MESSGERAET

Description

[0001] The present invention relates to a measuring instrument for measuring a physical quantity and a method for testing a measuring instrument.

[0002] Measuring instruments can be used to measure physical quantities. For example, measuring instruments are used to measure electrical power in a system, such as a multi-phase power line system.

[0003] Such a measuring device can issue error messages if the measured physical quantity meets a predetermined condition. For example, if voltage or current drops sharply, or if the frequency of an electrical signal varies unusually, the measuring device can indicate this.

[0004] To test such a measuring device, a source that provides the relevant physical quantity is typically used. For example, an electrical power source that provides an electric current and an electric voltage is used to test a measuring device that can measure an electric current or an electric voltage.

[0005] However, providing such a physical quantity can be complex and expensive. For example, if currents in the range of 500 A are possible for a charging station in the e-mobility sector for vehicles, and these currents are to be measured with a measuring device, then a current source that provides 500 A is also needed to test the measuring device.

[0006] Publication WO 2013 / 038176 A2 discloses a current measuring device with a measuring arrangement which is connected to a load supplied with a current signal and is designed to measure the current signal of the load.

[0007] Publication US 2017 / 045570A1 discloses a test device for testing an intelligent electronic device of an energy system.

[0008] Publication WO 98 / 17003 A2 concerns an integrated circuit comprising an A / D converter and a test circuit.

[0009] Publication EP 1 970 720 A2 discloses a semiconductor device comprising a semiconductor chip with a sensor element for measuring a physical quantity and with an evaluation device.

[0010] Publication EP 2 557 703 A1 relates to a semiconductor device and a method for testing a semiconductor device as well as a transmission circuit.

[0011] The publication DE 10 2014 111 758 A1 describes a method for checking a field device with a measurement signal processing system.

[0012] One objective of the invention is to provide an improved concept for testing a measuring instrument.

[0013] This problem is solved by the subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of the dependent claims, the description, and the accompanying figures.

[0014] The revelation is based on the realization that a signal synthesizer in the measuring device imitates the output of an analog-to-digital converter in order to simulate a measurement input for the processing unit.

[0015] The task is solved, according to a first aspect, by a measuring instrument for measuring a physical quantity. The measuring instrument includes a transducer configured to convert an input quantity applied to a measurement input into a measurement signal and provide it as an output quantity. The measuring instrument includes a processing unit configured to process the output quantity of the transducer. The measuring instrument includes a signal generator configured to generate a test signal, based on a specification, that corresponds to an output quantity of the transducer and an input quantity corresponding to the specification; the processing unit being connectable to either the transducer or the signal generator via a switching element.

[0016] The converter is an analog-to-digital converter. The measurement input can be a socket or a plug to which sensors can be connected. An input signal can be acquired via the measurement input to measure the physical quantity. The input signal acquired at the measurement input can be the input to the converter. The converter converts the input into an output. The output is the measurement signal, i.e., the signal that the converter forwards to the processing unit. The measurement signal can be a digital signal. The converter can include preprocessing, such as current reduction. The preprocessing is located upstream of the converter. The preprocessing can include a sensor or a current transformer. The physical quantity can also be fed directly into the converter. An analog signal is present at the input of the converter. This signal has characteristics that define the signal.

[0017] Testing can include a demonstration or training. For example, in addition to functional testing of the measuring device, the test mode can be demonstrated to a customer or used for staff training. This can prepare users more thoroughly for handling the measuring device and thus increase safety, as fewer operating errors occur.

[0018] The processing unit can include integrated electronics. The processing unit can include logic components. The processing unit is configured to process the measurement signal provided by the converter. For example, the processing unit generates a signal from the output of the

[0019] The converter transforms the measurement signal into a display signal that can be shown on a display. The processing unit can evaluate the measurement signal, i.e., determine whether limit values ​​are being met, unusual conditions are occurring, or other errors are present.

[0020] The converter generates a digital signal whose information content describes the characteristics of the converter's input signal. The signal generator generates a digital signal whose information content describes the characteristics according to the specifications. If the values ​​of the specifications represent the same signal present at the converter's input, then the output signals of the converter and the signal generator would be identical if both were to provide an output signal simultaneously.

[0021] The signal generator can be a synthesizer. It can be configured to generate a digital electrical signal. Signal generation depends on a predefined set of parameters. These parameters can include one or more that are necessary for determining and generating a test signal. The predefined set can be read in whole or in part from a memory located within the measuring device. For example, the predefined set from an external source, such as a computer system, can simply be a modification of individual predefined parameters of the signal to be generated. In this case, the predefined set can adjust only one of the several parameters required to generate a test signal. Alternatively, the predefined set can originate entirely from an external source, eliminating the need for memory. The predefined set can represent the characteristics of the signal at the input of the converter.This means the specification provides values ​​that represent the analog input signal present at the converter. The specification includes the parameters that represent the signal the converter receives at its input.

[0022] The switching element can be implemented in software or physically.

[0023] For example, the signal generator and processing unit can be part of a microcontroller. Switching can then take place within the microcontroller, particularly via software. In this case, a signal input of the processing unit can be switched.

[0024] The test signal generated by the signal generator corresponds to an output of the converter. That is, if the converter were to detect an input signal at its measurement input, convert this input signal into a measurement signal, and provide it as an output, then the test signal of the signal generator corresponds exactly to this output signal, provided the input signal supplied to the signal generator precisely describes the signal present at the measurement input of the converter.

[0025] For example, if the measuring device is measuring a standard German household socket, the input signal at the measuring input can be a voltage of 230 V and / or a current of 16 A. This is an alternating current with a frequency of 50 Hz. The converter transforms these measured values ​​into one or more measurement signals and provides this signal(s) as an output. The measuring device can then, for example, determine a display from the output signal and thus show the voltage value of 230 V and the current value of 16 A. The measuring device can also perform further processing steps and, if necessary, output results and warnings.

[0026] When the measuring device is operated in test mode, the signal generator can be set to a voltage of 230 V, a current of 16 A, and a frequency of 50 Hz. The signal generator then produces an artificial measurement signal as a test signal, thus simulating an output signal from the converter. This test signal corresponds exactly to the signal that the converter provided to the processing unit. Specifically, the test signal is provided directly to the processing unit. This means that when the signal generator is connected to an input of the processing unit, it receives the same signal—that is, a signal with the same characteristics—as the signal previously output by the converter.The display provided by the processing unit, that is, its output, corresponds exactly to the output of the processing unit as if the converter were connected to the processing unit. If the output of the processing unit is displayed, it will also show 230 V and 16 A.

[0027] The specification can include a voltage in percent or volts. The specification can include a current in percent or amperes. The specification can include a phase angle, a frequency for all lines, or even harmonics for voltages and currents. The specification can include a dynamic change. In this case, the corresponding value can be varied randomly within predefined limits.

[0028] The advantage here is that no precise and expensive current or voltage source is required to provide the corresponding signals. It is sufficient to input the specifications to be tested as a signal into the measuring device. The signal generator then internally produces a test signal within the measuring device that corresponds to the output of the converter. The device functions of the measuring device can be retained. These can include a web server, Modbus interface, display, average values, logging functions, limit switches, error messages, and other analog or digital interfaces. The complete process chain can be maintained, and the functions of the measuring device can be used as before.

[0029] According to one embodiment, the measuring device is configured to measure an electrical quantity. This allows for the measurement of current or voltage.

[0030] In one embodiment, the measuring device has six measuring inputs and is configured to measure a voltage and a current of a three-phase line; or the measuring device has two measuring inputs and is configured to measure a voltage and a current of a single-phase line, and the signal generator is configured to generate two corresponding test signals. In this case, the signal generator is configured to generate six corresponding test signals.

[0031] The measuring device can be used to measure a multi-phase system. Specifically, it can measure three lines of a multi-phase system. If the power in each phase of the system is to be measured, six measurement inputs are required: one voltage input and one current input for each phase. Internally, the measuring device's processing unit can then calculate the power from the voltage and current of the inputs assigned to each line. In this case, the signal generator also provides six outputs, each corresponding to a converter output. Other values ​​can also be measured, such as energy, distortion (especially THD), or other parameters. The connectors for connecting measuring cables or probes may differ from the measurement inputs mentioned here. In particular, a measurement input for current measurement may have two connectors.The measuring device can also have additional jacks for each measurement input. For voltage measurements, a measurement input also includes a reference point. For current measurements, a measurement input includes two jacks.

[0032] In one embodiment, the measuring device can be connected to a computer system and configured to receive the specification for the test signal from the computer system.

[0033] The computer system can be a PC, a laptop, or a smartphone. It can be connected to the measuring device via a wireless or wired interface. Input of the setting can be user-friendly via the computer system using a familiar data transmission method, thus reducing the effort required for setting the setting.

[0034] In one embodiment, the specification includes a phase and / or a frequency and / or a harmonic and / or an amplitude of a signal. The signal generator is configured to generate the test signal based on at least one of these parameters. The frequency can apply to the entire system being measured by the instrument. For each phase, the resulting RMS values ​​of the fundamental frequencies are specified for both current and voltage. Additionally, the phase shift angle can be specified for each phase.

[0035] If not only sinusoidal quantities are to be specified, the harmonics for current and voltage, and their angles, can optionally be specified as well. One or more harmonics can be specified for each.

[0036] The signal generator is designed to produce a test signal. The test signal can have default values, which are stored, for example, in the measuring device's memory. This allows the signal generator to produce a test signal even if not all parameters are specified externally. The external input, i.e., the specification, defines which parameters the test signal should represent. For example, the phase, frequency, harmonic, and / or amplitude of the test signal can be determined. If multiple specifications for a test signal or multiple test signals are entered, for example, to simulate a three-phase line, the individual test signals can be generated with a phase shift to correspond to a specific measurement value. Similarly, a harmonic of a signal can be simulated.

[0037] In one embodiment, the switching element includes an electronic switch. The measuring device is configured to switch the switching element depending on a control signal. The electronic switch can be implemented in software, thus eliminating the need for a physical switch. The switching element can be used to switch a signal input within a logic circuit, for example, in a microcontroller.

[0038] A computer system can be connected to the measuring device. The computer system can switch the measuring device from a measurement mode to a test mode, and the input of the processing unit is connected to the output of the signal generator instead of the output of the converter. This provides a safe switching mechanism, preventing accidental switch activation.

[0039] In one embodiment, the measuring device is configured to query a security feature, in particular a password and / or an electronic identifier, in order to switch the switching element.

[0040] To prevent the measuring device from switching incorrectly from measurement mode to test mode, the switching element can be protected. This can be achieved by requiring a password, for example an alphanumeric string, and / or an electronic identifier, such as a token or electronic ID card.

[0041] In one configuration, the measuring device is set up to communicate via a Modbus interface and receive the input signal via Modbus. Modbus is an interface that allows communication with programmable logic controllers (PLCs). The measuring device can be easily connected to this interface.

[0042] The measuring instrument includes an output interface, in particular a display. The processing unit is configured to output the processed output variable via this interface. This allows for easy reading of the measured values ​​when the output is displayed. When the measuring instrument is in test mode, the processed test signal can be displayed. The display can be an optical indicator or another type of interface.

[0043] In one embodiment, the switching element is integrated into a microcontroller. This has the advantage that the switching element can be implemented inexpensively.

[0044] According to a second aspect of the invention, the problem is solved by a method for testing a measuring instrument. The method comprises: Switching a switching element to connect a processing unit of the measuring instrument to a signal generator of the measuring instrument, wherein the signal generator is configured to generate a test signal based on a preset; receiving the preset for the test signal at a control input of the measuring instrument; outputting the test signal by the signal generator to the processing unit based on the preset; processing the output test signal by the processing unit; and outputting the processed test signal via an output interface of the measuring instrument.

[0045] In one embodiment, the method includes outputting the processed test signal to a display.

[0046] Outputting the test signal includes issuing a notification that the output is based on a test signal.

[0047] This can increase safety by preventing accidental operation of the measuring device in test mode.

[0048] In one embodiment, a safety check is performed before the switching element is activated. The switching element is only connected to the signal generator if the result of the safety check is positive.

[0049] The security check may involve entering a password, in particular an alphanumeric string, capturing biometric input, or an electronic identifier such as a token or electronic ID card. Only if this security check determines that the security criterion has been met, i.e., if it is determined that the user is authorized to switch the measuring device to test mode, will the measuring device be switched to test mode.

[0050] In one embodiment, a display indicating the switching position of the switching element is output when the switching element connects the signal generator to the processing unit.

[0051] This allows the circuit of the measuring device to be displayed in test mode, regardless of whether a test signal is generated or not.

[0052] In one embodiment, the method further comprises: switching the switching element from the signal generator to the converter if the switching element was previously switched in such a way that the signal generator was connected to the processing unit and a predetermined time period has elapsed. This ensures that the measuring device is not forgotten to be switched back into measurement mode.

[0053] The measuring device can then be switched back to measurement mode. Switching back can also occur during or by resetting or restarting the measuring device.

[0054] The measuring device may have a timer for this purpose.

[0055] In one implementation, the input parameter is modified to represent a faulty signal. The input parameter can be defined to represent a signal exhibiting faulty behavior. This allows for the simulation of an error in a signal that would be measured by the instrument. Similarly, the instrument's response to signal errors can be determined. In another implementation, the input parameter is provided in a faulty state. This can also be used for testing the instrument.

[0056] The invention is explained below with reference to exemplary embodiments and the figures. The figures show: Fig. 1 a schematic representation of a measuring instrument according to an embodiment; Fig. 2 a schematic representation of an arrangement of a measuring instrument according to an embodiment; and Fig. 3 a flowchart for a method according to an embodiment.

[0057] Fig. 1Figure 1 shows a measuring instrument 100. The measuring instrument 100 has a measuring input 101. In the described embodiment, the measuring input 101 comprises six terminals. In another embodiment, the measuring input 101 comprises one or a different number of terminals. Three lines, each carrying current and voltage, can be connected to the six terminals of the measuring input 101. In this way, the measuring instrument 100 can measure a three-phase power line, so that the measuring instrument 100 can display the power of each phase of a three-phase line. A terminal can comprise multiple sockets. In particular, a current terminal, via which a current measurement is possible, can comprise two sockets.

[0058] The measuring device 100 is a measuring instrument for measuring electrical power and other values ​​that can be derived from the measured data, such as electrical energy. Physical quantities, such as voltage, can also be displayed directly. In another embodiment, the measuring device 100 is a different type of measuring instrument, for example, for measuring electrical voltage, temperature, spatial distance, or another physical quantity.

[0059] The measuring device 100 includes a converter 102. The converter 102 is an analog-to-digital converter. The converter 102 converts an input signal applied to the measurement input 101 into a digital output signal. The converter 102 thus provides an output signal at an output 103 of the converter. The output signal is a measurement signal that reflects the measured physical quantity. In addition to the analog-to-digital converter, the converter 102 also includes preprocessing (not shown) to process the measured signals before they are fed into the converter 102, in particular to condition or transform the signal. The converter 102 with the preprocessing can represent an analog front end.

[0060] The measurement signal, that is, the output of converter 102 at its output 103, can be processed by a processing unit 104 of the measuring device 100. For this purpose, the processing unit 104 is electrically connected to the output 103 of converter 102. A switching element 105 is arranged between the processing unit 104 and the output 103 of converter 102 for this purpose. When the switching element 105 is in its first switching position, the output 103 of converter 102 is connected to an input of the processing unit 104. The output, i.e., the measurement signal, of converter 102 is thus present at the input of the processing unit 104. The processing unit 104 interprets the measurement signal and, if necessary, calculates further value parameters from it, which are to be recorded by the measuring device 100.The processing unit 104 can derive further physical quantities from the measurement signal or compare the physical quantity represented by the measurement signal with limit values ​​and, if necessary, generate an error message or a warning.

[0061] The measuring instrument 100 has an output interface 106. The output interface 106 serves to output the measured values ​​of the measuring instrument 100; in particular, the output interface 106 includes a display to show the measured physical quantity to a user. For example, an electrical voltage in volts and an electrical current in amperes can be displayed. In further embodiments, corresponding other physical quantities can be displayed. The processing unit 104 is configured to transmit the processed measurement signal to the output interface 106 and thus display the value measured by the measurement signal in measurement mode on the output interface 106.

[0062] The measuring device 100 has a signal generator 107. The signal generator 107 is connected to a control input 108. The control input 108 can conform to a communication standard, for example, it can be configured as a Modbus interface. A setting can be given to the signal generator 107 via the control input 108. The setting can include values ​​of a physical quantity. For example, a voltage value can be set to 230 V, specifically an RMS value of a voltage signal of 230 V, and a current, i.e., an RMS value of a current, can be set to 16 A. Similarly, a signal frequency can be set to 50 Hz; other values ​​are also possible. A harmonic or multiple harmonics, or a phase shift, can also be specified. The harmonic can relate to a current or a voltage.

[0063] Signal generator 107 is configured to output a test signal at an output 109 of signal generator 107. The test signal simulates the measurement signal of converter 102. The test signal represents a synthetic measurement signal.

[0064] The operation of the measuring device 100 can be simulated using the signal generator 107 and the test signal. For example, it can be used for demonstration or testing purposes to check whether the measuring device 100 reacts to certain signal changes or signals.

[0065] In a switching position not shown, the switching element 105 is in a first switching position and thus connects the converter 102 to the processing unit 104. This position is the measuring mode of the measuring instrument 100. A physical quantity can be measured via the measuring input 101. For example, a first voltage of 231.2 V, a second voltage of 230.9 V, and a third voltage of 229.8 V are measured via the measuring input 101. Corresponding currents can also be measured via other connections of the measuring input 101. The converter 102 converts these measured analog voltage values ​​into digital signals. The digital signals are available as an output quantity or as a measurement signal at the output 103 of the converter 102. Since the measuring instrument 100 is in measuring mode, these signals are also present at the processing unit 104. The processing unit 104 can use the values, i.e.,The system calculates the corresponding power from the voltage and current of each phase of a three-phase line, as well as other values. These processed values ​​can be output, and in particular displayed, via output interface 106.

[0066] If it is now necessary to test whether the measuring device 100 can detect a voltage drop on one of the lines, for example, it is not necessary to provide a line with a voltage and then induce a voltage drop on this line.

[0067] Thanks to the test mode of the measuring device 100, the measuring device 100 can be switched into test mode by switching the switching element 105 from the first switching state to a second switching state. In test mode, the converter 102 is not connected to the processing unit 104. Instead, the signal generator 107 is connected to the input of the processing unit 104 via the output 109 of the signal generator 107.

[0068] The control input 108 can be used to instruct the signal generator 107 to generate a test signal that the converter 102 would have produced if an input variable corresponding to the specified value were applied to the measurement input 101. For example, it can be specified that a voltage signal is at 231.2 V and experiences a voltage drop of 95% after approximately 10 seconds. The other values ​​correspond to the typical values ​​of a three-phase power line. These can be specified via the control input 108 or are provided by the signal generator 107 from an internal memory (not shown). The signal generator 107 then generates a test signal, functioning as a synthesizer. The signal generator 107 generates the test signal in such a way that the processing unit 104 cannot distinguish between a measured signal, which the converter 102 converts into a measurement signal, and the test signal of the signal generator 107.The processing unit 104 interprets the test signal as the output of the converter 102. The processing unit 104 processes the test signal in the same way as the output of the converter 102, i.e., the measurement signal. The processing unit 104 outputs the test signal, i.e., the supposedly measured value of the physical quantity, via the output interface 106. It can then be determined whether the voltage drop is displayed correctly or not, in the case that an error has occurred in the processing unit 104.

[0069] The test signal can be used to check the functionality of the measuring device 100. Furthermore, the test signal can be used for training or demonstration purposes to instruct users in the operation of the measuring device 100.

[0070] Fig. 2Figure 1 shows an arrangement 200 with the measuring instrument 100. The output interface 106 is visible on the measuring instrument 100. Six measuring leads 201, i.e., sensors, lead into the measuring instrument 100 and are connected to the measuring input 101 of the measuring instrument 100. The measuring leads 201 are connected to a first phase 202a, a second phase 202b, and a third phase 202c of a three-phase electrical line 202. Two terminals of the measuring input 101 are connected to each of the phases 202a, 202b, and 202c of the electrical line 202 via a current transformer 203. Two measuring leads 201 of each phase 202a, 202b, and 202c are configured to measure a current. In further embodiments, additional measuring lines 201 are used to additionally or alternatively detect further parameters, such as voltage.

[0071] Fig. 3shows a flowchart 300 for a method for testing a measuring instrument 100 according to an embodiment.

[0072] In step 301, the measuring device 100 receives a control signal at control input 108. This control signal indicates that the switching element 105 should be switched from the first switching position to the second switching position, and that the measuring device 100 should therefore switch from measurement mode to test mode. An administrator mode can be activated for this purpose. This may include a soft reset. The test mode can also be executed outside of administrator mode. However, administrator mode is required for switching between modes. In another embodiment, an administrator mode is not required.

[0073] The measuring device 100 then initiates a security prompt in step 302. Here, a user is prompted to enter a password via the control input 108. In another embodiment, the security prompt includes the acquisition of a biometric identifier, an electronic identifier, or the like. If the security prompt is negative, i.e., for example, if the password was entered incorrectly or no password was entered, the user who sent the control signal 108 to switch to test mode is not authenticated, and the procedure ends in step 303. The switching from the first switching position to the second switching position of the switching element 105 is not performed by the measuring device 100.

[0074] If the result of the security check in step 302 is positive, i.e., the user has been correctly authenticated, then in step 304 the switching element 105 is switched from the first switching state to the second switching state. Here, the switching element 105, in this case an internal switch of a signal input of a microcontroller, is switched electronically. In another embodiment, this could be a different switching element, such as a bipolar transistor. In yet another embodiment, a mechanical switching path is enabled.

[0075] When the switching element 105 switches to test mode, i.e., its second switching state, a timer simultaneously starts running. The timer measures the time, i.e., the duration for which the switching element 105 remains in the second switching state. Regardless of any other input signals, the switching element 105 switches back to its first switching state after the timer expires or after a predetermined time has elapsed. This can serve as a safety feature to prevent forgetting to switch the measuring device 100 back to measurement mode. In another embodiment, this automatic switching back does not occur. In this case, a timer can be omitted.

[0076] In step 305, a input signal is received. This input signal is provided to the signal generator 107 via control input 108. The input signal includes parameters that influence and / or define an electrical signal. Further necessary parameters for generating such a signal are either predefined by a memory or must be entered by the user via control input 108. The input signal determines the characteristics of an electrical signal. In particular, the input signal includes amplitude and / or frequency and / or phase and / or harmonics.

[0077] Based on the received input, signal generator 107 can output the test signal in step 306. The test signal is output by signal generator 107 via its output 109. Since switching element 105 is in a second switching state, output 109 of signal generator 107 is connected to the input of processing unit 104. Thus, the test signal, rather than a measurement signal, is now present at processing unit 104. However, processing unit 104 interprets the test signal as a measurement signal and processes it as if it originated from converter 102 and represented an actual measured physical quantity.

[0078] The processing by processing unit 104 takes place in step 307. Here, processing unit 104 interprets the test signal according to certain specifications; for example, a power value or another value is calculated from it.

[0079] In step 308, the processing unit 104 provides output to the output interface 106 to display the values ​​of the test signal on the display or to output them in another way. This allows the user to check whether the display corresponds to expectations according to the input specifications. In another embodiment, no output is provided if there is no error or other predetermined event.

[0080] In step 309, the measuring device 100 detects that the control input 108 is no longer supplying a signal. This means, for example, that a connector has been unplugged from the control input 108. The switching element 105 then switches back to its initial state and returns the measuring device 100 to its measurement state.

[0081] In another embodiment, this does not happen automatically. Here, a user must manually switch the measuring device 100 back. To do this, they must, for example, apply a control signal again via the control input 108 to switch the switching element 105 back. The measuring device 100 is actively switched back in this case, for example via a reset or a restart.

[0082] The output interface 106 can also be used to indicate that the measuring device 100 is in test mode. This indication can depend on the switching position of the switching element 105. Furthermore, it can be indicated that the display is based on a test signal. List of reference symbols

[0083] 100 Measuring instrument 101 Measuring input 102 Converter 103 Output 104 Processing unit 105 Switching element 106 Output interface 107 Signal generator 108 Control input 109 Output 200 Arrangement 201 Measuring line 202 Three-phase line 202a, 202b, 202c Phase 203 Current transformer 300 Flowchart 301-309 Process step

Claims

1. Measuring device (100) for measuring a physical variable, comprising: an A / D converter (102), which is configured to convert an input variable present at a measurement input (101) into a measurement signal and to provide the same as an output variable; a processing unit (104), which is configured to process the output variable of the A / D converter (102); and a signal generator (107), which is configured to generate a test signal on the basis of a specification, which test signal corresponds to an output variable of the A / D converter (102) associated to an input variable corresponding to the specification; wherein the processing unit (104) can be connected via a switching element (105) either to the A / D converter (102) or to the signal generator (107), wherein the measuring device (100) comprises an output interface (106) and the processing unit (104) is configured to output the processed output variable via the output interface (106); and wherein the specification can be read completely or partially from a memory, which is arranged in the measuring device (100), or wherein the specification originates completely from an external source, characterized in that the signal generator (107) is configured to output the test signal with an indication that the output is based on a test signal.

2. Measuring device (100) according to claim 1, wherein the measuring device (100) is configured to measure an electrical variable.

3. Measuring device (100) according to claim 2, wherein the measuring device (100) has six measurement inputs (101) and is configured to measure a voltage and a current of a three-phase line (202), respectively, and wherein the signal generator (107) is configured to generate six corresponding test signals; or wherein the measuring device (100) has two measurement inputs (101) and is configured to measure a voltage and a current of a single-phase line (202), respectively, and wherein the signal generator (107) is configured to generate two corresponding test signals.

4. Measuring device (100) according to any of the preceding claims, wherein the measuring device (100) can be connected to a computer system and is configured to receive the specification for the test signal from the computer system.

5. Measuring device (100) according to any of the preceding claims, wherein the specification comprises a phase shift and / or a frequency and / or a harmonic and / or an effective value of a signal and wherein the signal generator (107) is configured to generate the test signal on the basis of at least one of these parameters.

6. Measuring device (100) according to any of the preceding claims, wherein the switching element (105) comprises an electronic switch and the measuring device (100) is configured to switch the switching element (105) as a function of a control signal.

7. Measuring device (100) according to claim 6, wherein the measuring device (100) is configured to query a security feature, in particular a password and / or an electronic identifier, in order to switch the switching element (105).

8. Measuring device (100) according to any of the preceding claims, wherein the measuring device (100) is configured to communicate via a Modbus-interface and to receive the specification via the Modbus.

9. Measuring device (100) according to any of the preceding claims, wherein the switching element (105) comprises a microcontroller or a transistor, in particular a MOSFET or a bipolar transistor and is configured to perform an electronic switching.

10. A method for testing a measuring device (100) with an A / D converter, the method comprising: switching (304) a switching element (105) in order to connect a processing unit (104) of the measuring device (100) to a signal generator (107) of the measuring device (100), wherein the signal generator (107) is configured to generate a test signal on the basis of a specification; receiving (305) the specification for the test signal at a control input (108) of the measuring device (100); outputting (306) the test signal by the signal generator (107) based on the specification to the processing unit (104); and processing (307) the output test signal by the processing unit (104), outputting the processed output variable via an output interface (106) of the measuring device (100); wherein the specification can be read completely or partially from a memory, which is arranged in the measuring device (100), or wherein the specification originates completely from an external source, characterized in that the outputting (306) of the test signal comprises outputting an indication that the output is based on a test signal.

11. Method according to claim 10, further comprising: outputting (308) the processed test signal via an output interface (106) of the measuring device (100).

12. Method according to any one of claims 10 or 11, wherein before the step of switching (304) the switching element (105), a security query is performed (302), and the switching element (105) is only switched to the signal generator (107) if a result of the security query (302) is positive.

13. Method according to any one of claims 10 to 12, wherein a display for displaying the switching position of the switching element (105) is output when the switching element (105) connects the signal generator (107) to the processing unit (104).

14. Method according to any one of claims 10 to 13, wherein the specification comprises a change in a parameter in order to represent an erroneous signal.