Earth fault loop impedance sounds complicated, but the idea is simple. If a line conductor touches earthed metalwork, fault current flows round a loop and back to the transformer. The lower the impedance of that loop, the bigger the fault current, and the faster the protective device disconnects. Ze and Zs are just two measurements of that loop.
Ze: the external part
Ze is the impedance of the part of the loop outside your installation: the supply transformer, the supplier's line conductor and the earth return path back to the origin.
How to measure it: at the origin, with the installation safely isolated, disconnect the main earthing conductor from the main earthing terminal. This removes parallel paths through bonding. Test between line and the disconnected earthing conductor. Reconnect the earthing conductor before restoring the supply.
Typical maximum values declared by network operators:
| Earthing system | Typical maximum Ze |
|---|---|
| TN-C-S (PME) | 0.35 Ω |
| TN-S | 0.8 Ω |
| TT | 21 Ω (from the supplier; the earth electrode adds more) |
Zs: the whole loop
Zs is the total loop impedance at the furthest point of a circuit. It's Ze plus the resistance of the circuit's own line and protective conductors:
Zs = Ze + (R1 + R2)
You can measure it directly at the furthest point with a loop tester, or calculate it from Ze and your R1 + R2 reading from continuity testing. Doing both is a useful cross-check.
Is my reading OK?
BS 7671 gives maximum Zs values for each type and rating of protective device, so it will disconnect in time. Those tables assume conductors at their operating temperature. Readings taken on a cold circuit should be compared against 80% of the table value (the On-Site Guide and Guidance Note 3 print corrected values for this).
Worked example: a ring final protected by a 32 A Type B circuit breaker. Ze is 0.35 Ω and R1 + R2 is 0.35 Ω.
- Zs = 0.35 + 0.35 = 0.70 Ω
- BS 7671 maximum for a 32 A Type B: 1.37 Ω; 80% of that: about 1.09 Ω
- 0.70 Ω is under 1.09 Ω, so it passes.
Why it matters
If Zs is too high, fault current is too low to trip the breaker quickly. Metalwork could stay live long enough to give someone a dangerous shock. That's why it's checked on every circuit.
Common causes of high Zs
- Long cable runs or undersized protective conductors
- Loose or corroded connections, especially at the main earthing terminal
- A poor earth electrode on a TT system (which relies on RCDs instead)
Learn it properly
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