Reading TDR Waveforms: How to Tell a Joint from a Fault
The TDR screen shows a waveform. To the experienced eye, it's a story of the cable's entire life — every splice, every repair, every weakness. To the untrained eye, it's a confusing line with bumps on it.
The difference between the two is the difference between a 2-hour repair and a 2-day search. This guide teaches you how to read the trace.
1. The Anatomy of a TDR Trace
2. Polarity Tells You the Fault Type
The direction of the reflection tells you whether the impedance went up or down:
- Positive (upward) reflection — impedance increased: open circuit, broken conductor, or a cable with higher impedance
- Negative (downward) reflection — impedance decreased: short circuit, low-resistance fault, or a splice with lower impedance
This is why the same trace can instantly tell a veteran "open at 780 m" from "short at 780 m."
3. Joint vs Fault — The Three-Question Test
When you see a reflection, don't guess. Ask:
1. Is it at a documented position? If the reflection sits exactly at a known joint coordinate from the as-built records, it's almost certainly that joint. Joints are your friend — they help you calibrate distance.
2. How strong is it? A healthy joint produces a small reflection — a few percent of the launch pulse. A fault produces a strong reflection, often 50-100% of the pulse. If a reflection is disproportionately large, suspect a degraded joint or fault at that point.
3. Was it there before? This is the decisive question. If you have a commissioning baseline trace, compare: any NEW reflection that wasn't in the baseline is a fault or developing defect — even if it's small.
4. The Baseline Method (Professional Standard)
The professional workflow is not about interpreting a single trace in isolation — it's about comparison:
- At commissioning, save a baseline trace for every phase with its velocity factor recorded
- On every fault job, shoot the same trace and overlay it against the baseline
- Identify new reflections — these are the suspects
- Correlate with records — distance to the new reflection + route chart = dig point
This is why A10 series TDRs include waveform storage as a core feature: a TDR without trace comparison is a flashlight without a map.
5. Common Mistakes to Avoid
- Trusting a single trace — always shoot multiple times and compare
- Ignoring the velocity factor — a 5% VF error shifts every distance (see our velocity factor guide)
- Mistaking a large joint reflection for a fault — a degraded joint can look like a fault; check joint records first
- Forgetting to save the trace — the next engineer (or you, in three years) will need it
6. When the Trace Looks Clean
If the cable is dead but the TDR shows nothing, you're probably facing a high-resistance fault — above a few hundred ohms, it reflects too little energy to see. The solution is conditioning: burn down or bridge-measure the fault into a locatable state (see our high-resistance fault guide).
How TriEdge Helps
The A10 and A10 Advanced TDRs make trace comparison practical:
- 0–100 km range, ±0.5 m accuracy
- Waveform storage and comparison — overlay today's trace on the commissioning baseline
- Velocity factor presets + fine manual calibration
- Linux-based operating system for stable field operation
Factory-direct pricing, 40–60% below equivalent Western brands.
View A10 → · Full catalog (PDF)
FAQ
How can I tell if a reflection is a joint or a fault? Check three things: (1) is it at a documented joint position? (2) how strong is the reflection? (3) was it present in the commissioning baseline? A strong reflection at an unexpected position that wasn't in the baseline is a fault.
What does a positive reflection mean on a TDR? A positive (upward) reflection means impedance increased — typically an open circuit or broken conductor. A negative reflection means a short circuit or low-resistance fault.
Why does my TDR show nothing even though the cable is dead? You likely have a high-resistance fault (above a few hundred ohms), which reflects too little energy to measure. Condition the fault first with a bridge or burnout method.
How important is waveform storage? Critical for professional work. Trace comparison against a baseline is the most reliable way to distinguish new faults from normal cable features.