Light switch... live!!!

[glocko]

Just put a new light switch on for a neighbour, a metal one replacing a plastic one which was the pad type (ie all of the switch is the rocker rather than a little switch in middle.)
I put in an earthwire from the switch to the back box and after putting power back on the switch is showing as live by my little screwdriver tester thing.
All wires are secure including earth, I'm guessing that the problem may be in the wire coming through to the switch and the reason she hasn't had a shock is that the rocker pad hid the metal screws on her old switch.
Any ideas chaps?
Thanks

 

[JohnD]

Yes, throw away your neon screwdriver and buy a multimeter. About £8 in the high street.

BTW was there an earth wire in the cable that enters the backbox; and was this wire connected to the backbox and thence to the switch?
If not then you must use a plastic, not a metal, switch.

 

[glocko]

Yes the cable coming through is twin & earth with the earth connected to the back box, I put new earth from that to the switch.
Are there instances when you can't use metal boxes?
Ta

 

[JohnD]

in a hollow plasterboard wall it is easier to fit clip-in plastic boxes if there is nothing to screw into.

 

[RF Lighting]

It sounds to me like the CPC in the cable to the switch is not connected to earth.

It may be just the cable to the switch, or it maybe the whole lighting circuit or even the whole installation.

Can you take a photo of the cutout / meter / fusebox and post it on here for us to have a look at?

In the mean time you must NOT under any circumstances leave the metal switch in place until a reliable earth connection can be provided / verified at the switch.

 

[JohnD]

yum yum, we love pictures.

 

[securespark]

John,

It is plain to see (a fellow sufferer spots these things!) that over Christmas, you have been starved of piccys and therefore are in dire need of some photos to study. Here are an (admittedly) old pair of pictures to keep you going until something more worthwhile comes along...

Is that better?

If not, e-mail Dr Secure and he can administer further treatment...

 

[RF Lighting]

Come on Secure, you could have gone that extra bit futher and treated John to an MEM RCBO.

 

[Qedelec]

As a rough guess, I would say that the CPC is disconnected from the installation (probably at the ceiling rose) and is picking up an induced voltage from the phase and neutral running along side it. Only a proper examination with a meter will prove this is so but you might like to check the origin of the twin & earth.