There’s been a veritable plethora of email questions and Facebook comments about the causes of hot-skin voltage on an RV in the last few weeks. Many of these seem to have come from first-time RV owners, while at least one of them has been plugging their RV into the same residential outlet for the last 7 years. So I think it’s time for a review of what a hot-skin voltage is, how to detect it, and a few possible fixes.
What’s a hot-skin voltage?
Glad you asked. Anytime the voltage of your RV’s skin (and chassis, hitch, bumper, axles, etc.) develops more than 5 volts above earth ground, then it’s a hot-skin voltage. Sometimes this won’t be felt if you touch the RV with a dry finger while standing on dry ground. But if the voltage gets up to around 30 volts or more, your hands are damp, and you’re standing on wet dirt or concrete, I guarantee you’ll feel a tingle (at the very least).
First of all, very low hot-skin voltages have a hard time breaching your skin’s epidermis layer, which functions as a non-linear resistance. But if your hands are wet, then even a low voltage can create a current through your body.
You can prove this to yourself by touching across the terminals of a 9-volt battery with your dry fingers and not feeling anything. But touch it to your wet tongue (an old sound-guy trick to see if a battery is any good) and you’ll feel a substantial (and yet safe) tongue shock. That’s because there’s enough current in that instance to stimulate your nerves that say “ouch.”
Keep your heart safe
Of course, a shock across your tongue isn’t dangerous, even though it can give you a brief case of “numb tongue.” But if you were to touch an electrical hot surface with one hand while standing on the wet ground, then you’ve provided a fault patch for current to pass through your body. And your heart is right in the middle of your body, where it can become confused by the 60 Hz AC current. It only takes 10 mA (10 milliamperes or 0.010 amperes of AC) current to confuse your heart and put it into fibrillation. Without quick intervention with CPR followed by using an AED to restart your heart, that can lead to death.
First things first … a lost ground connection
First, you must have a lost ground wire in your RV’s shore power connection. This is technically called the EGC (for Equipment Grounding Conductor) and it needs to have a solid connection from the frame of your RV, through your shore power cord, through the pedestal connections, all the way back to the service panel’s neutral/ground bonding point.
If that EGC connection is secure, then it should be impossible for your RV chassis/skin voltage to exceed more than 5 volts to earth. The reason why it could easily measure maybe 2 or 3 volts to earth ground is that the power company’s neutral grounding bus can easily be several volts above earth ground due to unbalanced 3-phase currents in their distribution substations. But that’s an advanced topic you don’t need to worry about. Just know that if you read a few volts between your RV chassis/skin and a screwdriver stuck in the wet ground, that’s expected and safe.
Second thing. You need a fault current source (everything leaks at least a little)
The second part of the equation for a hot-skin voltage is that you need a fault current source of some kind. And this fault current can vary from a fraction of an mA (below 1 milliampere) up to a full 20, 30 or even 50 amps of available current from your circuit breakers. It all depends on how the fault current gets to your RV chassis.
Harmless leakage currents
As I’ve hinted above, everything you plug into an electrical outlet will leak a little current to its own chassis. And if that microwave, battery charger, or any other appliance is connected to your RV chassis via its own grounded plug, then that current will find a path through your RV chassis back to earth somehow. Normally these are very small (1 or 2 mA at most) leakage currents that will easily be drained away by your RV’s ground wire in the shore power cord. But if that ground wire path is broken for any reason, then instead of that fault current draining away harmlessly to “ground” it will turn into a voltage on your RV’s skin (and chassis). These small currents generally show up as a hot-skin voltage of 30 to 60 volts or so.
These small currents in your various RV appliances are additive, which can exceed the 5 mA trip point of the GFCI outlet on the side of your house that you plugged into to charge your RV batteries. There’s not necessarily anything wrong in your RV’s electrical system that’s causing your home outlet GFCI to trip. But you certainly don’t want to bypass your home outlet GFCI for any reason. I do have a workaround that’s both code legal as well as safe and practical. But that’s for a future article.
These generally show up as hot-skin voltages of around 60 to 80 volts, but with a dangerous amount of potential fault current. Since your body has a hand-to-hand or hand-to-foot impedance of around 1,500 ohms, you can calculate that a 60-volt hot-skin voltage with sufficient leakage current could generate around 40 mA (0.040 amperes) of current through you. And a prolonged 30 mA shock is almost certainly lethal.
Again, if you have a proper EGC ground connection from the frame of your RV to the service panel’s bonding point, then this current will drain away harmlessly and not allow a hot-skin voltage to develop.
The other possibility of a fault current is from a dead short between one of the hot-wires in the shore power connection and the skin or chassis of the RV. This can be due to a pinched wire somewhere inside of the RV, or even a screw accidentally driven through a wall into a power conductor. In that case the available fault current is the amperage rating of the circuit breaker, typically 20 amps on a branch circuit, or up to 30 or 50 amps if you’re unlucky enough to pierce the insulation of the feeder circuit from your RV’s shore power inlet.
Yes, that’s an actual picture from my lab bench where I’m experimenting with short circuits on a 30-amp circuit breaker to determine peak fault currents. Kids, don’t try this at home.
When a short circuit occurs, and you have a proper low-impedance ground wire path from your RV chassis to the service panel’s neutral bond, your circuit breaker should instantly trip; something we call “clearing” the breaker in the electrical trades. That’s because there will likely be a few hundred amperes of short circuit current feeding thought the breaker, which quickly trips the magnetic latch.
But if you have a high-resistance ground connection, then that short circuit can turn into a full 120-volt hot-skin with circuit breaker fault current. If you touch that with wet hands and feet, that can easily send 80 mA to 100 mA of current through your body (and heart), which can be lethal.
The quick hot-skin test
I pioneered this quick and safe hot-skin test some 10 years ago, and it’s really quite simple for anyone to do. Immediately after plugging your RV into any kind of shore power, take your Non-Contact Voltage Tester and power it up. Then confirm it’s working properly by touching the tip of the pen to one of the pedestal outlets that is on. It should beep at you, demonstrating that it’s operating properly. Then simply touch the NCVT pen to anything metal on your RV and see if it beeps.
Basically everything metal on your RV is bonded to the chassis, so you can use the metal steps, bumper, wheels, hitch, propane tanks or bike rack. If it beeps from that contact, then you probably have a 30- to 40-volt hot-skin. If it beeps from 4 to 8 inches away, then you probably have an 80-volt hot-skin. But it if beeps from 1 to 2 feet away from your RV, then you likely have a full 120-volt hot-skin. In all cases you want to disconnect from shore power immediately and remedy the cause of the hot-skin voltage.
Here’s where you can get a Southwire 40136N Non-Contact Voltage Tester like I use in my seminars and videos. It’s a great tool for safely discovering hot-skin voltages described above.
Well, that’s a lot of words already, so we’ll cover troubleshooting principles in more detail next week and show you a few ways to correct a hot-skin voltage. See you then.
In the meantime, let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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