The Boston Edison electric utility shut down its Pilgrim I nuclear power plant early yesterday after tests disclosed a possible safety problem in the electrical system.

The discovery of faulty insulation occured during tests ordered in November by the Nuclear Regulatory Commission (NRC) after a Washington-based group raised the issue of possible electric troubles in many nuclear plants. A spokesman for that group, Larry Tye of the Union of Concerned Scientists, said he was "curious why the Boston problem wasn't discovered earlier in the review process."

The possible hazard involves electrical connectors (special plugs or sockets) on cables of the safety system within the structure that contains the nuclear reactor.

Tests by the Bendix Corp., which manufactured the equipment, "found some evidence that some insulation on the connectors might not have been properly installed," according to Frank Ingram, spokesman for the Nuclear Regulatory Commission.

The shutdown was ordered shortly after 2 a.m. yesterday and is expected to last for several weeks, Ingram said, while the connectors are tested for reliability.

"There was no danger, no emergency, and we weren't ordered to do it. We just decided it was the best thing to do under the circumstances," said Harriett Stanley, a spokeswoman for Boston Edison. A total of 31 connectors will be tested, she said.

Failure of the electrical connectors would make it more difficult to turn on safety devices that would cool the reactor if it should somehow lose its normal cooling system, leading to a highly dangerous situation.

The Donald C. Cook Unit I nuclear power plant at Bridgman, Mich., on the southern tip of Lake Michigan, was closed from Nov. 18 to Dec. 2 because of problems with its electrical connectors. The Boston closure is the fifth made for safety reasons in the past three years.

The NRC ordered the November review of the nation's 65 nuclear power plants after the Union of Concerned Scientists called public attention to a series of tests at Sandia Laboratories in New Mexico last July. The tests had shown that certain kinds of connectors failed under the extreme heat, pressure and chemical conditions that would occur if a reactor should somehow lose its cooling system.

The Boston Edison official said Pilgrim I normally provides 34 per cent of the power supply to 550,000 customers in the 40 cities and towns of greater Boston. The slack will be taken up by increased generation from oil-powered plants, the official said.