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Thread: Residential troubleshoot

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    Residential troubleshoot

    I had an interesting troubleshoot yesterday that I have to return to finish. The kitchen/living-room/foyer lighting circuit has a short circuit that suddenly began a couple of days ago. I checked every switch box involved, making sure every switch was switched off, and found nothing. I placed a receptacle in series with the breaker and black wire, and plugged in the customer's shop vacuum as an audible test. The vacuum runs at full speed, with no variation, confirming a direct short.

    Nothing I did caused any change to the vacuum, so I then disconnected the white and bare conductors of that circuit's NM cable from the panel's neutral bus, and the vacuum still runs at full speed. This tells me the black wire is grounded externally from the circuit's conductors. The only box where another circuit is involved is the sink light/disposer switches, which I opened and pulled the switches out. Still no change. I figure the next step is to visually trace the circuit in the crawl.

    To me, disconnecting the neutral and EGC in the panel eliminates a staple, nail, or screw through the cable. I plan to return tomorrow with my helper to try the visual trace. I hope to minimize the time and cost to the customer any more than necessary to restore the power. Can anyone suggest a possibility other than the black wire being faulted to something solidly grounded under the house (or worse, inside a wall) that can cause this set of symptoms?
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by LarryFine View Post

    Nothing I did caused any change to the vacuum, so I then disconnected the white and bare conductors of that circuit's NM cable from the panel's neutral bus, and the vacuum still runs at full speed. This tells me the black wire is grounded externally from the circuit's conductors. The only box where another circuit is involved is the sink light/disposer switches, which I opened and pulled the switches out. Still no change. I figure the next step is to visually trace the circuit in the crawl.
    Did you open/separate the EGCs in that box ? Wherever your hot is shorted, it could be returning on the other circuits ground.

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    Quote Originally Posted by LarryFine View Post
    The only box where another circuit is involved is the sink light/disposer switches
    If that's true then I think there must be a current path that strays off the premises wiring. But you can check for another circuit having its neutral improperly tied to your problem circuit's neutral by checking if you continuity at the panel between the end of the lifted neutral and the neutral bar. [Edit: and as 414MHz suggests, you can check the lifted ground the same way.]

    Cheers, Wayne

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    Quote Originally Posted by 414Mhz View Post
    Did you open/separate the EGCs in that box ? Wherever your hot is shorted, it could be returning on the other circuits ground.
    Not yet. That's the next step.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by wwhitney View Post
    But you can check for another circuit having its neutral improperly tied to your problem circuit's neutral by checking if you continuity at the panel between the end of the lifted neutral and the neutral bar.
    Next next step.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Short from hot to panel metal perhaps? I would start by looking at where the cable leaves the panel first. That's easier than tearing boxes apart and a short to ground by sharp metal does happen.
    Cheers and Stay Safe,

    Marky the Sparky

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    180218-1637 EST

    Larry:

    From your description there is no current flowing back thru the neutral and/or the EGC for this circuit with the problem as measured at the main panel because these two wires are disconnected at the panel. Thus, the return current has to be thru a different path to the main panel or some other way to the transformer center tap.

    Therefore, there is a substantial stray magnetic field generated by a relatively open loop. Open loop does not mean an open electric circuit, but rather wires separate such that their magnetic fields do not cancel.

    With a coil of 1500 turns of #33 wire on a 5/8" core measuring about 5/8 x 1.25 x 1.5" on the outside I can get moderate voltages (millivolts) from an open loop with 5 A 60 Hz of current. About 5 mV at the 1 turn coil center. This test coil is about a 5 ft diameter circle. Next to my water supply line I read 15 mV with this coil.

    With a coil like this and a meter that can resolve 0.01 mV (10 microvolts) you might be able to get some idea of where the short is.

    Where could you find such a coil. A small 120 V transformer that was not potted and could be disassembled to get the primary, doesn't matter that secondary is also there. A 5 or 10 W 120 V primary transfotrmer might provide a reasonably number of primary turns.

    Within my home I read about 10 to 20 microvolts from this coil in most places. This coil in proximity to wires in my main panel will produce multi-mV readings. I would use a 1500 W space heater as a means of getting about 10 to 12 A at 120 V.

    Another technique that might help find the unwanted return path envolves measuring voltage referenced to the neutral-EGC buses in the main panel. #12 copper wire is about 1.6 ohms per 1000 ft, or 0.0016 ohms per foot. At a change of 10 A you will see a change of 16 mV per foot. Thus, with a voltage measurement and a changing 10 A current you may be able to find what other path or paths are being used. A steady 10 A may be easy enough with which to work.

    .

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    Quote Originally Posted by K8MHZ View Post
    Short from hot to panel metal perhaps? I would start by looking at where the cable leaves the panel first. That's easier than tearing boxes apart and a short to ground by sharp metal does happen.
    First thing I checked once I returned to the panel. I loosened the screws in the cable clamp to make sure it wasn't pinched, and it now moves freely. Thusly, my helper and I will hopefully be able to ID the cable from below.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Larry, I had a similar bolted short on a residential kitchen lighting circuit last spring. Turned out, the teenager, of the house, had hit a threeway hard enough to break the toggle pivot shaft which caused one of the two cams that operate the contacts to stop turning. It happened that the cam that broke loose was allowing its contact to close.

    Now, in a normal threeway this wouldn't show up as a bolted short, but, for what ever reason, the electrician in this high-end kitchen, several years earlier, had to remove the far threeway (the hot was fed to this one), and just have a single switch controlling this particular group of lights. The electrician did it, rather than changing out the threeway to a single pole switch, by feeding the hot back on what should have been a traveler. And, evidently, did this in a recessed light J-box. I never did verify where the hot to traveler splice was. . . but the error that the broken threeway toggle cam exposed was that the white traveler (that was left over) was unconsciously tied into the neutrals in the recessed light J-box.

    When the good cam was operated it shorted internal to the threeway switch to the traveler that was landed with neutrals. Welded everything together. But the toggle still moved, just a little wonky.

    In your case, a cross-circuit neutral tie would lead to the fault.

    While you have the series receptacle at the panel running the vacuum on the short, you could test amperage on other neutrals while turning the vacuum on and off.
    .
    Another Al in Minnesota

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    Quote Originally Posted by al hildenbrand View Post
    While you have the series receptacle at the panel running the vacuum on the short, you could test amperage on other neutrals while turning the vacuum on and off.
    Excellent idea! I will take my clamp-on with me. Thanx!
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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