Borrowing an earth

[SamKB]

Putting back an old metal wall sconce on a middle landing, that had been knocked loose, I made a point of connecting an earth (from the T&E) which didn't seem to have ever been connected. This made me a little unsure about the earthing of the circuit in general, and tracing the T&E back to a junction box, I discovered earth was not only not connected, but there was nothing to connect it to as the old twin cable feeding it had no earth. To complicate things further, the neutral was being borrowed from a neighbouring circuit, so the short twin cable from the switch was actually just a single live, from an old style dual switching circuit (thus the borrowed neutral).

Unfortunately the wiring going both up and downstairs is entirely embedded in the structure of the house, so pulling it out and putting in a proper 3&E modern dual switching circuit would involve major works, and I've checked the breakers and found that the same one is cutting both the circuit the neutral is borrowed from as well as the switch circuit in both positions, so it'll all have to stay there till the place is rewired. I'm just left with the problem of unearthed metal fittings.

So my question is: as there is a modern shower switch on the landing wall next to the light switch, is there any reason I shouldn't "Borrow" an earth from that, and just replace the short bit of cable from the switch to junction box with T&E (I've got some with the right colours), taking the earth to the junction box and from there to the metal fitting, and to another upstairs which is also earthed to the box with no connection ?
I know it would be preferable to rewire the whole thing, but given that isn't going to happen, are there any down sides to using the nice hefty recently fitted earth from the shower as an earthing point, or rather CPC to link back to the breakers and actually give some protection.
Yes I know I've switched between "Earth" and CPC, but you know what I mean.

 

[plugwash]

The worry is that at some point the shower circuit may end up getting disconnected and thus end up losing your borrowed earth.

Personally I think that this is probably a lesser evil than leaving a peice of class 1 equipment without an earth, but others may disagree with me.

 

[EFLImpudence]

I've checked the breakers and found that the same one is cutting both the circuit the neutral is borrowed from as well as the switch circuit in both positions, so it'll all have to stay there till the place is rewired.

To clarify: Do you mean they are on the same circuit and so not borrowed?

As for the earthing: better than nothing as already said.

 

[SamKB]

To clarify: Do you mean they are on the same circuit and so not borrowed?
.

Good point, I was thinking "Borrowed" as it was coming in from another part of the circuit, but I suppose if there is only one lighting circuit then it doesn't really matter where it connects to it.

 

[SamKB]

The worry is that at some point the shower circuit may end up getting disconnected and thus end up losing your borrowed earth.

Personally I think that this is probably a lesser evil than leaving a peice of class 1 equipment without an earth, but others may disagree with me.

Yes, I felt the borrowing was a lesser evil than the un protected metal light fitting which has already been demonstrated to be at risk of breakage.
Hopefully anyone removing the shower switch, (it's big and on the landing so you wouldn't take out the wiring and leave the switch) would notice the dangling earth wire they had left.
Also, it's the only shower, no bath and all the water heating is electric so there's no chance of replacing with a non electric one, but of course that doesn't guarantee anything.

 

[AndyPRK]

If running a separate cpc there is a requirement of minimum thickness for its mechanical protection.

In practical terms connecting that thickness to a lighting circuit is unpractical, so probably gets ignored.

 

[plugwash]

I always assumed that connecting to 4mm earth wires was the reason ceiling roses had such big earth terminals. A traditional round junction box shouldn't have any trouble with them either.

 

[SamKB]

If running a separate cpc there is a requirement of minimum thickness for its mechanical protection.

In practical terms connecting that thickness to a lighting circuit is unpractical, so probably gets ignored.

Are you suggesting that I shouldn't be connecting a thinner cpc to the thicker one ? I assumed that the thinner one was a path from the light fitting to the thicker one which linked back to the breaker so inherently a safe option. I wouldn't consider connecting the thicker one back to the breaker via a thinner wire.

Or are you just saying 4mm isn't specified as suitable for light wiring as it is assumed to be impractical, even though perfectly suitable ?

The connection to the 4mm would be at an earthing point in the shower switch.

 

[AndyPRK]

No I’m not saying that. You carry on