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Thread: Voltage on switch leg. Switch off.

  1. #1
    Join Date
    Feb 2007
    Location
    Highlands Ranch, CO
    Posts
    41

    Voltage on switch leg. Switch off.

    I had an inspector come out to a house and met with one of my guys on a final.

    He plugged into a 1/2 switched outlet (switch in off position) with a real fancy Ideal arc fault/GFI tester. The neon lights on the tester glowed dimly and he made the statement that there seemed to be voltage on the neutral. The regular GB outlet tester had no lights glowing at all.

    I went back to the house with my guy and we started checking out the circuit. We put the digital meter across the switch leg and the neutral and across the switch leg to ground and got about 40 volts. We went through the whole house checking the same way and found this to be true on every 1/2 switched outlet.

    We took a piece of 14/3 NMB about 20' long and hooked the black and white to 120 volts and what do you know? The red wire had about 40 volts on it.

    The last couple of days we've been checking every 1/2 switch that we ran across just to see. It always has about 40 volts on the switch leg with the switch off.

    As this does not seem to be due to any sort of wiring defect such as too tight on the stapling or a tab on the receptacle not cut all the way I'm assuming that the red wire gets partially energized by induction or something. I'm no engineer. Just a guess.

    There is no real potential present as you can touch the wire to the ground or neutral and there is no indication of a spark or anything.

    I was looking for some input from others who may have experienced this occurence.

  2. #2
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,287
    Some call it ghost voltage or phantom voltage

    Phantom Voltages

  3. #3
    Join Date
    Feb 2007
    Location
    Highlands Ranch, CO
    Posts
    41
    Thanks for the short reply to a long question.

    That document may come in very handy on Monday when I meet with the inspector again.

  4. #4
    Join Date
    Feb 2003
    Location
    Massachusetts
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    48,287
    No problem, good luck.

  5. #5
    Join Date
    May 2003
    Location
    Leesburg, VA
    Posts
    6,931
    I just did a report for a n EC having an issue with an electrical inspector exact same issue, if you'd like I can forward this to you.
    Brian John
    Leesburg, VA

  6. #6
    Join Date
    Jan 2007
    Location
    Newport, NC
    Posts
    1,881
    I had the same problem on a final a couple of weeks ago. We took apart the whole circuit ohming the wires and checking tension of the wire (it was a floor receptacle that I was able to remove and grab the wire under the floor). There was no signs of anything wrong, and when anything was plugged in (no load), the voltage went away. I eventually chalked it up to "phantom" voltage, but I am never fond of chalking it up.

    Edit to add: Thanks for the link, Bob.
    Thom

    There is no greater sign of wealth than the ability to throw away food.

  7. #7
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Thom, that's why many of us prefer using a low-impedance tester, such as a solenoid, for most testing and troubleshooting.

    I only use a volt-meter if I need to know the exact voltage, and then, I make sure I'm reading between loaded conductors.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  8. #8
    Join Date
    Sep 2008
    Location
    Fairmont, WV, USA
    Posts
    1,591

    Get a resistor

    I ordered Fluke SV225s for the guys on our crew. It is a 3Kohm resistor packaged in a little wart with two guarded male connectors to plug into your meter and two guarded female connectors to accept your test leads. It loads down circuits enough to eliminate phantom voltage readings and is rated CAT IV for up to 600V.

    I leave it in my meter except when reading resistance or debugging 24V fire alarm circuits as it can trip them.

    /s/ Jim WIlliams

  9. #9
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,287

  10. #10
    Join Date
    May 2003
    Location
    Leesburg, VA
    Posts
    6,931
    I have been working with EC's and inspectors with the phantom voltage issue for 15-20 years, shouldn't the inspectors be aware of this by know.

    Do most inspection agencies require continuing education?
    Brian John
    Leesburg, VA

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