Min CPC size

[JohnW2]

well obviously I'm going to source the CPC from somewhere else that isn't a light circuit (let alone switched from its current position) ... I think that is within the regs ?

That's an interesting question, and one which obviously always arises when a separate CPC is run because the lighting circuit(s) have no CPC. If you ran the separate CPC back to the CU, or MET, there presumably could not be any problem, but I can think of no regs that would be violated even if it were derived from some other circuit (e.g. a sockets one). Since it has the potential to cause some confusion during fault-finding (and maybe testing), it would probably be wise to put a note on/near the CU to explain what was going on.

Kind Regards, John

 

[Risteard]

I wouldn't personally say that PVC sheathing represents significant 'mechanical protection'.

The sheath on insulated and sheathed cable is considered to provide mechanical protection. If you do not believe that it offers sufficient mechanical protection then it is the wrong installation method.

However I do not believe that there is any dispute about what constitutes mechanical protection in terms of BS 7671.

 

[eveares]

well obviously I'm going to source the CPC from somewhere else that isn't a light circuit (let alone switched from its current position)


I think that is within the regs ?

I would say no, because if you took it from another circuit such as from a socket/junction box, etc... and that circuit (including the CPC) was then later disconnected at the CU you would effectively lose the earth to your light fitting.

Equally, if a circuit got damaged and lost it's CPC before the point (and after the point for RFC) where you took the CPC to your light, you would have the same problem.

I would assume the regs may say that you have to take it from the CU it's self or the the MET.

Even if the regs don't say so, it would be better to take the CPC from the CU/MET.

 

[Risteard]

It is perfectly legitimate to do that as long as the cpc on the circuit you take it from is adequate and the conductor you connect between them is adequate (which means 4mm^2 if not mechanically protected!).

 

[JohnW2]

I wouldn't personally say that PVC sheathing represents significant 'mechanical protection'.

The sheath on insulated and sheathed cable is considered to provide mechanical protection. If you do not believe that it offers sufficient mechanical protection then it is the wrong installation method.

I think you know what, in context, I meant - that the sheathing would not make a significant contribution towards the mechanical protection required by 543.1.1 (and the cited, and equally non-explicit, 543.3.1) for a 2.5mm² 'lone' CPC.

However I do not believe that there is any dispute about what constitutes mechanical protection in terms of BS 7671.

I don't know about 'dispute', but BS7671 says essentially nothing about what constitutes adequate mechanical protection, 543.3.1 leaving others to decide what represents "suitable protection against mechanical and chemical deterioration and electrodynamic effects". One might expect that the requirements might differ between CPCs and live conductors (or cables containing live conductors) since, in the latter case, much of the reason for requiring mechanical protection of cables is to avoid exposure of bare live conductors - something which is obviously not a concern with CPCs.

In the context of a lighting circuit, it could be said to be a little odd that the regs are happy with a 1mm² CPC (In T+E) which is sheathed but not insulated, but (in the absence of additional protection) require it to be 4mm² if it is a 'standalone' CPC, whether it is just insulated or insulated and sheathed. AFAICS, the regs do not require it to be even insulated.

Kind Regards, John

 

[JohnW2]

well obviously I'm going to source the CPC from somewhere else that isn't a light circuit (let alone switched from its current position) ... I think that is within the regs ?

I would say no, because if you took it from another circuit such as from a socket/junction box, etc... and that circuit (including the CPC) was then later disconnected at the CU you would effectively lose the earth to your light fitting. ... Equally, if a circuit got damaged and lost it's CPC before the point (and after the point for RFC) where you took the CPC to your light, you would have the same problem. ... I would assume the regs may say that you have to take it from the CU it's self or the the MET. ... Even if the regs don't say so, it would be better to take the CPC from the CU/MET.

Whilst there is some common sense in what you say, I don't think there is anything in the regs which says that. I've already indicated that the 'surest' method is to run the CPC from CU or MET, but I don't believe it would be non-compliant to take it from another circuit. As I said, if the CPC originates anywhere other than at the CU, it's desirable that a note should be put on/near the CU, not the least to warn people of what you say above - that disconnecting the CPC of one circuit may remove the CPC from another.

Kind Regards, John