Connect the housing and metal bracket of the electrical equipment to the neutral line, which is called the protective zero. In the power grid where the neutral point of the three-phase four-wire system is directly grounded, the protective zero connection is widely used. When the insulation damage of the electrical equipment causes a single-phase shell to be touched, the voltage of the equipment shell to the ground is the phase voltage, and a serious electric shock accident will occur if the human body touches it.

After the protection is connected to zero, the shell-touching phase current forms a single-phase closed loop through the zero line. Due to the small resistance of the neutral wire and the large short-circuit current, short-circuit protection devices such as fuses blown or automatic switches are activated in a short time, cutting off the power supply of the faulty equipment, thereby avoiding electric shock.

It must be noted that the protection principles of protective zero and protective grounding are different. Protective grounding is to limit the voltage of the leakage equipment shell to the ground so that it does not exceed the allowable safety range: and the protection zero connection is to make the leakage current form a single-phase short-range through the neutral line, causing the protection device to act, thereby cutting off the power supply of the faulty equipment.

Note that in the same transformer-powered system, protective grounding and protective grounding cannot be mixed, and it is not allowed to use protective grounding for some equipment. While another part of the equipment uses protective grounding. Because when a protective grounding device is used, one phase is in contact with the shell At this time, a voltage to ground will appear on the neutral line of the power supply, and a zero-connected device will generate a voltage to ground, causing more electric shocks.

The grounding resistance should be determined according to the allowable voltage of the enclosure after the equipment leaks. For low-voltage power grids below 1000 volts, the protective grounding resistance should not be greater than 4 ohms. The transformer capacity should not exceed 100 kVAh, and the grounding resistance should not exceed 10 ohms.

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