Protective earthing is required.
In order to prevent the metal casing of electrical equipment, the frame of the power distribution unit and line towers and other energized to endanger the safety of people and equipment and grounding. The so-called protective grounding is not charged under normal circumstances, but in the insulating material is damaged or other circumstances may be charged electrical metal parts, and the charged part of the insulated metal structure part) with a wire and the grounding body is reliably connected to a protective wiring method.
Earth protection is used for distribution transformer neutral point is not directly grounded (three-phase three-wire system) power supply system, to ensure that when the electrical equipment due to insulation damage and leakage of electricity generated by the ground voltage does not exceed the safe range.
Expanded information:
p>Protective grounding requirements related to the provisions:
1, limit leakage equipment to the ground leakage current, so that it does not exceed a certain safety range, once more than a certain value of the protector can automatically cut off the power supply.
2, with the help of zero line, so that the equipment in the insulation is damaged after touching the shell to form a single-phase metallic short circuit, the use of short-circuit current to prompt the rapid action of the protective device on the line.
3, in the normal and accident and cable system internal overvoltage, lightning overvoltage, the use of the earth as a current loop, the cable grounding potential is fixed at the permissible ground potential.
Baidu Encyclopedia-Protective Earthing