A stubborn fire in an electrical transformer yesterday morning forced the evacuation of about 1,000 employees from a federal building in downtown Washington.

No one was injured, but workers said word of the blaze was slow to spread, causing confusion about whether to evacuate the Ariel Rios Federal Building on 12th Street and Pennsylvania Avenue NW.

"I came out into the hallway to go to the restroom and there was a fireman with his breathing apparatus," said an Internal Revenue Service employee who works on the eighth floor. "It was pitch black. The fireman had a light on his helmet. The alarm never went off."

Several workers said the lights in the building went out and, about 20 minutes later, the fire alarm sounded.

Union officials representing some federal workers also said the building, which is more than 50 years old, has a history of safety problems, including asbestos as well as previous fires and power outages.

The fire broke out in an eighth-floor electrical vault, a self-contained room housing two transformers. The fire department received the alarm at 10:26 a.m., and within an hour more than 110 firefighters and hazardous material response teams had converged on the scene.

The fire was allowed to burn inside the vault for several hours, and most of the firefighters remained outside the building as a precaution.

Fire investigators said they were misled by a sign on the vault that indicated the presence of PCBs, or polychlorinated biphenyls, a potentially dangerous substance common to older electrical transformers. Fire officials later found out the transformers were new and did not contain PCBs.

Hazardous materials specialists were called from Montgomery County because D.C. firefighters lacked special protective suits for dealing with particularly serious PCB spills, authorities said.

The suits had been on order for some time and at the time of yesterday's fire, were at a shop "being worked on" to add desired numbers and lettering, according to Capt. Theodore Holmes, a Fire Department spokesman.

The long-awaited suits were finally delivered to firefighters yesterday after the blaze, a Fire Department source said.

The fire was extinguished by 1 p.m., but a lack of power in the building kept most workers out for the rest of the day.

Union officials said . evacuation procedures were not followed.

"The lights were out, there was a fire going on, and people were still in there," said Cheryl Eskew, a computer programmer for the IRS. "There were firefighters out here before people got out."

The building's fire alarm must be activated by hand, said Donald Plummer, the building manager. Still, Plummer said, most people were out three to five minutes after the fire was noticed.

The blaze was reported by workers on the fifth floor, who noticed smoke coming out of the air conditioner vents, said Assistant Fire Chief Thomas C. McCaffrey. The first firefighters determined it was in the eighth-floor vault, he said.

Terry M. Nuriddin, director of negotiations for the National Treasury Employees Union, said the proximity to the transformers and an asbestos problem posed dangers to workers. The General Services Administration, which manages federal properties, acknowledged there have been some problems but said they pose no danger to workers.

The GSA said the problems, which include some asbestos and lack of sprinklers, are not out of the ordinary considering the age of the building.

Staff writer Linda Wheeler contributed to this report.