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What causes a hot-skin shock voltage?
Here's the basics of how and why hot-skin voltages happen on an RV...
I read your article last week about RV hot-skin, and have to admit that I’ve felt a few shocks from my RV over the years. Can you explain where this comes from? Can’t the RV manufacturers do something to make our campers safer? —Pete (the Dragon)
Yes, what causes RV hot-skin voltage is the next thing to consider. I’ll post a link to my RVelectricity article next week describing hot-skin voltage conditions and how to find them.
And here’s a short video about NCVT (non-contact voltage tester) testing from my RV Electricity seminar in Funkstown, MD. This 3-minute video shows the basics of NCVT testing and what happens if you (or your action figure) get between an energized object and anything that’s grounded: Video Shorts.
Let’s address your last question first. Yes, the RV manufacturers have, in fact, installed something to help keep your RV safe from hot-skin/stray-voltage. It’s called the ground wire in your RV’s electrical system and it’s connected to the ground contact in your shore power plug.
More specifically it’s called the Equipment Grounding Conductor, or EGC, which we’ll call it in this article simply because the EGC is only peripherally connected to earth ground (the dirt under your feet). Interestingly, the EGC really doesn’t rely on the grounding rod to protect your RV from an internal short-circuit. That’s the job of the EGC/Ground being properly “bonded” back inside of the incoming electrical panel, generally referred to as the service panel.
So when we say that your RV is properly “grounded,” what we really mean is that the skin and chassis of your RV is properly “bonded” (connected) back to the neutral/ground bonding point back in the service panel (the main electrical panel feeding the campground or your own home).
Much more on the principles of earth-grounding and bonding in a later article. But for now realize when the NEC (National Electrical Code) or your RV technician says that something is grounded, this doesn’t mean it’s actually connected to the earth with a grounding rod. For now let’s talk about what causes these hot-skin voltages to occur.
In order for your RV skin (and chassis) to develop any kind of appreciable voltage, it first needs a failed EGC (the Equipment Grounding Conductor – more commonly called the ground wire). It’s the job of this ground wire is to drain away any fault or leakage currents which can turn into voltage potential.
First let’s define that a leakage current is generally only a few to a few hundred milliamperes of current, not enough to power a light bulb, but enough to kill you under the right conditions. Again, if you have a proper EGC/ground connection, then these small (and normal) leakage currents are caused by your battery converter-charger, microwave, air conditioner and nearly everything else electrical in your RV. You can also get some extra leakage currents from water in a junction box feeding power around your RV or failing insulation in a wire that’s getting a little old and tired.
But if your EGC/ground connection is compromised somehow from something like a broken/loose dog-bone adapter, a broken EGC wire in the pedestal, or even someone breaking off the ground lug on an extension cord or using a ground-lift adapter (like you see above), then these small leakage currents can turn into a voltage potential, which typically measures around 1/2 of the line voltage, or 60 volts. This is what’s likely happening if you feel a small shock from your RV that you might choose to ignore. But ignore it not, because that’s a hint that you have a failed EGC/ground connection somewhere between the chassis of your RV and the power company service panel.
Now for the intermediate/mid current faults. If you have a failed heating element in your electric water heater it can provide up to 2 or 3 amperes of fault current, and will typically create around 80 volts of hot-skin voltage. And that 80 volts with 2 amperes of fault current should be drained away harmlessly by a properly functioning EGC/ground wire. This isn’t enough to trip a regular circuit breaker in a pedestal or your home, unless it’s a GFCI (Ground-Fault Circuit Interrupter, or Breaker). Again, if your RV’s EGC/ground connection has failed, then touching your RV hot-skin can be deadly.
Finally, here’s the BIG one. If you’ve driven a screw or nail into the wall of your RV and penetrated a power wire, or the insulation on a power wire has been cut by draping over a sharp piece of metal in a wall or under the floor of the RV, then you can have a direct contact point between the incoming hot-wire and the chassis of your RV. This will create an available fault current of 20, 30 or even 50 amperes, at a potential of 120 volts. If your EGC/ground wire is properly bonded back to the power company’s service panel, then all that should happen is the lowest rated circuit breaker should trip and the power go off.
You need a missing EGC ground for a hot-skin to happen
However, if your EGC/ground wire is compromised, then that 50 amps of current at 120 volts is available to shock and kill you. Now, you’ll be long dead before your body will actually draw 50 amps of current since you’re essentially a 1,000 ohm resistor. But that 100 mA or so of fault current flowing through your body is easily enough to put your heart into fibrillation and cause death in minutes.
So if you feel ANY kind of shock from your RV, unplug from shore power immediately and find the problem. Remember, the next shock you get could be your last, so don’t ignore any electrical tingles you might feel, no matter how small they seem at first.
Let’s play safe out there…. Mike