Why Can't My TDR Find the Fault? 6 Real Reasons (and the Fixes)
You've tested the cable twice. The TDR trace looks clean. But the network trips every night, and you know the fault is there.
Frustrating? Absolutely. And more common than most engineers admit. Here are the 6 real reasons a TDR shows nothing — and how to reveal what it's hiding.
Reason 1: The Fault Has High Resistance
The #1 cause. A TDR works by detecting impedance changes. A high-resistance fault (above a few hundred ohms) reflects so little energy that the signal disappears into the noise floor.
Why it happens: aged insulation, water trees, thermal degradation — these create faults with high resistance that don't conduct at the TDR's low test voltage.
The fix: condition the fault first with a high-voltage supply (up to 32 kV). The G35 Fault Location Power Supply burns the fault down to a low-resistance arc that the TDR can then see. Test AFTER conditioning, not before.
Reason 2: The Fault Is Voltage-Dependent
Some faults only conduct at operating voltage. At the TDR's safe 24V test signal, the insulation holds — the fault behaves like it doesn't exist.
Why it happens: partial discharge damage, water-tree bridges that only break down under stress.
The fix: apply HV conditioning to force the breakdown, then re-test. If the fault reappears only at high voltage, it's the classic "invisible under test, deadly in service" failure.
Reason 3: The Fault Is Intermittent
A fault that flashes over occasionally — under load, at temperature, or during surges — may not be present when you test.
Why it happens: loose connections, thermal cycling, moisture migration.
The fix: test at different times and conditions. Use a multi-pulse generator that fires repeated sequences — each discharge is another chance to catch the flashover. The A10 Advanced captures 10 waveforms per discharge, making intermittent faults far harder to miss.
Reason 4: Wrong Velocity Factor
The TDR computes distance using the cable's velocity factor. If it's wrong, reflections appear at wrong distances — and on a long cable, the fault reflection can be pushed into the noise or misidentified.
Why it happens: default velocity factor ≠ your cable type. XLPE is 0.63-0.66; PILC is 0.53-0.58. Using the wrong value shifts everything.
The fix: calibrate the velocity factor against a known cable length before trusting any reading.
Reason 5: The Fault Is in a Cable Section You're Not Testing
A TDR only sees the cable between its connection points. If the fault is on a branch, a tap, or a section beyond a joint you didn't account for, the reflection can be ambiguous.
Why it happens: cable networks have branches, taps, and undocumented connections.
The fix: trace the full cable route first with a locator, map the branches, and test section by section. Confirm you're testing the right cable with an identifier in shared ducts.
Reason 6: The Trace Is There — You Just Can't Read It
Sometimes the fault reflection IS on the trace, mixed with noise, cable joints, and the far-end reflection. Inexperienced operators dismiss it.
Why it happens: joints look like small reflections; noise looks like small reflections; the fault can be any of them.
The fix: compare traces over time — a real fault changes; joints don't. Test from both ends — the fault shows at complementary distances. If your TDR has waveform storage, save and compare.
The Complete Reveal Workflow
A complete fault location system bundles these capabilities — TDR, HV conditioning, multi-pulse, and pinpointing in one kit.
FAQ
Why is my TDR showing no fault on a known-bad cable? Most likely a high-resistance or voltage-dependent fault — invisible at TDR test voltage. Condition it with HV first, then re-test.
Can an intermittent fault hide from a TDR? Yes. If the fault isn't conducting at the moment of the test, the TDR sees a clean cable. Multi-pulse generators increase the chance of catching it.
Do I need to replace my TDR? Usually not. The TDR is fine — the fault needs conditioning. An HV supply and multi-pulse generator reveal what the TDR alone misses.
Why does my TDR reading keep changing? An unstable reading often means an intermittent or partial fault that's changing state. Compare multiple traces and check from both ends.