Monitoring of the contact region in a plug device

Abstract

In a method for the determination of an electrical contact property in a contact region between a first contact element of a first plug device and a second complementary contact element of a second plug device, a property of a current path including the first contact element, the contact region, and from the second plug device only the second contact element is evaluated and an induction voltage is generated in the current path for a measurement in the current path to thereby enable to draw a conclusion about the electrical contact property.

Claims

1. A method for the determination of an electrical contact property in a contact region between a first contact element of a first plug device and a second complementary contact element of a second plug device, said method comprising: evaluating a property of a current path which includes the first contact element, the contact region, and from the second plug device only the second contact element; and generating an induction voltage in the current path for a measurement in the current path to thereby enable to draw a conclusion about the electrical contact property.

2. The method of claim 1, further comprising adding in the current path at least two sensors which are arranged in the first plug device and contact the second contact element when the first and second contact elements are plugged together.

3. The method of claim 1, further comprising synchronizing the evaluation of the property of the current path with a course of an operating current across the contact region.

4. The method of claim 1, further comprising: disposing in the current path a sensor which is arranged in the first plug device and contacts the second contact element when the first and second contact elements are plugged together, and forming in the current path a capacitance of more than about 10 pF to allow an alternating current flow in the current path.

5. The method of claim 1, further comprising measuring a voltage drop across the current path that occurs when an operating current flows across the contact region.

6. The method of claim 4, further comprising establishing a defined contact property between the sensor and the second contact element.

7. The method of claim 6, wherein the defined contact property between the sensor and the second contact element is established by applying a breakdown voltage.

8. The method of claim 1, wherein the current path forms a secondary coil on a magnetic core, and further comprising arranging a primary coil on the magnetic core.

9. The method of claim 1, wherein the second contact element is a plug pin and wherein the current path includes at least two contact fingers which both contact the plug pin.

10. A method for the determination of an electrical contact property in a contact region between a first contact element of a first plug device and a second complementary contact element of a second plug device, said method comprising: evaluating a property of a current path which includes the first contact element, the contact region, and from the second plug device only the second contact element; and adding in the current path at least two sensors which are arranged in the first plug device and contact the second contact element when the first and second contact elements are plugged together.

11. A method for the determination of an electrical contact property in a contact region between a first contact element of a first plug device and a second complementary contact element of a second plug device, said method comprising: evaluating a property of a current path which includes the first contact element, the contact region, and from the second plug device only the second contact element; and synchronizing the evaluation of the property of the current path with a course of an alternating operating current across the contact region by carrying out measurements in a range of zero crossings of the alternating operating current.

12. A method for the determination of an electrical contact property in a contact region between a first contact element of a first plug device and a second complementary contact element of a second plug device, said method comprising: evaluating a property of a current path which includes the first contact element, the contact region, and from the second plug device only the second contact element; disposing in the current path a sensor which is arranged in the first plug device and contacts the second contact element when the first and second contact elements are plugged together; and forming in the current path a capacitance of more than about 10 pF to allow an alternating current flow in the current path.

13. A plug device, comprising: a first plug device including a first contact element; a second plug device including a second contact element which complements the first contact element and contacts the first contact element, when the first and second contact elements are plugged together in a contact region; a current path including the first contact element, the contact region, and from the second plug device only the second contact element; and means for generating an induction voltage in the current path for a measurement in the current path to thereby enable to draw a conclusion about an electrical contact property in a contact region between the first contact element and the second contact element.

14. The plug device of claim 13, further comprising at least two sensors configured to contact the second contact element when the first and second contact elements are plugged together.

15. The plug device of claim 13, further comprising a sensor disposed in the current path and arranged in the first plug device, said sensor contacting the second contact element when the first and second contact elements are plugged together, wherein the means includes a magnetic core configured to surround the first contact element and/or the sensor and/or the contact region.

16. The plug device of claim 13, further comprising an evaluation circuitry configured to evaluate the electrical contact properties of the current path.

17. A plug device, comprising: a first plug device including a first contact element; a second plug device including a second contact element which complements the first contact element and contacts the first contact element, when the first and second contact elements are plugged together in a contact region; a current path including the first contact element, the contact region, and from the second plug device only the second contact element; and at least two sensors configured to contact the second contact element when the first and second contact elements are plugged together.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) In the following, exemplified embodiments of the invention will be explained in greater detail with reference to the figures. It is shown in:

(2) FIG. 1 the sockets of a first plug device (coupling) and a plug as second plug device with plug pins that are complementary to the sockets;

(3) FIG. 2 schematically a current path which includes a socket, a plug pin, and a socket-side sensor which contacts the plug pin within the socket;

(4) FIG. 3 schematically two embodiments of a current path which includes a plug pin, a socket and a plug-side sensor;

(5) FIG. 4 schematically a current path which includes a socket, a plug pin and two socket-side sensors;

(6) FIG. 5 schematically a current path which includes a socket, a plug pin, and a socket-side sensor which contacts the plug pin outside the socket;

(7) FIG. 6 the current path of FIG. 2 with a superimposed equivalent circuit diagram;

(8) FIG. 7 the equivalent circuit diagram for a direct resistance measurement;

(9) FIG. 8 the equivalent circuit diagram of FIG. 7 supplemented by an electric circuit for the operation, configured for measuring the voltage drop in contact region under operating conditions;

(10) FIG. 9 the equivalent circuit diagram of FIG. 8 supplemented by a voltage source for generating a breakdown (fritting) in the contact region of a sensor;

(11) FIG. 10 a plan view upon the arrangement of two contact fingers inside and two contact fingers outside a magnetic core for generating an induction voltage;

(12) FIG. 11 a side view of the arrangement of FIG. 10;

(13) FIG. 12 the equivalent circuit diagram for the arrangement of FIGS. 10 and 11;

(14) FIG. 13 exemplary curves of real part and imaginary part of the input impedance of the system of FIGS. 10 and 11 as a function of the contact resistance in the contact region;

(15) FIG. 14 an alternative arrangement of a magnetic core around a plug pin and between two contact regions;

(16) FIG. 15 a perspective view of the arrangement of a magnetic core relative to a plug pin, with the magnetic core enclosing the contact region between the plug pin and a contact finger of the socket (not shown);

(17) FIG. 16 a longitudinal sectional view through the arrangement of FIG. 15 with simultaneous illustration of the socket;

(18) FIG. 17 a section along the line A of FIG. 16;

(19) FIG. 18 a section along the line B of FIG. 16;

(20) FIG. 19 schematically a current path which includes a plug pin, a socket and a socket-side pressed-on sensor for a measurement with conductive and/or capacitive conductivity of the current path;

(21) FIG. 20 schematically a current path which includes a plug pin, a socket and a socket-side sensor as well as a detection circuitry coupled mechanically to the sensor for the plugged state;

(22) FIG. 21 a modification of the arrangement of FIG. 11 with a measuring coil around the current path.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(23) For the sake of clarity, FIG. 1 shows by way of a perspective view from a coupling only the sockets BU as the first plug device SV1. In the example shown, each of the five sockets BU includes a cylindrical base, from which four projecting contact fingers KF extend in parallel relation and enclose a cylindrical cavity. Furthermore, the Figure shows a plug as a second plug device SV2 with plug pins ST that complement the sockets. As the plug SV2 is plugged together with the coupling SV1. the plug pins ST come into electrically conductive contact in contact regions with the sockets BU.

(24) Due to aging processes, corrosion, contamination, and the like, a relatively high contact resistance can occur in the contact regions, causing an unacceptably high amount of heat loss. Therefore, it is desirable to determine or monitor the electrical contact properties in the contact regions, especially the contact resistance in the contact region between a plug pin ST and the associated socket BU.

(25) The components necessary for such a determination should hereby be accommodated if possible only in one of the two plug devices SV1, SV2, so that monitoring is independent of the type and origin of the used complementary plug device. In most of the exemplary embodiments described hereinafter, the required components are accommodated in the coupling as the first plug device SV1, while any plug may in principle be used as second plug device SV2. However, the corresponding explanations also apply (with the corresponding adjustments) to reversed roles of coupling and plug.

(26) FIG. 2 shows a schematic partial cross section through a first contact dement in the form of a socket BU, into which a plug pin ST is inserted as a complementary second contact element. The socket BU can be a sleeve that is closed all-round or made of several contact fingers that protrude in parallel relation from a base. The contact fingers can be separated at their free, distal end (cf. FIG. 1) or be electrically connected to each other. Between the plug pin ST and the socket BU or its contact fingers, there is (at least) one contact region via which the electrical operating current flows during operation. The contact properties of this contact region, in particular the contact resistance thereof, shall be determined according to the invention with components arranged in the coupling.

(27) The afore-described structure corresponds to the situation with conventional plug devices and forms the starting point also in the following FIGS. 3, 4, 5 as well as 19 and 20.

(28) In the embodiment according to FIG. 2, provision is made from this starting base for arrangement of an electrically conductive sensor F at the bottom of the socket BU, with the tip of the sensor contacting the plug pin ST when the latter is fully inserted into the socket. Contact is hereby established with slight normal contact force, whereas insertion and puffing forces remain almost unaffected. The sensor F can be implemented, for example, with spring wire made of nickel silver or bronze (both materials have very good spring properties, good to very good corrosion resistance, and do not necessarily need a coating). Furthermore, the sensor is electrically insulated from the socket BU and guided to the outside at a connection b.

(29) A second connection a is formed to the material of the socket BU. A current path is thus formed between the connections a, b, and includes the following components: the socket BU; the plug pin ST, which contacts the socket in the contact region of interest; the sensor F, which contacts the plug pin ST in a separate second contact region.

(30) A suitably designed evaluation circuitry (not shown) can be connected to the connections a, b in order to determine the properties of the current path and, in particular, those of the contact region of interest.

(31) FIG. 3 shows an embodiment in which the measurement of the contact properties is realized on the plug side. For this purpose provision is made for two connections a, b, of which one (b) is connected to the plug pin ST and separately therefrom the other (a) is connected with a sensor F which contacts in the plugged state the socket BU from the outside.

(32) The inset in the Figure (bottom right) shows an alternative embodiment in which the plug pin ST′ has a recess or groove extending in axial direction. A sensor F′, which can contact the socket BU from the inside, is arranged in this groove, insulated from the plug pin ST′.

(33) FIG. 4 shows an embodiment similar to that of FIG. 2, but provision is made here in addition to the first sensor F1 for a second sensor F2 which contacts the plug pin ST in a separate third contact region and which is guided to a connection c that is electrically separated from the socket and the first sensor F1. In this case, a suitable evaluation circuitry can make use of all three connections a, b and c.

(34) FIG. 5 shows an alternative construction to FIG. 2, in which the sensor F is not located on the bottom of the socket BU, but rather contacts the plug pin ST outside of the socket.

(35) FIG. 6 shows an equivalent circuit diagram of the formed electrical current path for the arrangement of FIG. 2. The current path between the connections a and b extends via the base and the contact fingers of the socket, from there via the contact resistance R.sub.0 of interest in the plug pin, from there via the further contact resistance R.sub.F in the second contact region between plug pin and sensor to the sensor, and finally ends in connection b. Within the solid material of the socket or plug pin, the resistance can normally be neglected compared to the contact resistances R.sub.0, R.sub.F in the contact regions.

(36) FIG. 7 shows once again the equivalent circuit diagram of FIG. 6, with a measuring device being additionally connected between the connections a and b. As indicated in the Figure, this could involve an ohmmeter to measure the resistance in the current path, which is essentially additively comprised of the two serially connected contact resistances R.sub.0 and R.sub.F. However, since both resistances are unknown beforehand, such a measurement of the sum (R.sub.0+R.sub.F) would not allow conclusions about the contact resistance R.sub.0 of interest.

(37) The arrangement of FIG. 4, in which two sensors, F1 and F2, and thus three (unknown) contact resistances R.sub.0, R.sub.F1 and R.sub.F2 are present, offers a way out of this situation. The three independent measurements of the total resistance R1=(R.sub.0+R.sub.F1) between a and b, the total resistance R2=(R.sub.0+R.sub.F2) between a and c, as well as the total resistance R3=(R.sub.F1+R.sub.F2) between b and c can then be used, however, to determine the contact resistance R.sub.0 of interest as R.sub.0=(R1+R2−R3)/2. Prerequisite for this is that the three contact resistances R1, R2, R3 can be measured with sufficient accuracy.

(38) FIG. 8 illustrates a further measuring principle which can be achieved with a single sensor (see FIGS. 2, 3, 5) and which is carried out in the operating state of the plug device. In this regard, the operating circuit is indicated in the right part of the Figure and includes an AC voltage source Q, the contact resistance R.sub.0 of interest, and a consumer R.sub.x. The operating current I.sub.B flowing in this electrical circuit generates at the contact resistance R.sub.0 a voltage drop U.sub.B. The latter can be measured with a voltmeter at the connections a, b.

(39) The unknown contact resistance R.sub.F between the sensor and the plug pin does not interfere with such a voltage measurement when the contact resistance is sufficiently small compared to the internal resistance of the voltmeter. Because the voltage drop U.sub.B generated in the operating circuit at the contact resistance R.sub.0 generates across the connection a, the voltmeter, the connection b, and the contact resistance R.sub.F a measuring current which is small due to the high internal resistance in the voltmeter. In addition, a high internal resistance compared to the contact resistance R.sub.F ensures that essentially the entire voltage drops at the voltmeter.

(40) FIG. 9 shows an extension of the circuitry of FIG. 8, with which the mentioned condition can be achieved that the contact resistance R.sub.F at the sensor should be small compared to the internal resistance of the voltmeter. For this purpose, a breakdown voltage U.sub.D of sufficient magnitude is applied to connections a and b via a switch S. In detail, the following exemplary sequence can be established:

(41) Initially, for a period of approx. 1 second, the switch S is closed (“phase 1”). A voltage U.sub.D of approx. 50 V has the effect that insulating oxides, foreign layers, etc. present on aged contacts are penetrated or fritted, so that afterwards via the contact and via a series resistor R.sub.v of approx. 500Ω a current of approx. 100 mA (=50 V/500Ω) flows. After breakdown/fritting, the voltage at the transition between sensor and plug pin is at a maximum in the order of magnitude of the melt voltage of the material of the contact surface. For nickel, for example, the melting temperature is 1453° C. and the associated melting voltage is 0.65 V. The contact resistance R.sub.F,frit sensor to plug pin, created by the fritting, is in the order of magnitude of 1 V/100 mA=10Ω.

(42) For checking purposes, a voltage measurement can be carried out when the switch S is closed. Otherwise, the purpose of phase 1 is only to establish between the sensor and the plug pin a contact resistance R.sub.F,frit, which is at least one order of magnitude lower than the high internal resistance with which the voltage measurement is carried out.

(43) At the beginning of the subsequent “phase 2”, the switch S is opened. A voltage measurement now yields the sought voltage drop U.sub.B between plug pin and socket during operation of the plug device.

(44) When the operating current I.sub.B is known from another point, an evaluation circuitry is able to calculate the contact resistance R.sub.0 from voltage drop U.sub.B and current I.sub.B. But even without knowledge of the operating current, the voltage drop represents a valuable information: In the case of a plug device, a voltage drop plug pin to socket of for example up to 25 mV is still regarded as a stable operation, but permanent operation can be called into question from approx. 50 mV onwards (this corresponds with a 125 A-plug device to a contact resistance plug pin to socket of 0.4 mΩ; other limit values for the voltage are possible).

(45) Since the crucial feature involves the measurement of the voltage drop in phase 2, phase 1 may be dispensed with entirely, when the sensor is appropriately designed.

(46) Phase 2 can be followed immediately by phase 1. However, a rest phase (“phase 3”) can also be taken between the two phases.

(47) An alternative course of a measurement in an arrangement with two sensors can look as follows (with reference to FIG. 4):

(48) Phase 1: Momentary current across a and b to “prepare” the transition between first sensor F1 and plug pin ST.

(49) Phase 2: A current is briefly impressed across connections a (socket BU) and c (second sensor F2) and is in the order of magnitude of the later operating current (across socket BU and plug pin ST). The voltage drop between a and b (first sensor F1) is measured.

(50) Phase 3: If phase 2 has resulted in an “OK”, e.g. the operating voltage/the operating current is connected via a contactor. Otherwise a warning message is triggered.

(51) The method can thus be used to check the properties of the transition plug pin to sleeve before connecting the operating current.

(52) An advantage of the described methods is their functional independence on the mode of contact plug pin to socket (“X-contact” according to WO 2016/184673 A1, torsion spring, etc.). Furthermore, they can be carried out during ongoing operation and the operating current does not falsify any measured values.

(53) FIGS. 10-18 illustrate a further method which uses a voltage induced in a current path. Such a method can be carried out in particular with the sockets BU shown in FIG. 1, which have two (or more) contact fingers KF in parallel relation to one another on a common base without making contact at the distal end.

(54) FIG. 10 shows a schematic plan view of a socket BU with four separate contact fingers KF1, KF2, KF3, and KF4, which extend over a circular ring sector of approximately 90°, respectively. Exemplary dimensions (diameter, gap distance) are indicated in mm. Two of these contact fingers, KF1 and KF3, are surrounded by a soft magnetic core MK (e.g. made of nanocrystalline or amorphous material, a ferrite, or a similar magnetic material), with the circumferentially closed core MK in turn being wrapped around a portion of a multi-layer primary coil PSP, for example from lacquered copper wire, lying outside the socket. The primary coil PSP is connected to an evaluation circuitry AS, via which suitable current and/or voltage profiles can be impressed.

(55) According to the principle of a transformer, current fluctuations in the primary coil PSP generate a changing magnetic field which is bundled and forwarded by the core MK. As can be seen from the side view of FIG. 11, the contact fingers KF1 and KF3, enclosed by the core MK, and the external contact fingers KF2 and KF4 by virtue of the plugged-in plug pin ST form a closed current path about the core MK, which can also be understood as a secondary coil with one turn. The changing magnetic field in the core MK therefore leads to an induction voltage U.sub.ind and thus to corresponding induction currents within this current path (see the arrow tips indicated in FIG. 10 in the contact fingers KF1, KF3 and arrow ends in the contact fingers KF2 and KF4). These induction currents must flow across the contact regions between the contact fingers and the plug pin ST and thus constitute the desired sensor for the contact properties or the contact resistance in the contact regions.

(56) The described structure involves ultimately two series-connected parallel connections of two contact fingers each according to the equivalent circuit diagram of FIG. 12, with the material resistances being neglected for the sake of simplicity. The total resistance in this induction circuit of the “secondary coil” is calculated with
R.sub.13=R.sub.01R.sub.03/(R.sub.01+R.sub.03)
R.sub.24=R.sub.02R.sub.04/(R.sub.02+R.sub.04)
as
R.sub.tot,ind=R.sub.13+R.sub.24

(57) In contrast to the circuitry of FIG. 12, the contact fingers are connected in parallel in normal operation of the plug device. The total resistance is then
R.sub.tot,ind=R.sub.13R.sub.24/(R.sub.13+R.sub.24)
If all four contact resistances are the same, i.e.
R.sub.01=R.sub.02=R.sub.03=R.sub.04=R.sub.0,
then
R.sub.tot,ind=R.sub.0 and R.sub.tot,B=R.sub.0/4, i.e. R.sub.tot,B=R.sub.tot,ind/4 applies.

(58) If the four contact resistances assume unequal values (in practice statistically distributed), the measured value R.sub.tot,ind determined with the method described here can exceed the value of R.sub.tot,B, but is therefore “on the safe side”.

(59) In a modified approach, the evaluation circuitry AS could be configured on the primary winding PSP in such a way as to not only determine the complex input impedance Z and thus R.sub.tot,ind, but also to measure with I.sub.B13 the portion of the operating current flowing via the contact fingers KF1 and KF3 enclosed by the magnetic core MK in FIG. 10 (cf. transformer/current transformer principle). When the evaluation circuitry also knows the current I.sub.B24 across the other contact fingers KF2, KF4 or the operating current I.sub.B itself (for example through measurement), the following relations can be utilized:
I.sub.B=I.sub.B13+I.sub.B24
I.sub.B24R.sub.24=I.sub.B13R.sub.13
R.sub.tot,ind=R.sub.13+R.sub.24
R.sub.tot,B=R.sub.13R.sub.24/(R.sub.13+R.sub.24)

(60) The last two relations have already been used before. When this system of equations is solved for R.sub.tot,B, this leads to
R.sub.tot,B=R.sub.tot,indI.sub.B13I.sub.B24/(I.sub.B13+I.sub.B24).sup.2

(61) Using this formula, the evaluation circuitry AS can yield not only information about R.sub.tot,B, but R.sub.tot,B itself.

(62) To determine R.sub.tot,ind, the evaluation circuitry AS of FIGS. 10, 11 measures the (complex) input impedance Z of the primary coil PSP, which input impedance depends, i.a., on R.sub.tot,ind.

(63) In this regard, FIG. 13 shows the result of a simulation with typical parameters for the used components. The real part Re (Z (R.sub.0)) and the imaginary part Im (Z (R.sub.0)) of the complex input impedance Z are shown as a function of the contact resistance R.sub.0 (with above assumption that all individual contact resistances are the same). It is readily apparent from the graphic that the input impedance Z of the measuring circuit changes significantly when the contact resistance R.sub.0 changes, An evaluation circuitry AS connected to the measuring circuit is thus able to determine the sought contact resistance on the basis of the input impedance.

(64) The simulation assumes that the magnetic core MK is designed in such a way that the magnetic flux density of 1 Tesla is attained when excited with 127 A. If the excitation is even stronger, saturation of the magnetic circuit can be expected. This saturation can occur, for example, when, during heavy start-up for several seconds, currents flow considerably above the nominal current, of which currents the two contact fingers that are enclosed by the core account for about half. The downstream electronics will therefore measure falsified Z values in the range of the operating current peak value, but will detect meaningful values in the vicinity of the operating current zero crossing. The evaluation circuitry AS can therefore optionally be designed in such a way that it carries out a measurement synchronized with the operating current (50 Hz), in particular only near the zero crossing of the operating current.

(65) The described method with the induction of a measuring current can, optionally, also be carried out using a sensor. For example, a sensor F according to FIG. 2 could assume the role of the contact fingers lying outside (or inside) the magnetic core. Furthermore, a split plug pin could replace several contact fingers.

(66) The principle on which the above approaches are based is that per induction the contact points between socket and plug phi can be examined with the help of a magnetic core, when being able to form a dosed path (curve, line) guided through electrically conductive material and traversing the area spanned by the magnetic core, only with simultaneous participation of contact dements of the first and second plug devices.

(67) These conditions can also be achieved hi alternative constellations, as already illustrated (Note: For sake of clarity, the turns of a primary winding, which wrap around the magnetic core in a manner similar to that of a transformer, are not shown hi FIGS. 14 to 18).

(68) FIG. 14 shows the arrangement of a magnetic core MK which runs around a plug phi ST within a socket BU, with contact regions being present between plug pin ST and socket BU on both sides next to the core MK in axial direction, A current path of the desired type then extends from the plug pin ST via a first of the contact regions plug pin/socket to the socket BU (or its contact finger), and from there via the second of the contact regions plug pin/socket back to plug pin ST.

(69) Another approach is depicted in FIGS. 15 to 18. In this case, only the contact region between plug pin ST and socket BU effectively runs through the area of the magnetic core MK.

(70) As can be seen from the perspective view of FIG. 15, the shape of the magnetic core MK, which is rectangular in plan view, bears upon the plug pin ST in a semi-cylindrical manner.

(71) In the axial sectional view of FIG. 16, this three-dimensional shape of the magnetic core MK appears as a rectangle. FIG. 16 further illustrates the socket BU, which includes two parallel contact fingers KF1 and KF2. In the area of an outer sleeve spring HF wrapped around 360°, the contact fingers KF1, KF2 touch the plug pin ST in first and second contact regions, respectively. The first one of these two contact regions, between the first contact finger KF1 and the plug pin ST (on the left in FIG. 18 in cross section along the line “B”), is enclosed by the magnetic core MK.

(72) The current path in which the desired induction voltage is generated runs from the plug pin ST via the first contact region KF1/ST, through the first contact finger KF1, via the bottom of the socket BU and/or the sleeve spring HF to the second contact finger KF2, from there via the second contact region KF2/ST back to the plug pin ST.

(73) An advantage of this embodiment is its applicability with sockets having free-standing contact fingers as well as (as shown) with sockets having distally connected contact fingers.

(74) In the examples presented so far, there was no further discussion as to whether the involved current paths were continuously conductive or (only) capacitively conductive. In any case, all examples work with a conductive conductivity, in which a permanent direct current can be conducted through the current path.

(75) However, it is also possible that an involved current path is only capacitively conductive at at least one point, i.e. a capacitance in series connection. Such a current path is unable to conduct permanent direct current, only alternating current. Apart from that, the described examples work unchanged.

(76) The use of a current path that is possibly only capacitively conductive is illustrated in FIG. 19. This shows schematically a current path which includes a plug pin ST, a socket BU, and a socket-side sensor F. In contrast to the arrangement of FIG. 2, provision is made within the socket BU for an electrically insulating pressure pad DK which presses the sensor F onto the plug pin ST. In this way, a sufficiently tight contact between sensor F and plug pin ST should be ensured. Even when oxide layers or the like should prevent conductive electrical conduction from the sensor F to the plug pin ST, a sufficiently high capacitance is formed between the sensor F and the plug pin ST to permit a usable high (alternating) current flow in the current path. Measures like, for example, the described fritting of the contact would therefore not be necessary.

(77) The electrical equivalent circuit diagram of such an arrangement is shown superimposed in FIG. 19, with C.sub.F being the capacitance and R.sub.F being the ohmic resistance between sensor F and plug pin ST. R.sub.iso designates the insulation resistance between the sensor F and the socket BU and is typically in the order of at least 5 MΩ.

(78) Optionally, in such an embodiment, only the conditions at the transition between sensor F and plug pin ST may initially be determined in a separate measurement, i.e. C.sub.F and R.sub.F, for example by a measurement without operating current and/or a measurement without inserted plug pin ST. The result of this determination can then enter into the implementation and/or evaluation of the subsequent actual measurement in order to improve its reliability.

(79) Another expansion of the invention is illustrated in FIG. 20. This approach involves an independent detection of the condition whether a plug pin ST is plugged into the socket BU at all. This makes it possible to differentiate between the cases “no plug pin inserted” and “infinitely high contact resistance”.

(80) The plugging state is preferably detected mechanically in order to be independent on the current feed. As indicated in FIG. 20, the sensor F can, for example, be mechanically coupled to the switch of a detection circuitry DS. The sensor F is moved as an inserted plug pin ST touches, and this movement is converted mechanically into an opening (or closing) of the switch of the detection circuitry DS. The switch status can then be externally queried again.

(81) FIG. 21 shows a modification of the arrangement of FIG. 11, with only the differences being described hereinafter and otherwise reference is made to the above description.

(82) In the modified arrangement, an alternating magnetic field is again induced in the magnetic core MK via the primary coil PSP by connecting an alternating voltage source U.sub.p to the primary coil PSP (this alternating voltage source is a part of the evaluation circuitry AS, not shown in detail in FIG. 11).

(83) The (reactive) effects of a current flow induced by the magnetic alternating field of the core in the current path of interest are, however, no longer ascertained via the primary coil PSP in this arrangement. Instead, a further coil is arranged around the current path and is connected to the evaluation circuitry AS as measuring coil MS. A measurable electrical quantity (e.g. voltage) is induced in the measuring coil MS directly by the current flow in the current path that the measuring coil surrounds.

(84) As indicated in the Figure, the entire current path can run through the measuring coil MS, when the measuring coil MS encloses all of the contact fingers of a current direction (here KF2 and KF4 or KF1 and KF3) encloses. As an alternative, only part of the current path could be detected, for example when the measuring coil MS were to run only around a single contact finger.

(85) Finally, the most important aspects of the invention, which can be implemented individually or in combination, are listed again, with bracketed reference symbols referring to exemplary embodiments of the Figures:

(86) A) Method for the determination of the electrical contact properties (RÜ) in the contact region between a first contact element (BU) of a first plug device (SV1) and a second complementary contact element (ST) of a second plug device (SV2), with the properties of a current path being evaluated, which current path includes the first contact element (BU), the contact region, and from the second plug device (SV2) only the following: the second contact element (ST) and, optionally, further contact elements of the second plug device (SV2),
wherein
a) an induction voltage (Uind) is generated in the current path so that measurements can be made in the current path to allow conclusions about the contact properties of interest; and/or
b) the current path includes at least one sensor (F, F1, F2) which is arranged in the first plug device (SV1) and contacts the second contact element (ST) when plugged together; and/or
c) the current path includes at least two sensors (F, F1, F2) which are arranged in the first plug device (SV1) and contact the second contact element (ST) when plugged together; and/or
d) the current path includes at least one sensor (F, F1, F2) which is arranged in the first plug device (SV1) and contacts the second contact element (ST) when plugged together, and wherein a breakdown voltage (UD) is applied between the sensor (F, F1, F2) and the second contact element (ST); and/or
e) the evaluation is synchronized with the course of an operating current (IB) across the contact region; and/or
f) the current path includes at least one sensor (F, F1, F2) which is arranged in the first plug device (SV1) and contacts the second contact element (ST) when plugged together, and wherein a capacitance of more than approximately 10 pF is provided in the current path to allow an alternating current flow in the current path; and/or
g) the voltage drop (UB) is measured across the current path and occurs when an operating current (IB) flows across the contact region; and/or
h) the current path forms the secondary coil on a magnetic core (MK) on which a primary coil (PSP) is also arranged.
B) Plug device (SV1) with at least one first contact element (BU), which contacts in a contact region a complementary second contact element (ST) of a second plug device (SV2) when plugged together, with the plug device having a current path, which includes the first contact element (BU), the contact region, and from the second plug device (SV2) only the following: the complementary second contact element (ST) and, optionally, further contact elements of the second plug device (SV2),
wherein
a) the plug device includes means for generating an induction voltage (U.sub.ind) for a method according to point A); and/or
b) the plug device includes at least one sensor (F, F1, F2), which contacts a complementary second contact element (ST) of the second plug device (SV2) when plugged together; and/or
c) the plug device includes at least two sensors (F, F1, F2) which contact a complementary second contact element (ST) of the second plug device (SV2) when plugged together; and/or
d) the plug device includes at least one sensor (F, F1, F2), which contacts a complementary second contact element (ST) of the second plug device (SV2) when plugged together, and includes means for generating a breakdown voltage (UD); and/or
e) the plug device includes a magnetic core (MK), which surrounds the first contact element (BU) and/or a sensor (F) and/or the contact region; and/or
f) the plug device includes an evaluation circuitry (AS) for evaluating the properties of a current path according to a method according to point A).