Electrical Safety: Devices

Safety devices stop dangerous currents before they cause fires or electric shocks. Think of electricity like water in pipes: if the flow gets too strong or leaks out, safety devices shut it off.

Mains wires and safe switching

• Live (brown): carries high voltage from the supply.
• Neutral (blue): completes the circuit back to the supply.
• Earth (green/yellow): safety path for fault current.

The switch must be in the live wire so the appliance is disconnected from the high voltage when off.

Fuses

A fuse is a thin wire in the live path. If the current becomes too large, the wire melts (blows) and breaks the circuit. It protects the wiring and appliance from overheating. It is single-use and must be replaced with the correct rating.

• Choose a fuse just above the normal operating current (common plug fuses: 3 A or 13 A).
• Always fitted in the live side of the plug.

Circuit breakers (trip switches/MCBs)

A circuit breaker is an automatic switch that opens when current is too high. It acts faster than many fuses and can be reset after the fault is fixed. It protects circuits from overloads and short circuits.

RCDs (residual current devices)

An RCD compares current in live and neutral. If some current leaks to earth (for example through a person), the currents differ and it trips very quickly. RCDs help protect against electric shock, but fuses/MCBs are still needed to protect against overload.

Earthing and double insulation

Earthed metal cases: If a live wire touches the metal case, the earth wire gives a low-resistance path to ground. A large fault current then flows and the fuse/MCB disconnects the supply quickly.

Double-insulated appliances have plastic cases and two layers of insulation. They do not need an earth wire. In these, a fuse (if present) protects the circuit and cabling.

Common misconceptions

• “Switching the neutral makes it safe.” Incorrect. Only a live switch ensures the appliance is not connected to high voltage when off.
• “Fuses protect people.” Mainly they protect wiring and appliances. RCDs provide shock protection.
• “The earth wire carries current all the time.” No. It carries current only during a fault.