LIGflTXIXG PROTECTION OF BURIEn TOLL CABLE 255 



vided with steel tape armor for protection against mechanical damage. 

 While such armor may also reduce voltages due to low-frequency induction, 

 mainly because of the high permeability of the steel, this is not true in the 

 case of lightning voltages. The magnetic field in the armor due to lightning 

 current in the cable is rather high, and the corresponding permeability fairly 

 low. The armor resistance is, furthermore, quite high compared to that of 

 the sheath, so that the effect of the armor may be neglected in considering 

 lightning voltages. The tape or armor is usually separated from the sheath 

 by paper and asphalt, but is bonded to the sheath at every splice point. 

 Strokes to ground, or to the cable, may give rise to large currents in the 

 armor and thus to excessive voltages between the armor and the sheath 

 some distance from bonding points. The resulting arc may fuse a hole in 

 the sheath or dent it, due to the explosive efTect of the confined arc, and 

 insulation failures may be experienced on this account. Such failures are 

 not considered here since they are usually confined to a single point and are 

 thus of less importance than insulation failures due to excessive voltages 

 between the core conductors and the sheath, which may be spread for a 

 considerable distance along the cable. 



For protection against corrosion, buried cables are usually jute-covered 

 (asphalt, paper and jute) and in some cases have thermoplastic or rubber 

 coating. The leakance of jute-covered sheaths is usually large enough so 

 that the cable may be assumed to be in direct contact with the earth and 

 the leakance is, furthermore, increased at the time of lightning strokes by 

 numerous punctures due to excessive voltage between sheath and ground. 

 This effect is large enough so that even rubber-covered cable may be re- 

 garded as in direct contact with the soil in the case of direct strokes and 

 sometimes also for strokes to ground in the vicinity of the cable, as discussed 

 later. 



In order to calculate the voltage between the sheath and the core con- 

 ductors of a buried cable, due to a surge current entering the sheath or the 

 ground in the vicinity of the cable, it is convenient to consider at first a 

 sinusoidal current. The voltage due to a unit step current may then be 

 obtained by operational solution and, in turn the voltage for a current /(/) 

 of arbitrary wave shape, by means of either one of the integrals: 



Vij) = [ /'(/ - t)S{t) dr 

 h 



= \ J{i - T)S\r) dr 



(1) 



where Sif) is the voltage due to unit step current and S'{t) the time deriva- 

 tive of this voltage. The second of the above integrals is more convenient 

 in the present case. 



