The late-afternoon explosion rocked south Jacksonville, Fla., and when firefighters arrived minutes later the plume of smoke from the American Electric Co. could be seen for 20 miles.

It was the disaster firefighters knew could occur at the company, which stored old electrical transformers and the PCB coolants drained from them.

By nightfall, firefighters had saved a nearby 10,000-gallon PCB storage tank but the company's entire operation was contaminated by the deadly PCBs and the even more toxic dioxins emitted by the blaze. The fire department, too, was virtually out of business. Most of its equipment had been contaminated, and everything from protective clothing to hoses had to be destroyed.

The Jan. 29, 1984, blaze was the fifth-worst PCB fire in the nation, but area fire officials say the dangers of any PCB fire are much the same. Still, the Jacksonville firefighters had an advantage -- they knew where the PCBs were located.

Local fire departments lack even that elementary information -- they do not have complete lists of where PCB transformers are located.

However, the explosions and electrical fires over the weekend at the Washington Hilton, as well as a federally ordered cleanup of PCB transformers at the Smithsonian Institution have prompted fire officials to take action.

D.C. Fire Chief Theodore Coleman yesterday created a special task force to study problems in battling the Hilton blaze and the PCB problem in general. Fire department spokesman Ray Alfred said the department needs to identify "what steps need to be taken" when PCBs might be involved.

No PCBs were involved in the fires Saturday and Sunday at the Washington Hilton, but firefighters arriving at the fire on Saturday waited about 1 1/2 hours for specific information on the electrical gear and for workers from the Potomac Electric Power Co. to cut electricity to the structure.

Officials of other area fire departments said yesterday that their firefighters approach electrical fires with extreme caution.

"The general rule is that we treat [all electrical fires] as suspected PCB fires until they are confirmed otherwise," said Capt. John Kimball, head of Fairfax's hazardous materials unit. "We would expose ourselves at the absolute minimum necessary to save lives."

"With high-voltage electrical fires, the danger is not just the fires themselves and PCBs," explained Lt. Scott Erbele of the Arlington Fire Department. "The electrical insulation produces hundreds of different deadly chemicals and gases including hydrogen chloride and hydrogen cyanide. They kill firefighters just as much as PCBs."

None of the major jurisdictions in the area -- the District, Alexandria and Arlington, Fairfax, Montgomery and Prince George's counties -- has a comprehensive list of PCB transformers in its area.

Officials of the Environmental Protection Agency, which regulates PCBs and transformers that contain the coolant, said their only lists are of buildings they suspect may have PCB transformers because of their age and size. They estimated there are 140,000 high-voltage transformers -- 480 volts or greater -- still in use in the United States.

Because of the danger created by burning PCBs, new regulations issued by the EPA require all PCB transformers to be listed with local fire departments by Dec. 1. The new regulations require replacement of all high-voltage PCB transformers in public or commercial buildings by Oct. 1, 1990, and modifications in lower-voltage PCB transformers that would cut power to them immediately in case of an electrical problem.

But lists kept by various agencies in the Washington area indicate thousands of PCB transformers are in service in this area.

The General Services Administration's central listing shows 779 of the 998 transformers in federal buildings in the Washington area contain PCBs -- which because of their fire retardant qualities were the primary coolant for transformers manufactured from 1929 until 1977.

Among the government buildings with the PCB transformers are the White House, the Old Executive Office Building, the State Department and the Pentagon.

District government buildings contain 427 PCB transformers, the University of Maryland has about 100 at its College Park campus and Pepco has 30 located in Montgomery and Prince George's counties.

There are 47 PCB transformers in Metro stations, 10 in the National Gallery of Art, 57 in Smithsonian Institution Buildings, but none in the Archives Building.

The Smithsonian was cited by the EPA for about 40 violations involving the use, disposal, storage, marking and record-keeping of PCB transformers and materials. The EPA conducted an emergency inspection of the Smithsonian's transformers after the hazardous materials unit of the D.C. Fire Department discovered leaking PCB transformers during a June 27 visit.

Officials of the hazardous materials unit visited the Smithsonian to determine the location of PCB transformers after learning that the fire department's listing of a single PCB transformer in the Smithsonian castle was incorrect.

As at the Smithsonian, transformers are usually located in electrical vaults and protected by steel doors that make them inaccessible to the public and present no major imminent danger. But fires create monumental problems. Not only do burning PCBs emit deadly dioxins, water used to fight a PCB fire also becomes contaminated and runoff must be prevented from entering storm sewers or seeping into the ground.

Fire departments in the District, Alexandria, and Fairfax, Montgomery and Prince George's counties have hazardous materials teams on duty around the clock.

New EPA regulations require that vaults and other areas with PCB transformer locations be marked on the outside by Dec. 1. Fire officials said Virginia Power Co. locations that do not contain PCB transformers are already marked with blue stickers.

"Unless they see the blue stickers, the firefighters treat them as containing PCBs," said Lucy Crockett of the Alexandria Fire Department.

None of the hazardous materials teams handles those calls exclusively. Some handle all types of fires and others operate as rescue squads. Most larger fire departments have several hazardous materials teams.

In the District, where the largest number of PCB transformers is located, there is only one hazardous materials unit and members double as members of a rescue squad.

"The HAZMAT unit is a rescue squad that does this part time," said Tom Tippett, president of the International Association of Fire Fighters, Local 36. "We are going to push up on the Hill, before the council and with the mayor that this unit be full time.

"It's time for this city to catch up with other major cities."