EC&M Tech Talk — Rules for Measuring Earth Resistance

In Episode 34 of EC&M Tech Talk, Electrical Trainer Randy Barnett covers three main points when it comes to earth resistance, also known as ground resistance. First, the National Electrical Code in Sec. 250.53 addresses a specific maximum resistance between a rod, pipe, or plate electrode and ground. However, there is no requirement to measure this resistance if a second ground rod is installed. The keys to understanding the need for minimum ground resistance are found in the performance requirements in Sec. 250.4. Dissipating voltage surges and stabilizing voltage during normal operation require proper grounding.  

The second topic in this Tech Talk  looks at practical applications that exceed NEC requirements. Randy provides an overview of grounding requirements for steel structures that support radio and antenna equipment, along with referencing the appropriate Telecommunications Industry Association standard. Substation grounding is much more complex than typical facility distribution grounding. While different options exist when designing a substation, the most common grounding method is the construction of a grid system. In both examples above, the earth resistance is typically first measured. The resistance from the grounding electrode system into the earth may be required to be a low as 10 ohms or less. Proper measurement and testing are required to ensure proper system operation and maintain safety. 

The final topic Randy covers shows the two most common methods to test earth resistance, which are the clamp-on method and the fall-of-potential method. He uses graphics to explain each method. Other test methods are also listed that may be used for specific applications along with the IEEE guide providing these methods.