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Thread: Some one please explain the dangers in this practice

  1. #21
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    Quote Originally Posted by kwired View Post
    I agree. Modern (and for past 50 or more years) tires would likely have been conductive enough to help eliminate static charge buildup, but are not low enough resistance to pass any significant current at 120 volts.
    The significant current part is what is nagging at me.

    While I was typing my post above I was thinking about how little current it would take to give him a strong whack with the sweat and the metal contact points.

    So I added the question about the water.

    It struck me odd that he could get a shock in the way he described but I am not willing to be a human guinea pig to test my theory.

  2. #22
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    Quote Originally Posted by iwire View Post
    Please do not take this the wrong way I am sure you got the shock you said you did but I feel there had to be a path to ground beyond rubber tires and wood blocks.

    Was it all wet?
    No, but I was. If the trailer is at 0V, the compressor at 120V, and you ran a bond between the two, the trailer would be at the same potential = no shock. I may have only been shocked a cycle or two, it always feels longer.

    The trailer was at a lower potential than the compressor. There may have been a hitch chain touching the concrete slab but it's not like there is a CEE here.
    Electricians do it until it Hertz!

  3. #23
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    I megged a Dodge tire
    this morning. Dry. Tread to sidewall about​ half way around. 260 meg at 1000v.
    Tom
    TBLO

  4. #24
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    Quote Originally Posted by ptonsparky View Post
    I megged a Dodge tire
    this morning. Dry. Tread to sidewall about​ half way around. 260 meg at 1000v.
    You'd have to test it parked on your garage or driveway with all four treads moist and measure microamps between the hot pin and bare metal of the vehicle. Surface area means a whole lot. Moisture on tire is likely conductive because of residues from road salts and minerals within concrete/asphalt. The ohms compute from 120v AC and micro amps is a much more realistic impedance than how you measured it. I haven't tested this personally. I doubt it's enough to trip a GFCI but probably enough to feel it. There is both resistive and capacitive component so AC ohms is lower than DC ohms.

    If you were to measure the resistance of a slice of carrot, there's a huge difference between what you get touching two points with probes or pinched in between coins and measured across the coins.
    Last edited by Electric-Light; 03-20-17 at 10:48 PM.

  5. #25
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    Quote Originally Posted by Electric-Light View Post
    You'd have to test it parked on your garage or driveway with all four treads moist and measure microamps between the hot pin and bare metal of the vehicle. Surface area means a whole lot. The ohms compute from 120v AC and micro amps is a much more realistic impedance than how you measured it. I haven't tested this personally. I doubt it's enough to trip a GFCI but probably enough to feel it.

    If you were to measure the resistance of a slice of carrot, there's a huge difference between what you get touching two points with probes or in between coins and across the coins.
    I have a couple EKG patches I was tempted to use up, but I'm saving them for something a bit more important than checking dry rubber tires.
    Tom
    TBLO

  6. #26
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    Quote Originally Posted by ptonsparky View Post
    I have a couple EKG patches I was tempted to use up, but I'm saving them for something a bit more important than checking dry rubber tires.
    You don't need one. Just measure the microamps between the hot pin in the outlet and one of the lug nuts. While not at work. This measurement test likely won't pass OSHA.

  7. #27
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    Quote Originally Posted by Electric-Light View Post
    You don't need one. Just measure the microamps between the hot pin in the outlet and one of the lug nuts. While not at work. This measurement test likely won't pass OSHA.
    Bring your vehicle right on over we will do it this afternoon.
    Tom
    TBLO

  8. #28
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    So why not just measure from lug nut to a grounding electrode? My bet is the resistance is still pretty high if on a dry concrete floor, high enough that a 120 volt fault through this path won't trip a GFCI.

    Add: keep in mind that measurement will be measuring resistance through all four tires to ground. It will likely be lower resistance then a two wheel trailer, and if his dodge has larger tires then the trailer had will also lower the resistance per tire.

    Even my Dodge has wider tires though about same diameter as those on a 14,000 pound trailer I pull with it, so I would guess the truck has lower resistance through the tires then the trailer has, otherwise assuming same tire composition.
    Last edited by kwired; 03-21-17 at 08:56 AM.

  9. #29
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    Quote Originally Posted by kwired View Post
    So why not just measure from lug nut to a grounding electrode? My bet is the resistance is still pretty high if on a dry concrete floor, high enough that a 120 volt fault through this path won't trip a GFCI.
    Capacitance. It might not trip, but probably enough to cause an unpleasant tingle. A good example is old school rapid start T12 fluorescent lighting. The center channel cover provides a ground plane that sits closely behind the lamps. The open circuit voltage of ballast is only about 350v, but the capacitance between the ballast cover and lamp wall is important enough that lamp starting can become unreliable if you're missing the cover. The surface area of tread to soil and bead to wheel of all four wheels is probably considerable. Tires are black because of powdered carbon.

    Add: keep in mind that measurement will be measuring resistance through all four tires to ground. It will likely be lower resistance then a two wheel trailer, and if his dodge has larger tires then the trailer had will also lower the resistance per tire. Even my Dodge has wider tires though about same diameter as those on a 14,000 pound trailer I pull with it, so I would guess the truck has lower resistance through the tires then the trailer has, otherwise assuming same tire composition.
    The effective impedance to the transformer ground can be dramatically different depending on soil conditions. My point is that DC resistance doesn't tell you anything about AC coupling especially as large surface area contacts get involved.

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