AJJewsbury said:

Wouldn't a typical open-PEN device -which wouldn't allow the PE contacts to be open while any of the live contacts were closed -  meet 543.3.3.101 (ii) or (iii)?

The switching device would, but it's not simply a switching device. If it were that simple, I'm sure it would already have been implemented in 717.411.4.1.

In this particular case, refrigerated unit, the potential losses are a key consideration for unwanted operation.

mapj1 said:

Putting the regs aside for a moment, electrically this is similar to a caravan, albeit a very cold one, where no one sleeps (though some caravan holidays have felt like sleeping in an ice box, so maybe not so different) and normally for that PME would not be permitted.

That is a key consideration.

mapj1 said:

The risks are about the same as a car on charge, as the body of the vehicle - I presume - is at mains earth potential while connected to shore power, and folk do presumably touch the thing as part of normal operations of loading and unloading.

I think you have said yourself why the risks are NOT "about the same as a car on charge" - more risk of human contact, larger contact area - generally more probability of human contact if something does go wrong.

Where trailer plug-in facilities are provided at trailer docks as represented in the photo, here, at least, they are normally on TN-S simply because the site normally has its own tx due to the significant current demand refrigeration plant requires.

lyledunn said:

at least, they are normally on TN-S simply because the site normally has its own tx

And whilst the basic EV charging guidance is "treat TN-S in the public supply like PME" the guidance also states that if you have TN-S from a private a transformer, this can (and should) be treated as TN-S and NOT PME.

AJJewsbury said:

but there's no explicit recognition for any of those situations.

Probably because those situations are considered prohibited by ESQCR and BS 7671 can't contradict legislation.

AJJewsbury said:

I guess you could go for a deviation,

But legislation takes precedence.

But BS 7671 still is used in several countries where ESCQR and it's NE bond clause does not apply, and sometimes within building networks, which can be pre-metering so not technically 'consumer' installation, and yet to ' 7671 as requested by the DNOs  - we should not be too presumptive.

And, in reality, legislation, or regs, or not, there are often in effect multiple NE bonds within private sites.

Mike.

mapj1 said:

But BS 7671 still is used in several countries where ESCQR and it's NE bond clause does not apply, and sometimes within building networks, which can be pre-metering so not technically 'consumer' installation, and yet to ' 7671 as requested by the DNOs  - we should not be too presumptive.

Use of BS 7671 internationally (that is, outside the UK) is not of concern. Those users know that BS 7671 aligns with UK legislation. We cannot account for other nations in our national standard. It would be presumptive of those who use our standard internationally to consider otherwise?

mapj1 said:

or not, there are often in effect multiple NE bonds within private sites.

It's worth expanding on that, because within an installation it can cause unwanted operation of RCDs, as well as circulating currents (EMC problems).

I would accept there are multiple PE to Earth connections - it may also be the case that some cables on the supply transformer side might effectively be a CNE conductor (dependent on how you view it, ie Private PNB, but there should be no further N-PE connections downstream of the main switchboard?

Well, agree, but I'm not sure how well the average Qatari electrician knows ESCQR .

Mike.

mapj1 said:

Well, agree, but I'm not sure how well the average Qatari electrician knows ESCQR .

Not much of a problem, I believe ... I understand most built-up areas are TN-S in Quatar, but they also use TT, so the PME (TN-C-S) bits aren't relevant.

In addition, there are local Qatar Electricity Wiring Code to apply on top of BS 7671 ... for example, line conductors RD, YE, BU, with BK neutral !

(I've had experience in electrical installations in the Middle East. Most difficult thing is balancing fire safety requirements which follow NFPA with electrical safety and mechanical requirements which follow British Standards plus CENELEC/IEC ... the two are not necessarily compatible)

Probably because those situations are considered prohibited by ESQCR and BS 7671 can't contradict legislation.

I'm not following you there Graham. The ESQCR only mentions caravans and boats. I don;t think it would float, and the usual definition of a caravan is something along the line of "a structure which is designed or adapted for human habitation and is capable of being moved from one place to another" - and I'm pretty sure refrigerated containers aren't intended for human habitation at all (just walking in and out occasionally to load/unload isn't habitation to my mind)..

For sure they carry similar risks to a habitable caravan - and engineering judgement way well err towards treating them similarly, but that's not quite the same as saying the law demands you must. Even section 717 permits PME in certain situations (717.411.4 (i) and (ii)) - as you say it could hardly do that if the ESQCR's blanket ban applied.

If anything a car or bus is much more like a caravan than a container (they are at least intended for human occupation, especially by unskilled and unsupervised persons) - if we consider PME banned for containers, we'd be on a slippery slope using it for EV charging.

  - Andy.

AJJewsbury said:

I'm not following you there Graham. The ESQCR only mentions caravans and boats.

The DTI Guidance on the ESQCR (page 20), states:

Special consideration should be given to the earthing and protection arrangements for certain installations where reliance on the connection of the consumer’s protective conductor with the distributor’s combined neutral and protective conductor could result in more significant risks.

AJJewsbury said:

If anything a car or bus is much more like a caravan than a container

Except that many caravans, and motor homes, when in use, are often found in muddy fields. With 'containers' it could be almost anything, including use for construction site offices in muddy fields ... but containers like the trailer will have much larger metal surfaces than a car.

Hence, though, why a HSL Report was commissioned ... to inform Section 722.

AJJewsbury said:

if we consider PME banned for containers, we'd be on a slippery slope using it for EV charging.

No, this is exactly the position we find ourselves in. Containers and other mobile/transportable units are effectively treated as "caravans", whereas the EV charging scenario is subtly different.

To be honest, there's not a lot of difference though. EV charging can only use PME now if either:

• There's an earth electrode that keeps the touch-voltage down (not often practicable unless you have a large steel structure connecting with the ground that's providing sub 1 ohm); or
• You use an open-PEN device

Of course, we are now charging very large EVs using large Mode 3 and Mode 4 charging options ...  usually, though, the power necessary means private transformer so TN-S ... or as an alternative, Mode 4 may well use IT earthing arrangement. Separating from PME below ground in built up areas is interesting.