What’s all this electrical “ground” stuff about?

 

Dear Mike,
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.

Dear Frank,
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.

##RVT826

Related

11 Thoughts to “What’s all this electrical “ground” stuff about?”

  1. Fletcher Hartman

    I’m confused. Which is quite common for me. But in your example of the extension cord hot wire making contact with the metal door frame and tripping the 20A breaker. I thought the breaker tripped with the over load of current that traveled from the breaker, thru the rv frame to the EGC and to ground via the grounding rod. You state in your article “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.” Isn’t the earth ground the grounding rod? Am I missing something reading the article? Please try and explain how I may be confused. Thanks

    1. Mike Sokol

      I’ll make a short video next week explaining this better, but for now consider what happens in your 12 volt DC system if a wire is shorted to the chassis. Current will flow from the positive pole of the battery thru the fuse, then thru the short in the wire to chassis, then thru the frame of the RV, and finally back to the negative pole of the battery to complete the circuit.. It will then blow the fuse due to too much current. And it doesn’t matter at all if your RV is connected to earth with a ground rod or not since the fault current never goes through the earth ground. Sounds confusing but it’s really very simple.

  2. MICHAEL FIEHLER

    I have a major concern; we host at a popular state park that has allowed some conditions to develop at several sites that allows water to stand after a rain that covers the pad and extends to around the power pedestal for a depth of several inches. The result is a camper is an island with their power cord in the water and going up to the electric plugs. This can last for 6-10 hours after a rain of only an inch or so. Park staff knows about this as the condition has existed for several years. My thought is that this is totally not safe. Park staff responds that it will drain and dry up. How best to address this?

    1. Mike Sokol

      This can be very dangerous, but maybe not for the reasons you think. So I’m going to cover this topic in an extended article soon. Thanks for the idea.

  3. George

    I always use a three prong plug-in type of tester that has three indicator lights that I plug-in to an interior duplex receptacle. On shore power it indicates all is well. When I run my generator it shows an open-ground. Is this something I should be worried about?? Tks.

    1. Mike Sokol

      Your generator has a floating neutral, so the fix is to add a G-N bonding plug I designed. Here’s my first article on the topic with a link to a video I made about it. http://noshockzone.org/generator-ground-neutral-bonding/
      There will be a commercial version available soon, so stay tuned for an announcement on where you can purchase one.

  4. booneyrat

    Goes right back to having a good quality EMS plugged into the pedestal,or hardwired in your coach,to help prevent electrical problems. While it will not fix the problem inside a pedestal,it can help protect your coach’s electrical system.

  5. Tom Fitch

    On a related note…would having the jacks down to the ground in a lightening storm help or hinder ones chances in the event of a lightening strike? Rubber tires don’t ground the vehicle do they?

    1. Mike Sokol

      The rubber tires have nothing to do with it. And it really doesn’t matter if your jacks are on the ground. However, unplugging your RV from shore power is definitely the best way to protect it from lightning damage. But an on-board generator or inverter should be safe to use.

  6. David Lee

    I love reading your responses to all the questions you get. I retired from the postal service as an electronic technician and worked around some of the varied conditions and examples you’ve covered. With that being said, I love learning new things about the electrical systems in my RV. Thanks for what you do.

    1. Mike Sokol

      You’re very welcome. I love learning new things too, which is why I write for so many publications. Electricity is one of those topics that a lot of people know a little about, while a few people know a lot about. And there’s a lot to know, so I keep studying and experimenting. Glad you’re along for the ride.

Comments are closed.