The resistance of a single earth can be calculated if the soil resistivity is known. An earth rod driven vertically, the electrode resistance can be calculated using the following formula.
R = electrode resistance, Ω
ρ = Soil resistivity, Ω-m
L = Length of electrode buried in soil, m
d = Outer diameter of earth rod, m
Note: In Australia the minimum depth of vertical type earth electrode is 1.2 m [Clause 18.104.22.168, AS/NZS 3000 - 2007]
The soil resistivity at the location of the earth rod needs to be investigated. The resistivity of soils varies with
- the depth from the surface,
- the type and concentration of soluble chemicals in the soil,
- the moisture content, and
- the soil temperature.
In other words, the soil resistivity is dependent on the electrolyte in the soil. The presence of surface water does not necessarily indicate low resistivity.
In the absence of soil investigative data, typical resistivity of various soil types is given in the following table.
Typical soil resistivity (IEEE 142)
|Soil Type||Average Resistivity Ω-m|
|Well-graded gravel||600 - 1000|
|Poorly graded gravel||1000 - 2500|
|Clayey gravel||200 - 400|
|Silty sand||100 - 800|
|Clayey sands||50 - 200|
|Silty or clayey sand with slight plasticity||30 - 80|
|Fine sandy soil||80 - 300|
|Gravelly clays||20 - 60|
|Inorganic clays of high plasticity||10 - 55|
Soil resistivity = 180 Ω-m
Earth rod = ø19mm x 1.8 m
Assuming that the earth electrode is fully driven into the ground, the resistance will be
In the event that the desired soil resistivity is not achievable using a single earth electrode, multiple electrodes could be used. The multiplying factors for multiple electrodes are given in the following table
Multiplying Factor of Multiple Electrodes(IEEE 142)
|Number of electrodes||Factor|
- IEEE 142 (Green Book) 2007IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
- Practical Grounding, Bonding, Shielding and Surge ProtectionG. Vijayaraghavan, Mark Brown, Malcolm BarnesNewnes 2004 - IDC Technologies
- AS/NZS 3000 - 2007 Wiring Rules