대기부품은 대기기간 중에 우발적 고장이 발생할 수 있으며(type I failure), 해당상황이 장기간 방치되는 것을 방지하기 위해 주기적인 검사를 하는 것이 일반적이다. 그러나 검사가 대기기간 중 발생한 고장을 확인할 수 있게 하는 반면, 검사를 시작할 때 대기하던 부품에 부하를 가하는 과정에서 고장을 유발할 가능성이 존재하며(type Ⅱ failure), 검사시간동안 대기부품을 작동시킴으로써 열화에 의한 고장발생(type Ⅲ failure)의 가능

This paper derives the limiting availability of the standby unit by considering three types of failures. Assuming the standby unit can fail during the standby period (type Ⅰ failure), the periodic test is employed to check the unit condition. Although the periodic test is capable of detecting failures during the standby period, it can cause test-induced failures for the standby unit at the start of the test (type Ⅱ failure). Also the aging process of the standby unit during the test period needs to be taken into account (type Ⅲ failure), which is not treated in the existing studies. To identify relationships between the optimal periodic test interval an various failure characteristics of the standby unit. several experiment are performed assuming Weibull distribution as an operating time distribution of the standby unit. The result of this paper can be applied to determine an appropriate test interval for the standby unit in priority redundant system to enure higher reliability characteristics.