Predictions of circuit interruption including non-equilibrium electron densities

Calculations and predictions of arc properties, for example for arc welding and also circuit interruption, are generally made by solving the energy balance equation to obtain temperature profiles, assuming local thermodynamic equilibrium. Results for high power circuit breakers have needed to attribute turbulence as the principal energy loss, with larger turbulence parameters for SF6 than for air to fit the superior experimental performance of SF6. In the present paper, predictions of circuit interruption are obtained by solving the electron continuity equation together with the energy balance equation for a simple low current arc, with no imposed convective flow and thus no need for turbulence. It is found that circuit interruption depends critically on electron-ion recombination coefficients and also electron-neutral attachment coefficients, large values of which reduce electron densities and thus electrical conduction. SF6 is found to be superior to air because of its very large attachment coefficient. The results suggest that in the search for a replacement gas for SF6, because of its Global Warming Potential of 24000 times that of carbon dioxide, material properties of high electron recombination and attachment are desirable, as well as the high enthalpy density which follows from the results where convective flow is the principal energy loss.