Borrowing a cpc from another cct for lighting cct coded C2 in PE magazine codebreakers.

I haven't see the article, but I agree it sounds a bit odd. I can see the logic that the two circuits are not entirely separate (especially if you had a situation where large protective conductor currents were present) but I don't that not being "individually isolatable" as being a contravention of the regulations (unless there was a particular requirement for them to be so). After all we allow single pole MCBs on TT systems that require the N to be broken for isolation - so strictly speaking we'd have to revert to the main switch or shared RCCB to isolate a final circuit.

c.p.c.s are often linked by shared extraneous-conductive-parts or supplementary bonding anyway.

I'd admit I have been a little uncomfortable when I've had to "borrow" a c.p.c. from another circuit - mostly because of the danger of someone working on the other circuit disconnecting the c.p.c. without realizing the it could still be in use via the other circuit - but my solution to that was just a label at the CU.

   - Andy.

If you have wired in singles in galvanised conduit, and used the conduit as cpc, it is shared by all the circuits.

We do not normally consider the CPC to be part of the working circuit. It is not nice if a  CPC follows a different route to the appliance as the L and N, as it may be accidentally broken and not noticed, but it is not a regulatory fail  as such - after all we could be using metal trunking as the CPC.  I detect over-zealousness again.

Under what conditions of isolation do you switch a CPC anyway ? (apart from gensets in parallel with the mains, which I presume this is not.)

Mike.

The article quotes regs 134.1.1 "good workmanship", "proper materials" and "manufacturers' instructions" so i don't see the relevance of that, and 314.4 "... The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated". So the potential danger is working on something live which you believe to be isolated, and just means the live conductors, surely not the CPC.

Also you are allowed to share CPCs in 543.7.2.201(c) between two identical circuits having high protective conductor currents 

hertzal123 said:

Thought this was allowed under 543.1.2  if the borrowed cpc was of adequate size?

But not permitted in general under Regulation 542.6.1 unless ADS is provided by RCD alone:

542.6.1 Where overcurrent protective devices are used for fault protection, the protective conductor shall be incorporated in the same wiring system as the live conductors or in their immediate proximity.

So if you really do "pinch the cpc from another circuit", it does not comply in many TN systems.

Chris Pearson said:

If you have wired in singles in galvanised conduit, and used the conduit as cpc, it is shared by all the circuits.

And is the "same wiring system", so there is no problem ... no code required in that case ... but this doesn't apply to pinching the cpc from another circuit wired in T&E etc.

OlympusMons said:

Also you are allowed to share CPCs in 543.7.2.201(c) between two identical circuits having high protective conductor currents

But in that case, each circuit would have a cpc of its own (to comply with Regulation 542.6.1), along with a "reinforcing cpc" from another circuit to meet the additional requirements of Regulation Group 543.7 ?



At the end of the day, though, a cautionary note for those who think the guidance quoted in the OP is poor advice. What if someone doesn't realise the cpc is taken from another circuit. The circuit from which the cpc is taken is isolated for maintenance, leaving some of the installation intact, and a fault occurs, and someone disconnects the cpc to work on it ... I have only at this point to quote CDM Regulations and Designer's Duties to bring the whole discussion back on an even keel ... only "pinch a cpc" if it's well documented and signposted.

So if you really do "pinch the cpc from another circuit", it does not comply in many TN systems.

I guess it's possible that one T&E can run in the immediate proximity of another T&E (like a G/Y run alongside SWA) - even extraneous-conductive-parts may be used as c.p.c.s (543.2.1 (vii)) -so certainly no requirement to be part of the same wiring system. In any event if there was an upstream RCD all it would take would be a flick of the pen by the designer to nominate that as the ADS device.

So what's the actual danger from such a non-compliance - compared with the same arrangement on a TT system (would be entirely acceptable) what's the additional danger? Does it really equate to a C2?

    - Andy.

gkenyon said:

Chris Pearson said:

If you have wired in singles in galvanised conduit, and used the conduit as cpc, it is shared by all the circuits.

And is the "same wiring system", so there is no problem ... no code required in that case ... but this doesn't apply to pinching the cpc from another circuit wired in T&E etc.

That's why I chose galvanised conduit. I think that you might well want a separate cpc for each circuit in PVC conduit (or other containment), but you would have to be very careful to identify which is which at the DB.

Thanks for all replies.

                          Regards,

                                    Hz

I like an obvious separate C.P.C. for each separate final circuit in most cases. This is illustrated in modern consumer units by having numbered N and E bars to allow line, N and C.P.C.s to be connected in order. This makes fault finding easier if the original cables have been correctly terminated. If say a lighting final circuit used another lighting final circuit's C.P.C. and the first is disconnected for any reason you lose two C.P.C.s. This could happen if we did not realise that two circuits rely upon one C.P.C.

Z.

That`s one thing I like about ganging switches on different lighting circuits. It often means that in reality although each circuit cpc path is checked on it`s own there are often a link or two at various parts of such circuits thereby givind i) cpc redundancy & ii) lowering effective R2 readings. Commonly 2 or 3 circuits have such intentionable or unintentionable interlinks in domestic properties.

Tin hat on and ducked down below the parapet !