The device is applicable to AC 50HZ power lines with rated voltage of 380V, and can operate independently on the grid. It is used for transformer load protection, line section switch protection and small substations, and for circuit automatic (manual) cut-off of overload current and fault protection. Using intelligent components to detect the operating current can accurately judge the relevant electrical operating conditions. In case of overload, short circuit, overvoltage, current imbalance, phase failure, leakage and other phenomena, the intelligent device can trip within 0.2 seconds according to the above conditions, and can automatically reclose about 10 seconds after tripping. If the fault disappears, the normal power supply will be restored, which can effectively protect the transformer and electrical equipment. Made of stainless steel, corrosion resistant, maintenance-free, long service life, strong breaking capacity, safe and reliable.