A power shortage struck Cambridge along with large portions of Massachusetts, Rhode Island and Vermont yesterday morning, depriving about 400,000 people of electrical power from 10 a.m. to noon.

The out age resulted from deliberate precautionary measures to ensure that two power transmission problems did not snowball into an uncontrolled black-out, Paul Harprey, a spokesman for the Rhode Island, Eastern Massachusetts, Vermont Energy Control (REMVAC), said yesterday.

Power Overload

At about 10 a.m. a major REMVEC transmission line failed in Connecticut while one half-hour later, one of Boston Edison's generating units in Everett "tripped" itself off during a test, Harprey said.

Consequently, one of New England Power Company's main generating units was overloaded, and power output had to be cut back by 75 per cent.

"At that point REMVEC sent out an order to electric companies in Eastern Massachusetts, Rhode Island and Vermont to cut back on their electrical load by interrupting service to a large number of customers," Harprey added.

Robert Petrie, a spokesman for Cambridge Electric Light Co., said yesterday, "We were asked by REMVEC to shed some load. We did this on a rotating basis, shutting down different areas for about an hour each."

The transmission problems that forced REMVEC officials to resort to a power cutback partially stemmed from the increased demand for electricity to cool buildings during the current hot spell.

Priscilla Korell, a spokesman for Boston Edison Co., said yesterday the greatest drain on the Boston power lines occurs during the summer.

REMVEC has developed precautionary plans such as load shedding in order to prevent incidents like yesterday's from developing into major blackouts.

The unusual heat over the past several days has spurred the use of air-conditioners and pool filters, causing the REMVAC line to trip itself because of overloading, Korell said.