About 90 hotel guests were evacuated yesterday morning when an underground electrical transformer exploded in the parking lot of the Holiday Inn in Rosslyn, sending a billowing tower of flames and black smoke high into the air.

No one was hurt in the incident, and there appeared to be no property loss except for damage to a pair of transformers in front of the hotel, at 1850 Fort Meyer Dr.

Denise Lindberg, a hotel guest, said she was having breakfast in the Holiday Inn dining room when she looked outside and saw the explosion in the parking lot below her. "I was just biting into my English muffin when suddenly there was an explosion and flames -- really loud," she said.

Virginia Power officials said the cause of the explosion might take days to determine. The transformers were purchased by Virginia Power from Potomac Electric Power Co. within the last year, officials said.

Virginia Power officials said some trace levels of polychlorinated biphenyls (PCBs), a carcinogenic chemical often present in transformer oil, were found in the vault holding the transformers. The level of PCBs measured in the vault was 170 parts per million, officials said.

According to federal guidelines, a PCB level of 50 to 500 parts per million is considered PCB-contaminated, but officials said the fire in Rosslyn did not appear to pose a significant health hazard.

Jack Pierce, an Arlington electrical inspector, said the building and transformers are about 20 years old and do not "have the safety things they should have."

Guests at the Holiday Inn, some of whom were awakened by what they called a tremendous explosion just after 10 a.m., stood outside watching as Arlington County firefighters spent about 90 minutes extinguishing the flames. All power except for the emergency generators was knocked out at the Holiday Inn, and managers at the hotel said their guests would be lodged at the Westpark Hotel next door until electricity is restored.

The explosion and fire also disrupted electrical service in the area of Rosslyn near Key Bridge.