224 BELL SYSTEM TECHNICAL JOURNAL 



these devices to power line exposures have been made. They are, how- 

 ever, being studied by the subcommittee to see whether the objections 

 mentioned above can be overcome and to determine their possible 

 field of application. 



Shielding. — Shielding on a telephone line may be effected by special 

 grounded conductors, by working conductors, or by cable sheaths. 

 Miscellaneous structures such as pipe lines or rails in the immediate 

 vicinity of an exposure also introduce more or less shielding. The 

 employment on a telephone line of a high conductance shield wire, 

 well grounded at the ends of the exposure and at intermediate points, 

 may reduce the induced voltage by as much as 40 per cent at a fre- 

 quency of 60 cycles. As bearing on the prevention of electric shock 

 from induced voltages on telephone lines, shielding has a disadvantage 

 in that it may, depending somewhat on the method of construction, 

 add to the chance of a lineman making contact with grounded metal. 



Use of Cable. — A metallic sheath enclosing the conductors of a 

 cable is a type of shielding. The lead sheath of a 2% in. diameter aerial 

 telephone cable, if effectively grounded at the ends, as when directly 

 connected to an underground cable sheath, reduces the voltages in- 

 duced in the conductors within the cable by about 50 per cent at 60 

 cycles. The additional shielding brought about by the surrounding 

 earth when such a cable is placed underground is negligible at low 

 frequencies, although underground construction has an advantage in 

 affording a low-resistance ground for the sheath. The large number of 

 conductors in a cable afford mutual shielding which varies from a negli- 

 gible to a considerable amount depending upon many factors, impor- 

 tant among which is the extent of the cable beyond the ends of the ex- 

 posure. If two or more cables are close to one another through an 

 exposure, each benefits by the shielding action of the others, so that the 

 shielding increases with the number of cables. 



If the lead sheath of the cable is surrounded by magnetic material 

 as by armoring or placing cable in iron pipe, the shielding may be 

 largely increased. With the form of iron tape armored cable referred 

 to in the Harrison-Silver paper, which is now in trial use, shielding at 

 60 cycles is about 80 per cent, assuming effective grounding. Armoring 

 a cable increases its cost substantially but has an advantage apart from 

 shielding in that the cable being protected by the armor against mech- 

 anical injury may be buried directly in the earth without conduit. 

 The armor is protected by impregnated wrappings but its life has yet 

 to be determined. The shielding afforded by this type of cable has been 

 studied experimentally under practical field conditions. Other instal- 

 lations and studies have been made abroad. It is probable that there 



