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RCBO shared between TN-S and TT earthing systems.

justinneedham
Have come across ~ 20 year old installation where RCBO in TN-S household CU acts both as additional protection for local sockets circuit, and fault protection for a remote garage which is spurred then TT'ed. 

I can't on the face of it see any reason (other than inconvenience in event of a fault), why not. (OK, BBB.. 531.3.5.3.1). 

  • 0
    well, electrically it is OK, in the sense that both ring and garage are protected against L-CPC and L- terra-firma faults.

    I think ideally as far as the regs, you'd have a dedicated RCD for the garage, but realistically they would race.
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    I can't on the face of it see any reason (other than inconvenience in event of a fault), why not. (OK, BBB.. 531.3.5.3.1).



    You might also consider the TT regulation 411.5.1 "Every exposed-conductive-part which is to be protected by a single protective device shall be connected, via the main earthing terminal, to a common earth electrode. ...." Although I can't see a physical reason for such a demand (at least since VoELCBs were dropped from the regs) and there could be some debate in cases such as this where part of the TT system (the protective device and the upstream part of the circuit) is outside the TT system as it were.


      -  Andy.

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