WO2000063932A1

WO2000063932A1 - System for diagnosing electric contact with mechanical displacement

Info

Publication number: WO2000063932A1
Application number: PCT/FR2000/000976
Authority: WO
Inventors: Denis Flandin; Hervé Carton
Original assignee: Crouzet Automatismes
Priority date: 1999-04-20
Filing date: 2000-04-14
Publication date: 2000-10-26
Also published as: FR2792766B1; AU4124500A; FR2792766A1

Abstract

The invention concerns a system for diagnosing electric contact with mechanical displacement for switching electrical loads. The diagnostic system comprises: a device for detecting (10) current inversion in the electric contact (12), with an analog output (SA) supplying, when the contact is closed, a voltage Vs proportional to the residual voltage of the contact Vc, that is proportional to the current (Ich) flowing in the contact (12) and in the electric circuit (CE); a microcontroller (18) receiving at an analog input (20) the voltage Vs in the analog output (SA) of the detecting device and at a logic input (22) a first logic signal (SO) supplying to the microcontroller data concerning the electric circuit zero crossing current, the microcontroller comprising first means for acquiring the maximum analog voltage level Vsm at the terminals of the contact (12) at each alternation, at the time a maximum current passes into the load (14). The invention is applicable to electric switches, electromechanical relays, hybrid relays, diagnostic system of an electric contact.

Description

ELECTRICALLY MOVABLE ELECTRICAL CONTACT DIAGNOSTIC SYSTEM

The invention relates to a system for diagnosing an electrical contact with mechanical displacement for switching electrical charges, comprising an electronic detection device intended to ensure optimum switching of charges on an electrical circuit (or network). The connection and disconnection of loads on an electrical circuit is ensured by switching systems among which are electrical switches, electromechanical relays, hybrid relays, and circuit breakers for network protection.

Switching loads on or off on a live electrical circuit produces phenomena that can disturb the electrical circuit and degrade the switching system. For example, the switching by an electromechanical relay of the loads on a live electrical circuit, and particularly when the loads are inductive, produces arcs between the contacts of the relay when they are opened or closed. On closing when the current is established in the electrical circuit and on opening when the current is interrupted in the same circuit.

These switching arcs degrade the quality and reduce the reliability of the contacts by increasing their contact resistance producing an abnormal heating or even destroying the contacts by the passage of current.

The closing of the electrical contacts with mechanical movement of a switch, following a switching order, can be carried out at any time during a period of the voltage of the electrical circuit or in the case of opening of the contact, at any time during a period of electric current in the circuit. These closings and openings at a random time during a period of voltage and current respectively, produces, in unfavorable switching cases, strong electric arcs between the contacts. For example closing the electrical contact, when the voltage of the electrical circuit is close to its maximum, will produce one or more strong electric arcs which will result, on the one hand, in the rapid degradation of the electrical contacts of the relay by sparking, and on the other hand, strong disturbances of the circuit which may have harmful effects for the loads connected to this circuit. A similar phenomenon occurs when the contact opens when the current in the electrical circuit is interrupted.

A device for detecting current reversal in an electrical contact with mechanical displacement comprises means for generating, when the electrical contact is closed, a first logic signal changing state when the residual voltage across the electrical contact, produced by the passage of current in the contact, change of polarity.

The device makes it possible, when the electrical contact is closed, to detect the passage of current in the electrical circuit and consequently in the electrical contact, by a value substantially equal to 0 amperes. Indeed, the closed electrical contact has a very low resistance, but not zero. This contact resistance produces a residual electrical voltage drop Vc across the contact substantially proportional to the current flowing through the contact. It is this effect which will be used to determine the inversion of the current in the electric circuit and its passage by a value substantially zero during the change of polarity.

In addition, the residual voltage across the contact can be used to measure the current flowing in the load. To this end, the detection device further comprises an analog output providing, when the contact is closed, with respect to a voltage reference, a voltage Vs proportional to the residual contact voltage Vc is proportional to the current flowing in the contact and in the electrical circuit.

Said means provide, when the electrical contact is open, a second logic signal changing state when the voltage of the electrical circuit at the terminals of the electrical contact changes polarity.

The detection device can be associated with a microcontroller receiving the voltage across the terminals of the electric contact with mechanical displacement in order to produce according to the invention a system for diagnosing the electric contact. In one embodiment of the diagnostic system, the first and second logic signals are supplied on the same output of the detection device.

The diagnostic system according to the invention comprising such a device for detecting the passage through zero of current finds a particularly advantageous application in the diagnosis of electrical switching systems such as electromechanical contactors, electromechanical relays, electromechanical relays whose contact is assisted by semiconductors (hybrid relays) or any other device comprising an electric contact with mechanical displacement. Other characteristics and advantages of the invention will appear on reading the description of an exemplary embodiment of diagnostic systems according to the invention in which:

- Figures 1a, 1b and 1c show a block diagram of a diagnostic system according to the invention having a current detection device connected to the terminals of an electrical contact.

- Figures 2a and 2b show as a function of time t respectively the voltage across the open electrical contact and the second logic signal at the output of the detection means.

- Figures 3a, 3b and 3c show as a function of time t respectively the current in the closed electrical contact, the first logic signal at the output of the detection means and the voltage across the terminals of the closed contact.

- Figure 4 shows a block diagram of a hybrid relay comprising the diagnostic system according to the invention. FIG. 1a represents a block diagram of a diagnostic system according to the invention having a current detection device 10 connected in parallel on the two terminals A and B of an electrical contact 12 in the open position. Figure 1b shows the same block diagram of Figure 1a with the electrical contact in the closed position. The electrical contact 12 is inserted in series with a load 14 in an electrical circuit CE with alternating current under a voltage U applied to two supply terminals C1 and C2 of the electrical circuit.

The means of the detection device 10 comprise a limiting resistor R1 in series with two diodes D1 and D2 mounted in head-to-tail parallel and a comparator 16 having two differential inputs E1 and E2 connected to the terminals of the diodes D1 and D2, a logic output SO of the comparator 16 providing a logic signal changing state with the change of the polarity of a voltage Vd at the terminals of the diodes D1 and D2.

We will then describe the operating principle of the detection device connected to terminals A and B of electrical contact 12.

When the electrical contact 12 is open (Figure 1a) the current I in the electrical circuit will be considered of negligible value compared to a current Ich when the contact 12 is closed supplying the load

14. In the case of the open electrical contact, the voltage V across the electrical contact is substantially identical to the voltage U of the electrical circuit.

The voltage V across the contact produces an alternating current il in the diodes D1 and D2 limited by the limiting resistor R1. The current it flows in one direction one of the two diodes in head-to-tail parallel, during one of the two half-waves of the voltage V across the terminals of the electrical contact 12, and in the other direction the other diode during the other alternating voltage V across the contact. At each alternation the diodes D1 and D2 will be alternately one in the on state and the other in the blocked state. The voltage Vd at the terminals of the diodes applied to the differential inputs E1 and E2 of the comparator 16 will be substantially the conduction threshold voltage of the diodes, ie approximately 0.7 volts for a silicon diode.

During a voltage alternation, the threshold voltage of the diodes applied to the differential inputs E1 and E2 of the comparator is of a polarity such that the output of the comparator provides a signal in a logic state (0, 1) and when Another alternation of opposite polarity, the threshold voltage being inverted, the comparator provides the complementary logic state. Figures 2a and 2b show as a function of time t respectively the voltage V across the electrical contact and the voltage at logic output SO of the comparator. A rising edge occurring at the logic output SO of the comparator when the polarity of voltage V changes in one direction, then a falling edge during the next polarity change, of the same voltage V, in the other meaning.

When the electrical contact is closed (Figure 1b), an electrical current Ich supplying the load is established in the electrical circuit through the electrical contact 12. The electrical contact 12 has a resistance r contact whose value is linked to different parameters among which we can cite, the pressure and the contact surface, the type of metal in contact or the coal state of the contact. The smearing of the electric contact with mechanical displacement results in the appearance of carbonized particles on the contact pads produced by the combustion of dust and other particles during the switching arcs. Charcoal produces an increase in the resistance r of the contact and consequently a heating of the contact during the passage of the current. The same current Ich in the contact produces a higher residual voltage Vc across the contact when the level of the coal increases.

This low value resistor r produces a difference in residual contact potential Vc between the terminals of the electrical contact when the current flows:

Vc = r.lch The contact resistance r can be accepted as purely resistive at the usual operating frequencies of electrical circuits of the order of 50 to 400 Hz.

In a normal operation of the closed contact crossed by the electric current, the potential Vd across the terminals of the diodes D1, D2 applied to the differential inputs E1 and E2 of the comparator 16 will be substantially identical to the residual contact potential Vc, this potential Vc being less than the threshold voltage of diodes D1 and D2 but of a level sufficient to effect the switching of the comparator with change of state at its logic output SO when the polarity of the residual contact potential Vc at the terminals of the contact changes.

For example, the residual voltage Vc at the terminals of the closed electrical contact of a power relay traversed by a maximum current of 20 amperes is of the order of 150 millivolts.

In this closed contact configuration, the change in logic state (0 and 1) at the output of comparator 16 occurs when the polarity of the residual contact potential Vc changes, or when the direction of current changes in the electrical circuit since this potential residual contact Vc is proportional to the current Ich in the electrical circuit (Vc = r.lch).

Figures 3a and 3b show as a function of time t respectively, the residual voltage Vc across the contact, image of the current Ich in the electrical circuit CE (and in contact 12), and the logic voltage at output SO of the comparator. A rising edge occurs at the logic output SO of the comparator during a change of polarity in one direction of the current Ich then a falling edge during the next change of polarity in the other direction.

The detection device 10 supplies the same logic output SO of the comparator 16 on the one hand, when the contact is open, a first logic signal changing state at each passage substantially by 0 volts of the voltage U of the electrical circuit, the device detecting the 0s of the electrical circuit, and on the other hand, when the contact is closed, a second logic signal changing state at each passage by substantially 0 amperes of the current in the load, the device detecting the 0s of current in charge.

The diagnostic device according to the invention uses the residual voltage Vc across the terminals of the closed contact to measure the current flowing in the load. To this end, the detection device comprises an analog output SA taken at the junction of the limiting resistor R1 and the diodes D1, D2. This analog output SA supplies, when the contact is closed, with respect to a reference potential Rf, a voltage Vs proportional to the residual contact voltage Vc, ie, proportional to the current Ich flowing in the contact and in the electrical circuit. FIG. 3c shows the voltage Vs across the terminals of the closed contact, as a function of time.

The detection device 10 is associated with a microcontroller 18 in order to produce the diagnostic system for the electrical contact. The microcontroller 18, represented in FIGS. 1a and 1b, receives at an analog input 20 the voltage Vs at analog output SA of the detection device and at a logic input 22 the logic level at output SO of the comparator providing the microcontroller with the passage information by 0 of current of the electric circuit. The microcontroller comprises first means for carrying out, when the contact is closed, an acquisition of the level of the maximum analog voltage Vsm at the terminals of the contact 12 at each half-wave at the time of passage through a maximum of the current in the load 14; for this purpose, the acquisition by the microcontroller of the maximum voltage Vsm corresponding to a maximum residual voltage Vcm at the terminals of the contact and at a maximum current in the electrical circuit, is effected after a waiting time dT starting on passing through a zero of current, the waiting time dT being substantially equal to a quarter of the period T of the alternating current in the circuit. The analog value of the maximum voltage Vsm across the contact can be interpreted in several ways.

If the maximum voltage Vsm exceeds a predefined value:

- either the current through which the contact flows is too high;

- either the contact resistance increased by wear; - either it is the product of both.

If the analog voltage Vsm is lower than the preset value:

- either the contact is in good condition and the current is less than or equal to the maximum allowed;

- either the contact is a bit worn and the current is lower than the maximum allowed.

In addition, during a contact closure period, the microcontroller can, by successive acquisitions of the current over time, monitor the evolution of the current in the load and initiate an action of disjunction of the electrical circuit. To this end, the microcontroller includes second means for storing successive acquisitions of the maximum current over time and for opening the electrical contact controlled by an actuating coil 24 of the contact 12, when the evolution of the current comes out of the drawing. of a smoke curve.

Several actions can be taken following the observations made previously:

- Either, we consider the minor fault (for example, beginning of wear of the contacts) and the only action to be carried out will be the activation of a wear indicator information signaling the need for a preventive action to replace the relay. - either the fault is major (advanced wear of the contacts, significant overload) and the microcontroller performs an action controlling the opening of the contact, equivalent to a trip.

In a highly reliable configuration, temperature information can also be taken into account by the microcontroller and interpreted together with the other information. Such a system comprising the microcontroller exploiting, inter alia, the information provided by the detection device according to the invention makes it possible to carry out diagnostic functions of the state of the electrical contacts and possibly to detect an overload of the electrical circuit and induced actions of break.

The evolution of the state of the contact can be followed very precisely by using additional information of current Ich in the electrical circuit CE. This current measurement is carried out by an additional means 26 for conventional current measurement other than the measurement of the residual voltage Vc at the terminals of the contact, for example using a shunt, a hall effect sensor or other. The comparison between the current measured by this other means 26 and that resulting from the measurement of the residual voltage Vc across the terminals of the electrical contact 12 makes it possible to determine the state of the contact and its evolution. For example, it is thus possible to determine the level of “coaling” of the contact.

FIG. 1 c represents the device of FIG. 1 b comprising the additional means 26 for measuring the current Ich.

In this device of FIG. 1 c, the microcontroller 18 receives from additional means 26, at another analog input 28, additional information Isp of current Ich in the electrical circuit, the microcontroller further comprising first means of acquiring current maximum in the load from the residual voltage Vc, means for acquiring the maximum current measured by the additional means 26. The microcontroller furthermore comprises means for calculating the difference between the two current measurements, the current measured by the additional means 26 on the one hand and that resulting from the measurement of the residual voltage Vc across the contact on the other hand. This current difference can be used to determine, for example with respect to a threshold which can be a function of the maximum current level, if the contact is defective and trigger an alarm or open the electrical contact 12.

Electromechanical type relays comprise one or more electrical contacts with mechanical displacement coupled to a mobile element of the magnetic circuit of an electromagnet. The control of the electromagnet is carried out by the supply of its coil which by producing a flux induction in the magnetic circuit causes the moving element to move and the electrical contacts of the relay to close or open.

The electromechanical relays have delays between the moment when the electromagnet is controlled by the supply of its coil and the actual closing or opening of its electrical contact. These delays are mainly due to the inertia of the moving element of the relay.

Some control systems for electromechanical relays take these delays into closing or opening of the contacts to take into account switching at favorable moments of voltage or current in the electrical circuit.

These delay times are liable to vary over time through aging, wear of the relay mechanism or by other causes. These variations in the delay times can be large enough for the closings and openings of the electrical contact to occur at instants too far from the favorable switching instants initially provided. The detection device according to the invention associated with the microcontroller makes it possible, by measuring the voltage at the terminals of the electrical contact, to determine the moment of closing and opening of the contact and to deduce therefrom the actual delay time existing between the moment of order and the moment of closing or opening of the contact. To this end, the microcontroller 18 comprises third means for measuring on the one hand, the closing delay time dTF between the moment of the supply of the actuating coil 24 of the contact 12 and the instant of closing of the contact. and on the other hand, the opening delay time dTO between the moment when the supply to the contact actuation coil is interrupted and the time when the contact opens.

The detection device can be used in switching systems comprising an electric contact with mechanical displacement to detect the zero crossing of voltage and current in the electric circuit in which the relay is inserted. The zero crossing information provided by the detection device will be used by the relay control system using the microcontroller comprising fourth means for closing the contact at an instant close to zero crossing of the voltage across the terminals of the contact and its opening at an instant close to zero current crossing in the circuit. The diagnostic system described will be programmed by software for the microcontroller 18 used for the diagnosis of the electrical contact in the electrical circuit CE, the software comprising means for making the microcontroller execute the following successive operations: - acquisition of the voltage level analog maximum Vsm at the terminals of contact 12, image of the maximum current in the electrical circuit CE;

- memorization of the successive acquisitions of the maximum current over time; - provision of information on the state of the contact based on said acquisitions.

The software can also further include a step of controlling the opening by the microcontroller of the electrical contact by an actuating coil, when the evolution of the maximum current exceeds the limits defined by a maximum current curve as a function of the time of opening of the electrical contact; this curve is commonly called the "smoke curve".

The diagnostic system according to the invention detection device is particularly suitable for uses in switching systems, for example in an electromechanical relay comprising an electric contact with mechanical displacement, a relay control circuit using the first logic signal of the detection device. detection. A microcontroller of the electromechanical relay control circuit can use the first logic signal to calculate an instant of generation of a contact opening command.

In a variant of the electromechanical relay, the contact comprises a parallel triac controlled by the microcontroller at an instant calculated as a function of said first logic signal.

By way of example, FIG. 4 shows a diagram of an electromechanical relay comprising a diagnostic system according to the invention having a detection device 30, an electrical contact 32 assisted by switching by a triac 34, and a control circuit of the relay using a microcontroller 36.

Contact 32 of the electromechanical relay is inserted in series with a load impedance 38 in an electrical network RE under voltage U alternative between two supply terminals C3 and C4. The triac 34, in parallel on the contact 32 is controlled in the state passing through the control circuit, during the closings or openings of the contact largely eliminating the switching arcs at the terminals of the contact. A regulated power supply 40 supplies, from the voltage U of the electrical network RE and relative to a line L1 connected to the terminal C4 of the network, a supply voltage Va1 of a coil 42 for actuating the contact 32 and a regulated supply Va2 of the detection device 30 according to the invention and of the microcontroller 36. The line L1 is in this case considered to be at the reference potential of the detection device.

A logic control signal Xr is applied to a logic control input 44 of the microcontroller 36 through a photo-coupler 46 of galvanic isolation connected to a control input of the relay EC. A logic state of the logic control signal Xr causes the relay to close and the complementary logic state to open it.

The microcontroller 36 provides on the one hand, by a first logic output 48 through an optotriac 50 a conduction signal of the triac 34 and on the other hand by a second logic output 52, through a follower transistor T1 a current d supply of the coil 42 actuating the electrical contact 32.

The detection device 30 comprises the limiting resistor R1 in series with in series with the two diodes D1 and D2 mounted in head-to-tail parallel and a comparator 56 comprising two differential inputs.

The inverting input (-) of the comparator 56 with differential inputs is connected through an input resistor R2 at the connection point of the diodes D1 and D2 and the limiting resistor R1, and the non-inverting output (+) at the point of variable potential of a resistive divider bridge comprising a potentiometer P1 in series with a heel resistance R4, the divider bridge being connected on the side of the potentiometer to the regulated supply Va2 and on the side of the heel resistance at the reference potential (line L1). A feedback resistor R5 connects the logic output SO of the comparator to the non-inverting input of comparator 56.

The logic output SO of the current and voltage zero crossing detection device 30 is connected to a logic detection input 58 of the microcontroller 36. One side of the diodes D1, D2 in head-to-tail parallel, of the detection device 30, is connected to the terminal of the contact 32 connected to the reference potential (C4), the other side of the diodes being connected through the limiting resistor R1 and a ballast resistor R6 at the other terminal of contact 32. The junction point of the limiting resistor R1 and the ballast resistor R6 being connected to the reference potential (line L1) by a zener diode Z1 limiting the zener voltage Vz the voltage across the detection device when the switch is open and the electrical circuit energized. The detection device provides the microcontroller, as described above, with passage information by 0 volts of the voltage of the electric circuit, when the contact is open and passage information by 0 amps of current in the same circuit, when the contact is closed. This information can thus be used by the microcontroller to close and open the electrical contact at the most favorable times to avoid arcs between its terminals. For example, the microcontroller can close the contact 32 when the voltage across the contact is close to 0 volts and open when the current in the electrical circuit is close to 0 amperes.

An analog input 60 of the microcontroller is connected, through a protective resistor R7, to the terminals of the diodes connected to the limiting resistor R1. The analog input 60 receives, when the contact 32 is closed, the residual voltage Vc across the contact produced by the flow of current (with Vc = r.lch, Ich being the current in impedance 38). This residual analog voltage Vc is exploited by the microcontroller 36 in order to make diagnoses on the state of the contact 32, to carry out disjunctions (opening of the contact) according to criteria of maximum voltage and speed of variation of this voltage or other possible functions . The diagnostic system according to the invention allows:

- monitoring of contact wear by measuring their contact resistance by the detection device and the possibility of making predictive diagnostics and self-tests;

- carrying out disjunction functions as a function of a smoke curve integrated in a computer (or a microcontroller); - detecting the closing of the contact by detecting the passage of the threshold voltage of the diodes;

- detecting the opening of the contacts by detecting a voltage below the threshold of the diodes;

Claims

1. Diagnosis system for an electrical contact with mechanical displacement (12) in an electrical circuit, characterized in that it comprises: - a detection device (10) for current reversal in the electrical contact (12, 32 ), the device having means (R1, D1, D2, 16, 56) for generating, when the electrical contact is closed, a first logic signal (SO) changing state when a residual voltage Vc across the electrical contact, produced by the passage of a current Ich in the contact, changes polarity, the device further comprising an analog output (SA) providing, when the contact is closed, with respect to a voltage reference (Rf, L1), a voltage Vs proportional to the residual contact voltage Vc is proportional to the current (Ich) flowing in the contact (12, 32) and in the electrical circuit (CE, RE). - a microcontroller (18) receiving at an analog input (20) the voltage Vs at analog output (SA) of the detection device and at a logic input (22) the first logic signal (SO) providing the microcontroller with the passage information by 0 of the electrical circuit current, the microcontroller comprising first means for acquiring the level of the maximum analog voltage Vsm at the terminals of the contact (12) at each half-wave, when a maximum of the current passes through a load ( 14).

2. diagnostic system of an electrical contact according to claim 1, characterized in that the microcontroller (18) receives from additional means (26), at another analog input (28), additional information Isp of current Ich in the electrical circuit, the microcontroller (18) comprising, in addition to the first means for acquiring the maximum current in the load from the residual voltage Vc, means for acquiring the maximum current measured by the additional means (26).

3. diagnostic system of an electrical contact according to claim 2, characterized in that the microcontroller (18) further comprises means for calculating the difference between the two current measurements, the current measured by the additional means (26) on the one hand and that resulting from the measurement of the residual voltage Vc at the terminals of the contact on the other hand, this current difference can be used to determine, for example with respect to a threshold which can be a function of the maximum current level, if the electrical contact (12 32) is defective and trigger an alarm or open the electrical contact.

4. diagnostic system of an electrical contact according to one of claims 1 to 3, characterized in that said means (R1, D1, D2, 16, 56) provide, when the electrical contact is open, a second signal logic changing state when a voltage V across the electrical contact changes polarity.

5. diagnostic system of an electrical contact according to one of claims 1 to 4, characterized in that the acquisition of the maximum current in the electrical circuit is carried out after a waiting time dT beginning with the passage by a 0 current, the waiting time dT being substantially equal to a quarter of the period T of alternating current in the circuit.

6. Diagnostic system according to one of claims 2 to 5, characterized in that the microcontroller comprises second means for memorizing successive acquisitions of the maximum current over time and for opening the electrical contact controlled by an actuating coil , when the evolution of the current goes outside the limits defined by a maximum current curve as a function of time.

7. The diagnostic system as claimed in claim 5, characterized in that the microcontroller comprises third means for measuring on the one hand, the closing delay time dTF between the moment of supply of the contact actuation coil and the instant of closing of the contact and on the other hand of the time of delay of opening dTO between the moment of the interruption of the supply of the contact actuating coil and the instant of opening of the contact.

8. Diagnostic system according to one of claims 4 to 7, characterized in that the microcontroller comprises fourth means for closing the contact at an instant close to zero crossing of the voltage across the terminals of the contact and its opening at a time close to zero current crossing in the circuit.

9. Diagnostic system according to one of claims 4 to 6, characterized in that the first and second logic signals are supplied on the same output (SO).

10. Dignostic system according to one of claims 4 to 8, characterized in that the means for generating the first and second logic signals comprise a limiting resistor R1 in series with two diodes (D1, D2) mounted in parallel head- spade and a comparator (16) having two differential inputs (E1, E2) connected to the terminals of the diodes (D1 and D2), a logic output (SO) of the comparator (16) providing a logic signal changing state with the change of the polarity of a voltage Vd across the diodes (D1, D2).

11. Diagnostic system according to one of the preceding claims, characterized in that the first, second, third, fourth means as well as the means for acquiring the current measured by the additional means and the means for calculating the current difference of the microcontroller are a series of programming instructions for the microcontroller.

12. Diagnostic system according to one of claims 3 to 11, characterized in that the additional means (26) of conventional current measurement is either a shunt, or a hall effect sensor or other.

13. Electromechanical relay, characterized in that it comprises a diagnostic system according to one of the preceding claims, the relay being assisted in switching by a triac 34.

14. Method for diagnosing a diagnostic system for an electrical contact (12) according to one of claims 1 to 12, characterized in what it includes means for executing by the microcontroller (18) the following successive operations:

- acquisition of the level of the maximum analog voltage Vsm at the terminals of the contact (12, 32), image of the maximum current in the electric circuit CE;

- memorization of the successive acquisitions of the maximum current over time;

- provision of information on the state of the contact based on said acquisitions.