Household Circuits

ICSE Class 10 · Physics · Chapter 9

What this chapter covers

Power distribution from the power station to your home
The main circuit: live, neutral and earth wires
House wiring and the ring system
Colour coding of wires
Switches and their position in a circuit
Staircase wiring
The fuse
The miniature circuit breaker (MCB)
Earthing
Three-pin plug and socket
Electrical safety precautions

Key definitions

Power distribution is the process of carrying electrical energy from the generating station to the consumers, stepping its voltage up for long-distance transmission and down again for safe use in homes, shops and factories.

The live wire is the wire that carries current from the supply to the appliance and is at a high potential, about 220 V, with respect to the earth.

The neutral wire is the return wire that completes the circuit back to the supply and is kept at zero potential because it is connected to the earth at the local substation.

The earth wire is a wire connected to a metal plate buried deep in the ground, joined to the metal body of an appliance so that any leakage current flows safely into the earth instead of through a person.

The main switch is a double-pole switch placed after the meter that disconnects both the live and the neutral wires of the whole house from the supply in one movement.

The distribution board is the panel near the main switch where the supply is divided into separate circuits, each protected by its own fuse or MCB.

The ring system is a method of house wiring in which the live, neutral and earth wires start from the distribution board, run around a set of rooms in a closed loop and return to the same point, with the sockets and appliances connected in parallel at points along the loop.

A switch is a device that makes or breaks a circuit, allowing the current to flow when it is on and stopping the current when it is off.

A two-way switch is a switch with three terminals, one common terminal and two other terminals, in which the common terminal is connected to either one or the other of the two terminals depending on the position of the switch.

Staircase wiring is an arrangement of two two-way switches by which a single lamp can be switched on or off from either of two places, such as the bottom and the top of a staircase.

A fuse is a safety device consisting of a short piece of thin wire of low melting point, connected in series in the live wire of a circuit, which melts and breaks the circuit when the current exceeds a safe value.

Fuse wire is a wire made of an alloy of tin and lead, which has a high resistivity and a low melting point, so that it heats up and melts quickly when a large current passes through it.

Key points

Electrical energy is generated at about 11 kV, stepped up to 132–400 kV for transmission, and stepped down in stages to about 220–230 V, 50 Hz, for homes.
High-voltage transmission reduces the current and therefore the I²R loss in the cables.
The live wire is at about 220 V, the neutral wire is at zero potential, and the earth wire carries current only during a fault.
The main circuit runs: pole fuse, kWh meter, main fuse or MCB, double-pole main switch, distribution board.
Household appliances are connected in parallel; the lighting circuit is rated about 5 A and the power circuit about 15 A.
In the ring system each wire forms a closed loop from and back to the distribution board, so current reaches each socket by two paths and thinner cables can be used.
Colour code: live red or brown, neutral black or light blue, earth green or green-and-yellow.
Switches and fuses are always placed in the live wire.
A two-way switch has a common terminal connected to one of two other terminals; two of them make staircase wiring, in which either switch can turn the lamp on or off.
A fuse is a thin tin–lead alloy wire with a low melting point that melts when the current exceeds its rating; its rating is chosen just above I = P/V.
An MCB trips on excess current and can be reset instead of replaced.
Earthing keeps a faulty metal body at zero potential and makes the fuse blow.
In a socket seen from the front, earth is at the top, live at the right and neutral at the left; the earth pin of a plug is longer and thicker.
Safety: correct fuses, earthed metal appliances, sound insulation, no overloading, dry hands, and the main switch off before any repair.

Questions answered

Name the stages through which electrical energy passes from the generating station to a house.

Generation (about 11 kV), step-up transformer (132–400 kV), transmission lines, step-down at grid and city substations (33 kV, then 11 kV), a distribution transformer near the street (220 V), and the service line into the house.

Why is electrical power transmitted over long distances at a very high voltage?

For a given power P = VI, a high voltage needs only a small current. The heat lost in the cables is I²R, so a small current means a much smaller energy loss, and thinner cables can be used.

Define the live wire, the neutral wire and the earth wire.

The live wire carries current to the appliance at about 220 V with respect to the earth; the neutral wire is the return wire kept at zero potential; the earth wire joins the metal body of an appliance to a plate buried in the ground so that leakage current flows into the earth.

List, in order, the parts of the main circuit from the service line to the distribution board.

Pole fuse, kWh meter, main fuse or MCB, double-pole main switch, distribution board.

Give two advantages of the main switch.

It cuts off the whole house at once in an emergency or before repairs; being double-pole, it disconnects both the live and the neutral wires.

Why are household appliances connected in parallel?

Each appliance then receives the full 220 V, can be switched on and off independently, and keeps working even if another appliance fails.

State three advantages of the ring system of house wiring.

Current reaches each socket by two paths, so thinner cables can be used; new sockets can be added easily anywhere on the ring; each appliance can have its own fuse of suitable rating.

Give the colours of the live, neutral and earth wires in both the older and the newer conventions.

Older: live red, neutral black, earth green. Newer: live brown, neutral light blue, earth green with yellow stripes.

Past board exam questions

ICSE 2026 sample paper · 1 mark

According to the old convention, the colour of the earth wire is:

black
green
yellow
red

Show answer

green

ICSE 2025 compartment paper · 1 mark

According to the NEW international convention, what is the colour coding for the live, neutral and earth wires in household circuits?

Live – red, Neutral – black, Earth – green
Live – green, Neutral – yellow, Earth – black
Live – brown, Neutral – blue, Earth – yellow
Live – red, Neutral – blue, Earth – yellow

Show answer

Live – brown, Neutral – blue, Earth – yellow

ICSE 2025 compartment paper · 2 marks

State the potential of the wire connected to the right hand side terminal of the three pin plug and also state its colour.

Show answer

The right hand side terminal of a three pin plug is connected to the live wire. It is at a high potential (220 V a.c., alternating with respect to earth). Its colour is brown (red in the old convention).

ICSE 2025 board paper · 2 marks

According to the NEW colour convention which colour of wire is connected to: (a) the metal body of the appliance (b) the switch of the appliance?

Show answer

(a) Green (the earth wire). (b) Brown (the live wire).

ICSE 2025 compartment paper · 3 marks

A bulb is connected by dual control (two-way) switches in two circuits, A and B. Answer the questions that follow. (In both circuits one terminal of the bulb is connected to the neutral wire N, and its other terminal to the common terminal of the left switch; the common terminal of the right switch is connected to the live wire L. Each two-way switch has an upper and a lower terminal. In circuit A, the upper terminal of the left switch is joined to the upper terminal of the right switch and the lower terminal to the lower terminal. In circuit B, the wires cross: the upper terminal of the left switch is joined to the lower terminal of the right switch, and the lower terminal of the left switch to the upper terminal of the right switch. In both circuits, at present, the left switch connects its common terminal to its upper terminal and the right switch connects its common terminal to its lower terminal.) (a) Which switch can successfully turn the bulbs ON or OFF? (Circuit A, Circuit B, or both) (b) At present, in which circuit is the bulb glowing? (c) If the L and N wires are swapped in the circuit (your answer to (b)), will the circuit still function?

Show answer

(a) Both circuits A and B — in each, the two traveller wires join the switch terminals in pairs, so changing the position of either switch completes or breaks the circuit. (b) Circuit B. Path: L → right switch common → lower terminal → crossed wire → upper terminal of left switch → left switch common → bulb → N, a complete circuit. In circuit A the right switch is at the lower wire but the left switch is at the upper wire, so the circuit is broken. (c) Yes, the bulb will still light and can be switched on and off, but the switches will then be in the neutral wire. This is unsafe: the bulb holder remains connected to the live wire even when switched off, and touching it can give a shock.

ICSE 2024 board paper · 1 mark

Identify the option that displays the correct wiring with correct colour code. In each option an appliance is connected to the mains by two wires; the options state the colour of the second mains wire and the wire in which the switch is placed:

Brown wire and blue wire; switch in the brown wire
Brown wire and yellow wire; switch in the brown wire
Brown wire and blue wire; switch in the blue wire
Brown wire and yellow wire; switch in the yellow wire

Show answer

Brown wire and blue wire; switch in the brown wire

ICSE 2024 sample paper · 2 marks

It is observed that the house circuits are arranged in a parallel combination. Give two advantages of this arrangement.

Show answer

1. Each appliance gets the full supply voltage (220 V). 2. Each appliance can be switched on or off independently; if one appliance fails or is switched off, the others continue to work.

ICSE 2024 board paper · 2 marks

A fuse is rated 5 A. Can it be used with a geyser rated 1540 W, 220 V. Write Yes or No. Give supporting calculations to justify your answer.

Show answer

No. Current drawn by the geyser I = P ÷ V = 1540 W ÷ 220 V = 7 A. Since 7 A is more than the fuse rating of 5 A, the fuse would melt (blow) as soon as the geyser is switched on.