Think of grounding as the foundation of a house that holds all of your electrical systems. Without it, the whole thing comes tumbling down.
Grounding tests fall into two categories: those you do to ensure your installation meets grounding specifications, and those you do to make sure it still meets grounding specifications. Why the first category? Your electrical system needs a low impedance path to ground for protecting people and equipment. Why the second? Because grounding systems deteriorate and so does their performance. Such deterioration is unavoidable, as it is the result of the ion exchanges that make grounding systems effective in the first place.
Here's one example that illustrates how much things can change. About 12 years ago, an engineer evaluated the lightning protection system at an airport. The original installation met the UL specifications for a Master Label. After years of service, the impedance to ground measurement was almost infinite. Excavation revealed the buried portion of the grounding system had corroded so only a few recognizable pieces remained.
What about original installations? Why not just drive a bunch of ground rods or bury a grid and be done with it? In some cases, that's just not effective. In other cases, it's not possible. The only way to know if your grounding system provides low enough impedance to ground is to test it.
What is ground testing? Ground testing is the verification that resistance between your grounding system and earth meets the specifications of the National Electrical Code (NEC) and other pertinent guidelines. The standard answer to NEC requirements is to drive a second rod and call the job done. However, doing so may not protect your installation for purposes beyond those of the NEC.
To test ground levels on your system, measure the resistance between a ground connection and earth. This is not as straightforward as it seems. Results are approximations; even though you are dealing with precise measurements. The variables that affect the measurement can, and usually do, change your readings.
For example, don't do a ground resistance test right after a major rainfall; the results will be overly favorable. You want to repeat your ground resistance test on a day when the soil is especially dry; your estimate of a "worst case" or "less than normal moisture" scenario. Besides this, you must understand how to do the test correctly and where to select the points of measurement.
You can use several methods to do a ground test. All of them use at least two reference ground points and a current source. The testing device circulates AC current through the ground under test. The testers that use the bridge method are common. These use currents on the order of milliamps. Some of the high-end testers use over 100A. There's no difference in accuracy between these, but each has its own advantages. Skill with the tester is more important than which to use. Now, let's look at what ground testing reveals.
Bonding problems. Remember, grounding is what you do to connect your system to earth. Bonding is what you do to connect your system components to the grounding system. Either one is useless without the other. Bonding is the means by which you can achieve effective grounding. Without good bonding, you lack the effective low-impedance path over which short- circuit currents can flow to ground. This leaves you with unsafe electrical equipment, metal raceway, and enclosures.
Let's look at a case history. A plant had many instances of arcing damage to equipment. Yet, it had ground rods driven next to each of its 10 main production machines. A ground test showed 25 ohms to earth ground from the building steel, 180 ohms to earth ground from the transformer. Amazingly, there was an open circuit from the machine to the main ground!
How can there be an open circuit with the machine's ground rod in place? That ground rod, for lack of bonding to the rest of the system, was irrelevant. The assumption was the 250 ft or so of soil between the system's earth ground and the machine's ground rod would provide a low-impedance path; no need to bond! The soil; even after a solid rain; proved to be a poor conductor. The grounding test showed the need for bonding jumpers and some other changes.
If you test for proper bonding, it's normally sufficient to do a visual inspection annually. If you replace bonding jumpers, add equipment, or modify the grounding system, it's good to repeat this test. Unfortunately, it cannot reveal improper neutral to ground connections.
Grounding system inadequacies. How many ohms to ground should you try for? You may have other concerns beyong the NEC. It tells you to look for 25 ohms and add another ground rod if you're above 25 ohms.
You may want your lightning protection system to provide increased security. Or you may want a low resistance, to provide a better environment for your electronic loads. What should that resistance be? That depends on what's normal for your area and type of soil, as well as what equipment you are trying to protect. Also consider the level of zero sequence current, if there's a safety ground or step touch potential concern.
You can perform tests to help determine a figure, or speak to a consultant familiar with your area. Use this number as a reference and look for gross differences between that number and what you are reading.
Think of grounding as the foundation of a house that holds your electrical systems. Without it, the whole thing comes tumbling down.
Wilcox is Area Manager for Electro-Test, Inc., Lee's Summit, Mo. and Chairs the IEEE's Power Engineering Society, Kansas City Section. Lamendola is a Technical Editor for EC&M and Chair of the IEEE, Kansas City Section.