and carried through distribution lines to downtown customers.

A total of four 33,000-volt transmission power lines run into the substation, and with two out, the remaining two lines could not provide enough electricity for the heavy summer loads, Pepco spokeswoman Venetia Miles said. To avoid a greater outage, Pepco disconnected some of the distribution lines that lead to customers.

While the lines were disconnected, the utility company called customers and asked them to curtail electrical use so power could be restored to everyone.

"Without the deliberate disconnection, the outage would have spread throughout the entire area served by the substation and there would have been additional damage, which would have taken longer to repair," Miles said.

Baltimore Gas & Electric Co. experienced a similar problem on Tuesday when a cable fault on a distribution line in the Glen Burnie area caused the line to fail. About 4,300 customers were without power for one to two hours.

Virginia Power reported no outages yesterday.

At The Washington Post's 15th Street headquarters, some employes were sent home, some lights were cut off, and air conditioning, computer and elevator use were reduced. The presses at the downtown office were idled as Pepco crews worked to restore power, forcing The Post to print all of its early edition papers at the company's presses in Southeast and in Springfield.

Andy Harteveld, The Post's Northwest plant manager, said customers' home delivery of the newspaper should not be delayed by the electrical problems.

Pepco spokeswoman Nancy Moses said late last night that power was restored to one of the two supply feeders about 11:40 p.m. Crews hoped to restore power to the other supply feeder before sunrise, she said.

At AFL-CIO, 815 16th St. NW, employes stumbled down dark stairways when the electricity to the building went off at 11:20. Christine Gautreaux sat in the lobby working on a rug-hooking project.

"I can't type, can't Xerox and can't see in some offices because there are no windows," she said. "Thank goodness I had something to do. This could get boring."

While full service to most buildings was restored by 12:30 p.m., other buildings received partial service and a few buildings were left with no electricity at all. The amount of service depended on whether a building received all of its electricity from the L Street substation or whether part of its service came from another location.

A 12-story office building at 1050 17th St. NW used electricity from its own emergency generator. Most workers elected to leave early.

But Y.C. Park and C.C. Kim, owners of Imperial Trade Liquor in the building, stayed in the dark store, trying hard to peddle cold beer and champagne and cheese.

"We asked when was the power going to come back and they said, 'No idea.' We asked why and they said, 'No idea,' " Park said. "We will stay open as long as we can stay inside. We still have pretty cold beers and champagne, but the ice is melting." Staff writers Kuae Noel Kelch, Anne Simpson and Elizabeth Tucker contributed to this report.