Electrical safety devices protect people and equipment from the dangers of excessive current and faults.
The three wires in a mains plug
| Wire | Colour | Function |
|---|---|---|
| Live (L) | Brown | Carries current to the appliance (alternates at mains voltage, 230 V in Malaysia/UK) |
| Neutral (N) | Blue | Completes the circuit; returns current to the supply |
| Earth (E) | Green and yellow | Safety wire; connected to the metal casing; carries no current in normal use |
Fuses
A fuse is a thin wire that melts and breaks the circuit when the current exceeds its rated value. This prevents overheating and fire.
Choosing a fuse: the fuse rating should be slightly above the normal operating current. For a 2.3 kW, 230 V kettle:
A 13 A fuse would be appropriate (the next size above 10 A).
Circuit breakers
A circuit breaker does the same job as a fuse but can be reset after tripping. It acts faster than a fuse. An RCD (residual current device) detects a difference between the current in the live and neutral wires (indicating current leaking to earth) and cuts the circuit in milliseconds.
The earth wire
If a fault causes the live wire to touch the metal casing of an appliance, the earth wire provides a low-resistance path to the ground. A large current flows through the earth wire, which blows the fuse (or trips the circuit breaker), disconnecting the appliance before anyone touches it.
Double insulation
Some appliances have plastic casings (no exposed metal parts) and do not need an earth wire. They are marked with a double-insulation symbol (a square inside a square).
Common errors and how to correct them
Saying the fuse is in the neutral wire. The fuse must be in the live wire so that the appliance is disconnected from the high-voltage supply when the fuse blows.
Stating the earth wire “stops the current.” The earth wire provides a low-resistance path for the fault current, which causes the fuse to blow. The fuse then stops the current.
How to apply this in an exam
Describe the fault scenario step by step: live wire touches casing, current flows through earth wire, fuse blows (or circuit breaker trips), appliance is disconnected. Always mention the live wire, earth wire and fuse in your answer.
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