Current transformer is a kind of current conversion equipment. It turns high-voltage and low-voltage large current into low-voltage small current to supply appearance and relay maintenance equipment, and separates appearance and maintenance equipment from high-voltage circuit. The secondary side current of the current transformer is 5 A, which makes the measurement appearance and relay maintenance equipment safe and convenient, and also makes it standardized in production.

The structure of current transformer is composed of iron core, primary winding, secondary winding, wiring terminal and insulation support. The number of turns of the primary winding of the current transformer is less, which is connected in series in the line requiring the measured current, flowing through the larger measured current, and the number of turns of the secondary winding is more, which is connected in series in the measuring surface or relay maintenance circuit.

Open circuit is not allowed for the secondary circuit of current transformer. During the operation of the current transformer, its secondary circuit is always closed, but because the impedance of the series winding of the measuring appearance and maintenance equipment is very small, the operation of the current transformer is close to the short circuit, most of the magnetization force generated by the primary current is compensated by the secondary current, the total magnetic flux density is small, and the potential of the secondary winding is also small. When the current transformer is open circuit, the impedance of the secondary circuit is infinite and the current is equal to zero. The primary current completely becomes the excitation current. A high potential occurs in the secondary winding, threatening personal safety and causing damage to the appearance, maintenance equipment and secondary insulation of the transformer.

It is necessary to ground the secondary circuit of current transformer to avoid primary insulation breakdown and secondary series high voltage, which will threaten personal safety and damage the equipment.