In order to ensure the safe operation of electrical equipment, certain points in the power system are grounded, called working grounding. For example, the neutral point grounding of power transformers and transformers are all working grounding. The common point of the star connection of the three-phase windings of the power transformer is the neutral point, and the neutral line (neutral line) drawn from the neutral point is used as a single The dual role of phase wires and electrical equipment safety protection.

In the three-phase four-wire low-voltage power system, there are many advantages of using working grounding. For example, grounding the neutral point of the low-voltage side of the transformer can prevent the voltage from the low-voltage side to the ground from increasing when the high-voltage side coil insulation of the power transformer is damaged, thereby ensuring the safety of people and equipment. At the same time, the neutral point can be prevented from displacement when the three-phase load is unbalanced, so as to avoid the three-phase voltage unbalance. In addition, zero-connection protection can also be used to cut off the power supply when the three-phase load is unbalanced to avoid the other two phase-to-ground voltage increases.

Connecting the metal shell or metal bracket of electrical equipment to the grounding device is called protective grounding. Protective grounding can effectively prevent electric shock accidents and ensure personal safety. Protective grounding is mainly used in power systems where the neutral point is not grounded. When the insulation of electrical equipment is damaged and the phase line touches the shell, the shell of the device is electrified (phase voltage), and there is a danger of electric shock if the human body touches it.

If the enclosure of electrical equipment has a protective ground, current flows through the grounding body and the human body at the same time. In a parallel circuit, the current is inversely proportional to the size of the resistance. The smaller the ground resistance, the larger the current passing through, and the smaller the current flowing through the human body. Generally, the grounding resistance is less than 4 ohms, and the human body resistance is generally above 1000 ohms, which is much larger than the grounding resistance, so the current flowing through the human body is very small, and there is no danger of electric shock.

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