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Current in ground wire?

GotAll10 | Posted in General Discussion on May 4, 2011 01:04am

I have 3 120v circuits coming from my main panel into the basement thru metal conduit.  The conduit is connected to the main panel.  I believe this is original to the house (about 1955)  As the conduit comes into the basement through the concrete block wall, it connects to a metal junction box.  I have used these 3 circuits for the basement lights, and replaced all the old wiring from this box on.  The junctions between the old and new wires are in this box.

Tonight I noticed that one of the ground wire connections in this box look loose so I unscrewed this wire nut.  The (live) hot and neutral wires were still connected.  When I touched the ground wire that goes out to the lights and the box at the same time I got a little tingle.  I measured 40 volts between the ground wire and the conduit.  This went away when I turned off the circuit breaker.  I checked the other 2 circuits and got between 10 and 25 volts between the ground wires and the conduit.

The main panel is grounded to the copper water pipe coming into the house.  Out of curiosity I un-did the connection between the pipe and the  #6 ground wire and measured 120v.

Other than calling my electrician tomorrow, any suggestions?

Thanks,

Paul

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Replies

  1. DanH | May 04, 2011 06:56am | #1

    It's possible, in the first case, that you were simply measuring/sensing "ghost voltage" -- the voltage that can accumulate on an unconnected wire due to capacitance.  It would normally measure in the 40-50 volt range in the circumstances you describe.

    However the current that "ghost voltage" can deliver is very, very small, not usually enough to produce a "tingle".  So it's worth checking that out.

    Probably of more concern is the 120V you detected between the panel and ground.  This suggests that the main neutral connection coming into the panel is bad, either a poor connection at the panel, or a bad connection somewhere going out to the pole and eventually the power transformer. 

    Do you have oddly flickering lights at times, where the lights actually get brighter for an instant, rather than getting dimmer (eg, when the furnace motor kicks in)?  That's a sure-fire sign of a bad neutral connection.

    A bad neutral can be quite hazardous to man, machine, and house, and definitely bears investigation.

    1. User avater
      GotAll10 | May 04, 2011 08:42am | #2

      No flickering lights, and no problems that I have noticed.  This wiring was done several months ago. 

      I'm going to start by turning off breakers to see if I can isolate the problem to 1 or more circuits.

    2. User avater
      GotAll10 | May 04, 2011 09:34pm | #7

      OOPS

      Back the bus up.  My problem is not as big as I thought.  I'm not sure I should admit this on a public forum, but I had my multi-meter set to read ohms, not volts.  It's been dropped a few times, so when reading 0 ohms, the dial doesn't swing all the way over and stops right where 120v is on another scale.  Now I have an exuse to buy a new tool, maybe on of the fancy digital clamp-over meters that measures current without contact.....

      Anyway, I do NOT have voltage between my main ground wire and the copper water pipe.  I do still feel the tingle on the ground of the lighting circuit, but I'm going to re-evaluate it after I get a new meter.

      1. DanH | May 04, 2011 10:11pm | #8

        I've done that once or twice.  Not as "interesting" as when you try to measure 120V on the ohms scale, but probably more confusing.

        Do note that your standard digital multimeter has a "high input impedance", meaning it puts very little load on the circuit and can give you those "ghost" voltage measurements, while a "wiggy" -- a low-tech device for measuring line voltage -- loads the circuit a bit and will not give such false readings.  In general the digital multimeter is a better tool, but you need to be sensitive to its limitations in situations like this.

      2. gfretwell | May 05, 2011 11:55pm | #10

        If you have 40 ohms between the ground and the water pipe you still have a problem.

        The electrical inspector in me says you should call an electrician if you don't figure all of this out pretty soon.

        1. User avater
          GotAll10 | May 06, 2011 08:55am | #11

          I get 0 ohms (no resistance) between ground and water pipe, just like if I touch the 2 leads of the meter together.  Normally there is a dial on the multimeter to adjust the scale to read 0 when the leads are touched together, but the dial is broken and the scale is off. 

  2. junkhound | May 04, 2011 10:14am | #3

    ground wire that goes out to the lights and the box at the same time I got a little tingle

    What type of lights?  Lights only on the line or some other appliance?   Some ballasts and some dimmers have small (0.02uF or less) capacitors to ground for electrical noise control, which would be enouht for a 'tingle'. 

    Any spark when you touch the ground to the box?  Can you meansure the ground current? 

    1. User avater
      GotAll10 | May 04, 2011 10:45am | #4

      Mostly can lights, some incandescent, some CFL.  Also a water softener and one outlet with a wireless router plugged into it.

    2. User avater
      GotAll10 | May 04, 2011 10:56am | #5

      No sparks, even with the 120v on the main ground.  I can't measure the current.  There was enough to give me a little tingle on the lighting circuit.  I haven't touched the main ground since I measured the voltage, but I have touched both in the past month when I hooked it up and didn't feel anything. 

  3. junkhound | May 04, 2011 01:14pm | #6

    I turned off every circuit breaker in the main panel and also pulled the main disconnect fuses, so no power to the house at all

    You are missing a ground wire someplace (like between water pipes and panel), plus....

    Reminds me of re-wiring an old knob and tube house in the late 1960s. 

    Meter was pulled from the base, so full disconnect.  Was cutting the old K&T wires out of the attic, and cut a neutral and got a good sized spark, aboutllike shoritng a car battery.

    6 houses in the neighborhood supplied from the pole pig in the alley area.  Xfmr ground wire at the pole was broken, and apparently none of the other houses had any decent ground, it all routed thru the house I was re-wiring, which had the only good ground in the area (concrete encased).

  4. gfretwell | May 05, 2011 11:51pm | #9

    It sounds like your ground electrode system is not working. Either the electrode is disconnected or it is not really grounding the system.

    You could start by driving a couple rods 6-8 feet apart and running a 6ga wire to the ground bus of your panel. Then verify your water piping is bonded to the ground electrode system through the ground bus.

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