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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?
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  • 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.


     


Reply
  • 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.


     


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