Abstract: Monitoring apparatus for monitoring partial discharge activity in an AC electrical system comprises: a pair (60) of high- and low-frequency sensors; a timing module arranged to receive a cyclic signal from the low-frequency sensor and to provide a timing signal derived from the timing of successive cycles of the cyclic signal; and a monitoring unit (66) arranged to digitise a signal from the high-frequency sensor, to capture the digitised signal as consecutive data blocks of varying duration, and to trigger the capture of each data block in response to the timing signal. A condition monitored electrical system comprises a variable-speed drive (VSD) and a motor (M) operated by the drive, wherein the pair (60) of sensors is arranged at an end of a cable supplying power from the drive to a winding of the motor.

Abstract: Monitoring apparatus for monitoring partial discharge activity in an AC electrical system comprises: a pair (60) of high- and low-frequency sensors; a timing module arranged to receive a cyclic signal from the low-frequency sensor and to provide a timing signal derived from the timing of successive cycles of the cyclic signal; and a monitoring unit (66) arranged to digitise a signal from the high-frequency sensor, to capture the digitised signal as consecutive data blocks of varying duration, and to trigger the capture of each data block in response to the timing signal. A condition monitored electrical system comprises a variable-speed drive (VSD) and a motor (M) operated by the drive, wherein the pair (60) of sensors is arranged at an end of a cable supplying power from the drive to a winding of the motor.