# How to identify your earthing system

> Three things tell you the system. If the main earthing conductor is connected to the supply **neutral/PEN terminal at the cut-out**, it is **TN-C-S (PME)** — the normal UK domestic arrangement. If it connects to the **cable sheath or armour** (a separate earth from the supply), it is **TN-S**. If there is **no supply earth at all — a local earth rod with an RCD as the main protection** — it is **TT**. The critical rule: an installation that *looks* TN-S should be treated as **TN-C-S unless the DNO confirms TN-S in writing**, because altered networks often contain combined neutral-earth sections. Do not open the cut-out — that is sealed DNO equipment; this is education, and confirmation is a competent person's job.

**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

- Earth to the supply **neutral/PEN terminal at the cut-out** → **TN-C-S (PME)** — the usual UK domestic supply.
- Earth to the **cable sheath/armour** (a separate supplier earth) → **TN-S** — now comparatively rare.
- **No supply earth — a local earth rod plus an RCD as main protection** → **TT**.
- If it *looks* TN-S, **treat it as TN-C-S unless the DNO confirms TN-S in writing** — altered networks hide combined neutral-earth sections.
- Why it matters for V2L: the earthing system decides how a floating V2L output is referenced on changeover — a PME supply is the least forgiving, and the design must be proven by test by a competent person.

**Where this stops:** This explains what an earthing system looks like and the cues a competent person reads. It is not a licence to open the service cut-out or carry out work — identification informs the competent person's design; it is not the design.

## Why you would want to know — and where to stop

Your earthing system is the arrangement that gives the metalwork in your home a safe connection to earth, so a fault disconnects the supply instead of leaving something live. For V2L it matters because it decides how a **floating** V2L output is referenced when you switch the house onto the car — and a PME supply is the least forgiving case. This page is about **what to look for**, so you can have an informed conversation with an installer. It is not an instruction to open anything.

> **Do not open the service cut-out** — The cut-out (the supplier's main fuse) and the incoming connections are **sealed DNO equipment**. Opening them is not occupier work and is potentially dangerous. You can identify the system from what is already visible, but confirming it — and any measurement — is a competent person's job, and the DNO is the authority on the supply type.

## The three cues that tell you the system

Almost all UK domestic supplies are one of three types: **TN-C-S (PME)**, **TN-S** or **TT**. The single most useful cue is **where the main earthing conductor — the cable running to your main earthing terminal — actually connects.**

- **Earth connected to the supply neutral/PEN terminal at the cut-out → TN-C-S (PME).** The supplier combines neutral and earth in one conductor (the PEN) and splits them at the cut-out; your earth is taken from that split. Often marked 'PME' or with a TN-C-S label. This is the normal arrangement for UK domestic supplies.
- **Earth connected to the cable sheath or armour → TN-S.** The supplier provides a separate earth all the way back to the substation, commonly via a lead-sheathed cable. The earth is a distinct connection to the sheath, separate from the neutral. Now comparatively rare.
- **No supply earth at all — a local earth rod, with an RCD as the main protection → TT.** The supplier provides only line and neutral; the installation makes its own earth with an electrode driven into the ground. The consumer unit usually carries a 'TT System — RCD Required' notice.

### TN-C-S (PME) — earth from the cut-out

On TN-C-S the distributor runs a single combined neutral-and-earth conductor (the **PEN**) and splits it into separate neutral and earth at your cut-out. Your main earthing terminal is bonded to that split point. The tell-tale is the earthing conductor going to the **supply neutral/PEN terminal**, frequently with a 'PME' marking. The reason it is the least forgiving system for V2L is the open-PEN hazard: if that combined conductor breaks upstream, earthed metalwork in the house can rise toward 230 V relative to true earth.

**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._

> **§722.411.4.1** (BS 7671, confidence: inference) — On a PME supply you must not simply rely on the distributor's earth for an EV unless a listed protective measure is applied, because an open PEN can raise metalwork to a dangerous voltage. This is why correctly identifying a PME supply is the first step before any V2L earthing design. _Reference only; standard text not reproduced._

### TN-S — a separate supplier earth

On TN-S the supplier keeps neutral and earth separate the whole way from the substation, and provides a dedicated earth terminal — classically a connection to the lead **sheath or armour** of the service cable, often soldered. The cue is an earth conductor connected to the cable sheath, clearly distinct from the neutral, with no link between neutral and earth at the cut-out.

**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._

> **If it looks TN-S, treat it as TN-C-S until the DNO confirms** — Many supplies that *look* TN-S are effectively TN-C-S on a network that has been repaired or altered with combined neutral-earth sections. Current IET guidance is to **treat an apparent TN-S as TN-C-S unless the DNO confirms TN-S in writing.** Do not design a V2L changeover on an assumed TN-S supply.

> **Confidence: inference** — An apparent TN-S supply should be treated as TN-C-S unless the DNO confirms TN-S in writing. (Stated in current IET guidance and reflected in the project's earthing-systems reference: altered networks frequently contain combined neutral-earth cable, so the conservative (and safer) default is TN-C-S until the distributor declares otherwise.)

### TT — your own earth electrode and an RCD

On TT the distributor provides only line and neutral; the installation makes its **own** earth via a local electrode (an earth rod). Because the fault path runs through soil, the earth-fault loop impedance is relatively high, so an **RCD is essential** as the means of disconnection. The cues are an earth rod with an earthing conductor to the main earthing terminal, **no** earth taken from the supply, and an RCD as the main protective device — usually with a 'TT System — RCD Required' label. TT is common with rural overhead supplies and is also a deliberate arrangement for some EV charge points.

**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._

> **Chapter 54 (§542, §544)** (BS 7671, confidence: inference) — Earthing arrangements (§542) and protective bonding (§544) — the requirements that define how the means of earthing, the main earthing terminal and main bonding are formed for each system type you are identifying. _Reference only; standard text not reproduced._

## Measurement as a cross-check, not a verdict

A competent person can measure the external earth-fault loop impedance (Ze) at the main earthing terminal as a cross-check. Indicative reference values are quoted as roughly 0.35 Ω for TN-C-S and roughly 0.8 Ω for TN-S, while TT can range from a few ohms to hundreds of ohms. But these are *typical declared* figures, not a definitive identifier — the supply type is settled by inspecting the connections and by the DNO's declaration, which the distributor is required to provide.

> **Not confirmed (safety-critical):** The Ze figures above (about 0.35 Ω for TN-C-S, about 0.8 Ω for TN-S) are indicative DNO/IET reference values only. They must be confirmed by site measurement and DNO declaration, and never used as the sole means of identifying the supply type.

## What this means before you go near V2L

Identification is the first step, not the design. Once the system is confirmed, the competent person decides how a floating V2L output is earthed on changeover — for an outbuilding, a TT island with its own electrode designed to BS 7430; indoors, an isolation transformer that keeps the single PME/MET earth. The contested floating-V2L-on-PME approach must be **designed and proven by test by a competent person, and the vehicle manufacturer does not sanction this use.** Knowing your system is what lets you brief an installer correctly — and recognise if one gets it wrong.

### How this is made and proven compliant

**What governs it:** BS 7671 Chapter 54 (§542 earthing arrangements, §544 protective bonding) — the means of earthing being identified; BS 7671 §722.411.4.1 (PME / open-PEN protective measures) — why the identified system drives the EV/V2L earthing decision; Electricity Safety, Quality and Continuity Regulations (ESQCR) — the DNO's duty to declare the supply earthing type and earth-fault loop impedance

**Who may do it:** Confirming the earthing system — including measuring external earth-fault loop impedance (Ze) and inspecting the cut-out connections — is work for a competent person. The service cut-out is sealed DNO equipment and must not be opened by the occupier. Where it matters, the DNO confirms the supply type in writing.

**How compliance is demonstrated:** Visual identification of where the main earthing conductor connects (supply neutral/PEN terminal, cable sheath/armour, or local electrode); External earth-fault loop impedance (Ze) measured at the main earthing terminal with main bonding disconnected — as a cross-check, not a sole proof of type; DNO written confirmation of the supply earthing arrangement where an apparent TN-S must be relied upon; Consumer-unit labelling consistent with the identified system (e.g. a 'TT System — RCD Required' notice)

## FAQ

### How do I tell if I have a PME (TN-C-S) supply?

Look at where the main earthing conductor connects. If your earth is taken from the **supply neutral/PEN terminal at the cut-out** — often with a 'PME' marking or a TN-C-S label — it is TN-C-S (PME). This is the normal arrangement for UK domestic supplies. Confirming it is a competent person's job; do not open the sealed cut-out.

### My installer says it looks like TN-S — is that good enough?

No. Many supplies that *look* TN-S are effectively TN-C-S on an altered network. Current IET guidance is to **treat an apparent TN-S supply as TN-C-S unless the DNO confirms TN-S in writing.** Do not let a V2L changeover be designed on an assumed TN-S supply.

### What does a TT supply look like?

TT has **no supply earth**: the installation makes its own earth with a local electrode (an earth rod), and relies on an **RCD** as the main protection because the soil fault path is high-impedance. The consumer unit usually carries a 'TT System — RCD Required' notice. It is common with rural overhead supplies.

### Can I just measure it to find out, instead of asking the DNO?

Measurement of external earth-fault loop impedance (Ze) is a useful **cross-check** a competent person can carry out, but it is not a definitive verdict on supply type — the indicative figures overlap and networks get altered. The DNO has a legal duty to declare the supply earthing type, so written confirmation is the authority where it matters.

### Why does my earthing system matter for using my EV as backup?

Because it decides how a **floating** V2L output is referenced to earth when you switch the house onto the car. A PME (TN-C-S) supply is the least forgiving — the open-PEN risk is why BS 7671 restricts relying on the distributor's earth for an EV. The earthing design that follows must be proven by test by a competent person; the manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

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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. Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 (as amended) — DNO duty to declare supply earthing type and loop impedance — https://www.legislation.gov.uk/uksi/2002/2665/contents/made
4. V2L Workshop earthing reference (internal) — UK earthing systems and grid-to-EV source selection; identification cues and the apparent-TN-S caveat