The earth resistivity extremely varies between 1 to 10,000-ohm meters and in most situations, the soil resistivity is found to be non-uniform. Variation of the soil resistivity in depths is more predominant as compared to the variation with same in the horizontal distance and this happens due to stratification of earth layer.
Measurement of earth resistivity at the sites will reveal the characteristics of soil whether it is homogenous or nonuniform.
Because of moisture content in the soil, there is a wide range of variation in soil resistivity so it is advisable to take the testing during the dry season for considerable values.
In substation or generating station, at least eight test directions shall be chosen from the center of the station to cover the entire site and it will be increased depending upon how larger the site will be.
For the transmission line, the measurements shall be taken along the direction of the line throughout the length approximately once in every 4 kilometers.
Wenner’s four-electrode method is widely used for measuring soil resistivity in site location.
In this method, four electrodes are driven in a straight line at equal intervals into the earth. A current is passed to the outer two electrodes and the earth at the same time, the voltage will be observed between the inner two electrodes.
The resistivity is going to be proportional to the ratio of the voltage to current accordingly.
If the depth of burial of the electrodes in the ground is negligible compared to the spacing between the electrodes, then the resistivity value is
Four terminal megger earth tester is the most frequent method used for measuring soil resistivity, which comprises the current source and meter in a single instrument and directly reads the resistance.
The modified equation for soil resistivity is below,
ρ - Resistivity of soil in ohmmeter
s = Distance between the successive electrodes in meters,
R = Megger reading in ohms.
By Wenner’s four-point method, four electrodes are driven into a straight line at equal intervals in the chosen direction of the selected test site.
The depth of the electrode in the ground shall be of the order of 10 to 15cm, the megger is placed on the steady and approximately level base, and the four electrodes connected to the megger terminal.
The readings are taken while turning the crank is about 135 rev/min. The measuring values are substituted in the equation to find out the soil resistivity.
In case, the resistance of the potential electrode (inner electrode) is comparatively high, a correction of the test
the result would be necessary on its value.
The corrected value of the earth resistivity would be:
ρ = Corrected value of earth resistivity,
ρ' = Uncorrected value of soil resistivity,
Rp = Megger value for corrected connection.
Rv = Resistance of the voltage circuit of the instrument used to obtain R (as indicated inside the scale cover of the meter)
For this purpose, the instrument is connected to the electrode as,