Safety-related load switching device and method for operating a safety-related load switching device

Abstract

Safety-oriented load switching device for electrically switching an automation component (5), comprising a first current branch (6) and a second current branch (7) extending from a respective supply-side supply connection (8, 9) to a respective load-side load connection (10, 11), wherein a switching arrangement (15, 16) is formed in each current branch (6, 7) comprising a parallel connection of a switching element (17, 18) designed for opening and closing the respective current branch (6, 7) and a resistance element (19, 20), and with at least one measuring point (23, 24; 52, 53) arranged on the load side in the current branch (6, 7) between the switching arrangement (15, 16) and the load connection (10, 11), wherein at the measuring point (23, 24;52, 53) a potential measuring device (25, 26) is connected, which is electrically connected to a reference point (27, 28) and which is designed to provide a potential-dependent measuring signal, wherein an electrical resistance of the resistive means (19, 20) is selected such that the automation component (5) is switched off when one of the switching means (17, 18) is open, wherein on the load side between the first current branch (6) and the second current branch (7) there is a balancing arrangement (32; 46) which is designed for load-side potential equalization between the current branches (6, 7) and wherein on the supply side between the first current branch (6) and the second current branch (7) there is a voltage divider (47) which is designed for supply-side provision of a reference potential.;

Description

The invention relates to a safety-related load switching device for electrically switching an automation component, comprising a first current branch and a second current branch extending from a respective supply-side supply connection to a respective load-side load connection, wherein a switching arrangement is formed in each current branch comprising a parallel connection of a switching element configured for opening and closing the respective current branch and a resistive element, and with at least one measuring point arranged on the load side of the current branch between the switching arrangement and the load connection. The invention further relates to a method for operating a safety-related load switching device. From EP 2 519 960 B1, a device for monitoring the electrical circuit of a consumer circuit containing at least one electrical load is known. This device connects two supply voltage terminals to two load terminals, to which the at least one load is connected, via current paths each having a switch. Control means are provided for opening the two switches in the current paths. An additional voltage source is provided, the potential of which can be applied to one of the load terminals by means of a further switch. The control means are also provided for closing the further switch during test intervals. Signaling means are provided for generating fault signals when the further switch detects a current exceeding a predetermined or predeterminable current flow. DE 10 2010 004 524 A1 discloses a device for electrical circuit monitoring of a consumer circuit containing at least one electrical consumer, wherein two supply voltage terminals are connected to the two consumer terminals via current paths each having a first semiconductor switch, and current sensing means are provided for sensing the current in each of the two current paths, wherein the current sensing means each have a first semiconductor switch serving as a high-current shunt and a parallel-connected arrangement of a second semiconductor switch and a low-current shunt connected in series therewith, wherein the two terminals of the low-current shunts are each connected to the current sensing means. The object of the invention is to provide a safety-oriented load switching device and a method for operating a safety-oriented load switching device in which the detection of a fault-related electrical connection of at least one of the current branches with an electrical potential, in particular a further supply potential or a ground potential, can be achieved with reduced circuit engineering effort. This problem is solved according to a first aspect of the invention for a safety-related load switching device of the type mentioned at the outset with the features of claim 1. It is provided that a potential measuring device is connected at the measuring point, which is electrically connected to a reference point and which is designed to provide a potential-dependent measuring signal. It is known from the prior art to provide a series connection of a voltage source and a measuring resistor in parallel to a switching device and to draw conclusions about the functionality of the switching device to be monitored based on a voltage drop across the measuring resistor. In contrast, the load switching device according to the invention does not require such a voltage source. Instead, the existing electrical supply source, which is already connected to the supply terminals and intended for the electrical supply of the automation component connected to the load terminals, is used to provide the electrical potential(s) necessary for testing the load switching device within the load switching device. This applies in particular even when both switching elements are open and no power supply is provided to the load that can be connected to the supply terminals.In this case, a current flows through the resistors arranged in parallel to the switching elements and through the load connected to the load terminals. This current results in a voltage drop across the resistors and the load, establishing an electrical potential at the measuring point. This potential depends on the electrical resistances of the resistors and the load, provided that the load switching device, the connected load, and the electrical connecting lines to the load terminals are functioning correctly. However, if an unwanted electrical connection exists between one of the current branches, the load, or the connecting line to the load and an external electrical potential, a potential shift occurs at the measuring point. This shift is detected and may trigger an error message.According to the invention, the electrical resistances of the resistive elements are selected to be significantly higher than the electrical resistance of the load. This ensures, on the one hand, that the current flow when the switching elements are open is not at a level at which a function of the load, which could be, for example, a drive motor, would be expected. On the other hand, this ensures that any voltage drop across the circuit formed by the first resistive element in the first current branch, the connected load, and the connected second resistive element in the second current branch occurs almost exclusively across the two resistive elements, thereby establishing a measurable electrical potential at the measuring point.Furthermore, this increases the sensitivity of the measuring device to a fault-related electrical connection of at least one of the current branches with an electrical potential independent of the supply source, in particular another supply potential or a ground potential. This potential is measured by the measuring device relative to a reference potential applied at the reference point and can then be output by the measuring device. While a predefined potential difference between the measuring point and the reference potential is established when the load switching device is functioning correctly, a first fault can lead to a deviation of the measured potential difference from the predefined potential difference. This can be due, for example, to a malfunction in one or both of the switching devices, which may be electronically controlled semiconductor switches. Alternatively, damage in the first and / or second current branch may cause an unwanted electrical contact with another electrical potential, such as a supply potential or a ground potential. In this case, too, a deviation occurs between the expected potential and the potential measured at the measuring point relative to the reference potential, which can be considered an indication of a fault. For example, the measuring device can be designed as an analog or digital voltmeter that displays the potential difference between the measuring point and the reference point. Preferably, the potential measuring device is connected to an evaluation unit that is configured to determine the measurement signal within a predefined measurement time interval and to output a status signal that depends on the determined measurement signal. The evaluation unit can be designed as an analog circuit, in particular as a comparator circuit, which compares the potential difference determined by the measuring device with a predefined potential difference and generates a status signal if the two potential differences deviate, provided the deviation exceeds a predefined threshold.Alternatively, the evaluation unit can be implemented as a digital circuit or in the form of a microcontroller program. This unit first digitizes the potentials determined by the measuring device and then compares them with a stored potential value. Here, too, a status signal is output that depends on the deviation between the two potential differences. In any case, the status signal is determined within a predefined time window or measurement signal interval to, for example, filter out switching effects that can occur when the switching devices open due to the load characteristics. These switching effects include, in particular, significant potential fluctuations caused by the capacitive and inductive components of the load.Accordingly, it may be planned to evaluate the potential difference only after a predetermined period of time has elapsed and to determine the desired status signal from it. The status signal can be provided to a higher-level machine control system and can, for example, contain information indicating that the load switching device is fault-free or that, due to at least one faulty electrical connection, the load switching device has a fault in at least one of its current branches with an electrical potential, in particular another supply potential or a ground potential. Preferably, the status signal is provided at regular intervals or upon request from the higher-level control system.It is particularly advantageous if the status signal is provided in phases in which the switching elements of the load switching device are to be in an open state, since it is possible to detect with less measurement effort whether a fault-related electrical connection of at least one of the current branches with an electrical potential, in particular another supply potential or a ground potential, exists. According to the invention, a balancing arrangement is provided on the load side between the first and second current branches, which is designed for load-side potential equalization between the current branches. The purpose of the balancing arrangement can, for example, be to shorten the time period during which switching-off effects have a significant influence on the determination of the potential difference.A reduction of this time period is particularly desirable, since the load switching device according to the invention is preferably used to operate loads with a potential hazard to people and / or machines, and in the event of a malfunction of at least one of the switching elements or another type of fault-related electrical connection, at least one of the current branches with an electrical potential, in particular a further supply potential or a ground potential, further measures must be taken as quickly as possible to prevent further energy supply to the load. Accordingly, the compensating arrangement has the task of compensating capacitive and / or inductive components of the load as quickly as possible, so that an electrical potential is present at the measuring point in the shortest possible time, which depends on the resistive components of the two resistive elements and the load. Preferably, the balancing arrangement comprises a resistor arrangement and / or a freewheeling diode. This allows for the simple balancing of capacitive and / or inductive components of the load. Furthermore, according to the invention, a voltage divider is provided on the supply side between the first current branch and the second current branch, which is designed to provide a reference potential on the supply side. This reference potential can, in particular, be used as a reference potential for the at least one potential measuring device connected at the measuring point. Thus, the actual supply voltage is not relevant for fault detection by means of the load switching device. Preferably, the evaluation unit includes a lock-in amplifier designed for the synchronized measurement of weak electrical signals and capable of detecting even slight deviations between an expected potential difference profile and an actual potential difference profile. This is particularly advantageous when, during operation of the load (i.e., with the switching devices closed), it is necessary to briefly open the switching devices to check whether a fault-related electrical connection exists between at least one of the current branches and an electrical potential, especially another supply potential or a ground potential. The switching devices are disconnected in such a way that the operation of the load is not compromised, which can be technically implemented, particularly when semiconductor switches are used for the switching devices.However, since such a short-term shutdown of the switching devices, especially in the range of a few milliseconds, would only result in minimal changes at the load-side measuring points, an additional compensation arrangement, located on the load side, is provided. Furthermore, the synchronization of the short-term shutdowns for testing purposes by the lock-in amplifier ensures high measurement sensitivity. Preferably, the balancing arrangement comprises a series connection of resistors and a measuring point arranged between these resistors. Preferably, the balancing arrangement and the supply-side voltage divider form a Wheatstone bridge circuit, with which the load-side influence of external potentials on one of the two current branches can be detected using a comparatively simple measuring technique. It is particularly advantageous if the resistances of the resistive elements in the current branches and / or the resistor arrangement are chosen to be the same. According to the invention, the electrical resistance of the resistive elements is selected to be sufficiently high compared to a load intended for connection to the load terminals and / or the equalizing resistor arrangement. This ensures that current flow through the load when the switching elements are open cannot lead to undesired operation of the load. Preferably, the reference point is configured to provide a reference potential, in particular a ground potential, a supply potential, or a measuring point potential. Using the reference potential provided at the reference point, the potential difference relative to the respective measuring point can be measured as a voltage value, thus determining the potential at the measuring point. The reference point can optionally be the electrical potential of the supply source, a ground connection, or a second measuring point, which is located, in particular, on the other current branch or on the supply side. According to a second aspect of the invention, the object of the invention is achieved by a method as specified in claim 6. This method provides that an evaluation device connected to the potential measuring device provides a function signal if a measurement signal of the potential measuring device, determined before the switching means are about to close, lies within a tolerance interval around a predefinable signal level, and that the evaluation device provides an error signal if the measurement signal lies outside the tolerance interval around the predefinable signal level. An alternative method involves the evaluation unit comparing the electrical potential at the load-side measuring point with the electrical potential at the supply-side measuring point at the end of a predefined measurement interval after the switching devices have opened. The switching devices then close after an opening time interval has elapsed, and this measurement interval is shorter than the opening time interval. This alternative method allows for the detection of possible fault-related electrical connections between at least one of the current branches and an electrical potential, particularly another supply potential or a ground potential, during virtually uninterrupted operation of the load.The opening time interval is selected based on the load characteristics such that the interruption of the load's power supply caused by the brief opening of the switching devices does not lead to a relevant change in the load's operating conditions. To enable the most meaningful measurement possible within the available measurement period, the measurement signals are acquired immediately before the switching devices close. This allows for the greatest possible influence on the measurement signals from any fault-related electrical connections of at least one of the current branches to an electrical potential, particularly another supply potential or a ground potential, as a basis for further processing of the measurement signals. Preferably, the opening time interval is significantly less than 1 millisecond. Advantageous embodiments of the invention are shown in the drawing. Figure 1 shows a first embodiment of a safety-related load switching device with a connected supply source and a connected load in an open state of the switching means; Figure 2 shows a second embodiment of a safety-related load switching device as a modification of the load switching device according to Figure 1; Figure 3 shows a third embodiment of a safety-related load switching device, which is optimized for detecting a fault-related electrical connection of at least one of the current branches with an electrical potential, in particular another supply potential or a ground potential, during operation of the load; and Figure 4 shows a signal waveform diagram for the second embodiment of the load switching device shown in Figure 2. A first embodiment of a safety-oriented load switching device 1 shown in Fig. 1 is provided for arrangement between a supply source 2, shown by way of example as a voltage source, and an electrical load 5, shown by way of example as a parallel connection of a load resistor 3 and a load capacitor 4, wherein the load 5 comprises two supply lines 12, 13 for electrical coupling with the load switching device 1. The load switching device comprises a first current branch 6 and a second current branch 7 running parallel to it. Each of the current branches 6, 7 extends from a supply terminal 8, 9 to a load terminal 10, 11. A switching arrangement 15, 16 is provided in each of the current branches 6, 7, which is exemplified as a parallel connection of a switching element 17, 18 and a resistive element 19, 20. The switching elements 17, 18 are preferably designed as semiconductor switches, in particular as field-effect transistors, and can be individually switched between an open position and a closed position by a control device (not shown) via an electrical control signal. In the open position, only an electrical connection between the supply terminal 8, 9 and the load terminal 10, 11 is provided in the respective current branch 6, 7 via the respective resistive element 19, 20.In the closed position, a current flows between the supply terminal 8, 9 and the respective assigned load terminal 10, 11 in parallel via the switching means 17, 18 and the resistance means 19, 20. The function of the load switching device 1 is to detect a fault-related electrical connection on the load side of at least one of the current branches 6, 7 or the supply lines 12, 13 with an electrical potential, in particular another supply potential or a ground potential. Such a fault-related electrical connection can occur, for example, due to a defective switching device 17, 18 or due to a load-side ground fault or supply fault.For example, a switching device 17 or 18 designed as a semiconductor switch may have damage to its semiconductor structure due to production defects or temporary electrical overload, which leads to the fact that, despite appropriate control of the switching device 17 or 18, a complete separation of the respective current branch 6, 7 does not take place and thus a current flow in the current branches 6, 7 is enabled that is above the current intended for a safe switching off of the load.Alternatively, in a load-side current branch section 21 or 22 formed between the respective switching device 17 or 18 and the respective load connection 10 or 11 of the respective current branch 6 or 7, an electrical connection with an additional electrical potential, for example a different supply voltage potential or a ground potential, may exist, for example due to damage to the electrical lines that form the respective current branches 6, 7. A measuring point 23, 24 is provided on each load-side current branch section 21, 22, to which a potential measuring device 25, 26 is connected. Each of the potential measuring devices 25, 26 is configured to determine a potential difference between the respective assigned measuring point 23, 24 and a reference point 27, 28, at which an electrical reference potential, in particular a ground potential, is present. Each of the potential measuring devices 25, 26 is configured to provide a measurement signal to an evaluation unit 29, which in turn is configured to process the measurement signals and output a status signal at a signal interface 30. By way of example, it is provided that the signal interface 30 is connected via a bus line to a higher-level control unit (not shown), in particular a programmable logic controller (PLC).The higher-level control device may also be provided for controlling the switching devices 17, 18 in order to carry out the operation of the load 5 according to a predetermined operating sequence. When the load switching device 1 is operated as shown in Fig. 1, i.e., with the switching elements 17, 18 open, current flow between the supply terminal 8 and the supply terminal 9 is possible without an electrical fault, exclusively via the resistive element 19, the load resistance 3, and the resistive element 20. The resistive elements 19, 20 are each designed to have an electrical resistance that is considerably greater than the load resistance 3. Accordingly, an electrical potential is established at the measuring points 23 and 24, which depends almost exclusively on the resistance values ​​of the resistive elements 19, 20.As an example, it is provided that the electrical resistances of the resistors 19, 20 are selected identically, so that the electrical potentials at the measuring points 23 and 24 correspond at least approximately to half the supply voltage provided at the supply terminals 8, 9. If the electrical potentials at the measuring points 23 and 24 correspond closely, particularly within a predefinable tolerance range, and the corresponding measurement signals of the respective potential measuring devices 25, 26, which are compared with predefined signal levels in the evaluation unit 29, the evaluation unit 29 can provide a function signal at the signal interface 30 that indicates the proper functioning of the load switching device 1. If, however, an undesired electrical contact with an electrical potential exists on one of the load-side current branch sections 21, 22 or on one of the connecting lines 12, 13, differing electrical potentials arise at the measuring points 23, 24 when the switching devices 17, 18 are open. These differing potentials lead to differing measurement signals from the potential measuring device 25, 26, which can be detected in the evaluation unit 29. If deviations of the measurement signals from the potential measuring device 25, 26 lie outside a predefinable tolerance interval, the evaluation unit 29 is designed to output an error signal. The load switching device 1 according to Fig. 1 is designed to be used with a load 5 that is de-energized before each switching-on process and can pose a hazard when energized. To ensure a certain level of safety, it must be ensured before the load 5 is switched on that it can be switched off again at that time and with a certain probability at a later time. When the load 5 is switched off, the hazard emanating from it in the switched-on state is eliminated. The second embodiment of a load switching device 31 shown in Fig. 2 differs from the embodiment shown in Fig. 1 only by means of compensating arrangements in the form of a resistor 32, which is described in more detail below, and a freewheeling diode 33. Otherwise, the same components are used in the load switching device 31 as in the load switching device 1, so the same reference numerals are used for these components as in the load switching device 1 and a further description of these components is omitted. The function of the balancing arrangements 32, 33 is to bring about a rapid discharge of the load capacitor 4 or a rapid reduction of any induced voltage, which may be caused by an inductive component of the load 5 (not shown), after the load 5 has been operated and the switching devices 17, 18 have been opened. The balancing arrangements 32 and 33 can be provided in combination or alternatively to each other. The third embodiment of a load-switching device 41 shown in Fig. 3 is designed, like the load-switching devices 1, 31, for an arrangement between a supply source (not shown) and a load (also not shown) and accordingly has supply connections 8, 9 and load connections 10, 11. Furthermore, the load-switching device 41 comprises, in the same way as the load-switching devices 1, 31, the switching arrangements 15, 16 with the associated switching elements 17, 18 and the resistive elements 19, 20. In contrast to the load-switching devices 1, 31 described in more detail above, the load-switching device 41 provides equalization arrangements 46, 47 between the load-side current branch sections 42, 43 and the supply-side current branch sections 44, 45. These equalization arrangements are each configured as a series connection of two resistors 48, 49 or 50, 51, respectively, with a measuring point 52 or 53 being provided between each of the resistors 48, 49 or 50, 51. By way of example, the two measuring points 52 and 53 are connected via measuring lines 54, 55 to an evaluation unit 56, which is designed to determine a potential difference between the two measuring points 52 and 53 and to provide a status signal dependent on the determined potential difference. For use in conjunction with a test shutdown of an otherwise switched-on load 5, the evaluation unit 56 is preferably equipped with a lock-in amplifier that synchronizes the measurement process with the test shutdown process. The resistors 48 to 51 form a Wheatston bridge circuit, which enables a particularly sensitive measurement for fault-free operation, independent of the actual value of the supply voltage. This allows verification of the proper function of the load switching device 41, especially with regard to the presence of faulty switching elements 17 or 18 or other faulty electrical connections, even during operation of the load 5. For this purpose, it is provided that during the operation of the load 5 a brief opening of the two switching devices 17, 18 is carried out, whereby this opening takes place within a predefinable opening time interval, which is preferably less than 10 ms. The potential difference is preferably determined by the evaluation unit 56 shortly before the end of the opening time interval, as the greatest possible difference between the electrical potentials at the measuring points 52, 53 occurs at this time. To ensure a reliable status signal, the evaluation unit 56 may be configured to provide a status signal only when several potential differences, determined successively, fall outside a predefined measuring interval in the same way. Accordingly, the status signal can only be provided after the load 5 has been briefly switched off several times using the switching arrangements 15, 16. The signal waveform diagram shown in Fig. 4 for the embodiment of the load switching device 31 shown in Fig. 2 is, for the sake of simplicity, divided into a total of four signal waveform blocks arranged one above the other, which are explained in more detail below. For illustrative purposes only, the signal levels occurring during the test sequence are arranged in a time grid with equal time intervals; however, this does not necessarily reflect reality but serves only for the sake of simplification. The lower signal path block 60 shows the control signals 61, 62 for the two switching devices 17, 18, which are exemplary semiconductor switches and electrically controllable. For illustrative purposes, the two switching devices 17, 18 are controlled by a square wave signal, the signal level of which is switched between a lower level, which does not result in a switching function of the respective switching device 17, 18, and an upper level, which does result in a switching function of the respective switching device 17, 18. The two control signals 61, 62 are provided by a control device (not shown) for the load switching devices 1, 31, 41 according to Figures 1, 2 to 3. In signal waveform block 63, located above signal waveform block 60 in Fig. 4, disturbance signals 64 and 65 are shown to represent a malfunction of the switching devices 17 and 18. For example, disturbance signal 64 is designed to simulate a fault in the switching device 17 that occurs during a period when the control signal 61 is at a lower level and the associated switching device 17 is not normally activated. Accordingly, disturbance signal 64 leads to an undesired bypass of the switching device 17 and thus to a fault that is to be detected by the load switching device 31. The same applies to disturbance signal 65, which simulates a fault in the switching device 18. In the signal waveform block 66 above, the voltage across the load 5 is shown, which, purely as an example, rises from a lower to an upper voltage level when the two switching devices 17, 18 are activated. It is assumed that the full voltage is present across the load 5 immediately after the two switching devices 17, 18 are activated. In contrast, when the two switching devices 17, 18 are deactivated, the voltage across the load 5 decreases over time due to the load capacitor 4. The time interval between the deactivation of the two switching devices 17, 18 and the voltage across the load 5 approaching the lower voltage level depends in particular on the capacitance of the load capacitor 4, the load resistance 3, and the electrical resistance of the equalizing arrangement 32. In the signal waveform block 67 above, the signal levels 68 and 69 of the potential measuring devices 25 and 26 are shown, assuming, for illustrative purposes, that the resistive elements 19 and 20 each have the same electrical resistance. Accordingly, when the switching elements 17 and 18 are open (i.e., deactivated and functioning correctly), the load switching device 31 operates as a voltage divider. It is further assumed that the electrical resistances of the two resistive elements 19 and 20 are significantly greater than the electrical resistance of the load resistor 3. Consequently, the voltage drop of the supply voltage provided by the voltage source 2 occurs almost exclusively across the two resistive elements 19 and 20. Assuming that the same reference potential, in particular a ground potential, is present at both reference points 27 and 28, the signal levels 68 and 69 are established.In an initial time interval between t0 and t1, the two control signals 61, 62 exhibit a low level value, so that the associated switching devices 17, 18 in the load switching device 31 are high-impedance and practically non-conductive when the switching devices 17, 18 are functioning correctly. Thus, an almost identical electrical potential is established at the measuring points 23, 24, differing practically only by the negligible voltage drop across the load resistor 3, which has a low resistance compared to the other resistors. When the two switching devices 17, 18 are activated at time t1 with the corresponding control signals 61, 62, which from this time onward have a high level, the associated switching devices 17, 18 in the load switching device 31 are low-resistance and practically conductive, assuming the switching devices 17, 18 are functioning correctly. Therefore, there is no significant voltage drop between the supply voltage terminal 8 and the measuring point 23, and the electrical potential at the measuring point 23 corresponds at least almost to the electrical potential of the supply source 2. In this operating state of the load switching device 31, the voltage drop occurs almost exclusively across the load resistor 3, so that the ground potential of the supply source 2 is at least almost present at the measuring point 24. Accordingly, the signal levels 68, 69 are established between t1 and t2. At time t2, the switching devices 17, 18 are switched off because the signal levels of the control signals 61, 62 have returned to their low levels. Due to the electrical charge stored in the load capacitor 4, the potential difference between the two electrical potentials at the measuring points 23, 24 is initially maintained, at least partially, after time t2 when the two switching devices 17, 18 are switched off. This potential difference then gradually decreases due to the discharge of the load capacitor 4, so that after an unspecified period of time, the same electrical potential is present at both measuring points 23, 24. The discharge time of the load capacitor 4 is essentially determined by the capacitance value of the capacitor 4 and the electrical resistance of the parallel circuit consisting of the load resistor 3 and the equalizing arrangement 32, which can be a purely ohmic resistance.For example, it is provided that the resistance of the compensating arrangement 32 corresponds at least almost to the resistance of the resistance means 19 or 20. Following a further activation of the switching devices 17, 18 at time t3, a disturbance occurs on the switching device 17 shortly after the subsequent deactivation of the switching devices 17, 18 at time t4F. This disturbance is symbolized by the disturbance signal 64 in the signal waveform block 63 and simulates a malfunction of the switching device 17. This malfunction of the switching device 17 could, for example, be a so-called "through-delay" of the switching device 17, which is implemented as a semiconductor switch. In this case, a junction within the switching device 17 can no longer maintain its blocking function due to overload or aging, and current flow through the switching device 17 is allowed even without an external control signal 61.The resulting loss of function of the switching device 17 leads to a voltage jump towards the supply potential immediately after the occurrence of the disturbance signal 64 at time t4F at both measuring point 23 and measuring point 24. This voltage jump is caused by the fact that, from time t4F onwards, due to the assumed malfunction of the switching device 17, at least almost the full supply voltage is present at measuring point 23. In this situation, a voltage drop only occurs across the parallel connection of the compensating arrangement 32 and the load resistor 3, as well as the resistor 20 connected in series with it.Since the resistor 20 has a particularly high resistance, practically the entire voltage drops across it, and the electrical potentials at measuring points 23 and 24 equalize after a certain time. They differ from the supply voltage level at the supply terminal 8 only by the voltage drop across the parallel connection of the equalizing arrangement 32 and the load resistor 3. This shift of the two electrical potentials at measuring points 23 and 24 can be detected by the evaluation unit 29 of the load switching device 31 and results in the output of an error signal at the signal interface 30. In practice, such an error signal leads to further measures that must be taken by a higher-level control unit, for example, switching off the power supply to the respective load switching device 31. At time t4N, the simulation of a fault in the switching device 17 is terminated, causing the regular electrical potential to be restored at measuring points 23, 24, as it occurred, for example, at the end of the time interval between t2 and t3. Following a regular switch-on phase between t5 and t6, a simulation of a malfunction of the switching device 18 occurs at time t6F, as represented by the disturbance signal 65. In this case, an unintentional short circuit of the load-side current branch section 22 to ground is simulated, causing signal levels 68 and 69 to be pulled towards ground potential and to differ only slightly from ground potential at the supply terminal 9 due to the voltage drop across the parallel connection of load resistor 3 and equalization arrangement 32. This change in signal levels 68 and 69 can also be detected by the evaluation unit 29 and likewise leads to the output of an error signal at the signal interface 30. At time t6F, the simulation of the malfunction of the switching device 18 ends and the regular situation for the two signal levels 68, 69 is restored.

Claims

Safety-oriented load switching device for electrically switching an automation component (5), comprising a first current branch (6) and a second current branch (7) extending from a respective supply-side supply connection (8, 9) to a respective load-side load connection (10, 11), wherein a switching arrangement (15, 16) is formed in each current branch (6, 7) comprising a parallel connection of a switching element (17, 18) designed for opening and closing the respective current branch (6, 7) and a resistance element (19, 20), and with at least one measuring point (23, 24; 52, 53) arranged on the load side in the current branch (6, 7) between the switching arrangement (15, 16) and the load connection (10, 11), wherein at the measuring point (23, 24;52, 53) a potential measuring device (25, 26) is connected, which is electrically connected to a reference point (27, 28) and which is designed to provide a potential-dependent measuring signal, wherein an electrical resistance of the resistive means (19, 20) is selected such that the automation component (5) is switched off when one of the switching means (17, 18) is open, wherein on the load side between the first current branch (6) and the second current branch (7) there is a balancing arrangement (32; 46) which is designed for load-side potential equalization between the current branches (6, 7) and wherein on the supply side between the first current branch (6) and the second current branch (7) there is a voltage divider (47) which is designed for supply-side provision of a reference potential.; Safety-oriented load switching device according to claim 1, characterized in that the potential measuring device (25, 26) is connected to an evaluation device (29; 56) which is designed for determining the measurement signal within a predefinable measurement time interval and for outputting a status signal dependent on the determined measurement signal. Safety-oriented load switching device according to claim 1, characterized in that the compensating arrangement (32, 33; 46) comprises a resistor arrangement and / or a freewheeling diode. Safety-oriented load switching device according to claim 1, characterized in that the compensation arrangement (46) comprises a series connection of resistors (48, 49) and a measuring point (52) arranged between these resistors (48, 49). Safety-oriented load switching device according to one of the preceding claims, characterized in that the reference point (27, 28) is designed to provide a reference potential, in particular a ground potential or a supply potential or a measuring point potential. Method for operating a safety-related load switching device (1; 31; 41) comprising a first current branch (6) and a second current branch (7) extending from a respective supply-side supply connection (8, 9) to a respective load-side load connection (10, 11), wherein a switching arrangement (15, 16) is formed in each current branch (6, 7) comprising a parallel connection of a switching element (17, 18) configured for opening and closing the respective current branch (6, 7) and a resistance element (19, 20), and with at least one measuring point (23, 24; 52, 53) arranged on the load side in the current branch (6, 7) between the switching arrangement (15, 16) and the load connection (10, 11), wherein at the measuring point (23, 24;52, 53) a potential measuring device (25, 26) is connected, which is electrically connected to a reference point (27, 28) and which is designed to provide a potential-dependent measurement signal, wherein an electrical resistance of the resistance means (19, 20) is selected such that the automation component (5) is switched off when one of the switching means (17, 18) is open, and with an automation component (5) connected to the load terminals (10, 11), wherein an evaluation device (29; 56) connected to the potential measuring device (25, 26; 56) provides a function signal when a measurement signal of the potential measuring device (25, 26; 56) determined before an impending closing of the switching means (17, 18) lies within a tolerance interval around a predefinable signal level, and that the evaluation device (29;56) provides an error signal if the measurement signal is outside the tolerance interval around the predefinable signal level, whereby with the switching devices (17, 18) open, no power supply is provided for the operation of the automation component connected to the supply terminals (8, 9), and a current flow takes place via the resistor devices (19, 20) arranged in parallel to the switching devices (17, 18) and via the automation component (5) connected to the load terminals (9, 10). Method for operating a safety-related load switching device (1; 31; 41) according to claim 2, characterized in that the evaluation device (29; 56) compares an electrical potential at the load-side measuring point (52) with an electrical potential at the supply-side measuring point (53) at the end of a predefinable measuring time interval after opening the switching means (17, 18) when the load (5) is operated, wherein the switching means (17, 18) are closed after an opening time interval has elapsed and the measuring time interval is shorter than the opening time interval.

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