What are step and touch voltages?
When earth fault current enters the soil through a substation earth grid, the ground surface around it rises in voltage, highest over the conductors and falling away with distance. A person touching earthed steel (which is at the full grid potential) while standing on ground at a lower potential experiences a touch voltage. A person walking across the gradient experiences a step voltage between their feet. The highest touch voltage inside a grid is usually in the middle of a corner mesh, called the mesh voltage.
How do you make an earth grid safe?
- Lower the grid resistance: a bigger area, more conductor, rods into lower-resistivity layers.
- Reduce the mesh size: closer conductor spacing flattens the surface potential.
- Add a surface layer: crushed rock or asphalt adds resistance in series with the feet and raises the tolerable limits.
- Clear faults faster: tolerable voltages rise with 1/√t.
- Control the boundary: a grading ring outside the fence and bonding the fence correctly.
- Cut the current into the grid: cable screens and overhead earth wires carry part of the fault current back to the source (the split factor).
ExpertSoil is rarely uniform. Wenner tests at several spacings usually show two or more layers. A low-resistivity bottom layer makes long rods very effective, while a high-resistivity top layer acts like a surface layer for step voltages but makes rods essential. Also check transferred potentials on LV neutrals, telecom cables and pipes leaving the site.
Distribution substations inside buildings
For building substations, the MV earth fault current is often limited by the neutral earthing resistor at the primary substation, and the building's foundation and structure form a large global earthing system. Many codes allow a combined HV and LV earth when the EPR stays below set limits, otherwise the HV and LV earths must be separated. Check the utility's earthing rules and the MV earth fault current and clearance time they give you.
Regional notes
GCC
IEEE 80 and BS EN 50522 are common references, with utility rules for earth pit construction, testing and the maximum earth resistance of distribution substations. Dry desert sand has very high resistivity near the surface, so deep rods or chemical earthing are often used.
Australia and New Zealand
AS 2067 with the ENA EG-0 risk-based method is used. EG-0 sets touch voltage limits based on the probability of contact and fault frequency, rather than one fixed value.
Asia
IEEE 80 is widely used, with national codes such as IS 3043 in India.