A substation earthing grid is safe when the voltages a person could experience during an earth fault — step and touch potential — stay below tolerable limits. Design starts with measured soil resistivity, models the grid and the fault current it must carry, and verifies the results against IEEE 80 or the applicable standard.
Key Takeaways
- Low grid resistance alone does not guarantee safety.
- Measured, layered soil models are more reliable than assumed values.
- Surface layers such as crushed rock raise tolerable limits.
- Fault current, duration and grid geometry all drive the result.
The Real Safety Criterion
During an earth fault the grid rises in potential relative to remote earth (the ground potential rise, GPR). Danger comes from the voltage differences a person bridges — between hand and feet (touch) or between two feet (step). A grid can have low resistance and still produce unsafe touch voltages at its edges or near fences.
Start with the Soil
Soil resistivity is measured on site using the Wenner four-pin method and interpreted as a layered model. We measure with our own Metrel MI 3290 GX earth analyser and feed the data directly into the XGSLab earthing model.
Tolerable Limits
IEEE 80 tolerable step and touch voltages depend on fault duration, body weight assumption and the resistivity of the surface layer. A high-resistivity surface such as crushed rock significantly raises the tolerable limit.
Designing the Grid
Conductor spacing, depth, rods and perimeter design are adjusted until the calculated step and touch potentials sit below the limits everywhere a person can stand — including fence lines and gates.
Verify After Construction
Earthing system testing after installation confirms that the built grid performs as designed.
Frequently Asked Questions
Which standards apply?
IEEE 80 is widely used for substations; IEC 60364 and CEA regulations apply as relevant to the Project and jurisdiction.
Why not just add more rods?
Rods lower resistance, but safety depends on the potential distribution. The grid layout and surface layer often matter more.
