After six months of silence on the investigation of Fairfax County Sheriff James D. Swinson, the County Board of Supervisor voted yesterday to conduct a public inquiry into how Swinson runs his department particularly the county jail.

The board invited Swinson, whose department has been under investigation since February for alleged misuse of inmates, and Commonwealth's Attorney Robert F. Horan, who has been conducting the investigation, to attend the session.

Swinson, who called the allegations against him "chicken stuff," said yesterday he would come to the session, but that he wouldn't answer questions related to Horan's investigation.

"I'm telling you," Swinson said yesterday in an interview, "I'm not going to drag any dirty linen through the board room. I'm not going to any political Donnybrook. If that starts, I'll get up and leave."

Horan said yesterday he would attend the session, but that he would make no comments concerning Swinson's ability to run the sheriff's department. "It would be foolish of me in get involved with questions that don't concern criminal matters," Horan said.

The decision to conduct the inquiry into the sheriff's department was made on a 5-to-4 vote that split along party lines, with the exception of Supervisor Joseph Alexander (D-Lee), who called the as yet unscheduled session "an inquisition."

Republican supervisors Marie B. Travesky (Springfield), John P. Shacochia (Dranesville) and Board Chairman John F. Herrity voted against the meeting. Travesky and Shacochia called the meeting "political harassment," Swinson, a Republican, is up for reelection to a fifth term as sheriff next year.

Alan H. Magaine (D-Mason) brought up the idea of a public meeting with Swinson, he said, because it is "obvious that there are some problems in the sheriff's department."

Magazine said he wants to ask Swinson why the sheriff was unware that inmates in the jail, which he oversees, allegedy were doing private work for sheriff's deputies, as Horan has said happened "frequently" at the jail for "four or five years."

Magazine also wants to ask the sheriff, he said, why David J. Voetsch, a convicted murderer, has been kept at the county jail rather than in the state penitentiary system. Magazine questioned "the lines of communications" in the jail and said he hoped the meeting with the sheriff would do something to "straighten them out."

The board will concentrate on "administrative problems" in the jail during the meeting rather than on criminal charges against members of Swinson's department, Magazine said.

Horan said yesterday, as he has before, that his investigation, the results of which are being compiled for release, has come up with no criminal charges against Swinson. Horan said, however, that deputies in the jail "frequently" used inmates to do work for them outside the jail. The prosecutor said this practice continued for "four or five years" and that he was surprised by Swinson's claim that he doesn't know anything about the misuse of inmates.

Swinson, who yesterday referred to Horan, a Democrat, as "grandstand Horan," said the only problem with his jail is that the Board of Supervisors won't supply him with enough men to run it according to stat standards.

The sheriff's department, which has a $2.2 million annual budget, gets about half its money from the county and half from state sources, according to county spokesman Edmund L. Castillo.

Swinson admitted that there has been some wrongdoing in his department in the past, but he called it trivial and said. "Suppose a deputy takes an inmate out this afternoon and has him do something. How the hell am I supposed to know is if I fall upon it or have somebody tell me about it."

Swinson said his failure to know about some misuse of inmates "has nothing to do with my administrative abilities."

The board also passed a resolution yesterday which said the sheriff should be appointed, rather than elected as is now required under the Virginia constitution.The resolution was sent to the General Assembly in Richmond requesting a constitutional amendment.