A feedthrough’s maximum voltage rating is determined in air at one atmosphere pressure. For reasons beyond the scope of these Notes, different gases can have very different breakdown voltages (BVs). For example, a paper from Rensselaer Polytechnic shows helium’s BV is roughly half air’s BV, for some given conditions. As a gross generalization, the simpler the gas’s structure, the lower its BV. The atomic inert gases, particularly helium, have a low BV under most backfill pressures. Complex gas molecules have high BVs and tend to quench discharges—which is why SF6 is used in transmission power line switches and breakers.
Sub-Category: Electrical Feedthroughs