EP0767981B1

EP0767981B1 - A device for protection against overcurrents in electric circuits

Info

Publication number: EP0767981B1
Application number: EP95922857A
Authority: EP
Inventors: Per Olof KARLSTRÖM; Tomas Hansson
Original assignee: Seldim I Vasteras AB
Current assignee: Seldim I Vasteras AB
Priority date: 1994-04-22
Filing date: 1995-06-15
Publication date: 1999-05-19
Anticipated expiration: 2015-06-15
Also published as: SE9401379D0; DE69509774T2; DE69509774D1; SE9401379L; SE514775C2; WO1995034931A1; EP0767981A1

Description

This invention relates to a device which is intended to prevent overcurrents in electrical circuits, comprising at least one electrically conductive elastomeric body and two electrodes which are intended to supply circuit current through said body, and constructed to repel one another under the influence of a short circuit current, where each electrode is in abutment with the elastomeric body at a corresponding position, either directly or through the intermediary of an intermediate part, and in which abutment pressure is obtained by means of a pressure device which tends to deform the elastomeric body at respective abutment sites.

It is already generally known that electrically conductive elastomeric bodies with electrodes can be used to protect against overcurrents in electrical circuits. In this respect reference can be made to Swedish Patent Applications No. 9201223-6 and No. 9203234-1, corresponding to WO-A-93/21677 and WO-A-94/10734 respectively.

When an overcurrent passes through a device of this type, combustion of electrical conducting particles occurs at the abutment pressure point of the usually convex elastomeric body against an electrode. The relaxation time of the device (i.e. the time necessary for the device to revert from a high- resistive state to a low-resistive state and stability after the overcurrent) is often very long, normally several hours. Residue products, produced on the surface of the elastomeric body during the combustion caused by the passing overcurrent, may remain on the electrodes resulting in increased contact resistance. Where the combustion on the surface of the elastomeric body is high enough after one or several current limitations, the surface of the elastomeric body may no longer give the desired, acceptable contact resistance after the relaxation period. This results in a dramatic reduction of the electrical load capacity. In extreme cases electrical arcs may even occur at the contact area and electrical breakthrough may occur in the elastomeric body itself. In such cases the device will be rendered completely useless. These problems can now be avoided by means of a device as described in the introduction part of Claim 1 where the elastomeric body and the electrodes in respect of their contact areas, are located in a liquid environment - which especially can be in paste consistency - with residue solving properties, and also a contact means, in the form of at least one raster-like layer, is arranged to be in contact with at least one electrode and an abutting elastomeric body and/or between two abutting elastomeric bodies, said contact means providing an increased contact effect in that the layer will sink into the abutting elastomeric body when pressure is applied. In this way the effect of the aforementioned residue is minimized while at the same time electrical arcing is prevented.

One further disadvantage, which occurs when several contact areas of one or several elastomeric bodies are connected in series, is that only one of the contact areas is activated during overcurrent. This means that the voltage source is not satisfactorily distributed whereby the activated area is unnecessarily over-stressed. The voltage strength of the device in a high-resistive state becomes low so that it remains dependant on the activated area.

It has been shown in devices which are relatively sensitive, for example based on repulsion devices of the type described in the aforementioned Swedish Patent Application No. 9201223-6, or which completely lack repulsion arrangements, that a current peak will immediately lead to the device becoming high-resistive. This current peak leads to high over voltages and so results in isolation problems in the device.

One other aim of the invention is to provide a device giving protection against overcurrents in electrical circuits as mentioned in the introductory paragraph, which device makes it possible to eliminate or greatly reduce the aforementioned current peak. This eliminates the aforementioned problem in connection with known devices of this type. The aim of the invention is therefore to ensure that the device will become more tolerant and be far less affected by overcurrents and so function satisfactory for a very long time.

The invention will now be described in more detail in the form of various examples with reference to the attached diagrams.

Fig. 1 shows a side view of a cross-section of a type of current limiting device used in the invention.

Fig. 2 shows the same view as in Fig 1. but with a larger scale section which shows how the current striction is obtained.

Fig. 3 and Fig. 4 show in two sections two embodiments of the current limiting device used in the invention, whereby current striction is obtained.

Fig. 5 shows a diagram of current/time for a known current limiting device and for the current limiting device used in the invention.

Fig. 6 shows a side view of a cross-section of another embodiment of the new current limiting device.

Fig. 7 shows a side view of a cross-section of yet another variant of the new device where certain components of the arrangement are illustrated on a larger scale.

Fig. 8 shows a side view of a cross-section of a further embodiment of the device used in the invention.

Fig. 9 and Fig. 10 show a further variant of the device where Fig. 9 shows a side view of a cross-section and Fig. 10 shows a longitudinal side view of the device.

Fig. 11 shows a modification of the current limiting device shown in Fig. 9 and Fig. 10.

Fig. 12 shows a side view of a cross section of the current limiting device, which arrangement comprises several current limiting devices stacked one on top of the other.

Fig. 13 shows the current limiting device shown in Fig. 12 connected to an electrical circuit.

Fig. 14. is a diagram illustrating voltage distribution of the different current limiting devices in the arrangement shown in Fig. 13 and

Fig. 15 shows a cross-section-of a special preferred embodiment of the current limiting device according to the invention.

In the various Figs, corresponding parts are marked by the same reference numerals even though the parts do not always look exactly the same.

With all of the following embodiments of the invention, it is particularly important that, in connection with their mutual pressure points, the elastomeric body and the electrodes are found in liquid, for which paste has been used in the present case. This also applies to the embodiments shown in Fig. 1 - 4 and 6 - 13 even if it is not particularly clear from the Fig., since the aforementioned Figs. are primarily intended to illustrate other important details in connection with advantageous arrangements. One example of a basic arrangement which shows that the elastomeric body and electrodes are in fact in liquid, follows from Fig. 15.

In the arrangement illustrated in Fig. 1 the main components are a centrally arranged electrically conductive elastomeric body 10 together with two

electrodes

12 and 14, one on each side. When the device is connected to a circuit there is a current supply between

electrodes

12 and 14 through the elastomeric body 10. The Fig. does not show in detail the pressure device creating an abutment force between the

respective electrodes

12 and 14 and the elastomeric body 10: this is illustrated by the F arrow. It should be noted that the

electrodes

12 and 14 are always arranged in this way so that during a short circuit they repel each other. This can happen for example because the

electrodes

12 and 14 which are placed against each other have a continuous form, the elastomeric body 10 being placed between said electrodes, adjoining one of their ends. The current will therefore pass in anti-parallel through the

electrodes

12 and 14 as shown with the arrows in Fig. 1. Thus, when an overcurrent passes the

abutment areas

16 and 18 are reduced between the

respective electrodes

12 and 14 and the elastomeric body 10. At the

abutment areas

16 and 18 between

electrodes

12 and 14 and the elastomeric body there is, as illustrated in Fig. 1, for enlargement of the interaction contact, electrically

conductive nets

20 and 22, which are formed in connection with the respective electrode surfaces. The directly applied abutment pressure F through electrode 12 normally ensures that there is a good contact between the

consecutive parts

10, 12 and 14, whereby it is possible for the circuit current to pass through the device since it is connected to an external electrical circuit. The

nets

20, 22 partly sink into the elastomeric body 10 due to the influence of pressure force F contributing to said contact effect between the

respective electrodes

12 and 14 and the electrically conductive particles (not shown here) of the elastomeric body 10. When, after the overcurrent has passed through, the electrically conductive particles have been combusted on the contact surface between the elastomeric body and the meshes, the pressure will cause the

nets

20, 22 to sink deeper into the elastomeric body and so create new contact with intact electrically conductive particles therein. In this way the device reverts to the initial state, when the current has passed and the circuit has re-opened.

With the new current limiting device it is now possible to considerably reduce the relaxation time; in this case to a matter of seconds. When the

nets

20, 22 are completely immersed in the elastomeric body the surfaces of the electrodes, between the meshes of the

nets

20, 22, come in contact with the undamaged elastomeric surfaces which contributes additionally to increase the lifetime of the device. The device's sensitivity also increases through the introduction of

meshes

20, 22 which results in a decrease of the current peaks.

Fig. 5 illustrates the current/time curve for a device which does not include the net described above - continuous line. The aforementioned current peak is clearly evident but is missing completely in the curve - dotted line - which in the same Fig. relates to the version of the invention comprising a net. One possible explanation of the increased sensitivity which is achieved by using the net can be the interaction which arises through current striction in connection with the said net. This relationship is shown with desirable clarity in Fig. 2 where current lines are compressed in electrically conductive mesh threads 24, which are included in

nets

20 and 22, respectively.

A current striction can be achieved to a greater extent by using the current limiting device shown in Fig. 3 where the passage through which the current is devised to pass in the device, is reduced at at least one contact position 13 between the net 20 and the electrode 12 and the elastomeric body 10, respectively. In this connection the outer parts of the electrode 12 facing the elastomeric body 10 is covered by insulation 11. The contact device 20 is placed centrally in respect of the adjoining electrode 12 and the elastomeric body 10. In this way the increased current density in the elastomeric body 10 is obtained. Since the electrode 12 and the net 20 have very good thermal conductivity and great thermal mass the temperature increase under normal conditions is negligible. There will be no increase in resistance at normal momentary overloads, for example when starting an engine, etc. When a short circuit occurs, when larger overcurrents occur, considerably more energy is provided to the contact area 13 through heating (joules) than that given off. In this way the temperature increases dramatically creating an abrupt increase in resistance. In the described arrangement, there is also increased sensitivity for short circuit current in the device with retained qualities during normal use.

A further modification of the device is given in Fig. 4, where the passage of the current which is designed to pass through the device, is reduced by at least one contact area 13 between the electrode 12 and the net 20. With this configuration, as with the arrangement shown at Fig. 3, insulators 11'-11" are arranged at the end parts of electrode 12, whereby the electrode projects centrally between the insulators. The net 20 is stretched over both insulations 11' and 11" and the centre section of the electrode 12. In this way the current through the net 20 can partly pass transversally as shown in the Fig. To further protect the sensitivity of the arrangement the net 20 should preferably comprise a material with considerably higher resistivity than the resistivity of the electrically conductive filling material of the elastomeric body 10. The material of the net 20 can be stainless steel, for example, while the filling material can be silver or similar low resistive material.

Fig. 6 shows that the current protection device has a current limiting element, which comprises three separate

elastomeric bodies

10a, 10b and 10c where the two outer

elastomeric bodies

10a and 10c are surrounded by separate electrical conducting meshes 20a and 22a, respectively. The circuit may be completed with an external resistor, represented by the dotted lines, which prevents over-voltages and guarantees current through the device even after the occurrence of current limiting. With the design shown in Fig.7, an arc-limiting material 26, for which Al(OH)₃ in powder form can be used advantageously, is placed between the mesh threads 24 of the

nets

20 and 22. The aim of this arrangement is to prevent arcing, which in turn leads to a reduction in the combustion of the material when the device is subjected to overcurrents.

Fig. 8 refers to an arrangement of the invention, where the

nets

20 and 22, of the type discussed above are also used as integrated electrodes in the current limiting device. Each electrode making up the

nets

20 and 22 is, in this case, placed in a rigid circular frame 28 which can be similar to the fastening of strings in a tennis racket. In this way two integrated functions are achieved.

The device shown in Fig.9 and 10 is arranged in cylindrical symmetry, one outer net 30 here being one of the electrodes. The other electrode 32 is centrally arranged. The arrangement offers technical production advantages since it can later on be produced and handled as "metre goods".

Fig. 11 shows the device with one electrode 34 which in this case uses a "net equivalent". The thread-like electrode 34 has been wound on an elastomeric core 10 which surrounds a central electrode 36. Between the points 38 and 40 the optimal resistance is high, preferably more than 10 mOhm. When the abutment area changes to a state of high resistivity there is a continuous resistance in the circuit. In this way the transients are suppressed and commutations to parallel resistors, if any, are made easier.

With the arrangements shown in Fig. 12 and Fig. 13 a large number of current limiting elements in the form of elastomeric bodies (10₁...10₂....10_N) are stacked on top of each other. Nets 40₁...40_n+1 connected to the elastomeric bodies 10₁ ...10_N are mutually linked in parallel through resistors R_P1, R_P2...R_PN. In this way the voltage division between the consecutive current limiting elements is achieved. This increases the voltage isolation level when the device is in a state of high resistivity. This in turn gives the advantage that the respective elements are not "stressed" as much as when only one is activated.

The diagram in Fig. 14 shows the voltage U_P as a function of the consecutive current limiting elements j. The voltage division in Fig. 14 shows a case where the majority of the elastomeric bodies in Fig. 13 are in a state of high resistivity. In this illustration, for example, the elastomeric body 3 is still in a state of low resistivity. An external voltage source E is used, a current then passing through the device.

With the embodiment of the current limiting device shown in Fig. 15 is disclosed that both the electrically conductive elastomeric body 10, the

electrodes

12 and 14, the contact arrangement (indicated by the electrically conductive nets 20 and 22), the pressure device (indicated by arrow F) and any additional components of the arrangement, are arranged in a preferably hermetic container 42. In Fig. 15 the direction of current circuit flow through

electrodes

12 and 14 is also marked. The container 42, which is made of non-electrically conductive material (for example ceramic or plastic) contains a liquid 44, entrapped in such a way that the environment for the contact areas between the elastomeric body 10 and the

electrodes

12 and 14 always is provided by said liquid 44, unaffected by the current limiting device's position in the room.

The liquid 44, which can be water or water with added glycerine, should have such properties that it dissolves any residue between the various components so that a low contact resistance is achieved and to a great extent retained between the

electrodes

12, 14 and the elastomeric body 10.

Since the

electrodes

12, 14 and the elastomeric body are located in said liquid 44, slag residue is effectively prevented from building up on the surface of the

electrodes

10, 12 during combustion. It is, of course, very important that said liquid 44 has arc preventing properties.

Further modifications of the above examples of embodiments of the current limiting device will be possible within the scope of the claims. Thus, as illustrated in the arrangements comprising the nets, the contact devices can also be made of an electrically conductive layer, being conductive essentially only perpendicular to the layer plane. The layer can be made of directionally arranged threads and of an electrically insulting material arranged therebetween and made of silicon rubber, for example. The contact device can also be produced, for example, through etching, plasma thermal coating etc. on the surface of the electrode. Further, the contact device can comprise two separate nets arranged against each other where the size of the mesh in one of the nets differs from the mesh size of the other. It is even possible to disperse the arc preventing material in the electrically conductive elastomeric body or to disperse said material in a thin metal layer in the electrically conductive elastomeric body, which in this case, can have a cylindrical form.

Claims

A device for protection against overcurrents in electric circuits, comprising at least one electrically conductive elastomeric body (10) and two electrodes (12, 14) which are intended to supply circuit current through said body, and constructed to repel one another under the influence of a short circuit current, where each electrode is in abutment with the elastomeric body at a corresponding position, either directly or through the intermediary of an intermediate part, and in which abutment pressure is obtained by means of a pressure device which tends to deform the elastomeric body (10) at respective abutment sites,
characterized in that the elastomeric body (10) and the electrodes (12, 14) in respect of their contact areas, are located in a liquid environment (44) - which especially can be in paste consistency - with residue solving properties, and also in that a contact means, in the form of at least one raster-like layer (20,22), is arranged to be in contact with at least one electrode and an abutting elastomeric body and/or between two abutting elastomeric bodies, said contact means providing an increased contact effect in that the layer will sink into the abutting elastomeric body when pressure is applied.
A device according to Claim 1, characterized in that the liquid (44), which has considerably higher resistivity than the elastomeric body (10), contains glucol.
A device according to any one of Claims 1 and 2, characterized in that the liquid has arc preventing properties.
A device according to any one of Claims 1 - 3, characterized in that the liquid (44) is contained in a hermetic container (42) which also includes the elastomeric body (10), the electrodes (12, 14) and the pressure device.
A device according to any one of Claims 1 - 4, characterized in that the passage for the current, which is intended to pass through the device, has a reduced portion at least at one contact point (13) between the contact means (20), an electrode (12) and the elastomeric body (10).
A device according to any one of Claims 1 - 5, characterized in that the passage for the current, which is intended to pass through the device, has a reduced portion at least at one contact point (13') between an electrode (12) and the contact means (20).
A device according to any one of Claims 1 - 5, characterized in that the contact means is made of a layer which is electrically conductive substantially only perpendicular to the layer plane.
A device according to any one of Claims 1 - 6, characterized in that the contact means is made of a separate mesh net, which for example is pressed, etched, weaved or knitted.
A device according to any one of Claims 1 - 6 and 8, characterized in that contact means (20a, 22a) are arranged to cover respective elastomeric bodies (10a, 10c).
A device according to any one of Claims 8 and 9, characterized in that threads included in the contact means are made of brass, copper or nickel, preferably with a silver coating.
A device according to any one of Claims 8 -10, characterized in that threads included in the contact means are isolated from each other.
A device according to any one of Claims 1-6, characterized in that the contact means has a part integrated in an electrode.
A device according to Claims 12, characterized in that the contact means is produced on the surface of the electrode, in particular through etching or plasma spraying.
A device according to any one of Claims 1-4, characterized in that the aforementioned layer (20, 22) constitutes an electrode.