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Posted by John Grabowski on January 25, 2008, 9:14 am
> You need to check voltage from the hot wire on the switch to the two
> neutrals that are spliced together. If you get 120 v, you have an open
> neutral between that box and the first lighting outlet. If you don't get
> 120v, you have an open neutral wire somewhere upstream of the switch box.
I
> would check nearby outlets and switches for the bad connection. It could
be
> an upstream GFCI, if one exists, but not likely as that would disconnect
the
> hot leg to your circuit.
>
>
>
> >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?
> >
> > Could it be a problem in the breaker box, itself? I did happen to
> > notice, about six months ago, that her breaker box was open, slightly
> > (not the door, the whole front panel!). But it appears to be closed-
> > up OK, now. The breakers look very old, though. Also, everything
> > else in the house appears to be working as it always has. And that
> > circuit does not have its own breaker.
> >
In addition to what RBM said, I would suggest that you stop using that
multimeter and just use a pigtail socket and standard light bulb to test the
wires. The results will be more definitive. All of those odd voltages that
a multimeter displays just confuses a do-it-yourselfer.
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