A federal judge refused late yesterday to block implementation of the Potomac Electric Power Co.'s new drug and alcohol rules after utility officials said they would not randomly test or search workers pending arbitration of a union grievance covering the regulations.

The new rules, issued March 13 after negotiations between the company and the union representing 3,300 Pepco employes broke down, said that the firm "when it deems appropriate" would test workers for drug use. The rules also said that, as a condition of employment, workers had to submit on demand to a body or vehicle search.

Under the rules, employes who refused to be tested or searched would be fired immediately.

U.S. District Judge Louis F. Oberdorfer said in denying a request for a preliminary injunction by the International Brotherhood of Electrical Workers, Local 1900, that the conditions agreed to by the firm's officials "make explicit that the company will continue to adhere to certain previous practices which protected the employes' privacy and employment rights."

But Oberdorfer said union officials could renew their request to block implementation of the rules if the company does not comply with the conditions it agreed to.

William H. Jones, Pepco vice president for corporate affairs, said last night that "the important thing . . . is that the rules can go into effect."

Union officials said they would not comment on the ruling until they had a chance to study it.

Last week, U.S. District Judge Harold F. Greene barred implementation of the new rules for five days, calling them "Draconian . . . and hysterical measures" that forced employes to "undergo invasions of privacy which are almost unheard of in a free society."

Oberdorfer said during a hearing yesterday that he considered an injunction "the equivalent of a bolt of lightning" and one that he didn't like to use it because it forced him to intervene in the operation of companies.

"Because no new injuries, other than temporary loss of employment, are threatened as a result of the rules" as they would be implemented under the agreement, Oberdorfer said, there was no need to stop the rules from going into effect pending a decision on the union's grievance.

According to testimony during yesterday's hearing, the main change in the rules that can go into effect is stricter disciplinary action -- including immediate dismissal -- against employes who test positive for drug use or refuse to submit to a test.

Local union President Terry Cross testified that before the new rules were instituted March 13, an employe who tested positive for drug use usually was given a 20-day suspension and then a one-year probationary period during which the employe had to submit to random tests.

Under the new rules, Pepco industrial relations manager William Wolverton testified, any employe who tests positive for phencyclidine (PCP), heroin, morphine, cocaine or other "hard" drugs will be discharged on the first offense.

An employe who tests positive for marijuana will be suspended for five days, then retested. If the employe fails the second test, he or she will be discharged, except in rare instances.