On Sat, 15 Sep 2007 15:48:18 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .
On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .

I'm measuring 17 Vac across my bathroom light fitting when the
light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?

What are you using to measure the voltage? If you are using a
modern
high impedance meter such as a Fluke, then what you are measuring
is
the capacitive coupling between a live wire and an isolated one,
and
it is quite normal. If on the other hand you are using an older
meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and
34
Vac on another fitting?

OK, you have a conducting circuit comprising your 'unconnected' wire,
the tiny capacitances to adjacent conducting wires, and the very high
impedance of the measuring instrument. Changes in the amount of
capactive coupling will vary the perceived voltage at the Fluke. So it
is probable that your 17v reading is on a wire with (about) half the
capacitive coupling than your wire associated with the 34v reading.
Neither are anything to worry about and are quite normal.

I don't wish to sound condescending or rude, but it is an example of a
little knowledge being a dangerous thing. You need to understand the
big picture and the limitations and scope of your tools.

AWEM


A little knowledge would be a dangerous thing if I ignored
perceived anomolies rather than find out what they are. But anyway
thanks for your input.