Supply to Container PME to TT

Discussion in 'Electricians' Talk' started by Norm2020, Oct 24, 2021.

  1. Norm2020

    Norm2020 New Member

    I have a container (Insulated etc for office space) that's about 5 m away from the unit i want to supply it from, this unit has a PME supply.

    So intension was 35m of 3 core 10mm Swa from 3Ph board in the main unit, all 3 cores connected at the supply end on C40A. Feeding an isolator on the outside of the container and then through the back with singles to the board on the inside. As PME not allowed cpc and armouring would be isolated at the isolator via cutting back taping and use of stuffing gland. Earth rod with 6mm earth through isolator straight to board (Plus 6mm bonding to the container from the earth rod). Board is Metal 3 way protected by 63A 30Ma RCD.

    1. Have i interprtated the regs correctly as not done any TT before.

    2. I understand why TT is needed incase the loss of the PEN conductor on a PME supply. But under the TT system if there is a fault in the supply cable and the line touches the case of my fuse board surely this will make the board housing and anything connected to it live as there is no earth fault path back to the supply board?

    Hope that made sense any advice appreciated
     
  2. Tony Goddard

    Tony Goddard Screwfix Select

    Normal practice to isolate the earth would be to terminate the SWA in the normal way into a plastic adaptable box or plastic isolator with no connection made between the gland and onward circuits, this has the benefit of preserving the mechanical integrity and bonding together the armour strands each end of the line, but not onwards.

    16mm earth cable is routinely used as the main earth and 10 for bonding.

    The earth cable should not be switched (you say through isolator) I'm assuming you mean passing through the enclosure thereof as opposed to through the switch!

    The board housing will be earthed to ground via the spike, this will carry the fault current back to the supply and activate the fault protection, the PME supply is itself earthed, just not on your premises so all roads lead down to ground!!

    Presumably you have access to a tester to ensure your earth loop and RCD operating times are within spec, sometimes more than one spike is needed depending on the location.
     
  3. Bob Rathbone

    Bob Rathbone Screwfix Select

    The container, I assume is metallic, this will create an excellent 'Faraday cage' or an area of equipotential bonding. Internally it should be treated as you would a domestic installation, bonding the MET to the metal frame of the container. That will ensure that inside the container all exposed and extraneous metalwork will be at the same potential during an earth fault. Now for outside, this is unusual as the whole metal box is exposed to touch by anyone standing on the ground outside, the closest parallel would be a caravan. As Tony suggests do not use the earth from the supply, instead install an earth electrode and a 30mA RCD. Install the electrode within 1 metre of the entry door to create and control the voltage gradient between the container shell and true earth, you may wish to install additional electrodes at the sides of the container where they can be touched by those outside, again about 1 meter away from the container. Others may think this is excessive use of extra electrodes, but if safety is your primary goal, it is not.
     
  4. The Happy Builder

    The Happy Builder Screwfix Select

    My wholesaler would send me some two core SWA, no need to have an extra core.
     
    nigel willson likes this.
  5. PhilSo

    PhilSo Screwfix Select

    " So intention was 35m of 3 core 10mm Swa from 3Ph board in the main unit, all 3 cores connected at the supply end on C40A. "

    3 phase supply on 2 core...?

    PhilSo (confused):(
     
  6. Bob Rathbone

    Bob Rathbone Screwfix Select

    Would require a 4 core if 3 phases were to be brought out to the container, I reckon it's just a single phase supply, but who knows?
     
  7. MGW

    MGW Screwfix Select

    The container is in essence same as a caravan, it has a metal outer so with loss of PEN anyone going in or out of the container would get a shock, so yes same as EV charging point it needs some thing to make it safe with loss of PEN. Be it auto disconnect including earth if voltage outside the 207 to 253 volt range per phase, or TT, something needs doing, and as long as you can't touch one item connected to TN-C-S at same time as some thing connected to TT then TT is the way to go.

    Read this PDF on supply to outbuilding from IET OK out of date Autumn 2005, but still most of what it says is valid. In old BS 7672:2008 it was section 717 that gives examples.
     
  8. Comlec

    Comlec Screwfix Select

    Thank you @MGW. I am fed up posting that link.:)
     

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