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A highly accurate fiber-optic current sensor for direct currents up to 500 kA is presented. Applications include the control of the electrolysis process for the production of metals such as aluminum, copper, zinc, magnesium, and others. The sensor offers significant advantages with regard to performance and ease of use compared to state-of-the-art Hall-effect-based current transducers. The sensor makes use of the Faraday effect in an optical fiber loop around the current-carrying bus bars. A novel scheme of a polarization-rotated reflection interferometer and fiber gyroscope technology is used to measure the magneto-optic phase shifts. An appropriate technique has been developed for packaging the sensing fiber in a flexible strip of fiber-reinforced epoxy for loop diameters of up to several meters. Sensor accuracy and repeatability are well within 0.1% over a wide range of currents and temperatures. The sensor calibration is valid, regardless of the given magnetic field distribution, and remains stable under repeated manipulation of the flexible sensing strip.