The impedance of N-wire is at the milliohm level, and its unbalanced voltage of load neutral point is the voltage drop of N-wire current on N-wire impedance with small value. Even if the three-phase load is seriously unbalanced, it is enough to clamp the neutral point potential of load on the neutral point potential of power supply. While the ground resistance is at the ohm level, which is several hundred times larger than the N-wire impedance, so it is impossible to clamp the neutral point potential of load on the neutral point potential of power supply.