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Fuses, Big or Small

• A fuse is a safety device. It contains a thin wire which is designed to melt when too large a current passes through it.
• The fuse prevents too much current which could otherwise cause overheating and possibly fire.
• A fuse is a switch which is always ON but excess current melts the wire. This turns the switch permanently OFF.

Note the 200,000 Amp fuse in the image. This probably came from a power generating station or an electric railway sub-station.

Circuit Diagram Symbol

Construction (Quick Blow Fuse)

Fuses are used to protect circuits by preventing large currents from flowing. In the UK, power connectors for devices like kettles and fan heaters use a 13 Amp fuse. If more than 13 amps pass through the circuit, the fuse wire melts and the circuit is turned off. This protects the other wiring from overheating and possibly causing a fire. Smaller fuses (three amp) are used for low power appliances like table lights and radios. Always use the smallest possible fuse that will not fail under normal (or switch on) current operating conditions.

Construction (Slow Blow Fuse)

This is similar to a normal fuse but the metal blob prevents the wire heating and melting so quickly. This is useful if the circuit has a switch-on surge current.

• Motors take more current while they are accelerating up to full speed.
• Power supplies take more current until the smoothing capacitors are fully charged.