How to Improve the Accuracy of Cable Fault Testing

I. Pre‑test Preparation (Foundation; many errors stem from inadequate pre‑work)

Collect complete cable archive data

Safe power‑off and sufficient discharge

Distinguish cable conditions

II. Accuracy Improvement in Pre‑location Stage

• Adopt cross‑verification with multiple pre‑location methods; do not rely on a single method
  • Low‑resistance faults & open‑circuit faults: Time‑Domain Reflectometry (TDR, low‑voltage pulse method)
  • High‑resistance leakage & flashover faults: Impulse high‑voltage flashover method

• Identify interference waveforms and eliminate false reflection points

  
  

  Joints, branches and cable terminals generate reflected waveforms which are easily misread as faults. Distinguish joint reflections from fault reflections by referring to actual cable length and joint positions.
• Perform repeated tests with multiple pressurization cycles

  
  

  Some high‑resistance faults are unstable with intermittent conduction. Apply repeated impulses and check whether waveforms can be stably reproduced. If waveforms differ every time, the fault status is changing; do not adopt readings from a single test.

III. On‑site Anti‑interference to Eliminate External Errors

• Avoid strong electromagnetic interference sources

  
  

  Keep away from live high‑voltage lines, frequency converters, motors and high‑current busbars. If separation is impossible, use acoustic‑magnetic synchronous anti‑interference instruments that only recognize high‑voltage impulse synchronous signals to filter ambient noise.
• Standardize grounding and minimize grounding resistance

  
  

  Poor grounding of the test system causes unstable high‑voltage loops and waveform distortion. Drive ground electrodes deep and avoid loose connections.
• Avoid adverse environmental impacts

  
  

  Sound transmission attenuates greatly on rainy or water‑logged ground. Surface dampness modifies earth current. Conduct tests under stable environmental conditions where possible. If field work in rain is unavoidable, prioritize magnetic‑signal‑aided judgment instead of sound‑only detection.

IV. Key Points for Precise Pinpointing

• Adopt dual constraints: pre‑located distance + actual cable route

  
  

  Do not fully trust the pinpointing instrument. Confirm the actual cable path first. Pinpointing shall only be performed within the range around the pre‑calculated fault distance. Signals far beyond this range are most likely false points.
• Prioritize acoustic‑magnetic synchronization; do not rely purely on auditory detection

  
  

  Sound‑only detection is highly susceptible to misjudgment caused by road or construction‑site noise.

  
  

  Acoustic‑magnetic synchronization uses the time difference between the earlier‑arriving magnetic signal and the later‑arriving acoustic signal for automatic fault judgment, greatly reducing human auditory errors.
• Multi‑point re‑verification

  
  

  Move the probe back and forth several meters before and after the estimated fault point and compare acoustic‑magnetic time differences. The time difference reaches its minimum directly above the fault and increases on both sides, so as to confirm the fault center. Do not stop testing once one signal point is found.

V. Reduce Errors from Personnel and Equipment

• Regular instrument calibration

  
  

  Periodically calibrate TDR units, high‑voltage signal generators and pinpointing devices. Run simulation tests with short test cables before field work to verify proper instrument performance.
• Operators shall not rely solely on automatic instrument interpretation

  
  

  Automatic algorithms may misinterpret waveforms. Operators shall review raw waveforms manually and understand waveform characteristics instead of only reading the auto‑calculated fault distance displayed by equipment.
• Eliminate hidden branch interference

  
  

  If T‑connections or branch boxes exist and branches are not disconnected, superposed reflected waveforms from branches will completely distort distance readings. Disconnect all branch loads before testing.

VI. Typical Causes of Inaccurate Cable Fault Testing

1. Incorrect wave‑velocity parameter setting
2. Cable branches and transformers not disconnected
3. Residual cable charge not fully discharged
4. Unstable flashover‑type faults with continuously varying resistance values
5. Sound‑only pinpointing under strong electromagnetic interference
6. Complete dependence on instrument readings without cross‑checking the actual cable route