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This article presents the results of the studies on positive streamers behavior in an insulation system of non-uniform electric field distribution where point-to-sphere electrode system was divided on two parts by placing the pressboard barrier in the middle of oil gap. The studies were performed for two synthetic esters as per IEC 61099, natural ester as per 62770, blended low viscosity ester and mineral oil as per IEC 60296. The experimental techniques used in the measurements were both electrical and optical ones. The results showed that independently of the liquids tested lightning strength of the insulation system increased circa 3 times in relation to the system without pressboard barrier. In turn in the waveforms of light emitted by the streamers propagating in esters no differences were observed between them. However in comparison to the mineral oil duration of streamer propagation is longer in esters and intensity of emitted light is higher. In general the results indicated quite similar properties of the streamers developing in all the liquids tested in the condition of the experiment.