Why do plugs have three pins when current needs only two wires?
Two wires, live and neutral, carry the current. The third is the earth wire. It connects the metal body of an appliance to a metal plate buried deep in the ground near the house. If a fault lets current leak to the body, the earth wire gives it a low-resistance path to the ground. The body stays at the potential of the earth, so a person who touches it may not get a severe shock. The third pin on a plug joins the appliance's metal body to that earth wire.
The three wires and their colours
Electricity reaches a house through overhead poles or underground cables. NCERT names three wires, each with its own colour of insulation:
| Wire | Insulation colour | Job |
|---|---|---|
| Live (or positive) | red | brings the supply in |
| Neutral | black | completes the path back |
| Earth | green | safety, for metal-body appliances |
In India the potential difference between live and neutral is 220 V. The supply is alternating current at 50 Hz.
At the meter board the wires pass through a main fuse, then the electricity meter, then the main switch, and only after that do they connect to the line wires inside the house.
Two circuits, and why everything is in parallel
Often, two separate circuits are used. One is rated 15 A, for appliances that draw a lot of power, such as geysers and air coolers. The other is rated 5 A, for bulbs, fans and similar loads.
Each appliance is connected across the live and neutral wires, in parallel, and has its own switch. This matters for two reasons. Every appliance gets the same potential difference, 220 V. And switching one off doesn't cut the others. If your room's bulbs were in series, removing one would plunge the whole room into darkness.
The earth wire: protection for you
Picture a toaster with a metal body. Inside, a wire's insulation wears through and the live wire touches the casing. Without an earth wire, the metal body sits at a high potential. Touch it while you're standing on the floor and current can flow through your body to the ground. That's an electric shock.
Now add the earth wire. Its green-insulated conductor connects the toaster's body to a metal plate deep in the earth. The leaking current now has a low-resistance path to the ground, and the body stays at the earth's potential. NCERT's wording is careful: the user may not get a severe shock. It does not say "never".
The book lists the electric press, toaster, table fan and refrigerator as examples of metal-body appliances that use the earth wire. The three-pin plug is how that connection is made in practice: the third, usually longer and thicker pin goes to earth. That detail about the pin is general knowledge, not something the textbook states.
The fuse: protection for the circuit
A fuse has a different job. It protects the circuit and the appliances from overloading, which means too much current flowing. NCERT gives three ways overloading can happen:
- The live and neutral wires come into direct contact, because the insulation is damaged or an appliance is faulty. This is a short circuit, and the current in the circuit increases abruptly.
- The supply voltage rises accidentally.
- Too many appliances are connected to a single socket.
The fuse is connected in series in the live wire. When the current gets too large, the fuse wire heats up by Joule heating and melts. The circuit breaks and the current stops.
Keep the two straight. The fuse protects the circuit. The earth wire protects the user.
A worked check: the 2 kW oven
Here's NCERT's own exercise. An electric oven rated 2 kW runs on a 220 V domestic circuit rated 5 A. What happens?
I = P/V = 2000 W ÷ 220 V ≈ 9.09 A.
That's well over 5 A, so the circuit is overloaded and the fuse melts. An oven like this belongs on the 15 A circuit.
Check yourself
A student says, "The green wire is the live wire." True or false? (Answer at the bottom.)
Questions students ask
What are the colours of the live, neutral and earth wires?
In NCERT, live is red, neutral is black and earth is green.
Why are household appliances connected in parallel?
So that each one gets the same potential difference and can be switched on or off on its own.
What is the function of the earth wire?
It gives leaking current a low-resistance path to the ground and keeps a metal body at earth potential, so the user may not get a severe shock.
What happens to the current in a short circuit?
It increases abruptly, because live and neutral touch directly. The fuse then melts and breaks the circuit.
Does the fuse protect people from electric shock?
Not directly. It protects the circuit and the appliances from overloading. The earth wire is the safety measure for the user.
Answer: False. Green is earth. Live is red and neutral is black.
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