Rain-Triggered Isolation Fault at a Public Carport
A 208 kWDC carport array at a municipal fleet yard has tripped on inverter 'isolation fault' three times in six weeks, each time within 24 hours of rain. The inverter is a 100 kW three-phase string unit with integrated GFDI and a 0.5 A residual current monitor. The last trip recorded insulation resistance of 0.38 MΩ before the inverter disconnected. The array has eight strings of 18 modules on two canopies, one 12-channel combiner, and exposed DC conduit runs along the canopy underside. After each trip the system reset and ran normally until the next rain. Your firm has a two-person crew, a 1000 V megohmmeter, a clamp meter, and a thermal camera; the site is open to the public during business hours. The municipality wants the array back on before a heat wave. Rain is forecast again in 48 hours.
1. What diagnostic sequence should the crew follow, and what should they look for given the rain correlation?
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Lock out the inverter and open all string disconnects before touching DC conductors. Segment the array methodically: test each of the eight strings to ground at the combiner with a megohmmeter at an appropriate test voltage (commonly 1000 V DC here, not exceeding module ratings), then test the combiner-to-inverter feeder and the inverter DC input.
A 0.38 MΩ trip reading is below the typical 1 MΩ threshold, so expect a real low-resistance path. Because the fault is rain-triggered, focus on water intrusion: connectors, junction boxes, conduit entries, and module edges. Look for damaged backsheets, pinched cables, missing gland nuts, and rodent damage. A thermal scan while energized can localize leakage current.
Record every reading and compare against dry-weather values. Do not repeatedly re-energize to 'see if it holds' - each reset stresses the insulation and the GFDI, and a moisture fault can progress to arcing.
2. Why is resetting the inverter not an acceptable response to the repeat trips?
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Resetting clears the symptom, not the cause. Moisture ingress can carbonize and progress to arcing or a ground fault with fire potential, and repeated trips erode confidence in the GFDI's protection. The municipal schedule pressure does not change the electrical risk, particularly with public access beneath the canopies.
If the fault cannot be located before the next rain, isolate the affected string or strings and leave the remainder running only if the inverter design and code permit it, with clear labeling, notification to the owner, and documentation in the maintenance record. Otherwise keep the system de-energized until the repair is complete.
3. How should the crew manage public access and the 48-hour weather window?
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Use cones, barriers, and signage around the work area, and schedule intrusive work off-hours or with a spotter to keep the public away. Follow arc-flash boundaries and PPE requirements for DC testing. Photograph and log conditions before and after testing.
Prioritize finding and isolating the fault before the next rain rather than restoring full output. Give the municipality a written finding, the strings left isolated, and a repair plan with a date. If a string must remain isolated, confirm the inverter can operate safely with reduced input and that monitoring alarms are configured to reflect the temporary configuration.