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FOR many years, the insulation strength of power but their magnitudes generally do not exceed 3.5 times transformers and other electrical equipment was normal voltage.' On many systems, they are less than determined chiefly by the performance in service. 2 times normal voltage. Dynamic overvoltage due to When failures occurred, the insulation was made heavier hydroelectric generator runaway or other abnormal unat the points of weakness, and gradually the number of loading conditions may reach values as high as 2 times weak points was reduced in the particular lines of equip-normal. Apparatus designed in accordance with AIEE alment and ratings involved. VVhen new lines or ratings ternating voltage dielectric tests has proved to be entirely were initiated, operating experience was frequently nec-adequate for such overvoltages arising from system opessary to perfect the design. Little information was eration. available regarding the insulation strength of equipment Lightning is considered to be the principal source of under impulse conditions, and there was practically no insulation failure because of the very high values of voltselected co-ordination between the equipment strength age and the steep wave fronts involved. Lightning and the performance of surge protective equipment.voltages may be the result either of a direct stroke or a The principal cause of equipment failures has been traveling wave, and the effect on insulation depends upon lightning, and much attention has been given during the magnitude, the duration, and the rate of rise and the past few years to the study of lightning phenomena decay of the surge potential. It is apparently more and the behavior of insulation material under lightning economical and practical to take advantage of the prostress. At the same time, the study of protective equip-tective equipment available and to utilize apparatus of ment has made available a knowledge of its behavior under an insulation strength properly co-ordinated with this lightning surge conditions. Great strides have been made protective equipment than to attempt to build apparatus in this direction, so that it is now possible to select in-in the present operating voltage ranges which will be able sulation levels with a degree of confidence as to subsequent to withstand lightning surges without the use of some performance, provided that proper correlation of these auxiliary protective device, either separate from or inphenomena and behavior characteristics is effected.J. H. FOOTE iS supervising engineer and J. R. NORTH iS genkeral engineer wihin the report of the AIEE lighng aretrsbomt 1. For all numbered references see list at end of paper, tee. 2