The principle of current transformer is based on the principle of electromagnetic induction. The current transformer is composed of a closed core and a winding. 

Its primary winding turns are very few, and it is connected in the circuit of the current to be measured, so it often has all the current of the circuit flowing through it, the secondary winding turns are more, and it is connected in series in the measuring instrument and protection circuit, when the current transformer works, its Secondary Circuit is always closed, so the impedance of the coil in series between the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to short circuit.