# Island-mode earthing & neutral switching — Section 551

> When an EV runs your home as a backup source with the grid disconnected, the installation is in **island mode**. BS 7671 §551.4.3.2.1 requires the islanded source to have its own **independent means of earthing** — you cannot rely on the distributor's (DNO) earth alone, because it may be disconnected during network maintenance. A two-pole changeover switches the **neutral as well as the line**, so the neutral-earth reference is correct in each state: the substation's bond on grid, and a single source-side bond when on the car. The changeover is **break-before-make** (open transition) so the two sources never parallel. This is a contested area on a PME (TN-C-S) supply 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

- Island mode = your home runs off the EV with the grid disconnected. The islanded source needs its **own** earth reference.
- BS 7671 **§551.4.3.2.1**: a switched-alternative / island source needs an **independent means of earthing** and must not rely solely on the distributor's earth.
- The DNO earth may be **disconnected during network maintenance**, so an islanded installation cannot depend on it — that is the whole point of the rule.
- A **two-pole changeover switches the neutral as well as the line**, so each source has its correct neutral-earth reference (grid bond on grid; one source-side bond on V2L).
- **Break-before-make (open transition)** keeps the two sources from paralleling — see §551.7. Designed and **proven by test** by a competent person; the maker does not sanction this use.

**Where this stops:** This explains the §551 island-mode earthing principle and why the neutral is switched. It is not a wiring recipe — the independent earthing arrangement and the neutral switching must be designed, installed and proven by test by a competent person.

## What island mode is — and why earthing changes

**Island mode** is the state where your installation runs off a local source — here an EV's V2L output — with the grid disconnected. On a normal grid supply the earth reference comes from the distribution network: the supply transformer's neutral is earthed at the substation, and on a PME (TN-C-S) supply that reference is delivered to your installation as the combined neutral-and-earth (PEN). The moment you island, that arrangement can no longer be trusted to be there — so the islanded source must carry **its own** earth reference.

> **The one-line rule** — Every source needs exactly one neutral-earth reference. The grid's is at the substation. An islanded V2L source must be given its own, independent of the distributor's earth — because you cannot assume the distributor's earth is connected while you are islanded.

## The §551 rule: an independent means of earthing

BS 7671 deals with sources like this in **Section 551 (low voltage generating sets)** — the same part that covers standby generators, inverters and other switched-alternative or standby sources. The specific requirement that governs island-mode earthing is **§551.4.3.2.1**.

> **§551.4.3.2.1** (BS 7671, confidence: inference) — Where the source keeps supplying in island mode or as a switched alternative, an independent means of earthing is required — it must not rely solely on the distributor's earth. The distributor's earth may be disconnected during network maintenance, so the island cannot depend on it alone. _Reference only; standard text not reproduced._

The reasoning is concrete, not theoretical. When the network is being worked on, the distribution network operator (DNO) may **disconnect the supply neutral/earth** at the cut-out or upstream. If your islanded installation were leaning on that earth for its fault protection, the protection could be absent at exactly the moment a fault occurs. So the islanded source is given its own earth reference that does not depend on the DNO being connected.

> **Confidence: inference** — An islanded / switched-alternative source must have an independent means of earthing and must not rely solely on the distributor's earth. (The §551.4.3.2.1 citation/edition is checked (public-primary) against both the BS 7671 standards card and the BS EN ISO 8528 card; the clause substance is pending a licensed/source-text check. Cited by reference only; the standard's wording is not reproduced.)

## Neutral switching — why the changeover moves the neutral too

If each source has its own neutral-earth reference, the changeover must move the **neutral as well as the line** — a **two-pole** changeover. Switch only the line and you would leave the two neutrals (and therefore the two earth references) joined across the changeover, which defeats the whole point of giving the islanded source its own reference and can leave a neutral-earth bond in circuit in the wrong state.

- **On grid:** the installation takes its neutral-earth reference from the supply (the substation bond / PME arrangement). No source-side bond is in circuit.
- **On V2L (island):** the line AND neutral are transferred to the car's output, and a **single** neutral-earth bond is made on the source side — the equivalent of the substation transformer's earthed star point — so RCDs have a reference to operate against.
- **Exactly one bond at a time:** never two neutral-earth bonds in parallel. The source-side bond exists only while on V2L (typically made through a changeover auxiliary contact), and is removed again on return to grid.

> **A floating output passes a socket tester but has no RCD protection** — An EV V2L output reported as floating (no internal neutral-earth bond) will look healthy on a plug-in socket tester yet give an RCD nothing to operate against until the source-side bond is made. Switching the neutral and making one correct bond is what turns it into a referenced, RCD-protected supply. Confirm the actual adapter's behaviour on the bench.

**Figure: V2L / PME changeover earthing circuit.** On a PME (TN-C-S) supply the V2L island gets its own earth reference — the CPC stays continuous, the neutral-earth bond is switched in only on V2L, and a local electrode provides the earth.

_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._

## Break-before-make — keeping the two sources apart (§551.7)

The changeover must be **break-before-make** (an open transition): it disconnects one source completely before it connects the other, so the grid and the car are never live together for an instant. That keeps the arrangement out of the **parallel-operation** regime, where extra requirements apply.

> **§551.7** (BS 7671, confidence: inference) — Additional requirements apply where a source can operate in parallel with the public supply, including a protective device where energy flow is bidirectional. A break-before-make changeover avoids paralleling and keeps most domestic V2L out of this regime. _Reference only; standard text not reproduced._

> **Confidence: inference** — A break-before-make changeover keeps a domestic V2L backup out of the §551.7 parallel-operation requirements. (The BS 7671 card carries §551.7 as inference — it is the correct framing for non-paralleling island operation, but the precise applicability to a given V2X/V2G interface is a design judgement to confirm against the licensed text.)

Mechanically this is delivered by an **interlocked** changeover — two contactors that physically cannot close together, or a changeover switch with a single common throw. An EV inverter cannot synchronise to and parallel the grid in any case, but the interlock is the hard guarantee, not a behaviour you assume.

**Figure: TT-island earthing — bond and aux-contact detail.** For an outbuilding made TT: a local electrode feeds the earth bar and all CPCs continuously; the V2L neutral-earth bond is made through a KM2 auxiliary contact, so it exists only on V2L.

_The TT-island arrangement. A local earth electrode connects to the outbuilding earth bar; all circuit protective conductors (E/CPC) connect to that bar continuously and are never switched. The PME (distributor's) earth is NOT exported to the outbuilding — only line (L) and neutral (N) run over. The V2L neutral-earth (N–E) bond is made through an auxiliary contact on the V2L contactor (KM2), so the single bond exists only while on V2L. An RCD protects the island. The point: one continuous local earth, one switched source bond, no exported PME earth._

## The genset analogue and the EESS guidance

None of this is new thinking — it is exactly how a standby generator is integrated. **BS EN ISO 8528** is the product-standard family for engine-driven AC generating sets, and although it does not apply to an EV or its V2L output, it is the established **design analogue**: a standby source is changed over and earthed so that its earth reference is correct in each switch state. An EV used as a backup source is, electrically, a switched-alternative source of the same kind.

> **Confidence: inference** — BS EN ISO 8528 is a design analogue (not a governing standard) for an EV used as a V2L backup source. (The BS EN ISO 8528 card carries the genset analogue as inference and is explicit that the series excludes road-vehicle sources — its value is the changeover/earthing mental model, with §551 the binding UK rule.)

On the home (consumer-unit) side, the **IET Code of Practice for Electrical Energy Storage Systems** is the closest applicable guidance: it treats island-mode operation directly, including forming a local earthed (TN-S-like) arrangement with a neutral-earth bond and a consumer earth electrode while islanded. A vehicle acting as a backup source behaves much like a fixed battery system, so this is the right reference for the home side — design and prove by test.

> **PME / open-PEN persists even when islanded** — The distributor's PEN/earth stays physically connected even when you are running off the car, and some open-PEN detection devices may not operate effectively while islanded. This is why floating-V2L-on-PME is contested and must be designed and proven by test by a competent person — never treated as a settled recipe. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

## What A4:2026 does — and does not — change here

This page is written against **BS 7671:2018 + A4:2026** (the consolidated 18th Edition incorporating Amendment 4, published 15 April 2026). The IET names low-voltage generating sets among the A4-affected areas; the exact clause-level redraft of §551 is pending the licensed A4 text, so confirm the §551 clause structure before relying on the numbering. Equally, the public IET/BSI change material checked here does not identify a §722 / V2X / PME / open-PEN change in A4:2026; existing §722 / PME/open-PEN material appears to carry forward from earlier amendments, subject to licensed-text confirmation — so do not assert a change.

> **Not confirmed (safety-critical):** Whether A4:2026 redrafted the §551 clause structure, or altered any V2X / PME / open-PEN requirement. The §551.4.3.2.1 citation and its independent-means-of-earthing requirement are carried as inference (citation/edition checked public-primary; substance pending a licensed/source-text check); the A4 clause-structure detail is not publicly confirmed — verify against the licensed A4:2026 text before citing the structure as A4-specific.

> **Not confirmed (safety-critical):** The specific chapter number (reported elsewhere as "Chapter 57") for the new A4:2026 stationary secondary battery installations chapter. The chapter exists; the number is not confirmed against the published standard.

### How this is made and proven compliant

**What governs it:** BS 7671 §551 (low voltage generating sets) — especially §551.4.3.2.1 (independent means of earthing for island / switched-alternative operation) and §551.7 (parallel operation / bidirectional protection); BS 7671 Chapter 54 (§542 earthing arrangements, §544 protective bonding) for the island-mode earth and its single neutral-earth bond; IET Code of Practice for Electrical Energy Storage Systems (island-mode earthing guidance) and BS EN ISO 8528 (the standby-genset analogue) as supporting design references

**Who may do it:** Design, installation, inspection and testing by a competent person. Adding a changeover, an island-mode earthing arrangement or a consumer-unit alteration is normally notifiable under Part P (England; Wales, Scotland and NI differ).

**How compliance is demonstrated:** Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate; Two-pole switching of line AND neutral proven, with the source-side neutral-earth bond present only when on the V2L source; RCD operation proven by test in BOTH grid and island (V2L) modes; Break-before-make (open transition) of the changeover confirmed so the two sources cannot parallel; For a local electrode route, electrode resistance (Ra) measured low and stable enough for disconnection; confirm the adapter's actual neutral-earth behaviour on the bench

## FAQ

### Why can't I just use the distributor's earth when I'm running off the car?

Because in island mode the distributor's earth may be disconnected during network maintenance. BS 7671 **§551.4.3.2.1** requires an islanded / switched-alternative source to have an **independent means of earthing** and not to rely solely on the distributor's earth. Designed and proven by test by a competent person.

### Why does the changeover have to switch the neutral, not just the line?

So each source has its correct neutral-earth reference in its own state. A two-pole (line and neutral) changeover keeps the two earth references separate; switching only the line would leave the neutrals joined across the changeover and a bond in circuit in the wrong state. Exactly one neutral-earth bond is in circuit at a time.

### What is break-before-make and why does it matter?

It means the changeover disconnects one source completely before connecting the other, so the grid and the car are never live together. This open transition keeps the arrangement out of the **§551.7** parallel-operation regime. It is enforced with a mechanical interlock, not assumed.

### Is this the same as how a standby generator is wired?

Conceptually, yes. **BS EN ISO 8528** (engine-driven generating sets) is the design analogue — a standby source is changed over and earthed so its earth reference is correct in each state. It does not govern an EV or its V2L output; **BS 7671 §551** is the binding UK rule for the fixed installation.

### Does Amendment 4:2026 change the island-mode earthing rule?

The independent-means-of-earthing requirement at **§551.4.3.2.1** is carried here as inference — the citation/edition is checked (public-primary), but the clause substance is pending a licensed/source-text check. Whether A4:2026 redrafted the §551 clause *structure* is **Not confirmed** here; confirm against the licensed A4:2026 text before relying on the numbering.

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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), §551 generating sets — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (cited by clause only (§551.4.3.2.1, §551.7, §542/§544); standard text not reproduced)
2. IET — Amendment 4 updates to the 18th Edition — https://electrical.theiet.org/amendment-4-updates-to-18th-edition (confirm §551 clause-structure changes against the licensed A4:2026 text)
3. IET Wiring Matters — "Island mode earthing arrangements" (new guidance in the 2nd edition of the IET CoP on EESS), March 2021 — https://electrical.theiet.org/wiring-matters/years/2021/84-march-2021/island-mode-earthing-arrangements-new-guidance-in-the-second-edition-of-the-iet-code-of-practice-on-electrical-energy-storage-systems/ (island-mode N-E bond relay + consumer earth electrode guidance)
4. IET Code of Practice for Electrical Energy Storage Systems (3rd Edition, 2024) — https://shop.theiet.org/code-of-practice-for-electrical-energy-storage-systems-3rd-edition (home-side island-mode reference; cited by reference only)
5. BS EN ISO 8528 series — reciprocating IC-engine driven AC generating sets (the standby-genset design analogue) — https://www.iso.org/standard/68539.html (analogue only — does not govern an EV V2L output; cited by reference only)
6. V2L Workshop technical reference (internal) — verified design facts and confidence flags