Earth Loop Impedance Too High? How to Find the Cause and Fix It

Earth fault loop impedance (Zs) is the total impedance of the path that earth fault current takes when a live conductor comes into contact with an earthed part. This path starts at the transformer winding, runs through the phase conductor to the point of the fault, through the fault itself, through the CPC (circuit protective conductor) back to the distribution board, through the main earthing terminal, through the means of earthing (supply company earth or earth electrode), and back to the transformer.

The reason Zs matters is simple: when an earth fault occurs, the fault current must be large enough to operate the protective device (MCB, fuse, or RCD) within the maximum disconnection time specified by BS 7671. The lower the Zs, the higher the fault current, and the faster the protective device trips. If Zs is too high, the fault current is too low, and the protective device may not trip at all — leaving the fault energised and creating a risk of electric shock and fire.

Zs is made up of two components: Ze (the external earth fault loop impedance — everything from the transformer to the consumer unit) and R1+R2 (the impedance of the circuit phase conductor and CPC from the consumer unit to the point of measurement). The formula is Zs = Ze + R1+R2.

BS 7671:2018+A4:2026 sets maximum Zs values for each type and rating of protective device. These values ensure that in the event of an earth fault, the protective device will disconnect the supply within the required time — 0.4 seconds for circuits supplying socket outlets and portable equipment, and 5 seconds for circuits supplying fixed equipment.

When comparing your measured Zs against the tabulated maximum, apply the 80% rule. The BS 7671 tabulated values assume conductors at their maximum operating temperature. When you test a circuit that is not carrying load (cold), the conductor resistance is lower. To account for this, multiply the tabulated Zs by 0.8 — your measured (cold) value must be below this reduced figure. For example, the 32 A Type B maximum of 1.37 ohms becomes 1.10 ohms for comparison with cold test results.

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When your Zs reading exceeds the BS 7671 maximum for the protective device, the fault lies somewhere in the earth fault loop. The cause could be in the external supply (high Ze), in the circuit wiring (high R1+R2), or both. Here are the most common causes, starting with the most frequently encountered.

In the majority of cases where Zs readings are marginally above the BS 7671 maximum, the cause is one or more loose or high-resistance connections in the earth fault loop. Every terminal, joint, and clamp in the path adds a small amount of resistance. When several connections are slightly loose or corroded, the cumulative effect can be enough to push Zs over the limit.

The fix is straightforward: re-make the connection. Clean the terminal, strip back to bright copper if the conductor is tarnished, tighten to the manufacturer's recommended torque, and retest. A single re-tightened connection can reduce Zs by 0.1 to 0.5 ohms — often enough to bring a marginal circuit back into compliance.

Conductor resistance is proportional to length. On long cable runs — common in large houses, converted barns, commercial units, and circuits feeding outbuildings — the R1+R2 component of Zs can be too high for the protective device, even with perfect connections and a healthy supply earth.

Consider a radial socket circuit using 2.5 mm squared twin and earth cable running 25 metres from the distribution board to the furthest socket. The R1+R2 for 2.5/1.5 mm squared cable is approximately 22.4 milliohms per metre (at 20 degrees Celsius), giving a total R1+R2 of 0.56 ohms. With a Ze of 0.35 ohms (typical for a TN-C-S supply), the Zs at the furthest point is 0.91 ohms — which passes for a 32 A Type B MCB. But extend that run to 40 metres and the R1+R2 becomes 0.90 ohms, giving a Zs of 1.25 ohms — which exceeds the 1.10 ohm cold limit.

Solutions for long cable runs include:

The circuit protective conductor (CPC) — the earth wire — is often the weakest link in the earth fault loop. In standard twin and earth cable, the CPC is one size smaller than the line and neutral conductors. This smaller cross-section means higher resistance per metre, and R2 often contributes more to Zs than R1.

For 2.5 mm squared twin and earth cable, the CPC is 1.5 mm squared. The R1+R2 per metre is made up of approximately 7.41 milliohms (R1, the 2.5 mm squared line conductor) plus approximately 12.10 milliohms (R2, the 1.5 mm squared CPC) — the CPC contributes 63% of the total circuit impedance. If you used a 2.5 mm squared CPC instead, the R1+R2 would drop from 19.51 to 14.82 milliohms per metre — a 24% reduction.

On circuits where high Zs is caused by the CPC size, the solution is to install a supplementary CPC of adequate size alongside the existing cable. This additional earth conductor runs in parallel with the existing CPC, reducing the combined R2 and therefore the overall Zs. The supplementary CPC must be correctly connected at both ends (at the distribution board earth bar and at the accessory earth terminal) and must be adequately sized per BS 7671 Table 54.7.

The approach to fixing high Zs depends on which component of the earth fault loop is causing the problem. Start by measuring Ze at the origin of the installation and R1+R2 for the circuit. This tells you whether the issue is in the external supply, the circuit wiring, or both.

After completing any remedial work, retest Zs at the furthest point of the circuit and record the new reading on the schedule of test results. Issue the appropriate certificate for the remedial work — an Electrical Installation Certificate or Minor Works Certificate depending on the scope of work.

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Supplementary equipotential bonding is sometimes proposed as a solution to high Zs, but it is important to understand what it does and does not achieve. Supplementary bonding connects exposed-conductive-parts (metal pipework, metallic baths, radiators) to the circuit protective conductor, reducing the touch voltage in the event of an earth fault. It does not reduce Zs or increase fault current — it protects by ensuring that all metalwork in the area rises to the same potential during a fault.

Under BS 7671 Regulation 415.2, supplementary bonding can be used as an additional protective measure in locations of increased risk (such as bathrooms). However, it is not a substitute for adequate Zs. The circuit must still meet the disconnection time requirements, or alternative measures (such as RCD protection) must be provided.

In practice, if Zs is too high for the MCB to disconnect within the required time, the recommended approach is:

Record all supplementary bonding on the EICR or EIC and note the reason for its installation. Elec-Mate's AI fault diagnosis tool can recommend the appropriate protective measures based on your measured Zs, the protective device type, and the installation characteristics.