Since the hardware reliability of microcomputer protection relays has been greatly enhanced， the main factors that influence the protection system reliability are protection principles and configurations. To assess the reliabilities of different protection principles and configurations， a novel reliability model is proposed. Based on the Markov method and considering two failure modes（false-function and fail-function）， the new reliability model is derived. The method can be used to calculate the probability of false-function and fail-function. The method also provides a mathematical way to improve the system risk assessment and reliability analysis. The results of case study prove the correctness of the model.