Abstract: A system is provided for early detection of leakage currents through ground connections of loads such as three-phase motors connected to isolated secondary windings of supply transformers. An alarm device is energizable when leakage current is a small fraction of a predetermined value which might present a potential hazard, the point of energization being adjustable by an operator until the predetermined value is reached. Thereafter, it is adjustable only by a supervisor, after operation of a key switch. Circuitry is provided to obtain sensitive, stable and reliable operation.