Island-mode earthing & neutral switching — Section 551

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 §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.

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.

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.

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.

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.

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.

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.

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.