The standby has been widely applied to improve the reliability of system. And the standby unit is usually activated only when the active units fail under the common switching policy. But it would not always make the system most reliable. In this paper, based on a two-unit standby system without repair, we introduce the active switching policy in which the standby unit is activated at either a pre-fixed time or the failure time of active unit. Considering the perfect and imperfect switching, the survival function and mean time to failure of system are derived using the general time-to-failure distribution under the active and common switching policy, respectively. Further, if the lifetimes of units follow the exponential distribution, the cases where the active switching policy is superior are specified clearly. For the Weibull distribution, an application example is presented and it demonstrates that the active switching policy sometimes is still superior. Besides, the essence of the active switching policy and the reason why it is more effective are simply discussed. The study proves that it is likely to make the system more reliable by adopting other switching policy rather than the common one.
['Xiang Jia', 'Hao Chen', 'Zhijun Cheng', 'Bo Guo']
A comparison between two switching policies for two-unit standby system