A U.S. District Court jury found the Potomac Electric Power Co. and the District government negligent yesterday in failing to repair a traffic light at a busy intersection and awarded $540,000 to the family of a man killed in an accident there.

Lewis Long, an Air Force master sergeant stationed at Andrews Air Force Base, was killed at 13th and L streets NW on Aug. 2, 1982, after a car struck the Volkswagen van in which he was a passenger.

"The accident occurred at 4 a.m. on a Monday . . . . We found the first complaint on the lights was made at 4:30 p.m. July 31," said Norman Singer, the attorney for Long's widow in the wrongful-death suit.

Pepco and the District have filed complaints against each other in a dispute over whether the power company or the city was responsible for repairing the traffic light that weekend.

"We don't have people on duty on the weekend," said Mike Zielinski, an assistant deputy corporation counsel for the District.

The city maintains that it buys power from Pepco and that part of that arrangement is for the electric company to handle emergency problems during hours when city employes are not on duty, said Zielinski.

"The city is evaluating the jury's verdict to determine what to do next," he said.

Pepco spokesman Tom Welle said the company had "no comment on the case until the judge has made decisions on the cross-complaints between the city and Pepco."

The jury absolved the driver of the Volkswagen van of any responsibility for the accident, according to Singer. The second vehicle was a stolen car. The unidentified driver of that car fled the accident scene.