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Searching for SF6-substitute gaseous dielectrics is currently an urgent task for high-voltage circuit breaker (HVCB) insulation and arc extinction applications. Once leaked, SF6 will affect the Earth's climate on the order of decades to millennia. In this work, we developed a test method that supplies a series of extended pulse voltages to obtain the dynamic dielectric strength of C3F7CN/CO2 and C3F7CN/N2 gas mixtures that are potential SF6 alternative gases in an HVCB during its no-load closing-opening processes, by measuring the dynamic breakdown voltage between the contacts. We also investigated the closing pre-strike and opening insulation characteristics of these gas mixture HVCBs at different voltage polarities, C3F7CN concentrations and pressures during the processes. We experimentally found an obvious “low-voltage-breakdown” phenomenon and much higher dispersion in the dynamic breakdown voltage of C3F7CN gas mixtures compared with pure SF6. Moreover, the dispersion in closing was significantly higher than that in opening, leading to the closing pre-strike characteristics of an HVCB filled with C3F7CN mixtures that are inferior to SF6. The results not only provide a criterion via data but also evaluate the interruption gap design and give insight into the expected performance of the interruption chamber during breaking capacity tests. [ABSTRACT FROM AUTHOR]