Sulfur hexafluoride (SF

6

) gas has been widely used in electrical equipment due to its excellent insulation and arc extinguish properties. However

it has strong greenhouse effect. With the increasingly serious problem of global warming

it’s urgent to find new environmentally friendly gases that can replace SF

6

. Perfluoroisobutyronitrile (C

4

F

7

N) possesses outstanding insulation performance and good engineering application prospect. However

there is no research on short line fault (SLF) interruption characteristics of circuit breaker with for C

4

F

7

N/CO

2

/O

2

gas mixture. In this paper

the SLF interruption performance of a 126 kV/3 150 A/40 kA live tank circuit breaker with C

4

F

7

N gas mixture is studied. The SLF interruption performance of the circuit breaker filled with C

4

F

7

N/CO

2

/O

2

gas mixture

CO

2

/O

2

gas mixture and SF

6

gas is compared under different current decline rates at current zero (d

i

/d

t

). The L90 short line fault interruption test results show that under the same conditions

SF

6

gas exhibits the best interruption performance and can be able to interrupt the test current within a wider arc duration range. The interruption performance of C

4

F

7

N/CO

2

/O

2

gas mixture and CO

2

/O

2

gas mixtures at 0.75 MPa under d

i

/d

t

=12 A/μs is close to that of SF

6

at 0.5 MPa under d

i

/d

t

=20 A/μs. Based on the test results

an improvement scheme of the interrupter structure is proposed and verified by test. This study in this paper provides a reference for the development and desig

n of circuit breaker with C

4

F

7

N gas mixture.