At present the reliability study of relay protection mainly focuses on assessing the reliability of the protection system itself. But the quantitative influence of protection system on the reliability of primary equipment is rarely analyzed, which is necessary for the large power networks. A Markov model is established to evaluate this effect. The unavailability of primary equipment attributed to protection system is obtained and sensitivity of some reliability parameters is analyzed. The reliability of large-scale power system may be further evaluated accurately based on the proposed method. The results of IEEE-RTS79 prove the correctness of the proposed method.