Residual current protective switch for a direct current voltage network

Abstract

The subject of the invention is a residual current protective switch (1) for a direct current voltage network (3), which comprises a primary coil (5), a secondary coil (7), a diametrically polarised permanent magnet (10), a magnet stabiliser (12), and a circuit breaker (13). The primary coil (5) and the secondary coil (7) have substantially identical electromagnetic and mechanical properties and are spatially arranged symmetrically with respect to the central position in which the diametrically polarized permanent magnet (10) is arranged, such that the sum of the magnetic field of the primary coil (5) and the magnetic field of the secondary coil (7) in the central position is zero when the current (I1) through the primary coil (5) is equal to the current (I2) through the secondary coil (7).

Description

[0001] Residual current protective switch for a direct current voltage network

[0002] The invention relates to protection of electrical networks, more specifically to a residual current protective switch for protecting direct current voltage networks. These types of protective switches cause the network to disconnect from the voltage source when a difference is detected between the current flowing from a voltage source to a network load and the current flowing from a network load to a voltage source. A difference between these two currents means, for example, that part of the current from the network flows to the earthing, i.e. that an unwanted contact with the earthing has occurred in the network, which is why it is necessary to disconnect the voltage source from the network.

[0003] Residual current protective switches are known that are used in alternating current circuits containing a toroidal core with a primary winding, a secondary winding and a detection winding, and a circuit breaker. A phase conductor runs from the voltage source through the primary winding to the network, while a neutral conductor runs from the network through the secondary winding to the voltage source. The primary winding and secondary winding are wound around the toroidal core in opposite directions, so that when the same current flows through them, the magnetic fields of both windings in the toroidal core cancel each other out. A detection winding is also wound around the toroidal core, in which voltage is induced if the magnetic field in the toroidal core is not equal to zero. A circuit breaker is connected to the detection winding, which circuit breaker is configured, for example, as a relay that cuts off the phase conductor and neutral conductor of the voltage source from the network when a certain voltage is present. So, if there is a difference between the current in the phase conductor and the current in the neutral conductor, a magnetic field is generated in the toroidal core, which is not equal to zero, inducing a voltage in the detection coil, which causes the voltage source to be disconnected from the network via a relay.

[0004] Type B residual current protective switches (Type B Residual Current Circuit Breaker) are known for detecting not only differences in the alternating current component in the circuit but also differences in the direct current component up to a certain frequency. An example of such a switch is a switch bearing the commercial designation F200 B-Type manufactured by ABB Asea Brown Boveri Ltd. (https: / / new.abb.com / low-voltage / products / system-pro-m / residual-current-devices / f200-b-type). Such protective switches contain many electronic components for filtering the signal, obtaining a direct current component of the current, amplifying the direct current signal to make it suitable for detection, and detecting the amplified direct current component of the current.

[0005] The residual current protective switch according to the present invention represents a new solution to such direct current switches. The advantage of the switch according to the present invention is its simple construction and production since it does not contain many mechanical and electronic components. This results in increased robustness and reliability of operation. Due to the simplicity of manufacture and a small number of components used, the manufacture of the switch according to the present invention is cost effective. A further advantage of the switch according to the invention is also the low resistance of the switch, resulting in low overheating and low losses due to overheating. Low overheating contributes to increased reliability of the switch operation. In certain embodiments, once the switch is produced, the design of the switch according to the present invention allows adjusting the sensitivity of the switch, i.e. the difference between the currents in the conductors that causes the circuit to be disconnected. This simplifies the production process of a single type of switch, which can be adapted to multiple applications by adjusting its sensitivity. In various embodiments, the switch according to the present invention may be used in direct current circuits with small or large currents, for example in generating electricity, such as solar panels, charging electric batteries, such as charging stations for electric cars, and in all direct current electric motors, for example in cars or other vehicles.

[0006] The residual current protective switch for a direct current voltage network according to the present invention comprises a primary coil, a secondary coil, a diametrically polarised permanent magnet (hereinafter also a permanent magnet), a magnet stabiliser, and a circuit breaker.

[0007] The protective switch according to the present invention will now be described in more detail and illustrated by embodiments and in figures:

[0008] - Figure 1 is a schematic representation of an embodiment of a protective switch according to the present invention and its connection to a network.

[0009] - Figure 2a and Figure 2b show a diametrically polarized permanent magnet in two embodiments having two different equilibrium positions with respect to a primary coil and a secondary coil when a magnet stabilizer is configured as an area of the core, around which the primary and secondary coils are wound.

[0010] - Figure 3 shows a diametrically polarized permanent magnet that gets offset from the equilibrium position due to the difference between the current flowing through the primary coil and the current flowing through the secondary coil.

[0011] - Figure 4a and Figure 4b show a diametrically polarized permanent magnet in two embodiments having two different equilibrium positions with respect to the primary coil and the secondary coil when the magnet stabilizer is configured as an iron element.

[0012] - Figure 5 shows an embodiment of a circuit breaker in an embodiment of the protective switch according to the present invention.

[0013] - Figure 6 schematically represents an embodiment of the protective switch according to the present invention, in which the primary coil and the secondary coil are divided into two parts: a core winding wound around the core, and a coreless winding not wound around the core.

[0014] A primary coil 5 and a secondary coil 7 substantially have identical electromagnetic and mechanical properties. The primary coil 5 is connected to a positive conductor between a voltage source 2 and a network 3, while the secondary coil 7 is connected to a negative conductor between the voltage source 2 and the network 3. The primary coil 5 and the secondary coil 7 are spatially arranged symmetrically with respect to the central position such that the sum of the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7 in the central position is zero when the current 11 through the primary coil 5 is equal to the current I2 through the secondary coil 7. For example, in the embodiment represented in Figure 1 , the axis of the primary coil 5 and the axis of the secondary coil 7 are aligned with each other, the distance of the primary coil 5 to the central position being the same as the distance of the secondary coil 7 from the central position. In preferred embodiments, the central position is positioned at a spot that satisfies the necessary condition of spatial symmetry, i.e. an equal distance both from the primary coil 5 and the secondary coil 7, and the additional condition which is that the yield of the magnetic field of a respective coil 5, 7 at this spot is maximal, so that any difference between the yield of the magnetic field of one coil 5, 7 relative to the yield of the magnetic field of the other coil 5, 7 at this spot is as distinctive as possible.

[0015] A diametrically polarized permanent magnet 10 with a longitudinal axis 11 that extends substantially on the limit between the north pole and the south pole of this permanent magnet 10 is arranged in the central position in a way that this permanent magnet 10 is allowed to rotate around the longitudinal axis

[0016] 1 1 and that the permanent magnet 10 gets offset around its longitudinal axis 11 if the sum of the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7 is not zero, with respect to the equilibrium position of the permanent magnet 10, when the sum of the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7 is zero.

[0017] A magnet stabilizer 12 is positioned at a certain radial distance R from the permanent magnet 10 and attracts one of the poles of the permanent magnet 10. The magnet stabilizer 12 is preferably spatially positioned such that its distance from the primary coil 5 is the same as the distance from the secondary coil 7. The main function of the magnet stabilizer 12 is to ensure a unique equilibrium position of the permanent magnet 10 when the primary coil 5 and the secondary coil 7 do not generate a magnetic field, or rather when the sum of the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7 in the central position is zero. The magnet stabilizer 12 also forces the permanent magnet 10 to return to the equilibrium position when there is no longer any difference between the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7. The magnet stabilizer

[0018] 12 is made of one or more materials that attract the north or the south pole of the permanent magnet 10, for instance ferromagnetic materials, such as iron, nickel, cobalt; or alloys such as Alnico (aluminium- nickel-cobalt, permalloy, or silicon iron; or ceramic and sintered materials, such as ferrites or NdFeB (neodymium-iron-boron), or natural materials such as magnetite (FesO4).

[0019] In various embodiments, by changing the strength of the magnet stabilizer 12 and / or a distance R between the magnet stabilizer 12 and the permanent magnet 10, the force needed for the permanent magnet 10 to get offset from the equilibrium position to the cut-off offset that causes disconnection due to a difference in the magnetic field of the primary coil 5 and the secondary coil 7 can be regulated. With all other parameters remaining the same, a larger distance R requires a smaller difference between currents 11 and I2 for the cut-off offset of the permanent magnet 10 and consequently for the disconnection. The higher the strength of the magnet stabilizer 12, with all other parameters remaining the same, the larger difference between the currents 11 and I2 is needed for the cut-off offset of the permanent magnet 10 and consequently disconnection. In this way, an embodiment of the protective switch 1 according to the present invention can be dimensioned in a way for the permanent magnet 10 to have a desired cut-off offset from the equilibrium position at a certain difference between the current 11 through the primary coil 5 and the current I2 through the secondary coil 7. The embodiments of the protective switch 1 according to the present invention, in which the distance R can be adjusted, are suitable for use in a plurality of different applications despite the same construction, where cut-off occurs at various differences between the currents 11 and I2 by adjusting the distance R.

[0020] A circuit breaker 13 is adapted to interrupt the connection on the positive conductor, preferably also on the negative conductor, between the voltage source 2 and the network 3 when the offset of the permanent magnet 10 from the equilibrium position is big enough, i.e. when it reaches the cut-off offset. The circuit breaker 13 may be configured in known ways. In one of the embodiments, shown in Figures 1 and 5, the circuit breaker 13 is configured as a combination of a fork optoelectronic sensor 15 and a relay 17. When the fork optoelectronic sensor 15 detects a cut-off offset of the permanent magnet 10 as a result of the difference between the currents 11 and I2, it generates a cut-off signal at the output. The fork optoelectronic sensor 15 detects the offset of the permanent magnet 10 via the position of a lever 16 attached to the permanent magnet 10 and they get offset together. The relay 17 is connected to the output of the fork optoelectronic sensor 15 and adapted and connected in a way that, when a cut-off signal is detected, it disconnects the connection between the voltage source 2 and the network 3 on the positive conductor, preferably also on the negative conductor.

[0021] In other embodiments which are not shown in the figures, the circuit breaker 13 may also be provided with a reflective optoelectronic sensor instead of a fork optoelectronic sensor, wherein, instead of the lever 16, a mirror is attached to the permanent magnet 10, which mirror gets offset together with the permanent magnet 10, this interrupts a light beam detected by the reflective optoelectronic sensor and a cut-off signal is generated at its output.

[0022] Figures 1 and 2a represent an embodiment of the protective switch 1 according to the present invention, which comprises an iron core 18, around which the primary coil 5 and the secondary coil 7 are wound. In other embodiments, other specific ferromagnetic materials can be used for the core instead of iron, such as silicon steel, cobalt, ferrites, permalloy.

[0023] Figure 1 also illustrates the connection of the components of the protective switch 1 according to the present invention to an entire circuit comprising the voltage source 2, the network 3 or rather a load 4 in the network, a test key 21 and the circuit breaker 13 which is configured as the fork optoelectronic sensor 15, and the relay 17. The lever 16 is attached to the permanent magnet 10, which lever offsets from the equilibrium position together with the permanent magnet and co-operates with the fork optoelectronic sensor 15, which is represented in more detail in Figure 5. When the fork optoelectronic sensor 15 detects a sufficiently large offset of the lever 16 from the equilibrium position, i.e. cut-off offset, a cut-off signal is generated at the output. The relay 17 is connected to the output of the fork optoelectronic sensor 15 and when it receives a cut-off signal, it interrupts the connection between the voltage source 2 and the network 3 on both the positive conductor and the negative conductor.

[0024] In the illustrated embodiment, the axis of the primary coil 5 and the axis of the secondary coil 7 are aligned with each other. This embodiment, which is represented in more detail in Figure 2a, includes the core 18 which is shaped in the area around the central position in a way to enclose at least one pole of the permanent magnet 10, in the instant case the north pole, a distance D1 between the north pole of the permanent magnet 10, when it is in the equilibrium position, and an area 20 of the core 18 that is closest to the permanent magnet, is smaller than distances D2 between the two edge boundary areas between the north and south poles of the permanent magnet 10, when it is in the equilibrium position, and the two areas of the core 18 closest to them. In this embodiment, the area 20 of the core 18, from which the distance D1 to the north pole of the permanent magnet 10 is measured, acts as the magnet stabilizer 12, so the distance D1 is at the same time the distance R. In the embodiments represented in Figures 1 , 2, this area 20 is separated by a gap 19, so one part of the area 20 is located at one side of the gap 19, for example on the left side of the gap 19, while another part of the area 20 is located on the other side of the gap 19, for example on the right side of the gap 19.

[0025] The core 18 is preferably divided into two sections 18, 18’ by at least one air gap 19. In the represented embodiment, the core 18 is divided into two sections 18’, 18” by two air gaps 19 because the core 18 splits into an upper part of the core 18 and a bottom part of the core 18 in orderto enclose the permanent magnet 10 around both poles. One air gap 19 splits the core 18 in the upper part of the split core 18 and the other air gap 19 splits the core 18 in the bottom part of the split core 18. The air gap 19 is positioned in the core 18 symmetrically with respect to the primary coil 5 and the secondary coil 7. In the embodiment represented in Figures 1 and 2a, both air gaps 19 are positioned symmetrically with respect to the primary coil 5 and the secondary coil 7. One or more air gaps 19 in the core 18 contribute to the magnetic field of the primary coil 5 and the magnetic field of the secondary coil 7 extending into the space around the core 18 so that it can influence the permanent magnet 10. This is why the air gaps 19 in the core 18 are positioned close to the permanent magnet 10, in the upper part in the area 20 of the core 18, which is located at the distance D1 , and in the bottom part in an area 20’ of the core 18, which is located at a distance D1 ’. In the absence of a magnetic field from the primary coil 5 and the secondary coil 7, the permanent magnet 10 assumes its equilibrium position due to the distance D1 being smaller than the distance D2. The permanent magnet 10 will remain in the equilibrium position or will assume this position also when the current 11 through the primary coil 5 and the current I2 through the secondary coil 7 are the same. In the represented embodiment, the protective switch 1 according to the present invention comprises a further magnet stabilizer 12’, i.e. the area 20’ of the core 18 that is located at the distance D1 ’ from the south pole of the permanent magnet 10 when the latter is in the equilibrium position. For this area 20’ of the core 18 being able to act like a further magnet stabilizer 12’, the distance D1 ’ must be shorter than the distances D2. The distances D1 and D1 ’ are preferably the same. The core 18 enclosing the permanent magnet 10 has one further function, it namely serves as a magnetic shield that reduces the impact of potential external magnetic interference on the permanent magnet 10, so it increases the reliability of operation of the protective switch 1 of the present invention. What was explained above about the area 20, applies by analogy also to the area 20’, namely in the embodiments represented in Figures 1 and 2, said area 20’ is separated by a gap 19, so one part of the area 20’ is located at one side of the gap 19, for example on the left side of the gap 19, while another part of the area 20’ is located on the other side of the gap 19, for example on the right side of the gap 19.

[0026] Figure 2a more clearly shows the distances D1 , D1 ’ and D2 between the core 18 and the permanent magnet 10 when the latter is in the equilibrium position. Moreover, the winding directions of the primary coil 5 and the secondary coil 7, i.e. the directions of the currents 11 and I2, are also visible. If the currents 11 and I2 are identical, which represents a situation in which both currents are zero, the strength of the magnetic field generated by the coils 5, 7 in the central position is substantially zero. The permanent magnet 10 will therefore assume the equilibrium position determined by the magnet stabilizer 12 and the further magnet stabilizer 12’.

[0027] Figure 2b shows an embodiment that is substantially identical to the embodiment represented in Figures

[0028] I and 2a, a difference being in that the permanent magnet 10 in the equilibrium position is rotated by 180 degrees with respect to the equilibrium position of the embodiment from Figure 1 and Figure 2a.

[0029] Figure 3 shows the position of the permanent magnet 10 of the embodiment from Figures 1 and 2a, when the magnet gets offset from the equilibrium position since the current 11 exceeds the current I2 in this situation. Due to a difference between the currents 11 and I2, the yields of the magnetic field of the primary coil 5 and the secondary coil 7 in the central position do not get cancelled out. Since the current

[0030] II is stronger, the sum of the yields of the magnetic field of the primary coil 5 and the secondary coil 7 in the central position, which is not zero, will cause the permanent magnet 10 to get offset from the equilibrium position to the right side.

[0031] Figures 4a and 4b show the equilibrium positions of the permanent magnet 10 in two embodiments, in which the core 18 was not used and the magnet stabilizer 12 is configured as an iron element 12. Figure 4a shows an embodiment of the equilibrium position of the permanent magnet 10, where the iron element 12 is arranged close to the south pole and symmetrically with respect to the primary coil and the secondary coil. Figure 4b shows an embodiment of the equilibrium position of the permanent magnet 10, where the iron element 12 is arranged close to the north pole. As the iron element in Figures 4a and 4b has substantially the same position, the permanent magnet in Figure 4b is rotated by 180 degrees with respect to the permanent magnet in Figure 4a. Figure 5 is a schematic representation of an embodiment of the circuit breaker 13 comprising the fork optoelectronic sensor 15. The lever 16 is attached with one of its ends to the permanent magnet 10 and projects with the other of its ends into the fork optoelectronic sensor 15. As the permanent magnet 10 gets offset, the lever 16 also gets offset and its other end moves away from the area of the optoelectronic sensor 15. This is detected by the sensor that generates a cut-off signal at the output.

[0032] The magnet stabilizer 12 may also be configured as a magnet positioned at a certain distance R from the permanent magnet 10 and spatially arranged symmetrically with respect to the primary coil 5 and the secondary coil 7 to ensure the symmetrical equilibrium position of the permanent magnet 10.

[0033] In a further embodiment, now shown in the figures, the magnet stabilizer 12 is configured in a way that the distance R between the stabilizer and the permanent magnet 10 can be adjusted in known ways, which makes it possible to set the sensitivity of the protective switch 1 to a difference in the currents 11 and I2. The smaller the distance R, the larger the difference in the currents 11 and I2 that is needed for the permanent magnet 10 to get sufficiently offset, to reach the cut-off offset, which causes, via the circuit breaker 13, the voltage source 2 to get disconnected from the network 3. The advantage of such an adjustable protective switch 1 according to the present invention lies in that it allows the production of a single type of switch, which can be adapted to a variety of different applications prior to use.

[0034] Figure 6 schematically represents an embodiment in which the primary coil 5 is divided into a core winding 5’ that is wound around a first section 18’ of the core 18, and a coreless winding 5” that is not wound around the core 18 and is located in the space between the first section 18’ of the core 18 and the permanent magnet 10. The secondary coil 7 is also divided into a core winding 7’ that is wound around a second section 18” of the core 18, and a coreless winding 7” that is not wound around the core 18 and is located in the space between the second section 18” of the core 18 and the permanent magnet 10. Between the first section 18’ of the core 18 and the second section 18” of the core 18 there are two air gaps 19 because the first section 18’ of the core 18 and the second section 18” of the core 18 enclose the space in which the permanent magnet 10 is located from two sides. The distance D1 between the area 20 of the core 18 and the north pole of the permanent magnet 10 is smaller than the distances D2 between the two boundary areas of the permanent magnet and the two areas of the core 18 closest to them. Also, the distance D1 ’ between the area 20’ of the core 18 and the south pole of the permanent magnet is smaller than the distances D2. This is the reason why the areas 20, 20’ of the core 18, which are located at the distances D1 and D1 ’ from the permanent magnet 10, act as the magnet stabilizer 12 and a further magnet stabilizer 12’. The distances D1 and D1 ’ are preferably the same. The purpose of a coreless winding 5” of the primary coil 5 and a coreless winding 7” of the secondary coil 7 is in that the difference in the magnetic fields due to a difference in the currents 11 and I2 comes spatially as close as possible to the central position, in which the permanent magnet 10 is located, i.e. that the smallest differences between the currents 11 and I2 can be detected. In this embodiment, a lacquered wire having a thickness of 1.5 mm was used for both the primary coil 5 and the secondary coil 7; a strong neodymium magnet of a diameter of 6 mm was used for the permanent magnet 10; the distances D1 and D1 ’ were 10 mm and the distance D2 was 14 mm; the number of windings of the core windings 5’, 7’ of both the primary coil 5 and the secondary coil 7 was 30 and the number of windings of the coreless winding 5”, 7” of both the primary coil 5 and the secondary coil 7 was 10. A difference of 70 mA in the currents caused an offset of 10 degrees which was sufficient for the circuit breaker 13 to interrupt the connection on both the positive conductor and the negative conductor.

[0035] In preferred embodiments, the protective switch 1 according to the present invention comprises also rotation limiting means (not shown in the figures) that prevent the permanent magnet 10 from rotating beyond a certain maximum angle in one or the other direction. The rotation limiting means prevent the permanent magnet 10 from getting offset by an angle of 45 degrees or more due to a big difference in the currents 11 and I2. If such huge offset would occur, the magnet stabilizer 12 could start attracting another pole of the permanent magnet 10 instead of one pole of the permanent magnet 10 and this could change the equilibrium condition of the permanent magnet 10, namely by 180 degrees, which is, of course, not desired. The maximum offset angle of the permanent magnet 10 that is still permitted by the rotation limiting means depends on a concrete application of the protective switch 1 and is preferably within the range of between 5 degrees to 10 degrees. Such a maximum angle is sufficient to achieve the desired effect, namely that the circuit breaker 13 within this angle detects the cut-off offset due to a defined difference between the currents 11 and I2 and interrupts the connection on the positive conductor and preferably also on the negative conductor. For example, the circuit breakers 13 which are provided with the fork optoelectronic sensor 15 have a sufficient maximal offset angle about 10 degrees, while the maximal angle may be smaller, for instance 5 degrees, in the circuit breakers 13 provided with a reflective optoelectronic sensor.

[0036] In general, it is desired for the switches of this type - based on the required characteristics in terms of foreseeable and rapid disconnection at a certain difference between the currents 11 and I2 - to have the following properties: as low resistance as possible in order to reduce losses and overheating, and as small inductive yield as possible. In order to achieve these properties in a specific application of the protective switch 1 according to the present invention, it is necessary to dimension the thickness of the wires and the number of windings in the primary and secondary coils appropriately, depending on the specific design of the embodiment.

[0037] In a further embodiment of the protective switch 1 according to the present invention (now shown in the figures), the core 18 is used that encloses at least one pole of the permanent magnet, preferably both poles, and is split into two sections 18’, 18” by at least one gap 19, preferably two gaps 19, the distances D1 , DT and D2 being substantially the same. As a result, the areas 20, 20’ of the core, which are closest to the north or south pole, cannot act as the magnet stabilizer 12, 12’. This is why the protective switch 1 in this embodiment comprises the magnet stabilizer 12 despite the core 18, said stabilizer being configured, for instance, as a special element 12, for example an iron element 12, or is made of other suitable materials as indicated above.

Claims

CLAIMS1. A residual current protective switch (1) for a direct current voltage network (3), characterized by comprising a primary coil (5), a secondary coil (7), a diametrically polarised permanent magnet (10), a magnet stabiliser (12), and a circuit breaker (13); the primary coil (5) and the secondary coil (7) having substantially identical electromagnetic and mechanical properties; the primary coil (5) being connected to a positive conductor between a voltage source (2) and a network (3), while the secondary coil (7) is connected to a negative conductor between the voltage source (2) and the network (3); the primary coil (5) and the secondary coil (7) being spatially arranged symmetrically with respect to the central position in which the diametrically polarized permanent magnet (10) is arranged, such that the sum of the magnetic field of the primary coil (5) and the magnetic field of the secondary coil (7) in the central position is zero when the current (11) through the primary coil (5) is equal to the current (I2) through the secondary coil (7); wherein the diametrically polarized permanent magnet (10) with a longitudinal axis (1 1) that extends substantially on the limit between the north pole and the south pole of this permanent magnet (10) is arranged in the central position in a way that it can rotate around said longitudinal axis (11) and that the permanent magnet (10) gets offset around said longitudinal axis (11) if the sum of the magnetic field of the primary coil (5) and the magnetic field of the secondary coil (7) is not zero, with respect to the equilibrium position of the permanent magnet (10), when the sum of the magnetic field of the primary coil (5) and the magnetic field of the secondary coil (7) is zero; wherein the magnet stabilizer (12) is positioned at a certain radial distance (R) from the permanent magnet (10) and attracts one of the poles of the permanent magnet (10) to ensure a unique equilibrium position of the permanent magnet (10) when the primary coil (5) and the secondary coil (7) do not generate a magnetic field, or rather when the sum of the magnetic field of the primary coil (5) and the magnetic field of the secondary coil (7) in the central position is zero; the magnet stabilizer (12) being made of one or several materials that attract the north or south pole of the permanent magnet (10); the circuit breaker (13) being adapted to interrupt the connection on the positive conductor, preferably also on the negative conductor, between the voltage source (2) and the network (3) when the offset of the permanent magnet (10) from the equilibrium position reaches the cut-off offset.

2. Protective switch (1) according to claim 1 , characterized in that the axis of the primary coil (5) and the axis of the secondary coil (7) are aligned with each other and the distance of the primary coil (5) to the central position is the same as the distance of the secondary coil (7) from the central position.

3. Protective switch (1) according to any one of claims 1 to 2, characterized in that the central position is positioned at such a spot that satisfies the condition of an equal distance both from the primary coil (5) and the secondary coil (7), and the additional condition which is that the yield of the magnetic field of a respective coil (5, 7) at this spot is maximal.

4. Protective switch (1) according to any one of claims 1 to 3, characterized in that the magnet stabilizer (12) is spatially positioned such that its distance from the primary coil (5) is the same as the distance from the secondary coil (7).

5. Protective switch (1) according to any one of claims 1 to 4, characterized in that the magnet stabilizer(12) is made of one or more of the following materials ferromagnetic materials, such as iron, nickel, cobalt; or alloys such as Alnico (aluminium-nickel-cobalt, permalloy, or silicon iron; or ceramic and sintered materials, such as ferrites or NdFeB (neodymium-iron-boron), or natural materials such as magnetite (Fe3O4).

6. Protective switch (1) according to any one of claims 1 to 5, characterized in that the distance (R) is adjustable.

7. Protective switch (1) according to any of claims 1 to 6, characterized in that the circuit breaker (13) is configured as a combination of a fork optoelectronic sensor (15) and a relay (17), the fork optoelectronic sensor (15) generating a cut-off signal at its output when the cut-off offset of the permanent magnet (10) is detected; the fork optoelectronic sensor (15) detecting the offset of the permanent magnet via the position of a lever (16) attached to the permanent magnet (10) in a way to get offset together; the relay (17) being connected to the output of the fork optoelectronic sensor (15) and adapted and connected in a way that, when a cut-off signal is detected, it disconnects the connection between the voltage source (2) and the network (3) on the positive conductor, preferably also on the negative conductor.

8. Protective switch (1) according to any one of claims 1 to 6, characterized in that the circuit breaker(13) is provided with a reflective optoelectronic sensor, wherein a mirror is attached to the permanent magnet (10), which mirror gets offset together with the permanent magnet (10), this interrupts a light beam detected by the reflective optoelectronic sensor and a cut-off signal is generated at its output.

9. Protective switch (1) according to any one of claims 1 to 8, characterized by comprising a core (18), around which the primary coil (5) and the secondary coil (7) are wound; the core being made of iron or other suitable ferromagnetic materials, such as silicon steel, cobalt, ferrites or permalloy.

10. Protective switch (1) according to claim 9, characterized in that the core (18) is shaped in the area around the central position in a way to enclose at least one pole of the permanent magnet (10), a distance (D1) between this north pole of the permanent magnet (10), when it is in the equilibrium position, and an area (20) of the core (18 that is closest to the permanent magnet is smaller than distances (D2) between the two boundary areas between the north and south poles of the permanent magnet (10), when it is in the equilibrium position, and the two areas of the core (18) closest to them; the area (20) of the core (18) acting as a magnet stabilizer (12).

11. Protective switch (1) according to any one of claims 9 to 10, characterized in that the core (18) is preferably divided into two sections (18’), (18”) by at least one air gap (19); the air gap (19) in the core (18) being preferably positioned symmetrically with respect to the primary coil (5) and the secondary coil (7) and preferably close to the permanent magnet (10).

12. Protective switch (1) according to claim 11 , characterized in that the core (18) is divided into two sections (18’), (18”) by two air gaps (19), the core (18) being split into an upper part of the core (18) and a bottom part of the core (18) in orderto enclose both poles of the permanent magnet (10); one air gap (19) splitting the core (18) in the upper part of the split core (18) and the other air gap (19) splitting the core (18) in the bottom part of the split core (18).

13. Protective switch (1) according to any one of claims 1 to 12, characterized by comprising a further magnet stabilizer (12’).

14. Protective switch (1) according to claims 12 and 13, characterized in that the magnet stabilizer (12’) is configured as an area (20’) of the core (18) that is located at a distance (D1 ’) from the south pole of the permanent magnet (10) when the latter is in the equilibrium position; the distance (D1 ’) being smaller than the distances (D2).

15. Protective switch (1) according to any one of claims 11 to 14, characterized in that the primary coil (5) is divided into a core winding (5’) that is wound around a first section (18’) of the core (18), and a coreless winding (5”) that is not wound around the core (18) and is located in the space between the first section (18’) of the core (18) and the permanent magnet (10); and the secondary coil (7) is divided into a core winding (7’) that is wound around a second section (18”) of the core (18), and a coreless winding (7”) that is not wound around the core (18) and is located in the space between the second section (18”) of the core (18) and the permanent magnet (10).

16. Protective switch (1) according to claim 9, characterized in that the core (18) encloses at least one pole of the permanent magnet (10), preferably both poles, and is split into two sections (18’, 18”) by at least one gap (19), preferably two gaps (19), the distances D1 , D1 ’ and D2 being substantially the same; the magnet stabilizer (12) being configured as a separate element (12) that is not part of the core (18).

17. Protective switch (1) according to any one of claims 1 to 16, characterized by comprising rotation limiting means that prevent the permanent magnet (10) from rotating beyond a certain maximum angle in one or the other direction.

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