A current transformer is a device for measuring a current flowing through a power system and inputting the measured current to a protective relay system. Electrical power distribution systems may require the use of a variety of circuit condition monitoring devices to facilitate the detection and location of system malfunctions.

A current transformer consists of primary and secondary coils of wire wrapped around a core, usually made of steel or a nickel alloy. Transformer windings are electrically insulated from each other and from the core. The winding connected to the power supply is called the primary winding. The transformer winding in which current is induced is called the secondary winding.