2 www.megger.comIntroduction ........................................................................................................1Safety ..................................................................................................................5SECTION IEarth Resistivity ..................................................................................................6How Earth Resistivity is Measured...............................................................6Practical Example of Test Method ...............................................................8Type of Soil Affects Resistivity .....................................................................9Resistivity Decreases with Moisture and Dissolved Salts ..........................10Effect of Temperature on Earth Resistivity ...............................................12Seasonal Variations in Earth Resistivity.....................................................12Determining a Good Electrode Location ..................................................14SECTION IIMeasuring Earth Resistance for Electrical Grounding Systems .....................16Factors That Can Change Your Minimum Earth Resistance .....................17Some Basic Definitions ...............................................................................17Factors Influencing Requirements for a Good Grounding System ..........19National Electrical Code Maximum Values ...............................................20Nature of an Earth Electrode.....................................................................21Principles Involved in Earth Resistance Testing .......................................23Basic Test Methods for Earth Resistance ...................................................26Effects of Different Reference Probe Locations .......................................30Lazy Spikes ..................................................................................................35Supplementary Tests ..................................................................................36How to Improve Earth Resistance .............................................................37SECTION IIIAccurately Measuring Earth Resistance for Large Ground Systems ..............42Testing Challenges in Large Ground Systems ...........................................42Addressing the Testing Challenges in Large Ground Systems .................43TABLE OF CONTENTS
