On Tue, 02 Mar 2004 01:20:31 GMT, in uk.d-i-y Martin Angove
strung together this:

Yes, the incoming water (and gas and oil) pipework must be bonded to the
main earth, on the consumer's side, within 600mm (IIRC) of the meter or
stop valve and before any branch pipework, but I can't recall any
requirement to test that there is continuity between *supplementary*
bonded metalwork in the bathroom and the main earth.

The point of supplementary bonding is *not* to make sure all metalwork
stays at earth potential, rather it is to ensure that it all stays at
the *same* potential, whatever that may be. The slightly dubious safety
reasoning behind this is that if there is an earth fault in (say) your
shower, this may cause the touchable metal parts of the unit
("exposed conductive parts") (not that modern showers have any) to rise
above earth potential. By cross bonding these metal parts to other metal
parts ("extraneous conductive parts" e.g. the taps) with which you may
be in contact, in theory no current will flow (through you) as
everything is at the same potential. Hopefully, of course, current
*will* be flowing down the CPC in the shower supply cable and will pretty
rapidly switch off the supply.

If you provide an explicit link from the supplementary bonding straight
back to the main earth, this link could quite easily be of lower
resistance than the CPCs involved (supp. bonding is usually in 4mm2
cable, CPCs of 6mm2 shower cables are 2.5mm2 and of lighting cables,
1mm2) and therefore anything thus bonded *could* be at a lower potential
than the cause of the fault... and the fault current will flow through
the bonding rather than the CPC which is where it should be flowing.
Of course if everything is hunky-dory this is highly unlikely.

But as you say, the point of the supplementary bonding is to make all
the exposed metalwork in the bathroom the same potential. If this was
linked to the main earth termminal then the metalwork would still be
at the same potential, just lower.
...

SJW
A.C.S. Ltd.