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Silicone rubber insulators were tested in salt-fog under different voltage and conductivity levels. The autocorrelation function was calculated for both the fundamental and third harmonic components of leakage current (LC). It has been observed that distinct differences exist in the behaviour of both the fundamental and that of the third harmonic components of the LC during the early aging period (EAP). Although the fundamental component of the LC begins to grow immediately after starting the test, the third harmonic requires a much longer period of time to begin. Dry band arcing is highly correlated with distortion in the LC and hence to its third harmonic component. But it has been observed that the level of the fundamental component of LC at which the third harmonic component started to increase is different from one case to another. Hence, in this work, time series modeling is used to quantify the difference between the fundamental and the third harmonic components of the LC during the EAR As such, it is more appropriate to use the autocorrelation function of the third harmonic component of LC as an indication of dry-band arcing rather than a simple threshold value.