LIGHTNING CURRENTS IN BURIED CABLE 



281 



obtained by use of Cun-e 2 in Fig. 1, which is a theoretical curve derived 

 from Curve 1. Thus for a total of 2.1 for 10 thunderstorm days, the inci- 

 dence of strokes exceeding 60 ka is 2.1-0.2 = 0.42. In Fig. 2 are shown 

 crest current distribution curves for cable currents due to direct strokes 

 obtained in this manner, together with distribution curves for currents due 

 to both direct strokes and strokes to ground not arcing to the cable. The 

 latter curves may be obtained by methods similar to those used in evaluat- 

 ing curves for the lightning trouble expectancy of buried cable, which are 



0.01 0.02 0.05 0.1 0.2 0.5 1.0 2 3 4 5 6 8 10 20 30 50 tOO 



PERCENTAGE OF LIGHTNING STROKES IN WHICH CURRENT EXCEEDS ORDINATE 



Fig. 1 — Distribution of crest currents in lightning strokes. 

 Curve 1. Currents in strokes to transmission line ground structures, based on 4410 

 measurements, 2721 in U. S. and 1689 in Europe. 



Curve 2. Currents in strokes to buried structures derived from Curve 1. 



shown in Fig. 3 for cable having a dielectric strength of 2 kv between the 

 sheath and the cable conductors. ^ The latter curves may, in fact, be used 

 to find the incidence of cable currents of various crest values due to direct 

 strokes and strokes to ground, by calculating the cable currents required 

 to produce 2 kv between the sheath and the core corresponding to various 

 sheath resistances shown in Fig. 3. Thus for a sheath resistance of 2 

 ohms per mile and an earth resistivity of 1000 meter-ohms, this current is 

 14.2 ka (see Section 1.3) and for a sheath resistance of 1 ohm, it is 28.4 

 ka, etc., as plotted in Fig. 2 for an earth resistivity of 1000 meter-ohms. 



From the above it follows that a verification of the distribution curves in 

 Fig. 2 by observations of lightning currents in buried cable would apply 



