DC arc faults in solar systems: the silent fire-starter

DC arc fault is the silent cause of solar fires. Unlike AC, once formed, it doesn't self-extinguish. Can burn for minutes at 7200 °F before anyone notices.

What it is and why it's dangerous

When two conductors slightly separate under load, a spark ionizes the air and forms a continuous arc. In AC the arc self-extinguishes 120 times per second (zero crossing). In DC no zero crossing: arc burns indefinitely.

Typical causes

1) Bad-crimped MC4 loosening with vibration. 2) Terminal screw loosened by thermal expansion. 3) USE-2 cable chewed by rodent. 4) Burned bypass diode in panel. 5) Internal microcrack opening over time.

Indirect symptoms

Burned smell with no visible cause. Black soot stain on junction box. String production drops 30-50% with no reason. Faint crackling near electrical boxes. Any of these requires immediate shutdown.

AFCI: the lifesaver

Arc Fault Circuit Interrupter. Electronic device monitoring DC circuit signature. Detects arc in milliseconds and disconnects. Mandatory in USA per NEC 2014+ for any residential install. EU: still voluntary but recommended.

Rapid Shutdown

Rapid-shutdown systems (Tigo, SolarEdge, Enphase) drop array voltage to <30 V in 30 seconds at the flip of a switch. Useful for firefighters. NEC 2017+ requires it in USA. EU: voluntary.

Basic prevention

1) MC4 crimping with correct tool and pull test. 2) Terminal screws tightened to torque (3-5 Nm typical) at install + annual review. 3) Visual inspection every 6 months looking for loose MC4s. 4) Annual thermal camera to detect hot spots.

Combine with correct electrical protections.