A fiber-optic current sensor (FOCS) is a device designed to measure direct current. Utilizing a single-ended optical fiber wrapped around the current conductor,^[1] FOCS exploits the magneto-optic effect (Faraday effect).^[2] The FOCS can measure uni- or bi-directional DC currents up to 600 kA, with an accuracy within ±0.1% of the measured value.^{[3][4]: 2 [5]}

This article contains content that is written like an advertisement. It's not horrible, but quotes extensively and uncritically from ABB marketing material.. (December 2015)

The fiber-optic current sensor uses an interferometer to measure the phase change in the light produced by a magnetic field. As it does not require a magnetic yoke, the FOCS is smaller and lighter than Hall effect current sensors, and its accuracy is not reduced by saturation effects.^[6] The inherent insulating properties of the optical fiber make it easier to maintain electrical isolation. It also does not need recalibration after installation or during its service life.^[4][5]

The optical phase detection circuit, light source and digital signal processor are contained within the sensor electronics; this technology has been proven in highly demanding applications such as navigation systems in the air, on land and at sea.^[7][4]: 5

As FOCS are resistant to effects from magnetic or electrical field interferences, they are ideal for the measurement of electrical currents and high voltages in electrical power stations.^[8][9]

In 2013, ABB introduced a 420 kV Disconnecting Circuit Breaker (DCB) that integrates FOCS technology replacing many conventional current transformers, thereby simplifying the engineering and design of the substation. By reducing the materials needed (including insulation), a 420 kV DCB with integrated FOCS can reduce a substation's footprint by over 50% by minimizing the need for materials, in comparison to conventional solutions involving live tank breakers with disconnectors and current transformers.