The Validity of the Similarity Law for the Electrical Breakdown of $\hbox{SF}_{6}$ Gas

The aim of this paper is to investigate the justification and the limitation of the geometrical similarity law application on the electrical breakdown of $\hbox{SF}_{6}$ gas. For this purpose, the measurements of the dielectric breakdown voltage for similar systems insulated with $\hbox{SF}_{6}$ gas were done. The applied dc voltage source had an 8 V/s rate of rise. Standard double exponential overvoltage pulse (rise time $T_{1} = 1.2\ \mu\hbox{s}$, fall time $T_{2} = 50\ \mu\hbox{s}$) of the amplitude $U_{1}^{\max} = 320\ \hbox{V}$, $U_{2}^{\max} = 4.80\ \hbox{kV}$, and $U_{3}^{\max} = 640\ \hbox{kV}$ was used. On the basis of the obtained experimental results and theoretical considerations, the conditions under which the geometrical similarity laws are applicable on discharges in gases are determined. It was concluded that there is no correlation between the geometrical similarity of the electrode surface topography and corresponding breakdown voltages. It was shown that the extension of the similarity law, introducing the electron mean free path as a linear dimension of the system, gives a good quantitative agreement. Also, in case of the pulse breakdown voltage, it is necessary to apply the breakdown possibility rise law.