Optimal Reliability Systems - Common Cause Failures

System reliability is a major challenge in system design. Unreliable systems are not only major source of user frustration, they are also expensive. Unfortunately, with the large component count in today's large-scale systems, failures are quickly becoming the norm rather than the exception. The reliability characteristic such as probability of survival, mean time to failure, frequency of failures and mean down time depend on the design and topological layout of the system. Common cause failure is an important phenomenon for a system with failure dependent parts. This paper deals with a single unit system that is operating in an environment exposed to the hazards of common cases failures, under some general assumptions the optional replacement policies were developed with the help of proposed measure of cost differences for old and new system.