Switch contact mechanism

A switch assembly is provided that comprises a switch having a normally open contact and a normally closed contact which is electrically connected to the normally open contact and which is located either within the same housing as the normally open contact or a separate housing. The housing(s) may be mounted onto a latch assembly which, in turn, is mounted onto a switch operator, thereby closing the normally open contact. If the latch assembly becomes mechanically disengaged from the switch operator, thereby rendering the normally closed contact non-operational, the normally open contact will open, thereby opening the circuit to a machine performing a controlled function. The user, noticing the stoppage of operation, will then be alerted of a malfunction within the switch assembly.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to switch assemblies and, more particularly,
 relates to a method and apparatus for monitoring a contact in a switch
 assembly.
 2. Discussion of the Related Art
 Electrical switches, such as pushbuttons or rotary switches used for the
 control of industrial equipment, are typically mounted onto a front panel
 of a cabinet so that the manipulated portion of the switch (termed the
 "operator") projects out from and is accessible at the front of the
 cabinet.
 For a pushbutton switch, a hole may be punched in the cabinet of sufficient
 diameter to accommodate the pushbutton and a surrounding threaded shaft.
 The shaft and pushbutton are inserted through the hole, and a threaded
 retaining nut is placed over the shaft and tightened to securely affix the
 switch to the panel. The panel is thus sandwiched between the switch body
 and the retaining nut.
 The end of the switch operator protruding inside of the panel may be
 snapped or otherwise mounted onto one side of a latch assembly, and a
 contact block or a plurality of contact blocks are mounted onto the other
 side of the latch assembly. The contact blocks are electrically connected
 to the circuit or circuits that the switch is to control.
 Contact blocks typically comprise housings that contain normally open
 and/or normally closed contacts. A normally open contact may be used, for
 example, when a user wishes to activate a specified function by actuating
 the operator, thereby closing the normally open contact. When the operator
 switch is deactivated, a plunger returns to its normal position, thereby
 opening the normally open contact and terminating the controlled function.
 A normally closed contact may be used when a user wishes to stop an ongoing
 function. One common example of a normally closed contact is an Emergency
 Stop (EStop) function which is activated when the user wishes to
 immediately terminate the controlled function due, e.g. to a malfunction
 in the process or the development of a situation that may cause damage to
 the product line or the operating equipment. In this situation, when the
 switch operator is actuated, the normally closed contact opens and remains
 open until the operator is returned to its normal state, thereby closing
 the normally closed contact and resuming the controlled function.
 In such systems, the user assumes a risk that the normally closed contact
 may become mechanically disengaged from the switch operator. Such a
 situation may occur, for example, if the latch assembly is damaged or not
 properly mounted onto the switch operator and therefore becomes detached
 during operation. Alternatively, the contact block may be damaged or
 improperly mounted. Even though, in these situations, the contact block is
 mechanically disconnected from the switch operator, the normally closed
 contact remains closed, thereby permitting the continuous operation of the
 controlled function. As a result, when the normally closed contact is
 functioning as an E-Stop, for example, the controlled function will remain
 in operation even though the contact block is no longer mechanically
 engaged with the switch operator.
 Currently, one known way to ensure an operable state of a normally closed
 switch is to test it by intermittently activating the switch operator. If,
 after activation, the controlled function is nonresponsive, then the user
 will become aware of a problem in the switch assembly and may take
 corrective measures. However, this method of detection is quite
 inefficient and results in considerable unnecessary down-time, thereby
 increasing cost. Furthermore, this method is unreliable as situations may
 arise that require the activation of an E-Stop that has become
 non-operational since the last test.
 The need has therefore arisen to implement a method and apparatus for
 detecting when the normally closed contact becomes mechanically disengaged
 from the switch operator in an efficient and reliable manner.
 OBJECTS AND SUMMARY OF THE INVENTION
 It is therefore a first object of the present invention to provide a switch
 assembly having a switch that: 1) monitors a normally closed contact to
 determine when the contact becomes mechanically disengaged from a switch
 operator, and 2) permits normal operation of the controlled function.
 It is a second object of the invention to permit the switch and normally
 closed contact to be mounted either within the same housing or in separate
 housings.
 It is a third object of the invention to provide a single switch that is
 able to monitor a plurality of contacts.
 In accordance with a first aspect of the invention, the switch comprises a
 normally open contact that is electrically connected in series to the
 normally closed contact to be monitored, and to the function that the
 switch assembly is to control. When the switch is connected to the switch
 operator, preferably via a latch assembly in a known manner, the normally
 open contact is closed, thereby completing the circuit for the controlled
 function. To perform a specified operation of the function, the switch
 operator is actuated to open the normally closed contact. If, during
 operation, the normally closed contact becomes mechanically disengaged
 from the switch operator, the normally open contact will open, thereby
 opening the circuit and terminating operation of the controlled function.
 The user, noticing the stoppage, will then be alerted that a problem
 exists in the switch assembly and may take corrective action.
 In accordance with a second aspect of the invention, the normally open
 contact and normally closed contact may either reside in the same housing
 or in separate housings. If both contacts are in the same housing, the
 switch preferably comprises a column that is disposed within the housing
 and that comprises a plunger that is permitted to engage a switch operator
 stem. A contact spring within the column is interposed between the two
 contacts in the housing and biases each contact towards its closed
 position. A return spring, disposed within the housing, biases the
 normally open contact towards its open position and, because it provides a
 greater force than the contact spring, maintains the normally open contact
 in its open position. Additionally, the return spring biases the column
 upwards so that the plunger extends outside the housing to engage the
 stem.
 When the latch assembly is mounted onto the switch operator, the stem
 depresses the column via the plunger, and the column compresses the return
 spring. The contact spring then closes the normally open contact, which is
 now in mechanical communication with the switch operator. When the
 operator is activated, the column is further depressed and the normally
 closed contact, also now in mechanical communication with the operator,
 opens to perform a specified operation to the controlled function. If the
 housing becomes disconnected from the switch operator, thereby
 mechanically disengaging the normally closed and normally open contacts
 from the switch operator, the plunger will return to its normal position,
 thereby biasing the return spring to open the normally open contact,
 opening the circuit, and terminating the controlled function.
 Additionally, if the plunger breaks, the return spring again will bias the
 column upwards, thereby opening the normally open contact and terminating
 the controlled function.
 Alternatively, the normally open and normally closed contacts could be
 disposed within separate housings. In the housing containing the normally
 closed contact, a plunger that is connected to a column and partially
 disposed within the housing is connected to the switch operator such that
 the normally closed contact is opened when the switch operator is
 actuated. A second plunger is partially disposed within the housing
 containing the normally open contact such that, when the housing is
 connected to the switch operator, the plunger closes the normally open
 contact. If the latch assembly becomes mechanically disconnected from the
 switch operator, the second plunger will also become disconnected, and the
 normally open contact will return to the open position and open the
 circuit.
 In accordance with a third aspect of the invention, a single switch
 operator may control a plurality of contacts that work in tandem on a
 latch assembly that is mounted onto the switch operator. Again, the
 plurality of contacts may either be all disposed within the same housing
 or in different housings. If the contacts are in the same housing, a
 plurality of contact springs are employed in conjunction with stops within
 the column to actuate each contact. If the contacts are disposed in
 different housings, the housings are mounted onto the latch assembly. The
 switch will again operate in the manner described above if it becomes
 mechanically disengaged from the switch operator.
 Other objects, features, and advantages of the present invention will
 become apparent to those skilled in the art from the following detailed
 description and the accompanying drawings. It should be understood,
 however, that the detailed description and specific examples, while
 indicating preferred embodiments of the present invention, 5 are given by
 way of illustration and not of limitation. Many changes and modifications
 may be made within the scope of the present invention without departing
 from the spirit thereof, and the invention includes all such
 modifications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Pursuant to the invention, a switch for monitoring a normally closed
 contact in a switch assembly is provided. The switch comprises a normally
 open contact that is electrically connected in series to the normally
 closed contact to be monitored. The two contacts are then electrically
 connected in series to the output controlled by the normally closed
 switch. In one embodiment, the normally open contact and normally closed
 contact are disposed within a single contact block that is preferably
 mounted onto a latch assembly that receives a switch operator. The contact
 block comprises a housing that includes a plunger that is depressed when
 the housing is mechanically connected to the switch operator. A movable
 column comprising the plunger actuates a conductive spanner to close the
 normally open contact, thereby completing the circuit, when the housing is
 mechanically connected to the operator. A second spanner is actuated by
 the column to open the normally closed contact when the operator is
 actuated. When the housing becomes mechanically disengaged from the
 operator, the plunger returns to its normal position, thereby opening the
 normally open switch and opening the electrical circuit. In another
 embodiment, the normally open contact and normally closed contact are
 disposed within separate housings that are preferably mechanically
 connected to the switch operator via a latch assembly. A plunger within
 one housing biases the normally open contact to a closed position when the
 housing is connected to the switch operator. Therefore, when the latch
 assembly becomes mechanically disengaged from the switch operator, the
 housing with the open contact will also become disengaged, thereby opening
 the normally open contact and terminating the controlled function.
 Likewise, the circuit will open when the switch operator is actuated,
 thereby actuating a plunger within a second housing to open the normally
 closed contact.
 Referring to FIG. 1, a switch 20 constructed in accordance with a first
 embodiment of the invention takes the form of a contact block including a
 housing 22. A normally closed contact 24 and a normally open contact 26
 are disposed within the housing 22 and comprise respective spanners 28, 30
 having respective contacts 32, 34 that, when closed, engage respective
 contacts 36, 38 on leads 40,42 that terminate in respective terminals 44,
 46. The spanners, leads, and terminals are formed from a conductive
 material such that a circuit is completed when both contacts 24, 26 are
 closed and the terminals are electrically connected in series to a
 controlled machine (not shown). While the normally open contact 26 is
 described in accordance with a preferred embodiment of the invention, any
 connection in a circuit that is normally open and that may be actuated to
 a closed position during normal operation may be used. Likewise, the
 normally closed contact 24 could comprise any connection in a circuit that
 is normally closed that may be actuated to an open position during normal
 operation.
 The leads 40, 42 are inserted into internal slots 57 within the housing 22,
 and the terminals 44, 46 extend through the housing and are electrically
 connected in series by one of any known means. The housing 22 includes a
 movable column 50 that interlocks with a lower stop 52 to enclose a middle
 stop 54 and an upper stop 56 that interact with a contact spring 58 and a
 return spring 60 to maintain the spanners 28, 30 in their respective
 positions and orientations illustrated in FIG. 1 (see also FIG. 5). The
 column 50 and the stops 52, 54, 56 preferably comprise a plastic or other
 nonconductive material(s). The column 50 also includes a plunger 62 that
 forms the uppermost portion of the column and that extends beyond an upper
 wall 64 of the housing 22 when not mechanically engaged with a switch
 operator. While FIG. 1 depicts terminals extending outwardly from the
 housing, any known manner of connecting the normally closed contact 24 and
 normally open contact 26 in series in accordance with the schematic
 representation of FIG. 2 may be used.
 Referring again to FIGS. 1 and 5, when the switch 20 is not mounted onto a
 switch operator, the spanner 28 rests between contacts 36 and stop 56, and
 spanner 30 rests between stop 52 and stop 54. Contact spring 58, disposed
 within the column 50, rests between stops 54, 56, thereby biasing the
 spanners 28, 30 towards respective contacts 36, 38 on leads 40, 42,
 retaining the normally closed contact 24 in the closed position. Return
 spring 60 is sandwiched between a bottom wall 66 of the housing 22 and the
 bottom of stop 52. Because the return spring force is greater than the
 contact spring force, the return spring 60 biases the stop 52 upwardly
 until the spanner 30, sandwiched between stops 52, 54, is forced away from
 lead 42 into its normally open position. The force of the return spring 60
 also biases the column 50 upwardly so that the plunger 62 is in a normal
 position, extending slightly beyond the upper wall 64 of the housing 22.
 The stops 52, 54, 56 and bottom wall 66 may contain small generally
 cylindrical nubs (not shown) having a diameter slightly smaller than the
 diameter of the spring 58 or 60 to which they connect to prevent the
 springs 58, 60 from sliding when installed.
 Referring now to FIG. 5, the spanners 28, 30 contain respective notches 68,
 70 that engage respective protrusions 72, 74 on the stops 54, 56 to
 prevent slippage of the spanners with respect to the stops. Stop 52
 contains projections 53 that engage cutout portions 55 in the column 50 to
 retain the stop 52 in place. Additionally, the column 50 contains a
 longitudinal protrusion 76 on each inside wall 78 that engages respective
 notches 80, 82 in stops 54, 56 to guide the spanners and stops and prevent
 twisting or binding. The column 50 also comprises a protrusion 84 on the
 exterior of outer wall 86 that mates with a slot 88 within the housing 22.
 The interaction between the protrusion 84 and slot 88 ensures proper
 movement of the column 50 within the housing 22 during operation, and also
 ensures that the column is not pushed out of the housing by the return
 spring 60.
 As a result of this construction, when the switch 20 is mechanically
 disengaged from a switch operator, spanner 28, in conjunction with contact
 spring 58, ensures that contact 24 is normally closed, and spanner 30, in
 conjunction with return spring 60, ensures that the contact 26 is normally
 open and that the plunger 62 is in a normal position extending outside the
 upper wall 64 of the housing 22 as shown in FIG. 1 and 4.
 FIG. 2 is a schematic electrical representation of the switch of FIG. 1,
 and shows normally closed spanner 28 in a closed position, and normally
 open spanner 30 in an open position when the switch 20 is mechanically
 disengaged from a switch operator. When the normally open spanner 30
 closes, the circuit becomes closed, thereby rendering the controlled
 function operational, as will now be described.
 Referring now to FIGS. 3 and 4, a switch assembly 90 is shown that
 comprises the switch 20 and that is mounted onto a latch assembly 92 via
 tabs, screws, or in any other known manner. The latch assembly 92 is then
 mounted onto a switch operator 94. While latch assembly 92 is shown in
 FIGS. 3 and 4 to comprise a housing 96 and collar 98, the latch assembly
 could include any apparatus that may be used to mechanically connect a
 contact block with a switch operator.
 The switch operator 94 includes a pushbutton 100 located at a head 102 at
 one end of a cylindrical shaft 104. The pushbutton 100 attaches to a stem
 106 passing generally inside the shaft 104 to communicate the action of
 the pushbutton to the plunger 62. A sheet panel 108, preferably made of
 sheet metal, has a hole (not shown) for receiving the shaft 104. External
 threads 110 are formed on the portion of the shaft 104 passing through the
 hole. The head 102, remaining on the outside of the panel 108 when the
 shaft 104 is inserted into the hole, is drawn against the panel by a
 retaining nut 112, placed over the shaft inside of the panel and tightened
 on the threads 110. The panel 108 is thus sandwiched between the nut 112
 and an inner face of the head 102. An elastomeric washer 114 may also be
 positioned between the head 102 and the panel 108 on the outside of the
 panel to provide a seal against the outside environment. While an
 electrical switch operator comprising a pushbutton has been described, it
 should be noted that any type of switch operator may be used. For example,
 another type of operator sold by the assignee under the NEMA designation
 comprises a shaft and actuator that is inserted from behind a panel, and a
 threaded mounting ring is inserted onto the shaft and secured in the front
 of the panel.
 Once the switch 20 is mechanically connected to the switch operator, the
 normally open contact 26 is in mechanical communication with the operator.
 Specifically, the operator stem 106 forces the plunger 62 and column 50
 into a first depressed position against the force of the return spring 60,
 wherein upper surface 116 of the plunger is generally flush with upper
 wall 64 of the housing 22. The contact spring 58 biases the stop 54
 downwardly and presses the spanner 30 against the lead 42, thereby closing
 the circuit when the terminals and the controlled function are
 electrically connected. As a result, when the switch 20 is mechanically
 connected to the switch operator 94, both the normally closed contact 24
 and normally open contact 26 are closed, thereby permitting the normal
 operation of the function controlled by the normally closed contact 24, as
 will now be described. The normally closed contact 24 could be employed
 for many functions that require a cessation of a given function. One
 example is an E-Stop. While the normally closed contact 24 is not limited
 to an E-Stop, it will be referred to as such for the sake of simplicity
 throughout this disclosure.
 Referring now to FIG. 6, the normally closed contact 24 is also in
 mechanical communication with the switch operator. Specifically, when the
 pushbutton 100 is depressed, the pushbutton stem 106 forces the plunger 62
 in the direction of arrow A and towards a second depressed position. As
 this occurs, the upper surface 118 of the column 50 biases the spanner 28
 away from the contacts 36, thereby opening the normally closed contact 24
 and opening the circuit. Because the contact spring 58 is compressed, it
 continues to press the normally open spanner 30 against the contacts 38.
 The controlled function is thereby terminated by the activation of the
 pushbutton 100. When the pushbutton 100 is released, the stem 106 raises
 upwardly under the force of a spring (not shown) within the switch
 operator 94, and the return spring 60 biases the column 50 upwardly such
 that the plunger 62 is returned to its normal extended position. The
 contact spring 58 biases spanner 28 toward contacts 36, thereby closing
 the contact 24 and resuming operation of the function.
 In operation, the normally closed contact 24 and normally open contact 26
 are both closed when the housing is mechanically connected to switch
 operator 94. When the contacts 24, 26 are electrically connected to a
 machine performing the controlled function, the function is fully
 operational until either the switch operator 94 is actuated, or the latch
 assembly becomes detached from the operator. The normally open contact 26
 opens at this time, thereby cutting off current to the machine performing
 the controlled function. The function will then cease to operate, which
 will alert the user of a malfunction. The overall reliability is thereby
 increased and, because the E-Stop will no longer need to be tested to
 ensure operability, the efficiency of the controlled function is also
 increased.
 FIGS. 7 and 8 show switches 120, 220 as having different contact
 configurations. In these Figures, for the sake of simplicity, those
 reference numerals that are incremented by 100 identify elements
 corresponding to similar elements in FIGS. 16, but having different
 structure. The reference numerals corresponding to the other elements have
 remained unchanged.
 In FIG. 7, switch 120 comprises a spanner 130 that is sandwiched between
 stops 52 and 54. When the housing 22 is mechanically disengaged from a
 switch operator (not shown), normally closed contact 124 is closed, as
 described above, and stop 52 ensures that spanner 130 is disconnected from
 lead 140. As described above, when the housing 22 mechanically engages the
 switch operator, the plunger 62 becomes depressed to its first position.
 Stop 54 then biases spanner 130 downwards in the direction of arrow B.
 Angled ends of spanner 130 then contact mating angled ends of leads 140
 and bias the contacts 134 on the leads toward contacts 138 in the
 direction of arrow C. Contacts 138 are located adjacent ends of leads 142,
 which terminate in terminals 146. As a result, when the plunger 62 is in
 its first depressed position, and when terminals 146 are electrically
 connected to the machine performing the controlled function, a closed
 circuit comprises terminals 146, lead 142, lead 140, and spanner 28. The
 user therefore need not manually electrically connect normally open
 contact 126 to normally closed contact 124, as this circuit is
 automatically completed when the plunger 62 is depressed.
 Spanner 130 preferably comprises a nonconductive material(s) in this
 embodiment to prevent open contact 26 from being in parallel electrical
 connection with closed contact 124. (Alternatively, as shown in FIG. 7A, a
 conductive spanner 230 could be mounted onto both sides of nonconductive
 stop 152 such that the opposite sides of the spanner would be insulated
 from each other by the stop.) Leads 140 comprise an elastic conductive
 material such that, when the plunger 62 returns to the normal extended
 position, the leads return to the position shown in FIG. 7, whereby they
 are disconnected form contacts 138. Additionally, as described above, when
 the plunger 62 is further depressed (e.g. upon activation of an operator),
 spanner 28 is biased away from contacts 36, thereby opening the circuit.
 In FIG. 8, the switch 220 comprises a normally closed contact 224 and
 normally open contact 226. Spanner 28 is opened and closed as described
 above. When plunger 62 is depressed to the first position, a stop 254 is
 moved downwardly in the direction of arrow D and moves contacts 234 in the
 direction or arrow E until contacts 234 contact contacts 238. Contacts 238
 are located on leads 234, which terminate in terminals 246. When the
 housing 22 is mechanically connected to a switch operator, and when
 terminals 246 are electrically connected to a controlled function, the
 closed circuit comprises terminals, 246, leads 242, leads 240, and spanner
 28. Leads 240 are preferably formed from a conductive elastic material(s)
 such that, when plunger 62 is returned to its normal extended position,
 thereby removing stop 254 from lead 240, the lead returns to the open
 position shown in FIG. 8.
 Alternatively, a spring could be inserted into housing 22 that bias leads
 140, 240 into the normally open position. In this arrangement, depressing
 the plunger and moving the leads 140, 240 in the directions of arrows C
 and D, respectively, would compress the springs 58, 60 and close the
 normally open contacts 126, 226 as described above.
 In another embodiment, as shown in FIGS. 9-11, switch 320 comprises a
 normally open contact 326 within housing 322, while normally closed
 contact 324 is disposed within a separate housing 323. The reference
 numerals in these Figures are incremented by an additional 100 to indicate
 elements corresponding to those elements in FIGS. 1-8. Because housing 322
 employs several common elements with housing 323, the reference numerals
 pertaining to those elements are the same.
 In this embodiment, both housings 322, 323 are mounted onto latch assembly
 92 via tabs 325 or in any other known manner, thereby retaining the
 housings in mechanical communication with one another.
 In the housing 322, contact spring 358 is disposed within a column 350
 between an upper surface 318 of the column 350 and a spanner 330. The
 spanner 330 is disposed between contact spring 358 and stop 352. Stop 352
 may either be an integral part of the column 350, or a removable stop that
 fits into place within the column. A return spring 360 rests against
 bottom wall 366 of the housing 322 at one end and the stop 352 at the
 other end. When the latch assembly 92 is mechanically disengaged from the
 switch operator 94, the return spring 360, having a greater force than
 contact spring 358, biases the stop 352 upwards into an open position away
 from contacts 338, and moves the plunger 362 to a normal extended position
 outside the housing 322. When the housing 322 is mechanically connected to
 the switch operator 94, the operator stem 106 biases the plunger 362
 downwardly to a first depressed position until the stop 352 compresses the
 return spring 360, and the contact spring 358 biases the spanner 330
 towards contacts 338 on lead 342, thereby closing the normally open
 contact 326. When the pushbutton 100 is actuated, the plunger 362 is
 depressed to a second position, thereby further compressing springs 358,
 360, which serve only to retain spanner 330 in a closed position against
 contacts 338. As a result, contact 326 will only open when it becomes
 mechanically disengaged from the switch operator 94.
 In the housing 323, contact spring 358 is disposed within the column 350
 between stop 352 and a spanner 328. The spanner 328 is disposed between
 contact spring 358 and upper surface 318 of column 350. Stop 352 may
 either be an integral part of the column 350, or a removable stop that
 fits into place within the column. A return spring 360 rests against
 bottom wall 366 of the housing 323 at one end and the stop 352 at the
 other end. A contact spring 358 rests against stop 352 at one end and
 normally closed spanner 328 at its other end. The return spring 360 and
 contact spring 358 interact to press the spanner 328 against contacts 336
 on lead 340, and to force the plunger 363 upward and away from the housing
 323. When the latch assembly 92 is not connected to an operator, return
 spring 360 biases stop 352 upwardly towards the contact spring 358,
 thereby biasing the spanner 328 towards contacts 336, and maintaining the
 normally closed contact 324 in the closed position.
 When the latch assembly 92 is mounted onto switch operator 94, the operator
 stem 106 biases the plunger 362 downwardly to close the contact 326. In
 order to prevent the stem from interfering with the plunger 363 and
 opening the contact 324, plunger 363 is shown shorter than plunger 362. As
 a result, when the latch assembly 92 is mounted to the operator 94, the
 stem 106 will bias plunger 362 downwards such that both plungers 362, 363
 will extend approximately the same distance from the housings 322, 323.
 Therefore, the plunger 362 in its first depressed position extends outside
 housing 322 the same distance that plunger 363 extends outside of housing
 323. As a result, when the switch operator 94 is actuated, plunger 363 is
 depressed, thereby biasing the upper surface 318 of the column 350 against
 the spanner 328 in a direction away from contact 328, and opening the
 normally closed contact 324. Alternatively, upper surface 318 could be
 located further upwards from spanner 328, thereby forming a gap between
 the upper surface and the spanner. This would allow the gap to close when
 the upper surface 318 is biased towards spanner 328 when the latch
 assembly 92 is mounted onto the switch operator 94.
 As a result, when terminals 346 of the housings 322, 323 are electrically
 connected in series by one of any known techniques, and the latch assembly
 92 is mounted onto switch operator 94, the controlled function becomes
 operational. The function is then halted when either the pushbutton 100 is
 actuated, thereby opening the normally closed contact 324, or when the
 latch assembly 92 becomes mechanically disengaged from the switch operator
 94, thereby also mechanically disengaging the normally open contact 326
 from the operator and opening the normally open contact.
 Additional contact blocks may also be connected to the configuration of
 FIG. 9 in accordance with an embodiment of this invention so long as they
 are connected in series with the switch 320 and mechanically connected to
 the switch operator 94, via latch assembly 92, such that the normally open
 contact 326 opens when the added contact block is mechanically disengaged
 from the operator 322, 323. While the housings are described as being
 connected to the switch operator 94 via a latch assembly 92, the switch
 and monitored contact may be implemented via any known manner of
 connecting the contact blocks in tandem to a switch operator.
 Alternatively, if a user is concerned with the possibility of a contact
 block becoming detached from the latch assembly 92, the switch 320 could
 be mounted onto the housing of the contact block to be monitored in a
 side-by-side orientation such that the switch would mechanically disengage
 the switch operator if the added contact block becomes mechanically
 disengaged, thereby opening the normally open contact and terminating the
 controlled function, as described above.
 Many changes and modifications may also be made to the invention without
 departing from the spirit thereof. The scope of these changes will become
 apparent from the appended claims.