Draw a schematic diagram of a common domestic circuit. Explain the role of the fuse and the earth wire, and give the causes of overloading.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open. Power reaches homes through live (red), neutral (black) and earth (green) wires; the potential difference between live and neutral is 220 V.
2 · Draw the circuit. [Mains, main fuse, meter, main switch, distribution box, 5 A and 15 A circuits with parallel appliances and switches on the live wire; earth wire to a plate in the ground]
3 · Explain the fuse. It is in series with the live wire. It melts when the current is too large and breaks the circuit.
4 · Explain the earth wire. It joins the metal bodies of appliances to the ground and gives a low-resistance path, so the user does not get a severe shock.
5 · Give the causes of overloading.
Live and neutral touching (short circuit)
Too many appliances on one socket
Accidental rise in the supply voltage
6 · Real life. A geyser, heater and iron on one extension board can blow the fuse or trip the MCB.
7 · Close in one line. Parallel wiring with fuses and earthing keeps a home's supply both useful and safe.
Scoring tip: Label L, N and E on the diagram, and show appliances in parallel with switches on the live wire.
