Hot-skin voltage troubleshooting

Hot-skin voltage troubleshooting

Dear Mike,
Thank you for the videos and literature online to learn more about electrical safety within and while using the RV and unknown outlets; it is amazing information indeed.

I wanted to contact you as I have (as so many have) been “tickled” due to a hot-skin condition on my 1991 Travelmaster Class C motorhome, but my condition seems a little bit different. Thus I would like to explain the scenario a bit, and inquire as to what areas you would recommend that I “review” to ensure full safety for myself and family during use of the RV.
 
Background: Purchased RV 4 months ago (used with bad transmission) from local person. Got the unit operational and plugged into outdoor outlet of my home to test RV functionality one system at a time. Upon connection, received several tingles upon touching RV surfaces. Began switching off breakers and found that the disengagement of the circuit that powers the RV refrigerator stopped the hot-skin condition. Please note that (as far as can be determined) the RV refrigerator is non-functional.
 
What areas should I begin investigating to locate a potential source for the hot-skin condition if the refrigerator circuit is activated? Any direction would be extremely helpful.
 
Best regards —Mark 

Dear Mark,

So here’s what’s happening. Any hot-skin voltage needs two conditions. First, there has to be a source of the leakage current, which obviously is coming from your refrigerator. But secondly and most importantly, you have to have a broken ground wire somewhere in the circuit path back to the service panel.

That means that in order for any voltage to develop on the RV chassis (skin), you must have a broken or high-resistance connection between the frame of your RV and the ground wire in the outlet you’re plugged into. Here’s a diagram of how the power comes into the campground service panel from the transformer on the pole. You’ll see that there’s a pair of hot lines that supply the 120-volt power, as well as a neutral wire for the return current and an EGC/Ground wire for any fault currents.

Next, this is what your RV’s grounding is supposed to look like inside. Note that the incoming EGC/Ground wire is bonded (connected) to the chassis of the RV. And I’ve also added a leakage current between one of the incoming hot wires and the chassis somewhere. In your case this is coming from some damaged insulation or a pinched wire in your refrigerator. This is where the fault current starts from. And as you can see, it SHOULD go right out the EGC/Ground wire back to the service panel and be shorted to the neutral/ground bond. But that’s obviously not happening in your case. So something is wrong with your ground wire.

So it has to be that you have a loose or corroded or missing ground connection SOMEWHERE. The reason for this statement is that if you DO have a properly connected safety ground (EGC for Equipment Grounding Conductor in the NEC handbook), then it’s impossible for internal leakage currents in your RV to create any significant hot-skin voltage. When I say significant, I mean anything more than 2 or 3 volts in reference to earth ground. Any more voltage than that means you’ve lost your EGC/Ground connection back to the incoming service panel’s Ground-Neutral-Earth bonding point. Small internal leakages from each appliance are quite normal and allowed by both UL and the NEC. But you MUST have a low-resistance EGC/Ground (Safety Ground Wire) to drain these currents away or they will end up as a hot-skin voltage. And that voltage can be deadly to both humans and their pets.

The usual suspects are any extension cords, shore power cords, power adapters, and even the Ground-To-Chassis bonding connection inside of your RV’s circuit breaker panel. But most of the time at home it’s a poor ground in the home outlet itself. I would start with testing your outlet for proper grounding with a basic 3-light tester, then move on to testing each power cord connection for continuity in its ground wire. As noted above, you can’t have any hot-skin voltage on your RV if you have a solid ground connection. Without that ground ANY leakage inside of your RV will electrify the RV skin, and not trip the circuit breaker like it’s supposed to do.

Once you have the incoming power sorted out, then you need to measure the skin voltage of your RV. It should be very close to zero volts when measured to earth-ground with a volt-meter.

Now, finally you can figure where the leakage current is coming from in your RV’s refrigerator. What I do is put a clamp-ammeter on the return ground wire, then start disconnecting things one at a time until the leakage current stops. It’s most likely a wire in your refrigerator with the insulation damaged from heat or vibration, and replacing the offending part will correct the fault current. But again, the really important thing is to make sure you have a solid ground wire for your RV so that leakage current can never turn into a dangerous hot-skin voltage.

Let’s play safe out there….

 

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

##RVT847

 

Related

6 thoughts on “Hot-skin voltage troubleshooting

  1. Jim Foreman

    When we were living in a motorhome, I built a box that went between my rig and the power pedestal. It had its own ground rod and checked for and corrected an open common, open ground and reversed polarity.

    1. Brian

      A ground rod will not provide an equipment ground and will not clear a fault. Equipment grounding must be a complete, low resistance path all the way back to the utility transformer in order to clear a fault, dirt is far too resistive to accomplish this. Ground rods have their place but certainly will not eliminate or even reduce a hot skin condition, the ground rod and “box” will simply become energized, increasing the amount of metal now energized.

      1. Brian, that’s all 100% correct. To expand on your explanation a bit, the primary job of the ground rod is to provide a fault path for any nearby lightning strikes. This is so it can short that lightning current spike to earth close to where it hits, not let it keep going back up the power line to find another ground path.

        Secondly, the ground rod’s job is to keep keep your local grounding system close to earth potential. Without it, the transformer isolation on the power pole could allow the neutral and your local ground-plane to float above earth voltage. I’ve actually seen this once in a church that was hit by lightning. It blew out the ground connection to the pole, and glassified the building ground rod in the sand. (I coined the phrase popciclefication). The impedance of the glassified ground rod was so high that the church’s entire electrical measured some 40 volts above earth potential. And anytime there was a lightning bolt in the clouds above the church, the guy touching the mixing console could feel it tingle in his fingertips. He was repeating the Ben Franklin kite experiment unknowingly. Plus they kept getting lightning side flashes inside the building between the A/V gear and structural steel, which is where the lightning ended up going because there wasn’t a ground rod to divert the current to earth before it started dancing around in a building.

        This is one failure example that illustrates the two things lightning rods do for us. A local ground rod at an RV will do NOTHING to prevent a hot skin condition. The dirt is a pretty poor conductor, and certainly won’t have a low enough impedance path to quickly clear a circuit breaker when something shorts to the frame of the RV. That’s the job of the EGC (Equipment Grounding Conductor) which, as you note, must be bonded to the power company’s transformer neutral.

  2. John Moreman

    I notice all plug in testers are used in 120 vac circuits. My rv used a 230 vac circuit wit two hot wires. Does the plug in tester test both hots vs neutral and ground circuits?

    1. Wolfe

      Technically, the 3-wire testers only check HNG on one phase (might even be different phase for different outlets). That said, if the other 3 wires are all correct in a 4 wire system, process of elimination suggests the 4th would be right or disconnected.

      The better test, of course, is to test the pedestal with a voltmeter to each contact pair — 120V in an “L” for 30A or 120V in a “diamond” on 50A (240V horizontal across the middle, 0V vertically).

      https://www.youtube.com/watch?v=p37z_G9CLRo

    2. You are correct that standard 3-light testers only check one of the hot lines, simply because the adapter you need to use for the 50-amp outlet only connects to one of the hot lines. I can’t find any commercially available 3-light testers that will check both hot lines in a 50-amp/240-volt outlet. Of course you can build one yourself if you’re an engineer, but that doesn’t help the normal, non-engineering types out there.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.