Considerable attention has been devoted by power utilities in recent years to the evaluation of power-system reliability. Most of the attention has been directed towards generation capacity, transmission and distribution system adequacy and few publications are available on protection systems. This paper reviews briefly the important concepts of reliability theory that will form a basis for analysing protective systems. The main purpose is to presume the equipment reliability data and to apply reliability concepts to evaluate the probability of failure of a system consisting of redundant relays, but with a single circuit breaker trip-coil. The study presents a development of protective system analysis by presenting some control configurations commonly used for protective systems, then it develops logic diagrams for these configurations and computes system reliabilities.