Solar Arc-Fault Recurs Only in Wet Weather: Decision Tree

Why this matters

A DC arc-fault circuit interrupter (AFCI) trip is the system telling you it detected the signature of a series or parallel arc in the array wiring, which is a genuine fire-ignition risk. When the trip recurs only in wet weather, two possibilities exist: a real moisture-driven fault (water tracking across a degraded connector, cracked insulation, or a compromised junction creating a low-current arc or leakage), or AFCI nuisance tripping where wet-weather electrical noise crosses the detection threshold without a true arc. The first is a fire hazard that must be found and fixed. The second is rarer and must never be assumed first. Getting the order right keeps you from clearing a real arc as nuisance, which is how PV fires start. Wet-weather correlation is a strong clue toward moisture ingress at a specific point.

Symptom presentation

The inverter logs a DC arc-fault and shuts the affected input down, requiring a manual or automatic restart per NEC 690.11. The trips cluster during or shortly after rain, heavy dew, or high humidity, and the system runs clean in dry conditions. The customer sees repeated outages on rainy days. There may be a visible scorch, discoloration, or melt at a connector or junction if a real arc is present, though early-stage faults leave little visible evidence.

Quick checks

Pull the inverter event log and confirm the code is specifically arc-fault, not ground-fault or overvoltage (different problem, different tree). Note the exact times and correlate with weather. With the array safely de-energized at the disconnect, inspect every MC4 and field connection on the affected string for water intrusion: cloudy or wet connector interiors, corrosion on pins, missing or pinched O-rings, and connectors resting in standing water or against the roof where they pool. Check for cracked or UV-degraded insulation, rodent damage, and any field-made connection or mismatched-brand MC4 mating (a common arc source). Inspect cable management; cables draped in a water channel are suspects.

Isolation tree

Start by confirming arc-fault vs ground-fault from the log; if it is ground-fault, switch to the ground-fault tree. Confirmed arc-fault: branch on visible evidence. Any scorch, melt, or discoloration at a connection? Yes branch: you found a real series-arc site, that connection is the fault. No visible evidence: go to connector integrity. With PV off, open and inspect each connector on the affected input; any water inside, corroded pins, or mismatched-brand mating is a probable arc/leakage source in the wet. Found a compromised connector: that is the fault. All connectors clean and dry: go to insulation and routing. Inspect the full cable run for cracked insulation, abrasion against racking or roof edges, and any point where water tracks across conductors; a pinhole in insulation in a wet location can arc. Nothing found and trips still wet-correlated: only then consider AFCI sensitivity, and confirm by reviewing the manufacturer guidance, never by simply disabling the AFCI.

Confirming diagnosis

A real moisture-driven arc is confirmed when you locate a water-intruded, corroded, or scorched connection or a damaged insulation point on the affected string, and the trips stop after repair across subsequent wet days. The wet-only correlation plus a physical defect at one location is conclusive. A series arc often leaves carbon tracking or a pitted pin once opened. Genuine AFCI nuisance is confirmed only after a thorough physical inspection finds no defect, the inverter manufacturer documents a known wet-weather sensitivity for the model and firmware, and a firmware update or a manufacturer-sanctioned detection adjustment resolves it without compromising real-arc detection.

Remediation

Replace any water-intruded, corroded, or scorched connector with new connectors of the correct matched brand and the manufacturer crimp tool; never mix MC4 brands in a mated pair, which is itself a frequent arc cause. Replace cable with cracked or abraded insulation, and re-route cables out of water channels with proper standoffs and UV-rated clips. Reseat every connector to full engagement and re-secure cable management so nothing sits in pooling water. Only after physical causes are excluded and the manufacturer confirms a known nuisance pattern should firmware be updated; the AFCI function itself must remain enabled and at the listed detection capability.

References

• NEC 2023 Article 690.11 Arc-fault circuit protection (direct current)
• UL 1699B Photovoltaic DC Arc-Fault Circuit Protection standard
• NEC 2023 Article 690.31 Wiring methods and connector requirements
• Inverter manufacturer arc-fault troubleshooting and firmware bulletin (cite the installed model)