Soil Resistivity - RWS200 - RWCC - AWS310-SITE - AWS310 - FOC201 - AWS430 - RME111

Grounding and Lightning Protection in Vaisala Outdoor Installations Technical Reference

Document code
M211786EN
Revision
B
Language
English (United States)
Product
RWS200
RWCC
AWS310-SITE
AWS310
FOC201
AWS430
RME111
Document type
User guide

Soil resistivity depends on the content of electrolytes in the soil. Geological differences and seasonal variations on a site can be big.

In particular frozen soil and dry soil are very poor conductors:

  • In frozen soil, position the ground electrode deep enough to reach non-frozen moist soil even during the winter or dry season. Do this if you need a low resistance to ground for electrical safety reasons.
  • In very dry soil, add soil enhancing chemicals, also known as backfill compounds, around the ground electrode to raise the moisture level, lowering the resistance to ground.

For installation examples in different soil types, see Installing Ground Electrode.

Table 1. Resistivity Values of Soil and Water Types
Type of Soil or Water Resistivity (Ωm) Usual Range (Ωm)
Sea Water 2 0.1 … 10
Clay 40 8 … 70
Ground well and spring water 50 10 … 150
Clay and sand mixtures 100 4 … 300
Shale, slates, sandstone, etc. 120 10 … 1000
Peat, loam and mud 150 5 … 250
Lake and brook water 250 100 … 400
Sand 2000 200 … 3000
Moraine gravel 3000 40 … 10 000
Ridge gravel 15 000 3000 … 30 000
Solid granite 25 000 10 000 … 50 000
Ice 100 000 10 000 … 100 000
Table 2. The Effect of Moisture Content
Moisture Content (% of weight) Resistivity of Clay Mixed with Sand (Ωm)
0 over 10 000 000
2.5 1500
5 430
10 185
15 105
20 63