For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.
It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.
It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.
P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.
All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in different directions.
Soil resistivity =2πLR.
R= Value of Earth resistance in ohm.
Distance between the spikes in cm.
π = 3.14
P = Earth resistivity ohm-cm.
Earth resistance value is directly proportional to Soil resistivity value
Measurement of Earth Resistance (Three point method):
In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test.
Terminals P2 & C2 are connected to the two separate spikes driven in earth. These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes.
If we rotate generator handle with specific speed we get directly earth resistance on scale.
Spike length in the earth should not be more than 1/20th distance between two spikes.
Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.
Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e. 0.62 x 60 ft = 37 ft.
Four Point Method:
In this method 4 spikes are driven in earth in same line at the equal distance. Outer two spikes are connected to C1 & C2 terminals of earth tester. Similarly inner two spikes are connected to P1 & P2 terminals. Now if we rotate generator handle with specific speed, we get earth resistance value of that place.
In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.C.
GI Earthing Vs Copper Earthing:
As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).
Where r = Resistivity of Soil Ohm-meter.
A=Area of Earthing Plate m3.
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d).
Where L= Length of Pipe/Rod in cm
d=Diameter of Pipe/Rod in cm.
The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with earth.
The material resistivity is not considered important role in earth resistivity.
Any material of given dimensions would offer the same resistance to earth. Except the sizing and number of the earthing conductor or the protective conductor.
Pipe Earthing Vs Plate Earthing:
Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick. soil resistivity of 100 ohm-m,
The resistance of Plate electrode to earth (R)=( r/A)X under root(π/A) = (100/2.88)X(3.14/2.88)=36.27 ohm
Now, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long. soil resistivity of 100 Ohm-m,
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d) = (100X100/2X3.14X300) X loge (4X300/5) =29.09 Ohm.
From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode.
As per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of equal surface area.
Length of Pipe Electrode and Earthing Pit:
The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but after that soil resistivity is mostly uniform.
After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number of rods in parallel are to be preferred to a single long rod.
Amount of Salt and Charcoal (more than 8Kg) :
To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.
Some substance like Salt/Charcoal is highly conductive in water solution but the additive substance would reduce the resistivity of the soil, only when it is dissolved in the moisture in the soil after that additional quantity does not serve the Purpose.
5% moisture in Salt reduces earth resistivity rapidly and further increase in salt content will give a very little decrease in soil resistivity.
The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, the moisture content at 10 percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of this (i.e.) 5% of 144kg, that is, about 7.2kg.
Amount of Water Purring:
Moisture content is one of the controlling factors of earth resistivity.
Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidly with the decrease in moisture content.
If the moisture content is already above 20% there is no point in adding quantity of water into the earth pit, except perhaps wasting an important and scarce national resource like water.
Length Vs Diameter of Earth Electrode:
Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth electrode diameter with the object in mind of increasing surface area in contact with the soil.
The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drive than smaller diameter electrode.
The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than has its diameter.
Maximum allowable Earth resistance:
Major power station= 0.5 Ohm.
Major Sub-stations= 1.0 Ohm
Minor Sub-station = 2 Ohm
Neutral Bushing. =2 Ohm
Service connection = 4 Ohm
Medium Voltage Network =2 Ohm
L.T.Lightening Arrestor= 4 Ohm
L.T.Pole= 5 Ohm
H.T.Pole =10 Ohm
Tower =20-30 Ohm
Treatments to for minimizing Earth resistance:
Remove Oxidation on joints and joints should be tightened.
Poured sufficient water in earth electrode.
Used bigger size of Earth Electrode.
Electrodes should be connected in parallel.
Earth pit of more depth & width- breadth should be made.
