A novel approach to AC air arc interruption phenomena viewed from the electron density at current zero

Abstract

Direct measurements were made of the time-dependent electron density around the current-zero period in a scaled-down air blast breaker with an interruption current level of a few kiloamperes, by means of the Thomson scattering of ruby laser light. The effects of interruption parameters such as the electrode material (copper and graphite) and the gas flow rate on the magnitude of the electron density in AC arcs were quantitatively investigated. The results showed that, in an arc current of 1.5 kA, the electron density at current zero in a copper electrode arc was significantly enhanced by a factor of 4-5 compared with that in a graphite electrode arc. Spectroscopy measurements have shown that such an increase in the electron density is brought about by the admixture of copper vapour with a concentration of 1.5% into high-temperature air at current zero. The electron density, however, was suppressed to around half the magnitude by the increase in the forced convection of air from 100 to 250 l min-1. The inter-correlation between arc interruption performances of the test circuit breaker and the electron density at current zero is discussed. Statistical analysis showed that the success rate of arc interruption of the circuit breaker decreases almost straightforward with increase in the current-zero electron density on the normal probability chart.