Solar Ground-Fault Trips Only After Morning Dew: Decision Tree

Why this matters

A ground-fault detector/interrupter (GFDI) trip means current is leaking from the DC array to ground, a shock and fire hazard the system is required to detect under NEC 690.41 and 690.5. When it trips only on dewy mornings then clears as the array dries, you are almost certainly looking at low insulation resistance: a marginal insulation defect, water-intruded connector, or damaged cable jacket where moisture lowers the leakage path resistance enough to cross the GFDI threshold, then recovers as the sun bakes the moisture out. This is a real fault that will worsen and can eventually trip in dry weather or ignite, so it must be found, not dismissed. The morning-dew correlation is one of the most useful diagnostic clues in PV because it points you straight at insulation resistance testing rather than chasing the inverter.

Symptom presentation

The inverter logs a ground-fault (often coded as GFDI, isolation, or Riso fault) shortly after sunrise on humid or dewy mornings, holds the array offline, then auto-recovers or restarts successfully once the array surface dries an hour or two later. Production is normal the rest of the day. The customer reports a brief morning outage that disappears, sometimes for weeks of dry weather, then returns with the next wet morning. Some inverters report a numeric insulation-resistance (Riso) value at startup; on trip mornings that value dips below the trip threshold.

Quick checks

Pull the event log and confirm the code is ground-fault/isolation, not arc-fault (different tree). If the inverter reports a Riso value at startup, watch it on a dewy morning; a healthy array reads high (hundreds of kilohms to megohms), a faulting one drops toward the threshold. With the array de-energized and following lockout, inspect for the usual moisture entry points: connectors lying in standing water or against the roof, cracked or rodent-chewed insulation, junction boxes with failed seals, and any field splice. A damaged spot where conductor copper or shield can reach a grounded surface through a wet path is the target.

Isolation tree

Confirm ground-fault, not arc-fault, from the log. Confirmed: branch on whether you can string-isolate. De-energize and disconnect strings one at a time at the combiner, then perform an insulation-resistance (megohmmeter) test on each string conductor to ground. Which string reads low? That string carries the fault. Within the faulting string, branch by section: split the string and re-test halves to bracket the leakage to a run, then to a connector or module. Found a low-reading segment: inspect it physically for the wet path. Common findings, water-intruded connector (cloudy interior, corroded pins, missing O-ring), cracked or abraded cable jacket against racking, a module junction box with a failed seal, or a damaged conductor at a roof penetration. No single string reads low under dry test: re-test during or right after dew, because the fault may only appear wet, then localize while the leakage is present. Equipment-grounding continuity and a properly bonded array are prerequisites; verify the grounding path is intact so the GFDI can actually sense.

Confirming diagnosis

Confirmed when megohmmeter testing isolates one string (and ideally one segment) reading low insulation resistance to ground, and that segment contains a physical moisture-entry defect. The morning-dew timing plus a measurable Riso drop plus a located wet path is conclusive. If the inverter logs Riso, the value crossing below threshold on dewy mornings and recovering when dry corroborates the moisture mechanism. A truly intermittent fault may require testing while the array is still wet to capture the low reading, since a dried defect can read acceptable.

Remediation

Replace the water-intruded connector with new matched-brand connectors and the manufacturer crimp tool; replace any cable with cracked or abraded insulation, and re-route out of water channels with proper standoffs. Reseal or replace a failed module junction box per the module manufacturer. Repair compromised insulation at roof penetrations and ensure conductors do not chafe against grounded racking. After repair, re-run the insulation-resistance test (ideally under wet conditions or by wetting the suspect area) and confirm Riso is well above the trip threshold, and verify the equipment-grounding and bonding path is continuous before returning to service.

References

• NEC 2023 Article 690.41 PV system ground-fault protection and 690.5 detection requirements
• NEC 2023 Article 690.45 and 250.119 Equipment grounding conductor sizing and bonding
• IEC 62446-1 PV systems commissioning and insulation-resistance test method
• Inverter manufacturer isolation/Riso fault troubleshooting bulletin (cite the installed model)