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The purpose of this paper is to develop a framework for the application of proportional hazard modelling to plant maintenance. Two regimes are investigated; a good-as-new regime where the hazard rate of the system is refreshed by either failure or preventive maintenance action. and a bad-as-old regime where only preventive maintenance action refreshes the hazard rate. The output of the analysis is the recommendation of optimal preventative maintenance plans under both of these regimes. Data for a local firm are used to illustrate the models. The inclusion of proportional hazard modelling is shown to yield improved maintenance plans in both regimes. A proposal for an adaptive scheme is made such that the maintenance plan can be adjusted as changing plant conditions warrant it.

An age model is proposed for modelling imperfect repairable systems operating under a non‐homogeneous Poisson framework. The imperfect repair model investigated here effectively includes good‐as‐new, imperfect repair and bad‐as‐old regimes that have appeared separately in reliability engineering literature. To incorporate the impact of the equipment's operating environment, proportional intensities assumptions are integrated into the age model. Maximum likeihood estimates are derived for the parameters of interest.

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In this paper, an analysis is conducted of the equipment failure regime known as bad‐as‐old. In such a regime, the repair of the machine after an unexpected emergency breakdown does not reset the hazard rate. Only a planned overhaul refreshes the hazard rate. This bad‐as‐old regime is modelled as a non‐homogeneous Poisson process. The structure is comprised of a base‐line hazard rate, here assumed to be Weibull, plus a covariate structure, since such (covariate) factors are found to influence equipment failure times. The results are compared to similar results1 when the underlying failure regime is assumed to be good‐as‐new. The analysis performed here is also compared to a previous analysis2 in which the base‐line hazard rate was of a non‐parametric form. Several tests are presented as well as an examination of the residuals in order to verify the appropriateness of the model chosen.

In this article, we survey the developments in the generalised models of repairable systems reliability during 1990s, particularly the last five years. In this field, we notice the sharp fundamental problem that voluminous complex models were developed but there is an absence of sufficient data of interest for justifying the success in tackling the real engineering problems. Instead of following the myth of using simple models to face the complex reality, we select and review some practical models, particularly the stochastic processes behind them. The Models in three quick growth areas: age models, condition monitoring technique related models, say, proportional intensity and their extensions, and shock and wearing models, including the delay-time models are reviewed. With the belief that only those stochastic processes reflecting the instinct nature of the actual physical processes of repairable systems, without excessive assumptions, may have a better chance to meet the demands of engineers and managers.

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In this article, we survey the developments with respect to generalized models of repairable systems during the 1990s, particularly for the last five years. In this field, we notice the sharp fundamental problem that voluminous and complicated models are proposed without sufficient evidence (or data) for justifying a success in tackling real engineering problems. Instead of following the myth of using simple models to face complicated reality, and based on our own research experiences, we select and review some practical models, in the quickly growing areas: age models, condition monitoring models, and shock and wear models, including the delay-time models. Further, we also notice that there is an attempt to develop synthetical models from a different point of view. Therefore, we comment the relevant developments with strong emphasis on stochastic processes reflecting the intrinsic nature of the actual physical dynamics of those repairable system models.