CABLE DESIGN AND MANUFACTURE 205 



the basic requirements and the methods of controlHng their quahty on a 

 samphng inspection procedure. The requirements for the materials were 

 estabhshed to insure that their use would not jeopardize the life of the 

 cable. Since cable life is critically related to the integrity of the insulation, 

 all materials had to be scrutinized for their tendency to cause environ- 

 mental cracking. These tests were necessarily made on an accelerated 

 basis. Since no correlation exists at present between accelerated tests 

 and long term (20 year) life tests, only conservative design selections can 

 be justified. 



Close tolerances such as dzO.0002 inch for the diameter of the solid 

 center wire in the central conductor were specified for all copper com- 

 ponents of the coaxial. In addition, these components had to be free from 

 slag or other inclusions, and the wire drawing and rolling of the tape had 

 to be controlled to assure smooth surfaces, edges of prescribed shape, and 

 freedom from filamentary imperfections. Compounds used in drawing 

 and rolling operations were selected to minimize the possibility of con- 

 taminating or causing cracking of the polyethylene. Residual quantities 

 of compound on the wire or tapes were removed prior to annealing, which 

 was controlled to prevent the formation of oxides and to assure clean 

 and bright copper. 



The dielectric constant range of the polyethylene-butyl compound was 

 limited to 2.25 to 2.29. These limits were determined by the limited ac- 

 curacy of the measuring equipment available at the time. Restrictions 

 covered the allowable amount of contamination since its presence in 

 other than minute quantities might reduce the dielectric strength or de- 

 grade the power factor of the compound. 



In addition, the melt index (a factor related to molecular weight) of 

 the final insulating compound composed of polytheylene resin, butyl 

 rubber, and antioxidant, was held to 0.15 to 0.50. The melt index of or- 

 dinary polyethylene used for insulation generally, is 2.0 or higher. 

 Choice of the low index assured the maximum resistance to environ- 

 mental cracking. 



The cutching and fixing processes used in the manufacture of bedding 

 jute were adjusted to limit the alkalinity of the jute because of the ad- 

 verse effect of alkaline materials on polyethylene compound. Oils used 

 in the spinning of the jute were selected to obtain types which were not 

 strong cracking agents for polyethylene, and the quantities used were 

 reduced to the workable minimum. The presence of such impurities as 

 bark and roots was restricted to provide the desired fiber strength. The 

 impregnation of the jute w^as controlled to ensure adequate distribution 



