How to Measure Distance to Fault in Underground Cables
When a cable fails, the first question is always: how far down the line is the fault? The answer to this question determines where your crew digs — and whether the repair takes 2 hours or 2 days.
Measuring distance to fault is the foundation of the entire cable fault location process. Here's how it works, how to do it accurately, and the mistakes that cost crews the most time.
The Principle: Radar for Cables
Distance to fault is measured using Time Domain Reflectometry (TDR) — the same principle as radar.
- The TDR sends an electrical pulse down the cable
- The pulse travels at a known speed (the velocity of propagation)
- When the pulse hits a fault, part of it reflects back
- The TDR times the round trip and calculates distance
The formula is simple:
Distance = (Speed × Time) / 2
The division by 2 accounts for the round trip — the pulse goes to the fault and the reflection comes back.
The Formula in Practice
A TDR measures the round-trip time. The speed depends on the cable's dielectric:
Example: a pulse takes 10 microseconds to return from a fault on an XLPE cable (velocity 0.65 × c).
- Pulse travels at 0.65 × 300,000 km/s = 195,000 km/s
- Round-trip distance = 195,000 km/s × 10 μs = 1.95 km
- Distance to fault = 1.95 / 2 = 975 meters
The #1 Accuracy Killer: Velocity Factor
If you set the wrong velocity factor, EVERY distance reading shifts by the same percentage.
- A 2% velocity error on a 1 km cable = 20 meters of wrong excavation
- A 2% velocity error on a 10 km feeder = 200 meters off
Professional TDRs like the A10 Cable Fault Tester allow velocity factor adjustment in 0.01 steps. Best practice:
- Use the manufacturer's velocity factor for your cable type
- Or measure it: test a known cable length and adjust until the TDR reads correctly
- Recalibrate when switching between cable types
Reading the Result Correctly
The TDR displays distance along the cable, not along the ground surface. This distinction matters:
- The cable may curve, cross roads, or pass under buildings
- "975 meters of cable" may be only 850 meters on the ground — or 1,100
- Cable slack and buried loops add to the discrepancy
Never dig based on TDR distance alone. Always confirm with pinpointing (acoustic-magnetic detection) before excavating.
Two-End Testing for Confidence
When both cable ends are accessible:
- Test from End A: "fault at meter 975"
- Test from End B: "fault at meter 1,025" (of a 2,000 m cable)
The two readings should agree within tolerance. Discrepancies indicate:
- Cable taps or branches
- Non-uniform cable sections
- A mislabeled or undocumented cable
Two-end agreement gives high confidence in the reading before anyone digs.
Distance to Fault: Complete Workflow
The complete fault location process bundles these steps into integrated systems.
FAQ
What is distance to fault measurement? A TDR-based measurement that tells you how far along a cable a fault is located, by timing pulse reflections.
How accurate is TDR distance measurement? Professional TDRs achieve ±0.5 m or ±0.5% of length, whichever applies. Accuracy depends heavily on correct velocity factor calibration.
Why does my measured distance not match the surface distance? Because the cable doesn't run straight — it curves, loops, and crosses terrain. TDR measures cable length, not ground distance. Pinpointing resolves this.
Can I measure distance to fault on a live cable? Standard TDRs require de-energized cables. Specialized live-network systems are available for utilities.