Abstract: A method essentially consisting in measuring the duration of the go and return travel of a light impulse between an accessible end of an optical cable and a break in the said cable. The distance of the break from the latter end can be calculated if the refractive index of the light conducting material is known. The equipment essentially comprises a source of light impulses forming optical signals, an injector element coupling the source to the cable, a separator dividing each optical signal into two signals respectively traveling on a first and a second path, the latter of which includes the above-mentioned go and return travel, detector elements detecting the signals at the outputs of their paths, and a viewing element displaying the detector outputs on a common screen.