What is soil resistivity?
Before designing protective earthing,it is necessary to understand the nature of the resistance of the ground and the various factors that govern it. When an electrode is driven into the ground,the region around the electrode is known as the resistance area or potential gradient area, and the fault current tries to flow away from the electrode in all directions. It follows that the rise of grounding potential or the flow of current in the ground depends upon the resistivity of the soil into which the earthing conductor is driven. The soil conductivity is a purely electrolytic phenomenon, and its resistivity is governed by nature and may vary from 1 to 10,000 ohm-m.
Water has very poor conductivity and is very useless to lay the grounding conductor in, but its presence in soil helps in dissolving the salts, thereby improving soil conductivity. Soil conductivity increases with an increase in moisture content. When the moisture exceeds 20%, the change in resistivity is negligible.
The temperature of the soil also plays a vital role in varying its conductivity. When the temperature is higher than 0 °C, its effect on soil resistivity is negligible. At 0 °C, water in the soil starts freezing, and resistivity increases.
The resistivity of soil is also considerably affected by the composition and amount of soluble salts. The magnitude of the current also affects the resistivity of the soil. The soil resistivity at a particular location also varies with depth. Lower layers of soil have greater moisture content and lower resistivity. If the lower layer contains hard and rocky soil, resistance may increase with depth.
Measurement of Soil Resistivity
For the design of a grounding system, a detailed investigation of the soil resistivity is necessary. The resistivity of soil is usually measured by the four-spike method,in which four spikes arranged in a straight line are driven into the soil at equal distances. A known current is passed between electrodes C1 and C2, and the potential drop V is measured across electrodes P1 and P2. The current I develops an electrical field that is proportional to current density and soil resistivity. The voltage V is proportional to this field. Thus, soil resistivity is proportional to the ratio of voltage V and current I (i.e., V/I) and is given by:
ρ =soil resistivity in Ω-m
s=horizontal distance between two successive spikes(electrodes) in m
b=depth of burial in meter.
For measurements in the field, earth based on the above principle are available. They are calibrated to indicate Rg i.e V/I directly. A number of readings with different values of spacing and the test set-up in different directions are taken. The average value of soil resistivity ρ so obtained should be used in the design.
If the measurements are to be carried out using the mains supply, an isolating transformer should be connected between the mains supply and test set-up so that residual ground currents do not affect the results.