Electronic residual current protection appliances do not have high requirements on the output power of the transformer. The section size of the core can be designed to be smaller. For residual current protection appliances with a small rated current, the transformer has a small diameter, such as high permeability iron-nickel soft magnetic alloy, the cross-sectional area can be designed between 10-20mm2; when the rated current is large, the transformer diameter Larger, the average magnetic circuit is also long, and the cross-sectional area of the core should be appropriately increased, which can be designed to be between 30-50mm2. For electromagnetic residual current protection appliances, transformers are required to have higher sensitivity and higher output power. Therefore, the cross-sectional area of the core must be increased. For example, residual current protection appliances with a rated current of 40-63A, such as high magnetic permeability The iron-nickel soft magnetic alloy with high rate, the cross-sectional area of the core is about 100-200mm2. For A-type residual current protection appliances, to increase the output during pulsating current, in addition to other corresponding measures (for example, changing the shape of the hysteresis loop, increasing the output during pulsating DC), the size of the cross-sectional area of the transformer core can be increased. The specific core size should be considered comprehensively according to the overall structure of the residual current protection appliance, the conductor size of the primary circuit and the requirements of detection sensitivity.