The overheating protection triggers when excessive temperatures occur in the drum core. The current is interrupted at a temperature of approx. 65°C. Such high temperatures occur when the cable has not been sufficiently unwound and when heat is generated by too much current flowing through the connected consumers. This is also referred to as the coil effect.
The maximum load capacity of your cable drum can be found on the device label on the back of the cable drum:
(example)
It should be noted, however, that overheating can also be caused by other heat sources, such as solar radiation or machines in closed rooms. The heat generated during use therefore adds up with the ambient temperature.
You can read more about the topic here.