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The electrical distribution system on the Isle of Man is linked to the UK power system via a 90 kV AC subsea interconnect cable. The total length of the interconnector is approximately 108 km of which about 104 km is subsea cable. Because of the importance of this installation the Manx Electricity Authority requires a strategy to deal with any fault that may arise with this cable. In the event of a fault one of the first requirements would be to determine a reliable estimate of the actual fault location on which the remaining actions would then depend. It was found that the extremely long length of cable posed its own difficulties in this respect and that fault location equipment designed for much shorter cables routes would not be suitable. The Manx Electricity Authority therefore initiated a programme of work to build a theoretical model to predict the magnitude of the time domain reflectometery, TDR pulse likely to be effective over the whole length of the cable and then carry out field trials of TDR measurements during a maintenance outage. Using specialist pulse generators and a high-speed storage oscilloscope during this outage it was possible to locate the remote end and a known subsea joint at about 60 km from the test point. The joint was located to within 150 m of the position determined from the installation records, an accuracy of 0.14% of the total length.