Earth Grid Resistance Calculator
Estimate the resistance to earth of a buried earthing grid from soil resistivity, grid area and total buried conductor length.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this for an early estimate of the resistance to earth of a substation or plant earthing grid. It models the grid as an equivalent disc with the same area and adds a term for the length of buried conductor.
Limits of this calculator
The model assumes uniform soil and a grid buried at shallow depth. Real soil is layered, so measure resistivity on site and use the design value from the earthing study. Step and touch voltages need a separate study.
Cross-check: 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Laurent-Niemann form without the depth refinement.
What this calculator checks
- Grid resistance from soil resistivity, area and conductor length (Laurent-Niemann form).
What remains unverified
- Burial depth refinement, measured resistivity and layered soil.
- Step and touch voltage, ground potential rise and a full earthing study.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEEE 80; IEEE 81; IEC 60364-5-54, IEEE 80-2013 (inactive-reserved legacy guide); IEEE 81-2025; IEC 60364-5-54:2011 + A1:2021 Ed 3.1 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Substation grounding-grid safety/design and field measurement are relevant. The calculator is a simplified resistance estimate only; it does not evaluate touch/step voltage, grid geometry, current division or fault duration. |
| Cross-check test cases | 8 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Soil resistivity | 100 Ω·m |
| Area covered by the grid | 900 m² |
| Total buried conductor length | 600 m |
- Equivalent radius r = √(900 / π) = 16.926 m
- Rg = 100 / (4 × 16.926) + 100 / 600 = 1.6437 Ω
Assumptions
- Uniform soil
- Grid buried at shallow depth
- Laurent-Niemann form; the Sverak refinement adds a depth term
Measure soil resistivity on site (Wenner method). Step and touch voltage need a separate study.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Soil resistivity | Ω·m | greater than 0 Ω·m | |
| Area covered by the grid | m² | greater than 0 m² | |
| Total buried conductor length | m | greater than 0 m |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Grid resistance | Ω | Shown to 3 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Soil resistivity (Ω·m) | Grid resistance (Ω) |
|---|---|
| 25 | 0.411 |
| 50 | 0.822 |
| 100 | 1.644 |
| 200 | 3.287 |
| 500 | 8.219 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEEE Std 80 (safety in AC substation grounding)
- IEC 60364-5-54 (earthing arrangements and protective conductors)
Soil resistivity varies with season and depth; use measured values and a site-specific design for any installation. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter soil resistivity, area covered by the grid and total buried conductor length. Change the unit of a value with its selector and the result stays the same.
What soil resistivity should I enter?
Use a measured value, for example from a Wenner four-pin survey, taken in the worst season for the site. Soil tables give only a rough range and vary widely with moisture and temperature.
Why does adding conductor lower the resistance only slightly?
Most of the resistance depends on the grid area. Extra buried conductor helps through the second term, but with diminishing returns, so a larger grid area is usually more effective.
Is a low grid resistance enough to be safe?
No. Safety depends on the step and touch voltages for the fault current and clearing time, which need their own calculation.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Earthing Formulas: Earth Rod and Fault Loop: Earthing formulas for the resistance of a vertical earth rod, rods in parallel
- Step and Touch Voltage Formulas: Substation earthing formulas: the tolerable touch and step voltage for a fault duration
- Lightning Protection Formulas: Rolling Sphere: Lightning protection formulas: rolling sphere radius by protection level, protected radius of a vertical air terminal
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