Earthing & bonding fundamentals for V2L

The one principle behind every V2L earthing decision

Every electrical source needs exactly one neutral-earth reference. On the public supply that reference is made for you: the star point of the substation transformer is earthed, and the network brings that reference to your home (on most modern UK supplies as a PME / TN-C-S arrangement). A car's V2L output is a *separately derived* source. On the Hyundai Ioniq 5 it is reported to be floating — there is no internal bond between its neutral and earth — so until you supply one, there is nothing for an earth fault to flow back to and nothing for an RCD to operate against.

One bond — and only one

Supplying that reference means bonding the V2L neutral to earth. It feels wrong — you have spent a career keeping live conductors away from earth — but it is correct: it is the exact equivalent of the substation transformer's earthed star point, just for your separately derived source. The rule is one source, one bond. The bond is made on the source side, and it is in circuit only when you are on V2L (typically through an auxiliary contact on the V2L contactor), so that you never have two earth references fighting in parallel — one at the substation, one at the car — at the same time.

The protective conductor is continuous — you never switch the earth

A changeover switches line and neutral between grid and V2L. It must never switch the protective conductor (the earth / CPC). Earth stays continuous from every exposed-conductive-part back to the main earthing terminal at all times, in both modes and during the transfer itself. What changes between modes is *where the single neutral-earth reference is made* — never whether an earth exists. Switching a protective conductor would leave metalwork unearthed for the instant it matters most.

Main and supplementary bonding

Bonding is what keeps everything that *can* be touched at the same potential, so there is no dangerous voltage difference to bridge. Main protective bonding ties incoming metallic services (water, gas, structural steel) to the main earthing terminal. Supplementary bonding ties exposed- and extraneous-conductive-parts together locally where required. On a PME supply this matters even more, because a broken PEN on the network can lift all 'earthed' metalwork together — bonding ensures it rises *together*, removing the touch-voltage difference rather than leaving one part live relative to another.

Bonding-conductor sizing is set by Chapter 54 and its tables, against the size of the supply conductors — cited here by reference only, not reproduced. It is part of what the competent person calculates and records.

Where the board is decides the arrangement

The single-reference principle is the same everywhere; how you realise it depends on where the essential board sits. There are two clean routes, and the full decision tree (with the diagrams) lives on the island-mode and method pages.

Separate outbuilding → make it TT

Drive a local earth electrode and take every circuit protective conductor to the outbuilding's earth bar — continuous, never switched. Do not export the dwelling's PME earth to the outbuilding (broken-PEN risk); only line and neutral run over. Make the single V2L neutral-earth bond through a contactor auxiliary contact, so it exists only on V2L, and protect with a Type A RCD. This only holds if no metal pipe, structure or cable armour bridges the two earth systems — a stray metallic path silently re-imports the PME earth you were trying to keep out.

Inside the dwelling → isolation transformer, keep the one earth

You cannot TT a single indoor sub-board: it creates a simultaneous-reach hazard with the rest of the PME-earthed installation — you could touch the TT-referenced metalwork and the PME-referenced metalwork at once. Indoors you keep one earth. An isolation transformer in the V2L feed galvanically separates the floating output; the single neutral-earth bond is made on the transformer secondary (on V2L), and the protective conductor stays continuous to the main earthing terminal. No second electrode, no neutral diversion, no indoor TT island.

Proof is by test, not by reasoning

None of the above is settled by being correct on paper — it is settled by test. The earthing and bonding are proven, in both grid and V2L modes, before the arrangement is relied on.

• RCD operation confirmed by test in both grid mode and V2L mode — a floating output gives no RCD protection until the single bond exists.
• Continuity of every protective conductor confirmed end to end; the CPC is never interrupted by a switch.
• One bond, only on V2L — confirm exactly one neutral-earth reference is live in V2L mode, and none of it is left in parallel with the grid reference.
• TT island: electrode resistance (Ra) measured low and stable enough for reliable disconnection within the required time.
• No metallic bridge between the two earth systems (pipework, structure, cable armour).
• Bench-confirm the actual V2L adapter's neutral-earth behaviour before relying on it as floating or bonded.

Where this fits

This page is the principle. For the branching design — outbuilding TT versus indoor isolation transformer — see the island-mode earthing and isolation-transformer pages; for why a floating output is the starting condition, see the floating-versus-bonded page; and for the whole journey end to end, see the cornerstone.