326 



THE INDIA RUBBER NA/'ORLD 



[July i, 1906. 



A STANDARD FOR RUBBER COVERED WIRES. 



IN a paper on Standardizing Rubber Covered Wires and 

 Cables, by Mr. John I.angan,* it is pointed out tliat 

 the really vital point in all tests of such cables is to 

 make sure of the rubber in the insulation. The writer 

 presents some specifications and tests for wires and cables, 

 based upon general principles which are summarized briefly 

 herewith. 



A badly insulated wire or cable will imperil the success of 

 any system. Xot only must the insulation itself be per- 

 fect : it must be properly tested, installed, and protected. 

 Cables, some a]>i)ear to think, are all alike. If they are 

 onlj- of copper, rubber covered cables are called for and pur- 

 chased by some people with no other requirement than that 

 they conform to the rules of some " code, '' and it all goes as 

 '■ rubber covered, " even if the compound be of cheap rubber 

 substitutes. Such insulation cannot last long, since, pos- 

 sessing no vitality, it soon falls prey to variations of tem- 

 perature and climatic conditions. 



The fault here indicated lies not with the manufacturer, 

 but with the rules, which impose no provision as to quality. 

 All they require is a certain diameter of insulation. No one 

 doubts the good intentions of the framers of these rules, but 

 as indicating how wide of the mark they go in fulfilling any 

 requirement of good insulation, consider what the rules say 

 about testing wires before installation : 



Each foot of the completed covering must show a dielectric 

 strength sufFicient to resist for 5 minutes the application of 3000 

 volts per j.';j of an inch thickness of insulation. 



This would require a conductor with I'.i inch of insulation 

 to stand a test of 60,000 volts — something impossible with 

 the best Para, much less with the cheap rubbish employed 

 on some wire. But supposing it possible, who is to enforce 

 the rule ? Some of the larger users of cables, especiallj' near 

 the wire factories, send competent engineers to make tests 

 at the works, but this is not alwaj-s practicable, and wires 

 are delivered and go into use without an}' tests except meas- 

 urements. But nothing can be more misleading than to 

 adopt the voltage test alone in determining dielectric ef- 

 ficiencj'. 



What constitutes good insulation, and how can it be known? 

 The consensus of opinion is that India-rubber is the best of 

 all insulating materials. But rubber of it.self is valueless as 

 an insulating medium. Its tendency to oxidize is a feature 

 of disintegration which precludes its use alone for this pur- 

 pose. But in conjunction with other ingredients, and when 

 vulcanized, it becomes absolutely waterproof and practically 

 indestructible. It does not matter how good the rubber is, 

 if it is not properl}' vulcanized. 



There are many different grades of rubber. Chemically, 

 thej- are distinguished by the amount of their resinous con- 

 tents, and physically by their tensile strength. Both prop- 

 erties are closelj' related, for rubber which is chemically 

 poor is also physically weak. The chemical test affords 

 the reason for the high regard in which Para rubber is 

 held. 



Now since good rubber is in itself very strong and very 



• Presented at the 206th meeting of the Amenc.in Institute of Klectrical Kngi - 

 necrs. New \"ork. 



elastic, these characteristics .should be present in any insu- 

 lated wire in proportion to the amount of rubber in the 

 insulation. But this is not all : the immense influence 

 e.xerted by vulcanization must be considered. Over vulcan- 

 ized rubber will become hard and brittle ; if under vulcan- 

 ized, it becomes brittle and inert, and in either case the 

 insulation will not meet the retiuirements of a tensile te.st. 

 Good insulation is clearly indicated by its prompt return 

 after being stretched several times to, say. three or four 

 times its length, which, roughly speaking, implies a 

 tensile strain of not less than Soo pounds to the .square 

 inch. 



It has been proved repeatedly that where there is 30 per 

 cent, of fine Para rubber in the insulation, this physical test 

 is easily obtained. But a combination of a lesser amount of 

 Para and a large amount of a cheaper rubber will initially, 

 at any rate, ]>roduce the same test as 30 per cent, of tine Parii. 

 Analysis of insulation compounds which gave a satisfactory 

 test showed them to contain 15 per cent, of Para and 30 per 

 cent, of cheaper rubber. In other words, it takes 45 to 50 

 per cent, of a cheaper combination of rubber to do what 30 

 per cent, of Para would do. 



It will be seen from the above that while the ph^'sical 

 test is eflective to a certain extent, it does not compel the 

 use of fine Para. It is only by means of chemical tests that 

 the percentage of resinous content can be ascertained. The 

 .best grade of rubber has hardly more than i per cent, of 

 resin, while some others contain as high as 20 per cent. 

 But insulation containing 30 per cent, of fine Para may have 

 from 3 to 5 per cent, of resin. This is because during vulcani- 

 zation, for reasons not fully understood, the amount of resin 

 in fine Para increases to about 3 per cent., besides which the 

 addition of some extractive matter to this normal amount is 

 considered by the manufacturers a good thing for the insula- 

 tion. But the total in the completed or braided wire should 

 never exceed 5 per cent. ; for beyond this limit the chemist 

 Gannot differentiate the grade of the rubber, and thus the 

 value of the analysis is compromised. For this rea.son, 

 tests should be made in the completed wire, rather than on 

 the unfinished sample. 



Referring to the unreliability of the voltage test the writer 

 points out that certain oils will stand extremely high pres- 

 sures, which shows why cheap compounds maj- stand verj- 

 high initial tests and yet in a short time break down in actual 

 service. The reason is that such oils, incorporated in a com- 

 pound, soon evaporate, when the temporary virtues thej' 

 possess disappear also and the insulation falls into decay. 

 As a criterion of merit it is pointed out that a high insula- 

 tion resistance is immeasurablj' better. At anj- rate this 

 much is certain, that in insulating compounds having 30 per 

 cent, fine Para there is always associated with them a very 

 high insulation resistance, whereas in cheap compounds the 

 reverse is equally evident. A high insulation test should, 

 therefore, wherever possible, always supplement a physical 

 test, as it tends to elevate the grade of the compound. When 

 a chemical analysis is contemplated the necessity for this is 

 not so obvious; but where it is not the two, if associated, 

 will produce the most satisfactorj- results. 



