DETERMINATION OF SAFE WORKING 50HZ 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. Howev er, at high temperatures, conductor (electrode:) material can diffuse into the ceramic causing reductions in its electrical strength and res istivity. Experimental work carried out to assess the effect of time o n the 50 Hz AC electrical of various ceramics at temperatures 800 and 1140 degrees 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 ce ramic insulation systems.