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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!
  • 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.
  • 0
    Thanks Mike,

    I had a suspicion that might be the case. Is there no issue from a DNO perspective of joining the METs or is that generally accepted practice in this situation? It's not something I've come across before.

    Thanks,

    Tim
  • 0
    I should add that I don't think TT is an option. It's central London, with no real practical location for a rod anywhere.
  • 0
    gibb01:

    Is there no issue from a DNO perspective of joining the METs or is that generally accepted practice in this situation? It's not something I've come across before.

     


    There may be, but more importantly BS 7671 also acknowledges the problem, in Regulation 542.1.3.3

     



    542.1.3.3 Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.





    How will you rate the conductor bonding the two installations together? Can you be sure they come from the same main in the street (they probably do, but ...)?


    One thing to note, you don't state whether the two installations are totally separate, or whether they share any space in the building in common. But I guess it's the case that someone could run an extension lead from one part of the building to another. If either of those is the case, and there is a risk that exposed-conductive-parts supplied from both installations could be touched simultaneously, then it does need addressing, as one of the basic rules for fault protection as part of automatic disconnection of supply is not met (Regulation 411.3.1.1, particularly my highlight below):



    411.3 Requirements for fault protection
    411.3.1 Protective earthing and protective equipotential bonding

    411.3.1.1 Protective earthing

    Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.
    Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

    Conductors for protective earthing shall comply with Chapter 54.

    A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.


  • 0
    Thanks for the reply.  Yes the two installations do share space, with some circuits running from the basement to the ground floor & vice versa. 

    Given they are both 100a TP&N I'd assume installing a 16mm G&Y as per table 54.7 between the two METs will suffice & adding a 10mm G&Y to the incoming water to the basement from the Ground floor MET (Which is already bonded via the basement MET) 


    Many thanks,

    Tim
  • 0
    gibb01:


    Given they are both 100a TP&N I'd assume installing a 16mm G&Y as per table 54.7 between the two METs will suffice & adding a 10mm G&Y to the incoming water to the basement from the Ground floor MET (Which is already bonded via the basement MET) 

     


    If the supplies are from the same main in the street, your assumption is probably correct.


    There is a reason, though, that 542.1.3.3 does not refer to Table 54.7 (or indeed anywhere else in Chapter 54).


    If the supplies are not from the same LV main ... now, or at some point in the future ... then who knows what diverted neutral currents there will be? Perhaps this is why the bonding is missing upstairs?


    If the supplies are from a separate transformer, this is where things start to get tricky, as you may well need to take into account HV fault currents etc. which is well outside the remit of BS 7671.



    To be honest, asking the DNO is the most correct course of action ... However, they may well consider it a change of use that's not been notified, and they may want to propose some changes (at a cost).


  • 0
    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).
  • 0
    Many thanks for all your replies! Looks like an email to UKPN is on the cards..


    Thanks,

    Tim
  • 0
    As others have noted the two lots of earthing are effectively already intertwined and probably linked by odd things such as water heater CPCs to common pipework , so effective separation  is not a viable option.

    The question becomes, when  a current tries to flow between them, does it go via anything so thin that is does any damage.. The DNO are the only folk who can estimate the likely  current, and that, in conjunction with the distance,  will determine how big a chunk of copper is needed. good luck.

    Mike.

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