. The Bell System technical journal . r shield wires. Extra Core Insulation One method of reducing failures caused by lightning strokes to buriedcables is to increase the insulation between the cable conductors and the 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. Thecable itself, cable stubs, loading cases, and gas alarm contactor terminalsare all provided with sufficient extra insulation to double the dielectricstrength between cable conductors and sheat
. The Bell System technical journal . r shield wires. Extra Core Insulation One method of reducing failures caused by lightning strokes to buriedcables is to increase the insulation between the cable conductors and the 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. Thecable itself, cable stubs, loading cases, and gas alarm contactor terminalsare all provided with sufficient extra insulation to double the dielectricstrength between cable conductors and sheath. For a cable like that onwhich the measurements referred to before were made, such a measurewould increase the stroke current which would damage the cable from30,000 to 60,0000 amperes and would reduce the number of direct lightningstrokes that could cause failure by direct arcing to the sheath to about 20 Z 1 .6 ^5 .25 *j/inlle Q 9 H a n o >> 3 a <B n P. o ■p <B CD ^ 9 r^ Tl 01 i 1^ ^ Vl n o H 01 a U o O •ri u m (d. 100 200 500 1000 2000 5000 10,000Earth fiesistivity - Meter-ohms Figure 12—Theoretical lightning trouble expectancy curves showing number of timesinsulation 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 linere})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 times. Shield Wires Another method, employed in addition to the extra insulation whereexcessive lightning damage would otherwise be expected, is to bury shieldwires over the cable. These conduct away part of the lightning currentand thus reduce the amount that flows along the sheath. These wiresmay 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 294 BELL SYSTEM
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