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Posted by Ralph Mowery on January 25, 2008, 5:58 am
>A friend asked me to help her repair her bathroom light switch, which
> had stopped working. By the time I got there, she had already
> installed a new switch, but it still wouldn't work. .It's a one-way
> switch, controlling two wall fixtures.
>
> The house (which is in the USA) is about sixty years old and has two-
> prong plugs, everywhere. The light switch has two screws, with a
> black wire to each screw. There are two white wires in the switch
> box, spliced together (and they had obviously been that way for many
> years). One lamp fixture has two black and two white wires, with the
> two blacks connected to the fixture's black wire and the two whites
> connected to the fixture's white wire. The other fixture has one
> white and one black going to it. So far, so good. Apparently, the
> fixtures' wiring is somewhat newer than the rest of the house's
> wiring, because they also each have a ground, which is connected to
> each box, mounting plate, and fixture housing.
>
> I couldn't see anything that was obviously wrong. And the light
> fixtures and the old switch had been working for years. So I used my
> multimeter (Tektronix DMM916) and did some very basic measurements,
> but will probably need to go back and do some more. Anyway, here is
> what I have measured, so far:
>
> With the switch on, and no bulbs installed in the fixtures, the
> voltage (VAC RMS) across the switch terminals is very low, i.e.
> approx .04 VAC. At each fixture, white-to-ground measured 54 VAC,
> black-to-ground measured 120 VAC, and black-to-white measured 34 VAC.
>
> WITH bulbs installed, and the switch ON, both white-to-ground and
> black-to-ground measured 120 VAC, and black-to-white measured anywhere
> from 14.6 VAC to 18VAC on the first fixture and about .04 VAC on the
> second fixture.
>
> But, after emptying some dead insects out of the first fixture, it,
> too, measured .04 VAC from black-to-white. (I neglected to measure it
> again, without bulbs, to see if the 34VAC from black-to-white had then
> changed.) The black-to-white measurements without bulbs also matched
> the measurements between the socket bases and threaded bulb holders.
>
> With the switch OFF (both before and after emptying-out the insects in
> the first fixture), the voltage across the switch terminals (two
> blacks) was about 70 to 76 VAC (varied between measurements).
>
> Also, with the switch off, the resistances, at the fixtures, between
> any two of black, white, and ground all appeared to be infinite, as
> did the resistances between the socket bases and threaded portions..
>
> Can anyone figure out what's going on, from that? Or, what else
> should I measure, or try?
>
>
It seems that the white wire is not connected somewhere. It should go all
the way back to the braker box and hook to the neutral side of the wiring.
It may have came apart in the switch box when the switch was replaced.
At no time should the white wire have any voltage on it to the ground. With
the meter you may see some of the induced voltage , but it should not be 120
volts. Sometimes the white wire will be used in a switched circuit, but
should be marked with tape to indicate that..
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