Method and measuring system for checking electrical contact-connection

Abstract

In order to be able to easily determine correct contact-connection between measuring contacts unambiguously and in spite of interference, the invention proposes a method for checking electrical contact-connection of an electrical system (2) and also proposes an associated measuring system (1), wherein a profile of a measurement voltage (um) brought about by the test signal (it) and a profile of a measurement current (im) brought about by the test signal (it) are measured (101) in each case and wherein an amplitude of the measurement voltage (um) and of the measurement current (im)) and a phase shift of the profile of the measurement voltage (um) and of the profile of the measurement current (im)) are determined (102) in each case from a measurement result.

Description

[0001] Method and measuring system for checking an electrical contact

[0002] Technical field

[0003] The present invention relates to a method for verifying the electrical contact of an electrical system with a measuring system, which is connected to measuring points of the electrical system via measuring contacts. A periodic test signal with a predetermined frequency is applied to the electrical system, generating a measuring current and a measuring voltage. The invention further relates to an associated measuring system for verifying the electrical contact.

[0004] State of the art

[0005] Before maintenance and / or repair work can be carried out on an electrical system, it is essential, primarily for the protection of people, to disconnect the electrical system from the electrical grid or power supply and verify that the system is de-energized. Electrical systems where verifying the absence of voltage is particularly important include, for example, components of a high-voltage system (HV system) in electrified vehicles, which are at least partially powered electrically by an electric drive motor. Electrified vehicles, such as electric vehicles, hybrid vehicles, etc., have a rechargeable electrical storage device—usually in the form of a high-voltage battery or traction battery—which powers not only the electric drive motor but also other components, such as...Power electronics components (e.g., an inverter for controlling the electric drive motor, a voltage converter or DC-DC converter, etc.), on-board chargers (OBCs), and other auxiliary components (e.g., pumps, heating elements, etc.) are electrically connected. The high-voltage (HV) system of an electrified vehicle is usually operated at a voltage above 60 V (direct current), typically in the range of several hundred volts (e.g., between 200 V and 1 kV). Therefore, disconnecting the HV system from the high-voltage battery before maintenance and / or repair of an electrified vehicle is a particularly important safety measure. This is achieved, for example, by disconnecting the HV system from the high-voltage battery before maintenance and / or repair using a manual disconnect switch – the so-called service disconnect plug.

[0006] However, simply disconnecting the electrical system from the electrical grid or power supply – for example, by activating the disconnect switch in an electric vehicle – does not guarantee that the electrical system is de-energized. Despite disconnection from the grid or power supply, electrical energy may still be stored in the electrical system's energy storage devices (e.g., capacitors). For instance, even after disconnecting the high-voltage (HV) system from the HV battery in an electric vehicle, residual voltage may remain, particularly in power electronics components, due to faulty capacitor discharge. Furthermore, other electrostatic charges can also lead to a low residual voltage in the electrical system.

[0007] However, even a voltage measurement on an electrical system does not guarantee the absence of voltage, even if no voltage is detected during the measurement. Due to potential contact errors between the measuring contacts of a measuring system and the measuring points on the electrical system under test, a false indication of the absence of voltage or a low residual voltage may occur, even though a (high) voltage may still be present on the electrical system. Electrostatic charge in the electrical system can also lead to a low voltage reading due to the high impedance input of a voltage meter. Therefore, a very low voltage reading during a voltage measurement on the system under test can also result from a faulty contact or from an actual residual voltage present in the system.

[0008] To reliably verify that an electrical system is de-energized, and thus enable further work on the electrical system, such as maintenance or repair of an electrified vehicle, a contact test of the measuring system's contacts is necessary. In the event of faulty contact, a lower voltage, or in the worst case, no voltage at all, could be measured than is actually present between the measuring points of the electrical system, posing a potential danger to anyone working on the electrical system. Conventional measuring devices do not offer this function. Therefore, contact testing has traditionally been performed visually. This means the user visually checks whether the measuring contacts are properly making contact with the electrical system or the measuring points on the electrical system under test. In many applications, e.g.,In electrified vehicles, however, the limited space and / or the location of the measuring points often make visual verification of the contact impossible or impossible to verify with certainty. Furthermore, visual contact does not always guarantee actual electrical contact.

[0009] One way to check the contact is, for example, by using the resistance measurement function often found in measuring systems. For this purpose, a current or voltage source is integrated into the measuring system, which can generate a test signal – for example, a test current or a test voltage. By measuring the current flow through the measuring contacts of the measuring system, it can be concluded whether a contact has been made. However, this method can only detect purely resistive and, moreover, only low-resistance components. Therefore, a contact fault cannot be reliably and unambiguously distinguished from a correctly contacted, voltage-free, and high-resistance system.

[0010] From AT 518595 B1, a method is known for verifying whether measuring contacts, on which a voltage measurement is to be performed, have been properly contacted. For this purpose, the measuring system comprises a voltmeter and a charging circuit which, after closing a discharge switch, is connected to the voltmeter and discharges through the internal resistance of the voltmeter. With the discharge switch closed, a measuring switch is then closed, connecting the voltmeter to the measuring contacts, and the contact between the electrical system and the measuring system is determined. The voltage waveform measured at the voltmeter is evaluated to detect contact, faulty contact, or no contact, as well as any voltage present in the system under test. However, the method known from AT 518595 B1 can be affected by interference (e.g.,Interference signals, hum voltages, etc. in the measuring system, existing DC voltage, etc.), and especially if the system to be tested has rather large capacitances, can lead to incorrect evaluation results.

[0011] Furthermore, JPH11-142451 A discloses a method and a measuring arrangement for verifying the contact of a capacitor, particularly a capacitor with a small capacitance, via measuring terminals. For this purpose, the measuring device includes, for example, a sine wave generator for generating an alternating current signal, which is passed through a coil in the measuring arrangement and the measuring terminals to the capacitor under test. Before the measuring terminals are connected to the capacitor under test, the frequency of the alternating current signal is varied such that a series resonant circuit formed by the inductance and the internal capacitances of the measuring arrangement is in resonance (i.e., that, for example, a measured impedance reaches a minimum or the phase becomes essentially zero).The capacitor under test is then connected to the test leads, and its impedance and phase are measured again using an AC current meter, maintaining the frequency of the AC signal at which resonance was detected in the test setup. If a significant change in the measured impedance or phase is observed compared to the impedance or phase without the capacitor under test connected, proper contact is assumed. However, the resonant frequency of the AC signal is very high due to the relatively small stray capacitances in the test setup, necessitating additional capacitances. Even with this method, interference in the test setup, such as noise signals, voltage disturbances, etc., can distort the results, especially if it affects the AC signal used for the measurement.

[0012] Description of the invention

[0013] It is therefore an object of the present invention to provide a method and a measuring system with which it can be determined in a simple manner, without doubt and resistant to interference, whether measuring contacts on which a voltage measurement is to be carried out have been properly contacted.

[0014] These and other problems are solved by a method and a measuring system according to the independent claims. Advantageous embodiments of the present invention are described in the dependent claims.

[0015] According to the invention, the problem is solved by a method of the type specified above, wherein a course of a measuring voltage caused by the test signal and a course of a measuring current caused by the test signal are measured, and wherein an amplitude of the measuring voltage and the measuring current and / or a phase shift of the course of the measuring voltage and the course of the measuring current are determined from a measurement result of the correlation.

[0016] The main aspect of the solution proposed according to the invention is that interference, such as interference signals or interference voltages, is suppressed, and therefore even the smallest capacitances are detected with very high output impedances of the test signal. Furthermore, the method monitors the voltage at the electrical system under test (e.g., the high-voltage system of an electrified vehicle) to ensure beyond doubt that the measuring contacts for the voltage measurement are properly in contact with the measuring points.

[0017] It is also advantageous if information is provided to a user and / or at least an action is triggered when, during a comparison of the phase shift of the measured voltage waveform and / or the phase shift of the measured current waveform with a respective reference phase shift, it is determined that a predefined tolerance is not exceeded. This allows a user to be easily alerted to a faulty electrical connection. At least one action could be, for example, repeating the measurement or aborting an ongoing contact check. The respective reference phase shift could, for example, be...The phase shift can be determined during calibration of the measuring system, for example by measuring the measurement voltage waveform caused by the test signal when the measuring system is unloaded. The respective reference phase shift can then be, for example, a phase shift of the measurement voltage waveform and / or the measurement current waveform relative to the test signal when the measuring system is unloaded.

[0018] Furthermore, it is advantageous to determine the impedance of the electrical system from the measured amplitude of the measured voltage and current, as well as the measured phase shift of the voltage and current waveforms. From the determined impedance and the known test frequency, the resistance and capacitance of the electrical system, for example, can be easily derived.

[0019] Preferably, a sinusoidal current signal is used as the periodic test signal, which is supplied, for example, by a signal source, in particular a current source or a regulated current source. However, other periodic test signals are also possible. t , such as a rectangular current signal, is conceivable.

[0020] The measurement of the current and / or voltage profile can ideally be performed using discrete-time sampling.

[0021] The aforementioned task is also solved by a measuring system for verifying the electrical contact of an electrical system, wherein the measuring system can be connected to measuring points of the electrical system via measuring contacts. The measuring system includes a signal source for generating a periodic test signal with a predefined frequency, which can be applied to the electrical system via the measuring contacts. The measuring system also includes a voltmeter for measuring the waveform of a test voltage induced by the test signal and an ammeter for measuring the waveform of the test current induced by the test signal.Furthermore, the measuring system includes an evaluation unit which is configured, at least, to determine the amplitude of the measured voltage and current, and / or the phase shift of the measured voltage and current waveforms, from the measurement results of the ammeter and the voltmeter. In a preferred embodiment, the measuring system includes a resistor arranged in series with the signal source. This resistor protects the measuring system, and in particular the signal source, from high voltages. Ideally, the signal source is designed as a current source, especially a regulated current source.

[0022] Furthermore, a display unit is provided, which can output information to the user. For example, the display unit can alert the user that the electrical system has a faulty connection. Actions (e.g., repeating the measurement, aborting the contact check, etc.) can also be displayed to the user via the display unit.

[0023] Ideally, the measurement data acquisition by the current measuring device and the voltage measuring device as well as the evaluation unit are designed to be microcontroller-based, which allows the execution of the method according to the invention to take place, for example, in near real time.

[0024] Brief description of the characters

[0025] The present invention is explained in more detail below with reference to Figures 1 and 2, which show exemplary, schematic, and non-limiting advantageous embodiments of the invention.

[0026] Fig. 1 shows a measuring system for checking the electrical contact of an electrical system.

[0027] Fig. 2 shows a sequence of the procedure for checking the electrical contact of an electrical system.

[0028] Implementation of the invention

[0029] Figure 1 shows an exemplary and schematic measuring system 1 for checking the contact of an electrical system 2. The absence of voltage on the electrical system 2, e.g., an electric drive system or high-voltage system of an electrified vehicle, is to be verified, for example, using measuring system 1, before work (e.g., maintenance, repair) is carried out on the electrical system 2. For this purpose, measuring points 31, 32, e.g., prepared measuring points, are provided on the electrical system 2. These measuring points 31, 32 are contacted by the measuring contacts 41, 42 of the measuring system 1 for voltage measurement or for verifying the absence of voltage on the electrical system 2. The measuring contacts 41, 42 can, for example, be designed as test probes which are connected to measuring points 31, 32 on the system 2 to be checked (e.g.,In the case of an HV system of an electrified vehicle, the measuring contacts 41 and 42 are connected to a positive high-voltage terminal and a negative HV terminal of the HV system or component under test. However, the measuring contacts 41 and 42 can also be designed as terminals and / or clamps, for example, to be connected to the appropriate measuring sockets 31 and 32 (e.g., HV+ socket, HV- socket) or clamped to corresponding measuring points 31 and 32 (e.g., the vehicle chassis).

[0030] The electrical system 2 to be tested, such as an HV system or a component (e.g., HV cable, charging socket, power electronics, etc.) of the HV system, is shown in Figure 1 as an example of a parallel circuit consisting of a capacitor C and a resistor R, where the resistor R (e.g., an insulation resistance of system 2) and the capacitor C (e.g., internal capacitances in electrical system 2) symbolize an impedance of electrical system 2. Furthermore, the electrical system 2 to be tested can be connected to an electrical power supply 6 (e.g., the high-voltage battery of an electric vehicle) via a switch 5 (e.g., actuated by the service disconnect plug). During normal operation of electrical system 2, the switch 5 is closed, so that electrical system 2 is connected to the power supply 6.For work on electrical system 2, switch 5 is usually opened to disconnect electrical system 2 from power supply 6.

[0031] Furthermore, the measuring system 1 has a signal source 7 from which a periodic test signal i is generated. t is generated at a predetermined frequency. The signal source 7 of the measuring system 1 can, for example, be designed as a current source, in particular as a regulated current source, from which, for example, a relatively low sinusoidal alternating current (e.g., in the pA range) is drawn as a test signal i. t The measurement system 1 is generated at the specified frequency (e.g., 1 kHz). Furthermore, the system has a resistor R connected in series with the signal source 7. s through this resistance R sThe measuring system 1, and especially the signal source 7 or the current source 7, is protected, for example, from high external voltages that can occur when contacting a non-de-energized electrical system 2 (e.g., switch 5 was left open, residual voltage in capacitor C, etc.). The resistor R s is designed as a high-impedance resistor, preferably in the MQ range (e.g. 3 MQ), and can limit the voltage in the measuring system 1.

[0032] The test signal i t For contact verification, a current is applied to the system 2 under test via the measuring contacts 41, 42 and a measuring current i is triggered. m and a measuring voltage u m This is evident. With proper contact between measuring contacts 41, 42 and the measuring points 31, 32 of the electrical system 2, the test signal i would be t generated measuring current i mvia the electrical system - i.e. via the impedance or a component i in each case r , i c of the measuring current i m through the resistance R and the capacitance C of the parallel circuit. Furthermore, the test signal i t a measuring voltage u m caused. Since each part i r , i c of the measuring current i m For example, if current flows across the resistance R and the capacitance C of electrical system 2, a voltage drop occurs across electrical system 2 or between measuring points 31 and 32, which is called the measuring voltage u. m can be measured by the measuring system. To determine the course of the measured current i m To measure, the measuring system 1 includes a current meter A, which is connected in series to the signal source 7 and the resistor R. sThe measuring system 1 also includes a voltage measuring device V, which is connected in parallel to the measuring contacts 41, 42 and to the series circuit consisting of the signal source 7 and the resistor R. s and is arranged with the current meter A. The voltage meter V can be used, for example, to measure the course of the measured voltage u. m The current i is measured between the measuring contacts 41 and 42, or at the system 2 under test. m by the current measuring device A and the measurement of the course of the measuring voltage u m The voltage measurement V can be performed, for example, by means of discrete-time sampling. This means that the waveform of the measuring current i is recorded. m or the measuring voltage u m is available as a sequence of time-discrete sampled measurements, which were determined by sampling the respective curve at a sampling frequency.

[0033] In the case of an open contact – e.g., if no electrical system 2 is connected to the measuring contacts 41, 42 during the calibration of measuring system 1 or if it is idle, or in the case of a defective contact of the electrical system 2 – the test signal i t This also caused a voltage drop between the measuring contacts 41 and 42. This voltage drop can also be referred to as the measuring voltage u. m measured by the voltage meter V. A signal i is generated by the test signal. t generated measuring current i m or its course can again be measured using the current measuring device A, where the measuring current i m The value is approximately zero when the contact is open or when idling.

[0034] For an evaluation of the measured curves of measuring current i m and measuring voltage u mFor checking the contact, the measuring system 1 has an evaluation unit 8. The evaluation unit 8 can be implemented as a microcontroller-based measurement unit, similar to the current meter A and the voltage meter V. The evaluation unit 8 is configured to derive the amplitude of the measuring current i from the measurement results of the current meter A and the voltage meter V. m and the measuring voltage u m and / or a phase shift in the course of the measuring current i m as well as the course of the measuring voltage u m to determine. That is, the evaluation unit 8 extracts the course of the measuring current i measured by the current measuring device A. m as a measurement result of the current measuring device A, in order to calculate the amplitude of the measuring current i. m and / or the phase shift of the measured current i m to determine, or rather, to convert the course of the measured voltage V measured by the voltage measuring device in order to calculate the amplitude of the measured voltage u.m and / or the phase shift of the measuring voltage u m to determine this. For this purpose, the evaluation unit 8 can, for example, correlate the course of the measuring current i measured with the current measuring device A. m with the test signal i t and with the test signal i, which is phase-shifted by 90 degrees t and a correlation of the course of the measured voltage u measured with the voltage measuring device V m with the test signal i t as well as with the test signal i, which is phase-shifted by 90 degrees. t perform. Furthermore, the evaluation unit 8 can be configured to, for example, determine the phase shift of the measuring current i. m and / or the determined phase shift of the measured voltage u mThe respective reference phase shifts are compared. These reference phase shifts were determined, for example, during the calibration of measuring system 1 by means of an open-circuit measurement – ​​i.e., without a load or electrical system 2 connected to the measuring contacts 41, 42. This open-circuit measurement determines a phase shift, which is caused, for example, by the capacitances between the measuring leads (i.e., the leads between measuring system 1 and the measuring contacts 41, 42). The evaluation of the open-circuit measurement and the determination of the reference phase shifts can also be performed by the evaluation unit 8.

[0035] Furthermore, a display unit 9 can be provided, on which, for example, information (e.g., warnings, instructions, etc.) can be displayed for a user of the measuring system 1 during the contact check. Information, particularly in the form of a warning, can be displayed on the display unit 9, for example, if a deviation occurs between the phase shift of the measured current i. m and a respective comparison phase shift or a deviation between the phase shift of the measured voltage curve u m and a respective comparison phase shift falls below a specified tolerance. That is, the phase shift of the measuring current i m and / or measuring voltage u m with regard to the test signal i tThe measured value corresponds approximately to the respective phase shift, e.g., determined during idle operation. In this case, it is assumed that the electrical contact of electrical system 2 is defective or faulty. This information is intended to alert the user. In this case, it may also be possible for at least one action, such as repeating the measurement or aborting the contact check, to be triggered, e.g., by evaluation unit 9. The user can be informed of this, e.g., via display unit 9.

[0036] Figure 2 illustrates an example of the procedure for checking the electrical contact of the electrical system 2. The procedure is carried out when the measuring system 1 is connected to the electrical system 2. For this purpose, the measuring contacts 41, 42 of the measuring system 1 are connected to the measuring points 31, 32 of the electrical system. The periodic test signal i generated by the signal source 7 is transmitted via the measuring contacts 41, 42. t applied to the electrical system 2. A current source, in particular a regulated current source, is used as signal source 7, which generates a sinusoidal alternating current i. t with a predetermined frequency as a test signal i t generated. However, other periodic test signals are also generated. t , such as a rectangular current signal, is conceivable.

[0037] The test signal i t calls up a measuring current i m, which flows via the measuring contacts 41, 42 to the electrical system 1, as well as a measuring voltage u m , which drops between the measuring contacts 41, 42. In a measuring step 101, the course of the measuring current i is recorded. m The current meter A of measuring system 1 measures the current. Furthermore, in measuring step 101, the course of the measuring voltage u is also recorded. m Measured by the voltage measuring device V of measuring system 1. The measured course of the measuring current i m as well as the measured course of the measuring voltage u m have the same frequency as the test signal i t which of the specified frequencies of the test signal i t corresponds. The course of the measuring current i m and especially the course of the measuring voltage u m differ from each other and from the test signal i t at least in amplitude and / or phase. Furthermore, the measured curves can i m , u mThe measurement may be affected by disturbances (e.g., ripple voltages within measurement system 1, DC signal noise, uncorrelated noise, etc.). The measurement of the waveform of the measuring current i m and the course of the measuring voltage u m This can be done, for example, simultaneously and using discrete-time sampling.

[0038] In a determination step 102, an amplitude of the measuring current i is then derived in the evaluation unit 8 from a measurement result of the current measuring device A. m and / or a phase shift in the course of the measuring current i m determined. Furthermore, in determination step 102, in evaluation unit 8, an amplitude of the measured voltage u is derived from the measurement result of the voltage measuring device V. m and / or a phase shift in the course of the measured voltage u m determined.

[0039] For example, the measured course of the measuring current i can be displayed in the evaluation unit 8. m with the test signal i tand with the test signal i, which is phase-shifted by 90 degrees t correlated. Likewise, the measured course of the measuring voltage u m with the test signal i t and with the test signal i, which is phase-shifted by 90 degrees t can be correlated. This correlation allows the similarity of the course of the measured current i to be determined. m or the course of the measured voltage u m with the test signal i t as well as a phase shift between the course of the measuring current i m or the course of the measuring voltage u m and the test signal i t to be analyzed. In a time-discrete sampling of the waveforms of the measured current i m and measuring voltage u m The correlation corresponds to a discrete Fourier transform, which only applies to the specified frequency of the test signal i. t is carried out. For example, the test signal i is used. t or the test signal i that is phase-shifted by 90 degrees tpoint by point with the measured course of the measuring current i m or the measuring voltage u m - i.e., the time-discrete sampled measured values ​​of the measuring current i m or the measuring voltage u m - multiplied and over at least one period of the test signal i t or the test signal that is phase-shifted by 90 degrees i t summed up. Through correlation, interference and interference signals, such as hum voltages in measurement system 1, DC signal noise, uncorrelated AC noise, etc., can be suppressed, especially since the measured waveforms of measurement current i m and measuring voltage u m only on the specified frequency of the test signal i t They can be correlated. By suppressing interference signals and influences through correlation, it is possible to achieve the desired results despite the very high resistance R. sIn measuring system 1, the output resistance of the test signal is determined by the relatively small capacitances C of the electrical system 2, thus confirming correct contact.

[0040] In a discrete Fourier transform, for example, the respective correlation with the test signal i represents t the real part of the measuring current i m or the measuring voltage u m in the frequency domain at the position of the specified frequency of the test signal i t The correlation with the test signal i, which is phase-shifted by 90 degrees. t represents, for example, an imaginary part of the measuring current i m or the measuring voltage u m in the frequency domain at the position of the specified frequency of the test signal i t From this, the amplitude of the measuring current i can then be very easily determined by the evaluation unit 8. m or the measuring voltage u m and / or the phase shift of the measuring current i m or the measuring voltage um compared to the test signal i t - e.g., a sinusoidal alternating current with a known, predetermined frequency.

[0041] Furthermore, in investigation step 102, the evaluation unit 8 can determine the amplitude of the measuring current i. m and the amplitude of the measured voltage u m as well as from the phase shift of the measured current i m and the phase shift of the measured voltage curve u m A complex impedance of electrical system 2 is determined. From the impedance and the known, predetermined frequency of the test signal i, tThe resistance R and capacitance C of electrical system 2 can then be derived, for example, if a parallel connection of the resistance R and the capacitance C is assumed, as shown in Figure 1. Furthermore, in determination step 102 in the evaluation unit 8, a phase shift between the course of the measuring current i can also be determined. m and the course of the measuring voltage u m e.g. from the determined phase shifts of measuring current i m and measuring voltage u m relative to the test signal i t to be determined.

[0042] In an optional comparison step 103, the determined phase shift of the measured voltage u is then calculated in the evaluation unit 8. m compared with a corresponding reference phase shift. If there is a deviation of the determined phase shift of the measured voltage waveform u m relative to the test signal i tIf the phase shift of the measured voltage waveform is at least equal to or greater than a specified tolerance, then correct contact of the electrical system 1 can be assumed. If this specified tolerance is exceeded by the deviation between the determined phase shift of the measured voltage waveform u m relative to the test signal i t If, however, the reference phase shift is not met, a defective or faulty contact of the electrical system 1 can be assumed. In an optional output step 104, corresponding information can, for example, be displayed to the user on the display unit 9. Additionally or alternatively, it is possible to trigger at least one action, such as repeating the measurement, aborting the contact check, etc.

[0043] The comparison phase shift can be determined, for example, during the calibration of measurement system 1. The test signal i is used for this purpose. twith open measuring contacts - i.e., with unloaded measuring system 1, without a load or an electrical system 1 being connected to the measuring contacts 41, 42 - applied and the course of the resulting measuring voltage u m measured. From the course of the measuring voltage u m Then, as in investigation step 102, a phase shift of the measuring voltage u will be observed. m Determined under no-load conditions. This phase shift of measuring system 1, which can be caused, for example, by the capacitance of the measuring leads, can then be used as a reference phase shift for the measured voltage u. m can be used when checking the contact of electrical system 1.

Claims

Patent claims 1. Method for checking the electrical contact of an electrical system (2) with a measuring system (1) which is connected via measuring contacts (41, 42) to measuring points (31, 32) of the electrical system (2), wherein (1) a periodic test signal (i t ) is applied to the electrical system (2) via the measuring contacts (41 , 42) at a predetermined frequency, and wherein the test signal (i t ) a measuring current (i m ) and a measuring voltage (u m ) causes, characterized in that each course of a signal is caused by the test signal (i t ) induced measuring voltage (u m ) and a course of a signal driven by the test signal (i t ) caused measuring current (i m ) are measured (101), and that each measurement result yields an amplitude of the measured voltage (u m ) and the measuring current (i m) and / or a phase shift in the course of the measured voltage (u m ) and the course of the measuring current (i m ) with respect to the test signal (i t ) are determined (102).

2. Method according to claim 1, characterized in that information is output to a user and / or at least one action is triggered (104) when comparing the phase shift of the measured voltage curve (u m ) and / or the phase shift of the measured current waveform (i m ) with a respective comparison phase shift is determined (103), that there is a deviation between the phase shift of the measured voltage waveform (u m ) and the respective comparison phase shift and / or from a deviation of the phase shift of the measured current (i m ) and the respective comparison phase shift falls below a specified tolerance.

3. Method according to one of claims 1 or 2, characterized in that the determined amplitude of the measuring voltage (u) m ) and the determined amplitude of the measuring current (i m ) as well as the determined phase shift of the measured voltage curve (u m ) and from the determined phase shift of the measured current (i m ) an impedance (R, C) of the electrical system (2) is determined (102).

4. Method according to one of claims 1 to 3, characterized in that the periodic test signal (i t ) a sinusoidal current signal is used.

5. Method according to one of claims 1 to 4, characterized in that a measurement of the course of the measuring current (i m ) and / or the course of the measured voltage (u m ) is performed using discrete-time sampling (101).

6. Measuring system (1) for checking an electrical contact of an electrical system (2), wherein the measuring system (1) is connectable to measuring points (31, 32) of the electrical system (2) via measuring contacts (41, 42), and wherein the measuring system (1) has a signal source (7) for generating a periodic test signal (i t ) with a predetermined frequency, which can be applied to the electrical system (2) via the measuring contacts (41, 42), characterized in that the measuring system (1) is a voltage measuring device (V) for measuring a waveform of a voltage driven by the test signal (i t ) induced measuring voltage (u m ) and a current meter (A) for measuring the waveform of the test signal (i t ) caused measuring current (i m ) and that the measuring system (1) further comprises an evaluation unit (8) which is at least configured to derive an amplitude of the measuring voltage (u) from each measurement result m) and the measuring current (i m ) and / or a phase shift in the course of the measured voltage (u m ) and the course of the measuring current (i m to determine.

7. Measuring system (1) according to claim 6, characterized in that the measuring system (1) has a resistance (R) s ) which is arranged in series with the signal source (7).

8. Measuring system (1) according to one of claims 6 to 7, characterized in that the signal source (7) is designed as a current source, in particular as a controlled current source.

9. Measuring system according to one of claims 6 to 8, characterized in that a display unit (9) is provided, via which information can be output to a user.

10. Measuring system according to one of claims 6 to 9, characterized in that the measurement data acquisition is based on the current measuring device (A) and the voltage measuring device (V) as well as the evaluation unit (8) microcontroller.

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