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    Calculates locally in your browser · IEEE 80 step and touch limits
    Power systems / Earthing

    Grounding grid calculator

    A substation earthing design is a voltage comparison, not a resistance target: the ground potential rise during a fault must stay below the touch voltage a person can survive. This gives the IEEE 80 limits, the grid resistance and the rise, plus the cable pulling tension for the same job.

    Soil100 Ω·mFault5.0 kAGridUnsafe
    Touch voltage limit
    681 V
    IEEE 80 §8
    Ground potential rise
    4953 V
    Rg 0.99 Ω
    Step voltage limit
    2231 V
    Cs 0.70
    Cable pull tension
    8.47 kN
    sidewall 4.2 kN/m
    Grounding and pulling
    Voltage limits against the rise, and the tension through the bend.
    Grounding and pulling
    QuantityValueUnit
    Surface derating Cs0.700—
    Step voltage limit2231V
    Touch voltage limit681V
    Grid resistance0.991Ω
    Ground potential rise4953V
    Pull tension out8.47kN
    Sidewall pressure4.24kN/m

    GPR = Ig·Rg must stay below the touch voltage limit; mesh voltage needs the full Km/Ki calculation.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    GPR against soil resistivity
    Both the resistance and the allowable rise move with ρ.
    GPR against soil resistivity0.0008875.551.775e+42.663e+43.550e+44.438e+425 Ω·m25 Ω·m50 Ω·m50 Ω·m100 Ω·m100 Ω·m200 Ω·m200 Ω·m400 Ω·m400 Ω·m800 Ω·m800 Ω·mGPR (V)
    Canonical equations
    The exact relationships implemented by this engine.

    E_touch = (1000 + 1.5·Cs·ρs)·0.116/√ts · IEEE 80, 50 kg body.

    E_step = (1000 + 6·Cs·ρs)·0.116/√ts · Six times the surface resistance term.

    Rg = ρ/(4√A) + ρ/L · Simplified Schwarz form.

    engineering methodology

    A clear path from inputs to results.

    A substation earthing design is a voltage comparison, not a resistance target. During a fault the whole grid rises above remote earth, and what matters is whether that rise exceeds the touch voltage a person standing on the surface can survive.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the soil resistivity and the surface layer.
    2. Set the fault current, clearing time and grid geometry.
    3. Compare the ground potential rise with the touch limit; check the cable pulling tension below.

    The formula

    Etouch=(1000+1.5Csρs)0.116tsE_{touch} = (1000 + 1.5 C_s \rho_s)\frac{0.116}{\sqrt{t_s}}

    Reference: IEEE 80 §8 step and touch criteria; simplified Schwarz grid resistance.

    Worked example

    1,000 m² grid in 100 Ω·m soil, 5 kA, 0.5 s

    Inputs
    • 100 mm of 3,000 Ω·m crushed rock on the surface.
    Calculation path
    1. 1

      Cs = 0.70. Touch limit = (1000 + 1.5 × 0.7 × 3000) × 0.116/√0.5 = 681 V. Rg = 100/(4√1000) + 100/500 = 0.99 Ω. GPR = 4,950 V.

    Result

    GPR 4,950 V against a 681 V touch limit — the grid fails and needs more conductor.

    Interpreting the result

    Adding buried conductor lowers Rg only slowly — the ρ/(4√A) term dominates — which is why the practical fix is usually a surface layer of crushed rock rather than more copper.

    Assumptions & limitations

    • Step and touch only; mesh voltage needs the full IEEE 80 Km/Ki calculation.
    • Cs valid for a surface layer up to 0.3 m.
    • Uniform soil resistivity.
    • Conductor sizing to §11 is a separate adiabatic check.

    Frequently asked questions

    Why is the touch limit lower than the step limit?

    The step path goes foot to foot through the earth, so the surface layer resistance is counted six times instead of 1.5.

    What is Cs?

    The derating factor for a thin surface layer; it approaches 1 as the layer gets thicker.

    Does a lower resistance always help?

    It lowers the GPR, but the limits move with the surface layer, so both sides matter.

    Can I use this for a building earth?

    The voltage limits apply, but a building electrode is not a grid and needs the relevant standard for its earthing arrangement.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.