Some otherwise unexplained short circuits and electrical fires may have been caused by tiny fibers of carbon or graphite released into the air by damage to a certain kind of plastic, according to a space agency report that has just been declassified.

The study was made public because the plastic, lightweight and superstrong, is likely to become widely used in automobile and aircraft bodies in the near future, a National Aeronautics and Space Administration spokesman said.

At the moment, it is used in skis, golf club shafts, tennis rackets and other hard-plastic sports equipment, as well as in experimental airplane and helicopter propellers, packaging and military materials, said the spokesman, David Williamson.

"If released from the composite (plastic) materials, the fibers can present a hazard to electrical machinery," the NASA technical memorandum said. The carbon/graphite fibers, specially produced under high-heat conditions, are supreme conductors of electricity. Strands of them across unprotected electrical circuits can cause overheating, short circuits or arcing, the report continued.

"Industrial plants producing and using free fibers have nearly all experienced malfunctions of electrical and electronic equipment and, in some cases, resultant fires," it said. Williamson said there had been "12 or 15" such incidents, beginning in 1970, two years after the composite plastic first was commercially available.

"An English plant blew its fuses . . . and overhead crane went out and nobody could figure out what shorted the power supply. It gradually became known that you had to watch those fibers," he said.

The fibers can be released into the air by burning, or by blows that pulverise small amounts of the plastic, Williamson explained.

The fibers can float for long periods in the air because of their microscopic size, only 8 microns thick and varying in length from 5 to 15 microns, he said. A piece of ordinary paper is 100 microns thick.

The particles pose no danger to human health, however, because they are chemically inert and too small to clog in the lungs, Williamson said.

"It's possible, very possible" that many fires whose origins are unexplained may have been caused by the carbon/graphite fibers, he said, but "it's not very probable," because of the relatively small amount produced. Six U.S. firms and those in six other countries together manufacture 350 tons a year.

Ford Motor Co. President Lee Iacocca revealed in October that his firm had already installed the graphite fiber plastic in some vehicles experimentally. He told a Chicago audience that a prototype car planned for 1979 would shed 1,300 pounds of body weight by using the fiber plastic "to the maximum degree possible" in the body, chassis and power elements.

A Ford spokesman said recently that the company had not known of the NASA report and would have to study it before making any comment on its plans for the light car.

"What poses the problem is a sudden takeoff in use that might occur in automaking as people want lightweight cars," Williamson said. Accidents, automobile fires and sloppy disposal of materials in auto plants could release large amounts of the fibers into the air, "turning off the air-conditioners, burning out the TV and the power systems all over," he said.

The report had been classified because the first large-scale uses of the hard plastic composite were in military aviation, Williamson said, and the effects of the fibers on electric circuits had military implications.

When wider use became a possibility, it was declassified in order that a solution might be found, he said.

There are no plans to ban carbon/graphite fibers, but a program described as government-wide has been organized to search for ways either to protect electrical circuits by coatings or air filters, or to produce composite plastic that will not release the fibers when damaged, Williamson said.

Daniel Landa of the Department of Commerce, which made the document public, said the light weight and strength of the composite plastic ould be "a major factor" in solving weight problems for automobile makers seeking to produce fuel-saving cars.

Meanwhile, discarded tennis rackets, skis and so on should be disposed of by landfill and not by burning, NASA said.