The excitation current of the primary circuit of the residual current transformer is very small, and it works under the condition of weak magnetic field. For the residual current protection device with the rated residual operating current of 0.006A, the minimum working magnetic field strength is only about 0.080A/m. When working, the main circuit is often impacted by a large starting current of hundreds of amperes to a thousand amperes or a short-circuit current of tens of thousands of amperes, and the residual current transformer is in an extremely saturated state. under these circumstances. The transformer cannot affect the operating characteristics due to the residual magnetism generated by the inrush current. Therefore, the core material of the residual current transformer generally adopts iron-nickel soft magnetic alloy material with high initial permeability, high maximum permeability and low coercivity. The iron-nickel soft magnetic alloy material has extremely high initial permeability and maximum permeability, extremely low coercivity, low saturation magnetic flux density, and good temperature stability.