Safety controllers (such as SCHMERSAL’s AES and AZR Series) are connected between machine guarding interlock/E-Stop switches and the machine’s stop control elements (such as a motor contactor or control relay). These controllers contain dual, self-checking safety system monitoring circuits and positive-guided output relays. Each is designed to monitor faults in the safety system’s interlock/E-Stop switches, the safety circuit interconnection wiring, and their own internal monitoring circuits and output relays. Detection of a fault in the machine’s safety circuit or of an open machine guard, disables the module’s output signal(s) facilitating machine stoppage, and/or prevents the restarting of the machine until the fault has been corrected. In addition to detecting open guards and/or actuated EStop switches, safety controllers are capable of detecting the following types of safety system faults. Some microprocessor-based safety controllers, such as SCHMERSAL’S AES Series, also feature integrated system diagnostics with visual LED outputs which indicates fault type and location — thus minimizing machine downtime.