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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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    You have made another point, that this is in central London. Here the supply arrangements are significantly different to most other places, in that many of the supplies are"Grided" to provide the necessary power levels and reliability. This makes it much more unlikely that neutral loss is a problem, the system is much more like a giant ring circuit with multiple points of supply. I suggest the DNO is the best point of contact, and if the water supply is steel, your METs are probably well connected elsewhere. You could get an idea by measuring the voltage between them, it may well be virtually zero, in which case two supplies are unlikely due to differing neutral voltage drops depending on load. (PME expected).
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    You have made another point, that this is in central London. Here the supply arrangements are significantly different to most other places, in that many of the supplies are"Grided" to provide the necessary power levels and reliability. This makes it much more unlikely that neutral loss is a problem, the system is much more like a giant ring circuit with multiple points of supply. I suggest the DNO is the best point of contact, and if the water supply is steel, your METs are probably well connected elsewhere. You could get an idea by measuring the voltage between them, it may well be virtually zero, in which case two supplies are unlikely due to differing neutral voltage drops depending on load. (PME expected).
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