“Fall of Potential” Method:
The Earth resistance is measured using the “Fall of Potential” Method. The method works by injecting a constant current between two spikes which are inserted into the ground and measuring the voltage at points between them (as shown in the figure)
The “Fall of Potential” Method is a three terminal test. The electrode whose earth resistance is to be measured is disconnected from the system or earthing grid. The earth meggar has a current terminal, a voltage terminal and a common terminal. The common terminal is connected to the electrode,
Equal lines of potential
When an electrode is inserted into the ground and current flows through it, the potential around the electrode takes the form of concentric circles of equal potential.
It is essential that the equal lines of the common terminal and the current terminal do not overlap. Therefore distance between the ground electrode to be tested and the current terminal is vital. The distance should be sufficient so that the equal lines of potential of the common terminal and the current terminal do not overlap.
Method of Measurement:
The readings are taken at points close to the ground electrode and then gradually away from it. The resistance readings in ohms are plotted against the distance in a graph. The graph should take the form as below. At around 62% of the distance between the ground electrode and the current terminal, the graph levels off. This reading is taken as the value of the earth resistance. This point should be outside the equipotential zones of both the current terminal and the ground electrode