Floating vs bonded V2L outputs — the neutral-earth bond
Many EV V2L outputs are floating: neither output conductor is referenced to earth inside the car. A floating output will light a socket tester and run an appliance, but an RCD has nothing to operate against, so there is no shock protection until a neutral-earth reference is made — the dangerous 'looks fine' case. Bonding the V2L neutral to earth feels wrong but is correct: it is the source's earth reference, exactly as the substation transformer's earthed star point references the grid. The rule is one source, one bond, made only when on V2L. On a PME (TN-C-S) supply this is contested and must be designed and proven by test by a competent person; the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.
In short
- A floating V2L output has no neutral-earth reference — neither conductor is tied to earth inside the car.
- It passes a socket tester and powers an appliance, but an RCD has nothing to operate against — no shock protection until a bond is made.
- Bonding the V2L neutral to earth feels wrong but is correct — it is the source reference, like the substation transformer's earthed star point.
- The rule is one source, one bond, made only on V2L — never two earth references in parallel.
- The Ioniq 5 output is reported floating, but adapters vary — bench-verify the actual neutral-earth behaviour before relying on it.
- On PME this is contested: designed and proven by test by a competent person; the manufacturer does not sanction this use.
Where this stops: This explains why a floating output is dangerous and why the bond is correct. It is not a wiring recipe — where, how and whether to make the bond is a design-and-test decision for a competent person.
Some details below depend on sources still being verified against the published standard, so we mark them Not confirmed rather than guess:
- Whether the specific Ioniq 5 V2L adapter presents a floating output is a field report, not a confirmed manufacturer fact — bench-verify the actual neutral-earth behaviour before relying on it. Reports vary by adapter. (safety-critical — not treated as settled until verified)
- What (if anything) Amendment 4:2026 changed for §722 / V2X / PME / open-PEN is not confirmed against the published standard. The existing PME/open-PEN framework appears to carry forward; do not assert an A4-specific change. (safety-critical — not treated as settled until verified)
Answer first
Many EV V2L outputs are floating — neither output conductor is tied to earth inside the car. That output will light a plug-in socket tester and run an appliance, so it *looks* fine. But a residual current device (RCD) works by comparing line and neutral current and tripping on the difference that leaks to earth; with a floating source there is no earth path for that leakage to follow, so the RCD has nothing to operate against and there is no shock protection until a neutral-earth reference exists. Supplying that one reference — bonding the V2L neutral to earth — feels wrong but is correct.
The dangerous 'looks fine' case
A floating output passes a socket tester and powers your kit, yet offers no RCD protection. Working kit is not proof of a safe earth. The protection is absent until the bond is made — and on a PME supply, making it correctly is a design-and-test job, not a DIY one.
What the diagram shows: Two states side by side. On the left, the V2L source as delivered is floating: line (L) and neutral (N) have no neutral-earth (N–E) reference, so a residual current device has nothing to operate against — the output passes a socket tester but offers no shock protection. On the right, a single N–E bond is made on the source side (only when on V2L): this establishes the earth reference so the RCD can operate. Make exactly one bond, never two in parallel. Bonding the V2L neutral to earth feels wrong but is correct — it is the source's earth reference, like the substation transformer's earthed star point. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
What 'floating' means
The grid you normally use is earth-referenced at the source: the supply transformer's star point is bonded to earth at the substation, so 'neutral' sits at roughly earth potential and a line-to-earth fault returns through a defined path. A floating V2L inverter output has no such reference — both conductors are isolated from earth. Voltage still exists *between* the two output conductors (that is why the appliance runs), but neither is fixed *relative to earth*. There is no defined fault-return path, so automatic disconnection of supply cannot work.
Automatic disconnection of supply (ADS) relies on a fault current large enough, through a defined earth path, to operate a protective device in time. A floating source defeats ADS at the root: with no neutral-earth reference there is no fault-return path for an earth fault to drive, so neither the RCD nor the overcurrent device can clear it.
Reference only — verify against the current edition; standard text is not reproduced.
Reported in field accounts and consistent with the inverter design, but it is not a confirmed manufacturer specification and behaviour varies by adapter. Treat any V2L output as floating until proven otherwise, and bench-verify the actual neutral-earth behaviour of the specific adapter before relying on it.
The floating behaviour of the specific Ioniq 5 V2L adapter is a field report, not a confirmed manufacturer fact — bench-verify before relying on it. Reports vary by adapter.
Why bonding the neutral to earth is correct, not wrong
It feels wrong because everyday electrical safety teaches you *never* to connect a live conductor to earth. But the neutral of a separately derived supply is not an ordinary live conductor you are shorting out — it is the conductor you choose to be the earth reference. The grid does exactly this at the substation: the transformer's star point is the neutral, and it is deliberately bonded to earth. A floating V2L output is a separately derived source that is simply *missing* that reference, so you supply one. Make exactly one bond, on the source side, in circuit only when on V2L. Never leave two earth references in parallel and never bond on both grid and V2L at once.
The principle in one line
Every source needs exactly one neutral-earth reference. The grid's is at the substation; the floating V2L has none, so you provide it — one source, one bond, made only on V2L.
Once that single bond exists, an earth fault on the V2L side has a defined return path, fault current flows, and the RCD sees the imbalance and trips. Without it, the same RCD is inert — which is exactly why a floating output can pass a socket tester and still be unprotected.
An EV connection point generally needs at least Type A RCD protection, plus detection of smooth DC residual current (an RDC-DD to BS IEC 62955, or a Type B RCD). Selecting the right RCD type only matters once a neutral-earth reference exists for the device to sense against — the bond is the precondition for any of this protection.
Reference only — verify against the current edition; standard text is not reproduced.
Where the bond goes — and why PME is contested
Knowing the bond is correct is not the same as knowing *where* to make it. On a PME (TN-C-S) supply you cannot simply tie the V2L neutral to the existing installation earth and call it done, because the PME earth carries its own open-PEN risk. The safe arrangement depends on where the load sits, and on each route the single bond is made through a contact that closes only on V2L.
On a PME (TN-C-S) supply you must not rely on the distributor's earth for the EV side unless a listed protective measure is applied — an open PEN can raise exposed metalwork to a dangerous voltage. This is the contested heart of re-referencing a floating V2L output on PME: it must be designed and proven by test, never presented as a recipe.
Reference only — verify against the current edition; standard text is not reproduced.
Separate outbuilding → make it TT
For a genuinely separate outbuilding, drive a local earth electrode and take all the 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 an auxiliary contact on the V2L contactor, 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.
What the diagram shows: 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. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
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. Indoors, keep one earth and put an isolating transformer in the V2L feed: it galvanically separates the floating output, and you make one neutral-earth bond on the transformer secondary (on V2L). The earth stays continuous to the main earthing terminal — no second electrode, no indoor TT island. An isolating transformer to BS EN IEC 61558-2-4 is the part that fits 230 V whole-circuit backup.
What the diagram shows: Indoors, an isolating transformer sits in the V2L feed. Its primary takes line (L) and neutral (N) from the floating V2L source; its secondary is galvanically separated. A single neutral-earth (N–E) bond is made on the secondary, giving the home-side circuit a defined earth reference. The installation keeps its one PME/MET earth continuous to the main earthing terminal — there is no second electrode and no TT island indoors (which would create a simultaneous-reach hazard). The transformer breaks the floating-source problem while preserving the single house earth. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
The non-obvious traps
- A floating output passes a socket tester but offers no RCD protection until a neutral-earth reference exists — the dangerous 'looks fine' case. Working kit is not proof of a safe earth.
- 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 — two earth references in parallel (bonded on grid *and* V2L at once) is its own fault. The bond is switched so it exists only on V2L.
- Don't assume the adapter is floating or bonded — bench-verify. A bonded adapter on PME can re-import the PME earth into a TT island; a floating one leaves you unprotected until you reference it.
- 'Floating on PME' is contested — only ever valid designed and proven by test by a competent person; the manufacturer does not sanction this use.
Sign-off, not DIY
Before it goes live: bench-verify the adapter's neutral-earth behaviour; confirm exactly one bond, present on V2L and absent on grid; prove RCD operation by test in V2L mode; certify under BS 7671 Part 6. The bond is correct engineering — but making it on a PME supply is a competent-person, notifiable job.
What (if anything) Amendment 4:2026 changed for §722 / V2X / PME / open-PEN is not confirmed against the published standard. The existing PME/open-PEN framework appears to carry forward; confirm before publishing.
How this is made and proven compliant
- BS 7671 §411 (automatic disconnection of supply — the protective measure a floating source defeats)
- BS 7671 §722.411.4.1 (PME / open-PEN protective measures — why re-referencing on PME is contested)
- BS 7671 §722.531.3.101 with the RCD/RDC-DD product standards (BS IEC 62955; BS EN 62423 for Type F/B) — see the protective-devices register
- BS EN IEC 61558-2-4 (isolating transformer — the indoor route to re-reference the floating output while keeping the one PME/MET earth)
Design, installation, inspection and testing by a competent person. Adding the bond, an inlet circuit, a changeover or any consumer-unit alteration is normally notifiable under Part P (England; Wales/Scotland/NI differ). The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.
- Confirm the actual adapter's neutral-earth behaviour on the bench before relying on it — do not assume floating or bonded
- RCD operation proven by test in V2L mode (and in grid mode, where the board is dual-source)
- Exactly one neutral-earth bond proven present on V2L and absent on grid — never two references in parallel
- Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate
Confidence: Inference rolled up across the clauses cited above (the strictest state wins).
Frequently asked questions
What does a 'floating' V2L output mean?
Neither output conductor is tied to earth inside the car, so there is no neutral-earth reference. Voltage still exists between the two conductors — an appliance runs — but neither is fixed relative to earth, so there is no defined fault-return path and an RCD has nothing to operate against until a bond is made.
If a socket tester lights up correctly, isn't the supply safe?
No. A floating output can pass a plug-in socket tester and run your kit while offering no RCD protection. Working kit is not proof of a safe earth — the shock protection is absent until a neutral-earth reference exists. This is the dangerous 'looks fine' case.
Why am I bonding the V2L neutral to earth — isn't connecting neutral to earth wrong?
It is correct here. The neutral of a separately derived source is the conductor you choose as the earth reference, exactly as the grid bonds its transformer star point to earth at the substation. A floating V2L output is simply missing that reference, so you supply one. Make exactly one bond, on the source side, in circuit only on V2L.
Is the Ioniq 5 V2L output definitely floating?
It is reported floating and that is consistent with the design, but it is not a confirmed manufacturer specification and behaviour varies by adapter. Treat any V2L output as floating until proven otherwise, and bench-verify the specific adapter's neutral-earth behaviour before relying on it.
Can I just make the bond myself?
No. On a PME (TN-C-S) supply, re-referencing a floating V2L output is contested and safety-critical — an open PEN can raise metalwork to a dangerous voltage, and the bond must sit in the right place (TT island outdoors, isolation transformer indoors) and be switched to exist only on V2L. It must be designed and proven by test by a competent person, is normally notifiable, and the manufacturer does not sanction this use.
- Last reviewed
- 14 June 2026
- Written against
- BS 7671:2018 + A4:2026
- Reviewed by
- Martin (qualified UK electrician)
- Next review due
- 14 December 2026
General information, not project-specific design advice. Standards are cited by reference only and never reproduced. How we source this.
References & sources (5)
- BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — cited by clause only (§411, §722.411.4.1, §722.531.3.101); standard text not reproduced
- IET — Amendment 4 updates to the 18th Edition — A4 changes to §722 / V2X / PME not confirmed at clause level
- IET Wiring Matters — RCDs for electric vehicle supply equipment (EVSE) — RCD type selection context (Type A/F/B + RDC-DD)
- BS EN IEC 61558-2-4:2025 — isolating transformers (BSI Knowledge) — indoor route to re-reference a floating output; cited by designation only
- V2L Workshop technical reference (internal) — verified design facts and confidence flags