RzB wrote:

Hmm - OK I'm Confused... and on re reading the regs I'm still confused..
I expected one of the Gurus to chip in that 4mm was needed not 2.5mm.

I had thought that 4mm cable in a nice green/yellow cover running round
the bathroom was the way to go..

It is, usually. You "can't go wrong" if you use 4.

In sections 547-03-02 & 03 what is the meaning of "sheath" and
"mechanical protection".. These don't get spelled out in the definitions.

Is "Sheath" the green/yellow plastic stuff?

No, that's insulation. All separate protective conductors of 6 mm^2 or
less have to be insulated [543-03-02].

Sheath is a second layer of PVC, usually grey, like the outer of
twin-and-earth cable or meter tails.

What is "mechanical protection" in this context? The side of a bath?
Steel conduit? And why should the type of protection dramatically
affect the diameter of the bonding...

I've always assumed it to mean "enclosed in some recognised form of
wiring system" - i.e. conduit (steel or PVC) or trunking (including
mini-trunking). The intent of the regs is to prevent separate thin
easily-broken wires being used. IOW 4 mm^2 is considered sufficiently
robust not to get damaged, even if not enclosed.

Hmm - I'm obviously confused - could somone please spell out
547-03-02 & 03 in simple terms!

547-03-02: For electrical purposes the size of bonding between
electrical circuit earths (CPCs of e.g. shower and lighting circuits)
and extraneous-conductive-parts (e.g. heating pipes) must be at least
half the size of the relevant CPC, but subject to a minimum of 4 mm^2
for reasons of mechanical robustness if not enclosed.

547-03-03: Same thing, but between two extraneous-conductive-parts.
Electrical: 2.5 min, but 4 if not enclosed.

There is another constraint in 413-02-28 which imposes resistance limits
for supplementary bonding. In principle this could mandate larger sizes
than 547-03-xx, - but not in a normal-size bathroom.

HTH
--
Andy