# Scheduled changeover — the interlocked 2-pole contactor architecture

> To switch between grid and V2L on a **schedule** — not on power failure — you do not use a cheap voltage-sensing automatic transfer switch. You use a **timed smart relay** (in maintained mode) to command a **pair of two-pole contactors** that are **mechanically interlocked**: KM1 for grid (the de-energised, fail-safe default) and KM2 for V2L. The interlock is the hard guarantee that the two sources can never be paralleled. The changeover switches **both line and neutral**, operates **break-before-make** (the load drops for a moment, the source swaps, then re-energises), and falls back to grid on any loss of control power. The relay carries only the small coil current; the contactors carry the load. It must be designed and proven by test by a competent person; the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

**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

- Scheduling is **not** what an auto-ATS does — an auto-ATS senses *source loss*; you need a *commanded* changeover between two healthy sources.
- Correct architecture: **timed smart relay (maintained) → interlocked 2-pole contactor pair** — KM1 grid (de-energised default), KM2 V2L.
- The **mechanical interlock** is the hard anti-paralleling guarantee — an EV inverter cannot parallel the grid, so the two contactors must never close together.
- **Two-pole** switches line **and** neutral; **break-before-make** (open transition) means the load drops momentarily during transfer — never a bridged supply.
- The relay carries only **coil current**; the contactors carry the **load**. Confirm coil inrush is within the relay's contact rating; fuse the control circuit (e.g. 3 A).
- **Fail-safe to grid** on loss of control power. Designed and **proven by test** by a competent person; the manufacturer does not sanction this use.

**Where this stops:** This explains the architecture and the product standards behind it. It is not a wiring recipe — the design, build, installation and testing of the changeover are for a competent person.

## Why an automatic transfer switch is the wrong tool for a schedule

A cheap voltage-sensing automatic transfer switch (ATS) does one thing: it watches the normal supply and transfers to the alternate source when it sees the normal one **fail**. It decides for itself, on a couple of seconds of voltage loss, and auto-reverts when the supply returns. Its status terminals are *indicators*, not a control input — there is nothing on it to command. That behaviour is right for backup-on-failure, but it is the wrong behaviour for **scheduled load-shifting**, where you want to move between two perfectly *healthy* sources at set times of day (run the essential board off the car during expensive hours, return to grid before the next charge).

> **Scheduling is a commanded changeover, not a fault response** — Moving between two healthy sources on a timer is not what an auto-ATS does. That is a job for a **commanded, interlocked contactor changeover** — a timed relay that *tells* the changeover which source to select.

## The correct architecture — power and control

Use a **timed smart relay** in *maintained* mode to command a pair of two-pole contactors: **KM1** is the grid contactor and **KM2** is the V2L contactor. KM1 is wired as the **de-energised default**, so the resting, fail-safe state selects the grid. A daily schedule (for example, close to V2L during the expensive-rate window and open back to grid before the car needs charging) drives the relay; the relay's dry contact only *steers* which coil is energised. The relay carries the small **coil current**; the **contactors carry the load**. Keep the actual draw well within the V2L rating — sizing the essential board to roughly 80% of the source is sensible headroom.

**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._

At terminal level the smart relay is powered from the grid side and uses a single-pole changeover (SPDT) dry contact: the common feeds from grid line through a **control-circuit fuse** (for example 3 A), the normally-closed output drives the **KM1 (grid)** coil — the de-energised default — and the normally-open output drives the **KM2 (V2L)** coil. Because grid is the de-energised state, any loss of control power drops the changeover back to grid: **fail-safe**.

**Figure: SONOFF MINI-D — terminal-level control wiring.** Terminal-by-terminal: the relay is powered from grid L/N, its dry SPDT contact is fed via a 3 A fuse, and the de-energised state selects grid (fail-safe).

_A terminal-level view of the relay's nine terminals. The relay is powered from grid line (L) and neutral (N). The dry changeover contact has COM fed from grid L through a 3 A fuse (the relay's control-circuit requirement); NC routes to the grid contactor coil; NO routes to the V2L contactor coil. The S1/S2 and DC+/DC- terminals are unused. With the relay de-energised the COM–NC path selects grid, which is the fail-safe default._

> **Confidence: inference** — A small dry-contact smart relay can command this changeover but must not switch the load itself. (The relay's contact handles only the contactor coil current, while every load path here is a 63 A-class circuit. Confirm the contactor's coil inrush is within the relay's contact rating, and fuse the control circuit. This is a design inference from published coil/contact ratings, not a manufacturer wiring instruction — bench-confirm the actual parts.)

## The mechanical interlock — the hard anti-paralleling guarantee

The single non-negotiable rule is that the grid and the vehicle must **never** be connected together, even for an instant — an EV inverter cannot parallel the grid, and a brief overlap would be a fault. Control logic alone is not enough to guarantee that. The two contactors must be **mechanically interlocked** so that, physically, KM1 and KM2 cannot close at the same time. The interlock is the hard backstop; the relay logic merely chooses which one to close.

> **Never parallel the two sources** — A switched alternative source must not be paralleled with the distributor's network. The mechanical interlock — not software, not timing — is what makes one-source-at-a-time a physical certainty. Where a changeover is assembled from discrete contactors, the interlock and the auxiliary contacts are part of the design that must be proven by test.

> **Electromechanical contactors — interlock & endurance** (BS EN IEC 60947-4-1, confidence: inference) — The product standard a designer points to for the contactors: utilisation categories, electrical and mechanical endurance, and (where the changeover is built from discrete parts) the coils, auxiliary contacts and mechanical interlock that physically prevent KM1 and KM2 closing together. Endurance matters because a load-shifting scheme cycles the changeover daily. _Reference only; standard text not reproduced._

## Break-before-make, two poles

The transfer is **open-transition**: break-before-make. The load is dropped for a moment, the source is swapped, then the load is re-energised — the two supplies are never bridged. This is exactly the model the transfer-switching product standard is framed around, and it is what makes an interlocked changeover safe. The changeover must also be **two-pole**, switching **line and neutral** together, because the V2L output is a separately derived supply with its own neutral reference: leaving a shared neutral connected across the changeover would defeat the separation the earthing arrangement depends on.

> **Transfer switching equipment — open transition** (BS EN IEC 60947-6-1, confidence: inference) — Transfer switching equipment moves a load between a normal and an alternate supply; the standard's scope is framed around the load being interrupted during transfer (open transition). That is the formal basis for insisting a V2L changeover is break-before-make unless a competent designer has specifically engineered and proven otherwise. _Reference only; standard text not reproduced._

> **§537 (isolation and switching)** (BS 7671, confidence: inference) — Governs isolation and switching across the installation — the regulatory home of the changeover and of the lockable isolation a competent person uses before working on either source. The product-standard conformity of the switchgear does not by itself make the changeover compliant; the installation must still be designed and verified to BS 7671. _Reference only; standard text not reproduced._

Conformity of a contactor, isolator or off-the-shelf ATS to its product standard does **not** by itself make a V2L changeover compliant. The product standards say the box is *built and rated* correctly; BS 7671 says it is *wired and protected* correctly — including the contested PME/earthing arrangement covered on the earthing pages. Both are required, and both are proven by a competent person.

## How it sits with the earthing

The changeover and the earthing are one design, not two. A KM2 (V2L) auxiliary contact is the natural way to make the single V2L neutral-earth bond exist **only** while on V2L — the floating output's source reference, in circuit when KM2 is closed and removed when it opens. The full outbuilding sheet below shows power, control and earthing on one page; the earthing decision itself (TT outbuilding vs isolation transformer indoors) is covered on the dedicated earthing pages, and is always designed and proven by test by a competent person. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

**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._

- **Grid is the de-energised default (KM1)** — the resting state and any loss of control power select the grid. Fail-safe.
- **The relay commands; the contactors carry the load** — confirm coil inrush is within the relay's contact rating and fuse the control circuit (e.g. 3 A).
- **Two-pole, switching line and neutral** — a separately derived V2L supply needs its neutral switched, not shared.
- **Break-before-make** — the load drops momentarily during transfer; the sources are never bridged.
- **The mechanical interlock is the backstop** — software and timing choose; the interlock physically guarantees one source at a time.

### How this is made and proven compliant

**What governs it:** Switchgear product standards: BS EN IEC 60947-6-1 (transfer switching equipment), 60947-4-1 (contactors), 60947-3 (isolators / switch-disconnectors) — see the switchgear deep-dive; BS 7671 §537 (isolation and switching) and §536 (coordination); §551.7 (parallel-operation provisions, avoided by an open-transition changeover); Anti-paralleling duty for a switched alternative source: the ESQCR principle that an alternative source cannot be paralleled with the distributor's network, applied via BS 7671

**Who may do it:** Design, build, installation, inspection and testing by a competent person. Adding an inlet circuit, a changeover/transfer switch or a consumer-unit alteration is normally notifiable under Part P (England; Wales/Scotland/NI differ).

**How compliance is demonstrated:** Mechanical interlock proven by physical test — KM1 and KM2 cannot be made to close together; Break-before-make (open-transition) confirmed: the load is de-energised during transfer, never bridged across both sources; Two-pole switching of line AND neutral verified in both grid and V2L positions; Fail-safe to grid confirmed on loss of control power (the de-energised state selects KM1); Contactor coil inrush confirmed within the smart relay's contact rating; control circuit fused (e.g. 3 A); Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate

## FAQ

### Why can't I just use a cheap automatic transfer switch on a timer?

Because an auto-ATS senses *source loss*, not a schedule, and has no control input to command. For scheduled switching between two healthy sources you need a timed smart relay driving an interlocked two-pole contactor changeover.

### What is the mechanical interlock for — isn't the timer enough?

No. The timer and relay only *choose* which contactor closes. The **mechanical interlock** physically prevents KM1 and KM2 closing together, so the grid and the car can never be paralleled even briefly — that one-source-at-a-time guarantee must be a physical certainty, not a software promise.

### Why two-pole, and why break-before-make?

**Two-pole** switches line and neutral together, because the V2L output is a separately derived supply with its own neutral reference. **Break-before-make** (open transition) means the load is dropped for a moment during transfer, so the two supplies are never bridged — the safe model the transfer-switching standard is built around.

### Does the smart relay carry the household load?

No. The relay carries only the small **contactor coil current**; the **contactors** carry the load. Confirm the contactor's coil inrush is within the relay's contact rating and fuse the control circuit (for example 3 A). Using a small dry-contact relay to switch a 63 A-class load directly is wrong and unsafe.

### What happens on power failure or a tripped relay?

The changeover is wired so grid is the **de-energised default** (KM1). Any loss of control power, or a failed relay, drops the changeover back to the grid — it is fail-safe to grid, not to the car.

### Does this need DNO notification?

An island-only changeover that genuinely cannot parallel or export is generally outside the grid-parallel ENA G98/G99 regime, but whether a given changeover truly prevents parallel operation is a design-and-test matter for the competent person and the DNO. The exact G98/G99 locators are Not confirmed here — confirm before relying on the carve-out.

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_Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._
_Last reviewed: 15 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 60947-6-1 — Transfer switching equipment (BSI Knowledge) — https://knowledge.bsigroup.com/products/low-voltage-switchgear-and-controlgear-multiple-function-equipment-transfer-switching-equipment-3 (cited by designation/clause only; current UK edition :2023; IEC :2026 parent on UK-adoption watch; standard text not reproduced)
2. BS EN IEC 60947-4-1 — Electromechanical contactors & motor-starters (BSI Knowledge) — https://knowledge.bsigroup.com/products/low-voltage-switchgear-and-controlgear-contactors-and-motor-starters-electromechanical-contactors-and-motor-starters-4 (cited by designation/clause only; current UK edition :2025; standard text not reproduced)
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/ (§536/§537/§551 cited by clause only; standard text not reproduced)
4. V2L Workshop switchgear standards register (internal) — editions, currency caveats and confidence flags
5. V2L Workshop technical reference (internal) — verified design facts and confidence flags for the power/control architecture