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.

I'm also reviewing this type of installation at the moment. The installation which I'm reviewing has a PME supply and the refrigeration trailers are parked in an outdoor yard. The installation will not be managed to comply with (i). Given that the trailers are parked in an outdoor yard, I don't believe the installation complies with (ii). Would you agree? Also, would the use of a O-PEN protection device permit the use of the PME supply? Any comments would be much appreciated.

would the use of a O-PEN protection device permit the use of the PME supply?

That's a good question. At the moment BS 7671 only recognised open-pen detection for EV charge points. There are many other similar situations - from boats and caravans to mobile units, to outdoor things in general - but there's no explicit recognition for any of those situations. I guess you could go for a deviation, and claim that the open-pen unit provides no less safe approach than direct BS 7671 compliance, but it would be on your head then - and it's not always obvious that alternatives (e.g. TT) might not be safer in some circumstances. Counter arguments might include that the 70V limit for EVSEs is higher than the 50V limit for most other situations, some open-PEN designs (especially single phase ones) can't detect all possible open-PEN conditions even when the PEN-Earth voltage does exceeds 70V, and quite separate limitations on use of PME - e.g. legal prohibition for boats and caravans in the ESQCR or just your local DNO rules.

   - Andy.

Thank you for that Andy, especially at this late hour. I did note that 717 does not contain the O-PEN detection provision and suspected it's use may have possible implications. 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? I appreciate a review with the DNO would be advisable.

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.

Patrick Benson said:

Also, would the use of a O-PEN protection device permit the use of the PME supply? Any comments would be much appreciated.

Not according to BS 7671.

An open-PEN protection device switches the protective conductor. According to BS 7671, a protective conductor may only be switched:

• Where permitted by Regulation 543.3.3.101; or
• Where an open-PEN protection device (OPDD) is used for an EV charging equipment installation (see 722.411.4.1).

Since connection of a mobile/transportable unit is neither of the above, use of an OPDD would NOT conform to BS 7671.

The same situation exists where an OPDD is considered for use for a heat pump or other fixed electrical equipment ... usually, the device contravenes BS 7671 (Regulation 543.3.3.101). The sole exception of the device being specified for EV charging equipment installations (722.411.4.1).

Even where 722.411.4.1 applies, it is only the charging point that is specified for protection by the OPDD.

Since connection of a mobile/transportable unit is neither of the above

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)?

   - Andy.

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. 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.

This would normally be solved in the caravan case by wiring the 'shore power' sockets as TT. But that may mean the whole site has to be TT if you cannot get far enough away from bonded services and bits of the 'indoor' installation. (which is possible - farms with supplies of hundreds of amps are often TT, but it needs some more planning.)

for small loads (trucks full of radio gear for example) a transformer is used, either in the truck or static, to in effect create a new TNs

At the moment the O-PEN solution is still quite novel, and in regs at least has only really been considered for the EV case, although as noted, the actual electrons don't know that, and the risk is very similar.

If the O-PEN operated the vehicle would be disconnected, but any metal bits of building would still be live so its not that complete in terms of removing the danger.

Mike.

Thank you for your response, much appreciated.