Mains supply: live, neutral and earth wires
Power reaches a house from the electric pole through a main cable. It has a live wire (red insulation, or brown in new wiring) and a neutral wire (black, or blue in new wiring). The live wire is at a high potential; the neutral is kept near zero. In India the potential difference between them is 220 V and the supply is AC with a frequency of 50 Hz.
The cable first goes to the electricity meter (it counts energy in kWh), then through a main fuse or MCB into the distribution box. A third wire, the earth wire (green, or green-yellow), is joined to a metal plate buried deep in moist ground near the house.
Why appliances are connected in parallel
Every appliance is connected between the live and the neutral wire, side by side, which is a parallel connection. This gives three benefits:
- Each appliance gets the same full 220 V.
- Each has its own switch, so one can be on while another is off.
- If one bulb fuses, the others keep working.
In a series connection the voltage would be shared, bulbs would glow dim and one fault would switch everything off. The total current from the mains is the sum of the currents of all appliances that are on, so switching on more appliances means a bigger current in the main wires.
Separate 5 A and 15 A circuits
Houses usually have a 5 A lighting circuit for bulbs, tube lights, fans and TV, and a 15 A power circuit (thicker wires, bigger sockets) for heaters, geysers, irons, ACs and fridges. Keeping them separate means a heavy load cannot overheat the thin lighting wires.
Electric fuse and MCB
A fuse is a short piece of wire with a low melting point (for example an alloy of tin and lead, or thin tinned copper), connected in series with the circuit. If the current goes above the safe value, the heating effect (H = I²Rt) melts the fuse wire and the circuit breaks.
Fuse rating is the largest current it can carry without melting, such as 1 A, 3 A, 5 A, 10 A or 15 A. Choose a rating a little above the normal working current I = P / V.
Why the fuse is in the live wire
When the fuse melts, the appliance must be cut off from the high potential. If the fuse were in the neutral wire, the appliance would still be joined to the live wire and could give a shock even after the fuse blows.
MCB (miniature circuit breaker)
An MCB is an automatic switch that opens when the current is too high. Unlike a fuse, it does not need a new wire: you simply push its lever back up after fixing the fault. Modern houses use MCBs in the distribution box.
Earthing: protection for metal-body appliances
Heaters, irons, geysers, fridges and washing machines have metal bodies. The earth wire is joined to the metal body through the third (thick) pin of the plug. If a loose live wire touches the body, the fault current flows through the low-resistance earth wire into the ground instead of through a person. This large current also blows the fuse or trips the MCB. The body stays at the potential of the earth (zero), so touching it is safe.
Overloading and short circuit
Overloading happens when too many high-power appliances run on one circuit or one socket, or the supply voltage jumps suddenly. The current becomes more than the wires can carry and they heat up.
A short circuit happens when the live and the neutral wires touch directly, usually because insulation is damaged. The resistance of the path becomes almost zero, so by I = V / R the current becomes very large at once, causing sparks and fire.
In both cases the fuse or MCB cuts the supply.
Electrical safety at home
- Use a fuse or MCB of the correct rating; never replace a fuse with a thick copper wire.
- Do not plug many appliances into one socket with multi-plug adapters.
- Replace cracked or worn insulation; keep wires away from water.
- Always use three-pin plugs for metal-body appliances and keep the earth connection working.
- Switch off the main switch before any repair; never touch switches with wet hands.
Choosing a fuse rating using P = VI
Find the normal current I = P / V with V = 220 V. Then pick the nearest standard fuse rating just above it. Exam questions often ask: can this appliance run on a 5 A circuit? Or how many bulbs can one circuit carry? See the solved examples below.
Key formulas and definitions
- P = V × I, so I = P / V (V = 220 V for Indian homes)
- Fuse rating: the standard value just above the normal current
- Total current in a house = sum of currents of all appliances that are on (parallel)
- Heat in fuse wire: H = I² R t
- Energy used: E (kWh) = P (kW) × t (h)
Worked examples
1. A 60 W bulb works on 220 V. What fuse rating should it have: 1 A, 5 A or 15 A?
I = P / V = 60 / 220 = 0.27 A. The smallest rating above this is 1 A, so use a 1 A fuse.
2. An electric iron is rated 1000 W, 220 V. Find the current and a suitable fuse.
I = 1000 / 220 = 4.55 A. A 5 A fuse is just above it, so a 5 A fuse is suitable.
3. A room heater is rated 2 kW, 220 V. Can it run on the 5 A lighting circuit?
I = 2000 / 220 = 9.09 A. This is more than 5 A, so the lighting fuse would blow. Use the 15 A power circuit (a 10 A fuse suits the heater).
4. A geyser of 1500 W works on 220 V. Which fuse is safest: 5 A, 10 A or 15 A?
I = 1500 / 220 = 6.82 A. 5 A is too low (it will blow). 10 A is just above 6.82 A, so choose 10 A. A 15 A fuse would let a dangerous current pass without melting.
5. Two 100 W bulbs, a 60 W fan and a 120 W TV run together on 220 V. Find the total current. Is a 5 A circuit enough?
Total power = 100 + 100 + 60 + 120 = 380 W. I = 380 / 220 = 1.73 A. This is below 5 A, so the 5 A circuit is enough.
6. A kitchen circuit has a 15 A fuse. A 2 kW heater, a 1 kW toaster and a 1.5 kW kettle are switched on together at 220 V. What happens?
Total P = 4500 W. I = 4500 / 220 = 20.5 A. This is more than 15 A, so the circuit is overloaded and the fuse melts (or the MCB trips).
7. How many 100 W bulbs can be lit together on a 220 V line with a 5 A fuse?
Largest safe power = V × I = 220 × 5 = 1100 W. Number of bulbs = 1100 / 100 = 11 bulbs.
Common mistakes
- Putting the fuse or switch in the neutral wire. It must be in the live wire, or the appliance stays live after the fuse blows.
- Thinking appliances at home are in series. They are in parallel, so each gets 220 V and works on its own.
- Mixing up short circuit and overloading. A short circuit is live touching neutral (R almost zero); overloading is too much load or a voltage jump.
- Choosing a fuse with a much higher rating ‘to be safe’. A higher rating is less safe; it lets a harmful current flow without melting.