LIGHTXIXG PROTECTION OF BURIED TOLL CABLE 295 



damage to the sheath covering in the case of strokes to ground near the 

 cable, particularly in the case of thermoplastic or rubber-covered cables. 

 As mentioned in section 1.10, considerable current may enter the sheath of 

 an insulated cable in the case of strokes to ground near the cable, due to 

 numerous punctures in the insulation of the sheath. If conditions along 

 the cable were uniform, the current through each puncture would be so 

 small that the insulation would not be damaged. Due to variations in the 

 resistivity of the soil and the presence of buried metallic structures, how- 

 ever, concentrated arcing may occur, and the insulation may then be 

 damaged in spots, so that corrosion may be initiated. Shield wires reduce 

 the voltage between the sheath and ground and thus the likehhood of 

 damage to the sheath insulation. 



It can be demonstrated theoretically, and has been proved by measure- 

 ments, that when current enters shield wires next to a buried cable in good 

 contact with the earth, the current in the sheath and the voltage between 

 sheath and cable conductors is negligible. The reason for this is that 

 current induced in the sheath by the shield wire current is equal and oppo- 

 site to the current entering the sheath by virtue of leakage through the 

 ground. Negligible voltages between sheath and core conductors would 

 thus be obtained if shield wires were installed at such a distance that light- 

 ning strokes would be intercepted and direct strokes to the cable prevented. 

 To prevent arcing to the cable of strokes to the shield wires, the separation 

 between cable and shield wires would have to be at least 6 feet when the 

 earth resistivity is 1000 meter-ohms, and greater for higher resistivities. 

 Such wires cannot, therefore, be as easily installed in one plowing operation 

 with the cable as shield wires at a smaller spacing. They are, therefore, 

 not considered here, although they have been installed in one instance in a 

 fairly short section where repeated lightning damage had been experienced. 



When the sheath, as well as the shield wires, is in intimate contact with the 

 earth, the propagation constant for the shield wires is the same as that for 

 the sheath. In the case of a direct stroke to the cable or the shield wires, 

 the voltage between sheath and shield wires will be so large that they will 

 be in contact with each other at the stroke point by virtue of arcing. The 

 current in the sheath is then: 



T( •) — _ Z22 ~ Z12 -Tx (^o^\ 



2 Zii -\- Zii — 2Zi2 



where / is the total current at x — 0, and 



Z\\ = R\-\r iwLn = Unit length impedance of sheath 

 2o2 = i?2 + /C0L22 = Unit length impedance of shield wires 

 Z12 — icoLi2 = Unit length mutual impedance of sheath and shield 



wires 



