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Bonding questions

Former Community Member
Former Community Member
Hi,


I have some bonding questions which I'm hoping that the experts on here can help with.


Example - TN-C-S installation with 25mm2 tails, 16mm2 main earthing conductor, 10mm2 main bonding conductors to incoming, buried metal water and gas pipes and 4mm2 supplementary bonding in the bathrooms.


Assuming the pipes have good conductivity to earth, am I correct in assuming that (1) during normal operating conditions, a portion of the 'return' current will flow via the bonding conductors & pipes (depending on the relative resistances of that path versus the main PEN conductor path) and (2) if the PEN conductor is damaged then all of the 'return' current will flow via the bonding conductors & pipes?  If so, I'm wondering why 10mm2 bonding conductors are deemed to be sufficient, particularly in the latter case where a potentially large current is flowing and it is perhaps not obvious that a fault exists?


I also started thinking that, depending on the layout of pipework and how the supplementary bonding is done (e.g. multiple pipes coming off of the incoming water main that end up being supplementary bonded at some end point) that current could also be flowing in the supplementary bonding conductors in cases 1 and 2 above.  Which then leads me to the question of why 4mm2 supplementary bonding conductors are deemed to be sufficient...


In my mind, the only way to be sure would be to measure the relative resistances of all of the various earth paths, but the regs quote the standard bonding conductor sizes without reference to this.  Sorry if this is a silly question or if I'm missing something fundamental, but it's had me scratching my head recently.  Any guidance gratefully received.


Regards,

Graham
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  • 0
    Good question - basically the main bonds are sized only for normal operating conductions - they're a decent fraction of the PEN conductor and experience shows that a fairly large proportion of N currents will follow the PEN. Broken PENs can cause a lot of trouble - if you really wanted to size bonds for that you'd have to consider some of your neighbour's N currents not just your own, and could end up with a size similar to what's running down the street - which probably wouldn't be practical.


    I'd argue that the resistance of buried metallic service pipework to true earth isn't an overriding consideration though - since it's more than likely connected back to the supply N point by a (or more likely many) continuous metallic path(s) - via neighbours' installations.


    The limit for supplementary bonding is actually 2.5mm² or half the size of the corresponding c.p.c. if it's connecting an exposed-conductive-part, or the smaller c.p.c. if interconnecting two exposed-conductive-parts - the 4mm² limit is purely for mechanical robustness considerations where it doesn't have other protection. But in a similar way that main bonding should be 'shunted' by the supply PEN (so the PEN always takes a significant proportion of the current), if supplementary bonding is exposed to diverted N currents, it should similarly be 'shunted' by the main bonding (you shouldn't be getting diverted N currents from within the installation).


    For sure you can construct theoretical arrangements where the bonding conductors look to be undersized, but in the experience of millions of homes across many decades suggests it's not a practical risk. (A bit like the unbalanced ring circuit problem.)


       - Andy.
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  • 0
    Good question - basically the main bonds are sized only for normal operating conductions - they're a decent fraction of the PEN conductor and experience shows that a fairly large proportion of N currents will follow the PEN. Broken PENs can cause a lot of trouble - if you really wanted to size bonds for that you'd have to consider some of your neighbour's N currents not just your own, and could end up with a size similar to what's running down the street - which probably wouldn't be practical.


    I'd argue that the resistance of buried metallic service pipework to true earth isn't an overriding consideration though - since it's more than likely connected back to the supply N point by a (or more likely many) continuous metallic path(s) - via neighbours' installations.


    The limit for supplementary bonding is actually 2.5mm² or half the size of the corresponding c.p.c. if it's connecting an exposed-conductive-part, or the smaller c.p.c. if interconnecting two exposed-conductive-parts - the 4mm² limit is purely for mechanical robustness considerations where it doesn't have other protection. But in a similar way that main bonding should be 'shunted' by the supply PEN (so the PEN always takes a significant proportion of the current), if supplementary bonding is exposed to diverted N currents, it should similarly be 'shunted' by the main bonding (you shouldn't be getting diverted N currents from within the installation).


    For sure you can construct theoretical arrangements where the bonding conductors look to be undersized, but in the experience of millions of homes across many decades suggests it's not a practical risk. (A bit like the unbalanced ring circuit problem.)


       - Andy.
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