Contract talks between Potomac Electric Power Co. and Local 1900 of the Internationl Brotherhood of Electrical Workers adjourned at 6 p.m. yesterday, with company officials reporting "continued progress" in resolving outstanding economic differences with the union.

The adjournment came at the behest of Federal Mediation annd Conciliation Service officials, who said the union needed time to study Pepco's lastest contract proposals, offered late yesterday afternoon.

Bargaining was scheduled to resume at 10 a.m. today.

The adjournment also postponed for another day any possible strike action by Pepco's 3,500 unionized workers. A strike could have started at 6 p.m. yesterday, when the union's 48-hour notice of its intent to terminate formally its old three-year contract took effect.

The old agreement between Pepco and Local 1900, which has been representing Pepco workers since 1979, actually expired May 31. But a contract provision allows the parties to continue the agreement past expiration, pending 48 hours' written notice of intent to terminate the pact by either side. Local 1900 officials gave Pepco representatives such notice at 6 p.m. Tuesday.

The two sides have been trying to work out differences over proposed job classification changes, wages and sick leave.

On Wednesday, sources close to the negotiations said the bargainers were making "some progress" in removing outstanding issues. Company spokesman held to that line yesterday afternoon.

But one union official, speaking "not for attribution," told The Washington Post at 5:30 p.m. yesterday that "things look bad." The union official declined elaboration. And both union and company officials declined to give details on the company's latest offer.

Pepco rank-and-file members last week voted 2,886 to 6 to authorize a strike if "no substantial progress" was made at the bargaining table. Such votes are as routine in collective bargaining as the term "substantial progress" is deliberately vague.

However, Pepco officials' strike fears have been heightened by a lengthy IBEW walkout at another utility company. The Public Service Electric and Gas Co. of New Jersey, based in Newark.

Fourteen IBEW locals representing 4,280 worker struck the New Jersey utility May 1 in a wage dispute.