The Potomac Electric Power Co., looking for causes of two major power outages last week that disrupted service to thousands of consumers, has concluded that the explosion of a new million-dollar transformer was a highly significant breakdown. It considers the other accident, in which an underground cable caught fire, to be more routine.

The total cost of the two incidents to Pepco -- and ultimately to its customers -- has not been established. The incidents were unrelated and neither is expected to have any lingering effect on customer service, according to Lee Himes, manager of substations.

While Pepco is investigating both accidents, they are concentrating on finding the precise cause of the explosion and fire that rocked a transformer near the Southwest Washington riverfront at 2:20 p.m. Thursday and cut power to 13,000 homes and businesses in Northwest, Bethesda and Chevy Chase for 2 1/2 hours.

At 4:03 p.m. Saturday an underground power line beneath Columbia Road began burning, damaging lower voltage cables and forcing Pepco to turn off power to about 3,000 customers in the area for up to seven hours.

"The cable failure was dramatic and it affected a lot of customers, but from the classification of a cable failure, it is not that unique an event," Himes said. He said the company has "a few cable failures a week of all kinds." Typically, those failures don't result in any loss of service or only a limited loss of service, he said.

But in contrast to the cable failure, the transformer explosion was a first for Pepco, said Himes, an engineer who has been with Pepco for 24 years. "We have never had that happen before," he said.

Himes said that transformers within the electric untility industry fail "on a rather infrequent basis." When they do fail, the damage typically is hidden inside the unit.

The damage to the Pepco transformer, however, is clearly visible. Heat from the fire was so intense that it deformed the steel structure supporting an oil-filled expansion tank and caused the tank to fall about 10 feet to the ground.

Harold Reddick, director of engineering for the D.C. Public Service Commission, toured the Buzzard Point substation the day after the explosion and found there was "substantial damage." Reddick said Pepco must file a report on the incident with the Commission.

Engineers for Pepco and for Siemans-Allis Manufacturing Co., the Atlanta-based company that manufactured the transformer, are now dismantling it to determine what triggered the explosion and fire. But it could be several weeks before a final conclusion is reached, officials said.

The transformer is an electromagnetic device used to lower the voltage of electricity coming from Pepco's power generating plants so it can be disseminated to various substations throughout the system. It was added to the Pepco system to increase the company's ability to bring low-cost, coal-fired generation into downtown Washington and to hold down the company's overall operating cost.

Pepco was able to restore power to customers after the Buzzard Point transformer explosion by using other transformers in the system. Himes said the damaged transformer will be repaired if that is possible; if not, it will be replaced.

"The whole problem should be resolved by the spring of 1983," he said.

In the meantime, Pepco is searching for a substitute transformer.