Arcing

Why a small fault burns

A circuit carrying its normal current through good connections spreads heat along the whole conductor and sheds it harmlessly. Introduce a poor connection and the resistance at that one point rises sharply, so the same current now dumps its heat into a few millimetres of terminal.

An arc across an actual gap goes further: the air itself ionises and the temperature at the arc is extraordinary. All of this while the current stays low enough that the breaker sees nothing unusual, because the wire along its length is not overloaded.

Where arcs start

A terminal screw that was never fully tightened, or that loosened over years of thermal cycling. A cable pierced by a nail or a screw during a renovation. Insulation crushed under furniture until the conductors nearly touch. A connection made outside a box by someone in a hurry. Aluminum wire on a device not rated for it.

The common thread is that none of these announce themselves, and most are inside a wall.

What you can actually perceive

A buzz or crackle from a switch or outlet, which is the arc itself. A smell of hot plastic or a sharp ozone-like smell. Warmth on a faceplate. Lights that flicker on one circuit rather than the whole house. Scorch marks or browning around a receptacle.

Any one of them is enough to stop using that circuit. The reason to be firm about that is that arcing is progressive: the heat degrades the connection further, which increases the resistance, which increases the heat.

Why AFCIs exist

This is the gap they were invented for. An overcurrent breaker cannot see an arc because an arc does not draw excessive current, and a GFCI cannot see it because the current is still returning on the neutral. An AFCI watches the waveform for the arc's signature instead.