25. Earthing

Earth Resistance 

Earth Resistance of an earth is sum of three separate resistances

a) The resistance of the conductor joining earth electrode to the installation.

b) The contact resistance between the surface of the earth electrode and the soil 

c) Resistance of the body of soil surrounding the earth electrodes.

The first two resistances are negligible compare with third.

So resistance of the earth is primarily depends upon nature of soil and not by electrode itself.

d) Material used for a standard electrode system should be corrosion resistant.Under ordinary soil conditions, use of galvanized iron or mild steel electrode is recommended.

If soil corrosion is likely to be excessive, it is preferable to use either copper or copper clad electrode.

The electrode shall be free from paint,enamel or greese.

e) Earth tester normally used for measurement of earth resistivity comprises of the current source and meters in a single instrument and directly read  the resistance value.

Resistivity of soil when checked by megger.          

then resistivity of soil P= 2πSR= 2*22/7*S*R  

P = Resistivity of soil in ohm meters

S= Distance between successive electrodes in meters

R= Megger readings in ohms.

Regular Checks of Earth

1) Block earths and their connection shall be examined at intervals of not more than 01 month by JE/Signal and not more than 03 months by SSE/Signal incharge.

2) Block earths shall be tested for Resistance at intervals of not more than 12 months by Sectional JE/SSE.If the Resistance is more than 10 ohms, action shall be taken to reduce Resistance by providing additional earths in parallel.

(1) Rocky soil has high resistivity 

(2) Resistivity of dry ground is more compare to moisture.

(3) Resistivity of cold ground is more compare to warm ground.

(4) Igneous rocks are more resistive compare to minerals.

(5) If we add salt content,Resistivity of soil reduce.

Resistivity of wet, moist and dry soil is 10,100,1000 ohm/meter.

All choose the soil with less resistivity.

Earth Resistance of the rod

R= 0.75× o/L if      25 <L/d <100

    = o/L    if       100 <L/d <600

     = 1.2 × o/L if      600<L/d <3000

o = Resistivity of earth in Ohm.m

L= Length of the electrode in m

d= Diameter of the electrode in m

While Earth testing distance between Main earth and P2 electrode must be 50 to 100 ft. Similarly distance between P2 and C2 must be 50 to 100 ft.

Depth of additional two rods must be 30 to 50 Cm.

Top of the earth should be 30 cm above ground.

The electrode shall be directly driven upto depth of at least 2.5 m.If a rock is encountered at a depth less than 2.5 m,the electrode can be inclined by upto 30 degree from vertical.

Minimum seperation between two earths - 04 m as per telecom manual.

Minimum Clearance of equipment earth from earth of AT/ transformer should be 20 m.

Artificial treatment of Soil to reduce Resistivity-

A Earth pit of  dia is 600 mm and depth is 2.5 m is to be formed. This pit shall be filled alternatively with layers of common salt and charcoal. Each layer of about 2.5 cm thick,upto height of about 20 cm from ground level.

Qty of salt = 1/200 of soil removed from pit.

For future improvement Calcium chloride and Sodium carbonate can be added