Spares are commonly used to improve system performances. They are allocated to original components during system missions. The optimal allocations depend on system configurations and lifetimes of components and spares. Various methods for finding optimal allocations have been proposed in the literature. For sake of brevity, lifetimes of components are commonly assumed to be independent. This paper deals with series systems, a common configuration, under a general setting, i.e. component lifetimes are dependent and heterogeneous. Moreover, the spare is also allowed to switch among original components to impose more flexibility for spare managements. This allowance occurred usually in network servers and electrical generators which manage by a dispatching center. Explicit expressions for system reliability functions are derived in detail. Since system lifetimes are random phenomena, stochastic orders are utilized for comparison purposes. Various illustrative examples are also given.

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