A practical reference to earth fault loop impedance testing, BS 7671 Table 41.3 values, the 80% rule, temperature correction and the mistakes that fail EICRs.

Zs is one of the most important measurements on an EICR, and also one of the most commonly misunderstood. Get the value right and the circuit passes. Get it wrong – or miss the temperature correction – and you might record a pass that should have been a fail, or fail a circuit that actually complies. This guide covers what Zs actually is, the maximum permitted values from BS 7671 Table 41.3, why the 80% rule exists, and the temperature corrections that catch people out.

Zs is the total earth fault loop impedance measured at the furthest point of a circuit. It is the sum of the external loop impedance Ze (from the supply transformer, through the distribution network, to the origin of your installation) plus the internal impedance of the line conductor (R1) and the circuit protective conductor (R2) from the origin to the point of measurement.

Zs = Ze + (R1 + R2)

The value matters because it determines how much current will flow under an earth fault, and therefore whether the protective device will disconnect fast enough to prevent electric shock. If Zs is too high, fault current is too low, and the MCB or RCBO will not trip within the required time specified in BS 7671 Table 41.1. Regulation 411.3.2.2 states that Table 41.1 disconnection times apply to final circuits with a rated current not exceeding 63 A where socket-outlets are present, or 32 A for circuits supplying only fixed equipment. For 230 V single phase, that means 0.4 seconds on TN systems and 0.2 seconds on TT systems. Distribution circuits and circuits above those limits are permitted up to 5 seconds on TN systems and 1 second on TT systems.

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The tabulated maximum Zs values from BS 7671:2018+A2:2022 Table 41.3 apply to BS EN 60898 circuit-breakers and the overcurrent characteristics of BS EN 61009-1 RCBOs at a nominal voltage Uo of 230 V, with the Cmin factor of 0.95 applied as introduced by the 17th edition Amendment 3. Values are derived from the requirement in Regulation 411.5.4 of BS 7671: Zs × Ia ≤ Uo × Cmin, where Ia is the current causing instantaneous operation of the device (5×In for Type B, 10×In for Type C, 20×In for Type D). Rearranged, this gives Zs ≤ (Uo × Cmin) / Ia, which is how the tabulated maximum values are calculated.

Source: BS 7671:2018+A2:2022 Table 41.3. Values assume conductors at their maximum permitted operating temperature (typically 70°C for thermoplastic cables).

This is where most Zs errors happen. The values in Table 41.3 assume the conductors are at their maximum operating temperature – typically 70°C for thermoplastic insulated cables. In real life, when you are testing, the cables are usually at ambient temperature (around 10°C to 20°C). Cold conductors have lower resistance than hot ones, so your measured Zs at ambient temperature will always be lower than it would be under full load fault conditions.

The industry accepted rule, published in the IET On-Site Guide and Guidance Note 3 and derived from Appendix 3 of BS 7671, is that measured Zs should not exceed 80% of the tabulated value. This accounts for the conductor temperature difference. For a 32 A Type B MCB, Table 41.3 gives a maximum of 1.37 Ω. The maximum acceptable measured value is therefore 1.37 × 0.8 = 1.10 Ω.

80% values rounded to two decimal places. For Type C, divide Type B values by 2. For Type D, divide Type B values by 4.

Where an RCD is used to provide fault protection (most commonly on TT systems, but also on TN systems under Regulation 411.4.204), the condition in Regulation 411.5.3 must be satisfied:

R_A × IΔn ≤ 50 V

Where R_A is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts, and IΔn is the rated residual operating current of the RCD. Note 2 to Regulation 411.5.3 permits Zs to be used where R_A is not known, giving the more familiar form Zs ≤ 50 V / IΔn. The maximum Zs values corresponding to this requirement are tabulated in BS 7671 Table 41.5:

Table 41.5 values are the maximum Zs that satisfies the RCD requirement for automatic disconnection – they do not replace the Table 41.3 values where an overcurrent device is also providing fault protection. Note 2 to Table 41.5 cautions that a value exceeding 200 Ω may not be stable. On TT installations the earth electrode resistance Ra is typically the dominant component, and the On-Site Guide recommends this should be as low as practicable, with 200 Ω deemed satisfactory.

The 80% rule is a simplification based on an assumed ambient test temperature of 10°C. If you are testing in a loft in August at 35°C, or a basement in January at 5°C, the correction needs adjusting. Table B8 of the On-Site Guide and Table A6 of Guidance Note 3 give precise correction factors for different ambient temperatures. For most typical UK indoor testing between 10°C and 20°C the 0.8 factor is adequate.

Where you have a measured Zs that falls between the 80% value and the full tabulated value, do not automatically fail the circuit. Apply the correct temperature factor for the actual ambient temperature at the time of testing. If the corrected value still exceeds the BS 7671 limit, then you have a genuine non-compliance and the circuit fails.

Appendix 3 of BS 7671 explicitly states: “Wherever possible designers should use the manufacturer’s specific data.” The values in Table 41.3 are worst case limits based on the standards. Individual manufacturers often declare tighter trip tolerances, which means their devices operate at higher Zs values than the generic Table 41.3 figures.

For MCCBs to BS EN 60947-2, Table 41.3 does not provide values at all – manufacturer data is the only source. If you are using manufacturer-specific Zs values, record this in the “Remarks” column (column 31) of the schedule of test results, and reference the source document. The BS 7671 model schedule of circuit details specifically requires this under the footnote to column 12 (Maximum permitted Zs).

The iCertifi Circuit Designer handles all the BS 7671:2018+A2:2022 calculations automatically including Zs verification, cable sizing, voltage drop and derating factors – with the full working shown so you can justify every value to a QS or inspector.

These are the errors that repeatedly fail EICR quality checks and cost electricians rework:

• Testing against the full Table 41.3 value instead of the 80% value. Always apply the 0.8 factor unless you have explicit manufacturer data and temperature correction.
• Forgetting that bonded metallic services artificially lower Zs. Water pipes, gas pipes and structural steel in equipotential bonding provide parallel earth return paths. If these are later replaced with plastic, Zs increases. Where readings look unusually low, consider the parallel paths.
• Using the old pre-amendment-3 values. The 17th edition amendment 3 introduced the 0.95 Cmin factor which reduced all Table 41.3 values. Anything from before 2015 is out of date – a 32 A Type B circuit was 1.44 Ω under the old values, now 1.37 Ω.
• Not recording temperature at time of test. If a reading is borderline, the ambient temperature determines whether the 80% simplification is valid or whether you need the Guidance Note 3 correction factors.
• Confusing Ze and Zs. Ze is external loop impedance measured at the origin only. Zs includes the circuit and is measured at the far end.
• Recording Zs for an RCD-protected circuit without noting which formula applies. For fault protection by an RCD, the 50 V / IΔn formula in Regulation 411.5.3 applies. Where both an overcurrent device and an RCD provide fault protection, both checks need to be satisfied.

When a measured Zs exceeds the permitted maximum on an EICR, the observation is normally coded C2 – potentially dangerous – because automatic disconnection of supply cannot be relied upon to operate within the required time. A C2 makes the overall installation unsatisfactory. In some cases where the excess is marginal and further investigation is needed to confirm the reading, FI (further investigation required) may be appropriate, with the circuit retested under controlled conditions before a final code is assigned.

Record the measured Zs, the maximum permitted Zs, and the device type and rating in the schedule of test results. If the measured value is within 80% of the tabulated value, the circuit is confirmed compliant. If it falls between 80% and 100%, apply temperature correction and decide based on the corrected value.

The iCertifi BS 7671 electrical certificates app checks every measured Zs against the correct Table 41.3 value and the 80% rule automatically, flagging non-compliances before the certificate is signed. Board Vision reads the distribution board into the circuit schedule, and Test Vision reads test instrument displays directly so Zs readings populate the correct field without manual transcription.

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