Velocity Factor: The Hidden 5% That Determines Whether Your TDR Distance Is Right

Every cable fault locator engineer knows the embarrassment: the TDR says "the fault is at meter 780," you excavate at 780, and there's nothing there. The cable is fine at 780. The fault is actually at 830.

The distance wasn't wrong because the TDR was broken. It was wrong because the velocity factor was wrong.

1. What Velocity Factor Is

A TDR sends a pulse and measures how long it takes to bounce back. It then multiplies time × speed to get distance. The speed is the VF — the propagation velocity of the signal through that specific cable, relative to the speed of light.

The VF is not a universal constant. It depends on:

• Insulation type: XLPE, EPR, paper (PILC) and PVC cables all propagate at different speeds
• Insulation design: solid extruded vs mass-impregnated insulation changes the dielectric constant
• Cable age and condition: aged insulation can shift VF measurably over decades

Typical starting points (always verify by measurement):

2. The Error Math — Why 5% Matters

Distance error is directly proportional to VF error:

If your VF is off by 5%, your distance is off by 5%.

On a 2 km urban cable, a 100 m error means digging in the wrong street. On a 10 km feeder, it means days of wasted survey. This is why VF calibration is not a nice-to-have — it's the difference between a search box and a search corridor.

3. How to Calibrate the VF (Known-Length Method)

The most reliable method needs only one thing: a known length of the same cable.

1. Connect the TDR to a healthy conductor of the cable under test (or a spare length of identical cable).
2. Measure the cable length from records, or better, from a known physical reference — a joint position, a manhole, a termination.
3. Adjust the TDR's VF setting until the displayed distance matches the known length.
4. Save this VF as the working value for that cable circuit.

If no known length is available, use the next-best options, in order:

• Joint positions from as-built records: walk the route and use 2-3 documented joint locations to reverse-engineer the VF
• Factory test certificates: many cable suppliers state the measured VF on delivery tests
• Catalog values: only as a starting point — never trust them alone on long or aged cables

4. Field Habits That Keep You Honest

• Calibrate per cable circuit, not per cable type. Two XLPE cables from different manufacturers can differ by 1-2% in VF — that's 20 m on a 2 km run.
• Re-verify VF after major repairs. A new joint slightly changes the electrical length.
• Use waveform storage to validate. Compare a fresh trace against the commissioning baseline: if reflections don't align with known joints, suspect the VF before suspecting new faults.
• Document the VF used on every trace you save. A trace without its VF setting is ambiguous years later.

5. How TriEdge Helps

The A10 Cable Fault Flash Tester (0–100 km, ±0.5 m) and A10 Advanced include:

• Velocity factor presets for common cable types — quick start
• Manual VF adjustment down to fine resolution — for calibrated measurement
• Waveform storage — save traces with their VF settings and compare against baseline
• Linux-based operating system — stable, field-proven operation

Factory-direct pricing, 40–60% below equivalent Western brands.

View A10 → · Full catalog (PDF)

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FAQ

What is the velocity factor of a TDR? The speed at which the TDR pulse travels through a cable, expressed as a fraction of the speed of light. It converts measured time into distance.

Why is my TDR distance wrong? The most common cause is an incorrect velocity factor. Check whether the VF was calibrated for the specific cable circuit — not just the cable type.

How do I calibrate the velocity factor? Measure a known length of the cable (from records or a physical reference like a joint or termination) and adjust the TDR's VF until the displayed distance matches. Save that value for the circuit.

Does velocity factor change over time? Yes, gradually. Insulation aging and moisture ingress can shift VF over decades. Re-verify periodically, especially before trusting a measurement on an old cable.

Is velocity factor the same for all cables of one type? No. Manufacturing differences mean two XLPE cables can differ by 1-2%. Calibrate per circuit, not per type.