Earthing Grid Design: Step and Touch Voltage Calculation
earthing grid design: An earthing grid is a buried conductor mesh that carries fault current safely into the soil while keeping surface potential differences within limits that are harmless to people; the design is verified by step and touch voltage calculation to IEEE 80 or EN 50522.
Contents
A low earth resistance value alone does not prove that a substation is safe. The real question is how much surface potential difference the fault current creates, and what voltage that imposes between a person’s feet, or between hand and feet. Earthing grid design exists to bring those two quantities, step voltage and touch voltage, below the permissible limits.
Why a resistance figure is not enough
A common habit reduces earthing design to a single value in ohms. Yet two arrangements with identical resistance can differ completely in safety. What matters is how the current spreads through the soil and how steep the surface potential profile becomes. A sparse grid with wide meshes may achieve low resistance while still leaving a dangerous touch voltage in the centre of each mesh.
Input data: resistivity, current and duration
Design starts from three inputs. Soil resistivity is measured by the Wenner four point method and reduced to a two layer model. The current actually entering the soil is derived from the single phase to earth fault current after subtracting the share returning through cable screens and overhead earth wires. Clearing time is the sum of relay and breaker operating times; the longer it is, the lower the permissible voltage becomes.
Establishing the permissible limits
Permissible touch and step voltages are calculated from body resistance, foot contact resistance and current duration. A surface layer of high resistivity crushed rock, typically 100 to 150 mm thick, substantially raises foot contact resistance and therefore the permissible limits. It is one of the most effective and least expensive improvements available on site. The wet resistivity of the stone must be the value used in the calculation.
Grid geometry and conductor selection
| Parameter | Typical range | Effect |
|---|---|---|
| Mesh spacing | 3 to 10 m | Directly reduces touch voltage |
| Burial depth | 0.5 to 0.8 m | Smooths the surface gradient |
| Surface stone layer | 100 to 150 mm | Raises the permissible limit |
| Conductor size | 50 to 240 mm² copper | Thermal withstand plus corrosion allowance |
| Clearing time | 0.2 to 1.0 s | Longer times lower the limit |
Grids are typically laid with 3 to 10 metre mesh spacing at a burial depth of 0.5 to 0.8 metres. Potential rise is greatest at the perimeter and corners, so mesh spacing is reduced there and additional rods are driven along the boundary where required. Conductor cross section follows the thermal effect of the fault current; for copper the governing temperature is what the joints can withstand, not the melting point. Exothermic welding proves more reliable than bolted joints over the long term.
Verification, measurement and transferred potential
Once designed, the installation is measured on site: earth resistance by the fall of potential method, the surface potential profile and the continuity of bonds. Transferred potential also has to be examined, because a pipe, rail, telecommunication line or low voltage neutral leaving the grid can carry the raised potential far away. Such points require insulating sections, isolation flanges or a separated earthing arrangement.
Key takeaways: earthing grid design
- Low resistance is not proof of safety — the surface potential difference is what governs.
- A stone surface layer is the cheapest gain — higher foot resistance raises the permissible limit.
- Corners and edges are critical — potential rise peaks around the perimeter.
- Never ignore transferred potential — metallic paths leaving the grid export the hazard.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Which standard should be used for an earthing grid?
European practice follows EN 50522, while IEEE Std 80 is widespread in international engineering. Both aim to limit step and touch voltages but their acceptance criteria and calculation approaches differ, so the specification must state which one governs.
What can be done when soil resistivity is high?
Reduce mesh spacing, reach a lower resistivity layer with deep driven rods, enlarge the grid area and apply a surface stone layer. Chemical enhancement compounds give temporary benefit and are not a standalone solution.
Should the low voltage neutral be bonded to the substation grid?
It depends. Common earthing can export the earth potential rise into the low voltage installation. EN 50522 allows a common arrangement when the potential rise stays below defined limits and requires separation above them.
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Do you need an earthing grid calculation for your substation or plant site? Devpan produces the step and touch voltage verification and the grid detail from your resistivity measurements. Related Devpan solution: electrical panel design services.
