A fiber optic current sensor (FOCS) using the Faraday effect in a fiber coil is presented. The current sensor is realized on the base of the all-fiber low-coherent reflection interferometer. An Erbium-doped fiber superfluorescent radiation source and a sensing SPUN hi-bi fiber coil are applied in the interferometer. The detectivity of the sensor is about 70 mA/square root of Hz, the scale factor error is about ±0.5%, the range of measured currents is 01.A - 3000A, and the bandwidth is up to 10kHz. Some physical and technical sources of the FOCS scale factor errors are studied. It is shown that the normalized optical scale factor of the FOSC is virtually independent of the SPUN fiber length for a given coil diameter. The evaulative relation for the FOCS scale factor error owing to the spectrum instability of light wideband source is given.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.