Investigations of the properties of SF6as an arc quenching medium

The results of experiments on stationary and nonstationary arcs in SF
6
are summarized. High temperature gas properties, like the electrical and thermal conductivity as well as the theoretically predicted plasma demixing effects, are determined by electrical and spectroscopic measurements. Investigations on interrupted dc arcs give insight into the energy transport mechanism of the arcs. The transient temperature behavior of gas blast interrupted arcs is measured. Finally, the application of the investigations to circuit breaker arcs is discussed. The following principal results have been found: From the lower time constant of blown N
2
arcs in comparison to SF
6
arcs above 10 000 K it follows that the good quenching properties of SF
6
must be due to processes taking place below 10 000 K. This agrees also with the time-constant measurements in the interrupted cascade arc: at lower temperature--below approximately 8000 K-- the conductance decay in N
2
is very much slower than in SF
6
. Further, these measurements revealed that the steep descent in SF
6
is not caused by electron attachment but is due to energy transport mechanisms. Experimental work about the plasma properties on steady-state arcs, including demixing effects, is in good agreement with theory. Finally the ac experiments show that the temperature profile of the blown ac arcs has a nearly rectangular shape. Variations in current affect mainly the diameter of the arc whereas the temperature variations are fairly small.