The basic principle of FOCS and OCTs is to measure polarization rotation due to the Faraday effect. The Faraday effect is the rotation of the polarization state of light, β, when it passes through a magnetic field, B, induced by an electrical current. The larger the electric current, the greater the magnetic field, and hence the larger the polarization rotation.
FOCS/OCTs offer significant advantages over traditional current sensing technologies; the sensor element is naturally decoupled from the voltage line, there is minimal electrical interference on the signal line, they offer extremely fast response times with high measurement accuracy, the size and weight of the sensors is reduced in comparison with existing technologies and they do not explode during catastrophic failure, unlike oil-filled electrical insulation towers.