AFCI

The problem it solves

A loose terminal, a nail through a cable, a cord crushed under a chair leg. Each can produce an arc — current crossing a small gap — that generates intense heat at that one point while drawing far too little current to trouble a breaker.

That is the gap in protection. The wire never overheats along its length, so the overcurrent breaker never sees a reason to act, while a few millimetres of it are hot enough to ignite whatever is around them.

How it can tell

An arc has a distinctive electrical fingerprint: erratic, high-frequency noise on the waveform rather than the smooth sine of normal current. The breaker contains electronics that watch for that pattern and open the circuit when they find it.

The difficulty is that ordinary equipment produces arcing too. A motor with brushes, a dimmer, a switch closing — all arc harmlessly. So the device has to distinguish a dangerous arc from a benign one, and that discrimination is why AFCIs are more expensive than ordinary breakers and why they occasionally trip on something innocuous.

How it differs from a GFCI

They protect against different things and are not interchangeable. A GFCI protects a person from shock by detecting current leaving the circuit. An AFCI protects the building from fire by detecting arcing within it.

Combination devices exist that do both. Where each is required is a code question rather than a preference.

Nuisance tripping

It happens, and the honest position is that early generations were worse at it than current ones. A repeatedly tripping AFCI is worth investigating rather than swapping for a plain breaker, because sometimes it is a vacuum motor and sometimes it is a genuinely damaged cable inside a wall, and the device cannot tell you which.