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This study refers to the laboratory tests that have been conducted under 50 Hz applied voltage on a polluted plane model simulating the 1512 l outdoor insulator largely used by the Algerian Company of Gas and Electric Power (SONELGAZ). They were carried out in order to identify the pollution level of the insulator surface, first by measuring the leakage current (LC) waveforms (i), and then by measuring the voltage waveforms (ii). (i) The evaporation of the electrolytic solution has especially been taken into account when electric discharges appeared on the contaminated insulator surface by performing a spectral analysis of LC waveforms. This has been done with the Welch periodogram in order to understand the contribution of LC frequency harmonics in drying the wetted layer depending on the pollution level of the insulator surface. (ii) As for the continuous wavelet transform, it has been used for analysing locally the voltage waveform, to understand the effects of different pollution levels on voltage drops, and ultimately to establish a visual diagnostic tool for monitoring the pollution level and for preventing the breakdown flashover.