3-way switch problems

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I have what seems to be something wrong with a 3 way switch on the light at the top of my stairs. This light can be turned off/on from the bottom of the stair as well as from the top.
I was replacing the fixture at the top of the stair and turned the power off at the switch thinking that I would be safe to replace the fixture. After removing the screws holding the fixture to the ceiling I checked the light with my greenlee gt-11 voltage detector and was surprised to hear the unit ringing. I broke out the DVM and sure enough 92 volts between the white and black wires to the fixture.
Turning on the light the voltage goes to 120vac, off back to 92vac.
So something seems very wrong, I went to the panel and turned off the breaker and changed out the fixture. I am now glad I was lazy and did it the wrong way first.
I don't understand why the incandecent bulb did not light at 92 volts. I have replaced the bulb with a CF as that is what we are doing throughtout the house.
About the house: 1957 cape 2 wire hot/neutral with no ground for all parts of the house. Any new work has been done with romex in proper gauges for circuits. 5 years ago we had the 100Amp fuse panel replaced with a 200 amp breaker panel.
Any common problems that might cause this? Am I using electricity even though the light is off?
Thanks! e
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Your DVM is giving you false readings. If you actually had anywhere near that voltage, the lamp would have lit

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snipped-for-privacy@gmail.com wrote:

Think about how a 3 way switch works and the answer should be clear.
There's no "off" per se -- only the selection of which of two lines is energized, and (on the other end) which of the two is selected to be connected to the fixture.
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Of course there is an "off". The hot goes between the two switches an either completes a circuit to the lamp... or doesn't

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1. There should not have been any voltage between the wires with either switch off, ie, the light went off.
2. It's likely that one of your switches has a high resistance connection in the contacts even when the switch is off. 92 volts between the wires doesn't mean the light should have come on. At no load you may read 92 but if the hot wire, ie, switchleg has voltage on it from a high resistance connection to a 120 volt source, the voltage will drop to zero when you put a load across those wires because of the high resistance source.
If you want to troubleshoot further, try turning off the other switch and see if the voltage is still there. If it's not, replace the first switch you turned off when you replaced the light.
Remove both switches from their wiring and see if the voltage is still there. Use resistance scale on your dvm and check the switches. Measure between the one screw that is different color then the other two, and each of the other screws, one at a time and flip the switch on and off and confirm 0 ohms in one direction and OL-infinite in the other. If you get anything besides infinite in that direction, replace that switch.
You have some kind of short circuit connection in the switch or wires that is still feeding some voltage to that hot wire when the switch is off. Could be one of the switches or the wiring.
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RBM wrote:

It's more a "choice of trunk" than "on/off" -- one or the other is always energized.

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Not at the light fixture, which is where he was measuring voltage

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RBM wrote:

good point -- I hadn't focused on that

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On 28 Feb 2007 18:07:40 -0800, " snipped-for-privacy@gmail.com"

It's common.

I don't know but if so, it would be minute, not 92/110 times normal.
Apparently you have an induced voltage in that line, just like voltages in the secondary of a transformer are induced by the current in the primary. But the maximum amperage will be minute.
Go back and measure with a voltmeter that has about 50,000 ohms per volt, instead of the 11 megohms per volt that iirc digital meters have. This is normally the difference between an analog meter and a digital meter, except for FET VOM's which are analog but have high resistance. Are there many of those around still? They were only popular for 10 years iirc. If you have an old one, it will say FET on its face.
With 50,000 ohms per volt, the voltage will drop to below one volt, but I don't know how much below. REport back to us.
It's easy to have voltage when the resistance is too high to let any current through. Something like when you hold your finger over most of the garden hose and the water shoots out 30 feet, but when you let your finger off, it comes out 5 inches.

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Thanks everyone for all the great responses, as for my meter it is old, [made in early 90's] but does not say FET that I can see it is a Fluke 8020A, I do have an old analog VM from radio shack tucked away somewhere I will dig that up tonight and test again. I will measure at the lamp socket this time as I won't have time to pull the fixture until the weekend.
Measuring between the screw thread and the center point at the bottom of the socket with the light off then on should be the same as what I did before.
I also want to test the switches as they have all been replaced over the time that I have been in the house. My wife did not like the older beige switches with the heavy click action so they were all replaced by me with leviton white 115v switches.
thanks to everyone again, running off to work now will report back tonight e

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On 1 Mar 2007 04:16:25 -0800, " snipped-for-privacy@gmail.com"

I think I'm sort of wrong here in that 20,000 ohms per volt is more common, but 20K, 50K are about the same compared to 11Meg. For analog meters, the value is printed on face of the meter.

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I would love to see this 92v. Everyone talks about phantom voltages, but I have never seen one, nor has my voltage detector ever gone off on anything but an authentic hot wire. But on the other hand, if you really had 92v, the bulb would be glowing. So it probably is phantom, but I would like to see it.
Did you test the other wire? Maybe it is 92v also, so the bulb sees not voltage difference and doesn't light up. Highly unlikely, but not impossible.
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On 28 Feb 2007 18:07:40 -0800, " snipped-for-privacy@gmail.com"

This could happen if you have the neutral switched instead of the hot. The 120 - 90 drop could be because you are measuring with the lamps in the circuit. If you take the lamps out and you do have a switched neutral then tester would then read 0.
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wrote:

I thought you might be onto something, but couldn't actually see a circuit where this would happen. The unswitched hot would be at 120v. The switched neutral would be at zero because of the switch. So I can't see how switching the neutral would affect anything.
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wrote:

Not sure which situation you are referring to, but if you mean the hot wire is connected to the bulb, and you are measuring voltage on the other, the cold side of the bulb when the neutral is switched and the neutral is open, there cannot be 90 volts there for that reason. There would be a full 117 volts or whatever is measured at the receptacle or at the hot side of the lightbulb.
E=IR. When I (the current) = zero, then E, the voltage drop accross the resistor (R) is also zero. It doesn't matter how narrow the filament in the light bulb is, with the neutral open and no current** flowing through the lightbulb, there will be no voltage drop accros the lightbulb or accross the wire that feeds it from the receptacle.
**But there will be some current when you attach a meter***. How much current depends a lot on what kind of meter you use. ***I think this is a good example of the Heisenberg (sp?) Uncertainty Principle, which says iirc that when one attempts to measure something, he changes it. Not sure why that would always be true, but physics was a long time ago.
But this is a good time to talk about the difference between a meter that has 20,000 ohms per volt and one that has 11 million ohms (per volt?, I don't know) For house current, 20,000 is plenty, but the meters don't have the advantages of auto-polarity, and even auto ranging might be useful sometimss. The change to digital meters might have been started by the space program, but is driven by the fact that 20,000 ohms is not so good for electronics.
If you have a transistor that is connected by a 10 meg ohm resistor to ground, and you want mesure the voltage on the non-grounded end of the transistor, and you connect a 20,000 ohm meter across the resistor, you now have two resistors in parallel, one the original 10 megs, and the other the 20,000 ohm meter. So the total resistance is now about 20,000 ohms and current that might be flowing through the transistor has, barring other factors, increased tremendously, up to a factor of 500 since the resistance is now 1/500th of what it used to be. IF otoh, you used a 11 meg meter, the resistance (impedance might be a better term but they are both measured in ohms) of the two resistances in parallel is about 5 megs, not such a big change.
So people in electronics, especially with transistors and not tubes (but tubes too), needed higher impedance meters.
What is an advantage in electronics can be a disadvantage in electricity. If you have a pretty high voltage for a home (like 90 volts) but one with a very low maximum current, and you connect a digital 11 meg meter across it, the currrent draw will be very low, and the voltage will not drop much because the voltage drop is determined by E=IR and the resistance of the circuit is low compared to the resistance of the meter. So it's like a big pipe feeds a very teeny pipe, adn there is almost no current through the big pipe and no pressure drop.
If on the other hand one uses a 20,000 ohm meter, that will still be quite a restriction to current flow, but only 1/500th and there will be 500 times as much current, so the voltage drop will be 500 times as much.
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While your description of what is hapning is ok, much of what you are saying is not accurate. The 20,000 ohm meter is really that per volt of the full scale setting. That is if you have the standard Simpson 260 meter (concidered the best analog meter for electricians for years) and set it for 250 VAC there is about 5 meg of resistance in the circuit which is only about half the resistance of the digital meter. In the problem above if it is leakage or induced voltage it will read about half of what the digital meter will. Also if you click to the next lowest scale you will see the relative position of the meter will not change much if it is very low current leakage.
Also many of the transistor circuits are high current low voltage circuits and the tube circuits are high voltage low current circuits. The analog meter will usually be more accurate in reading the voltages there than they will be in the tube circuits. A 10 meg resistor in the circuit in parallel with the 11 meg meter will register a very large change, not a small one like you state. Anything over about 10 % is usually noticable and you have a 50 % differance .
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Well I have to say that you guys are well over my head, but as the OP I have more data that seems to validate what some have said concerning the DVM vs Analog meter. With my little micronta on 150V scale and the switch off, I barley get the needle to move perhaps 10V or so. With switch on it goes up to 119V as expected.
What is disconcerting to the guy like me who knows enough to be dangerous, is that in the off position my greenlee voltage detector goes nuts.
Some observations on my part: There are at least 2 if not 3 more wires in the box that the fixture is hung from, this house was built in 1957 and the breaker that I turn off to kill power to this light also powers the bathroom lights and the lights of one of the two upstairs bedrooms. [all two wire no gound circuits]
Another thing that may or may not mean anything, but I have also changed the breaker that the electrician installed with a GFCI breaker as there is an outlet in the bathroom near the sink that I felt needed protection.
So there you have it, I am going to test the switches as dreamchaser suggested tomorrow and post the results. thanks again
e

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On 2 Mar 2007 17:27:39 -0800, " snipped-for-privacy@gmail.com"

I did not follow much of what you posted, but I do know that a bad switch could not cause your problems. A bad connection at the switch boxes could be the cause, but not the switches themselves.
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wrote:

er....never mind. :) I totally spaced on that one. I confused your message with another thread I was following.
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I am not sure what the greenlee is but if it is similar to the voltage sticks I have seen, somethink like a big ink pen where the thing glows if near an electric wire.
Just try rubbing it on your cloths and watch it light up from just the small ammount of static electricity.
Those things require a lot of expierance to tell when you actually have a voltage that is really connected to the power lines or just picking up some static or induced voltage.
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