POLYETHYLENE INSULATED TELEPHONE CABLE 



1249 



Fig. 2(b) — Cross section of expanded polyethylene insulation from 19 gauge 

 conductor — 55 per cent air. MagniJ&ed 75 times. 



The upper curve in Fig. 3 was determined in this manner. However, 

 in multipair cable the dielectric constant is not amenable to calculation 

 and must be determined experimentally.^ The empirical relation between 

 the percent air in the insulation and the dielectric constant in the cable 

 for a typical design is shown in the lower curve in Fig. 3. Effective di- 

 electric constants as low as 1.40 have been achieved in expanded poly- 

 ethylene cables. 



Solid polyethylene insulated cables are more costly for given trans- 

 mission characteristics than those insulated with paper or pulp and, 

 therefore, are restricted to special uses where a system saving can be 

 obtained in spite of the higher first cost. One such use is for small aerial 

 toll cables in rural areas where, with paper-insulated cable, maintenance 

 costs are Ukely to be high. There are several factors which tend to in- 

 crease the maintenance costs in such cables. For instance, in small 

 isolated cables lightning troubles are common because of the high sheath 

 resistance.^ While the incidence of sheath breaks from other causes is 

 usually neither more nor less than in other aerial cables, maintenance 

 is more difficult and costly because of inaccessibility. Maintaining gas 

 pressure on these cables is expensive for the same reason. 



