Garage PV Fire: Arc Mapping With an Energized DC Source

Arc mapping is less reliable here because the PV array is an energized DC source that can sustain arcing even after the AC breaker trips. The multiple beads along a 6 m run may be caused by the fire burning insulation, by mechanical damage during firefighting, or by the original fault. The investigator should map each bead, note whether it is in a series or parallel configuration, and compare bead location with char depth and fire patterns. Beads at the fire origin are more significant than beads downstream. Because the system lacks rapid shutdown, the DC conductors remained energized, so arcing could continue and create misleading beads. NFPA 1033 requires evaluating electrical systems and arc mapping with an understanding of their limitations. Arc mapping alone cannot establish origin; it must be corroborated by the fire pattern, inverter data, and laboratory analysis.

The investigator should document the array, inverter, wiring, and conduit with photographs and a scaled diagram; record string voltages and insulation resistance if safe; and download inverter fault logs and monitoring data. They should test the table saw circuit and the AC panel, and examine the DC conductors for pre-fire damage versus fire-induced damage. Laboratory analysis of arc beads, including metallography, can help determine whether beads formed in air or under debris. The investigator should review the PV design and installation records, check for rapid shutdown compliance, and interview the homeowner about the table saw use. NFPA 1033 requires a systematic approach and evidence collection. The goal is to test competing hypotheses: a PV series arc, a table saw electrical fault, and an external fire. If the fire patterns begin at the inverter and the DC beads are downstream, the PV system may be the cause; if the fire began at the table saw and the PV damage is above, the PV may be a victim.

The missing rapid shutdown means the DC conductors could remain energized, increasing arc hazard and fire duration; it is a code-compliance issue but not proof of cause. The tripped AC breaker may have tripped because of the fire, not because it initiated the fire. The investigator should inspect the breaker for arc damage and determine whether it tripped before or after the fire. If the breaker tripped before the fire, that could indicate an AC fault; if after, it is a fire effect. The investigator should also check whether the inverter had a ground fault or arc-fault detection. NFPA 1033 requires the investigator to consider building systems and codes. The final report should separate code violations from cause and avoid using a violation as a substitute for causal analysis.