4. Insulating walls

4.1 General

In high-voltage insulation practice, liquids are used almost exclusively in conjunction with solid insulators (impregnated capacitors, transformers, etc.). Insulation must be absolutely free of gaseous inclusions, in which partial discharges are likely to occur as soon as the voltage they support exceeds the Paschen minimum (around 400 V), leading sooner or later to degradation and then breakdown of the insulation.

The solids used are paper and cardboard and, increasingly, synthetic polymers (e.g. polypropylene), which are impregnated with a suitably selected liquid. As there is always a double layer at the separation between an insulating solid and a liquid (see paragraph