Ze (external earth fault loop impedance) is the impedance, in ohms, of the part of the earth fault loop that lies outside the installation: from the main earthing terminal back through the supply's earth path and the transformer winding to the origin. It is measured once at the origin with the main earthing conductor disconnected, recorded on the supply characteristics page of every EIC and EICR, and compared with the distribution network operator's declared maximum of 0.35 Ω for TN-C-S, 0.8 Ω for TN-S or 21 Ω for TT supplies as quoted in IET Guidance Note 3.
- Ze is the supply's contribution to the earth fault loop; Zs for any circuit equals Ze + (R1 + R2).
- It is a live measurement made at the origin with the installation isolated and the earthing conductor disconnected.
- Declared maximum values: 0.35 Ω (TN-C-S), 0.8 Ω (TN-S), 21 Ω (TT).
- A high Ze limits the Zs achievable on every circuit and may need reporting to the DNO.
- Ze must be measured for each certificate, not copied from a previous one.
What is Ze in practice?
Every earth fault on a TN system flows back to the supply transformer through the distributor's network: the sheath or separate earth core of the service cable on TN-S, or the combined neutral and earth conductor on TN-C-S (PME). On a TT supply the return is through the general mass of earth via the installation's own electrode, so Ze is much higher.
Ze is a property of the supply, not the installation. The electrician cannot reduce it, but must know it because it sets the floor for every Zs in the building: if Ze is 0.3 Ω, no circuit can have a Zs below 0.3 Ω.
When is Ze measured?
Ze is measured and recorded on every Electrical Installation Certificate and every EICR, in the supply characteristics and earthing arrangements section. Minor Works Certificates have no Ze box, but the Zs recorded for the circuit depends on it. It is also measured before connecting equipment sensitive to the earthing arrangement, such as EV charging under BS 7671 Section 722.
| Earthing system | Typical declared maximum Ze | Typical measured range |
|---|---|---|
| TN-C-S (PME) | 0.35 Ω | 0.1 to 0.35 Ω |
| TN-S | 0.8 Ω | 0.2 to 0.8 Ω |
| TT | 21 Ω (electrode plus network) | A few ohms to 200 Ω, depending on the electrode |
Measured values above the declared maximum should be queried with the DNO, since they indicate a supply earth that is not performing as designed.
How is Ze measured?
The procedure in IET Guidance Note 3 is:
- Isolate the installation at the main switch and prove dead on the load side.
- Disconnect the main earthing conductor from the main earthing terminal (MET), removing the parallel paths through bonding and circuit protective conductors that would give a falsely low reading.
- Connect the loop tester between the incoming line conductor and the disconnected earthing conductor and take the reading; the instrument gives the prospective earth fault current at the same time.
- Reconnect the earthing conductor, confirm it is secure, and only then restore the supply.
Step 2 is why Ze cannot be taken from a socket-outlet: the reading there is Zs for that circuit. It is also why the installation must be isolated first, since a live installation with its earth removed is a serious hazard.
On a TT system the same method gives Ze including the electrode resistance. BS 7671 requires that resistance to satisfy RA × IΔn ≤ 50 V for the RCD providing fault protection, which is 1667 Ω for a 30 mA device, though GN3 advises a practical limit of 200 Ω for stability.
Common mistakes with Ze
- Measuring with the earthing conductor connected, which reads Ze in parallel with the bonding and gives a low, meaningless figure.
- Taking Ze at a socket-outlet. That is Zs.
- Copying Ze from the previous EICR. A TN-S sheath can corrode through between inspections.
- Recording the DNO's declared maximum instead of the measured value.
- Not isolating the installation before disconnecting the earthing conductor.
- Forgetting the PFC, which the same test gives and which must also be recorded.
Related terms
- Earth fault loop impedance (Zs)
- Prospective fault current (PFC)
- Main protective bonding
- Multifunction tester (MFT)
- Electrical Installation Certificate (EIC)
- EICR
How Certio helps
Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.