Earth Rod Resistance Calculator — Size Your Earth Electrode for TT Systems

Earth rod resistance (also called earth electrode resistance) is the resistance between an earth rod driven into the ground and the general mass of earth. It is measured in ohms and determines how effectively fault current can flow to earth through the electrode. In a TT earthing system — where the installation's earth is provided by the consumer's own earth electrode rather than the supply company's earth — this resistance is critical for safety.

The earth electrode resistance directly affects whether the protective device (almost always an RCD in TT systems) can disconnect the supply quickly enough in the event of an earth fault. BS 7671 requires that the product of the earth electrode resistance (RA) and the rated residual operating current of the RCD (IΔn) must not exceed 50 V. This ensures that touch voltages remain within safe limits during a fault condition.

Electricians working on rural properties, agricultural installations, temporary supplies, and any site without a PME or TN-S earth from the DNO will need to install earth rods and verify the resistance. The earth loop impedance calculator is the companion tool for checking overall Zs values once the earth electrode is installed.

A TT earthing system is one where the installation earth is provided by an earth electrode (typically a driven earth rod) installed at the property, rather than by the electricity distributor. The first "T" stands for "terre" (earth) on the supply side — meaning the supply transformer neutral is earthed. The second "T" means the installation's exposed-conductive-parts are connected to earth via a local earth electrode, independently of the supply earth.

TT systems are common in rural areas where the DNO supplies electricity via overhead lines without a PME (protective multiple earthing) terminal. They are also used for temporary electrical installations, agricultural buildings, caravan parks, marinas, and any situation where a reliable earth connection from the distributor is not available.

The key characteristic of TT systems is that the earth fault loop impedance is much higher than in TN systems because the return path includes the resistance of the earth electrode and the general mass of earth. This means that overcurrent devices (MCBs, fuses) alone cannot provide adequate disconnection times — the fault current is too low to trip them quickly. This is why BS 7671 requires RCD protection on every circuit in a TT system. Understanding earthing arrangements is fundamental to safe installation design.

The resistance of a single vertical earth rod driven into homogeneous soil is calculated using the formula:

This formula shows that the three factors affecting earth rod resistance are soil resistivity (rho), rod length (L), and rod diameter (d). Of these, soil resistivity is by far the most significant — it varies by a factor of 100 or more between different soil types. Rod length has a moderate effect — doubling the rod length approximately halves the resistance. Rod diameter has relatively little effect because it appears inside a logarithm.

In practice, this means that if you cannot achieve a low enough resistance with a single rod, your options are to use a longer rod, install parallel rods, or choose a location with lower soil resistivity. Increasing the rod diameter has very little benefit. These calculations feed into the overall fault current assessment for the installation.

Soil resistivity is measured in ohm-metres and varies enormously depending on soil composition, moisture content, temperature, and mineral content. The following table gives typical values for common UK soil types:

Soil resistivity varies significantly with depth. The top layer may be dry topsoil with high resistivity, while deeper layers may be wet clay with much lower resistivity. This is one reason why longer rods achieve lower resistance — they penetrate into moister, more conductive soil layers.

For accurate results on critical installations, soil resistivity should be measured on site using the Wenner four-electrode method. For typical domestic TT installations, the estimated values above combined with an on-site earth electrode resistance test are sufficient.

When a single earth rod cannot achieve a low enough resistance — typically in areas with high soil resistivity — additional rods can be installed in parallel to reduce the combined resistance. The rods must be spaced apart by at least their driven length to minimise interaction between their resistance zones.

For two identical rods spaced at their length or more apart, the combined resistance is approximately half the resistance of a single rod. For three rods, the combined resistance is approximately one-third. In general, for N well-spaced identical rods:

If the rods are spaced closer together than their length, the resistance zones overlap, and the combined resistance is higher than the simple division suggests. A correction factor must be applied. Elec-Mate's calculator includes these correction factors — you enter the number of rods and the spacing, and the calculator returns the corrected combined resistance.

The parallel rods must all be bonded together with a copper earth conductor, and the conductor must be protected against mechanical damage and corrosion. The bonding connections should be accessible for future testing. Correct cable sizing for the earthing conductor is essential. This is part of the overall EICR documentation for the installation.