TREXDS IX PRACTISE AS AFFECTI.XG COORDIXATIOX 167 



presence of the lead sheath, together with the twisting of the cable 

 conductors, the high degree of balance with respect to ground, and the 

 mutual shielding effect of the many circuits in one cable, practically 

 prevents noise currents from being induced directly into the cable cir- 

 cuits from outside electrical sources. The shielding etTect of the lead 

 sheath when suitably grounded also provides substantial reductions in 

 the voltages of fundamental frequency which may be induced along 

 the cable conductors at times of trouble on neighboring power systems. 



A telephone toll cable with its associated equipment costs about the 

 same per mile as a twin circuit power transmission line of the 110-kv. 

 class. This high cost has led to a large use of private right of way for 

 new^ extensions of these cables, particularly for aerial cable construction. 

 This, of course, has an added advantage from the coordination stand- 

 point in tending to keep these important telephone routes off the high- 

 ways, which are so much used for the distribution systems of both 

 utilities. In the more rapidly growing cable routes underground con- 

 duit construction is employed and these in most cases are located along 

 the highways. In these cases, however, the close proximity of several 

 cables in the same conduit run offers a considerable amount of mutual 

 shielding effect which reduces the susceptiveness of circuits in these 

 cables to values approaching that obtainable by a single tape armored 

 cable. 



This tape armored cable, w^hich recently has been placed in use in 

 this country, is designed for burying directly in the ground, and has an 

 increased degree of magnetic shielding. This is provided by two wrap- 

 pings of steel tape outside the lead sheath which are necessary for the 

 mechanical protection of the cable when ducts are not used. During 

 the past year about 160 miles of this cable were installed and it is ex- 

 pected to have a considerable field of use in the future. 



As indicated above, in all these types of cable construction the sus- 

 ceptiveness to noise induction is so greatly reduced that low frequency 

 induction generally becomes the limiting factor relative to the permis- 

 sible proximity of these cables to power circuits. The relative amounts 

 of induced voltages with these different types of construction in com- 

 parison with open wire construction, while naturally varying with local 

 conditions, are indicated in a general way in Table I. 



TABLE I 



Approximate Relative Volts on 

 Telephone Circuits per Ampere 

 of Inducint; Current at 

 Type of Construction 60 Cycles 



Open wire ^ -^ 



Single cable, aerial or underground — sheath well grounded O.o 



Buried tape armored cable — well grounded U-- 



Note: All values for cables assume full size, i.e., 2;\s-in. diameter. 



