Dielectric breakdown in gases typically involves the avalanche ionization of the gas molecules by
energetic electrons accelerated by the applied field. The mean free path between collisions must be
sufficiently long to allow the electrons to gain sufficient energy from the field to impact-ionize the
gas molecules. The breakdown voltage, Vbr, between two electrodes depends on the distance, d,
between the electrodes as well as the gas pressure, P, as shown in Figure 7Q10-1. Vbr versus Pd
plots are called Paschen curves. We consider gaseous insulation, air and SF6, in an HV switch.
a. What is the breakdown voltage between two electrodes of a switch separated by a 5 mm gap with
air at 1 atm when the gaseous insulation is air and when it is SF6?
b. What are the breakdown voltages in the two cases when the pressure is 10 times greater? What is
your conclusion?
c. At what pressure is the breakdown voltage minimum?
d. What air gap spacing, d, at 1 atm gives the minimum breakdown voltage?
e. What would be the reasons for preferring gaseous insulation over liquid or solid insulation?
