The electrical fire that crippled the main water treatment plant for suburban Maryland last July, leaving thousands without drinking water, was triggered by human error, the Washington Suburban Sanitary Commission acknowleded for the first time yesterday.

The WSSC had previously denied that its personnel were responsible, maintaining instead that a short in an electrical system caused the fire.

Robert S. McGarry, WSSC general manager told the county councils for both Montgomery and Prince George's counties yesterday, however, that the fire resulted when workers failed to recognize "the very serious hazard" created by weakened electrical batteries and neglected to inform plant operators to use "extreme caution" when starting up water pumps.

In a letter to Montgomery and Prince George's County officials, McGarry said, "I find that the extensive damage . . . would not have occurred if the overall management of maintenance and operations had been better."

The batteries operate the protective "fail-safe" system at the plant that safeguards against an electrical fire by automatically tripping circuit breakers when a dangerous surge of electrical power occurs.

McGarry said the July 6 fired at the Potomac River Water Filtration Plant on River Road occurred because this protective system was then being powered by batteries that were too weak.

In addition, McGarry said, the WSSC's electrical staff had been using ordinary vehicle batteries to power the protective system because they were cheaper and more readily available than those that had been specially designed for use at the Potomac plant.

Because of the fire, which occurred at about 7 a.m. when a plant operator turned on an additional pump to meet the morning water demand, the plant had to be shut down for a day and Montgomery and Prince George's counties were plunged into a five-day water crisis.

McGarry said the pump operators should have known how to recognize a fault in the electrical system and that the WSSC's top officials should have realized that "anything less than a perfect (battery) system is very dangerous."

The general manager reiterated yesterday that he did not know the vehicle batteries were being used at the time.

Edward Boone, head of the WSSC's electrical staff, and Richard Hocevar, director of maintenance and operation, refused to comment on McGarry's report.

The general manager's statements were based on finding by Trident Engineering Associates; INc., an Annapolis consulting firm hired by the WSSC to conduct an independent investigation of the fire.

McGarry told the council's members that the WSSC's electrical staff had to be called in to work on the battery power system the night before the fire occurred because power had dropped to slow.

"In hindsight, we should have shut down the plant" at that point, McGarry said.

"It takes a lot of courage to explain what really went wrong . . . that it was a mistake made by individual people who operate the system," Prince George's Councilman Frank Casula told McGarry at yesterday's meeting.

In response to a question by Montgomery Council President John Menke, McGarry said the possibility of the same emergency occurring again is remote now because the WSSC has made certain improvements at the Potomac plant.

A dual set of new batteries has been installed so that if one set fails, the other will take over. The defective battery charger that was in use when the fire occurred also has been replaced, and generators have been brought to the plant to supply power for lighting in emergency situations, McGarry said.

In addition the WSSC plans to step up its repair and improvement program for the Patuxent water treatment plant.

"This (the breakdown at Potomac) may have been a blessing in disguise," McGarry said.