Volcanic ash is a potentially highly disruptive form of contamination capable of causing insulator flashover across station and line insulators. Ash-induced insulator flashover is a disruptive electrical discharge over the surface of solid insulation caused by the high conductivity of volcanic ash when it becomes wet (e.g. from light rain conditions such as dew, fog, light rain, etc.).
The dielectric strength of ceramic insulators under critical ash contamination (complete surface coverage) is ~10 kV/unit or 30 kV per metre of creepage distance. It is estimated that 1 mm (1/32 in) thickness coverage of ≥40% of insulator creepage distance in wet ash is critical, and will lead to flashover problems.
The dielectric strength of non-ceramic (e.g. composite polymer, silicone rubber, etc.) insulators is higher than ceramic equivalents under volcanic ash contamination (recent literature suggests 35 kV per metre of creepage distance). Thus, non-ceramic insulators are the optimal choice of insulation in ashy environments.