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

> Indoors you cannot make one sub-board TT by driving an earth spike — a second earth system within arm's reach of the rest of the PME-earthed installation is a simultaneous-reach shock hazard, so you keep the one PME/MET earth. The fix is a **double-wound isolating transformer** (to BS EN IEC 61558-2-4 — the full-LV part, not the SELV part) in the V2L feed. It galvanically separates the floating V2L output so you can make exactly one neutral-earth bond on the transformer **secondary** (in circuit only on V2L), restoring a normal earth-fault path and RCD operation while the building's earth stays continuous to the main earthing terminal. The trade-off is cost, weight and standing losses on a 3.6 kW unit. It must be designed and **proven by test** by a competent person; the vehicle manufacturer does not sanction this use.

**Safety frame.** This is education, not an instruction to carry out work. Work connecting V2L equipment to fixed wiring is safety-critical and may be notifiable under Part P. It must be designed, installed, inspected and tested by a competent person to the current edition of BS 7671. Vehicle manufacturers generally do not sanction back-feeding fixed wiring from V2L outlets; follow manufacturer instructions.

## In short

- **Indoors, keep one earth.** You cannot TT a single indoor sub-board — a second earth system within reach of the PME-earthed installation is a simultaneous-reach hazard.
- **Use a double-wound isolating transformer** to **BS EN IEC 61558-2-4** (the full-LV isolating part) in the V2L feed — *not* the -2-6 safety-isolating (SELV) part.
- It **galvanically separates** the floating V2L output, so its secondary is a fresh, independent supply you can earth in a controlled way.
- Make **one** neutral-earth bond on the transformer **secondary**, in circuit only on V2L — that restores the earth-fault path and lets the RCD operate.
- The **building earth stays continuous to the MET** — no second electrode, no neutral diversion, no indoor TT island.
- Trade-off: a 3.6 kW isolation transformer is **costly, heavy and has standing losses** that can erode the saving on so small a load.
- Designed and **proven by test** by a competent person. BS 7671 §413/§418.3 (electrical separation) governs whether/how it is valid; the manufacturer does not sanction this use.

**Where this stops:** This explains why the indoor route is an isolation transformer and not an earth spike. It is not a wiring recipe — the design, installation and testing are for a competent person, and the method must be proven by test.

## 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.

> **Why not just run an earth spike indoors?** — A single indoor TT sub-board sits within reach of the surrounding PME-earthed metalwork. Under fault, the two earth systems can be at very different potentials — a simultaneous-reach hazard. The legitimate TT routes are a genuinely out-of-reach point (e.g. an outdoor EV point) or converting the whole installation to TT, not spiking one indoor board.

## 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.

> **Confidence: inference** — The Ioniq 5 V2L output is floating (no internal neutral-earth bond). (Reported in field accounts and consistent with the design; behaviour varies by adapter and should be bench-verified before it is relied on. The safe default is to treat the output as floating until proven otherwise.)

## 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.

**Figure: Isolation-transformer method (indoors).** An isolating transformer in the V2L feed galvanically separates the floating output; a single neutral-earth bond is made on the transformer secondary, keeping the one PME/MET earth continuous.

_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._

> **(whole part)** (BS EN IEC 61558-2-4, confidence: inference) — Specify the **full-LV** isolating transformer (Part 2-4), not the safety-isolating SELV transformer (Part 2-6). A 230 V essential board needs a full low-voltage secondary, not an extra-low-voltage one. Both sit on the BS EN IEC 61558-1 base for construction, insulation and dielectric strength. _Reference only; standard text not reproduced._

> **-2-4 vs -2-6 — don't conflate them** — **BS EN IEC 61558-2-4** is the full-LV isolating transformer (up to normal mains voltage) — the correct part for a 230 V V2L feed. **BS EN IEC 61558-2-6** is the *safety isolating* transformer with a SELV (extra-low-voltage) output — the wrong part here, because the load needs full LV, not SELV.

## 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.

**Figure: Dwelling — isolation-transformer earthing arrangement.** Inside the dwelling, keep the single PME/MET earth and use an isolation transformer on the V2L feed — never a second indoor earth electrode.

_The dwelling arrangement keeps one earth. The PME/MET (main earthing terminal) earth stays continuous to all circuit protective conductors (E/CPC). The V2L feed passes through an isolating transformer whose secondary carries a single neutral-earth (N–E) bond, defining the backup circuit's earth without a second electrode. Because the rest of the installation is PME-earthed, a separate indoor TT island is not used (it would create a simultaneous-reach shock hazard). The point: one earth indoors, the floating V2L output referenced through the transformer secondary._

> **§551.4.3.2.1** (BS 7671, confidence: inference) — A switched-alternative or island source needs an independent means of earthing and must not rely solely on the distributor's earth, which may be disconnected during network maintenance. The earthed transformer secondary supplies that independent reference for the V2L island. _Reference only; standard text not reproduced._

> **§413 / §418.3** (BS 7671, confidence: inference) — Electrical separation is the protective-measure framework that governs a separated source like this: §413 for a single item, §418.3 for more than one item under tightly controlled conditions and with a required warning notice. BS 7671 — not the transformer's product standard — decides whether and how the method is valid here. _Reference only; standard text not reproduced._

> **Not confirmed (safety-critical):** The exact §418.3 warning-notice wording is not reproduced here (BS 7671 is copyright) — consult the licensed standard. And whether electrical separation / secondary re-earthing is a valid protective measure for a given floating-V2L-on-PME setup is a competent-person design judgement, designed and proven by test, not a settled recipe.

## 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.

> **Outbuilding instead? Then it's the TT route** — If the essential board is in a genuinely separate outbuilding, you don't need the transformer — you make that building TT with a local electrode and never export the dwelling's PME earth. The indoor isolation-transformer method exists precisely because that TT route is *not* safe to use inside the PME-earthed dwelling.

> **Chapter 54 (§542, §544)** (BS 7671, confidence: inference) — Keeping the single PME/MET earth, and confirming exactly one secondary-side neutral-earth bond made only on V2L, are earthing-and-bonding (Chapter 54) decisions the competent designer makes and proves by test. _Reference only; standard text not reproduced._

## 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.

### How this is made and proven compliant

**What governs it:** BS EN IEC 61558-2-4 (full-LV isolating transformer, the indoor route) on the BS EN IEC 61558-1 base — defines the transformer as fit for purpose; BS 7671 §413 / §418.3 (electrical separation) — governs whether/how separation is a valid protective measure for the V2L feed; BS 7671 §551.4.3.2.1 (independent means of earthing for an island/switched-alternative source) and Chapter 54 (§542/§544 earthing and bonding)

**Who may do it:** Design, installation, inspection and testing by a competent person. Adding the V2L inlet, the changeover and the isolation-transformer feed is normally notifiable under Part P (England; Wales/Scotland/NI differ).

**How compliance is demonstrated:** Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate; RCD operation proven by test in **both** grid and V2L modes against the secondary-side bond; Confirm exactly one neutral-earth bond exists on the transformer secondary and only when on V2L (no second bond in parallel); Confirm the building earth remains continuous to the main earthing terminal — no second electrode, no neutral diversion; Confirm the actual adapter's neutral-earth behaviour on the bench before relying on it

## FAQ

### Why can't I just run an earth spike to one indoor board instead of buying a transformer?

Because a single indoor TT sub-board creates a **simultaneous-reach hazard**: a second, separate earth system sits within arm's reach of the surrounding PME-earthed metalwork, and under fault the two can be at very different potentials. Indoors you keep one earth and use an isolation transformer; the earth-spike (TT) route is for a genuinely separate outbuilding or a whole-installation TT conversion.

### Which BS EN IEC 61558 part do I specify — 2-4 or 2-6?

**BS EN IEC 61558-2-4** — the full-LV isolating transformer. A 230 V essential board needs a full low-voltage secondary. **BS EN IEC 61558-2-6** is the *safety isolating* (SELV, extra-low-voltage output) part and is the wrong choice for 230 V home backup. Both sit on the `BS EN IEC 61558-1` base standard.

### Where exactly does the neutral-earth bond go?

On the transformer **secondary**, and arranged to be in circuit only when you are running on V2L. There must be exactly one such bond — never a second one in parallel. That single bond is the secondary's earth reference, the equivalent of the substation transformer's earthed star point for the grid.

### Does the house earth change when I'm on the car?

No. The whole point of this method is that the **single PME/MET earth stays continuous** to the main earthing terminal. The transformer galvanically separates the floating V2L output and gives its secondary its own controlled bond, so you never introduce a second electrode or divert the neutral indoors.

### Is the isolation transformer worth it for such a small load?

That is the honest trade-off. A ~3.6 kW isolation transformer is costly, heavy, and has standing losses whenever it's energised — which can erode much of the saving from a small scheduled load-shift. Weigh it against an outbuilding TT route (if you have one) or re-deriving the supply with a battery inverter/charger. Either way it must be designed and proven by test by a competent person.

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_Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._
_Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._
_Status: reviewed. General information, not project-specific design advice._
_[How we source this](/methodology) — evidence hierarchy, confidence flags and source policy._

## Sources

1. BS EN IEC 61558-2-4:2025 — Safety of transformers… isolating transformers (BSI) — https://knowledge.bsigroup.com/products/safety-of-transformers-reactors-power-supply-units-and-combinations-thereof-particular-requirements-and-tests-for-isolating-transformers-and-power-supply-units-incorporating-isolating-transformers-for-general-applications-1 (cited by reference only; standard text not reproduced)
2. IET Wiring Matters — Mythbuster 12: separation and isolation — https://electrical.theiet.org/wiring-matters/years/2025/106-july-2025/mythbuster-12-a-galvanizing-tale-of-separation-and-isolation/
3. BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (§413/§418.3 electrical separation; §551; Chapter 54 — cited by clause only, not reproduced)
4. V2L Workshop technical reference (internal) — verified design facts and confidence flags