Vehicle electrical system with high-voltage branch, low-voltage branch and low-voltage side insulation fault recognition

Abstract

The vehicle electrical system (FB) is equipped with a high-voltage branch (HV) and a first low-voltage branch (NV), which is current-isolated from the high-voltage branch (HV) by means of insulation (IN). The low-voltage branch (NV) has at least one low-voltage line (NL) leading to the high-voltage branch (HV). The low-voltage branch (NV) has a voltmeter (SM). The voltmeter is connected to the at least one low-voltage line (NL) via signal transmission. Furthermore, the voltmeter is configured to detect whether the voltage value of the at least one low-voltage line (NL) relative to the ground potential (M) of the vehicle electrical system (FB) is above a voltage limit. The voltage limit characterizes a voltage value greater than the maximum signal voltage of the low-voltage line, which the low-voltage line has during fault-free operation.

Description

Background Technology

[0001] Vehicles equipped with electric drive systems have drive components (“high-voltage components”) that operate at high voltages to achieve high power, thereby enabling high traction power. The vehicle also has other electrically operated components, such as onboard computers, safety devices, auxiliary systems, bodywork, and other control equipment with signal processing and signal transmission tasks, which operate at lower voltages (low voltage), typically 12V-14V or even 24V.

[0002] High protection requirements exist for high-voltage components to prevent dangerous contact voltages to humans. These protection requirements, for example, relate to insulation measures in high-voltage branches where high-voltage components are located. One objective is to demonstrate measures that can further protect vehicle users from dangerous contact voltages. Summary of the Invention

[0003] This task is solved by the subject matter of claim 1. Other features, characteristics, implementations, and advantages are described in the dependent claims, specification, and... Figure 1 The conclusion is as follows.

[0004] Because it is recognized that if low-voltage lines, such as data, signal, or other lines carrying low voltage (low-voltage lines), originate from high-voltage branches and are subjected to high-voltage potentials due to faults in the high-voltage branches, danger may also arise from these low-voltage lines, it is proposed to identify insulation faults not only on the high-voltage side but also on the low-voltage side in order to take protective measures. The measures described herein are intended to reduce the danger caused by the high contact voltages that may be present on these lines.

[0005] It is proposed that in a vehicle electrical system equipped with a high-voltage branch and an isolated (first) low-voltage branch, a voltmeter is used to monitor the voltage at the low-voltage line leading from the high-voltage branch on the low-voltage branch side. The voltmeter detects when the voltage (or its value) of the low-voltage line is at a predetermined voltage limit. This allows it to be determined, by means of the voltmeter, whether a dangerous high-voltage potential has been carried from the isolated high-voltage branch through the low-voltage line due to a fault, and thus, if necessary, whether a dangerous contact voltage exists, especially on the low-voltage side.

[0006] A vehicle electrical system is proposed, comprising a high-voltage branch and a low-voltage branch, wherein the high-voltage branch is current-isolated from the low-voltage branch by means of insulation. Specifically, the insulation may have insulated bridging devices (optocouplers, transformers, instrument transformers, current transformers, insulated digital interfaces) and / or insulating materials (insulating layers of wiring, printed circuit boards, connectors, etc.). Insulation faults can exist, for example, in undesirable conductive bridges between the low-voltage and high-voltage branches, such as bridges that connect the aforementioned devices or the insulating materials that isolate the current between the branches from one branch to another.

[0007] The low-voltage branch has at least one low-voltage line leading to the high-voltage branch, for example, to monitor operating parameters there, such as the plug status, voltage, power, temperature, or current on the low-voltage branch side, or to control components in the high-voltage branch. The low-voltage branch is directed to the high-voltage branch through the aforementioned insulation and, in operation without insulation failure, has no current connection to the high-voltage branch (due to the insulation). The low-voltage branch has a voltmeter connected to at least one low-voltage line in a signal transmission manner. In this case, a non-current-isolated connection between the line and the voltmeter, i.e., a direct connection, a connection via a series resistor, or a connection via a voltage divider, is referred to as a "connection in a signal transmission manner."

[0008] A voltmeter is configured to detect whether the voltage value between at least one low-voltage line and the ground potential of the vehicle's electrical system is above a voltage limit. This voltage limit characterizes a voltage that would not normally occur under fault-free operating conditions, and particularly characterizes a voltage that occurs at the low-voltage line due to contact with a high-voltage potential. The voltage limit characterizes a voltage value caused by a fault, which is greater than the maximum signal voltage of the low-voltage line, which the low-voltage line has under fault-free operation. The maximum signal voltage of the low-voltage line corresponds, for example, to the maximum voltage level of the associated low-voltage line. The voltage limit may also characterize values ​​from which human hazards can be assumed, such as 45V, 50V, or especially 60V. The voltmeter serves as a detection device for voltages that are above the voltage of the low-voltage branch due to a fault in the high-voltage branch and that are applied at the low-voltage line. The voltage detected by the voltmeter relates to the potential difference between the low-voltage line and a reference potential, such as ground (of the low-voltage branch) or the supply potential of the low-voltage branch (e.g., +12V).

[0009] The prefix "low voltage" specifically indicates voltages not exceeding 60V. The prefix "high voltage" specifically indicates voltages exceeding 60V, such as at least 100V, 200V, 400V, or 800V.

[0010] Low-voltage lines can be sensor lines leading to current sensors, shunts, temperature sensors, voltage taps, or other components located within high-voltage electrical systems. Therefore, at least one low-voltage line can have (at least) one sensor line connected in a signal transmission manner to a sensor or a voltage tap of a high-voltage branch or an interlocking connection of a high-voltage branch. In particular, the sensor line can lead to an interlocking circuit arranged at a plug assembly in the high-voltage branch for monitoring the plug assembly. If the relevant monitoring unit is located in the low-voltage branch, a corresponding low-voltage line is drawn from the high-voltage branch to the low-voltage branch to connect the monitoring unit to the interlocking circuit. A voltmeter enables monitoring of the low-voltage line as well. In particular, non-current-isolated connections are referred to as "in a signal transmission manner," and these connections are drawn from the high-voltage branch via a voltage divider or directly. Even when operating inverters, rectifiers, or DC voltage converters of high-voltage electrical systems in a current-isolated manner, there is a line section from the low-voltage electrical system leading to the current-isolated element, which is monitored by a voltmeter and thus represents the low-voltage line as described herein.

[0011] The low-voltage line may also have control or data lines connected to the data source or destination of the high-voltage electrical system via signal transmission, such as CAN bus, battery management device or inverter, voltage converter or motor lines, which are led out from the high-voltage branch. For example, it may be a control line for an inverter, for a voltage converter, for a motor, for a high-voltage box or battery, which is located in the high-voltage electrical system or guides high voltage as the operating voltage.

[0012] Alternatively, at least one low-voltage line may be a low-voltage power supply line connected to a (sub)component of a high-voltage electrical system to supply low voltage to the (sub)component.

[0013] At least one low-voltage line is preferably connected to the input of the voltmeter via a series resistor. A varistor may also be connected to at least one low-voltage line, particularly having a breakdown voltage corresponding to a voltage limit. The varistor connects the low-voltage line to a reference potential, preferably to the ground potential of the vehicle's electrical system. The series resistor is connected to the input of the voltmeter, particularly in a switchable manner. In this case, a switch, such as a transistor or electromechanical switch, can be connected downstream of the series resistor, the switch leading to the input of the voltmeter. If a connection exists from the series resistor to the reference potential (e.g., ground) via a shunt resistor, the switch can only be closed temporarily, preferably repeatedly, and especially periodically, to avoid persistent discharge via the shunt resistor, for example, during the ruhephase of the vehicle's electrical system.

[0014] The evaluation of multiple low-voltage lines using a voltmeter can also be bundled together. In this case, multiple low-voltage lines are each connected to the input terminal of the voltmeter via a series resistor (as described, a varistor can be connected to the series resistor), i.e., connected to the same input terminal of the voltmeter. If only one of the low-voltage lines has too high a voltage (relative to ground or other reference potentials in the low-voltage branch), it can be detected by the voltmeter that at least one line in the line has a critical potential. The varistor is also used to discharge energy storage devices, such as Cy capacitors, in the high-voltage branch; in a symmetrical configuration, a Cy capacitor connected to a high-voltage potential with an insulation fault is discharged through the varistor, with charge transferred to another Cy capacitor. Therefore, the varistor can also be used to recharge one Cy capacitor to another Cy capacitor (at another high-voltage potential). This function of the varistor does not affect the identification of insulation faults by the voltmeter, because the voltage applied at the varistor is sufficient to identify excessively high voltage values.

[0015] The vehicle's electrical system, and particularly the low-voltage branch, can have a fast-shutdown signal switch. A voltmeter can be operatively connected to the fast-shutdown signal switch. In particular, the input terminal of the voltmeter can be operatively connected to the fast-shutdown signal switch. In other words, the low-voltage line is operatively connected to the signal switch to (indirectly) control the signal switch based on the voltage at the low-voltage line. If the voltage at the low-voltage line is above a voltage limit, the signal switch is closed and a signal is generated as follows. In other words, at least one low-voltage line can be connected to the control input terminal of the fast-shutdown signal switch in a signal transmission manner, for example, via a series resistor, so as to switch the fast-shutdown signal switch by means of a signal applied to the low-voltage line (especially closing the switch). By switching, a fast-shutdown signal is generated, which is transmitted to the fast-shutdown unit.

[0016] Preferably, the fast shutdown signal switch is connected between a ground potential or another reference potential of the vehicle's electrical system and the signal connection of the fast shutdown unit leading to the high-voltage electrical system. The fast shutdown signal switch can therefore be connected between a reference potential, such as ground, and the fast shutdown line. If a voltage exceeding the aforementioned limit exists at the low-voltage line, the signal switch is closed by the voltage level at the input of the fast shutdown signal switch (which, if necessary, corresponds to the voltage level at the input of a voltmeter). This signal switch then connects the fast shutdown line to a reference potential, such as ground, and thereby generates a signal in the fast shutdown line that causes fast shutdown.

[0017] The fast turn-off unit is also known as a "fast turn-off" (FTO) unit. The signal corresponds to the FTO signal, which is used for fast turn-off. Closing the fast turn-off signal switch on the fast turn-off line or in the fast turn-off unit of a high-voltage electrical system results in a signal that instructs the fast turn-off unit to immediately turn off or deactivate the high-voltage electrical system, for example, by discharging the high-voltage electrical system. The fast turn-off signal switch can be a transistor, with its control input (base, gate) connected to the output of a voltmeter. The fast turn-off signal switch is configured and connected such that it closes only when the voltage at the low-voltage line is above the voltage limit, and otherwise remains open. The input of the voltmeter is connected to the fast turn-off signal switch in an actuated manner. Therefore, the voltage or potential at the voltmeter controls the signal switch. In other words, the fast turn-off signal switch is controlled by the voltage across the shunt resistor. Thus, the voltage applied to the voltmeter is used, on the one hand, for detection by the voltmeter for subsequent evaluation, and on the other hand, to control the aforementioned signal switch, which closes according to the applied voltage. The quick-shutdown unit can be installed in the high-voltage branch, but it can also be installed in the vehicle's electrical system, or, if necessary, outside the aforementioned electrical system.

[0018] The vehicle electrical system may also have a signal unit. This signal unit is particularly located in the low-voltage branch. The signal unit is configured to issue an insulation fault signal when the voltage value exceeds a voltage limit. An insulation fault signal indicates that the voltage value at the low-voltage line exceeds a given voltage limit, and therefore an insulation fault (in the high-voltage branch) causes the low-voltage line to conduct voltage above the voltage limit (relative to a reference potential, such as ground) in a faulty manner. The insulation fault signal can be an electrical signal, which is transmitted to a higher-level control device or display, or also to the charging control device of the high-voltage branch. Alternatively, or in combination with this, the insulation fault signal can be an optical and / or acoustic signal.

[0019] The implementation of the vehicle electrical system includes a de-shaking device configured to suppress voltage values ​​exceeding a voltage limit during the de-shaking duration, either by temporarily suppressing a corresponding voltage signal emitted by a voltmeter or by suppressing an insulation fault signal during the de-shaking duration. If an excessively high voltage or insulation fault signal persists after the de-shaking duration, a voltage signal or insulation fault signal emitted by a voltmeter is emitted as described to identify the insulation fault.

[0020] The voltmeter can be an analog-to-digital converter (ADC). The ADC may have a measurement input (hereinafter referred to as the input terminal), which is connected to at least one low-voltage line in a signal transmission manner (especially to current ground or via a series resistor). The input terminal is preferably connected to ground or another reference potential of the low-voltage branch via a shunt resistor. A voltage divider is thus derived, connected between ground or other reference potential and the low-voltage line. The voltage divider has a series resistor and a shunt resistor as series connections for voltage division, and the voltage divider has a tap at the connection point between the series resistor and the shunt resistor, the tap being connected to the input terminal.

[0021] The shunt resistor is typically equipped with a resistance value significantly larger than that used for current sensing, and can be, for example, greater than 1 kOhm, 10 kOhm, or 100 kOhm. This series resistor, or these series resistors, can be greater than 100 kOhm, greater than 1 MOhm, or greater than 10 MOhm. The shunt resistor and series resistor reduce the voltage applied across them by a factor not exceeding 10%, 5%, or 1%, where such a reduced voltage is applied across the shunt resistor.

[0022] Furthermore, it can be specified that the sum of the resistance values ​​of either the shunt resistor or the series resistor is less than the resistance value of the current that would cause the nominal voltage of the high-voltage branch to exceed the trigger current of the insulation detector on the high-voltage branch side when the nominal voltage of the high-voltage branch is applied at the relevant low-voltage branch. Thus, excessively high potentials at the low-voltage line can be identified not only by the voltmeter described herein but also by the insulation detector. Attached Figure Description

[0023] Figure 1 , 2 3 is used to describe the vehicle electrical system described herein. Detailed Implementation

[0024] Figure 1 The vehicle electrical system FB shown has a high-voltage branch HV and a low-voltage branch NV. Branches NV and HV are separated from each other by means of insulation IN. Electrical insulation IN is symbolically shown and may correspond to a transformer and / or an electrical insulation layer.

[0025] Multiple low-voltage lines NL extend from the high-voltage branch into the low-voltage branch. Exemplary examples show signal lines (e.g., for transmitting signals from sensors located within the high-voltage branch), data lines (for transmitting control signals and / or communication or bus signals) originating from or entering the high-voltage branch HV, and power supply lines for transmitting low-voltage supply voltage between branches NV and HV. For example, these low-voltage lines may be 12V+ power supply lines, communication lines (e.g., CAN bus lines), HV interlock circuit lines, or signal lines to terminal 15 (i.e., the connected 12V+ potential) of the vehicle's electrical system.

[0026] The low-voltage line NL can be connected to the input terminal E of the voltmeter SM via a switch S and a series resistor R, as exemplarily shown using line SL. As exemplarily shown using lines DL and VL, the low-voltage line NL can be connected to the input terminal E of the voltmeter SM without switching via the series resistor R. Furthermore, as exemplarily shown using line XL, the low-voltage line NL can be connected to the input terminal E of the voltmeter SM via a parallel circuit of a series resistor R and a varistor V, the line XL leading to the low-voltage section of the components of the electrical system branch HV. The varistor has a breakdown voltage less than the contact voltage hazardable to humans, and therefore generates a current when this voltage is reached, which is detected by a high-voltage side insulation monitor. Thus, the current flowing through the varistor has triggered fault identification (on the high-voltage side insulation monitor side).

[0027] The voltmeter SM outputs a signal at output terminal A, which reproduces the voltage at input terminal E of the voltmeter. In the illustrated embodiment, this voltage is transferred to the signal unit ME of the vehicle electrical system FB. The signal unit ME is able to evaluate the signal at output terminal A, and in particular detect whether the voltage value reproduced by the signal allows the deduction of a voltage value relative to ground M at one of the low-voltage lines, which is above the voltage limit (e.g., 30V, 50V, or 60V). The signal unit ME is also able to consider the resistance values ​​of R and SH and their wiring as a voltage divider (i.e., voltage division via R and SH) when evaluating the voltage value emitted by the signal from output terminal A of the voltmeter SM. Therefore, the signal unit compares the value of the voltage applied at line NL relative to ground M (or another reference potential) with the voltage limit, and in particular issues an insulation fault signal when the voltage limit (e.g., 60V) is exceeded or is exceeded. If the low-voltage line operates at a signal level of 0V-xV, the voltage limit is above the maximum level of xV. For communication lines as line NL, x can be 5V, 10V, 12V, or 15V. Depending on the gate or base voltage required to control the transistor, x can be in the range of 10 volts, 12 volts, 15 volts, or 18 volts for a control circuit as line NL, such as for a high-voltage transistor in a high-voltage branch.

[0028] Furthermore, as shown in the high-voltage branch HV, a fast turn-off unit (FTO) can be installed. This fast turn-off unit can disconnect the high-voltage branch HV or the high-voltage energy source in the event of an insulation fault (e.g., detected by a high-voltage insulation monitor). The fast turn-off unit FTO can be installed within the high-voltage branch, as indicated by the reference numeral FTO, but it can also typically be installed within the vehicle's electrical system, or, if necessary, outside the aforementioned electrical system, as indicated by the reference numeral FTO'.

[0029] The circuit shown allows direct intervention of the fast shutdown unit FTO by controlling the fast shutdown signal switch T with a potential applied at line NV. The example specified in the diagram indicates that the fast shutdown signal switch T connects a reference potential, such as ground M, in a switchable manner to the signal input of the fast shutdown unit FTO or the line of the pilot signal (i.e., the line of the pilot fast shutdown signal of the vehicle's electrical system). For this purpose, a signal connection SV exists between switch T and the fast shutdown unit FTO. This connection can typically exist between switch T and the line of the pilot fast shutdown signal (FTO signal).

[0030] In the example shown, the fast-shutdown signal switch T is switched by means of the (common) potential of resistor R or by means of a voltage applied across SH (typically: using the voltage applied at line NL), as indicated by the arrow leading to T. The fast-shutdown signal switch T is a normally open contact or a latching transistor. Therefore, a signal from at least one of the lines NL is directly or indirectly fed to the control input TE of the fast-shutdown signal switch T so that it closes according to the signal when the signal corresponds to a voltage sufficient for switching. The switching point of the fast-shutdown signal switch T is set such that the fast-shutdown signal switch T closes when a voltage limit is reached at one of the lines NL.

[0031] By closing the fast shutdown signal switch T, the potential of the line that guides the FTO signal is set to a reference potential (here: ground M). This potential or level corresponds to the fault signal that triggers the fast shutdown unit FTO.

[0032] The fast-shutdown signal switch T can have a transistor (signal transistor) as a switch. Specifically, the fast-shutdown signal switch T can have a series resistor leading to the control input terminal (base) of the transistor. The end of the series resistor opposite to the base can be connected to a shunt resistor, for example, to the end of the shunt resistor opposite to the reference potential (ground M). The transistor can be connected to the reference potential (ground M) via a resistor (emitter resistor). Furthermore, a resistor connecting the base and emitter can be provided to adjust the operating point of the transistor. This results in a collector circuit with operating point adjustment for the fast-shutdown signal switch T. If the fast-shutdown signal switch T has a MOSFET, the corresponding wiring for the MOSFET can be provided. Additionally, other transistors (with additional wiring if necessary) can be provided, these other transistors being at a negative potential in one of the lines NL, the negative potential corresponding to a negative voltage value above the voltage limit. Therefore, other transistors can be provided in a complementary manner to the transistors described above.

[0033] Figure 2 and 3 This describes possible physical aspects of the vehicle electrical system FB. In addition to the first low-voltage branch NV, a second low-voltage branch NV' may also exist, wherein at least one low-voltage line extends from the first low-voltage branch NV into the second low-voltage branch NV'. If the low-voltage line has a dangerous potential, this danger is transmitted to the second low-voltage branch via that line, even if a housing GE is provided, wherein the second low-voltage branch is housed within the housing GE, while the first low-voltage branch NV is located outside the housing GE, and at least one low-voltage line NL', NL''" passes through the housing entrance of the housing GE. ZG extends into the housing and thus into the second low-voltage branch.

[0034] At least one low-voltage line has taps P and P', through which a voltmeter SM is connected. The taps are located at points on the line where the line begins to extend in multiple directions, i.e., at branching points. Whether taps P and P' are inside or outside housing P' (as shown with P), they are preferably located directly at the housing shell layer of housing GE or at the housing entrance ZG. Tap P can be arranged in an interface device located outside housing GE, see interface device U, or it can be arranged in an interface device located inside housing GE, wherein the interface device, such as a cable feeder or plug connection element, is directly adjacent to or together with the housing entrance. Therefore, taps P and P' are located directly at the beginning of the second low-voltage branch NV' or at the beginning of the housing space where taps P and P' are located. This ensures that overvoltages on the line can be monitored by means of a voltmeter when entering the second low-voltage branch NV'. In particular, a voltmeter, at least one series resistor, and / or a shunt resistor can also be directly arranged at the location of the tap. An interface device can be provided, in which taps P and P' are provided. The interface device can be located externally, see reference numeral U. Alternatively, or in combination therewith, the interface device can be located internally, see reference numeral U'.

[0035] Furthermore, the first low-voltage branch NV and the high-voltage branch HV can be located in the high-voltage housing HB, particularly within the housing of a high-voltage device, which can be referred to as a high-voltage box. The high-voltage housing HB or high-voltage box has a low-voltage line connection terminal AN. This can be a plug connection terminal for low-voltage signals and / or low-voltage supply voltages. At least one low-voltage line NL has a tap, through which a voltmeter SM is connected to at least one low-voltage line NL. This tap is constructed, in particular, as described above for voltage taps. Tap P is located directly in or at the low-voltage line connection AN (e.g., a plug connector), or alternatively in the interface device U”, which is vorgeleitet in front of the low-voltage line connection AN. The housing HB is preferably conductive and connected to ground. This ensures high protection against high contact voltages resulting from faults in the high-voltage box, as the tap is directly positioned at the outlet of the low-voltage line NL from the high-voltage box, and thus the line is directly monitored for excessively high contact voltages after the outlet. A voltmeter SM, at least one series resistor R, and a shunt resistor SR (and, if necessary, a varistor V) are also preferably arranged there.

[0036] One embodiment specifies that a voltmeter SM, at least one series resistor R, and a shunt resistor SR (or, if necessary, a varistor V) are housed within a housing, thus constituting a high-voltage safety device. Preferably, a tap is also housed within the high-voltage safety device, wherein alternatively, the high-voltage safety device has a connection terminal for connecting to at least one tap or for connecting to at least one low-voltage line. Furthermore, a signal unit ME may also be housed within the high-voltage safety device. Communication equipment may also be housed within the high-voltage safety device. At least one low-voltage line may be routed through the high-voltage safety device or may be located externally. The high-voltage safety device may be located within... Figure 2 and 3 The components designated by U, U', U”, or AN are placed in places where they are used, especially in place of or in combination with these components. Such high-voltage safety devices for monitoring low-voltage lines can be physically... A standalone device, which can also be connected to the opposite end of the low-voltage line to the section or end of the high-voltage branch HV.

Claims

1. A vehicle electrical system (FB) having a high-voltage branch (HV) and a first low-voltage branch (NV), which is galvanically separated from the high-voltage branch (HV) by means of insulation (IN), wherein the low-voltage branch (NV) has at least one low-voltage line (NL) which leads to the high-voltage branch (HV), wherein the low-voltage branch (NV) has a voltage meter (SM) which is connected in a signal-conducting manner to the at least one low-voltage line (NL) and is set up to detect whether a voltage value of the at least one low-voltage line (NL) with respect to a ground potential (M) of the vehicle electrical system (FB) is above a voltage limit, which characterizes a voltage value which is greater than a value of a maximum signal voltage of the low-voltage line, which has the maximum signal voltage in fault-free operation, wherein the at least one low-voltage line has a control or data line (DL) which is connected in a signal-conducting manner to a data source or sink of the high-voltage electrical system and / or has at least one low-voltage supply line (VL) which is connected to a component of the high-voltage electrical system, and wherein the at least one low-voltage line (NL) is connected to an input (E) of the voltage meter (SM) via a series resistor (R) or via a series resistor (R) to which a voltage-dependent resistor (V) is connected.

2. The vehicle electrical system (FB) according to claim 1, wherein the at least one low-voltage line has a sensor line (SL) which is connected in a signal-conducting manner to a sensor or to a voltage tapping of the high-voltage branch (HV) or to an interlock connection of the high-voltage branch.

3. The vehicle electrical system (FB) according to claim 1 or 2, wherein the at least one low-voltage line (NL) is connected to the input (E) of the voltage meter (SM) in a switchable manner.

4. The vehicle electrical system (FB) according to claim 1 or 2, wherein a plurality of low-voltage lines (NL) are each connected to the input (E) of the voltage meter (SM) via a series resistor (R).

5. The vehicle electrical system (FB) according to claim 3, wherein the series resistor (R) is connected to a ground potential (M) of the vehicle electrical system (FB) via a shunt resistor (SH). ​ ​ 6. Vehicle electrical system (FB) according to claim 1 or 2, having in addition a fast-off signal switch (T), wherein the input (E) of the voltmeter (SM) is connected in a controlled manner with the fast-off signal switch (T) or the at least one low-voltage line is connected in a signal-transmitting manner with a control input of the fast-off signal switch (T), in order thereby to control the fast-off signal switch (T) by means of a signal applied at the input (E) of the voltmeter or at the low-voltage line (NL), wherein the fast-off signal switch (T) is connected between the earth potential (M) or another reference potential of the vehicle electrical system (FB) on the one hand and a signal connection (SV) on the other hand, which leads to a fast-off unit (FTO) of the high-voltage electrical system.

7. Vehicle electrical system (FB) according to claim 1 or 2, wherein the low-voltage branch (NV) has a signal unit (ME), which emits an insulation fault signal if the voltage value is greater than the voltage limit.

8. Vehicle electrical system (FB) according to claim 7, wherein the signal unit (ME) is set up to emit an insulation fault signal if the voltage value is greater than the voltage limit at least over a predefined debounce duration.

9. Vehicle electrical system (FB) according to claim 1 or 2, wherein the voltmeter (SM) is an analog-digital converter, which has a measurement input, which is connected in a signal-transmitting manner with the at least one low-voltage line.

10. Vehicle electrical system (FB) according to claim 1 or 2, having in addition to the first low-voltage branch (NV) a second low-voltage branch (NV'), wherein the at least one low-voltage line (NL', NL") extends from the first low-voltage branch (NV) into a housing (ZG) of the second low-voltage branch (NV') and wherein the at least one low-voltage line (NL', NL") has a tap (P, P'), via which the voltmeter (SM) is connected to the at least one low-voltage line (NL', NL"), wherein the tap (P) is located directly at a housing shell or a housing inlet (ZG) of the housing (GE) and inside or outside the housing (GE).

11. Vehicle electrical system (FB) according to claim 1 or 2, wherein the first low-voltage branch (NV) and the high-voltage branch (HV) are located in a high-voltage housing (HB), which has a low-voltage line connection (AN), wherein the at least one low-voltage line (NL) has a tap, via which the voltmeter (SM) is connected to the at least one low-voltage line (NL), wherein the tap (P) is located in or directly at the low-voltage line connection (AN).

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