Determining the earth fault resistance at a terminal of a device

Abstract

In a method for determining an ohmic earth resistance (20,a,b) of a pole (10a,b) of a device (4), a measuring circuit (22) is connected to the pole (10a,b) which contains a voltage source (30) with source resistance (32) in parallel to a measuring resistor (34), a first and second measuring voltage (UMess1,2) are determined at the measuring resistor (34), and at least one value of the measuring circuit (22) is changed and the earth resistance (20,a,b) is determined from the two measuring voltages (UMess1,2) and the first and second values. The measuring circuit (22) contains the voltage source (30) with source resistance (32) in parallel to the measuring resistor (34). A measuring arrangement (40) includes the measuring circuit (22) and an evaluation unit (42) for carrying out the procedure. The device (4) with the poles (10a,b) contains the measuring circuit (22) or measuring arrangement (40).

Description

[0001] The invention relates to devices having poles or connecting poles, wherein at least one, several, or all of the connecting poles are intended to be kept potential-free. In other words, a ground fault of the corresponding poles must be avoided, i.e., an ohmic earth resistance from the pole to a reference potential (earth) must be greater than a minimum value. State of the art

[0002] From DE 10 2010 028 022 A1 a device and a method for monitoring a loudspeaker line are known, wherein a main module arranged at a first point of the loudspeaker line and at least one monitoring module arranged at a second, remote point of the loudspeaker line communicate, wherein the impedance between the loudspeaker line and a reference ground is changed by the transmitting communication participant in order to transmit a digital communication signal. Disclosure of the invention

[0003] One subject of the invention is a method according to claim 1. Preferred or advantageous embodiments of the invention and other categories of invention emerge from the further claims, the following description and the attached figures.

[0004] The method is used to determine, or results in, the determination of a value for ohmic earth resistance (and thus making it available for further processing, etc.). Earth resistance is the ohmic resistance that exists between a pole of an electrical device and a reference potential, in particular earth potential. The device therefore has at least one pole or “connection pole”. The pole serves to electrically connect the device to other elements, e.g. another device. For this purpose, for example, an electrical cable is connected to the pole, which then leads away from the device. At least one, and in particular several or all of the poles of the device must be kept potential-free as intended, i.e. the earth resistance of the pole must have a minimum value and, depending on legal / national / etc. specifications, must not be less than 50 kOhm or 100 kOhm.

[0005] The procedure therefore requires a corresponding device or the procedure is carried out on the device.

[0006] In this method, a connection point of a measuring circuit is electrically connected to at least one, in particular exactly one, of the poles. "Connection" here always means: directly or indirectly, in particular with the interposition of an electrical line, but no other electrical components such as resistors or capacitors, etc. The measuring circuit thus includes the connection point. A corresponding measuring circuit is therefore also required for the method, or is used in the method.

[0007] The measuring circuit contains a parallel circuit leading from the connection point to the reference potential. The parallel circuit contains a first branch. This branch contains a series connection of a voltage source and an ohmic source resistor. The parallel circuit contains a second branch connected in parallel to the first branch. This second branch contains an ohmic measuring resistor. Both branches are connected in parallel between the connection terminal and the reference potential, or a node having the reference potential or connected to the reference potential.

[0008] In particular, the measuring circuit does not contain any components other than those mentioned.

[0009] The method involves performing an initial measurement: A first measurement voltage is determined or measured across the measuring resistor at initial values ​​of the measuring circuit. "Initial values ​​of the measuring circuit" means that both a first value of the source voltage generated by the voltage source and a first value of the source resistance as well as a first value of the measuring resistor are present. All values ​​are non-zero.

[0010] The method then performs a second measurement: A second measurement voltage is determined or measured across the measuring resistor at two values ​​of the measuring circuit. "Second values ​​of the measuring circuit" means that both a second value of the source voltage generated by the voltage source, a second value of the source resistance, and a second value of the measuring resistor are present. All values ​​are non-zero.

[0011] The following applies to both measurements: the voltage source generates a source voltage in the form of a direct voltage.

[0012] From the first to the second measurement, or from the first to the second values ​​of the measuring circuit, at least one of the values ​​of the source voltage of the voltage source and / or the source resistance and / or the measuring resistance changes. This means that the first and second values ​​of the source voltage are different and / or the first and second values ​​of the source resistance are different and / or the first and second values ​​of the measuring resistance are different.

[0013] In this method, the earth resistance value is then determined from the two measured voltages and the first and second values. This is typically achieved by determining equations from equivalent circuit diagrams of the device and measuring circuit, and evaluating the equations.

[0014] To formulate the equations, voltages, resistances, and currents along a measurement path are considered in particular. The measurement path leads from the reference potential through the measuring circuit, via the connection point, the pole, and the earth resistance back to the reference potential. The measurement path is closed via the reference potential to form a circuit.

[0015] Thanks to the method according to the invention, it is possible to set up two equations that differ in terms of the values ​​of the measuring circuit. This makes it possible to eliminate an unknown in the equations. This is, in particular, the value of a DC voltage from a source of interference in the form of a DC voltage source. The only prerequisite is that the value of the DC voltage / interference voltage of the interference source to be eliminated is the same for both measurements.

[0016] In other words, two equations are created based on the two measurements at different values ​​of the measuring circuit, from which a known unchanged value or quantity can be eliminated. Therefore, the eliminated value or quantity does not need to be known; the earth resistance can still be determined safely and reliably.

[0017] For the purposes of the invention, the earth resistance is assumed to be a concrete, individual equivalent circuit element in an equivalent circuit diagram, but its value can also be infinite. This means that, strictly speaking, such an earth resistance does not have to actually exist. The earth resistance can be composed of various components. These can be present inside or outside the device. For the present considerations, the equivalent circuit diagram assumes a single concentrated element as the earth resistance, which then combines all actually present components.

[0018] The earth resistance is that of the device or pole itself and does not include any components of the measuring circuit connected to the pole. In this case, the measuring resistance has a finite value for both measurements, especially permanently, and therefore does not represent an open circuit. The second branch is therefore not an open circuit between the connecting pole and the reference potential.

[0019] At the end of the process, the determined earth resistance value is provided, e.g., transmitted for further processing or stored in a memory for readability, etc. In particular, the determined earth resistance value is compared with a predefined minimum threshold value. If the comparison shows that the determined earth resistance value is lower than the predefined minimum threshold value, an alarm, such as an acoustic and / or visual alarm, is preferably issued.

[0020] In a preferred embodiment of the invention, ground potential is chosen as the reference potential. This makes the method particularly easy to implement, since the measuring circuit only needs to be connected to ground potential.

[0021] In a preferred embodiment of the invention, the value of the source voltage remains constant from the first to the second values ​​of the measuring circuit. In other words, the value of the source voltage is the same and not equal to zero for both measurements. The method or measuring circuit therefore does not require a voltage source whose generated voltage is variable, making it particularly simple.

[0022] In a preferred embodiment of the invention, the values ​​of the source resistance and the measuring resistance in the measuring circuit are selected to be different from each other. This allows the measuring circuit to be particularly well adapted to the device in terms of circuitry.

[0023] In a preferred variant of this embodiment, the values ​​of the source resistance and the measuring resistance are swapped from the first to the second values ​​of the measuring circuit. This has the advantage that the impedance of the measuring circuit (input impedance viewed from the pole into the device) remains the same for both measurements. In other words, the input / output impedance of the device does not change from the perspective of the poles, despite the included measuring circuit and the changed values ​​of the measuring circuit between the two measurements. Thus, despite the measuring properties of the measuring circuit and even during the execution of the method, a device with a measuring circuit is available that has an unchanged electrical impedance with respect to its poles.

[0024] In a preferred embodiment of the invention, the measuring resistor and / or the source resistor contain at least two partial resistors connected in series. The value of the measuring resistor or the source resistor (if each is implemented with partial resistors, in this case "and / or") can then be varied by bridging one of the partial resistors using a switch. In other words, closing the switch can create a short circuit across the partial resistor, thus "removing" it from the measuring circuit in terms of its circuitry or resistance effect. This makes variable measuring resistors or source resistors particularly easy to implement.

[0025] In a preferred embodiment of the invention, an interference source is contained in a measuring path already explained above (this is considered in the equivalent circuit diagram), which leads from the measuring circuit / connection point via the earth resistance to the reference potential (and also through the measuring circuit from the connection point to the earth potential). The interference source is therefore located between the connection point and the earth potential outside the measuring circuit in the measuring path. The interference source has an interference voltage in the form of a direct voltage. In other words, a direct voltage source is contained in the measuring path, at least in its equivalent circuit diagram. The value of the earth resistance is then determined from the two measuring voltages and the first and second values ​​of the measuring circuit in such a way that the value of the interference voltage is eliminated during the determination. This is done, as explained above, by setting up two equations and eliminating the interference voltage, for example, by adding or subtracting the equations.As already indicated above, the earth resistance can still be determined from both measurements even in the presence of a corresponding interference source. This assumes that the interference voltage remains unchanged in both measurements, as explained above.

[0026] A further subject of the present invention is the above-explained measuring circuit according to patent claim 8. This serves to determine the value of the earth resistance of the pole of the electrical device as explained above. The earth resistance exists, as explained above, between the pole and a reference potential. The measuring circuit contains, as explained above, the connection point, which is electrically connectable or connected to the pole of the device. The measuring circuit contains the parallel circuit connected between the pole and the reference potential, consisting of the series connection of the voltage source and the ohmic source resistance on the one hand, and the ohmic measuring resistance on the other. At least one of the values ​​of the voltage source and / or the source resistance and / or the measuring resistance is variable.

[0027] The measuring circuit and at least some of its possible embodiments as well as the respective advantages have already been explained in connection with the method according to the invention.

[0028] The measuring circuit is particularly intended for use in the above-mentioned device in order to be able to carry out the method according to the invention with its aid.

[0029] A further subject of the present invention is a measuring arrangement according to claim 9. This arrangement contains the measuring circuit according to the invention and an evaluation unit. The evaluation unit is configured to carry out the above-mentioned method. The evaluation unit is, for example, a measuring and computing device with measuring capability for the quantities to be used or measured in the method (measurement voltage) and computing capability for determining the earth resistance.

[0030] The measuring arrangement is therefore suitable for carrying out the corresponding procedure in conjunction with the device explained above and thus for determining the value of the earth resistance.

[0031] A further subject of the invention is the device according to claim 10, as already explained above. This device has at least one pole that is intended to be kept potential-free. The device also contains the measuring circuit or the measuring arrangement explained above. The connection point of the measuring circuit is electrically connected to at least, in particular precisely, one of the poles.

[0032] The device and at least some of its possible embodiments as well as the respective advantages have already been explained in connection with the method according to the invention, the measuring circuit and the measuring arrangement.

[0033] In a preferred embodiment, the device is a component of or for an audio system, in particular a public address and / or voice alarm system. For example, the device is a main module or monitoring module of a loudspeaker line. The invention is particularly useful or suitable for such components.

[0034] In a preferred embodiment, the device has at least two poles which are or can be connected to one another via a direct current connection, particularly internally within the device. By connecting the measuring circuit to one pole, the earth resistance of both poles can be determined in a single process run as explained above (first and second measurements). If the measuring resistance is above the minimum required values, it can be ensured with the help of a single process run that both poles are potential-free as intended. If the determined earth resistance is below the minimum required value, it may not be possible to identify which of the poles has an earth fault, where and how. However, the lack of safety of the device in the form of the inadmissibly low earth resistance can be determined in a single process run / with a single measuring cycle (two measurements), and the device can be, for example,be taken out of service for safety reasons.

[0035] The determination method according to the invention also works if other components of a device system (e.g., the aforementioned audio system) are connected to the poles as intended. Using this method, the earth resistance of the entire system currently connected / configured to the poles can be determined with reference to the respective pole.

[0036] In a preferred variant of this embodiment, the DC connection contains the aforementioned interference source, which has the interference voltage in the form of a DC voltage, i.e., a DC voltage source. Thanks to the method / measuring circuit according to the invention, this source can be mathematically eliminated as explained, thus preventing interference with the determination of the earth resistance. It must simply be assumed that the voltage source supplies a DC voltage that has the same value for both measurements of the method.

[0037] The invention is based on the following findings, observations, and considerations and also includes the following preferred embodiments. These embodiments are sometimes referred to as "the invention" for simplicity. The embodiments may also contain parts or combinations of the above-mentioned embodiments or correspond to them and / or may also include previously unmentioned embodiments.

[0038] The invention is based on the idea of ​​improved earth fault monitoring.

[0039] The invention is based on the idea that some standards, such as the EN54 series or UL2572, require potential-free networks including monitoring, such as loudspeaker networks or control systems between different systems. Various devices known from practice have ground fault monitoring and check the connected loudspeaker networks for ground fault isolation.

[0040] For example, a device could have a high-impedance DC source connected to ground combined with a simple DC voltage measurement. The earth resistance, or resistance to ground, Rge, can be calculated using the voltage divider formula. Rge=Rq∗(UqUMess−1)

[0041] This monitoring method is very simple, inexpensive to implement and suitable for all networks to be monitored that do not contain an additional DC source in the measuring circuit.

[0042] The invention aims to improve the monitoring described above so that networks with DC source(s) can also be monitored. In other words, it should be possible to determine the earth resistance value even if there is a disturbance source in the measurement path that generates a DC voltage.

[0043] The invention is not limited to specific device groups. Any device requiring ground fault monitoring can use the described method.

[0044] In particular, the invention can be used for interface modules as devices.

[0045] Further features, effects, and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention and the accompanying figures. Each of these figures shows a schematic diagram: Fig. 1 an equivalent circuit diagram of an audio system with a device with a measuring circuit not according to the invention, Fig. 2 an equivalent circuit diagram of an alternative audio system with a device with an interference source and with a measuring circuit according to the invention, Fig. 3 a simplified equivalent circuit diagram of a device with a measuring circuit according to the invention, Fig. 4a an alternative equivalent circuit diagram of a device with a measuring circuit according to the invention, Fig. 4b and its simplification after redrawing, Fig. 5a for the device Fig. 2 a first and Fig. 5b second variant of earth resistances, which provide the same overall result according to the method according to the invention.

[0046] Fig. 1 shows the following: A further device 8 is connected to a device 500 not according to the invention via a line connection 6. For example, a high-impedance DC source 504 connected to ground 502 (generates voltage Uq) in combination with a simple DC voltage measurement (measurement voltage Umess at measuring resistor 506, value Rq) is conceivable for device 500. The value Rge of the earth resistance 20 or resistance to earth can be calculated using the voltage divider formula. Rge=Rq∗(UqUMess−1)

[0047] This monitoring method is very simple, inexpensive to implement and suitable for all networks to be monitored that do not have an additional DC source (interference source 18, see Fig. 2) included in the measuring circuit.

[0048] Fig. Figure 2 shows an electrical equivalent circuit diagram of a section of an audio system 2, in this case a public address and / or voice alarm system. This contains a device 4, which is connected to another device 8 via a cable connection 6. The device 4 contains two terminals 10a, b in the form of connection poles, each of which serves to connect a line 12a, b of the cable connection 6. Line 12a is a ground or GND connection, and line 12b is a signal or GBO (General Purpose Output) line.

[0049] Both poles 10a,b each have an earth resistance 20a,b or earth fault resistance with the respective values ​​Rge1,2, which is shown here in the form of a discrete ohmic resistance as an element of the equivalent circuit.

[0050] The device 4 contains a DC connection 14, i.e., a DC-conductive connection between both poles 10a,b, as indicated here by a dashed frame. The DC connection 14 contains an interference source 16, which has an interference voltage U1 in the form of a non-zero DC voltage. It also contains an ohmic interference resistor 18, which here has the value R1.

[0051] The device 4 contains a measuring circuit 22, which is also indicated here by a dashed frame. The measuring circuit 22 contains a connection point 24, which is connected here to pole 10a of the device 4. The measuring circuit 22 contains a parallel circuit of a first branch 26a and a second branch 26b, both of which lead in parallel from the connection point 24 to a reference potential 28, here earth potential. The earth resistances 20a,b also lead from the lines 12a,b or poles 10a,b to the reference potential 28. The first branch 26a contains a series circuit of a voltage source 30 (generates the DC voltage with value Uq) with a source resistance 32 (value Rq). Branch 26b contains a measuring resistance 34 with value Rm.

[0052] Fig. 2 shows, in contrast to Fig. 1 an example with a DC source (interference source 16, generates voltage U1) in the measuring circuit. By a simple measurement as described above with respect to Fig. As explained in section 1, an earth connection on the GND line (Rge1) can be reliably detected, but an earth connection on the GPO line (Rge2) cannot:

[0053] Here's a simple example: Device 4 provides a switchable 24VDC output (at poles 10a,b, voltage U1 generated by interference source 16) to activate a warning lamp (hereafter referred to as device 8, not shown). If Uq and U1 have the same value, no current flows through measuring resistor 34 (regardless of the value Rge2 of earth resistance 20b). Thus, UMess = Uq, and the mathematical value for Rge is "infinite."

[0054] It is irrelevant where the DC source (interference source 18) is located (in device 4, in the further device 8) or whether there are several DC sources in the measuring branch (not shown).

[0055] According to Fig. 2, monitoring may be carried out in accordance with Fig. 1 so that networks with DC source(s) can also be monitored.

[0056] Fig. 3 shows a simplified equivalent circuit diagram for Fig. 2. In other words, it can be achieved according to Fig. 3 to determine the value Rg of the earth resistance 20, even if there is an interference source 16 in the measurement path that generates a DC voltage Ug. This is achieved as follows:

[0057] Fig. 3 shows a further simplified equivalent circuit diagram of the device 4 from Fig. 2. The interference source 16 and other interference sources located on the line connection 6 or in the additional device 8 are combined into a single effective interference source 16 (value Ug). All interference resistances 18, which may also be present in the line connection 6 and / or the additional device 8, are combined into the earth resistance 20 (value Rg).

[0058] Mathematically, this results in Fig. 3 a formula with the two unknown variables Rg and Ug. The resistance to earth Rg can only be calculated if Ug is known or can be eliminated from the formula. UMeasure=Uq∗Rg+Ug∗RqRg∗Rq+Rg∗Rm+Rq∗Rm∗Rm

[0059] By performing two measurements, in which one or more of the known parameters Uq, Rq, or Rm are varied, the interfering DC source can be mathematically eliminated. The earth resistance 20 (resistance to earth, value Rg) can thus be calculated.

[0060] For symmetry and cost reasons, Rq and Rm can be varied for the two measurements. The voltage Uq remains constant. The following calculations refer specifically to this configuration.

[0061] Fig. 4a shows an alternative measuring circuit 22, which in a first step is simplified and converted into an equivalent circuit according to Fig. 4b is transferred.

[0062] Fig. Figure 4a shows a further modified equivalent circuit of an alternative solution: Here, the switchability of the source resistor 32 and the measuring resistor 34 is indicated. The voltage source 30 is a fixed voltage source whose voltage value Uq cannot be changed.

[0063] Source resistor 32 and measuring resistor 34 are each formed from two partial resistors 36a and 36b connected in series, with partial resistor 36a being bridged by a switch S1,2. By closing and opening switch S1,2, the value of source resistor 32 and measuring resistor 34 can be changed, respectively, with respect to their values ​​Rq1,2 and Rm1,2.

[0064] In reality, distributed ohmic resistances (values ​​Rgv, Rgi, Rge) are all to be understood here as components of a common effective earth resistance with value Rg.

[0065] Fig. 4b finally shows a further simplified equivalent circuit of the Fig. 4a. Here, all components contributing to the effective earth resistance Rg are represented by a single equivalent circuit element or a single earth resistance 20.

[0066] The following formula results for a resistance adder with two sources: UMeasure=Uq∗Rg+Ug∗RqRg∗Rq+Rg∗Rm+Rq∗Rm∗Rm

[0067] For two measurements with different values ​​Rq1,2 and Rm1,2 of the measuring circuit 22, the following formulas result: For the first measurement (UMess1), Rq1 and Rm1 result with switch S1 closed and switch S2 not closed. For the second measurement (UMess2), Rq2 and Rm2 result with switch S1 not closed and switch S2 closed.

[0068] The first values ​​of measuring circuit 22 are Uq, Rq1, and Rm1; the second values ​​of measuring circuit 22 are Uq, Rq2, and Rm2. In the example, the values ​​of source resistor 32 and measuring resistor 34 are selected such that Rq1 = 100 kOhm, Rm1 = 200 kOhm, and Rq2 = 200 kOhm, Rm2 = 100 kOhm. These values ​​are therefore interchanged and have different values. UMess1=Uq∗Rg+Ug∗Rq1Rg∗Rq1+Rg∗Rm1+Rq∗Rm1∗Rm1 UMeasure2=Uq∗Rg+Ug∗Rq2Rg∗Rq2+Rg∗Rm2+Rq∗Rm2∗Rm2

[0069] Now the unknown variable Ug can be eliminated and the formula can be solved for Rg: Rg=Umess1 − Umess2Rm1∗Umess1−Rm1∗Uq+Rq1∗Umess1Rm1∗Rq1−Rm2∗Umess2−Rm2∗Uq+Rq2∗Umess2Rm2∗Rq2

[0070] The calculation always yields the resulting earth resistance 20 (resistance to earth, value Rg). It is irrelevant which external point (GP1 (General Purpose Input), GPO, ground (GND), etc.) has a connection to earth (reference potential 28). Even multiple connections to earth are measured correctly.

[0071] In order to carry out the described method, ie to perform the two measurements of the measuring voltages UMess1,2 and to change the values ​​of the measuring circuit 22, the device also contains an evaluation unit 42 in the form of a measuring and computing device, only symbolically indicated in the figure. This, together with the measuring circuit 22, forms a measuring arrangement 40. These are symbolically shown in Fig. 2 shown.

[0072] However, it is not always possible to locate the external point of an earth connection because the internal resistances are contained in the resulting resistance and therefore cannot be distinguished from an external resistance of the same value.

[0073] This shows Fig. 5: In Fig. 5a contains an internal noise resistance 18 with a value of 10 kOhm. The external earth resistance 20a is infinitely large and therefore not shown. The external earth resistance 20b, on the other hand, is 0 kOhm. The total value Rg=10 kOhm is determined using the above formula.

[0074] In Fig. In 5b, however, the external earth resistance 20b is infinitely large and therefore not shown. The external earth resistance 20a, on the other hand, is 10 kOhm. According to the above formula, the same total value Rg=10 kOhm is determined here, although this is distributed across other real resistances / current paths.

[0075] In Fig.5 both variants therefore provide the same result with a determination of 10kOhm for Rge. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2010 028 022 A1

[0002]

Claims

[1] Method for determining a value (Rge,1,2) of an ohmic earth resistance (20,a,b) of a pole (10a,b) of an electrical device (4) intended to be kept potential-free to a reference potential (28), in which: - a connection point (24) of a measuring circuit (22) is electrically connected to the pole (10a,b), wherein the measuring circuit (22) contains a parallel circuit leading from the connection point (24) to the reference potential (28) consisting of a series circuit of a voltage source (30) and an ohmic source resistor (32) on the one hand and an ohmic measuring resistor (34) on the other hand, - a first measuring voltage (UMess1) is determined across the measuring resistor (34) at first values of the measuring circuit (22), - a second measuring voltage (UMess2) is determined across the measuring resistor (34) at second values of the measuring circuit (22), - wherein at least one of the values of a source voltage (Uq) of the voltage source (30) and / or of the source resistor (32) and / or of the measuring resistor (34) is changed from the first to the second values of the measuring circuit (22), - the value (Rge,1,2) of the earth resistance (20,a,b) is determined from the two measuring voltages (UMess1,2) and the first and second values. [2] Method according to claim 1, characterized by that the earth potential is chosen as the reference potential (28). [3] Method according to one of the preceding claims, characterized by that the value (Uq) of the source voltage of the voltage source (30) is not changed from the first to the second values of the measuring circuit (22). [4] Method according to one of the preceding claims, characterized by , that - in the measuring circuit (22) the values of the source resistance (32) and the measuring resistance (34) are selected to be different from one another. [5] Method according to claim 4, characterized by , that - the values of the source resistance (32) and the measuring resistance (34) are exchanged from the first to the second values of the measuring circuit (22). [6] Method according to one of the preceding claims, characterized by , that - the source resistor (32) and / or the measuring resistor (34) contain at least two partial resistors (36a,b) in series and - the value of the source resistance (32) and / or the measuring resistance (34) can be changed by bridging one of the partial resistances (36a) by means of a switch (S1,2). [7] Method according to one of the preceding claims, characterized by , that - a measuring path from the connection point (24) via the earth resistance (20,a,b) to the reference potential (28) contains an interference source (16) which has an interference voltage in the form of a direct voltage, and - the value (Rge,1,2) of the earth resistance (20,a,b) is determined from the two measuring voltages (UMess1,2) and the first and second values in such a way that the value (U1) of the interference voltage is eliminated during the determination. [8] Measuring circuit (22) for determining a value (Rge,1,2) of an earth resistance (20,a,b) of a pole (10a,b) of an electrical device (4), wherein the earth resistance (20,a,b) exists between the pole (10a,b) and a reference potential (28), - with a connection point (24) which is electrically connectable or connected to the pole (10a,b), - with a parallel circuit connected between the pole (10a,b) and the reference potential (28) comprising a series circuit of a voltage source (30) and an ohmic source resistor (32) on the one hand and an ohmic measuring resistor (34) on the other hand, - wherein at least one of the values of the source voltage of the voltage source (30) and / or the source resistor (32) and / or the measuring resistor (34) is variable. [9] Measuring arrangement (40) with the measuring circuit (22) according to claim 8 and with an evaluation unit (42) which is designed to carry out the method according to one of claims 1 to 7. [10] Device (4), - which has at least one pole (10a,b) which is intended to be kept potential-free, - with the measuring circuit (22) according to claim 8 or the measuring arrangement (40) according to claim 9, - wherein the connection point (24) of the measuring circuit (22) is electrically connected to at least one of the poles (10a,b). [11] Device (4) according to claim 10, characterized by that the device (4) is a component of an audio system (2). [12] Device (4) according to one of claims 10 to 11, characterized bythat the device (4) has at least two poles (10a,b) which are connected to one another via a direct current connection (14). [13] Device (4) according to claim 12, characterized by that the direct current connection (14) contains an interference source (16) which has an interference voltage in the form of a direct voltage.

Images