Earth wire needed?

Doctorx1958 · 29 Oct 2025

Hi, I have installed 2-way switching using 1.5 T&E. The common is a separate 1.5 T&E. I have clipped back the spare wire paired with the common and am wondering about the need to retain the earth running along side the common given there is already an earth in the other switching cable?
I guess that I should have used 3 core and E, but had non to hand at the tine!

EFLImpudence · 29 Oct 2025

Doctorx1958 said:
Hi, I have installed 2-way switching using 1.5 T&E. The common is a separate 1.5 T&E. I have clipped back the spare wire paired with the common

You might as well have connected both to the common terminal.

Doctorx1958 said:
and am wondering about the need to retain the earth running along side the common given there is already an earth in the other switching cable?

It's not actually required but all earth wires should be connected together.

Doctorx1958 said:
I guess that I should have used 3 core and E, but had non to hand at the tine!

No matter.

Jurassicspark · 29 Oct 2025

Doctorx1958 said:
I have clipped back the spare wire paired with the common and am wondering about the need to retain the earth running along side the common given there is already an earth in the other switching cable?

Just connect the earth wires together. I reckon that the other end of the COM wire for the cable is connected to the same terminal of the junction box, ceiling rose or the consumer unit?

Or is it dangling? In which you shouldn't connect the COM to the same terminal at the switch then.

Doctorx1958 said:
I guess that I should have used 3 core and E, but had non to hand at the tine!

This is going to get criticism.

What about if you used the 1.5 T&E with one core connected to the common terminal, the other to the L1 terminal and ~~the earth wire to the L2 terminal?~~

Which, of course, should not be done, just giving you an illustrative and hypothetical alternative, if it was allowed with T&E and if you were using flex , with the earth wire identified as a live switching wire, and the use of only class 2 accessories, which definitively would be quite bizarre.

sparkwright · 29 Oct 2025

Best practice in these circumstances to connect both earth wires. Also, not cut off the unused wire, as it may come in handy one day.

JohnW2 · 29 Oct 2025

Jurassicspark said:
What about if you used the 1.5 T&E with one core connected to the common terminal, the other to the L1 terminal and ~~the earth wire to the L2 terminal?~~ ....... Which, of course, should not be done, just giving you an illustrative and hypothetical alternative, if it was allowed with T&E and if you were using flex , with the earth wire identified as a live switching wire, and the use of only class 2 accessories, which definitively would be quite bizarre.

The biggest problem with using the CPC of T+E as a live conductor is not so much one of 'identification' but, rather, that it is 'bare' hence, at best 'single insulated' (by the sheath) - and some would even argue that the sheath did not count as 'insulation'.

For what it's worth, it has always been my personal view (with which some/many might disagree) that the argument for not doing it is much weaker in the case of 3-core flex (with an insulated 'earth' conductor), since the only issue then is that of 'identification' (which can be achieved with sleeving/whatever at the terminations).

Also, unless there is another cable, there is a requirement for a CPC to be run to every 'point' in an installation, even if it is not currently needed - and that could not be done if there were only one cable, whose 'CPC' was being used as a live conductor.

Jurassicspark · 29 Oct 2025

JohnW2 said:
and some would even argue that the sheath did not count as 'insulation'.

The sheath is mechanical protection and is not tested for insulation properties.

It is simply to prevent damage to the insulation which is contained within it.

So could that be considered as, dare I say, uninsulated live conductor?

JohnW2 said:
For what it's worth, it has always been my personal view (with which some/many might disagree) that the argument for not doing it is much weaker in the case of 3-core flex (with an insulated 'earth' conductor), since the only issue then is that of 'identification' (which can be achieved with sleeving/whatever at the terminations).

Jurassicspark said:
and if you were using flex , with the earth wire identified as a live switching wire

JohnW2 said:
Also, unless there is another cable, there is a requirement for a CPC to be run to every 'point' in an installation, even if it is not currently needed - and that could not be done if there were only one cable, whose 'CPC' was being used as a live conductor.

Which is why it was a hypothetical and an illustrative example.

But are we going to forget that the CPC in T&E is only about 2/3rd's of the CSA of the live conductors, so its current carrying capacity is lower, and if this was 1.0mm being installed, with old fashioned lights, I'm sure that would pose a problem.

JohnW2 · 29 Oct 2025

Jurassicspark said:
The sheath is mechanical protection and is not tested for insulation properties.

Quite so, but I think it's almost a 'technicality' - i.e. if it were 'tested as an insulator', I very strongly suspect that it would 'pass'! In fact, I presume it must satisfy some requirements as regards 'insulation properties', since it's hard to see how a 'conductive' sheath would be acceptable .

Jurassicspark said:
But are we going to forget that the CPC in T&E is only about 2/3rd's of the CSA of the live conductors, so its current carrying capacity is lower, and if this was 1.0mm being installed, with old fashioned lights, I'm sure that would pose a problem.

I'm not sure what sort of 'problem' you are 'sure' would arise. For a start 1 mm² T+E is the one case in which the CPC has the same CSA as the live conductors. Whether 1.0 or 1.5 mm² T+E (I can't imagine anything else being used for lighting), the CPC would be 1mm², and with a (Method C) CCC of 16A, that would be very much more than enough for switching of even multiple 'old fashioned' lights. Don't forget that, if the CPC were being used as a live conductor for switching, it would only be current taken by the light(s) that it was switching that it carried.

Risteard · 30 Oct 2025

JohnW2 said:
Quite so, but I think it's almost a 'technicality' - i.e. if it were 'tested as an insulator', I very strongly suspect that it would 'pass'! In fact, I presume it must satisfy some requirements as regards 'insulation properties', since it's hard to see how a 'conductive' sheath would be acceptable .

I'm not sure what sort of 'problem' you are 'sure' would arise. For a start 1 mm² T+E is the one case in which the CPC has the same CSA as the live conductors. Whether 1.0 or 1.5 mm² T+E (I can't imagine anything else being used for lighting), the CPC would be 1mm², and with a (Method C) CCC of 16A, that would be very much more than enough for switching of even multiple 'old fashioned' lights. Don't forget that, if the CPC were being used as a live conductor for switching, it would only be current taken by the light(s) that it was switching that it carried.

MIMS has a conductive sheath.

JohnW2 · 30 Oct 2025

Risteard said:
MIMS has a conductive sheath.

It does, but that's a totally different kettle of fish. It doesn't have even one layer, let alone two layers of 'solid' insulating material around the conductors - only some powder.

I was talking about T+E and, as I said, I would expect there o be a requirement for its sheath to be non-conductive.

Ragnar_AT · 30 Oct 2025

Cables are usually expected to be double-insulated or have an earthed metal sheath so I do suspect the sheath of T&E is tested.

One of the few exceptions is H03VV-F flat twin flex, which probably falls under „reinforced insulation“ but is almost extinct anyway.

JohnW2 · 30 Oct 2025

Ragnar_AT said:
Cables are usually expected to be double-insulated or have an earthed metal sheath so I do suspect the sheath of T&E is tested.

Sure. We very often call it 'double insulated', and I would be very surprised if the sheath wasn't essentially as good an insulator as the inner insulation (around the conductors). However, the pedantic amongst us say that we should call it "insulated and sheathed" (not "double insulated") and seem to believe that it is neither 'rated' nor tested in relation to its insulating properties (which I rather doubt is the case).

Risteard · 30 Oct 2025

Ragnar_AT said:
Cables are usually expected to be double-insulated or have an earthed metal sheath so I do suspect the sheath of T&E is tested.

One of the few exceptions is H03VV-F flat twin flex, which probably falls under „reinforced insulation“ but is almost extinct anyway.

Cables are not "double insulated" though. That is a common misnomer. They are insulated and sheathed. (Obviously precluding non-sheathed cables such as singles.)

JohnW2 · 30 Oct 2025

Risteard said:
Cables are not "double insulated" though. That is a common misnomer. They are insulated and sheathed. (Obviously precluding non-sheathed cables such as singles.)

Yep. You've presumably read what I wrote about an hour ago ...

JohnW2 said:
Sure. We very often call it 'double insulated', and I would be very surprised if the sheath wasn't essentially as good an insulator as the inner insulation (around the conductors). However, the pedantic amongst us say that we should call it "insulated and sheathed" (not "double insulated") and seem to believe that it is neither 'rated' nor tested in relation to its insulating properties (which I rather doubt is the case).

Risteard · 31 Oct 2025

JohnW2 said:
Yep. You've presumably read what I wrote about an hour ago ...

The sheath isn't "as good as insulation" though, even where it had insulating properties. It isn't tested for these.

JohnW2 · 31 Oct 2025

Risteard said:
The sheath isn't "as good as insulation" though, even where it had insulating properties. It isn't tested for these.

That's what a lot of people believe, but I wonder why? PVC is an incredibly good insulator, and the sheath (and of the underlying 'insulation') is far thicker than would theoretically be needed to provide adequate insulation at LV (when sheath was mechanically intact) - and I presume that appropriate testing would confirm that.

I imagine that the 'overkill' in relation to sheath thickness is primarily to reduce the risk of mechanical damage, which obviously could undermine its value as an insulator.

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