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TT or PME..best practice?

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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Parents
  • If you are deriving TT from an existing PME installation, there are two considerations, which may well be overlooked:
    1. Ensure separation from buried metalwork of the PME system (otherwise, of course, the TT system is effectively connected to PME). For caravan sites, Guidance Note 7 recommends the TT earth electrode to be at least 10 m from any buried conductive metalwork connected to the PME earthing system. This is based on Figure 16 of BS 7430.


      Whilst in your particular case this separation may well be achievable, in many small curtilage properties, it is not ... and in those cases, something else needs to be considered.

       

    • If the main installation has SPDs, then certainly for a length of 100 m, SPDs are also required on the outgoing TT feed - and recommended at the remote end.

Reply
  • If you are deriving TT from an existing PME installation, there are two considerations, which may well be overlooked:
    1. Ensure separation from buried metalwork of the PME system (otherwise, of course, the TT system is effectively connected to PME). For caravan sites, Guidance Note 7 recommends the TT earth electrode to be at least 10 m from any buried conductive metalwork connected to the PME earthing system. This is based on Figure 16 of BS 7430.


      Whilst in your particular case this separation may well be achievable, in many small curtilage properties, it is not ... and in those cases, something else needs to be considered.

       

    • If the main installation has SPDs, then certainly for a length of 100 m, SPDs are also required on the outgoing TT feed - and recommended at the remote end.

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