We know that the sandy earth cannot absorb water. Water immediately Nows down to earth through sand as there is comparatively more space between the sand particles.
Such soil dries up also quickly. In short, moisture cannot retain and the soil dries quickly.
Due to this, the earth's resistance increases. So when an earth fault occurs, the fault current cannot flow to the ground. When fault current passes through such soil, the sand becomes very hot due to the heating of electrodes due to more resistance.
The sand particles around the electrode are converted into quartz particles and a layer of glass is formed around the earth electrode which increases the resistance of the electrode so simple methods of earthing like plate earthing and pipe earthing fail in such situations.
The question of maintenance also arises when such type of earthing is employed. The protective multiple earthing method is employed where the value of the short circuit current is large. In this method, the neutral wire of the supply is connected to the equipment and the equipment is earthed at two or more points. Due to this, all the things are connected in parallel so the net earth resistance becomes less.
One difficulty in this is that the circulating current flows between the electrodes when the earth resistances of electrodes are not equal so there is a disturbance to the nearby communication lines.
Rod-type earthing is also employed in sandy beds in addition to the above method. In this method, a copper rod of 12.5 mm diameter or rod of galvanized iron of 16 mm diameter and of 2.5 m length is pierced vertically in the ground. Sometimes more than one rod is used and connected in parallel
Advantages :
(1) Net earth resistance decreases as more than one rod are connected in parallel.
(2) As more than one rod is used no difficulty arises when the connection of one rod becomes open
3) Fault current flows through the neutral wire when the earth fault occurs so the protective system operates quickly.
(4) Fault current is distributed in different electrodes so the value of fault current flowing through each electrode is reduced.
Disadvantages
(1) Circulating current flows between the electrodes when the resistances of electrodes are not equal. This causes disturbance to the nearby communication lines.
(2) If the neutral wire is disconnected from the neutral point, the potential on the equipment increases so there is the possibility of electric shock.
Method of earthing in rocky bed :
Rocks formed of lava are very hard. There is a wastage of manpower in digging holes for the pipe for earthing in such a situation.
The cost of labor also increases. Strip earthing is found effective and suitable in such a situation. In this method, copper strips of 25 mm x 1.6 mm cross-section or strips of galvanized iron of 25 mm x 5 mm cross-section are employed.
Rod can also be used in place of strips. In this the copper rods of minimum 3 mm cross-sectional area or galvanized iron rods having a minimum cross-sectional area of 6 mm' can also be used, Numbers of such rods or strips are embedded in the rock at the depth of 0.5 m. The upper ends of the strips are joined in parallel by the conducting wire. The value of the earth resistance decreases as the electrodes are connected in parallel and the fault current is distributed. Water gets stored in the cracks of the rocks for a longer period, so the low value of the earth resistance is maintained.
Method of earthing of extra-high voltage system :
Earthing of extra high tension lines and equipment working on such voltage requires special attention. When an earth fault occurs in such equipment the spreading of electric potential is radial as shown in the figure. Hence sufficient area around the equipment gets charged. So even if a person is away from the equipment he will receive an electric shock.
Method of earthing of underground cable :
The three-phase wires and neutral are kept very near to each other in an underground cable. There is insulation between these. So capacitors are formed between conductors.
The effect of capacitance is less if the length of the cable is not more.
The effect of capacitance becomes considerable when the length increases. So the electric potential is developed on the cable sheath. For this, the cable sheath is earthed at some intervals.
Pipe earthing is employed for these points and the plate earthing is employed for the earthing at the ends. Many times the sheath is connected to the neutral wire, While joining the two cables, the joining of the sheath also should be made by welding or soldering of bolts and nuts so that the continuity of earthing is maintained.
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