Abstract: To locate a leak in a length of pressure-assisted oil-filled electric power cable, the static fluid pressure is measured at the two ends of the length and the flow into or out of the cable is measured at one end for at least two different pressure supply conditions giving different flow rates while maintaining zero flow rate at the other end of the cable. The measurements are then repeated with zero measured flow rate at said other end. From these measurements, the relationship between flow rate at each end of the cable and the pressure drop between the ends due to flow can be determined and this enables the position of the leak to be calculated. Apparatus for making the necessary measurements is described.