In order to explore the arc extinguishing performance of the new environmentally friendly insulating gas C

4

F

7

N mixed with different buffer gases

we theoretically calculated the equilibrium composition and thermodynamic coefficients of C

4

F

7

N/CO

2

N

2

and air mixtures in the temperature range of 300～30 000 K

under different pressure conditions（0.01

0.1

0.3

0.5 and 1 MPa）

and in different mixing ratios（molar fraction）. Assuming that the arc plasma was in the local thermodynamic equilibrium state

the equilibrium compositions were obtained based on the minimum principle of

the system Gibbs free energy; the thermodynamic coefficients of the mixed gas were calculated by the statistical thermophysical method; meanwhile

the calculation results were compared with gases such as SF

6

. The results show that the decomposition temperature of CO

2

N

2

and air and the difference of constituent elements lead to large differences in thermodynamic coefficients of different mixed gases. Under the same conditions

CO

2

as buffer gas has stronger radial heat exchange capacity than N

2

and air. Therefore

the C

4

F

7

N/CO

2

mixed gas among the three mixed gases has a stronger thermal breaking ability.