Abstract: A first fault locater system includes a plurality of monitored switches, each connected across a separate electrically isolated input terminal in circuit relation with an optical signaling means which lights up when a fault is detected in the monitored switch. Each signaling device is optically coupled to a separate silicon controlled rectifier which becomes conducting in response to the optical signal. The output from each silicon controlled rectifier is fed through gating circuits to initiate operation of amplifying and reversing means which imposes a negative bias on each silicon controlled rectifier when a first fault is detected. The negative bias serves to disable each of the monitoring circuits except the circuit detecting the first fault, which remains operated, until reset.