What is a "Reflected Hot-Skin Voltage"?

It’s caused by a little known, but very dangerous, campground wiring failure!

Dear Readers,

Time for a reminder about a miswiring condition I first discovered some 50 years ago, and then found in campgrounds around 13 years ago when I began writing for the RV industry. It’s a hot-skin voltage that can be “reflected” to multiple campers in the same area of a campground. And it’s caused by two separate failures at the same time; a lost ground/bond conductor in a campground electrical system, and a fault current in an RV.

How does a reflected hot-skin voltage happen?

First things first. A hot-skin voltage is when the electrical potential of your RV skin (and chassis) is a significant voltage level above the earth ground around it. While it’s quite normal for there to be a few volts difference between your RV’s grounding system and earth ground, once you get above 5 volts difference, that’s a hint there may be something wrong with your RV’s grounding system via its shore power connection. If you touch an RV with a significant hot-skin voltage while standing on the ground, you can have a fault current go through your body (and your heart) – which can be very dangerous.

How to measure it…

While plugged into shore power (not powered only from a generator), if you stick a screwdriver into the damp dirt near your RV and measure between the metal shaft of the screwdriver and the metal frame of your RV with an AC multimeter, you should read a maximum of 5 volts AC.

Here’s a picture of me measuring 0 volts AC between the trailer’s stabilizing jack and the screwdriver shaft. I have the digital multimeter set to the 600-volt AC scale.

Note that this test could be dangerous to perform if your RV had a significant hot-skin voltage. So only touch the insulated meter probes with your hands, not the RV chassis itself, since it could have up to 120 volts of electrical potential if something is really wrong.

When does this voltage get dangerous?

While a 5-volt shock is likely not dangerous, note that once that hot-skin potential reaches 30- to 40-volts AC, it can become deadly if your hands and feet are wet and you touch the frame of your RV while standing on damp ground or concrete.

Just 10 mA (milliamperes or 0.010 amps) of fault current can be deadly if your heart is already at risk. And just 20 mA is enough current that you can’t let go of an energized object (like a screwdriver shaft or aluminum ladder) that you grabbed with your hand while standing on the wet dirt or concrete.

If you make contact with 30 mA or more of fault current, that just about guarantees your heart will go into fibrillation within seconds of contact. Without immediate medical aid, you can die from electrocution within minutes.

It’s all your fault (current)!

For a hot-skin voltage to occur, you first need some sort of fault current leakage in your RV. That can range from a few mA of low-current fault leakage from your charger or converter’s RF (radio frequency) filter capacitors, or 1 to 2 amps of a medium-current fault from a melted or corroded electric water heater element, up to 20 or 30 amperes of a high-current fault. That can be due to a direct short-circuit between a hot conductor and the chassis by something like a screw or nail driven through a piece of Romex wire or wiring insulation worn through by a piece of metal it’s been in contact with.

A low-current fault of a few mA isn’t enough to be dangerous to your heart and will give you a tingle. But the failed water heater element can easily provide more than the 30 mA of current through you that can put your heart into fibrillation. And the short circuit with 20 or 30 amps of available fault current at 120-volts AC will create a maximum current flow of around 100 to 150 mA through you. That just about guarantees death by electrocution if CPR isn’t immediately started and an AED used within minutes to put your heart back into normal rhythm.

But what about the RV ground?

Glad you asked. If you have a properly bonded ground in your RV’s shore power system, then this fault current can’t turn into a hot-skin voltage. Instead, it will return to the original source of the power company’s transformer.

• Note that an 8-foot grounding rod has way too high of an impedance to earth ground to get rid of anything but the lowest current leakages. While code implies that 25 ohms would be an excellent ground, even that much conductivity to earth ground isn’t enough to trip (clear) the circuit breaker in the event of a high-current fault. Nor will it reduce the hot-skin voltage caused by an internally shorted electric loopwater heater element.

Fault current needs a ground/bond connection, not a grounding rod!

To be safe from these fault currents, your RV needs a solid connection from its frame back to the campground (or home) incoming electrical service panel, where it must be “bonded” (connected) to the panel’s neutral-ground bonding point. This is named the “EGC” – for Equipment Grounding Conductor – which is what we call the “ground”.

That green or copper EGC wire is what allows the fault current to return safely to the power company transformer neutral bus. However, if that ground wire is interrupted for any reason, then the fault current can’t find a direct path back to the power company transformer and that’s when a hot-skin voltage occurs.

But what is this “Reflected Hot-Skin Voltage” about?

This is a bit tricky to understand, but it happens more than you might think. In a campground, the feeder conductors typically daisy-chain from pedestal to pedestal, fed by a common circuit breaker in the main service panel. If there’s an interruption in the grounding conductor path for any reason (corroded or loose connection in one of the pedestal, or a broken wire underground, or even a disconnect of the bond to the neutral in the service panel), all RVs that are downstream of the ground break will have their chassis connected together, but none of them will be grounded to the main service panel.

All for one, and one for all…

So, in that case, if a single RV has something like a fried electric element in the water heater, that 1 or 2 amps of fault current can easily turn into 80 to 120 volts of hot-skin potential. And that same voltage can be “reflected” to the other 6 or so pedestals on that feeder. So, if one RV has a fault current that turns into a hot-skin voltage, all the other RVs in the area can have the same voltage if there’s no service panel bond/ground.

Is a reflected hot-skin voltage the campground’s fault?

Well, yes, it is. If the campground doesn’t properly maintain and test their pedestals for proper grounding, then ANY RV that’s plugged into a pedestal can cause ALL the RVs on that feeder to develop a reflected hot-skin voltage. So, simply removing the offending RV from the campground does not correct the real problem of a non-existent service panel bond in the campground electrical system. But without a standardized campground maintenance and test SOP (Standard Operating Procedure), this dangerous condition can be unnoticed until an RV with a large fault current plugs in. And that’s what can endanger all the other campers in that area of the campground connected to that common feeder circuit.

Let’s play safe out there…. Mike