Grounding resistance is an important parameter in electrical equipment, which has rigid specification indicators. By analyzing the resistance specification requirements of electrical equipment, we can find that most grounding resistances are below 4 ohms. Do you know why?

Let's start with the definition of grounding resistance. Grounding resistance is the resistance encountered when the current flows from the grounding device into the earth and then flows through the earth to another grounding body or spreads to a distance. Its function is to discharge to the earth to ensure safety.

It directly reflects the good degree of contact between the electrical device and the "ground". For different grounding requirements, the resistance value specifications are also different, which can be summarized as follows:

1. The independent lightning protection grounding resistance should be less than or equal to 10 ohms;

2. The independent safety protection grounding resistance should be less than or equal to 4 ohms;

3. The independent AC working grounding resistance should be less than or equal to 4 ohms;

4. The independent DC working grounding resistance should be less than or equal to 4 ohms;

5. The anti-static grounding resistance is generally required to be less than or equal to 100 ohms.

6. The shared grounding body (combined grounding) should not be greater than the grounding resistance of 1 ohm.

It can be seen that the vast majority of grounding resistances are below 4 ohms, which is related to their grounding requirements. Generally, there are these grounding methods:

1. Protective grounding: The metal shell of electrical equipment, concrete, electric poles, etc., its insulation damage may cause the generation of live electricity, so grounding is required.

2. Anti-static grounding: Grounding to prevent static electricity, this type of grounding is used on flammable oil, natural gas storage tanks, pipelines, and electronic equipment.

3. Lightning protection grounding: Grounding by leading lightning underground.

4. Working grounding: Grounding of neutral points and arc suppression coils in power systems.

The role of these groundings is to prevent the generation of live electricity, lead the point into the earth, and achieve the role of ensuring the safety of human body and equipment. During the discharge process, a fault current will be generated, which is usually not more than 10A. According to Ohm's law, when the grounding resistance is 4Ω, the fault voltage generated when flowing through the grounding resistance is 4x10=40V. It is lower than the safe voltage value of 50V for the human body, so the maximum limit of grounding resistance is set at 4Ω.

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