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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

    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.

  • 0 in reply to mapj1

    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.

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  • 0 in reply to mapj1

    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.

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