Preventive Maintenance (PM) can be

Preventive Maintenance (PM) can be defined as the activity undertaken regularly at preselected intervals, while
the device is satisfactorily operating, to reduce the accumulated deterioration, while repair is the activity to bring the
device to a non-failed state after experiencing a failure (Chen and Trivedi 2002, Akturk and Gurel 2007, Samrout
et al. 2009). In this policy, a unit is preventively maintained at fixed time intervals kT (k¼1, 2, 3, . . .) independent of
the failure history of the unit, and repaired in intervening failure situation (where T is a constant). The main
objectives of using PM are to reduce the number of failures and their economic consequences by performing
maintenance actions at predetermined points of time, regardless of the condition of the equipment/component. To
implement PM, the fixed cost is higher, as it needs decision-support systems, insufficient historical data, and
inaccuracy in assessing the time to action, especially when the standard deviation is large (Yuniarto and Labib 2006,
Batun and Azizog˘ lu 2009, Renna 2011).