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SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative is required from an environmental perspective. The authors focus particularly on CO2 gas, which has a low global warming potential, as one of these potential alternatives. In order to apply this CO2 gas to actual equipment, the insulation characteristics for overvoltage waveforms generated in an actual field (called non-standard lightning impulse waveforms) must be obtained. The present study obtained and evaluated the insulation characteristics where a disconnector switching surge was superimposed over the residual dc component (bias voltage) caused by operating a switchgear under a non-uniform electric field, typically represented by metallic particles. Consequently, it was concluded that the breakdown voltage was lower for positive polarity under a non-uniform electric field in the presence of a bias voltage and that it was appropriate to conduct an experiment using a positive polarity waveform in the study of dielectric strength. Furthermore, it emerged that the breakdown voltage for a positive polarity waveform was 1.05 to 1.64 times higher than that for the standard lightning impulse waveform, even if the frequency and damping rate were changed.