What is a common test to verify distance-to-fault on a copper link?

• A. Megger insulation resistance test
• B. Bit error rate testing
• C. OTDR testing
• D. Distance-to-fault testing using a time-domain reflectometer (TDR) or length measurement

Answer: (D)

Distance-to-fault on a copper link is determined with time-domain reflectometry. You inject a fast electrical pulse into the conductor and watch for reflections caused by a fault or impedance change. The time it takes for the pulse to travel to the fault and back, combined with the cable’s known velocity factor, lets you calculate the distance to the fault. Some testers also translate that time directly into length, giving you the fault location.

Megger insulation resistance testing measures how well the insulation resists current, not where a fault is. Bit error rate testing checks data integrity, not physical location of a fault. OTDR testing is used for fiber and isn’t appropriate for copper links. The TDR approach (or length-based estimation using known velocity) is the standard method for locating faults on copper cables.