Ground Rod Testing (It Ain't Easy)!

It's not as simple as you might think, but probably unnecessary...

Dear Mike,

Is it possible to use normal measurement tools such as a multi-meter, surge protector, or Klein dual voltage no-contact tester, to simply measure the effectiveness of a “ground rod”? I know that not all soil conditions will ground the same. I try to be very thorough prior to connecting to shore power to prevent damage to my motorhome electronics. Because of the information you have provided in your articles, I have learned how to use my electrical tools with greater confidence. Thank you for sharing the information that you do! Sometimes I get lost in the narrative, and read s-l-o-w-e-r! —RAY

Hey Ray,

That’s the $64,000 question, as they used to say. And while it seems like there should be a simple way to measure how effective a ground rod is, the test procedure is a bit complicated. So complicated, in fact, that the vast majority of inspectors don’t bother to do it. That’s because there’s a workaround that nearly everyone does to avoid the complexity of this test. Here’s how it works and why you’ll probably never do it. And, more importantly, I’ll answer why you really don’t have to do this test.

Fall of Potential Test

By golly, this is one complicated test, so follow along carefully. First you need a special Fall of Potential meter that costs $1,000 or more just to get started. And as you can see from the diagram, you need to place short test rods at specific distances from the actual ground rod being tested. That’s usually 75 feet to 100 ft apart, depending on the soil conditions.

Next you apply current from the meter, which gives you a current and voltage reading on each of the terminals connected to the test rods. You’ll move the rods to different distances and repeat the test several times, at least, marking your readings down on a chart. The resulting graph will look something like this (below). You’re looking for a flattened area in the middle of the curve, which gives you an approximation of the actual impedance of the ground rod. If it measures UNDER 25 ohms, you don’t need a second ground rod. But if it measures OVER 25 ohms, then a second ground rod is required.

Finally, you have a number which approximates the impedance of the ground rod under those weather conditions, and yes, it changes day by day depending on water table, etc.

Do you really need to do a ground rod impedance test?

If this sounds like an expensive test you’re exactly right. In fact, it’s been decades since I’ve done an actual Fall of Potential Test that a client paid for. And here’s why… Unless you’re building a cell tower, electrical substation or military component assembly plant, you’re not going to pay hundreds or even thousands of dollars for an electrical engineer and crew to spend hours poking test rods in the ground and drawing up a graph. But in those specific cases, you really do need to spend the money.

The workaround in code is that if you don’t perform an actual test certifying the ground rod impedance is less than 25 ohms, then all you have to do is install a second ground rod at least 6 ft away from the first ground rod and bond the two together. And since a second ground rod costs around $10 and a Fall-of-Potential Test is many times that, everyone just pounds in a secondary ground rod and skips the test.

NEC Notes…

As far as spacing of the grounding electrodes goes, NEC 250.53 (B) requires a minimum of 6-ft between electrodes (rods). However, in NEC 250.56 they note that spacings greater than 6-ft will improve the efficiency of the electrode. I typically separate grounding rods by 8 ft.

Grounding vs Bonding reminder…

Finally, remember, the grounding rod isn’t what actually “grounds” your RV. That’s the job of the ground wire (the EGC or Equipment Grounding Conductor) in your shore power cord, and that green ground wire must connect back to the service panel’s neutral bonding point. See my diagram above for an overview of how this is supposed to be bonded at a campground. More on this later…

Let’s play safe out there…. Mike