Step voltage versus touch voltage at a substation earthing grid — why mesh spacing and a gravel surface layer matter

The guide on the conventional touch voltage limit UL covers the voltage threshold used within an installation to assess automatic disconnection of supply. This article covers a related but separate topic, specific to the earthing arrangement of a medium- or high-voltage substation (an own transformer station, common for example on a larger agricultural, horticultural or industrial connection): the distinction between touch voltage and step voltage, and why the physical design of the substation's earthing grid — not just its overall resistance value (see the earth resistance measurement guide for that separate measurement) — determines how safe the ground around it actually is.

Two different body current paths, near the same earthing system

When an earth fault current flows through a substation's earthing system, it raises the local potential of the earth around that system relative to "true," remote earth — and that raised potential does not drop instantly to zero at the edge of the buried electrode; it decays gradually outward with distance. Two different situations arise from that same potential gradient:

Touch voltage: the potential difference between an earthed metal part (for example a transformer enclosure or a fence around the substation) and a point on the ground at the distance a standing person can reach while touching that part — the current path runs roughly hand-to-feet, through the chest.
Step voltage: the potential difference between the two points on the ground where a person's two feet land during a single stride while walking across the potential gradient — no earthed metal part needs to be touched at all; the current path runs foot-to-foot, through the legs.

Both are driven by the same underlying phenomenon (a fault current raising the local earth potential), but they are assessed against different limit values in EN 50522, in part because the foot-to-foot current path is considered to present a different risk than the hand-to-feet path for a given voltage.

Why the gradient is steepest close to the electrode

The potential in the soil around an earthing system does not fall off in a straight line with distance; it decreases fastest close to the electrode and flattens out further away. A step voltage measured right at the edge of a single, concentrated earth rod is therefore considerably higher than a step voltage measured the same 1 m stride length taken several metres further out, even though both steps cover the same physical distance.

Why a substation grid is a mesh, not a single electrode

This is precisely why a substation earthing system is normally designed as a buried horizontal mesh grid of interconnected conductors covering the whole working area (often combined with vertical earth rods at strategic points), rather than relying on one or a few concentrated electrodes. A mesh grid deliberately flattens the potential gradient across the area a person can actually walk or stand on: because the earthing conductors are distributed under the whole surface rather than concentrated at one point, the potential across that surface stays comparatively uniform, so both the touch voltage at any accessible metal part and the step voltage anywhere within the mesh area stay within tolerable limits. Mesh spacing is typically tightened near the perimeter and around access points, where a person is most likely to be standing at the moment a fault occurs.

The gravel (or crushed-rock) surface layer as an additional mitigation

A layer of high-resistivity material — typically crushed stone or gravel, with a resistivity in the rough range of thousands of ohm- metres, laid over the entire substation working area — adds series resistance to the contact between a person's feet and the true earth surface. Because the body current for a given step or touch voltage depends on the total resistance in that current path, this added foot-contact resistance directly increases the tolerable (permissible) touch and step voltage for that specific location, without changing the electrical behaviour of the buried grid itself. It is a comparatively low-cost measure, commonly applied in combination with — not instead of — correct mesh grid design.

Practical relevance

When assessing an existing substation compound, or specifying a new one, both a low overall earthing resistance (see the earth resistance measurement guide) and a grid design that limits the local step and touch voltage across the accessible area are needed — a technically low total resistance value does not, on its own, guarantee that the potential gradient near the grid's perimeter or around an access gate stays within safe limits; and the condition of the high-resistivity surface layer (thickness, whether gravel has been eroded away or contaminated by soil over time) should be checked as part of that same assessment, since a degraded surface layer silently reduces the tolerable voltage the original design relied on.

Common mistakes

Assuming a low overall earthing resistance automatically means the step and touch voltages around the substation are safe — overall resistance and the local potential gradient across the accessible area are related but distinct properties of the same earthing system.
Relying on a single, concentrated earth electrode for a substation instead of a properly meshed grid — a concentrated electrode produces a much steeper, more hazardous potential gradient close to itself than an equivalent mesh grid covering the same area.
Treating the gravel/crushed-rock surface layer as purely cosmetic or for weed control, and not checking its condition during inspection — a degraded or missing surface layer removes a mitigation the original touch/step voltage design relied on.
Not tightening the mesh spacing near the perimeter and access points, where a person is statistically most likely to be standing at the moment a fault occurs.