LIGHTNING PROTECTION OF BURIED TOLL CABLE ' in 



thus more difficult to repair. For full-size cable in low-resistivity areas, 

 however, insulation failures are more likely to occur as a result of sheath 

 denting. For instance, along the 300-mile Kansas City-Dallas full-size 

 cable route, where the earth resistivity is in the order of 100 meter-ohms, 

 and where there are some 50 thunderstorm days per year, failures over a 

 period of about 15 years have occurred about .5 times per 100 miles per 

 year. Of these troubles 85% were due to sheath denting as a result of 

 arcing between the tape armor and the sheath. Based on (99), 100 miles 

 of cable would attract lightning strokes within an area of .5 square mile, 

 so that the cable would be struck about six times per 100 miles per year, 

 when the number of strokes per square mile per year is 2.4 per 10 thunder- 

 storm days (Section 2.2). The rate of lightning failures experienced on this 

 route may thus be accounted for by assuming that about 7% of the strokes, 

 i.e. currents in excess of 90 kiloamperes as obtained from curve 2, Fig. 1, 

 will produce sheath denting severe enough to cause insulation failure, while 

 about 1%, i.e. currents in excess of 140 kiloamperes, will cause insulation 

 failure due to excessive voltage. The latter value is in substantial agree- 

 ment with that calculated for a full-size cable when the earth resistivity is 

 assumed to be 100 meter-ohms. It is evident from the above examples 

 that in low-resistivity areas lightning troubles will not be a problem, and 

 this is also borne out by experience on other routes installed in such terri- 

 tory during the last few years. 



All cable installed in high-resistivity territory since 1942 has been provided 

 with doubled core insulation and with shield wires, in spite of which con- 

 siderable damage has been experienced on some routes, as between Atlanta 

 and Macon. This appears to have been due partly to the circumstance 

 that in many cases the insulation m splices and accessories has not been 

 equal to that obtained in the cable through the use of extra core wrap, and 

 that in some cases damage has been due to holes fused in the sheath due to 

 arcing between the sheath and the shield wires. As an example, along the 

 Atlanta-Macon route there are some 70 thunderstorm days per year and 

 the average effective earth resistivity is about 1300 meter-ohms. The 

 corresponding estimated rate of direct strokes to the cable is about 20 per 

 100 miles per year. It is estimated, in the manner outlined in section 3.0, 

 that only stroke currents in excess of 80 kiloamperes are likely to damage the 

 cable, so that on the basis of curve 2, Fig. 1, cable failures would be ex- 

 pected to occur about 2 times per 100 miles per year. The actual rate of 

 trouble experienced on this route over two years has been about five times 

 higher, so that some 50% of the strokes to the cable; i.e., currents in excess 

 of 30 kiloamperes or so, appear to have caused cable failures, most of which 

 occurred in splices and accessories. 



