Generator ground rod confusion and clarification

Generator ground rod confusion and clarification

Hi, Mike,
In your efforts to make things clear, you’ve confused me (doesn’t take much, apparently). I’ve read in several of your articles where you say that grounding your RV with a ground rod doesn’t do anything to protect against hot skin, that the resistance is too high.  OK, that makes sense.

Then in the March 5th RV Travel Daily Tips, you said if you were running a generator that powers multiple RVs, that the generator should be grounded with a ground rod in case there is a short in one of the other RVs, to prevent a hot skin condition on all the other RVs.  How can it be that the ground rod is effective here? In thinking this through, it feels like the short would have a higher current to dissipate which would make the high resistance of the ground rod more of a problem. What am I missing?

Thanks, as always. —Al

Dear Al,
If you’re running your own generator power distribution system to multiple RV campsites, a local ground rod provides two basic functions. First, if the generator was totally floating with respect to earth ground, and one of the RVs developed a line-to-chassis short via something like a pinched extension cord, then without an earth ground all the hot wires in all the RVs would be at ground potential, and all the neutral wires would be at 120-volts potential above earth. Plus there would be no return current path to trip the GFCI in the branch circuit. Then if an appliance plugged into one of the RVs develops a line-to-earth fault (like a power drill dropped into a water puddle), your entire group of RVs can all develop hot skin voltage.  And that would be VERY dangerous.

A ground rod will typically measure 20 to 100 ohms to earth, so 120 volts will create a few amps leakage through the earth. While that’s not enough to trip the 20-amp circuit breaker, it’s enough to reach the 6mA threshold of the GFCI and cause it to trip.

Secondly, if there’s any kind of lightning strike in the area, then all those power wires on the ground will act like a big antenna and shunt many thousands of volts into every RV plugged into your “distro” generator. That will result in side-flashes between the remote RVs and the earth, destroying most of the electronics plugged into the power distro. So the generator ground rod provides a more direct path for the lightning surface charge to ground itself.

But as I’ve often noted, if your RV is the only thing hooked on the generator, then a ground rod isn’t needed since you actually create your own localized ground plane. And also be aware that a ground rod is not a replacement for a solid EGC (Equipment Grounding Conductor with less than 1 ohm resistance) between the chassis of your RV and the campground service panel’s Ground-Neutral bonding point. See my diagram to trace out how this works.

Hope that helps. I know for many of you this will seem like a confusing answer, but I’m trying to simplify it as much as possible while still keeping the engineering intact. Don’t feel bad, since grounding and bonding confuses a lot of electricians I know.

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.

Sign up to receive an email every day of
articles we’ve published in the past 24 hours.
Enter your email address:

Delivered by FeedBurner

##RVT837

 

Related

2 thoughts on “Generator ground rod confusion and clarification

  1. Mike Sokol

    That’s not a generator… that’s a single phase of the power line on the street before it hits the transformer on power pole near your house. And all power poles have some sort of grounding plate that usually looks like a spiral on the bottom. In the US, all these “grounds” are bonded together and also tied to the neutral bus on the transformer feeding your house. And finally, your house has an additional 8-ft earth ground rod tied directly to your incoming service panel. Looks complicated, but remember what the neutral bond establishes is the short circuit fault path, while the earth ground rod takes care of voltage spikes from nearby lightning strikes.

  2. Gerry O'

    Your diagram shows a 7kV generator. How long/diameter, grounding rod would be required to safely ground the system?

Leave a Comment