Phantom Voltage?

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.
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switch.
no
This happens because you are using a digital meter with an extremely high input impedance which allows even minute capacitive coupling, as in an open switch, to "leak" voltage (well, technically current but lets not quibble). If you had an old-fashioned analog multimeter there might be some measurable voltage but it would be much lower. In either case if you put a load such as an incandescent lamp across the lines the voltage will be pretty much undetectable with either sort of meter.
--
John McGaw
[Knoxville, TN, USA]
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On Tue, 6 Jul 2004 14:49:51 -0700, John McGaw wrote

John -
I don't follow you. The way I read the original post, a new switch leg has been inserted on the line side of the existing pull-chain switch, and the OP is checking the voltage bewteen the line side of the new switch and neutral.
If he were checking voltage from the load side of an open switch to neutral then I would understand your explanation, but I don't see that here.
Additionally, he's reading a significantly different voltage between his test point and the EGC than he found to neutral...
I would be grateful for more explanation.
Toller -
Am I correct that the new switch leg was wired in "before" the pull-chain? And that you were checking the voltage at the hot/line side of the new switch? Was the new switch open or closed when you were performing these tests? Where was your neutral test point? Your ground test point?
Thank you both for your time,
- Kenneth
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It's difficult to briefly explain this situation to an non scientist or engineer. (sometimes it's even hard to convince them)
All physical measurement schemes effect the situation they are measuring. These effects can be gross or subtile. It is up to the experimenter to manage and interpret the results.
Let's consider a situation where there is a water leak and we would like to measure how much water is leaking out.
One technique would be to measure the weight of a dry jar, place the jar under the leak for a fixed period of time, measure the jar and water, then subtract the dry jar weight. This could be a reasonable technique in many situations, but for a small leak the water may evaporate before it reaches the jar and we will always measure zero.
--
In your situation there is a small amount of energy leaking out of any
electrical wire into the surrounding area. Voltmeters need a small
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This is the reason ametuers should stick to light-bulb testers instead of digital multimeters.
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Phantom loads are waisted juice from apliances that remain plugged in on standby and wall transformers , as Tvs radios, microwaves , CDs DVDs etc. Many can cost 1.5$ each month for each unit . New Energy Star rated can take 1/10th th power.
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No, it's not.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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No its not ? well Chris Lewis whats a phantom load, whats is a wall wart - transformer load in off mode. Phantom load.
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Phantom _voltage_ (what the OP was asking about) is what you measure on a wire that's not connected to anything.

Wall warts are ordinary loads, not in the slightest bit phantom. They don't induce voltages onto disconnected wires either.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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I have heard the term Phantom Load used often, referring to the power and cost they waste unknown to most.
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Unfortunate choice of terminology I think.
Best to remember that the OP was talking about a situation where he's measuring a _voltage_ on something that's supposedly not connected to _anything_.
Like "where on earth did that come from?"
He's measuring an induced voltage on an antenna (the disconnected wire) from a "60Hz radio transmitter" - the wiring in rest of his house (tho, likely most from the hot wire in the same sheath)...
This is how radio receivers _work_ - by detecting the voltage induced on their antennas from radio transmitters. And is why receiver sensitivity is rated in "volts" (tho, usually millivolts, not 10s of volts).
I wouldn't consider wall wart load "phantom" in any sense, other than "I forgot about that...".
Remember: there's no such thing as infinite resistance (aka "zero conductance"). If you have a sensitive enough meter, you can measure the current flow through an _open_ switch (thru the insulators). It may amount to a milli-watt-hour per millenia, but it's still there.
"Phantom voltage" (more properly termed "induced voltage") is simply of interest because it makes a DIY with a very-high-impedance DVM doubt their own sanity. So if you have one of those, you need to take it into consideration when you're measuring stuff...
--
Chris Lewis, Una confibula non set est
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toller posted for all of us....

This worries me H-G 120v! Switch seems to be on the neutral leg. I'm not a sparky so I could be wrong. Hope Jake or John Grabowski reads this.

--
Tekkie

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switch.
the
No no. The readings are at the switch, not the light.
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Probably - see the explanations from John McGaw and Barry, they describe inductive pickup pretty well, but without a better description of the circuit, it's hard to tell for certain.
It's virtually impossible to get any voltage measurement other than "zero or real close" or "120V or real close" on a 120V circuit. Other than thru inductive pickup.

Inductive pickup voltages can certainly repeat. Try a different meter, and i'll be different.

Can't really tell from the circuit description. If you're saying you're reading 61.8V on a line that is disconnected at _both_ ends, it really has to be inductive pickup (or damaged wire insulation).
What's the N-G voltage?
--
Chris Lewis, Una confibula non set est
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toller wrote:

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
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"My luck is so bad that if I bought a cemetery, people would stop dying."
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<snip> CL> Can't really tell from the circuit description. If you're saying you're re din CL> CL> 61.8V on a line that is disconnected at _both_ ends, it really has to be in uct CL> ve CL> pickup (or damaged wire insulation). CL> CL> What's the N-G voltage?
I'm guessing measurement between neutral and ground, which further seems to indicate mV rather than V being measured.
(BTW, checking the GFCI problem here still on the to-do list: barely hit 65 today here -- brrrr!!!)
- barry.martinATthesafebbs.zeppole.com
* Writing Rules: Avoid alliteration. Always.
--
RoseReader 2.52 P003186
The Safe BBS Bettendorf, IA 563-359-1971
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