296 



BELL SYSTEM TECHNICAL JOURNAL 



Some of the other observations, not reproduced here, were less consistent 

 than those shown, probably due to the combined effects of more than one 

 stroke; but they permitted fairly satisfactory determinations of crest cur- 

 rents and times to half- value. 



From Table I it appears that the minimum duration to half-value is 

 about 130 microseconds, the maximum about 1000 and that the average for the 

 three most severe strokes is about 150 microseconds. In general the dura- 

 tion appears to be longer the lower the crest currents, the average for cur- 

 rents of 20 ka and less being about 500 microseconds to half-value. 



5 o 

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o o 



Z Z 



I- iiJ 3 



6 ^ 



O 



_j 



654321 01 234567 



DISTANCE ALONG CABLE (FROM ESTIMATED STROKE POINT) IN KILOFEET 



Fig. 11 — Variation in crest current, voltage, and charge along cable for an extrapolated 

 stroke current of 16 ka, having a time to half-value of 190 microseconds. Dashed curve 

 shows theoretical variation of current for this time to half-value and an earth resistivity 

 of 1200 meter-ohms. 



In the above derivations a simple wave-shape was assumed, although 

 actually it is likely to be rather complex with many fluctuations. The use 

 of an equivalent simple wave-shape is, however, permissible in evaluating 

 the liability to lightning damage, since statistical results rather than the 

 wave-shapes of individual currents are of main importance. 



Regarding the cause for the long duration of the currents, it appears to 

 be inherent in meteorological rather than geological conditions, as for the 

 wave-shapes of lightning currents in general. The significance of meteoro- 

 logical conditions is also indicated by the observations discussed in Section 

 3.3. In the case of strokes to the cable, the latter provides a path of very 

 low impedance compared to that of the lightning channel, so that the wave- 

 shape is determined by the impedance of the lightning channel and the 

 distribution of charge along the leader and in the cloud. This is also true 



