Using your EV (V2L) as home backup — the changeover and earthing nobody explains

The plan most people start with — and why it's wrong

The common idea is to use a Wi-Fi smart relay to 'fool' a cheap automatic transfer switch (ATS) into selecting the car. It fails in three ways. First, the smart relay is a roughly 2 A dry contact, while every supply path here is 63 A — it cannot switch or interrupt them. Second, the auto-ATS decides for itself on *voltage loss*; its status terminals are indicators, not a control input, so there is nothing to command. Third, even if you forced it, you would island a 63 A board onto a roughly 15 A source — a gross overload — and onto a floating supply with no earth reference.

How it actually works

An auto-ATS is self-sensing: it transfers in a couple of seconds on source loss and auto-reverts when the source returns. That is the wrong behaviour for moving between two *healthy* sources on a timer. The V2L output itself is around 2.3–3.6 kW (≈10–15 A) at 230 V and cannot be used while the car is charging. On the Hyundai Ioniq 5 the output is reported to be floating — no internal neutral-earth bond — so without an external bond an RCD has nothing to operate against. Confirm this on the actual adapter; reports vary.

The correct approach — power and control

Use a timed smart relay (in maintained mode) to *command* an interlocked two-pole contactor changeover: KM1 the grid contactor (the de-energised, fail-safe default) and KM2 the V2L contactor. The relay carries only the small coil current; the contactors carry the load. A mechanical interlock is the hard guarantee against paralleling — an EV inverter cannot parallel the grid. On power loss the changeover falls back to grid (fail-safe). Keep actual draw within about 80% of the V2L rating.

The correct approach — earthing (the crux)

Every source needs exactly one neutral-earth reference. The grid's is at the substation; a floating V2L output has none, so you must supply one — at the source side, in circuit only when on V2L, never two bonds in parallel. How you do that depends on where the board is.

Separate outbuilding → make it TT

Drive a local earth electrode and take all the outbuilding's circuit protective conductors to its earth bar — continuous, never switched. Do not export the dwelling's PME earth (broken-PEN risk); only line and neutral run over. Make the single V2L neutral-earth bond through a KM2 auxiliary contact, so it exists only on V2L. Protect with a Type A RCD. This only works if 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 — it creates a simultaneous-reach hazard with the rest of the PME-earthed installation. Keep one earth. An isolation transformer in the V2L feed galvanically separates the floating output; make one neutral-earth bond on the transformer secondary (on V2L), and the earth stays continuous to the main earthing terminal. No spike, no indoor TT island.

The non-obvious traps

• Bonding the V2L neutral to earth feels wrong but is correct — it is the source reference, the equivalent of the substation transformer's earthed star point. One source, one bond, made only on V2L.
• A floating output passes a socket tester but offers no RCD protection until that bond exists — the dangerous 'looks fine' case.
• Never export a PME earth to an outbuilding — and a stray metal pipe or cable armour can silently re-import it.
• You can't TT one indoor sub-board — simultaneous reach with the rest of the PME installation.