High-Resistance Cable Faults: The Problem TDRs Can't See
High-resistance cable faults — insulation breakdowns above a few hundred ohms — are invisible to a standard TDR. The reflection is too weak to measure. Before you can pre-locate the distance, you must either measure the fault with a bridge method (Murray loop, Varley loop) or condition it with a burnout system that carbonizes the fault into a low-resistance path. Professional practice: bridge first (non-destructive, precise), burn down only when the bridge can't close the gap, then finish with TDR + acoustic pinpointing. A combined bridge-burnout unit like the MB10 (60 kV) lets one instrument handle the whole sequence.
Every cable fault locator engineer knows the frustration. The fault is there — the cable is dead, the insulation test says so — but the TDR screen shows nothing. No reflection. No distance. Just a flat trace.
The reason is physics, not equipment. A TDR works on reflected pulses, and a high-resistance fault (typically above a few hundred ohms) reflects almost nothing. The pulse passes through the fault point like it isn't there. Until the fault is converted into a low-resistance state, every TDR in the world is blind to it.
This article covers the two professional methods that solve the problem — bridge pre-location and burnout conditioning — and how they fit into a complete fault location sequence.
1. The Two Methods, Explained
Method A: Bridge Pre-Location (measure the fault, don't change it)
Bridge methods use a known-good conductor in the same cable as a reference and balance resistance ratios to compute the fault distance — no high voltage, no damage to the cable.
- Murray loop: compares the faulted conductor against a healthy loop to calculate distance. The classic, most widely used method for low-resistance and moderate faults.
- Varley loop: similar principle using a precision resistor box; works when a second healthy conductor is available.
Why start here: bridge pre-location is non-destructive. It tells you approximately where the fault is without stressing the insulation further. On aged cables, this matters — every unnecessary impulse or burn weakens the insulation around the fault.
Limits: bridge accuracy depends on conductor uniformity and contact quality at the far end. Very high-resistance faults (>100 kΩ) still resist accurate measurement — the bridge needs a stable, measurable path.
Method B: Burnout Conditioning (change the fault, then measure)
Burnout applies a sustained HV voltage with controlled current to the fault, carbonizing the breakdown path into a stable, low-resistance connection — typically tens of ohms. Once the fault is burned down:
- The TDR can now see it (strong reflection)
- The HV surge generator + acoustic pinpointer can now pinpoint it (audible discharge)
- The fault stays low-resistance long enough to complete the location
Why not always: burning stresses the insulation and can spread the damage zone. On cables where you plan to repair the damaged section anyway, this is acceptable — you're replacing the faulted length regardless. On cables where the fault may be at a joint you'd rather re-terminate than splice, burn carefully and keep current/time limited.
2. The Complete Professional Sequence
The order matters more than the tools. A disciplined sequence minimizes cable damage and vessel/crew time:
The golden rule: burn only as much as needed. The burnout system's current limiting isn't a safety feature — it's a precision tool. Minimal burnout = minimal cable damage = smaller repair.
3. What to Look For in a Bridge-Burnout System
When choosing equipment for high-resistance fault work, verify these capabilities:
- Bridge functions: Murray loop and Varley loop modes, with sufficient measurement range for your cable lengths (100 kΩ+ fault resistance handling)
- Burnout voltage and current control: rated for your network class (60 kV class covers most MV/HV distribution), with adjustable current limiting
- Burn-down monitoring: live insulation resistance feedback so you stop exactly when the fault is conditioned, not over-cooked
- Field robustness: built for van and field use, clear display in daylight, simple enough for shift crews
- Combined vs separate units: a combined bridge-burnout unit saves setup time on every job and reduces what the crew carries
4. TriEdge MB10 — The Bridge + Burnout Combination
The MB10 High-Voltage Bridge Burnout System (60 kV) is built for exactly this sequence:
- Bridge pre-location for high-resistance faults — measure before you burn
- Burnout conditioning with controlled current — convert the fault to a locatable state
- Completes the workflow alongside a TDR (A10/A10 Advanced) and acoustic pinpointer (C11) for steps 4–5
Complete HV bridge testing at factory-direct pricing, 40–60% below equivalent Western brands — ISO 9001 and CE certified.
View MB10 product page → · Full catalog (PDF)
FAQ
Why can't a TDR see high-resistance faults? A TDR measures reflected pulses. High-resistance faults (above a few hundred ohms) reflect almost no pulse energy, so no measurable reflection reaches the TDR. The fault must first be conditioned to a lower resistance.
What is the difference between a bridge and a burnout system? A bridge (Murray/Varley loop) measures the fault distance using a healthy conductor as reference — non-destructive. A burnout system changes the fault by applying controlled HV current to carbonize it into a low-resistance path — then the TDR can see it.
When should I burn down a fault instead of using the bridge? Use the bridge first. Burn down when the fault resistance is too high for accurate bridge measurement (typically >100 kΩ) or when repeated attempts fail. Burnout is appropriate when you're replacing the damaged section anyway.
Does burnout damage the cable? It concentrates controlled stress at the fault point by design. With current limiting, damage is confined to the fault zone — which you are repairing. Uncontrolled or repeated over-burning spreads the damage, so current control matters.
Can the MB10 replace a surge generator and TDR? No. The MB10 handles pre-location (bridge) and conditioning (burnout). You still need a TDR for precise distance and a surge generator + acoustic pinpointer for pinpointing. Together they form the complete kit.