Earthing

[Piotr1000]

Hi guys,is it ok to borrow earth from socket to light switch which is unearthed.It is all about changing plastic switch to metal one.The socket is underneath and probably the easiest way to do this if you do not want to rewire the whole circuit.Thank you in advance for all the answers.

 

[westward10]

Yes but it would have to be 4.0 as I assume it wouldn't be enclosed. Sounds like a whole load of hassle for a switch change, are cpcs absent throughout the circuit.

 

[timhoward]

Not ideal but 4mm cpc from socket to switch is ok. Don't love this as anyone working on sockets circuit with lighting live is inadvertently removing cpc to the switch. Leave a sticker on CU

 

[Piotr1000]

Thanks,yes there is no cpc through lighting circuit and customer wants brass switch in that room.

 

[loz2754]

Thanks,yes there is no cpc through lighting circuit and customer wants brass switch in that room.

Technically a 4mm² cable from a nearby circuit would be ok. But not a good idea, as already stated.

I would refuse the job personally. Too much risk of it going wrong. In fact I did, a few months ago, exactly the same type of request. The customer was fine with my refusal once I explained the risks.

 

[Zerax]

What's the logic/regs on using 4mm ?

 

[loz2754]

What's the logic/regs on using 4mm ?

Minimum size of CPC allowed unless part of another cable, or in conduit or trunking

 

[LastManOnline]

What's the logic/regs on using 4mm ?

It's subjective logic . A 2.5mm would be perfectly fine from an electrical point of view. From a mechanical protection point of view though the powers that be, deem a "lone" wire needs to be larger than it would be if enclosed in a cable. In principle though a mechanically secured 2.5mm (or even 1.5mm) would be perfectly safe.

 

[brianmoooore]

Last time I looked at the regs., 2.5mm2 is fine as long as its enclosed, as inside a wall cavity. 4.0mm2 is the minimum for an exposed cable.

 

[timhoward]

Last time I looked at the regs., 2.5mm2 is fine as long as its enclosed, as inside a wall cavity. 4.0mm2 is the minimum for an exposed cable.

I didn’t think “contained in an enclosure formed by a wiring system” included inside a wall. Section 2 definitions for “enclosure” and “wiring system” seem to agree?
So I think we end up with 4mm for non mechanically protected conductor.
It’s still far too much hassle for a switch change however we read the regs!

 

[LastManOnline]

I didn’t think “contained in an enclosure formed by a wiring system” included inside a wall.

You are correct in what you state. But this discussion highlights the failings in some of our regs. We are permitted to run a single 2.5mm earth along with 2 x 1.5 pvc/pvc singles from an attic down through a stud partition to a light switch. Yet in a similar stud on the opposite side of the room with an unearthed switch (OP, s scenario), we are now obliged to run a 4mm from a socket to said switch. Despite the exact same physical conditions. Something does not add up here!!

 

[timhoward]

You are correct in what you state. But this discussion highlights the failings in some of our regs. We are permitted to run a single 2.5mm earth along with 2 x 1.5 pvc/pvc singles from an attic down through a stud partition to a light switch.

I admit to being rather confused now! Tin hat ready....
If singles, I'm assuming you mean in some form of containment for double insulation which therefore isn't the same conditions and tail end of 543.1.1 wouldn't apply, and cpc can just match the line conductor and be 1.5mm.
If you meant a loose 2.5mm cpc doesn't that contravene the same regulation as it's not integral, not in conduit/trunking/ducting and not in enclosure/wiring system so needs to be 2.5 if mechanically protected or 4 if not?

 

[brianmoooore]

IMHO, inside the cavity of a stud wall is mechanically protected.

 

[timhoward]

IMHO, inside the cavity of a stud wall is mechanically protected.

Ah ok, I see how you got to 2.5mm now.
I thought the external influences bit (Impact) waffled about mechanical protection sufficient to protect against nails and screws or something similar. But I might have dreamed that up.
I still think the customer in this case needs to be told "sorry, no earth, no metal light switch"

 

[brianmoooore]

I still think the customer in this case needs to be told "sorry, no earth, no metal light switch"

I'm not disagreeing with that bit. The wiring's likely to be at least half a century old.

 

[LastManOnline]

I admit to being rather confused now! Tin hat ready....
If singles, I'm assuming you mean in some form of containment for double insulation which therefore isn't the same conditions and tail end of 543.1.1 wouldn't apply, and cpc can just match the line conductor and be 1.5mm.
If you meant a loose 2.5mm cpc doesn't that contravene the same regulation as it's not integral, not in conduit/trunking/ducting and not in enclosure/wiring system so needs to be 2.5 if mechanically protected or 4 if not?

I realise that I could have been a little clearer in my description. We may use a 1.5mm T&E (3 cores in one cable) or two 1.5 single pvc/pvc plus a 2.5mm earth (3 individual cores) to wire a lighting circuit. I may run these individual cores along an attic and down a stud partition to a switch. And it's all hunky dorrey.
Now the OP is required according to regs to run a 4mm earth (not 2.5mm) in a similar stud partition from a socket to a switch. Does, nt make sense to me

 

[timhoward]

or two 1.5 single pvc/pvc plus a 2.5mm earth (3 individual cores) to wire a lighting circuit. I may run these individual cores along an attic and down a stud partition to a switch. And it's all hunky dorrey.

I'm probably frustrating you but this still isn't going ding in my head.
I'm reading that as two double insulated singles, and one single insulated 2.5mm cpc. If I've got that wrong ignore the rest!
I'm happy to be corrected but my understanding is that the salient points are:
1) whether the sperate cpc is mechanically protected if loose in the attic (if no 4mm needed?)
2) While I take 522.6.204 [iv] to mean that a stud wall isn't mechanical protection, if a view was held that it is sufficient mechanical protection (as expressed by others) and the three cores are all loose in the wall then cpc can be 2.5mm. And so could the OP's case.

 

[nicebutdim]

I realise that I could have been a little clearer in my description. We may use a 1.5mm T&E (3 cores in one cable) or two 1.5 single pvc/pvc plus a 2.5mm earth (3 individual cores) to wire a lighting circuit. I may run these individual cores along an attic and down a stud partition to a switch. And it's all hunky dorrey.
Now the OP is required according to regs to run a 4mm earth (not 2.5mm) in a similar stud partition from a socket to a switch. Does, nt make sense to me

This would be a single with CPC or single and separate CPC?


Edit: Cross posted with above.


Aslo; as it'll take me a considerable time to find the correct regulation - does BS7671 stipulate mechanical protection of CPC or suitable containment?

 

[timhoward]

Aslo; as it'll take me a considerable time to find the correct regulation - does BS7671 stipulate mechanical protection of CPC or suitable containment?

In summary we only get into this territory if iii,iv and v apply, and then we end up with 2.5 if mechanically protected and 4 if not. So if 'contained' by either of the 3 methods the normal cpc sizing stuff can apply.
(My current view is that the wall is not enclosure/wiring system and not mechanical protection)

 

[LastManOnline]

I'm probably frustrating you but this still isn't going ding in my head.
I'm reading that as two double insulated singles, and one single insulated 2.5mm cpc. If I've got that wrong ignore the rest!

Not at all. And your description of the cables is spot on.

I'm happy to be corrected but my understanding is that the salient points are:
1) whether the sperate cpc is mechanically protected if loose in the attic (if no 4mm needed?)

There may possibly be a difference in UK and ROI regs here. If running a separate cpc in attic for a lighting circuit I must use a 2.5mm. If running a separate cpc for a socket circuit I must use a 4mm

2) While I take 522.6.204 [iv] to mean that a stud wall isn't mechanical protection, if a view was held that it is sufficient mechanical protection (as expressed by others) and the three cores are all loose in the wall then cpc can be 2.5mm. And so could the OP's case.

I would agree with all of the above. But I must say in conclusion that it's one of these "splitting hairs" discussions. "Should it be 4mm or should it be 2.5mm"? Bearing in mind that it's all based on a concept of a perceived mechanical threat to the integrity of the cpc. (completely unexplained in the regs as far as I am aware) What could happen to threaten a 2.5 cpc but would allow a 4mm cpc to remain unharmed? I think if I was a member the electrical fraternity responsible for producing the electrical regs I would not want it to be part of my legacy