Myth of Selecting Right Earthing Conductor

A widespread misconception exists that burying thicker or more conductive wires will automatically make an electrical substation safer from shocks. It seems logical that a massive copper cable would absorb a surge better than a thin steel one. However, when evaluating safety metrics like Earth Potential Rise, touch voltage, and step voltage, conductor material and thickness are surprisingly irrelevant.

Why Soil Always Wins

To understand this, the relationship between the buried grid and the surrounding earth must be examined. An earth grid acts primarily as a geometric cage designed to distribute electricity evenly. While copper and steel have different resistances, the resistance of any metal wire is practically zero when compared to the massive electrical resistance of the soil. Because the soil is the dominant bottleneck, altering the wire's thickness or material does not change how voltage ripples through the dirt. Safe touch and step voltages are achieved through the physical layout and mesh spacing of the grid, not the recipe of the metal.

Proof in the Simulation Models

This behavior is clearly illustrated in the simulation models. Three touch voltage maps are compared under identical fault conditions but using different grounding wires.

The first map evaluates a 70mm² copper conductor, yielding a peak touch voltage of 541.5 Volts. In the second map, the thickness is increased to a 100mm² copper wire, yet the peak touch voltage only drops by a tiny five volts to 536.8 Volts. In the third map, the copper is swapped entirely for a 70mm² galvanized steel conductor. Remarkably, the maximum voltage remains virtually unchanged at 540.6 Volts. Across all three visual outputs, the color-coded danger patterns are identical, proving that safety voltage indicators are completely insensitive to conductor modifications.

Where Size and Material Matter

If size and material do not alter shock safety, why are they scrutinized? The answer lies in a completely different parameter calle thermal stability. Thermal stability is the conductor's ability to handle fault-current heating without exceeding its temperature limit. During a ground fault, current through the earthing conductor generates Joule heating. The conductor cross-section must be large enough that its temperature rise over the backup protection clearing time stays within the material limit.

When a bolted single-line-to-ground fault hits a substation, fault current can surge to tens of thousands of amperes in milliseconds. All of that violent energy funnels through the buried earthing conductor — and for a brief moment, that buried wire behaves exactly like a fuse. It is not supposed to be one, but physics does not care about intent. The heat generated follows Joule's law — proportional to I²t — and if the conductor cannot absorb or dissipate that energy fast enough, the metal will reach its melting point and literally part ways. The engineering term for this is "thermal withstand," and it is one of the most fundamental checks in earthing system design.

The governing UK standard BS EN 50522, mandates that every earthing conductor must be dimensioned for thermal strength. For fault durations under 5 seconds, an adiabatic temperature rise is assumed — meaning all the heat stays in the conductor, none escapes to the surrounding soil. This is a deliberately conservative assumption. For example, a 70 mm² copper conductor in a single-spur configuration can safely carry 12 kA for 3 seconds before hitting its temperature limit. IEEE Std 80 goes even further and accounts for soil drying effects, which can degrade heat dissipation and compound the problem.

To make this concrete, consider two examples.

Example 1 — Copper vs. aluminum. Let’s say there is a 33 kV substation with a 16 kA fault and a 3-second backup protection clearance time. Copper, with its high conductivity and melting point around 1083°C, yields a required minimum cross-section of roughly 50 mm² using the BS EN 50522 Annex D adiabatic formula. Engineers would typically uprate this to 70 mm² for margin — perfectly safe. Now substitute aluminum. Aluminum has roughly 60% of copper's conductivity and a melting point of only 660°C. Its thermal material constant K is about 148 compared to copper's 226. For the same fault, the required cross-section balloons to over 120 mm². If someone mistakenly installed a 70 mm² aluminum conductor thinking it equivalent to copper, that conductor would exceed its melting temperature within the 3-second fault window, fuse open, and leave the fault current with no return path — a catastrophic failure. This is precisely why ENA TS 41-24 Table 5 lists only copper and steel; aluminum's poor thermal stamina disqualifies it for buried earthing duty.

Example 2 — Undersized steel. Steel is sometimes used as a lower-cost alternative, but its thermal capacity is dramatically lower. Its K constant is only 78 A·√s/mm², and BS EN 50522 caps its final temperature at 200°C — not because the steel itself would melt at that point (steel melts far higher), but because exceeding 200°C would bake and crack the surrounding soil, destroying the electrode-to-earth interface and causing the ground resistance to spike. Running the same 16 kA / 3 s fault through a 50 mm² galvanized steel conductor (the minimum size allowed by BS EN 50522) yields a temperature rise well past the 200°C limit. The steel would glow, its welded joints would weaken, and the earthing system would physically disintegrate under the thermal stress. The conductor, designed to be a permanent safety path, becomes a one-shot fuse.

This brings us to a key insight that often trips up newcomers: safety voltage calculations and thermal withstand calculations live in completely different worlds. Touch voltage and step voltage (EPR) depend almost entirely on the earth electrode's resistance to ground — the geometry of the grid, the soil resistivity, the surface layer material. Whether the grid is made of thin copper wire or thick copper bar matters negligibly for the voltage result. But thermal stability is ruthlessly sensitive to conductor choices — material type, cross-sectional area, connection method (welded vs. bolted).

So the bottom line: when engineers specify thicker copper cross-sections — often 70 mm², 120 mm², or even 240 mm² for large installations — it is a calculated response to the physics of adiabatic heating, governed by a chain of standards. The goal is simple: ensure that when the grid suffers its worst-case fault, the earthing system does not become the weakest link and physically vaporize mid-crisis.