Abstract: A system for detecting the presence of an average dc current component in a load comprising a current transformer 10 for supplying a signal representative of the current, and a comparison means 12 and 13, for example a microprocessor and associated RAM, for comparing values of the current taken at time intervals corresponding to a phase interval of substantially n.pi., where n.pi. is an odd integer, of the ac periodic supply signal, so as to determine the presence of a dc current component caused by rectification of the supply. In one embodiment, the signal from a voltage detecting means 14-18 can be used to synchronise the sampling of the current at the correct phase.