Insight into Standby Redundancy via Unreliability

Standby redundancy is the oldest form of redundancy using switches. After a failure of the working element, an identical spare is substituted by switching. Unfortunately, the Coverage (the probability of a successful reconfiguration) of the system is always less than unity and, furthermore, the spare element might fail before it is switched in. Using, certain assumptions, this paper provides simple formulae for the reliability of a standby system. Criteria for the Coverage and the best number of standby elements are presented. Simple expressions for a Mission Time Improvement Factor are derived. A general discussion of standby redundancy is given to place the assumptions in proper perspective.