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Supply to Outbuilding with Extraneous Pipework

statter
Hello

Situation is an outbuilding (1)  housing gas meter and longish length of exposed steel gas pipe (not plastic covered).

Power is used for internal lighting and external lights and socket for internal and external use and is being supplied via a 16A MCB from another outbuilding (2) which has a TT supply with 30mA RCD.  The exported TT earth is not connected in outbuilding 1.


The TT supply for outbuilding 2 comes via overhead L+N conductors from a house which has a TNC-S supply.  The arrangements in outbuilding 2 all look and test fine.


The gas pipe on the customer side of the meter goes underground from outbuilding 1 to this house and is steel throughout. 


Gas pipework is bonded to the BMET in outbuilding 1. There is also a 10mm2 G/Y earth cable from the BMET in outbuilding 1 to the MET at the source in the house. There is separate bonding of the gas pipe to the house MET where it enters the house.

So the outbuilding 1 supply effectively is TNC-S but the earthing is odd to say the least as it is separated from the supply.

The power requirements are low so 10mm2 should be quite adequate and the circuits have 30mA RCD overall.

This all feels quite unconventional but I cant see that it breaches any regulation?  There is a diagram by the intake showing the arrangement.

Long term the plan is to tidy this up which will not be simple but meanwhile what am I missing?


Peter
  • 0
    statter:

    Hello

    Situation is an outbuilding (1)  housing gas meter and longish length of exposed steel gas pipe (not plastic covered).

    Power is used for internal lighting and external lights and socket for internal and external use and is being supplied via a 16A MCB from another outbuilding (2) which has a TT supply with 30mA RCD.  The exported TT earth is not connected in outbuilding 1.


    The TT supply for outbuilding 2 comes via overhead L+N conductors from a house which has a TNC-S supply.  The arrangements in outbuilding 2 all look and test fine.


    The gas pipe on the customer side of the meter goes underground from outbuilding 1 to this house and is steel throughout. 


    Gas pipework is bonded to the BMET in outbuilding 1. There is also a 10mm2 G/Y earth cable from the BMET in outbuilding 1 to the MET at the source in the house. There is separate bonding of the gas pipe to the house MET where it enters the house.

    So the outbuilding 1 supply effectively is TNC-S but the earthing is odd to say the least as it is separated from the supply.

    The power requirements are low so 10mm2 should be quite adequate and the circuits have 30mA RCD overall.

    This all feels quite unconventional but I cant see that it breaches any regulation?  There is a diagram by the intake showing the arrangement.

    Long term the plan is to tidy this up which will not be simple but meanwhile what am I missing?


    Peter


    So there could be a TT Class 1  earthed appliance in outbuilding 1 AND a bonded metal gas pipe on a P.M.E. system? Is this your concern? Two earthing systems in effect.


    Does 541.2 apply along with the principle described in 542.1.3.3 I wonder?


    Z.


  • 0
    Thanks Z.

    No, I think outbuilding 1 is effectively TNC-S and not TT (the TT earth is not connected).

    I think my concern is that the path that the earth conductor for outbuilding 1 follows is different to the path that the L &N conductors follow.

    Having said that the diagram provided is very helpful for anyone who wants to know what the earthing arrangements are.

    The fact that a TNC-S earthed installation  is down stream (in L&N conductor terms) of a TT earthed installation is also at the very least unusual. This wouldn't normally arise as the installation of the separate earth would be not much easier than the installation of a separate supply (E&L&N).  In this case I suspect that the separate earth was originally an extra bonding conductor and has been repurposed, but I may be wrong.
  • 0
    Whilst this may appear slightly odd, it sounds OK. The gas pipe and 10mm provide a positive TNC-S Earth (with a reasonable local connection to the ground I expect from the pipe) and the fact this is a different route from the L&N is not a particular problem. You say building 2 is TT, I presume it has an Earth electrode which is satisfactory? As there are no exposed conductive parts connected to the building 1 Earth, it sounds safe enough, and both buildings have 30 mA RCD protection which is basically suitable as both additional protection (building 1) and Earth and additional protection (building 2). There is nothing here which contravenes the regulations directly but as you say is a little unusual. I think I would prefer building 2 to be connected to the TNC-S Earth as well as the electrode so that an extension from building 1 is not connected to a different Earthing system, but that is all. It is all as safe as you can make it with the gas pipe present.
  • 0
    statter:

    Thanks Z.

    No, I think outbuilding 1 is effectively TNC-S and not TT (the TT earth is not connected).

    I think my concern is that the path that the earth conductor for outbuilding 1 follows is different to the path that the L &N conductors follow.

    Having said that the diagram provided is very helpful for anyone who wants to know what the earthing arrangements are.

    The fact that a TNC-S earthed installation  is down stream (in L&N conductor terms) of a TT earthed installation is also at the very least unusual. This wouldn't normally arise as the installation of the separate earth would be not much easier than the installation of a separate supply (E&L&N).  In this case I suspect that the separate earth was originally an extra bonding conductor and has been repurposed, but I may be wrong.


    Yes, I would think that the 10.0mm2 G/Y single cable was originally a main bonding conductor for the metal gas pipe in outbuilding 1. It is normally the case that the supply L and N take the same route as the associated C.P.C. so I can see where the confusion comes in here. Electrically if the supply is just P.M.E. in building 1 with its associated earthing then all should be safe. We don't really want two earthing systems within the same building. If sufficient in size a cable can be a combined bonding conductor and C.P.C. at the same time.


    Z.


     


  • 0
    543.6.1 might be worth some consideration - "Where overcurrent protective devices are used for fault protection, the protective conductor shall be

    incorporated in the same wiring system as the live conductors or in their immediate proximity."


    Although in this case, I presume there's effectively RCD protection for the TN-C-S system in outbuilding 1 (via the TT system in outbuilding 2) - so you might be able to side-step that requirement.


       - Andy.

  • 0
    What sort of cables are used between the three buildings please?
  • 0
    AJJewsbury:

    543.6.1 might be worth some consideration - "Where overcurrent protective devices are used for fault protection, the protective conductor shall be

    incorporated in the same wiring system as the live conductors or in their immediate proximity."


    Although in this case, I presume there's effectively RCD protection for the TN-C-S system in outbuilding 1 (via the TT system in outbuilding 2) - so you might be able to side-step that requirement.


       - Andy.

     


    Thanks Andy, yes RCD should provide fault protection.  


     


  • 0
    Chris Pearson:

    What sort of cables are used between the three buildings please?


    House to OB2 is T&E (earth connected at source but not at remote end) joined using line taps to bare copper overhead on porcelain insulators (binding in is a real work of art).  back to T&E at remote end with earth connected to TT earth at destination.

    Two sets of rods achieve about 50 ohms for TT installation in OB2.


    From OB2 its SWA to OB1 with armour earthed at source and insulated at destination in OB1.


     


  • 0
    statter:
    Chris Pearson:

    What sort of cables are used between the three buildings please?


    House to OB2 is T&E (earth connected at source but not at remote end) joined using line taps to bare copper overhead on porcelain insulators (binding in is a real work of art).  back to T&E at remote end with earth connected to TT earth at destination.

    Two sets of rods achieve about 50 ohms for TT installation in OB2.


    From OB2 its SWA to OB1 with armour earthed at source and insulated at destination in OB1.


     




    417.3



    Z.


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