The target for a 380/220 V network and a typical private-house system is 4 Ω or less. The connecting strip is not included in the estimate and adds extra margin.
Soil resistivity
| Soil | ρ, Ω·m |
|---|---|
| Clay | 20 |
| Peat | 25 |
| Chernozem | 30 |
| Garden soil | 40 |
| Loam | 100 |
| Sandy loam | 300 |
| Sand | 500 |
| Rocky soil | 2000 |
Target values
4 Ω - for a 380/220 V network
(source neutral earthing) and the usual goal for a private house;
8 Ω - for 220/127 V; 10 Ω - repeated earthing of the PEN
conductor and some lightning protection cases; 30 Ω - lightning
protection of standalone structures. Soil resistivity figures are approximate -
they change a lot with moisture and freezing.
How it is calculated
A single rod:
R₁ = ρ / (2π·L) · ln(4L/d); a system of n rods:
Rs = R₁ / (n·η), where η is the utilisation factor (accounts for
mutual screening of the electrodes; depends on their number and spacing). This is
an estimate: the real resistance depends on moisture, burial depth and season.
If it does not pass
Ways to lower the resistance: more rods, longer rods and wider spacing, reaching a moist soil layer, choosing a more conductive soil (clay, loam). A finished system must be measured with a proper tester. Design and installation should be done by a qualified electrician.