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.
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.
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 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.
What the diagram shows: 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. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
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
- 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)
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).
- 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
Confidence: Inference rolled up across the clauses cited above (the strictest state wins).
Frequently asked questions
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.
- 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 (2)
- BS EN IEC 61558-2-4:2025 — isolating transformers (BSI) — cited by reference only
- BS 7671:2018+A4:2026 §413 / §418.3 (electrical separation) — IET/BSI — cited by clause only; notice wording not reproduced