a system for preventing accidents in the operation of a monitored apparatus carried by a user has terminal in direct contact with the body of the user and provided with a circuit for continuously or periodically transmitting an authorizing user data signal through the body of the user . a signal receiver assigned on the monitored apparatus or machine has an interface in contact with the user for receiving the signal , a circuit for continuously or periodically testing the received data signal , another circuit for outputting a clearance signal that allows operation of the monitored machine or apparatus after a successful test of the received authorizing user data signal , and yet another circuit for terminating output of the clearance signal following a successful test of the authorizing user data signal when a subsequent test of the data signal fails .