Earth Fault Loop Impedance: What the Numbers Mean

Why Zs Matters on Every Certificate

Earth fault loop impedance — Zs — is one of the most critical readings you'll record on an Electrical Installation Certificate or Minor Works Certificate. It tells you whether, in the event of a fault, enough current will flow to trip the protective device quickly enough to prevent electric shock. Get it wrong, and someone could be left exposed to a dangerous fault for far too long.

Despite its importance, Zs is one of the most commonly misunderstood values in domestic and commercial testing. This guide breaks down what the numbers actually mean, what the maximum permitted values are, and what to do when your readings land in that awkward borderline territory.

The Basic Principle

The earth fault loop is the complete path that fault current takes: from the transformer, through the line conductor, through the fault, back through the earth path, and back to the transformer. Zs is the total impedance of that entire loop, measured in ohms (Ω).

The lower the Zs, the higher the fault current. The higher the fault current, the faster your protective device (MCB, fuse, or RCD) will operate. BS 7671 sets maximum Zs values to ensure that protective devices disconnect within the required time — 0.4 seconds for final circuits in a TN system and 0.2 seconds for TT systems protected by RCDs.

The Formula

The relationship is straightforward:

Zs = Ze + (R1 + R2)

• Ze — External earth fault loop impedance (everything from the transformer to your intake position)
• R1 — Resistance of the line conductor from the consumer unit to the furthest point of the circuit
• R2 — Resistance of the circuit protective conductor (CPC) over the same route

You can either measure Zs directly at the furthest point of the circuit with a loop impedance tester, or you can calculate it from your Ze measurement and your R1+R2 readings. Both methods are valid, but the calculated method is often preferred for reasons we'll cover below.

Maximum Zs Values You Need to Know

BS 7671 (the 18th Edition Wiring Regulations, as amended) provides maximum Zs values in Tables 41.2 to 41.4. These values ensure disconnection within the required time. Here are the ones you'll use most often in domestic and light commercial work:

Type B MCBs (Most Common in Domestic Work)

• 6A — Max Zs: 7.28 Ω
• 10A — Max Zs: 4.37 Ω
• 16A — Max Zs: 2.73 Ω
• 20A — Max Zs: 2.19 Ω
• 32A — Max Zs: 1.37 Ω
• 40A — Max Zs: 1.09 Ω
• 50A — Max Zs: 0.87 Ω

Type C MCBs (Motors, Inductive Loads)

• 6A — Max Zs: 3.64 Ω
• 10A — Max Zs: 2.19 Ω
• 16A — Max Zs: 1.37 Ω
• 20A — Max Zs: 1.09 Ω
• 32A — Max Zs: 0.68 Ω

These figures include the C_min factor of 0.95 introduced by Amendment 3 to the 17th Edition — if you're working from an older memorised table (e.g. 1.44 Ω for a B32), those values are out of date.

BS 1361 / BS 88-3 Fuses (Found in Older Consumer Units)

• 5A — Max Zs: 9.93 Ω
• 15A — Max Zs: 2.30 Ω
• 30A — Max Zs: 0.91 Ω
• 45A — Max Zs: 0.57 Ω

Measured vs Calculated Zs

There are two ways to determine Zs, and understanding the difference will save you headaches on borderline results.

Direct Measurement

Using a calibrated loop impedance tester at the furthest point of the circuit. This gives you a real-world reading at the ambient temperature of the conductors at that moment. It's quick and straightforward, but the reading will vary depending on load and temperature conditions.

Calculation Method

Measure Ze at the intake (with main bonding disconnected, or use the supply company's declared value). Then measure R1+R2 for the circuit using a low-resistance ohmmeter at the consumer unit. Calculate: Zs = Ze + (R1+R2).

The calculation method is generally considered more reliable for certification purposes because your R1+R2 measurements are taken at a known temperature and can be corrected to operating temperature using a multiplier.

The 80% Rule of Thumb

Here's where many electricians trip up. The maximum Zs values in BS 7671 assume conductors at their maximum operating temperature (70°C for thermoplastic cables). When you measure on site, your cables are likely at ambient temperature — perhaps 10–20°C. As conductors heat up under load, their resistance increases, and so does Zs.

To account for this, the IET Guidance Note 3 recommends applying a 0.8 multiplier to the tabulated maximum Zs values when comparing against live test readings. In practice, this means:

Maximum acceptable measured Zs = Tabulated Zs × 0.8

For example, a 32A Type B MCB has a tabulated maximum Zs of 1.37 Ω. Your measured Zs at ambient temperature should not exceed:

1.37 × 0.8 = 1.10 Ω

If you're using the calculation method with R1+R2 values measured at ambient temperature, you should apply a correction factor of 1.2 to your R1+R2 readings to approximate their value at operating temperature.

What to Do with Borderline Readings

You're on site, you've taken your readings, and your Zs is hovering right around the maximum. What now?

Step 1: Verify Your Measurement

• Re-test. Make sure your instrument probes have a good connection.
• Check your instrument is in calibration and within its certification date.
• Test at the actual furthest point of the circuit, not just a convenient socket.

Step 2: Cross-Check with the Calculation Method

• Measure Ze and R1+R2 independently.
• Apply the temperature correction factor to R1+R2.
• Compare the calculated Zs against the tabulated maximum (not the 80% figure — the correction is already built into your calculation).

Step 3: Consider the Protective Device

• If the circuit is also protected by a 30mA RCD, BS 7671 (Regulation 411.4.204 and Table 41.5) permits a much higher maximum Zs — up to 1667 Ω for a 30mA device. In practice, treat any reading above 200 Ω with suspicion: an earth path that high may not be stable, so investigate rather than rely on it. And you must still verify RCD operation.
• However, remember that RCD protection doesn't replace the need for the overcurrent device to clear a fault — it's an additional layer. Best practice is to meet the MCB/fuse Zs limits as well.

Step 4: If It Genuinely Fails

• Check all connections in the earth path — loose CPCs at accessories are a common cause of high Zs.
• Check the main earthing conductor and earthing terminal.
• Verify the Ze with the supply company's declared value. If Ze is excessively high, the problem may be on the supply side — contact the DNO.
• Consider whether a larger CPC or shorter cable run is feasible.
• As a last resort, changing the protective device type (e.g., from Type B to Type B with RCD backup) may bring you within limits, but address the root cause first.

Recording Zs on Certificates

When completing your Electrical Installation Certificate or EICR in CertBox, record Zs values for every circuit in the schedule of test results. Include:

• The measured or calculated Zs value
• Which method you used (a note in the comments is good practice)
• The protective device type and rating for each circuit

On an EICR, if a Zs reading exceeds the permitted maximum, this is a Code C2 (potentially dangerous) observation, or possibly C1 (danger present) if the situation creates an immediate risk. Don't fudge the numbers — record what you measured, note the limitation, and recommend remedial action.

Common Mistakes to Avoid

• Forgetting the 0.8 correction — comparing a live ambient reading directly against the BS 7671 table value without adjustment.
• Testing at the nearest socket — Zs must be measured or calculated at the furthest point of the circuit, not the most convenient one.
• Ignoring high Ze — if your Ze is unusually high (above 0.35 Ω on a TN-C-S supply or above 0.8 Ω on TN-S), investigate before blaming the circuit wiring.
• Confusing Zs limits for different device types — a Type C MCB has a much lower maximum Zs than a Type B of the same rating. Always check the correct table.
• Not accounting for parallel earth paths — supplementary bonding and gas/water pipes can create parallel paths that artificially lower your Zs reading. The reading may look fine with them connected but could fail without them.

Key References

• BS 7671:2018+A2:2022 — 18th Edition IET Wiring Regulations, Chapter 41 (Protection against electric shock), Tables 41.2–41.4
• IET Guidance Note 3: Inspection & Testing — detailed guidance on measurement methods and correction factors
• GN3 Appendix B — Tables of corrected Zs values for on-site comparison

Published 2026-07-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.