Abstract: In order to test the leakage between cores of a multicore cable, a test voltage is applied by a first switch in turn to the cores of a cable whose other end is disconnected. A second switch selects the remaining cores in turn and, after each new selection is made by the second switch, the selected core is momentarily connected to a predetermined voltage, e.g., ground or earth potential, by a third switch. This allows the capacitor formed by the two cores to be rapidly charged so that current measuring devices for detecting leakage current do not give spurious readings caused by the charging current. Also, all pair-combinations of the cores may be checked rapidly, allowing cables with large numbers of cores to be tested efficiently.