The readiness of supervised protective systems for nuclear reactors is analyzed by the use of a four-state Markov process. The supervisions are short and reveal the capability of the system to initiate the protection action. When this capability—defined as the probability of initiating the protection action—is found to be below an acceptable level, the reactor is shut down. The analysis interrelates the probability level of readiness with the desired readiness of the protection system and its rate of supervision.