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.

　　A current transformer basically consists of three parts: a magnetic core, a primary winding wound around the core, and a secondary winding wound around the primary winding in the opposite direction. Current through the primary winding creates a magnetic flux across the magnetic core. The magnetic flux in the core induces a voltage across the secondary winding. A measuring device in parallel with the secondary winding creates an oppositely directed magnetic flux in the magnetic core because the current through the secondary circuit is in the opposite direction to the winding. This magnetic flux balances the magnetic flux generated by the current through the primary winding. For this, the secondary terminals of the current transformers must be short-circuited with the measuring device or the load. Otherwise, the magnetic flux in the opposite direction will not occur, and due to the unbalanced magnetic flux, the magnetic core may heat up and fail. Furthermore, this situation is dangerous for the user as it increases the voltage on the secondary side.

　　Current transformers are basically used for measurement purposes. 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.