The Environmental Protection Agency, prodded by a series of fires it had said were not likely to occur, yesterday said it wants to put additional controls on 140,000 electrical transformers using dioxin-containing polychlorinated biphenyls (PCBs).

The agency in 1982 exempted transformers from its ban on PCBs, saying the chemicals were fully enclosed and posed no threat to the public health.

It also said the cost of replacing the transformers with newer models that do not use PCBs as an insulator "would be billions of dollars, primarily as a result of the disruption of electrical service."

But, in the new proposed regulations, the agency says it was wrong in concluding that transformer fires would not likely produce toxic PCB byproducts capable of causing a health problem. A hearing on the proposed regulations is set for Dec. 26.

Dave Ryan, an EPA spokesman, said the agency is concerned that the vast majority of the PCB transformers are located in or near buildings where pollution from a fire could affect heavily populated areas.

Documented tests on laboratory animals show that various levels of PCBs can cause skin lesions, tumors and mutations. In addition, they release toxic dioxin compounds when exposed to fire.

The new rules also require:

* Immediate removal of all combustible materials stored near the transformers.

* Clearly marking the units and their location with PCB identification labels.

* Installation of additional electrical protection equipment in the transformers and the isolation of them from ventilation ducts by July 1988.