5. Thermal resistance of insulating materials

One of the most important stresses to which insulating materials in electrical equipment are subjected is, after electrical stress, that resulting from heating.

The effect of thermal stress is twofold:

• the first concerns the instantaneous behaviour of insulators at high temperatures;

• the second relates to the degradation that takes place progressively over time and affects their long-term behavior.

We'll look at these two aspects in turn, highlighting the different temperature limits that need to be considered, depending on whether we're dealing with short- or long-term heating.