The structure of a current transformer is very similar to that of a normal transformer. The iron core of the current transformer is composed of silicon steel laminations. A current transformer (CT) basically has one or more turns of a primary coil of a large cross-sectional area. In some cases, the strip carrying the high current may act as the primary. It is in series with the line carrying the high current.

　　The current transformer consists of a primary coil, a secondary coil, an iron core, an insulating bracket, and an outlet terminal. The iron core of the current transformer is made of laminated silicon steel sheets. Its primary coil is connected in series with the main circuit, and through the measured current I1, an alternating magnetic flux is generated in the iron core, so that the secondary coil induces the corresponding secondary current I2. If the excitation losses are ignored, then I1n1=I2n2, where n1 and n2 are the number of turns of the primary and secondary coils, respectively. The conversion ratio of the current transformer is K=I1/I2=n2/n1.

　　Since the primary coil of the current transformer is connected to the main circuit, the primary coil must be grounded with an insulating material suitable for the primary line voltage to ensure the safety of the secondary circuit and the human body. The secondary circuit consists of the secondary coil of the current transformer, the instrument, and the current coil of the relay in series. Current transformers can be roughly divided into two categories, current transformers for measurement and current transformers for protection.

　　The principle of the current transformer is based on the principle of electromagnetic induction. It's primary winding often carries all the current of the line through it. When the current transformer is working, its secondary circuit is always closed. Therefore, the measuring instrument and the protection circuit are connected in series with the coil. The impedance is very small, and the working state of the current transformer is close to a short circuit.