An instrument (1) and a method for detecting partial electric discharges involve acquiring a discharge signal (10), for example picked up by a direct-measuring impedance device (7) through a broadband HF acquisition channel (18), and acquiring the discharge signal (10) in a narrowband LF acquisition channel (180) complying with regulations, using on the LF acquisition channel (180) a trigger controlled in slave mode by a trigger of the broadband HF acquisition channel (18); they also involve acquiring another discharge signal (32) picked up by an indirect-measuring impedance device (8) through a second narrowband LF acquisition channel (180A) and comparing digital signals (34, 34A) generated in the first and second LF acquisition channels (180, 180A), in order to generate a balanced digital signal (36) without components representative of common mode electrical signals present in the measuring circuit.