The reduced critical electric breakdown field(E/N)

cr

of SF

6

and its substitute or mixtures gets very crucial for electrical breakdown evalution. Th

e(E/N)

cr

of SF

6

is investigated under 0.4 MPa and 0.8 MPa respectively

and over the temperature range of 300 to 3 000 K

by solving the two-term Boltzmann equation. The results show that the(E/N)

cr

of SF

6

decreases with increasing temperature

and the decreasing rate depends on the SF

6

decomposition rate; at temperature within 1 500 and 2 500 K

the(E/N)

cr

of SF

6

under 0.8 MPa is evidently higher than that under 0.4 MPa

however almost equal between 0.8 MPa and 0.4 MPa at other temperature. The calculations for(E/N)

cr

of SF

6

-CF

4

、SF

6

-CO

2

、SF

6

-O

2

、SF

6

-CH

4

、SF

6

-C

2

H

6

and SF

6

-CHF

3

under 300 K indicate

when x(SF

6

)in mixture is over 10%

the(E/N)

cr

of all these mixtures changes approximately linearly with x(SF

6

); among these gases

the(E/N)

cr

of CF

4

and O

2

approaches to the highest; the(E/N)

cr

of SF

6

-CF

4

and SF

6

-O

2

closely appears

and both higher than that of the others. And r

the ratio of the critical breakdown field of SF

6

to that of a gas mixture

is introduced to conclude that under proper pressure and mixing ratio

the critical breakdown field of a gas mixture becomes equivalent to SF

6

.