A guard door monitoring system comprising a guard door monitoring module for monitoring a state of a guard door to a safety area. The guard door monitoring system also comprises a control unit, configured to analyze a guard door signal generated by interaction between an actuator and a sensor, and to control a machine or system located in the safety area depending on the guard door signal. A locking device with a main body and a locking lever arranged on the main body is additionally provided. The locking lever can be moved relative to the main body from a release position into a blocking position in which the locking lever prevents the interaction between the actuator and the sensor, wherein the locking device comprises a receptacle for a lockable securing part, which, when introduced into the receptacle, keeps the locking lever in the blocking position.