Grounding protection is the protection of grounding, which connects the metal casing of electrical equipment to the grounding body to prevent electric shock when the casing is electrified due to insulation damage of the electrical equipment, and the operator comes into contact with the equipment casing. In low voltage systems where the neutral point is not grounded, under normal circumstances, the exposed metal parts of various power devices that are not charged should be grounded unless otherwise specified. For example:

⑴ Enclosures for motors, transformers, electrical appliances, portable and mobile electrical appliances.

⑵ Transmission device of power equipment.

⑶ Framework of distribution panel and control panel.

⑷ The outer shell of the cable sheath, power cable junction box, and terminal box.

Metal protective pipes for power lines, steel cables laid, and crane tracks.

Towers equipped with lightning arresters for power lines.

The casing and brackets of power devices such as switches and capacitors installed on power line towers.

The requirements for grounding resistance of power devices in low-voltage power grids are as follows:

In low-voltage power grids, the grounding resistance of power devices should not exceed 4 ohms.

In a low-voltage power grid powered by a single transformer with a capacity of 100 kV · A, the grounding resistance of the power device should not exceed 10 ohms.

In low-voltage power grids with a total capacity not exceeding 100 kV · A, transformers operating in parallel with the same grounding device should not have a grounding resistance of more than 10 ohms for the power device.

When it is difficult to achieve the above grounding resistance value in areas with high soil resistivity, the grounding resistance of low-voltage power equipment is allowed to be increased to 30 ohms.

To prevent electrical equipment from being exposed to electric shock hazards due to insulation damage, connecting the metal casing of the electrical equipment to the neutral point of the power transformer is called neutral protection. In low-voltage power grids where the neutral point is not directly grounded, power devices should adopt low-voltage neutral protection. In low voltage power grids where the neutral point is not directly grounded, power devices should adopt low voltage grounding protection. In low-voltage power grids powered by the same generator, transformer, or busbar, it is not advisable to use both grounding protection and neutral protection simultaneously.

Protective grounding and protective neutral are two technical measures to maintain personal safety, and their differences are:

Firstly, the protection principles are different. The basic principle of low-voltage system protection grounding is to limit the voltage of leakage equipment to ground, so that it does not exceed a certain safety range; The protective grounding of high-voltage systems not only limits the ground voltage, but also plays a role in promoting the action of protective devices in the system in some cases. The main function of neutral protection is to connect the neutral line to make the equipment concentrate and form a single-phase short circuit, which promotes the rapid action of the protection device on the line.

Secondly, the scope of application varies. Protective grounding is applicable to general low-voltage ungrounded power grids and low-voltage grounded power grids with other safety measures taken; Protective grounding can also be used for high-voltage ungrounded power grids. An ungrounded power grid does not need to use protective grounding.

Thirdly, the circuit structure is different. The protective grounding system only has a protective ground wire in addition to the phase wire. The protective grounding system must have a neutral wire in addition to the phase wire; When necessary, the protective neutral wire should be separated from the working neutral wire; Its important devices should also have grounding wires.