Measurement of Earth Resistance

There is a great importance of earthing plate resistance and its surrounding area. The earth electrode resistance is defined as the area surrounded by earthing electrodes.

• The E is main earthing electrode and A is auxiliary earth electrode as shown in the Figure A. 
• The positions of these electrodes are decided such that resistance area does not overlap. 
• There is one auxiliary electrode placed between electrode A and electrode E. 
• The current passes through transformer secondary winding through path E to A and there is a voltage drop between point E and B. 
• Therefore the earth resistance is equal to

R_E = Voltage drop between electrodes E and B / Current passes through 

         electrode E and A

      = V / I

• The position of the electrode movement is done from B to B1 and B to B2 in order to prevent overlapping of resistance area. 
• The value of earth resistance should be same for different positions of earthing electrode B ( position B1 and position B2 ) and it is called as earth resistance.
• When the earth resistance should not be same for different position of earth electrode of B, the position of electrode A is kept away from electrode E. 
• This process must be continuing until test reading should not be same.

Earth Tester

• This test is also performed by earth tester. 
• It is a type of Megger which provides DC supply to earth and alternating supply  to measuring instrument. 
• It consists of four terminals P1 , C1 , P2 and C2. The terminals P1 and C1 is connected together and it is connected to earthing electrode whereas the other terminal P2 and C2 is connected to auxiliary electrodes B and A. 
• The instrument indicates earth resistance when handle is rotated. 
• The test electrode in the form of rod, plate or plate. 
• The current electrode shall be placed at least 30 m away from the test electrode and potential electrode shall be placed midway between two electrodes. 
• The test is repeated by keeping distance between test electrode and current electrode up to 50 m and potential electrode is placed between them. 
• The diameter of mild steel auxiliary electrode usually consists of 12.5 mm and it is placed 1 meter inside the ground during testing.

Why Earth Tester is used for earthing measurement?

• There are stray current flows in the soil and it may produce error in the measurement. 
• If the testing current is same as that of stray current, it is difficult to eliminate error therefore it is better to use earth tester which is hand driven generator.
• This earth tester generates direct current. 
• It consists of rotary current reverse and synchronous rectifier mounted on the generator shaft therefore it supplies alternating current to test circuit and resulting potential is rectified for measurement by direct reading Moving coil ohm meter. 
• The wandering of instrument pointer indicates presence of stray current in the soil but it will disappear if the generator speed increase or decrease.

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