Structure of Plug
The inside of a plug looks like this 👇
- The brown wire is a live wire that carries current to the electrical appliance.
- The blue wire is a neutral wire that completes the circuit by providing a path back to the mains for the current.
- The yellow and green wire is a earth wire that provides an outlet for the current in case the appliance becomes live.
- Another important component of the plug is the fuse.
Fuse and Circuit Breaker
- The fuse contains a piece of wire that melts easily.
- When too much current passes through the wire, the wire melts, breaking the circuit.
- No more current can flow through the wire.
- The fuse protects the appliance and wiring in case of fault by preventing overheating or fire.
- Fuses come in standard ratings, such as 1A, 3A, 5A, and 13A.
- 5A fuse means that the fuse starts to melt when 5A of current flows through the wire.
- Therefore, you should use a fuse with a higher rating than the current needed by the appliance.
- A circuit breaker is a resettable fuse. In case of fault, the automatically operated switch will shut down and stop the flow of electricity.
Example
A washing machine has an electric motor and an electric heater. The resistance of the motor is 22Ω. The mains voltage is 230V.
(a) Show that the current in the heater is about 10A when it is working.
V = IR
I = V / R
I = 230 / 22 = 10.45A ≈ 10A
(b) When the motor is working, the current in it is 1.74A. Explain why it would not be sensible to replace the 13A fuse with a 2A fuse.
→ The total current needed by the motor and heater is approximately 12A. If you use a 2A fuse, the fuse would melt at 2A and break the circuit, and the washing machine will not operate. It is sensible to use a 13A fuse, which is slight over the current needed by the washing machine.
Earth Wire
- Many electrical appliances have metal casings, such as the washing machine.
- If the live wire becomes loose and touches the metal casings, current will flow through the appliance because metal can conduct electricity.
- If you touch the washing machine at this moment, electricity will flow through you, and you may get electric shocked!! 😱😱
- The earth wire, which is connected to the ground, has a very low resistance. The current will go through the earth wire to the ground instead of going through you.
Double Insulation
- Electrical appliances that do not have metal casings do not need a earth wire.
- Instead, they have a double insulation.
- The inside is encased by an insulating plastic, which is insulated again by an outer casing.
- Even if the live wire becomes loose, it will not cause electric shock.