Prince William County's recently improved 911 service failed for 45 minutes last week, leaving callers to the emergency number for police and fire assistance with a busy signal.

A severed telephone cable shut down the 911 number on Feb. 8 throughout Prince William as well as the cities of Manassas and Manassas Park, and a backup system to reroute calls was not working, Prince William officials said yesterday.

Contel of Virginia Inc., which supplies telephone service to Prince William, had assured the county that the 911 system, which was enhanced in September, was "redundant," meaning that service would continue even if a cable broke, said Katherine M. Lueckert, director of the county's Office of Telecommunications.

Lueckert said she knew of no deaths or injuries resulting from the failure of police or fire crews to respond during the 45 minutes that the emergency number was not working.

The outage began about 2 p.m. after an electrician working at Potomac Mills mall accidentally sliced through an optical fiber cable that carries all 911 calls from a Contel facility in Occoquan to a facility in Dale City, said Doug Bradley, a Contel manager.

Prince William officials said yesterday that they were outraged at Contel for assuring the county its 911 service was fail-safe.

"It's a pretty damn serious thing," said Supervisor John D. Jenkins (D-Neabsco), vice chairman of the county board. "We were down there for 45 minutes. Apparently nothing happened, but it could have."

Dispatchers were alerted to the problem immediately, Lueckert said, when the words "missed heartbeat" flashed on their computer screens. She said they began working with Contel on a makeshift rerouting system, which took about 45 minutes to devise. The full system was not back on line for about 2 1/2 hours, she said.

Bradley said the backup system is now working. He said he did not know how long it had been malfunctioning, but said it might have been since September, when the expanded 911 system went into service. Contel had been working on the backup copper cable within two days of the malfunction, he said.

Under the enhanced system, callers for the first time could get police service in addition to fire and rescue service by dialing 911. Also, a computerized system displays for dispatchers the telephone number and address of an incoming call.

Calls to 911 travel over lines to the Contel station in Occoquan, where they are screened by a computer and switched instantly to the Dale City facility, which routes them to appropriate dispatchers, Lueckert said. All the 911 lines should have backup lines, except those originating in rural Nokesville.

Lueckert said Prince William, Manassas and Manassas Park "went to extraordinary lengths" to make certain the improved 911 technology was working properly before the service was publicized to residents on Dec. 1.

"We were extremely fortunate that there were no emergencies" during the outage, Supervisor Edwin C. King (D-Dumfries) said yesterday. "But I don't think we can count on that again."