Live, neutral and earth wires, fuses, switches and how to choose a safe fuse rating.
Mains in India is 220 V, 50 Hz. The live wire carries the high potential; the neutral is near earth potential; the earth wire is a safety connection to the ground. CAUTION on colours: older Selina pages show live = red, neutral = black, earth = green. Current Indian practice follows IEC: live = brown (or red), neutral = blue (or black), earth = green-yellow. Always check the exam board's wording.
Which wire must the switch and fuse be placed in? Why?
The live wire. Then, with the switch off or the fuse blown, the appliance is cut off from the high potential and is safe to touch.
What is the purpose of the earth wire?
It joins the metal body to the ground. If the live wire touches the body, the large current flows to earth and blows the fuse instead of shocking the user.
A fuse is a short wire of low melting point and high resistance that melts when current exceeds its rating. Pick a rating just above the normal working current. Current is found from I = P/V.
Current drawn by a 100 W, 220 V bulb?
I = 100/220 = 0.455 A.
A 2 kW, 220 V heater: which fuse is suitable (1 A, 5 A, 10 A, 15 A)?
I = 2000/220 = 9.09 A. Choose 10 A: it is just above the working current. A 5 A fuse would blow at once; 15 A gives weak protection.
A 1.5 kW geyser on 220 V. Working current?
I = 1500/220 = 6.82 A, so a 10 A fuse.
How many 100 W, 220 V bulbs can a 5 A circuit carry?
Each takes 0.455 A, so 11 bulbs draw 5.0 A, exactly at the limit. For safety, use 10 bulbs or fewer.
Overloading is too many appliances on one circuit. A short circuit is live and neutral touching, so resistance is almost zero and current is huge. An MCB (miniature circuit breaker) trips automatically and can be reset; an ELCB cuts the supply when current leaks to earth.
Give one advantage of an MCB over a fuse.
It trips quickly and can simply be switched back on after the fault is cleared, whereas a fuse wire must be replaced.
Cost of running five 60 W bulbs for 6 h at ₹6 per unit?
Energy = 5 × 60 × 6 / 1000 = 1.8 kWh. Cost = 1.8 × 6 = ₹10.80.
1. Name the three wires in a domestic supply.
2. What is the potential and frequency of Indian mains supply?
3. Why is the fuse placed in the live wire?
4. Why is the fuse wire made of a material of low melting point?
5. Why must a copper wire not be used as a fuse wire?
6. What is a short circuit?
7. A 1.1 kW iron works on 220 V. Find the current and choose a fuse from 3 A, 5 A, 13 A.
8. Why are three-pin plugs used for heavy appliances?
9. What does an ELCB do?
10. Why are bulbs and fans in a house connected in parallel?