Earthing system on new garden centre installation

Discussion in 'Electricians' Talk' started by SeaBass, Dec 8, 2009.

  1. SeaBass

    SeaBass New Member

    Hi, new on here, so hello to all.

    I'm pricing on a new garden centre installation, i've done domestic for years, but haven't done much commercial, could do with some earthing advice on this install.

    Plot is about an acre.
    Supply is 100A/3PH/PME, this is a new overhead supply and comes into an external enclosure with the meter on the edge of the plot.
    I plan to install a 4P 100A switchfuse in this enclosure and run 35mm/4c/XLPE/SWA to the main building in an underground duct, which will be in the centre of the plot, about a 60M run.
    I plan to install a 24way 3 phase distribution board in the main building and distribute to local circuits with RCBO's and MCB's to other distribution boards in other buildings from this point.

    There will be various buildings all over the site, shop, coffee shop, internal and external planting buildings, water feature areas, etc..

    Anybody got an opinion on what earthing arrangement?
    1) Run a separate earthing conductor with the main supply SWA and export the PME supply all over the site, bonding what is nessesary.
    2) Use the PME supply to protect the SWA armour then isolate it when glanding in the main building and TT it? If this then do i have to install a separate earth rod at each building?

    Any other ideas / advice will be much appreciated.

    Thanks.
     
  2. smallvile

    smallvile New Member

    where will the DNO meters be what system are thet bringing in to the main fuses
     
  3. Charred Sparky

    Charred Sparky New Member

    Hi Sea Bass,
    I'm new to the trade and no expert, but I think your best way forward is to sit down and make some calculations. Obviously the easiest way is to export the earth around the site. But if the length of the run means the CPC resistance will hinder the disconnection times . Then you either upsize your cable choice or look at the alternative means. I'm not sure either if an normal sparky is qualified to PME a site. The calculations and practices I think? are outside or the reg's, but I might well be corrected on this.
     
  4. smallvile

    smallvile New Member

    i"ve always been told your not allowed to export a pme
     
  5. state-it

    state-it New Member

    There's absolutely no problem with your Option (2)

    Your 1st Option is OK by the book if there's no extraneous conductive parts in the outbuildings, which is unlikely.

    There's nothing wrong with TT-ing further down the line from a PME intake, and it keeps the sub-mains in nice discrete chunks.
     
  6. SeaBass

    SeaBass New Member

    Ta for the answers, i do understand the pitfalls of exporting the PME all over the site, i.e. potential dangerous situation with the loss of the PEN conductor with extraneous metal.

    I am erring towards the TT system. I just wondered if anyone had 1st hand experience of this type and scale of installation?
     
  7. M107

    M107 New Member

    1) You could export the pme around the site, after all any other services will be shared between buildings?
    If you do export the pme I'd be inclined to use a seperate large csa conductor as a combined cpc & main bonding conductor to each building, then connect local bonding between cu & the services of the building.

    2)Yes & connect meb's of each building to the TT met.
     
  8. JP.

    JP. Screwfix Select

    Away from the main building which is supplied by a sub main proper..then I think differentiation is needed between sub mains and sub circuits to the other buildings. As far as I aware there is no requisite to RCD protect a sub mains within the parameters of BS 7671..however there may well be a requisite to RCD (or RCBO) protect a sub circuit..maybe this important.
     
  9. M107

    M107 New Member

    Afternoon JP.

    If it's TT'd then submain or subcircuit will require rcd what ever.
     
  10. JP.

    JP. Screwfix Select

    I would be interested to hear Lucias views on this topic as it is a good one.
     
  11. JP.

    JP. Screwfix Select

    Afternoon M107, yes and this could cause problems I think. However I have limited knowledge with respect to this subject, so will be interested in the final method used by the OP.

    Have a good evening M8.
     
  12. SeaBass

    SeaBass New Member

    Cheers chaps,

    I'm looking at the drawings now - extraneous metal work all over the place with metal irrigation pipework and one of the buildings will be a converted metal container!!

    What about...
    PME protecting the armoured of the supply SWA from the intake (via switchfuse) to the main building.
    PME isolated at gland, TT rod(s) installed at main building.
    100ma (or higher if Ra reading allows it), 3PH RCD main switch installed in 3PH distribution board.
    30ma RCBO's to all local circuits in main building.
    MCB's to supply SWA and separate earthing conductor to each outbuilding, with additional Rod at each outbuilding building.
    Distribution board at each outbuilding with isolator and 30ma RCBO's for local circuits.
    ????
     
  13. spark&half

    spark&half Active Member

    Would it not be practicable to arrange for dno to install their supply into the building once built rather than having a meter in the middle of nowhere?

    Then you will have your PME supply in the building which could be split to various sub-db's via a busbar or mccb dis board.

    ;)
     
  14. state-it

    state-it New Member

    Had a right cock of an install a few years ago w.r.t. a trailer park site with lots of service buildings (bogs, showers, kitchens, offices, bar...and two converted metal containers...)

    Your scheme seems as fair as anything. Thank **** for RCBOs.

    To make a pukka job of the rods actullay dig an earth pit and put in an inspection box. Backfill it with bentonite if the Ra is a tad high.
    Looks so much better than a rod sticking out of the mud...
     
  15. SeaBass

    SeaBass New Member

    "Would it not be practicable to arrange for dno to install their supply into the building once built rather than having a meter in the middle of nowhere?"


    I agree - and that was the original plan - but it comes down to cost.
    EDF have apparantly quoted silly money to bring the overhead supply direct to the main building. I was about £5k cheaper using an external enclosure, switchfuse and SWA run.
     

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