Stray Voltage?

Vulcan Rider wrote:

Mike_Sokol wrote:
However, note that adding a ground rod next to your RV does virtually NOTHING to actually "ground" your RV. The confusion is that the same word "ground" is used for a lot of different things in the electrical business, many of which have nothing to do with earth ground.

This statement would seem to be in direct conflict with the previous paragraph that talks about the N-G-E connection that occurs at the service entrance.

In normal commercial power applications is it NOT recommended that an earth ground be applied at each distribution box also ? While connected to shore power, why would that not be a good thing to do with an RV too ?

I'm pretty sure that a "mobile home" parked in a fixed location and permanently connected to commercial power WOULD have an earth ground applied at the distribution box.....or the metal frame.

OK, so here's how it works. A ground rod typically has an impedance to earth of between 25 and 100 ohms. It's not designed to trip the circuit breaker in the event of a hot-to-chassis fault. And here's the math to back up that statement. Ohms Law tells us that voltage divided by resistance equals amperage, or E/R=I. If we have a dead short inside of your RV from a hot wire to the frame (screw through a wire or something similar), lets assume worse case of a 100-ohm ground rod impedance. In that case, 120 volts divided by 100 ohms = 1.2 amperes of fault current. While this can trip a GFCI, it won't do anything to trip a 20 or 30 or 50 amp circuit breaker. But that's exactly what the EGC (Equipment Grounding Conductor) is supposed to do, trip the main circuit breaker. Now lets run the math for a 25 ohm ground rod. So 120 volts divided by 25 ohms equals 4.8 amperes of fault current. Again, it won't trip the 20, 30 or 50 amp circuit breaker, what we call the OCD (Over Current Device). Now, let's run the numbers for the fault path through a properly built EGC wire connected back to the incoming neutral of the transformer. So 120 volts divided by 1 ohm equals 120 amperes of fault current, easily enough to trip any circuit breaker.

Note that the ground earthing path via the ground rod really doesn't have anything to do this overcurrent trip path. That's why we call it a Neutral-Bond. The ground rod is important, but for a completely different reason. Its job is to direct lightning strikes near the power line to earth (the ground), rather than letting them side-flash around your home's electrical system. Plus, the ground rod(s) at the service panel's job is to pull the potential of your local transformer's neutral line close to earth voltage. Without a ground rod at the service panel, your entire home (or RV's) electrical system could float above earth potential by a significant voltage which would be very dangerous.

Also, the ground rod at the service panel isn't the only ground rod in the power distribution system, since if you watch them put in power poles you'll see that every one has a coil of bronze wire on the bottom, acting like a LOT of ground rods to help get rid of lightning hits. So while extra ground rods at each pedestal may help prevent electrical damage from lightning strike, by themselves they don't "ground" your RV in any way that counts. According to the NEC, the "ground" of the outlet in the pedestal or home needs to connect back to the incoming service panel's Neutral-Ground bonding point, which is then "earthed" to ground rod for additional lightning safety. Code does not allow you to use a remote ground rod by itself unless it's connected (bonded) to the incoming Neutral-Ground-Earth bonding point.

So before you start pounding in an 8 ft ground rod next to your RV, know that you just can't start digging holes and putting in pointy objects where underground wiring and plumbing could be. You need to call a safety group like Miss Utility 48 hours in advance to get clearance before you can do any serious digging or pounding.

As a side note, the circuit breaker box of your RV is wired just like a sub panel in your house with an unbonded neutral, NOT like an home or campground's incoming service panel with a G-N-E bond. However, permanent structures such as "mobile" homes are generally built with their own incoming service panel, power meter, ground rod, and G-N-E bond. There's just different wiring methods for permanent vs. mobile power systems.

Somewhere I have a circuit diagram I drew that shows all this for my advanced classes, since even licensed electricians often have trouble figuring out the difference between grounding and bonding. I'll post it here when I find it later.

Mike Sokol
www.NoShockZone.org