The core of the residual current transformer generally adopts a ring structure, and a wound core or a punched core can be used. The size of the core is determined according to the structure of the residual current protection device, the size of the primary circuit conductor and the sensitivity requirements. 2) After winding or processing the iron core of high permeability iron-nickel soft magnetic alloy material, it must undergo high temperature heat treatment to meet the magnetic performance requirements. The iron core after high temperature heat treatment is very sensitive to mechanical stress. After being subjected to mechanical stress or impact, the magnetic performance will drop sharply, which will change the operating characteristics. Therefore, adequate protective measures should be taken to prevent the core from normal use during the design of the transformer. Affected by mechanical stress.