A DEVICE FOR DETECTING ARCS

Abstract: A device for detecting arcs in electric devices and installations comprises a substantially solid light detector body (7) of a light damping and light spreading material and an optical fibre (8) adapted to conduct light directed thereagainst through the light detector body (7). The light detector body (7) is provided with a recess (10) adapted to receive an end of the optical fibre (8).

Description: A DEVICE FOR DETECTING ARCS BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to a device for detecting arcs in electric devices and installations. It comprises in particular a substantially solid light detector body of a material damping and spreading light and an optical fibre adapted to conduct light directed towards the fibre through the light detector body. The invention also relates to a method for producing a device for detecting arcs in electric devices and installations. In electric devices and installations there is a need of detecting the creation of arcs. This need is met through so called arc guards. A conventional arc guard is a device comprising a light collector, which collects light from the possible arc and directs it towards a light conductor. The light conductor is preferably an optical fibre and forms a part of the arc guard. The light conductor conducts the light further to an arrangement for converting light into electrical signals, which in its turn are utilized for controlling the electrical device or installation, for example for activating a breaker for breaking the current to the entire or a part of the electrical device or installation. The arc guard preferably comprises also the converting arrangement. The light conducted by the optical fibre is damped as a consequence of absorption and light spreading in the fibre. The ab- sorption takes place in impurities, such as metals, water rests, and organic molecules. The absorbed light power is transformed into heat in the fibre. At the fibre end receiving light there is also reflection against the end surface, spreading losses when the fibre surface is uneven and absorption caused by dust and dirt on the fibre end. The total damping of the optical fibre is accordingly dependent upon the length thereof. The longer fibre the greater total damping. The light collecting ability of the light collector should therefore for a given arc be adjusted with respect to the length of the optical fibre. Accordingly, optical fibres being too long require a high light sensitivity of the light conductor. The prior art comprises a light detector body, i.e. light collector, consisting of a semi-transparent plastic. The light detector body has the form of a cylinder tapering towards and being rounded at the light receiving free end thereof. It is in the opposite end provided with a thinner, cylindrical foot portion adapted to be inserted in a container, in which one end of the optical fibre is fixated or secured. The holder defines a sleeve of opaque plastic material. The holder is arranged so that an air slot will be present between the fibre end and the foot of the light detector body. As a consequence of the air slot light directed towards the end of the optical fibre is, however, spread and damped, which results in a restricted sensitivity of the device. For improving the sensibility experiments have been done where the fibre end has been displaced to intermediate bearing against the detector end. A somewhat improved but still too restricted sensitivity was possible to be registered. With a device of the type described it is also difficult to adjust the light transmission from the light collector to the fibre end so that the sensitivity thereof will be high independently of the length of the optical fibre. SUMMARY OF THE INVENTION An object of the invention is to provide a device for detecting arcs, which have a high sensitivity and which are so constructed that the function thereof may easily be adapted to different lengths of the optical fibre, so that a high light sensitivity is ob- tained independently of the length of the fibre in the individual case. The device has to be uncomplicated and easy to produce. These objects are obtained with a device of the type defined in the introduction, which is characterized in that the light detector body is provided with a recess adapted to receive one end of the optical fibre. By placing the fibre end inside the light detector body a considerably increased sensitivity is obtained with respect to the case in which an air slot is there between the fibre end and the body or when the fibre end only bears against an outer surface of the detector body. The arrangement of the present invention enables a functional and geometrically uncomplicated design of the detector body. The light transmission through the detector body to the fibre end may be influenced by adapting the depth of the recess and the distance the fibre end is pushed into the detector body. According to a preferred embodiment the depth of the recess is adapted to the length of the optical fibre so that the sensitivity of the device is substantially the same independently of the length of the fibre. With a given size and geometry and given material properties of the detector body this is at least true for fibre lengths from about 1 to about 40 metres. For larger length differences further adaption alternatives may be required, such as adaption of the size, the geometry and the material of the de- tector body. According to a further preferred embodiment the recess defines a hole having an inner portion with an inner dimension substantially corresponding to the outer dimension of said end of the optical fibre, and a mouth portion having a larger inner dimen- sion than the inner portion. By such an arrangement the introduction of the fibre end in the detector body is facilitated. Furthermore, production of the detector body by injection moulding is facilitated, since a part of the core required for achieving the recess may be given a somewhat larger dimension, so that it may resist the outer forces to which it is exerted at the very injection moulding. Furthermore, it has turned out that the transition between the mouth portion and the inner portion of the hole is easy to deform by clamping influence from the outside so as to efficiently clamp and fixate the optical fibre in the detector body. According to a preferred embodiment the material comprises a crystalline or partly crystalline polymer. The advantage of crystalline or partly crystalline polymers is that they may as such have an arrangement of crystals spreading and damping light in a way suitable for the detector body. According to a further preferred embodiment the crystalline or partly crystalline polymer comprises polyoxymethylene. The polyoxymethylene (POM) has several advantages. It has a good resistance against moisture and chemicals, it is useful over a broad temperature range, it is resistant to thermal and oxidative decomposition and may be produced with narrow tolerances and a good dimension stability of the details produced. According to a further preferred embodiment the material comprises an amorphous polymer having an addition of a filler for damping and spreading light, in which the amorphous polymer comprises polycarbonate. Polycarbonate has an advantage of a high glass temperature (t g * 150°C) and it may withstand high temperatures before the optical properties are changed. It is re- sistant to weak acids, aliphatical hydrocarbons, paraffin, alco- holes (but not methanol), animal and vegetable oils and fats. A further object of the invention is to provide a method for pro- ducing a device for detecting arcs. The method has to be simple and enable a production of at least a part of the device constituting the light detector body to a comparatively low cost. The light detector body has to be constructed so that the ability thereof to conduct light to a light conductor in the form of an op- tical fibre may easily be adapted to the length of the optical fibre, so that the sensitivity of the device is made substantially independent of the length of the optical fibre. This object is obtained by means of a method of the type de- fined in the introduction, which is characterized in that at least a solid light detector body of a light damping and light spreading material is provided with a recess for receiving an optical fibre adapted to conduct light directed towards this through the light detector body. According to a preferred embodiment the depth of the recess is adapted to the length of the fibre so that the depth increases with an increasing fibre length. Series of different light detector bodies having different hole depths may accordingly be pro- duced with respect to the individual fibre lengths existing in the individual case. According to a further preferred embodiment the fibre surface of the end of the optical fibre- is melted before this is introduced into the recess. Unnecessary light damping of the fibre end as a consequence of different surface defects is thereby avoided, and the fibre may absorb more light, since it flows out somewhat and receives a larger surface in connection with such a melting. The fibre end may for the same reasons advantageously also be polished before positioned in the light detector body. Further features and advantages of the invention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention is now to be described for the sake of exemplifying with respect to the appended drawings, in which: Fig 1 is a lateral view of a light detector body and a holding member according to the prior art, Fig 2 is a side cross section view of the holding member and the detector body according to Fig 1 separated from each other, Fig 3 is a cross section view from the side of a light detector body according to the invention, and Fig 4 is a side-elevation of the light detector body according to Fig 3, connected to an optical fibre. DETAILED DESCRIPTION OF AN EMBODIMENT Figs 1 and 2 show a detector body 1 of a light damping and light spreading material, a holding member 2 of a non-transparent plastic material, and an optical fibre 3 adapted to conduct light to an arrangement not shown for conversion of electrical signals for controlling one or several breakers of an electrical device or installation. The light detector body 1 , the holding member 2, the light conductor 3 and said conversion arrangement form an arc guard of the electrical device or installation. It appears from Fig 2 that the light detector body 1 is designed as a plug adapted to be inserted into and fixed in a recess in one end of the holding member 2. A recess 5 for receiving the light conductor 3 is arranged in the opposite end of the holding member 2. A channel 6 is arranged between the two recesses 4 and 5, so that an air gap will be present between the light detector body 1 and the light conductor 3 when these are inserted in their respective recesses 4 and 5. Figs 3 and 4 show schematically an embodiment of the device according to the invention. The device comprises a light detector body 7, a light conductor 8 and an arrangement 9 for converting light signals into electric signals, preferably for controlling one or more breakers or switches of an electric device or installation (not shown). The light detector body 7 has the shape of a cylinder with a substantially semi-spherical shape at one end thereof. Substantially the same sensitivity of the device within a space angle of 2 π sr may be obtained thanks to this shape, i.e. a semi-sphere, for the light inciding against the detector body. A recess 10 extends from the opposite end of the light detector body 7 in the direction towards the semi-spherical end. The recess 10 is arranged for receiving the light conductor 8. The recess 10 defines a hole having an inner portion 1 1 with inner dimensions substantially corresponding to the dimensions of the end of the light conductor 8 to be positioned in the recess 10. The recess 10 also comprises a mouth portion 12, which has larger inner dimensions than the inner portion 1 1. When the light conductor 8 is inserted in the recess 10 it is preferably that far introduced that it reaches or is located close to the bottom of the inner recess portion 1 1 . Air gaps damping light are avoided. The depth of the recess 10 is adapted to the length of the light conductor 8. The light conductor 8 is preferably an optical fibre, the damping of which may be described through the equation logP(x) = logPo - αx / 10, where P(x) is the intensity of the light, x is the length in metres from the fibre end, P 0 is the intensity of the light when it has ex- actly entered the fibre (x = 0) and α is the damping coefficient in decibel/metre, which varies for different fibres and different wave lengths. The proposed detector body has a hole depth in the order of 10 mm for a fibre length in the order of 1-2 m, 12 mm for a fibre length of 4-10 m, 13 mm for a fibre length of 15-20 m and 15 mm for a fibre length of 25-40 m. The total length of the detector body is then about 20 mm and the radius of the semi-sphere is about 5 mm. The cylinder defining the detector body has along a substantial part of the length thereof reduced outer dimensions, with a diameter in the order of 8 mm. The mouth portion 12 has a diameter of about 4 mm and the inner portion 1 1 a diameter of about 2,5 mm. The transition between the inner portion 1 1 and the mouth portion 12 defines a truncated cone, which promotes guiding of the light conductor 8 when it is introduced into the inner portion 1 1. The detector body 7 is preferably mainly made of a semi- transpartent plastic material with ability to damp and spread light from for example an arc towards the end of the light conductor 8 arranged inside the light detector body 7. Said plastic comprises polyoxymethylene (POM) in the embodiment shown. The polymer is preferably UV-stabilized for being able to take light influence from primarily arcs. The device comprises for smaller hole depths a substantially opaque cover member 13 adapted to be applied onto and cover a part of the detector body when the depth of the recess 10 is below a given threshold value, so that detectors having a small hole depth will not get more sensitive from the side than from the front. The cover member 13 has in the preferred embodiment the shape of a sleeve or girdle surrounding at least a part of the portion of the detector body 7 having the reduced outer diameter. The cover member applied is shown in Fig 4. The light detector 8 is fixed inside the recess 10, either through gluing or clamping by a contact pressing tool of at least a part of the portion of the detector body 7 surrounding the recess 10, preferably at the transition between the inner recess portion 1 1 and the mouth portion 12. The device according to the invention, and in particular the detector body 7, is preferably adapted to detect arcs in an electric device or installation. The device comprises preferably only electrically non-conducting material in the region of and about the light detector body 7. It is hereby suitable for devices in which intermediate or high voltage applications are a part. A number of alternative embodiments will of course be apparent for a man skilled in the art without departing from the idea of the invention. The scope of protection of the invention appears from the appended claims, with support from the description and the appended drawings.

Claims: Claims 1 . A device for detecting arcs in electric devices and installations, comprising a substantially solid light detector body (7) of a light damping and light spreading material and an optical fibre (8) adapted to conduct light directed thereagainst through the light detector body (7), characterized in that the light detector body (7) is provided with a recess (10) adapted to receive one end of the optical fibre (8). 2. A device according to claim 1 , characterized in that the depth of the recess (10) is adjusted to the length of the optical fibre (8), so that the sensitivity of the device is substantially the same independently of the length of the optical fibre (8). 3. A device according to claim 1 or 2, characterized in that the recess (10) defines a hole having an inner portion (1 1 ) with an inner dimension substantially corresponding to the outer dimension of said end of the optical fibre (8), and a mouth portion (12) having a larger inner dimension than the inner portion (1 1 ). 4. A device according to any of claims 1 -3, characterized in that the material is a polymer. 5. A device according to any of claims 1 -4, characterized in that the plastic material comprises a crystalline of partly crystalline polymer. 6. A device according to claim 5, characterized in that the crystalline or partly crystalline polymer comprises polyoxymethylene. 7. A device according to any of claims 1 -4, characterized in that the plastic material comprises an amorphous polymer with an addition of a filler for damping and spreading light. 8. A device according to claim 7, characterized in that the amorphous polymer comprises polycarbonate. 9. A device according to any of claims 1 -8, characterized in that the plastic material is UV-stabilized. 10. A device according to any of claims 1 -9, characterized in that the detector body (7) is elongated and comprises a substantially semi-spherical end, said recess (10) being arranged at the opposite end of the body (7) and extending towards the semi-spherical end. 1 1 . A device according to claim 10, characterized in that it comprises a substantially opaque cover member (13) adapted to be applied onto and cover a part of the light detector body (7) when the depth of the recess (10) is below a given threshold value. 12. A device according to any of claims 1 -1 1 , characterized in that said end of the optical fibre (8) is polished. 13. A device according to any of claims 1 -12, characterized in that the device only comprises electrically non-conducting material in the region of and around the light detector body. 14. A method for producing a device for detecting arcs in electric devices or installations, characterized in that a substantially solid light detector body (7) of a light damping and light spreading material is provided with a recess (10) for receiving an opti- cal fibre (8) adapted to conduct light directed thereagainst through the light detector body (7). 15. A method according to claim 14, characterized in that the depth of the recess (10) is adapted to the length of the fibre (8), so that the depth increases with an increasing fibre length. 16. A method according to claim 14 or 15, characterized in that it comprises introduction of an end of the optical fibre (8) in the recess (10) and fixation of the fibre through clamping of a part of the light detector body (7) surrounding the optical fibre (8). 17. A method according to any of claims 14-16, characterized in that it comprises application of a substantially opaque cover member (13) onto a portion of the light detector body (7) when the depth of the recess (10) is below a given threshold value. 18. A method according to any of claims 14-17, characterized in that the fibre surface of the end of the optical fibre (8) is melted before introduction of the optical fibre (8) in the recess (10). 19. A method according to any of claims 14-18, characterized in that the end of the optical fibre (8) is polished before placing it in the recess (10). 20. A method according to any of claims 14-19, characterized in that the light detector body (7) is produced by injection moulding.

Inventors: MATTSSON SIGVARD, MATTSSON, SIGVARD
Applicants: ABB AB, MATTSSON SIGVARD, MATTSSON, SIGVARD
Application year: 2000
Main classification: H01H9
IPC codes: H01H9, H02B13