When measuring earth fault loop impedance (Zs) on-site with a multi-function tester, the measured reading cannot be compared directly against the maximum Zs tables in BS 7671 without temperature correction. Understanding the 0.8 multiplier ensures your circuits will trip within disconnection times even under full load.
The Physics: Conductor Resistance & Temperature
Copper has a positive temperature coefficient of resistance. As a cable heats up under load, its electrical resistance increases proportionally:
- On-Site Testing: Typically conducted when the installation is cold or lightly loaded (ambient temperature approx 20°C).
- Fault Conditions: Occur when circuits are operating at full rated capacity, where PVC-insulated conductors reach 70°C and XLPE conductors reach 90°C.
A circuit that measures 1.0 Ω at 20°C will increase to approximately 1.20 Ω at 70°C. If this temperature rise is ignored, a measured value that appears to pass on-site might fail to trip the protective device within 0.4 seconds during a genuine earth fault under full load.
The 80% Rule of Thumb Explained
BS 7671 Appendix 3 and the IET On-Site Guide provide maximum permitted Zs values calculated at conductor operating temperature (70°C).
To allow on-site testing without measuring ambient temperature and calculating individual temperature correction factors, Appendix 14 introduces the standard rule-of-thumb 0.8 factor (80% of tabulated maximum Zs):
Maximum Measured Zs Limit:
Measured Zs ≤ 0.8 × Tabulated Zs
Practical Example: 32A Type B MCB (0.4s disconnection)
- Tabulated Maximum Zs (Table 41.3 at 70°C): 1.37 Ω
- Applied 80% Rule of Thumb (1.37 × 0.8): 1.10 Ω
- Rule: If your meter reads ≤ 1.10 Ω at ambient temperature, the circuit passes without further calculation.
What to Do if Measured Zs is Between 80% and 100%
If your measured reading is 1.20 Ω (higher than 1.10 Ω, but below the tabulated 1.37 Ω):
- Do not immediately fail the circuit: The 0.8 factor is a conservative worst-case approximation assuming full ambient-to-operating rise.
- Perform precise temperature correction: Measure the actual ambient temperature and apply the formula from BS 7671 Appendix 14.
- Verify Supplementary Protection: If Zs cannot meet disconnection times, verify whether supplementary RCD protection (30 mA) is in place to provide fault protection under Regulation 411.4.200.