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Earthing arrangement for an external consumer unit

MC

I am trying to get some advice for a new consumer unit that I have been asked install at a commercial premises. The single phase consumer unit will be installed outside on a wall of the boundary of the property and will be fed from the three phase main distribution board in the main building which has a PME earthing arrangement and it is around 50-60 meters away. The new consumer unit will be installed in the correct rated IP rated enclosure and is fed by 16mm SWA cable protect by a 63 amp MCB. The new consumer unit will be used to supply power to a electric gate, lighting and some other services. 

I wanted to know what earthing arrangement other people would recommend to use for this external consumer unit. I would normally look at using a TT earthing system as it is outside. However, the site is an old substation so there will be a large amount of cables in the ground and also the area where they want the consumer unit is in a tarmac car park so putting a earth rod in could be hard work. I have tried looking for guidance on exporting an PME earth to an external outside area but I only seem to be able to find information on exporting it to an external building, not a consumer unit outdoors servicing circuits which will be outside. There is also a metal fence that surrounds the property close by. 

Is it a TT always for this situation? Could I transfer the PME, would standing outside on the ground effect this? Would I need to bond the structural metal work like the fence? 

Any help on what you earthing arrangement you would install would be great and maybe some reasons why and any regs that back this up would be great.

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  • 0

    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.

  • 0 in reply to mapj1

    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.

Reply
  • 0 in reply to mapj1

    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.

Children
  • 0 in reply to MC

    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.

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