Maintenance is a costly activity. It is one of the sizeable indirect costs that enter in to manufacturing and hence it should be given considerable attention by the operations managers .

Figure below shows that total cost decrease up to point M under preventive maintenance and beyond this point an increasingly higher level of preventive maintenance is not economically justified. The level of degree of maintenance at point M, which gives the minimum total maintenance cost is known as the optimal level of maintenance activity or ideal level of maintenance. It is seen from figure below that at the optimal level of maintenance preventive maintenance cost equals the break down maintenance cost.