ABSTRACT

Th e physicochemical properties of the electric arc determine the success or failure of the electric current by the circuit breaker. Th is work is devoted to the determination of the transport coeffi cients of air plasma contaminated by silver tin(IV) oxide (AgSnO₂) alloy electrical contact vapour in a low voltage circuit breaker. It aims to theoretically evaluate the infl uence of AgSnO₂ alloy electrical contact material on the electrical current breaking capacity of the low-voltage air circuit breaker through the electrical conductivity, thermal conductivity and dynamic viscosity of the plasma air-AgSnO₂. Th ey are determined over a temperature range from 500 K –30,000 K, at atmospheric pressure and at local thermodynamic equilibrium. Th e analytical expressions necessary to determine these transport coeffi cients of the air-AgSnO₂ mixture plasma are deduced from the Boltzmann equation using the approximate Chapman- Enskog method. Analyses of the obtained results show that, for the same given temperature, the electrical conductivity increases signifi cantly when the AgSnO₂ alloy vapour increases in the mixture for temperatures below 11,000 K. Th e dynamic viscosity of the plasma decreases as the percentage of AgSnO₂ alloy electrical contact vapour increases in the mixture, for temperatures below 12,000 K. In the temperature range from 500 K–20,000 K, the total thermal conductivity of the plasma decreases as the percentage of AgSnO2 alloy electrical contact vapour increases in the mixture. It results that increasing the percentage of electrical contact vapour in AgSnO₂ alloy will slow down the extinction of the electric arc created inside the circuit breaker, especially beyond 0.1% of vapour. Th is slowing down of the extinction of the electric arc can cause the circuit breaker to fail to cut off electrical current. 

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