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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
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  • 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.


     


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  • 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.


     


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