Switching device for protecting electrical load against overcurrent

Abstract

The invention relates to a switching device for protecting an electrical load against overcurrent, comprising: a load current input and a load current output; a load current path; a semiconductor switch allowing a current to flow through the load current path when the semiconductor switch is turned on and blocking the current from flowing through the load current path when the semiconductor switch is turned off; a control circuit arranged to turn on or off the semiconductor switch in response to the received switching signal; a current measuring device; a controller, which is designed to execute a single function or a combination of two or more functions of at least two predefined functions, which compare the current intensity and / or the temporal current profile of the current flowing through the load current path with defined threshold values, depending on the operating mode of the electrical load, and controlling the control circuit by means of the switching signal in such a way that the semiconductor switch is switched off as a function of one or more results of the executed one or more functions.

Description

Technical Field

[0001] The invention relates to a switching device and a method for protecting an electrical load from overcurrent. Background Art

[0002] The term "overcurrent" describes a state in an electrical circuit in which the normal load current is exceeded in the conductors of the circuit. Overcurrent can be caused by overload or short circuit, see for example DIN VDE 0100-430:2010-10. Overcurrents that are only slightly higher than the maximum continuous permissible current of the conductor will only heat the conductor to impermissibly high temperatures after a relatively long time. In the case of a short-circuit current, on the other hand, impermissibly high temperatures are reached in the conductor within only a fraction of an electrical half-wave (10 ms at 50 Hz alternating current).

[0003] Line protection switches are used in low-voltage power networks to protect conductors from damage caused by heating caused by overcurrent. To trigger in the event of an overload, a bimetallic strip can be used, which bends in the event of heating caused by the current flowing through and triggers the switch-off mechanism (thermal triggering). To trigger in the event of a short circuit, an electromagnet can be used through which current flows and which builds up a magnetic field within a few milliseconds, which triggers the switch-off mechanism (electromagnetic triggering). Summary of the invention

[0004] Due to the widespread electrification and automation worldwide and the high risk of overcurrent to people, animals and property, there is a great need for improved overcurrent protection. Therefore, the technical problem to be solved by the present invention is to improve overcurrent protection.

[0005] According to the invention, the above-mentioned technical problem is solved by a switchgear having the features specified in the invention and a method having the features specified in the invention.

[0006] Switchgear is used to protect electrical loads, such as electric motors, from overcurrents. For the sake of abbreviation, the term "switchgear" is used here and below to describe a switch and protection device that is connected upstream of a consumer connected to the power grid. Such a switch and protection device comprises at least one electrically switchable switching element and, in the event of a fault, disconnects the connected consumer or each connected consumer (or generally a load area) from the feed side. This is the protective effect of the switch and protection device. In the same way, with the help of the switch and protection device, it is also possible to cause the load area starting from the switch and protection device to be switched off by a switching command acting on the switch and protection device, and thus cause the consumer or each consumer connected to the switch and protection device to be switched off. This is the switching effect of the switch and protection device.

[0007] The switchgear has a load current input, to which a grid-side load current line can be connected in order to connect the load current input to a power source, such as a grid. The switchgear also has a load current output, to which a load current line can be connected in order to connect the load current output to an electrical load. The switchgear has a load current path, through which current can be conducted from the load current input to the load current output.

[0008] The switching device has a semiconductor switch connected in the load current path. Here, when the semiconductor switch is turned on, the semiconductor switch allows current to flow through the load current path; conversely, when the semiconductor switch is turned off, the semiconductor switch blocks the current from flowing through the load current path. The semiconductor switch is an electrically switchable switching element. For example, the semiconductor switch is an electrically switchable power semiconductor switch. The semiconductor switch can also be designed as a combination circuit of two power semiconductor switches that can conduct and block in both directions.

[0009] The switching device has a control circuit, which is configured to switch the semiconductor switch on or off in response to a received switching signal. The switching state of the semiconductor switch can be influenced via a control input (e.g., a gate input) of the semiconductor switch and by means of a control signal, which can be generated by a control circuit, which can be designed, for example, in the form of a control unit, and output at the control input of the semiconductor switch, and in operation of the switching device, the switching state is influenced by means of a control signal generated by the control circuit and output to the control input; the semiconductor switch is switched to an open or closed state by means of the control signal.

[0010] The switchgear has a current measuring device for detecting the current flowing through the load current path. As the current measuring device, a transformer, for example a Hall sensor, a measuring shunt, a compensating current transformer or the like can be used.

[0011] The switch device has a controller. The controller is configured to execute a single function of at least two predetermined functions or a combination of two or more functions according to the operating mode of the electrical load. The predetermined function compares the current intensity and / or the current trend over time of the current flowing through the load current path with a defined threshold value and outputs a comparison result. The controller is also configured to control the control circuit by means of a switching signal according to one or more results of the executed one or more functions so that the semiconductor switch is turned off.

[0012] According to the present invention, the above technical problem is also solved by a method for protecting an electrical load from overcurrent. The method has the step of collecting a current flowing through a load current path of a switching device. Here, the load current path guides the current from a load current input terminal to a load current output terminal, to which a load current line from a power supply is connected, and to which a load current line to the electrical load is connected.

[0013] The method has the step of providing at least two predefined functions which compare a current intensity and / or a current profile over time of a current flowing through a load current path with defined threshold values.

[0014] The method has the step of executing a single function or a combination of two or more functions of the predetermined functions. Here, the execution is carried out according to the operating mode of the electrical load.

[0015] The method also comprises the step of controlling the control circuit by means of a switching signal. Here, the control is performed in dependence on one or more results of the one or more functions performed. The control is performed in such a way that a semiconductor switch connected in the load current path is switched off. Here, when the semiconductor switch is switched on, the semiconductor switch allows current to flow through the load current path. And when the semiconductor switch is switched off, the semiconductor switch blocks current from flowing through the load current path.

[0016] The invention is based on the idea that the overcurrent protection is implemented as a SW or FW solution in the controller (SW = software; FW = firmware). By freely configurable execution of individual functions or a combination of two or more of the predefined functions, the triggering of the semiconductor switch can be influenced in a variety of ways, either logically or temporally. Thus, for example, in the event of a short circuit, an instantaneous, very fast shutdown can be set. By configurable execution of individual functions or a combination of two or more of the predefined functions, EMC (electromagnetic compatibility) interferences can also be suppressed or the limit value evaluation can be deliberately slightly delayed. In this way, flexible overcurrent protection can be ensured.

[0017] Advantageous embodiments and developments of the invention are described in the preferred exemplary embodiments. The method according to the invention can also be developed according to the preferred exemplary embodiments of the device, and vice versa.

[0018] According to a preferred design of the present invention, the predefined group of at least two functions has a first function, with which the current trend over time in the load current path is continuously detected, compared with a first current threshold value, and the result of the comparison is output as a result of the function. In addition, the predefined group of at least two functions has a second function, with which the current trend over time in the load current path is detected at a specific time point, the detected current value is compared with a second current threshold value, and the result of the comparison is output as a result of the function. In addition, the predefined group of at least two functions has a third function, with which the current trend over time in the load current path is detected at a specific time point, the current change over time of the detected current value is compared with a current change threshold value, and the result of the comparison is output as a result of the function. The interval between two consecutive time points for detecting the current value can be 1 to 2 μs; however, the interval can also be significantly longer, for example 100 μm. The advantage of this design is that different overcurrent detection algorithms can be activated depending on the operation of the load: thus, short circuits and overloads can be detected and distinguished from each other. Another advantage of this design is that it allows diagnosis in the event of a fault.

[0019] According to a preferred embodiment of the present invention, the switch device has a command signal input, from which the controller can receive a switching command for switching the semiconductor switch. As a result, it is also possible to shut down a consumer connected to the switch and protection device in a manner consistent with operation by means of the switch and protection device through a switching command acting on the switch and protection device. The advantage of this embodiment is that in this way the switch and protection device acts as a switch device, such as a relay or contactor for switching on or off a consumer in a manner consistent with operation.

[0020] According to a preferred design of the present invention, the switching device has an analog comparator, by means of which the first function can compare the current value of the current trend in the load current path with the first current threshold value. The analog comparator can be designed as a unit of the controller. The advantage of this design is that continuous current monitoring can be performed.

[0021] According to a preferred design of the present invention, the switch device has a digital comparator, with which the second function can compare the acquired current value with the second current threshold value and / or the third function can compare the temporal change of the acquired current value with the current change threshold value. The digital comparator can be designed as a unit of the controller. The advantage of this design is that very flexible current monitoring can be performed.

[0022] According to a preferred embodiment of the present invention, the switch device has a data memory in which functional algorithms can be stored, which define under what circumstances the controller outputs a switching signal to the control circuit. The advantage of this embodiment is that very flexible current monitoring can be performed.

[0023] According to a preferred design of the present invention, the method provides the following three functions: a first function, using the first function, the current trend over time in the load current path is continuously collected and compared with the first current threshold, and the comparison result is output as the result of the function; a second function, using the second function, the current trend over time in the load current path is collected at a specific time point, the collected current value is compared with the second current threshold, and the comparison result is output as the result of the function; a third function, using the third function, the current trend over time in the load current path is collected at a specific time point, the current change over time of the collected current value is compared with the current change threshold, and the comparison result is output as the result of the function. The advantage of this design is that very flexible current monitoring can be performed.

[0024] According to a preferred design of the present invention, in the method, if the result of the first function shows that the current in the load current path exceeds the first current threshold; or if the result of the second function shows that at least one of the current values ​​of the current trend over time in the load current path acquired exceeds the second current threshold; or if the result of the third function shows that the current change over time of the acquired current value exceeds the current change threshold, a switching signal for turning off the semiconductor switch is sent to the control circuit. The advantage of this design is that it can react to overcurrent very flexibly.

[0025] According to a preferred design of the present invention, the electrical load is a motor, and in the method, during the starting of the motor, the first function is activated, and in the rated operation of the motor, a combination of the first function and the second function or a combination of the first function, the second function and the third function is activated. The advantage of this design is that the overcurrent of the motor can be monitored very flexibly. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention is explained below with reference to a number of exemplary embodiments with the aid of the accompanying drawings. In the drawings, each is shown schematically and not to scale:

[0027] Figure 1 Protection equipment is shown;

[0028] Figure 2 A flow chart showing a first function is shown;

[0029] Figure 3A flow chart showing a second function is shown;

[0030] Figure 4 A flow chart showing a third function;

[0031] Figure 5 The current flow during motor starting and short circuit is shown;

[0032] Figure 6 The current flow of overload is shown;

[0033] Figure 7 The current flow of the short circuit is shown;

[0034] Figure 8 The current flow of overload is shown;

[0035] Fig. 9 Shown with Figure 2 The structure of the switch device 1 corresponding to the flowchart;

[0036] Fig.10 Shown with Figure 3 The structure of the switchgear 1 corresponding to the flowchart; and

[0037] Fig.11 Shown with Figure 4 The flow chart corresponds to the structure of the switching device 1. DETAILED DESCRIPTION

[0038] Figure 1 A switch and protection device 1, which for simplicity is referred to as a protection device, is shown for protecting an electrical load 16, such as a motor, from overcurrent. The switch device 1 has a load current input 2, to which a load current line 17 from a power source 15, such as a power grid, is connected. The switch device 1 has a load current output 3, to which a load current line 18 to the electrical load 16 is connected. The switch device 1 has a load current path 10, via which a current can be conducted from the load current input 2 to the load current output 3.

[0039] The switching device 1 has a semiconductor switch 5 connected in a load current path 10. When the semiconductor switch 5 is turned on, the semiconductor switch 5 allows current to flow through the load current path 10. When the semiconductor switch 5 is turned off, the semiconductor switch 5 blocks the current from flowing through the load current path 10. The switching device 1 has a control circuit 6 (also called a driver circuit, English: Driver Circuit), which is configured to turn on or off the semiconductor switch 5 in response to a received switching signal.

[0040] The switchgear 1 has a current measuring device 9 for detecting a current flowing through a load current path 10. A transformer, for example a Hall sensor, a measuring shunt, a compensating current transformer or the like can be used as the current measuring device.

[0041] The switchgear 1 has a controller 4. The controller 4 is configured to execute, depending on the operating mode of the electrical load 16, a single function of at least two predetermined functions A, B, C or a combination of two or more functions, which compare the current intensity and / or the current profile over time of the current flowing through the load current path 10 detected by means of the current measuring device 9 with defined threshold values ​​S1, S2, S3.

[0042] The controller 4 is configured to control the control circuit 6 by means of a switching signal 50 in such a way that the semiconductor switch 5 is switched off, depending on one or more results of the executed one or more functions A, B, C.

[0043] The control unit 4 has a microprocessor 40 and a data memory 43 in which algorithms of three predefined functions A, B, C are stored in a data format. The microprocessor 40 has a selection circuit 41 and an overcurrent detection circuit 42 .

[0044] The switchgear 1 also has a command signal input 13, via which a command signal 14 from an external command signal generator (e.g. a smartphone, a personal computer (PC), a control device or a switching center) can be directed to the controller 4. With such an externally generated command signal 14, the controller 4 can be triggered to generate a switching signal for switching the semiconductor switch 5 on or off in a manner independent of the measured value of the current measuring device 9 and to send it to the control circuit 6.

[0045] The switching device 1 is used to perform a method for protecting an electrical load 16 from overcurrent. For this purpose, a current measuring device 9 detects a current flowing through a load current path 10 of the switching device 1. Depending on the operating mode of the electrical load 16, a single function of the predefined functions A, B, C or a combination of two or more functions is performed. Here, the functions A, B, C are configured to compare the current intensity and / or the current trend over time of the current flowing through the load current path 10 with a defined threshold value. Depending on one or more results of the executed one or more functions A, B, C, the controller 4 controls the control circuit 6 by means of a switching signal 50 so that the semiconductor switch 5 is turned off: a) if the result of the executed one or more functions A, B, C shows that the semiconductor switch 5 should be turned off, the controller 4 controls the control circuit 6 by means of the switching signal 50 so that the control circuit 6 sends a control signal 60 to the semiconductor switch 5, which causes the semiconductor switch 5 to be turned off; b) if the result of the executed one or more functions A, B, C shows that the semiconductor switch 5 should not be turned off, the controller 4 does not control the control circuit 6 and the semiconductor switch 5 remains turned on.

[0046] Figure 2 An embodiment of the first function A is shown in the form of a flow chart. Fig. 9 The corresponding structure of the switching device 1 is shown.

[0047] The first function A compares the continuously detected current profile with a current threshold value in an analog comparator. The first function provides the fastest path to switch off the semiconductor switch, but does not allow a distinction between a short circuit and an overload.

[0048] In this case, the current measuring device 9 continuously detects 101 the current profile I in the load current path 10. The detected value is passed as an analog value 102 to an analog comparator 103. The analog comparator 103 is part of an overcurrent detection circuit 42 of the control device 4. A first threshold value S1 is input 104 into the switching device 1 via the interface 19 of the switching device 1, passed as a digital value 105 to a DAC 106 and from there as an analog value 106.1 to the analog comparator 103. In the analog comparator 103, the current profile I is compared with the first threshold value S1. If the current profile I does not exceed (“N”) 107 the first threshold value S1, the current measurement 101 is continued. If the current profile I exceeds (“Y”) 108 the first threshold value S1, the overcurrent detection circuit 42 of the control device 4 generates 109 a switching signal 50, which is transmitted 110 to the control circuit 6. Triggered by the received switching signal 50 , the control circuit 6 generates 111 a control signal 60 , which is transmitted 112 to the semiconductor switch 5 and causes the semiconductor switch 5 to be switched off there.

[0049] Figure 3An embodiment of the second function B is shown in the form of a flow chart. Fig.10 The corresponding structure of the switching device 1 is shown.

[0050] The second function B compares the value of the discontinuously sampled current profile with a current threshold value in a digital comparator. The second function provides a slower path to switch off the semiconductor switch than function A. The second function allows a distinction to be made between short circuits and overloads, see Figure 6 .

[0051] In this case, the current measuring device 9 samples (“samples”) 201 the current course I in the load current path 10 discontinuously at individual points in time. The acquired current value is conducted as an analog value 202 to an ADC 203 and from there as a digital value 204 to a digital comparator 205. The digital comparator 205 is part of the overcurrent detection circuit 42 of the controller 4. The second threshold value S2 is input 206 into the switching device 1 via the interface 19 of the switching device 1 and is conducted as a digital value 206.1 to the digital comparator 205. In the digital comparator 205, the acquired current values ​​I1(t1), I2(t2), I3(t3), ... are compared with the second threshold value S2. If the acquired current values ​​I1(t1), I2(t2), I3(t3), ... do not exceed (“N”) 207 the second threshold value S2, the sampling 201 of the current is continued. If the acquired current values ​​I1(t1), I2(t2), I3(t3) ... exceed ("Y") 208 the second threshold value, the overcurrent detection circuit 42 of the controller 4 generates 209 a switching signal 50, which is transmitted 210 to the control circuit 6. Triggered by the received switching signal 50, the control circuit 6 generates 211 a control signal 60, which is transmitted 212 to the semiconductor switch 5 and causes the semiconductor switch 5 to be turned off there.

[0052] Figure 4 An embodiment of the third function C is shown in the form of a flow chart. Fig.11 The corresponding structure of the switching device 1 is shown.

[0053] The third function C compares the temporal change of the discontinuously sampled current profile with a current threshold value in a digital comparator. The third function provides a slower path to switch off the semiconductor switch than function A, but allows a distinction between a short circuit and an overload.

[0054] In this case, the current measuring device 9 samples the current course I in the load current path 10 discontinuously at individual points in time (“sampling”). The acquired current value is fed as an analog value 302 to an ADC 303 and from there as a digital value 304 to a digital comparator 305. The digital comparator 305 is part of an overcurrent detection circuit 42 of the controller 4. The third threshold value S3 is input 306 into the switching device 1 via the interface 19 of the switching device 1 and fed as a digital value 306.1 to the digital comparator 305. In the digital comparator 305, the temporal change dI / dt of the acquired current values ​​I1(t1), I2(t2), I3(t3), ... is compared with the third threshold value S3. If the temporal change dI / dt of the acquired current values ​​I1(t1), I2(t2), I3(t3), ... does not exceed (“N”) 308 the third threshold value S2, the function C outputs the result that an overload 310 is present. If the temporal change dI / dt of the acquired current values ​​I1 ( t1 ), I2 ( t2 ), I3 ( t3 ), . . . exceeds (“Y”) 307 a third threshold value S2 , the function C outputs a result indicating that a short circuit 309 exists.

[0055] Figure 5 The current trend of the motor starting and short circuit is shown. During starting, the motor reaches the maximum current value I=8×I_Nenn at time point T1, where I_Nenn is the rated value of the motor current in rated operation. Since the operating mode of the motor is the starting of the motor, the selection circuit 41 of the controller 4 activates the first function A: only the "actual" short circuit (I> first threshold value S1) is to be identified. For this purpose, the first threshold value S1=10×I_Nenn is defined in the analog comparator. Therefore, the surge current of I=8×I_Nenn at time point T1 will not cause the semiconductor switch to be turned off, and only causes a short circuit at time point T2 when the current in the load current path exceeds the first threshold value S1.

[0056] Figure 6 The current flow of the overload is shown. The motor is in continuous operation. Since the operating mode of the motor is the rated operation of the motor, the selection circuit 41 of the controller 4 activates the second function B: As a result, the semiconductor switch can not be turned off directly when the threshold value is exceeded, but only when the threshold value is exceeded for a certain duration.

[0057] For this purpose, a second threshold value S2=6×I_Nenn is defined in the digital comparator. Therefore, an inrush current of I=10×I_Nenn at time T1 will not cause the semiconductor switch to shut down, because the inrush current only occurs in a relatively short period of time. By sampling the current profile at fixed predetermined time points t1, t2, t3, etc., the inrush current at time point T1 is not sampled at all. Only an overload event lasting a time interval of ΔT at time point T2 will cause the semiconductor switch to shut down.

[0058] Figure 7 Since the current values ​​sampled at the times t4, t5, t6 and t7 give a current change dI / dt which is higher than the third threshold value S3=(dI / dt)_limit, a short circuit exists at the corresponding functional output.

[0059] Figure 8 Since the current values ​​sampled at times t5 to t13 give a current change dI / dt which is below a third threshold value S3 =(dI / dt)_limit, an overload, for example a motor blockage, exists at the corresponding functional output.

Claims

1. A switching device (1) for protecting an electrical load (16) from overcurrent, the switching device (1) comprising: a load current input terminal (2) to which a load current line (17) from a power source (15) can be connected, and a load current output terminal (3) to which a load current line (18) leading to the electrical load (16) can be connected; - a load current path (10), through which a current can be conducted from the load current input terminal (2) to the load current output terminal (3); - a semiconductor switch (5), which is connected in the load current path (10), wherein: When the semiconductor switch (5) is turned on, the semiconductor switch (5) allows current to flow through the load current path (10), and when the semiconductor switch (5) is turned off, the semiconductor switch (5) blocks current from flowing through the load current path (10); - a control circuit (6), the control circuit (6) being arranged to switch the semiconductor switch (5) on or off in response to the received switching signal; - a current measuring device (9), the current measuring device (9) being used to collect the current flowing through the load current path (10); - a controller (8) which is configured to execute, depending on the operating mode of the electrical load (16), a single function of at least two predetermined functions (A, B, C) or a combination of two or more functions, the at least two predetermined functions comparing the current intensity and / or the current profile over time of a current flowing through the load current path (10) with defined threshold values, and - based on one or more results of one or more functions (A, B, C) performed, The control circuit (6) is controlled by means of a switching signal (50) in such a way that the semiconductor switch (5) is turned off.

2. The switchgear according to claim 1, in, A predefined group of at least two functions (A, B, C) has: a first function (A), by means of which the current profile (I(t)) in the load current path (10) is continuously detected, compared with a first current threshold value (S1), and the result of the comparison is output as a result of the function; - a second function (B), using which the current trend (I(t)) in the load current path (10) is acquired at a specific time point, the acquired current value (I1, I2, I3, ...) is compared with a second current threshold value (S2), and the comparison result is output as a function result; - A third function (C), using the third function (C), to collect the current trend (I(t)) in the load current path (10) at a specific time point, compare the time current change (dI / dt) of the collected current value (I1, I2, I3, ...) with the current change threshold (S3), and output the comparison result as the result of the function.

3. A switching device according to any one of the preceding claims, wherein the switching device has a command signal input terminal (13), and the controller (8) is capable of receiving a switching command (14) for switching the semiconductor switch (5) from the command signal input terminal (13).

4. A switching device according to any one of the above claims, wherein the switching device has an analog comparator (AK), using which the first function (A) is able to compare the current value of the current trend in the load current path (10) with the first current threshold value (S1).

5. A switching device according to any one of the above claims, wherein the switching device has a digital comparator (DK), using which the second function (B) is able to compare the collected current value (I1, I2, I3, ...) with the second current threshold (S2) and / or the third function (C) is able to compare the temporal change of the collected current value (I1, I2, I3, ...) with the current change threshold (S3).

6. A switching device according to any one of the preceding claims, wherein the switching device has a data memory (43), in which an algorithm of the function (A, B, C) can be stored, the algorithm defining under what circumstances the controller (8) outputs a switching signal to the control circuit (6).

7. A method for protecting an electrical load (16) from overcurrent, the method comprising the following steps: - collecting a current flowing through a load current path (10) of the switching device, wherein: The load current path (10) guides current from the load current input terminal (2) to the load current output terminal (3), a load current line (17) from a power source (15) is connected to the load current input terminal (2), and a load current line (18) leading to the electrical load (16) is connected to the load current output terminal (3); - providing at least two predefined functions (A, B, C), which compare the current intensity and / or the current profile over time of the current flowing through the load current path (10) with defined threshold values; - executing a single function or a combination of two or more of the predetermined functions (A, B, C) according to the operating mode of the electrical load (16); and - controlling the control circuit (6) by means of a switching signal in accordance with one or more results of the one or more functions (A, B, C) being performed so as to turn off the semiconductor switch connected in the load current path (10), wherein when the semiconductor switch (5) is turned on, the semiconductor switch (5) allows current to flow through the load current path (10), and when the semiconductor switch (5) is turned off, the semiconductor switch blocks current from flowing through the load current path (10).

8. The method according to claim 7, comprising: - Provides the following three functions (A, B, C): a first function (A), by means of which the temporal current profile (I) in the load current path (10) is continuously detected, compared with a first current threshold value (S1), and the result of the comparison is output as a result of the function; - a second function (B), by means of which the current trend (I(t)) over time in the load current path (10) is acquired at a specific time point, the acquired current value (I1, I2, I3, ...) is compared with a second current threshold value (S2), and the comparison result is output as a function result; - A third function (C), using the third function (C), at a specific time point, the current trend (I(t)) in the load current path (10) is collected, the temporal current change (dI / dt) of the collected current value (I1, I2, I3, ...) is compared with the current change threshold (S3), and the comparison result is output as the result of the function.

9. The method according to claim 7 or 8, wherein: - if the result of said first function (A) is that the current (I) in said load current path (10) exceeds said first current threshold value (S1); or - if the result of the second function (B) indicates that at least one of the current values ​​(I1, I2, I3, ...) of the acquired current profile over time (I(t)) in the load current path (10) exceeds the second current threshold value (S2); or - if the result of the third function (C) shows that the temporal current variation (dI / dt) of the acquired current values ​​(I1, I2, I3, ...) exceeds the current variation threshold (S3), A switch signal for turning off the semiconductor switch is sent to the control circuit (6).

10. The method according to any one of claims 7 to 9, in, The electrical load (16) is an electric motor, and wherein, during starting of the electric motor, the first function (A) is activated, and during rated operation of the electric motor, a combination (A, B) of the first function and the second function or a combination (A, B, C) of the first function, the second function and the third function is activated.

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