SE470118C - Device for protection against overcurrent in electrical circuits - Google Patents

Abstract

An overload protector includes at least one body (10) made of an electrically conductive elastomeric material, and two electrodes. The defining surface (10') of the elastomeric body is intended to lie against and be deformed against a surface on at least one other body (11, 12). A pressure device (14) is provided for exerting pressure onto the body (10). The pressure device is preferably resilient. In this way, the overload protector will pass from a low resistive state to a high resistive state very rapidly when overcurrents occur, whereafter the overload protector will return to its initial resistance and can be reused.

Description

15 20 25 30 35 ff70 118 2 switch contact system or indirectly via a special excitation device consisting of an electromagnetic trigger so-called "plunger or percussion pin device", which is also excited by the main current, an anchor in a magnetic circuit acting on the contact system and / or on a trigger for a spring mechanism, which performs on / off function.Remote control also occurs, for example, in contactors, to maintain 2 stable mechanical equilibrium positions, to or from Contact contact systems where electrodynamic currents act directly on the contacts are previously known, e.g. 1 519 559, GB l 489 010, GB l 405 377.

Hybrids where the two principles are used are described in the patent publication GB l 472 412 and through the article "A New PTC Resistor for Power Applications" by RS Perkins et al., Published in the journal IEEE Transactions on Components, Hybrids and Manufacturing Technology, V0l.CHMT -5, No.2, June 1982, pp. 225-230 and US 3,249,810 and DE 35 446 47.

A serious disadvantage of short-circuit protection according to points 1 and 2 above, especially with high and steep (= fast-growing) short-circuit currents, is the large built-in inertia. Thermal inertia is limiting for the short-circuit protection mentioned under point 1, while in arc-based switches it is mechanical the inertia, ie the mass inertia, which prevails at a desired rapid contact separation. In arc-based switches, due to said mass inertia, the arc is delayed on the contacts, so the arc voltage important for current limitation only reaches values after a relatively long delay time (ms), which limits the otherwise monotonically growing short-circuit current.

In addition, very high contact pressures, proportional to the rated current of the device squared, are required for the contacts to be able to carry the rated current during normal operating currents.

This prevents further rapid contact separation, since the contact pressure is opposite to the separating electrodynamic repulsive forces. 10 15 20 25 30 35 470118 The possibility of adjusting the sensitivity of the short-circuit protectors under points 1 and 2 above is very limited. As a result, extensive coordination work with superiors and subordinates, protection included in the electrical circuit, is required.

Standards have therefore been prepared, eg DIN 57636 Part 21 / VDE 0636 Part 21 § 7.12 and IEC 947-2, as incorrect coordination can mean selectivity problems, which are difficult to remedy (adjust) in existing facilities.

Short-circuit protection based on principles 1 and 2, are due to the above-mentioned disadvantages, especially inertia, less suitable as short-circuit protection or current transient protection for thyristors or electronic equipment, as these are sensitive to both high current derivatives and high energy development.

In capacitive circuits or inductive motor circuits with high presumptive short-circuit currents, high current derivatives and high short-circuit currents can also occur. Typical values of presumptive short-circuit currents are Ik = SO-10OkA and the corresponding current-time derivative from 22-44 kA / ms. A conventional fuse with the rated current 100 A then transmits a current peak of about 16 kA and liz-dt = 20 kA? S, which greatly exceeds the permitted values of the corresponding thyristors.

Chokes are therefore often connected in thyristor circuits to reduce current derivatives, whereby the short-circuit protectors above can be used.

Self-retrieval short-circuit protection consists mainly of so-called thermistors. The term PTC element is a common term for thermistors whose resistivity has a Positive Temperature Coefficient.

Electrically conductive polymer compositions, especially PTC compositions, and devices incorporating PTC compositions are known in the art. References can be made to e.g. Patents Nos. 2,978,665, 3,351,888, 4,017,715, 4,177,376 and 4,246,468 and U.K. l, 534.7l5. Recent advances have been described, for example, in German Patent Nos. 2,948,350, 2,948,28l, 2,949, l74 and 3,002,72l and in various applications 10 15 20 25 30 35 470 “H8 4 U-S. Serial Nos. 41,071 (MPO295), 67,207 (MPO299) and 88,344 (MPO701), as well as application patents such as U.S. Pat. Serial Nos. ll1, 984 (Mp = 7l2), 141.98? (MPO713), 141, 988 (MPO714), 141, 989 (MP0715), 141, 991 (MPO720) and 142,054 (MpO725).

A problem with PTC elements, when heated by the current flowing through them, is that at the temperature, when the PTC elements become self-regulating, the voltage is absorbed by a fragment of the PTC element, so that the fragment is subjected to very large stresses in said fragments. . The PTC element can thus be destroyed. PTC embodiments where this problem is eliminated are known, for example, from European patent EP 0 038 716. PTC elements for overcurrent protection are often made up of a polymeric material, eg high-pressure polyethylene, containing particles of an electrically conductive material, eg carbon black, and have a resistivity with a positive temperature coefficient.

Ceramic thermistors which exhibit PTC characteristics are known from patent publication GB-A-1570138. The most common ceramic thermistors are based on BaTiO3 or v20,.

An advantage of the polymer-based thermistor compared to the ceramic one is that its resistance is monotonically rising with temperature. It is also relatively inexpensive. Commercially available polymer-type thermistors are, however, designed for relatively low rated voltages and can therefore not be used without further ado in, for example, distribution networks. In addition, the configuration and electrode connections of the thermistors are generally such that the thermistors are exposed to high repulsive forces due to high short-circuit currents due to antiparallel current paths, whereby the electrodes are torn apart. It is further known that plastic-based PTC elements of the so-called sandwich type do not return to the initial resistance after the transition from a low-resistance to a high-resistance state. In more severe cases when the PTC elements are exposed to very large electrical stresses, such as short-circuit currents, blistering and cracking occur in central or other parts of the PTC element's polymer composition, so that it can no longer function, ie the element is destroyed. -if 10 15 20 25 30 35 470118 y fi Due to the above reasons, polymer-based thermistors have so far not come into significant practical use in electric power technology, but have mainly been used for protection of electronic equipment, where the thermal inertia limits the possible application area.

A significant difference between thermistors and fuses is that thermistors are reusable, "self restoring", after a short circuit, ie thermistors can be reused after a short circuit, which also applies to circuit breakers.

Elastomers are all polymers which have similar elastic properties to natural rubber. Within a relatively large allowable elastic range, elastomers can be compressed or stretched, to return to the initial state after the load has ceased. Electrically conductive elastomers are a class of rubber and plastics made electrically conductive either by the addition of metal mixtures, or by the orientation of metal fibers under the influence of electric fields, or by the addition of various carbon mixtures or ceramics, for example VZO3 materials dispersed as described in article "VZO3 Composite Thermistors" by D Moffat et al., published in Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 1986 pp. 673-676. In rubber several types of "carbon black" were used eg graphite, acetylene black, lamp black and furnace black with particle diameters from 10 ~ 300nm.Examples of suitable rubber materials which, after the addition of metal mixtures or carbon mixtures, become electrically conductive are butyl, natural, polychloroprene, neoprene, EPDM and most commonly, silicone rubber. as additives in elastomers are silver, nickel, copper, silver-plated copper, silver-plated nickel and silver-plated aluminum.

In the sensor technology, electrically conductive elastomers are used as pressure sensors. The electrical properties change when electrically conductive elastomer is deformed, for example by being subjected to pressure or tensile stresses, which manifests itself in a resistance change.

The most common types of carbon or metal-filled plastics are polyethylene and polypropylene. These are used today for heating cables and overcurrent protection, the latter, for example, the previously mentioned polymer-based PTC thermistors.

However, the mechanical properties of the plastic deteriorate after the addition of electrically conductive fillers. The material becomes brittle and hard and thus difficult to deform. These materials are therefore unsuitable as pressure sensors and require for PTC applications, in addition to a relatively complicated contacting technique. A further limitation of carbon-filled plastics is the relatively high resistivity typically 1 Ohmcm and more. Metal-filled plastics, on the other hand, can be manufactured with significantly lower resistivity, less than 0.50hmcm, but the stress resistance becomes very poor, which is why these materials are not suitable as overcurrent protection.

The resistance of electrically conductive elastomers can be made very low by mixing in metal powder, for example 2mOhmcm or lower. An advantage of elastomers is that they are very soft compared to carbon-filled polyethylene and polypropylene even at high levels of electrically conductive fillers, typical shoretal according to American standard ASTM D224O (Q / C) between 20-80.

OBJECT OF THE INVENTION The object of the present invention is to provide a relatively simple and inexpensive protection device which can limit the highest short-circuit currents occurring in low voltage networks even at very high current derivatives, and whose trip characteristics i.e. sensitivity can be easily adapted to the object of protection. This is achieved according to the invention by a device which has the features stated in claim 1. By deforming at least one electrically conductive elastomeric body of the electrically conductive elastomer body 470118 included in the current limiting element, by means of a pressure device, and integrating electrodes for conducting current through the current limiting element, a much more effective current limiting limitation is achieved than with conventional described under the heading "State of the art". This entails significant cost advantages, especially on the downstream side of the current limiting element. The device can replace both conventional fuses and so-called circuit breakers (MCBs), and has the advantages of both of these fuse types without their disadvantages, such as the limited life of the fuse and the circuit breaker.

The device acting as a current limiting element includes at least one electrically conductive elastomer body and 2 electrodes. The polymer composition of the elastomer body may be of a known type and does not form part of the present invention.

Examples of suitable elastomers which may be mentioned in particular are butyl, natural, polychloroprene, neoprene, EPDM and silicone rubber.

The conductive powdery material preferably consists of silver, nickel, cobalt, silver-plated copper, silver-plated nickel, silver-plated aluminum, carbon black, conductive soot or carbon black. Suitable particle size of the powdered material is 0.01-10 micrometers and suitable content of the powdered filler is 40-90% of the total weight of the powdered filler and elastomeric material. The resistivity of the electric elastomer body is preferably in the range 0.1 mohmcm - 10 Ohmcm. If more than one body of electrically conductive elastomer is included in the device, the bodies may be of the same or different elastomers and then with the same or different fillers and resistivity. The electrodes are of a conventional type, for example silver-plated copper. They have preferably been oriented so that when they are traversed by high currents, repulsive forces arise between said electrodes.

The pressure maintained on the electrodes, for example with a known pressure device described in US 3,914,727 or a conventional spring mechanism, on / off, for electrical couplers, deforms the convex abutment surface of the elastomer bodies where such an abutment surface is included. The deformation preferably amounts to at least 5%. Particularly preferred is a deformation of 5-30%, defined by the distance between the bodies adjacent to a considered elastomer body, i.e. if the distance, when the pressure is 0 and adjacent bodies abut the elastomer body, is d, and after the pressure has been applied the distance changed to O.7 ~ d, the body has been deformed 309. In particular, elastomeric bodies with a hardness between 30-50 IRHD in accordance with British standard BS903 / A26 are preferred, but materials with both lower and higher hardness may be used.

According to a particularly preferred embodiment of the invention, the pressure device is provided with pressure-applying devices capable of springing. Such a preferred design of the pressure device markedly facilitates a separation and thus a reduction of the transition area between the convex abutment surface of the elastomeric bodies where such an abutment surface is included and adjacent body.

According to a particularly preferred embodiment of the invention, the body of electrically conductive elastomer, if only one is included in the current limiting element, is inserted between an electrically insulating plate with a slot. The elastomer body is placed in the slot, which when the elastomer body is subjected to pressure, fills the slot space. This provides an electrical insulation that prevents electrical shocks in the event of a short circuit.

According to a further embodiment of the invention, an elastomer body is stacked on another elastomer body according to the invention in the same pressure device.

According to a further embodiment of the invention, the elastomeric body has been designed hollow, which means that the body can be deformed considerably more than 30%, determined by the diameter of the hole. The advantage of this solution is also that relatively hard elastomeric materials can be selected and the elastomeric body is still allowed to deform significantly. According to the present invention, it has been found possible to counteract or completely eliminate the disadvantages described under the heading "Prior art" such as insensitivity etc. of the short-circuit protectors. The current limiting element changes its resistance when high short-circuit currents occur at a lower energy development, whereby the thermal and mechanical inertia has decreased. Furthermore, the current limiting element returns to the initial resistance after the transition from low-resistivity to high-resistivity state, and is reusable even after it has been subjected to short-circuit currents. A possible explanation for the result obtained according to the invention may be the following.

During normal current passage, a low transition resistance is maintained between the elements which make contact with each other through the transition surface formed when the body with convex abutment surface, or the bodies if several are included, are deformed due to an external pressure device. When high short-circuit currents occur, the electrodes want to separate by current forces. In addition, so-called restrictive forces arise in the transition between the convex abutment surface, of the elastomeric bodies where such an abutment surface is included, and adjacent bodies, due to the design of the preferred abutment surface. As a result, the energy development increases more rapidly in the decreasing transition surface, which means that the resistance of the elastomer body increases sharply in the transition surface without the elastomer body otherwise being subjected to impermissibly high stresses. In addition, due to the cross-section of the preferred elastomer body, the current density is greatest along the line of symmetry of the cross-sectional area between the electrodes, which causes the material to be most stressed here, thereby preventing cracking and blistering in cross-section perpendicular to the current direction. By combining the physical properties described in the prior art such as, the pressure sensitivity of electrically conductive elastomers, transition surfaces, obtained electrodynamic repulsive action by suitable geometry design of electrically conductive elastomer bodies and electrodes together with a suitable choice of electrode material, for current limitation, the following advantages are achieved: a) b) d) e) f) 9) h) Significantly increased sensitivity at high current derivatives and short-circuit currents due to a resilient pressure device and preferred electrode design which together with the specially designed, electrically conductive, elastomer body wants to repel the electrodes.

The device can be made very low impedance due to deformation of the contact transition between electrically conductive elastomer body and electrode.

Reduced selectivity problems in electrical circuits where there is superior and subordinate protection.

The element returns to the initial resistance after the transition from low-resistive to high-resistive state.

Simple switching device, which may not need to be provided with arc shields, if the electrodes are mechanically connected to a conventional switch-on / switch-off mechanism for switches, which in addition then maintains the required pressure between electrode (= contact) and electrically conductive elastomer body.

Eliminated welding risk when a breaker device according to point e) above is inserted.

Vibration-insensitive and bounce-insensitive switch-on function.

Possibility to adjust the sensitivity if the pressure, which is maintained by the pressure device in a known manner, is adjustable and can be varied, whereby one and the same short-circuit protection can be used in an extended rated current range. i) Very small external dimensions because the resistivity of the electrically conductive elastomeric material can be obtained very low <l mohmcm j) Exclusive chokes can be eliminated in thyristor circuits.

DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail by describing exemplary embodiments with reference to the accompanying drawings, in which Figs. 1a-c Fig. 2 Fig. 3 Fig. 4 Fig. 5 shows a central section through three preferred embodiments of the invention, which essentially consist of electrically conductive elastomer bodies and electrodes. shows the resistance R as a function of the distance d between 2 electrodes, between which an electrically conductive elastomer body, with a cross section of a half-cylinder, with the radius r, is compressed. shows an embodiment of the current limiting element (100) connected in an electrical circuit according to the invention. shows the current course in the event of a short circuit with a protection device according to figure 3. shows a comparison between fiz-dt curves for a current limiting element (100) according to the invention and conventional protection such as fuse and power rider, MCCB. 12 shows a central section through an elastomer body according to the invention with associated electrodes, and a repulsion device - Figs. 8-19 show further variants of embodiments of current limiting elements according to the invention.

Figure 6 shows a current limiting element according to a device analogous to that shown in Figure 1b, containing a centrally arranged body (10) in the form of a homogeneous cylinder with a diameter of 3 mm and a length of 10 mm of a deformable electrically conductive elastomeric material, for example consisting of 80% silver powder. and 20% by weight of silicone plastic, as well as two parallel electrodes tangential to the body, placed on the opposite side of the body (10) (ll, l2). In this exemplary embodiment, the elastomer body (10) has a shore number of 40 according to BS 903 / A26. The electrodes (ll, l2) consist of 0.7 mm thick angled plates of silver-plated copper. The electrodes abut against the body (10) by means of a spring device (14), which in a known manner exerts a pressure against the electrodes (l1, l2) by spring action, and thereby deforms the abutment surfaces (10 ', 10 ") of the body against the respective electrode, a deformation of A repulsion device (13), described in eg GB 1 519 559 or GB 1 489 010 may be included in the device to increase the sensitivity, or that the electrode design itself, due to the design, gives rise to repelling electrodynamic current forces.

The repulsion device (13) may alternatively be a self-activating magnetic circuit, previously described in US 4,513,270, arranged to act only on one electrode, and directed so that the electrodes want to separate from each other due to magnetic forces or electrodynamic current forces. The resistance across the device is 2mOhm. When the device is exposed to high short-circuit currents, preferably more than 50A, especially more than 50OA, the current density in the deformed abutment surfaces (10 ', 10 ") increases, the resistance in the element increasing to 10 m / h or more. This is sufficient to limit short-circuit currents in low-voltage installations. through the preferred device in Figure 6 and the connection according to F) 10 15 20 25 30 35 41:. <<CI) 13 Figure 3, is limited and gives a current-time diagram according to Figure 4- Figure 7 shows a current limiting element according to Figure 6. and Fig. 1c, where the elastomer body (20) is not homogeneous, unlike in Fig. 6. A hole (9) has been inserted, which means that the deformation of the elastomer body can be increased to 30% or more determined by the dimensions of the hole. shoretal eg 80 is selected- It is particularly suitable to deform the body (20) so that the convex abutment surface (9 ') thus formed is in physical contact with the abutment surface (9 ").

Figure 8 shows an embodiment of the invention where two electrically conductive elastomer bodies (10a, 10b) have been stacked on top of each other, while in Fig. 9 the electrically conductive elastomer bodies (10a, 10b) have been placed next to each other.

Figures 10a-b illustrate a device according to the invention, in which an electrically conductive elastomer body (10) according to Figure 7 is arranged between 2, longitudinal electrodes (11, 12) parallel to the body (10). The pressure against the electrodes and the abutment surfaces of the elastomer body (10 ', 10 ") is provided by the previously described resilient pressure device.

Figure 11 illustrates a device according to the invention, where an electrically conductive elastomer body (10) is arranged between 2 electrodes (11, 12) according to Figures 10a-b. A ferromagnetic repulsion circuit (13) encloses the longitudinal electrodes (l1, l2) and the elastomer body (10) and amplifies the repulsive action on the electrode (ll) when the current limiting element is traversed by overcurrents. The pressure against the electrodes and the abutment surfaces (10 ', 10 "of the elastomer body) is provided by a previously described resilient pressure device. Figure 12 shows a device analogous to Figures 10a-b, with the difference that the electrically conductive elastomer body (10) is designed as a half-cylinder, and can be securely anchored with an electrically conductive adhesive to electrode (12) or abut freely.

Figure 13 illustrates a device according to the invention, in which two electrically conductive elastomer bodies (10a, 10b) are arranged between 2 electrodes (11, 12), between which two further elastomer bodies (100) and (10d), respectively, which enclose the electrodes (11, 12 ), has been placed. The pressure against the electrodes and especially the elastomeric bodies provided with convex end surfaces is provided by a previously known pressure device.

Figure 14 illustrates a further embodiment according to Figures 12 and 9 of the invention in which the elastomer bodies (10c, 16a) and (10e, 16b) enclosing the electrodes (11, 12) are constituted by electrically conductive (10c, 10e) and electrically insulating (16a), respectively. , 16b) elastomeric material. The elastomeric bodies (10c, 16a) and (10e, 16b), respectively, are advantageously double-molded, the elastomeric bodies becoming cohesive and the electrodes electrically insulated. Electrical connections to the electrodes are not drawn in the figure.

Figure 15 illustrates a device according to Figures 6 and 7 according to the invention, in which two pieces of electrically insulating polymer bodies (15a, 15b), of polyethylene, are arranged in parallel with an electrically conductive elastomer body (10).

When the device is subjected to a pressure, symbolized by the force F acting on the electrodes (11, 12), body 10 is deformed, and thus abuts against the boundary surfaces (15a ') and (15b') of the electrically insulating bodies. This provides an electrical insulation which prevents overcurrent in the event of a short circuit, at the same time as the electrically conductive elastomer body does not flow out, which is otherwise a common problem.

Figure 16 illustrates a device according to the invention in which the electrically conductive elastomer body (10) has several convex deformable bearing surfaces (10a ', 10b', 10c'10D '), consisting of several integrated elastomer bodies according to previous figures. The elastomer body (10) is continuous and homogeneous.

Fig. 17 illustrates a device according to the invention in which the electrically conductive elastomer body (10) has a convex deformable abutment surface in a "spline design". , consisting of several integrated elastomer bodies according to previous figures.

The elastomeric body (10) is thus continuous, and several convex surfaces can be activated by, for example, increasing the pressure with the pressure device (14).

Figures 18a-b illustrate a device according to the invention consisting of two electrically conductive elastomer bodies (20a, 20b), with convex deformable abutment surfaces (20a ', 20b') and 2 electrodes (11, 12). The electrodes are enclosed by concentric electrically conductive elastomer bodies (20a, 20b), which abut with the abutment surfaces (20a ', 20b') in physical contact with each other. A pressure device (14) exerts a pressure which deforms the abutment surfaces (20a ', 20b'). The electrodes (11, 12) are provided with connecting means (31) and (32), respectively.

Figure 19 illustrates a device according to the invention in which the electrically conductive elastomer bodies (10a1, 10a2, 10a3, 10a4) with their convex boundary surfaces are oriented perpendicular to the convex of the electrically conductive elastomer bodies (10bl, 10b2, 10b3, 10b4). 2 electrodes (l1, l2) are included in the device for conducting current through it, electrodes on which a pressure device exerts a pressure which deforms the abutment surfaces (10a1 ... 10b ...) - The invention is not limited to the embodiments shown but several variants are possible within the scope of the claims. For example, the number of electrically conductive elastomer bodies stacked on top of each other according to Figure 8 can be considerably more.

Claims (11)

10 15 20 25 30 35 40 / é 470118 Patent claim Device for protection against overcurrent in electrical circuits, which comprises at least one electrically conductive and consisting of elastomeric material (10) and two electrodes (11, 12) intended for supplying the circuit current through it, each - directly or via an intermediate part - in corresponding places abuts against the body (10), a pressure device (14) producing abutment pressure, characterized in that the electrodes (11, 12) are designed so that they, under the influence of short-circuit currents, strive to repel each other, and that the contact surface decreases with passing overcurrent in that the body (10) and / or at least one electrode / intermediate part in a pressure-loaded state is convex in a manner known per se but deformed by the pressure device at the respective contact point during pressure loading. Device according to claim 1, characterized in that the intermediate part also consists of elastomeric material. Device according to claim 1 or 2, k ä n n e - t e c k n a d has shoret numbers between 20-80. Device according to Claim 1, 2 or 3, in that the elastomeric bodies included in the device are characterized in that the pressure device (14) can spring. Device according to one of Claims 1 to 4, characterized in that the pressure device (14) consists of a spring mechanism with two mechanically stable equilibrium positions, for resp. from, and that at least one electrode (11a) is mechanically connected to the spring mechanism (14) for galvanic separation between electrode (11) and 12). Device according to one of Claims 1 to 5, characterized in that the abutment pressure applied by the pressure device (14) is adjustable. Device according to one of Claims 1 to 6, in that the electrodes (11, 12) are provided with ferromagnetic circuits (13) which want to amplify the characteristic between the electrodes. Device according to one of Claims 1 to 7, characterized in that elastomer bodies (20a, 20b) are expanded over the electrodes (11, 12). Device according to one of Claims 1 to 8, characterized in that the pressure device (14) provides 470 118 commenced compression of the body (10) amounts to at least 5%, when the body is homogeneous. Device according to claim 9, characterized in that compression of the body (10) achieved by the pressure device (14) amounts to 5% - 40%, when the body is homogeneous. 11. An overcurrent protection device according to any one of the patent claims (20) has a centered cavity (9). claims 1-8, that the elastomer body