Page 931 - Basic Electrical Engineering
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point at the substation and runs along the supply lines. This wire is earthed
along its run once in every 1.6 km.
13.9.5 Earthing
All metal parts of electrical appliances are always connected to the earth
through an earth wire. If the earth wire is not provided and any part of the
current-carrying live wire touches a metal frame, the frame will acquire the
same voltage as the live wire. Any person touching the body of the appliance
will get a severe electric shock. If a low-resistance earth wire is provided, in
case of any fault in the circuit, a large amount of current will flow through the
circuit. This will cause the fuse provided in the live wire to blow and protect
the circuit as well as the person touching the electrical appliance, gadget or
equipment, as the case may be.
As shown in Fig. 13.12, the live and the neutral wires are used to supply
power to an electric device, say a heater. The metal case of the heater is
connected to the earth wire of a three-core cable supplying power to the
heater. The earth wire from the distribution board is connected to the main
earth terminal of the building as has been shown. This earth terminal in turn
gets connected to the earth electrode at the substation through the general
mass of the earth (earth is the conducting media). Thus, in case of any electric
fault, if the live wire touches the metal case, it gets connected to the earth
electrode at the substation. At the substation, the neutral point is earthed.
Method of earthing
Earthing means connecting the earth terminal solidly and securely on the
ground through an electrode. Conductivity of earth depends on the moisture
content of the soil and its chemical composition. The soil near the earth
electrode should have low resistivity (i.e., high conductivity). Before placing
the earth electrode, the soil around is made highly conducting by adding
some agents like common salt, i.e., NaCl, calcium chloride, i.e., CaCl,
sodium carbonate, i.e., Na Co , soft coke, etc.
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