DIYbanter - View Single Post - Bathroom Earth Bonding

wrote:

Hmm, logically (or even illogically) speaking, would you want to
connect the earth bonding to the cpc of a local circuit, wouldn't this
potentially increase the risk of a live to earth fault, as you have
intentionally moved the protective earth bonding to the close vicinity
of the/any live terminals ?

Or am I just being silly ?!

Methinks you've missed the whole point of supplementary bonding, which
is to prevent the appearance of voltage differences - "touch voltages"
to use the jargon - as Owain has explained.

But if it is taken separately you are not intentionally connecting into
junction boxes with adjacent lives, it goes back to the star terminal
at the cu which only contains earths.

Ditto. The whole rationale here is *bonding* rather than *earthing*
/per se/. If a bathroom appliance develops an internal earth fault
(phase-CPC short) then there will be a significant voltage drop across
the length of the CPC until the protective device clears the fault.
During this time a potentially lethal touch voltage could exist between
the exposed metal parts of the appliance and (say) the water taps & bathtub.

Example: let's say the appliance is an electric shower unit wired in
10mm^2 T&E cable, which has a 4mm^2 CPC. If the supply to the final
circuit had negligible impedance then an earth fault in the appliance
would cause 10/14 of the mains voltage to appear on the shower unit's
case. The unfortunate user is then possibly standing in an earthed
metal bath with the shower outlet and pipework at ~160 V. This
condition could exist for up to five seconds (the max. allowed fault
clearance time for fixed equipment circuits) and the danger of that in a
wet-body situation should be obvious.

Feel free to shoot me down here, but I have just the same bonding
question. I have bonded all exposed metalwork together in the bathroom,
including to a cold water pipe. I don't particularly want to link into
any local circuits, I was going to take back to the TN-S earth point at
the CU.

Then you must be shot down in flames because what you are proposing
clearly violates the wiring regulations (BS 7671). Supplementary
bonding to exposed-conductive-parts (and therefore the CPCs) of
electrical equipment in a bathroom has been required since coming of the
14th edition of the IEE Regs (c. 1966). When the revised Section 601
(bathroom rules) came in to force in 2000 this was modified to require
bonding to the CPCs of *all* circuits feeding equipment within the
zones, _whether_or_not_ there are exposed-conductive-parts. Thus you
must still bond to the relevant CPCs, even if Class 2 equipment is
installed. This is clearly intended to provide for safety when
equipment is subsequently changed.

HTH
--
Andy