Strange cabling for kitchen under-cabinet

TMSG

I have two ancient lamps under the kitchen cabinets and I am planning to replace them with new ones. There's one cable coming from the on/off switch which splits into two for the two lamps. Now this incoming cable has three separate wires: a black, a red and a naked wire without any isolation. There's 240V on the red/naked and nothing (0V) on either black/naked or red/black. Is this sort of cabling to be expected or is it perhaps a case of some electrical cowboy doing a quick but bad job? (The cable runs from the switch through the walls to the back of the first cabinet.)

FaceHead

The 'naked' wire, is most likely intended to be earth, which should have yellow and green sleeving put over it to signify such.

There are really two options for what's going on with the red and black.

Either it's switched live [red] and neutral [black] from the switch (so you'd expect to see 230V difference when switched on, and 0V difference when switched off). This is what is implied by no sleeving. 

Alternately it could be the switch wire - that is permanent live to the switch, and switched live back - in which case the switched live, usually the black, should be sleeved/taped in the live colour (red in this pre-2005 convention). So when switched on you see 0V difference as they are both live, and when switched off you (probably? is there a bulb in the light?) also read 0V difference as the black wire is disconnected. This seems to fit your measurements. So when switched on, do you see 230V from the black to the earth wire?

Depending on how its set up the wires should be sleeved appropriately so the next person knows what's going on. 

Edit: taking a look inside the switch would help you see if it's the live+switched live setup, with the switch simply connecting the black and red wires together. 

TMSG

(The house was built in 1992 and I am 99.9% sure that the wiring from the switch to the cabinet is the original wiring.)

Yeah, I was first assuming that the naked wire (though w/o the green/yellow sleeves) was earth but it doesn't seem to be the case.

So if I check the red/naked pair, this indeed comes up with ~240V when switched on. The other two combinations all show 0V.

Belenus

Can you, safely and carefully making certain that all power is disconnected, remove one of the lights, open it up and see how it is wired. It may well be a metal or other light that required an earth and that should be obvious when you examine it.

Our 5 year old under cabinet lights are plastic and have no earth, just live and neutral. I presume they are double insulated.

grumbler

TMSG said:

Yeah, I was first assuming that the naked wire (though w/o the green/yellow sleeves) was earth but it doesn't seem to be the case.

So if I check the red/naked pair, this indeed comes up with ~240V when . The other two combinations all show 0V.

Are you saying that red-black isn't 240V? And by "switched on" what switch do you mean? If the one in the kitchen, why do you measure after the switch, not before?

TMSG

grumbler said:

TMSG said:

Yeah, I was first assuming that the naked wire (though w/o the green/yellow sleeves) was earth but it doesn't seem to be the case.

So if I check the red/naked pair, this indeed comes up with ~240V when . The other two combinations all show 0V.

Are you saying that red-black isn't 240V? And by "switched on" what switch do you mean? If the one in the kitchen, why do you measure after the switch, not before?

YES that is precisely what I am saying all along.

But OK I have apparently not succeeded in explaining what happened. I am trying make things as clear as I can.

1. I have a wall switch in the kitchen (original, never touched by me) which switches two small lights under the kitchen cabinets on and off. These lamps are v old but they worked though I have now removed them.
2. There is a cable coming out of the back of one the cabinets which splits in two for the two lights. This cable has three wires: one is red, one is black, one has no sleeve at all, just the naked metal wire.
3. I assumed that 240V would be between the red/black pair. This is not the case. There is nothing there and the meter says 0V.
4. Same thing for the black wire and the naked/no-sleeve wire: 0V.
5. But between the red wire and the naked/no-sleeve wire there are 240V which I found v puzzling and which was the reason for my OP.

ciderboy2009

Presumably you've tried with the switch both on and off?

How were the old lights wired before you removed them?

FaceHead

TMSG said:

grumbler said:

TMSG said:

Yeah, I was first assuming that the naked wire (though w/o the green/yellow sleeves) was earth but it doesn't seem to be the case.

So if I check the red/naked pair, this indeed comes up with ~240V when . The other two combinations all show 0V.

Are you saying that red-black isn't 240V? And by "switched on" what switch do you mean? If the one in the kitchen, why do you measure after the switch, not before?

YES that is precisely what I am saying all along.

If one is switched live and the other is the permeant live then you would expect to see 0V difference between the red and the black wire. A very common - even the most common - setup. In most light switch setups a piece of twin and earth flex is used to take permanent live to the light switch and bring back switched live - meaning that the flex is carrying live+switched live, not the live+neutral the colour coding off the shelf suggests. 

If this is what's going on, when the light switch is closed they are actually the same wire, hence the reading of 0V between two points on the same wire. When the light switch is open, the switched live is disconnected because you've removed the lamp / bulb, you are (potentially) testing between live and a wire floating in space (not grounded) and again expect and get 0V. 

When you test between the red (hypothesis this is permanent live) and the 'naked wire' (hypothesis this is earth) you expect to see, and indeed do see 230V. 

So black wire [potentially switched live] to the naked wire [potentially earth]. Expected behavior is 230V with the switch closed, and 0V with the switch open.  At 4. say there is 0V between black and the naked wire: is this with the light switch open, closed or both?

grumbler

It's worth opening the switch and looking how it's wired

Section62

TMSG said:

grumbler said:

TMSG said:

Yeah, I was first assuming that the naked wire (though w/o the green/yellow sleeves) was earth but it doesn't seem to be the case.

So if I check the red/naked pair, this indeed comes up with ~240V when . The other two combinations all show 0V.

Are you saying that red-black isn't 240V? And by "switched on" what switch do you mean? If the one in the kitchen, why do you measure after the switch, not before?

YES that is precisely what I am saying all along.

But OK I have apparently not succeeded in explaining what happened. I am trying make things as clear as I can.

1. I have a wall switch in the kitchen (original, never touched by me) which switches two small lights under the kitchen cabinets on and off. These lamps are v old but they worked though I have now removed them.
2. There is a cable coming out of the back of one the cabinets which splits in two for the two lights. This cable has three wires: one is red, one is black, one has no sleeve at all, just the naked metal wire.
3. I assumed that 240V would be between the red/black pair. This is not the case. There is nothing there and the meter says 0V.
4. Same thing for the black wire and the naked/no-sleeve wire: 0V.
5. But between the red wire and the naked/no-sleeve wire there are 240V which I found v puzzling and which was the reason for my OP.

I'd suggest you need to get an electrician in to do a proper inspection/test.

I can think of some reasons why you'd get the readings you have, but they would involve some fairly fundamental issues with the way in which the wiring has been done and would make guesswork a rather dangerous approach.

In the meantime, please bear in mind that a 0V reading between two wires doesn't mean there isn't a potentially lethal voltage present on either or both of them. A 0V reading doesn't always mean a circuit is switched off or isolated.

Also bear in mind that the standards and conventions (on which much of the advice on forums like this are based) are not universally followed.  E.g. whilst the unsheathed conductor in T&E is intended to be used as the CPC, there are some people out there who might have happily (but dangerously) used it as a 'live' conductor.

Apodemus

Noting that I am not an electrician...

If everything was wired correctly, you are managing to get the current to flow from live (red) to earth (no-sleeve), but not to neutral (black)?  Normally, connecting live to earth would have blown a fuse somewhere, so perhaps the neutral and earth wires have been connected wrongly at the switch?  Alternatively, you have a live, a switched live and a neutral.

My next stop would be to do the same investigation on the switch - checking continuity on the (disconnected) output side of the switch, through to the exposed end of the cable.