Volcanic ash contamination: limitations of the standard ESDD method for classifying pollution severity

The pollution severity of airborne contamination on high voltage insulation has traditionally been quantified by calculating the contaminant´s equivalent salt deposit density (ESDD). Volcanic ash is a rare but severe form of airborne pollution, and the high conductivity of wet volcanic ash (often >1.3 × 10^-4 S/cm) can cause pollution-induced insulator flashover. This paper presents the ESDD and non-soluble deposit density (NSDD) for four different fresh volcanic ash samples and two ash proxies measured at different thicknesses using a standardised plate test. Results show that there is a log-linear increase of ESDD with increasing NSDD. Tests indicate 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. Whilst the ESDD/NSDD method provides direct analysis of the ionic content of a contaminant, the procedure is time consuming, cannot accommodate the high NSDD of volcanic ash for site pollution severity classification and does not account for changes in the contaminant´s electrical conductivity under different environmental, chemical and physical conditions. Given these limitations, this study proposes an alternative, simple yet more comprehensive technique for investigating the electrical properties of volcanic ash by means of direct resistivity analysis.