Fuses and circuit breakers

Fuses and circuit breakers are both devices that 'break' if the current through them is too large.

In its simplest form a fuse is just a piece of wire that heats up as the current through it increases. Eventually the wire reaches such a high temperature that its melts. Thicker pieces of wire have a lower resistance than thinner ones and so the heating effect in a thinner wire is less than a thicker one (for the same voltage difference) and so thicker wires can stand a bigger current before they break.

The disadvantage a fuse wire is that it takes a finite length of time for the wire to heat up and break, and in that time damage may be dome to the equipment.

A circuit breaker is an electronic circuit that will switch off the current if it is too great. Such a circuit is designed to operate much more quickly than a fuse and so the damage is less because the current is switched off in a much shorter time.

Circuit breakers are often used to protect the user if the casing of the equipment becomes live. They break the circuit before the person using the equipment gets a near fatal shock.