Cable Fault Location . . . Not for the Faint of Heart (Part II)

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After the cable has been tested the results must be interpreted and the type of failure identified. Proper interpretation of test results and selection of a fault locating technique best suited for the parameters identified will save time, money, and stress to both the technician and cable under test.  


Fault locating techniques can be placed into one of two different categories: tracking techniques and pre-locating techniques. A tracking technique requires the technician to probe or test along the entire cable path in search of the fault. Tracking techniques in most situations also serve as pinpointing techniques. Pre-locating techniques allow a measurement of approximate distance to the cable fault.

Pre-locating techniques include:

 

Cable Type

Pre-locating Techniques

Best Conditions/Limitations

Telecommunication (Copper)

Time Domain Reflectometry (a.k.a; TDR, Pulse Echo, or Cable Radar)

 

Ideal for locating open circuits, low resistance short circuits, split pairs, crossed pairs, bridge taps, loading coils, and crosstalk.  Limitationsinclude: will not see through loading coils or digital repeater equipment. Typically will not detect high resistance short circuits (resistances above 800 ohms).

 

Resistive Fault Location / Resistive Ratio Bridge

Ideal for locating short circuits and high resistance short circuits.   

Cautions include: This technique requires a good cable pair (reference) and far end strapping between good pair and faulted pair.  Cable size and temperature is required for best results. Limitations include: Will not locate open circuits and water in the cable can affect results.

 

Capacitance / Open Meter

Ideal for locating distance to open circuits.

Limitations: Cannot locate short circuits.

 

Low Voltage Signal & Control

Time Domain Reflectometry (a.k.a; TDR, Pulse Echo, or Cable Radar)

 

Ideal for locating open circuits, low resistance short circuits, and splice/joints.  Limitations include: Typically will not detect high (resistance short circuits resistances above 400 ohms).

 

Resistive Fault Location / Resistive Ratio Bridge

Ideal for locating short circuits and high resistance short circuits. 

Cautions include: This technique requires a good cable pair (reference) and far end strapping between good pair and faulted pair.  Cable size and temperature is required for best results.

Limitations include: Will not locate open circuits and water in the cable can affect results.

 

Electric Power (Medium and High Voltage)

Time Domain Reflectometry (a.k.a; TDR, Pulse Echo, or Cable Radar)

 

Ideal for locating open circuits, low resistance short circuits, and splice/joints.  Limitations include: Typically will not detect high (resistance short circuits resistances above 200 ohms).

 

Arc Reflection / Arc Stabilization / Secondary Impulse Reflection

This technique incorporates both low voltage TDR and high voltage testing and is ideal for locating most faults in medium voltage power cable.  Caution:Systems are typically rated by output voltage and energy of the high voltage device (capacitive discharge/surge generator/or “thumper”).  An under-rated high voltage unit will hamper the performance of this technique.

 

 

Impulse Current

This technique relies on capturing a transient waveform created during cable breakdown.  Caution: Transient waveforms can be difficult to interpret and requires experience.

 


Most cable fault locating failures and wasted time can be attributed to technicians not properly interpreting test results, selecting the wrong tools for the job or taking short cuts in the process.  Use Protec Equipment Resource’s years of experience to help improve results interpretation and equipment / technique selection.  Feel free to ask us to assist in setting up a fault locating strategy and comprehensive procedure to eliminate wasted time and effort.

In Part III of this series we will explore Tracking and Pinpointing techniques.