Small voltage on earth

[matthew.larkin]

Should I be worried or not? I've got quite a lot of metal faced plugs / switches, and this weekend whilst generally pottering around I happened to touch my voltage testing screwdriver to one of these plates and discovered that it was showing something around 12v. This was consistent then across all of the other faceplates that I touched with it. I took the switch off and found that I got the same result on the earth cable.

The device I used was the little red screwdriver in this item (http:// tinyurl.com/32xxka (links to B&Q)) which displayed a flickering 12V symbol.

Is this normal?

Matt

[John Rumm]

Probably not...

Voltages must be measured relative to something - i.e. they are potential differences, rather than absolute numbers. Hence you have to ask yourself what that 12V is relative to?

Assuming this is one of these single touch volt sticks where you only need make one connection, the answer is that it is measuring relative to the potential of your body. Since that is unlikely to be at true earth potential, it does not actually tell you much about the actual potential of your earth with respect to true earth.

[matthew.larkin]

John

Thanks. That's what my gut feeling was.

I did try a multi-meter out on it too and couldn't reproduce the apparent voltage, but I'm wary of using a multimeter because to be honest I don't really understand what I'm doing. Its great for a bit of continuity testing and checking the odd battery, and I use this more than the "single touch volt stick" when I really want to convince myself that I've turned off the power if I'm swapping a switch or something similar, but other than that I'm a complete noddy!

I kind of worked on the basis that as no-one was shouting "ouch" as they touched the light switch that everything was likely OK!

Matt

[John Stumbles]

Could it be that that was because they were dead? ;-)

[matthew.larkin]

Nah, I'd have definitely spotted a number of dead bodies if they were lying the way of me getting to the plug sockets.

I'll check tonight though just to make sure

Matt

[Geo]

Probably the metalwork /is/ earthed and the measuring device is showing /you/ having the voltage. You will see the same effect if you earth one wire of your multimeter and grab hold of the other wire.

Geo

[Harry Bloomfield]

That is perfectly normal. Try touching something earthed, with your finger tip and seeing what reading you get on the screw driver. It is just the surrounding ac field being picked up by your body, from the mains circuits around you combined with a high impedance voltage measuring device (the screwdriver).