Bonding to single building with 2 TNCS supplies

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.

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.