254 BELL SYSTEM TECHNICAL JOURNAL 



resistivity is high. The Hghtning stroke will then arc directly to the cables 

 from the point where it enters the ground, often at the base of a tree. Fur- 

 rows longer than 100 feet have been found in the ground along the path of 

 such arcs. 



The current entering the sheath near the stroke point is attenuated as it 

 flows towards remote points. Since a high earth resistivity is accompanied 

 by a small leakage conductance between sheath and ground, the current 

 will travel farther the larger the earth resistivity. The current along the 

 sheath produces a voltage between the sheath and the core conductors, 

 which is largest at the stroke point. This voltage is equal to the resistance 

 drop in the sheath, between the stroke point and a point which is sufficiently 

 remote so that the current in the sheath is negligible. Since the current 

 travels farther along the sheath the higher the earth resistivity, this resist- 

 ance drop will also increase with the earth resistivity. The maximum 

 voltage between sheath and core is thus proportional to the sheath resistance 

 and, as it turns out, to the square root of the earth resistivity. Carrier 

 cables now being used are of smaller size and have a higher sheath resistance 

 than full-size voice-frequency cables, and for this reason they are liable to 

 have more lightning damage, particularly when the earth resistivity is high. 



To secure experimental verification of certain points of the theory pre- 

 sented here, staged surge tests were made on the Stevens Point-Minneapolis 

 cable, one of the first small-size buried toll cables to be installed. The 

 results of these tests, which have already been published, are here compared 

 with those obtained theoretically, on the basis of the earth resistivity meas- 

 ured at the test location. Lightning voltages on this cable route were also 

 recorded by automatic oscillographs and the results of these observations 

 are also briefly discussed together with the rate of lightning failures experi- 

 enced on this and other routes. 



The first part of the paper deals with voltages between the cable con- 

 ductors and the sheath due to sinusoidal currents and surge currents. The 

 second part deals with the liability of damage due to excessive lightning 

 voltages and with certain characteristics of lightning discharges of impor- 

 tance in connection with the present problem, such as the impedance 

 encountered by the lightning channel in the ground, the rate of lightning 

 strokes to ground and to buried structures and the crest current distribution 

 for such strokes. In the third part remedial measures are discussed, to- 

 gether with lightning-resistant cable. 



I. Voltages Between Cable Conductors and Sheath 

 1 . 1 General 

 Cable installed in the ground is designated "underground" when placed 

 in duct, and "buried" when not in duct. Buried cable is sometimes pro- 



