High-Resistance Cable Faults: The Complete Field Procedure from Detection to Dig Point

A feeder goes dead. The megger shows the insulation is broken — but the TDR screen is flat. No reflection. No distance. The fault is there; the TDR just can't see it.

This is the high-resistance fault — the most frustrating case in cable fault location, and the one where the field procedure matters most. Rushing to conclusions wastes days; following the sequence finds the fault in hours.

Step 1 — Confirm and Classify the Fault

Before anything else, establish what you're dealing with:

• Insulation resistance test (megger) — confirms the fault and its severity
• Identify the fault type: phase-to-earth, phase-to-phase, or open circuit
• Estimate resistance: below a few hundred ohms → TDR-visible; above → high-resistance, needs conditioning

A fault above roughly a few hundred ohms reflects too little pulse energy for the TDR to measure. Knowing this one fact saves hours of staring at a flat trace.

Step 2 — Pre-Locate with the Bridge (Measure First)

Before applying any HV stress, use a bridge method (Murray loop or Varley loop) to get a first distance estimate. The bridge uses a healthy conductor in the same cable as a reference:

• Non-destructive — no additional stress on already-weak insulation
• Precise for moderate resistance faults — often accurate enough to plan the repair
• Fast — one measurement, no conditioning needed

Bridge pre-location answers the first question — roughly where? — without risking further damage to the cable.

Step 3 — Condition the Fault (Burn Only as Needed)

If the bridge can't close the gap (fault resistance above ~100 kΩ) or the fault is unstable, use a burnout system to carbonize the breakdown path into a stable, low-resistance state:

• Apply controlled HV with current limiting
• Monitor the insulation resistance as it drops toward tens of ohms
• Stop exactly when the fault is conditioned — over-burning spreads the damage zone

The MB10 (60 kV) bridge-burnout system combines steps 2 and 3 in one instrument, so the crew carries one unit instead of two.

Step 4 — Confirm Distance with TDR

Once the fault is low-resistance, the TDR can finally see it:

• Shoot the trace with the correct velocity factor
• Pre-locate from both ends if possible — two agreeing readings are a conclusion
• Correlate the distance with the route chart to get a surface position

Step 5 — Pinpoint Precisely

Distance alone isn't a dig point. Use an HV surge generator + acoustic-magnetic pinpointer:

• Fire the surge generator in impulse mode; the fault discharges audibly
• The pinpointer detects the acoustic and magnetic signals at the surface
• The signal peaks directly above the fault — that's your dig point

Step 6 — Excavate, Verify, Repair

• Dig at the pinpointed position with appropriate care
• Verify the fault visually before cutting
• Repair (joint or re-terminate), then re-test with VLF and a fresh TDR baseline

The Complete Sequence at a Glance

The Golden Rules

1. Bridge before burn. Measuring first is non-destructive; burning first damages insulation you might not need to damage.
2. Burn only as much as needed. Current limiting isn't a safety feature — it's a precision tool.
3. Confirm before digging. A single reading is a hypothesis; two readings and a pinpointer peak are a conclusion.
4. Save the new baseline. The repair changes the cable — update the records and traces.

How TriEdge Helps

The complete high-resistance workflow maps directly to TriEdge equipment:

• MB10 (60 kV) — bridge pre-location + burnout conditioning in one unit
• A10 / A10 Advanced TDR — precise distance confirmation and baseline traces
• G35 (32 kV) / A30D (40 kV) — surge conditioning and impulse sources
• C11 acoustic-magnetic pinpointer — exact surface pinpointing

Factory-direct pricing, 40–60% below equivalent Western brands.

View MB10 → · Full catalog (PDF)

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FAQ

Why can't a TDR find high-resistance faults? Faults above a few hundred ohms reflect too little pulse energy to produce a measurable reflection. The fault must first be conditioned into a low-resistance state.

What is the fastest way to locate a high-resistance fault? Follow the sequence: bridge pre-location first (non-destructive), then burnout conditioning if needed, TDR confirmation, then surge + pinpointer for the exact dig point.

Does bridge pre-location damage the cable? No — the bridge method measures using a healthy conductor as reference without applying damaging HV stress. It should always be tried before burning.

How do I know when to burn down a fault? When the fault resistance is too high for accurate bridge measurement (typically above ~100 kΩ) or the fault is unstable. Burn with controlled current and stop as soon as the fault is conditioned.

Can the MB10 do both bridging and burning? Yes — the MB10 (60 kV) combines bridge pre-location (Murray/Varley) and burnout conditioning in one instrument, covering steps 2 and 3 of the procedure.