Earthing arrangement for an external consumer unit

What will this new board supply ? - in terms of the CU itself a fibreglass door on the cabinet may allow PME. Realise that PME is used for all sorts outside, busstops lamp posts and so on, and there are not that many piles of bodies found every morning, so with care it is not always unsuitable.

Mike.

I tend to agree with Mike - there's certainly no blanket ban or even consistent recommendation against using PME outdoors - and there are many examples of its use, from outdoor lighting to street furniture. Advise against its use tends to be for specific higher risk situations - e.g. caravans, boats, farms, EV changers etc.

That said, there is a recognised risk from a broken PEN, and often some additional precautions are taken (e.g. an additional electrode for street lighting columns) or a buried grid next to street cabinets, so you are right to be a little cautious I think.

In this case, it doesn't seem like the kind of equipment that people will normally be touching, and tarmac usually provides some insulation from Earth, so the risks don't seem particularly high. Ensuring that the CU has or is within an insulating enclosure  and using something other than steel conduit/trunking for the connections can reduce the risk further. Switching to TT doesn't entirely eliminate risks - in some ways it just substitutes some risks (shock due to broken PEN) for others (shock due failure of ADS due to faulty RCD) - so it's more a matter of which outweighs the other in particular circumstances.

Bonding to the fence is an interesting one.  Bonding outdoors can sometimes cause more problems that it solves, exporting nasty potentials into areas that would otherwise be safe from them, or often doing nothing more useful than merely moving the problem from one end of the fence to the other. In other situations (e.g. where there's a lot of bonded metalwork about anyway) it can help. If it was a simple lamp post the regs specifically suggest omitting bonding (714.411.3.1.2) but aren't particularly clear about other situations where the risks may be substantially the same. Simple reading of 411.3.1.2 might suggest that all extraneous-conductive-parts within reach of the installation need to be bonded whether indoors or out, but the historical interpretation has often been that it's only intended to apply inside buildings. IIRC there was a suggestion in the recent DPC that 411.3.1.2 be altered to apply only to buildings, but whether that's been carried through to the new edition of the regs we don't know yet (and it wouldn't apply at the moment anyway). So really one for 'engineering judgement' looking at the particular circumstance I think.

   - Andy.

AJJewsbury said:

Advise against its use tends to be for specific higher risk situations - e.g. caravans, boats,

No, use of a PME terminal for caravans and boats is really prohibited in ESQCR, although the particular Regulation within that legislation is really a duty on the distributor.

It is worth pointing out that most 230V countries that use something akin to PME do not have the  same prohibitions, and most do not report a noticeably higher accident rate in terms of shocks from metal work during neutral faults, although to be fair it is very hard to gain accurate statistics for the relative risks, as neither electrocution figures, not those for notifiable incidents are reported in a way that allows easy separation of PME specific issues, from shocks and overvoltages cased by other kinds of fault condition.

It can also be quite problematic to factor out other local practices in terms of electrical installation that may contribute to variations  in accident rates. Much as we shake out heads at some ropy installations, the UK actually has a very high standard of installation and safety compared to much of the planet.

Anyway, we need the OP to say what this is supplying. Personally I'd start from the premise of leaving  the fence well alone, unless mains powered lights or cameras are fitted to it, and to change that,  I'd need a compelling case for how bonding it to the MET makes things any safer.

Mike.

Thanks for the reply Mike,

I understand that PME is safe apart from in a situation of a broken PME. I was looking to see if there was a device similar to MattE device used for car chargers to monitor the cable and sense a broken PME. I was wanting to try and get some guidance on transporting a PME to an outside consumer unit but everything I look into seems to be just for out buildings or very vague on if its ok or not. A TT is just not ideal for this location.

Well the question I'd be asking is can a person earthed solidly to terra-firma touch any metal of the CU, or any CPC or things earthed to it, that are not either painted metal or small area contact.

Given the person will not be standing on bare earth,  rather on paving, you are starting in a good place, as there is some current limiting resistance already.

The short answer is that if it is not a stables, boat caravan or other high risk thingy, then you do not really need to TT, and could consider PME.

I'm not sure that open PE detection is worth it, it is only just worth it with cars parked on the front lawn.

Mike.

Thanks for the reply Andy.

I agree and I cannot see where the regulations say not to use it or advise against it apart from in the special locations you mentioned. However it seems to be in those locations due to being at higher risk of shock and as this consumer unit is going to be outside would this not count as being as a higher risk of shock compared to if it was inside in a out building?

 Exporting A PME Supply To Another Building - Professional Electrician (professional-electrician.com) 

This link above is where I have got some information and advice from but again its for an out building and not a consumer unit outdoors on a boundary wall. 

The installation is going to feed power to the electric gate, CCTV system, lights in the car park area and a commando socket. At a later date it could be used to supply two car chargers.

I was wanting to find in the regs or guidance notes some clear guidance if exporting a PME is safe apart from in the special locations in the regs. I am happy the a PME supply is safe apart from if you get a break in the neutral back to the transformer but I do not want to install this installation incorrectly and I also want it to be as safe as possible. 

I mentioned above about the Matte device car chargers use, this could be good to try and detect a break in a PME supply to an external board so then it will hopefully rule out any danger a broken PME could introduce.