Radiative Energy Transfer in Circuit Breaker Arcs

Summary

Computer studies have revealed that in high-current gast-blast arcs, energy losses at the arc centre are dominated by the radiation losses so that the arc is approximately isothermal. However, most of this radiation is in the far ultra violet region of the spectrum and is re-absorbed in the outer cold region of the arc. Thus, the electrical energy is largely expended in producing arc plasma rather than lost as radiation. Formulae which result for arc diameter as a function of current are in reasonable agreement with experiment. The model for radiation transfer provides a mathematical foundation for the Cassie arc model that for high-current gas-blast arcs the arc diameter is approximately proportional to the square root of the current and arc voltage and temperature are largely independent of current.

Radiation processes are also of primary importance in determining the onset of dielectric breakdown. Recent theoretical investigations have verified that in the breakdown process a conducting channel is propagated by space charge and radiation effects of exp(αd) ~ 10⁸ where α is Townsends ionization coefficient and d is the transit distance of an avalanche. Furthermore, absorption of radiation from the arc before current zero will produce an outer mantel of hot gas which will not rapidly cool by conduction because of its large radius. Because α/n increases rapidly with E/n, breakdown voltages are significantly reduced by any warm gas remaining in the inter electrode region after current zero; n is gas number density, E the electric field.