This paper presents a PC based instrument for ground impedance measurements. The instrument injects a transient electric current between the ground under measurement and an auxiliary ground, and it measures ground potential differences (GPDs) around the ground under measurement. The GPD measurements are processed with software which rejects external noise using correlation methods. Subsequently, estimation methods are used to extract the ground potential rise and ground impedance. The final result is the ground impedance versus frequency in a user selected frequency range. The instrument has been constructed and field tested by three utilities.

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