EP0073742A1 - Release mechanism for a circuit breaker - Google Patents

Abstract

A bimetallic strip (4), mounted in series with the apparatus to be protected, is integral with an inertia arm (5) pivoted about an axis (6) and carrying a mass (5a). The free end of this strip (4) is connected to a bistable escapement disc (8) by an actuating rod (10). This disc (8) is clamped radially inside the elastic rim of a cover (7) which retains it in one or other of its stable positions. An adjustable stop (13) serves to limit the displacement of the inertia arm (5), following the deformation of the strip (4). In the event of a sudden increase in the current, the inertia of the mass (5a) prevents the pivoting of the arm (5) before the strip (4) displaces the centre of the disc (8) beyond its critical position. <IMAGE>

Description

The present invention relates to a tripping mechanism of a circuit breaker capable of ensuring tripping as a function of the permissible overheating of the installation to be protected as well as in the event of a short circuit.

Most of the known circuit breakers of this type have two separate devices, one with a bimetallic strip for tripping as a function of the permissible heating of the installation, the other electromagnetic for actuating an exhaust mechanism in the event of a short circuit.

This double tripping command makes it possible to satisfy the requirements for slow and rapid tripping which make it possible to protect the installation controlled by the circuit breaker both from excessive heating and in the presence of very high currents resulting from a short circuit. On the other hand, the number of parts making up such a circuit breaker is high, which results in relatively complex assembly operations, in addition to the cost of manufacturing each part.

It has already been proposed to simplify the construction of circuit breakers by having the two abovementioned tripping functions fulfilled by a single device. It has thus been proposed to use memory alloys based on the hysteresis of a phase change, in order to produce a thermal trip device capable of reacting quickly in the event of a short circuit. These alloys are capable of suddenly releasing a high mechanical energy at the transition, which can directly be used to open the contact of the circuit breaker in the event of a short circuit. The practical advantages of this solution are not obvious. The composition of the alloy used has a great influence on the hysteresis domain and, consequently, on the heating values causing the transition. This results in problems of precision and reproducibility of the characteristic of slow triggering following heating. In addition, this solution does not lead to a significant simplification of the circuit breaker.

Circuit breakers with double tripping mechanism, thermic and magnetic, use a type of trigger formed by a friction escapement mechanism taken from clockwork mechanisms, in which the trigger pawl is held against a lug locking an exhaust lever with a force greater than the contact pressure force. If we wanted to use only the bimetal thermal release to release the exhaust lever, while retaining a sufficient safety margin against an accidental release caused by parasitic vibrations external to the circuit breaker, it would be impossible to obtain a rapid tripping in the event of a short circuit, since the opening of the contact is preceded by an escape period involving a displacement of the end of the bimetal with departure at zero speed followed by a ratchet displacement / exhaust lever also with zero speed departure. Since in such a trip device, the cut-off depends on the integral fi ² dt corresponding to the permissible overheating of the installation to be protected, in this particular case, and for a nominal current of 10 A, it has been calculated in "Leitungsschutz durch stronbegrenzende LS-Schalter, J. Kirch- dorfer, Sonderdruck aus Electro-Revue "that this escape period is of the order of Ji ² dt = 10 ⁴ A ² s corresponding to a displacement of 2 mm from the end bimetal. Therefore, it is excluded to consider the only use of a bimetal associated with a friction escapement to control the slow and fast releases.

Triggers with bistable spherical bimetallic discs are known, the peripheral edge of which is clamped inside an elastic side wall of a bowl. However, this device, the design of which is suitable for low displacements and high forces, does not have very precise and reproducible performances due to the buckling phenomena which occur during the transition between the two stable states.

The object of the present invention is to remedy at least in part the drawbacks of the abovementioned solutions, by proposing a triggering mechanism, capable on its own, of operating according to the rapid triggering mode consecutive to a sudden increase in current and according to the mode of slow triggering following overheating greater than the permissible heating of the installation.

To this end, the present invention relates to a tripping mechanism of a circuit breaker, capable of ensuring tripping as a function of the permissible overheating of the installation to be protected as well as in the event of a short circuit, characterized by the fact that it comprises a bistable element connected to an electrical contact and whose respective positions determine the closing, respectively the opening of this contact, an actuating member for driving this bistable element from one to the other of its positions , associated with one of the ends of a bimetallic strip, means of heating by Joule effect of this strip, mounted in series with the installation to be protected, the other end of this bimetallic strip being fixed to an arm whose l free end carries a mass of inertia, the whole forming a unit articulated around an axis parallel to the plane of the bimetallic strip, a stop being placed in the trajectory of this unit to limit the amplitude of its displacement consecutive to the deformation of the bimetallic strip.

The essential advantage of this mechanism is to provide a solution to the problem of rapid triggering by a purely thermal element. The combination of two functions in the same mechanism results in a significant simplification of the circuit breaker which results in another equally remarkable simplification of the exhaust mechanism.

The accompanying drawing illustrates, very schematically and by way of example, an embodiment and a variant of a circuit breaker provided with the tripping mechanism object of the present invention.

Fig. 1 is an elevational view of this circuit breaker in the engaged position.

Fig. 2 is a partial view of FIG. 1 illustrating the circuit breaker in the tripped position.

Fig. 3 is the same partial view illustrating the circuit breaker in the arming position.

Fig. 4 is another partial view of a variant of the trigger mechanism.

Fig. 5 illustrates the rapid tripping characteristic of the circuit breaker according to the invention compared to that of known circuit breakers.

The circuit breaker illustrated in fig. 1 has two connection terminals 1 and 2 to the installation to be protected, between which are mounted in series with a movable contact 3 and a bimetallic strip 4. One end of this bimetallic strip 4 is fixed to an inertia arm 5 carrying a mass 5a articulated around an axis 6 parallel to the plane of the bimetallic strip 4 and secured to the bottom of a bracket 7 fixed to the casing of the circuit breaker. The bottom of this stirrup is ribbed to make it rigid while its two arms are not and play the role of springs. Since the mass 5a is used to statically balance the inertia arm 5 while creating a moment around the axis 6 in dynamic regime, this mass 5a is chosen to be more important than that of the inertia arm 5 alone. A bistable exhaust blade 8 is compressed between the free ends of the arms of the stirrup 7 in notches 9 formed at these ends. This compression of the exhaust blade 8 has the effect of keeping it constant in one or other of its equilibrium positions. The free end of the bimetallic blade 4 and the middle of the exhaust blade 8 are connected by an actuating rod 10 slidably mounted through a support 11 secured to the stirrup 7. The entire crew formed of the bimetallic blade and of the inertia arm 5 articulated around the axis 6 is biased by an elastic return spring 12 with adjustable tension by a screw 12a, integral with the support 11 which bears against the free end of the bimetallic blade 4. Therefore, this free end of the bimetallic blade presses the actuating rod 10 against the middle of the bistable exhaust blade 8, with a pressure substantially lower than that which is necessary to tilt the middle of this blade exhaust in its second stable position. An adjustable calibration stop 13 constituted by a screw fixed in the bottom of the stirrup 7 is placed opposite one of the ends of the inertia arm 5 and serves to limit the movement of the mobile assembly formed. of this arm 5 and of the bimetallic blade 4 around the axis 6.

The bistable exhaust blade 8 and the mobile part 3a of the contact 3 are connected by a second rod 14 slidingly mounted in a fixed support 15. The end of the mobile part 3a of this contact 3, opposite the end adjacent to the fixed part 3b of this contact is slidably mounted in a slide 16 and this mobile part is biased by a return spring 17 which constantly tends to keep this contact 3 closed. A blown out fire chamber The magnetic arc of the arc 18 is adjacent to this contact 3. This chamber contains a packet of strips 19 intended to divide the arc so as to increase the overall arc voltage, at medium voltages and at high breaking capacities.

The movable part 3a of the contact 3 carries a cam 20 engaged with the end of a cocking lever 21 articulated around an axis 22. This lever 21 is secured to a bent arm 23 concentric with its axis of articulation 22. The elbow and the free end of this bent arm 23 have two lateral projections intended to come into engagement with the bistable exhaust blade 8 and located on either side of this blade. A trigger return spring 24 exerts on this cocking lever a pressure which tends to make it rotate in the opposite direction to that of a clockwise.

The tripping mechanism of the circuit breaker that has just been described is designed to allow rapid tripping in the event of a sudden increase in the intensity of the current consecutive in particular to a short circuit and to allow a slow tripping subsequent to heating. lines or devices to be protected exceeding an accepted value and corresponding to the integral 1 i ² dt. In the first case, the mass associated with the inertia arm 5 will not have time to move and the triggering will take place for low values of fi2dt. The setting of this value at which the triggering takes place can be obtained by adjusting the tension of the return spring 12 using the adjusting screw 12a.

It is also possible to replace this adjustment by modifying the mass 5a of the inertia arm either by lightening it or by adding an additional mass to it. We could provide for this purpose that the mass 5a is interchangeable. The advantage of the adjustment by the variation of the mass instead of varying the tension of the spring 12, resides in the fact that this variation in mass has no repercussion on the duration of triggering unlike the adjustment of the tension of the spring 12.

In a moderate overcurrent regime, the moving assembly formed by the bimetallic blade 4 and the inertia arm 5 carrying the mass 5a will pivot around the axis 6 in the opposite direction to that of the watch hands until that it is stopped by the stop 13.

We also note that this inertial solution makes it possible to solve the problem of variations in ambient temperature which, being slow, are absorbed by a displacement of the mass of inertia.

Before returning in more detail to the two behaviors of the tripping mechanism which is the subject of the invention, we want to finish the description of the operation of the circuit breaker.

So, in one or the other case of slow or fast triggering, the free end of the bimetallic blade 4 communicates in the middle of the bistable exhaust blade 8, a force via the actuating rod 10, until this exhaust blade 8 passes through its critical state. From this critical state, the compression exerted on the exhaust blade 8 by the elastic arms of the stirrup 7 assisted at the start by the force of the spring 12, brings the middle of this exhaust blade 8 into its second position illustrated in fig. 2. During this displacement, the middle of the exhaust blade 8 meets the rod 14 and the displacement of the latter causes the opening of the contact 3 by the tilting of its movable part 3a. During this tilting, the cam 20 releases the end of the cocking lever 21 which switches under the action of the trigger return spring 24 in the position illustrated in FIG. 2.

The trigger mechanism is reset by operating the cocking lever 21 to tilt it clockwise but beyond its engaged position, so that the elbow of the bent arm 23 passes the blade d bistable exhaust 8 beyond its critical position, until in its initial position. This arming position is illustrated in FIG. 3. It should be noted that, thanks to the cam 20, the closing of the contact 3 can only occur after the arming of the bistable exhaust blade 8 and that once the arming lever 21 returned to its stable "engaged" position defined by the cam 20. As a corollary, the cocking lever can only be maintained in the "engaged" position after having armed the bistable exhaust blade 8 and once the contact 3 closed.

During manual triggering, the slide 16 allows pivoting without opening of the mobile part 3a of the contact 3 until the bistable exhaust blade 8 is triggered by the sail lateral lie of the free end of the bent arm 23.

The triggering operation takes place in two phases, the first phase going from the rest state of position 1, illustrated in FIG. l, of the exhaust blade 8 in a critical state 0 of this blade, the second phase going from this position 0 to a position 2, illustrated in FIG. 2. The bimetallic strip 4 is essentially concerned with phase 1 of this triggering operation. It is reasonable to allow the trigger force to be set at 10 p, which would correspond to approximately 0.05 mm of displacement in free extension mode of the bimetallic strip. The heating of the bimetallic strip is then very reduced, let us assume a heating corresponding to ƒi ² dt = 200 A ² s.

The second phase of this triggering operation is provided by the elastic arms of the stirrup 7 between which the exhaust blade 8 is clamped. The exhaust disk from position 0 to position 2 is propelled with increasing force, for example from 0 to 1500 p for a displacement of 2 mm.

où ω est la fréquence angulaire (W = 314 1/s à 50Hz) La valeur de fi²dt vaut alors:

First of all ignoring the recoil of mass 5a, we will calculate the escape times corresponding to the passage from position 1 "engaged" from its rest state to its critical state 0. To simplify the calculation and taking into account very low times considered, it is admissible to replace the sinusoidal variation of the current by a linear rise of the same slope.

where ω is the angular frequency (W = 314 1 / s at 50Hz) The value of fi ² dt is then worth:

t_1-o correspondant au temps nécessaire à la lame d'échappement 8 pour passer de l'état de repos à l'état critique 0.Taking into account an integral ƒ i ² dt of 200A ² s for a displacement of 0.05 mm from a bimetallic blade of type 155 TB 1577 of 40 mm in length, 5 mm in width and 1 mm in thickness, we obtain about:

t _1-o corresponding to the time required for the exhaust blade 8 to pass from the rest state to the critical state 0.

To assess the time required to pass from critical point 0 to the "triggered" position 2 illustrated in FIG. 2, we will admit an average acceleration equal to half of the final acceleration and a stroke e from the middle of the exhaust blade 8 equal to 2 mm, its equivalent mass m being 0.5 g; f _m = 1500 p / 2 = 750 p = 7.5 N (average force applied).

The average acceleration a _m is therefore worth:

The passage time t _o-2 from positicn 0 to position 2 is given by:

The total opening time therefore corresponds to t _1-o + t _o-2 .

In the presence of the mass of inertia 5a, so that the escape of the exhaust blade 8 can occur in rapid release, it is necessary that the recoil of this mass of inertia, while the bimetallic blade is subjected to the stress consecutive to the heating, that is to say of amplitude much lower than the recoil corresponding to the same heating, but under a mechanical stress null (free extension).

With the same bimetallic blade as before, the tripping force will be adjusted 10 p and the maximum acceleration a _o of the mass of inertia equal to 20 g will therefore be 0.5 "g", or 5 m / s ² .

où a est l'accélération du temps t a_o est l'accélération maximale de recul de la masse d'inertie au passage au point critique 0, au temps t_1-o.Assuming the recoil of the mass 5a negligible compared to the free extension of the bimetallic strip 4, the acceleration of recoil will be proportional to I ² _eff t ³ according to formula (2) or again at:

where a is the acceleration of the time ta _o is the maximum acceleration of retreat of the mass of inertia at the passage to the critical point 0, at the time t _1-o .

By integration, we obtain the recoil distance e _r

(si ao = 5m/s² comme indiqué précédemment)The retraction of the mass 5a at the time of the escape of the blade 8 is therefore worth: er

(if ao = 5m / s ² as indicated above)

Using formulas (3) and (5) we can draw up the following table I:

From this table, we see that the fast exhaust condition is realized for all currents ≽300A, that is to say that for e _r << 50 µm.

The values corresponding to I _eff = 100 A are indicated in parentheses, the approximation of the linear rise of the current being no longer valid for a time of 6.7 ms. The recoil distance e is therefore in reality substantially greater than the 11 μm indicated. It can be estimated that the transition between slow and fast exhaust regimes occurs around 100 A. According to official standards, this transition corresponds to a nominal 10 A circuit breaker.

It should also be noted that the sink arm between the pivot axis 6 of the inertia arm and the mass 5a has a great importance with regard to the problem of the insensitivity of the circuit breaker with respect to vibrations. . In fact, there is a standard DIN 57641 (0641 / 6.78) regulating the vibration resistance of circuit breakers. According to this standard, the circuit breaker must hold while being placed on an oscillating platform subjected to accelerations of about 10 "g '' at a distance of 400 mm from the axis of rotation. The circuit breaker is located at an average distance of 300 mm from the axis and is therefore subject to accelerations of up to 7 to 8 "g", capable of causing an untimely tripping.

In the case of the triggering mechanism described, if the center of gravity of the moving assembly coincides with axis 6 and if a 10 mm lever arm separates axis 6 from mass 5a, we can calculate the equivalent acceleration of the mass subjected to the vibrations of the official test by multiplying the acceleration to 400 mm by. the ratio of the lever arms is 10 "g" x 10/400 = 0.25 "g" only. Thus, the described solution adjusted for a tripping force of 10 p could receive a mass of up to 40 g.

The inertial thermal trip object of the invention applies in principle to rapid tripping for all circuit breakers calibrated above 2 or 3A by adapting the dimensions and the connections of the bimetallic strip.

If rapid tripping can be considered useless in the case of currents below 0.2A, it is not the same for intermediate currents between 0.2 and 2A for which an interruption of a few ms is necessary, which cannot be ensured by the thermal trigger with indirect heating by a resistive tape wrapped around the bimetallic blade.

To solve this problem without introducing a magnetic trigger separate tick as do conventional solutions, it is possible to deposit a thin resistive layer on the surface of the bimetallic strip so as to cause its rapid heating. Such a thin layer can be deposited on the bimetallic strip by any known technique such as vacuum deposition or CVD.

A laboratory model was produced in accordance with the triggering mechanism described to verify its behavior in rapid triggering mode (medium current, low voltage). The measured results are quite comparable to those of the previous calculations, the results of which are shown in Table I.

The characteristics measured on this prototype have been plotted on the I / t diagram in fig. 5 in the form of a dashed line curve compared to the characteristics of a conventional circuit breaker of nominal 10A with thermal trip (curve 2) and with magnetic trip (curve 3). It emerges from this comparison that the characteristic of the model is much faster than the thermal part of the characteristic of the conventional trip unit and can be compared to the magnetic part.

These tests therefore confirm the technical interest of the proposed solution and the possibility of obtaining rapid or slow triggering with a single triggering mechanism capable of operating in two modes depending on the value of the overcurrent to inter-current. It follows a considerable simplification of the circuit breaker and consequently a lowering of its production cost, without the performance suffering. This is due to the bimetallic blade trigger associated with a mass of inertia, combined with a bistable exhaust member with zero tripping stroke which allows the bimetallic blade to be used for low values of fi ² dt, that is to say i.e. for low tripping forces. By way of indication, the circuit breaker which is the subject of the present invention can be manufactured with around forty parts to be assembled, while the comparable circuit breakers sold today consist of 50 to 70 parts.

Fig. 4 illustrates a variant of the calibration screw 13 placed on an elastic blade 25, the deformation of which is adjustable by means of a second calibration screw 26. This variant is particularly useful for protecting the motors. The first cali screw Bration 13 enclosed in the case B of the circuit breaker is used for factory calibration, while the second calibration screw 26 which can be operated from outside the case B is used to make an adjustment adapted to the power of the motor to be protected.

Claims (6)

1. Tripping mechanism of a circuit breaker capable of tripping as a function of the permissible overheating of the installation to be protected as well as in the event of a short circuit, characterized in that it comprises a bistable element connected to a electrical contact and whose respective positions determine the closing, respectively the opening of this contact, an actuating member for driving this bistable element from one to the other of its positions, associated with one of the ends of a bimetallic strip, means of heating by Joule effect of this strip, mounted in series with the installation to be protected, the other end of this bimetallic strip being fixed to an arm whose free end carries a mass of inertia, the whole forming a crew articulated around an axis parallel to the plane of the bimetallic strip, a stop being placed in the trajectory of this crew to limit the amplitude of its displacement consecutive to the deformation of the bimetallic strip. 2. Mechanism according to claim 1, characterized in that the said assembly is articulated along the said arm near its center of gravity and that the distance from the center of gravity of the said mass as well as the value thereof is chosen by depending on the desired moment of inertia. 3. Mechanism according to claim 1, characterized in that said bistable element is a flexible blade whose ends are compressed by elastic members, the middle of this blade being applied, in one of its bistable positions, against a stop located at a determined distance from the critical position of this blade beyond which it switches to its second bistable position. 4. Mechanism according to claim 1, characterized in that said bimetallic strip is directly connected in series with the installation to be protected. 5. Mechanism according to claim 1, characterized in that said bimetallic strip is covered with a thin resistive layer mounted in series with the installation to be protected. 6. Mechanism according to claim 1, characterized in that said stop placed in the path of said equipment is adjustable to allow the calibration of the triggering characteristics.

Images