Model G9SX-SM is designed to monitor back electromotive force of machine’s motor to judge whether machine has stopped operating. It includes off-delay timer that is set to unlock Guard Lock Safety Door Switch after operation has stopped. Model G9SX-LM, using proximity sensor to monitor standstill and speed conditions. Unit allows machine to operate with door of protective fence open only when rotational speed of hazard source is below preset threshold and operator is pressing enabling grip switch.