The residual current transformer is the detection element of the leakage protector. Its main function is to detect the residual current (earth fault current such as electric shock, leakage, etc.) of the main circuit through the core of the transformer, and transform the residual current of the primary circuit into a secondary circuit The output voltage. The residual current transformer is one of the most critical components in the leakage protector. The grid wire passes through the magnetic core of the RCT, and the RCT detects the phasor sum of the current of the protected circuit. Under normal circumstances, the currents of each phase are balanced, and the phasor sum of the primary current I through the RCT is equal to zero, which is known from Kirchhoff's current law. The sum of the magnetic flux Φ generated by the working current of each phase line in the toroidal core of the current transformer is also zero. When someone gets an electric shock or other ground leakage faults occur, due to the existence of leakage current, the phasor sum of each phase load current (including the neutral current) passing through the primary side of the current transformer is no longer zero.