How the current flows in the soil?
When fault occurs in the system the live phase gets connected to the earth point. The current starts flowing through the earth electrode to surrounding soil inside the earth pit. The earth fault current flows in a series of concentric shell, as given below.
The concentric shells near to electrode has lesser area and hence it has larger resistance. The each successive shell has more area and thus has lower resistance for earth fault current. The concentric shell at the large distance from the earth electrode carry the maximum current and the current density is minimum because of the larger area.
What happens if the soil resistivity is high?
The high resistivity of the soil impede the flow of current and the large voltage develops across the earth electrode and soil. This will result into higher voltage ( step or touch potential) on the grounding system. The higher voltage developed in the grounding system due to higher soil resistance may cause electric shock to person working near the earth pit.
The soil testing must get done before installing any type of grounding system.
What is soil resistivity?
The soil resistivity is the measurement of the soil resistance of the sampled soil. The resistivity of the soil depends on many factors like depth of the soil,moisture,chemical content and temperature.Soil resistivity values can range from 500 Ω cm with large amounts of electrolytes to over 1 million Ω cm in sandy dry soil.
Why is soil resistivity important?
Before site selection for grounding system, the soil resistivity must be checked because soil is the basic building block of grounding system. If the soil resistivity is high or it is rocky area the earth fault current will be not able to quickly pass in the ground and may cause damage to equipment. The risk of electrocution caused by rise in touch potential is also there if soil resistivity is high. If soil resistivity is high, additional measures can be taken to reduce the resistivity to ensure good grounding system.
On what factors does soil resistivity depend?
The soil resistivity depends on the following factors.
Moisture tends to increase soil conductivity. The greater moisture content in the soil is desired for grounding system.
Temperature is especially important in areas with colder climates. The resistivity of the soil increase if the temperature fall below the freezing temperature. At below freezing point even have more detrimental effects on the grounding system because the soil or cement based fill material can solidify.
Soil types with high organic content are hygroscopic in nature and it can hold the moisture for longer period of time, and it is considered as a good conductor.Contrary to this, sandy soil lose moisture faster and its resistivity is high.Whereas, sandy soils will lose moisture faster and have lower amounts of electrolytes. The rocky soil is the worst soil for grounding system as it does not have the moisture at all.
Depth and type of surface soil may change. As we go down the type of earth may change from organic mix to clay or rocky soil. The variation in the soil type with depth impact soil resistivity value.
What to do if soil resistivity is high?
The soil resistivity can be improved in areas with high resistivity soil , including rocky and sandy soil by adding conductive enhancement materials in the soil to overcome the problem of high soil resistivity.These products can be referred to as “grounding” or “earthing” enhancement materials.