SF

6

is currently the most widely used insulation gas in gas-insulated equipment. However

its global warning potential（GWP）is extremely high. C

4

F

7

N/CO

2

gas mixtures

a kind of environmental friendly alternative gas to SF

6

and with great development prospect

has attracted much attention. At present

the study on the switching impulse insulation characteristics of C

4

F

7

N/CO

2

gas mixture in practical application scenarios is less. In this paper

the switching impulse insulation test platform in the experiment is set up

and the switching impulse insulation characteristics of 252 kV GIL in C

4

F

7

N/CO

2

gas mixture with mole ratio of three C

4

F

7

N gas at different gas pressure is studied. And SF

6

at 0.5 MPa is set as the control group. It is found from analysis of discharge data that at the same pressure of C

4

F

7

N/CO

2

gas mixtures increase of the mole proportion of C

4

F

7

N can increase switching impulse insulation strength of GIL. In case of identical proportion of two components of C

4

F

7

N/CO

2

increase pressure of the gas mixture can also increase switching impulse withstanding voltage level. The switching impulse U

50%

of C

4

F

7

N/CO

2

with proportion of 5% C

4

F

7

N at 0.5 MPa can reach 72% of that of SF

6

at the same gas pressure. The switching impulse of C

4

F

7

N/CO

2

with proportion of 5% of C

4

F

7

N at 0.6 MPa is close to the switching impulse U

50%

of that with proportion of 9% of C

4

F

7

N at 0.5 MPa

which exceeds 80% of that of SF

6

at 0.5 MPa. Therefore

high gas pressure and low C

4

F

7

N gas ratio and low gas pressure and high C

4

F

7

N gas ratio are two alternative insulation gases for SF

6

.