The D.C. fire department's main communications link with its 33 firehouses was knocked out for more than 17 hours Thursday and early yesterday after telephone cables in its communications headquarters caught fire, department officials said.

The department immediately switched to an emergency backup system, using radios to communicate with its fire stations, and there was no danger caused by the breakdown, fire officials said. But the mishap marked the first time the department's primary communications link to the stations was cut, said Battalion Chief Michael Tippett, the department's spokesman.

Fire stations around the city and about 60 of the 1,900 fire department emergency boxes on city streets lost their main communications link to headquarters from 10 a.m. Thursday, when the fire started in the department's communications headquarters at 300 McMillan Dr. NW, to 3:30 a.m. yesterday, when the breakdown was repaired.

The fire, on the building's first floor, was quickly put out by firefighters from a nearby fire station and did no structural damage. But it sent billowing smoke throughout the building, headquarters for dispatchers who direct emergency vehicles in the city.

The fire was caused by a power surge in the department's communications network. The power surge was mistakenly initiated by workers for the D.C. Department of Transportation (DOT) who were conducting a routine test of power lines connecting traffic lights and street lights, Tippett said.

Although fire officials warned city employes that the fire department's main communications lines could not withstand the 700-volt test, according to fire department sources, the DOT workers, apparently inadvertently, sent the jolt through both the street light and fire department lines, sparking the fire.

DOT officials declined yesterday to comment on the mishap.

DOT employes spark the power surges every few months to test the capacity of traffic light and street light cables, as well as fire department communications lines, said fire department dispatcher Bob Simms. The transportation department maintains the emergency fire boxes on the street and the communications cables, in coordination with fire department personnel, fire officials said.

DOT workers initiated the power surge at a facility on the grounds of the former Blair School in Northeast Washington, one of the city's numerous gathering points for underground cables, Simms said.

When the department's main "vocalarm" communications link to the fire stations, via telephone lines and two-way intercoms, was knocked out Thursday morning, fire officials resorted to radio contact, one of several backup mechanisms. "It's designed so that if somebody bombed our headquarters, we could go on," Tippett said.

"There was no real danger to anybody," said Tippett. "But going over to the backup provided us with a good test in the event of an emergency."

The fire department dispatched its normal complement of about 300 ambulance calls and 100 fire calls on Thursday, Tippett said, but there were no grave emergencies.

The complicated repair of the system required splicing more than 200 lines in a network at the communications headquarters, Tippett said.

Coincidentally, at the very moment that the power surged and the fire started, the fire department's new chief, Theodore R. Coleman, was across town being sworn in.

"He must have been just walking down the aisle when it happened," Tippett said.