Availability may be generically be defined as the percentage of time that a repairable system is in an operating condition. However, in the literature, there are four specific measures of repairable system availability. We consider only the limit availability, defined with the limit of the probability A(t) that the system is working at time t, when t tends to infinity.
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The limit availability just seen is also called intrinsic availability, to distinguish it from the technical availability, which also includes the logistics cycle times incidental to maintenance actions (such as waiting for the maintenance, waiting for spare parts, testing...), and from the operational availabilitythat encompasses all other factors that contribute to the unavailability of the system such as time of organization and preparation for action in complex and specific business context1.
The models of the impact of preventive and corrective maintenance on the age of the component, distinguish in perfect, minimal and imperfect maintenance. Perfect maintenance (perfect repair) returns the system as good as new after maintenance. The minimal repair, restores the system to a working condition, but does not reduce the actual age of the system, leaving it as bad as old. The imperfect maintenance refers to maintenance actions that have an intermediate impact between the perfect maintenance and minimal repair.
The average duration of maintenance activity is the expected value of the probability distribution of repair time and is called Mean Time To Repair - MTTR and is closely connected with the concept ofmaintainability. This consists in the probability of a system, in assigned operating conditions, to be reported in a state in which it can perform the required function.
Figure 4.17 shows the state functions of two repairable systems with increasing failure rate, maintained with perfect and minimal repair.
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