This article investigates an optimal preventive standby activation policy for an m-out-of-n redundant system. We assume that for such a system, which starts operating at time t = 0, a standby component will be activated at either the fail- ure time of the system or at a predetermined time ????, whichever occurs first. We first obtain the system reliability function under this switching policy as a func- tion of ????. Then, we investigate the optimal switching time ???? so that the mean time to failure of the system is maximized. The results indicate that the existence of an optimal value of ???? depends on the lifetime of the standby redundancy and its virtual age in the standby state. Some illustrative examples are presented to examine the theoretical outcomes.

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