question

dj-dulux avatar image
dj-dulux asked

Earthing in the UK - connecting mobile units to PME

In the UK its against regulation to supply caravans / campers / motorhomes from a PME (TN-C-S) supply according to Section 721 of BS7671. Section 717 of BS7671 also starts that PME supply would need to be under supervision to supply a mobile unit like a food truck and this is applied by organisation like the BBC on OB trucks, the supply earth is checked before connection.

In one of my days jobs (sound systems) we meter the mains before connection to ensure no broken PEN (loss of neutral) which could, although unlikely cause metal work to raise to dangerous voltages.

When we quote for a job we do some level of risk assessment to see if we need to add features to an installation to ensure all is good. For example someone who has a caravan on the drive long term with a PME install would maybe need the outside socket turning into a TT installation, although that is not that easy in built up areas due to underground services which would also raise to dangerous voltage levels close to the TT rod, so likely doesn’t remove the issue 100%.

After much google searching this topic does not seem to get discussed too much and we see food trucks and caravans on drives all the time with a trailing lead to the house, so am I been over cautious? I have a job coming up on a mobile bar and it will get plugged into anything from farms to houses to pubs and the operator can not be expected to check the mains. Therefore I see 2 options:

  • Isolation transformer to make the bar an IT system (maybe also need insulation monitor too)
  • Use an over / under voltage monitor between L-N to disconnect all 3 conductors entering the vehicle in case on a PEN fault

Both options obviously cost money, the voltage monitor is cheaper than the transformer option...

Any thoughts from installers in the UK?


Thanks,

Simon...

Grounding
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1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

For such mobile installations, where there will be public access to the vehicle, I would install the isolation transformer, as an under-voltage detection might not trip in a marginal situation. This could lead to a potentially hazardous touch potential situation under worst case conditions. Cost would be higher, but the liability aspect for such an application is also much greater.

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dj-dulux avatar image dj-dulux commented ·

Thanks for the reply @Mike Dorsett, I think this is the best option and after some more calulations its pretty clear that the PEN method would not be that effective due to the relatively low power and transient nature of appliances in a mobile unit. In a car charger situation the sustained high power makes an issue obvious, in a mobile situation with maybe only a fridge running this would not be the case.

Thanks,

Simon...




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phoenix avatar image phoenix dj-dulux commented ·

The IET forum is a good place to ask this question. It comes up a fair amount around shipping containers on sites and lamp posts.

https://engx.theiet.org/f/wiring-and-regulations

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dj-dulux avatar image dj-dulux phoenix commented ·

Thanks @Phoenix, I have seen a few articles and replys to questions on the IET forum, including this article, IET Mobile Units, which has been a pretty good summary. Most replys on the IET I have read seem to say, it depends on many external factors. How close units are together, how realistic is it to maintain seperation from other metal work which maybe connected to PME system, services related to PME underground below the units etc all of which are capable of causing a difference in potential between the ground and unit.

I have been going back-and-forth to the regs and believe the isolation transformer with its own N-E link inside the bar is the most robust way to ensure it can plug into anything without issues or side effects.

Unless anyone can come up with a negative to this approch?

Thanks,

Simon...


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phoenix avatar image phoenix dj-dulux commented ·

I think you may have answered your own question. ' it depends'

A user of a caravan is assured a safe supply when they turn up at a site. That is the site's responsibility. Food trucks are similar when they go to a market - the council provide a safe supply. A food truck can pull a solid 3.5kVA with a couple of friers, extraction, fridges etc. That needs a dedicated supply from the venue. You could try a 13A plug off a ring main but expect trips.

I've worked on the odd food truck and a few caravans. All had valid I&T certificates. The manufacturer didn't worry about where it was plugged it.

It feels to me like you are worrying too much.


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