Over a three-year period, the Potomac Electric Power Co. sold 2,000 electrical transformers, including 75 containing hazardous levels of PCBs, to a storage company unlicensed to dispose of toxic substances, according to Maryland health department files.

The 75 were among an estimated 730 Pepco transformers found during a visit to the storage company site last May by state investigators. One transformer contained 8,460 parts of PCB per million parts of oil, according to state records.

Maryland and federal environmental laws require that a PCB waste generator dispose of wastes containing more than 50 parts PCB per million parts of oil with a federally approved facility.

Pepco sold the transformers to United Rigging and Hauling, Inc. of Beltsville, which burned and dumped the chemical wastes at an industrial park in Beltsville, the files from the state Department of Health and Mental Hygiene indicate.

The wastes from Pepco, along with some other wastes, amounted to the worst PCB contamination ever uncovered in Maryland, according to officials in the Maryland health department's Waste Management Administration.

About 5,000 persons live within a half-mile radius of the 10-acre site, but state officials are working to prevent the spread of PCB contamination from the site and say that residents are in no immediate danger.

PCBs (polychlorinated biphenyls) are man-made chemicals widely used as coolant in electrical equipment. For decades, the electrical industry has relied on PCBs to insulate electrical transformers to prevent fires.

Manufacture of the chemical was banned by the federal government in 1977 after it was found to cause cancer, birth defects and liver damage in laboratory animals.

Officials of the Washington-based electric power company, while acknowledging that Pepco sold transformers to United from 1981 to 1983, said it did so only after it tested the transformers and drained any mineral oil containing hazardous levels of PCBs from them.

Pepco spokeswoman Nancy Moses said the utility burned the drained oil with permission of the U.S. Environmental Protection Agency at a Morgantown facility in Charles County.

She said it was Pepco's understanding that United did not need state or federal permits or approval to dispose of transformers with PCB levels below 500 parts per million.

"Basically, we followed all the applicable rules and regulations when Pepco sold transformers . . .to United Rigging," Moses said.

Assistant Maryland attorney general Michael Power, responding to Moses, said, "We contend that the transformers were not drained. It's very clear that our interest in it is that they weren't empty."

What led to the high level of PCB contamination at United's Ammendale Road facility -- and who is responsible -- are among the questions being probed by a Prince George's County grand jury, according to government sources.

The panel has been listening for the past month to evidence presented by Maryland Attorney General Stephen Sachs' Hazardous Waste Strike Force, the sources said.

In May, state investigators armed with a search warrant found hundreds of haphazardly stacked electrical tranformers on the United property in the Beltsville Industrial Park adjacent to the 14,000-acre National Agricultural Research Center, according to the state files. Among them were the Pepco transformers and 50 belonging to Electrical Equipment Corp. of Lorton. Twenty-seven of the EEC transformers contained high levels of PCBs, state officials said.

EEC officials said the 50 transformers were being stored at United for resale and were not leaking.

Officials of United, which reclaims metals such as copper from junked equipment, have told state investigators that they burned oil and transformers on company property without federal or state approval, according to the state records. When burned, PCBs emit dioxins, unwanted chemical byproducts considered unsafe to humans at any level.

"I was not supposed to be getting hazardous waste. I was told that they the transformers all met EPA regulations," DeeWitt C. Sperau, executive vice president of United, said in a recent telephone interview.

Emergency cleanup by state health officials was prompted in part by concern that during dry weather, PCB-contaminated soil particles can become airborne and inhaled or ingested, according to the state records.

To protect nearby residents, EPA technicians are wetting the soil during rainless periods and loading contaminated soil into trucks for immediate disposal, an agency spokesman said. Because these measures are being taken, "the public need not worry about being contaminated by PCBs or being exposed to PCB-contaminated dust particles," EPA spokesman Harold Yates said yesterday.

Because of the public health threat and long-term environmental problems posed by PCBs -- including accumulation in the tissue of humans and other animals -- federal and state laws strictly regulate the chemicals from creation to disposal.

The laws hold generators such as Pepco, transporters and disposers such as United, and companies that store PCBs such as Electrical Equipment Corp. responsible for the safe handling of materials containing dangerous levels of the chemical.

Pepco and United earlier this month signed a consent agreement with the EPA and the Maryland Department of Health and Mental Hygiene, in which the companies agreed to remove the transformers and contaminated soil from the Beltsville site and said they would dispose of both at special EPA-licensed facilities. The parties have estimated the cost of the cleanup to be from $1.5 million to $3 million.

United has a fluctuating work force of about 75 to 100 employes, at least six of whom have testified before the grand jury, according to Sperau.

Discovery of the PCB contamination in Beltsville marks the third time in 14 months that hazardous materials have been uncovered in Prince George's County.

In June 1984, construction workers breaking ground for a town-house development near Calverton unearthed dozens of corroded and leaking barrels oozing waste sludges. Then, last September, 72 tons of low-level, radioactive nuclear debris was found being stored in a Beltsville warehouse in the same industrial park where United is located. It has since been removed.

At the PCB-contaminated site in Beltsville, environmental technicians have thus far filled an estimated 300 55-gallon drums with PCB-laced waste oil, according to state records.

"Pepco has acted as if they were innocent bystanders and I don't think they were," said Maryland State Sen. Arthur Dorman, a Democrat who represents the Beltsville area. "And the fact that they agreed to the cleanup that's costing up to $3 million leads me to believe that they tried to get away with something. Their experts, I'm sure, knew."

In an earlier interview, Moses said, "We have a group in the company that handles the sale of transformers and they knew, were told I presume, that United Rigging was a facility that could handle it."

Former District of Columbia People's Counsel Brian Lederer, who represented city consumers in utility cases heard by the D.C. Public Service Commission, said the issue of PCB disposal has not been widely explored by the agencies that regulate monopoly utilities, either in this area or elsewhere in the country.

In 1978 the federal government began regulating disposal of oil with PCB levels greater than 500 parts per million, requiring that it be buried in an EPA-approved landfill or incinerated at an EPA-approved facility. In 1979, new federal rules lowered the level EPA was to regulate from 500 parts per million to 50, said Suzanne Rudzinski, an EPA toxic specialist.

United, a Delaware corporation that began doing business in Maryland in April 1981, has never had state or federal approval to dispose of any hazardous wastes, including PCBs, according to state officials.

A permit that allowed United to haul wastes to an approved disposal facility -- but not to store, treat or dispose of them -- expired last year and the company did not renew it, said Sperau, United's executive vice president.

Grand jury subpoenas have been issued to Pepco officials, according to government sources.