What is ground resistance ?
Grounding resistance is the resistance encountered when current flows from a grounding device to the earth and then flows through the earth to another grounding body or spreads to a distance. It includes the resistance of the grounding wire and the grounding body itself, and the contact resistance and the resistance of the earth between two ground bodies or the resistance of the earth from the ground body to infinity.
The size of the grounding resistance directly reflects the good degree of contact between the electrical device and the “ground”, and also reflects the size of the grounding network. In single-point grounding systems and strong interference conditions, the auxiliary ground pole measurement method can be used for measurement.
There are mainly four types of grounding:
1. Protective grounding: The metal shell of electrical equipment, concrete, poles, etc. may be charged due to insulation damage. Grounding is provided to prevent this situation from endangering personal safety.
2. Anti-static grounding: Ground flammable oil and natural gas storage tanks, pipelines, electronic equipment, etc. to prevent the dangerous effects of static electricity.
3. Lightning protection grounding: In order to introduce lightning into the ground, the grounding end of the lightning protection equipment (lightning rod, etc.) is connected to the earth to eliminate the harm of lightning overvoltage to electrical equipment, personal property, and grounding. It is also called overvoltage protection grounding.
4. Working grounding: refers to connecting a certain point of the power system (such as the neutral point) directly to the earth, or connecting it to the earth metal through arc suppression coils, resistors, etc., such as the neutral point grounding of a transformer , transformer, etc.
Ground resistance specification requirements
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 ground resistance should be less than or equal to 4 ohms;
4. The independent DC working ground resistance should be less than or equal to 4 ohms;
5. Anti-static grounding resistance is generally required to be less than or equal to 100 ohms.
6. The common grounding body (joint grounding) should not be greater than the grounding resistance 1 ohm;
7. The instrument must be handled with care when carrying and using it to avoid severe vibration.
Note: The ground wire of the lightning rod belongs to lightning protection grounding. If the ground resistance of the lightning rod and the anti-static ground resistance are set as required, then the ground wire of the anti-static equipment and the ground wire of the lightning rod can be connected together, because the ground wire of the lightning rod The resistance is 10 times smaller than the electrostatic ground resistance. Therefore, when a lightning accident occurs, most of the lightning will be discharged from the lightning rod ground, and the current passing through the anti-static ground can be ignored.
Why is the ground resistance generally not greater than 4Ω?
Grounding resistance is the resistance encountered when current flows from a grounding device to 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.
When we use many household appliances , we often come into contact with the casing of the appliance, such as the door opening and closing of the refrigerator , the opening and closing door of the washing machine, etc. However, once an electrical appliance fails, the casing will carry a certain voltage. When a person touches the casing, an electric shock will occur, which is very dangerous. Therefore, it is very necessary to lead the electricity to the earth through a grounding resistor.
For ground resistance, we want it to be as small as possible. According to Ohm’s law, when the voltage is constant, the resistance is inversely proportional to the current; therefore, when the working voltage is determined, the smaller the ground resistance, the greater the current that can pass, and the better the current discharge effect. If there is a leakage , all the electricity will be transmitted from the grounded ground wire to the ground.
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Why is the maximum value of ground resistance 4Ω?
Under normal circumstances, when an electrical system equipment fails, the fault current will not be greater than 10A. Therefore, when the grounding resistance is 4Ω, the fault voltage generated when flowing through the grounding resistance can be calculated according to Ohm’s law: 4×10=40V. Standards formed through experiments stipulate that under normal and fault conditions, the voltage between any two conductors or between any conductor and the earth shall not exceed the AC (50~500Hz) effective value of 50V.
Such a fault voltage of less than 50V is relatively safe and is almost impossible to cause an electric shock accident to the human body. Therefore, the maximum limit of grounding resistance is set at 4Ω.
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important reason
Complying with laws, regulations and industry standards is an important reason for maintaining the corresponding resistance value of the ground resistor. The construction, operation and management of power systems must comply with a series of laws, regulations and industry standards. In the power industry, these regulations are lessons learned from countless practical tests, especially those involving safety. They are all pioneers in the power industry. We have learned practical experience and data through hard lessons, including the regulations on grounding resistance.
For example, the “Electrical Safety Code” stipulates the requirements for ground resistance of different types of electrical equipment. If the grounding resistance is greater than the specified value, it will cause safety hazards in the equipment, violate relevant standards and regulations, and more importantly, affect the life safety of surrounding people.
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Proper grounding method
Choosing a suitable grounding method can also ensure that the grounding resistance is not greater than 4Ω. In different places where electricity is used, appropriate grounding methods need to be selected. For example, for dry-type transformers, using the busbar method for grounding can reduce the busbar inductance , improve the anti-interference ability of the equipment, and thus keep the grounding resistance no greater than 4Ω.
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Influence of ground environment and grounding materials
Generally, factors such as soil resistivity, moisture level, temperature , etc. on the ground will affect the ground resistance. By selecting appropriate grounding materials and reasonable grounding methods, you can adjust the grounding resistance to ensure that the grounding resistance is not greater than 4Ω.
Application scenarios of grounding resistance
In terms of specific applications, there are many grounding scenarios. Common groundings include: Lightning protection grounding (also known as a common lightning rod): The smaller the grounding resistance, the faster the lightning discharges to the earth through the grounding wire when struck by lightning, that is, the faster the grounding resistance. Safety.
Electrical equipment safety ground (such as washing machine chassis grounding):
Many household appliances, especially large appliances such as refrigerators, washing machines, air conditioners, etc., need to be grounded (the chassis is grounded). When the chassis leaks electricity, the smaller the ground resistance, the more leakage electricity will be transmitted to the earth through the ground wire. But if the grounding resistance is too large (if it is greater than the resistance of the human body), when a person touches the casing, the human body becomes a grounding wire, and current flows from the human body to the earth, causing electric shock to the human body, which is quite dangerous.
There are also working grounding inside electronic equipment, etc. The smaller the grounding resistance, the better. Some electronic equipment will generate static electricity. If a grounding resistor is connected, the danger caused by static electricity can be prevented.
