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TT or PME..best practice?

Sparks79
Evening all,


I’m currently looking into running a submain approx 100m buried from a PME supply to a small outbuilding (timber frame, metal sheet clad) that is being used next to a glamping tent as a place to cook and shower etc. The sub main will be a 16mm SWA..I was initially going to run a 3 core SWA or potentially a 2 core SWA with a seperate 10mm CPC (slight cost reduction) with the later option. I’ve since thought that perhaps a TT install would suffice, and, as possibly in the future I might be asked to extend the power into the tent, which, would require a TT setup. Any thoughts of this from a safety point or preference, also, if deciding on a TT and running a 2 core SWA as a submain which would obviosuly be a cost saver compared to a 3 core, however, would it be best practice to still have a CPC available seeing as a trench is now open...future proofing....? just as a side note, the metal sheets will be bonded to earth and any plumbing in the building is plastic pipes, also, this isn’t a question about cable selection i.e. the sub mains might be 16 or 25mm once I know exactly what the client wants, this question relates to the earthing setup. Thanks in advance!.

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    (timber frame, metal sheet clad)



    Are the metal sheets accessible from inside the building? If they're not (e.g. because of a timber or plasterboard inner skin) then I wouldn't consider them extraneous-conductive-parts as far as the installation within the shed is concerned - so no need to bond.  Personally I wouldn't be happy bonding them to a PME earth, since they'll be accessibe when standing on the general surface of the earth outside. If they are extraneous (i.e. in contact with the soil) and accessible from within, then I'd bond but go for a TT system. Lots of individual sheets, especially if painted/coated are going to be a real pig to bond effectively - perhaps to the point that putting in an inner skin to make them inaccessible from the inside would be easier.

     

    or potentially a 2 core SWA with a seperate 10mm CPC (slight cost reduction)



    As the 10mm G/Y will be buried in the ground table 54.1 will apply (despite its title, it's for all protecitve conductors, not just earthing conductors, as per the last sentence of 543.1.1) - so unless it has mechanical protection it might have to be a minimum of 16mm².


      - Andy.
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  • 0

    (timber frame, metal sheet clad)



    Are the metal sheets accessible from inside the building? If they're not (e.g. because of a timber or plasterboard inner skin) then I wouldn't consider them extraneous-conductive-parts as far as the installation within the shed is concerned - so no need to bond.  Personally I wouldn't be happy bonding them to a PME earth, since they'll be accessibe when standing on the general surface of the earth outside. If they are extraneous (i.e. in contact with the soil) and accessible from within, then I'd bond but go for a TT system. Lots of individual sheets, especially if painted/coated are going to be a real pig to bond effectively - perhaps to the point that putting in an inner skin to make them inaccessible from the inside would be easier.

     

    or potentially a 2 core SWA with a seperate 10mm CPC (slight cost reduction)



    As the 10mm G/Y will be buried in the ground table 54.1 will apply (despite its title, it's for all protecitve conductors, not just earthing conductors, as per the last sentence of 543.1.1) - so unless it has mechanical protection it might have to be a minimum of 16mm².


      - Andy.
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