# 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. **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 - **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. ## 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. **Figure: SONOFF control of the interlocked changeover.** 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. _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._ ## 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. **Figure: Outbuilding TT — full sheet (power + control + earthing).** The complete outbuilding scheme on one page: interlocked contactor changeover, scheduled control, TT electrode and switched N-E bond — everything ends earthed. _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._ > **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 ## FAQ ### 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. --- _Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._ _Last reviewed: 15 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 60947-6-1 (transfer switching) — BSI — https://knowledge.bsigroup.com/products/low-voltage-switchgear-and-controlgear-multiple-function-equipment-transfer-switching-equipment-3 (cited by reference only; current UK edition :2023; IEC :2026 parent on UK-adoption watch) 2. BS EN IEC 60947-4-1 (contactors) — BSI — https://knowledge.bsigroup.com/products/low-voltage-switchgear-and-controlgear-contactors-and-motor-starters-electromechanical-contactors-and-motor-starters-4 (cited by reference only; current UK edition :2025)