Officials of Montgomery and Prince George's counties yesterday lifted the last remaining restrictions on water use in their areas as investigators said a communications breakdown apparently contributed to the spread of a fire that crippled the suburban Maryland water system last week.

Montgomery County Executive James P. Gleason and Prince George's County Executive Winfield M. Kelly Jr. revoked at 2 p.m. the remaining restrictions, which banned "outside" use of tap water for such purpose as lawn sprinkling and car washing except between 9 p.m. and midnight. But both executives wint on to encourage water conservation.

Washington Suburban Sanitary Commission spokeswoman Alice Cleary said water consumption during the weekend and the first two days of this week had been about 128.5 million gallons a day, "considerably" below the normal level of 172 million gallons.

The WSSC, which is continuing to repair its Potomac Water Treatment Plant in western Montgomery County, is asking its 1.2 million customers to continue to conserve by watering outdoors every other day rather than daily. The agency also asked all restaurants within its jurisdiction to serve drinking water only whe customers request it.

In an effort to prepare for possible future water emergencies, the WSSC release a complex, four-part plan, according to spokesman Arthur P. Brigham, that attempts to avoid inequities that surfaced during the past week's water restriction.

For example, commercial florists and nurseries that have their own water supplies would be exempt from any water usage ban, Brigham said. Car washes that used recycled water would also be exempt, he said.

As the investigation continued into the fire July 6 that put the treatment plan out of action, a source familiar with the probe said that the blaze apparently started with an electrical malfunction in one of the plant's water pumps.

Such fires are not unusual, WSSC officials have said, but their spread normally is prevented by a fall-safe system - circuit breakers that automatically shut off electricity in the plant.

The fail-safe system is powered by special batteries, which on the day of the fire were augmented with a make-shift arrangement of ordinary auto batteries. The fire spread, according to sources, because the batteries' voltage was too low to power the fail-safe system.

A source familiar with the investigation said a plant electrician became aware that voltage in the batteries was low and was directed by his superior to prevent plant operators from turning on any additional water pumps. But the electrician was unable to get this message to the plant employee who was operating the water pumps and who turned on an additional pump.

Investigators said it has not been determined why the voltage was low or what effect, if any, the use of car batteries had on the system.

Investigators said damage to the plant's equipment during the fire may prevent them from ever determining why the pump and the fail-safe systems malfunctioned.

Meanwhile, the Metropolitan Washington Council of Governments (COG), unanimously passed a resolution yesterday that would broaden the agency's powers to deal with water-related problems in the future.

If approved by each of COG's 17 members, the resolution would give COG the ability to coordinate various water use emergency plans, and also deal for the first time with such problems as oil spills or chemical pollution in local waterways, according to Walter A. Schreiber, COG's executive director.

Schreiber told an afternoon meeting that COG principal concern now is the low flow of the Potomac River, which supplies water for 75 per cent of Washington-area users. "The prognosis is not good," he said.

The Potomac normally flows at a rate of seven billion gallons a day but is now down to just under two billion galons per day, a rate that is "dangerously" low, he said.