Open PEN, PME and EVs — the Section 722 anchor
On a PME (TN-C-S) supply the combined neutral-and-earth (PEN) conductor can break in the network. If it does, exposed metalwork can rise toward live potential — a serious touch-voltage hazard. Because an electric vehicle is large, often outdoors and touched by a person standing on the ground, BS 7671 §722.411.4.1 says you must not simply rely on the distributor's PME earth for the EV unless one of a short list of open-PEN protective measures is applied. That same rule is the reason the floating-V2L-on-PME earthing question is contested: it is exactly the open-PEN problem in reverse, and it must be designed and proven by test by a competent person — the vehicle manufacturer does not sanction this use.
In short
- On a PME (TN-C-S) supply, the neutral and earth share one combined PEN conductor up to the cut-out — and if that PEN breaks, metalwork can rise toward line voltage.
- BS 7671 §722.411.4.1 says a PME earth must not be relied on for an EV unless a listed open-PEN protective measure is applied.
- The standard lists several permitted open-PEN protective measures (described by category, not reproduced): broadly, either hold the installation's earth close to true earth, or detect loss of the PEN and disconnect the supply — with the exact measures, their limits and any device conditions set out in the licensed standard and the IET CoP.
- This is the rule that drives the V2L earthing decision: a separate outbuilding is made TT on its own electrode; indoors you keep the single PME/MET earth and use an isolation transformer.
- Contested ground: floating-V2L-on-PME must be designed and proven by test by a competent person. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.
Where this stops: This explains the open-PEN hazard and what the standard requires. It is not a wiring recipe — the protective measure, its design, installation and testing are for a competent person.
Some details below depend on sources still being verified against the published standard, so we mark them Not confirmed rather than guess:
- What (if anything) Amendment 4:2026 changed for §722 / V2X / V2H / V2L / 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; the existing §722 / PME/open-PEN material appears to carry forward from earlier amendments, subject to licensed-text confirmation. Do not assert that A4 changed §722. (safety-critical — not treated as settled until verified)
- The exact indent lettering (e.g. (i)–(v)) of the §722.411.4.1 protective-measure list, and which indent the IET CoP 'narrowed', is amendment-dependent and Not confirmed here. This page describes a narrowing of the PME workaround as an inference, not a precise indent claim. (safety-critical — not treated as settled until verified)
- The number of the new stationary secondary-battery chapter added by A4:2026 (reported elsewhere as 'Chapter 57') is Not confirmed; the chapter's existence is reported by the IET (citation-level), with its scope and number pending a licensed/source-text check. It is not the governing clause for the V2L source side in any case.
The short answer
On a PME (TN-C-S) supply, neutral and earth share one combined PEN conductor in the network up to your cut-out. If that PEN conductor breaks (an 'open PEN'), the load current looks for another way home — through your earthing and bonding, through earthed metalwork, and potentially through a person. Exposed metal can rise toward line voltage. An electric vehicle is a large, often outdoor lump of earthed metal that people touch while standing on the ground, so it is squarely in scope of this hazard. That is why BS 7671 §722.411.4.1 says you must not simply rely on the distributor's PME earth for an EV unless you apply one of a short list of open-PEN protective measures.
Why this page is the anchor
Every V2L earthing decision on this site — TT outbuilding, indoor isolation transformer, switched neutral-earth bond — traces back to this one rule. Read this page to understand *why* those arrangements exist; read the cornerstone and the method pages for *how* a competent person builds them.
What 'open PEN' means, plainly
In a PME (Protective Multiple Earthing) arrangement, the distributor combines neutral and protective earth into one conductor — the PEN — and earths it at multiple points along the network. At your property the PEN splits into a separate neutral and an earth at the supply terminal. The scheme is safe while the PEN is intact. If the PEN fails somewhere upstream, your installation's neutral current has to return another way. Because your earth and neutral were joined at the cut-out, that return current now flows in the earthing and bonding system, lifting earthed metalwork — pipes, the car body, an EV's exposed-conductive-parts — toward live potential. Nothing inside your installation has 'gone wrong'; the danger is imported from a fault in the network.
What the diagram shows: 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. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
Electric vehicle charging installations is the special-location section. A car's standalone V2L socket feeding portable loads sits outside the fixed installation — but the moment that output is wired into the consumer unit it becomes fixed-wiring work, and §722's open-PEN/PME rules are in play.
Reference only — verify against the current edition; standard text is not reproduced.
What §722.411.4.1 actually requires
The clause does not ban PME outright. It says a PME earthing facility must not be relied on for the EV unless one of a listed set of open-PEN protective measures is applied. Without reproducing the standard's list, those measures fall into two broad strategies: hold the installation's earth close to true earth so an open PEN cannot lift it to a dangerous voltage, or detect the loss of the PEN and disconnect the installation from the supply before the hazard can be touched. The exact permitted measures, their limits and the conditions on any disconnecting device are set out in the licensed BS 7671 §722.411.4.1 and interpreted in the IET Code of Practice — consult those, not a web paraphrase, when selecting one.
A PME facility must not be relied on for an EV unless one of the listed open-PEN protective measures is applied; and a device used for the purpose must not re-close onto the hazard — it must not restore the dangerous connection automatically once it has tripped on an open PEN.
Reference only — verify against the current edition; standard text is not reproduced.
The 'no re-close onto the hazard' point is easy to miss
A protective device here must not behave like a self-resetting auto-changeover that quietly re-energises the dangerous path once conditions look normal again. If it trips on an open PEN, it must stay safe. This is a design-and-test requirement, not a setting you tick.
How the IET Code of Practice frames it
The IET Code of Practice for EV Charging Equipment Installation (5th edition, 2023) is the EV-specific companion to BS 7671. Its Section 10 — 'Vehicle as Storage' covers vehicle-to-home and island-mode operation, including earthing on a PME supply and the use of open-PEN detection devices. The 5th edition is understood to reflect a narrowing of the PME open-PEN workaround at §722.411.4.1 — that is, fewer or more tightly-bounded ways to lean on PME than installers once assumed.
This is a framing inference from the CoP card, not a precise indent claim. The exact indent lettering of the §722.411.4.1 list, and which indent was narrowed or removed, is amendment-dependent and shifts between BS 7671 versions. Treat it as 'the easy PME route has tightened — confirm the current indents against the licensed standard', never as a specific 'indent (i) was deleted'.
'Vehicle as Storage' — earthing where the vehicle acts as storage on a PME supply, and open-PEN detection devices. The contested floating-earth-on-PME approach must be designed and proven by test. The CoP is BS 7671:2018+A2:2022-aligned; an A4:2026-aligned EV CoP is not yet published.
Reference only — verify against the current edition; standard text is not reproduced.
Why this drives the V2L earthing decision
A car's V2L output is, on the units we have looked at, floating — it has no internal neutral-earth bond, so on its own it offers no earth reference at all. The instinct is to borrow the house's PME earth. But that is precisely what §722.411.4.1 warns against: importing the PME earth onto the V2L island re-imports the open-PEN hazard. So the same rule that restricts relying on PME for *charging* an EV also shapes how you earth an EV used as a *source*. The two standard responses on this site both follow directly from it:
- Separate outbuilding → make it TT. Drive a local earth electrode, keep all circuit protective conductors continuous to the outbuilding earth bar, do not export the dwelling's PME earth, and make the single V2L neutral-earth bond only when on V2L. The local electrode is the 'additional earth' answer to the open-PEN problem — provided no metal pipe, structure or cable armour bridges the two earth systems.
- Inside the dwelling → isolation transformer. You cannot TT a single indoor sub-board (simultaneous-reach hazard with the rest of the PME installation), so you keep the one PME/MET earth and use an isolation transformer to galvanically separate the floating output, with a single neutral-earth bond on the transformer secondary.
What the diagram shows: The diagram shows two sources feeding an essential board through a changeover. From the grid, line (L) and neutral (N) arrive at the changeover; the circuit protective conductor (E/CPC) runs continuously to the board and earth bar and is never switched. From the V2L source, L and N arrive at the other side of the changeover. A neutral-earth (N–E) bond is made only when the board is on V2L, providing the floating output its single earth reference. A local earth electrode connects to the earth bar. The point: every source has exactly one neutral-earth reference, and the protective conductor is continuous in both switch states. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
A source supplying in island mode or as a switched alternative needs an independent means of earthing and must not rely solely on the distributor's earth. This is §722.411.4.1's open-PEN concern seen from the source side: the island cannot lean on a PME earth that might be open or disconnected.
Reference only — verify against the current edition; standard text is not reproduced.
Contested ground — the standing wrapper
Floating-V2L-on-PME is a contested earthing arrangement. It is only ever valid when 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 settled recipe; it explains the rule that any competent design must satisfy.
What this page does NOT claim
We do NOT claim Amendment 4:2026 changed §722 or 722.411.4.1. The public IET/BSI change material checked here does not identify a §722 / V2X / PME / open-PEN change in A4:2026; the existing §722 / PME/open-PEN material appears to carry forward from earlier amendments, subject to licensed-text confirmation. Treat any 'A4 changed the EV PME rule' statement as Not confirmed until checked against the consolidated A4:2026 text.
We do NOT pin the exact indent lettering of the §722.411.4.1 list, nor state which indent the IET CoP narrowed. That lettering is amendment-dependent — confirm the current indents against the licensed BS 7671 before relying on a specific one.
How this is made and proven compliant
- BS 7671 §722.411.4.1 — PME must not be relied on for an EV unless a listed open-PEN protective measure is applied
- BS 7671 §551.4.3.2.1 — independent means of earthing for island / switched-alternative operation
- IET Code of Practice for EV Charging Equipment Installation, 5th ed., Section 10 ('Vehicle as Storage') — interprets §722 for vehicle-to-home use (A2:2022-aligned)
Selecting and proving an open-PEN protective measure, and the earthing arrangement that follows, is design, installation, inspection and testing by a competent person. Wiring a V2L output into the fixed installation (an inlet circuit, changeover or consumer-unit alteration) is normally notifiable under Part P (England; Wales, Scotland and NI differ).
- Open-PEN protective measure selected from the §722.411.4.1 list and its operation proven by test — including that it does not re-close onto the hazard
- Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate
- RCD operation proven by test in both grid and V2L modes
- For a TT route: earth-electrode resistance (Ra) measured low and stable enough for disconnection, with no metallic path bridging the PME and TT earth systems
- The actual V2L adapter's neutral-earth behaviour confirmed on the bench before it is relied on
Confidence: Inference rolled up across the clauses cited above (the strictest state wins).
Frequently asked questions
Does §722.411.4.1 ban PME for EVs outright?
No. It says a PME earth must not be relied on for the EV unless one of a listed set of open-PEN protective measures is applied. Broadly, those measures either hold the installation's earth close to true earth or detect loss of the PEN and disconnect the supply; the exact list and its conditions live in the licensed standard and the IET CoP, and the competent designer selects and proves the measure.
What is an 'open PEN' and why is it dangerous on PME?
On a PME (TN-C-S) supply, neutral and earth share one combined PEN conductor up to the cut-out. If that PEN breaks in the network, neutral current returns through the earthing and bonding system instead, lifting earthed metalwork — pipes, an EV's body — toward line voltage. The fault is in the network, not your installation, which is what makes it so dangerous: nothing local looks wrong.
Why does an EV-charging rule shape how I earth a V2L home-backup setup?
Because borrowing the house's PME earth for the floating V2L output re-imports the very open-PEN hazard §722.411.4.1 is written to prevent. The rule is the reason the two standard responses exist: a TT electrode for a separate outbuilding, or an isolation transformer indoors so you keep the single PME/MET earth. It must be designed and proven by test by a competent person.
Did BS 7671 Amendment 4:2026 change the EV PME rule?
Not confirmed. The public IET/BSI change material checked here does not identify a §722 / V2X / PME / open-PEN change in A4:2026; the existing §722 / PME/open-PEN material appears to carry forward from earlier amendments, subject to licensed-text confirmation. We do not assert that A4 changed §722 — confirm against the consolidated BS 7671:2018+A4:2026 text before stating any A4-specific change.
The IET CoP 'narrowed' the PME workaround — which indent was removed?
We treat this as an inference, not a precise indent claim. The 5th-edition CoP (Section 10) reflects a tightening of the PME open-PEN route, but the exact indent lettering of the §722.411.4.1 list is amendment-dependent and shifts between BS 7671 versions. Confirm the current indents against the licensed standard rather than relying on a specific letter.
- Last reviewed
- 14 June 2026
- Written against
- BS 7671:2018 + A4:2026
- Reviewed by
- Martin (qualified UK electrician)
- Next review due
- 14 December 2026
General information, not project-specific design advice. Standards are cited by reference only and never reproduced. How we source this.
References & sources (5)
- BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — §722, §722.411.4.1, §551.4.3.2.1 cited by clause only; standard text not reproduced
- IET — Amendment 4 updates to the 18th Edition — public change material checked here does not identify a §722 / V2X / PME change in A4:2026, subject to licensed-text confirmation; A4-specific EV PME change Not confirmed
- IET Code of Practice for EV Charging Equipment Installation, 5th Edition (2023) — Section 10 'Vehicle as Storage' — BS 7671:2018+A2:2022-aligned; cited by reference only
- IET — EV charging installations FAQs
- V2L Workshop standards reference (internal) — BS 7671 and IET CoP cards, with confidence flags