THERMAL DISTURBANCE PRODUCED BY AN ARC IN A MODEL GAS-BLAST CIRCUIT-BREAKER

The properties of the thermal region surrounding the electrical core of a gas-blast arc have been measured using laser shadowgram and deflection mapping techniques. The arcs burned in an 11 mm long, 19 mm diameter orifice and were sustained by peak currents of 3 or 8 kA at a frequency of 85 Hz. The gas blast was driven by air stored at 7.8 bar in an upstream reservoir. Tests have been performed to investigate the influence upon the thermal region of different upstream electrode gaps, upstream electrode materials and geometries, and peak arc currents. Axial and temporal variations in the position of the thermal region boundary have been recorded throughout the arcing cycle, including the current zero period, and the transient variations of gas density and temperature within the thermal region have been determined. The results presented should enable empirical laws to be derived which form the basis of a boundary layer integral analysis of gas-blast arc behaviour.