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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

  • The second building with the same supply is always a point of contention, and you need to consider if bonds to incoming extraneous parts are actually MAIN bonding conductors (to a distribution circuit), or if these are supplementary bonding. That is the point though Andy, they share extraneous parts and these must either be fairly high resistance apart or directly connected, in which case one does not need the bonding conductor. To get seriously large bonding currents the necessary circumstances are both very unlikely, or more likely impossible. Plastic service pipes have reduced the possibilities still further, and real Earth resistance hasn't changed. Try drawing out the possible fault circuits, and compare these with any buildings you know, are they similar, I rather doubt it?

    Even with PME the bonding conductors are quite small really, and the risk is negligible, dangerous events are extremely unusual, again probably due to service pipes being plastic. In a TT installation the RCD protection protects against foreseeable circumstances, or are you simply thinking that the local CPC potential may be raised?

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  • The second building with the same supply is always a point of contention, and you need to consider if bonds to incoming extraneous parts are actually MAIN bonding conductors (to a distribution circuit), or if these are supplementary bonding. That is the point though Andy, they share extraneous parts and these must either be fairly high resistance apart or directly connected, in which case one does not need the bonding conductor. To get seriously large bonding currents the necessary circumstances are both very unlikely, or more likely impossible. Plastic service pipes have reduced the possibilities still further, and real Earth resistance hasn't changed. Try drawing out the possible fault circuits, and compare these with any buildings you know, are they similar, I rather doubt it?

    Even with PME the bonding conductors are quite small really, and the risk is negligible, dangerous events are extremely unusual, again probably due to service pipes being plastic. In a TT installation the RCD protection protects against foreseeable circumstances, or are you simply thinking that the local CPC potential may be raised?

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