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  • Writer's pictureKlaudia Weres

5 Types of Thermal Anomalies You Need To Keep An Eye On

The longevity of PV panels often depends on a constant maintenance performed on site. Neglected problems may lead to malfunction of the modules which could cause a significant drop in energy production over the years. Since most of the flaws are invisible to the naked eye, this is where thermographic inspections come in to help spotting and resolving those disruptions before further damage affects the panels.

Here at Wesii, with the help/guide/in compliance with the international Regulation IEC, we have established a classification of the most common anomalies. Let’s dive in and have a quick look at each one of them!


A single hotspot is probably one of the most common anomalies that could affect a PV panel. The high increase of the temperature is caused by broken cells and it’s visible on TIR images as a punctual spot. An untreated hotspot may lead to irreversible cell damage, encapsulation and creation of a bypass diode.

Maintenance recommendation:

Check the module and make sure there is no shading or severe soiling


What we classify as multiple hotspot is nothing but a set of punctual hotspots grouped together in the same panel. They might be caused by normal dirt, bird droppings, broken glass or modules in short circuits (crystalline Si). Moreover, multiple hotspots have a similar pattern to the broken front glass, PID, cell defects and mismatch.

Maintenance recommendation:

Check the module and its cabling


Diodes are linear anomalies insisting on an entire bypass diode area of a PV module. Part of the module surface is homogeneously heated up and heat dissipation by the bypass diode which is operating, is visible as a vertical stripe.

Temperature difference of the glass on top of the junction box containing the operating bypass diode differs from construction state.

Maintenance recommendation:

Check the module, its junction box and bypass diode


Strings are probably the most severe panel malfunction. They present a temperature increase of all the modules of one or more disconnected strings. The module surface is homogeneously heated and ΔT of the junction boxes is similar to operational state.

Maintenance recommendation:

Check the module, the state of inverter operation, the condition of cabling, connectors and fuses


This type of anomaly is easily noticeable. It shows up on TIR images as a uniform temperature increase all over a single panel. It’s caused mostly by broken glass.

Maintenance recommendation:

Check the module

All the above recommendations are technical advice suggested in the international standards IEC TS 62446-3, Edition 1.0 2017-06


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