Type
|
Ohm.m
|
| Sea water |
0.5 |
| Alluvium clays |
5-80* |
| Clay |
5-20 |
| Fresh water body |
10-100 |
| Boggy soil, top soil |
20-40 |
| Marls (e.g. Keuper marl) |
10-30 |
| Loamy soils |
90-150 |
| Chalk |
50-100* |
| Wet Sandy soil |
200-400 |
| Dry Sandy soil |
1,000-2,000 |
| Concrete |
150-500 |
| Porous sandstones |
30-300 |
| Quartzites, compact and crystalline limestone |
100-1,000 |
| Slates and chales |
1,000-3,000 |
| Granite |
750-1,250 |
| Gneiss and igneous rock |
2,000+ |
*moisture dependent.
Soil resistivity will fluctuate throughout the seasons due to changes in temperature, rainfall (moisture retention), organic material, etc.
Accurate design of a grounding system requires an accurate assessment of the site’s soil conditions. However, even a small site will often have widely varying soil resistivity from one spot to another. Many measurements should be made, and samples of the soil must be taken from several test locations and analysed for both moisture and temperature.
Dealing With High Soil Resistivity (Soil Conditioning)
Soil resistivity is a function of several factors. These include the type of soil, moisture content, temperature, mineral content, granularity, and compactness. Usually, moisture and mineral content are the only factors that can be influenced by any practical control concept. The higher mineral content also reduces soil sensitivity to moisture content. It can therefore, be useful to consider increasing the mineral content when dealing with medium to high soil resistivities.
See Chem-rod
Refs: BS7430, BSEN62305,