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August 2018
Application Note: 3 Steps to Finding a Cable Fault

Application Note: 3 Steps to Finding a Cable Fault

22 August 2018

Megger finding cable faultsAn exhaustive review of the instrumentation involved in cable fault finding would require a week’s worth of class time and then some. But as a journey of a thousand miles begins with a single step, so does the understanding of instrumentation used for cable fault finding. 

First, the important pieces

Three critical pieces of test equipment must be considered when discussing cable fault finding:

  1. 1. Insualtion tester used before the fault locating test
  2.  
  3. 2. Time domain reflectometer (TDR) used during the test
  4.  
  5. 3. Route tracer used after the test

 

Using the Insulation Tester

When a known or suspected electrical problem exists (like smoke coming out of the ground), the faulted cable must be identified. For this step, an insulation tester (megohmmeter) or dielectric test set (high-pot) is used. These are DC test sets capable of medium- or high-voltage output.

The tester is connected across the insulation of the suspect cable, typically phase to ground, but other configurations may be utilized, especially for difficult problems. The applied voltage pulls leakage current through the insulation. No insulation is perfect in all situations (consider what can happen during a lightning strike), but ‘good’ insulation allows only nano-amps of leakage.

The tester uses Ohm’s law to calculate insulation resistance. The operator looks for cables or phases that produce low resistance measurements. Typically these are kΩ to low MΩ values. In the Megger line, MIT400 Series 1 kV handheld testers will do the job for circuits rated at building wiring levels, whereas for higher-voltage circuits, MIT525, 1025, and 1525 are the products of choice. For the highest-rated circuits, the 220 series of dielectric test sets may be used.

Time for the TDR

Megger TDR time domain reflectometerOnce the faulted circuit has been identified, a TDR locates the fault. This instrument works on the same principle as radar detection along a highway - the time it takes a signal to return to the source translated into distance in the case of the TDR (or speed, in the case of radar detection).

The instrument sends an energy pulse through the insulation between two conductors in a cable. Wherever there is a change in impedance, some of the energy echoes back. The time it takes for that to occur, coupled with the velocity of propagation (speed of the impulse through the insulating material), calculates the distance to the fault. The tester displays this first as a trace on the instrument’s display, somewhat similar to what you might see on an oscilloscope.

A perfect cable would produce a perfectly flat line, interrupted only by the end of the cable, which can easily be determined by opening and shorting the conductors. The trace will flip up (open) and down (short) as the cable ends are manipulated.

A faulted cable will produce an additional reflection at the point of the fault. The operator then moves a cursor across the screen to the beginning of the reflection and, if the VoP has been correctly set, the TDR will indicate the distance to the fault.

Different types of faults produce distinct reflection shapes, which the operator will be able to quickly recognize and interpret. In Megger’s catalog, CFL510G is a good starting point. Additional models offer additional features.

Because telecom, datacom, and control cabling is relatively simple and highly standardized, a TDR is all that is needed to locate faults. For power cable, however, the extreme variability limits the use of a TDR as a stand-alone solution. Power cable can develop what is termed “high resistance” faults (>100 Ω) that may require the addition of what is called a ‘thumper’ to the test equipment arsenal. Use of a TDR-thumper combination is another situation beyond the scope of this note.

The Cable Route Tracer

Megger route tracer cable faults finderHaving found the distance to the fault, the operator may now be faced with an “uh-oh” moment. You know it’s, say, 350 feet away, but that could be anywhere 360° around you. The fault could be in front of you, behind you, or to the left or right of you. Unless equipped with a reliable wire map…and good luck with that...you will then need a cable route tracer.

Cable route tracers are two-piece devices: a transmitter and a receiver. The transmitter puts a high-frequency tracing tone on the cable and can even be used on energized cable. The operator then walks the line holding the receiver, which visually and audibly indicates strength of signal and positions the operator directly over the cable.

Continue in the indicated direction for the distance found with the TDR, and you are then within a convenient working distance of the fault. For route tracing, Megger offers the AccuTrace Cable Route Tracer.

Although it may seem daunting to find a cable fault, by using the correct instrumentation you should be able to locate your fault in no time at all.

Tags: app note, application note, cable fault, finding cable fault, insulation tester, route tracer, TDR, test equipment, time domain reflectometer