The Temperature Rise Limits for Earthing Conductors

What the Limit is Actually Protecting

Earthing conductors are sized based on a worst-case scenario: a late-cleared fault. As ENA TS 41-24 Issue 2 explicitly states, conductor sizing "should be based on backup protection clearance time, i.e., the design should allow for failure of primary protection without damage to the earthing system." The conductor must withstand the maximum foreseeable fault current for a specific duration—typically three seconds—without exceeding a permitted final temperature, even when factoring in future network growth.

Crucially, this permitted temperature is not a material limit. Copper melts at approximately 1085°C, far above any permitted ceiling. Instead, these limits protect two vulnerabilities: the mechanical integrity of the joints (repeated thermal expansion and contraction works bolted connections loose over time) and the overarching guarantee that the installation can still protect personnel after a severe fault. It is a strict acceptance ceiling for one specific short-circuit design case.

Using Correct Currents and Clearance Times

Most compliance disputes stem from conflating two distinct sets of design assumptions:

• Safety Checks (Touch Voltage, Step Voltage, Earth Potential Rise): Evaluated using the ground return current (the fraction of fault current returning through the soil) combined with normal primary protection operating times.

• Thermal Checks (Temperature Rise): Evaluated using the full, unreduced earth fault current across all voltage levels, combined with backup protection clearance times (e.g., 3 seconds up to 132 kV, and 1 second for 275 kV and above). TS 41-24 Section 5.4.3 also mandates a margin for future growth, typically 15%.

The safety check evaluates what happens when protection works. The thermal check evaluates what happens when it fails. Mixing these figures is the most common structural error in earthing reports.

The Three Key Documents and Their Roles

One Example, Three Different Verdicts

Consider a 70 mm² copper conductor on a single spur connection, subjected to an 8.5 kA fault for 3 seconds, with an initial temperature of 30°C.

• Read from the table (Fails): TS 41-24 Table 5(a) for a single spur lists 70 mm² against 8 kA and steps to 95 mm² at 12 kA. There is no 8.5 kA row, table values cannot be interpolated, so a conservative reading moves the design up to 95 mm² and declares the installed 70 mm² insufficient. On this reference, the conductor does not comply.

• Calculate by formula (Passes): First, list the inputs and where each one comes from: the current I = 8500 A and the duration t_f = 3 s are the design fault current and the backup protection clearance time from TS 41-24 Section 5.4.5; the initial temperature θ_i = 30 °C is the value TS 41-24's tables and ratings are based on; the final temperature θ_f = 405 °C is the copper limit from Section 5.5.1, justified in this case by an exothermically welded, continuous run; and the material constants for copper come from BS EN 50522 Table D.1, K = 226 A·√s/mm² and β = 234.5 °C. Substituting those values into Formula, the BS EN 50522 Annex D formula calculates a minimum requirement of 69.3 mm². The installed 70 mm² reaches a final temperature of 394°C, safely below the 405°C ceiling. By formula, the installation complies.

• Change the joint (Fails Again): The only change is the permitted final temperature, from 405 °C to 250 °C (TS 41-24 Section 5.5.1, which applies the lower figure because repeated thermal cycling can loosen bolted connections). A lower permitted final temperature means each kilogram of copper can absorb less heat, so more copper is needed to carry the same current for the same duration. In the arithmetic, only the numerator of the logarithmic term changes — from 405 + 234.5 = 639.5 to 250 + 234.5 = 484.5 — which reduces the logarithm from 0.8828 to 0.6053 and raises the required cross-section by about 21%, to 83.7 mm². As no standard size exists between 70 mm² and 95 mm², the practical outcome is a jump to the next size up.

These discrepancies reflect where each document draws its conservatism. Tables are blunt, pre-rounded, and fast. Formulas are continuous but rely entirely on the exact assumptions supplied by the assessor. Because neither method is systematically more generous, marginal results should always be reported with their underlying assumptions explicitly stated, rather than glossed over as a comfortable pass.

Practical Project Implications

Because conductor sizes scale in large, standard steps, minor changes in fault current or jointing methods can jump the specification to the next tier, driving up copper costs. Jointing is a strict engineering constraint, not a construction preference; exothermic welds and bolted connections are not interchangeable at the design stage.

Layout topology also plays a critical role. A duplicate connection or ring allows each path to be sized for 60% of the fault current, whereas a single spur must shoulder 100%. Ultimately, traceability is paramount. A fault current figure untethered to a network operator's documentation, or a temperature baseline left unstated, is the fastest way for a compliant design to fail a technical review.

The entire exercise reduces to one core principle: the temperature limit is a guarantee that the earthing system will survive the failure of its primary protection. That guarantee is only as robust as the current, the clearance time, the initial temperature, and the jointing method written down beside it. Selecting and justifying the most appropriate calculation method does more than just tick a compliance box—it is the key to delivering a cost-effective earthing assessment, successfully avoiding the heavy financial penalty of unnecessary over-engineering.