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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.

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  • OK , you have made several assumptions there Andy, and whilst there is some possibility that they may be true, it is unlikely. The first is that the CPC is going to carry a large current, which it cannot because of its resistance and the fact that the diverted voltage driving the current will be small, unless there is a serious fault elsewhere. To cause a failure of the 1.5mm CPC a continuous current of somewhere around 50A would be required and a reasonable estimate for the cable resistance may be 2 Ohms, so the driving voltage would have to be 100V! This is obviously ridiculous and a voltage of perhaps 1V is possible, so a current of 500mA.

    If worried about this situation an insulating link in the water pipe is the obvious answer, and you must admit that it would be an unusual scenario.

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  • OK , you have made several assumptions there Andy, and whilst there is some possibility that they may be true, it is unlikely. The first is that the CPC is going to carry a large current, which it cannot because of its resistance and the fact that the diverted voltage driving the current will be small, unless there is a serious fault elsewhere. To cause a failure of the 1.5mm CPC a continuous current of somewhere around 50A would be required and a reasonable estimate for the cable resistance may be 2 Ohms, so the driving voltage would have to be 100V! This is obviously ridiculous and a voltage of perhaps 1V is possible, so a current of 500mA.

    If worried about this situation an insulating link in the water pipe is the obvious answer, and you must admit that it would be an unusual scenario.

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