Earthing Grid Resistance Calculator
Estimate the resistance to earth of a buried earthing (grounding) grid using the simplified Laurent-Niemann formula. Enter the soil resistivity, the grid's plan area, and the total length of buried conductor to get a quick grid resistance figure for early design checks.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the soil resistivity in ohm.m from a Wenner or driven-rod resistivity survey.
- Enter the grid area in m2 (the plan footprint covered by the grid).
- Enter the total length of all buried grid conductors in metres, then read the equivalent radius and grid resistance.
How it works
The grid is modelled as an equivalent circular plate of radius r = sqrt(area / pi). The Laurent-Niemann formula estimates resistance as R = rho / (4 x r) + rho / L, where the first term is the plate resistance and the second accounts for the finite length of buried conductor. Lower soil resistivity, larger area, and more buried conductor all reduce the resistance.
Worked example
Worked example. With soil resistivity 100 ohm.m, a 900 m2 grid and 300 m of buried conductor: r = sqrt(900 / pi) = 16.93 m, and R = 100 / (4 x 16.93) + 100 / 300 = 1.48 + 0.33 = 1.81 ohm.
Common mistakes
- Treating this simplified estimate as a substitute for a full IEEE 80 grid design and a measured test.
- Using a single surface resistivity value where the soil is strongly layered - deep resistivity often differs greatly.
- Assuming a low resistance alone means safety; step and touch voltages must also be checked.
Frequently asked questions
What grid resistance is acceptable?
Large substations commonly target below 1 ohm, but the real requirement is set by the earth-fault current and the allowable step and touch voltages, not by a fixed resistance number.
How accurate is the Laurent-Niemann formula?
It is a first-pass estimate for uniform soil. Use it for feasibility and sizing, then confirm with a detailed design tool and a field measurement.
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