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BELL SYSTEM TECHNICAL JOURNAL 



sustained over the insulation to such an extent that the circuit was not 

 broken at the proper moment. An examination of the various grades 

 of phenol fiber commercially supplied indicated that they varied widely 

 as to their resistance to arcing. Fig. 2 shows testing apparatus de- 

 signed to evaluate this characteristic. 



The sample under test was made into a sequence switch cam and 

 rotated on the fixture at a speed of 10 r.p.m. The set is wired to give a 

 circuit condition comparable with that causing failure in service, except 

 that slower speed and higher voltage are used to accelerate the test. 

 The position of the rear brush is so adjusted that after the material 

 has become carbonized through an arc of 15 degrees or a hole has been 

 burned through the insulation, the machine would be stopped by means 

 of a circuit breaker, shown in Fig. 3. This instrument makes the 



Fig. 3 — Assembly of apparatus for arcing test 



failure value independent of the operator's judgment, and has proven 

 so satisfactory that it has been employed for specification purposes. 



Fig. 4 shows insulators tested by this instrument; those at the top 

 having been rejected, and those at the bottom being satisfactory. An 

 improvement of 20 to 1 in arcing characteristics was obtained. This 

 was brought about by close cooperation with the Bakelite Research 

 Laboratories, which developed a special grade of resin to be used in the 

 manufacture of this material. In this case the materials engineer 

 developed a method of test for evaluating the particular quality de- 

 sired which enabled the supplier to improve his product in the desired 

 respect. 



Even though resistant to moisture in the ordinary sense, phenol 

 fiber absorbs a certain amount of moisture depending on the quality 

 of the material furnished. As this moisture is given up, the material 



