The magnitude and wave shape of the lightning stroke current have been recognized to be of primary importance in determining the impulse spark over of line insulation. Several years of field investigation have resulted in the determination of a lightning-stroke current probability curve. Although these stroke current probabilities were utilized, it was the practice to assume a single wavefront [1] in estimating performance, recognizing that the analysis really should include other wavefronts. It is now possible to separate all lightning strokes into 2-, 4-, or 6-,us fronts [4] in relation to the stroke current magnitude, as shown in Table II. Thus, for a particular value of stroke current, it is possible to determine the probability of sustaining a stroke current equal to or greater than this value and, also, the percentage of these strokes with 2-, 4-, and 6-as fronts.