A current transformer (CT) is a type of transformer used to measure alternating current (AC). It produces a reduced current in its secondary winding proportional to the current flowing in its primary winding. This reduced current can then be used for metering or protective relaying. Current transformers are commonly used in electrical power systems to measure the electrical current of a circuit and provide a signal proportional to the measured current. They are also used to provide an isolated signal for use with measuring or control instruments.