Max Zs / Earth Fault Loop Calculator

Look up the BS 7671:2018+A2:2022 maximum earth fault loop impedance (Zs) for a protective device, get the 80 % limit for readings taken at ambient temperature, and check a measured or calculated Zs against it.

Guidance only. Results are estimates based on the figures you enter and published tables. All electrical work must be designed, checked and tested by a qualified electrician to the regulations that apply where you are (e.g. BS 7671, NEC).

Protective Device

TN systems, U0 = 230 V. Table 41.1 / Reg. 411.3.2.2: 0.4 s for final circuits up to 63 A with sockets, and up to 32 A fixed equipment; 5 s for distribution circuits and other circuits (Reg. 411.3.2.3).

Max Zs (tabulated)
—
Max measured Zs (80 %)
—
For readings at ambient temperature (Appendix 3).
Implied Ia
—
U0 × Cmin ÷ Zs = current to operate within the time

Check a Circuit

Ω
External loop impedance (measured or DNO declared).
Ω
Measured continuity, end of circuit.
Ω
Overrides Ze + (R1+R2) if entered.
m
mm²
mm²
R1+R2 per metre at 20 °C
—
Copper, IEC 60228 conductor resistance.
R1+R2
—
Zs
—
Limit Used
—
Result
—

Next: check the CPC survives the fault with the Adiabatic CPC Calculator, or check breaking capacity with the Prospective Fault Current Calculator. Size the cable itself with the Cable Gauge Calculator.

RCD Protection (Table 41.5)

Where an RCD provides fault protection, Zs (TN) or RA (TT, Reg. 411.5.3) must satisfy Z × IΔn ≤ 50 V.

Rated residual current IΔn Max Zs / RA

These limits are very high, but an earth electrode resistance above 200 Ω may not be stable (IET On-Site Guide / Guidance Note 3). Aim for as low as practicable.

Max Zs Reference Table

Device In 0.4 s 0.4 s × 0.8 5 s 5 s × 0.8

Values in ohms. “—” = not tabulated in BS 7671 for that time. 80 % values are rounded down.

Notes & References

  • Tabulated values: BS 7671:2018+A2:2022 Table 41.2 (fuses, 0.4 s), Table 41.3 (circuit-breakers and the overcurrent part of RCBOs, 0.4 s and 5 s) and Table 41.4 (fuses, 5 s), for U0 = 230 V. They already include Cmin = 0.95 (Zs = U0 × Cmin ÷ Ia, Reg. 411.4.4 / Appendix 3), and are for conductors at their maximum operating temperature.
  • Circuit-breakers: Ia is 5 × In for Type B and 10 × In for Type C (same limit for 0.4 s and 5 s). Type D uses 20 × In for 0.4 s and 10 × In for 5 s.
  • 80 % rule: readings taken at ambient temperature are lower than the in-service value, because conductor resistance rises with load. BS 7671 Appendix 3 gives the rule of thumb Zs(m) ≤ 0.8 × U0 × Cmin ÷ Ia. Guidance Note 3 gives more exact temperature corrections if a reading is marginal.
  • Estimating R1+R2: copper conductor resistances are the IEC 60228 maximum values at 20 °C, as used in the IET On-Site Guide (e.g. 2.5/1.5 mm² = 7.41 + 12.10 = 19.51 mΩ/m). The 1.20 multiplier brings them to 70 °C (thermoplastic) and 1.28 to 90 °C (thermosetting). A calculated Zs at operating temperature is compared against the full tabulated value.
  • BS 1361: withdrawn and replaced by BS 88-3 fuse system C, which is what the current tables list. For older BS 1361 ratings (15 A, 30 A), devices not listed (e.g. MCCBs, larger fuses) or a U0 other than 230 V, use the manufacturer's time/current data and the Appendix 3 formula.
  • RCDs: Table 41.5 values are 50 V ÷ IΔn for non-delayed and time-delayed (S-type) RCDs.

This tool is guidance only. It does not replace BS 7671, the IET Guidance Notes, manufacturer's data, or the judgement of a competent person. Check values against your copy of the current regulations before certifying.