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Exporting TN-C-S earth in TN-S

Former Community Member
Former Community Member
I know this sounds weird, but let me explain before.


I have two dwellings both connected to one substation. One classified TN-C-S (dwelling A) and the other TN-S (dwelling B).


I have to run a new line for providing electricity inside the new plant room that will be built inside the dwelling B. The new line will be taken from a DB installed inside the dwelling A.


The reason because I need to do in this way, it is because of in the dwelling B I have no in enough power and I'm not allowed to run a new line from the substation.


My question is: if I run a new cable form the dwelling A to dwelling B, should I also connect the earth system of the dwelling A to the plant room in the dwelling B? I have to suppose yes. Obviously, I have to keep separate the two earth system, have not I?
  • Former Community Member
    0 Former Community Member
    A couple of questions to get your scenario straight in my head please.

    Are these actually dwellings?

    How large a plant room/supply are you talking about?

    What capacity are the two current supplies?

    Is this in the UK?

    Will there be a "brick wall" totally isolating the plant room and the TN-S "zone"?

    Will extraneous conductive parts pass from one earthing "zone" to the other?
  • Former Community Member
    0 Former Community Member
    A couple of questions to get your scenario straight in my head please.
    Are these actually dwellings? Yes Dwelling A is a restaurant e dwelling B is a big house
    How large a plant room/supply are you talking about? The Main supply for the plant room is around 100A three phase
    What capacity are the two current supplies? In the Dwelling A is 600A  in the dwelling B 100A
    Is this in the UK? Yes
    Will there be a "brick wall" totally isolating the plant room and the TN-S "zone"? The plant room is already existing and it has been built with bricks and it is located in the basement of the house
    Will extraneous conductive parts pass from one earthing "zone" to the other? unfortunately yes
  • Former Community Member
    0 Former Community Member
    Hi,

    So "dweling A" is not a dwelling at all it is a commercial building, i.e. a restaurant.


    Next question, why will the DNO not provide an upgraded supply to the house?

    You are taking the extra 100A from their existing transformer, so the capacity is already there, I suspect that you believe that the restaurant is not near their 600A limit, what if they are?

    Do you know that you have 100A of spare capacity in the restaurant supply?

    How do you know that in a short time the restaurant will not be sold, and they will require all of the supply capacity, and then disconnect the house?
  • Former Community Member
    0 Former Community Member
    Next question, why will the DNO not provide an upgraded supply to the house? Because the house and the substation are far away from each other. After around there is a huge park, with a lot of restricted​
    You are taking the extra 100A from their existing transformer, so the capacity is already there, I suspect that you believe that the restaurant is not near their 600A limit, what if they are? Indeed the restaurant has a 250A free so we could use part of that. ​
    Do you know that you have 100A of spare capacity in the restaurant supply? yes
    How do you know that in a short time the restaurant will not be sold, and they will require all of the supply capacity, and then disconnect the house? No, because the house the park and the restaurant are part of the same company. They are going to do this kind of job for the renovation of the entire site.​
  • 0
    There are a lot of issues here, perhaps not an easy one to answer at all without seeing the particular site and conditions.


    In terms of BS 7671, it could be very difficult to comply in this situation, depending on what the plant room was doing, where it was serving around the site(s), and the extent of simultaneously-accessible, or commonly-connected, extraneous-conductive-parts and exposed-conductive-parts. But I guess that could perhaps be an issue if the plant room is serving both installations in any case?


    In particular, the requirements of two important requirements in this regard:

    Regulation 411.3.1.1 Protective Earthing [for fault protection]


    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 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.




    and
    Regulation 542.1.3.3 [Installation earthing arrangements]




    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.






    Have you explored other options, such as converting both installations to TT and using a common earthing system? Although of course, this might not be cheap as additional protective devices and main switchgear assembly equipment, may be required.


     


  • Former Community Member
    0 Former Community Member
    Thank you Graham, I have been very busy over the last couple of days.

    I was hoping to try and guide the poster to his own enlightenment, but your response sums this up well.
  • 0
    Sorry Paul, size 11 site boots ...
  • Former Community Member
    0 Former Community Member
    Don't fret Graham it's fine, I have been criticsed many times online for my way of trying to guide people to enlightnment rather than tell them, and you certainly didn't do that!

    My skin is thick enough, you can apologise in person in July if you like! ;)

    However, absolutely NO bad feelings I promise.

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