An electrical malfunction shut down one of the two water treatment plants that serve Fairfax County yesterday afternoon, prompting fears that thousands of residents could be without water at a time when record-breaking temperatures have pushed demand to new heights.

The plant failure coincides with the seventh consecutive day of a heat wave that has twice pushed temperatures above 100 degrees and caused record demands for power and water. The temperature at National Airport reached a high of 96 degrees at 5 p.m. yesterday, tying the record set for the date in 1968, according to the National Weather Service.

Weather forecasters held out little hope of immediate relief, predicting that daytime temperatures over the weekend would remain in the mid-90s under generally sunny skies.

Fairfax County Water Authority officials said last night that a blown transformer crippled the Potomac Corbalis treatment plant, which is near Great Falls and provides water to about 300,000 residents in western Fairfax County.

"The worst-case scenario is that our storage tanks would fall down to zero and we would not have any water," said Jim Warfield, a spokesman for the authority. He added, "We've had these things happen before, but never when we were also experiencing the greatest demand on the system that we've ever had."

Water officials said they expected the plant to remain shut down for eight to 12 hours and appealed to Fairfax County residents by radio and television to restrict water use to essential needs.

Late last night Warfield said that "obviously people have cut back on their water use quite a bit," and that "we seem to be holding our own."

If an interim transformer is installed and the plant is running again by early morning, he said, "we should be okay." Otherwise, he added, "we could have a problem."

Earlier in the day, Fred Griffith, director of the authority, said that water use in the county had reached the highest levels ever.

The authority pumped 153 million gallons of water Wednesday, breaking the record of 147 million gallons set last year, according to Griffith. The authority pumped 152 million gallons on Thursday, he said.

Even before the plant was shut down, the county appealed to residents to conserve water, asking residents with odd-numbered addresses to restrict their lawn watering to odd-numbered dates and those with even-numbered addresses to limit theirs to even days.

The two main power companies serving the Washington area, Virginia Power and Potomac Electric Power Co., reported that demand this week has been higher than ever, with both utilities setting records on Tuesday.

A spokesman for Virginia Power said the utility was operating at close to capacity, but added, "The supply should be adequate."

The Weather Service said the high temperatures could ease somewhat Monday with the arrival of a cold front from Canada. By Tuesday or Wednesday, forecasters said, daytime highs should hover around a more comfortable 85 degrees.

The last time the daytime high registered below 90 degrees was July 17, when the mercury stopped at 86, according to the Weather Service.

Fairfax water officials said they were not yet sure whether the heat had anything to do with the transformer malfunction at the treatment plant. Officials said the transformer failed about 4:30 p.m., cutting off the flow of electricity to the pump that draws water into the facility from the Potomac River.

The county is served by one other treatment plant, in Lorton. Officials said that plant could handle some of the capacity of the Potomac plant but that it would not be adequate to serve the entire county.

Until the transformer is repaired, residents will have to rely on water that is being held in storage tanks. Warfield said last night that "the tanks are holding. If we can maintain that through the night, we'll be in good shape."

The Washington Suburban Sanitary Commission, which provides water to Prince George's and Montgomery counties, reported pumping a record-breaking 238 million gallons of water on Wednesday but did not issue any pleas for restraint.