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Change to 544.1.1 - do protective conductors between buildings with extraneous-conductive-parts no longer need to be sized for main bonding?

AJJewsbury

544.1.1 now includes the sentence "Where an installation serves more than one building, a main protective bonding conductor shall be selected in accordance with the characteristics of the distribution circuit protective conductor for that particular building."  At the same time 411.3.1.2 has changed to suggest that bonding applies separately to each building.

So if I had an outbuilding with modest power requirements, could I just run say a 2.5mm² T&E - with just a 1.5mm² - to it, connect the outbuilding's earth terminal to that building's extraneous-conductive-parts using (min) 6mm² and be compliant? No need to size the c.p.c. between the main and outbuildings according to main bonding as we used to do.

That feels a bit dodgy to me - potentially connecting two distinct extraneous-conductive-parts with nothing but a thin c.p.c. - as not all services always enter the main building first and these days insulating repairs/alterations can readily isolate one building but not another from the street mains.

I realize none of this applies to PME systems - but there are plenty of TT installations (some still sharing metallic pipework with neighbouring installations) and a few proper TN-S ones around.

    - Andy.

Parents

  • It is an interesting question - clearly if the water to the outbuilding comes from the house - perhaps verified if it can be turned off from a stop tap already on a pipe main bonded then there is no issue - but as drawn, where the water pipe and the mains are supplied from opposite ends of the installation then something akin to the problem of multiple PME supplies in the same block arises.
    For a long time the fact that the regs specify main bonding size conductors even where there is no credible high current path has rankled, as a waste of effort and copper, but equally a broad brush abandonment is just as unwise. What is really needed is far more nuanced guidance where the installed can only reduce conductor size after verifying the route or impedance of the non cabled metallic paths.

    (and while you could perhaps protect the inadequate CPC with a fuse or impedance to limit the current, but that too would be very much against the spirit of the regs  though I have often though t for FE it could be permitted so long as the safety earthing was separated.)
    Mike.

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  • It is an interesting question - clearly if the water to the outbuilding comes from the house - perhaps verified if it can be turned off from a stop tap already on a pipe main bonded then there is no issue - but as drawn, where the water pipe and the mains are supplied from opposite ends of the installation then something akin to the problem of multiple PME supplies in the same block arises.
    For a long time the fact that the regs specify main bonding size conductors even where there is no credible high current path has rankled, as a waste of effort and copper, but equally a broad brush abandonment is just as unwise. What is really needed is far more nuanced guidance where the installed can only reduce conductor size after verifying the route or impedance of the non cabled metallic paths.

    (and while you could perhaps protect the inadequate CPC with a fuse or impedance to limit the current, but that too would be very much against the spirit of the regs  though I have often though t for FE it could be permitted so long as the safety earthing was separated.)
    Mike.

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