The Nuclear Regulatory Commission postponed for a second time yesterday a decision on whether to shut down four operationg nuclear power plants built by the same firm that constructed Three Mile Island 2.

Staff members were meeting continually with utility executives, NRC Chairman Joseph Hendrie told a packed and expectant hearing room, and had asked for more time to study the executive's proposal. Staff chiefs had reported Wednesday that safety demanded the closure of the Rancho Seco plant of the Sacramento Municipal Utility District in California and Oconee 1, 2 and 3 South Carolina, owned by the Duke Power Co. of Charlotte, N.C. They were built by Babcock & Wilcox.

The commission rescheduled a decision for this afternoon.

Safety, however, was the topic of the dayas the emergency preparedness staff explained that the NRC has essentially no role in the operation of state and local nuclear accident planning other than to be kept informed.

Licenses for plants may be issued without any state or local evacuation or emergency plan, and in fact only 11 states have plans that comply with NRC guidelines, reported Harold Collins, assistant director of state preparedness programs.

"We just look for evidence of obvious incapability," he said. "There is no role spelled out for the NRC itself in any of these plans, either at the staff or the commission level," nor do the plans set any time limit in which anyone must be notified of accidents, Collins said.

The Three Mile Island accident on March 28 near Harrisburg, Pa., has galvanized many states into action, he continued. Maryland officials, for example asked for and got a meeting with Colins' office last week and will submit a plan for review within two or three months.

Virginia has "basically a good plan" for meeting nuclear emergencies but has had "some problems committing the [evacuation] drills and exercises, to writing," said Harold Gaut, planning coordinator.

The same is true for Nebraska, which had been "dead in the water" but began gearing up two weeks ago; Illinois, which had been "a long way from satisfactory" and was "footdragging" before Three Mile Island; Georgia and Michigan, which both just reorganized and will submit plans in about a year and Maine, which first asked for aids last week after years of clashing agency debate, Gaut said.

"Without Three Mile Island, that could have gone on for a decade," said Commissioner Peter Bradford, who is a former public service commissioner in Maine.

Collins explained that Pennyslvania has had "difficulties within its departments as to who is going to do what" and had submitted nothing since a 1975 preliminary outline the staff had called "a pretty good start." Last December, Gaut said, he received "through the side door" a further outline but has not yet received any official submission.

Other states having nuclear facilities but no NRC approved plan include Arkansas, which is expected to receive concurrence this week; Colorado, Massachusetts, Minnesota, North Carolina, Ohio Oregon, Vermont and Wisconsin, Gaut said.

"Obviously this is an area which hasn't received the attention it deserved, at all levels, including the commission level," observed Commissioner Victor Gilinsky. "We are focused on it now."

Chairman Hendrie noted that the neglect was not solely the fault of the states. "It was a pervasive philosophy . . . that these accidents were so unlikely that these kinds of efforts were a second, third, fourth-level priority," he said. "We're all at fault."

The commissioners expressed amazement that Collins and his staff had accomplished what they had with only a four-person office and promised more funding and staff.

At an earlier session on the same subject Wednesday, Wayne Houston, director of the NRC's accident analysis branch , told the commission that plans on paper often were too complex to work, and that studies had shown little difference in response among people informed or uninformed about the plans in advance of the need to carry them out.

"It's not clear that anything useful other than satisfying a person's curiosity or anxiety about the situation is accomplished by advising them in advance," Houston said. He then added that Three Mile Island had reoriented his office from its focus on pans for treating high radiation exposure cases to focusing on "the need to cope with public anxiety over exposure levels that are really rather low."

The goal of evacuation plans, he added, is not to eliminate radiation exposure-" that's impossible . . . just isn't logical"-but "to reduce unnecessary exposure." He said low exposure levels did not justify the disruption and possible injuries evacuation might cause.

Hendrie testified yesterday morning before the opening session of President Carter's commission investigating Three Mile Island. He told them he had thought before the accident that operator training was adequate to prevent such incidents, "but that is clearly not the case."

In Pennsylvania, meanwhile, the state Public Utilities Commision slashed $25 million for six months from a recent rate increase granted the Pennsylvania Electric Co., which owns one-quarter of the Three Mile Island plant. The increase was granted last year to cover its share of the plant, which is now closed. CAPTION: Picture, Environmental Protection Agency Administrator Douglas Costle testifies before John Kemeny, center, chairman of President Carter's commission investigating Three Mile Island, who is flanked by commission staff members, Ronald Natalie, left, general counsel, and Barbara Jorgenson, public information officer.