UK earthing systems explained: TN-S, TN-C-S, TT and what Ze should read
Identifying the earthing arrangement is the first thing you do at the intake and the thing most likely to be recorded wrongly. Get it wrong and every Zs limit, bonding decision and EV charger design that follows is wrong too.
Updated 2026-09-24
The three arrangements you will actually meet
BS 7671 names the arrangement with two letters. The first describes the supply earthing, the second describes how exposed-conductive-parts of the installation are earthed. In UK domestic and light commercial work you will meet TN-S, TN-C-S and TT, with IT reserved for specialist applications such as some medical and industrial installations.
- · TN-S — separate neutral and protective conductors throughout, earth usually taken from the sheath of the supply cable
- · TN-C-S (PME) — combined neutral and earth in the supply, separated at the cut-out into a neutral and an earthing terminal
- · TT — no supply earth at all, the installation relies on its own earth electrode
How to tell them apart at the intake
On a TN-S supply the earthing conductor is clamped to the lead or steel sheath of the incoming cable, usually with a separate earth terminal beside the cut-out. On TN-C-S the earthing conductor comes out of the supplier's cut-out or an adjacent terminal block, because the earth is being derived from the combined neutral.
On a TT supply there is no supplier earth. You will find an earth rod, often outside near the intake, and the installation will depend on RCD protection for fault protection. If you cannot find a supply earth and cannot find an electrode, you have a problem, not a TT system.
Typical maximum Ze values
Distribution network operators quote maximum external earth fault loop impedance figures for each arrangement. These are design limits, not pass marks — your measured value should normally be well below them.
- · TN-S — maximum 0.8 Ω, commonly measured at 0.2 to 0.4 Ω
- · TN-C-S (PME) — maximum 0.35 Ω, commonly measured at 0.05 to 0.25 Ω
- · TT — no DNO figure; the electrode resistance is typically expected below 200 Ω, with RCD protection mandatory
Why PME changes your bonding
On a TN-C-S supply the neutral carries fault and load current back to the transformer, and a break in that combined conductor can push the installation earth up to a dangerous potential. Main protective bonding is what holds all the extraneous-conductive-parts at the same potential when that happens, which is why the bonding conductor size on PME is generally 10 mm² rather than the 6 mm² you might get away with on TN-S.
It is also why a broken or missing main bond is normally coded C2 on an EICR rather than an improvement recommendation.
EV charging and the PEN fault problem
An EV charge point puts a person in contact with a large earthed metal object outdoors, which is exactly the scenario a PEN fault on a PME supply makes dangerous. Section 722 of BS 7671 requires either an earth electrode dedicated to the charge point, or a device that disconnects on detecting a PEN fault by monitoring the supply voltage.
Most modern chargers integrate the detection device. If you inspect one that does neither, that is a coded observation, not a note.
TT installations in practice
TT is common on rural supplies, farms, overhead-fed properties and older caravan sites. Because the earth path runs through soil, the loop impedance is far too high for an overcurrent device to clear an earth fault in time. RCD protection is therefore not optional — it is the fault protection.
Expect a time-delayed or S-type RCD at the origin with 30 mA devices downstream, so a final circuit fault does not take out the whole installation.
Recording it correctly
The earthing arrangement goes on every certificate and report, alongside measured Ze and the prospective fault current. Once you have recorded it, use the Certi maximum Zs tool to check your circuit readings against the right limits for the protective devices fitted.