Time Domain Reflectometry uses a radar principle to identify the location of certain faults along a cable, usually only effective on low resistance faults under 200 ohms. The TDR instrument will transmit a single pulse onto two parallel conductors in a cable and ‘listen’ for the return signal. Changes in impedance of the cable will result in reflections being returned back to the TDR. Time taken for the reflections to return back to the TDR is measured and displayed as a waveform map called a TDR trace. Each type of impedance change will result in a different effect to the trace on the TDR.