Odd Voltage Reading When Changing Light Switch

Postby timuren » Thu Oct 17, 2013 10:20 pm

Hi all,

I wonder if anyone can shed some light on the following. I replaced an old twin dimmer switch a couple of days back - both sides were buzzing when their respective lights were on.

However, when wiring in the replacement I noticed there were three wires going into the old switch, in addition to the earth, with the red shared across both "wiring boxes" for the respective switches.

When testing the three wires with a multimeter I got the following results:
Red + blue: 240v
Red + yellow (with red sleeve): 150v

Does this sound correct? I was surprised to see the 150v reading and am wondering if there might be something wrong with the wiring. Thoughts?

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Postby ericmark » Fri Oct 18, 2013 12:21 pm

Much depends on what one uses to measure voltage. Where you have wires close together there will be capacitance and induction links and any wire not tied to a voltage can read all sorts as a result. We as supposed to earth unused wires but that rarely happens.

If part of a two way switching arrangement clearly it would not be earthed down and would float.

We use to use test lamps rather than meters so we could have some load to remove this problem but to be frank I can't quite understand why three wires unless two way switching is used.
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