LIGHTNING PKOTECTION OF BURIED TOLL CABLE 



279 



source of lightning damage, however, as the voltages are likely to be in- 

 sufficient unless the insulation is below normal. 



The curves of lightning trouble expectancy calculated here are significant 

 only if troubles on a long cable route are considered over a period of several 

 years, so that the mile-years covered are in the order of 1000 or more. 



2.2 Incidence of Strokes to Ground 



To estimate the lightning trouble expectancy it is necessary to consider 

 the incidence of strokes to ground in the vicinity of the cable, the number 



Figure 8 — Map showing the average number of thunderstorm days per year. 



of such strokes that will arc to the cable and cause damage in this manner 

 and the number that will give rise to failure without arcing to the cable. 



Magnetic link measurements (14) indicate that high tension transmission 

 lines will be struck by lightning about 113 times per 100 miles per year, on 

 the average, the minimum incidence in one year being about one half and 

 the maximum about 1.6 times the average value. The above average inci- 

 dence is based on observations covering about 1600 mile-years and applies 

 for lines traversing areas where s<jme 35 thunderstorm days are expected 

 per year, as indicated by data issued by the U. S. Weather Bureau and 

 collected over a period of 30 years, '' and shown in Fig. 8. The above data 

 on strokes to transmission lines may be used to estimate the rate of lightning 

 strokes to ground, provided the width of the zone within which a transmis- 

 sion line will attract lightning can be determined. 



