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- 35 - FAULT LOCATION BY ONE-QUARTER WAVE LENGTH RESONANT METHOD H. B. Slade The Okonite Company Passaic, N, J. Standard methods of locating f a u l t s i n insulated cables are very w e l l - known, p a r t i c u l a r l y the use of the loop te s t s , Murray I^oop and Varley Loop, etc., also comparative capacity tests f o r opens and comparative inductance tests f o r shorts. The above methods are very common, and wherever^the f a u l t y cable i s par- a l l e l e d by a good cable, such methods are the most accurate where tests are made ^rora the ends of the cable and the distance to the f a v i l t must be computed. Wher- ever possible, of course, i t i s desirable to locate a cable f a v i l t t o an exact spot by a te s t on the cable i t s e l f , but i n many cases t h i s i s impossible f o r one reason or another. Where faxilty cables are not psralleled by good ones, the t e s t i n g has always been of a more complicated nature, and numerous methods have been proposed to accomplish the desired r e s u l t . At the present time, there are several recognized procedures including the pulse echo method wi t h the pulse and the echo shovn on the screen of a cathode- ray oscilloscope. Equipment has been manufactured f o r t h i s purpose and i s p r e t t y well developed, although expensive. Another method used i n the past was to measure the impedance of a f a u l t y cable over a considerable frequency range and determine from the spacing of the peaks the approximate location of the f a i i l t . This l a t t e r method i s the background of our t e s t i n g procedure except that we attempt to apply the correct frequency to give a one-quarter wave resonance or a phase s h i f t such that the impedance of the l i n e appears to increase to a maximum. Simply, the c i r c u i t involves a calibrated o s c i l l a t o r , a small coupling condenser and a vacuum tube v o l t meter. Basically, the p r i n c i p l e i s that with a badly terminated cable open or shorted, the current i s r eflected back to the sending end only s l i g h t l y diminished, and at certain c r i t i c a l frequencies depending on length, the reflected current arrives back 180° out of phase thereby giving the cable the appearance of high impedance or open c i r c u i t . This causes the voltage t o r i s e from the conductor to ground since the coupling condenser creates a constant current c i r c u i t . The resonant frequencies bear the approximate relationship 1, 3, 5, 7, 9, etc. f o r short c i r c u i t e d cables, and 2, 4., 6, 8, 10, etc. f o r open c i r c u i t e d cables. This method has been used successfully on f a u l t y cable where a p a r t i a l discontinuity occurs i n the conductor at the f a u l t , preventing accurate loop measurements and also was used on an ariiiored submarine cable which had been completely cut thereby again preventing loop measure- ments. The relationship between frequencies and distance to the f a u l t i s as follows! L = 2^6 X N Fj. X ^BIO i n feet from the t e s t end.
