The design of high integrity interlock systems involves more than the simple specification of high-quality components. Redundancy of components is often used as a strategy to improve the reliability of an interlock system. This paper uses quantitative reliability engineering methods to explore the relationships of component reliability, time between proof tests, redundancy and common cause failures to overall system performance. The paper demonstrates that addition to redundancy often, but not always, improves the reliability of an interlock system. The paper also demonstrates how common cause failures can greatly reduce the chances of an interlock system performing when needed.