Locating faults in underground power cables is one of the most valuable applications of a DC high-voltage generator. While low-resistance faults can be found with a simple time domain reflectometer (TDR), high-resistance faults and intermittent failures require the high-voltage capability of a DC generator. This article explains the principal fault location methods and the role of each instrument.

1. Classification of Cable Faults

Cable faults are classified by their resistance at the fault point:
- Low-resistance faults (0–100 Ω): Usually a direct short or cut. Simple TDR works well.
- Medium-resistance faults (100 Ω–1 kΩ): TDR may still detect reflection, but attenuation reduces accuracy.
- High-resistance faults (1 kΩ–10 MΩ): TDR alone cannot see the fault. High voltage is required.
- Intermittent faults: Fault appears only under certain conditions (moisture, vibration, thermal cycling). A DC high-voltage generator with continuous monitoring can help reproduce the fault.
The majority of field failures in aged XLPE cables are high-resistance faults caused by water treeing, which is why DC high-voltage generators remain essential field instruments.

2. Fault Burning with a DC High-Voltage Generator

Fault burning converts a high-resistance fault into a low-resistance one by controlled application of high current. Procedure:
1. Apply DC high voltage from the generator until the fault breaks down (arc formation).
2. Limit current to prevent complete cable destruction (typically 100 mA–2 A).
3. Maintain the arc for seconds to minutes to carbonize the insulation at the fault point.
4. Periodically test with TDR to check if the fault resistance has dropped sufficiently for accurate location.
Caution: Excessive burning can damage healthy insulation or create a second fault. Modern DC high-voltage generators include programmable current limits and automatic burn timers to prevent over-burning.

3. Arc Reflection Method

Arc reflection is the most accurate method for high-resistance faults. It combines a DC high-voltage generator, a surge capacitor, and a TDR:
1. The DC generator charges a surge capacitor to a preset voltage.
2. A spark gap or solid-state switch discharges the capacitor into the cable, creating an arc at the fault.
3. The TDR launches a low-voltage pulse immediately after the arc forms.
4. The arc momentarily provides a low-impedance path, reflecting the TDR pulse.
5. The TDR measures the time to reflection and calculates distance.
Arc reflection accuracy is typically ±0.5% of cable length or better. The DC high-voltage generator must provide a stable, repeatable charging voltage to synchronize with the TDR.

4. Surge Wave Method (Thumping)

The surge wave method uses the DC high-voltage generator to charge a large capacitor, which then discharges into the fault through a spark gap. This creates an acoustic "thump" at the fault point. A ground microphone or acoustic sensor detects the sound, allowing pinpoint location. Procedure:
1. Charge capacitor to a voltage high enough to break down the fault (typically 10–25 kV).
2. Discharge through the fault, creating an audible and mechanical shock wave.
3. Move along the cable route with the acoustic sensor.
4. Maximum sound intensity indicates the fault location.
This method is used after arc reflection gives approximate distance, to pinpoint the exact digging location. The DC high-voltage generator must deliver enough energy per discharge (typically 500–2000 Joules) to generate a detectable thump.

5. Time Domain Reflectometry Basics

TDR sends a low-voltage pulse down the cable and measures the time for reflections to return. Distance formula:
Distance = (Velocity of propagation × Time) / 2
Where velocity of propagation (Vp) is typically 0.6–0.8 times the speed of light, depending on cable insulation. For XLPE, Vp ≈ 0.67c; for PILC, Vp ≈ 0.75c.
TDR traces show:
- Open circuit: Positive reflection at fault distance.
- Short circuit: Negative reflection at fault distance.
- High-resistance fault: Small or no reflection – cannot be detected without high-voltage assistance.
The DC high-voltage generator's role is to convert the high-resistance fault into a low-resistance arc so TDR can see it.

6. Step Voltage Method for High-Resistance Faults

The step voltage method uses the DC high-voltage generator to apply a series of voltage steps to the cable. At each step, the leakage current is measured. A sudden change in current indicates breakdown at the fault. Distance is not directly measured, but the method confirms the fault's presence and approximate breakdown voltage. It is often used before arc reflection to determine the minimum voltage needed.

7. Pinpointing with Acoustic and Electromagnetic Sensors

After distance is determined by arc reflection or TDR, pinpointing locates the exact spot for excavation:
- Acoustic method: Detect the thump sound with a ground microphone.
- Electromagnetic method: Detect the magnetic field from the surge current using a ferrite antenna.
- Combined acoustic-electromagnetic: Simultaneously detect sound and magnetic signal; the time difference confirms the fault location.
The DC high-voltage generator must produce consistent, repeatable discharges for these methods to work. Variations in discharge energy cause inconsistent signals that confuse pinpointing.

8. Safety During Fault Location

Fault location involves high voltage and high energy. Essential safety practices:
- Isolate and lock out the cable at both ends before testing.
- Ground the cable at the far end during TDR testing (if not being used for reflection).
- Use a discharge rod after every high-voltage application.
- Keep personnel at least 3 meters from cable ends during thumping.
- Verify that the DC high-voltage generator's automatic discharge and grounding systems are functional.
- Never assume the cable is discharged – always verify with a contact voltage detector.
Arc and thump methods produce loud noises and mechanical shock – warn nearby personnel and protect hearing.

9. Equipment Configuration for Fault Location

A complete fault location system includes:
- DC high-voltage generator: 0–100 kV, 5–20 mA, with burn capability.
- Surge capacitor: 1–10 µF, rated for the generator's voltage.
- Spark gap or solid-state switch: Adjustable breakdown voltage.
- TDR: 100 MHz bandwidth or higher for accurate distance measurement.
- Acoustic sensor: Ground microphone with amplifier and headphones.
- Electromagnetic sensor: Ferrite antenna with receiver.
- Safety grounding kit: Discharge rods, grounding cables, voltage detector.
Some manufacturers offer integrated fault location systems combining all functions in one portable unit, reducing setup time and connection errors.

10. Troubleshooting Common Fault Location Problems

| Problem | Likely Cause | Solution |
| TDR shows no reflection after burning | Fault resistance still too high | Increase burn voltage or duration |
| Arc reflection distance inaccurate | Wrong velocity of propagation | Verify Vp from cable manufacturer data |
| Thump not audible at surface | Fault too deep or discharge energy too low | Increase capacitance or voltage |
| Multiple reflections on TDR | Splice or water in cable | Analyze trace with known splice locations |
| Generator trips during burning | Current limit set too low | Adjust current limit incrementally |
| Inconsistent arc formation | Moisture at fault or unstable contact | Dry cable ends, increase voltage slowly |

Conclusion

DC high-voltage generators are indispensable for locating high-resistance cable faults. Fault burning converts high-resistance faults into detectable low-resistance arcs. Arc reflection provides accurate distance measurement. Surge wave thumping pinpoints the exact location for excavation. Mastering these techniques requires practice and a thorough understanding of TDR trace interpretation. Always prioritize safety – high-voltage fault location involves stored energy that can be lethal. With proper equipment and procedures, a DC high-voltage generator reduces cable repair time from days to hours, minimizing customer outages and restoration costs.