The theme of the article is protection of electrical circuits against negative sequence currents both with sinusoidal and trapezoidal waveforms. The suggested solution is enhancement of instrument current transducers used in industrial relaying and control applications. The instrument negative sequence current transducer connected to Rogowski coils designed by the authors is presented in the article. To protect circuits with extremely non-sinusoidal currents (such as trapezoidal waveforms), instrument transducers with 2nd order low pass filter are suggested. In the presence of higher-order harmonics in the protected three-phase circuit the low pass filter will reduce the THD in the output voltage of instrument negative sequence current transducer (as compared to harmonics in currents of the protected circuit) and compensate for the differentiation effect typical of such transducer schemes. The paper may be of interest for investigators and engineers engaged in research, design and commissioning of protection and control equipment, current instrument and measurement devices used in industrial applications, and also for undergraduate and postgraduate students in electrical engineering. (C) 2016 Elsevier Ltd. All rights reserved.