Recently while reviewing some report data I came across a ground resistance reading of 24.3 ohms; which prompted a discussion. The term grounding in the electrical industry is one of the most misapplied terms in the trade. In the electrical world, by definition, when we talk about “ground” or “grounded” we’re usually talking about connecting “something” to the earth. In some parts of the world the term “earthing” is used to make that point clear.

Since the earth doesn’t come with a convenient electrical terminal to facilitate that connection we have to improvise. Ground electrodes can come in the form of ground rods, plates, underground water pipes, metal in-ground support structures, grids, pipe electrodes, ground rings, and similar structures as allowed by the National Electric Code. Once we install or identify that “terminal” how do we know if it’s an effective connection? Is it sufficient for our purpose? Will it keep our people safe? Protect our electrical system from overvoltages, and dissipate lightning? Will it effectively clear faults? Will it minimize the electrical “noise” which effects sensitive electronic equipment?

In most applications, the lower the ground resistance, the safer the electrical system. So, what is a good ground resistance value? For a solidly grounded system, the National Electrical Code requires the addition of a second grounding electrode when the ground resistance is above 25 ohms. The IEEE grounding standards points out that this value should not be interpreted as a satisfactory value. IEEE-142 identifies effective ground resistance in the 1 ohm to 5 ohm range for industrial substations, buildings, and large commercial installations. IEEE 80 is the standard for substation grounding and lists values in the range of 1/2 ohm to 1 ohm for generating plants and large substations. Typically, sensitive electronic and medical equipment calls for ground resistance in the 1 to 5 ohm range.

In order to evaluate the ground resistance, the resistance of the soil to grounding rod/electrode interface must be considered; as well as the resistance of the grounding conductors and the connections. Soil conditions, minerals, chemicals, moisture content and other factors play into the results measured by a ground resistance meter. The results must be field tested to determine the specific ground resistance at a given site. Periodic maintenance testing of the ground system should be performed, and a trend established as soil and site conditions change over time. Some organizations, such as the Mine Safety and Health Administration (MSHA) require annual testing.  Recently during an investigation at a customer site, where they were experiencing problems, the ground resistance at the grounding electrode was measured at above 75 ohms.

Knowing what testing technology and methodology is best for a given situation depends on the application but can range from simple clamp on measurements, to two point electrode measurements often used in urban environments, to the more complex three point or fall of potential method, the four point method used for conductive depth readings, and a few other variants. Partnering with an experienced engineering or testing firm can help assess your needs and select the best method to ensure a safe and reliable electrical system for personnel, equipment, and your specific application.