Insulator Flashover

Ash-induced insulator flashover is attributed to the high conductivity (Ï’), low resistivity (ρ) of volcanic ash when wet. Dry volcanic ash is non-conducting (ρ > 1.56 x 107 Ωm), however, the conductivity of volcanic ash increases abruptly with the adsorption of water. Further increase in conductivity has been observed with increasing soluble salt content and compaction. All grain sizes (<32 µm to 1.4 mm) can exhibit high conductivity values (ρ < 100 Xm) and therefore have similar potential to cause flashover on HV insulation.

Equivalent Salt Deposit Density (ESDD) and Non-Soluble Deposit Density (NSDD)

According to pollution severity classification in IEC 60815-1 (2008), volcanic ash is a rare but severe form of Type A pollution. Recent literature indicates that a 3 mm thick deposit (NSDD between 158 and 231 mg/cm2) of fresh volcanic ash yields an ESDD between 0.02 and 0.7 mg/cm2, suggesting that ash can have high contamination severity and therefore potential to cause pollution-induced insulator flashover.