Soil Resistivity test is performed for accurate design calculation and no of electrodes required for electrical system. Soil resistivity testing results are the key factor for designing complex grounding system. All types of soils conduct electrical current, some soils having good electrical conductivity i.e. poor resistivity while the majority has poor electrical conductivity i.e. high resistivity. The resistivity of soils varies widely throughout the world and changes dramatically within small areas even at the same point in different direction i.e. anisotropically. Soil resistivity is primarily influenced by the type of soil (clay, shale, etc.) and its moisture content, the amount of electrolytes (mineral and dissolved salts) and very nominally by temperature. 

The resistance of an earth electrode is related to the resistivity of the soil in which it is placed and driven, and thus soil resistivity calculations and measurements is a crucial aspect when designing earthing installations.

Why Testing the Soil Resistivity Important?

A major aim of grounding system is to set up a shared reference potential for the building structure, power supply system, electrical conduits, plant steelwork, and the instrumentation system. To achieve this objective, a suitable low resistance connection to earth is desirable. However, this is often difficult to achieve and depends on a number of factors:

Soil resistivity

Stratification

Size and type of electrode used

Deepness to which the electrode is covered

Dampness and chemical composition of the soil

The purposes of soil resistivity testing is:
To obtain a set of measurements which may be interpreted to yield an equivalent model for the electrical performance of the earth, as seen by the particular earthing system.
Geophysical surveys are performed using these values as an assistance in finding depth to bedrock, core locations and other geological phenomena.
The degree of corrosion in underground pipelines is determined. A drop in resistivity is proportional to an indent in corrosion in subversive pipelines.
Soil resistivity influences the plan of an earthing system absolutely and is the major factor that decides the resistance to earth of a grounding system. Thus before designing and installing a new grounding system, the determined location should be tested to find out the soil’s resistivity. Read more