C

4

F

7

N gas mixture is currently the most potential environmental friendly gas insulating medium whichcan replace SF

6

.In this paper

the influence of oxygen and temperature on the partial over thermal decompositioncharacteristics of C

4

F

7

N/CO

2

/O

2

gas mixture is tested and studied.It is found by the study that the main products ofthermal decomposition of C

4

F

7

N/CO

2

/O

2

gas mixture include CF

4

C

3

F

8

C

3

F

6

CO

COF

2

CF

3

CN

C

2

F

5

CN

C

2

N

2

.When the addition of O

2

is 2%

the volume fraction of C

3

F

6

increases

while the volume fraction of other productshave different degrees of reduction.When the addition of O

2

is higher than 8%

the rate of formation of CF

4

、C

3

F

8

、CO and COF

2

goes up.Both yields and effective gas production rate of CF

4

C

3

F

8

C

3

F

6

CF

3

CN

and C

2

F

5

CN havedifferent degree of reduction in case of the over thermal temperature reaching 500℃and continue to increase incase of exceeding 500℃.With comprehensive consideration of the insulation performance and electrical and ther-mal decomposition characteristics of the gas mixture

it is appropriate to add 4%～6% O

2

to the C

4

F

7

N/CO

2

/O

2

gasmixture in practical engineering applications.COF

2

and C

2

N

2

can be used as characteristic products to characterizethe severity of over-thermal fault of C

4

F

7

N/CO

2

/O

2

gas mixture insulation equipment and C

2

F

5

CN can be used as thedecomposition products of the marked change in the natu

re of the fault.