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
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 $500 just to get started. And as you can see from the diagram, you need to place little 1-foot test rods at specific distances from the actual ground rod being tested. That’s usually 50 to 75 feet apart, depending on the soil conditions.
Next you apply current from the meter, which gives you a current 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 (above). 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.
If this sounds like an expensive test you’re exactly right. Here’s a detailed version of the test if you really want to know the details. 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 an electrical substation or military component assembly plant, you’re not going to pay a few hundred dollars for an electrical engineer to spend an hour poking test rods in the ground and drawing up a graph. But in those two specific cases, you really do need to spend the money. The workaround in the latest code is that if you don’t perform an actual rod test certifying the ground rod impedance is less than 25 ohms, then all you have to do is install a second ground rod and bond the two together. And since a second ground rod costs around $10 and a fall-of-potential test is easily 20 times that, everyone just pounds in a secondary ground rod and skips the test.
Finally, remember, the ground rod isn’t what “grounds” your RV. That’s the job of the ground wire in your shore power cord, and that green ground wire must connect back to the service panel’s neutral bonding point. See my diagram on the right for an overview of how this is supposed to be connected. And next time you’ll hear the rest of the story of why that actually works.
Until 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.