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The echo ranger : a fault locator for power cables Naylor, Thomas Kipling 1948

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THE ECHO RANGER A Fault  L o c a t o r f o r Power C a b l e s by  Thomas K i p l i n g  Naylor  A Thesis Submitted i n P a r t i a l F u l f i l m e n t o f The R e q u i r e m e n t s f o r t h e D e g r e e o f  MASTER OF APPLIED SCIENCE In  t h e Department of  MECHANICAL AND ELECTRICAL  ENGINEERING  Approved:  In  c h a r g e o f m a j o r work,  ^ieacr'of  Department".  THE UNIVERSITY OF BRITISH September,  19k&  COLUMBIA  6) CONTENTS SUMMARY  -Page ( i i i )  I  Introduction  1  II  Review of Literature  3  III Investigation  6  A.  b  Theory of propagation 1. 2. 3. 5»  B.  Theory Terminating Impedances , E f f e c t of change of v e l o c i t y  Description Accuracy Range C i r c u i t mechanism Operation a. b. c.  6. IV  V  9 14 19  21 Z$ 28 30  Apparatus 1. 2. "3. 4. 5.  6 8  21  Reflections 1. 2. 3.  C.  General d i f f e r e n t i a l equation Pulse shape Solution f o r an i n f i n i t e l i n e having constant parameters Examination of parameters Approximate solution including skin e f f e c t .  I n i t i a l adjustment Trace expansion Measurements '  Sample c a l c u l a t i o n .  30 - 31 32 33 41 41 43 44 43  Discussion  47  A.  Advantages  47  B.  Limitations  47  C.  Suggested improvements  48  D.  Preliminary note on f l a s h over  49  Conclusions  52  A.  Theoretical  32  B.  Experimental.  53  L i t e r a t u r e cited  VII  Acknowledgements  58  VIII  Diagrams  59  A.  59  Apparatus 1. 2. 3. 4.  B.  Page  54  VI  Assembly Block diagram C i r c u i t diagram L i s t of parts  Echoes from a power cable I 3 S 9 feet long 1. 2. 3. 4. 5. 6. 7.  Panoramic viexir (compressed sweep) I n i t i a l pulse Echo from a Joint Transformer-tap echo Transformer-tap re-echo or a second j o i n t Multiple echo r e f l e c t e d from sending end Open-circuited f a r end of cable.  60 61 62 63 64 64 65 65 66 66 67 67  THE ECHO RANGER A F a u l t L o c a t o r f o r Power C a b l e s , t3 S T PL A e ~ T S U M M A R Y ft  The l o c a t i o n o f f a u l t s i n l o w - a t t e n u a t i o n  coaxial  c a b l e s and open-wire l i n e s by t h e use o f t h e echo-ranging t e c h n i q u e s o f r a d a r prompted t h i s i n v e s t i g a t i o n  o f a method t o  a c c u r a t e l y l o c a t e f a u l t s i n underground power c a b l e s .  As t h e  p r o p a g a t i o n - v e l o c i t y o f d i s t u r b a n c e s on a smooth l i n e o r c a b l e i s c o n s t a n t , t h e time d e l a y between t h e t r a n s m i s s i o n o f a p u l s e i n t o a c a b l e and t h e r e c e p t i o n o f an echo from an i n t e r n a l discontinuity continuity.  i s p r o p o r t i o n a l to the distance to the d i s The l o w i n d u c t a n c e and h i g h d i e l e c t r i c l o s s e s i n  power c a b l e s a t t e n u a t e and d i s t o r t t h e p u l s e s .  This d i s t o r t i o n  l i m i t s t h e a c c u r a c y and range o f equipment w h i c h must measure time i n t e r v a l s t o t h e n e a r e s t 3 x 1 0 " " ° seconds. Basically,  t h e Echo Ranger c o n s i s t s o f a p o r t a b l e  l o w - v o l t a g e impulse g e n e r a t o r and a d e l a y e d high-speed oscilloscope.  combined w i t h a t i m i n g O s c i l l a t o r  sweep on a commercial s p l i t - b e a m  A high-power hydrogen t h y r a t r o n d e l i v e r s  0.1-  m i c r o s e c o n d p u l s e s o f f i v e k i l o w a t t s (peak) t o the c a b l e . A l t h o u g h t h e range o f t h e apparatus  now c o n s t r u c t e d  i s o n l y two m i l e s on power c a b l e , f a u l t s a t l e a s t f i v e m i l e s away s h o u l d be v i s i b l e .  The minimum r e s i s t a n c e o f a d e t e c t a b l e  s e r i e s f a u l t i s about f i v e ohms and t h e maximum r e s i s t a n c e o f a d e t e c t a b l e shunt f a u l t i s about 2000 ohms.  Without m o d i f i c a t i o n ,  the Echo Ranger can be used on overhead l i n e s up t o f o u r m i l e s long.  On a 1 0 4 4 - f o o t p i e c e o f RG-gU p o l y e t h y l e n e c a b l e , 100-ohm shunt f a u l t s 20.2 On a t h r e e - c o n d u c t o r  two  f e e t a p a r t were l o c a t e d w i t h i n 0.63$.  o i l - f i l l e d l e a d - s h e a t h e d power c a b l e  1389 f e e t l o n g , a t r a n s f o r m e r tap 424 f e e t away and a 320 f e e t away were l o c a t e d w i t h i n  Joint  1.2$.  The a p p a r a t u s can be r e a d i l y m o d i f i e d to d e l i v e r l 6 - k i l o v o l t 5- egawatt (peak) p u l s e s to i n i t i a t e ^ a n Vrcf^at m  incipient faults.  The power to h o l d the a r c must come from a  superimposed power supply such as a k e n o t r o n set o r the normal line  voltage. F u r t h e r r e f i n e m e n t s w h i c h i n c r e a s e the a c c u r a c y  range w i t h o u t s a c r i f i c i n g s i m p l i c i t y o f o p e r a t i o n c o u l d be a p p l i e d to  advantage.  T.K. Naylor, U. B . C. September, 1943.  and  THE EOHO RANGER A Fault Locator f o r Power Cables.  1  INTRODUCTION  Recent work on the l o c a t i o n of f a u l t s i n lowattenuation coaxial cables and open-wire l i n e s by using the echo-ranging  techniques of radar has prompted t h i s investigation  of a method to accurately locate f a u l t s i n underground power cables. EchQ-ranging depends on the constancy of the v e l o c i t y of propagation of disturbances on a smooth cable.  The time delays  between the transmission of pulses fed into one end of the cable and the a r r i v a l of r e f l e c t i o n s from Internal d i s c o n t i n u i t i e s are proportional to the distances between the sending end and the discontinuities.  Then, i f the distance to one of the d i s -  continuities i s accurately known, the locations of the remainder can be e a s i l y calculated. Although much power cable i s t r u l y coaxial, skin effect and high d i e l e c t r i c losses absorb most of the components of the pulses used i n echo ranging.  high-frequency The  resulting  d i s t o r t i o n of the pulses l i m i t s the accuracy and range of the measuring equipment.  ,  The intermittent or steady d i s c o n t i n u i t i e s i n the smooth caMe may or f a u l t s .  be junction points, open c i r c u i t s , short c i r c u i t s ,  The insulation may be carbonized and the  more or l e s s burnt away at a f a u l t .  conductor  The apparatus should be portable, rugged, sensitive, accurate, simple, and inexpensive.  Provision should be made to  prevent extraneous disturbances such as induced voltages on the sheath, electrochemical emf's at wet f a u l t s , or accidental energizing of the core from a f f e c t i n g the apparatus or the operator.  I f possible, the measuring currents should not destroy  the evidences of the cause of the f a u l t . I f the f a u l t w i l l appear only when rated voltage i s applied, apparatus should be devised to locate the f a u l t while i t i s arcing over. To locate a f a u l t within 10 feet on a 10-mile cable, a time delay of 10 ' seconds must be measured to the nearest —21 3 x 10~ seconds.  3.  11  REVIEW OF LITERATURE  • Much excellent work has been done on methods of 1 l o c a t i n g cable f a u l t s .  Mr. Savage recently summarized many  effective methods. In bridge or loop methods, the resistances or capacitances of an unknown length of faulted cable and a known p  length of good cable are compared. The abrupt change i n the magnetic f i e l d around the cable at a d i s c o n t i n u i t y can be detected by search..coils taken along the route of the cable. Shielding by the steel armouring n u l l i f i e s t h i s method. Chopped or modulated currents fed into •5  the cable produce a traceable magnetic f i e l d . ^ The thump produced by Imterrupted  arcing at the f a u l t  can be heard i f sensitive microphones are placed on the ground over the burled cable.'  The rumbling of t r a f f i c w i l l swamp weak  sounds. Although portable radio receivers are sometimes used to pick up the s t a t i c from the arc, the shielding effect of the 5 i s appreciable. 1  See reference  2 7 page 5 & 0 - 9 3  2  See reference  2 7 page 5 8 0 . 9 3 paragraph 2 . 2  3  See reference  2 7 page 5 & 0 - 9 3 paragraph 2 . 3  \  See reference  3  5  See reference  1 1 page 1 3 2  page 2 2  sheath  I f the lead sheath i s punctured, the odour of burnt Insulation permeates the s o i l and cable conduits near the f a u l t . The distance to a single f a u l t can be determined by a method i n which the l i n e a r l y varied frequency of the applied sine wave i s beaten against the echo.  The frequency of the beat w i l l  depend on how much the transmitter frequency changed while the echo was t r a v e l l i n g to and from the discontinuity.^" Standing waves produced when the cable i s a multiple of a quarter wavelength of the applied sinusoidal voltage provide quite a simple and accurate means of locating gross f a u l t s on p long cables. On overhead open-wire l i n e s and low-distortion speech or t e l e v i s i o n cables, echo-ranging devices using single pulses or bursts of high-frequency o s c i l l a t i o n s have proved to be quite accurate and sensitive.^  Their rapid development during the war  parallelled the evolution of sensitive radar equipment. A method of l o c a t i n g f a u l t s on overhead l i n e s by using the a r r i v a l times of echoes and re-echoes of the surges produced at a f l a s h over has been proposed by Messrs. Stevens and It Stringfield.' Messrs. Margoulies and Fourmarier have used 9 0 - ^ v  1 2 3 4  See reference 26 page (22 See reference(20 page 46 (5 See reference(IZjpage it (2o also reference 1 page 5^1 See reference 2 9 77  impulses to locate high-resistance f a u l t s on overhead l i n e s .  1  When the f a u l t e d section of cable has been exposed, the shunt f a u l t can be located within one inch 'In ten miles by a core-to-sheath potential-difference test developed by 2 Mr. Savage.  The vacuum-tube voltmeter connected between the  core and sheath at one end of the cable drops to zero and then reverses as the battery passes the f a u l t . In a somewhat l e s s accurate method, the battery i s connected to the end of the cable while the voltmeter probes are 3 moved along the sheath. Intermittent carbonized f a u l t s can sometimes be located i f the cable i s vibrated and the f a u l t i s used as a  4 carbon microphone. As most cable f a u l t s appear at taps, Joints, manholes, or places where the earth has been disturbed, rough methods of f a u l t l o c a t i n g often s u f f i c e .  However, precise measurements  must be made i f the f a u l t i s i n a length of cable buried d i r e c t l y i n the earth or under pavement.  1  See reference.  19  2  See reference  27 page 5^0-93  3  See reference  27 page 5^0-93. paragraph 2.4  4  See reference  27 page 58O-93  paragraph  2.5.4  paragraph 2.S.1  Ill A.  INVESTIGATION  Theory of Propagation.  General D i f f e r e n t i a l Equation .  ~fr?c  Consider a short length of l i n e ^2aF  =W*^"  - ^ i , ,  L_  :  where resistance inductance capacitance conductance operator  ^t, = ^& ~ c = y =  r  ohms per unit route length henries per unit route length farads per unit route length mhos per unit route length  =  9e am In the l i m i t as  and  —  -jir  Take and  3L  = ye  y^c—*- O  +  c  (y •+ />) c  of fS)  and substitute i n (^)  of (y)  and substitute i n (3)  and.  1.  =  See reference 6 page 9 7  e  €)  Treat (s) and '(^)aa ordinary l i n e a r d i f f e r e n t i a l equations.  Then the solutions are:  Where A, B, C, and D are a r b i t r a r y functions of t_alone  Substitute  and (S)ln  coefficients of £  let  &  =  and ^ a n d  compare  and then (5  v  *«*  (3)  -JET  &  _ /ST/^ESZ.  U+4/>  " /2-.  /  .  Then  = <£,  A  • -t £  &  '8  e  . 2.  Pulse Shape. I f the applied voltage i s sinusoidal, the steady-state  solutions of the general propagation equations result when i s replaced by ^cd .  By means of Fourier series, a piecewl'se  continuous function can be expressed as a sum of sines and cosines. As  cd w i l l be d i f f e r e n t for each term of the sum, the phase s h i f t  and attenuation of each component w i l l not be the same along the line.  As pulses are e s s e n t i a l l y discontinuous and f i n i t e , d i s -  t o r t i o n w i l l be excessive unless the frequency range Is kept very narrow.  The i d e a l pulse must also be simply produced at a high  enough amplitude to override background interference. Although the peaked wave shape of the discharge current from a condenser does not cover as narrow a band of frequencies as the error function, i t can be quite simply produced at high amplitude.  The wavefront  i s steep enough f o r precise measurements  to be made to the toe of the pulse, yet the peak i s not so badly distorted that i t i s unrecognizable.  A narrow pulse vrf.ll pass  quite e a s i l y through the high-voltage capacitors which couple the apparatus to a l i v e l i n e . In practise, the wave shape of the applied voltage approximates  where  ft?  i s about 6 x 10  The factor, 10  yv,  seconds.  , i s used because the time of r i s e i s about  3.  Solution f o r an I n f i n i t e Line (Constant parameters) ffor f i n i t e solutions of the general d i f f e r e n t i a l  when  zxi—a-  Then  o  ' •  ^  .  Applying a voltage  e  ^  A  £ a t  ^Ct)  Let  equatio  -  /  O  i  ^  OPERATOR FORMULAE  ^  S  where  jf®  -  *  (/-J)/®-  ''  (shifting)  - i y i s a unit function delayed u n t i l  1.  See reference 10 page 5 5  2.  Bee.reference  8" page 220  7  /o  Multiply both sides of (QJ by ^  (y)  frfc  Solve equation  -  5?  of ($)  /eft- s^e  ^  s'^.  ^/  a  c  ^  e  by using equation(^jto s h i f t g^^to  y  » ~; - - ^  Apply formula ^2) to equation ^ J .  For a graph of  s  JT@)  ~  *J>Q@^  s^<x-ir)  and  ^  the l e f t .  V 3^  s  . #•  ^p-  see reference 1^ page 22^  The f o l l o w i n g  s o l u t i o n f o r £ ^ i s based on a method  outlined i n reference 6  pages 1 0 0 - 2 .  From equation (y)of the d e r i v a t i o n o f the g e n e r a l d i f f e r e n t i a l equation we. have  '_ ,  In Equation  _  -I  shift  ^_  c5"  one term t o the r i g h t .  r^e Substitute  equation (=^v i n t o  The d e r i v a t i v e o f a product  equation  c o n t a i n s two d e r i v a t i v e s  (a)and^(b)  y^  Also Hence  j Z Lf  j  k  &  -  e ^ l f s ^ ' y  '  e ' ^ [ J f ^ * ) ^ *&>J  See r e f e r e n c e 1E> page 42  M-sC]  *7  :  '*S.  IX Then e q u a t i o n s 2">, 2 5 ,  2 6 , and 23 combine t o g i v e  The f i r s t i n t e g r a n d v a n i s h e s everywhere except at T~~/i^  The  second i n t e g r a n d does riot c o n t r i b u t e , u n t i l  hence  As  £  ~£ ~  ^  ^  /^^c)  i s  no  '  t  m u l t i p l i e d by a u n i t f u n c t i o n to  cause i t t o v a n i s h f o r  7^0  , then  —  G  ^  33.  For graph,of  1.  •^~ (^') ~ ~ ^ . ^ / ( ^ ^ ) /  R e f e r e n c e 1 5 pages 3 9 5 - 7  s e e  r e f e r e n c e ill- page 2 2 ^  J3-  Approximate wave shape a f t e r a r r i v a l * E x p r e s s e q u a t i o n (33) as a f u n c t i o n o f / " " " 7  ^j^~^lk~  As a f i r s t a p p r o x i m a t i o n  ^  ' J°-  O^0^2-  , 2.  N o r m a l l y the p u l s e s are c o m p a r a t i v e l y s h o r t , i . e . #7 i s l a r g e and  M > ° < -  /•  ^  R e f e r e n c e 14 page 224  Examination of  Parameters.  Power c a b l e I s made up of one o r more s t r a n d e d copper c o n d u c t o r s s e p a r a t e d from one a n o t h e r and from the e n c l o s i n g l e a d sheath by i n s u l a t i o n w h i c h may be o i l - o r b i t u m e n - i m p r e g n a t e d paper, rubber, varnished cambric, or i n e r t gas.  For mechanical  p r o t e c t i o n the l e a d sheath may be armoured w i t h an o u t e r l a y e r of steel  strands.  To a f i r s t a p p r o x i m a t i o n , the c a b l e may be c o n -  s i d e r e d to be c o a x i a l .  F o r more p r e c i s e c a l c u l a t i o n s ,  g e o m e t r i c a l f a c t o r s d e t e r m i n e d by Simmons  Ware and Reed  2  1  the  s h o u l d be a p p l i e d .  g i v e f o r m u l a s f o r r , 1, g , and c f o r  l o w - f r e q u e n c y o r d i r e c t c u r r e n t w h i c h i s assumed to be u n i f o r m l y d i s t r i b u t e d over the c r o s s - s e c t i o n o f each c o n d u c t o r . frequencies, r , 1, and g . frequency,  s k i n e f f e c t and d i e l e c t r i c a b s o r p t i o n l o s s e s  change  As the p e r m i t t i v i t y i s r e l a t i v e l y independent  the c a p a c i t a n c e  ~~~ ^ Jit^ —  A/  where x  At h i g h  can be c o n s i d e r e d  constant.  f a r a d s p e r meter  _yC""  i s the v e l o c i t y of l i g h t , 3 x 10  m/sec  i s the r e l a t i v e p e r m i t t i v i t y o f the i n s u l a t i o n . 1  See r e f e r e n c e  2  See r e f e r e n c e 30  2.8 pages  /Oj //"  of  As skin effect w i l l concentrate most of the current at the outer surface of the core and the inner surface of the sheath, the self-inductance due to f l u x linkages i n the metal conductors w i l l be reduced. l i n k the core.  However, the f l u x i n the i n s u l a t i o n w i l l This interconductor inductance -7  where /*f  can be calculated  henries per meter  i s the r e l a t i v e permeability of the d i e l e c t r i c .  most d i e l e c t r i c s ^  still  For  i s unity.  At high frequencies, skin effect w i l l increase the resistance by decreasing the e f f e c t i v e cross-section of the conductors.  The variations i n the resistance and self-Inductance  w i l l be evaluated according to the method outlined i n "Surge Phenomena"."'" From solution of the equations:  Ramo and Whinnery  where  show that, f o r a single conductor,  i s the radius of the conductor i n meters  radians per second i s the absolute permeability of the conductor or henries per meter  6  1  See reference  page Z---  2  See reference -2-5" page -2_/2-  2  2  y £  i s the d-c resistance i n ohms of a one-meter length of the conductor.  ^  i s the r e s i s t i v i t y of the conductor i n meter-ohms la the effective resistance i n ohms per meter length of the conductor  ^7. i s the Internal self-inductance of the conductor i n henries per meter. For  the very large values of Cd encountered i n surges,  the Be3sel functions can be approximated by formulas l i s t e d on page 157 of reference /O  . <z<£>  ST  '  j - i )  =  ,  f  A  T——  f t ' * .  +  2-  As  f o r sinusoids and ^  are equivalent  e D i e l e c t r i c losses i n the i n s u l a t i o n of power cables account f o r e m o s t of the apparent conductivity.  Ordinary  conduction losses increase with temperature; but at the  / 7  temperatures normally encountered, they are s t i l l very small.  1 Mr.. H.H. Race  has found that the t o t a l l o s s per cycle i n the  o i l used i n "solid-type" cables f i r s t increases and then decreases with frequency.  The frequency at which maximum l o s s occurs  increases with temperature.  The apparent gain i n accuracy does  not warrant the added complication of approximating an equivalent shunt conductance, f o r the operational equations w i l l be solved only approximately. If the v a r i a t i o n i n g i s neglible, then  Insulation  Velocity of propagation  1.0  9&3  polyethylene  2. 25  655  o i l - f i l l e d paper  3-5  525  compound  3-7  510  varnished cambric  4.5  460  rubber  6  400  gas  •3. See reference g^L page^S'  f t . peryte-Bec  /8  For eccentric single-core cables, Brown^gives the surge impedance as:  1  See  reference  /3  5.  Approximate Solution Including Skin Effect, ^  1  (Jacottet Method)  2 In.general,  f o r an I n f i n i t e l i n e  where From section \  a,  /—T,—  /  y?  As a f i r s t approximation, assume (f i s negligible. /  •4.  iU~r  Let  1 •2-  Expand hy the binomial  theorem 6-  For a large rate of change of 6  i.e.,- large ^  , ignore a l l  but the f i r s t two terms.  Let  ^  Then  ^  a  -  ~TA^^~ t~ 7  £  £  -  ff  jcrg)  1.  Compare solution i n reference 6, page 122.  2.  See equation 3 of Section 3.  7  8.  £0-  For  =• ^  e - /r -fc x  s  3-  £  sT**^  lo. If.  /A-  4-  (As before,  NOW  e'^-yij  «=• O )  -y(z^\y r  ••• < - */£  1.  For e*f< 6 see reference lK page 25.  2.  Reference 10 page 199  3-  Reference 10 page IKS  £-«r  J  £  /s.  -fuz  4T7-4  B. 1.  Reflections  Theory, For a f i n i t e l i n e closed at x « o by an i m p e d a n c e ^  and at  by an impedance  we have  \ U  1  T  >-  J,  In general  J  e At'•  =  A  /  + 3  if  let  r  Rearranging 0 Rearranging let Then  1.  from®,  -^4  '"4  /*-! '3  =  r-  ~ < ^  See reference 115 page 1°9  /4  ^2-  3  Hence from£g?<£©  Then  fvom®^©  (/-/t^/ £~ - )^^/) ^ a  =  A "  (r£~ ^X('*0  ^  X  (/~  Expand denominator as  =  /+  ^  AJL  -f  ^  •••  Similarly  For  an i n f i n i t e l i n e =  ~& -  £  ^y/ -Jx  *  £  ~~  6  4 - r  /3  ^~  j  (('+0  "  ^  JZ3-  The voltage and current In a f i n i t e l i n e are sums of disturbances which have t r a v e l l e d successively longer distances by being r e f l e c t e d at the d i s c o n t i n u i t i e s at the ends of the line.  While running on the smooth l i n e , however, the d i s -  turbances act as I f each were alone on an I n f i n i t e l i n e .  This  statement i s apparent when i t i s noted that the equation f o r the increment i n _g or i due to the second term i s of the same form as that of the i n f i n i t e l i n e except that x i s replaced by (2-A-  x).  As the solutions of the operational equations f o r the  I n f i n i t e l i n e are zero before t = ^ , then the solutions of the f i r s t echo equations w i l l be zero before t = 2^-x . v Consequently,  each echo i 3 delayed by a time which varies d i r e c t l y  with the t o t a l length of i t s route.  The problem then resolves  I t s e l f into producing the i n i t i a l pulse and accurately timing the a r r i v a l of the echoes.  2L  I  IF -J-  I f the l i n e continues on past the shunt impedance, then replace zfr^  i n the above formulas by  The input voltage to .if*^. , viz: 5 -  where  7~~Z  3 £  and  _^  w i l l be the same as across  e  then the input current to j£  is  As the denominator only adds i n the multiple echoes from the sending end and  — — ^ —  the discontinuity, then  /CyU.^  operator and  i s the voltage incident on i s the voltage r e f l e c t i o n  i s the r e f r a c t i o n or transmission operator.  Tables of these operators f o r various c i r c u i t arrangements may be found i n reference  j/*^  2.  Terminating  Impedances  The type of echo and the attenuation produced by r e f l e c t i o n and r e f r a c t i o n operators can be estimated i f the operator impedances are replaced by surge impedances. The voltage-reflection operator:  If the cable i s open-circuited,  =  / I f the cable i s short-circuited,  ST^- &  I f the cable i s terminated i n i t s surge impedance,  This approximation holds quite closely providing the incident pulses are of short duration."'" Hence f o r cable terminations l e s s than the cable surge Impedance, e.g., shunt f a u l t s or taps, the p o l a r i t y of the echo w i l l be the reverse of the incident pulse;  whereas f o r cable  terminations greater than the surge Impedance, the echo w i l l be of the same p o l a r i t y .  The transmitted or refracted pulse w i l l  resemble the incident pulse.  For maximum e f f i c i e n c y of  r e f l e c t i o n , the cable should be short- or open-circuited and not terminated i n i t s surge impedance. Now consider a uniform cable containing a discontinuity somewhere along i t s . length. As  — JEF^: (same type of cable)  then the voltage r e f l e c t i o n operator  1  See reference  £  f-7 '^ f  /-f-z  ^  and the voltage r e f r a c t i o n operator  / If reflected.  g  3  i s large, then very l i t t l e energy w i l l be  Hence a r e l a t i v e l y high resistance f a u l t may  go  unnoticed unless i t s echo i s highly amplified. If  — is  small, very l i t t l e energy w i l l be r e f r a c t e d  past the low-impedance f a u l t .  Echoes from d i s c o n t i n u i t i e s  beyond the f a u l t w i l l be doubly attenuated as they must pass the discontinuity twice i n order to return to the detecting apparatus. As the echo r e f l e c t e d from an open c i r c u i t i s of the same p o l a r i t y as the Incident pulse, the voltage at the open c i r c u i t at the instant of a r r i v a l of the pulse w i l l be double the voltage of the a r r i v i n g pulse.  Consequently, the most  sensitive detector should present an open c i r c u i t to the cable. As the thyratron i s a u n i l a t e r a l impedance which varies with time, the value of • .JLL w i l l depend on the p o l a r i t y of the f  incoming pulse, the voltage on the g r i d of the thyratron, and the deioniz-atlon time of the thyratron.  However, unless the  shunt f a u l t i s less than several hundred feet from the  sending  end of the cable, the thyratron acts l i k e a high impedance to incoming pulses. D i s t r i b u t i o n boxes w i l l appear as low impedances unless the t o t a l admittance of a l l the outgoing cables equals the admittance of the single incoming cable.  As  transformers  are generally connected to the tap j o i n t by a flve-^to ten-foot  cable, the termination f i r s t appears to be a drop i n impedance. •The large inductive loop between the fuses and the transformer presents a high series impedance which w i l l give an echo even though the stray capacity of the transformer winding e f f e c t i v e l y shorts the f a r end of the inductive loop. Grounding reactors between insulated sections of the sheath may have enough stray capacity to appear to short the sheath discontinuity.  However, t h i s p o s s i b l i t y has not yet been  experimentally checked. A distant good j o i n t i s too small a d i s c o n t i n u i t y to be detected without highly sensitive apparatus.  However, nearby  Joints or j o i n t s having a series resistance of several ohms can be detected as r i s e s i n impedance.  Crushed f a u l t s should give a  minute echo showing a s l i g h t decrease i n impedance.  However, two  d i s c o n t i n u i t i e s close together, e.g., the junctions to good cable at both, ends of a wiped Joint, produce echoes of opposite p o l a r i t y which p a r t l y cancel one another to leave short pips which are very quickly attenuated. Multiple echoes from complicated d i s t r i b u t i o n grids should be mapped while the cable i s unfaulted.  I f t h i s pre-  liminary work has not been done, the positions of the known echoes can be calculated and compared with the oscilloscope trace to determine any discrepancies which indicate f a u l t s . diagrams proposed by L.V. Bewley  1  Lattice  simplify the problem of  Identifying multiple echoes on l i n e s or cables whose v e l o c i t y , impedance, and attenuation vary from section to section.  1  See reference 9B  page ^"^5  J28  3.  E f f e c t of Change of Velocity As the length of the trace on the oscilloscope screen  i s a function of time or ~ , care must be taken to interpret Intervals between echoes i n terms of the v e l o c i t y of propagation of  the section of cable bounded,by the echoes.  I f a piece of  f l e x i b l e ^Q-oim polyethylene cable connects the apparatus to a 50-ohm rubber ot varnished-cambric power cable, the r e f l e c t i o n from the junction would be almost undetectable;  yet the  d i f f e r e n t v e l o c i t i e s of propagation w i l l compress the time-distance scale of the polyethylene cable to 7 5 $  o  f  the scale of the power  cable, A similar phenomenon occurs when the cable sheath i s 1 not  connected to the grounded sheath terminal of the Echo Ranger.  The sheath then acts only as a capacitive voltage-divider between the  core and ground.  Consequently,. most of the energy i n the  applied pulse travels at the v e l o c i t y of l i g h t i n free space u n t i l the pulse reaches the f i r s t place where the sheath i s grounded. the  An echo from the junction w i l l be produced because  pulse w i l l be entering a cable from an a e r i a l l i n e .  This  change of parameters may also appear at the insulated sheathj o i n t s sometimes used to control the induced currents which heat the  sheath.  Only the sending end of the sheath must be grounded,  for  the whole of the i n i t i a l pulse i s then impressed d i r e c t l y  between the core and sheath without an intervening inductive loop between l i n e and ground.  1  See reference  page  2-3  When t h e 'sheath's; a n d c o r e s a r e c o n t i n u o u s , j u n c t i o n s between,cables having d i f f e r e n t  characteristic  be d e t e c t a b l e f r o m  o f t h e echo.  the p o l a r i t y  impedance a t t h e echo w i l l in  be i n d i c a t e d  1  end o f t h e c a b l e .  See r e f e r e n c e  i s fed  i n impedance a t t h e  a p p e a r when t h e p u l s e i s f e d i n f r o m t h e 1  other  will  A drop i n  when t h e p u l s e  f r o m one end o f t h e c a b l e , w h e r e a s a r i s e  same j u n c t i o n w i l l  impedances  1 page, ^ko  1  3o  C, 1.  Apparatus,  Description Basically, the apparatus consists of a low-voltage  impulse generator combined with a timing device for measuring the i n t e r v a l between the transmission of the i n i t i a l pulse and the reception of echoes from d i s c o n t i n u i t i e s i n the cable.  The  input pulse approximates a 0 by 0.0} wave which reaches a peak of about $00 volts when feeding a 5 ~ -° <  ,  nm  line.  The impulse generator consists of a high-power hydrogen thyratron which discharges a condenser d i r e c t l y into the cable. A compact 2-kv power supply recharges the condenser while the echoes are being received. (See block diagram) The timer or marker pips and the echo pattern are compared on a Cossor double-beam oscilloscope adjusted to produce a single sweep each time a pulse i s sent into the cable. As the marker pips, common sweep, and i n i t i a l pulse are synchronized within 0.002yu-sec. of one another,  small parts of  the trace can be expanded without affecting the r e l a t i o n s h i p between the marker pips and the echoes. the high-speed  Hence the s t a r t i n g of  sweep can be delayed whenever echoes from distant  parts of the cable are to be examined i n d e t a i l . The apparatus,  exclusive of the oscilloscope, i s  23-g- inches high, 10^- Inches wide, and 7-| inches deep. weight i s 36^ l b . 60-cycle  The  The unit consumes 175 watts from a 115-volt  line. As the input impedance of a cable may  be as low as  50 ohms, the coupling device must provide a high burst of current  31 i n order to impress a pulse of a reasonably high voltage. Cathode followers are l i m i t e d by low transconductance peak plate-current unless enormous tubes are used.  and low Ordinary  Marx-type impulse generators are too bulky f o r easy p o r t a b i l i t y , although their hlgh-poiirer steep-wavefront could probably be used to advantage on long cables.  Mercury  or argon thyratrons are too slow i n i o n i z i n g to provide steep-fronted pulses.  pulses  extremely  Also, the deionization time i s so long  that the thyratrons act l i k e a short c i r c u i t to Incoming inverted echoes.  The sluggishness i n i o n i z i n g and deionizing has been  overcome i n small hydrogen thyratrons, such as the Sylvanla  5C22,  which can d e l i v e r into a 50-ohm load 2-microsecond pulses  of 5- gawatt peak power $00 times a second.^ me  In this apparatus, the 5^22 i s used as a switch to discharge a small condenser into the cable.  As the pulse can  reach a maximum of only SO kw with 2 kv driving i t , the pulse r e p e t i t i o n frequency was stepped up to about three k i l o c y c l e s . However, as the pulses are very short -  jx-aec. - the average  current through the thyratron i s only 1.7 nia.  Consequently  only  a small high-voltage "power-supply i s needed; 2.  Accuracy On RG-SU Polyethylene Coaxial Cable 1023 feet long, a  single 100-ohm shunt f a u l t 71.6 feet from the sending end appeared to be 73*6 feet away - an error of 0.2$. On a 1044-foot piece of RG^U coaxial cable with tx*o 100-ohm shunt f a u l t s 71-6 feet and 92.4 feet respectively from the sending end, the f a u l t s appeared to be distant  1.  See reference /2- page  feet  32-  and 9 3 . 5 feet respectively respectively).  . - (Errors of 0 . 6 3 $ and 0,06#  Note that the f a u l t s were only 20.8" feet apart.  On a three-conductor o i l - f i l l e d lead-sheathed power cable 1389 feet long, a transformer tap K&K feet from the sending end was located within 25 feet before the non-linearity of the start of the trace was r e a l i z e d .  In a l a t e r test with  improved apparatus the error was reduced to l 6 feet ( 1 , 1 5 $ e r r o r ) . A Joint, 320 feet from the sending end gave a small echo 11 feet short of i t s true position; When the oscilloscope was connected to the untapped core while the pulse generator f e d the tapped core of the above power cable, the transformer appeared to be 21 feet beyond i t s true position. I t has not yet been determined whether the frequency v a r i a t i o n of the marker-pip o s c i l l a t o r or the d i s t o r t i o n of the echo i s responsible f o r these errors. 3.  Rang;e Although the 5 ~* icrosecond brightened trace l i m i t s 0  n  the maximum range to two miles, the size of amplified  multiple  echoes indicates that f a u l t s three miles away on a f i v e - mile cable should be readily v i s i b l e .  I f both ends of the cable  are available, the e f f e c t i v e range could be doubled, depending on the position of the f a u l t .  Ultimately,  distant f a u l t s w i l l be masked by multiple mediate d i s c o n t i n u i t i e s . •  '  the weak echoes from echoes from i n t e r -  33  C i r c u i t Mechanism a.  Multivibrator. The master timer i s a double-pentode multivibrator\rhich produces nearly square waves under l i g h t load without appreciably a l t e r i n g the period of the square waves.  The screen grids of the  6AG7 s (V^ n d V ) act as the anodes of a triode r  a  2  multivibrator which are shielded from the power-.. handling plates by the grounded suppressor g r i d s . The plate current i s then almost independent of the plate voltage.  The t o t a l width of the asymmetrical  square waves of 52 and 22>5^i~sec. respectively corresponds to a r e p e t i t i o n frequency of 297° second.  P  e r  The amplitude (peak to peak) of the output  i s 267 v o l t s . b.  Trigger Network. The pulse-shaping network f o r t r i g g e r i n g the thyratron i s similar to the f i r s t of  shock-excited stage  the cascade trigger c i r c u i t used i n the Marine 2  Type-26g Radar Set. A negative-going square wave from Vg i s fed onto the  g r i d of  which immediately cuts o f f the steady  plate current through the inductance L^  The high  t  Inductance causes the plate voltage of V, to r i s e to 3  try  to keep the current constant through the c o i l .  the  Inductance has appreciable stray capacity, i t  begins to o s c i l l a t e to produce a da'mped sine wave of See reference l 6 page 534 See reference 21  Diagrams 4 0 , 45,  47.  As  34about 3° k » c  However, the 3 °  g r i d c i r c u i t of V  inductance i n the  received a shock when the negative-  going square wave was impressed on i t .  (Note the  r e s u l t i n g pip i n the multivibrator output). natural frequency of  i s about 5 ° kc,  across i t and the g r i d of V  As the  the voltage  w i l l complete the f i r s t  3  h a l f cycle several microseconds before the voltage across L^,  Consequently the g r i d of  w i l l be going  p o s i t i v e when the plate voltage nears a maximum.  The  sudden flow of pla,te current damps out the remaining o s c i l l a t i o n s of the plate inductance.  Also, as the  g r i d i s driven p o s i t i v e , i t draws enough g r i d current to damp out the succeeding o s c i l l a t i o n s of the 3 ° ^ inductance. As the thyratron (V^) requires at least^ 20-ma g r i d current to f i r e i t at the low plate voltage o f 2 kv., a. cathode follower (V^) i s inserted between the plate of and the thyratron g r i d . suddenly overloaded  As the cathode follower i s  when the grid,space  thyratron, the g r i d of  ionizes i n the  draws current to f l a t t e n the  peak of the damped sine voltage appearing on the plate of V .  The negative charge, stored on the coupling  capacitors while the grids were drawing current, w i l l  c.  hold  and V". well below cutoff when the square wave  from  finally.goes positive,  Thyratron. When the thyratron (V^) f i r e s , a 0 . 0 0 C 4 - u f condenser i s suddenly discharged  into the cable.  As the  3& cable appears as a resistance of only 50 ohms, the 0.5-^ih discharge c i r c u i t w i l l be much under-damped i f a smaller condenser i s used. C »  R*  f o r c r i t i c a l damping. *  Also, the thyratron i s slowly discharging the condenser while the g r i d plasma i s i o n i z i n g . thyratron acts l i k e a vacuum triode.)  (The  As a l a r g e r  condenser i s more slowly charged, a smaller charging resistance i s needed.  Hence, the small current drawn  by the thyratron does not appreciably lower the voltage applied to the condenser immediately before firing.  A s t i l l l a r g e r condenser takes too long to  discharge and requires too large a power supply. The amplitude of the pulse fed to the cable can be decreased by the i n s e r t i o n of l a r g e r f i l t e r i n g  :  r e s i s t o r s i n the high-voltage power supply. Voltage Divider and F i l t e r . In order to provide a condenser-charging  path  Independent of the cable, a tapped 2000-ohm metallized r e s i s t o r i s shunted across the cable terminals.  This  r e s i s t o r also serves as a voltage d i v i d e r to feed ^>Q% of the i n i t i a l pulse and echoes from -the cable to the plates of the oscilloscope. As the Input  capacitance  of the oscilloscope Is across the lower half of the r e s i s t o r , the ultrahigh-frequency components of the pulses are shunted to ground so that they cannot. Increase the d i s t o r t i o n of the trace on the screen.  36 For  weak echoes, the two-megacycle video amplifier i n  the  Cossor oscilloscope can be used providing i t i s not  overloaded, (e) Beam Intensifying. In order to black out the return trace, a 65-volt peak-to-peak  ( i . e . 5^»5 v o l t s above zero and 8.5  below) square wave from of  volts  i s f e d onto the grid (lower tag)  the Gossor cathode-ray tube through a 0.01-uf 1200-volt  coupling capacitor.  The g r i d r e s i s t o r i s 0.1 megohm. As  t h i s square wave i s going p o s i t i v e when the thyratron i s f i r i n g , the trace showing the i n i t i a l pulse and subsequent echoes i s i n t e n s i f i e d .  During the flyback, the square wave  i s going negative so that the beam i s blacked out.  As the  wave i s not t r u l y flat-topped, the focus and b r i l l i a n c e vary along the sweep, (f)  Triggered Sweep. When the self-repeating t r i g g e r voltage Is shortc i r c u i t e d , the Puckle time base i n the Cossor Model 339 oscilloscope can be started and stopped by an external trigger voltage applied to the "synch", terminal.  I f the  "synch", terminal i s driven negative, the sweep condenser i s discharged and the beam f l i e s back to the l e f t side of the screen where i t remains u n t i l the "synch", terminal i s driven p o s i t i v e and the normal sweep b e g i n s .  1  In order to delay the start of the high-speed sweep u n t i l the echoes return from the distant parts of. the cable, the  1  square-wave trigger voltage i s f e d through a tapped  See reference 3 page /£?  3/ four-microsecond  delay l i n e .  Owing t o t h e l o a d o f t h e  d e l a y l i n e on t h e m u l t i v i b r a t o r , t h e d i s t o r t i o n  along  the d e l a y l i n e , and t h e m u l t i p l e echoes i n t h e d e l a y l i n e , t h e f r o n t o f t h e t r i g g e r p u l s e i s t o p p l e d o v e r so t h a t t h e a m p l i t u d e now v a r i e s d i r e c t l y w i t h t i m e .  As  the v o l t a g e t o c a n c e l the n e g a t i v e charge on t h e g r i d o f t h e b u f f e r tube i n t h e P u c k l e time base i s p i c k e d o f f t h e "synch", p o t e n t i o m e t e r ,  t h e "synch."  i s used as  a v e r n i e r t o v a r y t h e time a t w h i c h t h e g r i d o f t h e b u f f e r tube i s d r i v e n p o s i t i v e .  As a t o t a l d e l a y o f  10 m i c r o s e c o n d s i s o b t a i n e d f o r t h e f a s t e s t sweep o f 2.5  cm p e r y j - s e c , t h e e f f e c t i v e t r a c e l e n g t h i s  expanded t o 2>5  c m  »  a* 2.5. cm p e r yu-sec.  The whole l i n e c a n be u n r o l l e d a c r o s s t h e s c r e e n by r o t a t i n g t h e s y n c h .  c o n t r o l so t h a t measuring o f  time i n t e r v a l s by c o u n t i n g t h e marker p i p s i s facilitated.  E i t h e r another d e l a y l i n e can be i n s t a l l e d  o r t h e sweep speed can be d e c r e a s e d  to take i n f a u l t s .  between \ m i l e and 2 1/& m i l e s d i s t a n t .  Hence, t h e  d e v i c e can be used t o l o c a t e f a u l t s w h i c h a r e between f i v e f e e t and two m i l e s from t h e end o f t h e c a b l e , (g)  Marker P i p s . Marker p i p s a r e used as a means o f measuring time i n t e r v a l s on t h e h o r i z o n t a l t r a c e .  equal  With the  common X sweep and double beam on t h e Cossor,  t h e marker  p i p s appear on t h e 1^ t r a c e s i m u l t a n e o u s l y w i t h t h e echo p i p s on t h e  trace.  As t h e marker p i p s a r e l o c k e d i n  s t e p w i t h t h e i n i t i a l p u l s e f e d t o t h e c a b l e , any p o r t i o n o f t h e sweep can be expanded and t h e p o s i t i o n o f t h e  38' expanded echo can be more accurately interpolated between adjacent marker pips. For i s used.  simplicity, only one frequency of marker p i p The o s c i l l a t o r frequency of k,k5 M.C. provides  pips 0.225 -u-sec. apart.  Although these pips are d i s -  crete at the low speed at which the r i g h t end of the 50 - -u-sec. brightened trace just appears on the screen, they are only 19 mm apart on the fastest sweep where 1 mm equals 0.012 ji-aec. A modified Hartley o s c i l l a t o r overdamped during the flyback time and shock excited at the beginning of the brightened trace provides the basic marker frequency. •The o s c i l l a t i o n s are damped out by a shunting triode (V£ ) which conducts during flyback while f o r c i n g the A  o s c i l l a t o r triode (Vgg) down near cutoff by means of the common cathode r e s i s t o r ( R ^ ) . 2  The small cathode bypass  condenser ( C ^ ) increases the amplitude of the f i r s t few cycles by aiding the g r i d c o i l i n delaying conduction of(Vgg)during the f i r s t h a l f cycle and by reducing degeneration during the succeeding cycles. of  The amplitude  the steady o s c i l l a t i o n s i s c o n t r o l l e d by the de-  coupling condenser and r e s i s t o r i n the common p l a t e supply lead.  This i s o l a t i n g f i l t e r prevents p l a t e -  supply variations from disturbing the o u t p u t  frequency.  The sine wave i s then clipped by the asymmetrical cathode follox^er ( y ) . V  buffer triode (V  A  A second cathode-follower  ) drives a d i f f e r e n t i a t i n g c i r c u i t  39  w h i c h produces sharp peaks a t t h e b e g i n n i n g o f each d i s c o n t i n u i t y i n the r e c t i f i e d s i n e wave f e d .from V T h i s peaked wave impressed  on a s m a l l i n d u c t a n c e  7A  .  (L^)  produces 25-volt rounded p i p s O.O3 jx-sec. wide and 0.225 j sec. a p a r t .  Each p e r i o d i s f u r t h e r s u b d i v i d e d by a  p a r a s i t i c o s c i l l a t i o n o f t h e X sweep which,produces b r i g h t s p o t s 0.03 jx-sec. a p a r t on the t r a c e s . By i n c r e a s i n g the r e s i s t a n c e o f t h e condenserc h a r g i n g c i r c u i t , t h e t h y r a t r o n can be made t o f i r e a t o n l y each second t r i g g e r p u l s e .  I n t h i s way, t h e degree  o f i n t e r - m o d u l a t i o n o f the X.^ and Y echo p u l s e s from t h e Y  1  g  v o l t a g e s by t h e  p l a t e s can be d e t e r m i n e d  from a  comparison o f t h e n o n - c o i n c i d i n g t r a c e s o f a l t e r n a t e sweeps b o t h o f w h i c h remain on the s c r e e n .  I ti s  found t h a t , b o t h b e f o r e and a f t e r t h e c a b l e p u l s e i s r e c e i v e d , the t i m i n g p u l s e s a r e u n d i s t u r b e d .  However,  w h i l e t h e c a b l e p u l s e i s g o i n g n e g a t i v e , the t i m i n g p i p s are d i s p l a c e d about O.O3 u-sec t o t h e r i g h t . t h i s displacement  However,  i s recovered while the pulse i s  d r o p p i n g back t o z e r o .  As t h i s e f f e c t i s not n o t i c e a b l e  u n t i l t h e c a b l e p u l s e has reached an a m p l i t u d e o f 4 mm (12 v o l t s ) ^ t h e t o e o f t h e p u l s e i s not d i s t u r b e d .  Hence  the f r a c t i o n o f a t i m i n g i n t e r v a l i m m e d i a t e l y b e f o r e the c a b l e p u l s e should be compared w i t h t h e p r e c e d i n g and not the f o l l o w i n g i n t e r v a l .  4-0  (h)  Power Supplies. As i t was o r i g i n a l l y intended to bias one of the tubes 100 v o l t s negativetyyya 4 0 0 - v o l t input power-supply was b u i l t .  condenser^  As this tube was l a t e r  discarded, the extra voltage was dropped through a series r e s i s t o r .  This method'provided an additional  f i l t e r section, the condenser of which was i n s t a l l e d i n the timer chassis close to the multivibrator.  The  plate supply delivers 90 ma at 300 v o l t s to the timer. As the thyratron needed only two ma at two fcv, a 1500-volt RMS oscilloscope transformer was used. The filament winding f o r the r e c t i f i e r tube was connected Internally to the plate winding^because the p o s i t i v e side of the output In an oscilloscope i s grounded.  However, i n order to ground the thyratron  heater to produce a negative i n i t i a l pulse, the negative side of the power' supply must also be grounded.  An Insulated 2 . 5 - v o l t filament supply was  available i n the spare centre-tapped 5-volt filament winding on the thyratron heater transformer.  A red  p i l o t lamp l i g h t s when the high-voltage transformer i s on, whereas a green p i l o t lamp l i g h t s when only the heaters and 300-volt supply are energized.  Operation (a)  I n i t i a l Adjustment. After connecting the oscilloscope and the pother input to the pulse generator, turn on the oscilloscope and only the top heater switch of the pulse generator.  Only the green l i g h t should show.  I f the l i g h t i s red, turn o f f the 2000-volt plate supply (bottom  switch), otherwise the thyratron  cathode w i l l develop hot spots and soon f a i l .  While  waiting 3 0 0 seconds f o r the thyratron cathode to heat uniformly, adjust the oscilloscope and pulsegenerator controls. 1.  On the generator, turn the Sweep Delay to zero and the Pulse Amplitude  2.  to the centre.  On the oscilloscope^* turn the Condenser or coarse v e l o c i t y to number 9.  3.  Turn the Velocity and Amplitude  to the maximum  clockwise position. Turn the Trigger to the maximum counterclockwise position. 5.  The Sync, control should be turned almost  fully  clockwise. 6.  Adjust the Focus and B r i l l i a n c y controls so that the timing wave i s c l e a r l y seen on the screen. Focusing i s s i m p l i f i e d i f the timing wave on Y^ i s above the zero l i n e of Y,.  See reference  9  page H.O  •  7»  ^ 2 .  I f only one trace i s v i s i b l e , rotate either Y j - or Y^- s h i f t u n t i l the other trace f l o a t s into view.  S.  Now adjust the Sync. and X - s h i f t to get the i n i t i a l part of the trace expanded at the l e f t side of the screen.  9.  I n i t i a l l y the pulses from the cable should be connected to Y^ d i r e c t l y and the selector switch should be set at Plates A.C.  Later on, i f  amplification i s desired, the pulses should be fed Into  and the selector sx^itch should be  turned to 2HFY1. 10.  Connect the cable to the Sheath-Core terminals by short twisted or coaxial leads containing as small an inductive loop as possible.  11.  When the  5-minu.te  on the 2000-volt  heating time has elapsed, turn  supply to the thyratron.  The  p i l o t l i g h t should change from green to red.  Do  not touch the cable leads while the 2000-volt supply i s energized.  A downward-going pulse  should appear near the l e f t side of the screen. 12.  Now turn the Velocity control counterclockwise to compress the sweep u n t i l the t o t a l number of timing pips indicates that the t o t a l cable length appears on the screen.  (Allow  approximately  6o c i r c u i t - f e e t of cable between p i p s ) . i r r e g u l a r i t i e s appear, Increase the pulse  I f no  4-3-  amplitude u n t i l the open end o f the cable (downward pip) i s v i s i b l e .  For cables longer than h a l f  a mile, use the amplifiers controlled by A^ and A r , G-ain. Do not overload the amplifiers,  f o r the pulses w i l l . b e distorted and uninterpretable. •The maximum positive amplitude with the b u i l t - i n video amplifiers i s about half an inch. The Y^ marker pips also s l i g h t l y modulate the Y^ cable trace to produce regularly spaced bumps. As the magnitude of these bumps i s independent of the.amplified signals, no ambiguity  should a r i s e .  However, to i d e n t i f y very weak signals, the markerpip lead should be temporarily disconnected from Y^. I f the f a u l t i s very close to the sending end of a long cable, the multiple echoes can be damped out by a 5°-  t  0  100-ohm carbon r e s i s t o r connected  across the cable  terminals.  A 100-ohm carbon  r e s i s t o r i n series with the core of the cable w i l l give longer pulses which are more easily interpreted when the f a u l t i s l e s s than 50 feet away. A small echo from a known d i s c o n t i n u i t y can be more p o s i t i v e l y i d e n t i f i e d i f the cable i s alternately shorted and opened at t h i s d i s c o n t i n u i t y . Trace  Expansion. After the f a u l t has been approximately located  on the compressed sweep, the trace can be expanded f o r a detailed study of the echoes.  <44  After allowing 250 c i r c u i t - f e e t per microsecond, set the sweep-delay switch to the desired range. Set the condenser switch to p o s i t i o n ten. Readjust the focus and b r i l l i a n c y i f necessary. By turning the v e l o c i t y and "sync." controls slowly clockwise, one can then expand the sweep to cover up to half a mile.  For more distant f a u l t s ,  the X-shlft and v e l o c i t y controls must be turned slowly counterclockwise. Measurements. The distances between echoes on the trace can be measured i n terms of the 0.225-microsecond i n t e r v a l s between the timing pips appearing simultaneously on the screen.  These larger  i n t e r v a l s are further subdivided by a. succession of bright and dim spo'ts produced by a p a r a s i t i c o s c i l l a t i o n of the X sweep. A l l measurements should be made to the beginning of the pulse, not the peak.  I f two  adjacent  pulses partly cancel one another, the second pulse begins where the f i r s t pulse changes slope. As the high-amplitude  negative-going pulses  temporarily expand the sweep whereas p o s i t i v e going pulses compress the sweep, an error of about ten feet w i l l creep i n . Although the marker pips are also distorted, they maintain the same time i n t e r v a l between t h e i r peaks because the o s c i l l a t o r  i s isolated.  However, Just count f u l l i n t e r v a l s  and then compare the additional f r a c t i o n a l Interval :•• to the immediately preceding i n t e r v a l . To increase the accuracy, i t i s advisable to work from both ends of the cahle and take the weighted mean of the two f a u l t positions so found. 6.  Sample Calculation Refer to the photographs of the oscilloscope traces showing the echoes from a power cable. expanded traces with the compressed  By comparing the  trace one can see  that pips 2 and 3 are badly distorted. echoes return almost at pips 6,  From figure 1,  9, 11, 17, and.23.  From  the remaining figures (which are much more d i s t i n c t on the screen than i n the photos) the echoes are more closely logged. Echo logged at  Intervals from beginning  0.93  0  5.91  K3& g.Ol  n.05(?) 10.12  i6.sk-  16.01  23.05  22.12  x: 139*- as 1  Calculate L distance Expected known (by i'ratlo) Echo distance 0 feet  505  325  transformer tap [joint (re-echo  6^7 653  1009  nultlple echo  97S"  Par end of cable  139*1-  139^  22.12  Error  -11 f t , 0.75$ 16  63s  (assumed)  %  0 ft.  beginning of cable Joint  Error  Hence x » 31^  -q  1.15  -17 •  1.2  32  2.2  Propagation v e l o c i t y = =  2 (length) . (intervals)(time of i n t e r v a l ) 2 (1394 (22.12)10.225) 5 6 0 f t . per micro sec.  Velocity r e l a t i v e to light  =  560 56.9$  +7 IV A.  DISCUSSION Advantages  Although the Echo Ranger has i t s l i m i t a t i o n s , i t possesses many desirable features: 1.  Multiple steady, variable, or Intermittent d i s continuities can be accurately located,  2.  The general nature and severity of the f a u l t Is Indicated.  3.  Only one end of the cable Is required.  4.  Only r a t i o and proportion are used i n the simple calculations.  5.  The whole l i n e i s v i s i b l e f o r rough checking of arithmetic.  6.  Known d i s c o n t i n u i t i e s can be used as reference points.  7.  Few controls i n addition to those on a moderately priced commercial oscilloscope simplify operation.  25.  The measuring current w i l l not destroy the f a u l t .  9.  As the. device requires l i t t l e power from a l i g h t socket, i t can be run from b a t t e r i e s and an inverter.  10.  Stable operation f a c i l i t a t e s reading, measuring, and checking.  11.  The apparatus i s compact and e a s i l y ca.rried.  12.  Temperature variations should not s i g n i f i c a n t l y affect the accuracy.  13.  The propagation v e l o c i t y of surges can be e a s i l y determined.  B, 1,  Limitations  High-resistance shunt f a u l t s , i . e . greater than 2000 ohms, cannot be detected without a d d i t i o n a l amplifiers or flashover equipment.  4-e 2.  A combined series and shunt f a u l t equalling the surge impedance of the cable appears as an i n f i n i t e l i n e and w i l l not produce echoes.  3.  Multiple f a u l t s may be missed i f they are .just beyond a major f a u l t . However, the accuracy of. l o c a t i o n of the f i r s t f a u l t i s not impaired.  4.  As the lower l i m i t f o r series f a u l t s i s about f i v e ohms, a poor j o i n t may not appear.  5.  A "roadmap" may be required to i d e n t i f y echoes from complicated networks such as d i s t r i b u t i o n systems.  6.  The distance between any two d i s c o n t i n u i t i e s , such as the ends of the cable, must be accurately known. On overhead l i n e s , the v a r i a t i o n In v e l o c i t y i s small because the d i e l e c t r i c i s a i r .  C.  Suggested Improvements.  Several additional problems appeared while the Echo Ranger was being developed. apparatus are also necessary.  Further refinements of the A method of separating arc  surges from the echo-ranging pulses when self-healing f a u l t s are being flashed over could take advantage of the longer time constant of the fault-generated surges.  Overloading of  the amplifiers could be prevented by a variable-gain stage which i s allowed to operate at maximum s e n s i t i v i t y only during a controllable period of the trace. \The i n i t i a l pulse could be cancelled by a clipper tube or a bridge.  The range and  the sweep delay should be increased to handle cables up to 10 miles long and overhead l i n e s up to ~$00 miles.  The  o s c i l l a t o r frequency should be s t a b i l i z e d by a c r y s t a l . Transient disturbances of the frequency at the beginning of the trace should be investigated. I f every f i f t h or tenth  ^9 marker pip were increased i n amplitude, sweeps would he more easily measured.  highly compressed A c i r c u i t similar to  the present timing o s c i l l a t o r would provide a c i r c u l a r sweep locked to the i n i t i a l pulse.  A delayed s p i r a l could then  be used to determine the number of complete revolutions between the a r r i v a l s of echoes.  Remove the i n i t i a l triode  current from the cable while the thyratron i s i o n i z i n g . Methods of reducing intermodulation should be Investigated. A cathode follower to i s o l a t e the sweep delay-line w i l l prevent defocusing of the trace.  The hydrogen thyratron  could be used instead of gaps i n an accurately controlled high-voltage surge generator. D,  Preliminary Note on Flashover.  I f the f a u l t s are self-healing or have a high resistance, high voltage can be applied to f l a s h over the f a u l t while the Echo Ranger i s monitoring the cable.  When this high steady  d-c voltage Is applied so that the echo-ranging  pulse  voltages add to i t , the arc starts more readily.  The cable  w i l l f l a s h over at a variable time a f t e r the pulse a r r i v e s at  the weak spot because of the time taken to completely  ionize the flash-over path. of  Owing to the intermittent nature  the discharge, the ruptured o i l has time to escape and be  pa.rtly replaced by fresh o i l .  Bubble formation and release  w i l l increase the agitation.  When the water vapour b o l l s out  of a wet f a u l t , the breakdown volta.ge w i l l r i s e u n t i l the arc  i s extinguished. When the f a u l t flashes over, the arc begins to discharge  the cable on both aides of i t .  Once the discharge wave  reaches the sending end of the cable, the supply w i l l begin to feed current into the cable. power supply, the  3 t o r e d  With a poorly-regulated  energy i s dissipated faster than i t  can be replenished and the arc goes out long before the next timed pulse arrives. I f a large enough power supply i s not available, highvoltage echo-ranging pulses applied while the cable i s normally energized but i s o l a t e d from the a-c supply by a current-limiting reactor would probably be enough to f l a s h over a weak spot and hold the arc long enough f o r i t s p o s i t i o n to be read o f f the oscilloscope screen.  Complete  burn-down i s unnecessary as the arc should have a low enough Impedance over part of each a-c cycle to give a. • recognizable echo. The amplitude, of the intermittent arc-discharge Wave i s so high that the r e l a t i v e l y small echoes superimposed on i t produce v e r t i c a l l i n e s which are too f a i n t to be interpreted.  As the arc surges are not locked to the sweep,  they d r i f t f a i n t l y across the screen leaving bright and steady the synchronized echo-ranging  pulses which were sent  and received while the arc was temporarily extinguished. When the arc path i s approaching  complete breakdoxtrn, a  bright pip begins to grow on the screen i n d i c a t i n g where the fault l i e s . Multiple echoes from only the arc surges could be used to locate the f a u l t s i f . t h e r e s u l t i n g pattern on the  oscilloscope screen could be interpreted. 'is needed on t h i s aspect of f a u l t location.  Further research  £2-  V.  CONCLUSIONS  A.  From Theory.  The beginning of the disturbance, the toe of the pulse, does not reach a. point a distance x awayuntil has elapsed. Current and voltage waves propagate at the same v e l o c i t y . At the wave front,  ——.  2±  After the a r r i v a l of the wave front, the shape of the wave depends on the t o t a l distance the wave t r a v e l l e d , the type of discontinuity from which i t was r e f l e c t e d , and the severity of the d i s c o n t i n u i t i e s i t passed. Each echo travels as i f i t were alone on an I n f i n i t e l i n e . The v e l o c i t y of propagation varies inversely as the square root of the d i e l e c t r i c constant of the i n s u l a t i o n and i s sensibly Independent of temperature. The p o l a r i t y of the echo from a t o t a l impedance lower than the surge impedance of the cable w i l l be the reverse of the Incident pulse.  A n e g l i g i b l e echo w i l l be returned  from a terminating impedance equalling the surge Impedance. An impedance higher than the cable surge impedance w i l l return an echo of the same p o l a r i t y as the incident pulse. For maximum e f f i c i e n c y of r e f l e c t i o n , the cable should be short- or open-circuited. Skin effect causes a distant echo to start apparently l a t e r than i t should. t h i s error.  However, amplification w i l l reduce  53 10.  The attenuation i s much larger i n cables than i n open-wire l i n e s because the inductance i s decreased without a corresponding decrease i n resistance.  11.  After the i n i t i a l rounding of the pulse, further d i s t o r t i o n takes place l e s s rapidly.  B. 1.  From Experiment.  The surge impedance of cables i s of the order of f i f t y ohms.  2.  The propagation v e l o c i t y i n o i l - f i l l e d cable i s approximately  •3.  $60 feet per microsecond.  The accuracy of l o c a t i o n of an i n t e r n a l d i s c o n t i n u i t y i s within  0.6%  on RG8U polyethylene cable and i s  within 1.2$ on o i l - f i l l e d power cable. k.  Multiple shunt f a u l t s are discernable i f they are more than 30 feet apart,  5.  Amplified echoes must be shown i n t h e i r entirety or compared with unamplified echoes, i f a p a r t i c u l a r wavefront i s to be p o s i t i v e l y i d e n t i f i e d .  6.  The pulse from the Echo Ranger i s high enough to a i d the applied steady voltage i n i n i t i a t i n g a flash-over at an Incipient f a u l t .  7.  The conclusions from theory have been t r i f l e d .  5-4  VI. 1.  LITERATURE CITED  Abraham, L.G., Lebert, A.W.,. Maggio, J.B.,. Schofct, J.T., "Pulse-echo Measurements on Telephone and Television F a c i l i t i e s " , Technical Paper 47-26, Transactions of the American I n s t i t u t e of E l e c t r i c a l Engineers, New York, 1947,  2.  v o l . 66, pp. 54-1-6.  Beck, E., "Sheath Grounds Affect Travelling Waves In Cables", E l e c t r i c a l Engineering, New York, v o l . 5 pp. 2 3 2 - 9 , A p r i l ,  3.  2  1933.  Benson, F. S., H i l l , (J.L.,. Machen, C.R.,  "Sonic Detector",  E l e c t r i c a l World, Albany, N.Y.,. v o l . 125, pp. 2 2 - 5 , June 22, 4.  Bewley, L.V.,  no. 25,  1946.  "Travelling Waves on Transmission Systems",  Transactions of the American I n s t i t u t e of E l e c t r i c a l Engineers, New York, 1931,  vol. 50,  part 2, pp. 532-50 5.  Blankmeyer, W.H.,  "Power Line Fault Locator^ E l e c t r o n i c s ,  New York, v o l . 17, 6.  p. l 6 6 , January, 1944.  The B r i t i s h E l e c t r i c a l and A l l i e d Industries Research Association,  "Surge Phenomena - Seven Years  11  Research f o r the Central E l e c t r i c i t y Board", Reference S/T 3 5 , London, 1 9 4 l . 7.  Brown, G.H.,  "Impedance Determinations of Eccentric Lines"  i n Markus and Zeluff, ed., E l e c t r o n i c s f o r Engineers, f i r s t Inc.,  ed., McGraw-Hill Book Co.,  New York, 1945.  55 g.  Bush, V.,  "Operational C i r c u i t Analysis" New York, John Wiley and Sons, 1929.  9.  Cossor, A.C., Ltd,,  "Cossor Double Beam Oscillograph  Model 339 Instruction Manual" CB.55E,  sixth ed.,  A.C. Cossor, Ltd., London, February, 19^-6. 10.  Coulthard,. W.B. ,• "Transients i n , E l e c t r i c C i r c u i t s " , London, S i r Isaac Pitman and Sons, Ltd., 19k6.  11.  Danner, G.L.,.  "R.F. O s c i l l a t o r Aids Locating of Cable  Faults", E l e c t r i c a l World, Albany, N.Y.,.. v o l . 123, p. I 3 2 , January 20, 12.  Heins, H.,  "Hydrogen Thyratrons", Electronics, v o l . 19, No. 7, p. 96, July,  13.  Hpyle, W.G.,  19^5.  1946.  "Transmission Line Fault Locator", National Research Council of Canada, Report PRA-I35, September, 19I+6.  \\.  Jahnke, E., and Emde, P.,  "Tables of Functions with  Formulae and Curves," fourth ed., New York, Dover Publications, 15.  19^5.  Karman, T.von.,. and Blot, M*A.,  "Mathematical Methods i n  Engineering", f i r s t ed., New York, McGraw-Hill Book Co., Inc., 19^0. 16.  Kiebert, M.V.jr., and I n g l i s , A.F., "Multivibrator C i r c u i t s " , Proceedings of the I n s t i t u t e of Radio Engineers, Mew York, vol.. 33, No. S, p..53 r, August, i  17.  L e s l i e , J.R., and Kidd, K.H.,. "The Linascope",  19^5.  The Hydro-  E l e c t r i c Power Commission of Ontario, Research and Testing Department, December, 19^7 • (see also A.I.E.E. Technical Paper lJ-8-207)  IS,  Margenau, H.,  and Murphy, G.M.,. "The Mathematics  of  P h y s i c s and C h e m i s t r y " , New Y o r k , D. Van N o s t r a n d Co.,. I n c . , 1 9 4 7 . 19.  M a r g o u l i e s , S., and F o u r m a r i e r , P.,  of  "The L o c a l i z a t i o n  F a u l t s on Overhead L i n e s by Means o f Impulse Waves", paper 3 ° 7  o f  Conference'Internationale  des Grands Re^seaux E l e c t r i q u e s k Haute T e n s i o n , P a r i s , F r a n c e , G a u t h i e r - V i l l a r s , June 2 4 t o July 3, 20.  Nalos, E.J.,  1942.  "High Frequency.Method  o f L o c a t i n g Power Cable  F a u l t s " , M a s t e r ' s T h e s i s i n the Department  of  E l e c t r i c a l Engineering, U n i v e r s i t y of B r i t i s h Columbia, August, 21.  1947.  N a t i o n a l R e s e a r c h C o u n c i l o f Canada, E l e c t r i c a l E n g i n e e r i n g and Radio B r a n c h , " T e c h n i c a l Manual f o r Merchant Marine Type 2 6 2 Radar w i t h C i r c u i t ( D r a f t Copy) 1 9 4 7  22.  Noakes, F.  Diagrams",  (?)  " H i g h Frequency Method f o r L o c a t i o n o f F a u l t s I n Poxirer C a b l e s . " E l e c t r i c a l News, v o l . 5 3 * » ' ^3* PP» 4 6 - 7 no  J u l y 1, 23.  P u c k l e , O.S.,. Inc.,  24.  Race, H.H.,  1944..  "Time Bases", New Y o r k , John W i l e y and  Sons,  1943.  " V a r i a t i o n s xirith Temperature  and Frequency o f  D i e l e c t r i c Loss i n a Viscous M i n e r a l I n s u l a t i n g O i l " P h y s i c a l Review, M i n n e a p o l i s , Minn., v o l . 3 7 , pp. 4 3 0 - 4 4 6 , 25.  February 1 5 ,  Ramo, S.,. and Whinnery, J.R.,  1931.  " F i e l d s and Waves i n Modern  Radio? New York, John W i l e y and Sons, I n c . , 1 9 4 6 . 26.  R o b e r t s , F.F.,  "New  Methods f o r L o c a t i n g Cable F a u l t s  P a r t i c u l a r l y on High Frequency Gables", Journal of the I n s t i t u t i o n of E l e c t r i c a l Engineers, London, vol. 27.  93, no. 26, part 3, pp. 325-95, November, 1946.  Savage, J.H.  "Localization of Faults i n Low  Voltage  Cables, with Special References to Factory Techniques", Journal of the I n s t i t u t i o n of E l e c t r i c a l Engineers, London, v o l . 92, pp. 23.  part 2,  3°,  no.  5^0-93, December, 1945.  Simmons, D.M.,. "Calculation of the E l e c t r i c a l Problems of Underground Cables", The E l e c t r i c Journal, v o l . 29, May to November,  29.  Stevens, R.F.,  1932.  and S t r i n g f i e l d , T.W.,  "Transmission Line  Fault Locator using Fault-Generated Surges," A.I.E.E. Technical Paper  42-202, presented at the A.I.E.E.  P a c i f i c General Meeting at Spokane, Washington on August 26, 30.  Ware, L.A.,  1942.  and Reed, H.R.,  "Communication Circuits",.  second ed.,. New York, John Wiley and Sons, Inc.,  1944.  Vll.  ACKNOWLEDGMENTS  Vancouver, B.C., September 1 0 , 19^2.  Dr. H.J. MacLeod, Head, Department of Mechanical and E l e c t r i c a l Engineering, University of B r i t i s h Columbia, Vancouver, B r i t i s h Columbia. Dear S i r : In presenting t h i s Thesis, I wish especially  to thank Dr. Frank Noakes f o r h i s encouragement  and guidance throughout the investigation. The B r i t i s h Columbia E l e c t r i c Railway Company Limited Research Scholarship which made the work possible was gratefully accepted.  Respectfully yours,  Thomas K. Naylor.  Vlll  A.  THE (one  DIAGRAMS  Apparatus  ECHO RANGER ninth  full  size)  Oscilloscope and Pulse Generator with coaxial cable attached.  PULSE GENERATOR  Trigger Network  fU Mdstir  Timer  T  Hydrofen Thyrafron  Vo/fctfe Divider de filler  CaJ>/e  A* /  A.  Multivibrator  2HFYI  Connection  |—  Siny/e Sneef>  Vernier  Sweep De/a-y  ST /Veo  Syrr-  3ri///'anee  JL BRIGHTENING  CaSS  PULSE  or  #339  Osc/V/osm/re s  MARKER-PIP GENERATOR  Shock Osc///af6r  4^  Marker  Vie Pulse - 'modu/dfeoi —<MMH/lr Rectifier- 8* Osciflafhr P ifferen-fitCfor  THE  ECHO-RANGER PLATE BLOCK  11  DIAGRAM  Pea. £ my Co//  THE  ECHO-RANGER PLATE 111 CIRCUIT DIAGRAM  -  U  f  S  .  C  .  \  "JN  63-  Parts L i s t f o r Echo-Ranger (See wiring diagram) CONDENSERS  RESISTORS Resistor # 1 2  I 9  10 ii 12  11 15  lS  H IB  Ohms 2500 7500 20 K 20 K 220 K  ^70  K  10 K 1500 11 K 6g K 100 K 50 K 20 K lj-00 1000 1000 500 K 3 eg 2 meg m  Watts  carbon  11 11 11 11  2 1.  I  II 11  2 meg 0.1 meg 0 . 1 meg 15 K goo  I  WW C WW  10 i  10 2  £  1 2 2 2 2 2 1  2  2 1  1 2  I  n  19 20 21 22  Condenser #  Type  9  c c c c c c c c c c  10 1112  c c c c c  20 21  0.1 meg 1000  c c  27  39  K  c  2g  1000  c  29 30  100 1350  II II ig 19  microfarads 0.05 0.05 0.001 0.0001 0.01 0.001 0.015 1.0 0.00025 0. OOOM-  0.00k  0.5 0.5 0.00025 0.1 o.oooif-5 0.01 0.001 g g g  Voltage  Type  600 600 500 500 600 500 600 600 500 2500 1200 3000 3000 500 600 500 500 500 %0  Paper  600 600  11 Mica 11  P M P Oil M M M  Oil Oil M P M P M Electro I— lytic  11  TUBES 6AG7  v  6AG7 c  3 V,-  6AG-7  6L6 Sylvania 5C22 or ij-035  Vg V Vg ?  V  q  B A  6SN7-GT 6SN7-GT  2X2 5Y3&T/G  INDUCTANCES  2  c  10  WW  L^  TRANSFORMERS  30 mh iron-core R.F.C.  Two g5 mh iron-core R.F.C. i n series T Hammond II36X 6 . 3 - 5 6.3 -5 L^ Hammond 10H - 150 ma 5-3 CT 215 1 5 0 0 Cathode Ray Trans. liu 31 turns #1^S.C.E. -pdiam. form.-coll 3^-" long. 275 4-00-0-li-OO; 5 - 3 Delay* 1| u-sec. T-ll*46.3 -5 Line ) J T^ #2g D.C.C. on 1 5/g" form, space(L-ll+7-152 i n c l . ) War Assets wound; overlap top two c o i l s (C-172-176 i n c l . ) Dubin Elec* tronics. V  L„  a  L  v  ;  v  V  v  a  a  B.  Echoes from a Power Cable Length  1 3 2 9 feet  Type  O i l - f i l l e d paper  Sheath  Lead  Core  three #3 copper  Outside Diameter  I . 0 3 inches  Voltage  23OO volts (At least)  Bed  Conduit i n concrete  Manholes  A l l dry  Grounding  No reactors  Terminations  Open-circuited pot-heads  1.  Panoramic view (compressed sweep) 0.225-microsecond marker i n t e r v a l s .  £4.  2.  I n i t i a l Pulse 0.225-microsecond inte»als.  Echo from a Joint 0.225-microsecond intenals.  4.  Echo from t h e t r a n s f o r m e r t a p . 0.225-microseoond  5.  intervals.  J r a n s f o r m e r - t a p re-echo or an echo from a second, j o i n t . 0.223-microsecond  intervals.  Multiple echo r e f l e c t e d from sending end, 0.225-microsecond i n t e s a l s .  Echo from open-circuited f a r end of cable 0 . 225-micro second internals.  

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