Receive analog signal representing real-time electrical current of electrical power transmission/distribution line; sample and digitise analog signal to generate power line current data; generate data frames of subsets of the power line current data for successive overlapping time slices, process data frames to generate frequency domain data representing frequency components for different frequency ranges; process frequency domain data to generate energy data representing average energies of frequency ranges over corresponding time slice; generate two-dimensional feature for machine learning, first dimension represents plurality of the successive time slices, second dimension represents different frequency ranges, values of the two-dimensional feature represent logarithms of corresponding average energies of frequency components of electrical current over corresponding time slices; apply deep learning classifier to two-dimensional feature to detect high-impedance fault in electrical power transmission/distribution line, and if detected: send fault signal to relay or circuit breaker to cut power to the electrical power transmission/distribution line.