RVelectricity – How to diagnose a hot-skin voltage

By Mike Sokol

Dear Readers,
A few days ago this message showed up on my Facebook RVelectricity Group about getting shocked while touching the RV and standing on the wet ground.

Within minutes there were a number of incorrect (and potentially dangerous) suggestions as to what might be causing it and how to fix it.

No, a reversed hot and neutral conductor will not cause a hot-skin if the RV is wired correctly. No, a bad electric water heater element will not cause a hot-skin voltage if your RV’s ground wire is intact and connected to a properly bonded outlet. And no, a ground rod will definitely NOT ground your RV.

I immediately shut down all comments and asked the poster to contact me directly. It did take us a little sleuthing to find the problem, but I thought this would make a great tutorial on how hot-skin voltages occur and what to do to fix them.

How it all begin

My interest in hot-skin voltage goes back to before I began writing for the RV industry when a pro-sound colleague of mine asked me why he was feeling a shock when touching the million-dollar tour bus of some rock star he was working for. A Google search connected me with Gary Bunzer, the RV Doctor, who then went on to explain to me the RV industry standards for hot-skin voltage testing on Recreational Vehicles.

After a deep-think I figured out better ways to test for hot-skin voltage, and soon developed my own test gear to experiment with it. Gary quickly adopted my test procedures and even invited me to speak about them during his own RV Doctor seminars. A friendship was quickly formed and we discussed all kinds of RV electrical issues over the next 10 years. Boy, I really miss Gary since his death from COVID-19 last April.

What’s the definition of a hot-skin RV

Anytime the voltage potential on the RV skin (and its chassis, wheels, bumper, tow-vehicle, etc.) gets to the point where a human being can feel a shock by touching the RV while standing on the wet ground, that’s a hot-skin condition.

Usually the low end of this noticeable hot skin is around 20 volts or so with a few mA of fault current, and in the vast majority of cases it’s not deadly. But that doesn’t imply it’s safe. Once it reaches 30 volts or so with at least 30 mA of potential fault current, then it gets to be dangerous. And 50 to 60 volts with anything more than 30 mA of fault current is definitely life-threatening.

What causes a hot-skin voltage?

In order for a hot-skin voltage to develop you need two conditions: a broken ground wire in your shore power connection, and a source of a potential ground-fault current. This ground wire is officially called the EGC for Equipment Grounding Conductor, and it must be connected to the campground or home electrical service panel’s neutral/ground bonding point. If that EGC/Ground wire is broken or missing, there’s nowhere for any fault currents to go, which is what allows a hot-skin voltage to develop.

But wait, there’s more…

If you have a properly connected (bonded) ground wire in your shore power hookup, it’s nearly impossible for a hot-skin voltage to develop. But if the EGC ground wire is broken or missing, then the smallest ground fault current will turn into a hot-skin voltage. So you have two separate but related things: hot-skin voltage and potential ground fault current.

What happened this time?

After measuring the voltage between the chassis of the RV and a screwdriver stuck in the wet dirt, we found a 38-volt hot-skin condition. Some investigation showed he had connected the RV shore power to an outside GFCI receptacle through a dogbone adapter with an old extension cord with a missing ground pin. Yes, the picture is a close-up of the actual extension cord that caused his hot-skin shock.

Once he replaced the extension cord with a new contractor-grade 12-gauge cord with a proper ground, the voltage between the RV chassis and the screwdriver stuck in the wet ground dropped to 0 volts. The reason for the drop in voltage is that the ground wire in the exterior home outlet was able to get rid of the ground fault current, so it never turned into a hot-skin voltage. And we know the available ground fault current was less than 5mA since it didn’t trip the GFCI outlet.

But what if it tripped the GFCI?

If it did trip the GFCI outlet with a properly grounded extension cord, then that tells us there’s something else causing a large amount of ground fault current. For example, an old microwave oven with an overheated transformer can have up to 10 mA of ground fault leakage, and should be replaced.

If you have a few surge-protector power strips in your RV, they each can leak up to 3mA of ground fault current and still pass UL testing. Plus, water in an unsealed wiring box can easily create 10 to 20 mA of ground fault current and needs to be corrected.

Certainly, if you measure 1 or 2 amperes of ground fault current (which will immediately trip any GFCI), then you likely have an electric water heater element that’s corroded or burned out and needs to be replaced. This happens a lot in the spring when RV owners anxious for their first trip turn on the electric power to their water heater without filling it with water first. Only takes a few seconds to burn out the Nichrome heating element like that.

Final analysis

So if you measure less than 3 or 4 mA of ground fault current through a properly terminated shore power cord’s EGC (Equipment Grounding Conductor), then there’s probably nothing wrong with your RV’s wiring. Certainly, finding 1 or 2 mA of ground fault current on all the RV appliances added together is perfectly normal and safe. But anything more than 5mA of fault current is a red flag that something in your RV is leaking fault current to the chassis and must be investigated and corrected.

How to measure ground fault current

I’ve just found this interesting line splitter from Italy with Edison plugs, and it also includes a third test loop for measuring ground fault currents using a 1 mA sensitivity clamp ammeter, without having to resort to disconnecting any EGC ground wires (which is always potentially dangerous). More on how to use this gadget in a future article, once my test unit arrives next week. In the meantime, here’s the link to it on Amazon if you want to play along. If Amazon is out of them, you can get one at TEquipment. But please wait for my article and video since any ground fault testing can be dangerous if done incorrectly.

Reader poll

Please participate in this poll (it may take a moment to load). Feel free to leave comments below.

Let’s play safe out there. I’m ready for 2021 … Aren’t you?

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.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

##RVT981