GB2074383A - Electrical snap switches - Google Patents

Abstract

An electrical snap switch has a housing 1, 2 with a stationary contact 8 and a pivotally movable contact member 12 which engages the contact 8 in the closed position of the switch. The switch also comprises a slidable operating member 10 which, during opening of the switch, moves the line of action of biasing spring 13 to cause the contact member 12 to move with a snap action away from the contact 8. In the abnormal event of the contact member 12 becoming welded to the contact 8, or the spring 13 failing, opening of the switch is ensured by the contact member 12 being forced away from the contact 8 by the advancing operating member 10 which during opening of the switch moves in the same general direction as the direction of separation of the contact member 12 away from the contact 8. The contact member 12 may bridge one pair of stationary contacts 8 in one position and another pair of stationary contacts 9 in the other position. <IMAGE>

Description

SPECIFICATION Electrical snap switches This invention relates to electrical snap switches of the type having a movable contact member which pivots, with an overcentre toggle action provided by a biasing spring, between first and second positions of the contact member. The invention is applicable to the sort of switch (hereinafter called "the first sort") in which the contact member bridges a pair of stationary contacts of the switch in the first position of the contact member and does not bridge the stationary contacts in the second position thereof, and to the sort of switch (hereinafter called "the second sort") in which the contact member engages a single stationary contact of the switch in the first position but does not engage the stationary contact of the switch in the second position.For the most part, the invention will be described as applicable to the first sort of switch, but it should be borne in mind that the invention is also applicable to the second sort of switch.

Such switches, which are sometimes called "quick make and break switches", or "snap action switches" are commonly used in protective, monitoring or supervisory circuits and reliability of operation of the switches is therefore important. In such an application of the switch of the first sort, it is usual for the switch to be normally closed (with the stattionary contacts bridged by the movable contact member) and for the switch to be opened when an event being protected, monitored or supervised occurs. It is thus important that the switch trips reliably to the open condition and that such opening is not impaired by the stationary contacts and the movable contact member becoming welded together or by failure of the biasing spring. In an effort to provide such switches with the necessary reliability on opening it has been proposed, e.g.

in German Patent Specification No.

1,935,225, to provide a mechanical back-up arrangement which forces the movable contact member away from the stationary contacts should the biasing spring fail or have insufficient strength to part welded contacts.

However, in prior arrangements of electrical snap switches, the operating member which slides to trip the switch generally moves in the opposite direction to the direction of movement of the movable contact member, with the result that some reversing mechanism incorporating for example bellcrank levers or see-saw levers is necessary to reverse the direction of movement of the operating member so that this movement can be used to force the movable contact member away from the stationary contacts, if necessary. The present invention stems from the realisation that a simpler and more reliable switch results if, on tripping the switch, the operating member moves in generally the same direction as the contact member so that the mechanical backup can be more simply and reliably provided.

As applied to a switch of the first sort, the invention provides an electrical snap switch comprising a housing supporting a pair of stationary electrical contacts, a movable contact member pivotable between a first position in which the contact member bridges the pair of stationary electrical contacts and establishes electrical connection therebetween and a second position in which the contact member is spaced from the pair of electrical contacts, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move over-centre with a toggle action during movement of the contact member between the first and second positions thereof, and an operating member which is movable with respect to the housing and undergoes an operative movement which alters the line of action of the spring means to cause the contact member, in normal operation of the switch, to move from the first position thereof to the second position thereof to trip the switch, wherein during said operative movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from the pair of stationary contacts towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.Hence, the movable abutment which provides the mechanical back-up or positive break feature moves in the same direction as the direction of movement of the contact member when the switch is tripped, rendering unnecessary the complication of the reversing mechanisms of the prior art.

A switch (of the first sort) according to the invention preferably has a further pair of stationary contacts which are bridged by the contact member when the latter is in the second position thereof. Since the positive break afforded by the movable abutment is not operative to separate the movable contact member from the further pair of stationary contacts, when the switch is used the firstmentioned pair of stationary contacts will be connected in a circuit where positive break of the contacts is crucial, leaving the further pair of contacts available for use in a circuit which has less demanding requirements.

As applied to a switch of the second sort, the invention provides an electrical snap switch comprising a housing supporting a stationary electrical contact, a movable contact member electrically connected to an electrical terminal on the housing and movable between a first position in which the contact member engages the stationary contact and establishes electrical connection between the stationary contact and said terminal and a second position in which the contact member is not in engagement with the stationary contact, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move over-centre with a toggle action during movement of the contact member between the first and second positions thereof, and an operating member which is movable with respect to the housing and undergoes an operative movement which alters the line of action of the spring means to cause the contact member, in normal operation of the switch, to move from the first position thereof to the second position thereof to trip the switch, wherein during said operative movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from the stationary contact towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment, which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.

A switch according to the invention and of the second sort may have a further stationary contact which the contact member engages in the second position thereof.

In the preferred embodiments (both of which are of the first sort of switch) to be described hereinafter by way of example, a lever is pivotally mounted in the housing, the spring means being connected between the lever and the contact member and the lever being engaged and pivotally displaced by the operating member on said operative movement thereof, this pivotal displacement of the lever altering the line of action of the spring means to cause the contact member to move from the first position thereof to the second position thereof. In the second embodiment to be described hereinafter, the lever is mounted in such a way as substantially to prevent dithering of the contact member.Dithering occurs when the contact member occupies a position of unstable equilibrium near the centre position and under low contact pressure, a condition which leads to arcing between the contact member and the stationary contacts. Dithering of the contact member is substantially prevented by arranging for the switch to be tripped in two stages, in the first of which the lever undergoes limited pivotal movement with respect to the operating member as a result of the line of action of the spring means moving through the pivot axis of the lever, such limited pivotal movement of the lever taking the line of action of the spring means through the pivot axis of the contact member which then moves to the second position thereof in the second of the two stages.Thus, once the operating member has been advanced sufficiently far for the lever to undergo its limited pivotal movement, tripping of the switch will immediately occur in normal operation. As a result it is improbable that the switch would be held at or near its break or trip position where dithering is likely to occur.

Two preferred embodiments of electrical snap switch according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of the first embodiment of switch, Figure 2 is a sectional view taken on the line ll-ll of Fig. 1, Figure 3 is a sectional view taken on the line Ill-Ill of Fig. 2, Figures 4 to 6 are fragmentary views similar to part of Fig. 2 but showing the first embqdiment of switch in three different operative positions, Figures 7a, 7b and 7c show an operating member of the first embodiment of switch in plan, side and end views, respectively, Figures 8a and 8b show a movable contact member of the first embodiment of switch in front and side views, respectively, Figures 9a and 9b show a spring of the first embodiment of switch in side and end views, respectively, Figure 10 is a plan view of the switch forming the second embodiment, Figure 11 is a sectional view taken on the line IX-IX of Fig. 10, Figure 12 is a sectional view taken on the line XlI-Xll of Fig. 11, Figures 13 to 17 are fragmentary views similar to part of Fig. 11 but showing the second embodiment of switch in different operative positions, Figures 1 8a, 18b and 18e show an operating member of the second embodiment of switch in side, plan and end views, respectively, and Figures 19a and 1 9b show a contact member of the second embodiment of switch in front and side views, respectively.

The first embodiment of electrical snap switch shown in Figs. 1 to 9 has a housing formed in two parts, namely a base member 1 and a cover member 2 both of which are made of an electrically insulating material such as a melamine phenolic material. The base member 1 has a base wall 3 (Figs. 2 and 3) and upstanding side walls 4 which define a recess within which the cover member 2 and the remaining operative parts of the switch are housed. The base member 1 and cover member 2 are secured together by means of a set screw 5. The cover member 2 is of generally cruciform shape in plan view (Fig. 1) and the four resulting quadrants defined by the cruciform shape are occupied by pairs of electrical terminals 6 and 7. Each of the four terminals has a terminal screw and a downwardly projecting terminal leg carrying an electrical contact.Hence, the first pair of terminals 6 provide a first pair of stationary contacts 8, one of which is visible in Fig. 2, and the second pair of terminals 7 provide a second pair of stationary contacts 9, one of which is also visible in Fig. 2.

The main movable components of the switch comprise an operating member 10 made of an electrically insulating material and slidably received between the base member 1 and the cover member 2; a movable contact member 1 2 made of an electrically conducting material and pivotally mounted in the cover member 2; a serpentine leaf spring 1 3 which biases the movable contact member 1 2 so that the latter either interconnects the first pair of stationary contacts 8 or the second pair of stationary contacts 9; and a lever 14 made of sheet steel, the lower end of which is pivotally mounted in a groove 1 5 of the cover member 2 and the upper end of which forms an anchorage point for the upper end of the spring 13.

Referring to Fig. 3 and Figs. 8a and 8b, it will be seen that the movable contact member 1 2 is generally U-shaped when viewed in the direction of the central longitudinal axis of symmetry of the switch, i.e. when viewed on the plane indicated by the section line Ill-Ill in Fig. 2. The contact member 1 2 has two side limbs 1 6 projecting upwardly from an interconnecting web 17, the aligned upper edges 1 8 of the side limbs being pivotally received in aligned grooves 1 9 (Fig. 3) formed in oppositely projecting portions 20 of the cover member 2.The pivot axis of the contact member 1 2 is shown by the letter P in Figs. 2 to 6, On each surface of the movable contact member 12, and adjacent the lower edges of the side limbs 16, are disposed circular contact pads 22 for making electrical contact with the respective pairs of stationary contacts 8 and 9.

As best shown in Fig. 3, the lever 14 is of generally rectangular shape but has two laterally extending projections 23 (for the purpose to be described hereinafter) and an upwardly projecting tab 24 which locates within an aperture 21 (Figs. 9a and 9b) formed in the upper end of the spring 1 3. The lower end of the spring 1 3 has, as particularly shown in Figs. 9a and 9b, a projecting tab 25 located within a recess 26 (Fig. 8a) formed in the lower edge of the web 1 7 of the contact member 1 2. The spring 1 3 is a leaf spring bent into a serpentine shape such that it urges the web 1 7 of the contact member 1 2 towards the upper end of the lever 14, so as to provide the spring bias for the contact member 12.

The shape of the operating member 10 can be seen from Figs. 7a, 7b and 7c. The operating member 10 has a stem 27 with an elongated aperture 28 for the passage of the screw 5 and a bifurcated end which leads during an operative movement of the operating member towards the right as viewed in Fig. 2. The bifurcation in the operating member 10 defines two laterally spaced arms 29 which respectively present laterally spaced surfaces 30 which constitute a movable abutment. The arms 29 also present further spaced surfaces 32 which, during operation of the switch, engage and move the lateral projections 23 of the lever 14. The lower ends of the arms 29 form guides 33 which slide within spaced tracks 34 formed in the base wall 3 of the base member 1.

A typical application of the switch is for the monitoring of the position of machine guards such that when a guard is opened the contacts of the switch are opened to break a supervisory circuit and prevent running of the machine. The switch is therefore of the normally closed type, the contact member 1 2 normally bridging and electrically interconnecting the first pair of stationary contacts 8, this condition being shown in Fig. 2. To open the switch, the operating member 10 is slid towards the right as viewed in Fig. 2. Such movement causes the spaced surfaces 32 of the operating member 10 to engage and displace the lever 14, the lower end of which pivots in the groove 1 5 in a clockwise direction as viewed in Fig. 2.Pivoting movement of the lever 14 alters the position of the upper end of the spring 1 3 and, therefore, alters the line of action A-A of the spring 1 3. When the line of action A-A of the spring 1 3 passes through the pivot axis P of the movable contact member 1 2 the switch is in the grip position shown in full lines in Fig. 4. It will be appreciated that the trip position is an unstable one in which the switch does not remain for any finite period of time since the spring 1 3 tends to move the contact member 1 2 with a snap action so that it moves across with an arcuate motion towards the right as viewed in Fig. 4 until the contact member 1 2 engages the second pair 9 of stationary contacts. The contact member 1 2 is now in the position shown in broken lines in Fig. 4.

Should the contact pads 22 of the movable contact member 1 2 become welded to the first pair of stationary contacts 8, for example as a result of arcing, or should the spring 1 3 fail, the advancing surfaces 30 of the operat ing member 10 will engage the web 1 7 of the movable contact member 1 2 at laterally spaced positions therealong to cause the contact member 1 2 to be forced away from the first pair of stationary contacts 8. Hence, the two surfaces 30 of the operating member 10 form a movable abutment which provides a mechanical back-up or positive break which ensures that operative movement of the operating member 10 results in opening of the first pair of contacts 8.During tripping of the switch, the surfaces 30 which provide the mechanical back-up thus move in the same direction as that of the pads 22 of the contact member 1 2.

The line X-X in Figs. 2 and 4 to 6 represents the vertical plane passing through the pivot axis P of the contact member 1 2. The line X-X can be used as a reference against which to judge the longitudinal displacement and the angular variation of the line of action A-A of the spring 1 3.

Fig. 5 shows the switch after the operating member 10 has completed its operative movement. The operating member 10 must be held in this position if the latter is to be retained because the spring 1 3 exerts a force on the lever 1 4 tending to move the operating member 10 towards the left as viewed in Figs. 5 and 6, in order to reset the switch. Fig. 5 represents the relative positions of the components when the contact member 1 2 is in its second position bridging the second pair of electrical contacts 9.Fig. 6 shows the switch when it is about to be reset, i.e. when the line of action A-A of the spring 1 3 is just passing to the left of the pivot axis P of the contact member 1 2. After the contact member 1 2 has snapped back into contact with the first pair of stationary contacts 8, the position of the contact member 1 2 is as shown in broken lines in Fig. 6, after which the operating member 10 continues to compiete its travel towards the left under the influence of the spring 13, until the normal position of rest shown in Fig. 2 is restored.

The lever 14 could be dispensed with, in which case the upper end of the spring 1 3 would be attached to the operating member 10. However, in such instance the force of the spring 1 3 acting on the member 10 would increase the frictional resistance to sliding of the member 10, and for this reason the presence of the lever 14 is preferred.

Reference will now be made to Figs. 10 to 1 9 which show the second embodiment of electrical snap switch. In Figs. 10 to 19, components equivalent to those in Figs. 1 to 9 bear the same reference numerals increased by one hundred. The second embodiment of switch has a housing formed by a base member 101 and a cover member 102 similar to (but not exactly the same as) the base member 1 and cover member 2 of the first embodiment described with reference to Figs. 1 to 9. As before, the base member 101 and the cover member 102 are secured together by a screw 105 and the switch has a first pair of terminals 106 and a second pair of terminals 107.The first pair of terminals 106 are respectively in electrical communication with a first pair of stationary contacts 108, one of which is shown in Fig. 11, and the second pair of terminals 107 are in respective electrical connection with a second pair of statioflary electrical contacts 109, one of which is also shown in Fig. 11. The movable contact member 11 2 is again U-shaped (Fig. 1 2 and Fig.

19a) and has two side limbs 116, the aligned upper edges 11 8 of which are pivotally re-, ceived in laterally spaced grooves 11 9 (Fig.

12) in the cover member 102. In this case, each side limb 11 6 of the movable contact member 11 2 is retained in its corresponding groove 11 9 by an associated one of two pins 50. P denotes the pivot axis of the contact member 11 2 in the grooves 11 9.

Instead of the planar lever 14 pivotally mounted about its lower edge of the first embodiment, the second embodiment has a lever 114 bent into the profile shown in Fig.

11. Also, instead of the serpentine leaf spring 1 3 of Figs. 1 to 9, the embodiment of Figs.

10 to 19 has a leaf spring 113 bent into a generally C or U shape, the respective limbs of the spring 11 3 being attached to the intermediate limb 11 7 of the movable contact member 11 2 and to the lower end of the lever 11 4. An an intermediate point along its length, the lever 11 4 has laterally projecting portions 52 (Fig. 1 2), the aligned upper edges of which are pivotally received in respective grooves 53 formed in the cover member 102.

The pivot axis of the lever 114 in the grooves 53 is indicated by R in Figs. 11 to 17. The aligned lower edges of the projections 52 of the lever 114 form edges against which the corresponding limb of the spring 11 3 engages and pivots during operation of the switch. the spring 11 3 urges the lower end of the lever 114 away from the web 11 7 of the contact member 112.

The shape of the operating member 11 0 can be seen from Figs. 18a, 18b and 18c.

The operating member 110 again has a stem 1 27 with an elongated aperture 1 28 for the passage of the screw 105 and a leading end presenting laterally spaced surfaces 1 30 which constitute a movable abutment. A central upwardly extending projection 54 of the operating member 110 terminates in a bosslike projection 55 serving to locate one end of a helical compression spring 56 which acts between the operating member 102 so as to provide a force urging the operating member 110 towards the left as viewed in Fig. 11, for the purpose of resetting the switch. The central projection 54 is formed to provide a downwardly facing groove 57 into which projects, with clearance, the upper end of the lever 11 4.

In the normal rest position of the switch shown in Fig. 11, the movable contact member 11 2 bridges and electrically interconnects the first two stationary contacts 1 08. In this first position of the contact member 11 2, the line of action of the spring 11 3 is directed along the line A-A shown in Fig. 11. The spring 11 3 thus retains the movable contact member 11 2 in the normally closed position and also maintains the lever 11 4 so that the upper end thereof engages the left hand face of the groove 57 in the operating member 110.As the operating member 110 is moved towards the right, the lever 11 4 is caused to pivot clockwise until the situation shown in Fig. 1 3 is reached. At this point, the line of action A-A of the spring 11 3 passes through the pivot axis R of the lever 114 in the grooves 53 and the switch is on the point of being tripped. As with Fig. 4, the position shown in Fig. 1 3 is not a stable position but merely one through which the switch passes during operation.

As soon as the line of action A-A of the spring 11 3 passes to the lefthand side of the pivot axis R of the lever 11 4 in the grooves 53 the lever 114 will pivot about its axis R to the limited extent permitted by the clearance of the upper end of the lever 11 4 in the groove 57.This limited pivotal movement of the lever 11 4 will cause the line of action of the spring 11 3 to move the line A'-A' shown in Fig. 1 3. This displacement of the line of action of the spring 11 3 is sufficient to take the line of action of the spring 11 3 through the pivot axis P of the contact member 11 2 in the cover member 102, with the result that the contact member 11 2 snaps over with a toggle action to the alternative or second position in which it bridges the second pair 109 of stationary contacts. In this position the line of action of the spring is indicated by the line A-A in Fig. 14.The switch is thus tripped in two stages, during the first of which the lever 11 4 undergoes its limited pivotal movement to alter the line of action of the spring 11 3 sufficient to move the line of action through the pivot axis P of the contact member 112, and during the second of which the contact member 11 2 moves over-centre with a snap action. This two-stage movement means that it is extremely difficult for the switch to be held in a dithering condition, i.e.

one in which the contact member 11 2 is held close to the trip position.

After the switch has been tripped, the components occupy the positions shown in Fig.

1 4. Should the movable contact member 11 2 become welded to the first pair of stationary contacts 108, or should the spring 11 3 or lever 114 fail in its intended operation, the spaced surfaces 1 30 of the operating member 110 serve as a mechanical back-up, or positive break. That is to say, the surfaces 1 30 will engage and displace the contact member 11 2 so that it is forced away from the first pair of stationary contacts 1 08. As with the first embodiment, the surfaces 1 30 providing the mechanical back-up move in the same direction as the pads 1 22 of the member 11 2 during tripping of the switch.

Fig. 1 5 shows the relative positions of the components after the member 110 has undergone its full operative movement and the movable contact member 11 2 has been placed in engagement with the second pair of stationary contacts 109. To reset the switch, the operating member 110 is allowed to slide to the left under the influence of the helical compression spring 56 and Fig. 1 6 shows the position just prior to resetting of the switch, when the line of action A-A of the spring 11 3 passes through the pivot axis R. When this happens, the lever 114 moves anti-clockwise as viewed in Fig. 1 6 to take up the clearance between the upper end of the lever 11 4 and the left-hand face of the groove 57.This movement of the lever 114 moves the line of action of the spring 11 3 through the pivot axis P, causing the contact member 11 2 to snap over into contact with the contacts 108.

After the contact member 11 2 has snapped back into contact with the first pair of stationary contacts 108, the position is as shown in Fig. 17, after which the operating member 110 continues to complete its travel towards the left under the influence of the expanding helical compression spring 56, until the normal position of rest shown in Fig. 11 is restored.

In both embodiments, the members which pivot one on another are formed so that a sharp line contact between the relatively pivotable parts is defined, this ensuring that pure pivoting without sliding is achieved, thereby reducing friction and wear along the pivot axis. Also, in both embodiments the contact member makes abutting face-to-face engagement with the stationary contacts, so that the plane of abutment is substantially perpendicular to the direction of movement of the contact member with respect to the stationary contacts, as distinct from a sliding engagement common in tumbler switches.

Both of the embodiments described with reference to the accompanying drawings are of the first sort of switch, i.e. one in which the movable contact member bridges a first pair of stationary contacts in the first position of the contact member. As previously indicated, the positive break feature afforded by the invention can with advantage be applied to a switch of the second sort, which may have two or three terminals.

In a two terminal switch of the second sort, the first terminal is in permanent electrical connection with the movable contact member, and the second terminal is electrically connected to a stationary contact of the switch. Hence, in the first position of the contact member, the first terminal is placed in electrical connection with the stationary contact, and in the second position of the contact member the first terminal is not in electrical connection with the stationary contact. This switch may be termed a two-terminal, single break, normally closed switch.

In a three terminal switch of the second sort, the first terminal is in permanent electrical connection with the movable contact member and the second and third terminals are respectively electrically connected to spaced first and second stationary contacts of the switch. Hence, in the first position of the contact member, the first terminal is placed in electrical connection with the first stationary contact and in the second position of the contact member the first terminal is placed in electrical connection with the second stationary contact. The first terminal may thus be regarded as a common terminal of the switch since it is placed in alternative connection with the second or third terminal. This sort of switch may be termed a three terminal single break changeover switch.

The embodiment of the invention shown in Figs. 1 0 to 1 9 is identical to the embodiment disclosed in the applicants' co-pending Application No. 8110871 of even date.

Claims (12)

1. An electrical snap switch comprising a housing supporting a stationary electrical contact, a movable contact member electrically connected to an electrical terminal on the housing and movable between a first position in which the contact member engages the stationary contact and establishes electrical connection between the stationary contact and said terminal and a second position in which the contact member is not in engagement with the stationary contact, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move over-centre with a toggle action during movement of the contact member between the first and second positions thereof and an operating member which is movable with respect to the housing and undergoes an operative movement which alters the line of action of the spring means to cause the contact member, in normal operation of the switch, to move from the first position thereof to the second position thereof to trip the switch, wherein during said operative movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from the stationary contact towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment, which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.

2. An electrical snap switch comprising a housing supporting a stationary first electrical contact and a stationary second electrical contact, a movable contact member electrically.

connected to an electrical terminal on the housing and movable between a first position in which the contact member engages the first contact and establishes electrical connection between the first contact and said terminal and a second position in which the contact member engages the second contact and establishes electrical connection between the second contact and said terminal, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move overcentre with a toggle action during movement of the contact member between the first and second positions thereof, and an operating member which is movable with respect to the housing and undergoes an operative movement which alters the line of action of the spring means to cause the contact member, in normal operation of the switch, to move from the first position thereof to the second position thereof to trip the switch, wherein during said operative movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from the first contact towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment, which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.

3. An electrical snap switch comprising a housing supporting a pair of stationary electrical contacts, a movable contact member movable between a first position in which the contact member bridges the pair of stationary electrical contacts and establishes electrical connection therebetween and a second position in which the contact member is spaced from the pair of electrical contacts, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move overcentre with a toggle action during movement of the contact member between the first and second positions thereof, and an operating member which is movable with respect to the housing and undergoes an operative movement which alters the line of action of the spring means to cause the contact member, in normal operation of the switch, to move from the first position thereof to the second position thereof to trip the switch, wherein during said operative movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from the pair of stationary contacts towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment, which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.

4. An electrical snap switch according to Claim 1, 2 or 3, wherein the movable contact member is pivotally mounted in the housing.

5. An electrical snap switch according to Claim 4, wherein a lever is pivotally mounted in the housing, the spring means being connected between the lever and the contact member and the lever being engaged and pivotally displaced by the operating member on said operative movement thereof, this pivotal displacement of the lever alternating the line of action of the spring means to cause the contact member to move from the first position thereof to the second position thereof.

6. An electrical snap switch according to Claim 4 or 5, as appendant to Claim 3, wherein the contact member is generally Ushaped when viewed in the direction of said operative movement of the operating member, the U-shape being provided by two side limbs projecting upwardly from and connected to an intermediate web of the contact member, the side limbs having aligned upper edges forming pivot edges by which the contact member is pivotally mounted in the housing, and the lower regions of the side limbs carrying contact pads for engagement with the electrical contacts supported by the housing, the spring means having one end thereof connected to the intermediate web of the contact member.

7. An electrical snap switch according to Claims 5 and 6, wherein a lower edge of the lever is pivotally mounted in the housing and the other end of the spring means is connected to an upper edge of the lever, the spring means urging the web of the contact member towards the upper edge of the lever to bias the contact member.

8. An electrical snap switch according to Claim 6 or 7, wherein the end of the operating member which leads during said operative movement is bifurcated and has two arms laterally spaced on respective sides of a central recess, the arms having laterally spaced surfaces which constitute said movable abutment and which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engage the web of the contact member at laterally spaced positions.

9. An electrical snap switch according to Claim 8, wherein the arms have further laterally spaced surfaces which engage and displace the lever to trip the switch.

10. An electrical snap switch according to any one of the preceding claims, wherein the spring means are constituted by a leaf spring bent into a serpentine shape.

11. An electrical snap switch according to any one of the preceding claims, wherein, after the switch has been tripped, the spring means urge the operating member in the direction opposite to the direction of the operative movement, to reset the switch.

12. An electrical snap switch according to Claim 5, wherein the switch is tripped in two stages, in the first of which the lever undergoes limited pivotal movement with respect to the housing and with respect to the operating member as a result of the line of action of the spring means moving through the pivot axis of the lever, such limited pivotal movement of the lever taking the line of action of the spring means through the pivot axis of the contact member which then moves to the second position thereof in the second of the two stages.

1 3. An electrical snap switch according to Claim 12, wherein the lever is pivotally mounted in the housing at an intermediate position between upper and lower ends of the lever, an upper end of the contact member being pivotally mounted in the housing, the spring means being connected to the lower end of the lever and urging the lever end of the contact member away from the lower end of the lever, the upper end of the lever being received with clearance in a groove or recess in the operating member, said clearance enabling the lever to undergo said limited pivotal movement.

1 4. An electrical snap switch according to Claim 1 2 or 13, wherein the spring means are constituted by a U- or C-shaped leaf spring the two ends of which are respectively attached to the lever and the contact member.

1 5. An electrical snap switch according to any one of Claims 1 2 to 14, wherein the operating member is subjected to the influence of a return spring which urges the operating member in the reverse direction to the advancing movement of the operating member, so as to reset the switch.

1 6. An electrical snap switch according to any one of the preceding claims, wherein the operating member is movable with a rectilinear motion.

1 7. An electrical snap switch which is normally closed and which comprises a hous ing supporting two pairs of stationary electrical contacts, a movable contact member pivotable between a first closed position in which the contact member bridges one pair of stationary electrical contacts and establishes electrical connection therebetween and a second position in which the contact member is spaced from said one pair of electrical contacts and bridges the other pair of electrical contacts, spring means which bias the contact member either towards the first position thereof or the second position thereof and which move over-centre with a toggle action during movement of the contact member between the first and second positions thereof, and an operating member which is guided for rectilinear sliding movement with respect to the housing and undergoes an operative sliding movement which alters the line of action of the spring means to cause the contact member to pivot from the first closed position thereof to the second position thereof to trip the switch, wherein during said operative sliding movement the operating member advances in a direction substantially the same as the direction of separating movement of the contact member from said one pair of stationary contacts towards the second position of the contact member, and wherein operative movement of the operating member causes advancement in said direction of a movable abutment which is provided by a surface or surfaces on the operating member and which, in the abnormal event of the contact member failing to move from the first position thereof to the second position thereof under the influence of the spring means, engages the contact member and displaces the contact member from the first position thereof to the second position thereof.

1 8. An electrical snap switch according to any of the preceding claims, wherein the contact member abuts the stationary contact or contacts in face-to-face relationship so that the plane of abutment is substantially perpendicular to said direction of separating movement of the contact member.

1 9. An electrical snap switch constructed and arranged substantially as herein particularly described with reference to Figs. 1 to 9, or Figs. 1 0 to 19, of the accompanying drawings.