Simultaenous Contact

Just out of curiosity, does anyone know of any instances at all of option 722.413 being used for real? (i.e. use of electrical separation to supply EVSE - e.g. as per fig A722).

  - Andy.

For full fault protection you probably want both the two pole RCD drawn and a three pole RCD encompassing L, N and PC between the two pole RCD and the transformer / N--PC link.

It is unclear why the PC exists as it looks like an opportunity for fortuitious earthing which is the opposite of what we want here.

DShutt said:

It is unclear why the PC exists as it looks like an opportunity for fortuitious earthing which is the opposite of what we want here.

It'll be because the EV power protocols check for the presence of an working earth connection before allowing charging to commence. Without the PC (and N-PC link) the EVSE/car would refuse to play at all.

 For full fault protection you probably want both the two pole RCD drawn and a three pole RCD encompassing L, N and PC between the two pole RCD and the transformer / N--PC link.

In what situation would the 3-pole RCD "see" any imbalance? (given the N point isn't earthed)

   - Andy.

You need the PC (it connects to car metalwork) to N loop to be detected as low impedance for the car to decide it is safe to charge. (some cars struggle on TT supplies with a high electrode resistance)
There is also a pulsed DC signal/ pilot line between car and charger that uses the PC as its return and reference voltage. 

There are a great many things that would have been easier if the charging car could have been a class II (double insulated ) device in terms of its mains connection, much like more or less all other outdoor electrical items that are hand held and/or  easy to touch, and if the  comms between car and charger had been isolated, like ethernet or even fibre. 

There is a general problem with isolation using traditional high power transformers, that revolves around the unavoidable inter-winding capacitance, and its why the middle of the builder's 110V is earthed to make 55-0-55 rather than fully floated.  

There is instead a massive EMC problem with the smaller lighter switch- mode equivalents running at hundreds of KHz, using the squarest possible switching waveform to minimise the time of dissipation of being neither on nor off. 

The filters to deal interference with this tend to need an earth connection, and around 600W to 1kW is where current SMPS designs generally stop being class II and becomes class I, but like all things  electronic the limit of the possible is a bit of a moving target.

Mike

Hi,

AJJewsbury said:

It'll be because the EV power protocols check for the presence of an working earth connection before allowing charging to commence. Without the PC (and N-PC link) the EVSE/car would refuse to play at all.

Ah, of course.  I definitely think cars would be better off as class II devices and I think a transition is probably possible but I doubt there is the will.

AJJewsbury said:

In what situation would the 3-pole RCD "see" any imbalance? (given the N point isn't earthed)

An imbalance would be seen in case where there was capacitive coupling or insulation breakdown in the tranformer windings  Whether an RCD is the correct way of addressing that risk is a different question.

mapj1 said:

Probably not much change from a cube of side 40cm and a weight of ~ 75kg by the time you add the windings and the enclosure.

Not a bad guess! - e.g. https://www.farnell.com/datasheets/3744957.pdf

Toroidal ones seen to come in at about half the weight though - e.g. https://www.victronenergy.com/upload/documents/Isolation_Transformer_8000W/108939-Isolation_transformers-pdf-en.pdf 

It's not clear whether either actually meet BS EN 61558-2-4 though and even those make the charge point itself look sensibly priced.

   - Andy.

DShutt said:

For full fault protection you probably want both the two pole RCD drawn and a three pole RCD encompassing L, N and PC between the two pole RCD and the transformer / N--PC link.

It is unclear why the PC exists as it looks like an opportunity for fortuitious earthing which is the opposite of what we want here.

No, definitely not a 3-pole device. 

The PC is required to do two things:

(a) As others have said, permit EVSE (charging points) and EVs that incorporate protective conductor monitoring to actually permit charging; and

(b) As the EV is Class I equipment, and EVSE may be, or incorporate, Class I equipment, it is absolutely needed to permit the RCD to operate for a fault between a live conductor and an exposed-cponductive-part given (a).

This type of system, with protective conductors connected to a live conductor, and possible Earth (but not necessarily) is used quite frequently with temporary generating sets, and is described in BS 7430:2026 as 'IN-S'.

The RCD located immediately after the source, with the live conductor to PC referencing point on the source-side of the RCD, is absolutely vital for safety, to prevent a very nasty surprise, if the live conductor not referenced to the PC becomes accidentally Earthed (say if a cable is partially damaged).

No, definitely not a 3-pole device.

The PC is required for two reasons.

First, as others have said, it allows EVSEs and EVs with protective conductor monitoring to recognise a valid protective conductor and permit charging.

Second, because the EV is Class I equipment, and the EVSE may also be, or incorporate, Class I equipment, the PC is needed to allow the RCD to operate for a fault between a live conductor and an exposed-conductive-part.

This sort of arrangement, with protective conductors connected to a live conductor and possibly to Earth, but not necessarily so, is used quite often with temporary generating sets. It is described in BS 7430:2026 as “IN-S”.

The RCD immediately after the source is absolutely vital, with the live-conductor-to-PC referencing point on the source side of that RCD. Otherwise there is a real risk of a nasty surprise if the live conductor that is not referenced to PC becomes accidentally earthed, for example through a partially damaged cable.

Thanks Mike, that is a very useful explanation.

I agree that using an isolating transformer may remove the PEN-loss issue, but it does not automatically make the arrangement risk-free. The floating secondary is an important point, especially if the vehicle body and the secondary side can rise to an undefined potential relative to true earth.

The comparison with portable generators is helpful. Unless the secondary arrangement is properly referenced, monitored, and protected, a fault from the secondary side to earth could create a hazardous touch-voltage situation around the vehicle.

The size and practicality are also significant. A 7 kW transformer is not a small domestic accessory; once weight, enclosure, losses, ventilation, fault protection, and installation requirements are considered, it may become less attractive than using a compliant EV charging arrangement designed for open-PEN protection.

So I would say the transformer idea may be technically possible, but only if designed as a complete protective system, not simply as a way to avoid PEN-loss concerns. It would still need proper earthing strategy, fault protection, and compliance with the relevant wiring regulations.

Terry osifo said:

It would still need proper earthing strategy

Or rather unearthed strategy - the whole point being to avoid the problems from (differing) earth references. Separated systems (and variants such as IN-S) are well known and long recognised by wiring regs - as well as portable generators you can look at shaver sockets (and other situations where one item of current using equipment is used) section 413 of BS 7671, similar to multiple items (418.3), unearthed supplies (including by transformer fed from the grid supply & a  N-PC link feature) in mobile and transportable situations (section 717) and of course the specific situation here - in section 722.

Nothing involving LV electricity is ever entirely risk-free (especially in 2nd fault or double fault from single event situations)- the damaged flex on conductive soil is a classic risk even in conventional ADS systems - the usual mitigation being the addition of a 30mA RCD. Sections 722 and 717 take exactly the same approach where the separated installation extends outdoors.

   - Andy.

Thanks Andy, that is a useful clarification.

I take your point that in this case the objective is not to establish another earth reference, but rather to maintain separation and avoid the risks associated with differing earth potentials and an open PEN condition. The examples you have given from BS 7671, including Sections 413, 418, 717 and 722, show that separated and unearthed systems are already recognised approaches where the conditions are appropriate.

My concern was less about the principle of electrical separation itself and more about ensuring that the complete installation remains safe under foreseeable fault conditions. As you say, no LV system is entirely free from risk, and the regulations generally rely on a combination of separation, fault protection, RCD protection and other measures to reduce risk to an acceptable level.

I agree that if a separated supply arrangement is designed in accordance with the relevant requirements of BS 7671, then the discussion becomes one of demonstrating compliance and managing residual risks, rather than simply whether the transformer removes the PEN-loss issue in isolation.

Thanks for the references they are helpful in putting the proposal into the wider context of established wiring system design.

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Thanks Andy, that is a useful clarification.

I take your point that in this case the objective is not to establish another earth reference, but rather to maintain separation and avoid the risks associated with differing earth potentials and an open PEN condition. The examples you have given from BS 7671, including Sections 413, 418, 717 and 722, show that separated and unearthed systems are already recognised approaches where the conditions are appropriate.

My concern was less about the principle of electrical separation itself and more about ensuring that the complete installation remains safe under foreseeable fault conditions. As you say, no LV system is entirely free from risk, and the regulations generally rely on a combination of separation, fault protection, RCD protection and other measures to reduce risk to an acceptable level.

I agree that if a separated supply arrangement is designed in accordance with the relevant requirements of BS 7671, then the discussion becomes one of demonstrating compliance and managing residual risks, rather than simply whether the transformer removes the PEN-loss issue in isolation.

Thanks for the references they are helpful in putting the proposal into the wider context of established wiring system design.

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