# Isolation transformers for indoor V2L — BS EN IEC 61558 and the floating-output fix > Indoors you cannot drop an earth spike for one sub-board — keep the single PME/MET earth. Instead, a **double-wound isolating transformer to BS EN IEC 61558-2-4** in the V2L feed galvanically separates the floating output, and a single neutral-earth bond on the **secondary** restores a defined earth and RCD operation. Use the full-LV part **-2-4** (full low-voltage output), not the **-2-6** safety-isolating part (extra-low-voltage / SELV output) which is the wrong fit for 230 V backup. BS 7671 §413 / §418.3 (electrical separation) — not the 61558 series — governs whether and how this is valid. **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 - Use a **BS EN IEC 61558-2-4** double-wound isolating transformer (full LV) — **not** the **-2-6** safety-isolating (SELV) part. - It galvanically separates the floating V2L output so the **secondary** can carry one controlled neutral-earth bond. - The earth stays continuous to the **MET** — no second electrode, no indoor TT island (which would be a simultaneous-reach hazard). - BS 7671 **§413 / §418.3** (electrical separation) governs whether/how it is a valid protective measure — designed and proven by test. - Trade-off: a ~3.6 kVA isolating transformer is costly, heavy and has standing losses — it may erode the saving for a small load. **Where this stops:** This identifies the right transformer and the BS 7671 separation rules. Whether electrical separation is a valid protective measure for a given setup is a competent-person design judgement, proven by test. ## Why a transformer, not a spike, indoors A floating V2L output has no neutral-earth reference, so an RCD cannot operate. Outdoors you can make a separate TT island; indoors you cannot, because a single TT sub-board sits in simultaneous reach of the rest of the PME-earthed installation — a shock hazard. The answer is to keep the one house earth and break the floating-source problem with galvanic separation. **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._ ## The right part: -2-4, not -2-6 The **-2-4** part covers full-LV isolating transformers (double-wound, output that can exceed extra-low-voltage limits) — the correct choice for a 230 V backup circuit. The **-2-6** part covers safety isolating transformers whose secondary is limited to SELV (extra-low voltage) — that is the wrong fit, because the load needs full LV. Both sit on the base **-1** part for construction, insulation and dielectric strength. **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._ ## BS 7671 governs the design The 61558 series tells you the transformer is fit for purpose; **BS 7671** tells you whether and how electrical separation or re-earthing is a valid protective measure here — §413 for a single item and §418.3 for more than one, the latter with a required warning notice (not reproduced). The floating-output re-earthing rationale is engineering inference, not a single clause — so keep the design-and-prove-by-test wrapper, and confirm against the in-force BS 7671 edition. ### How this is made and proven compliant **What governs it:** BS EN IEC 61558-2-4:2025 (product fitness) + base BS EN IEC 61558-1:2019; BS 7671 §413 / §418.3 (electrical separation as a protective measure) **Who may do it:** A competent person decides whether electrical separation / controlled re-earthing is valid for the specific V2L setup, and designs and proves it by test. Notifiable under Part P (England). **How compliance is demonstrated:** Specify a 61558-2-4-compliant transformer rated for the load; One neutral-earth bond on the secondary, made only on V2L; earth continuous to the MET; RCD operation proven by test on the secondary side; separation arrangement verified ## FAQ ### Will any isolation transformer do? Use a double-wound isolating transformer to BS EN IEC 61558-2-4 (full LV), rated for the load — not a -2-6 safety-isolating (SELV) unit, which outputs extra-low voltage and cannot feed a 230 V backup circuit. ### Is the indoor transformer method always worth it? Not always. A ~3.6 kVA isolating transformer is costly, heavy and has standing losses, which can erode the saving for a small essential load. It is a design trade-off for a competent person. ### Does the transformer remove the need for BS 7671 design? No. The transformer is a product; BS 7671 §413/§418.3 decides whether the separation is a valid protective measure, and the arrangement must be proven by test. --- _Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._ _Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._ _Status: published. 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 — 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) 2. BS 7671:2018+A4:2026 §413 / §418.3 (electrical separation) — IET/BSI (cited by clause only; notice wording not reproduced)