Phantom voltage

Discussion in 'Electrical Forum discussion & Blog' started by jparrie, Aug 20, 2008.

  1. jparrie

    jparrie Automotive Locksmith

    While poking around with a digital volt/ohm meter looking for the hot conductor in a three way switch circuit, I noticed a reading of 38 volts on the red conductor. I get the same reading at either end of the conductor, and this is being done with both switches removed from the circuit. As soon as I ground either end of the conductor, the reading drops to 0, with no arcs or sparks:) I assume that this is a case of using the incorrect tool for the job, and the meter is just too sensitive?

    Or is there something else I should be looking at?

    What do you pro's use to test for hot? I have one of those inductive probes that is supposed to do the job, but it is always giving me false hot indications. I would like something quick and reliable.
  2. Billy_Bob

    Billy_Bob In the Trades

  3. jparrie

    jparrie Automotive Locksmith

  4. TigerDriver

    TigerDriver New Member

    Salem, VA

    First choice: an inductively coupled Fluke tester (e.g., Model 1AC-A1-II).
    Since they ignore anything under 90 VAC, I've yet to have a false positive. They're about $30.

    Second Choice: The cheapest Radio Shack volt/ohm meter you can buy (about $8 IIRC). Because of its low input impedence, you'll get no false positives, but it's always a two-handed operation and there's no audible indicator.
  5. Bob NH

    Bob NH In the Trades

    New Hampshire
    For voltages up to 300 Volts:

    Get a double bannana plug that allows terminating a couple of wires and has a female connection on the back to insert you leads as you now insert them in the meter. Connect a 250 kOhm 1/2 Watt resistor across the bannana plug.

    Stick that into the terminals of your meter when you are tracing voltages as you describe and plug your leads into the back of the double bannana plug.

    If you can't find the double bannana plug you can get a set of leads and solder the resistor across them, with appropriate insulation, and use those leads when you have a questionable measurement.

    You will not get any more false readings.
  6. Igor

    Igor New Member

    Some of the newer Fluke Digital meters have a low-impedance mode to avoid this problem. I'm not sure if other manufacturers have come out with similar products yet.
  7. Billy_Bob

    Billy_Bob In the Trades

    I don't!

    Actually for home and business electrical wiring, I've done so much work in construction and wiring that I can pretty much tell what is what just by looking.

    And I always work with the power off.

    So pretty rare that I need to test anything so far as home/business wiring goes.

    If I do need to test power to something (home/business wiring), my tester of choice is a table lamp, radio, or whatever is handy to plug in. This is usually to be darn sure power is off before I work on something.

    If I need to test a wire (low voltage, house voltage, car wiring, whatever...), there are many ways to do this. I might disconnect both ends of the wire from the circuit and use a digital ohm meter. Or an analog meter if there is a "diode" in the circuit. Or a digital meter which has a diode function. (Diodes only allow direct current to flow one direction.) They need more electricity flowing through the circuit to test them than a digital VOM provides. Analog meters will test them or a digital VOM with the diode symbol which looks like the bottom picture here...

    BTW - LED's are "light emitting diodes"! Stick the test leads on one way and you will get nothing. The opposite way and you get a reading.

    Most of my testing is for low voltage stuff because you can't plug anything in to test it. I mostly use a digital ohm meter for this. And I always disconnect both ends of the wire from the circuit before testing. If you don't do this, there could be a voltage on the wire which could damage the ohm meter. Or there may be other things in the circuit which would give you a false reading.

    The other day I was installing a new plug on a trailer. I used a digital voltmeter along with a wiring diagram of what is what on the trailer plug and turned on turn signals, etc. on the truck to verify I was making the correct connections.

    Then there is the case where someone runs a bunch of low voltage cables - say 10 four conductor cables going to different rooms and terminating at one spot. But they DIDN'T MARK THE CABLES! In that case I would go around to the rooms and strip each wire and leave them hanging. Then in one room short two wires in that cable. Then go back to the main spot and see which cable had a short on those two wires (using a continuity tester or ohm meter). Then mark that wire. Then on to the next wire...

    For problems with a 3-way switch circuit - say it is not working. The way I would go about this is by experience. Most likely problem is light bulb...

    Second would be a bad switch. Sometimes you can flip a switch and a bad switch will sound or feel different when flipping it. (If they are old switches, I'll just go ahead and replace both of them.)

    Then third most likely problem would be with the light fixture or the wiring just behind the fixture. Lots of heat in there which can do a lot of damage to older wiring. If the insulation crumbles, I'll replace the fixture - maybe some of the wiring to the fixture if needed.

    VERY unlikely there would be a problem with the wiring itself. Sometimes there can be a bad connection somewhere. But again I would not test, I would remove covers and take a peek inside. See what HORRORS are awaiting me!

    For phone lines, my tester of choice is a phone! There are two tests - dial tone and punch a number to be sure you get a tone (if wires backwards, you will not get a tone). For this "tester" you can get a cheap phone, plug it into a jack, then connect test leads to the jack connections.

    For finding where there is a cut or break in a wire, I'll disconnect power - remove wires from the circuit, then use a tone generator and a "through the wall detector". Like this one...

    Then for appliances which are "dead" - no power (circuit breaker on). I'll use a digital voltmeter and start where the electricity comes in from the power cord, then go from there. For this testing power needs to be on of course.

    For testing 120V appliances which are dead. 1st thing I do is find something which is working like a lamp or radio, then plug it into that outlet to be sure there is power to that outlet. Then test the appliance to be sure it is getting power through the cord and then go from there.

    For testing a wire running from one room to another, get a long string of wire, connect it to one end, in one room, then run it to the other room, then you can use your ohm meter for testing.

    A common problem when repairing electrical/electronic things is that people will suspect/replace whatever they don't understand. Like if there is an electronic "black box" or computer module. They don't understand how it works, so this is what they replace. In reality the things most likely to have problems are mechanical things. These wear out with use. So things like switches, relays, motors, etc.
  8. jparrie

    jparrie Automotive Locksmith


    Thanks for the excellent reccomendation. I picked up a 1AC-A1-II and it is exactly what I was looking for. No false reports, and is discriminating enough to pickup the hot side of a lamp zip cord and ignore the neutral side. Very nice!
  9. renaissanceEE

    renaissanceEE New Member

    Brings back memories I first saw the same thing two years ago at a buddy's house.
    Just picture two Electrical Engineers trying to fix a messed up 3-way in a 100 year old house, puzzling over why we were seeing 38 or so Volts on the red. Then it hit us, DUH it was just induced and floating high relative to the ground. Problem ended up being the previous owner used two different styles of very old three-way switches and failed to notice a change in layout of the terminals.
  10. Thatguy

    Thatguy Homeowner

    50' of ROMEX® @ 100 picofarads/foot @ 60 Hz gives a reactance of 500 kohms.
    If only one ROMEX® conductor is connected to 120v, a DVM with 20 megohm input impedance will read almost 120v on either of the other two leads, but a VOM @ 1000 ohms/volt on the 150v scale will read ~43v. Lower scales on this same meter will read less than 43v and I guess this changing reading is a clue that this 120v has a high source impedance.
    Last edited by a moderator: Dec 7, 2011
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