'Borrowed' earth for lighting circuit

[Neiliow]

The lighting circuits in my house dated from the days before earths were common on such things. Most of it has been replaced but the lighting spur to the garage still has no CPC in the cable.
I would like to replace the existing pendant bulbs with fluorescent fittings, 2 of which I have for the purpose. However these are metal and should be earthed. Replacing the whole cable to the garage would be tricky as it goes through a 2' thick wall into the inaccessible void between floors (old house!)
My question is - can I run an earth cable from an outlet on the ring which is close the the lighting circuit? Would this affect RCB operation? I think the lighting is on a separate RCB from the ring.
Thanks for any input.

 

[EFLImpudence]

Firstly, Do you mean RCD or MCB?

Residual Current Device
Miniature Circuit Breaker.

 

[AndyPRK]

Yea you can do that

 

[Neiliow]

Firstly, Do you mean RCD or MCB?

Sorry, got my abbreviations mixed up there! They are on separate RCDs; the consumer unit is split into 2 RCDs.

 

[EFLImpudence]

Ok. As Andy says. That will be fine.

 

[Neiliow]

Great, that'll save me a load of work. Thanks a lot.

 

[SimonH2]

Just be aware that there are regulations on the size of cable you need for a standalone CPC. From memory, if it's not enclosed in conduit or trunking then the minimum size is 4mm², or 2.5mm² if it's protected. From an electrical PoV it's overkill, but it's to do with mechanical strength.

 

[AndyPRK]

however, you try getting a 4mm cable into a light fitting !

 

[SimonH2]

I did think that as I was writing it

 

[securespark]

I did think that as I was writing it

It is possible. I've done it a few times!

 

[Neiliow]

Thanks to everyone for the additional information. The replacement fittings I am putting up are fluorescent striplights with large cable entries and chunky terminal block for the wires so 4mm² won't be a problem.

 

[JohnW2]

Just be aware that there are regulations on the size of cable you need for a standalone CPC. From memory, if it's not enclosed in conduit or trunking then the minimum size is 4mm², or 2.5mm² if it's protected. From an electrical PoV it's overkill, but it's to do with mechanical strength.

however, you try getting a 4mm cable into a light fitting !

I've often wondered about this. The relevant bit of 543.1.1 says ...

...... If the protective conductor:
(iii) is not an integral part of a cable, or
(iv) is not formed by conduit, ducting or trunking, or
(v) is not contained in an enclosure formed by a wiring system,
the cross-sectional area shall be not less than 2.5 mm2 copper equivalent if protection against mechanical damage is
provided, and 4 mm2 copper equivalent if mechanical protection is not provided

In terms of (iii), it says nothing about 'the cable' having to be a cable supplying L and/or N to the circuit being protected by the CPC, nor even that any other of the conductors in the cable actually have to be being used for anything.

Hence, it would seem to me that, when wishing to add a CPC to a lighting circuit, it would probably be compliant to use the CPC of, say, 1mm² T+E (ignoring its other conductors). A bit of a waste of copper (unless the brown and blue can be put to some good purpose) but (a) many people (particularly DIYers) are more likely to have 1mm² T+E available than 4mm² G/Y and (b) it avoid the problem mentioned above of potential difficulties in terminating a 4mm² conductor in some light fittings/accessories.

Just a though!

Kind Regards, John

 

[winston1]

How do you define a cable? Is not a single insulated wire a cable?

 

[EFLImpudence]

Or a single insulated and sheathed to use for the CPC.

I realise DIYers won't have this either so - get some.

 

[JohnW2]

How do you define a cable? Is not a single insulated wire a cable?

Good question and, somewhat surprisingly, BS7671 does not offer any definition.

However, to make any sense of the reg I cited, I think that one has to assume that their intention (and nearly everyone's interpretation of their intention) was to write "multicore cable" (but 'forgot to'!).