How a fuse works

The idea

A fuse is a wire of appropriate melting point placed in series with a device; a current larger than the specified value heats and melts the wire, breaking the circuit and stopping the unduly high current.

A safety wire

A fuse is a piece of wire placed in a circuit to protect it. Suppose too large a current starts to flow through a circuit with a fuse.

What does the fuse do?

• Its wire heats and melts, which breaks the circuit
• Its wire grows thicker and lets more current pass
• It changes the extra current into light

Melt the fuse

Pick a fuse and raise the current past its rating.

Stopping a high current

Heating is the key. A current larger than the rating makes the fuse wire so hot that it melts. The gap that results stops the current. The fuse was in series with the device, so the whole circuit was broken.

A fuse is a wire of appropriate melting point placed in series with a device. A current larger than the specified value heats and melts the wire, breaking the circuit and stopping the unduly high current.

Notes

A fuse is a wire of appropriate melting point placed in series with a device; a current larger than the specified value melts the wire, breaking the circuit and stopping the unduly high current.

Check yourself

How is a fuse joined to the device it protects?

What happens when a current larger than the specified value flows through a fuse?

Which property of the fuse wire makes it work as a safety device?

An iron draws a current of about 4.5 A. Which fuse from 1 A, 3 A, 5 A and 10 A should be used for it?

• In parallel with the device. The fuse is placed in series with the device, so the whole current passes through it.
• In series with the device — correct. Yes!
• Far away from the circuit. The fuse has to be in the circuit, in series with the device, so that the current passes through it.
• The fuse wire becomes stronger. A current larger than the specified value raises the temperature of the fuse wire until it melts.
• The fuse lets the high current flow on. The melted wire breaks the circuit, stopping the unduly high current.
• The fuse wire heats and melts, breaking the circuit — correct. Yes!
• It has an appropriate melting point — correct. Yes!
• It does not change when heated. The fuse works because the wire melts when a larger current than specified heats it.
• It has no resistance. A fuse wire is heated by the current passing through it, which is the heating effect of current.
• 3 A. A 3 A fuse would melt at the iron's normal current of about 4.5 A, since the current is larger than its rating.
• 5 A — correct. Yes!
• 10 A. A 10 A fuse allows currents far larger than the iron needs, so it would not protect the iron as well. The 5 A fuse is the one that suits a current of about 4.5 A.