Ingeniero

Getting Down To Earth

A practical guide to earth resistance testing
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Introduction Nothing is quite so common or abundantly available throughout the world as the earth’s soil. We are more apt to think of earth as something to be tilled for planting or to be excavated for a building foundation. Yet, it also has an electrical property -- conductivity (or low resistance) --that is put to practical use every day in industrial plants and utilities. Broadly speaking, “earth resistance” is the resistance of soil to the passage of electric current. Actually, the earth is a relatively poor conductor of electricity compared to normal conductors like copper wire. But, if the area of a path for current is large enough, resistance can be quite low and the earth can be a goodconductor. It is the earth’s abundance and availability that make it an indispensable component of a properly functioning electrical system. Earth resistance is measured in two ways for two important fields of use: 1. Determining effectiveness of “ground” grids and connections that are used with electrical systems to protect personnel and equipment. 2. Prospecting for good (low resistance)“ground” locations, or obtaining measured resistance values that can give specific information about what lies some distance below the earth’s surface (such as depth to bed rock). It is not the intent of this manual to go too deeply into the theory and mathematics of the subject. As noted in the references at the end, there are many excellent books and papers that cover these. Rather, the information hereinis in simple language for easy understanding by the user in industry. From years of experience in supplying instruments for the tests involved, Megger can provide advice to help you make specific tests. We would be pleased to have a representative call on you to discuss your problem.

©2005 Megger

www.megger.com GETTING DOWN TO EARTH 1

TABLE OF CONTENTS
Introduction . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 SECTION I Measuring Earth Resistance for Electrical Grounding Systems . . . . . . . . . . . 6 Factors That Can Change Your Minimum Earth Resistance . . . . . . . . . . . 7 Some BasicDefinitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Factors Influencing Requirements for a Good Grounding System. . . . . . 9 National Electrical Code Maximum Values . . . . . . . . . . . . . . . . . . . . . . . 10 Nature of an Earth Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Principles Involved in Earth Resistance Testing . .. . . . . . . . . . . . . . . . . 13 Basic Test Methods for Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . 16 Effects of Different Reference Probe Locations . . . . . . . . . . . . . . . . . . . 21 Lazy Spikes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Supplementary Tests . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 26 How to Improve Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 SECTION II Earth Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 How Earth Resistivity is Measured . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Practical Example of Test Method . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 34 Type of Soil Affects Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Resistivity Decreases with Moisture and Dissolved Salts. . . . . . . . . . . . . 36 Effect of Temperature on Earth Resistivity . . . . . . . . . . . . . . . . . . . . . . . 38 Seasonal Variations in Earth Resistivity . . . . . . . . . . . ....