# The wrong plan — why you cannot fool an ATS into selecting your car

> You cannot make a cheap voltage-sensing automatic transfer switch (ATS) select your EV on command. A cheap ATS senses **source loss**, not a schedule, and its status terminals are indicators, not a control input — so there is nothing to command. A Wi-Fi smart relay is only a roughly 2 A dry contact, while every supply path here is around 63 A, so it cannot switch or interrupt them. And even if you forced the changeover, you would island a 63 A board onto a roughly 15 A floating source — a gross overload with no earth reference. Scheduled switching needs a different device: a timed relay **commanding** an interlocked two-pole contactor changeover, designed and proven by test by a competent person.

**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

- A cheap automatic transfer switch (ATS) senses *source loss* and auto-reverts — it has no control input, so a smart relay has nothing to command.
- A Wi-Fi smart relay is a ~2 A dry contact; every supply path here is ~63 A — orders of magnitude too small to switch or interrupt.
- Forcing the changeover would island a 63 A board onto a ~15 A **floating** source: gross overload plus no earth reference.
- Scheduled switching is a different job — a timed relay **commanding** an interlocked two-pole contactor changeover, break-before-make.
- Designed and **proven by test** by a competent person. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

**Where this stops:** This explains why the popular shortcut fails and points to the correct device. It is not a wiring recipe — the design, installation and testing are for a competent person.

## The plan most people start with

The popular idea is neat on paper: buy a cheap 63 A automatic transfer switch (ATS), add a Wi-Fi smart relay, and use the relay to 'fool' the ATS into selecting the car on a timer — charge cheap overnight, then run a small board off the EV during expensive hours. It does not work, and it is worth understanding exactly why before you spend a penny. The short version: an ATS is the wrong *kind* of device for scheduled switching, the smart relay is far too small to switch the power, and forcing it would create two hazards at once.

> **Answer first** — A cheap ATS senses source loss, not a schedule, and has no control input — there is nothing for a smart relay to command. And a ~2 A relay cannot switch a ~63 A supply path anyway. Scheduled switching is a different job: a timed relay **commanding** an interlocked two-pole contactor changeover.

## Why it fails — three separate reasons

### 1. The ATS decides for itself — there is nothing to command

A cheap voltage-sensing ATS is self-sensing. It watches its normal (priority) input, transfers to the alternate source in a couple of seconds when that input *fails*, and auto-reverts when it returns. That behaviour is built around **source loss**, not a schedule. The status terminals on these units are **indicators** — they tell you which side is live — not a control input you can drive. There is simply no 'switch now' command to send. This is the behaviour the relevant product standard describes for transfer switching equipment.

> **60947-6-1** (BS EN IEC, confidence: inference) — Transfer switching equipment moves a load between a normal (grid) and an alternate (V2L) supply — manual, remote or automatic. The automatic variant acts on the state of its inputs (typically source loss), and its scope is framed around open-transition. A plain auto-ATS therefore reacts to a dead source, not to your timetable. _Reference only; standard text not reproduced._

Most of the affordable single-box ATS units on the market make this worse, not better. Their documented logic keeps **normal-source priority** in automatic mode — the grid wins whenever it is healthy — so even a unit that *does* expose a remote input will not freely 'pick grid or EV whenever I say so' in its ordinary auto behaviour. They are designed for backup, not for arbitrary scheduled source selection.

### 2. The smart relay is far too small to switch the power

A Wi-Fi smart relay (the kind people reach for) is a roughly 2 A dry contact intended for control-circuit signalling. Every power path in this arrangement is around 63 A — the consumer-unit scale. Asking that contact to switch or interrupt a 63 A supply is orders of magnitude beyond its rating; it would not make or break the load safely, and would likely weld or burn. A relay like this is fine for one job only: **commanding** a properly rated device, never carrying the load itself.

> **Confidence: verified** — A ~2 A dry-contact smart relay cannot switch or interrupt the ~63 A power and sense paths in this arrangement. (The contact rating (a few amps) is two orders of magnitude below the 63 A consumer-unit-scale paths. Confirmed across multiple unit manuals in the device research. The relay's only valid role here is low-current control signalling.)

### 3. Even if you forced it — overload and no earth reference

Suppose you somehow bridged the switching. You would now connect a 63 A consumer unit to an EV's V2L output of roughly 10–15 A (around 2.3–3.6 kW at 230 V). That is a **gross overload** — the whole house demanding far more than the car can deliver. Worse, an EV's V2L output is commonly **floating**: there is no internal neutral-earth bond, so an RCD has nothing to operate against. The output would pass a plug-in socket tester yet offer no real shock protection. You would have islanded onto an over-stressed, unreferenced supply.

> **Whole-house onto V2L is a gross overload** — A typical consumer unit is rated around 63 A; an EV's V2L output is around 10–15 A. The realistic job is a small *essential* board — lighting and one radial socket — not the whole house. And a floating output gives no RCD protection until a single neutral-earth bond is correctly made on the source side.

> **Not confirmed (safety-critical):** That the Hyundai Ioniq 5's V2L output is floating (no internal neutral-earth bond) is a field report and varies by adapter — treat the output as floating until bench-verified on the actual adapter.

## The correct device — command, don't fool

Scheduled switching between two *healthy* sources is a different job from an auto-ATS's source-loss reaction. The correct device is a **timed relay commanding an interlocked two-pole contactor changeover**: KM1 the grid contactor (the de-energised, fail-safe default) and KM2 the V2L contactor. The relay carries only the small coil current; the contactors carry the load. A **mechanical interlock** is the hard guarantee that the two can never close together — break-before-make, so grid and vehicle are never paralleled (an EV inverter cannot parallel the grid in any case).

> **60947-4-1** (BS EN IEC, confidence: inference) — Contactors and their utilisation categories and endurance — the product standard behind the KM1/KM2 changeover. Endurance matters here because a load-shifting scheme may cycle the changeover daily. _Reference only; standard text not reproduced._

**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._

Keeping grid and vehicle physically separate at all times is not just good practice — it is what keeps a domestic V2L island out of the grid-parallel regime, where a DNO and additional protection would come into play. A genuine interlocked, break-before-make changeover is the proof that the two sources can never bridge.

> **§551.7** (BS 7671, confidence: inference) — Where a source could run in parallel with the public supply, additional requirements apply. A break-before-make changeover avoids paralleling and keeps most domestic V2L out of this regime — but it has to be a true interlocked changeover, designed and proven by test. _Reference only; standard text not reproduced._

That is the whole picture on one sheet: the scheduled control, the interlocked two-pole power changeover, and — because the V2L output is floating — the earthing that gives the island a single neutral-earth reference. The full design (power, control and earthing) and the floating-output earthing decision are covered on the cornerstone and changeover pages; this page exists only to retire the ATS-fooling idea before you waste money on it.

**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._

> **What to take away** — Don't try to fool an ATS. Use a timed relay to **command** an interlocked two-pole contactor changeover, fail-safe to grid, break-before-make — designed, installed and proven by test by a competent person. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

### How this is made and proven compliant

**What governs it:** BS EN IEC 60947-6-1 (transfer switching equipment / ATS) and 60947-4-1 (contactors) for the changeover hardware — see the changeover deep-dive; BS 7671 §551 island / switched-alternative source provisions (esp. §551.7 anti-paralleling); BS 7671 §722 PME / open-PEN earthing for the V2L side, and Part P for notification (England; Wales/Scotland/NI differ)

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

**How compliance is demonstrated:** The changeover is proven open-transition (break-before-make) by a mechanical interlock — grid and vehicle can never close together; Contactor coil inrush confirmed within the commanding relay's contact rating; RCD operation proven by test in **both** grid and V2L modes; Confirm the actual adapter's neutral-earth behaviour on the bench before relying on it

## FAQ

### Can I use a smart relay to make the ATS switch to my EV?

No. A cheap ATS senses source loss and has no control input, and a ~2 A smart relay cannot switch a ~63 A supply path. Scheduling needs a timed relay **commanding** an interlocked two-pole contactor changeover — the relay drives only the coils, the contactors carry the load.

### Why won't a cheap automatic transfer switch just do what I tell it?

Because it is a self-sensing device, not a commanded one. It transfers when its priority (grid) input *fails* and auto-reverts when it returns. Its status terminals are indicators, not a control input, and most affordable units keep the grid as priority in auto mode. There is nothing to command.

### Some ATS units advertise a 'remote' input — doesn't that solve it?

Only partly, and not as a free-form selector. The best-documented affordable single-box unit does expose remote/manual control, but its published logic still gives the normal (grid) source priority in automatic mode. You would be using it as a remote/manual changeover, not a plain auto selector — and the safer, cheaper route remains a properly interlocked contactor pair.

### What happens if I just force it onto the car anyway?

Two hazards at once. You island a ~63 A consumer unit onto a ~10–15 A V2L output (a gross overload), and onto a typically **floating** output that gives no RCD protection until a single neutral-earth bond is correctly made on the source side. The output passes a socket tester but is not safe.

### So what is the right device?

A timed relay commanding an **interlocked two-pole contactor changeover**: KM1 grid (fail-safe default), KM2 V2L, mechanically interlocked so they can never close together (break-before-make). It must be designed and proven by test by a competent person; the manufacturer does not sanction back-feeding fixed wiring from V2L.

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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 series — LV switchgear and controlgear (transfer switching 60947-6-1; contactors 60947-4-1) (BSI) — https://knowledge.bsigroup.com/products/low-voltage-switchgear-and-controlgear-multiple-function-equipment-transfer-switching-equipment-3 (cited by clause/standard only; 60947-6-1 current UK edition :2023; IEC :2026 parent on UK-adoption watch)
2. BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (§551 switched-alternative/anti-paralleling provisions cited by reference only; standard text not reproduced)
3. V2L Workshop device research (internal) — affordable UK source-selection devices; prices/availability unverified (ATS self-sensing behaviour, normal-source-priority logic, and the contactor-pair route)
4. V2L Workshop technical reference (internal) — verified design facts and confidence flags