Earth Fault Loop Impedance Explained (Ze, Zs & Zdb)
Earth fault loop impedance is the total resistance of the path a fault current takes back to the transformer — and it must be low enough for the protective device to disconnect in time.
When a live conductor touches earth, current has to flow all the way round a loop — out through the fault, back through the earthing arrangement and the supply, and round to the point of the fault again. The impedance of that loop decides how much fault current flows, and therefore whether the fuse or breaker trips fast enough to make the installation safe.
This guide walks through the fault loop path for the three common earthing systems, defines the terms Ze, Zs and Zdb and how they relate, explains the equation Zs = Ze + (R1+R2), why the value must stay below a maximum, how you measure it on site, and the 0.8 rule-of-thumb that accounts for warm conductors.
Key takeaways
- Earth fault loop impedance is the total impedance of the path a fault current takes back to source.
- Ze is the external part (up to the origin); Zs is the whole loop at the point tested; Zdb is the loop impedance at a distribution board.
- Zs = Ze + (R1+R2), where R1+R2 is the resistance of the line and protective conductors of the final circuit.
- A low enough Zs means a big enough fault current to trip the device within the required disconnection time.
- Apply the 0.8 rule to measured or tabulated maximum Zs values to allow for conductors warming under load.
The fault loop path in TN-S, TN-C-S and TT
In a TN-S system the earth is provided by a separate metallic path (traditionally the cable sheath) all the way back to the supply transformer, so the loop is entirely metallic and impedance is typically low. In TN-C-S (PME), the neutral and earth are combined in the supply, giving an even lower external impedance because the return path is a substantial conductor.
TT is the odd one out: the installation earth is a local electrode driven into the ground, and the fault current returns partly through the mass of earth. That earth path has a much higher impedance, which is why TT installations rely on RCDs for earth-fault disconnection rather than on the loop impedance alone.
| Earthing system | Earth return path | Typical Ze |
|---|---|---|
| TN-S | Separate metallic earth (cable sheath) | Low — often below 0.8 Ω |
| TN-C-S (PME) | Combined neutral-earth conductor | Very low — often below 0.35 Ω |
| TT | Local earth electrode + mass of earth | High — often several ohms or more |
Ze, Zs and Zdb — what each one means
Ze is the external earth fault loop impedance: everything on the supply side of the origin of the installation, measured with the main earthing conductor disconnected so you read only the supply’s contribution. It is a fixed characteristic of the supply and is usually confirmed at the origin during initial verification.
Zs is the earth fault loop impedance of the whole loop at a given point — it includes Ze plus the resistance of the circuit conductors out to that point. Zdb is simply the loop impedance measured at a distribution board partway through the installation; it becomes the effective Ze for the final circuits fed from that board.
Record it where it matters
TradePlanr’s certificate app captures Ze at the origin and Zs at each circuit on the schedule of test results, so the measured values sit alongside the maximum permitted figure on the finished EIC or EICR.
The equation: Zs = Ze + (R1+R2)
Once you know Ze and the resistance of the final circuit’s conductors, you can predict Zs without measuring it live: Zs = Ze + (R1+R2). R1 is the resistance of the line conductor along the circuit and R2 is the resistance of the circuit protective conductor (CPC); together R1+R2 is what you measure end-to-end during continuity testing.
This is useful because you can verify a circuit will disconnect in time before energising it, and it explains why long circuits or thin CPCs push Zs up. If the measured Ze is 0.35 Ω and the circuit’s R1+R2 is 0.45 Ω, the expected Zs is 0.80 Ω, which you then compare against the maximum for the protective device.
Why Zs must be low enough — and how to measure it
The whole point of a low Zs is a high fault current. Fault current equals the supply voltage divided by Zs, so the lower the impedance the bigger the current, and the faster a fuse or MCB reaches its disconnection threshold. BS 7671 sets maximum disconnection times (typically 0.4 s for most final circuits up to 63 A on a TN system, 0.2 s for TT), and the maximum Zs tables are worked back from those times.
On site you measure Zs directly with a loop impedance tester connected line-to-earth at the point of use, with the installation energised. On RCD-protected circuits use the no-trip (low-current) test to avoid tripping the device. Compare the reading against the maximum Zs for that device — if it is under, the circuit disconnects in time.
The 0.8 correction for temperature
The maximum Zs values in the BS 7671 tables assume conductors at their full operating temperature, where copper resistance is highest and Zs is at its worst. But you usually test a cold installation, so a measured value taken cold will be a little lower than it would be in service.
The common rule of thumb is to multiply the tabulated maximum Zs by 0.8 to get a figure you can compare against a cold measured reading. If your cold measurement is below 0.8 times the tabulated maximum, you can be confident the circuit will still disconnect in time once the cables are warm and their resistance has risen.
Cold reading, warm limit
Don’t compare a cold Zs measurement straight against the full tabulated maximum — apply the 0.8 factor (or use a calculator that does it for you) so you’re allowing for the resistance rise once the circuit is loaded.
Frequently asked questions
What is the difference between Ze and Zs?
Ze is the external loop impedance — everything on the supply side of the origin. Zs is the total loop impedance at a point in the installation, equal to Ze plus the resistance of the circuit conductors (R1+R2) out to that point. You confirm Ze at the origin and Zs at each circuit.
Why do TT systems need an RCD when TN systems often don’t?
A TT earth returns fault current through an earth electrode and the mass of earth, giving a high loop impedance and too little fault current for an overcurrent device to trip quickly. An RCD detects the small residual current instead and disconnects in time, which is why RCD protection is effectively mandatory on TT.
Should I multiply the maximum Zs by 0.8?
Yes, when comparing against a value measured on a cold installation. The tabulated maxima assume conductors at operating temperature; applying the 0.8 factor gives a cold limit that leaves headroom for the resistance rise once the circuit warms up under load.
From guidance to action
Related guides
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
What Is Prospective Fault Current (PFC)?
PSCC vs PEFC, how to measure PFC, and why breaking capacity has to exceed it.
Earthing Systems: TN-S, TN-C-S and TT Explained
How TN-S, TN-C-S (PME) and TT earthing arrangements work, and how to identify each on site.
