The wiring of current transformers should follow the series principle: that is, the primary winding should be connected in series with the circuit under test, and the secondary winding should be connected in series with all instrument loads and current transformers. According to the measured current, select the appropriate ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the primary side high voltage from entering the secondary low voltage side once the insulation is damaged, causing personal and equipment accidents. An open circuit on the secondary side is absolutely not allowed. Once an open circuit occurs, the primary current I1 will all become the magnetizing current, causing a sudden increase in φm and E2, resulting in excessive saturation magnetization of the iron core, severe heating and even burning of the coil. The error increases. When the current transformer is working normally, the secondary side is used in series with current coils such as measuring instruments and relays. The impedance of the current coils such as measuring instruments and relays is very small, and the secondary side is similar to a short circuit.