Earthing arrangement for an external consumer unit

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.

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.

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.