TECHNICAL FIELD

The present invention relates to a device for overload and short-circuit protection in electric plants . The device comprises a circuit breaker and a current limiter arranged in series with the circuit breaker and comprising at least two parallel-connected current branches, of which one branch comprises a thermistor with a positive temperature coefficient, and the other comprises an impedance for current and voltage relief of the thermistor upon a short circuit. The protection device is primarily intended for use in low-voltage plants by which is meant here plants with an operating voltage of at most 1000 V.

BACKGROUND ART

A device of the above-mentioned kind, which includes a ther¬ mistor of a ceramic type, is previously known from patent publication DE-A-2 510 322. A disadvantage of ceramic ther¬ mistors is that they exhibit a negative temperature coeffi¬ cient when the temperature exceeds a certain value. In addi- tion, a ceramic thermistor will easily crack if traversed by a short-circuit current because of the thermal and mechani¬ cal stress to which it is subjected when it changes to its high-resistance state. In addition, it is relatively expen¬ sive.

Another type of thermistors intended for overcurrent protec¬ tion is known from, inter alia, patent publications EP-A-0 038 716 and EP-A-0 087 884. Thermistors of this type are composed of a polymeric material, for example high-pressure polyethylene, containing particles of an electrically con¬ ductive material, for example carbon black. A polymer-based thermistor of the above-mentioned kind possesses several advantages compared with a ceramic one. For example, its

resistance in the temperature range in question increases monotonously with the temperature and the resistance increase is considerably greater. In addition, it is much less expensive. However, a disadvantage with hitherto known thermistors of the polymer-based type is that the resistance of such a thermistor, after having changed from a low- resistance to a high-resistance state, does not return to the initial resistance. In more serious cases, when the thermistor is subjected to very great electrical stresses, such as short-circuit currents, the formation of blisters and cracks occurs in the central or other parts of the polymer composition of the thermistor so that it can no longer operate, that is, the thermistor is destroyed.

In the known polymer-based thermistors, the aim has been to secure the electrodes as efficiently as possible to the body of polymer composition to achieve the best possible electrical contact and hence minimize the transition resis¬ tance. To obtain the good anchoring of the electrodes, they are normally designed with an uneven surface structure on the side which faces the body of polymer composition, such that the polymer composition during the manufacturing of the thermistor may melt and penetrate into cavities in the elec¬ trode surface. The electrodes usually consist of metal foils and are applied by being pressed onto the body during heating.

SUMMARY OF THE INVENTION

The present invention aims to provide a relatively simple and inexpensive protective device of the above-mentioned kind _r which is capable of breaking the highest short-circuit currents occurring in low-voltage networks and which does not have the above-mentioned disadvantages which previous protective devices with thermistors suffered from. This is achieved according to the invention by a device which exhibits the characteristic features described in claim 1.

The protective device according to the present invention includes a polymer-based thermistor which, contrary to hitherto known thermistors of this kind, after changing from a low-resistance to a high-resistance state returns to the 5 initial resistance, and which is reusable also after having been subjected to short-circuit currents. Further, the thermistor changes its resistance at a lower energy development, that is, its current-limiting properties are improved.

10

According to the invention, the above-mentioned result is obtained by effecting at least one electrical contact between an electrode and a body of electrically conductive polymer composition or between two bodies of electrically

15 conductive polymer composition in the thermistor, by the electrode making free contact with the mentioned body or by the mentioned bodies making free contact with each other while a pressure device maintains a pressure directed perpendicular to the contact surface in question.

20

The pressure which is maintained on the electrodes perpen¬ dicularly to the parallel surfaces of the body or bodies of polymer composition preferably amounts to at least 0.1 MPa. Especially preferred is a pressure of 0.1 MPa-10 MPa.

25

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail by describing embodiments with reference to the accompanying 30 drawings, wherein

Figure 1 shows a circuit diagram for a protective device _{* .} according to the invention,

n 35 Figure 2 shows a curve of the resistance versus the tempe¬ rature for a thermistor included in the protective device according to Figure 1,

Figures 3 and 4 show in cross section two different thermis¬ tors consisting of two electrodes and one and two bodies, respectively, of electrically conductive polymer composition between the electrodes,

Figures 5 and 6 show in cross section two different embodi¬ ments of a thermistor, provided with a pressure device, of the type which is included in the pro¬ tective device according to Figure 1,

Figure 7 shows the variation of the current during a short- circuit breaking with a protective device accor¬ ding to Figure 1,

Figures 8a, 8b and 8c show in perspective different parts of a current limiter included in the protective device according to Figure 1, in dismantled condition,

Figure 9 shows in perspective part of the same device in a vertical section, and

Figures 10 and 11 show circuit diagrams for additional variants of the current limiter included in the protective device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device for overcurrent and short-circuit protection shown in Figure 1 comprises a circuit breaker and a current limiter connected in series with the circuit breaker. The circuit breaker may, for example, consist of a so-called automatic fuse, also called MCB (Miniature Circuit Breaker) or midget circuit breaker, with a rated current of, for example, 63 A. The circuit breaker comprises a contact device 1, a bimetallic tripping device 2 adapted to initiate the opening of the contact device at moderate overcurrents as well as an instantaneous tripping device 3 in the form of

a percussion armature magnet (a so-called kicker) to influence the contact device directly at a short-circuit current. The current limiter comprises a thermistor 4 with a positive temperature coefficient (PTC element) connected in parallel with an impedance 5, which in the embodiment shown consists of a linear resistor with a resistance of, for example, 100 mΩ,

Thermistor 4 is of the above-mentioned polymer-based type. Figure 2 shows the resistance R for such a thermistor versus the temperature T. In the normal operating range of the thermistor, which may extend up to, for example, 80°C, the resistance is low, for example 3 mΩ, and increases slightly with the temperature. If the temperature of the thermistor rises above the mentioned value, for example because of an overcurrent, the resistance increases more rapidly, and when exceeding a certain temperature T , referred to below as the transition temperature, which, for example, may lie at about 120°C, the thermistor suddenly changes from a low-resistance to a high-resistance state, in which the resistance amounts to, for example, 150 mΩ.

The thermistor according to Figure 3 comprises a centrally arranged body 10 of an electrically conductive polymer composition with a positive temperature coefficient, for example consisting of 67 per cent by volume polyethylene and 33 per cent by volume carbon black, in the form of a rec¬ tangular 1 mm thick disc as well as two electrodes 11 and 12, arranged at the parallel end surfaces 10' and 10" of the body (the flat sides of the disc) , with associated terminals 13 and 14, respectively. The electrode 11 consists of an 0.5 mm thick plate of nickel with an even surface structure on both sides. On its outside the plate is coated with a thin layer of copper. It makes contact with the body 10 only by abutting against the body, that is, it is not fixed to the body. The electrode 12 consists of an 0.3 mm thick copper foil which, on its side facing the body 10, is coated with an 0.1 mm thick layer of copper with uneven surface struc-

ture, which layer is applied by plasma spraying. The elec¬ trode 12 is fixed to the polymer by pressing it against the body 10 after the body has been heated such that the polymer material in liquid state may penetrate into irregularities on the inwardly-facing side of the electrode.

The device according to Figure 4 includes two bodies, 10a and 10b, of the same electrically conductive polymer compo¬ sition as that in body 10 in Figure 3, in the form of 1 mm thick rectangular discs with the parallel surfaces 10a',

10a" and 10b ¹ , 10b", respectively. These discs make electri¬ cal contact only by abutment, that is, without being fused or otherwise fixed to each other. The electrodes 11 and 12 may be of the same kind as the electrodes 11 and 12 in Figure 3. However, the electrode 11 may also be of the same kind as the electrode 12 and, like this electrode, be fixed to the body 10a. In the device according to Figure 4, the bodies 10a and 10b may be of different electrically conduc¬ tive polymer compositions and have different resistivities to modify the properties of the thermistor. The device may also be modified so as to comprise more than two bodies (10a and 10b) of conductive polymer composition with the same or a different resistivity. If the electrodes are fixed to adjacent bodies or electrically conductive polymer composi- tion, at least one of the bodies must make free contact with another one of the bodies. If one of or both of the elec¬ trodes make free contact with an adjacent body of electri¬ cally conductive polymer composition, all the polymer bodies may be fixed to each other, for example by fusion.

Figure 5 illustrates a device according to the invention in which a thermistor according to Figure 3 is arranged in a pressure device comprising two plates 15, 16, which are parallel to the parallel end surfaces of the body 10 (the flat sides of the disc 10) and to the electrodes 11 and 12. The terminals 13 and 14 are not shown in the figure. The plates are of electrically insulating material, for example resin reinforced with glass fibre. The pressure against the

electrodes and against the end surfaces of the body 10 is brought about by tightening a number of bolts 17. Between the bolt heads 17a and one of the plates 16 of the pressure device, stiff springs 17b are arranged which are not com- pletely compressed when the thermistor is pressurized and prepared for normal operation. In case of a short circuit, the springs may therefore be further compressed, which permits the electrode 11 to be separated from the body 10.

The device according to Figure 6 differs from the device according to Figure 5 in that the springs 17a are not included and in that a 1 mm thick mat 30 of rubber is arranged between the electrode 11 and the plate 15. Such a mat of rubber may possibly be arranged between the electrode 12 and the plate 16 as well.

Instead of the thermistor according to Figure 3, a thermis¬ tor according to Figure 4 may be arranged, in an analogous manner, in the devices according to Figures 5 and 6.

Figure 7 shows the variation of the current during a short- circuit breaking with a protective device according to Figure 1 at a network voltage of 500 V and a network short- circuit current of 30 kA. In normal operation the thermistor 4 (Fig. 1) has a low resistance, for example 3 mΩ, and the entire operating current flows therethrough. If a short circuit occurs, the current rises very rapidly, and even after about 1 ms from the occurrence of the short circuit (Fig. 7), the thermistor trips (i.e. changes from a low- resistance to a high-resistance state) . This causes a very fast limitation of the current, which is for the main part commutated over to the parallel resistor 5, which has a resistance of about 100 mΩ. In this way, the thermistor is relieved of both current and voltage, whereby breakdown of the polymer material in the thermistor is avoided. After an additional couple of milliseconds, the contact device 1 is opened by the action of the percussion armature magnet 3, whereby the current is broken after about 5 ms from the

instant of short circuit. Since the current is limited to a relatively low value because of the current limiter 4, 5 and, in addition, is substantially resistive, the breaking is relatively easy. Through the contact opening a rapid voltage relief of the thermistor 4 is obtained, and there¬ fore the thermistor need not be designed to continuously withstand the operating voltage.

A feasible explanation of the result obtained with the protective device according to the invention may be the following. When the thermistor is traversed by short-circuit current, the heating of the polymer composition is concen¬ trated at the initial stage substantially at contact sur¬ faces between those elements in the thermistor which make contact with each other, since the resistance is higher at the transition between the elements than in the polymer composition itself. This leads to the formation of gas in the material surface with ensuing pressure build-up between the elements, which, in addition, are influenced by electro- dynamic repulsion forces. This leads to a separation of the elements and hence to a reduction of the number of contact points between electrode and conducting particles in the body of the polymer composition making contact therewith, or between conducting particles in bodies of the polymer com- position making contact with each other. This leads to a current concentration at the remaining contact points, which causes molten phases to appear in the polymer composition at the contact surface and the thermistor to trip at the con¬ tact surface without the polymer composition otherwise being subjected to any stress with ensuing unfavourable effects. Since the compression pressure on the thermistor remains, when the contact surfaces even after the short circuit are hot, the original contact and the original transition resis¬ tance can be resumed between the elements making contact with each other.

The device according to Figures 8a, b, c and 9 comprises three identical thermistors, one of which being shown with

the parts separated in Figure 8b. The body 10 of electri¬ cally conductive polymer composition, for example of the same kind as in Figure 3, is arranged in a plastic frame 40 which extends around the body. The two electrodes 11 and 12 consist of 1 mm thick silver-plated plates of copper which make contact with the body 10. only by abutting against it, that is, they are not fixed to the body. Each electrode is provided with alternative conductor terminals 13a and 14a for cable connection and 13b (not shown) and 14b, respec- tively, for connection to a busbar. Outside the electrode 11 a wedge-shaped, loose plate 41 is arranged, and outside the electrode 12 a loose spring plate 42 of thin sheet metal is arranged. The spring plate is provided with a plurality of bent-out tongues 42a, which are cut out from the plate and which impart to the plate its resilient properties . The electrode plate 12 supports the parallel resistor 5. A package comprising the plates 41 and 42 and electrodes 11 and 12 arranged therebetween as well as with an intermediate body 10 of conductive polymer composition is arranged in each of three compartments 43a, 43b and 43c in an apparatus housing 43 with two intermediate walls 43d and 43e which form the compartments. The intermediate walls are parallel to two opposite parallel walls 43f and 43g in the apparatus housing. The apparatus housing is provided with recesses 44a and 44b for alternative connection of a conductor to the attachment 14a or 14b in the compartment 43a, with recesses 45a and 45b for alternative connection of a conductor to the attachment 14a or 14b in the compartment 43b, and with recesses 46a and 46b for alternative connection of a conduc- tor to the attachment 14a or 14b on an electrode 12 in the compartment 43c. On the opposite, not visible wall of the apparatus housing, there are corresponding recesses for the connection of conductors to attachments 13a and 13b on each electrode 11. The pressure between the electrodes 11 and 12 and the body in each one of the packages in the compartments 43a, 43b and 43c is effected by applying on the apparatus housing a cover 47 with wedge-shaped plates 48, 49 and 50 fixed on the cover, each of the wedge-shaped plate thus

being inserted into a respective one of the compartments 43a, 43b and 43c until contact is made with a plate 41 located there. The cover is arranged with mechanical attachments 47a, 47b, 47c and 47d for mounting. As will be clear from Figure 9, the apparatus housing is provided with plane-parallel reinforcement walls 51 and 52 on the pressure-absorbing sides. The device according to Figures 8a, b, c and 9 is intended to be connected into a three- phase line with a thermistor connected into each of the three phase conductors. The apparatus housing 43 with intermediate walls 43f and 43g, reinforcement walls 51 and 52, cover 47 and wedge-shaped plates 41, 48, 49 and 50 are all made of an electrical insulating material, for example polyamide, provided with a filler, for example in the form of short glass fibres.

The impedance for current and voltage relief of the thermistor 4 need not necessarily consist of a linear resistor 5, but several other variants are possible. Figure 10 shows a second embodiment in which the thermistor 4 is connected in parallel with a voltage-dependent resistor 6, preferably in the form of a ZnO varistor. Figure 11 shows a third embodiment in which the thermistor 4 is connected in parallel with a second thermistor 7 with a higher initial resistance, for example 10-50 mΩ, and with a relief resistor 5, which may be linear or voltage-dependent. The relief impedance may also consist of a thermistor with a negative temperature coefficient (NTC element) . In protective devices for higher rated currents, at least the electrodes 11, 12 of the thermistor 4 are provided with cooling means, for example in the form of cooling flanges, which are then arranged thermally connected but electrically insulated from the electrodes.