The present status of maintenance strategies and the impact of maintenanceon reliability

In this paper, the most frequently used maintenance strategies are reviewed . Distinction is made between strategies where maintenance consists of repl acement by a new (or "good as new") component and where it is represented b y a less costly activity resulting in a limited improvement of the componen t's condition. Methods are also divided into categories where maintenance i s performed at fixed intervals and where it is carried out as needed. A fur ther distinction is made between heuristic methods and those based on mathe matical models; the models themselves can be deterministic or probabilistic . From a review of present maintenance policies in electric utilities it is c oncluded that maintenance at fixed intervals is the most frequently used ap proach, often augmented by additional corrections. Newer "as needed"-type m ethods, such as reliability-centered maintenance (RCM), are increasingly co nsidered for application in North America, but methods based on mathematica l models are hardly ever used or even considered. Yet only mathematical app roaches where component deterioration and condition improvement by maintena nce are quantitatively linked can determine the effect of maintenance on re liability. Although more complex, probabilistic models have advantages over deterministic ones: they are capable of describing actual processes more r ealistically, and also facilitate optimization for maximal reliability or m inimal costs.