Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 9 replies
  • Subscribers 32 subscribers
  • Views 1450 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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!
Parents
  • 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).


Reply
  • 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).


Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines