Current Interruption in Air for a Medium-Voltage Load Break Switch

The current interrupting capability of a load break switch (LBS) depends on many design parameters, such as contact and nozzle geometry, contact movement, and gas flow. For developing more compact gas blow LBSs for air, it is necessary to find design recommendations where each parameter is addressed individually. In this paper, a current interruption test switch is built for this purpose. The interruption tests are conducted with a directly powered high-voltage circuit. The result shows the minimum gas flow required for current interruption for various basic nozzle geometries and at different contact positions. The study is particular relevant for the 24 kV/630 A class, and it is shown that 0.3 bar upstream pressure appears to be a threshold value for successful interruption. Some conclusions are also applicable for other medium-voltage ratings. An LBS should be designed so that at least one current zero crossing comes outside the nozzle when the switch is still blowing with full strength.