Wenner 4-Pin Readings
| Spacing a (m) | Reading R (Ω) | ρ (Ω·m) |
|---|
Rule note
Enter the equal pin spacing and the resistance read on the tester at each spacing. ρ = 2πaR. Pin depth should be under a/20. The method follows IEEE Std 81.
Soil & Target
Rule note
From the soil resistivity tab or a site survey.
Rule note
Applied to ρ to cover dry-season rise. Use 1.0 if the survey was done in the driest period.
Rule note
Set from your project specification or the applicable standard. For reference, FAA AC 150/5340-30 shows a maximum of 25 Ω for airfield lighting counterpoise and ground rods (check the current edition); lightning protection or NAVAID facilities may need lower values.
Rod Geometry
Rule note
Spacing of at least the rod length, ideally about twice, keeps mutual interference between rods low. Closer spacing makes extra rods much less effective.
Group Resistance by Rod Count
Test Set-up
Rule note
Should be at least 5× the electrode's longest dimension. The 62% rule assumes fairly uniform soil and probes in a straight line, and suits a single rod or small electrode. For a large grid or a long counterpoise run use a much longer probe distance.
Tester Readings
Rule note
Take the potential probe readings at 52%, 62% and 72% of the distance to the current probe. If the three readings agree, the 62% reading is a valid earth resistance.
How the three tools work
The formulas below are the ones used on the other tabs. Results are estimates for planning and verification.
Soil resistivity — Wenner 4-pin
Four pins are driven in line at equal spacing a. The tester injects current through the outer pins and reads the voltage across the inner pins. Resistivity is calculated at each spacing.
Readings that rise strongly with spacing suggest a higher-resistivity deeper layer. Falling readings suggest a wetter or more conductive layer. A spread of more than 50% across the spacings is flagged as layered or non-uniform soil.
Single earth rod
A single vertical rod with its top at ground level uses Dwight's formula. The design ρ is the entered ρ multiplied by the seasonal multiplier.
Rods in a line
For n rods in a straight line at spacing s, the group resistance adds the mutual resistance between every pair of rods (Sunde's expression for equal rods with tops at ground level). The tool finds the smallest n up to 60 that meets the target.
Rₘ(x) = ρ/(4πL) · [ 2·asinh(2L/x) − √(4 + (x/L)²) + x/L ]
Connection resistance and cable are not included.
Fall-of-potential, 62% rule
The potential probe is placed at 52%, 62% and 72% of the distance to the current probe. If the spread between the highest and lowest reading is 5% or less of the 62% reading, the probes sit outside each other's resistance areas and the 62% value is valid. If not, move the current probe further out and repeat.
Worked examples
| Case | Result |
|---|---|
| Wenner: a = 4 m, R = 25 Ω | — |
| Rod: ρ = 100 × 1.3, L = 3 m, d = 17 mm | — |
| Rod group for 10 Ω at s = 6 m | — |
| FoP: 4.62 / 4.71 / 4.78 Ω | — |
What this tool does not do
Results are estimates for planning and verification. Always confirm the final installed value with a calibrated earth tester, and follow the resistance limit given in your project specification, the aerodrome manual or the applicable standard. This tool does not replace the project design. See the Legal Notice.