A high performance automatic switch

Abstract: A toggle break magneto-thermic automatic switch in which an arming link (11) and a support lever (12) for a movable contact arm (10) form a toggle articulated in an arming-link-retaining dihedral formed by an inclined plane (31) of the said support lever (12) and a latch pawl (14) retaining tooth (29), actuated to break the toggle articulation by an electromagnetic striker (52) via the intermediary of a trip lever member (42) which interferes with the said latch pawl (14).

Description: The present invention relates to a miniaturised automatic switch (or breaker) according to the preamble of claim 1 (FR-A- 2 674 679). It is known that magneto-thermic switches are automatic switching devices in which two contacts are closed by a manual arming or setting device and must open in an automatic manner, by the effect of excess current which traverses the switch, in order to ensure protection of electrical systems against overloads and short circuits. To this end two types of protective action are in general envisaged: protection against overloads, protection against short circuits. Protection against overloads is ensured by a bimetallic element which, when heated by the current which flows through it, deforms until it acts on a release mechanism of the arming device and thus causes the contacts to open. Since the bimetallic element forms a cantilever beam having a certain flexibility, the end of which acts on the release device, the deformation induced in given overload conditions depends on the resisting forces exerted by the release device, which can vary over time and as a function of the working conditions. In order to ensure repeatability of intervention it is therefore necessary that the opposing resistant forces are minimised so that their variations will be negligible both in absolute value and in relation to the mechanical forces which can be exerted by the bimetallic element. The action of the thermal overload protection does not have to be particularly rapid but must be repeatable. The problem of protection against short circuits is very different. In this case the current flowing through the contacts can rapidly increase in the space of only a millisecond to levels of the order of thousands of amperes with catastrophic effects on the system and on the switch if allowed to continue for even only a few milliseconds. The first consequence, due to the contact resistance between the contacts, which however small is not zero, is the local heating and possible welding together of the contacts which prejudices the operation of the switch and its ability to act as protection to avoid further consequences. It is therefore essential that immediately a short circuit occurs, and, that is, as soon as the current reaches levels greater than normal, the contacts are opened with extreme rapidity. For this purpose a plunger type electromagnet energised by the current flowing through the switch projects a striker against the release mechanism causing release of the arming device. Simultaneously the striker acts, in general through an intermediate element, on one of the contacts moving it away from the other. The speed imparted to the striker by the magnetic field generated by the current, however high, is not infinite, and the energy transferred by the striker to the release device and the speed imparted to it depend on its inertia and on the resisting forces which oppose the release operation. It is therefore necessary that the release devices have a minimum inertia and the lowest possible resisting forces. At the same time the devices must be constructionally simple, must allow easy operations of assembly and setting up and must be small and compact in order to be able to be housed in miniaturised containers with dimensions which are in fact standardised, in which a plurality of other components must be housed such as terminals, rapid acting electromagnets, and arc quenching chamber. In order to satisfy these requirements simultaneously a wide variety of release devices have been proposed, among which a particularly effective one as far as the speed of operation is concerned appears to be the toggle-break release devices with inclined plane and retaining hook pawl, one example of which is described in French Patent publication No. 2 674 679. In these devices an arming link is pressed against an inclined plane formed in a support lever for a movable contact arm and holds the support lever and the movable contact arm in the armed position (with the contacts closed) against the action of a spring biasing the support lever and the contact arm to a rest position in which the contacts are open. The thrust exerted by the arming link on the inclined plane is not perpendicular to the plane so that the link tends to slide along the plane, but is held in place by a retaining hook pawl which absorbs that component of the thrust exerted by the link tangential to the inclined plane. This component, which is conveniently less than the thrust of the link, is transferred to a pawl pivot pin. Upon turning the retaining hook pawl about its pin the arming link, no longer restrained, can slide along the inclined plane and release the support lever from the previously-imposed constraint, thus allowing it to assume the rest position urged by the spring. The inclined plane formed by the support lever and the tooth of the retaining hook pawl form an articulation capable of opening, then breaking, the toggle constituted by the support lever and the arming link. Although release devices of this type are effective there remains the requirement of even better performance and of providing miniaturised automatic switches with even higher switching power and even shorter response times. The miniaturised automatic switch which forms the subject of the present invention satisfies this requirement in that an arming device of conventional type is associated with a release device of the type just described in which the retaining hook pawl is replaced by a latch pawl controlled by an electromagnetic striker via the intermediary of a trip lever or member. It is noted here that a hook pawl is a stop element stressed in tension whilst a latch pawl is a stop element stressed in compression. With this simple substitution it is possible not only to form a simple and particularly compact release device which leaves more space for other components within a casing of standardised volume, in particular rapid response electromagnet and arc quenching chamber, but also allows the achievement of shorter release device response times than are obtainable with hook pawl devices. A theoretical explanation is proposed for this fact which in many ways is unexpected and surprising given that hook pawls and latch pawls appear to be functionally equivalent. The advantages achieved in performance compensate largely for the relatively greater constructional complexity of the device which requires an intermediate actuating element for the latch pawl. The characteristics and advantages of the invention will become clearer from the following description of a preferred embodiment of the invention given with reference to the attached drawings, in which: Figure 1 is a general view of the whole of the structure of an automatic switch formed in accordance with the present invention; Figure 2 is an exploded perspective view of a preferred embodiment of the release device of the switch of Figure 1; Figure 3 is a front view of the release device of Figure 2 in a rest position; Figure 4 is a front view of the release device of Figure 2 in an armed position; Figure 5 is a front view of the release device of Figure 2 in an armed position with the electromagnetic protection acting; Figure 6 is a vector diagram showing the forces acting on some of the elements of the trip device of Figure 2; Figure 7 is a qualitative energy diagram relating to a latch pawl; and Figure 8 is a qualitative energy diagram relating to a hook pawl. With reference to Figure 1, a miniaturised automatic switch formed according to the present invention comprises a modular casing 1 (constituted by two coupled half shells one of which is removed to allow the interior to be seen) in which is housed a plurality of components, and in particular: first and second terminals 2, 3 for external electrical connections; a bimetallic strip 4; a pull-in electromagnet 5; a stack of arc quenching plates 6; a manual arming lever 7; a fixed contact 8; a movable contact 9 at the end of a contact arm 10 of conductive material; an arming link 11; a rapid release device formed by a support lever 12 for the contact arm 10, a biasing spring 13 for the contact arm 10 urging it to an open position, a latch pawl 14 and a trip lever or member 15; a mechanical unidirectional coupling guide for coupling the bimetallic strip 4 and the release device. The two terminals 2, 3 are electrically connected together via the series connection of the bimetallic strip 4, a flexible conductive braid 16 connecting the strip 4 to the contact arm 10, the contacts 8 and 9, when closed, and the electromagnet winding 5. The electromagnet winding is obviously connected at its ends to the contact 8, via a rigid copper projection 18 of the electromagnet, which supports the contact 8, and to the terminal 3 via a soldering projection 19 of the terminal 3. Upon opening of the contacts 8 and 9 the electrical connection is interrupted with the development, between the contacts, of an electric arc which commutes, when the two contacts 8, 9 are conveniently spaced from one another, on to a commutation electrode 20 connected to the terminal 2 and which conveys it towards the stack of quenching plates 6. In order for the automatic switch to be effective it is necessary not only that the contacts 8 and 9 are opened, but also that they are separated from one another as rapidly as possible in such a way that the commutation of the arc onto the electrode 20 takes place as soon as possible so as to reduce the duration of the arc to the minimum. Figure 2 shows the structure of the rapid release device in exploded perspective view for greater clarity. The support lever 12 for the contact arm 10 is provided at one end with a cylindrical bearing 21 to receive a pin 22 which acts as a support pivot for the lever 12 which is thus articulated to turn about the pin 22. The pin 22 is engaged at its ends in a fixed position in the casing 1. At the opposite end of the lever 12 two cylindrical pins 23, 24 are formed on opposite faces of the lever and have axes parallel to that of the pin 22, which pins act as pivot pins for the arm 10 and for the latch pawl 14 respectively. The arm 10 is provided at its end opposite the contacts 9 with a bearing 25 for receiving the pin 23. The bearing 25 is preferably a saddle-like cylindrical seating because this allows the centre of rotation of the arm 11 to be as close as possible to its end and therefore increases the lever arm within the same dimensions. A slot 26 formed in an intermediate position in the arm 10 allows the free passage of the pin 22 in the arm, prevents the escape of the pin 23 from the saddle-like bearing 25, and allows a predetermined and limited relative rotation between the arm 10 and the lever 12 on the pivot formed by the pin 23. A tooth 27 formed on the arm 10 in an intermediate position between the slot 26 and the saddle-like bearing 25 provides a positional engagement for the compression spring 13 which, reacting against the casing, urges the contact arm 10 (and consequently the lever 12) to a rest position, with the contacts open, defined by a convenient abutment of the casing 1. Integrally with the support lever 12 there is formed a tooth 30 provided with an engagement plane 31 cooperating with a tooth 29 of the latch pawl to form a dihedral which receives one end 32 of the arming link 11 pivoted at the other end on a drum 33 of a manual arming lever 7. The tooth 30 extends beyond the plane 31 to form a throat for housing the end of the link 32 when the actuation of the latch pawl allows the end 32 to slide on the plane 31 and to penetrate into the housing throat by the effect of relative movement between the end 32 and the support arm 12 caused by the spring 13 or, as will be seen further below, by the electromagnetic actuator 5. The latch pawl 14 is essentially constituted by a small lever provided at one end with a lug 28 for receiving the pin 24 and at the opposite end with the previously-mentioned retaining tooth 29. Conveniently the latch pawl 14 extends as a U-shape arm 34 terminating in an actuation tooth 35 for the said latch pawl, bent towards the lug 28 and at a radial distance from the lug equal to or less than the radial distance of the retaining tooth 29 from the lug 28. The U-shape arm 34 forms, with the tooth 30 of the support arm, a closed containment slot in which the end 32 of the link can slide. The latch pawl 14 is biased towards a rest and abutment position by a spring 36 having two ends respectively interfering with the latch pawl 14 and the body of the arm 12. A conveniently shaped plastics cap 37 is snap engaged over the top of the support lever 12 for the contact arm 10 and prevents the latch pawl 14 from separating from the pin 24. Advantageously the cap 37 can be made of coloured plastics or provided with coloured adhesive labels in such a way as to provide, via an open window 38 (Figure 1) in the casing 1, a visual indication of the position of the cap in the casing and therefore of the position of the support lever of the movable contact arm. The release device also includes a trip lever or member 39 operating as an actuation intermediary between the electromagnet striker and the latch pawl 14. In Figure 2 the trip member 39 is shown as being divided into two parts 39A and 39B for greater clarity. The trip member, which is conveniently made of insulating material, is formed by two parallel juxtaposed cheeks joined by a plate 42 to form a saddle partially housing the lever 12 and the arm 10. The two cheeks are provided with axially aligned apertures 40, 41 through which the pin 22 passes freely. It is clear that the assembly comprising the trip member 39, the arm 10, the lever 12, the latch pawl 14 and associated spring 36, the cap 37 and the pin 22 can easily be assembled as a unitary assembly to form a trigger device which can be installed with a simple operation into the casing 1. The trip member can turn relative to the support lever 12 and the arm 10 through a predetermined angle defined by the interference of a projection 43 of the support lever 12 with a tooth 44 of the cheek 39A and by the interference between a projection 45 of the plate 42 with the movable contact arm 10. A second tooth 46 of the trip member interferes with a tooth 47 of the slide 48 when the slide is displaced in the sense indicated by the arrow A of Figure 2 by the flexing of the bimetallic strip 4 (Figure 1) which acts on a tooth 49 of the slide. In order to actuate the latch pawl 14 the trip member is provided with a trip lever 50 terminating in a tooth 51. When the trip member is caused to turn clockwise (as viewed in Figures 1 and 2) about the pin 22 by the effect of a force applied to the plate 42 by the electromagnet striker 52, the tooth 51 contacts the actuation tooth 35 of the latch pawl and causes it to turn in an anticlockwise sense about the pin 24. Consequently the end of the link 32, no longer restrained on the inclined plane 31, lifts so that the entire release device becomes free to turn clockwise. If the striker 52 acts, the rotation is caused by the spring 13 acting on the contact arm 10 together with the action exerted by the striker 52 on the trip member and transferred from this to the contact arm 10 via the tooth 45. Opening is particularly rapid because no resisting couple is applied to the release device except for the moment of inertia of the device itself. If the thermal overload protection operates the displacement of the slide 48 causes a rotation of the trip member and the lever 50 about the pin 22 which causes release of the latch pawl 14. Rotation of the contact arm 10 and of the entire release device is caused solely by the action of the spring 13. In both cases the rotation of the lever 12 caused by the spring is transferred, by the interference of the projection 43 with the tooth 44, to the trip member 39, two projections 39C and 39D of which can be mechanically coupled in a known way to corresponding projections of the trip members of juxtaposed switch modules to cause them to trip as well. For an easier understanding of the operation of the trip release device described, Figures 3, 4 and 5 schematically show the device in different operating states. In Figure 3 the trip release device is in the rest position. In this condition the spring 13 presses the upper part of the contact arm 10 and the support lever 12 against an abutment stop 53 formed by the casing 1. The contact arm 10 is maintained stably in the open position. The arming lever 7 is in the disarmed position and the end of the link is held without force in the dihedral formed by the inclined plane 31 and the tooth of the latch pawl 14. The latch pawl is maintained in the stop position by its biasing spring and the tooth for actuation of the latch pawl, pressing against the tooth 51 of the trip member, tends to cause it to turn in an anticlockwise sense about the pin 22 maintaining it in the rest position defined by contact of the tooth 44 against the projection 43. By turning the arming lever 7 in the direction indicated by the arrow 54, the end of the link 11 is pushed against the inclined plane and, since the latch pawl stops it from sliding, the support lever is turned, together with the contact arm into the position indicated in Figure 4. In this condition the arming lever 7 contacts an end-of-stroke abutment and the end of the link 11, pivoted on the lever 7, passes beyond the radial line extending from the fulcrum of the lever 7 to the end 32 of the link 11 and the spring 13 stably maintains the release device in the armed position. This also ensures a predetermined contact pressure between the contacts 8 and 9 closed by the effect of the pivoting of the contact arm on the pin 23 and thanks to the freedom of rotation ensured by the slot 26. If L1 is the length of the lever arm between contacts 8 and 9 and the axis of rotation of the contact arm, with L2 being the force-application arm exerted by the spring with respect to the axis of rotation of the contact arm and FM is the thrust exerted by the spring perpendicular to the contact arm, the force P exerted between the contacts is given by P = FM · L2/L1 . Figure 5 shows the trip device when activated due to an excess current. The striker 52 is projected against the trip member 39 which turns about the pin 22 together with the trip lever 50. The tooth of the trip lever acts on the latch pawl 14 causing its release. From this moment the end of the link 11, no longer retained by the pawl, is free to slide on the inclined plane and allows the release device to rotate by the combined effect of the thrust exerted by the striker 52 and the spring 13. The release time of the latch pawl is of extreme importance to ensure the speed of response of the protection which must be as short as possible. Since the mechanical force which can be developed by an electromagnet is limited, it is necessary that the work required to effect release be as small as possible in order to minimise the time necessary to perform the work. Figure 6 is a schematic diagram showing the forces on the different components which define the work to be done to release the latch pawl. The numerals 31, 32 and 56 respectively indicate the contact plane of the support lever, the end of the link 11, which in general has a circular section of non-zero diameter, and the retention plane of the latch pawl 14. If F is the reaction exerted by the link 11 to oppose and balance the thrust of the spring, the force F can be split up into two components, FT perpendicular to the plane 31, and FR perpendicular to the plane 56. The component FT is entirely absorbed by the support arm and the biasing spring. It is evident that the closer the plane 31 is to being perpendicular to the force F the smaller is the angle α formed between F and FT and the smaller is the component FR. In practice the angle α cannot be less than a certain limit in order to ensure that the link 11 is not stuck by the effect of friction on the plane 31, and in general is not less than 20°. It is also evident that, for the same angle α, the condition for which the component FR is a minimum is that in which the plane 56 is perpendicular to the plane 31. The force FR applied to the latch pawl at the point of contact between the plane 56 and the end of the link 32 is entirely balanced by the reaction exerted by the latch pawl support pin 24 if the axis of the pin is aligned with the component FR. Since the forces which act on the latch pawl are completely balanced a minimum couple M, negligible to a first approximation, is sufficient to bias the latch pawl in the locked condition. Thus, to a first approximation, neglecting the couple M one can say that the virtual work to be done to release the latch pawl is given by L = D · µ · FR = D·µ·F·tga where D is the movement of the plane 56 relative to the end of the link 32 necessary to bring the corner 56 of the tooth of the latch pawl to the point of contact between the plane 56 and the end 32 and where µ is the coefficient of friction. By using material having a low coefficient of friction (bronze, steel) it is therefore possible to make trip devices which require a minimum work for release of the order of a few dynes (g · cm). This conceptual arrangement is only valid in both the case where the plane 56 belongs to the tooth of a latch pawl and where it belongs to the tooth of a hook pawl if the rotation of the pawl (regardless of the type) necessary to release the tooth is negligible. In practice this is not true and a predetermined rotation does have to take place, necessitated by the fact that the centre of rotation of the pawl is relatively close to the plane 56. It is therefore evident that in the case of a latch pawl, a contact plane such as 56 forms an abutment with the end 32 of the link in conditions of stable equilibrium and that angular displacements of the latch pawl from the equilibrium position are obtained only by performing work which separates the end 32 from the centre of rotation in some measure. The diagram of Figure 7 qualitively represents the work to be done to rotate the latch pawl through an angle from the stable position. On the other hand in the case of a hook pawl a contact plane such as 56 forms an unstable equilibrium condition for the end of the link represented by the energy diagram A of Figure 8. It is only by shaping the contact surface 56 as a cylindrical arc with a radius equal to the distance between the contact surface and the axis of rotation of the pawl that a condition of neutral equilibrium is obtained in both cases. This however is not sufficient to render the behaviour of the two elements equivalent. In fact, in the case of a latch pawl, the force FR is a simple compression force which does not modify the orientation of the contact surface but only the distance of the surface from the axis of rotation (by the effect of the elasticity, although minimum, of the material). On the other hand in the case of a hook pawl, the tension stress is inevitably associated with flexing moments which tend to encourage the instability condition to avoid which it is necessary to increase the biasing moment M to a considerable and non-negligible degree, or to incline the surface/contact plane in such a way as to introduce a component of thrust, which opposes the release of the latch pawl and increases the work to be done in order to effect release. For both arrangements the energy diagram is qualitatively modified as shown by diagram B of Figure 8. A comparison of Figures 7 and 8 will emphasise a further and not negligible difference between the behaviour of a latch pawl and a hook pawl. In the case of the latch pawl the work differential L/d needed to displace it from the rest condition is initially zero and increases in proportion to the magnitude of displacement. On the other hand, in the case of a hook pawl the work differential is high in the region of the rest condition and decreases with increasing distance there from. Since, in the case of a trip device the angle of rotation of the latch pawl/hook pawl is correlated to the time necessary for its actuation, although not necessarily in a linear manner, the use of a latch pawl allows a greater efficiency and speed of response because its dynamic characteristics are better adapted to those of an electromagnetic actuator which is able to develop a thrust and force increasing gradually with time as the magnitude of the air gap diminishes. Thus the use of a latch pawl in substitution for a hook pawl, in a toggle-break trip device of an automatic switch shows itself to be advantageous whether the tooth of the latch pawl has cylindrical or flat contact surfaces. The preceding description relates only to a preferred embodiment of the invention and many variants can be introduced. For example the coupling between the latch pawl and the trip member can be formed by means of a link on the arm of the latch pawl and the trip lever rather than by means of interference teeth and the latch pawl biasing spring can in this case be replaced by a biasing spring which acts on the trip member which, in turn, imposes a predetermined rest position on the latch pawl.

Claims: A miniaturised automatic switch having a casing (1) housing a pair of contacts (8,9), respectively a movable (9) and a stationary (8) contact, the said movable contact (9) being disposed at a first end of a contact arm (10) movable between a closed and an open position, comprising: a manual arming lever (7) coupled to a transmission link (11) to form a bistable mechanical commutator, a striker (52) actuated by an electromagnet (5,17), a support lever (12) for the contact arm (10), the contact arm being a pivoted on a first pin (22) carried by the said casing (1), the said contact arm (10) being articulated at its second end on a second pin (23) at the free end of the said support lever (12) and capable of at least a predetermined rotation relative to the said support lever (12), the said second pin (23) being radially opposite the said contacts (8,9) in relation to the said first pin (22), a spring (13) acting on the said contact arm (10) in a position intermediate between the first (22) and second pin (23), to bias the said support lever (12) and the said contact arm (10) to the said open position, a tooth (30) formed on the said support lever (12) in an intermediate position between the said first (22) and second (23) pin, the said tooth (30) forming a contact and sliding plane (31) for a free end (32) of the said link (11), inclined and not perpendicular to the direction of the thrust exerted by the said link (11), the said switch being characterised in that it includes: a latch pawl (14) articulated on a third pin (24) at the free end of the said support lever (12), the said latch pawl (14) having a retaining tooth (29) with a retaining surface (56) for the said free end (32) of the link (11) which is orthogonal, at the point of contact with the said end (32) of the link, to the said sliding plane (31), the said third pin (24) being disposed with its axis in a plane parallel to the sliding plane (31) and containing the point of contact of the said link (11) with the said retaining tooth (29), a trip member (42) pivoted on the said first pin (22) and actuated by the said striker, having an actuation arm (50,51) which causes rotation of the said latch pawl (14) and disengagement of the said end (32) of the link (11) from the said retaining tooth (29) and, means (36) for applying to the said latch pawl (14) a biasing couple towards the retained position. A switch as in Claim 1, in which the said means (36) for applying a biasing couple to the said latch pawl include a second spring (36) around the said third pin (24), the said second spring (36) biasing the said trip member (42) via the said latch pawl (14) to a predetermined angular rest position defined by the interference of interference means (44) of the said trip member with the said support lever (12). A switch as in Claim 2, in which the said trip member (42) includes an opening tooth (45) which interferes with the said contact arm (10) in its closed position, when the said trip member (42) is displaced from its rest position by the said striker (52). A switch as in Claim 3, including a bimetallic element (4) in series between the said movable contact (10) and an electric connector terminal (2) and a slide (49) actuated by the said bimetallic element (10), the said slide (49) being provided with a tooth (47) for interfering with a corresponding tooth (46) of the said trip member (42). A switch as in Claim 4, in which the said third (24) and second (23) pins extend on opposite sides of the said support lever (12), further including a resilient cap (37) snap engaged on the said second and third pin to prevent separation of the said latch pawl (14) from the said third pin. A switch as in Claim 5, in which the said cap (37) faces an aperture (38) of the said casing (1) and provides an indication of the position of the said contact arm (10). A switch as in any preceding claim, in which the said latch pawl (14) includes a U-shape arm (34), the said third pin (24) being fitted in a cylindrical bearing seat (28) provided at one end of the said latch pawl (14), the other end of the said arm (34) being bent towards the said cylindrical bearing seat (28) to form an actuation tooth (35) contactable by the said trip member actuation arm (50,51) at a point of the said actuation tooth (35) spaced from the said third pin (24) by a distance equal to or less than that of the said retaining plane (31) from the said third pin (24). A switch as in any preceding claim, in which the said movable contact arm (10) includes a saddle type bearing (25) for receiving the said second pin (23).

Inventors: PIANEZZOLA SERGIO, PIANEZZOLA, SERGIO
Applicants: BTICINO SPA, BTICINO S.P.A.
Application year: 1995
Main classification: H01H1
IPC codes: H01H71