The night string reverse-bias test feeds a small reverse current into each module string in the dark. The current is about 3% to 5% of the rated short-circuit current, and the reverse voltage the string reaches is measured. A string with an unusually high reverse voltage likely has modules with an open or unconnected bypass diode, or an unconnected substring in the junction box. To find the faulty modules, inspect the flagged string with an infrared camera at night while the reverse current is applied. Look for substrings that have warmed up slightly. The test targets junction box failure from cold solder joints (failure mode A) and bypass diode failure. Mode A cannot be seen in a normal daytime infrared inspection and goes unnoticed until the module is shaded. Resistive heating at a bad joint in mode A normally shows only under shading. Seeing it during this test is safer for the inspector, because a shaded module with this defect can heat within seconds and may char or burn. The test cannot tell a failed diode from an unconnected substring. That takes destructive analysis of the junction box. Use thermal inspection for the other connection failure modes (B and C). Because of the high fire risk, test all modules once a single failure is found.
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producesc1 verified · fact High string reverse voltage
At night, a small reverse current of 3% to 5% of rated Isc is fed into each string. A string that reaches an unusually high reverse voltage points to modules with an open bypass diode or an unconnected substring (failure mode A).
If a string with extraordinarily high reverse voltage is found
sec=2.4.2 h=a48c6a4339a6verified by: joshuahamsaverified on: 2026-10-20
c2 drafted · lead Warm substring in IR image
While reverse current is applied, a night IR inspection of the flagged string shows substrings warmed by a few kelvin. This locates the modules with unconnected bypass diodes or mode A failure.
sec=2.4.2 h=a48c6a4339a6fig=46 h=e4272b24daa9sec=h5 h=f6f94e604618detected_by c34