Application of arresters and the selection of insulation levels

FOR many years, the insulation strength of power transformers and other electrical equipment was determined chiefly by the performance in service. When failures occurred, the insulation was made heavier at the points of weakness, and gradually the number of weak points was reduced in the particular lines of equipment and ratings involved. When new lines or ratings were initiated, operating experience was frequently necessary to perfect the design. Little information was available regarding the insulation strength of equipment under impulse conditions, and there was practically no selected co-ordination between the equipment strength and the performance of surge protective equipment.