Method for determining an electrical resistance

Abstract

The invention relates to a method for determining an electrical resistance of at least one component (2, 16, 17, 20, 21, 22) of a circuit (23), which serves to supply electrical energy to a magnetic drive (3) of a circuit breaker (1), wherein multiple components (2, 16, 17, 20, 21, 22) are connected in series in the circuit, wherein a first component is a magnetic coil (2) of the magnetic drive (3) and a second component is a voltage source (17), wherein measured values (30-33) of a voltage (U) present in the circuit are detected (23, 32) at two different times, i.e. before (30) and after (31, 32, 33) the circuit (23) is closed, wherein a measured value of the current in the circuit (23) is detected at at least one of the two different times, and wherein an electrical resistance value of at least one component (2, 16, 17, 20, 21, 22) of the circuit (23) is calculated from the detected measured values of voltage and current at the two different times.

Description

[0001] Description

[0002] Methods for determining electrical resistance

[0003] Technical field

[0004] The present invention relates to a method for determining the electrical resistance of at least one component of a circuit that serves to supply electrical power to a magnetic drive of a power switch.

[0005] Technical background

[0006] Medium-voltage circuit breakers can be operated with high-performance magnetic drives. During a switching operation, in which the circuit breaker is mechanically opened or closed, a relatively high drive current flows through a magnetic coil of the magnetic drive to generate a magnetic field necessary for the switching operation. The magnetic drives can obtain the electrical energy for the switching operation from a capacitor storage device; see, for example, DE 102005013197 A1 (Siemens AG) 28.09.2006.

[0007] The entire circuit of the magnetic drive, hereinafter referred to simply as the circuit, typically comprises the following circuit components: capacitor storage, cable connections, connectors, circuit breakers, semiconductor switches, and solenoid coils. The total electrical resistance of the circuit, also simply referred to as circuit resistance, should be as low as possible to minimize ohmic losses in the circuit and ensure that the voltage supplied by the capacitor storage is applied to the solenoid coil with the lowest possible voltage drop. The higher the voltage applied to the solenoid coil, the faster the drive current rises, the faster the magnetic drive operates, and the faster the circuit breaker switches.

[0008] In practice, aging leads to an increase in circuit resistance, for example, due to an increase in the internal resistance of the capacitor, an increase in the contact resistance of the connectors, and an increase in the contact resistance of the circuit breaker. This increase in circuit resistance results in a voltage drop (voltage loss) in the circuit during operation. Consequently, the solenoid coil no longer receives the full voltage of the capacitor, but a voltage reduced by the voltage drop. This causes the solenoid drive to operate more slowly, the operating time of the circuit breaker to increase, or the circuit breaker may not reach its end position due to insufficient energy (failure to operate).

[0009] If the circuit breaker is operated at low ambient temperatures, another factor comes into play: at low temperatures, the internal resistance of the electrolytic capacitors typically used in the capacitor bank increases significantly. This can result in a considerable voltage drop within the capacitor bank, potentially causing the voltage available at the solenoid to be at a critically low level. Consequently, the rise of the drive current in the solenoid is considerably slower, and the operating time of the circuit breaker is extended.

[0010] It is now standard practice to monitor the current levels of the drive current in the circuit during switching operation, as well as the operating time of the circuit breaker. If predefined threshold values ​​are exceeded or fallen below, a warning message is generated so that maintenance can be carried out in a timely manner. Since a solenoid coil is typically wound from copper wire, whose electrical resistance constitutes a significant portion of the circuit resistance, a relatively large tolerance range for the circuit resistance must be provided due to the temperature dependence of the copper wire's electrical resistance. This prevents false alarms caused by temperature fluctuations. Therefore, relatively small changes in the circuit resistance caused by components other than the solenoid coil often go undetected.Therefore, aging effects of these other components of the circuit can unexpectedly lead to a disruption of the circuit, without this having been detected in advance by the monitoring.

[0011] Specifically, the internal resistance of the capacitors in the capacitor bank, which exhibits a large variation depending on the capacitor temperature, can increase significantly due to aging and at low temperatures. The usual cyclical measurements of the capacitor capacitance for detecting aging effects usually provide no indication of an increased internal resistance.

[0012] Summary of the invention

[0013] The object of the present invention is to improve the determination of the electrical resistance of at least one component of a circuit that serves to supply electrical power to a magnetic drive of a power switch. This object is achieved according to the invention by a method with the features specified in claim 1.

[0014] The method according to the invention serves to determine the electrical resistance of at least one component of a circuit that supplies electrical power to a magnetic drive of a power switch. Several components are connected in series in the circuit. A first component is a magnetic coil of the magnetic drive, and a second component is a voltage source, e.g., in the form of a capacitor. Voltage measurements in the circuit are taken at two different times, namely before and after the circuit is closed. A current measurement in the circuit is also taken at at least one of these two times. Finally, the electrical resistance of at least one component of the circuit is calculated from the voltage and current measurements taken at these two different times.

[0015] A magnetic drive is a common design for switching devices in medium-voltage systems, for example. In this design, one or more coils generate a magnetic field under voltage, which attracts another ferromagnetic metal piece, an armature. This attraction causes a movement that is transmitted to a movable contact in the switching device, pressing it against the fixed contact. Typically, two coils are used, which can be connected in parallel or series. This has the advantage that the magnetic force can be kept approximately constant, even if the input voltage varies due to different nominal voltages of the power supply in different countries or in different applications, as mentioned earlier. When energized, the two coils of the magnetic drive exert an attractive force on the magnetic armature.The transmission mechanism translates the movement of the magnetic armature into a movement of the movable contact towards the fixed contact and, for example, uses a toggle lever for this purpose. The magnetic drive has controllable switching devices for the coils.

[0016] A vacuum switching device according to the invention has, for example, a fluid-tight housing inside which a vacuum prevails (or an extremely low gas pressure < 10 -6(Pa). If a movable contact is quickly pulled away from a fixed contact, e.g., by means of a spring force, any resulting arc is rapidly extinguished, partly because there is hardly any ionizable medium for current flow. Vacuum switching devices are particularly well suited for switching alternating current because an arc always breaks at the zero crossing of the voltage. With the invention, an increased circuit resistance during the operation of the circuit breaker can be determined, and the components in the circuit can be identified as those causing the increased resistance. Previous methods do not offer this capability.

[0017] Embodiments of the invention

[0018] Advantageous embodiments and further developments of the invention are specified in the dependent claims.

[0019] According to a preferred embodiment of the invention, the voltage and current values ​​are measured during normal switching operation. An advantage of this is that the operation of the circuit breaker does not need to be interrupted; instead, the electrical resistance of at least one component of a circuit supplying electrical power to a magnetic drive of the circuit breaker can be determined during the normal switching operation of the circuit breaker.

[0020] According to a preferred embodiment of the invention, at least one temperature measurement of the magnetic drive and / or in the vicinity of the circuit is recorded and taken into account when calculating the electrical resistance value in order to eliminate the influence of temperature on the electrical resistance. An advantage of this is that the temperature dependence of the electrical resistance of components of the circuit is considered, thus making the determination of the electrical resistance of at least one component of a circuit more accurate.

[0021] According to a preferred embodiment of the invention, a difference between a voltage value before and a voltage value after closing the circuit results in a voltage swing AU, and an electrical resistance value AR of at least one component of the circuit is calculated from the measured voltage and current values ​​at the two different times: AU = AR * I. An advantage of this is that determining the electrical resistance of at least one component of a circuit is reduced to measuring a voltage swing: the voltage swing is an indicator of the circuit resistance in the drive circuit. The higher the voltage swing, the higher the circuit resistance. According to a preferred embodiment of the invention, a maintenance message is generated if the determined voltage change exceeds a predetermined threshold.The advantage here is that measures against a potentially imminent power failure can be taken in a timely manner, thus reducing the occurrence of emergencies.

[0022] According to a preferred embodiment of the invention, measured values ​​of a voltage applied in the circuit are acquired at a pair of measuring points, wherein the pair of measuring points is selected such that as many components in the circuit as possible are energized. An advantage of this approach is that a comprehensive diagnosis of the electrical resistance of a large part of the circuit is performed.

[0023] According to a preferred embodiment of the invention, measured values ​​of a voltage applied in the circuit are acquired at two pairs of measuring points, wherein a first pair of measuring points is selected such that the voltage measurement is performed directly at the voltage source, and a second pair of measuring points is selected such that any number of components in the circuit are traversed by current. An advantage of this is that the location of an increased electrical resistance in the circuit, and thus the affected component of the circuit, can be identified.

[0024] Exemplary embodiments of the drawing

[0025] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood through the following description of exemplary embodiments, which will be explained in more detail with reference to the drawings. The drawings are schematic and not to scale.

[0026] Fig. 1 shows a switching arrangement with a vacuum switching device,

[0027] Fig. 2 shows an electrical circuit,

[0028] Fig. 3 Stress curves over time,

[0029] Fig. 4 shows an enlarged section of Fig. 3,

[0030] Fig. 5 shows a flowchart of a process, and Fig. 6 shows a simplified circuit diagram of an electrical circuit.

[0031] Detailed description of the exemplary implementations

[0032] Fig. 1 shows a switching arrangement 1. The switching arrangement 1 comprises a vacuum switching device 2-14 with a vacuum switch 11-13 in a housing 14, in which a vacuum 13 prevails. Inside the housing 14, a movable contact 11 is pressed against a fixed contact 12 because a magnetic coil 2 of a magnetic drive 3 is energized in a first direction; thus, Fig. 1 shows a current-conducting state of the vacuum switch 11-13 with a switched-on magnetic drive 3. A switching rod 8, which extends through the housing 14, is connected to the movable contact 11, with, for example, a bellows (not shown) ensuring the movement of the switching rod 8 while simultaneously maintaining a vacuum seal. A spring device 10 is provided for mechanically separating the contacts 11 and 12. This spring device is supported on a backing plate 9 and is pre-tensioned in the closed state shown.

[0033] The switching rod 8 is connected to a toggle lever 5-7, wherein a first leg 6 of the toggle lever 5-7 and a second leg 5 of the toggle lever 5-7 are rigidly connected at an angle, but rotatably mounted about a joint 7 of the toggle lever 5-7. The second leg 5 is connected to a magnetic armature 4 made of metal, e.g., iron. The toggle lever 5-7, which acts as a transmission mechanism, translates a movement of the magnetic armature 4 into a movement of the movable contact 11 towards the fixed contact 12. Alternative designs also exist in which the magnetic armature 4 acts directly on the switching rod 8 of the movable contact 11, without a toggle lever 5-7.

[0034] The magnetic coil 2 can be electrically connected in a circuit to a voltage source in the form of a capacitor 17. For this purpose, the capacitor 17 is electrically connected to a semiconductor switch 15 via a first connector 21. The semiconductor switch 15 is electrically connected to a circuit breaker 16 via a cable connection 20. The circuit breaker 16 is electrically connected to the magnetic coil 2. The semiconductor switch 15 can be switched either to conduct or block current. In the conducting state of the semiconductor switch 15, a circuit 23 is formed in which current flows from the capacitor 17 through the magnetic coil 2 and back to the capacitor 17. In the blocking state of the semiconductor switch 15, the semiconductor switch 15 interrupts the circuit 23 and no current flows through the magnetic coil 2.Furthermore, depending on the control of the semiconductor switch 15, the direction in which current flows through the magnetic coil 2 can be reversed, so that the movable contact 11 is either pressed against the fixed contact 12 or separated from it, depending on the current direction; shown is an operating state in which the current-carrying magnetic coil 2 pulls the magnetic armature 4 into a housing of the magnetic drive 3 with a force F and thus the movable contact 11 is pressed against the fixed contact 12.

[0035] The circuit arrangement 1 also includes a charging voltage source 19, from which the capacitor storage 17 can be charged with electrical charge carriers; for charging, the charging voltage source 19 is electrically connected to the capacitor storage 17.

[0036] Fig. 2 shows the circuit 23 of Fig. 1 in detail. In circuit 23, the capacitor storage 17, the first connector 21, which is arranged in the housing of the capacitor storage 17, the semiconductor switch 15, a second connector 22, the cable connection 20, the fuse breaker 16 and the solenoid 2 are connected in series.

[0037] When the semiconductor switch 15 is in a conducting switching state for a first current direction 11, current flows from the capacitor storage 17 via a first current path 23a of the circuit 23, comprising a first current path 23a of the first connector 21, a first current path 23a of the semiconductor switch 15, a first current path 23a of the second connector 22, a first current path 23a of the cable connection 20, and a first current path 23a of the fuse break switch 16, to the solenoid 2, and from the solenoid 2 via a second current path 23b of the circuit 23, comprising a second current path 23b of the fuse break switch 16, a second current path 23b of the cable connection 20, a second current path 23b of the second connector 22, a second current path 23b of the semiconductor switch 15, and a second Current path 23b of the first connector 21, back to the capacitor storage 17.

[0038] When the semiconductor switch 15 is in a conducting switching state for a second current direction I2 opposite to the first current direction 11, current flows from the capacitor storage 17 via the second current path 23b of the circuit 23, comprising the second current path 23b of the first connector 21, the second current path 23b of the semiconductor switch 15, the second current path 23b of the second connector 22, the second current path 23b of the cable connection 20, and the second current path 23a of the fuse break switch 16, to the solenoid coil 2, and from the solenoid coil 2 via the first current path 23a of the circuit 23, comprising the first current path 23b of the fuse break switch 16, the first current path 23b of the cable connection 20, the first current path 23b of the semiconductor switch 15 and the first current path 23b of the first connector 21, back to the capacitor storage 17.

[0039] The capacitor storage 17, which acts as a voltage source, has an electrical capacitance 17C and an ohmic resistance 17R.

[0040] The first connector 21 has an ohmic resistance 21 R1 in its first current path 23a and an ohmic resistance 21 R2 in its second current path 23b.

[0041] The semiconductor switch 15 has four controllable H-bridge switching elements S1, S2, S3, S4, e.g., FETs or IGBTs (H-bridge = semiconductor; FET = field-effect transistor; IGBT = insulated-gate bipolar transistor). The H-bridge switching elements S1, S2, S3, S4 can be controlled such that the semiconductor switch 15 is either conducting or blocking current. The H-bridge switching elements S1, S2, S3, S4 are connected as an H-bridge; they can be controlled such that the current direction 11, I2 of the current flowing through the solenoid 2 is reversed (polarity reversal). The H-bridge switching elements S1, S2, S3, S4 can not only be switched on or off abruptly, but also controlled via PWM; this allows the current flowing through the solenoid 2 to be regulated (PWM = pulse width modulation). The two HL switching elements S2 and S4, more precisely the source connection (FET) and thethe emitter (IGBT) is electrically connected to a ground potential 15E (GND or OV potential) of circuit 23 (GND = Ground, English for mass (electronics), reference potential in electrical engineering).

[0042] The second connector 22 has an ohmic resistance 22R1 in its first current path 23a and an ohmic resistance 22R2 in its second current path 23b.

[0043] The cable connection 20 has an ohmic resistance 20R1 in its first current path 23a and an ohmic resistance 20R2 in its second current path 23b.

[0044] The fuse breaker 16 has an ohmic resistance 16R in its first current path 23a.

[0045] The magnetic coil 2 has an ohmic resistance 2R and an electrical inductance 2L. The circuit 23 has a plurality of voltage measurement points at which a voltage measurement can be performed in the circuit 23. A first voltage measurement point 21M is located in the first current path 23a between the first connector 21 and the semiconductor switch 15. A second voltage measurement point 22M+ is located in the first current path 23a between the semiconductor switch 15 and the second connector 22. A third voltage measurement point 20M+ is located in the first current path 23a between the second connector 22 and the cable connection 20. A fourth voltage measurement point 16M+ is located in the first current path 23a between the third voltage measurement point 20M+ and the ohmic resistance 16R of the fuse breaker 16.A fifth voltage measurement point 16M- is located in the first current path 23a between the ohmic resistance 16R of the fuse breaker 16 and the solenoid 2. A sixth voltage measurement point 2M+ is located in the solenoid 2, specifically in the first current path 23a between the fifth voltage measurement point 16M- and the series connection of the ohmic resistance 2R with the electrical inductance 2L of the solenoid 2. A seventh voltage measurement point 2M- is located in the solenoid 2, specifically in the second current path 23b between the series connection of the ohmic resistance 2R with the electrical inductance 2L of the solenoid 2 and the fuse breaker 16. An eighth voltage measurement point 20M- is located in the second current path 23b between the cable connection 20 and the second connector 22. A ninth voltage measurement point 22M- is located in the second current path 23b between the second connector 22 and the semiconductor switch 15.

[0046] There is also a current measuring device 24 available, with which a current measurement can be carried out in the circuit 23, for example at the first voltage measuring point 21 M.

[0047] There is also a temperature measuring device 25 available, with which a temperature measurement can be carried out in or on the magnetic coil 2.

[0048] Fig. 3 shows voltage curves 31-34 of a voltage U over time t. Fig. 4 shows an enlarged section of Fig. 3.

[0049] During switching operation, the voltage is measured immediately before and after switching on and off the drive current of the solenoid coil, and the voltage swing AU is determined, i.e., a decrease or increase in voltage U. The voltage swing AU is an indicator of the circuit resistance: the higher the voltage swing AU, the higher the circuit resistance AR: AU = AR * I

[0050] I = const = U / R; i.e., if R increases, U also increases.

[0051] The voltage provided by capacitor 17 is approximately Uo = 160 V. However, the capacitor voltage is not always exactly 160 V, but can be higher or lower. It will be higher, for example, if a capacitor test is performed and switching occurs during this process, and lower, for example, if switching occurs – which is permissible – before the 160 V is reached. Therefore, a measurement of the voltage provided by capacitor 17 in the de-energized state of the circuit (I = 0) is also necessary to have a voltage reference value.

[0052] Voltage curves 31-33 show the voltage U applied between the third voltage measurement point 20M+ and the ninth voltage measurement point 22M- in three different aging states. Voltage curve 34 shows the voltage U applied between the second voltage measurement point 22M+ and the ninth voltage measurement point 22M- in the same three different aging states.

[0053] Initially (t < approx. 10 ms) the HL switch 15 is switched in a current-blocking manner, so that there is no voltage present between the third voltage measurement point 20M+ and the ninth voltage measurement point 22M- as well as between the second voltage measurement point 22M+ and the ninth voltage measurement point 22M-. At the time the current is switched on at t = approx. 10 ms, the HL switch 15 is switched to conducting current, so that the voltage between the third voltage measuring point 20M+ and the ninth voltage measuring point 22M- drops to a value of U31 = approx. 155 V in a first aging state 31 with the lowest aging, to a value of U32 = approx. 153 V in a second aging state 32 with a medium aging, and to a value of U33 = approx. 151 V in a third aging state 33 with the highest aging.

[0054] In contrast, the voltage 34 between the second voltage measurement point 22M+ and the ninth voltage measurement point 22M- drops to a value of U34 = approx. 157 V in all three aging states. After the HL switch 15 is switched back to blocking current (t = approx. 60 ms), no voltage is present at the two voltage measurement points under consideration.

[0055] By measuring the voltage U between the third voltage measurement point 20M+ and the ninth voltage measurement point 22M-, the increased voltage drop AU and thus the increased resistance value AR can be detected during the switching process. The voltage across the first resistor 22R1 of the second connector 22, measured between the second voltage measurement point 22M+ and the ninth voltage measurement point 22M-, is not affected by the resistance increase. Therefore, the increased resistance value AR can be precisely attributed to either the first resistor 22R1 or the second resistor 22R2 of the second connector 22.

[0056] If an adjustable threshold value of the voltage swing AU or the increased resistance value AR is exceeded, a maintenance message is generated to request an operator to take action.

[0057] Voltage measurement can be performed in different ways.

[0058] According to one method of voltage measurement, a measurement is taken at a time-delayed point pair. The voltage measurement points are chosen so that as many components in the circuit as possible are carrying current, e.g., capacitors, wires, connectors, safety switches. In this case, the voltage is measured immediately before the semiconductor switch. For example, the voltage is measured from the first voltage measurement point 21M to the ground potential 15E, i.e., before the semiconductor switch 15.

[0059] It is also possible to determine the aging of the capacitor storage device 17 by measuring the voltage directly at its electrical terminals, in order to detect only an increase in its internal resistance. The special case of low ambient temperatures may need to be considered. If an increased internal resistance of the capacitor storage device 17 is detected, it can be heated by cyclically discharging and charging the device, thus reducing the temperature-induced increase in internal resistance. Discharge is achieved, for example, via the magnetic coil 2, which is switched on for a period of time that does not yet result in movement of the magnetic armature 4. Charging takes place as in normal operation via the charging electronics of the capacitor storage device 17.Alternatively, a rectified AC voltage source can be connected to the capacitor storage 17 to heat it. The voltage ripple causes a cyclical change in current direction, which heats the capacitor storage 17. A second method of voltage measurement involves a time-delayed measurement at two pairs of measuring points. The voltage is measured directly at the capacitor storage 17 and additionally as described above for the first method. This allows the location of the increased circuit resistance, and thus the component of the circuit affected by excessive aging, to be determined.For example, the voltage between the second voltage measurement point 22M+ and the ninth voltage measurement point 22M-, the voltage between the third voltage measurement point 20M+ and the ninth voltage measurement point 22M-, the voltage between the fourth voltage measurement point 16M+ and the eighth voltage measurement point 20M-, the voltage between the fourth voltage measurement point 16M+ and the fifth voltage measurement point 16M-, or the voltage between the sixth voltage measurement point 2M+ and the seventh voltage measurement point 2M- is recorded.

[0060] In order to be able to calculate the effect of temperature on the circuit resistance, a temperature measurement can be taken in the magnetic coil 2 using the temperature measuring device 25.

[0061] Fig. 5 shows a flowchart of a method according to the invention. In a first step 51, measured values ​​of a voltage applied to the circuit are recorded at a first time point, namely before the circuit is closed. In a second step 52, measured values ​​of a voltage applied to the circuit are recorded at a second time point, namely after the circuit is closed. In a third step 53, a measured value of the current in the circuit is recorded at at least one of the two different time points. In a fourth step 54, an electrical resistance value of at least one component of the circuit is calculated from the recorded measured values ​​of voltage and current at the two different time points.

[0062] Fig. 6 shows a circuit 60 in which a voltage source S provides a voltage Uo at the connection points M1, M2. An electrical load C, which itself has an electrical resistance R, is connected to the voltage source S via current lines 60a, 60b. c The device exhibits a reduced voltage AUc = Uo - AU at its terminals M3, M4 during operation, i.e., with the circuit closed, because there is a total voltage drop AU across the total electrical resistance R of the power lines 60a, 60b. The total electrical resistance R is composed of the electrical resistances Ri to Re of all circuit components, such as cable connections, connectors, switches, etc.

[0063] The same current flows through all components of circuit 60; therefore, the current I is the same at every point in circuit 60. The current is not affected by a fluctuating voltage from the voltage source S, e.g., a capacitor, because the current I in circuit 60 is regulated. The voltage source S can also supply a multiple of the required current I. Across each circuit component Ri to Re of circuit 60, the voltage drops by a specific voltage drop AU; = Rj x I.

[0064] The total voltage drop AU across the total electrical resistance R is the sum of all individual voltage drops AU;

[0065] By determining the individual voltage drops AU; it is possible to determine a change in resistance (increase or decrease) of each individual circuit component Ri to Re as a function of time.

[0066] Reference symbol list

[0067] 1 Switching arrangement

[0068] 2 magnetic coil

[0069] 3 Magnetic drive

[0070] 4 magnetic anchors

[0071] 5 legs, second

[0072] 6 thighs, first

[0073] 7 joint

[0074] 8 shift rod

[0075] 9 Support plate

[0076] 10 Spring mechanism

[0077] 11 Contact, movable

[0078] 12 fixed contacts

[0079] 13 Vacuum

[0080] 14 switch housings

[0081] 15 semiconductor switches

[0082] 15E Ground Potential

[0083] 16 circuit breakers

[0084] 17 Voltage source, capacitor storage

[0085] 19 Charging voltage source

[0086] 20 cable connection

[0087] 21 connectors, first

[0088] 22 connectors, second

[0089] 23 Circuit

[0090] 24 current meter

[0091] 25 Temperature measuring device

[0092] 30 voltage value

[0093] 31 Voltage value

[0094] 32 voltage value

[0095] 33 Voltage value

[0096] 34 voltage value

[0097] 51st procedure step

[0098] 52nd process step

[0099] 53 Procedure step

[0100] 54 Process Step C Consumer

[0101] F force, attractive

Claims

Patent claims 1. Method for determining the electrical resistance of at least one component (2, 16, 17, 20, 21, 22) of a circuit (23) that supplies electrical power to a magnetic drive (3) of a circuit breaker (1), wherein several components (2, 16, 17, 20, 21, 22) are connected in series in the circuit, wherein a first component is a magnetic coil (2) of the magnetic drive (3) and a second component is a voltage source (17), wherein measured values ​​(30 - 33) of a voltage (II) applied in the circuit are recorded at two different times, namely before (30) and after (31, 32, 33) the circuit (23) is closed, wherein a measured value of the current in the circuit (23) is recorded at at least one of the two different times, and wherein an electrical resistance value of at least one component (2, 16, 17, 20, 21 ,22) of the circuit (23) is calculated from the recorded measured values ​​of voltage and current at the two different times.

2. Method according to claim 1, wherein the measured values ​​of voltage and current are measured during normal switching operation.

3. Method according to one of the preceding claims, wherein at least one measured value of a temperature of the magnetic drive (3) and / or in the vicinity of the circuit (23) is recorded and taken into account in the calculation of the electrical resistance value in order to eliminate an influence of the temperature on the electrical resistance.

4. A method according to any of the preceding claims, wherein a difference between a voltage value before (30) and a voltage value after (31, 32, 33) closing the circuit (23) results in a voltage swing AU, and wherein an electrical resistance value AR of at least one component (2, 16, 17, 20, 21, 22) of the circuit (23) is calculated from the measured values ​​of voltage and current at the two different times: AU = AR * I 5. Method according to any one of the preceding claims, a maintenance message is generated if the detected voltage change exceeds a predefined threshold.

6. A method according to any one of the preceding claims, wherein the acquisition of measured values ​​(30-33) of a voltage (II) applied in the circuit (23) is carried out at a pair of measuring points, wherein the pair of measuring points is selected such that as many components (2, 16, 17, 20, 21, 22) in the circuit (23) as possible are traversed by the current.

7. A method according to any one of the preceding claims, wherein the acquisition of measured values ​​(30-33) of a voltage (U) applied in the circuit is carried out at two pairs of measuring points, wherein a first pair of measuring points is selected such that the voltage measurement is carried out directly at the voltage source, and a second pair of measuring points is selected such that any number of components (2, 16, 17, 20, 21, 22) in the circuit (23) are traversed by the current.

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