Current transformer (CT) is used for measuring electric energy and current, relay protection in the electric system. Features: 1) Type 1 (square circuit hole) capable of passing through either the cable or the purpose of monitoring 2) Type 2 (horizontal square hole) capable of passing through 1-3 bus bar is generally used for the purpose of monitoring 3) Type 3 (vertical square hole) capable of passing through 1-3 bus bar has the same characteristics of Type 1 and 2, and is also intended to measure the electric energy of power supply system (class 0.2). It can still be used at larger secondary impedance and act as the zero-sequence current transformer