The testing methods covered in this Guide include those applicable to: (1) The measurement of the resistance and impedance to earth of electrodes varying from small rods and plates to large grounding sys- 4 QJ terns of stations. (2) Ground potential surveys@ including the measurement of step and touch voltages@ and potential contour surveys. (3) Scale-model tests for laboratory determination of the ground resistance and potential gradients for an idealized design. (4) The measurement of earth resistivity. The methods covered herein are generally limited to those using direct current or periodically reversed direct current@ or commercial power frequencies. This Guide does not claim to cover all possible methods@ even within these limitations. Extreme precision is seldom possible because of the many variables encountered; therefore@ the measurements should be carefully made by the most suitable method available@ with a thorough understanding of the possible sources of error. Definitions of terms pertinent to the subject matter are given@ with references to their sources where known. Those approved or standardized by other bodies are used wherever possible. Measurements of ground resistance or impedance and potential gradients on the surface of the earth due to ground currents are necessary to: (1) Verify the adequacy of a new grounding system. (2) Detect changes in an existing grounding system. (3) Determine hazardous step and touch voltages. (4) Design protection for personnel and for power and communication circuits. Scale-model tests are useful in developing new designs for grounding systems and for comparing them with old ones. Earth resistivity measurements are useful for: (1) Estimating the ground resistance of a proposed station structure or grounding system. (2) Estimating potential gradients including step and touch voltages. (3) Computing the inductive coupling between neighboring power and communication circuits. (4) Designing cathodic protection systems.