9 min readConcept sheet2 and 3-mark questions, a 5-mark answer with the house circuit diagram, and 4-mark case-based questions

Domestic electric circuits and safety

Homes get power through live, neutral and earth wires. Appliances are joined in parallel, and a fuse cuts the supply during a short circuit or overloading.

Wire colours (textbook)Live: red; neutral: black; earth: green

Supply in India220 V between the live and neutral wires

Two circuits15 A for heavy appliances (geyser, air cooler); 5 A for bulbs and fans

Safety devicesFuse in series; earth wire for metal-bodied appliances

From the pole to your plug

1
The mains supply reaches the house through overhead wires on poles or underground cables: a live wire (red), a neutral wire (black) and an earth wire (green).
2
The live and neutral wires pass through a main fuse into the electricity meter.
3
Through the main switch, they are connected to the line wires of the house.
4
The wiring is split into separate circuits: one of 15 A rating for heavy appliances such as geysers and air coolers, and one of 5 A for bulbs and fans.
5
In each circuit, appliances are joined in parallel between the live and neutral wires, each with its own switch, so each gets the full 220 V.
6
The earth wire is joined to a metal plate buried deep in the ground near the house, and to the metal body of appliances such as irons, toasters and refrigerators.

Why the earth wire matters

It is used for appliances with a metal body: electric iron, toaster, table fan, refrigerator.
If the live wire touches the metal body by fault, the earth wire gives the current a low-resistance path to the ground.
This keeps the metal body at the potential of the earth, so a person who touches it does not get a severe shock.

Short-circuiting vs overloading

Short-circuitingThe live and neutral wires touch directly, for example when the insulation is damaged or an appliance is faulty. The resistance becomes very small, so the current rises suddenly to a very large value.

OverloadingToo much current is drawn from the circuit. Causes: live and neutral touching, too many appliances on one socket, or an accidental rise in the supply voltage.

In both cases the large current heats the wires (H = I²Rt) and can start a fire. The fuse in series melts and cuts off the supply. Many homes also use an MCB (miniature circuit breaker), a switch that turns off by itself and can be reset.

How to draw a domestic circuit

Draw three lines coming from a pole marked 'Mains supply': live (L), neutral (N) and earth (E).
On the live wire draw the main fuse, then a box marked Electricity meter, then a Main switch across live and neutral.
Draw a Distribution box with a fuse on the live line of each circuit.
From it draw two pairs of horizontal lines for the 5 A and 15 A circuits.
Join bulbs, a fan and a socket between live and neutral as parallel branches, each with a switch on the live side.
Draw the earth wire to a metal plate in the ground and to the third pin of the socket. Label every part.

In real life

The three-pin plug of an iron or fridge has a longer, thicker pin for the earth wire. Plugging a geyser, a heater and an iron into one extension board overloads the socket, which is why a fuse blows or an MCB trips.

Exam angle

2 marks: “What is the function of an earth wire? Why should appliances with metal bodies be earthed?” It gives leaking current a low-resistance path to the ground. This keeps the metal body at the earth's potential, so the user does not get a severe shock.

3 marks: “What is short-circuiting? Name the device that protects a circuit from it and explain how it works.” Live and neutral wires touch directly, so the current rises suddenly. A fuse in series heats up (I²Rt), melts and breaks the circuit.

2 marks: “Why are household appliances connected in parallel?” Each gets the same 220 V, can be switched on and off on its own, and keeps working if another fails.

Remember it

Red runs, Black returns, Green guards: live, neutral, earth.