This is not an easy question, because the actual unit itself in this case may comprise only Machinery as defined in the Supply of Machinery (Safety) Regulations, which is outside the scope of BS 7671, and BS EN IEC 60204-1 applies. However, the connection of the Machinery to the electrical installation is referenced out to BS 7671 (IEC 60364 series) from BS EN IEC 60204-1. So certainly, 717.411.4 would apply, if not other parts of facilitating the connection in 717.

Daniel Hickson said:

Would part (ii) be a satisfactory measure in the above scenario for a PME supply? 

Yes, if the unit is located within, or outdoors on an upper storey or roof, of a building or structure containing the electrical installation that supplies the unit, PME can be used.

Precautions should be taken to prevent the connection of a unit outdoors.

These type of unit would usually be used in a place of work, so when looking at a connecting point (e.g. suitable socket-outlet), you would need to involve the duty holder in respect of risk assessments, because they will be managing ongoing safety.

If the supply was TN-C-S PNB, could the supply earthing facility then be used possibly on the basis that the risk of loss of neutral is minimal?

It sort of depends on who owns the transformer - if it's the local DNO they they tend to slap on "PME conditions apply" labels to almost any earthing facility they supply - not necessarily because it's actually PME at the moment, but just to keep their options open to make it PME in the future (e.g. if they have to add new consumers to the same transformer). If it's privately owned by the consumer then DNO's policies can't get in the way - and anyway it's likely to be TN-S (PNB) in that case anyway (even if with long transformer tails) as combined earth & neutral conductors are banned in consumer's installations by the ESQCR.

   - Andy.

Very useful comments Andy, thanks again.

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.

The DTI Guidance on the ESQCR

So guidance suggesting special consideration, rather than legislative prohibition?

Containers and other mobile/transportable units are effectively treated as "caravans"

Other than 717 does permit PME to be used in some circumstances - one of which is having a suitable human on site who presumably is able to recognise a broken PEN situation and do something sensible about it  (presumably not plugging the units in or pulling the plug on any units on site if it happens after the units are installed). It could be argued that an open-PEN device simply automates that process (possibly with better reliability).

  - Andy.

The DTI Guidance on the ESQCR

So guidance suggesting special consideration, rather than legislative prohibition?

Containers and other mobile/transportable units are effectively treated as "caravans"

Other than 717 does permit PME to be used in some circumstances - one of which is having a suitable human on site who presumably is able to recognise a broken PEN situation and do something sensible about it  (presumably not plugging the units in or pulling the plug on any units on site if it happens after the units are installed). It could be argued that an open-PEN device simply automates that process (possibly with better reliability).

  - Andy.

AJJewsbury said:

So guidance suggesting special consideration, rather than legislative prohibition

Only for things that are not "caravan" or "boat".

AJJewsbury said:

Other than 717 does permit PME to be used in some circumstances - one of which is having a suitable human on site who presumably is able to recognise a broken PEN situation and do something sensible about it  (presumably not plugging the units in or pulling the plug on any units on site if it happens after the units are installed).

Agreed ... but that does not apply to 722.

The issue being how that particular exception is worded.

AJJewsbury said:

It could be argued that an open-PEN device simply automates that process (possibly with better reliability).

Not if auto-reset is included.

If the OPDD operates more frequently than real open-PEN events, that would be problematic (in fact, it already is, without considering the use of the installation supplied by it).

When the installation supplied is a refrigerated unit, or some medical applications, like the local MRI etc units in frequent use, this is a huge issue.

I guess, though that some of the points made assume that these issues have not already been considered ? Just because a standard is economic in its requirements, does not mean there has been an absence of robust discussion.