For detecting faults on low voltage alternating current electric power distribution cables, recorders are provided, each triggered by a selected occurrence, e.g. a decrease in signal amplitude or a change in the waveform, to cease recording, after a short delay, of the waveform data in digital form, so that a store holds digital data enabling the waveform of the last few cycles covering the period before and after the selected occurrence. Using a voltage gradient technique analysis of the data enables the position of the fault to be located.