Due to their stable physicochemical property and good arc-quenching capability

both CO

2

and CF

4

gases attract a lot of attention of researchers as a possible substitute for SF

6

. The transient nozzle pressure during the arc interruption process is necessary for the arc-interrupting properties and the optimization design of circuit breakers.We measured and analyzed the gas pressure variations at the upstream of PTFE nozzle in SF

6

-CO

2

and SF

6

-CF

4

mixt

ures for different mixed ratios by carrying out tests with a 126 k V puffer gas circuit breaker prototype. It is found that both the peak value of the gas pressure increment Δppeak and the gas pressure increment at current zero ΔpCZ in SF

6

-CF

4

mixture are clearly higher than those in SF

6

-CO

2

which indicates the stronger gas blowing effect of SF

6

-CF

4

mixture. With the increase of SF

6

concentration

the Δppeak in both gas mixtures of SF

6

and the ΔpCZ in SF

6

-CO

2

increase obviously

while the ΔpCZ in SF

6

-CF

4

varies slightly. In addition

the establishment of outlet pressure plays an important role in the thermal interrupting capability of a circuit breaker.