> (IIRC, the trick is that most breakers have two independent trip mechanisms: a thermal one which has a built-in heat-dependent delay, and a magnetic one which

> (IIRC, the trick is that most breakers have two independent trip mechanisms: a thermal one which has a built-in heat-dependent delay, and a magnetic one which is instant.) precisely it. the magnetic one uses the fact that ac makes an electromagnet and they tune it so it snaps away quickly if there's a short.