A
system (2) for detecting insulation defects in an underground
power cable (12) having one or more single conductors (16, 18, 20) surrounded by a conductive shield (22). The
system (2) includes one or more external clamp-on sensors (4, 4', 4'', 4''') fastened to the outside of the
power cable (12) or positioned in close proximity to the
power cable (12). The clamp-on sensors (4, 4', 4'', 4''') are configured to provide two or more current measurements from outside the power cable (12) without being electrically connected to any of the one or more conductors (16, 18, 20) of the power cable (12). The
system (2) includes a
signal processing unit (50). The sensors (4, 4', 4'', 4''') are configured to detect
partial discharge events (6, 6', 6'', 6''', 6''''). The
signal processing unit (50) is adapted to use a mathematical
statistical model (52) to process the measurements obtained by the sensors (4, 4', 4'', 4''') to identify whether the current measurements are caused by a
partial discharge event (6, 6', 6'', 6''', 6'''') caused in a leakage structure (46) of the power cable (12).