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V2L Workshop

Specifying a compliant changeover — transfer-switching & contactor standards

A V2L changeover must move the load between grid and the car without ever bridging them — open-transition, break-before-make. The closest product standard is BS EN IEC 60947-6-1:2023 (transfer switching equipment), whose scope is framed around exactly that interrupted transfer. Where the changeover is built from discrete contactors, BS EN IEC 60947-4-1:2025 governs them and the mechanical interlock; 60947-3 covers the isolators. Conformity to these proves the box is built and rated correctly — BS 7671 still governs whether it is wired and protected correctly.

In short

  • Open-transition (break-before-make) is the safe V2L model — the load is dropped, the source swapped, then re-energised, never bridged.
  • BS EN IEC 60947-6-1 (transfer switching equipment) is the closest single product standard — its scope is framed around interrupted transfer.
  • BS EN IEC 60947-4-1 governs the contactors and the mechanical interlock; size for the load and daily cycling.
  • BS EN IEC 60947-3 covers isolators/switch-disconnectors declared suitable for isolation.
  • Edition caveat: 60947-4-1:2025 is BSI-confirmed; 60947-6-1:2023 remains the current UK transfer-switching edition, with IEC 60947-6-1:2026 on UK-adoption watch.

Where this stops: This explains which standards a compliant changeover declares against. The selection, interlock and installation are competent-person work — conformity to a product standard is not the same as a compliant installation.

Not yet confirmed on this page

Some details below depend on sources still being verified against the published standard, so we mark them Not confirmed rather than guess:

Open-transition is the whole point

An EV inverter cannot parallel the grid, so the changeover must never connect both sources at once. The safe model is open transition: drop the load, swap the source, re-energise. BS EN IEC 60947-6-1's scope is framed around exactly this interrupted transfer — which is the formal basis for insisting a V2L changeover is break-before-make unless a competent designer has specifically engineered otherwise.

The smart relay only commands: its single SPDT contact energises either the grid contactor (KM1, the de-energised fail-safe default) or the V2L contactor (KM2) — never both, because they are mechanically interlocked.

What the diagram shows: A SONOFF MINI-D relay in maintained mode has one changeover (SPDT) contact: COM, NC and NO. Grid line (L) feeds COM. The NC (normally-closed) output drives the KM1 coil — the grid contactor — so on power loss the load falls back to grid (fail-safe). The NO (normally-open) output drives the KM2 coil — the V2L contactor. Both coil returns go to grid neutral (N). KM1 and KM2 are mechanically interlocked so they can never close together. A schedule (e.g. 05:30 to V2L, 23:30 back to grid) drives the relay. The relay carries only the small coil current; the contactors carry the load. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.

Built from contactors, or a packaged unit

You can specify a packaged transfer switch (declare to 60947-6-1) or build the changeover from a mechanically interlocked contactor pair (60947-4-1). The interlock is the hard anti-paralleling guarantee. Because a load-shifting scheme can cycle the changeover daily, the contactors' electrical and mechanical endurance ratings matter.

The complete outbuilding scheme on one page: interlocked contactor changeover, scheduled control, TT electrode and switched N-E bond — everything ends earthed.

What the diagram shows: A single-sheet assembly of the outbuilding TT scheme. Power: grid line/neutral (L/N) and V2L L/N feed an interlocked two-pole contactor changeover (KM1 grid / KM2 V2L) into a small essential board. Control: a scheduled smart relay drives the interlocked coils, fail-safe to grid. Earthing: a local electrode feeds the outbuilding earth bar and all CPCs continuously; the PME earth is not exported; the V2L neutral-earth bond is switched in via a KM2 auxiliary contact; an RCD protects the board. The single key point: the load is dropped, the source swapped, and re-energised — never bridged — and the installation is earthed in every state. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.

Not confirmed

IEC 60947-6-1:2026 exists at international level, but UK BS EN adoption was not located on BSI Knowledge. Keep citing the current UK BS EN IEC 60947-6-1:2023 transfer-switching edition until BSI changes.

Conformity is not compliance

A device conforming to a 60947 part is built and rated correctly. It does not by itself make the installation compliant — BS 7671 still governs the wiring, protection, isolation (§537) and the contested earthing arrangement, all proven by a competent person.

How this is made and proven compliant

What governs it
  • BS EN IEC 60947-6-1 (transfer switching), 60947-4-1 (contactors), 60947-3 (isolators), 60947-2 (breakers)
  • BS 7671 §537 (isolation & switching) for the installation
Who may do it

A competent person specifies the changeover, proves the interlock, and verifies the installation to BS 7671. Notifiable under Part P (England).

How compliance is demonstrated
  • Specify devices declaring conformity to the relevant 60947 part (confirm the current UK edition on BSI)
  • Prove the interlock is mechanical and break-before-make
  • Verify isolation and switching to BS 7671 §537; confirm breaker coordination on the V2L fault current
Confidence & currency

Confidence: Inference rolled up across the clauses cited above (the strictest state wins).

Frequently asked questions

Can I use a cheap automatic transfer switch for scheduled V2L?

No — a voltage-sensing auto-ATS transfers on source loss and has no schedule input. Scheduling needs a timed relay commanding an interlocked contactor changeover, which is what the switchgear standards here describe.

Why does break-before-make matter so much?

An EV inverter cannot synchronise to the grid, so bridging the two sources is dangerous. Open-transition (break-before-make) and a mechanical interlock ensure they are never connected together.

Which 60947 edition should I cite?

Cite BS EN IEC 60947-4-1:2025 for contactors and BS EN IEC 60947-6-1:2023 for transfer switching. IEC 60947-6-1:2026 exists, but UK BS EN adoption is not confirmed here.

Martin — Qualified UK electrician; BEng Mechanical Engineering; qualified vehicle mechanic.

Last reviewed
15 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)
  1. BS EN IEC 60947-6-1 (transfer switching) — BSI — cited by reference only; current UK edition :2023; IEC :2026 parent on UK-adoption watch
  2. BS EN IEC 60947-4-1 (contactors) — BSI — cited by reference only; current UK edition :2025