Scheduled changeover — the interlocked 2-pole contactor architecture

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).

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.

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.

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.

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.

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.

• 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.