Protective switching device and procedure

Abstract

Protective switching device (SG) for the protection of a low-voltage alternating current circuit, comprising: - a housing (GEH) with mains-side (L1, N1) and load-side (L2, N2) connections for conductors of the low-voltage alternating current circuit, - a mechanical isolating contact system (MK) connected to the mains-side connections (L1, N1) for galvanic interruption of the low-voltage circuit, which is connected in series with an electronic interruption unit (EU) which has, by means of semiconductor-based switching elements, a high-resistance state of the switching elements for interruption and a low-resistance state of the switching elements for current flow in the low-voltage circuit, wherein the electronic interruption unit (EU) is connected to the load-side connections (L2, N2), - a voltage sensor (SU) which is arranged between the mechanical isolating contact system (MK) and the electronic interruption unit (EU).for determining the voltage level between the conductors of the low-voltage circuit, - wherein the mechanical isolating contact system (MK) is manually operable so that contacts of the mechanical isolating contact system (MK) can be manually closed for current flow and opened for interruption in the low-voltage circuit, - a control unit (SE) connected to the voltage sensor (SU) and the electronic interruption unit (EU), - wherein the protective switching device (SG) is designed such that the voltage sensor (SU) determines the instantaneous voltage level, and a corresponding instantaneous voltage value (DU) is examined for a voltage dip, using an instantaneous expected value of the voltage (UE).and that an opening of the mechanical disconnecting contact system (MK) is detected when the instantaneous voltage drops from 10 to 50 volts in a single-pole interrupting mechanical disconnecting contact system (MK) or from 20 to 100 volts in a double-pole interrupting mechanical disconnecting contact system (MK).

Description

The invention relates to the technical field of a protective switching device for a low-voltage circuit with an electronic interruption unit and a method for a protective switching device for a low-voltage circuit with an electronic interruption unit. Low voltage refers to voltages up to 1000 volts AC or up to 1500 volts DC. In particular, low voltage refers to voltages higher than extra-low voltage, defined as 50 volts AC or 120 volts DC. Low-voltage circuits, networks, or installations refer to circuits with rated currents of up to 125 amperes, or more specifically, up to 63 amperes. Low-voltage circuits also include circuits with rated currents of up to 50 amperes, 40 amperes, 32 amperes, 25 amperes, 16 amperes, or 10 amperes. These current values ​​refer specifically to rated, rated, and / or tripping currents, i.e., the maximum current that normally flows through the circuit or at which the electrical circuit is typically interrupted, for example, by a protective device such as a circuit breaker, miniature circuit breaker, or miniature circuit breaker. Miniature circuit breakers (MCBs) are long-established overcurrent protection devices used in low-voltage electrical installations. They protect cables from damage caused by overheating due to excessive current and / or short circuits. A MCB can automatically disconnect the circuit in case of overload and / or short circuit. A MCB is a non-resetting safety device. Unlike miniature circuit breakers (MCBs), power circuit breakers are designed for currents greater than 125 A, and sometimes even as low as 63 A. MCBs are therefore simpler and more delicate in design. MCBs typically have a mounting option for installation on a DIN rail (also known as a top-mounted rail or TH35 rail). Miniature circuit breakers (MCBs) are electromechanical devices. Within a housing, they contain a mechanical switching contact or shunt trip for interrupting (tripping) the electrical current. Typically, a bimetallic element is used for tripping (interruption) in the event of a sustained overcurrent (overcurrent protection) or thermal overload (overload protection). An electromagnetic trip with a coil is used for momentary tripping when an overcurrent limit is exceeded or in the event of a short circuit (short-circuit protection). One or more arc-quenching chambers or devices are provided. Furthermore, they include connection elements for conductors of the electrical circuit to be protected. Protective switching devices with an electronic interruption unit are relatively new developments. These devices feature a semiconductor-based electronic interruption unit. This means that the electrical current flow of the low-voltage circuit is routed through semiconductor components or semiconductor switches that can interrupt the electrical current flow or be switched to conductivity. Protective switching devices with an electronic interruption unit also frequently feature a mechanical isolating contact system, particularly with isolating characteristics according to relevant standards for low-voltage circuits. The contacts of the mechanical isolating contact system are connected in series with the electronic interruption unit, meaning that the current of the low-voltage circuit to be protected is routed through both the mechanical isolating contact system and the electronic interruption unit. Such a switching contact system typically has an arc extinguishing system with which the arc generated during switching (switching arc) can be extinguished or reduced. The present invention relates to low-voltage alternating current circuits with an alternating voltage, typically a time-dependent sinusoidal alternating voltage with frequency f. The time dependence of the instantaneous voltage value u(t) of the alternating voltage is described by the equation: where: u(t) = instantaneous voltage value at time t and U = amplitude (maximum value) of the voltage A harmonic alternating voltage can be represented by the rotation of a phasor whose length corresponds to the amplitude (U) of the voltage. The instantaneous displacement is the projection of the phasor onto a coordinate system. One oscillation period corresponds to one full rotation of the phasor, and its full angle is 2π (2π) or 360°. The angular frequency is the rate of change of the phase angle of this rotating phasor. The angular frequency of a harmonic oscillation is always 2π times its frequency, i.e.: ω = 2n*f = 2π / T = angular frequency of the alternating voltage (T = period of the oscillation) The angular frequency (ω) is often preferred over the frequency (f), since many formulas in the theory of oscillations can be represented more compactly using the angular frequency due to the occurrence of trigonometric functions whose period is by definition 2π: In the case of angular frequencies that are not constant over time, the term instantaneous angular frequency is also used. For a sinusoidal, especially a time-constant, alternating voltage, the time-dependent value of the angular velocity ω and the time t corresponds to the time-dependent angle φ(t), which is also called the phase angle φ(t). This means that the phase angle φ(t) periodically traverses the range 0...2π or 0°...360°. That is, the phase angle periodically assumes a value between 0 and 2π or 0° and 360° (φ = n*(0...2π) or φ = n*(0°...360°), due to periodicity; simplified: φ = 0...2π or φ = 0°...360°). The instantaneous voltage value u(t) therefore refers to the instantaneous value of the voltage at time t, i.e., in the case of a sinusoidal (periodic) alternating voltage, the value of the voltage at the phase angle φ (φ = 0 ... 2π or φ = 0°...360°, of the respective period). Austrian patent application AT 519 847 A1 discloses an electrical switching device. This device comprises at least one semiconductor switch arranged in the current path, electrical measuring devices, and a control device that evaluates the measured values ​​determined by the measuring devices and is designed to control the semiconductor switches. Upon detecting certain measured values, the control device automatically initiates specific activations of the semiconductor switches. The conditions for controlling the semiconductor switches can be transmitted to the control device via a data interface by means of an initiation process. U.S. Patent Application US 2017 / 0256934A1 discloses a hybrid air-gap / solid-state protection device. A hybrid air-gap / solid-state protection device (PD) for use in an electrical power distribution system comprises an air-gap isolator connected in series with a solid-state device, a sensing and driver circuit, and a microcontroller. When the sensing and driver circuit detects an impending fault or an excessively high and unacceptable overvoltage condition in the PD's load circuit, it generates a control signal that rapidly shuts down the solid-state device. Simultaneously, the microcontroller generates a shutdown pulse for the air-gap isolator, which responds by creating an air gap in the load circuit. Together, the shut-down semiconductor device and the air gap protect the load and its associated load circuit from damage.They also serve to electrically and physically isolate the source of the fault or overload condition from the rest of the electrical power distribution system. The object of the present invention is to improve a protective switching device of the type mentioned above, in particular to enable (alternative) detection of switching operations of a mechanical disconnect contact system. Specifically, it aims to reduce contact erosion caused by the switching arc or to reduce the duration of switching arcs. This problem is solved by a protective switching device having the features of claim 1, and by a method according to claim 14. According to the invention, a protective switching device for protecting a low-voltage electrical circuit, in particular a low-voltage AC circuit, is provided, comprising: - a housing with mains-side and load-side connections for (at least two) conductors of the low-voltage AC circuit, - a mechanical isolating contact system connected to the (two) mains-side connections for galvanic interruption of the low-voltage circuit, which is connected on the other hand to an electronic interruption unit, which has a high-resistance state of the switching elements for interruption and a low-resistance state of the switching elements for current flow in the low-voltage circuit by means of semiconductor-based switching elements, wherein the electronic interruption unit is connected on the other hand to the (two) load-side connections, - a voltage sensor which is arranged between the mechanical isolating contact system and the electronic interruption unit.to determine the voltage level of the low-voltage circuit, - that the mechanical disconnect contact system is manually operable, so that the contacts of the mechanical disconnect contact system can be manually closed for current flow and opened for an interruption in the low-voltage circuit, - a control unit that is connected to the voltage sensor and the electronic interruption unit. According to the invention, the protective switching device is designed such that the voltage level is determined and, in the event of a voltage drop of - 10 to 50 volts in the case of a single-pole interrupting (only one conductor, e.g., both conductors of the low-voltage circuit, is interrupted) or - 20 to 100 volts in the case of a double-pole interrupting (both conductors, e.g., both conductors of the low-voltage circuit, are interrupted) mechanical disconnect contact system, an opening of the mechanical disconnect contact system is detected. Furthermore, an opening of the mechanical isolating contact system can also be detected if the voltage drops below 10 volts. According to the invention, it is advantageously possible to detect an opening (actuation) of the mechanical isolating contact system independently of its mechanical components, using only a voltage measurement in the low-voltage circuit. Advantageously, a very fast voltage measurement should be used. Advantageous embodiments of the invention are specified in the dependent claims. In an advantageous embodiment of the invention, the (instantaneous) voltage level is compared with an (instantaneous) lower threshold value. The (instantaneous) lower threshold value is calculated by multiplying the magnitude of an (instantaneous) expected voltage value by a scaling factor, which may in particular be a value in the range of 0.85 to 1, and subtracting a fixed voltage value, in particular a value in the range of 10 to 50 volts for a single-pole interrupting or 20 to 100 volts for a double-pole interrupting mechanical disconnect contact system. When the instantaneous lower threshold value is undershot, the opening of the mechanical disconnect contact system is detected. This has the particular advantage that a simple threshold comparison solution for the invention is provided. The scaling factor allows for a robust solution, for example with a scaling factor of 0.85, which avoids false detections, or a sensitive detection, for example with a scaling factor of 1 (i.e., scaling is omitted). In an advantageous embodiment of the invention, the (instantaneous) voltage level is compared with an (instantaneous) upper threshold value. The (instantaneous) upper threshold value is formed by dividing the magnitude of an (instantaneous) expected voltage value by a scaling factor, in particular a value from the range of 0.85 to 1, and adding it to a fixed voltage value, in particular a value from the range of 10 to 100 volts, particularly independent of the number of poles of the mechanical disconnect contact system. If the current upper threshold is exceeded, an opening of the mechanical isolating contact system (MK) is detected. This has the particular advantage of providing another simple threshold comparison solution for the invention. Switching-induced voltage increases are also detected during or as openings that can arise, for example, from inductances on the load side. Furthermore, additional protection or a protective function against overvoltages is provided. Furthermore, an area check or corridor check (lower and upper threshold) is performed, whereby openings are reliably detected. The scaling factor allows for a robust solution, with a scaling factor of e.g. 0.85, which avoids false detections, or a sensitive detection, with a scaling factor of e.g. 1, which eliminates the need for scaling. In an advantageous embodiment of the invention, upon detection of the opening (actuation) of the mechanical disconnect contact system, the semiconductor-based switching elements of the electronic interruption unit switch to a high-resistance state, in particular within a first time interval or less than a first time interval, which is in particular less than 100 µs, more specifically less than 50 µs or 10 µs, in order to avoid contact erosion caused by the switching arc or to reduce the switching arc duration of the mechanical disconnect contact system. This has the distinct advantage that the mechanical contacts can be switched or opened with virtually no power consumption, and contact erosion, which ages and damages the device, is reduced. This is because, upon detection of the voltage drop caused by the switching arc, the electronic interruption unit creates a (virtually) immediate high resistance, thus instantly reducing the further current flow. This also eliminates the need for an arc extinguishing system. Furthermore, the contact system can be simpler, as less material is required for contact erosion, and special alloys for the contact material are not necessary. In an advantageous embodiment of the invention, the protective switching device includes a communication unit. Upon detection of the opening of the mechanical disconnect contact system, information is transmitted by the communication unit. This has the particular advantage that a shutdown of the protective switching device is detected and communicated, for example to higher-level units or management systems. In an advantageous embodiment of the invention, the protective switching device includes a display unit. When the opening of the mechanical disconnect contact system is detected, information is displayed by the display unit. This has the particular advantage of indicating when the mechanical disconnect contact system is opening. In an advantageous embodiment of the invention, the protective switching device has a memory. When the opening of the mechanical disconnect contact system is detected, information is stored in the memory. This has the particular advantage that information on opening the mechanical disconnect contact system can be retrieved afterwards. In an advantageous embodiment of the invention, a further voltage sensor is provided between the mains-side connection and the mechanical isolating contact system. This has the particular advantage of providing an alternative way to detect an opening of the mechanical disconnect contact system and for further options. In an advantageous embodiment of the invention, the further voltage sensor performs a voltage determination only to the extent of whether a voltage is present at the mains-side connections. In particular, whether a valid mains voltage (e.g., 230V + / - 10%) is present. This has the particular advantage that a simplified voltage sensor can be used. This allows for further verification and differentiation as to whether the mains-side power source has been switched off or whether a mechanical actuation / opening process is taking place. In an advantageous embodiment of the invention, the additional voltage sensor determines the voltage level at that location. A voltage drop is determined by calculating the difference between the voltages of the two voltage sensors. This has the particular advantage of providing an alternative way to detect when the mechanical disconnect contact system is open. In an advantageous embodiment of the invention, the additional voltage sensor is galvanically isolated from the other units of the protective switching device. In particular, at least one optocoupler or capacitive elements can be provided. This has the particular advantage of avoiding a bridging of the separating function or separating properties of the mechanical separating contact system. In an advantageous embodiment of the invention, the protective switching device has a power supply (such as a power supply unit) with an energy storage device. The power supply with energy storage device is designed such that, after the contacts of the mechanical isolating contact system have opened, sufficient energy is provided for: - switching the semiconductor-based switching elements to the high-resistance state or ( / and) - transmitting information via the communication unit or ( / and) - displaying information via the display unit or ( / and) - switching the protective switching device to a defined state, including storing information, in particular about the time of activation and / or the magnitude of the voltage and / or current and / or frequency and / or temperature. This has the particular advantage that the functions according to the invention can still be provided even after a loss of energy supply. In an advantageous embodiment of the invention, an energy supply via the energy storage device is available for a period of 10 ms to 1 s, in particular 20 ms to 100 ms or 500 ms, after an opening process of the contacts of the mechanical isolating contact system. This has the particular advantage that the protective switching device, especially the control unit, can enter a defined state after the opening process, during which the regular power supply is also switched off. This allows it to store information and send information, particularly about the opening of the contacts and / or the switching off of the protective switching device. The control unit, especially its microprocessor, can thus shut down in a controlled manner or complete the switching-off process in a controlled way. According to the invention, a corresponding method for a protective switching device for a low-voltage circuit with electronic (semiconductor-based) switching elements is claimed, offering the same and further advantages. All embodiments, both in dependent form with reference to claim 1 or 14, and with reference only to individual features or combinations of features of claims, result in an improvement of a protective switching device for the rapid and reliable detection of an opening process of the mechanical disconnect contact system / reduction of contact erosion / switching arcs. The described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawing. The drawing shows: Fig. 1 a representation of a protective switching device, Fig. 2 a first embodiment of the protective switching device, Fig. 3 a second embodiment of the protective switching device, Fig. 4 a third embodiment of the protective switching device, Fig. 5 a fourth embodiment of the protective switching device, Fig. 6 a first voltage and threshold value curve over time, Fig. 7 a second voltage and threshold value curve over time. Fig. 1 shows a representation of a protective switching device SG for protecting a low-voltage electrical circuit with a housing GEH, comprising: - connections for conductors (L, N) of the low-voltage circuit, in particular first connections L1, N1 for a network-side, especially energy source-side, connection EQ of the protective switching device SG and second connections L2, N2 for a load-side, especially energy sink-side, connection ES (consumer-side connection) of the protective switching device SG, wherein phase-side connections L1, L2 and neutral-side connections N1, N2 can be provided; the load-side connection can have a passive load (consumer) and / or an active load ((further) energy source), or a load that can be both passive and active, e.g., in sequence; the protective switching device is constructed as a two-pole device in this example: the first pole for a phase conductor L1, L2,the second pole for a neutral conductor N1, N2 of the low-voltage (alternating current) circuit; - a mechanical isolating contact system MK connected to the mains-side terminals L1, N1, for galvanic interruption of the low-voltage circuit, which on the other hand is connected to an electronic interruption unit EU, which has a high-resistance state of the switching elements for interruption and a low-resistance state of the switching elements for current flow in the low-voltage circuit by means of semiconductor-based switching elements, wherein the electronic interruption unit EU on the other hand is connected to the load-side terminals L2, N2, that the mechanical isolating contact system MK and the electronic interruption unit EU are electrically connected in series, - the mechanical isolating contact system MK is, in particular exclusively, manually operable,so that the contacts of the mechanical isolating contact system MK can be manually closed for current flow and opened for interruption in the low-voltage circuit; for example, a handle for opening and closing the contacts can be attached to the protective switching device; furthermore, a mechanism for switching support can be provided between the handle and the contacts; a voltage sensor SU, which is arranged between the mechanical isolating contact system MK and the electronic interruption unit EU, for determining the voltage level of the low-voltage circuit there, in particular for (periodic) determination of the voltage level of the low-voltage circuit, so that pulsed / instantaneous voltage values ​​are available; a control unit SE, which is connected to the voltage sensor SU and the electronic interruption unit EU. The protective switching device SG is designed to detect the voltage level and, in the event of a voltage drop of 10 to 50 volts for a single-pole interrupting mechanical disconnect system or 20 to 100 volts for a double-pole interrupting mechanical disconnect system, it detects the opening or actuation of the mechanical disconnect system. Alternatively or additionally, it can also detect a voltage drop below 10 volts. Upon such detection of the opening or actuation of the mechanical disconnect contact system, the semiconductor-based switching elements of the electronic interruption unit can, for example, switch to a high-resistance state to prevent contact erosion of the contacts of the mechanical disconnect contact system caused by switching arcs. The protective switching device SG can include a communication unit. Upon detection of the opening or actuation of the mechanical disconnect contact system, the communication unit can transmit information. This information can relate to the actuation of the disconnect contact, but also to other information, such as the instantaneous current or voltage values, the time, or the temperature at the moment of actuation. The protective switching device may have a display unit. When the opening or actuation of the mechanical disconnect contact system is detected, information is displayed by the display unit. The protective switching device may have a memory. When the opening or actuation of the mechanical disconnect contact system is detected, information (e.g., about the time of detection, voltage and / or current level, frequency, temperature, etc.) is stored in the memory, which can be read out later. An additional voltage sensor can be installed between the mains-side connection (L1, N1) and the mechanical disconnect contact system MK. This sensor can determine whether a voltage is present at the mains-side connections, and in particular whether it is of a certain magnitude. The additional voltage sensor can then determine the voltage level at these points. The voltage drop can be calculated by differentiating the voltage readings of the two voltage sensors. The additional voltage sensor can be galvanically isolated from the other units of the protective switching device SG, in particular by at least one optocoupler or capacitive elements, in order to avoid bridging the existing mechanical isolating contact. The protective switching device has a power supply with an energy storage device, for example, a power supply unit NT with capacitive energy storage (capacitor(s)). The power supply unit NT is connected on one side to the conductors of the low-voltage circuit, preferably to the conductors between the mechanical disconnect contact system MK and the electronic interruption unit EU. On the other side, the power supply unit NT serves to supply power to the control unit SE and / or the electronic interruption unit EU, and optionally to the voltage sensor SU and / or current sensor SI.The energy supply with an energy storage device is designed such that, after an opening process of the contacts of the mechanical disconnect contact system MK, sufficient energy is provided for: - a change of the semiconductor-based switching elements to the high-resistance state or ( / and) - a transmission of information by the communication unit or ( / and) - a display of the information by the display unit - a change of the protective switching device to a defined state (preparing the shutdown of the control unit), including the storage of information, e.g. about the time of actuation, level of voltage and / or current, frequency, temperature, etc. In general, the system can be dimensioned such that an energy supply from the energy storage device is available for a period of 1 ms to 1 s, in particular 20 ms to 100 ms or 500 ms, after an opening process of the contacts of the mechanical isolating contact system. Additionally, for one embodiment of the invention, a current sensor SI can also be provided for (periodically) determining the magnitude of the current in the low-voltage circuit and / or the change in current over time, so that (pulse-wise) current values ​​and / or current change values ​​are available, as shown in the example according to Fig. 1. The current sensor SI is then connected to the control unit SE. Current change values ​​can be determined, for example, either in the current sensor SI itself or in the control unit SE. The electronic interruption unit (EU) is shown as a block in both conductors, as shown in Fig. 1. In a first variant, this does not mean that both conductors are interrupted. At least one conductor, in particular the live conductor or phase conductor, has semiconductor-based switching elements. The neutral conductor can be free of switching elements, i.e., without semiconductor-based switching elements. This means the neutral conductor is directly connected and does not become high-impedance. Therefore, only a single-pole interruption (of the phase conductor) occurs. If further live conductors / phase conductors are present, in a second variant of the electronic interruption unit EU, the phase conductors have semiconductor-based switching elements. The neutral conductor is directly connected and does not become high-impedance. This is the case, for example, in a three-phase AC circuit. In a third variant of the electronic interruption unit EU, the neutral conductor can also have a semiconductor-based switching element, i.e., when the electronic interruption unit EU is interrupted, both conductors become high-resistance. The electronic interruption unit EU can comprise semiconductor devices such as bipolar transistors, field-effect transistors, isolated gate bipolar transistors (IGBTs), metal oxide layer field-effect transistors (MOSFETs), or other (self-commutated) power semiconductors. In particular, IGBTs and MOSFETs are especially well-suited for the protective switching device according to the invention due to their low forward resistance, high junction resistance, and good switching characteristics. The MK mechanical disconnect contact system can, in one variant, interrupt a single pole. This means that only one of the two conductors, specifically the live conductor or phase conductor, is interrupted, i.e., it has a mechanical contact. The neutral conductor is then without contact, i.e., the neutral conductor is directly connected. If additional active conductors / phase conductors are provided, in a second variant the phase conductors have mechanical contacts of the mechanical disconnect contact system. In this second variant, the neutral conductor is directly connected. For example, in a three-phase AC circuit. In a third variant of the mechanical disconnect contact system MK, the neutral conductor also has mechanical contacts, as shown in Fig. 1. For example, for a two-pole or all-pole interruption. In the case of a three-phase alternating current circuit, the voltage dip is determined analogously, whereby measurements can be taken between two active conductors (phase conductors) or between active and neutral conductors. The invention relates to the detection of manual actuation, specifically opening, of the contacts of the mechanical disconnect contact system. The detection depends on the number of opening contacts. According to the invention, the detection of manual actuation is intended to be independent of the detection of mechanical actuation, i.e., purely electronic. The protective switching device SG has a mechanical isolating contact system MK, in particular in accordance with standard with standard-compliant isolating properties, for galvanic isolation of the circuit, in particular for standard-compliant disconnection (as opposed to switching off) of the circuit. The term "MK mechanical disconnect contact system" refers specifically to a (standard-compliant) disconnect function implemented by the MK disconnect contact system. The disconnect function encompasses the following points: - Minimum clearance according to the standard (minimum contact spacing), - Contact position indicator of the contacts of the mechanical disconnect contact system, - Actuation of the mechanical disconnect contact system always possible (no blockage of the disconnect contact system). The minimum air gap between the contacts of the isolating contact system is essentially voltage-dependent. Other parameters include the degree of pollution, the type of field (homogeneous, inhomogeneous), and the air pressure or altitude above sea level. There are corresponding regulations and standards for these minimum clearances or creepage distances. These regulations specify, for example, the minimum clearance for an inhomogeneous and a homogeneous (ideal) electric field, depending on the degree of pollution, in the case of air required for impulse voltage withstand capability. Impulse voltage withstand capability is the resistance to being applied when a corresponding impulse voltage is applied. Only if this minimum length (minimum distance) is present does the isolating contact system or protective switching device exhibit an isolating function (isolating property). For the purposes of the invention, the standards DIN EN 60947 and IEC 60947 are relevant for the separator function and its properties, and reference is made to them here. The isolating contact system is advantageously characterized by a minimum clearance between the open isolating contacts in the OFF position (open position, contacts open), depending on the rated impulse withstand voltage and the pollution degree. The minimum clearance is, in particular, between 0.01 mm and 14 mm (at a minimum). More advantageously, the minimum clearance is between 0.01 mm at 0.33 kV and 14 mm at 12 kV, especially for pollution degree 1 and particularly for inhomogeneous fields. Advantageously, the minimum air gap can have the following values: E DIN EN 60947-1 (VDE 0660-100):2018-06 Table 13 - Minimum air gaps Table 13 - Minimum air gaps UimpFall AInhomogeneous fieldFall BHomogeneous field, ideal conditions kV (see 3.7.63)(see 3.7.62) Pollution level 12341234 0,330,010,20,81,60,010,20,81,6 0.50,040,04 0.80.10.1 1,50,50,50,30,3 2,51,51,51,50,60,6 4,03333121,21,2 6,05,55,55,55,52222 8,088683333 12141414144,54,54,54,5 NOTE The specified minimum air gaps are based on the 1.2 / 50 µs impulse voltage at an air pressure of 80 kPa, which corresponds to the air pressure at 2 000 m above sea level. The pollution levels and field types correspond to those defined in the standards. This advantageously allows for the creation of a standard-compliant protective switching device dimensioned according to the rated impulse withstand voltage. The electronic detection of a manual actuation, specifically opening, of the contacts of the mechanical disconnect contact system, i.e., the detection of a voltage drop due to the arc voltage of the switching contacts, depending on the number of switching contacts, i.e., the number of poles, can be carried out, for example, as follows. In one variant, the protective switching device SG can be designed such that a differential voltage is calculated from the measured voltage values ​​and an expected voltage value, resulting in cycle-wise differential voltage values. Each differential voltage value is compared, particularly in terms of its magnitude, with a first threshold value. If at least two consecutive differential voltage values ​​are exceeded, an opening process of the mechanical isolating contact system is detected. To prevent further current flow with switching arc, the electronic interruption unit EU is activated, which switches to a high-resistance state. The expected value of the voltage can be determined, for example, by a so-called phase-locked loop (PLL). A PLL is an electronic circuit arrangement that influences the phase and, consequently, the frequency of a variable oscillator via a closed control loop in such a way that the phase deviation between an external periodic reference signal and the oscillator or a signal derived from it is as constant as possible. This allows, among other things, the determination of the fundamental frequency and its amplitude of the supplied mains voltage, i.e., the determined voltage values, i.e., the (undisturbed or filtered) expected value of the (mains) voltage. The expected voltage value output by the PLL can then be compared with the measured voltage value, particularly in a time- or phase-synchronous manner, so that a difference between the values ​​is present. Often, a PLL itself can perform such a function, i.e., output the difference, i.e., a differential voltage value. The differential voltage is then compared, particularly in terms of its magnitude, with the threshold value. Alternatively, the expected value of the voltage can be stored in a table, whereby the respective voltage values ​​are then compared in phase synchrony or a phase-synchronous difference is formed, so that differential voltage values ​​are available. The differential voltage values ​​can be determined, for example, in the voltage sensor SU itself or in the control unit SE, so that differential voltage values ​​are available at each cycle. Fig. 2 shows a representation of a shutdown logic ASL, as it can be implemented, for example, in the control unit SE according to Fig. 1, either functionally or circuit-wise. The differential voltage values ​​are fed (clock-wise) to a first comparator COMP1 to compare each differential voltage value with the first threshold value. The output of the first comparator COMP1 is further: - firstly, directly, - secondly, via a first intermediate memory Z1, which temporarily stores exactly one comparator output value (clock-wise), - in the embodiment according to Fig.2. Thirdly, the first intermediate storage Z1 is connected to a second intermediate storage Z2, which stores exactly one comparator output value (clock by clock), with a first AND gate AND1, so that if, in the example, three consecutive differential voltage values ​​are exceeded, an interruption of the low-voltage circuit is initiated, for example by the interruption signal TRIP, which is transmitted from the control unit SE to the electronic interruption unit EU. Further intermediate storage devices can be provided analogously (exceeding four, five, ... differential voltage values). With this arrangement, the voltage drop of 10 to 50 volts in the event of a single-pole interruption of the mechanical disconnect contact system, or of 20 to 100 volts in the event of a two-pole interruption of the mechanical disconnect contact system, can be determined. Further functions can be implemented by the control unit SE or the shutdown logic ASL, as shown below. Fig. 2 further shows an embodiment in which a current measurement unit connected to the control unit is provided, so that the change in current over time in the low-voltage circuit is measured periodically, resulting in pulsed current change values. The measurement of the current change over time can be performed, for example, in the current sensor SI itself or in the control unit SE. The current change values ​​are fed to a second comparator COMP2, which compares each current change value with a second threshold value.The output of the second comparator COMP2 is furthermore connected on the one hand directly and on the other hand via a third intermediate storage Z3, which temporarily stores exactly one comparator output value (clock by clock), to a third AND gate AND3, so that if two successive current change values ​​are exceeded, an interruption of the low voltage circuit is initiated, for example by the interrupt signal TRIP, which is transmitted from the control unit SE to the electronic interrupt unit EU. In general, additional intermediate storage units may be provided (exceeding three, four, five, ... current change values). Fig. 2 further shows an embodiment in which the outputs of the comparators COMP1, COMP2 are logically linked via a second AND gate AND2, so that when the first and second threshold values ​​are exceeded, ideally the same clock or the preceding / following or adjacent clock, an interruption of the low-voltage circuit is initiated, for example by the interruption signal TRIP, which is transmitted from the control unit SE to the electronic interruption unit EU. Fig. 2 further shows an embodiment in which the outputs of the first, second and third AND gate AND1, AND2, AND3 are combined via an OR gate OR, so that - if at least two successive differential voltage values ​​are exceeded or - if at least two successive current change values ​​are exceeded or - if the first and second threshold values ​​are exceeded within the first time window, an interruption of the low-voltage circuit is initiated, for example by the interruption signal TRIP, which in this case is output by the OR gate OR. The (periodic) determination of differential voltage values, or, if applicable, current change values, or voltage values ​​and, if applicable, current values, for example in the voltage sensor SU or current sensor SI, or in the control unit SE, is performed at a clock frequency greater than or equal to 10 kHz and less than or equal to 10 MHz, specifically greater than or equal to 10 kHz and less than or equal to 1 MHz. Thus, differential voltage values, or, if applicable, current change values, or voltage values ​​and, if applicable, current values ​​are available at intervals of 100 µs to 0.1 µs, specifically from 100 µs to 1 µs. This allows the differential voltage values ​​or, if applicable, current change values ​​of the same clock cycle to be compared with the threshold values, provided the values ​​are clocked at the same interval. Alternatively, if there is a time interval (clock difference) between the voltage and current values, the differential voltage values ​​or, if applicable, current change values ​​can be compared with temporally corresponding clock cycles.The time interval should not be greater than one sampling cycle. This means that, depending on the sampling rate, the first time window is exactly one sampling cycle long. For example, at a sampling rate of 100 kHz, the clock cycle is 10 µs, meaning the interval between two samples is 10 µs, and therefore the first time window is at most 10 µs. According to the invention, a protective switching device SG can integrate only one voltage measurement into the device. This can be in the form of voltage sensors SU, for example. The voltage sensor SU preferably has a bandwidth greater than the switching frequency. The same applies to the optional current sensor. The current sensor SI can, for example, have a measuring resistor or shunt. The voltage sensor SU is used (solely) for detecting manual switching operations of the mechanical disconnect contact system. The measurement processing and the shutdown logic can be implemented, e.g., in the control unit SE, using analog technology, in a computer such as a microcontroller (uC) or a Field Programmable Gate Array (FPGA), or partly in analog technology and partly in a computer. In another variant for determining the voltage dip, which is described below, instantaneous voltage values ​​DU (phase-related voltage values) are determined by the voltage sensor SU to determine the level of the voltage of the low-voltage circuit. The protective switching device SG, in particular the control unit SE, is designed such that instantaneous (phase-angle-related) threshold values ​​are available. For example, in a simple case, the sine wave of the voltage, for example with an RMS value of 230 volts and an amplitude of 325 volts, could be stored as an instantaneous threshold value for each phase angle, with its expected instantaneous voltage value minus a discount of, for example, 10%, or a value between 10 and 30%, or a fixed discount of at least 10 volts, whereby a value of at least 10 volts is used as the minimum instantaneous threshold value (to prevent false tripping). With a reduction of, for example, 10%: 10 volts at 0° (minimum instantaneous threshold), ..., 146.25 volts (162.5 volts - 10%) at 30°, ..., 206.8 volts (229.8 volts - 10%) at 45°, ..., 253.3 volts (281.4 volts - 10%) at 60°, 292.5 volts (325 volts - 10%) at 90°, etc. The instantaneous threshold values ​​can be per individual phase angle, a phase angle range (multiple phase angles), e.g. every 2°, or a phase angle segment (a part of a phase angle), e.g. every 0.5°. The current threshold values ​​are adjusted according to the voltage drop expected during the manual actuation process. The instantaneous voltage values ​​DU are compared, preferably in magnitude, with the instantaneous threshold values ​​in relation to the phase angle. An activation is detected if the instantaneous threshold values ​​are undershot or exceeded. If absolute value calculation is omitted, the system checks for falling below the instantaneous threshold values ​​during the positive half-wave and for exceeding them during the negative half-wave. Alternatively, the magnitudes of the instantaneous voltage values ​​DU can be determined. These magnitudes are then checked against falling below the instantaneous threshold values, thus advantageously eliminating the need to consider the sign. The comparison is phase-angle-related; that is, the instantaneous voltage value at, for example, a 30° phase angle is checked against the instantaneous threshold value at a 30° phase angle. Alternatively, an instantaneous differential voltage value can be calculated (continuously) from the current threshold value (and possibly its magnitude) and the current voltage value DU (and possibly its contribution). This instantaneous differential voltage value is compared to an (absolute or instantaneous) differential voltage threshold value, and activation is detected when the threshold is exceeded (or fallen below – in the case of a signed comparison). Alternatively, a corridor check can also be performed, i.e., if the current voltage value deviates by more than a certain percentage, in particular 5 to 15%, or a certain + / - voltage amount, in particular 10 to 40 volts, from the expected threshold or expected voltage value, the activation is detected and advantageously an (immediate) initiation of the (electronic) interruption of the low-voltage circuit is initiated. The detection of an actuation is achieved, for example, by a (first) signal TRIP, which is sent from the control unit SE to the electronic interrupt unit EU, as shown in Fig. 1. The undershooting or exceeding can be advantageous for an initial time period or for a phase angle range or phase angle section in order to verify the detection of the actuation. Fig. 3 shows a representation according to Fig. 1, with a further detailed embodiment. Here, the control unit SE comprises two subunits: a first subunit SEA, preferably analog, and a second subunit SED, preferably digital. The first subunit SEA includes a voltage comparator CU. This comparator receives, on the one hand, the instantaneous voltage values ​​DU from the voltage sensor SU, preferably in absolute value. On the other hand, the instantaneous threshold values ​​SWU from the second subunit SED are also supplied to the voltage comparator CU. The voltage comparator CU compares the instantaneous voltage values ​​DU with the instantaneous threshold values ​​SWU and, as described, outputs a voltage interruption signal TU when the threshold is exceeded or fallen below, to detect activation and possibly initiate an interruption of the low-voltage circuit. The voltage interruption signal TU can be fed to a logic unit LG, which combines it with other interruption signals and outputs the (first) signal TRIP to the electronic interruption unit EU for semiconductor-based interruption or high-impedance interruption. In one embodiment, the voltage comparator CU stores the current threshold values ​​SWU in order to have the values ​​constantly available. The instantaneous voltage values ​​DU are also fed to the second subunit SED. In a preferred embodiment, the instantaneous voltage values ​​DU are digitized there by an analog-to-digital converter (ADC) and fed to a microprocessor (CPU). The CPU then determines or calculates the instantaneous threshold values ​​SWU. The instantaneous threshold values ​​SWU determined by the second subunit SED, and in particular by the microprocessor CPU, are then fed back to the first subunit SEA, specifically to the voltage comparator CU, to perform the comparison described above. Advantageously, the determination of the instantaneous threshold values ​​SWU in the second subunit SED can be carried out digitally, or at a slower processing speed than the continuous comparison of voltage values ​​and threshold values ​​in the first subunit SEA. In an advantageous embodiment, in which a current sensor SI is provided that outputs the magnitude of the current, i.e., instantaneous current values ​​DI, the first subunit SEA can include a current comparator CI. The instantaneous current values ​​DI from the current sensor SI are supplied to the current comparator CI. At the same time, instantaneous current threshold values ​​SWI are supplied to the current comparator CI by the second subunit SED. The current comparator CI compares the instantaneous current values ​​DI with the instantaneous current threshold values ​​SWI and, analogously, outputs a current interruption signal TI when the current exceeds the threshold value, thus initiating an interruption of the low-voltage circuit. The current interruption signal TI can be fed to the logic unit LG, which combines it with other interruption signals and outputs the (first) signal TRIP to the electronic interruption unit EU for semiconductor-based interruption or high-impedance interruption. In one embodiment, the current comparator CI stores the current threshold values ​​SWI in order to have the values ​​constantly available. The instantaneous current values ​​DI can also be fed to the second subunit SED. In a preferred embodiment, the instantaneous current values ​​DI are digitized there by the analog-to-digital converter ADC and fed to the microprocessor CPU. The CPU then determines or calculates the instantaneous current threshold values ​​SWI. The instantaneous current threshold values ​​SWI determined by the second subunit SED, or more specifically by the microprocessor CPU, are then fed back to the first subunit SEA, in particular to the current comparator CI, to perform the comparison described above. Advantageously, the determination of the instantaneous current threshold values ​​SWI in the second subunit SED can be done digitally or with a slower processing speed than the continuous comparison of current values ​​and threshold values ​​in the first subunit SEA. Fig. 4 shows a further embodiment or variant according to Fig. 1 and Fig. 3. Fig. 4 shows part of a simple variant of the first subunit SEAE and part of an alternative variant of the second subunit SEDE. The simplified version of the first subunit SEAE includes the voltage comparator CU, to which the instantaneous voltage values ​​DU, particularly their magnitude, and the instantaneous threshold values ​​SWU, also particularly their magnitude, are fed. In this example, the voltage comparator CU directly outputs the first signal TRIP to interrupt the low-voltage circuit, analogous to Fig. 3. The magnitude calculation can be performed by one or more units not shown. The alternative version of the second subunit SEDE includes a network synchronization unit NSE. This unit receives the (analog) instantaneous voltage values ​​DU. From these (analog) instantaneous voltage values ​​DU, which are a sinusoidal alternating voltage of the low-voltage circuit, the network synchronization unit NSE determines the amplitude U, the phase angle φ(t), and an expected time value of the voltage UE. The expected value of the voltage UE is a type of filtered, regenerated, or generated equivalent instantaneous voltage value DU. The expected value of the voltage UE, as well as the amplitude U and the phase angle φ(t), can be determined, for example, by a so-called phase-locked loop (PLL). A PLL is an electronic circuit arrangement that influences the phase and, consequently, the frequency of a variable oscillator via a closed control loop in such a way that the phase deviation between an external periodic reference signal (instantaneous voltage values) and the oscillator or a signal derived from it is as constant as possible. This allows, among other things, the determination of the fundamental frequency and its amplitude of the supplied mains voltage, i.e., the determined voltage values, i.e., the (undisturbed or filtered) expected value of the (mains) voltage. The amplitude U, phase angle φ(t) and expected time value of the voltage UE determined by the network synchronization unit NSE are fed to a threshold unit SWE.The threshold unit SWE modifies the expected value of the voltage UE to an instantaneous threshold SWU, where, for example: - a fixed voltage amount can be subtracted from the expected value of the voltage UE, for example the voltage dip to be tested, or - a voltage amount dependent on the phase angle can be subtracted from the expected value of the voltage UE, for example proportional to the voltage dip to be tested, or - a fixed percentage of the voltage can be subtracted from the expected value of the voltage UE, for example proportional to the voltage dip to be tested, or - a phase angle dependent percentage of the voltage can be subtracted from the expected value of the voltage UE, for example proportional to the voltage dip to be tested, in order to obtain the instantaneous threshold SWU. The instantaneous threshold SWU can also be adjusted by the amplitude U of the instantaneous voltage, i.e., a high instantaneous threshold is present at a high amplitude of the voltage and a low instantaneous threshold is present at a low amplitude of the voltage, in order to reliably detect the voltage dip. The instantaneous threshold values ​​SWU can be transmitted from the threshold unit SWE to the voltage comparator CU synchronously with the instantaneous voltage DU by virtue of the presence of the phase angle φ(t) in the threshold unit, so that a phase-related (phase-angle-related) comparison between instantaneous threshold value and instantaneous voltage value can be carried out in the voltage comparator CU. Fig. 5 shows an alternative embodiment according to Figs. 3 and 4, with the difference that the expected value of the voltage UE is not fed to the threshold unit SWE, but to a difference unit DE. Furthermore, the instantaneous voltage value DU is not fed to the voltage comparator CU, but to the difference unit DE. The difference unit DE calculates the difference between the expected value of the voltage UE and the instantaneous voltage value DU, with the calculation being phase-angle accurate. This generates a (phase-angle-dependent) differential voltage value DW, which is fed to the voltage comparator CU. The voltage comparator CU, on the other hand, receives the instantaneous threshold value SWU, which in this example is provided by the threshold unit SWE as a phase-related and amplitude-influenced differential voltage threshold value. In this example, the expected voltage value UE provided by the grid synchronization unit, particularly in the form of a PLL, is compared with the instantaneous voltage value DU, specifically in a time- or phase-synchronous manner, resulting in a difference of values ​​DW. Often, a PLL itself can perform such a function, i.e., provide the difference, i.e., a differential voltage value DW. The differential voltage value DW is then compared, particularly in terms of its magnitude, with the current threshold value SWU, in this case a differential voltage threshold that corresponds to the voltage dip to be tested. Alternative designs are also conceivable. The voltage's behavior over time is examined with regard to voltage dips. A very rapid detection is possible through phase-accurate comparison with the expected voltage. The phase angle resolution determines the speed of threshold calculation. With a phase angle resolution of 1°, meaning a threshold value is available for every full phase angle of the voltage, i.e., an instantaneous threshold value is available approximately every 55.5 µs. The switch-off is preferably performed via an analog comparator, i.e., continuously, and is therefore significantly faster than the phase angle resolution. Alternatively, the following time course applies to fully digital processing. The phase angle resolution determines the detection speed. With a phase angle resolution of 1°, meaning a threshold value exists for each full phase angle of the voltage (i.e., an instantaneous threshold value occurs approximately every 55.5 µs), this means that a shutdown can occur after a minimum of approximately 60 µs. Shorter shutdown times can be achieved with higher phase angle resolutions. In this example, the values ​​are then processed at a minimum of 18 kHz. Alternatively, the expected value of the voltage can be stored in a table, whereby the respective voltage values ​​are then compared in phase synchrony or a phase-synchronous difference is formed, so that differential voltage values ​​are available. Fig. 6 shows a diagram with voltage [V] in volts on the left vertical axis and time t [ms] in milliseconds on the horizontal axis. It depicts one period of a sinusoidal alternating voltage (the mains voltage), in this example the voltage DU of the low-voltage AC circuit for one period, in this case 20 ms (f = 50 Hz). Also shown is the magnitude of the sinusoidal alternating voltage abs(DU) over time t [ms]. Furthermore, the course of a first (voltage) threshold SWU, specifically a lower threshold SWU_low, for one period of voltage over time is shown. Furthermore, the course of a second (voltage) threshold SWU, specifically an upper threshold SWU_high, is shown for one period of voltage over time. The lower and upper threshold values ​​SWU_low and SWU_high can form a threshold corridor (corridor), as shown in Fig. 6. In an advantageous embodiment, the magnitude of the expected voltage value is used to calculate the lower (voltage) threshold curve SWU_low. The threshold curve SWU_low over time, i.e., the instantaneous lower threshold values, is calculated in this case from the (instantaneous) expected value of the voltage magnitude, in the simplest case the instantaneous mains voltage value, multiplied by a scaling factor, in particular a value from the range 0.85 to 0.99 or 1; and by subtracting a fixed voltage value, in particular a value from the range 10 to 40 or 50 volts. In an advantageous embodiment, the magnitude of the upper (voltage) threshold SWU_high is calculated analogously. In this case, the (instantaneous) upper threshold SWU_high is calculated from the (instantaneous) magnitude of the expected voltage value (mains voltage curve), divided by a scaling factor, in particular a value from the range 0.85 to 0.99 or 1; and the addition of a fixed voltage value, in particular a value from 10 to 100 volts, particularly independent of the number of poles (number of poles or contacts / interrupting conductors) of the mechanical disconnect contact system MK. The lower and upper (voltage) threshold curves together form a corridor. If the level of the (instantaneously) measured / determined voltage leaves this corridor, an opening (actuation) of the mechanical disconnect contact system is detected. Fig. 7 shows a representation according to Fig. 6, with the difference that the time course of the (voltage) threshold values ​​SWU_low, SWU_high is shown in detail for the voltage zero crossing, in this case at t = 10 ms. In addition to the representation of the time course of the voltage DU, its magnitude abs(DU) is also shown, as in Fig. 6. To avoid false triggering, a minimum distance to the determined voltage level is required. In an advantageous embodiment of the invention, particularly for small instantaneous voltages (<20V, <30V, <40V) near zero crossing, the instantaneous lower threshold value or the lower voltage threshold curve SWU_low should lie below the zero line, as shown in Fig. 7. A voltage dip would not be detected in this case, but this does not represent a disadvantage for the solution. Although the invention has been illustrated and described in detail by the exemplary embodiment, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

Claims

Protective switching device (SG) for the protection of a low-voltage alternating current circuit, comprising: - a housing (GEH) with mains-side (L1, N1) and load-side (L2, N2) connections for conductors of the low-voltage alternating current circuit, - a mechanical isolating contact system (MK) connected to the mains-side connections (L1, N1) for galvanic interruption of the low-voltage circuit, which is connected in series with an electronic interruption unit (EU) which has, by means of semiconductor-based switching elements, a high-resistance state of the switching elements for interruption and a low-resistance state of the switching elements for current flow in the low-voltage circuit, wherein the electronic interruption unit (EU) is connected to the load-side connections (L2, N2), - a voltage sensor (SU) which is arranged between the mechanical isolating contact system (MK) and the electronic interruption unit (EU).for determining the voltage level between the conductors of the low-voltage circuit, - wherein the mechanical isolating contact system (MK) is manually operable so that contacts of the mechanical isolating contact system (MK) can be manually closed for current flow and opened for interruption in the low-voltage circuit, - a control unit (SE) connected to the voltage sensor (SU) and the electronic interruption unit (EU), - wherein the protective switching device (SG) is designed such that the voltage sensor (SU) determines the instantaneous voltage level, and a corresponding instantaneous voltage value (DU) is examined for a voltage dip, using an instantaneous expected value of the voltage (UE).and that an opening of the mechanical disconnecting contact system (MK) is detected when the instantaneous voltage drops from 10 to 50 volts in a single-pole interrupting mechanical disconnecting contact system (MK) or from 20 to 100 volts in a double-pole interrupting mechanical disconnecting contact system (MK). Protective switching device (SG) according to claim 1, characterized in that the magnitude of the instantaneous voltage (DU) is compared with an instantaneous lower threshold (SWU low), wherein the instantaneous lower threshold (SWU_low) is formed from the magnitude of the instantaneous expected value of the voltage (UE) multiplied by a scaling factor, in particular 0.85 to 1, and subtracted by a fixed voltage value, and that when the instantaneous lower threshold (SWU_low) is undershot, an opening of the mechanical isolating contact system (MK) is detected. Protective switching device (SG) according to claim 1 or 2, characterized in that the magnitude of the instantaneous voltage (DU) is compared with an instantaneous upper threshold (SWU_high), wherein the instantaneous upper threshold (SWU_high) is formed from the magnitude of the instantaneous expected value of the voltage (UE) divided by a scaling factor, in particular 0.85 to 1, and added to a fixed voltage value, in particular 10 to 100 volts regardless of the number of poles of the mechanical isolating contact system (MK), and that an opening of the mechanical isolating contact system (MK) is detected when the instantaneous upper threshold (SWU_high) is exceeded. Protective switching device (SG) according to one of the preceding claims, characterized in that, upon detection of the opening of the mechanical isolating contact system (MK), the semiconductor-based switching elements of the electronic interruption unit (EU) switch to a high-resistance state, in particular within a first time period, in particular less than 100 µs, more specifically less than 50 µs or 10 µs, to avoid contact erosion caused by a switching arc or to reduce the duration of the switching arc of the mechanical isolating contact system. Protective switching device (SG) according to one of the preceding claims, characterized in that the protective switching device (SG) has a communication unit such that, upon detection of the opening of the mechanical isolating contact system (MK), at least one piece of information is transmitted by the communication unit. Protective switching device (SG) according to one of the preceding claims, characterized in that the protective switching device (SG) has a display unit such that, when the opening of the mechanical isolating contact system (MK) is detected, information is displayed by the display unit. Protective switching device (SG) according to one of the preceding claims, characterized in that the protective switching device (SG) has a memory in which information is stored in the memory when the opening of the mechanical isolating contact system (MK) is detected. Protective switching device (SG) according to one of the preceding patent claims, characterized in that a further voltage sensor is provided between the mains connection and the mechanical isolating contact system (MK). Protective switching device (SG) according to claim 8, characterized in that the further voltage sensor performs a voltage determination only to the extent that a voltage is present at the mains-side connections, in particular whether a valid mains voltage is present. Protective switching device (SG) according to claim 8, characterized in that the further voltage sensor determines the voltage level there, and that a voltage drop is determined by a difference calculation of the voltages of the two voltage sensors. Protective switching device (SG) according to claim 8, 9 or 10, characterized in that the further voltage sensor is galvanically isolated from the other units of the protective switching device (SG), in particular comprising at least one optocoupler or capacitive elements. Protective switching device (SG) according to one of the preceding claims, characterized in that the protective switching device (SG) has a power supply with an energy storage device, which is designed such that, after an opening process of the contacts of the mechanical isolating contact system (MK), sufficient energy is provided for: - a change of the semiconductor-based switching elements into the high-resistance state or - a transmission of information by the communication unit or - a display of information by the display unit or - a change of the protective switching device into a defined state, including the storage of information, in particular about the time of actuation and / or the magnitude of the voltage and / or current and / or frequency and / or temperature. Protective switching device (SG) according to claim 12, characterized in that an energy supply by the energy storage device is available for a period of 1 ms to 1 s, in particular 20 ms to 100 ms or up to 500 ms, after an opening process of the contacts of the mechanical isolating contact system (MK). Method for a protective switching device (SG) with mains-side (L1, N1) and load-side (L2, N2) connections for conductors (L, N) of a low-voltage AC circuit, wherein the protective switching device has a mechanical isolating contact system (MK) connected to the mains-side connections (L1, N1) for galvanic interruption of the low-voltage circuit, which on the other hand is connected in series to an electronic interruption unit (EU) which has, by means of semiconductor-based switching elements, a high-resistance state of the switching elements for interruption and a low-resistance state of the switching elements for current flow in the low-voltage circuit, wherein the electronic interruption unit (EU) on the other hand is connected to the load-side connections (L2, N2), wherein the mechanical isolating contact system (MK) is manually operable.so that the contacts of the mechanical isolating contact system (MK) can be manually closed for current flow and opened for interruption in the low-voltage circuit, that the magnitude of the instantaneous voltage between the conductors (L, N) between the mechanical isolating contact system (MK) and the electronic interruption unit (EU) is measured and checked for a voltage drop, using an instantaneous expected value of the voltage (UE), and if a drop in the instantaneous voltage of 10 to 50 volts occurs in the case of a single-pole interrupting or of 20 to 100 volts in the case of a double-pole interrupting mechanical isolating contact system (MK), an opening of the mechanical isolating contact system (MK) is detected. Method according to claim 14, characterized in that the magnitude of the instantaneous voltage (DU) is compared with an instantaneous lower threshold (SWU_low), wherein the instantaneous lower threshold (SWU_low) is formed from the magnitude of the instantaneous expected value of the voltage (UE) multiplied by a scaling factor, in particular 0.85 to 1, and subtracted by a fixed voltage value, and in that an opening of the mechanical disconnect contact system (MK) is detected when the instantaneous lower threshold (SWU_low) is undershot. A method according to claim 14 or 15, characterized in that the magnitude of the instantaneous voltage (DU) is compared with an instantaneous upper threshold (SWU_high), wherein the instantaneous upper threshold (SWU_high) is formed from the magnitude of the instantaneous expected value of the voltage (UE) divided by a scaling factor, in particular 0.85 to 1, and added to a fixed voltage value, in particular 10 to 50 volts for a single-pole interrupting or 20 to 100 volts for a double-pole interrupting mechanical disconnecting contact system (MK), and that an opening of the mechanical disconnecting contact system (MK) is detected when the instantaneous upper threshold (SWU_high) is exceeded.

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