Using a ground resistance tester: Measurement principles ... - Hioki

What is a ground resistance tester?

The resistance between a grounding electrode and the earth is generally known as ground resistance. More precisely, ground resistance is the total of the grounding conductor’s resistance, the contact resistance of the grounding conductor and earth, and the earth’s resistance. Ground resistance differs from ordinary resistors in that it has the following special characteristics:

•    Polarizing action
Because the earth behaves like an electrolyte, it exhibits polarizing action such that a DC current produces electromotive force in the opposite direction, making accurate measurement impossible. Consequently, ground resistance is generally measured using a square wave or sine wave at a frequency of several dozens of hertz to 1 kHz.

•    Special measurement setup
Ground resistance is the resistance between a grounding electrode and the earth. It cannot be measured without inserting the electrode into the ground. Since earth has comparatively low resistivity, a voltage drop occurs near the electrode from which the current used to make the measurement flows. Consequently, in order to accurately measure the resistance value of each grounding electrode (the E electrode, S [P] electrode, and H [C] electrode), you must move about 10 m away.

•    Existence of disturbances
Measurement of ground resistance is subject to disturbances such as ground potential and the effects of the auxiliary grounding electrodes. Ground potential caused by leakage current from devices connected to the grounding electrode is superposed on the signal that the ground resistance tester detects, affecting measured values. In addition, if the auxiliary grounding electrodes have a high ground resistance, the measurement current will decrease, making measurement more susceptible to the effects of noise such as ground potential.

The FT is designed to be resistant to these external effects, allowing accurate measurement even under poor conditions.

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Ground resistance tester measurement principles

An AC power supply’s voltage is applied between the H (C) and E electrodes, and the AC current I that flows as a result is measured by an ammeter. In addition, the voltage V that occurs between the S (P) and E electrodes when the current I flows is measured using an AC voltmeter.

Then the ground resistance RX of the E electrode is calculated from the measured current I and voltage V. It is not possible to accurately measure the voltage between the H (C) and E electrodes, or the voltage between the H (C) and S (P) electrodes.

Types of grounding installations and grounding reference resistance values