September i, 1906.] 



THE INDIA RUBBER WORLD 



391 



LEAD JACKETED UNDERGROUND CABLES OPPOSED. 



IN a paper on Present Underground Cable Practice,* Mr. 

 Wallace S. Clark mentions that practically all cal)les 

 of the class under consideration have continuous 

 metallic sheaths, and asks : Is this the best engineer- 

 ing? 



Low tension cables are run in ctmduits with some portion 

 of the circuit groun<led. The continuous sheath on these 

 cables is an invitation to stray currents and conseciucnt 

 electrolysis. In railway practice, grounding the sheath at 

 each manhole was announced as a cure for electroljsis, but 

 instead of a cure it has been found in some cases to be a 

 cause of trouble. The amount of current carried by the 

 sheath is greatly increased, producing a drop in electro- 

 motive force between ground strips sufficient to cause a flow 

 of current to earth at some intermediate point in the dtict 

 and in sufficient volume to give trouble. In the case of a 

 burn-out, the continuity of the sheath aggravates the 

 trouble. The volume of current carried by these low ten- 

 sion conductors is so large that in many cases circuit- 

 breakers or fuses will not operate with the current due to the 

 short circuit. 



Omitting the shcdth will cure all these ills. To do this 

 would bar paper, lead-jacketed cables absolutely, and would 

 increase the depreciation account if some type of cable insu- 

 lation needing lead only, as wood needs paint, were used. 

 .\batuloniug the lead entirely is an economic possibility with 

 only very large conductors, where it may be cheaper to re- 

 new the insulation on a non-leaded cable, saj' once in 10 

 years, than to renew a lead jacket cable once in 20 years. 

 These figures are, ofcour.se, merely used for comparison If, 

 therefore, we are compelled to use a lead sheath, the writer 

 believes that it should be interrupted by some form of insu- 

 lating joint on low tension cables. 



If this plan is carried out, a serious difficulty is the inabil- 

 ity to test the insulation of the cable. This may be met by 

 the use of an insulated wire — proof or pressure wire — in tlie 

 outer layer of strands forming the copper core. Such wire 

 should be insulated with some material like treated paper 

 susceptible to the absorption of moisture. Suggestions for 

 the various uses of such a wire are given. For the purposes 

 of initial tests when cable is installed, the joints in the 

 sheath may be bridged by fine fuse wires, which are after- 

 wards removed. 



With high tension lines some of the troubles due to the 

 metallic sheath on low tension cables less marked. The 

 load is usually more uniform and subject to less violent 

 fluctuation, especially where sub stations with batteries are 

 in use, allowing protective devices to be set so as to operate 

 more proniptlj'. Further, in the case of a network such 

 cables are usually protected against a reversal of current, so 

 that the arc at the fault is not maintained by energy derived 

 from the network or sub station. 



The metal sheath on high tension cables must be earthed 

 to prevent danger to life, and also risk of puncturing the 

 insulation by cumulative static charge. 



In the matter of sheaths, for a number of years the writer 



• Preseiilcfl at the 2o6th meelilig of the American Institute of Klt-ctrical 

 Engineers, Ni.\v York- 



has been advocating multiple conductor cables for arc cir- 

 cuits, instead of several cables in the same duct in trunk 

 lines. The running of a lot of small cables in one duct is 

 not good practice ; a burn-out on one cable is likely to injure 

 others in the duct, and the withdrawal of a defective cable 

 for repairs is apt mechanically to injure the other cables. 

 Of course, one conductor in a duct is ideal, but l)arred by 

 cost in small sizes. 



The writer considers the question, Should we use a heavy 

 wall of a cheap .so-called rubber compound, or a lighter wall 

 of better quality? Thick insulation has, among other 

 points, these against it: (i) Increased size of cable, involv- 

 ing increased cost of the sheath, duct space and handling ; 

 (2) thicker wall for heat generated in conductor to flow 

 through, resulting in higher operating temperature in the 

 copper core ; (,^) and, most serious of all, the fre<iuent accep- 

 tance of a poor quality of com])ound having a very short life. 



The last leature is the cause of the ill repute in which so 

 many engineers hold rubber insulated conductors. There 

 appears to be confusion in the minds of some engineers as to 

 high insulation, resistance, high puncturing resistance 

 and durability, which do not of necessity bear any rela- 

 tion to one another. An insulating material may have 

 any one or any two of these, and be deficient as to the re- 

 maining quality or qualities. In reaching this conclusion 

 the writer covers somewhat the same ground as Mr. John 

 Langan, in a paper on rubber insulation, abstracted in The 

 Inuia RuiiHKU WoKi.i) (July I — page 326). But in any 

 event a reasonable amount of good rubber in the present 

 state of the art is necessary to insure durability. 



A table of puncturing voltage, insulation resistance, and 

 electrostatic capacity tests is given to show that these factors 

 are not very good guides as to the durability of the insula- 

 tion. In the case of the use of insulation compounds having 

 three different amounts of rubber, the relative deterioration in 

 one year in elastic limit was respectively 66 per cent., -jo per 

 cent, and 20 per cent., though there was by no means a cor- 

 responding difference in the results obtained from the volt- 

 age and other tests. 



There is little accurate knowledge as to the limit of dura- 

 bility of which insulated cables are capable. An idea of the 

 life or a rubber cable leaded and operating at 11,000 volts, 

 25 cycles, is affi)rded bj' certain cables of the Cataract Power 

 and Conduit Co. (Buffalo, New York). There are two 3 con- 

 ductor cables, with rubber insulation .,'', inch on each con- 

 ductor, no over all jacket. Each cable is 32,052 feet in 

 length, of which about two-thirds vi-as installed in 1897 and 

 the remainder early in 1S99. Yet it appears that there is no 

 indication of any electrochemical or other electrical action 

 weakening the ability of the insulation to withstand the 

 working pressure. These. Mr. Clark believes, are the oldest 

 working rubber-insulated 1 1,000- volt three-phase cables any- 

 where in use. 



Further, these cables, originally operating alone, are now 

 in multiple with some 32 miles of 3-conductor cables, and 

 probably subjected to more severe strains than when first 

 installed. From a study of the data he has collected regard- 

 ing these cables, the writer believes that cables for very 



