The invention relates to a detector arrangement for detecting an object in an area in which Inductive Power Transfer (IPT) takes place. Furthermore, the invention relates to an IPT arrangement for transferring energy from a primary side conductor assembly that produces an electromagnetic field, while an electric current flows through the conductor assembly, to a secondary side receiving device, that receives the electromagnetic field and produces an electric current by magnetic induction. In addition, the invention relates to a method of detecting an object in an IPT area. The invention also relates to a method of manufacturing a detector arrangement for detecting an object and to a method of manufacturing the transfer arrangement mentioned before. In particular, the invention relates to IPT systems which are operated at high transfer of powers, in particular in the range of 1 to 500 kW, for example 3 to 200 kW. The object is, in particular, an object having an elevated temperature.

WO 2010/031595 A2 discloses an arrangement for providing a vehicle, in particular a track bound vehicle, with electric energy, wherein the arrangement comprises a receiving device adapted to receive an alternating electromagnetic field and to produce an alternating electric current by electromagnetic induction.

The present invention can be applied in particular to the field of energy transfer to any land vehicle, in particular track bound vehicles, such as rail vehicles (e.g. trams), but also to road automobiles, such as individual (private) passenger cars or public transport vehicles (e.g. busses). Preferably, the primary side conductor arrangement of the generating device which produces the alternating electromagnetic filed is integrated in the track, road or parking area of the vehicle so that the electric lines of the primary side conductor arrangement extend in a plane which is nearly parallel to the surface of the road, track or parking area on which the vehicle may travel or may be parked.

Unlike conventional transformers for transferring energy from a primary side to a secondary side, there is a gap between the primary side conductor assembly and the secondary side receiving device that receives the electromagnetic field produced by the conductor assembly. During operation of the arrangement, foreign electrically conducting material may be present in the gap, such as metal coins. In this case, the electromagnetic field produced by the conductor assembly will induce electric currents (in particular eddy currents) in the material, the electric currents will be damped due to the electric resistance of the material and, as a result, the temperature of the material will increase. In particular in case of high transfer power (such as in the range of more than 10 kW, especially of more than 100 kW), which may happen while the electrochemical energy storage of a parked vehicle is charged, the temperature of such material may increase to some hundred degrees centigrade. After charging, the vehicle may move on and there is a significant risk that the hot material causes damage and injuries. In addition, the foreign material reduces the efficiency of the IPT from the primary side to the secondary side.

WO 2012/004092 A2 describes a device for detecting presence of a foreign object in an inductively coupled power transfer environment. The device is provided with a thermally conducting sheet on a charging unit. A device to be charged can be coupled to the primary for charging only if it is within the area of the thermal conducting sheet. When a metallic foreign object is placed in the charging region it acts as a parasitic load and its temperature starts to increase. Since the object is in contact with the thermal conducting sheet, the temperature of the sheet will also increase. This is detected by temperature sensors provided to the thermal conductive sheet and triggers a controller to switch off the primary coil and stop the charging.

One disadvantage of the solution described in WO 2012/004092 A2 is that the thermally conductive sheet which is used for temperature detection is made of brittle material.

Furthermore, location detection of the region of elevated temperature is not possible with a single thermally conductive sheet. In addition, the response time with respect to the detection of an object having an elevated temperature is large.

It is therefore an object of the present invention to provide a robust detector arrangement for detecting a foreign object in an IPT area, a transfer arrangement for transferring energy from a primary side conductor assembly to a secondary side receiving device, a method of detecting a foreign object in an IPT area, a method of manufacturing the detector arrangement and the transfer arrangement, which reduce or eliminate the risk of damage and injuries and which can be used in rough environment, e.g. at the bottom of a road vehicle or at the surface of a road, parking area or railway. In particular, an object having an elevated temperature shall be detected. According to a basic idea of the invention, an optical waveguide is used for detecting an elevated temperature and/or a temperature rise, in particular a temperature rise per time interval or the time derivative of the temperature. During operation of the optical waveguide, electromagnetic waves (in particular light in the visible range) are guided by the optical waveguide during propagation of the waves and the temperature influences the propagation of the waves within the optical waveguide (i.e. the propagation and in particular the backscattering of waves depends on the temperature). Therefore, by detecting and/or analysing the waves which have passed the optical waveguide from one end to an opposite end and/or from one side to another side of the optical waveguide, and/or by detecting and/or analysing fractions of the waves which are scattered and/or reflected by the material of the optical waveguide, the temperature and/or temperature rise is determined.

According to another idea of the invention, the optical waveguide is used for detecting a foreign object in the IPT area by detecting a deformation of the optical waveguide. In this case, a foreign object which exerts forces onto the optical waveguide, due to its weight, can be detected. Therefore, it is preferred that the optical waveguide is placed at the bottom of and/or below the IPT area.

Furthermore, it is possible that the optical waveguide is used for both purposes, i.e. for detecting an elevated temperature or a temperature rise on one hand and for detecting a deformation by weight forces on the other hand. In particular, the foreign object may move into the IPT area while having ambient temperature. As soon as the foreign object exerts its weight forces onto the optical waveguide, this will cause a deformation. During the same time the temperature of the object rises, provided that the object comprises an electrically conducting material. The elevated temperature or the temperature rise will amplify the effect on the light propagation within the optical waveguide. This means that the detection of deformation can facilitate and accelerate the detection of the foreign object. In addition, foreign objects can be detected which are not affected by a temperature rise.

In particular, the optical waveguide may have an elongated geometry. Examples of an elongated geometry are optical fibres, optical tubes and optical rods. Optionally, the elongated optical waveguide may be coated by a coating material, such as polyimide.

In particular, a single elongated optical waveguide can comprise curves so that a

corresponding large area next to the optical waveguide can be monitored for temperature and/or temperature rise and/or can be monitored for deformation. In particular, due to the curves, the elongated optical waveguide (in particular an optical fiber) can be wound in a spiral or, alternatively, may comprise several sections which extend in parallel to each other and which are connected to each other by the curves. The distance between the centre lines of parallel sections may be in the range of 10 to 20 mm, in particular 15 mm. In this manner, a high resolution of temperature detection can be achieved if the temperature profile along the elongated optical waveguide is determined. In addition or alternatively, the area onto which external forces can be exerted is increased, so that an increased area can be used for detecting the foreign object.

An optical waveguide having an elongated geometry comprises a first end of the elongated geometry or a first end of a section of the elongated geometry and a second end of the elongated geometry or of the section. Electromagnetic waves, which are coupled into the elongated geometry or into the section at the first end, propagate to the second end. The propagating waves are damped and/or reflected and/or the scattered and corresponding fractions of the waves can be detected at the second end, respectively at the first end.

A corresponding wave detector can be used for detecting the waves or their fractions, in particular their intensity. In this description, "light" is used as a synonym for electromagnetic waves which propagates through the optical waveguide. Light in the visible range is the type which is preferred for the purposes of the present invention. According to a further aspect of the invention, an evaluation logic for evaluating the type and/or intensity of the light detected by the light detector is combined with the light detector. The evaluation logic determines the temperature and/or temperature rise from the evaluated type (in particular frequency or frequency distribution) and/or intensity of the detected light. In addition or alternatively, the evaluation logic determines the deformation from the evaluated type (in particular frequency or frequency distribution) and/or intensity of the detected light.

In particular, the evaluation logic can be connected (directly or indirectly) to a control of the primary side conductor assembly. If a predetermined criterion is fulfilled, the evaluation logic and/or the control trigger(s) the switching of the primary side conductor assembly or the reduction of the power produced by the primary side conductor assembly. For example, the predetermined criterion is fulfilled if the detector arrangement has measured a temperature at or above a predetermined threshold value. Alternatively or in addition, the predetermined criterion may be fulfilled if the detector arrangement has determined a temperature rise at or above a predetermined threshold value. In addition or alternatively, the predetermined criterion is fulfilled if the detector arrangement has measured a deformation above a predetermined threshold value. The optical waveguide can be placed within the area of the IPT, i.e. in the area between the primary side conductor assembly and the secondary side receiving device of the IPT system. Furthermore, the optical waveguide does not or does not significantly influence the IPT. The reason is that the optical waveguide is made of material which is not electrically conductive or is semi-conductive. In particular, a material is considered as non-conductive if the conductivity is smaller than 10 ^"8 S/m. Examples of suitable optical waveguide materials, are glass, plastic materials (in particular polymers) and semiconductors.

A method of detecting temperature by evaluating backscattered optical signals within an optical waveguide is described in EP 0692705 B1 , but not for the use in an IPT system. However, the whole contents of EP 0692705 B1 concerning the measurement of

temperature is incorporated by reference and any embodiment of the method described in EP 0692705 B1 can be applied in connection with the present invention. In particular, as described in the document, the light of a light source can be modulated in its amplitude and tuned in frequency with respect to time. The modulated light can be coupled into the optical waveguide which is the backscattering medium, and can be coupled into at least one reference channel, as described in the document. The backscattered light is coupled out in a wavelength-selective manner according to a reference band and a measurement band and corresponding to two measurement channels, as described in the document. The frequency ranges which have been coupled out can be filtered through band-pass filters. A notch filter may be inserted in both measurement channels to evaluate the light at the average wavelength of the light source, as described in the document. The reception signals of the two measurement channels and the reference channel light can be converted opto- electronically and can be evaluated in a network analyser, which is a kind of evaluation logic mentioned above. Fourier transforms of the two measurement channels can be normalised with the Fourier transform of the reference channel light. Signals which are corrected for segment-dependent properties of the backscattered signals can be set in relation to one another for the two measurement channels. The document describes further details of the method and, in particular, an example of the measurement of a temperature profile. The term "profile" in this context means that the temperature along the extension of the elongated optical waveguide can be determined. It is preferred with respect to the present invention to measure the temperature profile. The method can be performed in the same manner, but for the purpose of detecting a deformation of the optical waveguide.

The inventive step underlying the present invention is based on the finding that especially the use of an optical waveguide for example in the manner described in EP 0692705 B1 is particularly beneficial in IPT systems. The frequencies of the electromagnetic fields which are used for power transfer in IPT systems, typically in the frequency range of some kHz to less than 1 MHz, do not interfere with electrically non-conductive material of the optical waveguide and do not interfere with the light which is guided by the optical waveguide.

Therefore, the optical waveguide can be placed in the active area of such an IPT system and can be used for temperature detection.

Other methods of temperature detection which have been described before cannot be applied in an IPT system or have significant drawbacks, for example:

Conventional thermocouples or temperature dependent electrical resistances comprise metal parts which would be heated themselves in the IPT area and would disturb the power transfer. Infrared (IR) radiation sensors are subject to environmental pollution of the optical parts. In addition, if the surface of the road, parking place or railway is to be monitored, the IR sensor must be placed above the surface level and, therefore, is an obstacle. The same disadvantages as for IR sensors apply to laser scanners. In addition, laser scanners detect any object in the IPT area and cannot differentiate between hot objects and cold objects. Inductive coils for detecting electrically conducting material would interfere with the IPT system. In addition, the additional coils would be heated.

The use of an optical waveguide made of electrically non-conductive material or of semiconductor material has the advantages that it does not interfere with the electromagnetic field of the IPT system, can be produced at low cost (for example compared to a laser detector system or compared to IR-cameras) and can be adapted to determine the temperature profile and/or the profile of temperature rise and/or the deformation along the extension of the optical waveguide.

Preferably, the optical waveguide is embedded in a layer of electrically non-conducting material which mechanically stabilizes the optical waveguide and its arrangement. For example, the optical waveguide may be arranged in the shape of a serpentine (i.e. sections of the elongated optical waveguide extend next to each other in a meandering manner) or may be curved in a different manner. Plastic material (for example a polymer), which may optionally be reinforced by fibers (such as glass and/or mineral fibers) is preferred as embedding material which embeds the optical waveguide.

The layer of embedding material, including the embedded optical waveguide has the advantage that the optical waveguide is protected and the layer facilitates the process of arranging the optical waveguide as part of the IPT system. Furthermore, the detecting area is increased.

In particular, a detector arrangement is proposed for detecting a foreign object, in particular an object having an elevated temperature, in a power transfer area of an inductive power transfer (IPT) system, the arrangement comprising:

• a primary side electric conductor assembly of the IPT system for producing an

electromagnetic field in order to transfer power and/or a secondary side receiving device of the IPT system for receiving the electromagnetic field and for producing an electric current by magnetic induction and

• an optical waveguide being arranged in the area in which the electromagnetic field propagates during operation of the IPT system and being adapted to guide light within the optical waveguide through the area.

In addition, an inductive power transfer arrangement is proposed for transferring power from a primary side conductor assembly, that produces an electromagnetic field while an electric current flows through the conductor assembly, to a secondary side receiving device, that receives the electromagnetic field and produces an electric current by magnetic induction, wherein the transfer arrangement comprises the detector arrangement of any embodiment of the detector arrangement described, including the primary side conductor assembly and the secondary side receiving device, wherein the optical waveguide is located in an area between the primary side conductor assembly to the secondary side receiving device.

Furthermore, a method is proposed of detecting a foreign object, in particular an object having an elevated temperature, in a power transfer area of an inductive power transfer system, which transfers power from a primary side electric conductor assembly to a secondary side receiving device by an electromagnetic field produced by the primary side electric conductor assembly, wherein the method comprises:

• guiding light through an optical waveguide being arranged in the area in which the electromagnetic field propagates during operation of the IPT system,

• detecting a fraction of the light, which has been guided through the optical waveguide,

• evaluating the detected fraction of the light and, thereby, determining a temperature and/or temperature rise of the optical waveguide and/or determining a deformation of the optical waveguide and thereby detecting that the foreign object exerts a force onto the optical waveguide. In particular, the optical waveguide is placed at the bottom of and/or below the area in which the electromagnetic field propagates for the purpose of inductive power transfer.

Also, a method is proposed of operating an IPT arrangement, wherein an electromagnetic field is produced by a primary side conductor assembly and propagates to a secondary side receiving device that receives the electromagnetic field and generates an electric current by magnetic induction and wherein the method of detecting a foreign object is performed according to any of the embodiments of the detecting method described.

Finally, a method is proposed of manufacturing a detector arrangement for detecting a foreign object, in particular an object having an elevated temperature, in a power transfer area of an IPT system, the method comprising:

• providing a primary side electric conductor assembly of the IPT system adapted for the production of an electromagnetic field in order to transfer power and/or providing a secondary side receiving device of the IPT system adapted for reception of the electromagnetic field and for production of an electric current by magnetic induction and

• providing an optical waveguide and arranging the optical waveguide in the power transfer area in which the electromagnetic field propagates during operation of the IPT system, wherein the optical waveguide is adapted to guide light within the optical waveguide through the area.

In particular, the optical waveguide can be arranged above the primary side conductor assembly, especially if the primary side conductor assembly is integrated or placed on a road, parking place or a railway. This is preferred, since it also allows for detection of weight forces. Alternatively, the optical waveguide can be placed under the receiving device on the secondary side of the IPT system. In both cases, the optical waveguide is placed in between the primary side conductor assembly and the secondary side receiving device while the IPT system is operated.

In particular, as mentioned before, the detector arrangement may comprise a light detector which is combined with the optical waveguide so that light which is coupled out from the optical waveguide is detected by the light detector. For example, the light detector may comprise at least one photo diode.

In addition or alternatively, the detector arrangement may comprise a light source which is combined with the optical waveguide so that light generated by the light source is coupled into the optical waveguide during operation of the arrangement. In particular, the light source can comprise at least one laser diode. Optionally, a spectral filter may be arranged in the optical path between the laser diode(s) and the optical waveguide, for example as described in EP 0692705 B1 in connection with Fig. 1 of the document.

Preferably, the light detector and/or the light source and the optional spectral filter(s) is/are arranged in a common unit which is located separately of the optical waveguide. Preferably, this separate unit is located outside the active area of the IPT system, which is the area in between the primary side conductor assembly and the secondary side receiving device. Furthermore, it is preferred that the separate unit is shielded against electromagnetic radiation produced by the IPT system. For example, the separate unit may comprise a housing of electrically conducting material, such as a metal housing, for example made of aluminum. If the primary side conductor assembly is combined with the optical waveguide, the separate unit may be buried in the ground.

As mentioned above, the optical waveguide is preferably embedded in a layer of electrically non-conducting material. In particular, the layer is designed in such a manner that it does not deform due to its own weight if it is carried or handled in a different manner, especially during manufacture of the detector arrangement. Therefore, mechanical stress is removed from the optical waveguide. Preferably, the layer is tight against the intrusion of water, moisture and dirt to the optical waveguide. On the other hand, external forces (such as weight forces of the foreign object) which are exerted onto the optical waveguide, either directly or indirectly via the layer material, can be detected.

Preferably, the layer of electrically non-conducting material with the embedded optical waveguide forms the surface of a one-piece unit comprising the primary side electric conductor assembly or forms the surface of a one-piece unit comprising the secondary side receiving device. In particular, the surface is the outer surface of the one-piece unit towards the area in between the primary side conductor assembly and the secondary side receiving device which is typically occupied by air.

Examples of the present invention will be described with reference to the attached Figures. The Figures show:

Fig. 1 schematically a side view of an IPT system with a primary side conductor assembly buried in the ground and a vehicle having a secondary side receiving device, Fig. 2 schematically a side view of an arrangement comprising a primary side conductor assembly and a secondary side receiving device, wherein a optical waveguide unit for detecting the presence of an object having an elevated temperature is attached to the secondary side receiving device,

Fig. 3 an arrangement similar to the arrangement shown in Fig. 2, wherein the optical waveguide unit is attached to the primary side conductor assembly, and, therefore, allows for deformation caused by weight forces of a foreign object,

Fig. 4 schematically, an IPT system having a primary side conductor assembly and a

secondary side receiving device, wherein two possible positions of an optical waveguide unit and a hot object are shown,

Fig. 5 shows a partial cross-section through an embodiment of an optical waveguide in the form of an optical fiber embedded in electrically non-conductive material,

Fig. 6 a top view in the manner of a radiograph on an embodiment of an optical waveguide unit connected to a separate unit comprising a light source and a light detector. Fig. 7 shows a more detailed embodiment of a primary side conductor assembly and an optical waveguide unit forming a one-piece unit on the primary side.

The vehicle 1 which is schematically shown in Fig. 1 comprises a secondary side receiving device 2 for receiving an electromagnetic field produced by a primary side conductor assembly 3 which is buried in the ground. The surface of the ground (which may be formed partially by the layer embedding the optical waveguide) is denoted by 4. For example, the vehicle 1 drives on wheels 5a, 5b on the surface 4 of the ground. Alternatively, the wheels may roll on rails during operation, if the vehicle is a rail vehicle.

The primary side conductor assembly 3 produces the electromagnetic field, for example while the vehicle is driving. Alternatively, the vehicle 1 may stop or may be parked while the electromagnetic field energy is transferred to the receiving device 2 of the vehicle.

Especially if the vehicle may drive during transfer of electromagnetic energy to its receiving device 2, it is preferred that the optical waveguide unit 7 of the present invention is attached to the receiving device 2, as shown in Fig. 2. However, the example shown in Fig. 2 is not restricted to the use while the vehicle drives. In any case, the optical waveguide unit 7 may comprise or consist of an optical waveguide (in particular a single optical fiber) and material which embeds the optical waveguide.

Alternatively, in particular if a vehicle is provided with electromagnetic energy while it stops or is parked, the optical waveguide unit 7 may be attached to the primary side conductor assembly 3 as shown in Fig. 3. Any vehicle, to which the secondary side receiving device 2 is attached, is not shown in Fig. 2 or Fig. 3. This arrangement has the advantage that the optical waveguide unit 7 can also detect weight forces which cause a deformation of the unit 7. In this case, the optical waveguide unit 7 is preferably placed outside of the area in which the vehicle may exert weight forces to the ground by wheels. Alternatively, the locations of the wheels of the vehicle can be detected so that they can be distinguished from a foreign object. For example, several optical waveguide units 7 can be placed next to each other and an evaluation logic evaluating the signals from the different optical waveguide units 7 may evaluate which forces are exerted by wheels and which forces are exerted by a foreign object. The evaluation logic may, in particular, take into account that the wheels of the vehicle move on a merely straight path on the ground.

Fig. 4 shows again two possible locations for an optical waveguide unit 100, namely on the primary side above the primary side conductor assembly 201 or on the secondary side under the secondary side receiving device 202. In addition, Fig. 4 shows an object 203 which is electrically conductive and, therefore, would be heated during operation of the IPT system, or would exert its weight forces. If the optical waveguide unit 100 forms the upper surface of the primary side, the object 203 is in contact with the surface and, therefore, temperature detection or temperature rise detection is particularly fast. In any case, it would deform the optical waveguide unit 100.

However, reliable temperature detection or temperature rise detection is also possible if the optical waveguide unit 100 is placed under the receiving device 202. For example, a lower threshold temperature can be set in this case for the detection of a hot object. If the optical waveguide reaches the threshold temperature (which may be in the range of 30 to 60 °C), or if one section of the optical waveguide reaches the temperature, the hot object 203 is detected and corresponding actions can be taken, such as switching off the electrical current through the primary side conductor assembly 201 .

Fig. 5 shows a part of a cross-section of an embodiment of an optical waveguide unit, for example the unit 100 of Fig. 4 or the unit 7 of Fig. 2 or Fig. 3. Fig. 6 shows a top view (in the manner of a radiograph) on such an optical waveguide unit. The outline of the active area of the IPT system is denoted by 105 in Fig. 6. Therefore, the separate unit 106 comprising a light source 107 and a light detector 108 is located outside the active area. The light source 107 is connected to a first section 101 a of the elongated optical waveguide (in particular an optical fiber). This first section 101 a connects the separate unit 106 with the optical waveguide 101 in the active area. At the opposite end of the elongated optical waveguide 101 in the active area, a further section 101 b connects the active area with the separate unit 106, in particular with the light detector 108.

Other than shown in Fig. 6, the light detector and the light source can be connected to the same end of the optical waveguide unit, for example as shown in Fig. 1 of EP 0 692 705 B1 .

Within the active area, the optical waveguide 101 is arranged in serpentines. This means that parallel sections are connected in each case by one curve of 180 degrees. Alternatively, at least one of the curves can be divided in two curves of about 90 degrees which are connected to each other by a straight section of the optical waveguide. The distance between the parallel sections may be, for example 15 mm.

During operation of the IPT system, the light source in the separate unit 106 produces light which is coupled into the first section 101 a and propagates through the optical waveguide 101 in the active area. Depending on the temperature and/or deformation, fractions of the light is scattered, in particular backscattered.

A light detector can be connected either to the first section 101 a or to the further section 101 b or to both sections 101 a, 101 b for detecting and further evaluation of the detected light. Details of an example of the detection and evaluation are described in EP 0 692 705 B1 in connection with Fig. 1 to Fig. 3 of the document.

The cross-section shown in Fig. 7 comprises an optical waveguide unit 100 and a primary side conductor assembly 201 which are part of a one-piece unit 210 on the primary side of an IPT system. The conductor assembly 201 is embedded in concrete 207. The optical waveguide unit 100 is also embedded in the concrete 207 but forms part of the upper surface of the one-piece unit 210, while the conductor assembly 201 is fully embedded in the concrete. Cross-sections of some electric conductors 208 of the conductor assembly 201 are shown. In other areas (not shown) of the primary side conductor assembly 201 , the electric conductors 208 comprise sections which extend between the right hand side and left hand side in Fig. 7. Although the optical waveguide unit 100 is embedded in the concrete 207, it may be deformed. In addition or alternatively, the optical waveguide unit 100 may be supported by the material of the one-piece unit in a manner which allows for deformation towards below (if the one-piece unit is supported only locally) and/or if the supporting material of the one-piece unit is elastic. Other than shown in Fig. 5, the optical waveguide (in particular the optical fiber) can be placed nearer to the upper surface of the unit if the upper surface is oriented to the potential hot object or may be placed nearer to the lower surface of the unit if the lower surface is oriented to the potential hot object. In this manner, the response time with respect to the detection of a hot object is reduced. For example, the distance of the optical fiber to the surface which is oriented to the potential hot object may be in the range of 1 /4 to 1/3 of the distance to the opposite surface of the optical waveguide unit.

In practice, an optical fiber used as optical waveguide may have a fiber length of 100 to 200 meters, wherein the optical fiber is preferably arranged to form serpentines or a spiral. For example, by determining the temperature profile along the extension of the elongated optical waveguide (for example the temperature profile may consist of 500 to 1 .500 measurement points) not only the presence of a hot object can be detected, but also the location. The same applies to the detection of deformation.

For example, the temperature rise at one location or of the temperature profile can be observed. At the beginning of the operation of an IPT system, the temperature of the optical waveguide may be 25 'Ό and may be constant along the extension. If an electrically conducting object is present in the active area, a corresponding position of the optical waveguide directly below or directly above the object may increase to 29 ^< Ό within 10s, while other sections of the optical waveguide remain at 25 °C and some sections close to the hottest section of the optical waveguide may reach a temperature in between these two values. A threshold value for the temperature rise may be defined and may be 0.4 K/s or even smaller. Since the temperature rise at the hottest section or hottest point of the optical waveguide has reached the threshold value, it is decided that there is a hot object, respectively an object of electrically conducting material.

A corresponding action can be taken on detection, such as triggering a warning signal and/or affecting the control of the primary side conductor assembly.