Dear Gary,
How do you perform a hot skin test on a recreation vehicle? Also I’ve heard you should still hot skin test a fiberglass sided RV by checking faucets, door frames and windows. How do you do that on a slick side motorhome? – Jay Jones, (Silverdale, WA)
Answer:
Jay, it is indeed true that every RV should have a hot skin test performed if there is any question about its electrical integrity, including those with fiberglass or plastic siding. Receiving a slight shock or tingle as you step into a motorhome is a good clue that something is amiss.
For a complete hot skin test all you need is an accurate volt-Ohm meter, preferably digital. Here’s how the pros do it. First, be sure all the RV circuit breakers are turned on. Plug the shoreline cord into an external source of 120-volts AC. Set the voltmeter to the 250-volt scale. To test for an electrical short, position the red test lead on a bare metal portion of the RV; the frame, entry step, door frame, etc. Connect the negative test lead of the voltmeter to earth ground such as a water pipe or the ground conductor in the panelboard distribution box of the external power source. Run the test multiple times using several different metallic components on the RV.
- A “0” reading indicates no leakage or voltage on the skin.
- Any voltage reading indicates a definite electrical short and further testing is in order.
Always perform this test at least three times at different metallic locations on the motorhome and be sure to reverse the leads on the voltmeter to be sure no voltage is leaking or shorting. The hot skin test is necessary even if the motorhome is made of fiberglass or other non-metal components. The frame, chassis, copper tubing and metal trim pieces on the coach can still conduct voltage.
Hot skin issues with RVs are typically high resistance electrical shorts which do not usually trip breakers or blow fuses. This type of short may result from the insulation on the conductors rubbing thin or breaking down through normal wear or abuse. Problems occur, but not a dead short. In some cases, the white, neutral wire may be cross-connected to the ground conductor. This may cause reverse polarity which would then by-pass circuit breakers resulting in no over-current protection.
If the hot skin test reveals a high resistance short, disconnect from all sources of 120-volts AC and do not plug the motorhome in until all shorts have been eliminated. It is recommended that a professional RV service technician or an electrician make the necessary repairs before powering up that motorhome on AC again.
(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)
Ed Price
Although you are essentially correct, as an engineer who regularly designed and tested devices for power safety compliance, I wish you would use the term “fault” instead of the oxymoron “high resistance short.”
A common source of leakage current to ground is from stray capacitance between conductors and device chassis or metal housings. AT DC, you will read a safe, high resistance between a conductor and the chassis. But if a few inches of AC wire is very close to a grounded metal plate, there will be some capacitance between those two conductors, and at 60 Hz, the impedance of even 1 nanofarad will cause enough current to let you feeel the fault current through your fingertips. It’s still a long way from the “ouch & jump” level, but still significant.
Another test you can do is intentionally break the safety ground conductor in your shore power connection. This will show you what might happen if the safety ground line were to fail. Of course, put it back when you are done!
Bob Wallace
Dear Gary;
I had, or have a problem. I pluged in an extension for power and we were getting shocks when we touched the RV. I found out the cord had NO ground. After replacing the cord all was ok. Also when i check the battery by disconecting the neg. post and making contact with a light tester, it comes on, showing a short .The engine battery will die if i leave it hooked up for a long time. I just disconnect for now.I took it to a dealer and told them about the shocks, they said it checked out ok. is there a way to check this out??
Thanks
Bob Wallace
Keith
Another cause of that tingly sensation is induced voltage from proximity to overhead transmission lines. If the RV is properly grounded it shouldn’t be an issue, but you might get a shock before you connect or see a small arc on the ground when connecting your RV to shore power.
Dan
Continuing to use your voltage meter; should you find that you are seeing voltage to ground (leakage) a simply way to potentialy determin the circuit causing trouble is to shut each breaker off one at a time. The culprit will be the circuit that reduces the voltage to ground.
Drew
Gary and Dennis- thanks!
This info was what I was trying to relay to Larry Cad a couple of months ago when he discussed grounding and bonding generators, etc. He kept insisting that I was off-subject and my points made no sense. Maybe it takes someone else to help clarify these things sometimes.
Merry Christmas,
Drew
DENNIS
The crossed wiring can cause a fatal electric shock.. Always check the power plug at any site to be sure it is wired properly before plugging the unit in.. My 73 airstream has a warning light on the side and it lights red if there is malfunction in the circuit.. If the hot is connected to the neutral it will make the skin and frame hot to the earth and if things are wet it can cause the fatal shock thru rubber shoes even..If ever in doubt, touch the unit with the back of your hand, not the fingers or palm as the electric current will cause a muscle reaction and pull the back of the hand away from the metal, where it would lock your hand onto the electricaly charged item if you grabbed it with the fingers or palm .. Always be safe and check before plugging any unit in…
Troy Seagondollar
So if it is positive what is the worst case scenario?