Abstract: A series of signals are derived from respective transducers, fault detection switches, cubicle door interlock switches and the like at appropriate places in an industrial plant or apparatus. The signals are taken to a system which monitors them for the presence of a fault indicating signal or other signal of interest. The system has a series of scanner devices which are controlled by a logic circuit so as to scan the signals one after the other in a repetitive sequence. Since the transducers, switches and such may be linked one with another so that, when one is producing a fault signal, others later in the sequence may also produce such a signal, the logic system operates so that when a fault signal is located, a further scanning sequence is gone through so that there is located the first transducer or switch in the sequence which is producing a fault signal. This transducer or switch is then identified by a numeric or alpha-numeric display device.