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Monitoring of high voltage power cables to prevent major failure is best achieved by use of continuous on-line monitoring. Those diagnostics able to be applied on-line are very limited in number and essentially reduce to partial discharge monitoring. This paper describes the use of on-line PD sensors to monitor a variety of operational 11 kV cable systems and also to investigate typical PD characteristics using laboratory tests. The sensor used was a high frequency current transformer with bandwidth 1-100 MHz. The tests showed that recording of the time domain waveform and the derived frequency domain response gave the best performance in terms of providing PD data able to be related to insulation degradation. Interference was a major problem on site and it was necessary to investigate a number of methods of interference rejection to enable PD magnitudes of useable levels of about 50 pC to be achieved. It was found that use of the power spectrum of the PD waveforms and wavelet analysis were the best techniques. In particular wavelet analysis proved most suitable as it allowed recovery of the signal waveform for use in degradation analysis.