TDR (time-domain reflectometer) cable fault locators are electronic instruments that locate faults in metallic cables (such as a twisted wire). They use time-domain reflectometry (reflection of sound waves) to detect issues in cables. TDR cable fault locators are commonly used to locate issues in a connector, printed circuit board, or other electrical paths.
TDR cable fault locators work by measuring the reflection of waves along a conductor. In order to measure the reflections, the TDR will transmit a signal to the conductor and listen for any reflected sound waves as they return. If the conductor is operating correctly there will be no reflection displayed on the screen of the cable fault locator and the incident signal of the cable will be discharged. If there are reflections present, then the signal will be reflected back to the TDR fault locator and diagnosed for errors that will be displayed on the screen.
TDR cable fault locators are commonly used for in-place testing of very long cable, where the length of cable to be tested makes them impractical to remove.