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. A sensor unit (60) for condition monitoring of a high-voltage electrical system comprises a split-core current transformer with a low-frequency winding, arranged for sensing a high-voltage AC waveform in a cable about which the transformer is positionable, and a high-frequency winding arranged for sensing pulses in the cable that originate from partial discharge activity.