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Bonding

Guy Chapman
I have an extension going on to my house and it has engineered joists. I am going to run cables in steel tray, for tidiness, and I am assuming this would be considered an ECP and should be bonded (that makes obvious sense to me). It will carry ring and lighting circuits. I cannot imagine that more than 10mm2 would be required for this, but every time I read new guidance they seem to have upped the ante. What do people think? And do you use link jumpers between sections of tray, or are the bolts sufficient? I've not seen anything to suggest that copper jumpers are needed, and we don't use them in datacenters (the day job), but that's just CAT7 and fibre.


Also because Reasons there are no spare ways on the earth block on the board. Do I have to source an earth block of more than 8 ways or is it legit to join two blocks together with 16mm2? I don't want to look like a pillock and have the sparks refuse to energise the circuits when done.


Thanks
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    Guy


    Tray does not require bonding as should only have insulated and sheathed cables on it.
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    Got to ask why would you want to use tray in the first place. I would think it’s going to be a right hassle to install with no benefits.
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    It is good practice to bond tray etc, and while not extraneous, there is the EMC question these days. Although this is not main bonding, 10mmsq is a practical size. The DNO require links on steel rafters on a PME system, and again, although not essential in this case, the links finish off a quality job.


    Regards, UKPNZap
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    Cables across the joists, I wanted them more or less on the centre line and don't fancy tying to the webs. It seemed like an easy thing to do (and is trivial to install during the build).
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    Only extraneous-conductive-parts require bonding.


    If it is not then it would be earthing, which will introduce an unnecessary hazard.


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    You know what, I think you are right. I think I need a copy of Guidance Note 8 to be certain but as it doesn't come in from outside the building, and the cables are all insulated, then it does not seem to meet the definition of extraneous-conductive or exposed conductive. I confused the two, I reckon. But UKPN makes a good point too. You can make a case for earthing it (as there are plausible fault conditions due to mechanical damage that could lead to the part becoming live). There's no data, I run ELV physically separated, so no real EMC issue so the real question is, would earthing the tray introduce a potential hazard. I guess that would be an ecumenical matter.


    I found this rather useful discussion: https://www.tradeskills4u.co.uk/posts/metallic-cable-tray


    I think the answer is: it doesn't need to be bonded, it probably doesn't need to be earthed, there might be additional risk if it was earthed, so maybe don't. This turns out to have been a much more interesting question than I thought, and I thank everyone for their thoughtful and helpful input.
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    geoffsd:

    Only extraneous-conductive-parts require bonding.


    If it is not then it would be earthing, which will introduce an unnecessary hazard.


     


    There is Functional Bonding ... if provided for the purposes of EMC, as other contributors of this thread have posted, Regulation 444.5.3.1 uses the term "bonding". BS IEC 61000-5-2 uses the same terminology.


    The reasoning is :



    • it is provided for the purposes of equalization of potential to reduce the impact of radiated disturbances - although of course, it's not protective equipotential bonding.

    • there is not necessarily a connection to Earth involved in this type of functional bonding, although in most installations, a common bonding network (CBN) for EMC mitigation purposes is effectively connected to Earth via at least one point (the MET, via extraneous-conductive-parts bonded there and continuing inside the installation, or via exposed-conductive-parts).

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