Abstract: Defects in a cable are located by causing the defect to issue voltage pulses propagating in opposite directions; one pulse is reflected at one end and arrives at the other cable end later than the other pulse. The delay is indicative of the location of the defect and is ascertained by forming the auto-correlation function of a.c. signals which appear at that other cable end. For a phase setting of the auto-correlation function equal to that delay, the function has a pronounced peak. Calibration on the basis of correlating a stimulus with its reflection provides a maximum delay serving as a reference to locate defects by the auto-correlation method.