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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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  • 0

    Zoomup:




    UKPN:

    This shed, because of its construction should be protected by an independent earth electrode and rccd. The origin, at the fusebox, PME, protects the cable. The DNO req a min distance of 2 metres from the electrode to PME bonded material. The separate earth lead is a good idea in this case for future use, the cpc of the cable being too small to meet the PME regs.

    Regards, UKPN




    What P.M.E. regs will not be complied with if just the S.W.A. cable armour is used to supply the building, justifying the need for a separate "earth lead". There will be no main bonding to carry out as the incoming water pipe is plastic? 


     

    We are told that this shed has a tin roof, and walls. Should they be bonded or not? If they don't actually reach the ground, and the frame is not conducting, it is difficult to see how they are ECPs, so don't bond.


    PME earths can be "exported" (even after Brexit ? ) but only provided that the CPC is sufficient to provide main bonding.


    If these structures are caravans or tents, then as others have said above, the special locations rules apply.


    Incidentally, I wouldn't worry about the extra cost of 3-core as opposed to 2-core, or even the CSA size of the cable - it is small in comparison with the cost of digging the trench, etc. And BTW, it is difficult to terminate 25 mm2 SWA into a 25 mm2 terminal ('cos the core isn't round) let alone a 16 mm2 one!

  • 0
    What is the building the supply is being taken from used for and how is it constructed?


    Andy Betteridge
  • 0

    Chris Pearson:




    Zoomup:




    UKPN:

    This shed, because of its construction should be protected by an independent earth electrode and rccd. The origin, at the fusebox, PME, protects the cable. The DNO req a min distance of 2 metres from the electrode to PME bonded material. The separate earth lead is a good idea in this case for future use, the cpc of the cable being too small to meet the PME regs.

    Regards, UKPN




    What P.M.E. regs will not be complied with if just the S.W.A. cable armour is used to supply the building, justifying the need for a separate "earth lead". There will be no main bonding to carry out as the incoming water pipe is plastic? 


     

    We are told that this shed has a tin roof, and walls. Should they be bonded or not? If they don't actually reach the ground, and the frame is not conducting, it is difficult to see how they are ECPs, so don't bond.


    PME earths can be "exported" (even after Brexit ? ) but only provided that the CPC is sufficient to provide main bonding.


    If these structures are caravans or tents, then as others have said above, the special locations rules apply.


    Incidentally, I wouldn't worry about the extra cost of 3-core as opposed to 2-core, or even the CSA size of the cable - it is small in comparison with the cost of digging the trench, etc. And BTW, it is difficult to terminate 25 mm2 SWA into a 25 mm2 terminal ('cos the core isn't round) let alone a 16 mm2 one!


     




    I think that the O.P refers to the metal parts of the shed being earthed not bonded. Perhaps with its own earth electrode(s).  I would not bond the metal external shed parts at all, it is not necessary and could introduce a hazard like main bonding a metal walled caravan.


    Are there any extraneous gas pipes in the shed?


    Z

  • 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.
  • 0

    Sparkingchip:

    What is the building the supply is being taken from used for and how is it constructed?


    Andy Betteridge 




     

    Could or should the whole installation be TT earthed and the TNC-S earth disconnected?


    Andy B.
  • 0
    If the S.W.A. is made to B.S. 3646 the armouring of 25mm2 would be a copper equivalent of approx. 26mm2. But, often the S.W.A. cable does not conform to that standard.

    https://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf


    And.....https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=47544&enterthread=y&STARTPAGE=2

    Z.
  • 0
    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.
  • Former Community Member
    0 Former Community Member

    Zoomup:

    If the S.W.A. is made to B.S. 3646 the armouring of 25mm2 would be a copper equivalent of approx. 26mm2. But, often the S.W.A. cable does not conform to that standard.

    https://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf


    And.....https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=47544&enterthread=y&STARTPAGE=2

    Z.



    The armour wouldn't have a copper equivalent of 26mm2 - only a thermal equivalent of copper ie it's K2/K1 rather than an assessment of copper and steel conductivity - which is roughly a factor of 10 difference - or  a copper equivalent of about 6mm2 in your example case if you are looking for it to be a bonding conductor.


    regards


    OMS


     
     

  • 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

  • 0

    Zoomup:




    Chris Pearson:




    Zoomup:




    UKPN:

    This shed, because of its construction should be protected by an independent earth electrode and rccd. The origin, at the fusebox, PME, protects the cable. The DNO req a min distance of 2 metres from the electrode to PME bonded material. The separate earth lead is a good idea in this case for future use, the cpc of the cable being too small to meet the PME regs.

    Regards, UKPN




    What P.M.E. regs will not be complied with if just the S.W.A. cable armour is used to supply the building, justifying the need for a separate "earth lead". There will be no main bonding to carry out as the incoming water pipe is plastic? 


     

    We are told that this shed has a tin roof, and walls. Should they be bonded or not? If they don't actually reach the ground, and the frame is not conducting, it is difficult to see how they are ECPs, so don't bond.


    PME earths can be "exported" (even after Brexit ? ) but only provided that the CPC is sufficient to provide main bonding.


    If these structures are caravans or tents, then as others have said above, the special locations rules apply.


    Incidentally, I wouldn't worry about the extra cost of 3-core as opposed to 2-core, or even the CSA size of the cable - it is small in comparison with the cost of digging the trench, etc. And BTW, it is difficult to terminate 25 mm2 SWA into a 25 mm2 terminal ('cos the core isn't round) let alone a 16 mm2 one!


     




    I think that the O.P refers to the metal parts of the shed being earthed not bonded. Perhaps with its own earth electrode(s).  I would not bond the metal external shed parts at all, it is not necessary and could introduce a hazard like main bonding a metal walled caravan.


    Are there any extraneous gas pipes in the shed?


    Z


     




    Hi Z, currently no extraneous gas pipes, I believe if any gas is introduced it will be from a gas bottle for a gas hob, no pipes in the ground.

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