Unlike standard AC circuits where current crosses zero 100 times per second naturally quenching arcs, direct current (DC) from solar PV strings maintains continuous high voltage (up to 1,000 V DC). Improper isolation or incompatible connectors create intense plasma arcs that burn through enclosures and ignite roof structures.

The Nature of DC Arcing Hazards

A rooftop solar PV array cannot be turned off at the source during daylight hours. As long as light strikes the modules, conductors remain live with dangerous DC potentials.

  • Continuous Plasma Arc: An electric arc in a 600 V DC string will sustain across air gaps exceeding 20 mm, reaching temperatures over 3,000°C.
  • MC4 Connector Mismatch: Cross-mating connectors from different manufacturers (e.g. Stäubli MC4 with clone brands) is the #1 cause of rooftop PV fires due to microscopic resistance gaps and thermal runaway.

DC Rotary Switch Disconnector Selection (Regulation 712.536.2.2)

Every solar PV installation must have a dedicated DC switch-disconnector installed adjacent to the inverter:

  • Utilization Category: Must be rated for DC-PV1, DC-PV2, or DC-21B at the maximum string open-circuit voltage ($Uoc(stc)$) corrected for cold temperatures (-10°C).
  • Snap-Action Mechanism: Standard AC rotary switches must NEVER be used on DC circuits; they lack the magnetic arc chutes and rapid spring-assisted snap-action needed to break DC current.
  • Polarity and Enclosure: Ensure correct positive/negative terminal connections and UV-resistant IP65 enclosures.

Firefighter Safety & Emergency Shutdown

Section 712 Best Practice:

• Clearly label all DC cable enclosures with permanent warning labels: "WARNING: DUAL SUPPLY - DC CABLE ENERGIZED BY DAYLIGHT".

• Install automated motorized fireman switches or module-level power electronics (MLPE / micro-inverters) to de-energise rooftop string voltages to extra-low voltage (< 30 V) when the main AC power is cut.