arc fault

An arc fault occurs when electrical current jumps across a gap between two conductors, creating a luminous plasma channel. This happens when insulation fails, wires are damaged, or connections corrode, leaving a path of low resistance through air or another gas. The arc generates temperatures exceeding 1,000 degrees Celsius at the point of contact, hot enough to ignite surrounding materials like wood framing, insulation, or dust.

Arc faults are distinct from short circuits. A short circuit is a solid connection between two conductors of different potential; an arc fault is separated by air and characterized by intermittent sparking. This makes arc faults harder for standard circuit breakers to detect because the current may fluctuate rather than spike suddenly. A 15 ampere arc fault at 120 volts can still ignite wood or drywall.

Arc fault circuit interrupters, or AFCIs, are protective devices designed to detect and interrupt these events in tens of milliseconds. They monitor waveform distortion caused by arcing and trip the breaker when characteristic signatures appear. Residential electrical codes now require AFCI protection on most branch circuits, particularly bedroom outlets and areas where cables are vulnerable to damage.

Common causes include damaged extension cords, pinched Romex cable in walls, corroded aluminum wiring connections, and faulty appliance cords. Bedroom fires from arc faults were the primary reason for AFCI mandates beginning in the 1990s. Arc faults can initiate long before they become visible; a failing connection may arc intermittently for weeks before causing ignition.

Series arcs, where current attempts to flow through a broken conductor rather than jumping to ground, are particularly dangerous because they generate less heat and less current than parallel arcs (where one conductor contacts ground). Series arcs may never trip a conventional breaker, making their detection the key application for AFCI technology.