High Voltage 'Non-Persistent' Fault Finding Cable and ... - Dia-Test

High Voltage 'Non-Persistent' Fault FindingCable and Cable Fault Locating - Part 4This is the last of a four part series on cable and fault locating technologies that are in commonuse today. This installment addresses using high voltage surge generators also known as'Thumpers'. The previous articles have centered on what are basically low voltage test methods,cable locators, a-frames, and TDRs are all limited to between 5 and 50 volts. This means thefaults must be 'persistent' or always in a faulted state. In the electrical industry, there are somefaults that just don't show up under these conditions.We often need thousands or tens of thousands of volts to make the fault fail. This is where thethumper comes in. It is a portable source of high voltage. Common models put out 32 kV ormore but equally important to this specification is the amount of power that is available. Morepower will make a louder noise and make certain faults show up. The need to build up and storethis power means most thumpers put out a DC voltage.Thumpers have been around for the better part of the 20 th century but as with all technology,features and methods have improved for greater accuracy, easier use and interpretation of results.The original thumpers were just that, they repeatedly connected the high voltage to the cableunder test (CUT). Linemen would walk along the path of the cable listening for the 'whumph'(or sometimes a 'dtik' or a 'plunk') under ground. Different ground conditions, vehicle trafficpatterns and fault types can make these sounds hard to discern.Faults aren't always the same, sometimes they happen with relatively little voltage, perhaps only500v if the splice was wet. But when the water vaporizes because of the resulting arc, it nowmay take 10 kV for that same breakdown to fail. To find the fault the voltage usually gets turnedup and up until the fault appearsRepeated thumping can have some unfortunate side effects to the CUT. Faults that require ahigher voltage to break down often subject the cable to voltages above the design voltage. Whilethe existing fault was located, other areas of the cable may have been weakened. Statistically,thumped cables fail sooner. For this reason, thumping should be a last resort. Anotherconsideration of thumpers is that they are potentially very dangerous if used by unqualifiedpersonnel. The entire CUT must be isolated and protected from contact with apparatus andpersonnel.Advancements in thumper systems started with better listening devices. Mechanical methodslike a stethoscope were the first techniques. Antennas and meters were added as the flow ofcurrent makes an electro-magnetic (EM) field. In theory, most of the current is dissipated at thefault and the current (and hence the EM field) is reduced past the fault. The effectiveness of thistechnique is reduced when the fault is a short to another conductor in the same cable. Thecurrent returns on the other conductor in the same magnitude and opposite direction and the EMfields effectively cancel one another. Listening is still a good choice and all modern productsemploy a mechanical microphone or geophone that is placed directly on the ground over thecable. The amplifier and headphones connected to the microphone minimize the external noiseand make the thump easier to hear.

Safety systems were quickly integrated into thumpers. Self-discharge systems, grounding,manual discharge hot-sticks, key switch lock outs and other techniques are implemented tominimize the chances of injury. Like most aspects of line work though, there are still many waysto get hurt. Get full training and follow all of the manufacturers and your companies' guidelines.The next advancement was a visual 'pre-locator', making the thumper more like a high voltageTDR. The pre-locator will usually show the fault with an accuracy better than 10% - 15%. Thiswill cut the distance the lineman needs to walk by 80-90%, allowing more attention to be paid tothe suspected area and the faults to be more easily located.Remember that the theory of a TDR is that it transmits a pulse of energy that travels orpropagates along a cable. A portion of the energy will reflect back to the sending end wheneverit passes a relative change in the impedance of the cable. The time the reflections take to returnis proportional to the distance. If we know the approximate speed of the pulse in the cable andmultiply it by the time the reflection takes to return, the distance to the anomaly is easilycalculated. Most TDR instruments automatically do the math, displaying the distance in feet ormeters.With thumpers there is alarge difference, thatbeing the impedancechange causing thereflection is the arc ofelectricity. The arcdoesn't happen the instantthe pulse gets to the fault,it requires the air betweenthe conductors to ionizeand become conductiveand this can add severalmilliseconds onto thetime. As you can see inthe picture to the right,there is a long dead spacebetween the left cursor onthe launch pulse and thefirst reflection. The actual distance to the fault from the launch end is the distance between anytwo of the large successive peaks. The peaks are the one pulse bouncing back and forth fromlaunch to fault and back.

The next advancement in thumper technique and technology is commonly called the SecondaryImpulse Method (SIM). This was developed primarily to make trace interpretation easier. Withthe high voltage pulses, the dynamics of the pulse and cable create many reflections that are partof the cable and the fault may be difficult to detect. Using a low voltage TDR and the thumpertogether in an integrated system, a low voltage TDR pulse is taken of the cable and stored in adisplay memory. Then the thumper is triggered to send a HV pulse and while the arc is burning,the TDR sends the same low voltage pulse which is overlaid upon the first trace. The arc is avery low impedance point that causes the TDR pulse to reflect as it would with a short circuit(which it is). The picture below shows the two traces displayed on top of one another with thedashed cursor at the launch point and the solid cursor on the short indication. Now the distanceto the fault is easily read off as 134 meters.Earlier we talked about the voltage and power available for the test. The formula for power is P= C * V 2 (power in Joules, C = capacitance and V = voltage) and the metric units are Joules.This shows using lower voltage will result in the power decreasing by the square, so less isavailable to ionize the air and support the thump arc. Newer thumpers have variable capacitance

that can be increased at lower voltages to maintain the same available power. It is entirelypossible that a fault that isn't apparent at a particular voltage will be detectable at a lower voltagewhen there is more stored charge to sustain the arc. This has the advantage of less voltage stresson the CUT.Since the thumper has all this voltage generation, control and safety interlocks, most can be usedas a high voltage proof-tester or Hi-Pot as well. The system can be connected to an installedcable or one undergoing a proactive reliability test. The output is enabled and stepped up to avoltage above the normal working voltage of the cable and held for a set amount of time.Success of this test usually means it is fit for service.Advanced controls include amplification. Control over the vertical amplification of thedisplayed trace allows smaller faults with weak reflections to be detected.A screen zoom function is desirable to allow more accurate placement of a cursor whenmeasuring distances. The resolution of the screen will affect the accuracy of cursor placementand the ability to zoom in to improve the resolution will give greater accuracy.Two channels and/or memory settings give additional trouble-shooting ability. Due to theirdesign, or environment (temperature and moisture for example), some cables may show a 'noisy'trace that is difficult to interpret. Comparing a trace from a non-arcing pulse to an arcing pulsewill often allow the fault to be detected.Training. The best equipment manufactured will not give the required information if theoperators don't understand how to effectively use it. Make sure training and applications supportis available, included, and utilized when purchasing test equipment.I would be very remiss if I ended this series without thanking Nick Garrioch of Eecol Electric inWinnipeg for his help. I get along much better with the laws of physics than the laws of English.If anyone has cable or cable fault locating questions, please e-mail me at the address below and Iwill send you a reply. With your permission, I would like to share some of these situations infuture articles.

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