Grounding resistance of power equipment in effectively grounded systems
The grounding resistance for power equipment in an effective grounding system must meet specific standards to ensure safety and reliability. The standard cycle for checking the grounding resistance is generally no more than six years, but this can be adjusted based on the results of a visual inspection of the grounding network.
The requirements for grounding resistance are as follows:
- R ≤ 2000/I or R ≤ 0.5Ω (when I > 4000A)
- I represents the short-circuit current flowing into the ground through the grounding grid (in amperes).
- R represents the maximum grounding resistance, considering seasonal variations (in ohms).
When measuring grounding resistance, if the soil resistivity in the area is relatively uniform, various compensation methods can be used. Otherwise, a transport method should be applied. In areas with high soil resistivity, the grounding resistance may be allowed to exceed the specified value if it's technically and economically unreasonable. However, measures must be taken to ensure that:
- Contact voltage and step voltage do not exceed acceptable limits.
- No high-potential or low-potential areas occur outside the grounding network.
- 3–10kV valve arresters do not operate under fault conditions.
Before each preventive test or every three years, the grounding conductor’s resistance value should be checked and its thermal stability verified.
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Grounding resistance for power equipment in non-effectively grounded systems
For non-effectively grounded systems, the grounding resistance requirements vary depending on the voltage level of the equipment. The standard cycle for checking grounding resistance is also generally no more than six years, with possible adjustments based on the results of a grounding network inspection.
Key requirements include:
- For equipment rated at 1kV and below: R ≤ 120/I
- For equipment above 1kV: R ≤ 250/I
- In all cases, the grounding resistance should generally not exceed 10Ω.
Where:
- I = Short-circuit current flowing into the ground via the grounding grid (in amperes)
- R = Maximum grounding resistance, taking into account seasonal variations (in ohms)
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Grounding resistance of overhead line towers with ground wires
For overhead line towers equipped with ground wires, the grounding resistance requirements depend on the tower height and soil resistivity. The recommended testing cycle is:
- Towers within 1–2 km of a power plant or substation: 1–2 years
- Other line towers: no more than 5 years
For towers less than 40 meters in height, the grounding resistance should follow these guidelines:
- If soil resistivity is ≤ 100 Ω·m: ≤ 10 Ω
- If soil resistivity is between 100–500 Ω·m: ≤ 15 Ω
- If soil resistivity is between 500–1000 Ω·m: ≤ 20 Ω
- If soil resistivity is between 1000–2000 Ω·m: ≤ 25 Ω
- If soil resistivity is > 2000 Ω·m: ≤ 30 Ω
However, if the soil resistivity is very high and it's difficult to achieve the required resistance, additional grounding methods such as 6–8 radial grounding electrodes or continuous extended grounding conductors (up to 500 m total length) can be used. For towers 40 meters or higher, the grounding resistance must not exceed 20 Ω.
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