Page 141 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Electrodes shall, as far as practicable, be embedded in earth below the permanent moisture level.
The length of the rod and pipe electrodes shall not be less than 2.5 m.
Except where rock is encountered, pipes and rods shall be driven to a depth of atleast 2.5 m. The length of the
electrodes shall be atleast 2.5 m, and the inclination not more than 300 from the vertical.
Plate electrodes (Fig 5): Plate electrodes,when made of galvanised iron or steel,shall not be less than 6.3 mm
in thickness. Plate electrodes of copper shall be not less than 3.15 mm in thickness. Plate electrodes shall be of
a size, at least 60 cm by 60 cm.

Fig 5

Plate electrodes shall be buried such that the top edge is at a depth not less than 1.5 m from the surface of the
ground.
Where the resistance of one plate electrode is higher than the required value, two or more plates shall be used in
parallel. In such a case, the two plates shall be separated from each other by not less than 8.0 m.
Plates shall preferably be set vertically.
Plate electrodes is recommended in generating stations and substations.
If necessary, plate electrodes shall have a galvanized iron water pipe buried vertically and adjacent to the
electrode. One end of the pipe shall be atleast 5 cm above the surface of the ground, and it need not be more
than 10 cm. The internal diameter of the pipe shall be atleast 5 cm and need not be more than 10 cm. The length
of pipe, if under the earth’s surface, shall be such that it should be able to reach the centre of the plate. In no case,
however, shall it be more than the depth of the bottom edge of the plate.
Methods of reducing the resistance of an earth electrode to an acceptable value:

The earth electrode resistance is found higher in rocky or sandy areas where moisture is very low.
The following methods are suggested to bring down the earth electrode resistance to an acceptable value.
1 After installing the rod or pipe or plate in earth, the earth pit ( the area surrounding the rod / pipe / plate) should
be treated with layers of coke and common salt to get a lower value of earth resistance.

2 Pouring water in the earth pit at repeated intervals lowers the earth electrode resistance.

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CITS : Power - Electrician & Wireman - Lesson 20-25 CITS : Power - Electrician & Wireman - Lesson 20-25