The current transformer is composed of a closed core and windings. Its primary winding has a few turns, which is stringed in the line of the current that needs to be measured. Therefore, it often has all the current flowing through the line. The secondary winding has a large number of turns. It is serially connected to the measuring instrument and the protection circuit, and the current mutual inductance When the transformer is working, its secondary loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to short circuit. For a long time, the current mutual inductance transformer has been used to measure precision current as a standard device in the equipment. Even in harsh environments and high temperature conditions, this device is very accurate, easy to use and reliable.