[en] Expressions for the average unavailability of various common logic configurations are developed, taking into account the probability of failure of components due to human error during inspection and repair. The probability of a component being down due to human error is taken to depend upon the number of components that have gone through the tests consecutively prior to the inspection of the component under consideration. The derived expressions are compared with those employed in WASH-1400 and are used to evaluate the average unavailability of a subsystem of a pressurized water reactor. The unavailability expressions are then used to show how the optimal (according to a given criterion) test interval and test duration time can be estimated in order to meet a given unavailability goal