Two employes of Potomac Electric Power Co. were seriously burned yesterday morning when electrical equipment they were servicing at a downtown Washington office building exploded and set their clothes on fire, authorities reported.

While a building engineer tried to extinguish the flames and rip the fiery clothes off one of the men, witnesses said they heard other explosions from the penthouse of International Square, 1850 K St. NW, that rocked the structure and forced the evacuation of the 13-story building and adjoining offices at 1825 and 1875 I St. NW.

"It shook the whole building," said Gail McGinley, a secretary in the law offices of Donovan, Leisure, Newton and Irvine on the 12th floor, directly below the blast.

"There were two explosions by the time I hit the stairs, and you could hear more explosions down the whole stairwell," McGinley said.

The injured workers were taken by fire department ambulance to the Washington Hospital Center Medstar unit.

They were identified as Wesley W. Lott, 41, of Waldorf, who was listed in critical condition with second and third degree burns over 50 percent of his body, and David W. Shatrowsky, 27, of Greenbelt, who was in serious condition with second degree burns over 13 percent of his body, a hospital spokeswoman said.

In addition, according to a fire department spokesman, a 27-year-old woman was taken by ambulance to the George Washington University Medical Center, where she was treated for a minor foot injury suffered when she fell while evacuating the building.

Wesley Gilmer, 58, a building engineer at International Square, one of the District's largest office buildings, said he was sitting in an office next door to the transformer when the explosion occurred about 11:45 a.m.

"I had just opened a sandwich when I heard the explosion. It sounded like a cannon," he said, and he initially thought it might be a terrorist bombing.

"Then I heard the fire alarm and I opened the door and these two guys were on fire and running around," he said. Gilmer said he immediately got a fire extinguisher and began spraying the men.

"One guy got his jacket off, but the other guy's nylon jacket just melted," and Gilmer said he helped him tear it off. "Underneath, his T-shirt was smoldering, and I pulled that off. He was still hollering to take the rest of his clothes off," said Gilmer, who said he suffered minor burns to his hands.

"It was horrible," he said. "I'm still shaking."

Police closed K Street between 18th and 19th streets NW for about an hour, causing minor traffic tie-ups during lunch hour, while fire officials and officials from Pepco investigated the blast.

Pepco spokeswoman Nancy Moses said Lott, who has worked for the company for 18 years, and Shatrowsky, employed there since 1981, were "checking the voltage levels on one of our 480-volt, energized network protectors" at the time of the explosion, which she described as "an electrical flash that kind of created a fireball."

She said the building has five "network protectors" that act as a sort of circuit breaker, automatically monitoring power between five 13,000 volt feeders into the building and transformers that reduce the voltage to levels acceptable for commercial use.

Fire officials said there was minimal damage to the building.

Moses said the $20,000 network protector that exploded will be replaced, and that Pepco officials are investigating what caused the explosion.