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Thread: Stray voltage on gas service line

  1. #11
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    Quote Originally Posted by hbiss View Post
    Usually 24V control circuits float- one side is not grounded. However I would have to look at the wiring diagram of the specific piece of equipment to say that with certainty.
    They started grounding the 'common' (opposite the red wire) of the 24v control circuit years ago.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #12
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    Ok. So if the gas line was not bonded and the ungrounded side of the igniter circuit faulted to the gas line it could result in the situation described. I see your point. But wouldn't the gas line within the (I'm thinking of furnace) be also grounded to the sheet metal by virtue of it's supports?

    -Hal

  3. #13
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    Quote Originally Posted by hbiss View Post
    Usually 24V control circuits float- one side is not grounded...
    The only controls I work with are HVAC. Our 24V controls are indeed grounded, usually the blue wire, so that would include a residential furnace.

  4. #14
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    Can you think of any way it could put an intermittent 12-24 volts on the gas line? I can't.

    -Hal

  5. #15
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    If one of the other T-stat wires became grounded outside the furnace.

    As I said before, you can try killing circuits while monitoring the voltage across the gap.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #16
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    Quote Originally Posted by hbiss View Post
    Can you think of any way it could put an intermittent 12-24 volts on the gas line? I can't.
    Nope, but I'm not trying really hard, either.
    Last edited by Little Bill; 01-13-19 at 09:01 AM. Reason: added "not"

  7. #17
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    Quote Originally Posted by LarryFine View Post
    If one of the other T-stat wires became grounded outside the furnace.

    As I said before, you can try killing circuits while monitoring the voltage across the gap.
    But how would you explain the fluctuation? Certainly killing the power to the furnace, stove or dryer will either prove or put that theory to bed right away.

    But given the facts presented the most likely cause is a high resistance service neutral in one of the units. We are essentially measuring the voltage between the neutral bus in the panel of unit #1 and the neutral bus in the panel of unit #2. Different loads in the unit switching on and off cause different voltage drops across the poor connection as referenced to the solid ground of the other unit.

    -Hal

  8. #18
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    Quote Originally Posted by hbiss View Post
    We are essentially measuring the voltage between the neutral bus in the panel of unit #1 and the neutral bus in the panel of unit #2. Different loads in the unit switching on and off cause different voltage drops across the poor connection as referenced to the solid ground of the other unit.
    That's why a couple of us have suggested a neutral issue in one of the houses.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  9. #19
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    Bonding the gas line story!

    Years ago in the 1980s in NYC the gas utility installed isolation couplings before the meter in everyone’s homes & business service. This was to prevent electrolysispreventing erosion at the gas main. A couple years when I was working at JFK airport a contractor installing a lightning rods on the roof of a building and bonding everything also put a bond across the gas main isolation coupling. I called the gas company and there was this big meeting with the contractor, gas company and port authority. The contractor won the argument and the jumper bond remained across the isolation coupling.

  10. #20
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    Lightning is unpredictable and with a system that you designed to carry the energy of a strike to ground, you want to take every effort to keep it contained on your conductors. You don't want the possibility of side flashes. Bonding the metallic gas service is like bonding CSST taken to a higher level. CSST can pinhole, I can see a large enough flashover burning a hole in iron pipe or the coupling.

    -Hal

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