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Ron_Nielson's avatar
Ron_Nielson
Explorer
Aug 19, 2015

How to Discover Source of Hot Skin

My trailer is parked under a metal carport on a dirt pad covered with rock. It has been raining and the ground under the rock is damp or wet, depending on where you are under the carport. Last evening, I was laying on my back on the rock, poking around the trailer skirting and I felt an electrical tingle where my arm was touching the metal trailer skirting. I had never noticed any problem like this in the past, not to say it didn't exist, but I had never experienced it. Got out my circuit tester, the plug-in type with the lights, and start testing the receptacles in the trailer, the extension cord from the garage to the trailer, and the garage. Everything tests 'fine'. The trailer has a hard wired surge protector, Surge Guard brand, and the indicator that there might be a problem on the protector is not lit up. I also have an Electracheck monitor to alert me to problems, bot nothing detected there either. Immediately disconnected from shore power.

This morning, I plugged in the trailer to shore power after verifying that the male end of the shore power cord was wired correctly. I get out the multi-meter and test for AC voltage between the trailer skirting and placed the ground-side tester probe into ground (mud under the rock) and I get a reading of .245 volts. When the ground point is changed to the carport supports which have metal rebar driven into the ground about 3 ft, reading rises slightly to .260 volts AC. Checking with extension cord un-plugged, reading was .130V AC. (Definitely didn't expect to see that.) Did a recheck of all the receptacles with the light-type circuit tester and everything tests OK. Turned off battery switch - no change.

What is the problem here? I wouldn't think you would feel about 1/4 of a volt, but I did, at least when the ground is damp. How could there be AC Voltage (as small as it is) when the trailer is not plugged into shore power? Where to start? How does one figure this thing out?

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  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Aug 19, 2015
    Mr. Wizard one of our resident experts says "pounding ground stakes is just plain nuts". Guess he needs some additional electroshock therapy to help change his mind. Or see an ambulance carry a corpse off (that was a very kind older lady) in a campground in Guaymas, Sonora.

    Or the ship's carpenter who arrived early on The Polar Star and connected L3 to a lateral looking to power his woodworking tools. The trawler was on the hard supported by 18X18's and all welding leads were locked up. The only thing that saved his *!*!*!*! was a wooden ladder. When the shipfitters arrived one of them let fall a long piece of angle between earth and hull. The results were dramatic. It tripped a three phase 200 amp breaker. There was a lot of screaming going on in that yard. The owner scabbed a s**** angle iron between hull and several feet deep in the sand in the yard.

    Lately, jolts lay me up in bed with a irregular heart rhythm - sometime for days. My cardiologist says shocks for his patients with pacemakers is absolutely to be avoided. For me with 2nd degree heart block, it can lead to total heart block. When giving general advice about shock, it is sorta nice to consider some folks are more vulnerable than others. Years ago I got stupid around 480. It felt like someone bent a piece of 1" re-bar over the top of my head. I was unable to stand for 10+ minutes. I am afraid of electricity :)
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Aug 19, 2015
    Damp human skin. Twelve volts direct current "burns" inside forearm. Try it with your car battery terminal posts sometime.You need to establish the value, the voltage doing this and then establish whether it is AC or DC. Measure voltage both ways. Remember wet earth may or may not be the ultimate potential, so connect one probe to household neutral then touch the RV chassis. This would be the "actual" potential.

    If A.C. (which is 99.999% likely. Switch and test individually breaker by breaker which circuit is involved. Then within that circuit disconnect appliances one by one then test.

    One head scratcher was leakage of L2 to earth. The hot skin was the completion of the fault from earth to RV true neutral/earth bond.
  • MrWizard's avatar
    MrWizard
    Moderator
    Aug 19, 2015
    check the ground wires
    inside the receptacle the RV is plugged into
    inside the the rv 120v breaker panel
    and finally inside the house service panel (switch ALL power off first)
    and check the ground wire in the extension cord and the contacts in the receptacle

    the hot and neutral are bonded at the service panel, effectively being one wire, when everything is tight and connections good
    when some connection becomes loosen,
    you start getting hot frame/tt ground problems

    IF you want to fix all that
    drive an 8ft copper ground rod into the ground (maybe in a corner near the carport supports)
    and connect a cable and clamp to it
    clamp the cable to the TT when you park it
    100% earth ground for the TT, no more tingles
  • Ron_Nielson's avatar
    Ron_Nielson
    Explorer
    Aug 19, 2015
    Harvard

    The trailer has no electrical wires overhead. Only elec wire within 50 ft of the trailer is in the garage where a 20 amp circuit powers the garage and the extension cord plugged into the garage. There are power wires running across the front of property about 200 ft away, the usual stuff that feeds houses on a street (I wouldn't THINK it would be 6000 V). No really high voltage overhead stuff nearby that I can find.

    I don't understand how there can be current in the ground wire (or neutral or hot wire for that matter) when the trailer is not attached to shore power, with the main breaker shut off, and the battery switch turned off. Is the trailer generating it's own AC power? Why doesn't the circuit tester find a problem? Or are you indicating that there may be some voltage, but so small that it shouldn't be worried about?

    PS - I like your planes!
  • Harvard's avatar
    Harvard
    Explorer
    Aug 19, 2015
    When you have 3 conductors (Hot Neutral Ground) in close proximity to each other the 3 wires will have a capacitance between each other. The Ground conductor, if allowed to float, would try and rise to (120 / 2) volts because of the stray capacitance.

    Also, if your RV is ungrounded and parked under a single phase high voltage (6000 VAC) line the RV chassis will float some where above 0.0 VAC.

    In a perfect world with no faults and a good ground conductor the lesser the hot skin voltage is going to be but there is going to be some depending on circumstance. IMO

    Another point, if there is any current in the ground conductor then there would be a voltage drop in the ground wire which might also show up as a hot skin.

    What if the ground conductor were open circuit when you were lying under the RV but fixed itself when you started taking voltage measurements.

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