What is Zs and why does it matter?

Zs is the total earth fault loop impedance — the impedance of the complete fault current path from the supply transformer, through the line conductor, through the fault, and back via the earth path. BS 7671 requires Zs to be low enough that the protective device (MCB/fuse) disconnects within the required time (0.4s for socket circuits, 5s for distribution circuits). If Zs is too high, the fault current will be too low to trip the device quickly, creating a shock hazard.

How do I measure Ze and R1+R2?

Ze (external earth fault loop impedance) is measured at the origin of the installation with the main earthing conductor disconnected — this gives the impedance of the supply network. R1+R2 is measured using an insulation/continuity tester on the circuit with the supply isolated: R1 is the line conductor resistance and R2 is the CPC (circuit protective conductor) resistance. You can also measure Zs directly at the furthest point of the circuit, but the Ze + R1+R2 method allows verification of each component.

Why is temperature correction needed?

When you measure R1+R2 during testing, the conductors are at ambient temperature (typically 10-20°C). Under fault conditions, the conductors heat up rapidly — thermoplastic (PVC) cables are rated to 70°C. Copper resistance increases by approximately 0.4% per degree Celsius. The correction factor ensures your measured Zs value reflects the worst-case impedance at operating temperature, which is what matters for protective device disconnection.

What are the BS 7671 maximum Zs values for Type B MCBs?

For Type B MCBs per Table 41.3 (0.4s disconnection): 6A = 7.67Ω, 10A = 4.60Ω, 16A = 2.87Ω, 20A = 2.30Ω, 25A = 1.84Ω, 32A = 1.44Ω, 40A = 1.15Ω, 50A = 0.92Ω, 63A = 0.73Ω. These values assume a 230V nominal supply. Type C values are half of Type B, and Type D values are half of Type C, because they require progressively higher fault currents to trip magnetically.

Why use an 80% safety margin on maximum Zs?

The 80% rule (known as the 'rule of thumb') accounts for supply voltage variations, measurement instrument accuracy (typically ±2-5%), and the fact that supply impedance can vary with load conditions. BS 7671 Guidance Note 3 recommends that measured Zs should not exceed 80% of the tabulated maximum. This provides a safety buffer ensuring compliance even under worst-case conditions.

TradeCalculator

Zs Earth Fault Loop Impedance Calculator

Calculate total earth fault loop impedance (Zs) from Ze and R1+R2, apply temperature correction, and validate against BS 7671 Table 41.2-41.4 maximum Zs values.

Ze (External Earth Loop Impedance)

Measured at origin with main earth disconnected

R1+R2 (Circuit Impedance)

Line + CPC resistance at ambient temperature

MCB Type

Determines magnetic trip multiple

MCB Rating

Nominal current rating of the MCB

How We Calculate This

This calculator determines total earth fault loop impedance using the formula:

Zs = Ze + (R1 + R2) × Ct

Where Ct is the temperature correction factor:

Ct = 1 + 0.004 × (Operating temp - Ambient temp)

BS 7671 Maximum Zs Values

Type B MCBs: Trip at 3-5 × In — highest Zs allowed
Type C MCBs: Trip at 5-10 × In — half the Zs of Type B
Type D MCBs: Trip at 10-20 × In — quarter the Zs of Type B
0.4s disconnection: Required for final circuits up to 63A (Table 41.1)
5s disconnection: Permitted for distribution circuits
80% rule: Measured Zs should not exceed 80% of tabulated maximum

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.