Ze is the earth fault loop impedance outside your installation: from the main earthing terminal back through the supply's earth path, the transformer winding and the line conductor to the origin. Distributors usually declare typical maximums of 0.35 Ω for TN-C-S (PME), 0.8 Ω for TN-S and 21 Ω for TT (plus your electrode). Regulation 313.1 of BS 7671:2018+A4:2026 requires the supply characteristics, Ze included, to be determined, and you measure it at the origin with the installation isolated and the earthing conductor disconnected. Every circuit's Zs is built on top of it.
- Ze is the supply's share of the earth fault loop; for a final circuit, Zs = Ze + (R1 + R2).
- Typical declared maximums: 0.35 Ω TN-C-S, 0.8 Ω TN-S, 21 Ω TT (excluding the consumer's electrode).
- Measure it live at the origin, with the main switch off and the earthing conductor disconnected from the MET, so bonding doesn't give a false low.
- A declared or copied value proves nothing about today's earth. Measure it on every EIC and EICR.
- On TT, the reading at the origin includes your electrode, so note that on the certificate and check RA × IΔn ≤ 50 V (Regulation 411.5.3).
- A high Ze raises every Zs in the building. On TN supplies, report it to the DNO.
What is Ze, in plain terms?
When a line conductor faults to earthed metalwork, current flows from the transformer, down the line conductor, through the fault, back along the circuit protective conductor to the main earthing terminal, then home to the transformer star point. Ze is everything in that loop on the supply side of the MET: the transformer winding, the line conductor of the service cable and the return path.
What that return path is depends on the earthing arrangement (see TN-S, TN-C-S and TT explained):
- TN-S: a separate earth, usually the lead sheath or armour of the service cable.
- TN-C-S (PME): the combined neutral and earth (PEN) conductor of the supply network.
- TT: the general mass of earth, through your installation's electrode and the distributor's earthed star point.
You can't improve Ze from inside the installation. What you can do is know it, because it sets the floor for every Zs reading. If Ze is 0.3 Ω, no circuit in the building will read below 0.3 Ω.
What are the typical maximum Ze values?
Distribution network operators publish typical maximum values, and the same figures appear in the IET On-Site Guide and in scheme technical guidance (ELECSA's guidance via Voltimum, for example, gives exactly these three).
| Earthing arrangement | Typical declared maximum Ze | What you usually measure | Return path |
|---|---|---|---|
| TN-C-S (PME) | 0.35 Ω | Roughly 0.1 to 0.3 Ω | Combined PEN conductor |
| TN-S | 0.8 Ω | Roughly 0.2 to 0.8 Ω | Cable sheath or separate earth core |
| TT | 21 Ω, plus your electrode | Dominated by your electrode: a few ohms to 200 Ω or more | Earth, via the consumer's electrode |
Two things people get wrong here. First, the declared figure is a design value, not a pass mark you write on the certificate. Second, the 21 Ω for TT is the supply's part only. The consumer's electrode belongs to the installation, so a reading taken at the origin of a TT installation is really Ze plus your electrode resistance. IET EngX contributors suggest recording the measured value and noting that it includes the electrode, which is what most of us do.
How do you measure Ze safely?
The method in IET Guidance Note 3 has been the same for years:
- Isolate the installation at the main switch, secure the isolation and prove dead on the load side.
- Disconnect the earthing conductor from the MET. This removes the parallel paths through main bonding to gas, water and structural steel. Leave the free end where it can't touch anything and you can get a probe on it.
- Test between the incoming line and the disconnected earthing conductor at the supply side of the main switch, with your loop tester on a no-trip or high-current setting as appropriate. There's no RCD in that loop, so high-current is fine on most domestic origins.
- Note the PEFC too. The instrument works out the prospective earth fault current from the same loop. You still need the prospective short-circuit current (line to neutral); the higher of the two goes on the form as Ipf (see prospective fault current explained).
- Reconnect the earthing conductor and check it's tight before closing the main switch. Then put a label back if you disturbed one.
The Electricity at Work Regulations 1989 apply to this, because it's live testing (Regulation 14 needs it to be justified and properly controlled). Use a fused lead set or a proprietary test lead with shrouded probes, and don't leave the earthing conductor off a second longer than you need.
Why bother disconnecting it? Because a TN-S sheath that has corroded through, or a PME connection that has gone high resistance, can be completely masked by the gas and water bonding. With everything connected you might read 0.4 Ω and think the job is fine, when the "earth" is actually the water main. The Voltimum/ELECSA guidance makes the point bluntly: a previous value or one obtained by enquiry doesn't confirm the earthing is present and low in ohms. Only a measurement does.
What if I can't isolate?
On a working commercial site or a home with life-support equipment you may not be able to switch the whole installation off. Don't disconnect the earthing conductor with the installation live. Agree a limitation with the client, take a loop reading at the origin with everything connected, and record it as exactly that, not as Ze. Come back to it when the shutdown can happen.
Ze, Zs and Zdb: how do they fit together?
For a final circuit fed from the main board:
Zs = Ze + (R1 + R2)
Regulation 643.7.3 lets you determine Zs by measurement or by calculation from a reliable measured Ze and R1 + R2. That's why getting Ze right matters: if you use Ze in a calculation, an error ripples through every row of the schedule.
On larger installations, each distribution board has its own Zdb, the loop impedance at that board, which includes Ze plus the sub-main. On the A4 model forms the schedule records Zdb per board rather than repeating Ze, and the supply characteristics section carries Ze for the origin. If you're switching to the A4 layout, our schedule of test results guide walks through it column by column.
A worked example: why Ze changes the design
Take a 32 A Type B radial in 2.5/1.5 mm² twin and earth, 30 m long. Using the GN3 value of 19.51 mΩ/m for 2.5/1.5 copper at 20 °C, R1 + R2 = 0.59 Ω.
| Supply | Ze | Zs at 20 °C | GN3 80% check (1.37 Ω × 0.8 = 1.10 Ω) |
|---|---|---|---|
| TN-C-S | 0.35 Ω | 0.94 Ω | Passes |
| TN-S | 0.80 Ω | 1.39 Ω | Fails, and also exceeds the 1.37 Ω Table 41.3 value |
Same cable, same length, same breaker, different result. On a TN-S supply at the top of its declared range, that circuit needs a shorter run, a bigger cpc, or a different protective device. That's the practical reason to measure Ze before you size anything. More on the limits in our maximum Zs values guide.
What does a high Ze reading mean?
A reading above the declared maximum, or well above what that street normally gives, is telling you something about the supply earth. The usual suspects:
- TN-S: the lead sheath has corroded, the earth clamp on the sheath has loosened, or the sheath has been cut during groundworks.
- TN-C-S: a high-resistance joint on the PEN conductor or at the cut-out. Take this seriously. A deteriorating PEN can, at its worst, put the whole installation's earthed metalwork at a dangerous voltage.
- TT: the electrode has dried out, corroded or been disconnected, or the connection to it is poor (see earth electrode testing on TT systems).
What to do:
- Re-test with the earthing conductor properly disconnected and good contact on the line terminal. A poor probe contact adds ohms.
- On TN supplies, report it to the DNO. The supply earth is their equipment and their problem to fix. Don't touch the cut-out seals.
- Check the consequences for every circuit: does each Zs still meet its maximum? Our high Zs troubleshooting guide covers the circuit side.
- On an EICR, code what you find on the basis of danger, using Electrical Safety First's Best Practice Guide 4 as your reference. An open-circuit or effectively absent supply earth is not a borderline reading; treat it as the urgent problem it is and make it safe.
Ze on a TT system: what changes?
On TT, there's no metallic return path from the installation to the transformer. Fault current goes through your electrode, through the ground and back through the distributor's earthed star point. So:
- The reading at the origin is dominated by your electrode resistance, RA.
- Fault protection relies on an RCD, not on the circuit breaker clearing the fault in time. BS 7671 Regulation 411.5.3 requires RA × IΔn ≤ 50 V. For a 30 mA RCD that works out at 1,667 Ω; for a 100 mA time-delayed device, 500 Ω.
- The On-Site Guide advises that electrode resistances above 200 Ω may not be stable, because soil dries out and freezes. Most of us aim well under that.
- Record the measured loop reading at the origin with a note that it includes the consumer's electrode, and record RA separately if you measured it with an earth electrode tester.
If you're putting in a TT system for an outbuilding or garden supply off a PME house, see our outbuilding supply guide.
Common mistakes with Ze
- Measuring with the earthing conductor still connected and writing the result down as Ze.
- Taking Ze at a socket-outlet. That's Zs for that circuit.
- Copying Ze off the last EICR or the DNO's declared figure. A sheath can fail between inspections.
- Disconnecting the earthing conductor before the installation is isolated.
- Forgetting to reconnect it, or reconnecting it loosely. Check it with a tug and a screwdriver every time.
- Recording a TT reading without noting that it includes the consumer's electrode.
- Forgetting that Ze gives you PEFC, but not PSCC. You need both to get Ipf right.
Where does Ze go on the certificate?
On the EIC and EICR, Ze goes in the supply characteristics and earthing arrangements section, alongside the earthing arrangement, the number and type of live conductors, nominal voltage, frequency and Ipf. On the A4 model forms, the distribution board details carry Zdb and Ipf at each board. The Minor Works certificate has no Ze box at all, but the Zs you record for the altered circuit still depends on it.
Ze is also worth knowing before you fit equipment where the earthing arrangement matters. EV chargepoints are the obvious one: Section 722 has specific requirements about using a PME earth for charging points outdoors, so you need to know what you're dealing with first (EV charger certificates and Section 722). Main protective bonding sizes on PME also depend on the supply neutral conductor size, covered in our main protective bonding guide.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is £4.99 a month, charged when you sign up.