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#176079 03/19/08 08:42 PM
Joined: Dec 2002
Posts: 32
wiking Offline OP
Junior Member
I was on a job troubleshooting some a pair of three way switching systems when I noticed that when I checked the voltage on the "unenergized" switch leg I was reading 34 volts. The guy running the job said it was induced voltage and not to worry about it. I didn't think about it again until I was on a job and turning on the the two breakers that feed a 12/3 mc that fed two circuits of recepticles. I turned one on and just to do some checking I checked the wire that was on the red wire of the mc and although the breaker was off, it had 34 volts on it.
I touched the wire and put my finger to ground and could feel nothing. I remembered someone telling me about a kind of phantom instrument reading or something and wondered if anyone had any explanations on why this would be.
Could it be a neutral or proximity problem, or do you agree with the opinion I got that it was "induced"?

Joined: Dec 2000
Posts: 4,291
High impedance, digital meters see a "ghost" voltage because they put virtually no load on the unenergized conductor.

The trick is to use a solenoid type voltage tester.

Joined: Jul 2004
Posts: 9,828
Likes: 22
In the electronics world very high impedance is a wonderful thing. You meter does not affect what the circuit is doing. In "power" circuits, this high impedance is not so great. You can see induced voltages on "floating" wires that have no effect. I am very surprised digital meters meant for the "electrical" trades don't have a built in load resistor (maybe switchable) that makes these "phantom voltage" readings go away. A megohm would probably do it and still have negligible affect on your readings.

Greg Fretwell
Joined: Dec 2001
Posts: 2,498
Quality digital meters do have a switchable load resistor, at least in Europe (Benning Duspol for example). Problems like this are very common with DIYers using cheap DMMs or neon screwdrivers on 230V lines and then complain about the weird readings.

On the other hand, sometimes a neon screwdriver that lights up in both holes of a receptacle does indicate severe trouble... once found a broken neutral that way.

Joined: Mar 2005
Posts: 1,213
A good ole fashioned analog meter comes in handy for these occassions. We've made it standard procedure to doublecheck with a lower-impedance meter whenever a fluke picks up suspected ghost voltages.

Joined: Dec 2007
Posts: 35
When do you suspect phantom voltages?

Obviously when the breaker is off and you measure a voltage?

Can you suspect Phantom voltage in other situations? How high can the Phantom voltage go? 120?

If the circuit is off, then what is inducing the voltage? Can it be a shared neutral?

Sorry about the multiple questions, I am curious about this phenomena..

Joined: Jul 2004
Posts: 9,828
Likes: 22
You will see induced voltage any time you have a wire in a cable or raceway that is not connected to a load, power or ground at either end. The 3 way traveler is one example. When the switches are set so the load is on, the other traveler is not connected to anything but it is magnetically coupled to the current carrying traveler.

Greg Fretwell
Joined: Aug 2001
Posts: 7,520
Even if other energized conductors are not actually carrying current to a load, you will still get a reading from the capacitive coupling. Two conductors running parallel separated by just their insulation form a very effective capacitor. The longer the cable run, the higher the capacitance and the higher the voltage reading.

You'll see the same capacitive effect if you use an older-style analog megger for insulation testing. When you press the button, the needle will "kick" over to the right, then gradually drop back to the left. The longer the cable, the greater the "kick" and the longer it will take to drop back to its steady value. The D.C. you are applying to test is charging the capacitance of the cable.

Joined: Nov 2005
Posts: 827
Likes: 1
A number of years ago, I came up with the expression, "All the world is a voltage divider" I think it started with explaining the voltages to ground on battery strings measured with Fluke 77s Vs Simpson 260s. To gain greater understanding, first grab your meter manual and check the specs for the Mohms and pF that reflect it's input impedance. The cut sheets don't usually show these specs but they should be in the manual's expanded specs. Next, draw a circle with a sine wave in it that represents an AC voltage source. Place an "N" and the ground symbol below the source and write 120 VAC or Vrms beside it. To the right, draw a vertical resistor in parallel with a vertical capacitor. Add a ground symbol below them, the values from the manual, and the "M" in a circle meter symbol pointing across them. These things represent what you know. Next, draw a line up and then to the right of the source to a rectangle, then from the right side of the rectangle, over and down to the RC combo. In the rectangle, write inductive coupling, capacitive coupling, and resistive losses??? These are the values that you don't know, yet measure the effects of on your meter.

Now, I guess you decided to perform the death defying 34 volt to ground touch. Yet, you sought out the tingle without the enlightenment. Had you had a meter probe tip in each hand while touching the wire and ground, you would have gained another useful measurement. Had you then switched to ohms and touched the tips with the same pressure, you would know about the amount of parallel resistance that you added to the RC mix.

In a simple, purely resistive world, we could see that we dropped about 1/4 of the voltage with our meter loading. We would assume that if our meter was 10Mohms, that the other 3/4 of the 120 volts was getting dropped over about 30Mohms of lumped effects. But in the real world of AC circuits and reactive components, the voltages across all of the series components will likely add up to more than the total due to phase shifts involved.

I never really liked the term "phantom voltage" because thinking it isn't really there could turn you into a ghost! So could someone killed by induced voltage, be accused of over reacting???

Joined: Jul 2004
Posts: 9,828
Likes: 22
We ran into this a lot when IBM gave everyone a Fluke 8060. Suddenly they had changed all those "zero" readings on floating wires to what I called a random number generator.
My "support" calls spiked.
We also quickly found out the classic difference between "precision" and "accuracy" when guys started comparing their road worn flukes on the same source. After a while it became apparent calibration was a must if you really wanted to believe all the numbers that fluke gave you. Most had an inaccurate reading, "precise" out to 3 decimal places.
A lot of guys ordered the Triplett analog meter that was much smaller than the Fluke and as accurate for most real world measurements

Greg Fretwell
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