Fault locating methods for High and low tension cables

Cable Fault Detection Methods in Electrical Systems

In electrical systems, cables are prone to faults due to insulation failure, moisture ingress, overloading, mechanical damage, or aging. Detecting and locating the fault accurately is crucial because replacing long underground cables can be expensive and time-consuming.

Here are the most widely used methods of cable fault detection:

---

1. Megger Testing (For LT Cables)

• Purpose: Checks insulation resistance of low-tension cables.

• Process:

  • Apply 500V/1000V DC using a Megger.

  • Infinite reading → Cable is healthy.

  • >100 MΩ → Acceptable (may have slight moisture, which usually dries when load is applied).

  • 50–100 MΩ → Usable for light loads.

  • ≈0 Ω → Faulty cable (short between conductors or between conductor and armour).

🔹 Additional Use: Resistance measurement between two terminals using Megger.

---

2. Hi-Pot Testing (For HT Cables)

• Purpose: Tests insulation strength of high-tension (HT) cables.

• Process:

  • Apply DC voltage for 5–15 minutes.

  • According to IEEE-400:

    • For 15 kV class cable → 56 kV (acceptance test), 46 kV (maintenance test).

• Limitation: High stress can weaken old cables if overused.

---

3. Traditional Fault Location Methods

(a) Divide Rule

• Cable is cut into sections.

• Test insulation resistance of each piece.

• The faulty section is isolated for further testing.

• ⚠️ Drawback: Involves cutting cable, hence destructive.

(b) High Voltage “Thumping” Test

• Apply 25–32 kV to the faulty cable.

• Fault produces loud noise or vibration at ground level → helps locate fault.

• ⚠️ Limitations:

  • Causes heating and deterioration of cable.

  • Not safe for aged cables.

---

4. Advanced Fault Detection Technologies

(a) Time Domain Reflectometry (TDR)

• Working: A low-energy pulse is sent through the cable → reflection from fault gives distance estimate.

• Advantages:

  • Non-destructive.

  • Quick for long underground HT cables.

• Limitations:

  • Accuracy only up to 1% of range.

  • Cannot detect high-resistance (>200 Ω) faults-to-ground.

---

(b) High Voltage Radar Methods

Used for precise fault location in underground HT cables.

1. Arc Reflection Method

   • Combines TDR + Thumper + Filter.

   • Reduces stress on cable by limiting surge voltage.

   • Provides approximate fault distance.

2. Surge Pulse Reflection Method

   • Uses current coupler + oscilloscope + thumper.

   • Pros: Can detect difficult/distant faults.

   • Cons: High surge may damage cable; requires skilled interpretation.

3. Voltage Pulse Reflection Method

   • Uses voltage coupler + analyzer + dielectric test set.

   • Helps find faults that occur at voltages above 25 kV (beyond thumper capacity).

---

Key Takeaways

• LT cables → Megger is sufficient.

• HT cables → Hi-Pot, TDR, and Radar-based methods are preferred.

• Old methods (Divide Rule, HV Thumping) are destructive and risky for modern systems.

• Modern methods (TDR, Arc Reflection, Pulse Reflection) provide accuracy with minimal damage.