toller said:
I am installing a switch leg on a ceiling light that now has a chain switch.
When I measure voltage at the new switch it is H-N and H-G 120.2v with the
chain pull on; just like expected.
With the chain pull off, it measures H-N 61.8v and H-G 120.2v.
Is that 61.8v the famous phantom voltage I have read about so many times
here? It seems weird that if it doesn't exist that it reads exactly 61.8v
every time I measure it; but of course it can't be real because there is no
circuit.
What causes this?
I only measured at all because I wanted to make sure my ground was good;
which it obviously is.
You should have been a little more specific about what you did and how
you made your measurements. But here's the most likely scenario.
You didn't say which way the "new" swich was set when you measured that
"phantom voltage", but based on what you told us, I'm pretty certain it
had to be "off" and also that when you say "H" and "N" you are
describing the "black" and "white" wires in the new switch leg cable,
AND, there really ISN'T be a "N" (neutral) in the new switch box.
I read your description as saying you started with an existing ceiling
light whose only control means was a pull chain switch, perhaps part of
one of those ubiquitous ceramic or plastic ceiling sockets, or maybe it
was a pull switch mounted directly on some part of a more sophisticated
light fixture.
The term "switch leg" implies that you've brought a piece of two
conductor (plus ground) cable from the ceiling box the light is mounted
to down to a wall box containing a conventional SPST toggle switch.
You should have "cut into" the black wire going to the existing light
and the wire colors on the ceiling end of the new cable should have been
this; black to the existing black supply wire in the ceiling box and
white to the black wire of the fixture (or the screw terminal on a
ceiling socket from which you removed the existing black wire.) The
ground wire should have been connected to the ceiling box or pigtailed
to a ground witre in that box if the box in nonmetallic.
If the SPST switch you used happened to have had its terminals marked
"line" and "load" (some are) then you should have connected the black
leg wire to the "line" terminal and the white wire to the "load"
terminal, though it would work just as well if those two connections
were reversed. The ground wire should connect to the ground screw on the
new switch.
Now, if that's what you did, there won't be any "neutral" in your new
switch box, just a continuously hot lead, the black wire, which will be
at 120 volts relative to ground and a switched lead, the white wire
which will be at 120 volts relative to ground whenever that new switch
is closed, regardless of which way the pull chain switch is set.
When both the pull chain switch AND the new wall switch are "open", the
white wire in the switch leg will be "floating", and all along it's
length it's sitting right next to that black wire which has 120 vac on
it. It is also right next to the ground wire which is at ground potential.
If the capacitance per unit length between the three wires is about
equal, then the cable will act as a "equal arm" capacitive voltage
divider and the unloaded voltage on the white wire will be about half of
the 120 vac on the black wire, so 61.8 volts is about right measured
relative to either ground OR the black wire. Your meter must have a high
enough input impedance so that it doesn't significantly load that
capacitive voltage divider.
But, with the wall switch still open, and the pull switch closed, the
resistance of the light bulb will hold the white wire down at ground
potential. (which is also what the neutral power feed lead in the
ceiling box is at.) The miniscule current through the capacitive
coupling in the cable isn't enough to develop a measurable voltage
across the resistance of the light bulb, so you'd measure a full 120
volts between the black and white wires at the switch with a high
impedance meter.
Get it?
Jeff
