Mysterious Tripping GFCI receptacle

Discussion in 'Electrical Forum discussion & Blog' started by cacher_chick, Dec 13, 2011.

  1. cacher_chick

    cacher_chick Test, Don't Guess!

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    With all the talk of how GFCI's work and where or why they should be used, I would appreciate some discussion on how to diagnose a problem. I have a good multi-meter and know how to use it. I could even dig out my labscope if I had some good ideas of what to look for.

    I have 2- 20 amp branch circuits running through the same raceway (EMT).
    One branch feeds a single GFCI receptacle and nothing else.

    The other branch goes to a single pole switch which is mounted in the same box as the receptacle.
    This switch feeds a bank of troffer lights.

    Not that it should matter, but there are ground pigtails from the receptacle and switch ground terminals to the box. There are ground pigtails connected to the troffers. The EMT is the ground conductor back to the panel.

    The lights work fine. If I plug something into the receptacle, it works fine.

    BUT

    When I turn the lights OFF - about one in ten times, the GFCI will trip.

    The lights and the GFCI receptacle are fed from separate breakers. They do NOT share a neutral.
    Nothing is plugged into the GFCI.

    We want to test, not guess. Where do you start?
    Last edited: Dec 13, 2011
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    When the lights get turned off, the field collapses and it can act like an inductor, putting a spike on the line. It might be enough to couple to the other leads on the GFCI. Does it have a load side that is just capped off? Have you tried one without load terminals or pigtails (assuming you don't need them)? Would be interesting to record the info, but those test tools are pretty expensive, a typical scope is lousy at catching single events or triggering on them as well.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

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    He said that it was a GFCI receptacle so there would not be any induced voltage in the device itself and any induced voltage in the supply conductors would have no effect on the device. Besides induced voltage will not cause the device to trip, there must be current flowing and unless there is something plugged into the device no current will flow
    He might be hitting the test button when he turns off the light.
    Last edited: Dec 14, 2011
  4. leejosepho

    leejosepho DIY scratch-pad engineer

    Ah yes, good ol' operator error ... :eek:
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Some of the GFCI have pigtails for the load side. If not being used, they get capped. That pigtail may be enough from an adjacent power conductor in the box to give grief.
  6. DonL

    DonL Jack of all trades Master of one

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    If that GFCI is equipped for grounded neutral detection (Most Good ones are), It may be that it is affected because of the Ground wiring. Since that is the only Common conductor used by both items.

    To test some theory, You can connect your O-Scope to the GFCI and look at the 60Hz sinewave. Turning the light ON and OFF, You should be able to see the Spike on the sinewave that is tripping it.

    You can also Connect the Scope Probe Ground to the Ground in the box and probe tip connected to the neutral, Set the scope for DC Coupling, Line Trigger and as low a voltage that you can go and still get a visible straight line. Then turn the light OFF and ON, If the Trace Jumps, That is what the GFCI is seeing.

    A MOV on the line may help, But if it is a well built GFCI there should be one built in to it.

    A light bulb is Not a Pure Resistive Load but, I am not sure how much of a Field Collapse you would get from a light bulb, But many bulbs could play in. EM fields on Long wire runs are not that uncommon, Especially when the wires are run in parallel, and also affect the Ground as well as the Neutral and the Hot.

    An O-Scope is your friend.


    As always, Be Careful Playing with Electricity.


    "Some of the GFCI have pigtails for the load side. If not being used, they get capped. That pigtail may be enough from an adjacent power conductor in the box to give grief."

    Very True, They are better put in to their own box. Even if it does not have pigtails, Wires running close to the GFI transformer could cause problems.


    Is that a lighted switch by any chance ?
    Last edited: Dec 14, 2011
  7. leejosepho

    leejosepho DIY scratch-pad engineer

    I might have that kind of problem with a GFCI outlet on my rear patio. That circuit had been fine in the past but I had disconnected it before doing our recent service upgrade, and now that GFCI trips the breaker in the main panel whenever I try to reset it. Its pigtail supplies power to a switch supplying power to an outside light, but the outlet now trips even with that switch off.
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

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    If it is tripping the breaker it sounds like a fault somewhere in the circuit, or as some laypeople would it is shorted out.
  9. DonL

    DonL Jack of all trades Master of one

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    Sounds like someone got their wires crossed.

    Maybe the load side pigtails are using the same Wire Nut.


    That is funny JW, But you are right...
  10. leejosepho

    leejosepho DIY scratch-pad engineer

    The short has to be in the GFCI outlet or its box. The wire running out there had been connected before and that circuit was fine while fed from an existing junction box in the attic ... and now the new feed to the new junction box where that wire has been connected works just fine at another outlet being fed from that same new junction box. I plan to check that all out later today, but first I must deal with a leaking washing machine that kept my wife and I up late doing some mopping last night ...
  11. ballvalve

    ballvalve General Engineering Contractor

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    Turn your meter to "MIC" - made in China. That 12 year old that soldered the innards didnt get her rice at the communal factory that morning. Forgot to wear her red scarf. She also was held back on her 28 cent a day pay.

    Now you and we all pay for those empty factory buildings housing the homeless.
    Last edited: Dec 16, 2011
  12. cacher_chick

    cacher_chick Test, Don't Guess!

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    It seems pretty clear to me that the GFCI is being effected by the switching off of the florescent troffer lights.
    I can reproduce this by flipping the switch on and off several times.

    Jim could be onto something, as the "load" terminals of the GFCI are feeding another receptacle, which also has nothing plugged into it. The GFCI might be one of "Ballvalve's favorites", which could also be part of the problem.

    I will look at it further this weekend and see if I can nail anything down.
  13. ballvalve

    ballvalve General Engineering Contractor

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  14. DonL

    DonL Jack of all trades Master of one

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    I think that I have one of them in my bathroom.

    If I key my 2 Meter (144MHZ) walkie anywhere near it, it trips every time.
  15. ballvalve

    ballvalve General Engineering Contractor

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    The Chinese may have inserted a bug into all GFCI's so they can shut them all down before the invasion. You could be on to it.

    I think the pentagon has several teams devoted to such industrial sabotage. Like the smart meters that will bring our entire grid down with a few hackers from Romania, via Asia.
  16. DonL

    DonL Jack of all trades Master of one

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    Sounds Just like the open backdoor on the Internet Routers made over there...
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