Each maintenance type has cost and time items that mainly affect on its average unit cost. For The CM model, the main critical parameter is , time to retrain system after its unscheduled stop. is the cost that increase considerably the average cost for PM. With preventive maintenance, in fact, the component is replaced whatever its eventual residual life. In the CBM model, finally, the main costs are related to inspection activities.
Before applying these models to select the more suitable maintenance type, some observations must be made. In particular PM is a valid alternative to the CM, two conditions must occur:
1) The overall cost of PM is lower than the total cost of CM (otherwise it should always be waited for the occurrence of the fault);
2) The failure rate of component decreases: in the case of constant failure rate, in particular, the occurrence of a fault is modelled as a Poissonian event as (negative exponential model). Since the phenomenon is memory-