What Is a TDR Cable Fault Locator and How Does It Work?
If you work with underground power cables, you've heard the term TDR — Time Domain Reflectometer. It's the single most important tool in cable fault pre-location. But what exactly does it do, and how does it work?
This article explains the principle, the types of faults it detects, and what a TDR reading actually tells you.
The Principle: Radar for Cables
A TDR works on the same principle as radar or a sonar depth finder — except it sends electrical pulses instead of radio waves or sound.
Here's the process:
- The TDR sends a short electrical pulse down the cable
- When the pulse meets a change in the cable's impedance — a break, a short, a fault, even a cable joint — part of the pulse reflects back
- The TDR measures the time between sending the pulse and receiving the reflection
- Since the pulse travels at a known speed through the cable (about 2/3 the speed of light, depending on the cable), the TDR calculates the distance: Distance = Speed × Time / 2
The result is displayed as a waveform, with the distance to each reflection point marked along the trace.
What a TDR Can Detect
What a TDR cannot see: high-resistance faults (above a few hundred ohms) that don't create a measurable reflection — these require conditioning with a high-voltage supply first.
Reading a TDR Trace
A TDR trace shows:
- The launch pulse — the initial pulse at the near end (distance 0)
- The far end — the end of the cable, shown as a reflection at the full cable length
- Fault reflections — bumps or dips on the trace between the two ends
- Cable joints — small reflections that help you correlate distance with known joint locations
Most modern TDRs mark the fault distance automatically. Professional units also let you:
- Adjust the velocity factor (propagation speed) to match your cable type
- Zoom into waveform sections for precise measurements
- Save and compare traces over time to monitor cable degradation
The Limitation: Distance Is Not Location
A TDR gives you a distance along the cable — "the fault is at meter 780." It does not tell you where that point is on the ground.
The cable could be straight, curved, or looped underground. That's why TDR pre-location is always followed by pinpointing with an acoustic-magnetic pinpointer, which finds the exact surface location.
The professional workflow is:
- Pre-locate with a TDR — "meter 780"
- Walk to the area around meter 780
- Pinpoint with an acoustic-magnetic pinpointer — "dig exactly here"
Choosing a TDR
Professional-grade features to look for:
- Range — 0–100 km covers distribution and transmission feeders
- Accuracy — ±0.5 m or better
- Velocity factor presets — quick setup for common cable types
- Waveform storage — compare traces over time
- Field durability — IP-rated casing, long battery life, bright screen
The A10 Cable Fault Flash Tester from TriEdge Technologies offers 0–100 km range, ±0.5 m accuracy, and a Linux-based stable operating system — at factory-direct pricing 40–60% below equivalent Western models.
FAQ
What does TDR stand for? Time Domain Reflectometer — an instrument that measures distance by timing electrical pulse reflections.
How accurate is a TDR? Professional units offer ±0.5 m accuracy, sufficient to narrow the search area before pinpointing.
Can a TDR detect all cable faults? No. High-resistance and intermittent faults need to be conditioned (burned down) with high voltage before a TDR can see them.
Is a TDR the same as a cable locator? No. A TDR measures distance along a cable. A cable locator traces the cable's physical path and depth on the ground. They solve different problems.