This invention relates to fault detection in electrical circuits. The invention provides an input module for electrically isolating an input signal received via a field sensor to be transmitted to a plurality of processors, in which the input module comprises a plurality of optical coupler/controller circuits; and each optical coupler/controller comprises a light emitting diode drive sub-circuit driving a light emitting diode; and an input sub-circuit; and in which a sensor voltage representing said input signal and a supply voltage are connected to each input sub-circuit; and each of said light emitting diode drive sub-circuits are connected in series between the supply voltage and ground.