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
