Abstract:
A lockable safety switch mechanism having a lockable switch mechanism that cooperates in an offset or skewed manner with an electrical switch. The lockable switch mechanism includes a switch plunger that is displaceable along a predetermined axis between a first position and a second position. A contour is formed along the switch plunger and cooperates with one or more locking mechanisms. A fork cooperates with the locking mechanism so as to selectively interfere with free movement of the switch plunger depending on the interaction between the contour and the locking mechanism. A link extends from one of the locking mechanism and the fork and interacts with a plunger of an electrical switch contact carrier so that an axis of movement of the switch plunger can be offset or skewed relative to an axis of movement of the electrical switch plunger.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom Patent Application No. 0715957.7 filed on Aug. 16, 2007 and the disclosure of which is incorporated herein. 
     BACKGROUND 
     The present invention relates to a safety switch, and in particular a safety switch having a lockable switch mechanism. 
     Safety switches are often used to control the supply of electricity to electrically powered machinery. Typically, a safety switch is located on a doorpost of an enclosure inside which is located kinetic machinery. On the door to the enclosure is located an actuator which is engageable with the safety switch. When the door to the enclosure is opened, the actuator is not in engagement with the safety switch. When the actuator is not engaged with the safety switch, electrical contacts within the safety switch are kept apart such that electricity may not be supplied to the machinery within the enclosure. Thus, a user may enter and move around the enclosure with a reduced risk of injury, since the machinery is not operating. If the door to the enclosure is closed, the actuator is brought into engagement with the safety switch. The contacts in the safety switch are then brought into contact with each other such that electricity may be supplied to the machinery within the enclosure. This sort of arrangement, which is often referred to as a safety interlock, is used in a wide variety of applications. 
     A safety switch having a lockable switch mechanism is described in U.S. Pat. No. 6,872,898. That safety switch comprises a mechanism which comprises a plurality of elements that co-operate to lock a switch plunger in position, or allow it to move. Part of the locking mechanism comprises a solenoid and a solenoid plunger. The solenoid plunger is moveable in the solenoid and abuts against a contact block plunger of a contact block. When the solenoid plunger is energised, the solenoid plunger moves, which in turn causes or allows movement of the contact block plunger. The contact block plunger is moveable to move bridging contacts into or out of electrical connection with fixed contacts of the contact block to allow or prevent a safety switch of which the switch mechanism is a part to allow or prevent the conduction of electricity (e.g. to machinery in a machine guard). 
     The locking arrangement disclosed in U.S. Pat. No. 6,872,898 works well. However, existing safety switches which use this arrangement have a number of disadvantages. The way in which elements of the safety switch are positioned restricts the overall shape of the safety switch. Furthermore, due to the large number of co-operating elements of the safety switch, the design and manufacturing tolerances that need to be met to produce a reliable safety switch are very small. 
     It is therefore an object of the present invention to obviate or mitigate at least one of the disadvantages of the prior art, whether identified herein or elsewhere. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a safety switch mechanism that includes a lockable switch mechanism comprising a switch plunger which is mounted in a housing and is displaceable relative to the housing along a predetermined axis between a first unlocked position and a second position. The switch mechanism includes a locking mechanism for locking the switch plunger in the second position and a switch mechanism which is actuated by movements of the switch plunger between the first and second positions. The locking mechanism comprises at least one first locking member which is biased against a surface of the switch plunger and at least one second locking member which is displaceable between locked and released positions. The surface of the switch plunger against which the first locking member is biased defines a profile that is arranged such that movement of the switch plunger from the second to the first position causes the profile to displace the first locking member and the second locking member when in the locked position preventing displacement of the first locking member by the profile to thereby prevent movement of the plunger from the second to the first position. The switch mechanism includes a contact block having a set of fixed contacts and a contact block plunger. The contact block plunger includes at least one bridging contact and is moveable in the contact block to move the bridging contact into and out of electrical connection with the set of fixed contacts. The second locking member is attached to the contact block plunger via a linking member. 
     At least in part by attaching the second locking member to the contact block plunger, the safety switch mechanism of the present invention may be easier to reliably construct than similar prior art mechanisms. It is further appreciated that the overall shape not be restricted to being elongate, as described in more detail below. 
     Preferably, the contact block is provided with a biasing means which biases the contact block plunger such that the bridging contact is biased away from the fixed contacts. Preferably, the biasing means is only able to push apart the bridging contact and the fixed contacts when the linking member breaks, deforms, or becomes detached from one or both of the second locking member and the contact block plunger. 
     Preferably, each first locking member comprises a locking pin extending transversely relative to the axis of displacement of the switch plunger. The locking pin is spring biased towards the switch plunger in a direction perpendicular to the switch plunger axis. Two locking pins may be provided on opposite sides of the switch plunger. The locking pins may be mounted in a housing assembly that defines an aperture through which the switch plunger extends. The locking pins are preferably spring-biased towards each other from opposite sides of the aperture by springs supported in the housing assembly. The housing assembly may comprise a frame which receives the locking pins and springs and a cover plate that retains the locking pins and springs within the assembly. 
     The profile may be defined by an annular shoulder extending around the switch plunger. The shoulder may be tapered so as to readily lift the locking pins away from the switch plunger if the mechanism is not in the locked condition. One or more of the locking members may comprise a locking arm which is displaceable in a direction parallel to the switch plunger axis and, when in the locked position, extends on the side of the first locking member remote from the switch plunger to prevent displacement of the first locking member in a direction away from the switch plunger axis. Two locking arms may be provided to lock respective locking pins against displacement relative to the switch plunger axis. The locking arms may extend from one end of a solenoid plunger which is arranged at one end of the switch plunger and is displaceable along the switch plunger axis by a solenoid winding within a solenoid housing. The solenoid may be arranged so that, when energised, the locking arms are displaced from the locked position, or alternatively may be arranged so that, when energised, the locking arms are displaced to the locked position. 
     A compression spring may be arranged between the switch and solenoid plungers to bias the plungers apart, and a compression spring may also be arranged between the solenoid plunger and the solenoid housing to bias the solenoid plunger towards the switch plunger. The switch plunger may be axially displaced by rotation of a cam from a datum position by insertion of an actuator into the mechanism. Withdrawal of the actuator is prevented unless the cam is rotated back to the datum position, and such rotation is prevented by the locking mechanism if each of the one or more second locking members is in the locked position. 
     The contact block maybe positioned alongside the lockable switch mechanism. 
     Movement of the contact block plunger may be arranged to be parallel to movement of the switch plunger. 
     The contact block plunger or contact block may be provided with guides or channels for guiding movement of the contact block plunger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic cut-away view of a locking switch mechanism of a safety switch in accordance with an embodiment of the present invention with the switch in an unlocked condition; 
         FIG. 2  illustrates the mechanism of  FIG. 1  after the insertion of an actuator to switch the mechanism and locking of the mechanism; 
         FIG. 3  is a partial perspective view of some of the components of the mechanism of  FIGS. 1 and 2  showing those components in the positions adopted when the switch is unlocked as shown in  FIG. 1 ; 
         FIG. 4  is a side view of the components of  FIG. 3 ; 
         FIG. 5  is a partial perspective view of the components shown in  FIGS. 3 and 4  with those components in the switch locked position corresponding to  FIG. 2 ; 
         FIG. 6  is a side view of the components shown in  FIG. 5 ; 
         FIG. 7  shows the mechanism of  FIGS. 1 to 6  after insertion of an actuator but before locking of the mechanism; 
         FIG. 8  illustrates the application of a force to withdraw the actuator when the mechanism is locked; 
         FIG. 9  illustrates the mechanism after unlocking of the mechanism and partial withdrawal of the actuator; 
         FIG. 10  is a perspective view of assembled components of the locking mechanism; 
         FIG. 11  is an exploded view of the assembly of  FIG. 10 ; 
         FIG. 12  is a sectional view through a solenoid plunger of the mechanism of  FIGS. 1 to 11 ; 
         FIG. 13  is a perspective view of a solenoid locking fork of the mechanism of  FIGS. 1 to 12 ; 
         FIG. 14  is a sectional view through the solenoid locking fork of  FIG. 13 ; 
         FIG. 15  is a schematic cut-away view of a locking switch mechanism in accordance with another embodiment of the present invention with the switch in an unlocked condition; 
         FIG. 16  illustrates the mechanism of  FIG. 15  after the insertion of an actuator and locking of the mechanism; 
         FIG. 17  is a perspective view of a locking fork of the mechanism of  FIGS. 15 and 16 ; 
         FIG. 18  is a simplified perspective view of an exemplary safety switch equipped with a locking switch mechanism; 
         FIG. 19  is a cross-section view of the assembly shown of  FIG. 18 ; and 
         FIG. 20  is a cross-section view of another safety switch and locking mechanism according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the illustrated lockable switch mechanism comprises a housing  1  in which a plunger  2  is slidable and which supports a head assembly  3  supporting a rotatable cam  4 , the cam  4  being rotatable about a pin  5 . The plunger  2  comprises a metal core supporting an outer casing  6  which is slidably received in a sealing cap  7 . The plunger  2  is symmetrical about its longitudinal axis and is slidable relative to the housing  1  along that axis. 
     The end of the plunger  2  remote from the cam  4  is received in a bore  8 . A compression spring  9  is located within the bore  8  and biases the plunger  2  in the direction indicated by arrow  10 . The bore  8  is formed in the end of a solenoid plunger  11  which is received within a solenoid housing  12 . Energisation of a solenoid winding (not shown) in the solenoid housing  12  drives the solenoid plunger  11  to the right in  FIG. 1 . Denergisation of the solenoid results in the solenoid plunger  11  being moved to the left with respect to the orientation shown  FIG. 1  by a compression spring  13  ( FIG. 2 ) which is located between the solenoid housing  12  and a locking fork  14  which is engaged in a groove extending around the end of the solenoid plunger  11  in which the bore  8  is formed. 
     Two locking pins  15  are positioned on either side of the plunger  2 . The locking pins  15  are biased by springs  16  against the plunger  2 . The locking pins  15  and springs  16  are retained within a housing assembly made up from a frame  17  and a cover plate  18 . It will be seen that with the plunger  2  in the position shown in  FIG. 1 , the pins  15  are held at a distance from the axis of the plunger  2  such that they obstruct the passage of arms  19  supported by the locking fork  14  in the direction of the arrow  10 . 
       FIG. 2  shows the assembly of  FIG. 1  after the insertion of an actuator  20  into the head assembly  3  so as to cause rotation of the cam  4 . Such rotation of the cam  4  enables the plunger  2  to move towards the pin  5 . As a result a profile  21  in the form of an annular shoulder on the plunger  2  is moved to the left of the locking pins  15 . The locking pins  15  are biased towards each other so as to remain in contact with the plunger  2 , thereby enabling the arms  19  of the locking fork  14  to pass the locking pins  15 . 
     The actuator  20  and cam  4  are shaped such that insertion of the actuator into the head assembly  3  causes the cam to rotate from a datum position or the position of the cam  4  as shown in  FIG. 1 . The actuator defines projections (not shown) which engage in recesses defined by the cam  4  (as shown in  FIG. 2 ) so that once the cam  4  has been rotated from the datum position, the actuator  20  cannot be withdrawn from the head assembly  3  unless the cam  4  has been rotated back to the datum position. An actuator and cam mechanism of this general type is described in U.S. Pat. No. 5,777,284. 
       FIGS. 3 and 4  show a perspective view of a portion of the assembly in the unlocked condition. In  FIG. 3 , the solenoid plunger  11  has been moved to the position it assumes when the solenoid is energised and the plunger  2  is in the position in which it is displaced by the cam  4  as far as possible towards the solenoid housing  12 . As a result the spacing between the pins  15  is such that even if the solenoid is then deenergised the arms  19  cannot move past the pins  15 . The pins  15  therefore impose no restraint on the axial displacement of the plunger  2 . In contrast, as shown in  FIGS. 5 and 6 , if the cam  4  is then rotated to displace the plunger  2  so that the pins  15  can drop down the profiled shoulder  21  defined by the plunger  2 , the springs  16  urge the locking pins  15  towards each other so as to engage behind the shoulder  21 . Deenergisation of the solenoid then results in the arms  19  being extended past the pins  15 , restraining the pins  15  against movement away from each other. Any attempt therefore to drive the plunger  2  towards the solenoid housing  12  will be resisted as a result of the pins  15  jamming between the profile  21  and the arms  19 . 
       FIG. 7  shows the assembly after displacement of the plunger  2  towards the cam pin  5 . Unless the solenoid is energised, the arms  19  of the locking fork  14  will engage around the pins  15  as shown in  FIGS. 5 and 6 . In the configuration shown in  FIG. 7  however the solenoid has been energised, displacing the arms  19  to the right. There is then nothing to stop the locking pins  15  being moved apart against the biasing force provided by the springs  16 . Thus if the actuator  20  was to be withdrawn from the head assembly  3  this would result in the displacement of the plunger  2  to the right in  FIG. 7 , such movement being permitted as the tapered surface of the shoulder  21  would push against and force apart the two locking pins  15 . 
     Referring to  FIG. 8 , this shows the assembly if an attempt is made to withdraw the actuator  21  when the assembly is in the configuration shown in  FIG. 2 , or with the pins  15  locked in position by the arms  19 . Pulling on the actuator  20  causes the cam  4  to rotate in the clockwise direction in  FIG. 8  thereby applying an axial force to the plunger  2  and causing the plunger to attempt to move in the direction indicated by arrow  22 . Such displacement is however resisted by the locking pins  15  which bear against the profile  21 . The arms  19  prevent the pins  15  moving apart and thus prevent further axial displacement of the plunger  2 . 
     In contrast, if the solenoid is energised so as to displace the arms  19  to the position shown in  FIG. 7 , and the actuator  20  is pulled out of the head assembly  3 , rotation of the cam  4  is not resisted by contact between the pins  15  and the profile  21  and as a result the plunger  2  can be displaced in the direction of arrow  23  as shown in  FIG. 9 . 
       FIG. 10  illustrates the housing assembly for the locking pins  15  and springs  16  and  FIG. 11  shows the components of the assembly of  FIG. 10  in exploded form. Pins  15  flank an opening generally associated with plunger  2 . Frame  17  and cover plate  18  cooperate so as to support one or more pins  15  and springs  16  therebetween. 
       FIG. 12  is a sectional view through the solenoid plunger  11  showing the bore  8  and the groove extending around the end of the plunger  11  in which the bore  8  is provided, that groove being engaged by the locking fork  14  shown in  FIGS. 13 and 14 . 
     Referring to  FIGS. 13 and 14 , the locking fork  14  which supports the locking arms  19  has a C-shaped body defining an inwardly projecting edge  24 , that edge being received in the groove or slot formed around the end of the solenoid plunger  11  shown in  FIG. 12 . The inner faces of the fork arms  19  are tapered such that, on energisation of the solenoid, the arms  19  are released easily from engagement with the pins  15 . 
     Given the structure of the plunger and locking fork combination, it is a relatively easy matter to assemble the combination. In an alternative arrangement it would of course be possible to fabricate the plunger  11  and the locking fork  14  including the locking fork arms  19  as a single piece component. 
     In the embodiment of  FIGS. 1 to 14 , energisation of the solenoid is necessary to release the locking mechanism. Preferably, the solenoid is not energised accept when it is desired to release the locking mechanism. In the event of a power failure when the mechanism is locked, it is not possible to unlock the mechanism and therefore it is not possible to release the actuator from the cam. The actuator can only be released after the supply of power is restored. In some applications, this can be a significant disadvantage.  FIGS. 15 to 17  illustrate a second embodiment, in which this disadvantage is avoided by relying upon a solenoid which is energised when the switch is locked and de-energised when the switch locking mechanism is released. 
     Referring to  FIGS. 15 to 17 , components of the second embodiment which are equivalent to components of the first embodiment shown in  FIGS. 1 to 14  are identified by the same reference numerals. Thus, in the second embodiment a plunger  2  is biased against a cam  4  by a compression spring  9 . The plunger  2  is located between a pair of locking pins  15  which are biased against the sides of the plunger  2  by springs  16 . The plunger  2  defines a shoulder  21  behind which the locking pins  15  engage when the plunger  2  is displaced towards a pin  5  about which the cam rotates.  FIG. 15  shows the locking mechanism before insertion of an actuator into the assembly so as to rotate the cam. In this configuration the locking pins  15  cannot engage behind the shoulder  21 .  FIG. 16  shows the mechanism after displacement of the plunger  2  as a result of rotation of the cam  4 . In this configuration the pins  15  are biased inwards by the springs  16  so as to engage behind the shoulder  21 .  FIG. 16  shows the locking pins  15  after displacement of a locking fork  14  so that locking arms  19  extend outside the locking pins  15 , thereby preventing the locking pins  15  from moving outwards. In the condition shown in  FIG. 16 , the plunger  2  cannot therefore be moved to the right in  FIG. 16  as such movement would be prevented by inter-engagement between the shoulder  21  and the locking pins  15 . 
     The locking fork  14  is mounted on solenoid plunger  11  and is biased towards the cam  4  by a compression spring  13 . If the solenoid is de-energised, the spring  13  ensures that the locking arms  19  are displaced away from the locking pins  15 . The mechanism is therefore unlocked in that axial movement of the plunger  2  is not obstructed. If the solenoid is energised, the plunger  11  is driven to the right with respect to the orientation shown in  FIG. 16  such that, providing the plunger  2  is in the position shown in  FIG. 16 , the locking arms  19  can engage outside the locking pins  15 , thereby locking the mechanism. 
     With the arrangement illustrated in  FIGS. 15 and 16 , the switch will remain locked only so long as the solenoid is energised. When it is desired to unlock the mechanism, the solenoid is simply de-energised. With such an arrangement it will be appreciated that, in the event of a power failure, the mechanism is automatically unlocked. In some applications this is a significant advantage. In contrast, with the mechanism illustrated in  FIGS. 1 to 14 , unlocking of the mechanism requires energisation of the solenoid and therefore in the event of a power failure it would not be possible to release the actuator  20  from the cam  4 . 
       FIG. 17  illustrates the structure of the locking fork  14  of the embodiment of  FIGS. 15 and 16  in greater detail. It will be noted that the locking arms  19  are mounted on an L-shaped extension  25  of the locking fork  14 , the locking fork  14  defining a C-shaped body defining an inwardly projecting edge that is received in a slot formed around the end of the solenoid plunger  11 . 
     In  FIGS. 1 to 17 , various embodiments of the locking mechanism of the safety switch have been described. The locking function is also supplemented by an electrical power supply interlock. That is, when the switch plunger is locked in position by the locking mechanism, the ability of the safety switch to allow or prevent the conduction of electricity is determined by the electrical power supply interlock. For example, when the plunger is locked in position to prevent removal of the actuator from the switch (and therefore, for example, the opening of the door or an enclosure) the safety switch may be moved to a conducting state, such that power may be supplied to machinery located in a machine guard. Conversely, when the plunger is not locked in position the actuator may be removed from the switch, causing the safety switch to move to a non-conducting state, such that power may be not supplied to machinery located in a machine guard. 
     The electrical interlock principle described above is well known in the art. An implementation of the electrical interlock is depicted in  FIGS. 18 and 19 .  FIGS. 18 and 19  depict an exemplary safety switch which utilises the locking mechanism described in relation to  FIGS. 1 to 17  above in conjunction with a contact block  100 . Elements of the locking mechanism described in relation to  FIGS. 1 to 17  and which also appear in  FIGS. 18 and 19  are therefore given the same reference numerals. 
     In  FIGS. 18 and 19 , it can be seen that an end of the solenoid plunger  11  is in contact with the end of a contact plunger  110 . The contact plunger  110  is moveable in the contact block  100 , and along the same axis of movement as the solenoid plunger  11 . The contact block plunger  110  is provided with a plurality of moveable bridging contacts  120  which extend through the body of the contact block plunger  110 . The bridging contacts  120  are biased by springs  130 . The contact block plunger  110  is moveable to move the bridging contacts  120  into or out of electrical connection with fixed contacts  140  provided in the contact block  110 . The fixed contacts  140  may be connected to a power supply or machinery (not shown). 
     When the contact block plunger  110  is moved to bring some or all of the bridging contacts  120  into electrical connection with the fixed contacts  140 , the safety switch is able to conduct electricity. The arrangement of the fixed contacts  140  and moveable contacts  120  may be chosen and/or configured such that the safety switch may only conduct electricity when the locking pins  15  are locked in position by the locking arms  19 , i.e. when the actuator (not shown) cannot be removed from the safety switch. For example, it can be seen from the Figures that the contact block plunger  110  is biased against an end of the solenoid plunger  11  by a spring  150 . When the solenoid plunger  11  is moved by energising of the solenoid (not shown, but described above) to unlock the locking mechanism, the contact block plunger  110  is moved to bring some of the bridging contacts  120  out of electrical connection with the fixed contacts, thus preventing the safety switch from conducting electricity. 
     Although the locking and electrical interlock mechanisms described in relation to  FIGS. 1 to 19  work well, existing safety switches which use such mechanisms have can be improved upon. It can be seen from  FIGS. 18 and 19  that elements forming the physical and electrical interlocks are commonly arranged in a linear fashion. This means that a safety switch which incorporates these mechanisms needs to be elongate to accommodate these mechanisms. Furthermore, due to the large number of co-operating elements forming the physical and electrical interlock mechanisms, the tolerances in the design and fabrication of co-operating elements needs to be small. It is difficult to consistently meet these small tolerances. If the tolerances are not met, the mechanisms may not work well, or may not work at all. For instance, referring to  FIG. 19 , if the end of the solenoid plunger  11  is, for example, 0.5 mm too far away from the end of the contact block plunger  110 , there may be an unacceptable delay in the making or breaking of contacts in the contact block  100 . It is possible that the gap between the end of the contact block plunger  110  and solenoid plunger  11  may prevent the moveable contacts from being moved into or out of electrical connections with the fixed contacts  140 . 
     The present invention provides a solution to the problems of the prior art.  FIG. 20  shows a safety switch mechanism according to an embodiment of the present invention. The safety switch mechanism has the features of the lockable switch mechanism described in  FIGS. 1-17 , and also the electrical interlock features described with reference to  FIGS. 18 and 19 , and therefore like features are given the same reference numerals. In contrast to the mechanisms described in relation to  FIGS. 18 and 19 , however, the solenoid plunger  11  is no longer arranged to be in contact with an end of the contact block plunger  110 . Instead, a linking member  200  physically connects the locking arm  19  to the contact block plunger  110 . This means that movement of the locking arm  19  directly effects movement of the contact block plunger  110  and the contacts carried by the contact block plunger  110 . The number of tolerances that have to be considered for features which co-operate is therefore reduced, since there is no relative movement between the locking arm  19  and the contact block plunger  110 . This may make the mechanism of  FIG. 20  easier to reliably construct. Furthermore, by attaching the contact block plunger  110  to the locking arm  19  via a linking member  200 , the elements of the safety switch mechanism no longer have to be disposed a linear manner. It can be seen, for example, that the contact block  100  can now be placed alongside the locking mechanism, rather than in-line with it. This means that the shape of the safety switch which incorporates a mechanism according to an embodiment of the present invention does not have to be as elongate as those of the prior art. An additional advantage in the flexibility of the positioning of the contact block  100  is that more room may be available in existing or new safety switch housing for movement of the solenoid plunger  11 . This means that a larger solenoid (not shown) could be used to move the solenoid plunger with greater speed and/or force, thereby improving the locking mechanism. 
     The linking member  200  can be formed from any suitable material, for example plastics or metals. The linking member  200  could be integrally formed with the contact block plunger  110 , and then attached to the locking arm  19 . Alternatively, the linking member  200  could be integrally formed with the locking arm  19 , and then attached to the contact block plunger  110 . Alternatively, the linking member could be attached to an independent element which is attached to both the locking arm  19  and the contact block plunger  110 . The linking member may be a strip or rod of material, or maybe a more complex structure. In  FIG. 20 , it can be seen that the movement of the contact block plunger  110  is parallel to the movement of the solenoid plunger  11 . Understandably, contact block plunger  110  need not be oriented in parallel association with solenoid plunger  11 . The linking member could comprise or co-operate with a pivot or the like, such that axial movement of the solenoid plunger  11  causes movement of the contact block plunger in a direction other than parallel to the solenoid plunger  11 . For example, the contact block plunger  110  may be made to move perpendicularly with respect to the movement of the solenoid plunger  11 . 
     The spring  150  (or other biasing member) of the contact block  100  can be arranged to bias the contact block plunger  110  in such a way as to cause the bridging contacts  120  to be biased away from electrical connection (e.g. contact) with the fixed contacts  140 . In normal use, the compression spring  13  dominates the spring  150 , such that when an actuator is brought into engagement with the cam, the cam rotates and the switch plunger, locking arm  19 , linking member  200  and contact block plunger  110  all moved to the right (in the orientation shown in  FIG. 20 ). The bridging contacts  120  are brought into contact with the fixed contacts  140  and the safety switch is able to conduct electricity. However, if the linking member  200  breaks, or becomes detached from one or both of the contact block plunger  110  and locking arm  19 , the spring  150  is no longer in any sort of contact or competition with the compression spring  13 . The spring  150  is thus now able to move the contact block plunger  110 , and push apart the bridging contacts  120  and the fixed contacts  140 , thereby preventing the safety switch from conducting electricity. That is, if the linking member breaks, deforms, or becomes detached from one or both of the locking arm  19  and the contact block plunger  110  the switch fails to a safe (non-conducting) state. 
     Preferably, the spring  150  is only able to push apart the bridging contacts  120  and the fixed contacts  140  when the linking member breaks, deforms, or becomes detached from one or both of the locking arm  19  and the contact block plunger  110 . 
     The linking member need not be attached to the locking arm, but could be attached to a structure which supports the locking arm, e.g. a locking fork (described above). In generic terms, the linking member is attached to the second locking member. 
     The contact block plunger  110  and/or the contact block  100  could be provided with guides and/or channels to guide the movement of the contact block plunger. 
     In the above embodiments, the locking arm has been described as being moved coaxially with respect to the switch plunger. Other orientations, such as crossing, perpendicular, or non-coaxial, are envisioned. The second locking member may move in any suitable direction to effect the locking in position of the switch plunger. For example, the second locking member may move in a direction perpendicular to the axial movement of the switch plunger. 
     In the above embodiments, the second locking member had been described as a locking arm. It will be appreciated that other elements may also serve as the second locking member or a part of the second locking member, for example wedges, or curved segments or the like. Similarly, the first locking members have thus far been described as pins. It will be appreciated that structures other than cylindrically shaped pins may serve as the first locking members. For example, the first locking members may be elliptical in cross section, or triangular. The first locking members may be wedges, or curved segments or the like. 
     It will be appreciated that the above embodiments have been given by way of example only. Various modifications may be made to these and indeed other embodiments without departing from the invention as defined by the claims that follow.