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Bonding to single building with 2 TNCS supplies

gibb01
Hi,

I'm sure this has been answered before - but we've just been carrying out an EICR on a small commercial building that has a 100A 3 Ph TNCS Supply to the basement & separate 100A 3Ph TNCS supply to the ground floor. We believe it used to be to separate tenancies & has since been occupied by a single tenant. The installation has a single incoming water supply to the basement that is bonded to the basement MET, However there is no bonding from the ground floor MET onto the same water supply. 

I assume it's a simple fix of adding a 10mm from the ground floor - but just wanted to see what other views are in case i'm missing something obvious. 

Not that I believe it makes too much difference, but we believe the supplies to be fed via separate TXs as the client has mentioned one supply has stayed live whilst the other has not during a power cut. 

Thanks in advance!
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    It is not simple.

    If the two do share a substation earth, even if not the same transformer, then there is a risk of diverted neutral currents through the building frame and shared services.

    If it is not practical to do without one supply or the other, then either the two METS should be very solidly bonded together, or one or other supply arranged as TT.

    Mike.
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  • 0
    It is not simple.

    If the two do share a substation earth, even if not the same transformer, then there is a risk of diverted neutral currents through the building frame and shared services.

    If it is not practical to do without one supply or the other, then either the two METS should be very solidly bonded together, or one or other supply arranged as TT.

    Mike.
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