By Mike Sokol
Advanced Electrical Concepts
As promised, this is Part 2 of an article series on Hot-Skin voltage. If you haven’t done so already, please read about the basics of Hot-Skin voltage in Part 1 HERE so we’re all at the same starting point. You’ll also need to read my RV 101 article in magazine format which was linked to last week. Read it HERE. Once you read all that, you’re ready to proceed with advanced troubleshooting concepts of RV hot-skin/contact-voltage.
Ready… Set… Go!
This article is rated with an Advanced difficulty level since it involves complex troubleshooting concepts with potentially dangerous voltages. Do NOT attempt these tests unless you’re properly qualified to work around live voltages.
Here’s a quick review of the basics
1. ALL electrical appliances plugged into a wall outlet have SOME current leakage between the incoming line power and their own chassis.
2. All “double-insulated” appliances (including cell phone chargers, slow cookers, and double-insulated power tools) will have a non-grounded, 2-prong plug and be almost completely isolated from the line voltage (more or less).
According to the NEC (National Electrical Code) and UL (Underwriter Laboratory) standards, any appliance with a non-grounded plug must have LESS than 0.75 mA (milli-amperes) of line-to-chassis leakage current. Let’s call this a Very-Low Current Fault.
3. All other appliances with a 3-prong (grounded) plug may have line-to-chassis leakage currents of up to 3.5 mA (milli-amps) and still be within NEC and UL guidelines. Let’s call this a Low-Leakage Current. The round prong on the 120-volt plug is the “safety ground contact” which much have a low-resistance (impedance) path back to the service panels G-N-E (Ground-Neutral-Earth) bonding point to be effective.
4. An appliance with a partial-short failure (such as an electric water heater with a break in its hermetically sealed electric element) will leak 1 to 2 amperes of fault current from the AC line to the water supply (and your RV chassis). Let’s call this a Mid-Current Fault.
5. An appliance or wiring box with a dead-short between the line and the chassis (such as a pinched wire or a screw/nail driven into the wall which penetrates the wiring insulation) can provide full circuit breaker current between the line and the chassis, up to 20 amperes. Let’s call this a High-Current Fault.
6. If that appliance chassis is bonded (connected) to your RV’s safety ground with a proper low-resistance (also called impedance) connection, and your RV’s safety ground (and chassis) is properly bonded to the G-N-E (Ground-Neutral-Earth) connection back at the entrance service panel, then ALL of the above fault currents will be returned back to the service panel (and the transformer on the pole) and be rendered harmless. Note that these fault currents DO NOT return to the earth beneath your feet through the grounding rod. A service panel’s grounding rod’s job is to protect the system from lightning strikes and help maintain the local ground plane’s voltage close to earth potential.
7. The Very Low (0.75 mA) and Low (3.5 mA) Leakage currents will be easily drained away without you even knowing about it, probably not even tripping a GFCI you may (should) be plugged into. These ground fault currents are all within UL and NEC allowed leakage values and quite normal.
8. The Mid-Current Fault Leakage (water heater failure) would certainly be more than enough to trip a 5 mA GFCI breaker, but not enough to trip any 15- or 20-amp circuit breaker or fuse. However, you should still NOT have any hot-skin voltage on your RV as long as your low-resistance safety ground path back to the service panel is intact.
A proper low-resistance grounded RV will NOT develop a hot-skin even under this failure condition since these High-Current faults should still be drained to the service panel’s G-N-E bonding point, thus tripping the circuit breaker. Once again, the ground rod at the service panel has NOTHING to do with providing this fault current path.
The Main Event
You’ll notice there are two common points in all of the above scenarios. First, a proper (low-resistance) safety ground path between the chassis of the RV and the service panel’s G-N-E (Ground-Neutral-Earth) bonding point WILL NOT ALLOW any dangerous hot-skin voltages to exist on your RV. So if you DO have a hot-skin voltage, then your low-resistance safety ground path has failed. And secondly, the grounding rod really has nothing to do with getting rid of these ground-fault currents. So adding a ground rod to your RV or the campground’s power pedestal WILL NOT provide a low-resistance ground fault path and can allow hot-skin voltages to exist on your RV. And putting your leveling jacks down on the dirt definitely WILL NOT ground your RV.
That being said, the one thing that you must be certain to provide for your RV is a low-resistance (impedance) EGC (Equipment Ground Conductor) path from the chassis of your RV to the ground prong of your shore power plug. This low-impedance fault path includes all extension cords, pig-tail and dog-bone adapters, as well as any twist-lock disconnects for your shore power cord. It also includes your generator transfer switch as well as the RV’s circuit panel’s Ground Bonding point which connects your RV chassis to the incoming safety ground wire. A safety ground failure (high-resistance connection) at any of these points will allow these leakage currents to energize the skin/chassis of your RV with up to 120-volts AC. Then touching your RV while contacting the earth at the same time will cause current to flow through your body. If enough current flows through your heart (10 to 30 mA) then it can go into fibrillation and death will occur in minutes without intervention (CPR and someone restarting your heart with a defibrillator).
While just using an ohmmeter between the ground prong on your shore power cord and the RV chassis seems simple enough, unfortunately it doesn’t provide enough amperage to actually test the ground fault path for its ability to carry fault current. That’s why there are fancy Ground Loop Impedance Test (GLIT) meters which pulse a single cycle of 60-Hz 120-volt through the ground systems to test impedance integrity. See left for a SureTest Analyzer which retails for around $300. These are probably too expensive for a casual RV owner and do take some interpretation of the dozens of measurements it can perform in a few seconds, but I feel this should be part of a yearly test of ALL campground pedestal outlets.
However, while these Ground Loop Impedance Testers are great for installed circuits, they do depend on there being 120 volts in the RV’s electrical system for testing, and thus can induce a hot-skin voltage condition on your RV during the test cycle itself. That’s right, the act of testing with a fault load can create a hot-skin voltage condition which could shock you if you were touching the RV and earth/ground at the same time.
But there’s a safer way to do this that uses the RV’s own 12-volt battery. Back when I was a young pup Electrical Engineer building nuclear missile guidance systems (no kidding) we used something called a Kelvin Bridge to measure very low-ohm resistors with ppm (parts per million) accuracy. I’ve come up with an RV version I call a DC Ground Fault Tester, which is essentially half of a Kelvin Bridge and uses 12 volts DC from the RV’s house battery along with a tungsten brake light bulb to limit the current flow. See below for the basic diagram. No, this is not a commercial product and no, I won’t build one for you.
SAFETY ALERT: THERE MUST BE NO 120-VOLT AC POWER APPLIED TO YOUR RV DURING THIS DC CURRENT RESISTANCE TEST. AND YOUR METER MUST BE SET TO MEASURE DC VOLTAGE (20 VOLT SCALE) AND DEFINITELY NOT ON THE RESISTANCE (OHMS) SCALE.
Here’s how it works. By clipping a battery clamp to the positive terminal of the RV’s house battery, you provide a 12-volt DC source to one side the standard brake light bulb. The other battery clamp is connected to the safety ground component under test, which should draw around 2 amperes of current and light the bulb brightly. The Digital Multi-Meter (DMM) is optional but will allow you to calculate an actual resistance value if you like (it should be under 1 ohm total to be code compliant). After the bulb is in place and shining brightly, you first measure the voltage across the battery, then compare it to the voltage measured across the bulb. Those two voltages should be very close to each other, within 1/10 of a volt or so. Any more than 1/10 of a volt difference between the measurements points to a poor ground connection somewhere in the path.
Begin testing at your RV’s shore plug’s ground prong. If it’s dim or doesn’t light at all, then you’ve got an open or high-resistance ground probably caused by a broken wire inside the shore power plug or a corroded ground bond inside your transfer switch or RV breaker box. Once you establish a bright light bulb (which implies a low-resistance safety ground) then flex the shore power cable and twist-lock plug on your RV and watch the light bulb for any flickering. Flickering is bad and suggests an intermittent ground connection.
If that tests OK, then plug on each of your shore power extension cords one at a time and test each ground prong for a solid light as well. Finally, plug in any pig-tail or dog-bone adapters onto your shore power plug (along with any extension cords) and test all the way to the farthest ground prong. Wiggle everything while watching the brake light bulb for a solid light. Any flicker or dimming of the bulb suggests you have an intermittent or high-resistance connection somewhere in your shore power cord that must be found before putting the RV back in service.
The beauty of this DC Ground Current Test is that it uses only 12 volts DC, which is not dangerous to human beings but does provide sufficient current (2 amps) to find any corrosion or loose connections. Plus you can easily wiggle all shore power connections while watching the light bulb for a flicker, which is the easiest way to find an intermittent safety ground connection. The first diagram above is the “pro” version with battery clips and a Digital Meter measuring point. But it’s also possible to build a “consumer” version which plugs into any 12-volt DC plug inside your RV and gives you the same current/light test. See above/right for the wiring diagram of the consumer version.
The only thing to watch out for with this test is that a brake light bulb can get quite hot in just a minute, so I think that using something like a trailer brake light housing would be safer than a bare bulb. To reiterate, this is a 12-volt DC test, which must be performed only while your RV is disconnected from all 120-volt shore power and generators. Intermixing 120-volt AC and 12-volts DC during this test could be very dangerous (read deadly) to the person doing the test, but the 12 volts from your battery should be perfectly safe to use for this test, even with bare hands.
Finally, if after passing your DC current test and plugging back into shore power your RV still has a hot-skin voltage, then whatever you’re plugging into has either an open ground, reflected hot-skin voltage, or RPBG (Reverse Polarity Bootleg Ground). That’s a future installment of this hot-skin article series, so watch for it later.
As noted in Part 1 of this article, the easiest way to test your RV for a hot-skin voltage is by using a standard sensitivity (90 to 600 or 1,000 volt) NCVT (Non Contact Voltage Tester), which is available from any of the big box hardware stores. I’ve included a picture of the Southwire 40136N NCVT for example, which only costs around $15.
OK, that’s all the training your can handle in this session. Part 3 (Expert Only Tech Level) of this series on hot-skin troubleshooting is reserved for my 8-hour RV technician classes. But I’ll do another Advanced Level electricity topic in next month’s RV Electricity Newsletter. See you then.
In the meantime, let’s play safe out there…
Mike Sokol is an electrical and professional sound expert with 40+ 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.