On Tue, 3 Jan 2006, Lindy Mayfield wrote:

> I was installing ceiling lights in the new house and wanted to be 
> "smart" and double check to see if the exposed wires were hot or not. 
> So I connected my multimeter first just in case.  I grew up in the US 
> so I am not very familiar with connecting things here in Europe and I 
> wanted to be careful.
>
> I was a little surprised when my meter showed first around 118 or so 
> volts AC and then when the switch was flipped it jumped up to about 
> 230.  This kind of made me nervous, so of course I made the shorts 
> test with what I thought was the off position before proceeding. 
> (The shorts test is when I touch the wires together and then check my 
> shorts for signs of electrocution.)
>
> Needless to say I survived, but I was wondering why I was getting a 
> ~120 volt reading when the light switch was off?

You would have gotten 0 with the lightbulb in. The floating wire in the 
wall (when the switch is off) gets phase ac coupled into it and at the 
same time leaks (ac) to ground. So you can see a voltage that's about 
half of the mains on almost any floating wire in a conduit that also 
carries a phase.

This is one of the reasons why it is totally forbidden to put signal 
cables into power conduits.

Peter
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