Earthing and lightning protection

Earth rod resistance vs length

See how the resistance of a single vertical rod falls with every metre and when driving deeper stops paying off.

VisualisationMethod: BS 7430Built for an earthing systems manufacturer: case study
Resistance of a single earth rod versus its length (driven depth)

Hover over or tap the chart

Illustrative chart calculated from the BS 7430 formula for one earth rod: R = ρ / (2πL) × [ln(8L/d) − 1], for ρ = 100 Ω·m and d = 14.2 mm, lengths from 1.5 to 25.5 m in 1.5 m steps (one rod section). The model assumes uniform soil; the actual resistance depends on soil layers and must be measured.

Table of values
Rod length L [m]Resistance R [Ω]
1.560.90
3.034.13
4.524.18
6.018.90
7.515.59
9.013.32
10.511.65
12.010.37
13.59.36
15.08.53
16.57.85
18.07.27
19.56.78
21.06.35
22.55.98
24.05.64
25.55.35