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Operating principle

How can lightning current be measured? How do surge voltages occur? How do surge voltages get in your devices and systems? You've probably wondered about the answers to these questions. The following pages provide comprehensive information on lightning current detection.

Structure of the measuring section

The measuring section consists of a transparent medium (dielectric) with polarisers or polarising filters fitted at either end. The measuring section is positioned at an angle of 90° to the current flow direction in the down conductor. In this way, the propagation direction of a light wave in the measuring section lies parallel to the magnetic field of the surge current in the down conductor.

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Polarisers

Linear polarizer  

Linear polariser

Polarisers or polarising filters are optical elements which produce polarisation. To do this, electromagnetic waves are separated into linear, elliptical or circular polarised light through absorption or beam splitting. In this case, the light is linearly polarised in order to use the Faraday effect. This means that only linearly polarised light passes through the polarising filter.

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Magnetic effect on the plane of polarisation

Magnetic effect on the plane of polarization  

Magnetic effect on the plane of polarisation

The light wave causes the electrons in the dielectric to oscillate. The magnetic field changes the movement of the electrons within the dielectric. This in turn influences the plane of polarisation of the light. In principle, the plane of polarisation can be rotated in any direction.

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Magneto-optic effect in the LM-S

The graphical model shows all the important elements and variables of the magneto-optic effect in a lightning monitoring system. A light wave Φ with a defined light intensity is guided onto the measuring section via fiber optics.

The polarising filter P1 at the input of the measuring section linearly polarises the directed light. The light wave polarised in this way causes the electrons in the medium to oscillate and travels through the measuring section medium in the plane of polarisation. The plane of polarisation can be influenced magnetically.

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The magnetic field of a surge current rotates the plane of polarisation of the light wave within the medium about the longitudinal axis. The direction of rotation depends on the direction of the magnetic field lines and therefore the direction of the current flow. For example, surge currents from negative and positive lightning create magnetic field lines with different directions.

The greater the current I, the stronger the magnetic field B and the greater the angle of rotation β. The magnetic field B1 causes clockwise rotation and magnetic field B2 causes anticlockwise rotation of the light wave.

The second linear polarising filter P2 is positioned at the output of the measuring section at an angle of 45° to the input polarising filter. Only 50% of the light from a uninfluenced light wave thereby passes through the output polarising filter. The amount of light that passes through the output polarising filter is dependent on the rotation of the light wave. This results in a measurable light signal which can be evaluated.

Measuring result and evaluation

Measuring result and evaluation  

Schematic diagram: change in the amount of light that passes through the output polarising filter

Positive lightning results in clockwise rotation of the polarised light signal. The amount of light that passes through the second polarising filter increases and amounts to between 50 and 100%. When the angle of rotation of the light signal reaches 45°, this corresponds to a measured value of 100% for a positive lightning strike.

Negative lightning results in anticlockwise rotation of the polarised light signal. The amount of light that passes through the second polarising filter decreases and amounts to between 50 and 0%. When the angle of rotation of the light signal reaches -45°, this corresponds to a measured value of 100% for a negative lightning strike.

The amount of light that makes it through the output polarising filter is measured. The typical parameters of the detected lighting surge current are derived from the progression of the amount of light over time. These are the maximum amperage, polarity, lightning current rate of rise, charge, and specific energy.

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Influencing variables

The most important influencing variables are the medium material, the wavelength of the light, the length of the light path through the medium, and the strength of the magnetic field. Further theoretical principles and influencing variables are explained below. Click on the relevant headings to learn more.

The electric field vector E describes the progression and the position of the affected light wave. It is represented by an arrow (see graphical model).

Any weakly or non-conductive, non-metal substance whose charge carriers generally cannot move freely is referred to as a dielectric. This can be a gas, a liquid or a solid. These substances are typically non-magnetic and are acted upon by electrical or electromagnetic fields.

The Verdet constant V corresponds to the rotation per unit of the magnetic flux density. It describes the strength of the Faraday effect for the dielectric to be evaluated. Its value depends on the wavelength of the electromagnetic waves in the medium.

The angle of rotation β, about which the plane of polarization rotates, is calculated by:
 
                                                    β = V x d x B
 

d is the length of the light path through the medium, B is the magnetic flux density, and V is the Verdet constant.

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