One of the factors giving rise to failure of steel reinforced aluminium power line conductors (ACSR) is corrosion due to the loss of zinc galvanising from the steel. As the zinc coating deteriorates, so galvanic corrosion of the aluminium sets in, resulting in weakening of the conductor, overheating and eventual failure. The corrosion occurs within the conductor and is caused by the ingress of pollution and moisture to the interface between the aluminium and the galvanised steel strands. (Ref 1,2). Originally, conductors were manufactured from dry strands but in later conductors the strands (except the outer layer) were bitumen or grease coated to improve resistance of degradation. Methods of identifying internal corrosion in ACSR includes infrared scanning of energised lines, conductor diameter measurements and visual examination. Corrosion is usually well advanced before it is identified by these methods. In order therefore to obtain early warning of internal corrosion, a specialised corrosion detector based on the measurement of eddy currents was developed by the Central Electricity Generating Board. (Ref 1). This Overhead Line Corrosion Dector (OHLCD) has been brought to the market place by CORMON Ltd and this paper describes the construction and operation of the commercial system.