fallofpotentialmetode

The fall-of-potential method, also known as the fall-of-potential earth resistance test, is a practical procedure used to determine the resistance of an earth electrode system for electrical grounding. It is widely applied to verify the safety and effectiveness of grounding for electrical installations, substations, and telecom networks.

Principle and concept

The method measures the potential difference caused by a known current flowing through the soil from two outer current electrodes to the earth under test. An electrode under test, such as a grounding rod or grid, carries a current I from outer current electrodes placed along a line. A potential electrode (or two in some configurations) measures the voltage between itself and the electrode under test. As the distance of the current electrodes from the test point increases, the measured potential approaches the true earth resistance, hence the term “fall of potential.”

Configurations and procedure

In the common three-point arrangement, the electrode under test E is placed in the ground. Two current electrodes C1 and C2 are arranged on a straight line with E at specified distances. A potential electrode P is placed between E and the near current electrode. A stable current I is injected between C1 and C2, and the voltage V between P and E is measured. The earth resistance is calculated as R = V/I. To improve accuracy and minimize effects of local soil inhomogeneities, distances to the current electrodes may be increased and the measurement repeated. A four-point variant uses two potential electrodes to better average the potential field.

Limitations and considerations

Accuracy depends on soil conditions, moisture, temperature, and the absence of nearby large conductors or buried services. Inhomogeneous soil or insufficient electrode spacing can distort results. Standards and procedures vary by region, and multiple distance measurements are often recommended to confirm convergence.

Applications

The method is used to size and verify grounding systems for safety compliance, electrical installations, substations, and other systems where a reliable earth reference is essential. See also earth resistance measurement and related testing methods.