The Charles County branch of the NAACP has asked the Maryland Human Relations Commission to investigate allegations that the county sheriff's office has discriminated against blacks in promotions, disciplinary actions and job assignments.

The NAACP branch president, Gerald R. Watson, said that Sheriff David D. Fuller recently presented four white deputies for promotion to the rank of lieutenant or captain, and that promotions for black officers were limited to the lesser ranks of sergeant and corporal.

Fuller said yesterday that the charges were "ludicrous," that blacks are well represented in every rank except lieutenant, and that he has not received any specific complaints alleging racial discrimination.

"Nobody will say what the problem is," Fuller said. "It's something we can't see. Usually people will say what their bitch is, but nobody has. It doesn't make sense." According to Fuller all promotions are made in accordance with the state employe evaluation system.

Only one of 13 officers above the rank of sergeant is black, Watson said, while 17 of the department's 96 officers are black. A U.S. District Court in 1974 ordered the department to raise the number of black officers to 22 percent, or 21 members, a number in proportion to the county's black population.

Watson acknowledged that the department had made some progress in its hiring of black officers. Five blacks were in the department in May 1973, according to a spokesman, Sgt. Charles J. McDevitt.

In a filing Wednesday, the NAACP asked that state human relations investigators examine 14 areas of concern to the black deputies, including the number of black deputies and employes at the detention center, job assignments and promotion rates for black officers, according to Watson.

The complaint also asked investigators to examine practices and policies of affirmative action and equal opportunity within the sheriff's department and detention center.

Some black officers complained that their workloads differed from those of white officers of equal rank in the same job, Watson said. The NAACP asked officials to scrutinize "the equality of job assignments of black deputies."

On July 8, 11 black deputies resigned from the local Fraternal Order of Police. Watson said the deputies resigned because they did not share equally with white members of the organization in benefits, travel, and standing.