"Lurch" wrote in message ...

[Mike Hall]
Andy, but therin lies the crux of my question! - If I've joined all
3 cpc's, (2 halves of the ring and the lighting circuit) then the
cpc for the ring is now not a simple ring in itself.

It has to be in the form of a ring with both ends connected to the earth bar
in the CU. This does not forbid cross-connection with the CPC of another
circuit - see reg. 543-02-09 (and see below re. supplementary bonding in
bathrooms).

[Andrew Gabriel]
I think that's fine. I can't quite remember exactly what the regs
say, but there are cases of ring circuits where the CPC clearly
isn't a ring, such as when it's provided by conduit or the outer of
mineral insulated cable. I think the important point is the CPC
impedance at any point must be no worse than if it was a ring, or
you are going to have to do calculations to ensure you are still
withing allowable earth fault loop impedance for whatever your
protective devices are.
No amount of extra connections between CPC's can make it worse,
actually quite the opposite.

Agreed.

[Lurch]
Although, I personally would seperate them as I have had occasions
where I have done something that is within the regs but extremely
uncommon and then spent more time and effort trying to convince
people that it's correct than it would've taken me to do it the
common method in the first place.

Although I agree with the general drift of that (don't adopt unusual circuit
arrangements in domestic installations) I don't agree that it applies here.

Cross-connection between the CPCs of different circuits can be expected to
occur in some situations - e.g. between 'upstairs' and 'downstairs' lighting
circuits where they meet at one or more 2-gang 2-way switches (one switch on
each circuit) for hall and landing. Another case where cross-connection is
actually _required_ by the regs is in a bathroom (within the zones) where
you have some combination of shower, fused conn. unit on ring (for heater or
towel rail etc.), and luminaire and/or shaver outlet. Here the CPCs of the
relevant circuits must be cross connected by supplementary bonding. Since
the 2001 edition of BS 7671 came into force this bonding is required whether
or not the equipment connected to the said circuits has any
exposed-conductive-parts - in fact the bonding is still required if all the
items are Class 2.

--
Andy