The steep-fronted impulse voltage test equipment was developed for studying the discharge characteristics and mechanisms of SF

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gas under impulse voltages. The influences of electrode radius on breakdown characteristics of the distance fixed SF

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gap were researched. It was found that the discharge behaviors of SF

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gap presented the so-called critical radius effect occurred in long gap discharge in air. With the decrease of electrode radius

the 50% breakdown voltage of the gap decreased till the critical radius Rcr was reached

and then remained practically constant for further reduction of r. It was found that Rcr increased with the decrease of gas pressure and wave front time. When the pressure was fixed

the critical radius v

alue of negative VFTO or lightning impulse was bigger than that of positive VFTO or lightning impulse

the critical radius value of negative or positive VFTO was bigger than that of negative or positive lightning impulse. For negative voltage

Rcr is physically close to the length of filamentary streamer zone. While for positive voltage

Rcr is close to the length of homogeneous streamer zone. The critical radius phenomenon is related with the streamer-precursor-leader discharge process in SF

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gas

which is explained by the proposed critical charge criterion of leader breakdown.