A surge arrester is classified thermally stable in IEC 60099-4 [2] if it is able to cool down after one or more voltage surges are applied. Fig. 5 shows the principle behavior.
As long as the cooling heat flow power ˙Q exceeds the power losses P for a given temperature, the arrester is thermally stable. Beyond the stability limit, the thermal dissipation
capability of the arrester is smaller than the power losses. A destructive thermal runaway occurs. Due to the electrical excitation, losses occur depending on the arrester temperature and voltage surge amplitude. This injected energy must be transferred through the surface of
the arrester stack and further on via the air gap through the porcelain to the ambient air (cooling heat transfer ˙Qcd). The difference between the power loss P and ˙Qcd is the effective
power flow ˙Qeff from the ZnO material to the ambiance, resulting in a temperature decrease