Grounding resistance is not the only qualification for good grounding, but a numerical value for electrical safety grounding is sometimes a contractual requirement. To verify whether a contractual requirement is met, for example a maximum of 10 Ω grounding resistance, you need to measure the grounding resistance.
The best time to make the measurement is right after the rainy season.
| Soil resistivity measurements do not give correct results in frozen soil. Make the measurements when the soil in not frozen. |
The measurement helps you to evaluate and design the required ground electrode for a certain resistance value.
There are several methods and pieces of measurement equipment for measuring grounding resistance:
- Use a widely preferred method, which is the four-electrode Wenner method (ASTM G57-06), to measure soil resistivity in the intended grounding location.
- Use soil type tables for rough estimates of the grounding resistance of different electrode sizes.
- Outsource grounding resistance measurements to a local power utility company.
Multi-meters and clamp-type loop resistance meters can be used to measure one ground electrode at a time in a large system with several interconnected ground electrodes located far from each other. This is very seldom the case on a Vaisala installation site.
For the operational details of measuring equipment, see the user manual of your specific grounding resistance instrument.