Household Circuits Class 10 ICSE — Selina Chapter 9
Household circuits class 10 ICSE physics (Selina Concise, Chapter 9) explains how electricity travels from the generating station to your home and how that supply is kept safe. For the physics behind voltage, current and power, see the Class 10 Physics book hub. This chapter covers transmission voltages, ring wiring, fuses, switches, earthing, and the three-pin plug. Each section below follows the book’s own order, with the formulas and reasoning the board expects.
Download Household Circuits Class 10 ICSE Physics Chapter 9 PDF
Download the Household Circuits Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 16 pages, 11 sections, 51 questions.
What is inside Chapter 9
16 pages · 11 sections · 51 exercise questions
| Section | Title |
|---|---|
| 9.1 | TRANSMISSION OF POWER FROM THE GENERATING STATION TO THE CONSUMER |
| 9.2 | POWER DISTRIBUTION TO A HOUSE |
| 9.3 | HOUSE WIRING (RING SYSTEM) |
| 9.4 | FUSE |
| 9.5 | SWITCHES |
| 9.6 | CIRCUITS WITH DUAL CONTROL SWITCHES (STAIRCASE WIRING) |
| 9.7 | EARTHING (GROUNDING) |
| 9.8 | THREE-PIN PLUG AND SOCKET |
| 9.19 | COLOUR CODING OF WIRES IN A CABLE |
| 9.10 | HIGH TENSION WIRES |
| 9.11 | SAFETY PRECAUTIONS WHILE USING THE ELECTRICITY |
- EXERCISE-9(A): 15 questions
- MULTIPLE CHOICE TYPE: 3 questions
- EXERCISE-9(B): 31 questions
- MULTIPLE CHOICE TYPE: 2 questions
9.1 TRANSMISSION OF POWER FROM THE GENERATING STATION TO THE CONSUMER
Electric power is generated far from where it is consumed. At the generating station, it is produced at 11 kV and 50 Hz alternating current. Generation above 11 kV causes insulation difficulties. Generation below 11 kV forces a very high current. Alternating current is used because direct current cannot be stepped up or down. Its polarity changes 100 times per second.
From P = VI, for a fixed power P, the current is I = P/V. Higher voltage means lower current. The heating loss in line wires is I^2Rt, so lower current wastes far less energy as heat. That is why this ICSE chapter stresses high-voltage transmission.
The voltage is stepped up from 11 kV to 132 kV at the grid sub-station. It is stepped down to 33 kV at the main sub-station for heavy industries. Then 11 kV at the intermediate sub-station for light industries. Finally 220 V at the city sub-station for domestic consumers.
Worked pattern: transmitting 2.2 MW at 11 kV draws I = 2200000/11000 = 200 A. At 220 kV it draws only 2200000/220000 = 10 A. Since heating scales with I^2, the loss falls by (200/10)^2 = 400 times.
9.2 POWER DISTRIBUTION TO A HOUSE
The supply reaches a house through overhead wires or underground cable. The cable has three wires: live (L), neutral (N) and earth (E). Neutral and earth are joined at the local sub-station, so both sit at the same potential (0 V). The live wire carries current from the source to the distribution board. Neutral provides the return path.
Before the meter, a company fuse of about 50 A is connected in the live wire at the pole. Only supply company staff may handle it. Its rating depends on the load for which the connection is taken. The kWh meter is mounted on the outside wall of the house.
From the meter, connections go to a main switch and a main fuse. The main switch is a double pole switch with earthed iron covering. It breaks live and neutral together. The main fuse (or MCB) protects only the live wire. A modern consumer unit holds the double pole switch (or ELCB) plus one MCB per circuit.
9.3 HOUSE WIRING (RING SYSTEM)
In the ring system, a separate ring serves each portion of the house. Wires leave the distribution box, run around all the rooms, and return to the box. The box holds an MCB of about 30 A. Each appliance taps the live wire through its own fuse and switch. Its other terminal connects directly to neutral. Its metal case connects to earth.
The ring system has four advantages. Current reaches each appliance by two paths, so the ring wire needs lower current capacity and costs less. Each appliance has its own fuse, so one blown fuse does not stop the others. All plugs and sockets can be the same size. A new appliance connects straight to the ring, as long as total current stays under 30 A.
Parallel connection gives each appliance the full 220 V and lets each work independently. Series connection divides voltage among appliances, raises resistance, cuts current, and forces all to operate together. So series wiring is never used for house circuits.
Worked pattern: a 220 V, 40 W lamp has resistance R = V^2/P = (220)^2/40 = 1210\ \Omega. Two such lamps in series across 220 V each get 110 V and draw 110^2/1210 = 10 W each, so 20 W total. In parallel, each gets 220 V and draws 40 W, so 80 W total. Parallel delivers four times the power.
9.4 FUSE
A fuse is a safety device that limits current in a circuit. It protects the circuit and its appliances from damage. It works on the heating effect of current. The current rating is the maximum current a fuse permits before it melts. The temperature rise follows \Delta T \propto I^2/r^3. It grows with the square of the current and falls with the cube of the radius. It does not depend on the wire’s length.
Construction: the fuse wire stretches between two terminals on a porcelain holder that fits into a socket. Only the live wire connects to it. Working: when current exceeds the rating, the wire melts and breaks the circuit. The material is a lead-tin alloy with a low melting point of about 250^\circ\mathrm{C}. Copper and aluminium are unsuitable because they melt near 1080^\circ\mathrm{C}.
The current rating (equation 9.2) is total power of appliances in the circuit divided by the voltage of the supply. Worked pattern: a 220 V circuit running a 1.1 kW mixer and a 1.65 kW heater draws I = (1100 + 1650)/220 = 12.5 A. The fuse must be the next standard value above this, so 15 A. For the relation between current, resistance and power, see the previous chapter on current electricity.
Rating levels from the book: 5 A for lighting circuits, 15 A for power circuits, cartridge fuses from 1 A to 13 A. An MCB trips in about 25 ms. Motor-driven appliances draw more at starting than while running. A 1.5 kW water pump takes 10.4 A at start and 6.8 A running, so its fuse must sit above the starting current.
9.5 SWITCHES
A switch is an on-off device for current. It is connected in the live wire. A single pole switch disconnects only the live wire from an appliance. A double pole switch disconnects live and neutral together and is used as the main switch at the distribution board.
| Switch type | What it disconnects | Where it is used |
|---|---|---|
| Single pole | Live wire only | With each appliance |
| Double pole | Live and neutral together | Main switch at the distribution board |
Why the switch must sit in the live wire: with the switch off, the appliance is at zero potential and safe to repair. With the switch off in the neutral wire, the appliance stays connected to the high potential terminal through the live wire. Touching it can give a fatal shock. The switch seems to work, but the danger is hidden.
A switch must never be touched with wet hands. Water forms a conducting layer between the hand and the live wire inside the switch, so current passes through the body.
9.6 CIRCUITS WITH DUAL CONTROL SWITCHES (STAIRCASE WIRING)
A dual control switch is a special double pole switch. Its terminal b connects to terminal a (state ba) or terminal c (state bc). One switch sits at the bottom of a staircase, the other at the top. Either switch can turn the bulb on or off. The bulb is on only when both switches select the same wire.
| S1 position | S2 position | Bulb |
|---|---|---|
| ba | ba | On |
| ba | bc | Off |
| bc | ba | Off |
| bc | bc | On |
Human example: a person going up switches on at the bottom with S1, changing ba to bc. Both switches now at bc, the bulb glows. At the top, they switch off with S2, changing bc to ba. The positions now differ, so the circuit breaks and the bulb goes off.
9.7 EARTHING (GROUNDING)
Local earthing is done near the kWh meter. A hole 2-3 m deep holds a copper rod in an insulating pipe. A copper plate of 50 cm × 50 cm is welded to the lower end. The plate is surrounded by a charcoal and salt mixture for good ground contact. Water is poured through the pipe to keep the ground damp. The rod’s upper end joins the earth connection at the kWh meter.
Safety purpose: if short circuiting sends excess current through the line wires, it passes to earth through the earth wire. Without local earthing, the line wires overheat and may cause a fire.
Appliance earthing: the earth wire connects to the metallic case of appliances we handle, such as refrigerators and geysers. On a fault, the live wire touches the case. Heavy current flows to earth through the case, the fuse blows, and the user feels no shock. Paint insulates, so it must be removed where the earth contact is made.
9.8 THREE-PIN PLUG AND SOCKET
A three-pin plug has three brass pins in an ebonite case. The top pin is for earth (E), the left pin for live (L), and the right pin for neutral (N). The pins are split at the ends for spring action, so they fit tightly in the socket holes.
The earth pin is longer, so the earth connection is made before the live connection. If the appliance is defective, current passes to earth and the fuse blows before the user is exposed. The earth pin is thicker, so it cannot be inserted into a live or neutral hole by mistake.
The socket has three holes lined with hollow brass tubes that form the terminals. Top hole: earth. Right hole: live. Left hole: neutral. Two precautions matter: hands must be completely dry, and plug pins must fit tightly. A loose connection causes sparking and burning.
A two-pin plug is used with low-power appliances such as table lamps and night lamps. It carries only live and neutral.
9.19 COLOUR CODING OF WIRES IN A CABLE
The insulation on the three wires of a cable carries different colours. The book records two conventions: the old one and the new international one.
| Wire | Old convention | New convention |
|---|---|---|
| Live | Red | Brown |
| Neutral | Black | Light blue |
| Earth | Green | Green or yellow |
The colour coding helps a fitter connect the switch, fuse, and socket through the proper wire. Without it, a wiring mistake could put a live wire where a neutral should be. Memory aid for the new code: ‘Brown for Live, Blue for Neutral, Green for Earth.’ For the plug, remember ‘Earth at Top, Live on Left, Neutral on Right.’
The old red-black-green code still appears in older buildings. New installations follow the international brown-light blue-green (or yellow) code. Both codes serve the same purpose: correct and safe connections.
9.10 HIGH TENSION WIRES
Each wire in a cable can carry only up to a specific current limit. If the current exceeds this limit — from short circuiting or voltage fluctuations — the wire burns due to excessive heating. A fire can follow. To prevent this, high voltage and heavy current use a special wire called the high tension wire.
A high tension wire needs low resistance for heavy current and a large surface area to radiate heat. So several thin insulated wires are twisted together instead of one thick wire. The large surface area lets the wire radiate the heat produced more readily to the surroundings.
9.11 SAFETY PRECAUTIONS WHILE USING THE ELECTRICITY
Two dangers exist while using electricity: fire and electric shock. Fire comes from overheating of line wires, caused by broken insulation or short circuiting. Shock has five causes: poor insulation, wet hands, no appliance earthing, no fuse, and no local earthing.
Five precautions map one-to-one against the shock causes. Insulation must be of good quality and checked regularly. Never touch appliances with wet hands. Earth every metallic case. Fit a proper fuse in the live wire of each appliance. Maintain proper local earthing near the kWh meter.
The method: current drawn is I = P/V. The fuse rating is the next standard value above the drawn current. A 3 kW motor at 220 V draws 3000/220 = 13.6 A, so it needs a 15 A fuse.
Worked example: a 220 V circuit has a 5 A fuse. Three 60 W bulbs are running. How many 40 W tube lights can be added? Current from the bulbs: I = (3 \times 60)/220 = 0.82 A. Excess current available: 5 - 0.82 = 4.18 A. Current per tube light: 40/220 = 0.18 A. Number of tube lights: 4.18/0.18 = 23.2, so 23 tube lights can be added.
What to practise from Household Circuits Class 10 ICSE
The book closes Chapter 9 with four practice sets.
- Exercise 9(A) — 15 questions on transmission voltages, ring wiring, and parallel versus series connections.
- Multiple Choice Type — 3 questions on fuse placement, appliance connections, and the kWh meter.
- Exercise 9(B) — 31 questions on fuse material, switch placement, earthing, plugs, colour coding, and safety precautions.
- Multiple Choice Type — 2 questions on fuse ratings and the switch wire.
The sets build five skills: ring-wiring diagrams, fuse and switch placement reasoning, three-pin plug labelling, colour coding, and safety explanations. Reason questions recur in ICSE board papers: why the fuse and switch sit in the live wire, why the earth pin is longer and thicker, why appliances connect in parallel, and why a copper wire must never replace a fuse.
| Common mistake | Correct rule |
|---|---|
| Fuse or switch in the neutral wire | Put both in the live wire, before the appliance |
| Fuse rating chosen from running current | Use the starting current for motor-driven appliances |
| Fuse replaced with a copper wire | Copper melts near 1080°C and will not blow in time |
This chapter follows the Class 10 ICSE physics syllabus. The official CISCE syllabus and examination details are on the CISCE website.
Household Circuits Class 10 ICSE frequently asked questions
What are the most important household circuits class 10 ICSE topics for the board exam?
Focus on the transmission voltage steps (11 kV to 220 V), ring wiring with parallel connections, and the fuse rating formula I = P/V. Earthing, the three-pin plug, and staircase wiring also appear regularly. The switch and fuse in the live wire is the reason question that recurs most often.
Why must the fuse and the switch be connected in the live wire and not in the neutral wire?
With the switch or fuse in the live wire, breaking the circuit leaves the appliance at zero potential, so it is safe to touch. In the neutral wire, the appliance stays connected to the high potential terminal through the live wire. Touching it can give a fatal shock.
How is the current rating of a fuse calculated for a circuit or an appliance?
Divide the total power of the appliances by the supply voltage: I = P/V. Choose the next standard fuse rating above this value. For motor-driven appliances, use the starting current, which is higher than the running current.
Why is the earth pin of a three pin plug longer and thicker than the live and neutral pins?
The longer earth pin makes the earth connection first, so a faulty appliance blows its fuse before the live pin connects. The thicker pin cannot be forced into a live or neutral socket hole.
How do dual control switches switch a staircase bulb on and off from two places?
Two dual control switches, one at each end, each connect their terminal b to a or c. The bulb is on when both switches select the same path. Operating either switch toggles the bulb between on and off.
Reference: ICSE Class 10 Physics textbook material, chapter 9: Household Circuits — CISCE-aligned.