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FAQ - FOCS

Please find below the frequently asked questions and answers regarding FOCS.

General


What's the measuring principle?

The ABB FOCS is an unique Fiber-Optic Current measurement product using the Faraday effect in an optical fibre.

The sensing fiber encloses the current-carrying bus bars. The Faraday effect is the phenomenon that in a medium such as glass right and left circularly polarized light waves travel at different speeds if a magnetic field is applied along the propagation direction. As a result the waves accumulate a phase difference.

Two light waves with orthogonal linear polarizations travel from the optoelectronics module, which includes a semiconductor light source, via an interconnecting fiber to the single-ended sensing fiber looped around the bus bar. A single loop is commonly sufficient for high dc currents. At the entrance of the sensing fiber a fiber-optic phase retarder converts the orthogonal linear waves into left and right circularly polarized light. The circular waves travel through the coil of sensing fiber, are reflected at the end of the fiber and then retrace their optical path back to the coil entrance. Here, they are again converted into orthogonal linearly polarized waves.

Since the circular waves travel at somewhat different speeds through the sensing fiber if a dc current, I, is flowing the two returning light waves have accumulated a phase difference. The phase difference is proportional to the line integral of the magnetic field along the sensing fiber and is therefore a direct measure for the current.

The returning waves are brought to interference in the optoelectronics module. The signal processor then converts the optical phase difference into a digital signal. A particular advantage of operating the sensing coil in reflection is, besides the simplicity of the arrangement, the fact that the sensor signal is largely immune to mechanical perturbations such as shock and vibration. While the non-reciprocal Faraday optical phase shifts double on the ways forward and backward, phase shifts caused by mechanical disturbances are reciprocal and cancel each other.

The technology of the optoelectronic detection circuit is the same as used in fiber gyroscopes and has been well proven in demanding navigation systems. Fiber gyroscopes use the Sagnac effect seen by two counter-propagating light waves in a fiber coil to measure rotation velocities.


How accurate is the FOCS?

The accuracy is 0.1% of the reading in the range from 1 to 120% of the rated current.

Is an analysis of magnetic field distribution required to assist in selecting and placing the sensor head?

No. The system is immune to electromagnetic interference and also requires no magnetic centring.

What is the minimal current resolution?

Minimal dc current resolution depends on the FOCS configuration, i.e. for a 500 kAdc type the smallest value is below 100A.
Lower rated FOCS have lower current resolutions.

Mechanical


Do parallel busbars or busbar corners influence the accuracy of the FOCS?

No.

How close or far from the bus must the fiber optic cable be installed?

The sensor head housing is mounted around the bus bar (without an civil bus bar work). The size of the housing thus determines the distance of the fiber cable from the bus. The sensor performance does not depend on this distance.

How difficult is it to install the FOCS?

Installation is easy, since the sensing fiber is packaged in a single-ended flexible cable by a proprietary ABB technique. This cable is accommodated in a modular sensor head housing of fiber re-enforced epoxy.
The installation is done by assembling the sensor head housing (delivered in parts) around the bus bar and inserting a single loop of the current-sensitive cable section into the housing. There is no possibility and also no need to touch and or manipulate the optical sensing fiber itself.
Preferably, installation is performed when all supplies are de-energized.

What is the mounting position of the measuring head?

Preferably, the plane of the sensing head should be perpendicular to the bus bar. However, no negative effects occur from non-perpendicular orientation.

Is a special structure needed for installation?

No. The sensor is small in size and weight. It can easily be attached around the bus bar and can also easily be moved to another location if required.

What is the usual timescale for installation?

About half a day.

Electronic


What is the FOCS power consumption?

The FOCS is supplied with 24Vdc and the power consumption is about 20W, therefore the heat dissipation is negligible small. Other supply voltages on request.

What measuring outputs does the FOCS provide?

The FOCS provides today's state-of-the-art Industrial IT standard outputs.
Please refer to the FOCS technical data.

Calibration


How easy is calibration performed?

Each ABB FOCS is tested and calibrated in our works before shipment. During the calibration measurements the sensor will be subjected to temperature cycles and varying load current.

Can an electrician do the calibration or will we need ABB support?

Upon delivery, the FOCS is completely tested and calibrated. There is no need for calibration after initial installation. Re-calibration can only be done by ABB.

Is on-site calibration required?

No. The FOCS is pre-calibrated and there is no need for re-calibration during its lifetime.

Will it be necessary / possible to re- calibrate the FOCS after some time on site?

Yes, it is possible to re-calibrate the FOCS on site after some years of operation. Installing temporarily reference equipment performs this task. Preferably re-calibration is performed when all supplies are de-energized.
The re-calibration period depends on various environmental influences. ABB recommends to re-calibrate essential measuring equipment every 3 to 5 years.

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