Protection systems exhibit two important types of failures: failures to operate and unwanted operations. Both failure modes have been introduced in a model description designed for the evaluation of the influence of preventive maintenance on protection system reliability performance. The model description is constructed as a continuous time Markov chain. The system states and the transitions be1,ween states are generated in a general and systematic manner which makes it possible to apply the model description to a lot of different situations. Through an example it is shown that (the quality) of preventive maintenance can have significant influence upon 1 he reliability performance of a given system.