The Merit Systems Protection Board has reopened cases of more than a dozen federal workers who were fired or demoted this year for unacceptable performance.

MSPB's action could have a major bearing on the application of President Carter's new civil service reform law. It went into effect in January. The law is designed, in part, to streamline federal procedures to make it easier to fire incompetents.

MSPB was set up to give civil servants a place to appeal disciplinary action imposed by their agencies for a variety of reasons, including failure or inability to do a job.

In past years the govenment has taken actions of some kind -- firing, suspension or grade and pay cuts -- against about 20,000 workers annually for a variety of reasons, including poor performance, misconduct, or abuse of leave.

The Board's action puts a "hold" on a number of its earlier decisions. They involve employes in Air Force, Va, the Labor Department, Social Security Administration, Treasury, Commerce, Interior and the Department of Transporation. They involve agency disciplinary action for unacceptable performance.

In most of the cases the MSPB has upheld the agency. In a few, it reversed the firings or demotions either on procedural grounds or on merits of the cases themselves.

Most employes involved in the cases already are off the federal payroll. they will remain so pending a decision in the case of Wells vs. Harris now before the MSPB. It concerns the legality of interim regulations issued by the Office of Personnel Management that govern agency procedures for taking action against poor performers. OPM is the successor agency to the old Civil Service Commission.

The Wells vs. Harris case was brought by the American Federation of Government Employes. AFGE, the biggest federal union, contends that firings or demotions cannot be ordered by agencies until they have their own individual performance appraisal systems in operation, rather than interim regulations issued by OPM.

AFGE feels that agencies must negotiate with unions in determining so-called "critical elements" used in evaluating performance standards.The union also says those critical elements are important parts of a job, such as the ability to type in order to be a typist. Performance involves number of words typed per minute, accuracy or the like.

The next step is for the MSPB to decide if the OPM regulations are legal, and if so, whether they have been, and are being applied properly by all federal agenices.

The Wells vs. Harris case involves Social Security, biggest agency within HEW. It already has taken disciplinary action against approximately 30 workers for alleged poor performance.

Similar actions have been taken by other agencies and the MSPB decision -- depending on how it goes -- will determine whether agencies can forge ahead with disciplinary actions using OPM regulations or whether they must wait until their own programs have been approved and activated. The deadline for agencies having their own appraisal systems is October 1981.