This article explores the measurement of electric current using optical fibers, primarily through the Faraday effect, also known as the magneto-optic effect. A fiber-optic current sensor (FOCS) is a device designed to measure direct current. Radiation absorption creates electronic excited states that are trapped by localized defects for extended periods of time. Heating the material enables the trapped states to interact with phonons and decay into lower-energy. High current dc measurement systems are a well-established element of process control and regulation in the electro-chemical industry. However, their complexity and the difficulties of commissioning has led ABB to seek an improved solution.