Isolation-transformer method (indoors) — keep the one PME/MET earth

The short answer

Indoors you keep one earth. You cannot make a single indoor sub-board TT by driving an earth spike, because a second, separate earth system within arm's reach of the rest of the PME-earthed installation is a simultaneous-reach shock hazard. Instead you put a double-wound isolating transformer in the V2L feed. It galvanically separates the floating V2L output, so its secondary becomes a fresh, independent supply you can earth in a controlled way — one neutral-earth bond on the secondary, in circuit only on V2L. The building's earth stays continuous to the main earthing terminal throughout.

The problem: a floating output and a single PME earth

An EV's V2L AC output is typically a floating (unreferenced) source — there is no defined neutral-earth bond inside it, so on its own it has no earth reference for an earth-fault loop and an RCD has nothing to operate against. A floating output will pass a plug-in socket tester yet offer no RCD protection until a bond exists. Indoors you also can't borrow the obvious fix from the outbuilding case — you can't make a local TT island — so the floating output has to be referenced without introducing a second earth system into the dwelling.

The fix: a double-wound isolating transformer in the V2L feed

A double-wound isolating transformer has no electrical connection between its primary and secondary windings — only a magnetic one. Putting one in the V2L feed galvanically separates the floating V2L output from the house wiring. The secondary is then a fresh, independent 230 V supply with no inherited earth reference, which is exactly what you want: you can give it a controlled earth on the house side without tying the car's output to the PME system or creating a second earth electrode.

One bond, on the secondary, only on V2L

Every source needs exactly one neutral-earth reference. The grid's is at the substation; the floating V2L has none. With the transformer in place you make that single bond on the transformer secondary, and you arrange it to be in circuit only when running on V2L (the equivalent of the substation transformer's earthed star point, but for the separated secondary). The dwelling's earth stays continuous to the main earthing terminal — there is no second electrode, no neutral diversion, and no moment when two bonds sit in parallel.

The trade-off

Galvanic separation indoors comes at a price. A transformer sized for the full V2L output — around 3.6 kW — is costly, physically heavy, and runs with standing (no-load and load) losses whenever it is energised. For so small an essential load, those losses and the capital cost can erode much of the saving a scheduled load-shift was meant to deliver. That is a genuine reason to weigh the indoor isolation-transformer route against simply siting the backup at a genuinely separate outbuilding (where the TT route is available) or re-deriving the supply with a battery inverter/charger.

What to prove before it goes live

• The transformer is a full-LV double-wound isolating type to BS EN IEC 61558-2-4 (on the 61558-1 base) — not a -2-6 SELV safety-isolating type.
• Exactly one neutral-earth bond exists, on the transformer secondary, and is in circuit only when running on V2L — never two bonds in parallel.
• The building earth stays continuous to the main earthing terminal — no second electrode and no neutral diversion introduced indoors.
• RCD operation is proven by test in both grid and V2L modes against the secondary-side bond.
• The actual V2L adapter's neutral-earth behaviour is bench-confirmed, not assumed.
• The work is designed, installed and tested by a competent person and notified where required; the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.