## Fiber optic current sensor

December 26th, 2011

In the electric power industry and the electrochemical industry is the measurement problem of strong currents of a few hundred kA.The company  ABB  has developed for this purpose the Fiber-Optic Current Sensor (FOCS).

The method of measuring the current in the FOCS is based on the magneto-optical effect, named after the English physicist Michael Faraday, Faraday effect.

The Faraday effect is the phenomenon of the rotation of the polarization plane of the linearly polarized light wave passing through a medium, in the presence of a magnetic field. Linear light wave may be represented by a pair of co-propagating light waves from the left and right circular polarization. In a magnetic field, the two circular waves travel at different speeds, and thus accumulated  the phase difference between them, resulting in the rotation of the resultant wave with linear polarization at an angle $\Delta \phi$. In the current sensor, the  light extends in a closed circuit, which includes an optical fiber conductor. In reflection mode, the phase difference of  $\Delta \phi$ can be determined with the help of the circulation of the magnetic field in a closed loop:

$\Delta \phi = 4VN \oint \vec H d\vec l = 4VNI$

where: V – Verde constant, which depends on the material; N – the number of turns, which describes a light wave around the conductor; I – current.

Since the light path is closed, the signal value depends on the current strength and the number of turns of the optical fiber and does not depend on the geometrical parameters, such as loop diameter or shape. FOCS type fiber sensor shown in Fig. 1A. Fig. 1B illustrates how to use the sensor for measuring current: a coil of the optical fiber is inserted on the live parts.

А)                                                  Б)

Fig.1. General view of the fiber optic sensor FOCS (1A) and the method of application (1B).