Maryland telephone bills will go up about 20 cents a month as a result of a $9.1 million rate increase for Chesapeake and Potomac Telephone Co. approved yesterday by the Maryland Public Service Commission.

At the same time, Potomac Electric Power Co., went to court to try to block a Maryland PSC order to roll back its rates by $3.6 million a year.

The telephone rate increase, amounting to 1.845 per cent, will be added to monthly charges for basic service and to intra-state long distance charges, said a S & S spokesman.

Interstate toll calls, pay phone charges and any other charges of less than 20 cents will not be charged under the new rates, which are expected to go into effect Dec. 12.

C & P had asked for the rate increase to cover the cost of pay raises in a three-year contract with the Communication Workers of America that was signed in August.

The higher telephone charges will enable the company to continue to make the 9.25 per cent rate of return on investment authorized by the PSC last May.

The rate increase is C & P's second in Maryland this year. In May, a $19 million rate increase package added about 25 cents to basic bills and boosted the price of pay telephone calls to 15 cents.

Telephone company officials yesterday said no immediate need for further increase in telephone charges is foreseen.

Originally C & P asked a 2.5 per cent increase, but it scaled down that request on Nov. 1 to comply with a new PSC interpertation of how wage increases could be passed through to customers.

Although telephone bills will be going up, Maryland electric rates will remain unchanged while Prince George's County Circuit Court considers a lawsuit against the PSC.

Pepco sued the regulatory agency yesterday, appealing a Nov. 15 decisions ordering Pepco to reduce its rates by $3.6 million.

The $3.6 million is the amount Pepco would be allowed to make on its investment in the Douglas Point nuclear power plant, which the utility company now has decided not to complete.

The PSC ruled that electric customers did not have to help pay for the unfinished and unneeded atomic power plant.

pepco's lawsuit contend that if the PSC is going to adjust the untility's investment rate base downward by removing the money spent on the abandoned power plant, it should also adjust the rate base upward to show new investments.

Neither the PSC nor Pepco has estimated how the case will affect Maryland electric rates.