Abstract: The electronic trip device comprises in series for each phase of a power system a sampler, a squaring circuit and a finite impulse response (FIR) filter. A maximum circuit is connected to the output of the FIR filters and to the input of an infinite impulse response filter. A comparison circuit for comparison to thresholds is connected to the output of the IIR filter. It is thus possible to obtain both a good precision and good filtering of the harmonics of the frequencies of conventional power systems (50 Hz, 60 Hz and 400 Hz), and to reduce the necessary computing power, and consequently the cost of the trip device.