The sensing principle of fiber-optic electric current sensors is based on the Faraday effect. Electric current flowing in a conductor induces a magnetic field, which rotates the plane of polarization of the light traveling in a sensing path encircling the conductor. There are two types of sensing techniques. One is a polarimetric-type measuring the polarization direction change, the other is an interferometric-type detecting the phase difference between counter-propagating circular-polarization lights through a sensing fiber loop. An all-fiber Sagnac interferometer-type fiber-optic electric current sensor was developed and successfully tested. The advantages of this sensor are its performance and light structure, ease of handling, and robustness to an external magnetic field caused by an adjacent electric current. The basic characteristics of the fiber-optic current sensor were compared with those of conventional Hall effect current sensors. The fiber-optic current sensor showed sufficient response less than 1 ms and resolution of 10 A for practical application in a short circuit test, and its strong robustness to an external magnetic field was shown experimentally. The sensor was tested in a possible future measurement and protection applications in railway power systems. The field test configurations and results are described.