9.2 Earth Electrode Resistance on TT Systems

TT Systems and the Earth Electrode

A TT system earths the installation via an earth electrode (a rod or plate driven into the ground), not via the distribution network operator's (DNO) earth terminal. Because soil resistance is high and variable, the earth-electrode resistance (Ra) is usually too high for ADS (automatic disconnection of supply) to be achieved by an overcurrent protective device alone — so an RCD is almost always required on TT installations to meet the disconnection time.

The TT arrangement is common where the DNO does not provide an earth (typical for overhead supplies in rural areas) or where the consumer opts for an independent electrode. The electrode forms the return path for earth-fault current through the general mass of earth, and the quality of that path is what Ra quantifies.

Why Measure Ra

Measuring Ra confirms the electrode provides a stable, low-enough resistance for the RCD to operate and that touch voltage on exposed-conductive-parts stays safe during an earth fault. An unstable or high-resistance electrode can leave dangerous touch voltage on accessible metalwork long enough to cause harm, and can prevent the RCD from achieving its disconnection time under real fault conditions.

Ra also varies with season and soil moisture — a reading taken in wet winter conditions may not hold through a dry summer. This is why the stability guidance exists as well as the absolute Zs limit.

Acceptable Values

Two distinct limits apply, and they are not interchangeable:

• RA ≤ 200 Ω — BS 7671 Regulation 411.5.3 carries a note that a value of RA exceeding 200 Ω may not be stable, and GN3 and the On-Site Guide repeat it. Above 200 Ω, seasonal variation in soil moisture can push an already-high value higher still. This is a stability ceiling, not a Table 41.5 figure, and it is not conditioned on the rating of the RCD.
• TT compliance via Table 41.5 (Reg 411.5.3): Ra × IΔn ≤ 50 V, i.e. Zs ≤ 50/IΔn. For a 30 mA RCD, Zs ≤ 1667 Ω (50 ÷ 0.03). On TT, Zs is dominated by Ra (Zs ≈ Ra + R1+R2, and R1+R2 is small), so Ra must be well within 1667 Ω for the RCD to satisfy the disconnection criterion.

The 200 Ω stability limit is the practical target you design and verify to; the 1667 Ω figure is the regulatory maximum that the RCD/Zs combination must not exceed. An electrode at 400 Ω might technically allow the RCD to operate (below 1667 Ω) but would be flagged as unstable and unsuitable for a reliable installation.

Measurement Methods

Two methods are used. The fall-of-potential method is the accurate dead test; the 2-wire method is a quick live approximation.

Method 1: Fall-of-Potential (3-Point / 62% Method) — the Accurate Dead Method

This is the accurate method, performed with the installation dead (isolated). An earth-electrode tester is used with three connections:

• E — the earth electrode under test, temporarily disconnected from the installation.
• C — a current probe driven into the soil at a distance from E.
• P — a potential probe moved along the line between E and C.

The tester drives a known current between E and C through the soil, and measures the voltage between E and P. Ra = V / I. As P is moved from E towards C, the measured resistance rises and then flattens into a plateau before rising again near C. The reading in the flat zone is the true electrode resistance, independent of probe position.

Why the 62% Position Gives the True Value

The resistance-vs-distance curve has two regions of influence: near E, the electrode's own voltage gradient dominates and the reading climbs steeply; near C, the current probe's voltage gradient dominates and the reading climbs again. Between these two spheres of influence lies a flat plateau where the reading is independent of small probe movements — this is where the true Ra is measured. The plateau sits at approximately 62% of the E–C distance, which is why the method is commonly called the 62% method.

If P is placed too close to E or too close to C, the reading will be falsely low or falsely high. Verifying the plateau by taking readings at, say, 50%, 62%, and 70% of the distance is good practice — if all three agree, the flat zone has been found.

Method 2: 2-Wire Live Approximation

With the main bonding conductor temporarily disconnected, measure Ze (external loop impedance) using a loop impedance tester. On a TT system with the bonding disconnected, Ra ≈ Ze, because the only earth return path is the electrode itself. This is a quick check, less accurate than fall-of-potential: it includes the DNO neutral-to-earth path and is subject to network noise and parallel earth paths. It is useful as a fast verification but should not replace the fall-of-potential method for an initial verification or a formal EICR.

Safety When Testing the Electrode

Disconnecting the main bonding conductor or the electrode conductor for testing temporarily removes the earth fault path — the installation is without its normal ADS protection for the duration of the test. Take the following precautions:

• Work dead where possible — isolate the installation before disconnecting the electrode.
• Clearly label the disconnection point so the connection is not inadvertently left off.
• Restore the electrode/bonding connection immediately after measurement, and verify continuity before re-energising.
• Do not leave the installation unattended while the electrode is disconnected.

Recording Ra

Record the measured Ra and the method used (fall-of-potential or 2-wire) on the EIC (Electrical Installation Certificate) for new work, or on the EICR (Electrical Installation Condition Report) for periodic inspection. Where Ra exceeds 200 Ω, record it as a limitation or a recommendation to improve the electrode (drive deeper, add a second rod, or treat the soil), even if the RCD still achieves the Table 41.5 Zs value.