ABSTRACT
A highly reliable system can be realized by employing redundancy. A standard parallel system, which consists of N identical subsystem, is the most typical redundant model. It was shown illustratively that the redundant system can be utilized for a specified mean time by either changing the replacement time or increasing the number of subsystems (Barlow and Proschan 1965). In addition, qualitative disparities between system reliability and subsystem (Barlow and Proschan 1965), and reliabilities of many redundant systems were discussed (Ushakov 1994). A variety of redundant systems with multiple failure modes (Pham 2003) and dependent failures of units (Blokus 2006) were considered. Furthermore, some optimization methods of redundancy allocation for series-parallel systems were surveyed (Zia and Coit 2010). The variety of redundant systems affects maintenance policies. Lately, a new classification of three preventive maintenance models was suggested (Wu and Zuo 2010) and optimal maintenace policies for 2-phase systems with worn states were investigated (MacPherson and Glazebrook 2011).
