Thermographic inspection of PV systems — outdoor infrared thermography (IEC TS 62446-3)

The guide on thermal cameras covers the general theory behind infrared measurements: emissivity and reflected temperature. The guide on PV hotspots and bypass diodes covers the electrical failure mechanism behind a hotspot. This article covers the specific field procedure that brings both topics together for PV systems: outdoor infrared thermography per IEC TS 62446-3 ("Photovoltaic (PV) systems — Requirements for testing, documentation and maintenance — Part 3: Outdoor infrared thermography").

The minimum irradiance threshold: 600 W/m²

IEC TS 62446-3 requires that a reliable outdoor inspection only be performed at an in-plane irradiance of at least 600 W/m². Below that threshold, the power — and therefore the heat dissipated by an electrical fault — is too low to still cause a measurable temperature difference that can be distinguished from normal measurement noise and ambient influences. An inspection carried out under insufficient irradiance that reports "no anomalies found" therefore proves nothing: any real faults present may simply have stayed below the detection threshold of those measurement conditions.

Relative, not absolute, comparison

A common misconception is to judge the temperature difference of a suspected hotspot against ambient (air) temperature. IEC TS 62446-3 instead requires a comparison against a healthy reference module or area under the same conditions (same irradiance, ambient temperature, wind speed, angle of incidence) — because the absolute operating temperature of a healthy PV module already varies significantly with irradiance, wind, and ambient temperature. Only the difference (ΔT) relative to a comparable, healthy reference point filters out that background variation and isolates the effect of the actual anomaly.

Normalising for irradiance level

The same ΔT value does not mean the same thing everywhere: a ΔT of 10 °C at an irradiance of 500 W/m² points to a more severe underlying problem (for example a higher effective resistance or mismatch) than the same ΔT of 10 °C at 1000 W/m² — at higher irradiance there is simply more power available to cause the same temperature rise, so the same ΔT at lower irradiance indicates a relatively larger underlying anomaly. Judging the severity of an anomaly therefore always requires that the irradiance measured at that moment is recorded and taken into account — not just the raw ΔT reading from the camera.

The spatial pattern matters at least as much as the ΔT value

IEC TS 62446-3 provides a matrix of typical thermal patterns, each pointing to a different underlying cause regardless of the exact ΔT value:

A single, hot cell: usually points to a local cell defect or micro-crack within that one cell.
A hot substring (the part of a module protected by one bypass diode): usually points to a failing or permanently conducting bypass diode for that substring — see the PV-hotspot guide for the underlying mechanism.
A uniformly heated, full module: points more towards a junction box or connector problem for that module than a cell-internal defect.
A uniformly heated, full string: points to an electrical problem at string level (for example a poor connection or an incorrectly sized component), not an individual module defect.

The pattern of heat distribution is therefore at least as diagnostic as the absolute ΔT value: the same ΔT can point to a small, local problem on a single cell and to a far more serious electrical fault on an entire string.

Note: IEC TS 62446-3 also sets requirements for the measurement equipment (resolution, sensitivity), the inspection procedure, the reporting, and the qualification of the thermographer carrying out the inspection — this article covers the core principles of irradiance, reference comparison, and pattern recognition, not the full, detailed procedural requirements of the standard.

Practical relevance

When planning a thermographic inspection round of a PV system, it is important to measure and record the on-site irradiance (not just estimate it from the weather forecast), photograph healthy reference modules under the same conditions, and, when assessing any anomaly found, report not just the ΔT value but also the spatial pattern (cell, substring, module, string) and the irradiance at that moment — otherwise the severity classification is neither reproducible nor comparable with a later inspection.

Common mistakes

Inspecting at an in-plane irradiance below about 600 W/m² and concluding the installation is fault-free — at insufficient irradiance, real anomalies may disappear beneath the measurement noise.
Comparing ΔT against ambient air temperature instead of against a healthy reference module under the same conditions.
Basing the severity of an anomaly purely on the raw ΔT value, without weighing the irradiance at that moment — the same ΔT is more severe at lower irradiance.
Ignoring the spatial pattern of the thermal anomaly (cell versus substring versus module versus string) and looking only at the temperature peak — the pattern is often more diagnostic of the underlying cause than the ΔT value alone.