The protection object of over voltage and under voltage protection devices is electrical equipment. The key protection object of electric leakage protection is the person. The over voltage protector is to detect the power supply voltage and decide whether to act or not, and the leakage protector is to detect zero sequence current and decide whether to act or not.

Over voltage and under voltage protection devices, over current protectors and electric leakage protectors of the three are used each other in series, which inevitably have the front and back points when the circuit is connected. In principle, the arrangement is all right. But the circuit power is with overcurrent protector (air switch), power opened air switch can make the undervoltage protection with undervoltage tripping, so the protector is placed in the open space in front of (not affected by power failure). There is no need for the leakage protector to continue working, so it is in the end.

Over-under voltage protection - - - mainly to the equipment, control voltage in a certain range,  give a warning or disconnect circuit if beyond the range.