In an apparently unprecendented action, the Potomac Electric Power Co. has sued the District of Columbia's utility regulatory agency in an effort to get an immediate decision on its request for a $45 million, 16 percent rate increase.

Pepco is suffering "financial disaster" because the Public Service Commission has not made a decision on the request nearly 16 months after Pepco made it and five months after all legal arguments in the case were completed, Pepco told the D.C. Court of Appeals in its suit last Thursday.

"In this day and age, when everybody is concerned with regulatory efficiency, 16 months of silence is just not it's not comprehensible," said a senior Pepco officer who asked not to be identified.

The officer said nobody at Pepco can remember any past case in which the company - or any utility company in the nation - sued a regulatory agency to demand immediate action.

-It's not a simple case. We're working on it," the PSC chairman, Elizabeth Hayes Patterson, said yesterday. Patterson said she could not comment further on the company's suit, norcould she say when the commission's decision would be forthcoming.

The case is a complex one involving major issues that will affect the prices Washingtonians pay for electricity in the years ahead. At the same time that it is considering these issues, the three-member commission also is working on a gas price increase request, time-of-day electricity pricing, a proposed consumer bill of rights for the city and many other matters.

The suit represents an "improper attempt . . . both to reargue the case and to influence the commission with additional assertions and observations long after the record has been closed," wrote D.C. People's Counsel Brian Lederer in a legal brief opposing a previous effort Pepco made to prompt the commission to action.

Lederer, the public official who represents consumers in utility cases, said he would make the same argument with respect to Pepco's suit. Lederer added, however, that his office also is concerned about the lag in comission decision-making - but for a different reason.

"There's strong evidence in the record that their rates are too high," said Lederer, who argued in the hearings on the request that instead of an increase Pepco should be cut back its D.C. rates $21 million, or 7 per cent.

Such a cutback would not be unprecedented. Last May the Maryland Public Service Commission not only turned down Pepco's requested $24 million increase in that state but ordered the company to cut customers' bills by $248,000.

Pepco's current rate increase request in the District of Columbia is its sixth in eight years. During that period the company has been granted - not counting the presenr request - $109 million in rate increases out of a total $177 million requested.

Pepco's suit argues that Pepco suffered financial disaster during 1975 and 1976 because of the commission's "regulatory lag" - and that this "disaster . . . is again being experienced by Pepco for the same reason Pepco's financial results in the District of Columbia are day-by-day becoming more serious . . ."

The suit says that the company's rate of return on its investment in plant and equipment - the rate base - for 1978 will be 7.57 per cent for the D.C. portion of its business, far below the 9.06 rate authorized by the commission.

Likewise, the company's return on average common stock equity investment for Washington will be only 8.5 percent, much less than the 13 percent rate the commission has authorized the company to pay investors to attract their capital in the private financial marketplace.