Using the earthing system of one building to another

Question

I have a question about earthing systems. 
 In this scenario, I have a main commercial building that is fed from a new substation and is likely (as expected) to be a TNCS arrangement with a neutral earth link in the main LV switch panel for the customer. From this LV switch panel, it feeds another building as part of the sub-mains distribution, probably close to 80m away. The schematic shows parallel multicore cables supplying the building with a separate CPC and, in addition to this, a separate bond to a dedicated earth bar (i.e. 1No CPC and 1No bond). I presume this is acceptable, and even though this may look like a TNS in the second building, it is an extension of the TNCS with the CPC going back to the first building's mains switch panel where the neutral earth link is...... 
 
 What are people's thoughts?

mapj1 · Accepted Answer

Given that in anywhere that ESCQR operates, you cannot distribute as TNC after you have connected your first piece of consumers equipment, it is not only right, but essential, that the supplies to outbuildings on submains are carried as the 'S' part of the TNC-S system. Within those buildings you would treat the requirement for bonding and so on, as you would for the main building i,e, plumbing bonds and so on in PME size cables, but apart from that it is all normal. 
 Mike.

AJJewsbury · Answer

TN-S is only TN-S if the PE is separate from the N all the way back to the source (transformer) - generally it's assumed that any DNO provided earth terminal these days will have "PME conditions" attached, even if it looks like TN-S at the intake. 
 DNOs even sometimes provide separate earth wires to multiple customers in a multi-occupancy building - connected to a common N-PE link outside - all fed from a conventional TN-C main - all still very much PME. 
 
 a separate bond to a dedicated earth bar (i.e. 1No CPC and 1No bond). 
 
 That bit looks slightly odd (if I'm reading it right) - BS 7671 requires both c.p.c.s and main bonding conductors to connect to the same MET within each building (to minimise the potential difference between exposed- and extraneous-conductive-parts) - so while two "earth" conductors isn't a problem as such, they really should be connected together at both ends (or go to earth bars which are then connected together). 
 - Andy.

gkenyon · Answer

The main bonding of the second building, and a protective bonding from the MET of the second building back to the MET of the original building, will still need to be sized in accordance with Table 54.8 using the CSA of the distributor's PEN conductor.

Using the earthing system of one building to another

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