Managers of the Rancho Seco nuclear plant near Sacramento, Calif., voted 3 to 2 early yesterday to keep it operating despite the accident at its near-twin, the Three Mile Island plant near Harrisburg, Pa.

They voted after seven noisy hours of testimony from citizens, most urging them to shut the plant at least temporarily.

Gov. Edmund Brown Jr. had asked the publicly owned Sacramento Mucipal Utility District to shut down until the trouble at Three Mile Island analyzed. Both plants were built by Babcock & Wilcox Co.

The hearing opened with a turnaway crowd at the utility district headquarters building, with 107 persons signed up to speak. At midnight, the municiapal district's chief engineer arrived from Washington, D.C., with word that the federal Nuclear Regulatory Commission had found from preliminary information on Rancho Seco that there was no reason to shut it down. The engineer, John Mattimoe, recommended keeping it running.

In a follow-up letter to Brown NRC Chairman Joseph M. Hendrie said there was "reasonable assurance" that the plant "can continue to operate without danger to the public health and safety."

Other nuclear power developments: Near Seneca, III., an employe at Commonwealth Edison's reactor construction site said somebody is trying to sabotage the plant by cutting pipes and breaking instruments.

A company spokesman downplayed the incidents, calling them vandalism, and said there was no danger to the public. Nevertheless, the utility has posted a $5,000 reward for information about plant damage and stolen equipment.

Leo Burke, manager of the site, called it coincidence that the reward was posted at the time of the Three Mile Island Incident, and said "there has been less sabotage and damage and nuisance vandalism on this job than on any I've ever seen."

But a source inside the plant with knowledge of he construction operation and more than 30 years experience in the construction industry described the incidents as "sabotage, pure and simple."

"Oil and water pipes have been cut," he said. "In one case, you had to climb up with a flashlight to find a pipe that had been neatly cut with a hacksaw."

Although some nuclear fuel already has been delivered to the site, Steve Goldman, Commonwealth Edison spokesman in Chicago, said there was no danger because the reactor was not yet prepared to go into operation. He said security would be tightened and all systems repaired and inspected prior to startup late this year or early 1980.

In York, Pa., the Daily Record reported that Nuclear Regulatory Commission inspectors in late 1977 rated the Peach Bottom nuclear power plant in York County "the least safe site" in the northeastern United States.

The ratings were made by 19 NRC inspectors. Their comments on Peach Bottom included: "The poorest management . . . many repeat items of non-compliance."

Edward Greenman, new resident NRC inspector at Peach Bottom, said things have since improved. "If the plant was unsafe, we would have done something about it," he said. The plant is operated by Philadelphia Electric Co.

In Ottawa, Canada's government owned nuclear energy corporation said faulty tubing that could leak radioactivity has been provided to three reactor construction projects by Babcock & Wilcox Canada Ltd. The company had no comment.

The plants are being built in Ontario, Quebec and New Brunswick.

In New York, anti-nuclear groups say they are planning rallies at nuclear power plants and are enlisting public support for a White House phone-in Monday as part of a national campaign to replace nucler power with alternative energy sources.

Demonstrations will be held over the next few days at several nuclear power plants. A national organization, Mobilization for Survival is arranging a telephone campaign to ask President Carter to enact an immediate moratorium, according to the Rev. Paul Mayer, a professor at New York Theological Seminary.

The groups want te government to draft "immediate plans" to replace today's estimated 13 percent nuclear-generated energy with sources such as coal, oil, and solar power, he said.

In Oregon, Gov. Vic Atiyeh asked that recently concluded hearings on a nuclear power project be reopened to consider information on the Three Mile Island accident. The proposed Pebble Springs plants would use the same reactor manufacturer as the Pennsylvania plant. The state Senate, meanwhile, with Atiyeh's support is considering a two-year moratorium on a permit for the plants while the Three Mile accident is being investigated.