Testing, Inspection, and Troubleshooting
What you must understand
Follow a fixed order: visual inspection, then de-energized tests, then energized tests. De-energized work includes continuity and equipment-grounding checks, polarity verification, and insulation resistance with a megohmmeter (disconnect surge arrestors and electronics first). Energized work includes open-circuit voltage, short-circuit or operating current, string-to-string comparison, and I–V curve tracing. Thermal imaging finds hot spots, failed bypass diodes, and loose terminations.
Every DC measurement must be interpreted against the irradiance and cell temperature at that moment. Comparing a string's Isc at 700 W/m² to the nameplate at 1000 W/m² is meaningless without correction. Troubleshooting is elimination: array, then wiring, then disconnects, then inverter, then grid.
Traps that appear in questions
- Failing a string because Isc is low under low irradiance.
- Megohmmeter testing with arrestors, optimizers, or inverters still connected.
- Measuring both conductors of a circuit with a clamp meter, where the fields cancel to zero.
- Comparing strings on differently oriented roofs.
- Chasing an inverter fault before confirming DC input is healthy.
Worked example
Nameplate Isc is 9.0 A at STC. A field measurement reads 7.2 A with a pyranometer showing 800 W/m². Expected current ≈ 9.0 × (800 ÷ 1000) = 7.2 A. The string is healthy - no defect. Rule of thumb: always scale expected current by irradiance before you judge a reading.