Hot Skin?

[fasteddieb]

In another thread on Breakaway Batteries, I brought up the fact that I had noticed 12VAC between the frame and earth ground on my 21BHS hooked up to shore power at a friend's house - it was enough voltage to give my chin a tiny "tingle".

As prologue, my trailer has had issues bofore, chronicled elsewhere, when as delivered it would trip GFI circuits. That was traced to "bonded" hot and common circuits in the air conditioner and allegedly fixed.

Will insert photos in a minute - I'm posting all this via cellular on my iPhone 6+.

Photo #1: 13.2 VAC between frame and earth ground:



Question: I have jumper cables with me. Would running one from the frame to something driven into the ground be a worthwhile safety measure for the time being?

[charliem]

[font size="3"]Answer to your last last question: YES. And thanks for starting the new thread. Great move. 
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[fasteddieb]

Update:

The voltmeter seems to "float" at around 2 VAC, so the ground on the extension cord may be fine.

I did just hook up a jumper cable from a stabilizer jack to a stake in the ground. The voltage across them showed at about 13v. With that hooked up the trailer to ground dropped to about 1VAC, maybe due to the resistance of the jumper cable to ground.

[charliem]

[font size="3"]Eddie,

Measure from neutral and hot to safety gnd on the socket at the extension cord end. Should be 120V from hot to gnd and 0V from neutral to gnd.
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[charliem]

[font size="3"]Some small potentials between "true gnds" happen due to earth currents. As someone once said, "The whole world vibrates at 60 Hertz." Ground the trailer, have a beverage, and don't put your chin on the frame  





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[fasteddieb]

Back home on our very long extension cord, frame to earth ground measured .5vac.

I will still probably get in the habit of driving a ground stake regardless, given my history!

[jmsokol]

[quote source="/post/10598/thread" timestamp="1431264136" author="@fasteddieb"]

[jmsokol]

[font size="3"]Eddie,

Measure from neutral and hot to safety gnd on the socket at the extension cord end. Should be 120V from hot to gnd and 0V from neutral to gnd.
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Not exactly true. While an unloaded branch circuit should measure very close to 0 volts between the neutral and ground, once a load is applied to the circuit sufficient to cause a voltage drop (from 120 volts down to 110 volts, for example) it's common for the neutral to contribute up to 1/2 of that voltage drop (out of phase with the hot wire, of course). So reading 1 or 2 volts difference between the ground and neutral is not only OK, it's expected. In fact, that's a test I do to determine if there's a bootleg ground upstream. If I can change the load on a branch circuit with a space heater and the ground-to-neutral voltage DOES NOT change, then I know for sure that the neutral and ground wires are double bonded or even bootleg grounded somewhere. And I confirm that by changing the load while observing the ground-to-earth voltage, which should remain constant at less than 1 volts. If the ground-to-earth voltage is modulated by the current draw of the branch circuit, then I know for sure there's either a bootleg/double-bond situation, or someone has swapped the ground and neutral wires in the installation. Seems crazy, but true even in modern industrial and residential wiring done by untrained workers installing outlets.

Mike Sokol
www.noshockzone.org
 

[fasteddieb]

Thanks, Mike.

Like I said, back home measured 1/2 volt frame to ground. So my assumption is that something was amiss with the ground wire at my friend's boat house, where I ran the extension cord from.

I had previously dealt with a bonded common and ground in my trailer, traced to the air conditioner and resolved.

Appreciate the feedback!

[daplumbr]

Mike, I really appreciate your clear, accurate descriptions of electrical issues. As I've mentioned in other posts, I thought your book the best I've ever read on electrical safety. Thanks for potentially life-saving information. A person I knew many years ago in Kentucky was killed by an electrical problem in an RV, so using AC electricity while camping needs to be taken seriously. 

[fasteddieb]

Mike,

I'm still not clear why a ground stake would not help.

If the trailer chassis was hot, would not the current take the path of least resistance into the ground through a wire, rather than through a person?

I thought that was the whole purpose of grounding things.

[jmsokol]

Mike,

I'm still not clear why a ground stake would not help.

If the trailer chassis was hot, would not the current take the path of least resistance into the ground through a wire, rather than through a person?

I thought that was the whole purpose of grounding things.

It's a misstatement that electricity follows only through the path of LEAST resistance. It actually follows ALL paths of resistance at the same time. It just divides itself up with more current flowing through lower resistance paths, and less current flowing through higher resistance paths. So let's assume a ground rod impedance of 100 ohms (pretty typical) and a wet hand-to-feet resistance of 1,000 ohms. Again, pretty typical numbers. Now lets create a 100-volt hot-skin condition and run the current numbers (I picked 100 volts as a typical hot-skin voltage and easy math).  

Ohms Law tells us that voltage divided by resistance equals amperage. Or V/R=A in simple math terms. In the case of the grounding rod, 100 volts on the skin/frame of the RV becomes 100 volts divided by 100 ohms = 1 amperes of current. So your ground rod will be creating a fault current of only 1 ampere. That's not enough to trip a 20 amp breaker, nor will it create any sort of sparks or smoke. It can literally do this for days or week without any indication of there being a serious shock hazard.

Now in the same situation above let's also have someone standing on the wet grass touching the door handle with damp hands. That secondary current path will be 100 volts hot skin divided by 1,000 ohms body resistance which equals 1/10 of an ampere (or 100 milli-amps or mA for short) through your body. So the total ground fault current will be 1 amp from the ground rod plus 1/10 of a amp from your body path. That adds up to 1.1 amperes total, which is still not enough current to trip a 20 amp circuit breaker.

But while the earth can easily survive 1 amp of current indefinitely, your body cannot survive 1/10 of an ampere current (100 mA) or more than a few seconds. Here's the basic shock/electrocution numbers. Anything more than 30 mA of current through your chest cavity will most likely be deadly unless you receive immediate CPR and an AED rescue within minutes.

1 mA - noticeable shock
5 mA - significant shock
10 mA - painful shock
20 mA - very painful shock that you can't let go of due to involuntary muscle contractions
30 mA - likely heart fibrillation within a few seconds of contact and death in minutes without CPR
50 mA - probable heart fibrillation with short contact
100 mA - definite heart fibrillation with short contact and likely nerve damage with a few minutes of contact. Death in minutes without CPR and an AED (Automatic External Defibrillator).

So what do ground rods really do? Well they're primarily there for lightning strikes to keep the currents from flowing through your wiring. Without a ground rod your home or electrical system could be at 100 or 1,000 volts above the earth ground and you might never know it until it's too late, which is terribly dangerous. More on this in my book, but what you really need for shock safety is a very low resistance (less than 1 ohm) ground connection back to the electrical panel's G-N-E (Ground/Neutral/Earth) connection point.

And there's much more on generator bonding and grounding, which you can read about in my No~Shock~Zone book on RV Electrical Safety available at [a href="http://www.amazon.com/gp/product/B00L2DWBD8?*Version*=1&*entries*=0"]http://www.amazon.com/gp/product/B00L2DWBD8?*Version*=1&*entries*=0[/a]

In the meantime, Let's play safe out there...

Mike Sokol
www.NoShockZone.org

[charliem]

[font size="3"]Mike (and Eddie),

One element you didn't address is the source impedance of the leakage path. If the skin is at 100 volts from a low impedance source everything you say is correct. However i believe most leakage paths are a relatively higher source impedance, through poor insulation, bypass capacitors, maybe some inductive coupling? If this is the case the skin would still measure 100 volts with a high impedance meter, but would drop dramatically with a 100 ohm "ground". If Eddie drives a ground stake near the camper, or better yet, finds a nearby metal water pipe, he would be much better off. Then the best test might be to drive a second stake near the wet grass and measure to it. I realize "ground" is a fictional and elusive thing. If I were in Eddie's bare feet and measured something approaching 100 volts I would have to assume extreme miswire or fault and take immediate corrective action. If, however, I measured in the 0-20 volt range with the high impedance DVM as Eddie did I would try the frame ground and remeasure.

We've probably dug too deep for the average camper, but your thoughts??
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[jmsokol]

[font size="3"]Mike (and Eddie),

One element you didn't address is the source impedance of the leakage path. If the skin is at 100 volts from a low impedance source everything you say is correct. However i believe most leakage paths are a relatively higher source impedance, through poor insulation, bypass capacitors, maybe some inductive coupling? If this is the case the skin would still measure 100 volts with a high impedance meter, but would drop dramatically with a 100 ohm "ground". If Eddie drives a ground stake near the camper, or better yet, finds a nearby metal water pipe, he would be much better off. Then the best test might be to drive a second stake near the wet grass and measure to it. I realize "ground" is a fictional and elusive thing. If I were in Eddie's bare feet and measured something approaching 100 volts I would have to assume extreme miswire or fault and take immediate corrective action. If, however, I measured in the 0-20 volt range with the high impedance DVM as Eddie did I would try the frame ground and remeasure.

We've probably dug too deep for the average camper, but your thoughts?? [/font]

You are correct, and I've written a few white papers about leakage source impedance and how it affects hot-skin shock and grounding conditions. However, as you're likely aware, while a high impedance hot-to-chassis fault current can likely be shunted to earth via an ground rod, a high-impedance/low-current fault can become a low-impedance/high-current fault at any time. And a ground rod will do NOTHING to reduce the hot-skin voltage potential in that situation.

That's why I consider ground rods to be false security. If indeed they do drop the voltage on a hot-skin condition, then it must have been high-impedance/low-current to begin with and not potentially life threatening. But then you lose the warning sign of a small shock to let you know that your EGC (Equipment Grounding Conductor) has been compromised in some way. Then when the wire finally shorts to ground you'll have a high-current fault source that can kill you.

Also, the NFPA-70E National Electrical Code does not allow isolated ground rods alone to act as your safety grounding point, for exactly the same reasons I've noted above. To be code compliant your RV's (and your house wiring's) safety ground needs to have a low-impedance (less than 1 ohm) path back to your electric service panel's ground/bonding point.

Yeah, I think this is a little too deep for 99% of the forum readers which is why I avoid this technical level most of the time unless I'm in discussion with other EE's and code writers. That's when it gets REALLY fun.

Mike Sokol

PS: I'm trying to find a funding source so I can go on the road to do a bunch of No~Shock~Zone seminars at campgrounds and dealerships around the country plus hand out thousands of copies of my book. So if anyone from the Livin-Lite factory is reading this thread, please contact me about how we can work together to make RV travel even safer.

Mike Sokol