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TABLE OF CONTENTSIntroduction........................................................................................................ 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........................................... 43Addressing the Testing Challenges in Large Ground Systems................. 44APPENDIX INomograph Guide to Getting Acceptable Earth Resistance.......................... 46APPENDIX IIClamp-On Method............................................................................................ 48APPENDIX IIIAttached Rod Technique (ART)........................................................................ 54Appendix IVMeasurement of the Resistance of Large Earth-Electrode Systems: IntersectingCurves Method1......................................................................................... 58Test at a Large Substation.......................................................................... 59General Comments..................................................................................... 60Appendix VSlope Method.................................................................................................... 62Appendix VIFour Potential Method..................................................................................... 65APPENDIX VIIStar Delta Method............................................................................................ 67Appendix VIIIDetermining Touch and Step Potential .......................................................... 70APPENDIX IXGround Testing Methods Chart....................................................................... 72Ground Testers Available from Megger.............................................. 74References...................................................................................................... 762 1-866-254-0962 www.megger.com Getting Down to Earth 3