The lightning impulse breakdown characteristics in gas mixtures under different electric field distributions are the design basis of gas-insulated equipment. Consequently

we investigated the lightning impulse breakdown characteristics of C

4

F

7

N/CO

2

gas mixtures under lightning impulse in a quasi-uniform electric field（the factor of inhomogeneous field f=1.6） and an extremely non-uniform electric field（the factor of inhomogeneous field f=5.3 and 10.3）. The research indicates that

the breakdown voltage of C

4

F

7

N/CO

2

gas mixtures increases linearly with increase of gas pressure in quasi-uniform electric field

and the saturated trend of breakdown voltage effect appears at high pressure. Its insulation strength relative to SF

6

reaches its maximum at 0.4 MPa

that is

with the increase of air pressure

the relative insulation performance of mixed gases does not improve significantly. With the increase of the electric field non-uniformity

the breakdown voltage decreases significantly. The positive breakdown voltage is much lower than the negative. The sensitivity coefficient S is defined to characterize the decrease of discharge voltage of insulating medium when electric field is concentrated. The comparative analysis shows that the sensitivity of impulse breakdown characteristics of gas to electric field non-uniformity is C

4

F

7

N/CO

2

（5～20%）>SF

6

>CO

2

.