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In this research, three kinds of silicone rubber based composite materials, SIR, NSIR and DSIR, were prepared. SIR is the reference silicone rubber without addition of antimicrobial biocide. Two different antimicrobial agents at a concentration of 3 wt% were added to the reference silicone rubber to form the other two types of materials. The materials were inoculated with a fungal spore suspension containing nutrients. Fungal growth could be observed visually in form of spots by naked eye, and biofilm was formed and detected by SEM observation on the surfaces of SIR and NSIR samples, whereas DSIR samples were completely free of the fungal growth. Then the electrical performance of both clean and biofilm contaminated samples, including the surface and volume conductivity, surface flashover voltage and leakage current before flashover, were measured. The obtained results indicate that as compared with SIR base material, the biocides have not changed the surface conductivity and surface flashover voltage of NSIR and DSIR materials. At the same time, the volume conductivity of NSIR remains almost the same as SIR, while that of DSIR increases by about two orders of magnitude. Biofilm could increase the surface conductivity and decrease the surface flashover voltage significantly. The time domain and frequency domain analyses of the leakage currents flowing before flashover event provide useful information on the severity of contamination by biofilm and the eventual risk for surface flashover.