I had a problem a few months ago where the campground pedestal lost its ground, which gave my RV a hot-skin voltage. But I had my jacks down on the ground, so why didn’t that ground my RV? Are there different grounds or what? —Frank S.
I get this question a lot, so you’re in good company. There are indeed a lot of different types of “grounds,” and not all are created equal. In fact, the entire industry misuses the term so much that there’s often no way to know what kind of ground they’re talking about. Let me try to demystify it a bit.
First of all, the so-called “ground” wire in your shore power cable isn’t technically a ground at all. It’s actually called a EGC or Equipment Grounding Conductor which is bonded (connected) back to the incoming service panel’s Ground-Neutral bonding point. And that G-N bonding point is where any fault currents are supposed to go. See the diagram below:
A fault current can be the result of any type of short circuit to your RV’s chassis. For instance, an extension cord trapped in a door can have its insulation cut and an energized copper wire laying on the metal door frame. That’s going to try to electrify the chassis of your RV with 120-volts, which will go everywhere including the RV skin, hitch, wheels, etc.
But that EGC’s (Equipment Grounding Conductor) job is to send that current back to the incoming service panel’s G-N bonding point. See the diagram at the left, where you can see that the bonding point in the service panel connects the ground rod, neutral and ground wire together. Since code calls for this EGC to G-N path to have less than 1 ohm resistance, it should immediately trip any circuit breaker, and will shut off the source of the current. With a little Ohm’s Law we can calculate that 120-volts across a 1-ohm load will attempt to push 120 amps of current through the bonding path (Voltage / Resistance = Amperage). And since we only have a 20-amp circuit breaker (in most cases), it will trip very quickly, protecting you from a hot-skin condition.
It’s a very simple and elegant system that’s not actually a “ground.” While it also requires that this G-N bonding point in the service panel is connected to the earth (dirt) via a ground rod, the earth-ground really doesn’t have anything to do with tripping the circuit breaker in this example. The earth-ground just isn’t in the current path and doesn’t affect the outcome.
Now let’s look at what happens if we’re missing this G-N bond either through corrosion, a cut wire or simply by using an extension cord without a “ground” pin in place. Even if you had a ground rod on the side of your RV you still wouldn’t be protected from a hot-skin condition caused by a short circuit in your wiring. That’s because a ground rod can have up to a 100-ohm resistance into the earth and still is considered to be within code. A little more Ohm’s Law math shows us that 120-volts divided by 100-ohms gives us only 1.2 amperes of fault current (120 volts / 100 ohms = 1.2 amperes). And a 20-amp circuit breaker won’t recognize this as a dangerous short and will keep feeding 120 volts to your RV’s chassis and skin. So you could easily be electrocuted by an RV that’s “grounded” with a ground rod, but missing its G-N bonding path back to the incoming service panel.
Finally, back to your question about jacks on the dirt “grounding” your RV. I’ve measured the resistance of a flat jack to earth a few times, and even when the ground is wet it’s only around 200 to 300 ohms. And on dry ground or sandy soil it’s above 1,000 ohms. So if there was a short circuit in your RV’s electrical system, the jacks on the ground would only get rid of less than 1 ampere of fault current, and maybe as little as a few 10’s of mA (milliamperes). And that’s just not enough for any circuit breaker to recognize as a short circuit. So it will happily keep feeding voltage and current to your RV, electrifying it to a dangerous 120-volts of hot-skin potential.
So, as you can see, that so-called “ground” wire in your shore power plug is actually a G-N (Ground Neutral) Equipment Grounding Conductor, and the earth-ground doesn’t come into play for protecting you from a short circuit in your RV’s electrical system.
On the other hand, a GFCI (Ground Fault Circuit Interrupt) can easily detect and disconnect you from any fault currents as low as 6 mA (6 milliamps or 0.006 amps). So we’ll cover more about GFCI protection next week. See you then…
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.