earth and live wire mixed up in light switch socket?

[Deleted member 212919]

I've been replacing some sockets and a light switch in my kitchen, and fear I might be going mad because in my light switch socket the earth wire seems to be live, as is the red wire, and black has no voltage. I could have sworn they were in correctly when I pulled the old light switch panel out.

Is it possible that something has gone wrong in the consumer unit that could be causing this?

 

[AndyPRK]

Using a voltage stick ?

 

[EFLImpudence]

Usual question - what are you using to detect this voltage?

What do you mean by 'in correctly'?

 

[Deleted member 212919]

I was using one of those screwdrivers that light up when you touch a live wire.

One thing I realize now, is that the light switch in question was always in the 'off' position when it was on (i.e. top bit poking out), and the on position when it was off. I'd assumed it had been put in upside down, but now I wonder if when it was in the off position it was somehow making a contact with the 'earth' wire that was attached to the back plate.

I guess the solution is to simply label the wires as erroneously coloured and put them in their right places. I'm not sure how to tell the difference between live and neutral though - or does it matter (doesn't seem to matter which way you stick a european two-pin plug in)?

All a bit disturbing really since I paid an electrician when I moved in to put in a new consumer unit and check all the fittings.

 

[EFLImpudence]

I was using one of those screwdrivers that light up when you touch a live wire.

Unreliable. You need a two pole(lead) tester to measure voltage.

One thing I realize now, is that the light switch in question was always in the 'off' position when it was on (i.e. top bit poking out), and the on position when it was off. I'd assumed it had been put in upside down

It will have writing embossed on it which will be the right way up.

It but now I wonder if when it was in the off position it was somehow making a contact with the 'earth' wire that was attached to the back plate.

Nope.

I guess the solution is to simply label the wires as erroneously coloured and put them in their right places. I'm not sure how to tell the difference between live and neutral though - or does it matter (doesn't seem to matter which way you stick a european two-pin plug in)?

You will not have a neutral at the switch.
They are Line(live) and Switched Live.

Neutral is not a name for a black or blue wire.
It is the name of the conductor which completes the circuit from the load back to the source.
Wires can be anything we want them to be; they do not know what colour they are.

All a bit disturbing really since I paid an electrician when I moved in to put in a new consumer unit and check all the fittings.

If the light works then likely the switch wires are alright.

Do you have a multimeter?

 

[Deleted member 212919]

Thanks, yes I do have a multimeter, so it should be possible to find out which of these three wires is which using this?

 

[EFLImpudence]

Firstly, with the POWER OFF, set meter to 600VAC.
Ensure dead by Probes to Red & Earth, then Black & Earth - ensure 0V.

Set to low ohms ( Ω )
You should really, by using a long wire, test between Earth in the back box and a known Earth - a socket screw if there are metal back boxes or a pipe or appliance metal carcase like an oven, kettle, toaster etc.
Should be continuity - meter beep or very low ohms.

Probes to Red and Black
Light switch OFF - Open circuit
Light switch ON - 0 ohms (beep).



Set meter to 600VAC (probably 600V; if not, then first above 250V)
Then with the power ON - be careful.
Light switch ON, Probes to:
Red & Earth - around 240V
Black & Earth - 240V

 

[Deleted member 212919]

hi, thanks for the great instructions. Here is what I found:

>Ensure dead by Probes to Red & Earth, then Black & Earth - ensure 0V.
yes, all 0V

>Light switch OFF - Open circuit
no reading
>Light switch ON - 0 ohms (beep).
about 6 ohms.

Light switch ON, Probes to:
>Red & Earth - around 240V
nothing
>Black & Earth - 240V
nothing

If I turned the switch off then I got 200v between the black and red. But nothing on either of them with the earth.

The light is actually working now, with black in L1 and red in COM, and Earth in the mounting box. But the earth wire still lights up my screwdriver.

 

[EFLImpudence]

>Light switch OFF - Open circuit
no reading
>Light switch ON - 0 ohms (beep).
about 6 ohms.

Very poor contact suggesting burnt - replace.

Light switch ON, Probes to:
>Red & Earth - around 240V
nothing
>Black & Earth - 240V
nothing

Both nothing ??? Had you turned the power on?
If yes, this suggests that the Earth wire is not actually connected to an Earth.
Did you do the long wire test to a known Earth?

If I turned the switch off then I got 200v between the black and red. But nothing on either of them with the earth.

That could be possible - apart from the suspect Earth.

The light is actually working now, with black in L1 and red in COM, and Earth in the mounting box. But the earth wire still lights up my screwdriver.

So, there must be 240V through the switch.

You need to confirm the Earthing.
The switch cable likely goes to the light so make sure that the Earth wire from the switch is connected.

 

[bernardgreen]

But the earth wire still lights up my screwdriver.

Most likely because the potential of you body above ground potential is high enough to make the neon strike.

The "neon screwdriver" will light if there is a potential difference between the person holding it and the conductor being tested. A person standing on a dry non conductive surface and close to live wires like those in the cable to a switch can be at a potential above ground due to capacitive coupling between the Live wires in the cable and the person's body.

The current that flows via this capacitive coupling is far to small to be harmful.

For this reason a neon screw driver may not light up when touching a Live wire if the person is close to Live wires.

 

[Deleted member 212919]

>Did you do the long wire test to a known Earth?
I did try with the metal body of my electric cooker, and no reading.

Come to think of it, I do now recall something the electrician said about my earth, and that I shouldn't use metal light switches.

 

[333rocky333]

a known Earth - a socket screw if there are metal back boxes

I would of thought ALL socket and FCU screws would be Earthed regardless of box type.
Plastic Switch screws yes only with metal boxes

 

[EFLImpudence]

a known Earth - a socket screw if there are metal back boxes

I would of thought ALL socket and FCU screws would be Earthed regardless of box type.
Plastic Switch screws yes only with metal boxes

Plasterboard boxes or plastic surface mount.

 

[JohnW2]

I would of thought ALL socket and FCU screws would be Earthed regardless of box type. Plastic Switch screws yes only with metal boxes

Plasterboard boxes or plastic surface mount.

I suspect that rocky was thinking of the fact that, once tightened, all socket and FCU screws should be earthed by virtue of the (hopefully!) CPC connected to the socket or FCU (even if they are screwed into something plastic).

Kind Regards, John

 

[EFLImpudence]

I would of thought ALL socket and FCU screws would be Earthed regardless of box type. Plastic Switch screws yes only with metal boxes

Plasterboard boxes or plastic surface mount.

I suspect that rocky was thinking of the fact that, once tightened, all socket and FCU screws should be earthed by virtue of the (hopefully!) CPC connected to the socket or FCU (even if they are screwed into something plastic).

What?

How would you achieve that - and why?