This paper proposes a two-phase maintenance policy for a group of identical repairable units. We define the time-interval (0, T] as the first phase, and the time interval (T, T+W] as the second phase. As individual units fail individual units have two types of failures. Type I failures (minor failures) are removed by minimal repairs (in both phases), whereas Type II failures (catastrophic failures) are removed by replacements (in the first phase) or are left idle (in the second phase). A group maintenance is conducted at time T+W or upon the kth idle, whichever comes first. The optimal policy is to select T*, W* and k* to minimize the expected cost per unit time for an infinite time span. Various special cases are considered. Numerical examples are given to illustrate the method.
