At least three atomic power plants have not classified as "safety-related" equipment the electrical devices that failed at the Salem nuclear power station in southern New Jersey and prevented the automatic shutdown of that reactor, it was learned yesterday.

The Nuclear Regulatory Commission yesterday termed the Feb. 22 and 25 accidents at Salem, in which the reactor failed to respond to an automatic signal and had to be shut down manually, as the most significant events "with implications for reactor safety since Three Mile Island."

The accidents marked the only time that a commercial power reactor has failed to shut down completely on automatic command. Plant operators each time manually shut down the reactor after a delay of about 25 seconds.

The Salem reactor at the time of each incident was operating at relatively low power. Harold R. Denton, NRC director of reactor regulation, said yesterday that the Salem plant probably could have survived such an accident even if the reactor had been operating at full power.

But Denton said that at other types of nuclear plants, a failure of the automatic shutdown system when the plant was running at full power could lead to a meltdown unless the operator recognized what was happening and shut down the reactor manually in less than 90 seconds.

The failure of the Salem reactor to shut down automatically has been blamed on key circuit breakers that were not correctly classified "safety-related" and had not been maintained adequately.

Yesterday, at a hearing on the Salem accident, the NRC made it clear that it does not intend to permit operation of the two large reactors on the Delaware River until "many, many problems" had been solved.

NRC officials also disclosed at that meeting that they had learned of three other plants where the circuit breakers were improperly classified--Ginna 1 near Rochester, N.Y., Yankee Rowe 1 in Rowe, Mass., and Haddam Neck near Meriden, Conn. All three, like Salem, are Westinghouse reactors.

Officials of these plants could not be reached last night for comment.

But Roger Mattson, director of systems integration for the NRC, said the plants had notified the commission that they had checked the circuit breakers--which are "essential features" of the system that shuts a reactor down in a problem--and now are treating the breakers as safety-related equipment.

Mattson noted, however, that the discovery raises the larger question of how the NRC is going to treat the broader implications of the Salem accident. The staff told the NRC yesterday that it planned to report on the "generic implications" on April 18.

The NRC was highly critical of Public Service Electric & Gas Co., the utility that operates the two Salem reactors, at yesterday's hearing.

It cited poor maintenance, inadequate training of operators, inadequate supervision, and an overall lack of management concern for dealing with the plant's problems.

"There is just not the discipline there needs to be at this plant," NRC Commissioner Victor Gilinsky said.

He questioned whether the NRC "would have given a license to this plant" in view of the revelations that have come out in the past two weeks about its operation.

But Richard Eckerd, a senior vice president of the utility, told the NRC that his company is working hard to overcome its problems and hopes that the most recent accident was an "isolated occurrence."

"If we do find significant problems, we are going to sit on this plant until we have straightened them out," Eckerd said.