740 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



associated stub cable and pot wiring should have a dielectric breakdown- 

 strength as high as that of the cables for which the loading was intended, 

 and preferably somewhat higher, to assure that the loading apparatus would 

 not be dielectrically weak points in the loaded cable systems. 



When dielectric-strength improvements in toll cable design started during 

 the late 1930's in order to reduce damage by lightning, especially on buried 

 and aerial cables, equivalent improvements were also made in the loading 

 apparatus. These cable and apparatus improvements were primarily con- 

 cerned with raising the dielectric strength of the insulation between core 

 and sheath. 



During recent years, the extensive installation of buried toll cables in 

 areas where the ground resistance is high has led to the use of cables having 

 very much higher dielectric strength (wire to ground) than those used during 

 the 1930's. The development of the copper-jacketed toll cable having a 

 thermoplastic protective covering between the lead sheath and the jacket, 

 which was capable of withstanding a dielectric-strength test of 10,000 volts d-c, 

 between the sheath and the jacket, made it necessary to apply an equivalent 

 insulation to the exterior of the buried loading coil cases. The more recent 

 development of the "Lepeth" sheathed toll cable has made it possible to 

 approach a dielectric breakdown-strength of the order of 25,000 volts d-c 

 between cable core and sheath. This is achieved by extruding a sheath of 

 polyethylene of suitable thickness over the cable core, and over this a thin 

 lead sheath. Loading coil cases were redesigned to match this construction^ 

 using an inner lining of thermoplastic insulation to provide equivalent 

 insulation between the coils and the case. The stub cables have dielectric 

 design-features corresponding with those of the Lepeth toll cable. 



Potting Costs 



The potting cost per coil, or loading unit, varies considerably with the 

 potting complement-size, and is a maximum in small complements. These 

 general relations apply for all types of coils. 



In the early designs, the average potting cost per coil was much smaller 

 than the coil costs. Over the years, the'case cost-reduction that has resulted 

 from coil size-reductions, increased complement-size, and other design 

 changes, has been smaller on a percentage basis so that in the present designs 

 the average per coil potting costs are somewhat greater than those of the 

 coils. Notwithstanding the changes in cost relations just mentioned, the 

 direct and indirect savings that have resulted from the potting development 

 work constitute a substantial fraction of the aggregate plant cost-reduction 

 which has been achieved by the use of coil loading. 



