How To Do Electrical Earthing System Design
Earthing System Design & Planning
A earthing design starts with a site analysis, collection of geological data, and soil resistivity of the area. Typically, the site engineer or equipment manufacturers specify a resistance-to-earth number. The National Electric Code (NEC) states that the resistance-to-earth shall not exceed 25 ohms for a single electrode. However, high technology manufacturers will often specify 3 or 5 ohms, depending upon the requirements of their equipment. For sensitive equipment and under extreme circumstances, a one (1) ohm specification may sometimes be required. When designing a earth system, the difficulty and costs increase exponentially as the target resistance-to-earth approaches the unobtainable goal of zero ohms.
Once a need is established, data collection begins. Soil resistivity testing, geological surveys, and test borings provide the basis for all earthing design. Proper soil resistivity testing using the Wenner 4-point method is recommended because of its accuracy. This method will be discussed later in this chapter. Additional data is always helpful and can be collected from existing earth systems located at the site. For example, driven rods at the location can be tested using the 3-point fall-of-potential method or an induced frequency test using a clamp-on earth resistance meter.
With all the available data, sophisticated computer programs can begin to provide a soil model showing the soil resistivity in ohm-meters and at various layer depths. Knowing at what depth the most conductive soil is located for the site allows the design engineer to model a system to meet the needs of the application.
Soil resistivity is the key factor that determines the resistance or performance of an electrical earthing system. It is the starting point of any electrical earthing design. As you can see in Tables 2 and 3 below, soil resistivity varies dramatically throughout the world and is heavily influenced by electrolyte content, moisture, minerals, compactness and temperature.
|Type of Surface Material||Resistivity of Sample in Ohmmeters|
|Crusher granite w/ fines||140 x 106||1,300|
|Crusher granite w/ fines 1.5”||4,000||1,200|
|Washed granite – pea gravel||40 x 106||5,000|
|Washed granite 0.75”||2 x 106||10,000|
|Washed granite 1-2”||1.5 x 106 to 4.5 x 106||5,000|
|Washed granite 2-4”||2.6 x 106 to 3 x 106||10,000|
|Washed limestone||7 x 106||2,000 to 3,000|
|Asphalt||2 x 106 to 30 x 106||10,000 to 6 x 106|
|Concrete||1 x 106 to 1 x 109||21 to 100|
|Soil Types or Type of Earth||Average Resistivity in Ohm-meters|
|Bentonite||2 to 10|
|Clay||20 to 1,000|
|Wet Organic Soils||10 to 100|
|Moist Organic Soils||100 to 1,000|
|Dry Organic Soils||1,000 to 5,000|
|Sand and Gravel||50 to 1,000|
|Surface Limestone||100 to 10,000|
|Limestone||5 to 4,000|
|Shale’s||5 to 100|
|Sandstone||20 to 2,000|
|Granites, Basalt’s, etc.||1,000|
|Decomposed Gneiss’s||50 to 500|
|Slates, etc.||10 to 100|