Repair work on transformers leaking the toxic chemical polychlorinated biphenyls (PCBs) will begin today at five Smithsonian Institution buildings, institution officials said yesterday. They also said that cost estimates for repairing defective electrical circuit breakers are being prepared.

An emergency spot inspection Friday by the Environmental Protection Agency of 25 of the Smithsonian's 57 PCB transformers found leaks in three buildings, the Museum of Natural History, the Museum of American History and the National Portrait Gallery/Museum of American Art. All the leaking transformers are in electrical vaults that are not accessible to the public.

The EPA had set a 48-hour deadline for the Smithsonian to begin work to clean up the PCB spills and to repair the leaks.

EPA officials said yesterday they have not determined what actions will be taken against the Smithsonian for failing to comply with federal standards for PCB transformers, which require that the transformers be intact and nonleaking.

A written report by the inspector is expected to be completed by today, EPA officials said.

Prior to 1977, PCBs were widely used as a transformer coolant because they are about 300 times more fire-retardant than mineral oil, another coolant. Most modern transformers are dry or use silicone as the coolant.

The leaking PCB transformers were discovered by the Hazardous Materials Unit of the D.C. Fire Department, which was called to the Museum of American History by an electrician.

PCB transformers are strictly regulated by the EPA because when PCBs burn they produce more deadly chemicals, including dioxin 2,3,7,8-TCDD, which the agency has described as "one of the most toxic substances known to man."

Alvin Rosenfeld, Smithsonian spokesman, said Substation Test Co. of Forestville, an electrical testing and engineering firm, will make the repairs on 34 transformers in the National Portrait Gallery/Museum of American Art, the Smithsonian castle, the Arts and Industries Building and the museums of Natural History and American History.

He said repair costs are not expected to exceed $80,000 and that the money will be shifted within the institution's restoration and repair budget.

At the same time, Substation will inspect PCB transformers and circuit breakers in the National Air and Space Museum, the Freer Gallery of Art, the Hirshhorn Museum and Sculpture Garden and the Renwick Museum.

The company will also prepare cost estimates for "the repair of circuit breakers identified as having defects requiring servicing by outside contractors," according to the Smithsonian's statement.

A Substation report submitted to the Smithsonian last month recommended replacement of 35 low-voltage circuit breakers in the Museum of American History, the Arts and Industry building and the Smithsonian castle after 10 were found defective.

The report showed that six had failed to trip, or break the supply of electricity, and described one of the circuit breakers as "not safe for service."

Rosenfeld said the repair estimates will be for 26 "items" that have been found to have "discrepancies involving the electrical system," including circuit breakers and other equipment.

Substation's inspection earlier this year covered approximately half of the Smithsonian's electrical system.

A report on the remainder of the system, submitted in 1984, also showed numerous defective circuit breakers.

Smithsonian officials said yesterday that 10 circuit breakers covered in the earlier report will be among those repaired.

Smithsonian officials also confirmed yesterday that John Payne, the electrician dismissed after reporting the leaks, has been placed back in his post. They said they had no other comment on the status of Payne, who held a temporary appointment.

Dwight Bowman, president of the American Federation of Government Employees, Local 2463, which represents about 400 Smithsonian workers, said yesterday, "While we are concerned and certainly recognize that PCB is a major problem, we also are looking at the way the Smithsonian is handling the situation and giving employes the proper equipment."

Last week's inspection also showed that the Smithsonian "did not have proper containment as required by EPA regulations" for managing PCBs.

Bowman said the union is reviewing protective equipment provided to personnel who must work with PCB fluids.

Smithsonian officials have said that employes are given protective clothing and half-face respirators, but employes who work with the substance have reported that they use only goggles and a face shield similar to a surgeon's facial mask.

The union safety committee is considering asking that employes who have handled PCBs be given complete medical examinations and may ask that they be awarded hazardous duty pay in the future, Bowman said.