LIGIITXIXG PROTJXTIOX OF BURIED TOLL CABLE 



263 



If the breakdown voltage of the core insulation is assumed to be 2000 

 volts, the above cable would be able to withstand a stroke current of about 

 30,000 amperes before the insulation is punctured. From Fig. 1 it is seen 



1000 



800 



600 



400 



200 



20 40 60 80 100 



Microseconds 



120 140 



Figure 3 — Comjjarison of measured, shown tjy dashed curve, and calculated voltage 

 between sheath and core conductors, shown h)- curves 1 and 2, for surge current as shown 

 and cable constants as given in Fig. 2. 



1 : Calculated from formula including skin-effect in sheath and finite velocity of propa- 

 gation. 



2: Calculated from formula based on d-c resistance of sheath and assuming infinite 

 velocity of i)ropagation. 



that in about 50 per cent of all strokes the crest current exceeds 30,000 

 amperes. 



When there are two cables, each will provide shielding for the other, and 

 the shielding effect may be calculated as for shield wires (Sec. i.?)). Fre- 

 quently the cables are of equal or nearly equal size and are close together. 

 It is then accurate enough to use the parallel resistance of the two sheaths 

 in calculating the voltage, which will be practically the same in both cables. 



