What is an Earth electrode?
The conductor which is buried in the ground to make
connection to the mass of the earth is called earth electrode. As per Indian
Standards the sprinkler pipe, pipes conducting gas, water or inflammable
liquid, conduit, metallic enclosures of cables, conductor for lightening
arrester should never be used for earthing.
Types of earth electrodes are as follows.
(1) Rod electrode
(2) Pipe electrode
(3) Plate electrode
(4) Strip electrode
(5) Coil electrode
(6) Concrete encased rod or wire
1. Rod electrode :
This electrode is in the form of a rod. It is buried
vertically in the ground. Its diameter should be 16 mm minimum if it is made of
iron or steel and should be 12.5 mm minimum if it is made of copper. There
should not be insulating coating of enamel or colour on its surface. It is buried at the depth of
1.25 m minimum under the ground.
2. Pipe electrode :
Pipe electrode comprises of galvanized pipe. Its internal
diameter should not be less then 39 mm and its length should be minimum 2.5 m.
Holes of 12 mm diameter are drilled at the spacing of 150 mm. These holes are
drilled alternately in the two planes.
This electrode is buried under the ground vertically at the
minimum depth of 1.25 m. There should not be a coating of colour or enamel over
the pipe.
3. Plate electrode :
This electrode is in the form of plate. This plate is made
of copper or galvanized iron. Its size is 600 mm x 600 mm for both the copper
and galvanized iron plate but the thickness is 3 mm when it is made of copper
and 6 mm when it is made of galvanized iron. It is buried under the ground
vertically at the depth of 1.5 m minimum. Earthing lead is joined with the
plate with the help of bolt and nut. The bolt and nut are made of copper if the
plate is of copper and made of galvanized iron if the plate is made of galvanized
iron. Earthing lead is brought out through the galvanized pipe.
4. Strip electrode :
This electrode is made up of bare copper strips. Its size is
25 mm x 16 mm minimum. Its cross sectional area should be minimum 3 mm2.
Its length should be such that it gives sufficient
low earth resistance. It is buried in the ground in the horizontal trench dug
up at the depth of 2.5 m.
5. Coil electrode :
In this type of electrode a coil of GI wire of 8 SWG size is
made. The diameter of coil is kept 50 mm. 115 turns are closely wound so that
the length of the coil becomes 450 mm. It is buried vertically under the ground
at the depth of 1.5 m minimum.
6. Concrete encased rods or wires :
Resistivity of concrete becomes approximately equal to 3000 ohm
cm at 20° C under the ground which is less than the average earth resistivity.
So the place at which the resistivity of the earth is more, the rod or wire is
encased in concrete and buried in the ground instead of using the bare rod or
coil. So the resistivity decreases. So the reinforcing rods of foundations of
the buildings and towers are widely used as the earth electrodes. For this
suitable connection should be made at the rod of foundation and brought out.
What is earthing lead main-earthing conductor, sub main
earthing conductor and earth continuity conductor ?
1. Earthing lead-Main earthing conductor :
Wire connecting the earth electrode to the main switch board
or distribution, transmission or service line is called the earthing lead or
main earthing conductor.
Cross sectional area or the earthing lead should not be less
than half of the main conductor. Its minimum size should be equivalent to
copper conductor of size 8 SWG.
2. Sub main earthing conductor :
The earth conductor which connects the main switch board to
the distribution board i.e. sub main board is called the sub main earthing
conductor.
3. Earth continuity conductor :
The earth wire which is run from the distribution board to
different switch boards is called the earth continuity conductor (ECC). Its
size should not be less than 14 SWG (2.894 mm2).