LIGHTNING PROTECTION OF BURIED TOLL CABLE 



293 



sheath, no extra insulation being required between individual cable con- 

 ductors. This has already been done for most new installations. The 

 cable itself, cable stubs, loading cases, and gas alarm contactor terminals 

 are all provided with sufficient extra insulation to double the dielectric 

 strength between cable conductors and sheath. For a cable like that on 

 which the measurements referred to before were made, such a measure 

 would increase the stroke current which would damage the cable from 

 30,000 to 60,0000 amperes and would reduce the number of direct lightning 

 strokes that could cause failure by direct arcing to the sheath to about 20 



Z 1 .6 ^5 



.25 '*j/inlle 



100 200 500 1000 2000 5000 10,000 

 Earth fiesistivity - Meter- ohms 



Figure 12 — Theoretical lightning trouble expectancy curves showing number of times 

 insulation failures due to excessive voltages would be expected per 100 miles for 10 thunder- 

 storm da\s, for cal)les having sheath resistances as indicated on curves. Dashed line 

 re})resents full-size cable. 



per cent of the total instead of 50 per cent (see Fig. 1, curve 2). The num- 

 ber of failures due to direct strokes would thus have been reduced 2.5 times. 



3.3 Shield Wires 



Another method, employed in addition to the extra insulation where 

 excessive lightning damage would otherwise be expected, is to bury shield 

 wires over the cable. These conduct away part of the lightning current 

 and thus reduce the amount that flows along the sheath. These wires 

 may be plowed in with the cable, as has been done on several new routes, 

 or may be installed afterward. When the wires and cables are plowed in 



