After years of discussing the pros and cons of cable television service, the Fairfax County Board of Supervisors has taken the first step toward introducing the cable system to the 400-square-mile county.

The board voted unanimously Monday to seek bids from consulting firms for the development of an ordinance to govern a local cable television franchise operation.

It also authorized hiring a special manager who would serve as a liaison between county staff and the consulting firm in developing the ordinance and a specific proposal for the type of cable service the county wants.

The supervisors expect to review the ordinance within six months, after which the county would begin to seek bids from cable television firms interested in developing a system for Fairfax County.

Five firms already have shown interest in providing Fairfax with cable television, among them American Television and Communications Corporation and the locally based Fairfax Telecommunications Inc., said Ronald Mallard, the county's director of consumer affairs, who is working on the cable television issue.

Cable television is a system that uses a high antenna to receive distant signals and transmit them directly by cable to system subscribers. It provides far more channels than commerical television and permits opportunity for local programming as well as offering what is available on commercial television.

The board's decision followed a discussion during which some supervisors expressed annoyance that cable television had taken so long to come to Fairfax, and others urged caution in the undertaking described by one supervisor as "one of the most important decisionw we will ever make."

"This (cable TV) is either the greatest thing coming down the pike for Fairfax or a horrible monster," said Supervisor Martha V. Pennino (D-Centreville).

The supervisors want to make sure they take full advantage of programming possibilities, which conceivably could include televising governmental meetings and public hearings, and reap some of the profits a well-run cable operation is expected to generate if they make an initial multi-million dollar investment to bring a cable network to the county.

For an initial investment of $7 million. Arlington County this summer began providing the first major cable television service in the metropolitan area on an experimental basis to about 1,000 customers through Arlington Telecommunications Corporation (Artec).

The corporation has paid the county $12,000 so far and next year will pay 4 percent of its gross receipts during 1978 under a "business privilege tax," said John D. Evans, vice president of Artec.

Arlington's cable programming includes local consumer education programs, recent feature motion pictures, continually updated satellite weather photos and clear reception of all Washington and Baltimore commercial television stations.

A representative of American Television and Communications Corporations estimated it would cost about $7 million to start a cable system that would serve about half of Fairfax County.

But if cable television reached 45 percent of Fairfax-television viewers, Mallard said estimates showed, the county could earn between $175,000 and $200,000 a year in gross receipts taxes on the franchise cable operation.

Evans, who said Artec also is interested in providing cable operations in Fairfax, said the county should "go slow" in setting up cable television, and recommended the county not invite bids until late in 1979, after it has had a chance to study Arlington's experience with the network.