During commissioning of a 10-kW roof with three strings, the inverter shows low power and one MPPT reads near zero volts in full sun. The lead suspects a dead string, crossed leads, or a hot joint in the combiner. Which commissioning test of string voltage, polarity and thermography best isolates the fault?

The test that measures each string’s open-circuit voltage and polarity at the combiner, then scans the terminations with a thermal camera under load, combines checks suited to the suspected faults. Open-circuit voltage can reveal a dead or disconnected string, polarity can expose crossed leads, and a thermal scan under load can identify a resistive or otherwise hot termination. Measuring voltage and polarity and comparing them with design values is a valid commissioning step that can find a missing or reversed string, but it provides no thermal evidence about a hot combiner joint. Thermographically scanning after shutdown removes the operating current that can make a resistive connection heat up, so a cool-looking unloaded termination does not rule out a hot joint in service. Measuring open-circuit voltage and scanning terminations while the array operates can reveal an abnormal string voltage and a connection that heats under load, but without a polarity check it may not reliably distinguish crossed leads from other voltage abnormalities; one stated fault possibility remains insufficiently tested. Exam caveat: measure open-circuit voltage with the string disconnected from the inverter, following the equipment maker’s isolation procedure; an energized operating-voltage reading is not an open-circuit test. Practical check: compare each string’s measured voltage with the expected value for its module count and current irradiance, and confirm the meter leads and test points before recording polarity.