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BELL SYSTEM TECHNICAL JOURNAL 



latter will be in contact with the sheath by virtue of arcing. Under this 

 condition, the voltage Fo(-v, /) between this conductor and the sheath will 

 increase with distance along the cable as shown in Fig. 5. A maximum is 

 reached a fairly short distance from the original fault, and beyond this 

 point the voltage slowly decreases. After a puncture of the insulation where 

 current enters the sheath, other failures may therefore occur, not neces- 

 sarily at the point where Voix, t) is largest, but sometimes at points nearer 

 or much farther away where the insulation may be weaker. A single 

 lightning stroke may thus cause insulation failures over a considerable dis- 

 tance along the cable. 



100 



80 



60 



40 



20 



.2 .4 .6 .3 



Distance Along Cable - Miles 



Figure 5 — Comparison of measured variation of voltage between sheath and core, along 

 cable, as shown by points and dashed curve, with calculated variation shown by curves 

 1 and 2. 



1 : Conductor not connected to sheath. 



2: Conductor connected to sheath at point where surge current enters sheath. 



1 .6 Direct Strokes — I 'oltage Due to Long Duration Current 



As mentioned before, a current of low value and long duration may exist 

 on the lightning channel after the main discharge. This current is usually 

 of such long duration that the resistance of the sheath must be considered 

 in calculating the current propagation along the cable. The propagation 

 constant in that case becomes 



r - [(i? + io^QG^ = [t + P 



(£)' 



(28) 



R, L and G being the unit length resistance, inductance and leakance of the 

 sheath-earth circuit. Neglecting propagation in the core-sheath circuit, 



