Every EIC and EICR asks for the earthing arrangement in the supply characteristics box, and getting it wrong is more common than it should be. In the UK the three you will meet are TN-S (separate earth from the supply cable sheath, typical maximum Ze 0.8 Ω), TN-C-S or PME (earth taken from the supply neutral, typical maximum Ze 0.35 Ω), and TT (no earth from the supplier; the installation provides its own electrode, typically 21 Ω declared by the DNO). Each one changes the bonding sizes, the Ze you expect, what protective devices can provide fault protection, and whether the earth can be taken outside. This guide covers how to identify each at the cut-out, the numbers, and what to write on the certificate.
- Identify the earthing type by looking at the cut-out: sheath clamp (TN-S), neutral-earth link with PME label (TN-C-S), or no supplier earth and a rod (TT).
- Expect Ze up to 0.8 Ω on TN-S, 0.35 Ω on TN-C-S; a TT Ze is dominated by the electrode and can be tens or hundreds of ohms.
- Main bonding is 10 mm² minimum on PME for most domestic supplies; 6 mm² minimum on TN-S and TT.
- On TT, RCDs provide fault protection and RA × IΔn must not exceed 50 V.
- A PME earth must not be exported to an EV charger, caravan or outbuilding without the measures in Sections 722 and 708, or a TT island.
What do the letters mean?
The designation comes from IEC 60364 and BS 7671 Regulation 312.2. The first letter describes the relationship of the supply to earth (T, one point directly earthed), the second the relationship of the exposed-conductive-parts of the installation to earth (N, connected to the supply's earthed point; T, connected to a separate earth), and any further letters the arrangement of neutral and protective conductors (S, separate; C, combined; C-S, combined in the supply then separated in the installation).
| System | Supply earth | Installation earth | Neutral and earth | UK prevalence |
|---|---|---|---|---|
| TN-S | Transformer star point earthed | Via the supply cable's metallic sheath or a separate conductor | Separate throughout | Older urban supplies with lead-sheathed cable |
| TN-C-S (PME) | Transformer star point earthed and neutral earthed at multiple points | Via the supply neutral (PEN conductor) at the cut-out | Combined in the supply, separate in the installation | Most supplies installed since the 1970s |
| TT | Transformer star point earthed | Via the installation's own earth electrode | Separate; no supplier earth | Rural overhead supplies, some older properties, and TT islands for EV and outbuildings |
| TN-C | Combined PEN throughout | Combined | Combined throughout | Not permitted in consumer installations (Regulation 8 ESQCR) |
| IT | Isolated or high-impedance | Separate electrode | Separate | Specialist (medical, some industrial); not for public supply |
How do you identify the earthing arrangement at the cut-out?
Look before you test. The cut-out and the meter tails tell you most of what you need:
TN-S. The earthing conductor runs from the MET to a clamp on the supply cable's lead sheath or steel wire armour below the cut-out, or to a separate earth terminal on the cut-out that is not connected to the neutral. There is no PME label. Old lead-sheathed paper-insulated cables in urban areas are the classic case. The DNO's earth may have been converted to PME since the cable was installed, so check for a label or ask the DNO.
TN-C-S. The cut-out has an earth terminal bolted or linked to the neutral block inside, and usually a yellow "PME" or "Protective Multiple Earthing" label. Supplies with a modern combined service head, and almost all new-build, are PME. The DNO has earthed the neutral at several points along the network.
TT. No earth terminal at the cut-out, or one with a label stating no earth is provided. The installation's earthing conductor goes to an earth electrode, usually a rod, sometimes a plate or tape, in a pit near the intake. Overhead supplies in rural areas are commonly TT because the DNO cannot guarantee the neutral's integrity. Some urban properties were left TT when the DNO declined to provide an earth.
Two cautions. First, an earthing conductor connected to a water pipe is not a TT electrode and is not a permitted means of earthing (Regulation 542.2.6): record it as an observation. Second, the earthing type cannot be determined by measurement alone. A low Ze could be a TN-S sheath in good condition or a TN-C-S link, and a rod in parallel with a supplier earth confuses the picture further. If the cut-out is sealed and unlabelled, ask the DNO for confirmation and note on the certificate that the arrangement is as advised or assumed.
What Ze should you expect on each system?
Measure Ze at the origin with the installation isolated and the main earthing conductor disconnected from the MET, so that bonding does not provide parallel paths. Compare with the DNO's declared maximum:
| System | DNO typical declared maximum Ze | Typical measured | Comment |
|---|---|---|---|
| TN-S | 0.8 Ω | 0.1 to 0.5 Ω | Rising values over time suggest sheath corrosion; report to DNO |
| TN-C-S | 0.35 Ω | 0.05 to 0.3 Ω | Above 0.35 Ω is a DNO fault; potentially a PEN problem |
| TT | 21 Ω (DNO side) | Anything from 5 Ω to several hundred Ω, set by the electrode | Measure RA with an electrode tester or loop tester; see Regulation 411.5 |
A Ze above the declared maximum is not an installation defect but must be recorded and reported to the DNO, because it affects the disconnection times of every circuit. Method and interpretation are in What is Ze? External earth fault loop impedance and Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained.
How does the earthing arrangement change the bonding?
Main protective bonding connects extraneous-conductive-parts (incoming metal gas, water, oil pipes, structural steel) to the MET. The size depends on the earthing system:
| System | Regulation | Main bonding conductor minimum |
|---|---|---|
| TN-S and TT | 544.1.1 | Not less than half the earthing conductor, minimum 6 mm² copper |
| TN-C-S (PME) | 544.1.1 and Table 54.8 | 10 mm² copper for a supply neutral up to 35 mm²; 16 mm² for 35 to 50 mm²; larger above |
The larger PME size exists because under a broken PEN the bonding conductors carry the diverted neutral current of the installation and its neighbours. A 6 mm² bond on a PME supply is a common EICR observation on 1970s and 1980s houses, usually C3 where the bond is otherwise sound, though inspectors differ. The earthing conductor itself is sized from Table 54.7 or by calculation: 16 mm² copper is the norm for domestic PME, with 10 mm² acceptable where the supply is small. What is main protective bonding? Explained covers what needs bonding.
Why is TT different for fault protection?
On a TN system the earth fault loop is metallic all the way, so Zs is low and the MCB clears a fault within 0.4 s. On TT the loop includes the electrode resistance and the general mass of earth, so Zs of 50 Ω or more is normal and no MCB will ever trip on an earth fault. BS 7671 Regulation 411.5.3 therefore requires RCDs for fault protection on TT, with the condition RA × IΔn ≤ 50 V. For a 30 mA RCD that means an electrode resistance of up to 1667 Ω; for a 100 mA time-delayed upfront RCD, 500 Ω. In practice the IET On-Site Guide recommends keeping RA below 200 Ω for stability, because electrode resistance rises in dry weather.
TT installations also need a 100 mA or 300 mA time-delayed RCD at the origin (or an RCD main switch) so that the meter tails and the board are protected, and a double-pole main switch because the neutral cannot be relied on to be near earth potential. Testing the electrode is covered in Earth Electrode Testing for TT Systems: Methods and Limits.
Why can't a PME earth go outside?
On TN-C-S the installation's earth is the supply neutral. If the PEN conductor breaks upstream, the neutral and everything bonded to it rise towards line voltage through the connected loads. Indoors, the bonding keeps everything at the same potential and the risk is contained. Outdoors, a person standing on wet ground touching an EV charger, a caravan hook-up, a hot tub or a metal garden light is between that raised potential and true earth.
BS 7671 handles this by restricting the export of PME earths:
- Section 722 (EV charging): a PME earth may only be used for outdoor charging equipment with a protective device that disconnects on PEN failure (open-PEN detection), an earth electrode arrangement to Regulation 722.411.4.1, or by converting the charging point to TT.
- Section 708 (caravan parks and marinas): PME earth shall not be used for the pitch supply; TT is required.
- Section 702 (swimming pools and hot tubs) and Section 705 (agricultural): additional measures and often TT.
- Outbuildings: BS 7671 does not prohibit exporting PME to a garden office, but the metalwork must be bonded and the risk assessed; many electricians TT an outbuilding with a rod, particularly where there is a hot tub or EV charger. See Outbuilding and Garden Supply: SWA, TT or PME Earth?.
A TT island (an outbuilding or EV charger with its own rod and RCD, and no connection to the PME earth) must be separated from the PME earth by more than a few metres and the SWA armour or CPC must not bridge the two.
What do you record on the certificate?
The supply characteristics and earthing arrangements section of the EIC and EICR asks for:
- Earthing arrangement: TN-S, TN-C-S or TT (tick one), and whether confirmed by inspection, enquiry to the DNO, or assumed.
- Ze in ohms and the method.
- Prospective fault current at the origin.
- Earthing conductor: material and size, and whether continuity was verified.
- Main protective bonding conductors: material, size, and to which services.
- For TT: electrode type, location and measured RA.
- Supply protective device: type and rating (for example BS 88-3 100 A).
- Number and type of live conductors, nominal voltage and frequency.
On an EICR, an incorrect or unverifiable earthing arrangement is one of the FI or C2 situations that GN3 highlights: no effective earth is C1 or C2 depending on whether metalwork could become live, and an earth to a water pipe is at least C2. How to record it is in How to fill in a schedule of test results, column by column.
How Certio helps
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