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P81 - Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System

The purpose of this guide is to present practical instrumentation methods that may be used for measuring soil resistivity, the impedance to remote earth, step and touch voltages, and current distributions in ground grids associated with electric utility facilities. These grids typically consist of interconnected grounding systems ranging in complexity from a few ground rods to large grids with many ground rods or wells, buried conductors, and external ground connections. External ground connections may include overhead shield/ground/neutral wires, underground cable sheaths/neutrals, counterpoises, grid tie conductors, metallic pipes, and other connections that provide additional paths to remote earth. This guide is intended to assist the engineer or technician in obtaining and interpreting accurate, reliable data. The factors that influence the choice of instruments are discussed along with a presentation of field techniques for various types of measurements. These factors include the purpose of the measurement, the accuracy required, the types of instruments available, the possible sources of error, and the nature of the ground or grounding system under test. It also describes test procedures that promote the safety of personnel and property, and it seeks to minimize operating interferences with neighboring facilities.The test methods and techniques used to measure the electrical characteristics of the grounding system include the following topics: a) Establishing safe testing conditions b) Measuring earth resistivity c) Measuring the power system frequency resistance or impedance of the ground system to remote earth d) Measuring the transient (surge) impedance of the ground system to remote earth e) Measuring step and touch voltages f) Verifying the integrity of the grounding system g) Reviewing common methods and procedures for performing ground testing h) Reviewing instrumentation characteristics and limitations i) Reviewing various factors that can distort test measurements
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