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Separate main bonding to outbuilding

Former Community Member
Former Community Member
Hi, I just wanted people’s thoughts on this please.

I have been to a job with a TN-S supply. A sub main feeds a separate building with an SWA, the armour being used as the CPC. I have calculated the armour has not the additional cross sectional area to used as the required main bonding. I understand from GN8 that the main bonding needs to be extended to the building as specified in size by BS7671. I realise that this main bond will run from the MET to the Earth marshalling terminal in the second building. My question is this:

The SWA is buried and cannot be dug up (well easily). There is a second duct that runs between the buildings which is empty, but it follows a different route to the SWA. I seem to remember reading that it is best to run a separate bonding cable in close proximity to the existing SWA, am I correct or did I dream this? Is it advisable not to run the additional cable via a different route? If so, my only option will be to TT the second building.


Thanks in advance.


KS
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    As they are extremely unlikely to carry both currents at once, the largest size is enough. Failure due to fault current is very unlikely anyway unless this is a continuous diverted neutral current. You can immediately understand this in practice as you will almost certainly never have seen an overheated main bond, and it is unlikely even if there has been a neutral "incident". Making the bond larger than the incoming neutral is obviously unnecessary, the cable enclosed neutral will melt first whatever the bond does. One needs to be careful of "what if's" as most of them have never been seen or happened.
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    As they are extremely unlikely to carry both currents at once, the largest size is enough. Failure due to fault current is very unlikely anyway unless this is a continuous diverted neutral current. You can immediately understand this in practice as you will almost certainly never have seen an overheated main bond, and it is unlikely even if there has been a neutral "incident". Making the bond larger than the incoming neutral is obviously unnecessary, the cable enclosed neutral will melt first whatever the bond does. One needs to be careful of "what if's" as most of them have never been seen or happened.
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