A Montgomery County Circuit Court jury took only 20 minutes yesterday to dismiss the $500,000 claim of a 49-year-old Rockville woman who said she was seriously injured in 1979 when she fell off an unsafe chair provided by her employer,Communications Satellite Corporation (COMSAT).

The quick verdict came after the jury heard a surprise witness for COMSAT say that the plaintiff, Naomi Keller, an electronics parts assembler in Clarksburg, had practiced falling off chairs for some time before she toppled off her assembly table chair on July 12, 1979.

On Thursday, Thomas A. Farrington, COMSAT's attorney, called Eva Nealon, 64, who identified herself as a close friend and former work associate of Naomi Keller, to the witness stand.

Nealon told jurors that Keller had been practicing falling off chairs at work "for years" and that Keller, who worked with her for an employment agency that farmed its employes out to other firms, had told her she planned to fake an accident and file a lawsuit "as soon as she found a company that was big enough."

Arthur V. King, the attorney for the Kellers, told the jury his client had sustained serious injuries to her back and suffered a dislocated jaw because COMSAT "failed to provide a safe work place." King claimed that Nealon's testomony had been untruthful and that while on the witness stand "she appeared to be on drugs."

But Farrington argued that King had not proved COMSAT was negligent.

"The verdict was grossly unfair," Charles Keller said as he and his wife walked slowly from the courtroom. following the verdict. Added Naomi Keller, "Up until the time Eva came to court I thought we were friends."