# UK earthing systems explained — TT, TN-S, TN-C-S (PME), TN-C, IT

> Almost every UK home is on one of three earthing systems. **TN-C-S (PME)** is the normal modern arrangement: neutral and earth are combined (PEN) in the supply, then split into separate neutral and earth at your cut-out — its weakness is the **open-PEN** hazard. **TN-S** keeps neutral and earth separate all the way from the transformer, and is now comparatively rare. **TT** means the supply gives you no earth at all, so the installation makes its own with a local electrode and relies on an RCD. **TN-C** and **IT** are **not** UK domestic supply arrangements — shown only so you can rule them out. Which one you have changes the whole V2L design: PME drives the contested floating-output earthing question, while a separate outbuilding can be made TT as an island. Identify the system, and confirm it with the DNO, before any design decision.

**Safety frame.** This is education, not an instruction to carry out work. Work connecting V2L equipment to fixed wiring is safety-critical and may be notifiable under Part P. It must be designed, installed, inspected and tested by a competent person to the current edition of BS 7671. Vehicle manufacturers generally do not sanction back-feeding fixed wiring from V2L outlets; follow manufacturer instructions.

## In short

- **TN-C-S (PME)** — combined neutral/earth (PEN) split at the cut-out; the normal modern UK domestic supply; carries the **open-PEN** touch-voltage hazard.
- **TN-S** — separate earth from the transformer all the way to the property; still exists but now comparatively rare; do not assume it without DNO confirmation in writing.
- **TT** — supply gives no earth; the installation uses its own electrode and an RCD; the cleanest basis for a separate-outbuilding V2L island.
- **TN-C** and **IT** — **not** UK domestic supply arrangements; shown so a reader can recognise and exclude them.
- It matters for V2L: PME makes the floating-output earthing contested (designed and proven by test by a competent person), while TT is the outbuilding-island route.

**Where this stops:** This explains what each earthing system is and why it matters for V2L. It is not a wiring recipe, and it does not tell you which system you have — that is identified by measurement and DNO confirmation by a competent person.

## Why your earthing system is the first thing to settle

Earthing is what makes an installation safe under fault: it gives fault current a path back to the source so a protective device (a fuse, MCB or RCD) disconnects quickly and limits the voltage on any metalwork you might touch. UK supplies do this in one of a few standard ways, each given a two-letter code. For a V2L home-backup arrangement the earthing system is not a detail — it decides whether the floating-output earthing question is straightforward or contested, and whether a separate outbuilding can be islanded. Settle which system you have first, then design.

> **The three you will actually meet** — Almost every UK dwelling is **TN-C-S (PME)**, **TN-S** or **TT**. **TN-C** and **IT** are shown further down only so you can recognise and rule them out — they are not UK domestic supply arrangements.

Reading the codes: **T** means a direct connection to Earth, **N** means the installation's exposed metalwork is connected to the earthed neutral of the source, **C** means the neutral and protective-earth functions are **combined** in one conductor, and **S** means they are **separate**.

## TN-S — a separate earth all the way from the transformer

In a TN-S supply the distributor brings a **separate** protective earth (PE) conductor — often the lead sheath or armour of the service cable — all the way from the supply transformer to your property, alongside line (L) and neutral (N). Neutral and earth stay separate the whole way (the **S** = separate). At the consumer unit the neutral goes to the neutral bar and the earth to the main earthing terminal; they are never joined. No load current flows in the earth, so it is a very stable reference; the weakness is that if that separate earth conductor corrodes or breaks, the installation can lose its earth without warning.

**Figure: TN-S earthing system.** TN-S: the distributor provides a separate earth conductor all the way from the supply transformer — neutral (N) and earth (PE) are separate throughout.

_A TN-S supply. At the supply transformer the star point is earthed. Three conductors reach the installation: line (L), neutral (N) and a separate protective earth (PE) — often the lead sheath or armour of the service cable. Neutral and earth are kept separate the whole way (the 'S' = separate). At the consumer unit, N goes to the neutral bar and PE to the main earthing terminal; they are not joined. Identify it on site by a separate earth conductor connected to the supply cable sheath, distinct from the neutral._

> **Do not assume TN-S** — Genuine TN-S is now comparatively rare, and many networks that 'look TN-S' have been altered into combined sections over the years. Treat a supply as **TN-C-S unless the DNO confirms TN-S in writing**.

## TN-C-S (PME) — combined, then split at the cut-out

TN-C-S is the normal arrangement for modern UK domestic connections, and is run as **PME** (Protective Multiple Earthing). From the transformer the neutral and earth functions are **combined** into a single PEN conductor (the **C** = combined). At your cut-out the PEN is split into a separate neutral and a separate protective earth (the **S** = separate, downstream), and the installation's main earthing terminal is bonded to that split point. The PEN is earthed at multiple points along the network — that 'multiple earthing' is what PME means and what mitigates a break.

**Figure: TN-C-S (PME) earthing system.** TN-C-S / PME: neutral and earth are combined (PEN) in the supply, then split into separate N and PE at the cut-out — the source of the open-PEN hazard.

_A TN-C-S supply, commonly run as PME (Protective Multiple Earthing). From the transformer the neutral and earth are combined into a single PEN (combined protective-and-neutral) conductor — the 'C' = combined. At the property's cut-out the PEN is split into a separate neutral (N) and protective earth (PE) — the 'S' = separate, downstream. The installation's main earthing terminal is bonded to that split point. The hazard: if the PEN conductor breaks upstream (an 'open PEN'), exposed metalwork can rise to a dangerous voltage — which is why BS 7671 §722.411.4.1 restricts relying on a PME earth for an EV. Identify it on site by the earth being taken from the supply neutral/PEN terminal at the cut-out._

> **The open-PEN hazard** — If the combined PEN conductor breaks upstream while line is still present, the installation's 'earth' can be pulled up toward line potential through connected load — bonded metalwork (taps, radiators, pipework) can rise toward 230 V relative to true Earth. The risk is worse outdoors, where a person stands on true Earth. This is the single most important earthing risk on a PME supply, and the reason BS 7671 restricts relying on a PME earth for an EV.

> **§722.411.4.1** (BS 7671, confidence: inference) — On a PME (TN-C-S) supply you must not simply rely on the distributor's earth for the EV side unless a listed open-PEN protective measure is applied — an open PEN can raise metalwork to a dangerous voltage. This is why the floating-V2L-on-PME approach is contested and must be designed and proven by test by a competent person. _Reference only; standard text not reproduced._

## TT — the installation makes its own earth

In a TT supply the distributor provides only line and neutral — **no earth at all**. The installation makes its own earth with a local **earth electrode** (an earth rod driven into the ground), and the main earthing terminal connects to that electrode, not to the supply. Because the earth-fault loop has to run through soil, the loop impedance is relatively high and fault current is often too low to operate a fuse or MCB in time — so protection depends on an **RCD**, backed by an electrode resistance (Ra) kept low and stable. TT is common on rural overhead supplies and is the standard fallback where the DNO cannot safely provide an earth terminal.

**Figure: TT earthing system.** TT: the installation makes its own earth with a local electrode; the supply provides only line and neutral. Protection relies on an RCD plus a low, stable electrode resistance.

_A TT supply. The distributor provides only line (L) and neutral (N); the installation provides its OWN earth via a local earth electrode (an earth rod) driven into the ground. The main earthing terminal connects to that electrode, not to the supply. Because the earth-fault loop runs through soil, fault-loop impedance is relatively high, so protection depends on a residual current device (RCD) plus an electrode resistance (Ra) low and stable enough to operate it (BS 7671 Table 41.5; BS 7430 for electrode design). Identify it on site by an earth rod and an RCD as the main protective device, with no earth taken from the supply cable._

> **TT considerations / keeping Ra low and stable** (BS 7430, confidence: inference) — For a TT installation the electrode resistance must be low and stable enough for the RCD to disconnect within the required time (read with BS 7671 Table 41.5). At a high electrode resistance the resistance tends to become unstable as soil dries or freezes, so the electrode is kept as low and as deep as practicable. _Reference only; standard text not reproduced._

## TN-C and IT — not UK domestic supplies (rule them out)

These last two are included only so you can recognise and exclude them. In a **TN-C** arrangement the neutral and earth are combined into one PEN conductor for the **whole** system, with no split at the installation — this is not used as a UK domestic supply arrangement, and ESQCR prohibits PEN conductors inside consumers' installations. In an **IT** system the source has no direct earth (or is earthed only through a high impedance) and the installation is earthed locally; a first fault does not produce a large current, so the supply keeps running and must be monitored. IT is used in specific settings — some medical, industrial-process or continuity-critical supplies — but is **not** a UK public low-voltage supply.

**Figure: TN-C earthing system (rare in UK dwellings).** TN-C: neutral and earth combined as a single PEN conductor throughout — not used as a domestic supply arrangement in the UK; shown for completeness.

_A TN-C arrangement. Neutral and earth are combined into one PEN conductor for the WHOLE system (the 'C' = combined throughout), with no separation at the installation. This is not used as a domestic supply earthing arrangement in the UK and is generally only seen in specific industrial/earthed-concentric situations. Shown for completeness so a reader can rule it out: a UK home will normally be TN-C-S, TN-S or TT, not TN-C._

**Figure: IT earthing system (not a UK public supply).** IT: the supply has no direct earth (or earthed via high impedance) and the installation is earthed locally — used in special applications, not as a UK public LV supply.

_An IT system. The source has no direct connection to earth, or is earthed only through a deliberately high impedance ('I' = isolated source); the installation's exposed-conductive-parts are earthed locally to their own electrode ('T'). A first earth fault does not produce a large fault current, so the system keeps running but must be monitored and the fault found before a second occurs. Used in specific settings (some medical, industrial-process or continuity-critical supplies) — it is NOT a UK public low-voltage supply arrangement. Shown so a reader can recognise and exclude it for a normal dwelling._

> **Not confirmed (safety-critical):** Whether BS 7671 Amendment 4:2026 changed anything specific for §722 / V2X / V2H / PME / open-PEN is Not confirmed — the public IET/BSI change material checked here does not identify a §722 / V2X / PME / open-PEN change in A4:2026, and existing §722 / PME/open-PEN material appears to carry forward from earlier amendments, subject to licensed-text confirmation; do not assert an A4-specific change until it is confirmed against the published standard.

## Why it matters for V2L

When the car becomes the source, the installation can no longer rely on the distributor's earth — so the earthing system you start from decides the route. The crux is that a V2L output is typically **floating** (no internal neutral-earth bond), so a reference has to be supplied at the source side, in circuit only on V2L.

- **TN-C-S (PME)** — the least forgiving. The open-PEN consequences are severe, and 'just add an earth rod' to a PME installation is not automatically safe; the floating-output earthing here is contested and must be designed and proven by test by a competent person, with the manufacturer's permission for fixed-installation/V2H use confirmed first.
- **TN-S** — technically easier than PME **if** the DNO confirms it is genuinely TN-S in writing; an island still needs two-pole switching, a consumer electrode and a correctly interlocked neutral-earth bond.
- **TT** — usually the cleanest basis. A separate **outbuilding made TT** (its own electrode, the PME earth never exported) is the standard island route; it aligns naturally with RCD/RCBO protection.

> **Contested on PME — never a recipe** — Referencing a floating V2L output on a PME supply is contested. It must be designed and proven by test by a competent person, and the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet. Nothing on this page is a wiring recipe.

Next: identify which system you actually have — see the [earthing-system identification](/safety/identify-your-earthing-system) page — then follow the route for your case: the [outbuilding TT-island method](/safety/tt-explained-and-tt-island), the [isolation-transformer method](/technical/isolation-transformer-method) for an indoor board, or the [open-PEN hazard](/safety/open-pen-pme-and-evs) deep-dive. The full picture is in the [V2L home-backup cornerstone](/guides/v2l-home-backup-cornerstone).

### How this is made and proven compliant

**What governs it:** BS 7671 Chapter 54 (§542 earthing arrangements, §544 protective bonding) — how each system is earthed and bonded; BS 7671 §722.411.4.1 (PME / open-PEN protective measures) — the EV/PME sensitivity that drives the V2L earthing question; BS 7430 — electrode design and keeping Ra low and stable, for the TT (outbuilding-island) route

**Who may do it:** Identifying the earthing system, and designing any V2L changeover around it, is for a competent person. The supply earthing arrangement is the DNO's decision and must be confirmed with the DNO; treat an apparent TN-S supply as TN-C-S unless the DNO confirms TN-S in writing. Fixed-wiring alterations are normally notifiable under Part P (England; Wales/Scotland/NI differ).

**How compliance is demonstrated:** Earthing system identified by visual inspection and external earth-fault loop impedance (Ze) measurement, cross-checked with the DNO; For TT: electrode resistance (Ra) measured low and stable enough for RCD disconnection (BS 7671 Table 41.5; BS 7430); Main protective bonding present and correctly sized for the system (BS 7671 §544); Any V2L island design proven by test in both grid and V2L modes by a competent person

## FAQ

### Which earthing system does my house have?

Almost certainly **TN-C-S (PME)** if it is a modern connection, possibly **TN-S** (now comparatively rare) or **TT** on a rural/overhead supply. You cannot tell reliably by looking — it is identified by a competent person from the cut-out arrangement, an external earth-fault loop impedance (Ze) measurement and DNO confirmation. Treat an apparent TN-S supply as TN-C-S unless the DNO confirms TN-S in writing.

### What is the difference between TN-S and TN-C-S (PME)?

In **TN-S** the neutral and earth are separate all the way from the transformer. In **TN-C-S** they are **combined** as one PEN conductor in the supply and only **split** into separate neutral and earth at your cut-out. The split is why TN-C-S carries the open-PEN hazard that TN-S does not.

### Why is the open-PEN hazard such a big deal for PME supplies?

If the combined PEN conductor breaks upstream while line is still present, the installation's 'earth' can be pulled toward line potential — bonded metalwork can rise toward 230 V relative to true Earth, and the danger is worst outdoors on true Earth. It is the reason BS 7671 §722.411.4.1 restricts relying on a PME earth for an EV, and the reason floating-V2L-on-PME is contested.

### Can I just add an earth rod to make my PME house a TT system for V2L?

Not as a DIY fix. Creating a TT arrangement from PME may not be safe unless you genuinely separate the new earth from buried metalwork still connected to the supply PEN — a stray pipe, armour or sheath can silently re-import the PME earth. A separate **outbuilding** is the clean TT-island case; a single indoor sub-board is not. It must be designed and proven by test by a competent person.

### Are TN-C and IT used in UK homes?

No. **TN-C** (combined PEN throughout, no split) is not a UK domestic supply arrangement, and ESQCR prohibits PEN conductors inside consumers' installations. **IT** (source not directly earthed) is used in special applications such as some medical or process supplies, not as a UK public low-voltage supply. They are shown here only so you can rule them out.

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_Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._
_Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._
_Status: reviewed. General information, not project-specific design advice._
_[How we source this](/methodology) — evidence hierarchy, confidence flags and source policy._

## Sources

1. BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (cited by clause only; standard text not reproduced)
2. IET — Amendment 4 updates to the 18th Edition — https://electrical.theiet.org/amendment-4-updates-to-18th-edition
3. BS 7430:2026 — Code of practice for protective earthing of electrical installations (BSI) — https://knowledge.bsigroup.com/products/code-of-practice-for-protective-earthing-of-electrical-installations (cited by topic; 2026 clause numbers to be confirmed before precise citation)
4. Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 (as amended) — https://www.legislation.gov.uk/uksi/2002/2665/contents/made (DNO supply-earthing duties; Reg 8(4) prohibits PEN conductors in consumers' installations)
5. V2L Workshop UK-earthing-systems technical reference (internal) — working summary and confidence flags