RELIABILITY OF A REDUNDANT SYSTEM WITH RENEWAL AND PREVENTIVE MAINTENANCE.

Two types of preventive-maintenance work are encountered in practice. These are called rigid and sliding preventive maintenance. In the case of rigid preventive maintenance, the operating element is removed for preventive maintenance after a definite (in general, random) time interval, independently of the state of the second element (which may at the instant of beginning preventive maintenance be in an operating state, on standby, or undergoing repair in the case of failure or preventive maintenance work). With sliding preventive maintenance, it is removed to be sent for preventive maintenance only if the second element is in an operating condition. Thus there may be cases where an element, when its preventive maintenance is completed, has to continue operation until the return of the next element from repair (or even prior to failure of the operating element). The basic problem is to find the distribution functions of the duration of failure-free operation of a duplicated system, that is, the time from the beginning of operation of the system until both elements are in a nonoperating state because of either a failure or preventive maintenance work. Earlier work which found a minimal advantage of a sliding preventive maintenance over a rigid one is here evaluated and new conclusions reached.