During a midday O&M visit for a 12-kW rooftop string with low output, you suspect a hot fuse holder and one warmer module section. Ambient is 28C, wind is calm, and the array is under steady sun. Which instrument practice should you use to screen with the IR thermometer and thermal camera?

Comparing nearby similar parts under steady sun and the same load is the right screening practice: the hot fuse holder and warmer module section stand out as temperature differences from like components experiencing similar current, irradiance, ambient conditions and wind. This comparison controls for factors such as emissivity, viewing distance and the 28°C background, which can make a single absolute temperature unreliable for screening. Logging calibrated absolute temperatures against maker limits is useful for acceptance checks, but as screening it substitutes a threshold check for the required like-for-like comparison and is sensitive to emissivity settings, spot size and reflections. Comparing similar parts while clouds pass quickly fails because changing irradiance and current can create transient temperature differences that mimic or hide resistive heating. Comparing only the fuse holder with nearby fuses in steady sun omits the warmer module section, although the visit calls for screening both the fuse-holder and module concerns. Exam caveat: the conditions work together—steady sun, the same load and a like-for-like comparison—so changing irradiance stability or the comparison basis can undermine the result. Practical check: in calm wind and full sun, maintain a consistent viewing angle and distance, capture the suspect and nearby similar parts, and record load, irradiance, ambient temperature and wind; flag a clear rise over the reference for investigation and trending.