Domestic Electric Circuits
It is provided outside the building under a covered place (varandah or poarch). It contains the meter (energy–meter) and the main switch.
From the street electric pole a thick rubber insulated cord reaches the main board. It contains two thick copper or aluminium wires, one covered with red and the other covered with black (or brown) plastic covering.
They from the live line wire (L) and neutral line wire (N) respectively. Live line has a potential of 220V whereas the neutral wire has zero potential (with respect to the earth). They enter the main board and are connected to the meter.
Wiring ahead is provided by the house owner himself. These wires are also red and black plastic covered. From the meter the wires enter the main switch. In the main switch, a fuse F is provided in the live wire.
A third wire is a thick bare wire of copper, called earth wire E. It is connected to an earth connection which consists of a thick copper plate P buried deep inside the moist earth.
Inside the building
It is a well known fact that inside the house, connections to all the devices are made in parallel, each having independent switch and fuse (if necessary). Thus, whenever some fault occurs in circuit of one particular device in one room, devices in other rooms do not suffer.
As shown in fig. connection to low power devices like bulb B and fan F are made with lines N and L only, putting switch in line L. For devices of more power and with whom the body remains in contact (like electric press or refrigerator), we use connections through a 3 pin plug–socket (show in fig. b) system.
A three pin plug P and three pin socket S are shown in diagram. The three points of the socket are connected to the three lines as shown in diagram. (fig. b) A fuse F is also introduced to avoid damage to the appliance.
The three pin plug uses a three wire cord which has three plastic wires inside a single rubber insulating cover. The wire are coloured red, black and green to serve an extension of live, neutral and earth wires for the appliance. The three wires are connected to the three holes in the socket as shown in diagram. When the plug is inserted in the socket, proper lines become connected to the appliance.
Function of earth wire
Due to long use some covered wires inside the appliance may become bare and make contact with metallic body of the appliance. In such a case the appliance gives a shock if not earthed. The earth wire keeps the potential of the appliance zero and shock is avoided.