An electrical flash fire last night in the basement of the Southwest Washington building that houses the U.S. Department of Education and NASA headquarters destroyed electrical switching equipment serving about a quarter of the building, officials said.

Ted Leininger, deputy assistant regional administrator for the General Services Administration, said late last night that it would be at least 24 house before the equipment could be replaced and power restored to the affected portion of the building at 400 Maryland Ave. SW. But, he said, all persons who work in the building should report to work as scheduled.

D.C. fire department officials said that the fire occurred about 6:45 p.m. inside a subbasement electrical vault that also contained transformers filled with polychlorinated biphenyls (PCBs).

Leininger said a brief flash fire occurred when GSA workers, who had been repairing leaks in a PCB transformer, attempted to restore power to the transformer.

Fire officials said two unsealed 55-gallon drums containing PCBs that apparently had been drained from the transformers for repairs, were about 10 feet from the electrical switchboard that caught fire. A GSA official said later that the barrels were empty at the time of the fire.

Experts from the Potomac Electric Power Co. were at the building late last night to examine the transformers and switch gear to determine if any toxic substances were emitted during the incident.

When PCBs are burned, they give off deadly byproducts such as dioxin, which the Environmental Protection Agency has called "one of the most toxic substances known to man."

Leininger said no toxic fumes from the brief fire spread from the transformer vault to other areas of the building. He said that air from the vault, which contains four PCB transformers and which is located on the north side of the building, is vented to the outside.

Leininger said experts may be called in to take wipe samples from the transformer vault to examine for toxic substances. One worker was taken to a local hospital for treatment of "flash burns," Leininger said. The man's condition could not be learned. Fire officials said two other workers who received minor burns in the incident refused treatment.

Leininger said that the 800 federal government employes who work in the building should report for work, and that individual agencies will decide whether to send their employes home for the day.

GSA and fire officials said the cause of the fire was unclear last night. Fire officials said that a "fireball" occurred, which Leininger said destroyed an electrical switchboard, burned out a copper conductor known as a bus bar and overheated the transformer. A Pepco spokeswoman said GSA workers cut power to the transformer, and Leininger said the workers used fire extinguishers to douse the blaze.