A proposal to beef up safety inspections at Northern Virginia construction sites with Fairfax County employes drew objections yesterday from labor unions and a noncommittal response from federal officials.

John Quackenbush, secretary-treasurer of the AFL-CIO's Washington Building and Construction Trades Council, opposed the Fairfax proposal for using county workers as safety inspectors, arguing that the employes are untrained for the job. "What does a plumbing inspector know about a crane? What does a plumbing inspector know about an elevator?" Quackenbush asked.

Roger A. Clark, national field coordination director for the U.S. Occupational Safety and Health Administration, responded to county complaints about allegedly inadequate safety inspections, saying, "Those are the issues that we need to address." Clark did not indicate how soon a more definitive reply would be provided.

The freewheeling debate took place at Labor Department headquarters in Washington yesterday after efforts by Fairfax officials to crack down on unsafe construction practices. At least 14 workers have died in construction accidents in Northern Virginia in the last two years.

The Fairfax Board of Supervisors has voted to stop doing business with construction firms that violate safety regulations and to sue federal and Virginia safety agencies for allegedly conducting inadequate inspections. Fairfax has offered 68 county employes to supplement the state's safety inspection staff. The state has assigned only safety inspectors to Northern Virginia.

Despite his objections to using Fairfax employes as safety inspectors, Quackenbush sharply criticized the Virginia inspection system. The unions contend that the Virginia inspectors should be replaced by a federal inspection staff.

The meeting between Fairfax and OSHA officials was the second in two days. Fairfax Supervisor Audrey Moore talked with OSHA administrators on Monday. She later expressed uncertainty about what steps the federal agency might take. Yesterday's session also ended inconclusively.

The ball is in your court," said Acting Fairfax Executive J. Hamilton Lambert. "Yes, it is," Clark replied.