Determination of safe working 50 Hz AC electrical stresses for ceramics at high temperatures

When used as electrical insulation, ceramics in general do not degrade electrically under the influence of time or AC electric stress. However, at high temperatures, conductor (electrode) material can diffuse into the ceramic causing reductions in its electrical strength and resistivity. Experimental work carried out to assess the effect of time on the 50 Hz AC electrical strength of various ceramics at temperatures between 800 and 1140°C is described. Results were obtained for lifetimes of up to 5000 hours. All the ceramics tested, dense alumina, pyrophyllite, alumina and magnesia cement, are available commercially. The influence of electrode materials (heating element alloys) on insulation life is evaluated. Recommendations are made on design procedures for oxide ceramic insulation systems