The Potomac Electric Power Co. and the union representing 3,300 of its employes broke off negotiations late yesterday and the union began a strike at 1 a.m. today, the first in the 89-year history of the giant utility.

Pepco, which serves 1.75 million residents of the District, Maryland, and a small portion of Arlington, said the strike would have no immediate effect on essential service and that nearly 2,000 managerial and clerical employes would provide essential services for the duration of the strike.

The dispute centers on wages, proposed reductions in health benefits, and other issues, according to mediator Charles Scott of the Federal Mediation and Conciliation Service, which sought unsuccessfully to avert the walkout during a four-hour afternoon bargaining session. No further talks were scheduled.

Night shift employes left their jobs starting at 1:05 a.m. today, the time when a strike could legally begin once the union exhausted a 48-hour notice period required under the previous three-year contract. The union, which represents power plant workers, linemen, meter readers and customer service representatives, had a near-unanimous strike vote last week and informed Pepco at 1 a.m. Sunday of its intent to cancel the contract.

In a statement issued early today, Pepco president Edward F. Mitchell expressed disappointment over the strike, calling it a "sad event," and said the utility "will do everything we can to maintain reliable service to all our customers."

Listing changes in operations necessitated by the strike, the company said most residential meters would not be read, and bills would instead be based on estimates, to be adjusted later.

New service connections would be limited, the company said, and except in life-threatening cases, restoration of service after power outages might be delayed.

"We feel that Pepco is making record profits and they are not willing to share it with the employes who help make those profits," said Terry Cross, president of Local 1900 of the International Brotherhood of Electrical Workers, "If they were treating us right, we would not be going out on strike."

Pepco has declined to discuss the issues in the dispute, asserting that it would not negotiate in the news media. However, the company said this morning that it had made a full, complete and reasonable contract offer, and Mitchell said that Pepco "remains available" for further meetings.

"The essential services of power plants will be running, essential line work will continue, phones are still running, and bills will still be sent out," Moses said. "It is part of our prudent management to ensure that our service will continue."

Pepco yesterday asked unionized employes to surrender their keys to company facilities, and return other company equipment. Pepco warned union members in a letter last week that attempts to interfere with electrical service is a federal crime and the IBEW said it "would not condone" any such actions.

Pepco's "high profitability" was among the issues discussed during mediation, Scott said. The union argued that the utility's strong financial position allowed for higher wage increases and fewer benefit reductions. "In this instance, the company said they spent all they could and could not extend their package any further," Scott said.

Pepco's annual report characterized 1984 as "a year of exceptional growth in earnings, dividends" and the price of its stock. Pepco revenues were $1.2 billion and its net profit of $168 million was a 20 percent increase over the previous year. The firm reported a further increase for the first half of 1985.

Cross said that the company has substantially increased profit margins at a time when its unionized employes have seen the value of their wages decline. Since the IBEW's first contract with Pepco in 1979, wage increases have lagged 7.2 percent behind the Consumer Price Index, the union said.

Neither side would discuss the final wage and benefit offers, but Cross said that the combination of a "small" wage increase and benefit reductions would leave employes with little gains. Pepco sought to increase the deductible for medical coverage to $325 from its current $150, and to require employes to pay a greater share of costs in the event of serious illness, he said.

Pepco Vice President William T. Torgerson, in a letter to employes last week, said the company was "proud of its long history of nonconfrontational employe relations" and said Pepco "intends to have work available for all employes who want to work."