Safety circuits used in machine guarding applications incorporate devices such as sensors and relays to de-energize equipment if any hazardous condition is detected. These conditions normally involve a machine guard not being closed, a presence sensor detecting personnel in an unsafe area or the triggering of an emergency stop function.
It is also possible for the actual safety devices used, such as relays, to experience an internal fault, and force guided (FG) relays can be used to detect this condition.