Abstract: A residual current detector capable of detecting an earth leakage current, e.g. for use in a circuit breaker, includes a toroidal core through which passes the live and neutral wires of an electrical installation, to form the primary windings of a differential transformer. At least one secondary winding is wound on to the core and driven with an alternating voltage by a generator. A phase sensitive detector identifies a residual current by detecting the change in phase of the voltage across the at least one secondary winding. The secondary winding may comprise two coils, a first coil driven by the generator and a second coil adapted to pick up an induced voltage from the toroidal core. The phase sensitive detector, which may form part of a phase locked loop, compares the phase of the signal from the second coil with that of the alternating voltage of the generator.