LIGHTNING PROTECTION OF BURIED TOLL CABLE 297 



When o-o = jSd , the voltage with shield wires differs from that obtained 

 without shield wires by the factor t). With two 165-mil wires 12 inches apart 

 and 10 inches above the cable considered before, «o = 1.3- 10^, /3o = 1.5- 10^ 

 and -q = .47. In this case ao differs only slightly from /3o so that the voltage 

 is reduced by the factor 77. Measurements made before and after the shield 

 wires were installed indicated a reduction factor of .40 (i.e. the voltage with 

 was .4 times the voltage without shield wires). The reasons for the smaller 

 observed factor is partly that the shield wires are in more intimate contact 

 with the earth than the sheath, and partly that the resistivity of the soil 

 above the cable, where the shield wires are located, is somewhat smaller 

 than the resistivity at the depth of the cable. 



With 104-mil wires, ao = 3.2-10', jSo = 2.3- lO' and 7? = .49. When the 

 reduction factor is determined more accurately by calculating the crest 

 voltage with shield wires from (109) and comparing it with the crest voltage 

 without shield wires, a value of .52 is obtained as compared with .47 for 

 165-mil wires. It is thus seen that, within certain limits, the voltage re- 

 duction provided by shield wires depends to a comparatively small extent 

 on the size of the wires, the resistance of 104-mil wires being about 2.5 times 

 that of 165-mil wires. 



3.4 Lightning-Resistant Cable 



As mentioned before, buried cable may be covered by jute, thermoplastic, 

 or rubber for protection against corrosion. The coating may be damaged 

 by gophers or by lightning, and severe corrosion may be experienced at 

 points where the coating is ruptured, particularly when thermoplastic or 

 rubber coating is used. Even when the earth resistivity is low and protec- 

 tion against core insulation failures due to excessive voltage would not be 

 required, the sheath coating may be damaged rather frequently. 



Shield wires may effect a substantial reduction in core insulation failures 

 and may also prevent damage to the coating in the case of strokes to ground 

 at some distance from the cable. In the case of direct strokes, however, 

 arcing between the shield wires and the sheath will damage the sheath coat- 

 ing and may also fuse a hole in the sheath, although there may be no insula- 

 tion failures due to excessive voltage between the sheath and the cable 

 conductors. 



Reduction of damage to the coating and to the sheath occasioned by light- 

 ning, rodents, or corrosion and protection against core insulation failures 

 occasioned by excessive voltage or crushing of the sheath may be secured 

 by providing the sheath with a thermoplastic or rubber coating and an out- 

 side copper shield. If various auxiliary equipment connected to the sheath, 

 such as load coils, gas pressure contactors and terminals are also properly 



