Arlington County officials moved last week to install an emergency 911 telephone system for county residents. The decision made Arlington the first jurisdiction in Northern Virginia to take formal action on an emergency system that is widely used elsewhere in the metropolitan area.

The Arlington County board gave its approved to the 911 system by voting to sign a letter of intent with Chesapeake and Potomac Telephone Company.

The 911 system, which could not be installed until May 1, 1980, at the earliest, would cost $10,923 to install and $259 a month to operate, said Arlington County Manager W. Vernon Ford.

High costs have kept the county from approving such a system in the past. "We always had thought the cost would be prohibitive," said board Vice Chairman Ellen M. Bozman. "So I'm pleased with the modest amount of $10,000 plus the $259 per month."

The 911 emergency phone number is currently used in several areas of the county, including the District and Prince George's and Montgomery counties. It automatically connects a caller with police and fire emergency dispatchers. It is designed to be a universal number in all localities so people, no matter where they are, can contact emergency services by dialing 911.

Northern Virginia police and fire departments have been interested in establishing a 911 line for several years, according to William Norton, a computer analyst for the Fairfax police. The problem, he explained, is that fire and police jurisdiction borders do not always coincide with the boundaries of the telephone company's central offices.

For example, the C&P Telephone Company's central office for Falls Church includes numbers in Fairfax, Falls Church and Arlington, he said. Each of these areas has a separate police and fire department.

Ford told the Arlington board that a 911 staff would have to coordinate calls from Falls Church. Fairfax County and Alexandria because telephone exchanges cross jurisdicational boundaries. He said that he believes the biggest problem would be calls from Falls Church, which does not have a central telephone office. However, Ford said the details can be worked out satisfactorily and an integrated communication system for the fire and police departments would be more effective and efficient than the current system.

The federal Law Enforcement Assistance Administration authorized a study of the cross-jurisdictional problems in Alameda County, Calif., and developed a special computer system that could help Northern Virginia, Norton said. The newly developed system would allow "selective routing," which means that the computer would identify the caller and immediately channel the call to the correct police jurisdiction, he said.

The District established the 911 emergency system in 1971, Prince George's County in 1973 and Montgomery County in 1974. Except for a small area of eastern Montgomery served by a Prince George's central telephone office, these three areas do not have the samd jurisdictional problems that Northern Virginia has encountered.

Capt. Brian Traynor, who is in charge of the D.C. police communications department, said the 911 line has been important to residents of the District. The police received more than 1 million calls on the line last year, he said.

Besides decreasing the amount of time to dial the police in an emergency, the 911 system also speeds up police response, Traynor said. The D.C. police department received 81,509 calls on the 911 line in one recent month, he said. All of those except 1,961 were immediately channeled to the dispatcher's headphones and did not have to wait for the phone to ring, which saves three seconds, he said. For those people who, because the dispatches is answering another 911 call, have to wait, the average amount of time wasted is 2.2 seconds, he said.

The system in use in the District before the 911 line had an average wait of nine seconds, "which can be an eternity" for someone in a crisis, Traynor said.

The 911 system also is beneficial because it allows people to dial the emergency number free at phone booths, Traynor explained.

"It makes every telephone booth a police all box," he said.