IEC TS 62446-3 thermal inspections, explained
If you commission or deliver aerial thermography on solar PV plants, IEC TS 62446-3 is the document everyone will reference. Here is what it actually covers, how its abnormality classes are typically interpreted in practice, and what a report aligned with it should contain.
What the technical specification is
IEC TS 62446-3 is the third part of the IEC 62446 series on testing, documentation and maintenance of grid-connected PV systems. Part 3 addresses outdoor infrared thermography of PV modules and balance-of-system components on operating plants — the inspection type performed from a drone. It is a technical specificationrather than a full international standard, which matters for wording: it describes an agreed methodology and reporting framework, and industry practice has built a common interpretation on top of it.
The TS covers four things that determine whether a thermal inspection is worth acting on: the measurement conditions under which imagery is captured, the equipment and its suitability, the inspection procedure itself, and how findings are classified and reported.
The three abnormality classes
Findings are graded into three classes. The class definitions below reflect the typical industry interpretation aligned with IEC TS 62446-3 — they are how practitioners apply the framework, not quotations of normative thresholds from the specification itself.
| Class | Typical interpretation | Typical response |
|---|---|---|
| 1 | No thermal abnormality — behaviour within normal variation. | No action; keep in the monitoring baseline. |
| 2 | A real thermal abnormality — commonly read as a sustained ΔT of roughly 10 °C or more against comparable modules under good irradiance. | Investigate and schedule repair within a defined window. |
| 3 | A safety-relevant abnormality — commonly read as a ΔT of roughly 40 °C or more, or any finding with a credible fire or shock pathway (junction boxes, combiners, broken glass) at lower ΔT. | Immediate action; treat as a safety item, not a maintenance item. |
Measurement conditions that make ΔT meaningful
A temperature differential is only diagnostic if the modules were working hard when the frame was captured. The TS requires a minimum in-plane irradiance of 600 W/m² for thermographic inspection — below that, fault-driven ΔT shrinks into the measurement noise and a “clean” scan proves nothing. In practice that means flying within a few hours of solar noon under clear or lightly hazy sky.
- Irradiance at or above 600 W/m² in the module plane, ideally declared from a site measurement (per the TS).
- Low wind — strong wind cools module glass and suppresses the ΔT you are trying to measure.
- Near-nadir viewing angle, keeping the camera close to perpendicular to the modules to avoid reflections and emissivity error.
- Adequate ground sampling distance — enough thermal pixels per module that a single hot cell is resolvable, which sets your flight altitude.
- Stable, steady capture: consistent altitude, no motion blur, and a thermal range/gain that preserves scene contrast.
What a compliant report includes
A report aligned with the TS is more than a set of annotated images. It documents the plant and the inspection scope; the capture conditions (irradiance, ambient temperature, wind, sky state) that make the readings defensible; the equipment used; and then, per finding: the location down to the module, the thermal evidence, the anomaly type, the measured ΔT, its classification and a recommended action. Just as importantly, it records limitations — frames that could not be assessed and why — so the reader knows what the inspection did not establish.
How ThermalVariations aligns
Our analysis is built around the same framework. Every uploaded frame passes a capture-quality gate before analysis — irradiance floor, camera angle, sun glint, focus and thermal contrast — so findings rest on frames that meet the measurement conditions above (see the capture guide). Detections are classified against a taxonomy of 17 anomaly types in the TS style, graded by measured ΔT using the typical industry interpretation, with fire-risk types escalated earlier. The report you receive carries per-module locations, flagged imagery, severity ranking and a capture-issues section. We provide analysis aligned with the TS; formal certification of a plant remains the domain of an accredited on-site inspection.
Need an IEC-aligned thermal analysis?
Flat-rate analysis: you fly the site, we grade every thermal frame and deliver a severity-ranked report. Estimate the cost for your site size in seconds — no account, no sales call.
ΔT figures and abnormality classes on this page describe the typical industry interpretation aligned with IEC TS 62446-3 — indicative engineering guidance, not normative text from the specification. Temperature differentials are only meaningful when captured at ≥600 W/m² irradiance (per the TS) and verified on site before repair work.