Earth Grounding Basics From Fluke

This article is from Fluke. Check Fluke Products Here. Grounding Basics Components of a Ground Electrode Ground conductor Connection between the ground conductor and the ground electrode Ground electrode Locations of Resistances The ground electrode and its connection The resistance of the ground electrode and its connection is generally very low. Ground rods are generally made of highly conductive/low resistance material such as steel or copper. The contact resistance of the surrounding earth to the electrode The National Institute of Standards (a governmental agency within the US Dept. of Commerce) has shown this resistance to be almost negligible provided that the ground electrode is free of paint, grease, etc. and that the ground electrode is in firm contact with the earth. The resistance of the surrounding body of earth The ground electrode is surrounded by earth which conceptually is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area resulting in the greatest degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This finally reaches a point where the additional shells offer little resistance to the ground surrounding the ground electrode. So based on this information, we should focus on ways to reduce the ground resistance when installing grounding systems. Single ground electrode.   Multiple ground electrodes  connected   Mesh network.   Ground plate. What Affects the Grounding Resistance? First, the NEC code (1987, 250-83-3) requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil. But, there are four variables that affect the ground resistance of a ground system: Length/depth of the ground electrode Diameter of the ground electrode Number of ground electrodes Ground system design Length/Depth of the Ground Electrode One very effective way of lowering ground resistance is to drive ground electrodes deeper. Soil is not consistent in its resistivity and can be highly unpredictable. It is critical when installing the ground electrode, which it is below the frost line. This is done so that the resistance to ground will not be greatly influenced by the freezing of the surrounding soil. Generally, by doubling the length of the ground electrode you can reduce the resistance level by an additional 40 %. There are occasions where it is physically impossible to drive ground rods deeper—areas that are composed of rock, granite, etc. In these instances, alternative methods including grounding cement are viable. Diameter of the Ground Electrode Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 10 %. Number of Ground Electrodes Another way to lower ground resistance is to use multiple ground electrodes. In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods need to be at least equal to the depth of the driven rod. Without proper spacing of the ground electrodes, their spheres of influence will intersect and the resistance will not be lowered. To assist you in installing a ground rod that will meet your specific resistance requirements, you can use the table of ground resistances, below. Remember, this is to only be used as a rule of thumb, because soil is in layers and is rarely homogenous. The resistance values will vary greatly. Ground System Design Simple grounding systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding and can be found outside your home or place of business. Complex grounding systems consist of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops. These systems are typically installed at power generating substations, central offices, and cell tower sites. Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances. Type of soil Soil resistivity Earthing resistance Ground electrode depth  (meters) Earthing strip  (meters) M 3 6 10 5 10 20 Very moist soil, swamplike 30 10 5 3 12 6 3 Farming soil, loamy and clay soil 100 33 17 10 40 20 10 Sandy clay soil 150 50 25 15 60 30 15 Moist sandy soil 300 66 33 20 80 40 20 Concrete 1:5 400 - - - 160 80 40 Moist gravel 500 160 80 48 200 100 50 Dry sandy soil 1000 330 165 100 400 200 100 Dry gravel 1000 330 165 100 400 200 100 Stoney soil 30,000 1000 500 300 1200 600 300 Rock 107 - - - - - -