An apparatus for detecting partial discharge in on-line high voltage electrical equipment containing a dielectric, such as high voltage transformers. The apparatus includes an ultrasonic transducer and an annular capacitive plate for detecting, respectively, the ultrasonic pulse and the radio frequency pulse generated by the occurrence of partial discharge in the equipment. The outputs of the transducers are analyzed by signal processing means to allow determination of occurrences of true partial discharge from the plethora of electrical noise normally present in on-line voltage equipment