Switching device

Abstract

The invention relates to a switching device (10) designed for a rated current and comprising two terminals (16) electrically connected by a first current path (22) and a second current path (24), which are connected in series. The first current path (22) has a mechanical switch (26) and the second current path (24) has a semiconductor switch (32). The mechanical switch (26) is driven by an actuator (42) comprising a first actuator unit (44) energized by a control unit (20) and a second actuator unit (46) connected in parallel to the second current path (24). The switching device (10) is configured such that the mechanical switch (26) is opened when a predetermined electrical current, less than three times the rated current, is supplied by means of the second actuator unit (46). The invention further relates to a method (48) for operating a switching device (10).

Description

The invention relates to a switching device and a method for operating a switching device. The switching device comprises two terminals which are electrically connected by means of a first current path and a second current path. Circuit breakers are typically used to protect an electrical line or device from a malfunction in the associated circuit, such as excessive voltage or current. This can occur, for example, if the device or line being protected is already damaged. In this case, the circuit breaker interrupts the current flow, preventing further damage. To interrupt the current flow, the circuit breaker has a switch. Thus, the circuit breaker is a type of switching device. For touch protection and ease of installation, the switch is housed within a casing. The switch is, for example, designed as a semiconductor switch. With a semiconductor switch, galvanic contact is not possible. As an alternative, the switch is therefore mechanically designed and operatively connected to an actuator, which may include an electromagnet. By energizing the actuator, the switching state of the switch can be changed. If a switch malfunctions during operation, the circuit breaker can no longer interrupt the electrical current, rendering it ineffective. To increase safety, it is therefore known to connect two switches in series. This allows the electrical current to be interrupted even if one switch malfunctions, using the other. However, if galvanic isolation is required for safety reasons, two mechanical switches must be used, resulting in excessively large dimensions and weight. Furthermore, to achieve functional safety, the two switches must be based on different technologies, complicating the design and increasing manufacturing costs. The invention is based on the objective of providing a particularly suitable switching device and a particularly suitable method for operating a switching device, advantageously reducing manufacturing costs and / or increasing safety. With regard to the switching device, this problem is solved according to the invention by the features of claim 1, and with regard to the method by the features of claim 8. Advantageous further developments and embodiments are the subject of the dependent claims. The switching device is primarily used for switching an electric current, specifically for creating and / or interrupting an electric current flow. For this purpose, the switching device advantageously has two states: an electrically conductive state, also known as the closed state. In this case, it is possible to conduct the electric current via the switching device. In the other state, known as the open or non-conductive state, an electric current flow through the switching device is not possible. For example, the switching device may be manually operated, making it a hand-held switch. Alternatively, or in combination with this, it is possible, for example, to operate the switching device electrically, and thus remotely.In another alternative, the switching device is, for example, automatically operated, preferably depending on certain conditions. The switching device is preferably a component of a circuit breaker or contactor. The circuit breaker serves, for example, to protect a device, and the circuit breaker is, for example, a device protection switch. Alternatively or in combination with this, the circuit breaker serves to protect a line and is thus a line protection switch. In particular, the circuit breaker is used in a DC circuit, i.e., between a load and a DC voltage source or the like, so that a circuit is formed. Preferably, an electrical DC voltage between 400 V and 650 V is present in the DC circuit, i.e., a higher DC voltage. Preferably, the circuit breaker is used to protect an actuator in an industrial plant. The actuator constitutes the load in this case.The circuit breaker, or at least the switching device, is most suitable for use in industrial automation. Specifically, the electrical voltages switched by the circuit breaker / switching device are 24 V, 48 V, 380 V, 650 V, and 760 V. Alternatively, the circuit breaker is used to protect street lighting, ship electrical systems, railway infrastructure, railway propulsion systems, or in the field of electrified aviation. Another alternative application is the expansion and integration of renewable energy generation systems, in island grids, in private homes, in greenhouses, in the electrification of road-based mobility (electric mobility), in agriculture, or in construction vehicles. The electrical (direct) voltage used is, for example, between 1500 V and 3000 V, or is 110 V, 380 V, 400 V, 800 V, 1000 V, 1500 V, 3000 V.In summary, as an alternative to its use in an industrial plant, the circuit breaker / switching device is used, for example, in an electric vehicle, such as a motor vehicle, an aircraft or a ship / boat. Alternatively, the switching device can be designed as a disconnect switch, also known as a isolator. For example, the switching device may include a mechanical lever by which its switching state can be changed. The switching device comprises two terminals to which, in the installed state, further components of the circuit, preferably the necessary cables, are connected. The terminals are suitable for this purpose, and in particular, are designed and configured accordingly. Preferably, a cable or busbar is connected to each terminal, with a cross-section of, for example, between 10 mm² and 100 mm² or between 25 mm² and 92 mm². The cross-section is, for example, 16 mm², 25 mm², or 35 mm². The cross-section is, for example, 6 mm², between 6 mm² and 16 mm², or between 16 mm² and 50 mm². In particular, the terminals are designed as cage clamps or at least include cage clamps. The two terminals are electrically connected via a first and second current path. The first and second current paths are connected in series. For example, only the first and second current paths are present / sufficient for the electrical connection of the two terminals. Alternatively, this series connection can be further enhanced with another current path. In summary, when an electrical current is conducted through the switching device, it is conducted via the first and second current paths, at least during normal operation. The first current path features a mechanical switch, which is electrically connected between the two terminals of the switching device. This allows the electrical current flow through the switching device to be controlled by means of the mechanical switch. The mechanical switch is suitably housed in a casing, preferably made of plastic. This provides protection against accidental contact with the mechanical switch and also protects it from environmental influences. Advantageously, any necessary terminals are integrated into the casing, allowing for electrical contact between the two current paths from outside the casing. The mechanical switch has a point for interrupting the electrical current, which includes a moving contact to which a fixed contact is assigned. It is possible to move the moving contact relative to the fixed contact, and in particular to bring them into contact or distance them. When the moving contact is in contact with the fixed contact, the mechanical switch is electrically conductive, allowing electrical current to flow through it. However, if the moving contact is separated from the fixed contact, an air gap is formed between them, or at least the mechanical switch is open and therefore not electrically conductive. The mechanical switch, for example, has only one point for interruption. However, it is particularly preferred that the mechanical switch be designed as a double interrupter and therefore have two moving contacts, each of which is associated with a fixed contact. Advantageously, the mechanical switch comprises a contact bridge that is movably mounted. In particular, the contact bridge is movably mounted transversely, and one of the moving contacts is arranged at each of the opposite ends. The switching device also includes a semiconductor switch. This is part of the second current path and is therefore electrically connected in series with the mechanical switch. If the semiconductor switch is non-conductive, the second current path is also non-conductive, and therefore no electric current can flow through it. However, if the semiconductor switch is conductive, the second current path is conductive, and the electric current carried by the switching device is conducted through the semiconductor switch. The switching device has a drive unit comprising a first drive unit and a second drive unit. Both drive units are electrically powered, so that the mechanical switch is actuated by energizing them. When either drive unit is energized, the switching state of the mechanical switch can be changed. For example, each drive unit can be used to move the mechanical switch to the open state, in which the moving contact is separated from the corresponding fixed contact. To open the mechanical switch, the operation of just one drive unit is sufficient. It is also possible, for example, to move the mechanical switch to the electrically closed state using each drive unit separately. This is preferably achieved using the first drive unit.For example, this is not possible using the second drive unit, which is why the design is simplified. Advantageously, each drive unit comprises an electrical coil. During operation, a magnetic component, such as a permanent magnet or a ferromagnetic component, is moved within the coil. Preferably, both drive units are assigned the same magnetic component, thus reducing material costs and size. The (magnetic) component is advantageously connected to the mechanical switch, preferably to the contact bridge. The drive is designed, for example, as a "moving magnet actuator." The moving magnet actuator also includes the magnetic component, which is mounted for movement. The moving magnet actuator comprises the first drive unit with one or more electrical coils. When energized, a magnetic interaction occurs between these coils and the (magnetic) component. The electrical coils are held in a fixed position. Because the electrical coil(s) are stationary, the design is simplified, and, with the exception of the components required for mounting, no further moving parts or electrical connections are needed between the moving components (i.e., the component) and the stationary components of the moving magnet actuator, which will be referred to simply as the actuator in the following. This also reduces friction. Preferably, the first drive unit comprises two electrical coils, which are, for example, identical in construction. At a minimum, however, the two electrical coils are offset from each other along a longitudinal axis and arranged concentrically to this axis. The component is located, in particular, on the longitudinal axis and is movably mounted along it. In one switching state of the mechanical switch, the component is located in an air gap between the two electrical coils and is held there, for example, by means of a magnetic short-circuit plate. In the other switching state, however, the component is offset along the longitudinal axis. The second drive unit also comprises corresponding electrical coils, with each coil of the second drive unit surrounding one of the coils of the first drive unit, so that they are concentric to each other. Thus, electrical interaction with the (magnetic) component is also possible via the second drive unit, resulting in a comparatively compact drive. Furthermore, the mechanical switch can be actuated separately by each of the two drive units. Since the number of moving parts in the moving magnet actuator, especially the component itself, is relatively small, and these parts are also relatively lightweight, the actuator's dynamics are comparatively high. This reduces inertia when actuating the mechanical switch. Consequently, the switching device enables relatively fast switching. The switching device also includes a control unit. The control unit supplies power to the drive, namely the first drive unit. In other words, it is possible to energize the first drive unit, and in particular an electrical coil associated with the first drive unit, by means of the control unit, for which purpose an electrical voltage is expediently applied. For this purpose, the first drive unit is electrically connected to the control unit. For example, the control unit also performs other functions, in particular the detection of a request to change the switching state of the switching device, for example a request to start or stop the flow of electric current by means of the switching device. For example, the switching device also monitors the electrical current carried through it. Suitablely, the control unit includes an interface for signal communication with other components, such as a possible circuit breaker and / or the circuit in which the switching device is used. In summary, the drive is at least partially powered by the control unit. The second drive unit is, in particular, independent of the control unit and electrically connected in parallel to the second current path. Preferably, no additional switch or the like is present, so that the electrical current flows through the second drive unit when the second current path is electrically non-conductive, but the first current path is electrically conductive. The second drive unit provides electrical resistance, at least due to any electrical coil it contains. Therefore, if the switching device, and consequently the semiconductor switch, is in an electrically conductive state, the electrical current between the terminals is primarily conducted through the semiconductor switch and not, or only to a comparatively small extent, through the second drive unit.Advantageously, the switching device is designed such that if the semiconductor switch is electrically conductive, the electric current supplied by the second drive unit is insufficient to actuate the mechanical switch. In other words, the second drive unit may generate a magnetic field, but this field is too weak to move any magnetic component against the mechanical friction of the switch or other forces acting on the mechanical switch, such as spring forces. The switching device is designed such that the mechanical switch is moved to the open position when a predetermined electric current is applied via the second drive unit. In other words, the predetermined electric current advantageously generates a sufficiently strong magnetic field to open the mechanical switch. Specifically, the second drive unit is designed as follows. Advantageously, the second drive unit is connected to the second current path in this manner. The switching device is designed for a rated current. In other words, it is possible to conduct an electric current up to the rated current through the switching device without causing damage. The duration for which this electric current can be conducted through the switching device is essentially unlimited. However, if the electric current is increased, the switching device will be damaged, for example, immediately or at least after a short period. Particularly preferably, it is possible to conduct an electric current greater than the rated current through the control device, at least for a short period, which is particularly less than 10 seconds or 5 seconds. If, on the other hand, the current is conducted for a longer period, the switching device will be damaged, at least if the electric current is greater than the rated current plus any tolerance, such as 20%.In other words, while it is possible to conduct an overcurrent or short-circuit current using the switching device, this is only possible for a relatively short period of time, after which the switching device is destroyed or at least undergoes a reversible change. Based on the rated current, it is known how the switching device can be used, and the individual components of the switching device are matched accordingly. Therefore, the manufacturer of the switching device can estimate which components are required. In summary, the mechanical switch and the semiconductor switch are designed such that they can carry the rated current for essentially an unlimited period of time. The second drive unit can also carry the rated current for an unlimited period of time. The control unit is also designed for the rated current and is electrically connected to at least one of the current paths, for example, via a current transformer. Thus, the control unit does not supply the rated current, but a reduced current; however, the current transformer is specifically designed for this. Alternatively, the control unit can be powered by a secondary voltage source, particularly via auxiliary terminals. The switching device is advantageously designed such that an electrical voltage of less than 60 volts, particularly 24 V or 12 V, is applied to the auxiliary terminals, thereby powering the control unit. The control unit then applies the electrical voltage present at the auxiliary terminals to power the first drive unit. The predetermined electrical current that actuates the mechanical switch when it is driven by the second drive unit is less than three times the rated current. In other words, if the predetermined current is applied between the two terminals, but the semiconductor switch is open, the drive unit, due to the second drive unit, will actuate the mechanical switch in such a way that it opens the mechanical switch. Consequently, the electrical current flow through the mechanical switch, and therefore also through the switching device, ceases. The predetermined electrical current is less than an overcurrent or short-circuit current.In other words, the predetermined electrical current, also simply referred to as the predetermined current, is lower than the overcurrent or short-circuit current, and the predetermined current is actually reached during normal operation of the switching device, i.e., when there is no fault, for example, in the switching device and / or the circuit in which the switching device is used. In summary, the second drive unit can therefore carry the predetermined electrical current, at least for a certain period of time. Ideally, the second drive unit is designed for the predetermined electrical current or at least for the rated current. Due to this design, it is possible to interrupt the electrical current via the switching device in different ways. One way is by directly energizing the first drive unit via the control unit. This occurs particularly during normal operation, i.e., when the switching device is intended to interrupt the electrical current as desired. However, if the first drive unit is damaged, the electrical current continues to flow through both the first and second current paths. In this case, the semiconductor switch can be activated. Specifically, it can be switched to a non-conductive state, preventing the electrical current from flowing through the second current path and commutating to the second drive unit. Therefore, the mechanical switch is still activated. Consequently, the two connections are galvanically isolated from each other, even if the first drive unit is faulty. This increases safety. It eliminates the need for two mechanical switches, two drives, and two control units. This reduces the overall size and manufacturing costs. For example, the semiconductor switch is designed for bidirectional switching. Alternatively, the second current path comprises two semiconductor switches, each connecting in series but antiparallel to the other, interrupting the electric current in only one direction. Consequently, each of the two semiconductor switches can interrupt the electric current in either direction via the second current path. This allows the switching device to be mounted in virtually any orientation and / or to interrupt any reverse current when used in a DC circuit. The second drive unit preferably comprises a rectifier through which the electrical coil(s) are electrically connected to the second current path. This ensures that the electric current always flows in the same direction through the electrical coil(s), regardless of the polarity of the electrical voltage applied to the terminals. The second drive unit can then be used to move the mechanical device, at least to the open position. For example, the second current path includes a rectifier through which the semiconductor switch is electrically contacted via its terminals. The rectifier ensures that the electric current through the semiconductor switch always flows in a single direction, regardless of the direction of the electric current between the terminals. This increases safety, and only one suitable semiconductor switch is required. Preferably, at least two such semiconductor switches are present, connected electrically in series. The switching direction of the two semiconductor switches is the same relative to each other. This creates a cascode circuit, where each semiconductor switch applies a reduced voltage to interrupt the electric current flow through the second current path.This reduces the requirements for the semiconductor switches and therefore also the manufacturing costs. Advantageously, the semiconductor switches are bypassed by a resistor, thus creating a voltage divider. Alternatively, or in combination with this, the semiconductor switch(es) are bypassed by a varistor, limiting the maximum applied voltage. Consequently, even an electrical overvoltage does not destroy the semiconductor switch(es). For example, the predetermined electrical current is less than twice the rated current. This allows the second actuator to supply an increased electrical current without opening the mechanical switch. However, the mechanical switch will still reliably open if an excessively high electrical current is supplied by the switching device, particularly the second actuator. Alternatively, or in combination with this, the predetermined electrical current is greater than half the rated current. Therefore, if only a comparatively low electrical current is supplied by the switching device and the semiconductor switch is in a non-conductive state, for example due to a malfunction or delayed actuation, the mechanical switch will not be activated.This occurs particularly when the electric current carried by the switching device is comparatively large and could lead to damage or undesired behavior. Preferably, the predetermined electrical current is equal to or slightly lower than the rated current. Consequently, it is possible to use the second drive unit to de-energize the switching device even during normal operation, particularly when, for example, with the exception of the first drive unit, there is no other malfunction or fault in the circuit in which the switching device is used. For example, the semiconductor switch is self-conducting or bistable. Preferably, however, the semiconductor switch is designed to be self-blocking. Advantageously, the semiconductor switch is an n-channel MOSFET or an IGBT. If, therefore, suitable control of the semiconductor switch does not occur or cannot occur—for example, due to a malfunction in the semiconductor switch, its control circuitry, and / or the control circuitry—it automatically switches to the non-conducting state. Subsequently, any electrical current flowing between the terminals is conducted via the drive unit, which then opens the mechanical switch. In other words, if the semiconductor switch malfunctions, the mechanical switch essentially opens automatically, thus increasing safety.This occurs regardless of whether the drive, in particular the first drive unit, also has a malfunction. For example, the semiconductor switch is operated by a separate component. However, it is particularly preferred that the control unit, which also powers the first drive unit, is used to operate the semiconductor switch. This reduces the number of components required. Furthermore, it is possible to control the semiconductor switch essentially simultaneously with powering the first drive unit, so that it is switched to the non-conductive state. For this purpose, the first drive unit, the control unit, and the semiconductor switch are expediently interconnected. This reduces effort and complexity.Suitablely, the semiconductor switch is designed to be self-locking, so that in the event of a malfunction or failure of the control unit, which then prevents suitable control of the drive, the second drive unit is actuated in such a way that the mechanical switch is opened. For example, the switching device is essentially formed by means of the control unit, the first current path, and the second current path, i.e., in particular by means of the control unit, the semiconductor switch, the mechanical switch, and the drive. Most preferably, however, the second current path includes a current sensor that is electrically connected in series with the semiconductor switch. In particular, the current sensor is designed as a shunt and has a measuring resistor. Advantageously, the semiconductor switch is connected to the current sensor and the control unit in such a way, or at least the switching device is designed such that when the first drive unit is energized, but the electrical current continues to flow through the second current path, the semiconductor switch is switched to the non-conductive state.Thanks to the current sensor, it is therefore particularly possible to check whether the mechanical switch has been opened after the first drive unit has been energized. Alternatively or in combination with the current sensor, a signal contact, which is positively driven, is connected to or operatively linked to the mechanical switch. This also makes it possible to determine the position of any moving contact relative to the fixed contact. Alternatively or in combination with this, a voltage sensor or similar device is present, which detects the electrical voltage across the mechanical switch. At the very least, the voltage sensor also allows determination of whether the mechanical switch is open and, in particular, whether there is a malfunction in the first drive unit. This enables multiple monitoring of whether the switching device is functioning correctly, further increasing safety. Preferably, the second current path includes a fuse connected electrically in series with the semiconductor switch. The fuse is, in particular, a cartridge fuse. Advantageously, the fuse is designed to trip upon reaching the predetermined current or a higher current, thus interrupting the current through the second current path. In summary, the current at which the fuse blows / trips is, in particular, lower than the predetermined current. When the current then commutates to the second drive unit, it is sufficient to open the mechanical switch. Due to the opening of the mechanical switch, the galvanic isolation is improved compared to using only the fuse, where any remaining debris can still allow current to flow. For example, only the second current path is electrically connected in series with the first current path. Alternatively, or in combination with this, only the second current path is connected to the second drive unit. However, it is particularly preferred that several second current paths are present, which are electrically connected in parallel to each other and to the second drive unit, and thus each in series with the first current path. As a result, the electric current carried by the first current path is distributed among the individual second current paths, which reduces the electric current carried by each of the second current paths. Each second current path has a semiconductor switch, and in particular, all semiconductor switches are connected to a common control line so that they are always actuated simultaneously.Due to the distribution of the electric current to the second current paths, the requirements for the semiconductor switches are reduced, which is why manufacturing costs are reduced despite the majority of semiconductor switches. If, for example, due to a further malfunction, not all semiconductor switches are switched to the non-conductive state, the electric current supplied by the second drive unit increases if at least some of the semiconductor switches are non-conductive, thus opening the mechanical switch. This provides additional redundancy and increases safety. It is particularly advantageous that each second current path is assigned its own fuse, with the tripping threshold for the fuses being adapted to the respective second current path. Thus, if, for example, the electrical current increases, but operation of the first drive unit is no longer possible, at least one of the fuses will trip. Consequently, no further electrical current can flow through this second current path, causing the electrical current flowing through the remaining second current paths to increase, which in turn causes the fuses in these paths to trip, at least successively. This continues until the predetermined electrical current is supplied by the second drive unit, at which point the mechanical switch opens. This further enhances safety. In one advanced training system, for example, a semiconductor switch is connected in parallel to the mechanical switch. Specifically, this semiconductor switch is switched to an electrically conductive state before the first drive unit is energized. The electrical current then commutates to the second semiconductor switch, which is subsequently switched back to a non-conductive state when the mechanical switch is already open. This enables arc-free switching of the switching device. The method serves to operate a switching device designed for a rated current and comprising two terminals electrically connected by a first current path and a second current path, which are connected in series. The first current path has a mechanical switch, and the second current path has a semiconductor switch. The mechanical switch is driven by a mechanism comprising a first drive unit, energized by a control unit, and a second drive unit connected in parallel to the second current path. The switching device is designed such that the mechanical switch is opened when a predetermined electrical current, less than three times the rated current, is applied via the second drive unit. In this method, a request to interrupt an electrical current flowing through the switching device is detected by the control unit. For example, a signal connection exists, particularly via an interface, with other components of a circuit in which the switching device is used. Preferably, the request is provided by a higher-level process control system or the like, especially during normal operation. Alternatively, the request is generated, for example, by the control unit itself, perhaps as a separate component. Alternatively, or in combination with these, the request is generated based on sensor measurement data, advantageously when a fault condition, such as an overcurrent or short circuit, has been detected. After the request is detected, the control unit energizes the first drive unit in such a way that the mechanical switch is opened, thus galvanically isolating the two terminals. Advantageously, the semiconductor switch is left in the electrically conductive state, at least initially. Therefore, the mechanical switch is used to interrupt the electrical current, particularly during normal operation. Galvanic isolation is preferably achieved, thus increasing safety. The process is suitably carried out, at least partially, by the control unit. Preferably, the process is carried out or started when the switching device is energized. Advantageously, the semiconductor switch is left in the electrically conductive state after, or at least during, the energizing of the first drive unit. Preferably, after the first drive unit has been energized, or at least after this has occurred for a certain period of time, it is checked whether the electrical current is still flowing. Advantageously, a current sensor connected in series with the semiconductor switch is used for this purpose. Alternatively, or in combination with this, it is determined that the electrical current continues to flow if a signal contact associated with the mechanical switch indicates that the mechanical switch is still closed. If the electrical current continues to flow, the semiconductor switch is switched to a non-conductive state, allowing the current between the terminals to be carried by the second actuator. This, in turn, actuates the mechanical switch. Thus, even if the first actuator is faulty, a safe shutdown occurs when required. The second actuator and the semiconductor switch are therefore essentially only used when the first actuator fails. This reduces their required service life and, consequently, their manufacturing costs. For example, the first drive unit is only energized when requested. Preferably, the first drive unit is energized by the control unit regardless of the request, such that the mechanical switch is opened. However, this only occurs if a specific condition is met. This condition is always met, for example, when a time period has elapsed that is longer than 10 minutes, 1 hour, or 1 day. Advantageously, this time period is less than 1 month or 1 week. After the first drive unit is energized, it is checked whether the electrical current has been interrupted. If so, the first drive unit is then energized by the control unit in such a way that the mechanical switch is closed again, so that the electrical current is once again supplied via the switching device. Due to the interruption of the electrical current flow, a load powered via the switching device is briefly without current, preferably for less than 1 second or 100 ms. If the load has a capacitance, such as a capacitor, this brief drop in electrical current is absorbed, allowing the load to continue operating. If the first drive unit is still energized, the semiconductor switch switches to a non-conductive state, energizing the second drive unit and thus opening the mechanical switch. In this case, the drive is subsequently locked, and an error message is issued. This verifies the functionality of the switching device and allows for early detection of any fault, particularly before a further malfunction occurs in the circuit. Advantageously, the (specific) condition is met periodically, so that it is periodically checked whether the mechanical switch can be opened without errors by means of the first drive unit. As a result, a malfunction is detected relatively early, which increases safety. Alternatively, or in combination with this, the specific condition is met when a malfunction / fault is detected. The switching device is particularly preferably used for interrupting DC voltage and / or DC current. Advantageously, the switching device is used in a motor vehicle or an industrial plant. The invention also relates to the corresponding use of the switching device, which is preferably a component of a circuit breaker. The invention also relates to a circuit comprising such a switching device. Here, the circuit advantageously includes a DC voltage source and a load. In particular, the circuit is a component of a motor vehicle. The DC voltage source is, for example, an energy storage device such as a high-voltage battery. Alternatively, the DC voltage source is a rectifier. Advantageously, the load is an electric motor or at least a drive unit comprising the electric motor. The switching device is expediently a component of a circuit breaker. The first drive unit is suitably operated according to a tripping characteristic. Preferably, the mechanical switch is opened by means of the first drive unit when an electric current has been conducted for a specific period of time, which is defined by the tripping characteristic. A sensor for detecting the electric current conducted through the circuit breaker is expediently provided. The sensor preferably comprises a shunt. The invention also relates to such a circuit breaker. The further training and advantages explained in connection with the switching device can also be applied analogously to the procedure / the use / the circuit / the circuit breaker as well as to each other and vice versa. An embodiment of the invention is explained in more detail below with reference to a drawing. In the drawing: Fig. 1 schematically shows an electrical circuit with a load and a switching device, Fig. 2 a simplified circuit diagram of the switching device, and Fig. 3 a method for operating the switching device. Corresponding parts are marked with the same reference symbols in all figures. Figure 1 shows a simplified schematic representation of a circuit 2 comprising a DC voltage source 4 with two poles 6. The DC voltage source 4 provides a DC voltage of 650 V, which is applied between the two poles 6 and supplies a load 8. The load 8 is connected to one pole 6 and directly to the other via a switching device 10. A capacitor 12 is connected in parallel with the load 8. The load 8 and the switching device 10 are connected via a signal line 14, allowing data / information to be exchanged between them. Figure 2 shows a simplified schematic circuit diagram of the switching device 10. The switching device 10 has two terminals 16, each connected to a line of the circuit 2. One terminal 16 is electrically connected to one of the poles 6 of the DC voltage source 4, and the other is connected to the load 8 and the capacitor 12. The terminals 16 are enclosed in a housing (not shown), which surrounds the other components of the switching device 10. The housing also incorporates two auxiliary terminals 18, which are electrically connected to a secondary DC voltage source (not shown). This source provides a DC voltage of 12 V or 24 V, which is therefore present at the auxiliary terminals 18 during operation. A control unit 20 is electrically connected to the auxiliary terminals 18 and is thus powered via these terminals. Furthermore, the control unit 20 is connected to the signal lines 14 of the control device 10 via an interface (not shown). Between terminals 16, a first current path 22 and several second current paths 24 are connected, with the illustrated example comprising two such second current paths 24. The second current paths 24 are electrically connected in parallel. The first current path 22 is electrically connected in series with each of the second current paths 24, and the two terminals 16 are electrically connected to each other by means of the first current path 22 and the second current paths 24. Thus, if an electric current is conducted through the switching device 10, it is conducted entirely via the first current path 22 and proportionally via each of the second current paths 24. The first current path 22 has a mechanical switch 26 designed as a double interrupter. For this purpose, the mechanical switch 26 has a substantially straight contact bridge 28, which is movably mounted perpendicular to its direction of extension. A moving contact is assigned to each of the opposite ends of the contact bridge 28, which, due to the movable mounting, can be mechanically pressed against a corresponding fixed contact. In this case, the mechanical switch 26 is closed and electrically conductive. By adjusting the contact bridge 28, it is possible to move the moving contacts away from the fixed contacts. In this case, the mechanical switch 26 is open and electrically non-conductive. A first voltage sensor 30 is connected electrically in parallel to the mechanical switch 26 and thus to the first current path 22. This first voltage sensor 30 is connected to the control unit 20 via a signal. The voltage sensor 30 can detect the electrical voltage across the mechanical switch 26. If this voltage is negligible, the mechanical switch 26 is closed. Conversely, if the measured voltage is higher, the mechanical switch 26 is open. The two secondary current paths 24 are identical in construction and each has two semiconductor switches 32 configured as n-channel MOSFETs. Thus, the semiconductor switches 32 are self-blocking, and when no electrical potential is applied to their gates, the semiconductor switches 32 exhibit high resistance. The gates of the semiconductor switches 32 are directly electrically connected to each other and to the control unit 20, which operates the semiconductor switches 32. Due to this electrical connection, all semiconductor switches 32 are either in the electrically conductive or electrically non-conductive state, provided there is no fault. The semiconductor switches 32 of each secondary current path 24 are connected in series, with their reverse biases opposite to each other. Therefore, when all semiconductor switches 32 are in the high-resistance state, an electrical current flow through the secondary current paths 24 is prevented in both directions. Electrically connected in series with the two semiconductor switches 32 of each second current path 24, namely between them in the diagram, is both a current sensor 34 and a fuse 36. Thus, each second current path 24 has the current sensor 34 and the fuse 36, which are electrically connected in series with the semiconductor switches 32 of the same second current path 24. Electrically, a second voltage sensor 38 and a suppressor diode 40 are connected in parallel to each of the second current paths 24. The suppressor diode 40 ensures that the maximum electrical voltage across the second current path 24, which can be measured by the second voltage sensor 38, is limited. If the semiconductor switches 32 are electrically conductive, the electrical voltage detected by the second voltage sensor 38 is comparatively low, although it is slightly higher than the electrical voltage detected by the first voltage sensor 30 due to the internal resistance of the semiconductor switches 32. The switching device 10 further comprises a drive 42 with a first drive unit 44 and a second drive unit 46. Each of the drive units 44, 46 includes an electrical coil that is operatively connected to a magnetic component that is attached to the contact bridge 28. The electrical coil of the second drive unit 46 surrounds the electrical coil of the first drive unit 44, which in turn surrounds the magnetic component. Thus, when either of the two electrical coils is energized, the magnetic component is moved, and consequently the contact bridge 28 is adjusted. The first drive unit 44, on the other hand, is electrically contacted with the control unit 20, so that the first drive unit 44 is energized by the control unit 20. It is possible to apply the electrical voltage present at the auxiliary terminals 18 to the first drive unit 44 via the control unit 20, whereby the polarity can be reversed. Consequently, it is possible to move the magnetic component in different directions by means of the electrical coil of the first drive unit 44, so that the contact bridge 28 can also be moved in different directions. In other words, it is possible to both open and close the mechanical switch 26 by appropriately energizing the first drive unit 44 via the control unit 20. The second drive unit 46 is connected in parallel to the second current paths 24 and includes a rectifier through which the electrical coil of the second drive unit 46 is electrically contacted with the ends of the second current paths 24. This ensures that the electrical coil of the second drive unit 48 always carries current in only one direction, so that a force can only be applied to the magnetic component in one direction. The magnetic component can only apply a force away from the fixed contacts to the contact bridge 28. In summary, the second drive unit 46 can only be used to open the mechanical switch 26, i.e., to move it into the open state.Due to the rectifier, the design of the electrical coil, and / or a corresponding resistance of the second drive unit 46, it is ensured that the contact bridge 28 is only adjusted when at least a predetermined electrical current is supplied by the second drive unit 46. Otherwise, the magnetic interaction between the electrical coil and the magnetic component is too weak. The switching device 10 is designed for a rated current. This is, for example, 10 A. Thus, the individual components of the switching device 10 are designed so that an electrical current of 10 A can be carried by the switching device 10 for a substantially unlimited period of time. Due to the parallel connection of the second current paths 26, the required rated current of the semiconductors 32 is 5 A. The fuses 36 are designed to trip at an electrical current of 5 A plus a tolerance of 10%. This ensures that no tripping occurs even in the event of a brief increase in the electrical current carried by the switching device 10. The predetermined electrical current, from which the mechanical switch 26 is moved to the open state by means of the second drive unit 46, is equal to the rated current and is less than twice the rated current and greater than half the rated current. In summary, the switching device 10 is thus designed such that the mechanical switch 26 is moved to the open state when the predetermined electrical current, which is less than three times the rated current, is supplied by means of the second drive unit 46. The switching device 10 is operated according to a method 48, which is carried out at least partially by means of the control unit 10 and is shown in Fig. 3. The method 48 is started when the load 8 is operated, i.e., during normal operation. In a first step 50, it is checked whether a certain condition 52 is present. This condition 52 is always present every 24 hours. If the specified condition 52 is met, a second operation 54 is performed. The execution of the second operation 54 depends solely on whether the specified condition 52 is met and is independent of any other circumstances. In the second operation 54, the first drive unit 44 is energized by the control unit 20 in such a way that the mechanical switch 26 is opened. Provided this occurs without error, the electrical current flowing between the terminals 16 collapses, and the electrical voltage across the mechanical switch 26 increases, which is detected by the first voltage sensor 30. At the same time, no electrical current is measured by the current sensors 34. Thus, it is redundantly verified that no electrical current is actually flowing between the terminals 16. If this is the case, i.e., if the first drive unit 44 has operated without fault, and the measurement data of the first voltage sensor 30 and the current sensors 34 correspond to each other and are therefore also operating without fault, a third operation 56 is carried out. In the third operation 46, the first drive unit 44 is energized in the reverse direction, so that the mechanical switch 26 is closed. Consequently, following the opening of the mechanical switch 26, the first drive unit 44 is energized by the control unit 20 in such a way that the mechanical switch 26 is again closed, i.e., after the electrical current has been interrupted. Thus, the electrical current is again supplied via the switching device 10.While no electrical current was conducted through the switching device 10, the load 8 was energized via the capacitor 12, so that the desired functionality could continue to be performed by means of the load 8. The time interval between the second and third work steps 54, 56 was also chosen to be correspondingly short. After the third work step 56, the first work step 50 is performed again, thus monitoring whether the specified condition 52 is present again, i.e., whether another 24 hours have passed. Consequently, the switching device 10 is checked daily. If, however, the electrical current was not interrupted during the second operating step 54 and / or the values ​​measured by the first voltage sensor 30 and the current sensors 34 do not correspond to each other, for example, because no electrical current can be measured by the current sensors 34 even though no electrical voltage is measured across the mechanical switch 26 by the first voltage sensor 30, a fourth operating step 58 is carried out. In this step, the semiconductor switches 32 are switched to the non-conductive state by means of the control unit 20. Due to such control, all second current paths 24 are now high-impedance, and no electrical current flows through the two second current paths 24.If a further malfunction occurs, and, for example, only the semiconductor switches 32 of one of the second current paths 24 are switched to the non-conductive state, the entire electric current is initially routed via the other of the two second current paths 24. The fuse 36 is not designed for such a large electric current n and trips, so that both second current paths 24 are now interrupted. As soon as both secondary current paths 24 are interrupted, the electrical current, which continues to flow between the two terminals 16, is guided by the second drive unit 46. Consequently, the mechanical switch 26 opens, and the electrical current completely ceases. The two terminals 16 are then galvanically isolated from each other. A corresponding warning is then issued, for example, an LED or similar indicator is activated. The process 48 then ends. As long as procedure 48 is being carried out, a fifth step 60 is also performed, in which it is monitored whether a request 62 exists. This request is transmitted via signal lines 14 from the load 8 to the switching device 10, specifically when the operation of the load 8 is to be terminated or at least interrupted. Request 62 is generated, for example, when the functionality provided by the load 8 is no longer required, or if a malfunction of the load 8 occurs. Thus, the switching device 10 also serves to provide safety. Request 62 specifies that the electrical current flowing through the switching device 10 is to be interrupted. In summary, the fifth step 60 therefore detects the request 62 to interrupt the electrical current flowing through the switching device 10 by means of the control unit 10. In a subsequent sixth step 64, the first drive unit 44 is energized by the control unit 10. The energization is such that a force is exerted on the contact bridge 28, causing the moving contacts to be separated from the fixed contacts. In other words, the first drive unit 44 is energized by the control unit 10 in such a way that the mechanical switch 26 is moved into the open position. Following this, the current sensors 34 and the first current sensor 30 are used to check whether the switching device 10 is still conducting electrical current. This is the case, for example, if the first drive unit 44 has a fault. In this case, the fourth step 58 is then carried out, and thus, after the first drive unit 44 is energized, the semiconductor switches 32 are switched to the non-conductive state. The second drive unit 46 is then energized, opening the mechanical switch 26. Consequently, even in the event of a malfunction, a reliable interruption of the electrical current is ensured. Even if the control unit 20 should have a fault, and the semiconductor switches 32 are no longer correctly controlled, the self-blocking semiconductor switches 32 ensure that the electrical current is still supplied to the second drive unit 46, thus opening the mechanical switch 26. The invention is not limited to the embodiment described above. Rather, other variants of the invention can also be derived by a person skilled in the art without departing from the subject matter of the invention. In particular, all individual features described in connection with the embodiment can also be combined with one another in other ways without departing from the subject matter of the invention. Reference symbol list 2 Circuit 4 DC voltage source 6 Pole 8 Load 10 Switching device 12 Capacitor 14 Signal line 16 Connection 18 Auxiliary connection 20 Control unit 22 First current path 24 Second current path 26 Mechanical switch 28 Contact bridge 30 First voltage sensor 32 Semiconductor switch 34 Current sensor 36 Fuse 38 Second voltage sensor 40 Suppressor diode 42 Drive 44 First drive unit 46 Second drive unit 48 Procedure 50 First operation 52 Specific condition 54 Second operation 56 Third operation 58 Fourth operation 60 Fifth operation 62 Request 64 Sixth operation

Claims

Switching device (10) designed for a rated current and comprising two terminals (16) electrically connected by means of a first current path (22) and a second current path (24) connected in series, wherein the first current path (22) comprises a mechanical switch (26) and the second current path (24) a semiconductor switch (32), wherein the mechanical switch (26) is driven by a drive (42) comprising a first drive unit (44) energized by a control unit (20) and a second drive unit (46) connected in parallel to the second current path (24), and wherein the switching device (10) is configured such that the mechanical switch (26) is moved into an open state when a predetermined electric current, less than three times the rated current, is supplied by means of the second drive unit (46). Switching device (10) according to claim 1, characterized in that the predetermined electric current is less than twice the rated current and / or greater than half the rated current. Switching device (10) according to claim 1 or 2, characterized in that the semiconductor switch (32) is designed to be self-locking. Switching device (10) according to one of claims 1 to 3, characterized in that the semiconductor switch (32) is operated by means of the control unit (20). Switching device (10) according to one of claims 1 to 4, characterized in that the second current path (24) has a current sensor (34) which is electrically connected in series with the semiconductor switch (32). Switching device (10) according to one of claims 1 to 5, characterized in that the second current path (24) has a fuse (36) which is electrically connected in series with the semiconductor switch (32). Switching device (10) according to one of claims 1 to 6, characterized by several second current paths (24) which are electrically connected in parallel to each other. Method (48) for operating a switching device (10) according to one of claims 1 to 7, in which - a request (62) to interrupt an electric current passed through the switching device (10) is detected by means of the control unit (10), - the first drive unit (44) is energized by means of the control unit (10) such that the mechanical switch (26) is brought into the open state. Method (48) according to claim 8, characterized in that after the first drive unit (44) is energized, the semiconductor switch (32) is switched to the electrically non-conductive state if the electric current continues to be carried. Method (48) according to claim 8 or 9, characterized in that, irrespective of the request (62), when a certain condition (52) is present, the first drive unit (44) is energized by means of the control unit (10) such that the mechanical switch (26) is brought into the open state, and that subsequently the first drive unit (44) is energized by means of the control unit (20) such that the mechanical switch (26) is brought into the closed state when the electric current has been interrupted, and that otherwise the semiconductor switch (32) is brought into the electrically non-conductive state.

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