Max Zs Calculator
Calculate maximum earth fault loop impedance for circuit breakers and fuses per BS 7671
Default shows 80% values for testing at ambient temperature (per Appendix 14)
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 6A | 5.82Ω | 2.91Ω | 1.45Ω |
| 10A | 3.49Ω | 1.75Ω | 0.87Ω |
| 16A | 2.18Ω | 1.09Ω | 0.54Ω |
| 20A | 1.75Ω | 0.87Ω | 0.44Ω |
| 25A | 1.4Ω | 0.69Ω | 0.35Ω |
| 32A | 1.09Ω | 0.54Ω | 0.27Ω |
| 40A | 0.87Ω | 0.44Ω | 0.21Ω |
| 50A | 0.69Ω | 0.35Ω | 0.17Ω |
| 63A | 0.55Ω | 0.27Ω | 0.13Ω |
| 80A | 0.44Ω | 0.21Ω | 0.11Ω |
| 100A | 0.35Ω | 0.17Ω | 0.08Ω |
Values from BS 7671:2018+A4:2026 Table 41.3 × 0.8 (Appendix 14 correction for ambient testing)
Other Protective Devices
Download the Complete Max Zs Cheat Sheet
All BS 7671 tables in a printable PDF. Free, no email required.
The maximum earth fault loop impedance (Zs) determines whether your protective device will disconnect quickly enough in fault conditions. BS 7671 requires disconnection within 0.4 seconds for final circuits supplying socket outlets, or within 5 seconds for fixed equipment circuits.
Understanding the Values
Default values shown are 80% of BS 7671 Tables 41.2, 41.3, and 41.4 (per Appendix 14) for testing at ambient temperature. Toggle to see 100% raw values.
BS 3871 for Older Installations
Many older installations use BS 3871 breakers (Type 1, 2, or 3). Our calculator includes these legacy values for EICR inspections.
Calculator Scope
This tool covers fixed lookup values for the listed protective devices. Manufacturer-specific MCCBs/RCBOs, RCD-only ADS cases, TT system judgement, and bespoke time-current curves must be checked against manufacturer data and the current regulations.
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