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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!.

Source URL: www.electriciansforums.net/.../
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    whjohnson:

    I'm not a TT fan. Too many non-constants.

    Bang a rod in. measure once and forget about it - that's the usual norm. No future monitoring, nothing. A 100ma Type S up front? But will it work when needed? How long will a Type B32A mcb sit there with a potentially lethal fault current before it knocks off?

    In urban locations it can be nigh on impossible to install, let alone maintain - I've seen too many with the earthing conductor cut off.." because it looks unsightly in the front garden flowerbed and it doesn't really do anything does it?"

    Round here there's next to no chance of getting a rod in - solid limestone means that it is often much cheaper to get a PME conversion and transfer liability to the supply company for future maintenance of the earthing facility.

    As for exporting PME? I do it via combining the armour and 3rd core to outbuildings at both ends.

    Nice low Zs at the destination and job done.

    Given the right conditions, I also bang in a rod and combine both earths at a common point just for belt and braces.




    Thanks for the input, we always struggle here in earthing, there’s granite rock everywhere, if your lucky enough to get a rod into the ground it’s generally inbetween rocks and has very poor contact with the ground, I know there’s other options with laying a copper horiziontally etc. I was curious with you mentioning also driving in an earth stake at both ends, the DNO seem to state they want TNC or a TT not a bit of both, what’s your take on this?


    Cheers

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  • 0

    whjohnson:

    I'm not a TT fan. Too many non-constants.

    Bang a rod in. measure once and forget about it - that's the usual norm. No future monitoring, nothing. A 100ma Type S up front? But will it work when needed? How long will a Type B32A mcb sit there with a potentially lethal fault current before it knocks off?

    In urban locations it can be nigh on impossible to install, let alone maintain - I've seen too many with the earthing conductor cut off.." because it looks unsightly in the front garden flowerbed and it doesn't really do anything does it?"

    Round here there's next to no chance of getting a rod in - solid limestone means that it is often much cheaper to get a PME conversion and transfer liability to the supply company for future maintenance of the earthing facility.

    As for exporting PME? I do it via combining the armour and 3rd core to outbuildings at both ends.

    Nice low Zs at the destination and job done.

    Given the right conditions, I also bang in a rod and combine both earths at a common point just for belt and braces.




    Thanks for the input, we always struggle here in earthing, there’s granite rock everywhere, if your lucky enough to get a rod into the ground it’s generally inbetween rocks and has very poor contact with the ground, I know there’s other options with laying a copper horiziontally etc. I was curious with you mentioning also driving in an earth stake at both ends, the DNO seem to state they want TNC or a TT not a bit of both, what’s your take on this?


    Cheers

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