Replacing light switch

[BryanW]

I replaced my 3 gang 2 way light switch with a new plastic one and I'm certain that each cable has gone where it should. But when I screwed it back on, turned the electricity supply on and tested the switch with the voltage tester, I have a 12v reading on the screws which I assume is coming from the case, which is earthed. When I tested the wires, they had 240v as expected so I'm stumped. Does anyone have an idea of where this might be coming from?

 

[Echo the husky]

What points are you testing to and from to get 12V?
What points are you testing to and from to get 240V?

 

[ericmark]

An earth can be up to 50 volts from true earth. However maybe switch not earthed? Was the house wired before 1964 as before then earth was not required on lights.

 

[BryanW]

Thanks for the replies. I tested the screws in the plastic faceplate - 12V when screwed in, nothing when not screwed into the metal case (just sitting in the plastic faceplate). I got 240V when I had the faceplate hanging and put the voltage tester near one of the live cables. The switch has two earth cables and was rewired about 4 years ago. I tested the screws in a few light switches nearby just in case and they were fine. Unfortunately, I didn't test the screws before I changed the faceplate so I don't know if this is a new problem or not.

 

[Echo the husky]

To measure a voltage there must be 2 points of contact with the meter. Where are you connecting the 2 probes when you get these readings?

As stated above there may be a voltage difference between neutral and earth. Readings like this are common when the earth wires are not actually connected to earth of the installation, due to a loose connection or some other break/damage in the CPC.

 

[ban-all-sheds]

I only tested the screws in the plastic faceplate - 12V when screwed in, nothing when not screwed into the metal case (just sitting in the plastic faceplate).

I was about to repeat the question about what you measured between, i.e. between the screws and what did you find a potential difference of 12V?

But then I read this:

... put the voltage tester near one of the live cables.

Nothing which you "detect" with that magic wand can be relied upon to have any meaning or worth.

1) "Voltage" is always between two points, and you need a 2-pole tester to measure it.

2) The magic wand lighting up, or not, will tell you nothing about whether you have the conductors in the correct switch terminals, whether you have a complete circuit that would enable the light to work, and so on.

And having written that, I found myself wondering what your problem actually is. Are your light(s) working as they should?

 

[BryanW]

I've since been reading about this voltage tester. I gather it's not a useful thing to use. The lights work perfectly. The cables all went in to the same terminals they came out of - COM to COM, L1 to L1 and so on.

 

[Echo the husky]

Oops, I missed the bit about testing near the cable. As BAS says the results will be meaningless. Just out of interest what tester are you using?

 

[BryanW]

It's this one. It was recommended at B&Q - http://www.diy.com/departments/bq-pen-type-voltage-tester/177924_BQ.prd. It has a digital readout which read 12V on the screws but from what you're saying, either I'm misinterpreting it or it couldn't possibly detect that.

 

[ban-all-sheds]

I've since been reading about this voltage tester. I gather it's not a useful thing to use.

It can be, in some very specific and limited circumstances.

It can also be made to light up (probably) by rubbing it on a sweater, which might be of use to entertain children.

The lights work perfectly. The cables all went in to the same terminals they came out of - COM to COM, L1 to L1 and so on.

Stop worrying about what happens when you wave the magic wand.

 

[BryanW]

Now that you mention that, I remember that it did show up a little lightening-bolt symbol whenever I held it between my thumb and forefinger... but I assumed that the 12V actually meant something.

 

[Taylortwocities]

Put it in the circular filing cabinet