1250 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



2.4 



2.2 



1-2.0 

 I 



(0 



O 1-8 

 o 



1.6 



HI 



5 1.4 



1.2 



1.0 



100 



10 20 30 40 50 60 70 80 90 



PER CENT AIR BY VOLUME IN CONDUCTOR INSULATION 



Fig. 3 — Change in dielectric constant of coaxial cable and multipair cable with 

 degree of expansion of polyethylene conductor insulation. 



In the case of a sheath break with no insulation damage in poly- 

 ethylene-insulated cables, there is no interruption in service due to en- 

 trance of moisture into the cable and repairs can be made on a routine 

 maintenance basis. Gas pressure maintenance is unnecessary and its 

 omission effects appreciable annual savings. Because these cables usually 

 contain not more than 51 pairs the first cost penalty for the use of solid 

 polyethylene, in terms of cents per foot, is small. Small aerial toll cables 

 in rural areas, therefore, are the most promising candidates for solid 

 polyethylene insulation. 



The soUd polyethylene insulation development has progressed to the 

 point where several cables have been made for field trials. The first of 

 these was at Cooperstown, New York, where approximately four miles 

 of 26-pair 19-gauge cable was installed aerially. The sheath on this cable 

 was a composite of aluminum and polyethylene commonly known as 

 "alpeth" sheath. The Cooperstown cable (see Fig. 4) was one of the 

 first to be installed by the pre-lashing method which has been developed 

 as a means for effecting economies in placing aerial cable. 



A second trial installation of solid polyethylene insulated cable was 

 made between Trout Lake and St. Ignace, Michigan, a distance of 

 twenty-eight miles. This cable contained 51 pairs of 19 gauge and was 

 also covered with alpeth sheath. Since that time solid polyethylene 

 cables have been installed in other locations where the anticipated main- 

 tenance savings were believed to justify the higher first cost. 



Development of expanded polyethylene insulation has been carried 



