Temperature measurements of large power arcs and the relation of temperature to dielectric recovery

Abstract

A study of the basic mechanisms of dielectric recovery of power frequency arcs in air is reported. The particular arc studied is that of a standard 6 inch rod gap conducting 300 or 800 amperes crest for 1/2 cycle of a 60 cycle per second current. The temperature of the arc space and the variation of the arc temperature with time are measured. The temperature is measured by spectrographic and velocity of sound techniques. The temperature at current zero is 5000 degrees Kelvin. The temperature is 720 degrees K. 77 milliseconds after current zero and 415 degrees K. 196 milliseconds after current zero. The temperature is correlated to the dielectric strength during the recovery period. During the recovery period the arc space is at atmospheric pressure. The gas density is reduced due to the high temperatures existing in the arc space. Evidence is given that the low gas density is reduced due to the high temperatures existing in the arc space. Evidence is given that the low gas density is the major factor causing the reduced dielectric strength. Residual ions in the arc space also contribute to reducing the dielectric strength. A criterion for recovery breakdown is given including the effect of the residual ions. The ion density after current zero decreases due to electron recombination with a recombination coefficient [alpha] = 2.0 x 10[superscript -9] cubic centimeter per ion-second. The loss of ions by diffusion appears negligible.