Effects of Insulation Thickness on the Aging of Organic Insulation in Air [includes discussion]

Organic insulations composed of paper or cloth base with phenolic or black asphaltic varnish impregnant are shown by extensive tests to have an aging rate in air dependent on the insulation thickness. A change in insulation thickness from that representative of turn insulation to that used for a major voltage barrier may increase its useful life in air by as much as ten times if in a dry atmosphere. The corresponding change in temperature class for a specific number of hours of life in air may be of the order of 30 degrees centigrade (C) to 50 C for this change in thickness, under the same favorable conditions. A possible explanation of this difference is offered, based on the progressive shielding of the inner laminae by the outer portions which gives the effect of the shielded portions operating in a nonoxidizing medium. The inhibition of evaporation effects may also be a factor.