RVelectricity: Can reversed polarity alone cause hot-skin voltage?

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By Mike Sokol

Everyone: Since I’m teaching a 3-day Master Class on RVelectricity at the RVillage Rally in Live Oak, FL, this week, I’m rerunning this No~Shock~Zone article which was originally published back in 2016.

I’ll publish Part 3B of my series on Troubleshooting Short Circuits next week. In the meantime, this article debunks the myth that a reversed polarity outlet alone can cause a hot-skin voltage condition.

Hey Mike,
I replaced the valve on my black water flush-out system today and noticed that when I touched any metal part of the frame of my trailer, I could feel an electrical current running through it! Not enough to really shock me, but certainly enough to cause some discomfort. It doesn’t matter where on the frame I touch, either – it feels the same. What is going on? Could reversed polarity on the battery or shore power plug be causing this? I have a 2010 Rockwood Roo 23 RS. —Gus

Gus,
OK, let’s get to the bottom of the problem quickly, since this is really a very simple thing. But first, let me go over what it is NOT.

It’s NOT reversed or flipped battery cables. And you certainly don’t want to be swapping battery cables at any time since that really DOES reverse DC polarity and could easily destroy electronics such as your inverters, televisions, stereo systems, etc. That’s because a battery has a positive and a negative pole, so reversing the cables actually reverses the polarity of the system, and electronics are very sensitive to polarity reversal and can be destroyed by it in a few milliseconds (a millisecond is 1/1000 of a second).

It’s NOT reversed polarity on the AC shore power plug, unless something else is miswired at the same time. That’s loosely defined as the Hot and Neutral wires being swapped or “reversed” in the extension cord or outlet. But the White Neutral wire is supposed to be isolated from the frame/skin of your RV according to the NEC (National Electrical Code) and RVIA (Recreation Vehicle Industry Association) build codes. If you’ve accidentally bonded (connected) the Neutral and Chassis Ground together, then it’s possible that a Reversed Polarity Outlet could energize the RV chassis/skin, but if the ground wire is intact it should trip the circuit breaker immediately.

It’s definitely some sort of compromised EGC (Equipment Grounding Conductor), more commonly called a safety ground, or simply “ground” by most consumers. This “ground” is supposed to drain away any small ground fault currents and trip a circuit breaker for any large ground fault currents.

By definition, a ground fault current is any sort of leakage between the incoming hot wires and the chassis of an appliance or your RV itself. There will ALWAYS be some ground fault current available in anything plugged into a power outlet, but it’s normally very small, typically less than 1 mA (1 milliamp or 1/1000 of an ampere). Most of the time there will be a balancing act between the hot and neutral leakage impedance, with the open ground hot-skin voltage biasing to around 1/2 of the line voltage.

So if your EGC (ground) wire is compromised without anything else being wrong in your RV, you’ll often measure 40 to 80 volts between the RV frame and a screwdriver stuck in the earth. While this sort of low-current ground fault may not be immediately deadly, you still need to take it seriously since it can turn into a high current ground fault in a heartbeat, and there will be nothing to stop it from killing you or a loved one.

OK, now let’s consider what can compromise your RV’s grounding system. There needs to be a solid connection between the frame/chassis of your RV all the way back to the electrical service panel feeding your home or the campground. So everywhere there’s a connection, it’s possible for a failure to occur.

That means it could be caused by a broken or loose or corroded connection in your shore power cable, extension cord, dog-bone adapter, pedestal outlet, or even the AC power feeding the pedestal or outlet itself. I’ve seen loose grounding screws inside of the RV’s circuit breaker box cause this, and even a broken ground screw on the back of the RV’s shore power jack on the side of the vehicle. To be code compliant, this EGC (ground) needs to have less than 1 ohm impedance back to the service panel’s G-N-E (ground to neutral to earth) bonding point.

Also, a grounding rod connected to the frame of your RV does NOT “ground” your RV. There are lots of reasons for this, but any  grounding rod in dry soil can easily measure up to 100 ohms impedance to earth. So while it might drain away a low-current ground fault leak from inverter RF noise reduction capacitors, it certainly CAN’T drain away a high-current ground fault.

That kind of dead short fault can be caused by something like a screw being driven through a wire in the wall, or insulation worn through on a wire by rubbing on the frame, or even a failed transformer in your microwave oven.

As a side note, leveling jacks on the ground (dirt) do nothing to actually “ground” your RV, so don’t get me started on that subject. That’s not how grounding works.

Finally, there’s one really dangerous outlet miswiring condition I sometimes find in pre-1970s garages, churches and music stages. I’ve named it an RPBG (Reverse Polarity Bootleg Ground) and it can’t be detected by any standard metering tests including 3-outlet testers or even metering between H-N (hot to neutral), H-G (hot to ground) and N-G (neutral to ground). You must have an external reference to earth to find it.

While there are complex ways to discover and measure a hot-skin caused by an RPBG, a simple check with an NCVT (Non-Contact Voltage Tester) can easily find it (and any other hot-skin condition over about 40 volts). See my article on this topic at Failures in Outlet Testing Exposed | Contractor content from Electrical Construction & Maintenance (EC&M) Magazine.

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.

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

##RVT935

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Bill
7 months ago

I do not know where the campground was but I have seen many in the vicinity of power lines and all power lines give off magnetic fields depending on distance and voltage involved it may induce small voltages in the trailer because if it is not grounded to the same surface as you are you may feel a shock. I worked in electric substations and received many shocks from vehicles parked near the lines.
Fences around substations are grounded at every other post and a cable is buried under gates to stop shocks from one side to the other, also some times if the wire is missing at night you can even see sparks from the changing ground grid.

Cindy
7 months ago

My Dad was an electrical instructor for many years and I know what he’d say. Start with what you did last and begin with your trouble shooting there. He might have a ground somewhere near the valve he replaced and it was damaged or disconnected while working on the trailer. Checking that first at least might narrow down the possible places where the breach occurred. Good luck.

Jim
7 months ago

I know electricity is not easy for me to understand but after reading this I’m no closer to finding the problem. It seems to me that the problem didn’t exist until he installed the new valve. There must be a way to check that connection to see if everything is correct or maybe just disconnect what he connected and see if he still has hot skin. I’m sure you are correct on all the things you said could cause the problem but I would have no idea how to check each one of those, so I guess I would have to review many of your other articles and hope I could understand how I could to do the checks and repairs.

Mike Sokol
7 months ago
Reply to  Jim

Jim, these articles are generally not constrained to a single solution. That is, I’m trying to teach general troubleshooting principles to everyone by using the original posted question as the jumping off point. And I try to write these principles in the order of my own problem-solving techniques so it will help others.