The protection range is limited whether the protection is connected to the ground or the ground. For example, the protection of zero connection is to connect the metal casing of the electrical equipment to the zero line of the power grid, and to install the fuse on the power side. When the electrical equipment has a shell fault (when it is touching with the shell), the single-phase short circuit of the relative zero line is formed. Because the short-circuit current is large, the fuse is quickly fused, and the power supply is disconnected for protection. Its working principle is to change the "touch shell fault" to "single phase short circuit fault", so as to obtain large short-circuit current and cut off the insurance. However, the site of the electrical touch shell fault is not frequent, often happens is the leakage fault, such as equipment, damp load is too large, too long, line insulation aging caused by leakage, the leakage current value is small, can quickly cut off the insurance, so the fault does not automatically eliminate long time existence. But this leakage current poses a serious threat to personal safety. Therefore, the need to install more sensitive residual current circuit breakers for supplementary protection.