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Domestic Electric Circuits

  1. Draw a schematic labeled diagram of a domestic wiring circuit which includes
    1. a main fuse
    2. a power meter
    3. one light and
    4. a power plug
  2. Why is it necessary to connect an earth wire to electric appliances having metallic covers?

  1. Electric cable or overhead wire: The electric power to a house is supplied either through overheads wires or through underground cables. The cable has three separate insulated wires
    1. live wire (or phase or positive)
    2. neutral wire (or negative) and
    3. earth wire.
The live wire has usually red insulation cover, neutral wire has black insulation cover and the earth wire has green insulation cover.
  • As per the new International Convention, live wire has brown coloured insulation cover whereas neutral and earth wires have light blue and green (or yellow) insulation covers. The potential difference between the live and neutral wire is 220V. the neutral and the earth wires as connected together at the local sub – station so that both of them are at zero potential.
  • Pole fuse: Before the electric lines enter a house, the agency supplying electricity, places a fuse (Called the pole fuse or company fuse) in the live wire. The current rating of this fuse depends upon the load sanctioned by the agency to that house.
  • Energy meter or k Wh meter: After the company fuse, the cable is connected to the energy meter which records the electricity consumption of the house in kilowatt hour (kWh). The earth wire from the meter is locally earthed in the compound of the house.
  • Main fuse: The live wire coming out from the output terminals of kWh meter has another fuse in it which is called the main fuse.
  • Main switch: Beyond the main fuse, the live and the neutral wires are connected to the main switch. It is a double pole switch and has an iron covering. The covering of the main switch is also locally earthed. The switch can cut off the live and the neutral wires from the household circuit by operating a single lever.
  • Distribution board: Power lines coming from the main switch are taken to the distribution board. It is from the distribution board that the wires go to the different parts of the house through fuses in the board.
  • House – wiring: There are two systems of wiring by which the power is distributed to a house: (A) the tree system (b) The ring system. We shall be describing the tree system of wiring through the ring system is rapidly replacing the tree system.

Electric Fuse

A fuse consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it. This is connected to the live wire. The other part of the fuse is a removable fuse grip G, which is also made of porcelain. The fuse grip has a fuse wire fixed in it. When the fuse grip is inserted in the fuse holder as shown in the figure, the circuit of our domestic wiring is completed. So under normal circumstances, the fuse wire is intact and electric current is available in our wiring.
When a short circuit takes place, or when overloading occurs, the excessive current heats up the fuse wire. Since the melting point of the fuse wire is much lower than the copper wires, the fuse wire melts and breaks the circuit as shown in the figure. When the fuse wire melts, electricity supply is automatically cut off before any damage can be done to the rest of the wiring.
It is important to know certain necessary facts about the fuse wire and its use in electrical circuits. First of all, we should know why we use a thin wire as a fuse and not a thick wire. We use a thin wire in a fuse because it has a much greater resistance than the rest of the connecting wires. Due to its high resistance, the heating effect of current will be much more in the fuse wire then anywhere else. This will melt the fuse wire whereas other wiring will remain safe. We should not use a thick wire as a fuse wire because it will have a low resistance and hence it will not get heated to its melting point easily. The fuse wire is usually made from a tin-lead alloy having low melting point so that it may melt easily. A copper wire cannot be used as a fuse wire because it has a high melting point due to which it will not melt easily.
The fuse wire must have proper thickness, which depends on the maximum current the household wiring can safely carry. The thickness of the fuse wire should be such that it is able to withstand only a little more current than drawn by the household circuit. Fuse wires are rated as 10A, 15A, 20A and so on. It is clear that a "10 ampere" fuse wire will be thicker than a "5 ampere" fuse wire. The fuse in the lighting and fan circuits of a small house is of 5 amperes rating, which means that the fuse wire will melt if the current exceeds 5 amperes value. The fuse used in the power circuit of a small house for operating an electric iron, immersion heater, geyser, toaster, etc., having a power of 1000 watts or more is of 15A capacity. A blown fuse should be replaced only after the cause of excessive current has been found and removed.
So far we have discussed fuses, which are placed on the main switch-board in our houses to protect the entire wiring of the house. Fuses are also used to protect the domestic electrical appliances from damage, which may be caused due to excessive current flow through them. Costly electrical appliances like TV sets and refrigerators have their own fuses, which protect them against damage by too much current. The fuse used for each electrical appliance should be slightly larger than the normal current drawn by it. For example, a TV set, which normally takes less than 1 ampere of current should be fused at 2 amperes and not at 10 amperes.
Diagram of the fuse used in electrical appliances

The fuse in an electrical appliance is shown in the above figure It consists of a glass tube T having a thin fuse wire sealed inside it. The glass tube has two metal caps at its two ends. The two ends of the fuse wire are connected to these metal caps. The metal caps are used for connecting the fuse in the circuit in a suitably made bracket. In a circuit diagram, the electric fuse is represented by the symbol shown in the figure.


Many electric appliances of daily use like electric press, heater, toaster, refrigerator, table fan etc. have a metallic body. If the insulation of any of these appliances melts and makes contact with the metallic casing, the person touching it is likely to receive a severe electric shock. This is due to the reason that the metallic casing will be at the same potential as the applied one. Obviously, the electric current will flow through the body of the person who touches the appliance.

To avoid such serious accidents, the metal casing of the electric appliances is earthed. Since the earth does not offer any resistance, the current flows to the earth through the earth wire instead of flowing through the body of the person. Moreso, due to very low resistance (almost nil) offered by the earth wire, the current in the circuit rises to a very high value, thereby melting fuse in that circuit off its electric supply.


Note; It should be noted that all the switches are put in the live wire, so that when we switch off an electrical appliances, then its connection with the live wire is cut off and there will be no danger of an electrical shock if we touch the metal case of the electrical appliance.


Why is series arrangement not used for domestic circuits?

Ans: In domestic circuits, series arrangement is not used because of the following reasons
  • The total potential difference available (usually 220 volts) is divided between various appliances in the circuit according to their resistances since the current flowing through all the appliances is the same. Thus, each appliance will not get the required potential difference for it to operate properly.
  • If one of the appliances is out of order, e.g., if a bulb gets fused, all the appliances in the circuit will stop working as the circuit gets broken.
  • All the appliances will work simultaneously whether we want them to work or not, thereby involving a lot of power wastage.
  • IF we switch off any one of the appliances, the circuit is broken and all the appliances will stop working. What are the two main precautions to be taken while using an electric supply?
  • Electric supply should be immediately switched OFF from the main switch in case person touches a live electic wire or if there is a fire in the house. Electric supply should also be switched OFF during natural calamities like earthquakes, floods, etc.

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