Why Do Fuses Blow?

Step 1 of 5

Electricity heats every wire it passes through.

The more electricity crowded into a wire, the hotter it gets. Push far too much through a normal cable and the plastic around it melts and then burns — inside a wall, unseen.

Step 2 of 5

So one spot is made deliberately weak.

The fuse wire is thinner than everything else in the circuit, so it is always the hottest point. Whatever goes wrong, it reaches melting first — by design.

Step 3 of 5

A short circuit is the emergency it is really for.

If a live wire touches the return wire directly, the electricity skips the appliance and floods through with nothing to slow it. The fuse blows in a thousandth of a second.

Step 4 of 5

The rating has to match what it protects.

Too big a fuse and the cable burns before it notices; too small and it blows every time the kettle starts. The number on it is chosen for the cable behind it, not the appliance in front.

Step 5 of 5

Breakers do the same job and reset.

A circuit breaker is a switch held closed by a magnet or a bending strip. Too much current trips it open, and unlike a fuse it can simply be pushed back on once the fault is fixed.

A fuse that keeps blowing is reporting something. Fitting a bigger one silences the report and leaves the fault exactly where it was.