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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/.../
  • 0
    Section 708 covers caravan/camping parks and similar installations. 708.411.4 allows a P.M.E. supply to a permanent building. E.S.Q.C.R. prohibits the connection of a P.M.E. earthing facility to any metalwork of a leisure accommodation vehicle, including a caravan.


    708.553.1.14 (if it applies in this case) prohibits the connection of socket outlet protective conductors to a P.M.E. earthing facility.


    Personally I like TT earthing for any outdoor electrical supply, with two R.C.D.s in series as R.C.D.s can become faulty. If a TN-C-S. supply is run TT earthing can always be arranged at the final point of usage.


    In most cases, the armour of a steel wire armoured cable is big enough to use as a circuit protective conductor.


    Z.
  • 0
    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.

  • 0
    Graham,

    do you mind expanding on item 2?
  • 0

    lyledunn:

    Graham,

    do you mind expanding on item 2?




    I think that Graham is implying that any sudden disconnection of the loaded 100m cable, may induce a large Voltage whilst the magnetic field collapses in it, that might damage connected equipment or even the insulation of the cabling. The S.P.D.s would adsorb that high Voltage and limit damage.


    Z.

  • Former Community Member
    0 Former Community Member

    Zoomup:




    lyledunn:

    Graham,

    do you mind expanding on item 2?




    I think that Graham is implying that any sudden disconnection of the loaded 100m cable, may induce a large Voltage whilst the magnetic field collapses in it, that might damage connected equipment or even the insulation of the cabling. The S.P.D.s would adsorb that high Voltage and limit damage.


    Z.


     



    Or that 100m of external cable is pretty good at picking up an LPS ground strike and will import same back into the origin.


    It's a common mistake where incoming circuits are considered and protected, but circuits leaving the facility are missed or ignored resulting in a pretty big hole in your carefully considered layered protection scheme 


    Just switching off may be less of a concern *


    Regards


    OMS

    *other opinions are available

  • 0
    I was thinking of this:Transient overvoltages caused by electrical switching events are very common and can be a source of considerable interference. Current flowing through a conductor creates a magnetic field in which energy is stored. When the current is interrupted or switched off, the energy in the magnetic field is suddenly released. In an attempt to dissipate itself it becomes a high voltage transient. The more stored energy, the larger the resulting transient. Higher currents and longer lengths of conductor, both contribute to more energy stored and subsequently released. This is why inductive loads such as motors, transformers and electrical drives are all common causes of switching transients. So would a buried steel wire armoured cable be at risk of ground strikes? Wouldn't the armouring protect the inner conductors somewhat?


    Z.


  • 0

    Zoomup:




    lyledunn:

    Graham,

    do you mind expanding on item 2?




    I think that Graham is implying that any sudden disconnection of the loaded 100m cable, may induce a large Voltage whilst the magnetic field collapses in it, that might damage connected equipment or even the insulation of the cabling. The S.P.D.s would adsorb that high Voltage and limit damage.


    Z.


     




    I was thinking more in terms of lightning and surge protection "zoning", what in the US are sometimes called "ground windows"

  • 0
    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
  • 0
    Thanks all for your replies, I must admit the suggestion of SPD’s wasn’t something I’d considered for this scenario, I’ve used them before where there’s sensitive electronic equipment present. I have a better grasp now on a plan of attack!


    Al
  • 0

    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? 


    Z.

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