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Megger                                                                             PO Box 118 Cherrybrook                 ...
Table of ContentsSection                                                                                  Page            ...
Table of FiguresFigure                                                               Page       Figure                    ...
Cable Characteristics                                                                                                SECTI...
Cable Characteristics                                                                                            SECTION I...
Fault Locating Procedures                                                                                                 ...
Fault Locating Procedures                                                                                                 ...
Fault Locating Procedures                                                                                               SE...
Cable Route Tracers/Cable Locators                                                                                        ...
Cable Route Tracers/Cable Locators                                                                                        ...
Cable Route Tracers/Cable Locators                                                                                        ...
Cable Route Tracers/Cable Locators                                                                                        ...
Cable Route Tracers/Cable Locators                                                                                        ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
How to See Underground Cable Problems                                                                                     ...
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
Fault finding in cables
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Transcript of "Fault finding in cables"

  1. 1. Megger PO Box 118 Cherrybrook NSW 2126 FAULT FINDING AUSTRALIA T +61 (0)2 9875 4765 F +61 (0)2 9875 1094 E ausales@megger.com Megger SOLUTIONS PO Box 15777 Kingdom of BAHRAIN T +973 254752 F +973 274232 E mesales@megger.com Megger Limited 110 Milner Avenue Unit 1 Scarborough Ontario M1S 3R2 CANADA T 1 800 297 9688 (Canada only) T +1 416 298 6770 F +1 416 298 0848 E casales@megger.com Megger SARL 23 rue Eugène Henaff ZA du Buisson de la Couldre 78190 TRAPPES T +01 30 16 08 90 F +01 34 61 23 77 E infos@megger.com Megger PO Box 12052 Mumbai 400 053 INDIA See us on the web at www.megger.com T +91 22 6315114 F +91 22 6328004 E insales@megger.com Megger MBE No 393 1-800-723-2861 C/Modesto Lafuente 58 28003 Madrid Tel: 1-214-330-3255 ESPAÑA T + 44 1304 502101 Fax: 1-214-333-3533 F + 44 1304 207342 c f l @ m e g g e r. c o m E espana@megger.com Megger Limited Archcliffe Road Dover CT17 9EN UK T +44 (0) 1304 502100 F +44 (0) 1304 207342 E uksales@megger.com Megger 4271 Bronze Way Dallas, TX 75237-1088 USA T 1 800 723 2861 (USA only) T +1 214 333 3201 F +1 214 331 7399 E ussales@megger.com WWW.MEGGER.COM WWW.MEGGER.COMThe word “Megger” is a registered trademarkMEG-231/MIL/3M/11.2003
  2. 2. Table of ContentsSection Page Section PageI Cable Characteristics V Surge Generators, Filters and Couplers Good Cable Insulation . . . . . . . . . . . . . . . . . . . .2 Surge Generators . . . . . . . . . . . . . . . . . . . . . . .19 When Cable Insulation is Bad . . . . . . . . . . . . . .2 Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 Why a cable becomes bad . . . . . . . . . . . . . . .3 Capacitance . . . . . . . . . . . . . . . . . . . . . . . . .20 Cable Faults Described . . . . . . . . . . . . . . . . . . . .3 Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20 Basic Surge Generator Operation . . . . . . . . . .21II Fault Locating Procedures Proof/Burn . . . . . . . . . . . . . . . . . . . . . . . . . .21 Locate Faults in Buried Primary Cable . . . . . . . .4 Surge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 Test the cable . . . . . . . . . . . . . . . . . . . . . . . . .4 Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 Fault resistance and loop test . . . . . . . . . . . .4 TDR tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Single Point Grounding . . . . . . . . . . . . . . . . . .22 DC hipot test . . . . . . . . . . . . . . . . . . . . . . . . .5 Arc Reflection Filters and Couplers . . . . . . . . .22 Analyze the Data . . . . . . . . . . . . . . . . . . . . . . . .5 VI Localizing Methods Fault resistance and loop test . . . . . . . . . . . .5 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 TDR tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Sectionalizing . . . . . . . . . . . . . . . . . . . . . . . .24 DC Hipot test . . . . . . . . . . . . . . . . . . . . . . . . .6 Resistance ratio . . . . . . . . . . . . . . . . . . . . . .24 Cable Route . . . . . . . . . . . . . . . . . . . . . . . . . .6 Electromagnetic surge detection . . . . . . . . .25 Localize - prelocate the fault . . . . . . . . . . . . . . .6 Single phase, coaxial power cable with neutral bridges over splices . . . . . . .25 Locate - pinpoint the fault . . . . . . . . . . . . . . . .6 Single phase PILC cable with bonded Locate Faults in Above Ground grounds in conduit . . . . . . . . . . . . . . . . . .26 Primary Cable . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Three-phase PILC . . . . . . . . . . . . . . . . . . . .26III Cable Route Tracers/Locators DART Analyzer/High-Voltage Radar . . . . . . . .27 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Arc reflection . . . . . . . . . . . . . . . . . . . . . . . .27 Selecting a Locator . . . . . . . . . . . . . . . . . . . . . .8 Differential arc reflection . . . . . . . . . . . . . . .28 Hookups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Surge pulse reflection . . . . . . . . . . . . . . . . .28 Using the Receiver . . . . . . . . . . . . . . . . . . . . . .10 Voltage decay reflection . . . . . . . . . . . . . . .29IV How to See Underground Cable Problems VII Locating or Pinpointing Methods Methods of Operation . . . . . . . . . . . . . . . . . . .12 Acoustic Detection . . . . . . . . . . . . . . . . . . . . . .30 Time domain reflectometry . . . . . . . . . . . . .12 Electromagnetic Surge Detection . . . . . . . . . .31 Differential TDR/radar . . . . . . . . . . . . . . . . .13 Electromagnetic/Acoustic Surge Detection . . .31 Descriptions and Applications . . . . . . . . . . . . .13 Earth Gradient . . . . . . . . . . . . . . . . . . . . . . . . .33 Low-voltage TDR/cable radar . . . . . . . . . . . .13 VIII Solutions for Cable Fault Locating Faults that a low-voltage TDR will display . . . . . . . . . . . . . . . . . . . . . .13 Underground Utility Locating and Tracing Equipment . . . . . . . . . . . . . . . . . .34 Landmarks that a low-voltage TDR will display . . . . . . . . . . . . . . . . . . . . . .13 Time Domain Reflectometers . . . . . . . . . . . . . .35 Controls and Inputs to the TDR . . . . . . . . . . . .14 Cable Fault Pinpointing Equipment . . . . . . . . .36 Velocity of propagation . . . . . . . . . . . . . . . .14 High-Voltage DC Dielectric Test Sets . . . . . . . .37 Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Suitcase Impulse Generator . . . . . . . . . . . . . . .37 Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Cable Analyzer . . . . . . . . . . . . . . . . . . . . . . . . .38 Cursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Power Fault Locators . . . . . . . . . . . . . . . . . . . .38 Zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Impulse Generators . . . . . . . . . . . . . . . . . . . . .40 Pulse width . . . . . . . . . . . . . . . . . . . . . . . . . .16 Distance Measurements . . . . . . . . . . . . . . . . . .17 Three-stake method . . . . . . . . . . . . . . . . . . .17 Fault Finding Solutions
  3. 3. Table of FiguresFigure Page Figure Page1 Good insulation . . . . . . . . . . . . . . . . . . . . . . .2 28 TDR used to measure distance to open in conductor . . . . . . . . . . . . . . . . . . . .162 Equivalent circuit of good cable . . . . . . . . . .2 29 TDR used to localize distance to splice . . . .173 Bad insulation . . . . . . . . . . . . . . . . . . . . . . . .2 30 TDR used to localize distance to T-tap . . . . .184 Ground or shunt fault on the cable . . . . . . . .3 31 TDR used to localize distance to5 Fault region simplified diagram . . . . . . . . . . .3 fault relative to a landmark . . . . . . . . . . . . .186 Open or series fault on the cable . . . . . . . . . .3 32 Three-stake method . . . . . . . . . . . . . . . . . . .187 Test for insulation (fault) resistance 33 Block diagram of surge generator . . . . . . . .19 using a Megger® insulation tester . . . . . . . . .4 34 Energy vs. voltage for a 4-µF, 25-kV8 Loop test for continuity using a surge generator . . . . . . . . . . . . . . . . . . . . . .19 Megger insulation tester . . . . . . . . . . . . . . . .4 35 Energy vs. voltage for a 12-µF, 16-kV9 TDR test for cable length . . . . . . . . . . . . . . . .5 surge generator . . . . . . . . . . . . . . . . . . . . . .2010 TDR test for continuity . . . . . . . . . . . . . . . . . .5 36 Energy vs. voltage for a constant11 How cable locators work . . . . . . . . . . . . . . . .7 energy 12-µF, 16/32-kV surge generator . . .2012 Cable under test . . . . . . . . . . . . . . . . . . . . . . .7 37 Acoustic shock wave from arcing fault . . . .2113 Using an ohmmeter to measure 38 Single point grounding . . . . . . . . . . . . . . . .22 resistance of the circuit . . . . . . . . . . . . . . . . .8 39 Inductive arc reflection diagram . . . . . . . . .2314 Hookup showing ground rod at 40 Resistive arc reflection diagram . . . . . . . . . .23 far end of cable under test . . . . . . . . . . . . . .9 41 Sectionalizing method . . . . . . . . . . . . . . . . .2415 Hookup with far end of cable under test isolated . . . . . . . . . . . . . . . . . . . . .9 42 Basic Wheatstone Bridge . . . . . . . . . . . . . . .2416 Current coupler connection to 43 Murray Loop Bridge application . . . . . . . . .24 neutral on primary jacketed cable . . . . . . . . .9 44 Application of Bridge/TDR . . . . . . . . . . . . . .2517 Inductive coupling to neutral on primary jacketed cable . . . . . . . . . . . . . . . . .10 45 Coaxial power cable with neutral bridges over splices . . . . . . . . . . . . . . . . . . .2518 Use of return wire to improve current loop . . . . . . . . . . . . . . . . . .10 46 Electromagnetic detection in single-phase PILC cable with bonded grounds . . . . . . . . .2619 Circling path with receiver . . . . . . . . . . . . . .10 47 Electromagnetic detection of faults on20 No interference, no offset between three-phase power cable . . . . . . . . . . . . . . .26 magnetic field center and center of cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 48 Arc reflection method of HV radar . . . . . . .2721 Depth measurement using null method 49 Arc reflection and differential arc with antenna at 45-degree angle . . . . . . . .11 reflection methods of HV radar . . . . . . . . . .2722 Offset caused by interference from 50 Surge pulse reflection method nontarget cable . . . . . . . . . . . . . . . . . . . . . .11 of HV radar . . . . . . . . . . . . . . . . . . . . . . . . . .2823 Aircraft radar . . . . . . . . . . . . . . . . . . . . . . . .12 51 Decay method of HV radar . . . . . . . . . . . . .2924 TDR reflections from perfect cable . . . . . . .13 52 Acoustic surge detection . . . . . . . . . . . . . . .3025 TDR used to measure length of cable 53 Electromagnetic pinpointing . . . . . . . . . . . .31 with far end open . . . . . . . . . . . . . . . . . . . .14 54 Acoustic/electromagnetic pinpointing . . . . .3226 TDR used to measure length of cable 55 SD-3000 display at positions 1, 2, & 3 . . . . .33 with far end shorted . . . . . . . . . . . . . . . . . .14 56 AC voltage gradient . . . . . . . . . . . . . . . . . . .3327 TDR measuring distance to a low-resistance fault to ground . . . . . . . . . . .15 57 DC voltage gradient . . . . . . . . . . . . . . . . . . .33 Fault Finding Solutions 1
  4. 4. Cable Characteristics SECTION IGOOD CABLE INSULATION WHEN CABLE INSULATION IS BADWhen voltage is impressed across any insulation When the magnitude of the leakage currentsystem, some current leaks into, through, and exceeds the design limit, the cable will no longeraround the insulation. When testing with dc high- deliver energy efficiently. See Figure 3.voltage, capacitive charging current, insulationabsorption current, insulation leakage current, and Why A Cable Becomes Badby-pass current are all present to some degree. For All insulation deteriorates naturally with age,the purposes of this document on cable fault especially when exposed to elevated temperaturelocating, only leakage current through the insula- due to high loading and even when it is not physi-tion will be considered. cally damaged. In this case, there is a distributedFor shielded cable, insulation is used to limit cur- flow of leakage current during a test or whilerent leakage between the phase conductor and energized. Many substances such as water, oil andground or between two conductors of differing chemicals can contaminate and shorten the life ofpotential. As long as the leakage current does not insulation and cause serious problems. Cross-linkedexceed a specific design limit, the cable is judged polyethylene (XLPE) insulation is subject to a con-good and is able to deliver electrical energy to a dition termed treeing. It has been found that theload efficiently. presence of moisture containing contaminants, irregular surfaces or protrusions into the insulationCable insulation may be considered good when plus electrical stress provides the proper environ-leakage current is negligible but since there is no ment for inception and growth of these treesperfect insulator even good insulation allows some within the polyethylene material. Testing indicatessmall amount of leakage current measured in that the ac breakdown strength of these treedmicroamperes. See Figure 1. cables is dramatically reduced. Damage caused by lightning, fire, or overheating may require replace- ment of the cable to restore service. µAmps Inductance Series Resistance L RS HV Test Set Z0 Capacitance Parallel Z0 C Resistance RPFigure 1: Good insulationThe electrical equivalent circuit of a good run ofcable is shown in Figure 2. If the insulation were Figure 2: Equivalent circuit of good cableperfect, the parallel resistance RP would not existand the insulation would appear as strictly capaci-tance. Since no insulation is perfect, the parallel orinsulation resistance exists. This is the resistancemeasured during a test using a Megger® Insulation mAmpsTester. Current flowing through this resistance ismeasured when performing a dc hipot test asshown in Figure 1. The combined inductance (L),series resistance (RS), capacitance (C) and parallelresistance (RP) as shown in Figure 2 is defined as HV Testthe characteristic impedance (Z0) of the cable. Set Figure 3: Bad insulation 2 Fault Finding Solutions
  5. 5. Cable Characteristics SECTION ICABLE FAULTS DESCRIBED Phase ConductorWhen at some local point in a cable, insulation hasdeteriorated to a degree that a breakdown occursallowing a surge of current to ground, the cable isreferred to as a faulted cable and the position of Spark Gapmaximum leakage may be considered a cata- Gstrophic insulation failure. See Figure 4. At this Fault Resistancelocation the insulation or parallel resistance has RFbeen drastically reduced and a spark gap hasdeveloped. See Figure 5.Occasionally a series fault shown in Figure 6 candevelop due to a blown open phase conductorcaused by high fault current, a dig-in or a failed Shield or Neutralsplice. Figure 5: Fault region simplified diagram mAmps Spark Gap G Phase Conductor HV Test Fault Fault Resistance Set RFFigure 4: Ground or shunt fault on the cable Shield or Neutral Figure 6: Open or series fault on the cable Fault Finding Solutions 3
  6. 6. Fault Locating Procedures SECTION IILOCATE FAULTS IN BURIED PRIMARY CABLE ■ At end A, connect the instrument between eachAfter all clearances have been obtained and the of the other phase conductors, if any, andcable has been isolated in preparation for cable ground and record the insulation resistancefault locating, it is strongly recommended that a readings.fixed plan of attack be followed for locating the ■ After connecting a short between the phase andfault. As in diagnosing any complex problem, fol- neutral at end B (Figure 8), do a loop test forlowing a set step-by-step procedure will help in continuity at end A using the ohms or continuityarriving at the solution or, in this case, pinpointing range on the instrument. If a reading of greaterthe fault efficiently. than 10 ohms is obtained when the cable has aAt the very start, it is a good idea to gather as concentric neutral, test the conductor and neu-much information as possible about the cable tral independently by using a nearby good cableunder test. Information that will help in the fault as a return path. This will help to determinelocating process is: whether it is the conductor or neutral that is the problem. A reading in the hundreds of ohms■ Cable type — is it lead covered, concentric neu- is a good indication of corroded neutral if work- tral (bare or jacketed), tape shield? ing on a bare concentric-type cable. If no nearby■ Insulation type — is it XLPE, EPR, Paper? good cable is available, use a long insulated con- ductor to complete the loop from end B. If a■ Conductor and size — is it CU, AL, stranded, reading of infinity is measured either the phase solid, 2/0, 350 MCM? conductor or the neutral is completely open■ Length of the run — how long is it? between end A and end B which could be caused by a dig-in or a fault that has blown■ Splices — are there splices, are the locations open the phase conductor. known? ■ Repeat all tests from end B and record all■ T-taps or wye splices — are there any taps, are readings. the locations known, how long are branches?After obtaining the cable description the acronym“TALL” can help you remember the procedure forfinding cable faults in buried cable. TEST ANALYZE LOCALIZE LOCATE End A End BTEST THE CABLE MEGGER InsulationFault Resistance and Loop Test TesterAlthough most faults occur between phase Faultand ground, series opens also occur such asa blown open splice or a dig-in. Phase-to-phase faults can also occur on multi- phaseruns. Helpful information can be gathered Figure 7: Test for insulation (fault) resistance using awith a Megger® Insulation Tester that has Megger® insulation testerboth megohm and an ohm (continuity)range.Make a series of measurements as follows: End A End B■ At end A, connect the instrument MEGGER between the faulted conductor and Insulation Tester ground as shown in Figure 7. Using an insulation resistance range, measure and Fault record this resistance reading. Shorting Strap or Grounding Elbow Figure 8: Loop test for continuity using a Megger® insulation tester 4 Fault Finding Solutions
  7. 7. Fault Locating Procedures SECTION IITDR Tests End A End BRefer to Section IV for details on the use of theTime Domain Reflectometer. TDR■ At end A, connect a TDR or DART® Cable Fault Analyzer (use the TDR mode) between the fault- ed conductor and neutral or shield as shown in Figure 9. Look for an upward reflection from the open end of the cable and measure the length to the open using the cursors. Figure 9: TDR test for cable length■ After connecting a short between phase and neutral at end B (Figure 10), look for the down- ward indication of a short circuit at the cable end on the TDR. If the TDR shows an alternating End A End B open and short when alternately removing and applying the ground at the end of the cable, the TDR phase and shield are continuous to the cable Fault end. If the short does not appear on the TDR and a high resistance was read during the loop Shorting Strap or Grounding Elbow test, either the phase or shield is open at some point before the cable end.■ If a downward reflection is observed on the TDR Figure 10: TDR test for continuity and the fault resistance measured less than 200 Ω in the test, the fault has been found. If a downward reflection is observed on the TDR and If tests indicate insulation (fault) resistance values the fault resistance measured greater than 200 Ω less than 10 ohms, it may not be possible to create in the test, there is likely a T-tap or wye splice at a flashover at the fault site when surge generator that location. methods are used. This type of fault is often referred to as a bolted fault. A TDR can be used toDC Hipot Test locate this type of fault.After a surge generator is connected to the cableunder test, do a quick dc proof test to be sure the If a measurement of very low resistance in ohms iscable is faulted and will not hold voltage. Make a made from one end and a high resistance innote of the kilovolt measurement when the fault megohms from the other end, it is likely that thebreaks down. This will be an indicator of what phase conductor or a splice is blown open.voltage will be required when surging in order If the loop test indicates a resistance reading inbreak down the fault when doing prelocation or the 10 to 1000 ohm range and particularly if thepinpointing. If there are transformers connected reading varies during the measurement, there isto the cable under test, a proof test will always very likely neutral corrosion on the cable. Thisindicate a failure due to the low resistance path to could affect success when performing localizingground through the transformer primary winding. and locating procedures. If the loop test measure-A dc proof test in this case is not a valid test. ment is infinity, indicating an open circuit, either the phase conductor or a splice has blown open orANALYZE THE DATA a dig-in has occurred.Fault Resistance and Loop Test TDR TestsIf the insulation resistance of the faulted conduc- If the TDR tests indicate a shorter than expectedtor is less than 50 Ω or more than one MΩ, the cable length with no change of reflections when afault will be relatively easy to prelocate but may short is applied to the cable end there is likely abe difficult to pinpoint. For values between 50 Ω blown open splice or phase conductor or a dig-inand 1 MΩ, the fault may be more difficult to has occurred. If the TDR tests indicate a longerlocate. Some reasons for the difficulty with these than expected cable run, a thorough route tracefaults is the possible presence of oil or water in may be in order to detect additional cable notthe faulted cavity or the presence of multiple indicated on maps.faults. Fault Finding Solutions 5
  8. 8. Fault Locating Procedures SECTION IIDC Hipot Test Voltage gradient test sets are effective in pinpoint- ing faults on direct-buried secondary cable but theIf the cable holds voltage during the dc hipot test, method depends on the fault existing betweenthe cable may be good. If the cable is faulted, conductor and earth. When the cable is in conduit,burning may be required to reduce the breakdown a different method must be used. When a singlevoltage required when surging or you have con- conductor is contained within a plastic conduit,nected to the wrong phase. shorts cannot occur unless water gains access through a crack or other entry point. When a faultCable Route develops, leakage current flows from the conduc-At this point, it is recommended that the cable tor through the break in insulation, and then fol-route be determined or confirmed by consulting lows the water to the break in the conduit toaccurate maps or actually tracing the cable route. earth. If voltage gradient is used, the location ofSee Section III. When attempting to localize or the crack in the conduit could be found, but thelocate the cable fault, prelocation measurements location of the fault in the insulation wouldand pinpointing techniques must be made over remain unknown.the actual cable path. Being off the route by as lit-tle as a few feet may make the locate an extreme- LOCATE FAULTS IN ABOVE GROUND PRIMARYly difficult and time-consuming process. CABLE Some faults can be found by searching for obviousLOCALIZE - PRELOCATE THE FAULT physical damage to the cable especially if the cableSelection of a localizing technique is based, at section is short. If necessary, connect a surge gen-least in part, on the character of the fault. Several erator and walk the cable and listen for the dis-techniques are fully described in Section VI. They charge. If the cable is very long it might take aare as follows: good deal of time to walk the cable while the■ Sectionalizing surge generator is on. To reduce the total time spent and to minimize high-voltage exposure to■ Bridge — single faults the cable, use a localizing technique before■ TDR/low-voltage radar — faults measuring less attempting to pinpoint the fault. than 200 Ω and all opens Once the fault is localized, a listening aid is used■ High-voltage radar methods — all faults arc to zero in on the thump that occurs when the reflection, surge pulse reflection and decay surge generator breaks down the fault. For metal- to-metal (bolted) faults on non-buried cable, an■ Electromagnetic impulse detection — all shorts electromagnetic impulse detector may help to pin- and some opens point the fault. The use of electromagnetic impulse detectors is discussed in detail in SectionLOCATE - PINPOINT THE FAULT VI.Locating, often referred to as pinpointing, is nec-essary before digging up direct buried cable. Afterthe fault has been localized, a surge generator isconnected to one end of the faulted cable andthen listening in the localized area for the telltalethump from the fault. When the thump is not loudenough to hear, it may be necessary to use a surgedetector or an acoustic impulse detector to pin-point the fault. 6 Fault Finding Solutions
  9. 9. Cable Route Tracers/Cable Locators SECTION IIIOVERVIEW ■ Is the cable shielded or unshielded?Before attempting to locate underground cable ■ Is the cable direct buried or in conduit?faults on direct buried primary cable, it is neces-sary to know where the cable is located and what ■ Are there metal pipes or other undergroundroute it takes. If the fault is on secondary cable, structures under, over or near the target cable?knowing the exact route is even more critical. ■ Is the target cable connected to other cables orSince it is extremely difficult to find a cable fault pipes through grounded neutrals?without knowing where the cable is, it makessense to master cable locating and tracing and to This information will help to select the mostdo a cable trace before beginning the fault locat- appropriate locator and to prepare to locate theing process. cable successfully. See Figure 12.Success in locating or tracing the route of electricalcable and metal pipe depends upon knowledge,skill, and perhaps, most of all,experience. Although locating can Receiver Antennabe a complex job, it will very likely Electromagnetic Field AC Current Flowbecome even more complex as Produced by Current Flowmore and more underground Transmitterplant is installed. It is just asimportant to understand how theequipment works as it is to bethoroughly familiar with the exactequipment being used. Nearby Cables and/or PipesAll popular locators/tracers consistof two basic modules:The transmitter — an ac generator Current Return Pathswhich supplies the signal currenton the underground cable or pipeto be traced. Figure 11: How cable locators workThe receiver — detects the electro-magnetic field produced by thetransmitted ac current flow. SeeFigure 11. Interfering Cables and PipesBefore starting, it will be helpfulto obtain the following informa-tion: Cable to be traced■ What type of cable is it?■ Is the cable the same type all the way along its length? Tee or Wye Splice■ Is the target cable the only cable in the trench?■ Are there any taps?■ Is the cable run single phase or Figure 12: Cable under test multiphase?7 MEGGER Fault Finding Solutions 7
  10. 10. Cable Route Tracers/Cable Locators SECTION IIIMany transmitters are equippedwith some means of indicating theresistance of the circuit that it istrying to pump current throughand can indicate a measurement Ohmmeterof the current actually being trans-mitted. Output current can bechecked in several ways as follows: If the resistance is too high, ground the far end■ By measuring the resistance of the circuit with an ohmmeter. When the resistance is less than approximately 80,000 Ω, there will typically be enough current flowing in the cable to allow a If the resistance is still too high, connect an insulated jumper wire for the return path good job of tracing. This is no guarantee that the transmitted current is passing through the Figure 13: Using an ohmmeter to measure resistance of the circuit target cable. The measured resistance may be affected by other circuits or pipes electrically connected to the target cable acting as parallel resistances. SELECTING A LOCATOR See Figure 13. Cable locating test sets, often referred to as cable tracers, may be grouped as follows:■ By observing the actual signal strength being transmitted by the transmitter. Many transmit- ■ Low frequency — usually less than 20 kHz some- ters provide a measurement or some indication times referred to as audio frequency (AF). of output current. A loading indicator on the ■ High frequency — usually higher than 20 kHz Portable Locator Model L1070 blinks to indicate and in the radio frequency (RF) range to about the approximate circuit resistance. A rate of four 80 kHz. blinks per second indicates a low resistance, almost a short circuit providing a very traceable ■ 60 Hz — most tracers provide this mode to allow signal. A rate of one blink every three seconds tracing of energized cables. shows a high resistance and a weaker signal. Low frequency (AF) is considered the general-pur-■ By observing the signal power detected by the pose selection because it is more effective in trac- receiver. Signal level indicator numbers are dis- ing the route of cables located in congested areas played digitally on most receivers and older due to less capacitive coupling to everything else models may display signal power with analog in the ground. Low frequency can be more effec- meters. The L1070 has both an analog style sig- tive over greater distances due to less capacitive nal strength bargraph plus a digital numeric leakage and because higher signal power is readout. Tracing experience gives the operator allowed by the FCC. The use of high frequency (RF) the ability to judge whether or not the numbers is typical in non-congested areas on relatively short are high enough. This is the most practical way lengths of cable or when a return path cannot be to check signal current flow. provided from the far end. If a proper return path is provided, either low or high frequencies can beRemember, the more current flow through the used effectively for very long distances. The L1070conductor the stronger the electromagnetic field allows selection of AF, RF, both AF and RF, or 60 Hzbeing detected by the receiver and the further as required by the specific application.from the conductor being traced the less field isbeing detected. 8 Fault Finding Solutions
  11. 11. Cable Route Tracers/Cable Locators SECTION IIIHOOKUPS contact. Connect the other lead (usually black) to aWhen a direct-buried secondary cable is to be temporary metal ground probe and check that thetraced, the transmitter is connected to the conduc- pin is making good contact with the earth. Whentor. When coaxial types of primary cable are the earth is dry it may be necessary to use a longertraced, the signal may be transmitted along either metallic ground stake or to pour water on thethe phase conductor or the neutral. ground rod to give it better contact with the earth. Place the ground rod off to the side as farWhenever possible use the direct connection away from the target cable as practical, but try tomethod with the test leads supplied with the loca- avoid crossing over neighboring cables and pipes.tor. This is often referred to as the conductive It may be necessary to vary the location of themethod. Connect one output lead (usually red) ground rod to obtain suitable results.from the transmitter to the conductor under testmaking sure that the alligator clip is making good For best results, install a temporary ground con- nection to the far end of the conductor being traced. See Figure 14. In this case either AF or RF can be used. If a ground cannot be applied to the far end, use RF and expect that the effective traceable length may be L1070 Locator E Portable R BIDDL R TM as short as 200 feet. See Figure 15. The only current flow in this situa- Transmiter tion is due to capacitive current flow and after some point the sig- nal disappears. If a direct connection is impossible,Figure 14: Hookup showing ground rod at far end of cable under a clamp coupler can be used totest induce the signal current onto the target cable. See Figure 16. If trac- ing a primary cable, place the loop around the neutral. When tracing secondary, connecting jumper wires L1070 Locator from the conductor to earth at E both ends of the cable may be nec- Portable R BIDDL R TM essary to obtain an adequate signal current flow through the target Transmitter cable. Remember that for sufficient current to flow to produce a strong traceable field there must be a loop or return path provided back to theFigure 15: Hookup with far end of cable under test isolated source. If a current coupler is not available, the transmitter module itself can be used to couple the signal inductive- ly from an antenna in the base of Jacket the transmitter into the cable. See E R L1070 Locator Portable Figure 17. The transmitter is set on BIDDL the earth directly over the target R TM cable with the arrow on the top Neutral panel in line with the cable. Use Transmitter the RF frequency selection and keep the transmitter and receiver at least 25 feet apart to avoid inter- fering signals generated directlyFigure 16: Current coupler connection to neutral on primaryjacketed cable through the air. Fault Finding Solutions 9
  12. 12. Cable Route Tracers/Cable Locators SECTION IIIKeep in mind that the best technique is to connect If all else fails and in a very congested area, com-the isolated far end of the target cable to a tem- plete the current loop by using a long insulatedporary ground rod beyond the far end of the jumper wire connected between one side of thecable. This will reduce the loop resistance, increase transmitter and the far end of the cable underthe transmitted current flow, and maximize the test. This technique has limitations as to lengthstrength of the signal to be detected by the receiv- but will definitely limit current flow to the targeter. See previous Figure 14. cable. See Figure 18. Remember to keep the route of the return wire well off to the side to avoidWhen the far end is parked and isolated, loop cur- interference.rent is entirely dependent upon capacitive cou-pling through the insulation or jacket of the cable Direct buried concentric neutral cable can beand through any faults to ground that may be traced by connecting the transmitter to the con-present. See previous Figure 15. ductor or the neutral. Remember that when con- nected to the neutral, the signal can more easily bleed over to other cables and pipes that may be connected to the ground. A stronger tracing signal can sometimes be developed when the transmitter is connect- Transmitter ed to the neutral. This is particularly true when using a current clamp or coupler as shown previously in Figure 16. USING THE RECEIVER To begin the tracing process, start by circling the connection point to the target cable at Neutral a radius of 10 feet or so to find the position with the strongest signal when using the peaking mode. See Figure 19. The L1070 receiver allows pushbutton selection ofFigure 17: Inductive coupling to neutral on primary either the peaking or nulling modes of trac-jacketed cable ing. See Figure 20. Some older models require a change in position of the antenna head from horizontal to vertical. Most receivers now also provide an automatic depth meas- TM R BIDDL E R L1070 Locator Portable urement, usually with the push of a button. Older units require posi- tioning of the antenna head at a Transmitter 45-degree angle and following the process shown in Figure 21.Figure 18: Use of return wire to improve current loop In the peaking mode of operation, a maximum signal level is obtained when the receiver is positioned directly over the target cable. In the nulling mode, a minimum signal is detected when directly over the target cable. Some units provide a simultaneous display of both TM R BIDDL E R L1070 Locator Portable modes. In general, if the object of tracing is simply to locate the approximate path of the target Transmitter cable, the peaking mode is recom- mended. If a more accurate trace is required such as prior to secondaryFigure 19: Circling path with receiver fault locating or splice locating, the nulling mode may be the better10 Fault Finding Solutions
  13. 13. Cable Route Tracers/Cable Locators SECTION IIIchoice. An analog bar-graph display, a digitalnumeric readout, a variable volume audible toneor all three may indicate the receiver signal level. X Feet X FeetWhile walking along the route with the strongestsignal level, note the value of signal strength. Alsowhile tracing, periodically check the depth. If thesignal level numbers drop as you proceed alongthe path away from the transmitter, there shouldbe a corresponding increase in depth. If the signal X Feetlevel increases as you proceed along the path,there should be a corresponding decrease indepth. If signal level decreases, even though thedepth does not increase, it could mean that youhave just passed a fault to ground or a wye splice.The transmitter current flow beyond a fault maybe significantly reduced to only capacitive leakage Figure 21: Depth measurement using null methodso the resulting drop in signal level may be with antenna at 45-degree angleenough evidence to conclude that a fault toground has been passed.When no interference is present, the combinedantennas in the receivers of newer locators willsense both a null and a peak magnetic field at theidentical spot directly over the target cable. True Location of Indicated Position ofInterfering conductors and pipes can cause the Target Targetmagnetic field around the target cable to becomeoval, or egg-shaped rather than circular and con-centric. This will cause an offset between the Conductor or Pipe Interfering Conductordetected and actual location. See Figure 22. This or Pipeproblem is often not possible to detect at the timethe locating is being carried out and is only discov-ered when digging begins. To prevent this, everyeffort should be made to prevent signal currentfrom bleeding or leaking onto other conductors inthe area, which is often impossible. Peak Mode Null Mode Antenna Figure 22: Offset caused by interference from non-target cable Conductor or PipeFigure 20: No interference — no offset betweenmagnetic field center and center of cable Fault Finding Solutions 11
  14. 14. How to See Underground Cable Problems SECTION IVMETHODS OF OPERATION The radar set, other than the electronics to pro-Cable analyzers provide a visual display of various duce the pulses of radio frequency energy, is basi-events on electrical cable and may serve as the cally a time measuring device. A timer starts count-control center for advanced cable fault locating ing microseconds when a pulse of radio frequencytest systems. Displays include cable traces or signa- energy leaves the transmitting antenna and thentures which have distinctive patterns. Signatures stops when a reflection is received. The actual timerepresent reflections of transmitted pulses caused measured is the round trip, out to the target andby impedance changes in the cable under test and back. In order to determine simply distance out toappear in succession along a baseline. When the target, the round trip time is divided by two. Ifadjustable markers, called cursors, are moved to the speed of this pulse as it travels through the airline up with reflections, the distance to the imped- in microseconds is known, distance to the targetance change is displayed. When used as a TDR, can be calculated by multiplying the time meas-approximate distances to important landmarks, ured divided by 2 times the velocity.such as the cable end, splices, wyes and transform- Distance = Vp timeers can also be measured. 2Time Domain Reflectometry The speed or Velocity of Propagation (Vp) of this pulse in air is nearly the speed of light or approxi-The pulse reflection method, pulse echo method mately 984 feet per microsecond.or time domain reflectometry are terms applied towhat is referred to as cable radar or a TDR. The This same radar technique can be applied to cablestechnique, developed in the late 1940’s, makes it if there are two conductors with the distancepossible to connect to one end of a cable, actually between them constant for the length of the runsee into the cable and measure distance to and a consistent material between them for thechanges in the cable. The original acronym, length of the run. This means that a twisted pair,RADAR (RAdio Detection And Ranging), was any pair of a control cable, any pair of a triplexapplied to the method of detecting distant aircraft cable, or any coaxial cable are radar compatible.and determining their range and velocity by ana- When applied to underground cable, 10 to 20 volt,lyzing reflections of radio waves. This technique is short time duration pulses are transmitted at aused by airport radar systems and police radar high repetition rate into the cable between theguns where a portion of the transmitted radio phase conductor and neutral or between a pair ofwaves are reflected from an aircraft or ground conductors. A liquid crystal or CRT display showsvehicle back to a receiving antenna. See Figure 23. reflections of the transmitted pulses that are caused by changes in the cable impedance. Any reflections are displayed on the screen with elapsed time along the horizontal axis and ampli- tude of the reflection on the vertical axis. Since the elapsed time can be measured and the pulse velocity as it travels down the cable is known, dis- tance to the reflection point can be calculated. Pulses transmitted through the insulation of typi- cal underground cable travel at about half of the speed of light or about 500 feet/µs. Movable cur- sors when positioned at zero and a reflection point provide a measurement of distance to that point in feet. The TDR sees each increment of cable, for example each foot, as the equivalent electrical circuit Distance impedance as shown in Figure 24. In a perfect length of cable, all of the components in every D = Velocity of Propagation (Vp) X Time (µs) foot are exactly like the foot before and exactly 2 like the next foot.Figure 23: Aircraft radar12 Fault Finding Solutions
  15. 15. How to See Underground Cable Problems SECTION IV DESCRIPTIONS AND APPLICATIONS Low-Voltage TDR/Cable Radar A low-voltage TDR is an appropriate method to localize faults and other impedance changes on electrical cable such as twisted pair, parallel pair, and coaxial structure. TDRs are available in small hand-held, larger portable, and rack mount con- figurations for a broad variety of applications.Figure 24: TDR reflections from perfect cable Low-voltage, high-frequency output pulses are transmitted into and travel between two conduc- tors of the cable. When the cable impedanceThis perfect run of cable will produce no reflec- changes, some or all of transmitted energy istions until the end of the cable appears. At the reflected back to the TDR where it is displayed.end of the cable the pulses see a high impedance Impedance changes are caused by a variety of dis-(an open circuit), causing an upward reflection. If turbances on the cable including low resistancethe cable end is grounded (a short circuit), the faults and landmarks such as the cable end, splices,pulses see a low resistance and a downward reflec- taps, and transformers. See Figures 25 through 31tion is caused. A low-voltage TDR is an excellent for typical reflections or cable traces.tool for the prelocation of series open circuits andconductor to conductor shorts. For cable shunt or Faults That a Low-Voltage TDR Will Displayground faults with a resistance higher than 200 Low resistance faults of less than 200 Ω betweenohms the reflection is so small it is impossible to conductor and ground or between conductors aredistinguish from normal clutter reflections on the displayed as downward reflections on the screen.cable. Unfortunately, almost all faults on primary Series opens, since they represent a very highunderground distribution cable are high resistance resistance, are displayed as upward going reflec-faults in the area of thousands of ohms or even tions. See Figures 27 and 28.megohms. Due to the reflection characteristics ofthese high resistance faults, they are impossible to Landmarks That a Low-Voltage TDR Willsee using only the low-voltage TDR. An alternate Displaytechnique such as arc reflection must be utilized to A TDR can localize cable landmarks, such as splices,prelocate these faults as discussed in Section VI. wye or T-taps, and transformers. See Figures 29 through 31. The TDR helps to determine the loca-Differential TDR/Radar tion of faults relative to other landmarks on theWhen a TDR such as the Megger Model CFL535F cable. This is especially true on complex circuits.which has two inputs and is programmed to allow Traces of complex circuits are necessarily also verya display of the algebraic difference between two complex and difficult to interpret. To make senseinput traces, a technique referred to as differential of these complex traces, it is extremely helpful toTDR can be used. If the two traces (L1 and L2) are confirm the position of landmarks relative to theidentical, the display will show a totally flat line. faults observed. See Figure 32.When using differential TDR, any differencebetween the two phases (L1 minus L2) will be easi- For every landmark that causes a reflection, therely identified on the display. This can be useful is slightly less transmitted pulse amplitude travel-when fault locating on a three-phase system ing from that point down the cable. This means onwhere the faulted phase can be compared to a a cable run with two identical splices, the reflec-good phase. The fault is likely where the differ- tion from the first splice will be larger than that ofence is and the cursor can be positioned to meas- the second down the cable farther. No conclusionsure the distance to that point. can be drawn based on the size or height of reflections at different distances down the cable. Fault Finding Solutions 13
  16. 16. How to See Underground Cable Problems SECTION IVCONTROLS AND INPUTS TO THE TDR An alternate method to determine an unknown velocity value is to:Velocity of Propagation 1. Set the right cursor to the upward-going reflec-Certain information must be provided to the TDR tion at the end of the cable section.before it can provide distance information. Mostimportant is velocity of propagation (VP), the 2. Determine the true length of the section ofspeed at which the transmitted pulse travels down cable under test.the cable under test. This value is used by the ana- 3. Adjust the velocity until the correct distance islyzer to convert its time measurement to distance. displayed.This velocity is primarily dependent on the type ofcable insulation although technically is also affect- If a known length of cable is available on a reel,ed by conductor size and overall cable diameter. the above procedure may be used. The longer theThe table to the right shows typical velocity values sample of cable the better for an accurate deter-for various primary cable types. mination of velocity. DART Analyzer or low-voltage TDR Open End BIDDLE DART R R R ANALYSIS SYSTEMFigure 25: TDR used to measure length of cable with far end open DART Analyzer or low-voltage TDR Shorted End BIDDLE DART R R R ANALYSIS SYSTEMFigure 26: TDR used to measure length of cable with far end shorted14 Fault Finding Solutions
  17. 17. How to See Underground Cable Problems SECTION IVThe units of velocity can be entered into the DART Velocity of Propagation TableAnalyzer or TDR in feet per microsecond (ft/µs),meters per microsecond (m/µs), feet per microsec- Insulation Wire Vp Vp Vp Vpond divided by 2 (Vp/2) or percentage of the speed Type kV Size Percent Ft/µs M/µs Ft/µsof light (%). EPR 5 #2 45 443 135 221The values in the Velocity of Propagation Table are EPR 15 #2 AL 55 541 165 271only approximate and are meant to serve as aguide. The velocity of propagation in power cables HMW 15 1/0 51 502 153 251is determined by the following: XLPE 15 1/0 51 502 153 251■ Dielectric constant of the insulation XLPE 15 2/0 49 482 147 241■ Material properties of the semiconducting XLPE 15 4/0 49 482 147 241 sheaths XLPE 15 #1 CU 56 551 168 276■ Dimensions of the cable XLPE 15 1/0 52 512 156 256■ Structure of the neutrals, integrity of the neu- trals (corrosion) XLPE 25 #1 CU 49 482 147 241■ Resistance of the conductors XLPE 25 1/0 56 551 168 276■ Additives in the insulation XLPE 35 1/0 57 561 171 280■ Propagation characteristics of the earth sur- XLPE 35 750 MCM 51 502 153 251 rounding the cable PILC 15 4/0 49 482 147 241With such a large number of variables and a num- XLPE 0.6 #2 62 610 186 305ber of different manufacturers, it is impossible topredict the exact velocity of propagation for a Vacuum — — 100 984 300 492given cable. Typically, utilities standardize on onlya few cable types and manufacturers and have soilconditions that are similar from installation toinstallation. It is highly recommended that faultlocation crews maintain records of propagationvelocities and true locations. Using this informa-tion, accurate, average propagation velocities canbe determined. DART Analyzer or low-voltage TDR Open End BIDDLE DART R R R ANALYSIS SYSTEM Low resistance fault to groundFigure 27: TDR measuring distance to a low-resistance fault to ground Fault Finding Solutions 15
  18. 18. How to See Underground Cable Problems SECTION IVRange When the test leads are especially long (such asRange is the maximum distance the TDR sees 125 feet long on most high-voltage radar systems),across the face of the display. Initially, select a it is often desirable for you to set the left cursor torange longer than the actual cable length so the the end of the test leads. When this offset is cali-upward-going reflection from the end can be brated, the distance indicated by the right cursoridentified. Move the right cursor to that upward will not include the length of the test leads. To doreflection and measure the total length. Does the this calibration in the field simply touch the endsmeasurement make sense? A range can then be of the test leads and position the left cursor at theselected that is less than the overall cable run but toggle point as the TDR sees an open and then athe TDR will only see out the distance of the range short. Press the Save Offset to set the left cursorsetting. zero to that point.Gain ZoomGain changes the vertical height or amplitude of When you have set the cursor at the reflection ofreflections on the display. It may be necessary to interest, the distance to that point on the cableincrease the gain on a very long cable or a cable run will appear in the distance readout. When awith many impedance changes along its path to zoom feature is provided, the area centeredidentify the end or other landmarks. Gain adjust- around the cursor is expanded by the zoom factorment has no effect on measurement accuracy. selected: X2 (times 2), X4 (times four), etc. It is often possible to set the cursor to a more preciseCursors position when the zoom mode is activated and the reflection is broadened.For all TDR measurements, the cursor is positionedat the left side of the reflection, just where it Pulse Widthleaves the horizontal baseline either upward ordown. Move the right cursor to the reflection of The width of the pulses generated by the TDR typ-interest just as it leaves the base line so that the ically ranges from 80 nanoseconds up to 10TDR can calculate its distance. If the left cursor is microseconds. As range is changed from shorter toset to the left of the first upward-going reflection, longer, the pulse width is automatically increasedits zero point is at the output terminals of the in width so that enough energy is being sentinstrument. If you do not recalibrate, it will be down the cable to be able to see some level ofnecessary to subtract your test lead length from all reflection from the end. The wider the pulse thedistances measured. Remember, the TDR measures more reflection amplitude but the less resolution.every foot of cable from the connector on the The narrower the pulse the more resolution butinstrument to the reflection of interest. less reflection amplitude. For the best resolution or in order to see small changes on the cable, a nar- row pulse width is required and in order to see the DART Analyzer or low-voltage TDR BIDDLE DART R R R ANALYSIS SYSTEM Open conductor faultFigure 28: TDR used to measure distance to open in conductor16 Fault Finding Solutions
  19. 19. How to See Underground Cable Problems SECTION IVend a wide pulse width may be required. The Location of the third marker (stake 3), the actualpulse width may be changed manually to override fault, may be found by using the proportionalitythe automatic selection. An effect termed pulse that exists between the fault distances, d1 and d2,dispersion widens the pulse as it travels down a and their error distances, e1 and e2. To locatelong run of cable so resolution may be worse stake 3, measure the distance d3 between stakes 1toward the end of a long cable. and 2 and multiply it by the ratio of distance d1 to the sum of distances d1 and d2. Stake 3 then isDISTANCE MEASUREMENTS placed at this incremental distance, e1, as meas- ured from stake 1 toward stake 2.Three-Stake Method e1 = d3 d1Measurements to a fault using a low-voltage TDRare strictly a localizing technique. Never dig a hole d1 + d2based solely on a TDR measurement. There are toomany variables that include: Alternatively, stake 3 can be placed at the incre-• The exact velocity mental distance, e2, as measured from stake 2• The exact cable route toward stake 1.• The accuracy of the TDR itself e2 = d3 d2The three-stake method is a means to get a rea- d1 + d2sonably accurate fault pinpoint using only theTDR. The method consists of making a fault dis-tance reading from one end (terminal 1) of the This third stake should be very close to the fault.faulted line and placing a marker (stake 1) at that A practical field approach (with no math involved)position as shown in Figure 32. With the TDR con- is to make a second set of measurements fromnected at the other end of the line (terminal 2), both ends with a different velocity. If the distancefind the fault distance for a second marker (stake between stakes 1 and 2 was 50 feet, by adjusting2). In actual practice, stake 2 may fall short of the velocity upward the new distance measure-stake 1, may be located at the same point, or may ments may reduce the difference to 30 feet. Withpass beyond stake 1. In any case the fault will lie enough tests at differing velocities the distancebetween the two stakes. It is important that the can be lowered to a reasonable backhoe trenchingsame velocity setting is used for both measure- distance.ments and the distance measurements are madeover the actual cable route. This may mean tracingthe cable. DART Analyzer or low-voltage TDR Open End BIDDLE DART R R R ANALYSIS SYSTEM SpliceFigure 29: TDR used to localize distance to a splice Fault Finding Solutions 17
  20. 20. How to See Underground Cable Problems SECTION IV Open End of Tap DART Analyzer or Tee (Y) Splice low-voltage TDR Open End BIDDLE DART R R R ANALYSIS SYSTEMFigure 30: TDR used to localize distance to a T-tap Open End Low Resistance Fault to Ground DART Analyzer or Tee (Y) Splice low-voltage TDR Open End Splice BIDDLE DART R R R ANALYSIS SYSTEMFigure 31: TDR used to localize distance to a fault relative to a landmarkTerminal Stake 1 Stake 3 Stake 2 Terminal 2 Fault d3 d1 e1 e2 d2Figure 32: Three-stake method18 Fault Finding Solutions
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