GB1604731A

GB1604731A - Inertia switch assembly

Info

Publication number: GB1604731A
Application number: GB129478A
Authority: GB
Original assignee: Inertia Switch Ltd
Current assignee: Sensata Technologies Ltd
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-16

Description

(54) AN INERTIA SWITCH ASSEMBLY (71) We, INERTIA SWITCH LIMITED, of Elvaco House, Egham, Surrey, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state menu: This invention relates to an inertia switch arrangement for opening or closing an electrical circuit in response to an acceleration or deceleration attaining a predetermined threshold value.

Inertia switch arrangements of this kind are known in which an inertia mass, held in an inoperative position by a spring or magnet, is freed by an acceleration of predetermined intensity to effect the opening or closing of an electrical circuit by appropriate means. However, such devices have the disadvantage that they can generally only operate in conjunction with a normally open or normally closed circuit in a single switching mode.

Accordingly, it is an object of the invention to mitigate this disadvantage.

According to the present invention an inertia switch arrangement comprises a movable inertia mass, a first electrical contact member movable between a first stable position in which it engages a second electrical contact member and a second stable position in which it engages a third electrical contact member, operating means engageable by the inertia mass and arranged so that on an applied acceleration or decele ratioii exceeding threshold value the first electrical contact member is moved from the first to the second stable position, and an electrical circuit for locking and unlocking at least one door of a vehicle, the first contact member being arranged to lock the door when in the first stable position and to unlock the door when moved to the second stable position.

The inertia switch arrangement may also include resetting means for returning the first electrical contact member to the first stable position. The resetting means may also be arranged to return the inertia mass to an initial position after actuation by an imposed acceleration or deceleration exceeding the threshold value. The operating means may comprise a first arm pivotally mounted on a supporting member which also carries the first electrical contact member for pivotal movement between the two stable positions, a second arm-over lying the inertia mass, and resilient biassing means connecting the first arm and the first contact member to bias the first contact member towards the second stable position when the operating means is actuated by the inertia mass. The resilient biassing means preferably comprises a tension spring.

The inertia switch arrangement may be enclosed in a housing having a first part containing the inertia mass and a second part containing the three electrical contacts, and provided with three electrical terminals connected to the electrical circuit and respectively electrically connected to the first, second and third electrical contact members, the three electrical terminals being accessible from the exterior of the housing.

The inertia switch arrangement may also include means for providing a visual indication that the operating means has been engaged by the inertia mass and has moved the first electrical contact member from the first to the second stable position. The visual indication means may comprise a manually operated button extending through a wall of the housing to provide said indication, and operable to return (the first electrical contact member to the first stable position. A thermally responsive device may be connected in series with the second and third contact members and arranged to open circuit the series connection when the temperature exceeds a predetermined value.

The inertia switch arrangement may also include thermally responsive means adapted to respond and move the first contact member from the second to the first stable position when the temperature exceeds a predetermined value.

The inertia switch arrangement may also include delay means arranged to prevent resetting of the first electrical contact member to the first stable position and being effective to prevent a door being locked for a predetermined time after the actuation by an imposed acceleration or deceleration exceeding the threshold value.

The delay means may include capacitance means arranged to be charged and disconnect a locking circuit for the door in response to a flow of current due to the movement of the first contact member to the second stable position. The capacitance means may be adapted to prevent a drain on a power source of the electrical circuit if a door operating switch is held operated for a time in excess of that required to lock or unlock a door.

In an alternative arrangement the delay means comprises thermally responsive means arranged to disconnect a locking circuit for the door in response to a flow of current due to the movement of the first contact member to the second stable position.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-sectional view of an inertia switch device: Figure 2 is a schematic circuit diagram of an automobile door locking arrangement in accordance with the invention and including the inertia switch device of Figure 1; Figure 3 is a fragmentary perspective views showing the essential features of another inertia switch device; Figure 4 is a schematic circuit diagram of another automobile door locking arrangement in accordance with the invention and including the inertia switch device in Figure 3; Figures 5 and 6 are schematic circuit diagrams of further automobile door locking arrangements in accordance with the invention; and Figure 7 is a perspective view of a further inertia switch device suitable for use in the automobile door locking arrangement shown in Figures 5 and 6.

In the drawings, like parts are indicated by like reference numerals.

Referring firstly to Figures 1 and 2, there is shown an inertia switch device 1 incorporated in an inertia switch arrangement for locking and unlocking doors of an automobile. The device 1 has a housing 2 in which are mounted an inertia mass 3, three fixed electrical contacts 4, 5 and 6, an electrically conductive movable electrical contact member 7 and switch means 8.

The housing 2 has one internal part 9 in which the inertia mass 3 is housed and a second internal housing part 10 containing the three electrical contacts 4 to 6 and the movable electrical contact member 7.

The housing 2 has an external rebate or recess 11 below, as viewed in Figure 1, the interior housing part 10.

The inertia mass 3 comprises a spherical steel ball which is normally restrained (in the lowermost position shown in Figure 1) by a magnet 12 in a frusto-conical seating 13. The force of the magnet 12 is such that it is overcome by the attainment of a predetermined threshold value of acceleration or deceleration acting on the device 1 and thus on the ball 3. When this threshold value is attained the ball 3 moves away from the seating 13 to either of the positions 3' or 3".

The three electrical contacts 4, 5 and 6 are parallel and extend from the housing part 10 through a wall or partition 14 into the recess 11. They terminate at the level of a bottom, as viewed, wall 15 of the housing 2. The contacts 4, 5 and 6 comprise respectively first, second, and further or third contacts. The further contact 6 has a part or bearing end 16 at 90" to the main part. The part 16 terminates in a bifurcated end portion, similar to that shown in Figure 3, each limb 17 of which has an indentation 18 in its upper and lower surfaces. All three contacts are made of metal strip. The first contact 4 usually carries current in the direction of the arrow X. The second contact 5 in use carries current in the direction of the arrow Y. The further contact 6 is the one which carries current into the device 1, in the direction of the arrow Z.

The movable electrical contact member 7 has a lower part of a generally 'S '-shape as shown in Figure 1, the upper limb of the 'S' joining a generally linear part 19 which is bifurcated (not shown) the limbs forming the bifurcation bearing on the underside of the bearing part 16 at the indentation or depression 18. The central recess between the limbs of the bifurcations in the parts 16 and 19 are aligned in the vertical sense, as viewed in Figures 1 and 3.

The switch means 8 comprises a generally 'L'-shape member, the arm 20 of which extends between the first and second housing parts 9 and 10 in the first of which it overlies the inertia mass 3. The arm 21 is wholly within the second housing part 10 and is bifurcated at its lower end, as viewed in Figures 1 and 3, each end being seated for pivotal movement in one of the indentations 18 in the limbs 17 of the bearing part 16 of the contact 6. There is resilient biassing means in the form of a tension spring 22 hooked at each end into a hole in the arm 21 (best seen in Figure 3) and a hole in the extension 19 of the movable contact member 7. The spring 22 lies between the limbs of the respective bifurcations and normally acts to urge the contact member 7 against the first contact 4.

There is also a reset means in the form of a finger or thumb operable button 23 which has a forked end 24 (Figure 3) in which one end of the arm 20 of the switch means 8 is received.

The use of the inertia switch device of Figure 1 will now be described in relation to a circuit 25 (shown in Figure 2) for electrically operated door locks of a vehicle such as an automobile. The circuit 25 is powered by a battery 26 and includes a dash board switch 27 of the "normally off" rocker type and four solenoid operated door locks 28, there being in this case four locks as it is assumed that the automobile has four doors. The circuit 25 has a centre power line 29 and a lock iine 30 and an unlock line 31 connected to the appropriate sides of the solenoids 28. The inertia switch 1 is connected so that it is normally open, that is the current enters via the contact 6 and normally leaves via the first contact 4.

The second contact 5 is connected to the unlock line 31. There is a thermal circuit breaker 44 in the circuit 25 between the battery 26 and the inertia switch 1.

In normal operation, the inertia mass 3 is in position on the seating 13 and the movable contact member 7 is in contact with the first contact 4. If it is desired to lock the four door locks, the dash board switch 27 is rocked downwardly to the left as viewed in Figure 2 so that contact is made between points 32 and 33 of the switch 27. Current then passes from the battery 26 along line 34 to point 35 on the centre power line 29, along that line to point 36, along line 37 to point 32 of the switch 27 to point 33, thence along line 38 to the "lock" line 30. All four solenoids 28 are then actuated to lock the doors. The switch 27 is then released. It will be appreciated that the switch 27 is only held on for a very short time and in that time the doors are locked simultaneously instantaneously.

To unlock the doors, the switch 27 is rocked to the right as viewed in Figure 2 to make contact between points 32 and 39 of the switch. Current then passes to the "unlock" line 31 from the battery 26 and thence to the door lock solenoids 28, which are actuated to unlock the doors.

Normally the door will be locked during a journey. If there is an accident, an acceleration is applied to the inertia mass 3 and the predetermined threshold value is attained or exceeded. The inertia mass 3 is then urged upwardly (as viewed in Figure 1) to position 3' or 3", in either one of which it strikes the arm 20 of the switch means 8 and urges it to the uppermost position 20' shown in dashed lines (Figure 1). This causes the limbs of the bifurcation of the arm 21 to pivot about their points of engagement in the indentations 18 in the bearing part 16 of the further contact 6.

This in turn stretches the spring 22 and moves it to a unstable position where it moves over-dead centre to restore equilibrium. In doing so it pulls or flicks the movable contact member 7 to the right from its first end position in contact with the first contact 4 to contact the second contact 5, its second end position. Current then flows directly from the battery 26 to the unlock line 31 along line 40 (Figure 2), bypassing the dash-board switch 27. The solenoids 28 are thus operated to unlock the door locks.

The thermal circuit breaker 44 protects the circuit 25 by breaking the circuit when a predetermined temperature rise occurs in the separate circuit breaker, so preventing self welding of the contacts 5 and 7.

The inertia switch 1 can be reset by pressing the button 23 downwardly so that the arm 20 and the inertia mass 3 are moved to their initial positions. Movement of the arm necessarily pivots the arm 21 too and this in turn moves the spring 22 over-dead centre again to flick the movable contact member 7 from the second stable position in contact with the second contact 5 to the firsts table position in contact with the first contact 4.

Referring now to Figures 3 and 4, there is shown a second inertia switch device 100 and a circuit 250. Figure 3 shows the inertia switch device 100 without its housing though it of course has one, with two internal housing parts 9 and 10 similar to those of the device of Figure 1.

The device 100 has an inertia mass 3, first, second and third contacts 4, 5 and 6, a movable contact member 7, a switch means 8. The third contact 6 has a bearing part 16 with a bifurcated end consisting of limbs 17 and indentations 18. The movable contact member 7 has an extension part 19. The switch means has an arm 20 and a bifurcated arm 21 which is connected by a biassing spring 22 to the extension part 19. All these parts are constructed and operate in a similar manner to their counterparts in the device 1 of Figure 1.

The inertia switch device 100 differs from the device 1 in that it has a thermally responsive device in the form of two bimetallic strips 101 and 102. These enable the circuit 250 to be protected against thermal overload so that the circuit does not need a separate thermal circuit breaker 44 of the kind shown in Figure 2.

The bimetallic strip 101 has a contact 103 operable to contact the arm 21 of the switch means 8 on the side of that arm 21 nearest the ball 3. The bimetallic strip 102 has a contact 104 for contacting the arm 21 on the side thereof remote from the ball 3. An electrically conductive braid 105 (with an insulating covering) electrically connects the two strips 101 and 102 to the top of terminal 6 i.e. the common contact. There is also an electrically insulating keeper or block 106 mounted under pressure of a spring 107 for slidable motion along the face of first contact 4 facing the contact 5.

The keeper 106 is connected to the reset button 23.

Turning now to Figure 4 the line 34 from the battery 26 is connected directly to the contact 6 of the inertia switch device 100, which is in the "normally open" condition. In this arrangement the contact 4 is directly connected to the line 37 feeding neutral point 32 of the dash board switch 27. The second contact 5 is directly connected to the "unlock" line 31 via line 41 from point 39 of the switch 27.

In operation, the switch 27 is normally off or open. To lock the doors, it is depressed to the left to make contact between points 32 and 33. Current flows from the battery 26 through the device 100 via contacts 6 and 4 through bimetal 101 and contact 103 to the centre power line 29 and to the point 32 of the switch 27 whence it passes via point 33 to tbe "lock" line 30. All four solenoids 28 are instantaneously and simultaneously operated to lock the doors. Normally the switch 27 is released before the temperature increases in the switch so that the bimetallic strips 101 and 102 are not operative. If however the switch 27 is held on, the strip 101 moves the arm 21 at contact 103 so moving the switch means 8 and movable contact 7 positively over-dead centre to the second stable position in contact with the contact 5. At the same time electrical connection is established via the braid 105 with the bimetallic strip 102, which after a delay operates to connect the arm 21 at contact 104 and urges the switch means 8 back over-dead centre to move the contact 7 back to its position contacting the first contact 4. Meanwhile however the keeper 106 has been urged upwardly by the spring 107 so that the contact 7 engages the keeper 106, not the first contact 4. The circuit through the switch 100 is broken and the whole circuit 250 can only be restored by resetting the switch device 100 by depressing button 23 to depress the keeper 106 to the position shown in Figure 3.

The movable contact member 7 then moves to its first stable position engaging the first contact 4.

In the event of for example an accident which moves the inertia mass 3 to strike the arm 20, the arm 21 moves the member 7 as before to contact the second contact 5 so that current passes from the battery 20, through the device 100 to the "unlock" line 31 via line 41 by-passing the switch 27 in the process. The solenoids 28 are operated to unlock door locks. Again the keeper 106 is urged upwardly by the spring 107 so that when bimetallic strip 102 operates to return the movable contact member to contact the first contact 4, it cannot make contact with it as it engages the insulating contact instead so that the switch is electrically isolated and no current can flow to the lock line 30 so that the door locks are not automatically re-locked. The device can be reset as before by depressing button 24.

In both the above described embodiments of the invention, the locks may be operated individually from outside the vehicle by key operated locks 42.

The use of an inertia switch arrangement in accordance with the invention will now be described in relation to another electrical circuit shown in Figure 5 for electrically operating the door locks of a vehicle such as an automobile. The circuit is powered by a battery 26 and includes a dash board switch 27 of the "normally off " rocker type and four centre-tapped solenoids 28 each arranged to operate an associated door lock (not shown), it being assumed that the vehicle has four doors. The inertia switch 1 is connected so that it is normally in the position shown, that is the current from the positive pole 50 of the battery 26 enters via the contact 6 and leaves via the contact 4 which is connected to a lock line 30 or to an unlock line 31 depending on the position of the dash board switch 27. The second contact 5 is connected to a diode 58 and to one end of the four solenoids 28 in common.

The negative pole 51 of the battery 26 is connected to the solenoid 52 of a solenoid operated switch 53 having normally open contacts 54 arranged to connect the centretap of the four solenoids 28 to the negative pole 51 of the battery 26. The lock line 30 is connected by way of a diode 55 and a capadtor 56 shunted by a resistor 57 to the end of the solenoid 52 not connected to the negative pole 51 of the battery 26. In normal operation the inertia mass 3 is in position on its seating 13 and the movable contact member 7 is in contact with the contact 4 as shown. Some of the electrical components may be accommodated in the housing 2 as indicated in broken line.

As shown in Figure 7, the inertia mass 3 in the form of a steel ball is accommodated in a frusto-conical seating 13 and restrained in the lowermost position in the seating 13 by a magnet 12. The housing 2, comprises two co-operating parts 65 and 66 arranged to be fitted together to enclose an internal part 9 which contains the inertia mass 3, the seating 13, the magnet 12 and the three electrical contacts 4, 5 and 6.

The manually operable button 23, which extends out of the housing 2 is arranged to engage one end of the arm 23 for resetting the electrical contacts.

The two co-operating parts 65 and 66 also enclose an internal part 10 which contains the solenoid operated switch 56 the capacitor 56 and a printed circuit board 67 on which are mounted the diodes 55, 58, 61 and 62, the resistor 57 and other electrical components. Terminals 68, 69 and 70, which extend through an opening 71 in the bottom of the housing 2 enable the components to be connected to the solenoids 28 and the battery 26.

When it is desired to lock the four doors the dash board switch 27 is rocked to the position in which it completes the circuit from the positive pole 50 via lock line 30, the diode 55, capacitor 56 and solenoid 52 to the negative pole 51. The current flowing to charge the capacitor 56 and flowing through the solenoid 52 closes the contacts 54, thus completing a circuit from the positive pole 50 by way of the inertia switch contacts 7 and 4 and the right-hand sides of the four solenoids 28, to the negative pole 51 of the battery 26. The flow of current through the right-hand sides of the four solenoids 28, which is indicated by the arrows 59, locks the four doors. The switch 27 is then released. If the switch 27 is held operated, the flow of current through the solenoid 52 will cease when the capacitor 56 is fully charged, so that the contact 54 will open and prevent the solenoids 28 drawing further current from the battery 26.

To unlock the four doors the switch 27 is rocked to the other position to connect the solenoid 52 across the battery 26 by way of the diode 58 and the capacitor 56.

The current flowing to charge the capacitor 56 and flowing through the solenoid 52 closes the contacts 54, thus completing a circuit from the positive pole 50 by way of the inertia switch contacts 7 and 4 and the left-hand sides of the four solenoids 28 to the negative pole 51 of the battery 26. The flow of current through the left-hand sides of the solenoids 28, which is indicated by the arrows 60, unlocks the four doors. The switch 27 is then released. If the switch 27 is held operated, the flow of current will cease when the capacitor 56 is fully charged, so that the contacts 54 will open and prevent the four solenoids 28 drawing further current from the battery 26.

Normally the doors would be locked during a journey, and if there is an accident, an acceleration or deceleration is applied to the inertia mass 3 so that the predetermined threshold value is attained or exceeded.

The consequential movement of the inertia mass 3 moves the movable contact member 7 into engagement with the contact 5 completing a circuit from the positive pole 50 by way of the diode 58, the capacitor 56, the solenoid 52 to the negative pole 51. The flow of current through the solenoid 52 to charge the capacitor 56 causes the contacts 54 to close and provide a flow of current through the left-hand sides of the four solenoids 28, as indicated by the arrows 60 which unlocks the doors. The flow of current through the solenoid 52 ceases when the capacitor 56 is fully charged so that the contacts 54 open and prevent the four solenoids 28 drawing further current from the battery 26. The resistor 57, which allows the charge on the capacitor 56 to leak away, provides a delay which prevents the doors being relocked within a predetermined period of time. The diodes 61 and 62 prevent arcing at the contacts during operation.

Turning now to the circuit arrangement shown in Figure 6 which has an inertia switch 1 accommodated in a housing 2 together with a capacitor 56 and a solenoid operated switch 53. The contact 4 is connected to the dash board switch 27 which is again a "normally off" rocker switch such as can be rocked in two alternative positions to lock and unlock the doors of a vehicle.

The four door locks are arranged to be operated by respective centre-tapped solenoids 28.

As before, the flow of current through the solenoid 52 to charge the capacitor 56 when the switch 27 is operated, causes the contacts 54 to close and provide energising current for the four door-locking solenoids 28. The capacitor 56, when fully charged, prevents the door-locking solenoids 28 from becoming a drain on the battery 26 if the switch 27 is held operated for a time in excess of that required to lock or unlock the doors. Moreover, after the inertia switch 1 has been operated to unlock the doors, the flow of current through the solenoid 52 ceases when the capacitor 56 is fully charged, so that the contacts 54 open and prevent the doors being relocked until the charge has leaked away. This delay prevents the doors being inadvertently locked following an accident.

WHAT WE CLAIM IS: 1. An inertia switch arrangement comprising a movable inertia mass, a first electrical contact member movable between a first stable position in which it engages a second electrical contact member and a second stable position in which it engages a third electrical contact member, operating means engageable by the inertia mass and arranged so that on an applied acceleration or deceleration exceeding a threshold value

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. mass 3, the seating 13, the magnet 12 and the three electrical contacts 4, 5 and 6. The manually operable button 23, which extends out of the housing 2 is arranged to engage one end of the arm 23 for resetting the electrical contacts. The two co-operating parts 65 and 66 also enclose an internal part 10 which contains the solenoid operated switch 56 the capacitor 56 and a printed circuit board 67 on which are mounted the diodes 55, 58, 61 and 62, the resistor 57 and other electrical components. Terminals 68, 69 and 70, which extend through an opening 71 in the bottom of the housing 2 enable the components to be connected to the solenoids 28 and the battery 26. When it is desired to lock the four doors the dash board switch 27 is rocked to the position in which it completes the circuit from the positive pole 50 via lock line 30, the diode 55, capacitor 56 and solenoid 52 to the negative pole 51. The current flowing to charge the capacitor 56 and flowing through the solenoid 52 closes the contacts 54, thus completing a circuit from the positive pole 50 by way of the inertia switch contacts 7 and 4 and the right-hand sides of the four solenoids 28, to the negative pole 51 of the battery 26. The flow of current through the right-hand sides of the four solenoids 28, which is indicated by the arrows 59, locks the four doors. The switch 27 is then released. If the switch 27 is held operated, the flow of current through the solenoid 52 will cease when the capacitor 56 is fully charged, so that the contact 54 will open and prevent the solenoids 28 drawing further current from the battery 26. To unlock the four doors the switch 27 is rocked to the other position to connect the solenoid 52 across the battery 26 by way of the diode 58 and the capacitor 56. The current flowing to charge the capacitor 56 and flowing through the solenoid 52 closes the contacts 54, thus completing a circuit from the positive pole 50 by way of the inertia switch contacts 7 and 4 and the left-hand sides of the four solenoids 28 to the negative pole 51 of the battery 26. The flow of current through the left-hand sides of the solenoids 28, which is indicated by the arrows 60, unlocks the four doors. The switch 27 is then released. If the switch 27 is held operated, the flow of current will cease when the capacitor 56 is fully charged, so that the contacts 54 will open and prevent the four solenoids 28 drawing further current from the battery 26. Normally the doors would be locked during a journey, and if there is an accident, an acceleration or deceleration is applied to the inertia mass 3 so that the predetermined threshold value is attained or exceeded. The consequential movement of the inertia mass 3 moves the movable contact member 7 into engagement with the contact 5 completing a circuit from the positive pole 50 by way of the diode 58, the capacitor 56, the solenoid 52 to the negative pole 51. The flow of current through the solenoid 52 to charge the capacitor 56 causes the contacts 54 to close and provide a flow of current through the left-hand sides of the four solenoids 28, as indicated by the arrows 60 which unlocks the doors. The flow of current through the solenoid 52 ceases when the capacitor 56 is fully charged so that the contacts 54 open and prevent the four solenoids 28 drawing further current from the battery 26. The resistor 57, which allows the charge on the capacitor 56 to leak away, provides a delay which prevents the doors being relocked within a predetermined period of time. The diodes 61 and 62 prevent arcing at the contacts during operation. Turning now to the circuit arrangement shown in Figure 6 which has an inertia switch 1 accommodated in a housing 2 together with a capacitor 56 and a solenoid operated switch 53. The contact 4 is connected to the dash board switch 27 which is again a "normally off" rocker switch such as can be rocked in two alternative positions to lock and unlock the doors of a vehicle. The four door locks are arranged to be operated by respective centre-tapped solenoids 28. As before, the flow of current through the solenoid 52 to charge the capacitor 56 when the switch 27 is operated, causes the contacts 54 to close and provide energising current for the four door-locking solenoids 28. The capacitor 56, when fully charged, prevents the door-locking solenoids 28 from becoming a drain on the battery 26 if the switch 27 is held operated for a time in excess of that required to lock or unlock the doors. Moreover, after the inertia switch 1 has been operated to unlock the doors, the flow of current through the solenoid 52 ceases when the capacitor 56 is fully charged, so that the contacts 54 open and prevent the doors being relocked until the charge has leaked away. This delay prevents the doors being inadvertently locked following an accident. WHAT WE CLAIM IS:

1. An inertia switch arrangement comprising a movable inertia mass, a first electrical contact member movable between a first stable position in which it engages a second electrical contact member and a second stable position in which it engages a third electrical contact member, operating means engageable by the inertia mass and arranged so that on an applied acceleration or deceleration exceeding a threshold value

the first electrical contact member is moved from the first to the second stable position, and an electrical circuit for locking and unlocking at least one door of a vehicle, the first contact member being arranged to lock the door when in the first stable position and to unlock the door when moved to the second stable position.

2. An inertia switch arrangement as claimed in Claim 1, including resetting means for returning the first electrical contact member to the first stable position.

3. An inertia switch arrangement as claimed in Claim 2, wherein the resetting means is also arranged to return the inertia mass to an initial position after actuation by an imposed acceleration or deceleration exceeding the threshold value.

4. An inertia switch arrangement as claimed in any preceding claim, wherein the operating means comprises a first arm pivotally mounted on a supporting member which also carries the first electrical contact member for pivotal movement between the two stable positions, a second arm overlying the inertia mass, and resilient biassing means connecting the first arm and the first contact member to bias the first contact member towards the second stable position when the operating means is actuated by the inertia mass.

5. An inertia switch arrangement as claimed in Claim 4, wherein the resilient biassing means comprises a tension spring.

6. An inertia switch arrangement as claimed in any preceding claim, including a housing having a first part containing the inertia mass and a second part containing the three electrical contacts, and provided with three electrical terminals connected to the electrical circuit and respectively electrically connected to the first, second and third electrical contacts members, the three electrical terminals being accessible from the exterior of the housing.

7. An inertia switch arrangement as claimed in any preceding claim, including means for providing a visual indication that the operating means has been engaged by the inertia mass and has moved the first electrical contact member from the first to the second stable position.

8. An inertia switch arrangement as claimed in Claim 7, wherein the visual indication means comprises a manually operated button extending through a wall of the housing to provide said indication, and operable to return the first electrical contact member to the first stable position.

9. An inertia switch arrangement as claimed in any preceding claim, including a thermally responsive device connected in series with the second and third contacts members and arranged to open circuit the series connection when the temperature exceeds a predetermined value.

10. An inertia switch arrangement as claimed in any one of Claims 1 to 8, including a thermally responsive means adapted to respond and move the first contact member from the second to the first stable position when the temperature exceeds a predetermined value.

11. An inertia switch arrangement as claimed in Claim 10, wherein the thermally responsive means comprises two bimetallic elements.

12. An inertia switch arrangement as claimed in any preceding claim, including delay means arranged to prevent resetting of the first electrical contact member to the first stable position and being effective to prevent a door being locked for a predetermined time after the actuation by an imposed acceleration or deceleration exceeding the threshold value.

13. An inertia switch arrangement as claimed in Claim 12, wherein the delay means includes capacitance means arranged to be charged and disconnect a locking circuit for the door in response to a flow of current due to the movement of the first contact member to the second stable position.

14. An inertia switch arrangement as claimed in Claim 13, wherein the capacitance means is arranged to prevent a drain on a power source of the electrical circuit if a door operating switch is held operated for a time in excess of that required to lock or unlock a door.

15. An inertia switch arrangement as claimed in Claim 13 or Claim 14, wherein charging current for the capacitance means is adapted to close electrical contacts forming part of a locking circuit for the door, and the cessation or reduction of the charging current is adapted to open said electrical contacts and disconnect said locking circuit for the door.

16. An inertia switch arrangement as claimed in claim 15, wherein the electrical contacts forming part of the door locking circuit comprise contacts operated by a solenoid through which flows the charging current for the capacitance means.

17. An inertia switch arrangement as claimed in Claim 12, wherein the delay means includes thermally responsive means arranged to disconnect a locking circuit for the door in response to a flow of current due to the movement of the first contact member to the second stable position.

18. An inertia switch arrangement as claimed in any preceding claim, wherein the electrical circuit includes a solenoid adapted to lock or unlock a door.

19. An inertia switch arrangement as constructed and arranged to operate substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

20. An inertia switch arrangement constructed and arranged to operate substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

21. An inertia switch arrangement constructed and arranged to operate substantially as hereinbefore described with reference to Figures 5 and 7 of the accompanying drawings.

22. An inertia switch arrangement constructed and arranged to operate substantially as hereinbefore described with reference to Figures 6 of the accompanying drawings.