# TN-C-S (PME) explained — and the open-PEN problem

> Most UK homes have a **TN-C-S (PME)** supply: the distributor brings the neutral and earth combined in one conductor — the **PEN** — and splits it into separate neutral and earth at the cut-out, where your installation's earth is taken. The weakness is the shared PEN: if it breaks somewhere upstream (an **open PEN**), load current has no proper neutral return, so it pushes through the earthing and bonding instead and can lift exposed metalwork — and a connected EV's body — toward line voltage. Because an EV is touched while you stand on real ground, **BS 7671 §722.411.4.1** says a PME earth must not be relied on for it unless a listed open-PEN protective measure is applied. This is also why floating-V2L-on-PME is contested and must be designed and proven by test by a competent person; the vehicle manufacturer does not sanction back-feeding fixed wiring.

**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)** is the supply in most UK homes: neutral and earth are combined as one **PEN** conductor, then split into separate N and PE at the cut-out — your earth comes from that split point.
- PME = **Protective Multiple Earthing**: the PEN is earthed at several points along the network to reduce the chance, and effect, of it breaking.
- The **open-PEN** fault is the hazard: if the PEN breaks upstream, return current diverts through the earth/bonding and can raise exposed metalwork — and an EV body — toward **230 V**.
- An EV is touched from real ground, so **BS 7671 §722.411.4.1** does not let you simply rely on the PME earth — a listed open-PEN protective measure is required.
- This is why floating-V2L-on-PME is contested: **designed and proven by test by a competent person**, and the manufacturer does not sanction back-feeding fixed wiring.

**Where this stops:** This explains how a PME supply is arranged and why the open-PEN fault is dangerous. It is not a design recipe — deciding and proving the open-PEN protective measure for an EV is for a competent person.

## What TN-C-S (PME) actually means {#tn-c-s-pme}

**TN-C-S** is the earthing arrangement on the great majority of modern UK domestic supplies, almost always run as **PME** — Protective Multiple Earthing. The two-letter-plus codes describe where the neutral and earth functions are combined and where they are separate. In TN-C-S the distributor brings the neutral and earth **combined** into a single conductor — the **PEN** (protective-and-neutral) — from the substation to your property (the 'C' = combined). At the service cut-out the PEN is **split** into a separate neutral and a separate protective earth for your installation (the 'S' = separate, downstream). Your installation's earth is taken from that split point, and the main earthing terminal (MET) is bonded to it.

'Protective Multiple Earthing' refers to the network, not your house: the distributor earths the PEN conductor at several points along its length — at the substation, at joints and at the cut-out. Multiple earth connections reduce both the chance of the PEN going open-circuit and the severity if part of it does. You can usually recognise a PME supply on site by a concentric service cable, a 'PME' or 'TN-C-S' label at the cut-out, and the earth being taken from the supplier's combined neutral/PEN terminal rather than from a separate cable sheath.

**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._

> **Chapter 54 (§542, §544)** (BS 7671, confidence: inference) — Earthing arrangements and main protective bonding. In a PME installation, main bonding ties incoming metal services and structural metalwork to the MET so they all rise together — bonding is what prevents dangerous touch-voltage differences between, say, a tap and a radiator. This is the framework the PME and open-PEN questions resolve into. _Reference only; standard text not reproduced._

> **PME is the norm, not a fault** — A PME supply is the standard, cost-effective arrangement on most UK homes, and is perfectly safe in normal operation. The issue is a single specific upstream fault — an open PEN — and what that fault means for anything you can touch while standing on real ground, an EV included.

## Why the combined PEN is the weak point

The strength of TN-C-S — one combined conductor doing two jobs — is also its weakness. In normal use, neutral load current flows back to the substation along the PEN, and the PEN also holds your earthed metalwork at (close to) true earth potential. Those two roles share one conductor. If that conductor breaks **upstream** of your property — a corroded joint, a damaged cable, a failed network connection — the neutral return current loses its proper path back to the source.

Current still wants to return, so it finds another route: out through your (and your neighbours') earthing and bonding, into the general mass of earth, and back to the substation. Because that diverted current now flows through the **earth/PE side** of the installation, it lifts the potential of everything connected to it. All your 'earthed' metalwork — and, critically, anything bonded to it through the MET — can rise toward **line voltage** (around 230 V) above true earth. This is the **open-PEN** (sometimes 'lost neutral' or 'broken PEN') hazard.

**Figure: Open-PEN hazard on a PME supply.** If the combined PEN conductor breaks on a PME supply, load current returns through the earth/bonding path and lifts exposed metalwork toward live potential — a touch-voltage hazard.

_On a PME (TN-C-S) supply the line (L) stays live while the combined protective-and-neutral (PEN) conductor is shown broken upstream. With the PEN open, the neutral return current has to find another path — through the installation's earthing and bonding — which raises exposed metalwork and a connected EV body toward line voltage, creating a touch-voltage shock hazard. This is why BS 7671 §722.411.4.1 restricts relying on a PME earth for an EV and requires a listed open-PEN protective measure._

> **Confidence: verified** — On a broken PEN, exposed-conductive-parts and bonded metalwork can rise toward line voltage above true earth, creating a touch-voltage shock hazard. (Standard, well-documented behaviour of a TN-C-S supply with an open PEN: with the neutral return path lost, load current diverts through the earthing/bonding system, raising its potential. It is the recognised driving force behind the 18th Edition's EV-charging earthing rules.)

> **Why an open PEN is dangerous indoors and out** — Inside, bonding ties pipework, radiators and appliance casings to the same rising potential, so they move together — bonding limits the difference between two things you might touch at once. The acute risk is at a boundary between the bonded installation and true earth: a kitchen tap and a damp floor, or — outdoors — a car body touched while standing on the ground.

## What this means for an EV

An EV is the textbook open-PEN problem because you touch the vehicle body while standing on real ground outdoors. If the car's protective earth is taken straight from the PME earth, then on a broken PEN the body can rise toward line voltage while your feet are at true earth — exactly the simultaneous-contact case bonding cannot fix. This is why the 18th Edition of BS 7671 added specific earthing rules for EV charging, and why the same logic governs feeding a V2L output into fixed wiring on a PME supply.

> **§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. One of a listed set of open-PEN protective measures must be applied — for example a TT arrangement with its own electrode, or a device that disconnects line, neutral and protective earth on PEN-loss (and does not re-close onto the hazard). This is the contested heart of floating-V2L-on-PME, and must be designed and proven by test by a competent person. _Reference only; standard text not reproduced._

The protective measure removes the dependence on the PME earth for the part you can touch from the ground. The two common routes are: (a) make the EV side **TT** — a local earth electrode provides the reference and the PME earth is not exported to it; or (b) fit an **open-PEN detection device** that disconnects line, neutral and protective earth if it senses the voltage rise of a lost PEN. Which is appropriate, and whether it actually works on the day, is a competent-person design-and-test decision — not something to pick from a forum.

> **Section 10 (earthing on PME)** (IET CoP EV Charging (5th ed.), confidence: inference) — The IET Code of Practice's vehicle-as-storage chapter covers earthing on a PME supply and the use of open-PEN detection devices — a broken PEN can make exposed metalwork live, so an approved open-PEN mitigation is needed. The contested floating-earth-on-PME approach must be designed and proven by test. _Reference only; standard text not reproduced._

> **Floating V2L on PME is contested — not a recipe** — Wiring a vehicle's floating V2L output into a PME-earthed installation reopens exactly this open-PEN question. It is only ever valid designed and proven by test by a competent person, and the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet. Treat any 'just run a spike' or 'just bond it' shortcut as wrong until a competent person has proved otherwise on site.

> **Not confirmed (safety-critical):** Whether Amendment 4:2026 changed anything in §722 / 722.411.4.1 for PME / open-PEN is not confirmed — the published change-lists do not name V2X/PME, and the framework appears to carry forward from earlier amendments. Confirm against the licensed A4:2026 text, and treat the precise indent lettering of the listed protective measures as not confirmed, before relying on either.

## How an electrician tells it apart on site

Identifying the supply correctly is the first safety step, because the protective measure depends on it. The distinctions, in plain terms:

- **TN-C-S (PME):** earth taken from the supplier's combined neutral/PEN terminal at the cut-out; concentric service cable; a 'PME' or 'TN-C-S' label. External loop impedance (Ze) is typically low — often a few tenths of an ohm — but the DNO's declared figure must be confirmed, not assumed.
- **TN-S:** a separate earth conductor from the supply cable sheath, distinct from the neutral, with no neutral-earth link at the cut-out. There is no shared PEN, so the open-PEN-from-the-supply mechanism does not arise the same way.
- **TT:** only line and neutral arrive; the installation provides its own earth electrode, and an RCD is the primary protection. This is also the basis of the TT route used to take an EV off the PME earth.

> **Not confirmed:** The Ze and electrode-resistance figures above are indicative typical values for orientation, not fixed limits. The actual supply must be measured and the DNO's declared earth-fault-loop-impedance confirmed before any design relies on it.

### How this is made and proven compliant

**What governs it:** BS 7671 §722.411.4.1 (PME / open-PEN protective measures for EV) within §722; BS 7671 Chapter 54 (§542 earthing arrangements, §544 main protective bonding); IET Code of Practice for EV Charging Equipment Installation, 5th Edition — Section 10 (vehicle-as-storage earthing on PME)

**Who may do it:** Identification of the supply type and the choice and proof of an open-PEN protective measure are for a competent person. Adding an EV connection point, an inlet circuit, a changeover switch or a consumer-unit alteration is normally notifiable under Part P (England; Wales, Scotland and NI differ).

**How compliance is demonstrated:** Supply earthing arrangement identified and the external earth-fault loop impedance (Ze) measured at the main earthing terminal; The selected open-PEN protective measure applied and its operation proven — a TT electrode arrangement, or a device disconnecting line, neutral and protective earth on PEN-loss, must not re-close onto the hazard; For a TT route: electrode resistance (Ra) measured low and stable enough for the RCD to disconnect, with no metallic path bridging the PME and TT earths; Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate

## FAQ

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

They describe the same supply from two angles. **TN-C-S** is the earthing arrangement — neutral and earth combined as a PEN conductor in the supply, then split into separate N and PE at your cut-out. **PME** (Protective Multiple Earthing) is how the distributor manages that PEN on the network — earthing it at several points to reduce the risk and effect of it breaking. In a UK home the two terms are used for the same supply.

### What is the open-PEN (or broken PEN / lost neutral) problem?

On a TN-C-S supply, one combined PEN conductor carries both the neutral return current and your earth reference. If that conductor breaks upstream, the return current diverts through the installation's earthing and bonding instead, which can lift all your 'earthed' metalwork — and a connected EV body — toward line voltage (around 230 V) above true earth. That is a serious shock hazard, particularly where you touch metalwork while standing on real ground.

### Why is a PME supply a problem for charging an EV?

Because you touch the car's metal body while standing on the ground outdoors. If the body's earth comes straight from the PME earth, a broken PEN can raise the body toward line voltage while your feet are at true earth — exactly the case bonding cannot protect against. **BS 7671 §722.411.4.1** therefore does not let you rely on the PME earth for an EV unless a listed open-PEN protective measure is applied.

### How is the open-PEN risk dealt with on an EV connection?

By a competent person applying one of the listed protective measures: commonly either making the EV side **TT** with its own earth electrode (so it no longer depends on the PME earth), or fitting an **open-PEN detection device** that disconnects line, neutral and protective earth if it senses the voltage rise of a lost PEN. Which is right, and whether it actually operates correctly, is a design-and-test decision — see the IET Code of Practice, Section 10.

### Can I just run an earth spike to fix this for a V2L setup?

No — not as a DIY shortcut, and not as a settled recipe. Mixing a local earth spike with a PME-earthed installation can create its own simultaneous-reach hazard, and feeding a vehicle's floating V2L output into PME-earthed fixed wiring reopens the whole open-PEN question. It is only ever valid **designed and proven by test by a competent person**, and the vehicle 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), §722 / §722.411.4.1 / Chapter 54 — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (cited by clause only; standard text not reproduced)
2. IET — Electric vehicle charging installations FAQs (open-PEN / PME guidance) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/faqs/electric-vehicle-charging-installations-faqs/
3. IET Code of Practice for EV Charging Equipment Installation, 5th Edition (2023) — Section 10, vehicle-as-storage earthing on PME — https://shop.theiet.org/code-of-practice-for-electric-vehicle-charging-equipment-installation-5th-edition (A2:2022-aligned; predates A4:2026 — cited by reference only)
4. V2L Workshop earthing-systems reference (internal) — TN-C-S / PME and the broken-PEN hazard