Some one please explain the dangers in this practice

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kwired said:
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.
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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. :D
 
iwire said:
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?
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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.
 
ptonsparky said:
I megged a Dodge tire
this morning. Dry. Tread to sidewall about​ half way around. 260 meg at 1000v.
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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.
 
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Electric-Light said:
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.
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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.
 
ptonsparky said:
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.
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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.
 
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.
 
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kwired said:
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.
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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.
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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.
 
kwired said:
If another grounded object should contact whatever is connected to this bootlegged EGC, you have parallel path(s) for neutral current to flow over objects not intended to carry current.

"stray voltage" problems are almost always a result of unintended current paths of the grounded conductor.

This sort of bootleg may go without any noticeable problem for years in some cases. If in a wood framed building with no other grounded objects in the vicinity you can have full 120 volts on the frame of an appliance and never have a nearby ground reference to cause you to notice it. Of course if you have full 120 and bootlegged EGC, you probably have completely open neutral and the appliance doesn't operate, but my point here is there is no shock hazard in that situation because there is no grounded object within reach.
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If a frame has 120v on it wouldn't a person get shocked just by touching the frame while standing on the floor/ground?
 
iwire said:
Kwired was talking about a wood floor, you would be isolated.
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I have been standing on wood before, just the other day actually, and got tingled by 120v. It wasn't very bad but enough to get my attention. I always thought that wood isn't a perfect insulator so even standing on a wood floor on the second story of a house a person could feel a 120v shock.
 
DBoone said:
I have been standing on wood before, just the other day actually, and got tingled by 120v. It wasn't very bad but enough to get my attention. I always thought that wood isn't a perfect insulator so even standing on a wood floor on the second story of a house a person could feel a 120v shock.
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In theory you can feel a slight shock just from capacitive current if the voltage is high enough. You make a pretty good small value capacitor just standing in free space.
 
DBoone said:
I have been standing on wood before, just the other day actually, and got tingled by 120v. It wasn't very bad but enough to get my attention. I always thought that wood isn't a perfect insulator so even standing on a wood floor on the second story of a house a person could feel a 120v shock.
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Connect an ammeter in series with the ungrounded conductor and place the other probe into the wood. You likely don't even read anything in the milliamp range in old dry wood. New construction lumber, maybe you will get some current, new lumber still has a fair amount of moisture in it.

I have grabbed live conductors many times in older installations that have no EGC, you only get shocked if you are also in contact with a grounded object. Have touched live conductors while working on top of wood poles, same thing, that one is riskier though if you have grounded objects on the pole or bare grounded conductors in the vicinity. Of course today's safety standards discourage any of that, but just saying the risk is pretty low when there is nothing grounded in the vicinity to contact simultaneously.
 
kwired said:
Connect an ammeter in series with the ungrounded conductor and place the other probe into the wood. You likely don't even read anything in the milliamp range in old dry wood. New construction lumber, maybe you will get some current, new lumber still has a fair amount of moisture in it.

I have grabbed live conductors many times in older installations that have no EGC, you only get shocked if you are also in contact with a grounded object. Have touched live conductors while working on top of wood poles, same thing, that one is riskier though if you have grounded objects on the pole or bare grounded conductors in the vicinity. Of course today's safety standards discourage any of that, but just saying the risk is pretty low when there is nothing grounded in the vicinity to contact simultaneously.
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:thumbsup:
 
kwired said:
Connect an ammeter in series with the ungrounded conductor and place the other probe into the wood. You likely don't even read anything in the milliamp range in old dry wood. New construction lumber, maybe you will get some current, new lumber still has a fair amount of moisture in it.

I have grabbed live conductors many times in older installations that have no EGC, you only get shocked if you are also in contact with a grounded object. Have touched live conductors while working on top of wood poles, same thing, that one is riskier though if you have grounded objects on the pole or bare grounded conductors in the vicinity. Of course today's safety standards discourage any of that, but just saying the risk is pretty low when there is nothing grounded in the vicinity to contact simultaneously.
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Murphy's Law dictates there will always be something grounded in the immediate vicinity, like ductwork, the ground, building steel, etc, and you will be sweaty whilst touching it. :lol:
 
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