A current transformer is a measuring transformer used to measure the current passing through an electrical circuit. Measuring large currents directly with measuring instruments is not only costly but sometimes very dangerous and difficult. Therefore, the current through the circuit must be reduced to a certain value before it can be measured. A current transformer in series with the circuit reduces the magnetic field created by the current passing through the primary circuit and transfers it to a second circuit called the secondary. Due to the circulating current in the secondary circuit, the amount of current flowing in the primary circuit can be measured by a measuring device connected in parallel with the secondary winding.

　　Current transformers work differently than standard voltage transformers. It contains two windings similar to a normal voltage transformer. Whenever an alternating current is supplied in the primary section, an alternating magnetic flux is generated which then excites the alternating current in the secondary section. In this form, the load impedance is very low. Therefore, the device operates under short-circuit conditions. Therefore, the current through the second section is based on the current in the primary winding, not on the load impedance.

　　Current transformers are basically used for measurement purposes. If a current transformer is not used, a large measuring device must be used, and a protective relay of a larger size must be manufactured to measure a large current value. This is an expensive and extremely dangerous option. The use of current transformers allows the use of smaller measuring equipment and protective relays. While this provides a more economical solution, it also enables safe operation. Because the measuring devices and protective relays connected on the secondary side are isolated from the primary voltage.