ICSE Class 10 Physics Chapter 09 Household Circuits

Chapter 9: Household Circuits

Syllabus

Household circuits - main circuit; switches, fuses, earthing, safety precautions, three pin plugs, colour coding of wires.

Scope of syllabus: House wiring (ring system), power distribution; main circuit (3 wires - live, neutral, earth) with fuse/MCB, main switch and its advantages, circuit diagram, two-way switch, staircase wiring, need for earthing, fuse, 3-pin plug and socket, conventional location of live, neutral and earth points in 3-pin plugs and sockets; safety precautions, colour coding of wires.

A. Transmission Of Power And House Wiring

9.1 Transmission Of Power From The Generating Station To The Consumer

Electric power is generated at the power generating stations which are usually located very far from the areas where it is consumed. At the generating station, the electric power is generated at 11 kV because generation at voltage higher than 11 kV causes insulation difficulties, while generation at voltage lower than 11 kV involves a very high current. Since it is not possible to step up or step down the d.c. voltage, the voltage generated is alternating of frequency 50 Hz (i.e., its polarity at the terminals changes 100 times a second, 50 times + and 50 times -).

The power from the generating station is transmitted to the consumer over the long distances not at 11 kV, but at a voltage higher than 11 kV so as to reduce the loss of energy in form of heat in the line wires used for transmission.

From relation \(P = VI\), for a given power \(P\), current \(I = P/V\) i.e., higher the voltage, lower is the current. Thus by supplying a given electric power at a high voltage, the current becomes low and therefore the loss of energy due to heating (\(= I^2Rt\)) in the line wires becomes less. Thus, the alternating voltage generated is first stepped up from 11 kV to 132 kV at the generating station (or called the grid sub-station) using the step up transformer. It is then transmitted to the main sub-station. At the main sub-station, the voltage is stepped down from 132 kV to 33 kV using the step down transformer and is then transmitted to the heavy industries and intermediate sub-station. At the intermediate sub-station, the voltage is again stepped down from 33 kV to 11 kV using the step down transformer and from here it is transmitted to light industries and city sub-station. At the city sub-station, it is further stepped down from 11 kV to 220 V using the step down transformer to supply it to the domestic consumers. The transmission of electric power from the generating station to the consumer is shown in Fig. 9.1.

Fig. 9.2 shows the simple block diagram of the transmission of electric power from the generating station to the consumer.

Fig. 9.2 Block diagram of transmission of electricity from power station to the consumer

9.2 Power Distribution To A House

To supply the electric power to a house from the city sub-station, either the overhead wires or a cable on poles or an underground cable is used. The cable has three wires: (1) live (or phase) wire (L), (2) neutral wire (N), and (3) earth wire (E). The neutral and the earth wires are connected together at the local sub-station so that the neutral and earth wires are at the same potential (i.e., at 0 V). The live wire, also called the phase wire, carries current from the source to the distribution board, while the neutral wire is for the return of current to the source.

Before connecting the cable from pole to the meter in a house, first a fuse of high rating (= 50 A) is connected in the live wire at the pole (or just before the meter). This fuse is called the company fuse (or pole fuse). Only the persons of the electric supply company are authorised to handle it. The rating of the fuse depends on the load for which the connection is taken from the company. After the company fuse, the cable is connected to a kWh meter. The kWh meter is usually mounted on the front (or outside) wall of the house. From the meter, connections are made to a main switch (or earth leakage circuit breaker ELCB) and to a main fuse (or miniature circuit breaker MCB). The main switch (or ELCB) is connected in both the live and neutral wires while the main fuse (or MCB) is connected only in the live wire.

The main switch is a double pole switch. It has an iron covering. The covering is earthed (i.e., connected to the earth wire E). The advantage of using the main switch is that it breaks the connections of the live as well as the neutral wires simultaneously from the main supply. The earth wire from the meter is locally earthed (in the compound of the house). From the distribution board, the wires go to the different parts of the house. Now a days a consumer unit is connected after the kWh meter. This unit contains a double pole switch or an earth leakage circuit breaker (ELCB) and MCB for each circuit in the house.

Fig. 9.3 shows the connections from the electric pole to the distribution box in a house.

9.3 House Wiring (Ring System)

In a house, the wiring is commonly done by the ring system. In this system, we have a separate ring for each portion of the house. In a portion, the wires starting from the distribution box run around all the rooms of that portion and then come back to the distribution box, thus forming a ring. The distribution box contains a MCB of rating about 30 A. Fig. 9.4 shows a ring system of wiring connecting a lamp, socket with switch and fan with regulator.

A separate connection is taken for each appliance from the live wire of the ring. One terminal of the appliance is connected to the live wire through a separate fuse and a separate switch, while the other terminal of the appliance is directly connected to the neutral wire. The earth terminal or metal covering of the appliance is connected to the earth wire. For each appliance, the wires used for connection should be of proper current carrying capacity.

Advantages Of The Ring System

The ring system has the following four advantages:

(1) In the ring system, the current from mains can reach to an individual appliance through two separate paths. Thus each appliance gets connected to the mains effectively through a thick wire. Therefore, the wire required for main ring is of a lower current carrying capacity than that which would be required for a direct connection to the mains. This considerably reduces the cost of wiring.

(2) Each appliance has a separate fuse. Therefore if due to some fault, the fuse of one appliance burns, it does not affect the operation of other appliances.

(3) In this system, all the plugs and sockets used can be of same size, but each socket should have its own fuse of rating suitable for the appliances to be connected with it.

(4) While installing a new appliance in a room, a new line up to the distribution box is not required, but it can be directly connected to the ring circuit in that room. The care is taken that the total current drawn from the mains in the ring circuit does not exceed the rating of the main fuse (viz. 30 A).

Connection Of All Appliances (bulb, fan, socket, etc.) With The Mains

We note that all the electrical appliances (say, bulbs, fans, sockets, etc.) are connected in parallel with the mains. In the live wire before each appliance there is a separate switch and a separate fuse connected in series.

Advantages Of Connecting The Appliances In Parallel

It has the following two main advantages:

(1) Each appliance gets connected to 220 V supply (= its voltage rating) for its normal working.

(2) Each appliance operates independently without being affected whether the other appliances are operated or not.

Disadvantages Of Connecting The Appliances In Series

Appliances are not connected in series to the mains for the following three reasons.

(1) The voltage of the source gets divided in all the appliances connected in series, in ratio of their resistances, so each appliance does not operate at its rated voltage.

(2) On connecting one more appliance in the same circuit, the resistance of the circuit will increase. Hence it will reduce the current in the circuit, so each appliance will get less power.

(3) All appliances connected in series operate simultaneously. None of the appliance can be operated independently. If one appliance is switched off (or not operated), no other appliance connected with it in series will then operate.

Teacher's Note

When you turn on a light switch at home, you are controlling parallel circuits - each light can be turned on or off independently because they are connected in parallel to the mains supply.

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