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ISO Containers Earthing & Bonding - TNCS (PNB) Supply

Former Community Member
Former Community Member
I have a project to provide a 63A TP supply terminating with a Type B DB within an ISO container. Final circuits from the DB will supply industrial sockets located in the ISO via RCBO's and an external fan.

The ISO container is located on several concrete pads and internally is fully (wooden) boarded out.

The supply is TNCS PNB.

My current proposals are for a connection via a SWA supply cable (4c) and separate CPC, fed via a 63A Type C MCB.

The CPC will terminate onto a EMT.

The ISO container will have a main protective bonding conductor to the EMT using finely stranded cable. In addition bonding via flexible 'straps' will be provided to the doors.

I have not allowed to TT the installation due to the PNB supply.


Looking for advice on whether a local earth rod or ring earth electrode is the best option to earth the container locally and also the resistance figure to achieve.


Thanks Mike

 


 

  • 0
    Is this PNB direct from the DNO (e.g. overheads), or PNB from a private transformer?


    If from the DNO, PME conditions apply, and strictly the "container" is usually covered by the scope of Section 717 - which prohibits connection to a PME supply under most conditions.
  • Former Community Member
    0 Former Community Member
    As every circuit inside is RCD protected, what is wrong with a time delayed RCD "up-front" and take away your question by making it TT?


    Regards


    BOD
  • Former Community Member
    0 Former Community Member
    Thanks for the responses. Graham, yes it is via an DNO supply, although it is buried supply cables with a TX on a pole located adjacent the Switch Room.

    If with this type of PNB supply PME conditions apply then yes TT connection using a time delay RCD (instead of the MCB), earth rod at the container, RCBO's to the outgoing circuits.

    If not PME then would the original proposal be meet the guidance ie PNB supply via private TX ?

    Thanks.

  • 0
    You can use the PME terminal with one condition being

    PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section except where:

    (i) the installation is continuously under the supervision of a skilled or instructed person(s), competent in such work, and...

    I have visions of a Spark dressed as a palace guard marching up and down outside the container! Can someone expand on this exception?
  • 0
    lyledunn:

    You can use the PME terminal with one condition being

    PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section except where:

    (i) the installation is continuously under the supervision of a skilled or instructed person(s), competent in such work, and...

    I have visions of a Spark dressed as a palace guard marching up and down outside the container! Can someone expand on this exception?


    It's historical and related to BBC outside broadcast units I understand.


    Take a look at the proposed change to that Regulation in the Amendment 2 DPC.


  • 0
    Mikey70:

    Thanks for the responses. Graham, yes it is via an DNO supply, although it is buried supply cables with a TX on a pole located adjacent the Switch Room.

    If with this type of PNB supply PME conditions apply then yes TT connection using a time delay RCD (instead of the MCB), earth rod at the container, RCBO's to the outgoing circuits.

    If not PME then would the original proposal be meet the guidance ie PNB supply via private TX ?

    Thanks.

     


    If the supply is an installation with a couple of sources such as a generator and privately-operated transformer, where the N-E bond is made at the switchboard (instead of separately at the generator and transformer) to prevent circulating currents, then that is technically PNB, but:

    (a) PME conditions do not apply; and

    (b) You can generally consider the installation downstream of the main switchboard as TN-S.


  • 0
    gkenyon:
    lyledunn:

    You can use the PME terminal with one condition being

    PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section except where:

    (i) the installation is continuously under the supervision of a skilled or instructed person(s), competent in such work, and...

    I have visions of a Spark dressed as a palace guard marching up and down outside the container! Can someone expand on this exception?


    It's historical and related to BBC outside broadcast units I understand.


    Take a look at the proposed change to that Regulation in the Amendment 2 DPC.




    All that R and D for the loss of neutral in EVSE looks like it will have some spin off! 


  • 0
    Will the container really be a 'mobile or transportable unit' as far as the wiring is concerned? E.g. is it likely to be moved elsewhere and just hooked up to another supply? If not, and the container is being treated more like a of  bit pre-fab building which is intended to remain in the same place indefinitely then it might make more sense to treat it more like an ordinary installation - e,g. no need for finely stranded conductors.


    PME would still be a point to consider though. As Graham suggested, PNB isn't usually really TN-C-S (but can be in some rare circumstances - e,g. where the Tx serves more than one customer) and by definition isn't PME (as there's no requirement for multiple earthing), but for safety we usually regard all UK DNO earthing facilities as 'potentially PME' in case the DNO change the arrangement at a later date. Creating a TT island for the unit(s) is an obvious answer, but can cause difficulties where they're within reach or are connected to metalwork that's connected to the main installation.


       - Andy.
  • 0
    AJJewsbury:


    PME would still be a point to consider though. As Graham suggested, PNB isn't usually really TN-C-S (but can be in some rare circumstances - e,g. where the Tx serves more than one customer) and by definition isn't PME (as there's no requirement for multiple earthing),

     


    Hi Andy,


    If it's a DNO-provided PNB, then usually PME conditions apply (see G12/4), especially where the N-E bond is made at the service head. In rare cases, where the N-E bond is made in one place only for all customers, the DNO may declare TN-S (usually older supplies).


  • 0
    AJJewsbury:

    Will the container really be a 'mobile or transportable unit' as far as the wiring is concerned? E.g. is it likely to be moved elsewhere and just hooked up to another supply?

     


    717.1 (ii) - but I agree if it's not readily "transportable" then perhaps 717 doesn't apply - but it's still a metal-frame structure, and as the forum user known as UKPN has quite rightly stated in another thread along these lines recently, in some cases where PME conditions apply, particularly rural, it may well be best to follow the same rules as Section 717 in any case.


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