Physical Science International Journal

The physico-chemical properties of the electric arc are decisive for the success or failure of the circuit breaker to cut the electric current. The effectiveness of the circuit breaker in cutting off the electric current is highly dependent on the thermal and electrical characteristics of the extinguishing medium. The present study theoretically evaluates the influence of pressure on the electrical current-breaking capacity of low-voltage air circuit breaker, through the thermodynamic properties and transport coefficients of air plasma contaminated with silver alloy and tin dioxide vapor. These physical quantities are determined at local thermodynamic equilibrium in a temperature range from 500 K to 30,000 K. We use the Gibbs free energy minimization method to determine the equilibrium composition and thermodynamic properties of the air-AgSnO₂ mixture plasma. The analytical expressions required to determine the transport coefficients of the air-AgSnO₂ mixture plasma are deduced from the Boltzmann equation using the approximate Chapman-Enskog method.

 Analysis of the results obtained shows that, for a given temperature, the numerical particle densities and mass density of the air-AgSnO₂ mixture plasma increase as the pressure of the medium rises. The mass enthalpy, sound velocity, energy density, thermal flux density and electrical conductivity of the air-AgSnO₂ mixture plasma decrease with increasing pressure. The peaks in the specific heat and thermal conductivity of the air-AgSnO₂ mixture plasma are shifted towards higher temperatures as the medium pressure increases. For temperatures above 9,000 K, the dynamic viscosity of the air-AgSnO₂ mixture plasma increases with increasing media pressure. As a result, the increased pressure in the medium can help extinguish the arc created when the electric current is cut off by the AgSnO₂ air circuit breaker with electrical contact.