The Justice Department has launched a probe into whether Potomac Electric Power Co. has been violating federal environmental laws in its handling and disposal of PCBs, according to informed sources.

The probe by the environmental crimes unit centers on how well Pepco has been testing used electrical transformers to determine whether they are contaminated with PCBs before they are disposed of, government sources said.

For more than a year, a Maryland grand jury also has been looking at Pepco in conjunction with the discovery at a Beltsville industrial park of the highest levels of PCB contamination ever discovered in the state. Used Pepco transformers, leaking the potentially cancer-causing PCBs, were stored at the site, which is United Rigging and Hauling at 6701 Ammendale Rd. About 5,000 persons live within a half-mile radius of the 10-acre property.

The federal probe is broader than the state grand jury investigation, according to government sources. Sources said the Justice Department is examining whether Pepco performs faulty testing on PCB-laced oil from transformers at the company's Benning Road facility in the District of Columbia.

PCBs, or polychlorinated biphenyls, are man-made chemicals widely used as a coolant in electrical equipment before their use was banned in 1977 because they were found to cause cancer, birth defects and liver damage in laboratory animals.

Arnold Weiner, a Baltimore attorney who represents Pepco, said he was informed over the last several days of the Justice Department's probe. Weiner said that Pepco has no culpability under federal laws because the company followed what it understood to be federal testing procedures and did not intentionally violate those laws.

"Federal law requires an intentional violation of the standards and in this case the company was making every effort to comply with the standards," Weiner said.

Over three years, Pepco sold 2,000 electrical transformers, including 75 containing hazardous levels of PCBs, to United Rigging and Hauling, a company that is not licensed to dispose of toxic substances, according to Maryland health department files. One transformer contained 8,460 parts of PCBs for every million parts of oil, according to state records.

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

On April 25, United Rigging and Hauling pleaded guilty to violating state pollution laws and was fined $75,000 in exchange for a promise from prosecutors that the company would not face criminal charges in the Pepco case. The hauling company agreed to "cooperate and testify fully" in the state grand jury investigation.

Arthur R. Pearlstein, attorney for United Rigging and Hauling, said yesterday, "I've been assured that neither United Rigging nor its president is under federal investigation."