The grounding device can perform its intended function during operation, and its grounding resistance should all comply with the requirements of the regulations. For various commonly used grounding devices, the allowable grounding resistance values (in ohms) are as follows:
(1) For the working grounding of transformers or generators with a power capacity of over 100 kVA, R = 4 Ω.
(2) For the working grounding of transformers or generators with a power capacity of 100 kVA or less, R = 10 Ω.
(3) For the zero-line repeated grounding in low-voltage power distribution systems with a capacity of 100 kVA or less, R = 10 Ω; when there are more than 3 places of repeated grounding, R = 30 Ω.
(4) For the protection grounding of non-electrically charged metal parts of electrical equipment, R = 4 Ω; for the protection grounding of equipment with an introduced line equipped with a fuse of 25A or less, R = 10 Ω.
(5) For the grounding of low-voltage line poles or the grounding of insulator feet of low-voltage incoming lines, R = 30 Ω.
(6) For the grounding of FZ type valve-type lightning arresters on the busbars of substations, R = 4 Ω.
(7) For the grounding of FS type valve-type lightning arresters, pipe-type lightning arresters, independent lightning rods (for individual cases, R = 30 Ω), industrial electronic equipment (including X-ray machines), the protection grounding is all R = 10 Ω.
(8) For the lightning protection grounding of chimneys, R = 30 Ω (including the lightning protection grounding of water towers or silos is also subject to this requirement); etc.
Standard grounding resistance specification requirements:
1. The grounding resistance of independent lightning protection grounding should be less than or equal to 10 ohms;
2. The grounding resistance of independent safety protection grounding should be less than or equal to 4 ohms;
3. The grounding resistance of independent AC working grounding should be less than or equal to 4 ohms;
4. The grounding resistance of independent DC working grounding should be less than or equal to 4 ohms;
5. The grounding resistance of anti-static grounding generally requires to be less than or equal to 100 ohms.
6. The common grounding body (combined grounding) should not exceed the grounding resistance by 1 ohm.
The grounding wire of lightning rods belongs to lightning protection grounding. If the grounding resistance of the lightning rod and the anti-static grounding are both set as required, then the grounding wire of anti-static equipment can be connected to the grounding wire of the lightning rod, because the grounding resistance of the lightning rod is 10 times smaller than that of the static grounding, so in the event of a lightning strike, most of the lightning will be discharged through the lightning rod grounding, and the current passing through the anti-static grounding can be ignored.
Grounding is divided into three types: protection grounding: the metal casings of electrical equipment, concrete, poles, etc., which may become charged due to insulation failure, is set for grounding resistance of less than 1 Ω. The protection grounding resistance of low-voltage electrical equipment is no more than 4 Ω, the protection grounding resistance of high-voltage equipment with small grounding short-circuit current (less than 500A) is no more than 10 Ω, the protection grounding resistance of high-voltage equipment with large grounding short-circuit current (more than 500A) is no more than 0.5 Ω, the grounding resistance of the neutral point of the transformer is no more than 4 Ω, and the repeated grounding resistance is no more than 10 Ω. If the soil resistivity is too high, methods such as external grounding body, chemical treatment of the soil, changing the soil type, deep burial, extending the grounding body, using network grounding devices can be adopted to achieve the purpose of reducing the grounding resistance.
Anti-static grounding: to prevent the influence of static electricity hazards on flammable oil, natural gas storage tanks and pipelines, electronic equipment, etc.
Lightning protection grounding: to introduce lightning into the ground, connect the grounding end of lightning protection equipment (lightning rods, etc.) to the earth to eliminate the hazards of lightning overvoltage to electrical equipment, personal property, etc., also known as overvoltage protection grounding.
Post time: Aug-21-2026