Thermally-actuated switch assembly

A thermally-actuated switch assembly is capable of controlling an associated device in the event of a sensed temperature exceeding a predetermined threshold. A first switch in the form of for example a fuse is used to sense the temperature to which the device is sensitive. The first switch is carried by a housing which also carries a second switch. The second switch is arranged such that it is actuated if the housing is mounted on a support in a predetermined orientation. The first and second switches are connected to an output and arranged such that the output indicates an alarm condition if either the first switch is actuated or the second switch is not actuated. Thus one switch is used to sense the temperature, and the other switch is used to sense whether or not the switch assembly as a whole is correctly positioned.

CROSS-REFERENCE TO RELATED APPLICATIONS
 Not applicable.
 STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 Not applicable.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a thermally-actuated switch assembly, and
 in particular to such a switch assembly which may be used to control a
 damper in an air distribution duct.
 2. Description of the Related Art
 It is known to provide thermally-actuated dampers in ducts. For example, a
 damper may be fitted in a duct adjacent to a part of a duct system which
 penetrates a fire barrier or wall. Such dampers are arranged to close the
 duct in the event of temperatures within and/or outside the duct exceeding
 a pre-determined threshold. Dampers of this type are specified where it is
 necessary to ensure the integrity of the fire wall and thereby prevent the
 spread of a fire through a duct penetrating the fire wall. With such an
 arrangement the damper is normally open but closes in the event of an
 excess temperature being sensed. Other dampers are known which operate in
 the opposite sense, that is they are normally closed but open when exposed
 to an excess temperature. Such dampers may be used in situations where
 there is a requirement for providing a smoke vent which only opens in the
 event of a fire.
 One known damper consists of a steel casing which is interconnected between
 two sections of a duct. The casing houses a number of interlocking steel
 blades which can be rotated through 90 degrees between a first position in
 which the blades are edge-on to the direction of the ducts and a second
 position in which the blades extend transversely with respect to the
 direction of the duct. In the first position the blades are spaced apart
 and air can flow between them. In the second position the blades overlap
 and form an effective barrier across the duct. Generally the blades are
 held in the first (open) position to allow air flow through the ducts. The
 damper blades are held in the open position by means of a spring return
 actuator. The actuator incorporates a motor which is mechanically coupled
 to a damper shaft rotation of which controls the position of the damper
 blades. When power is supplied to the actuator, the motor is energised and
 turns the damper blades to the open position. The motor as it rotates the
 blades to the open position also tensions a spring. Once the actuator has
 been fully reset so that the blades are in the open position, the motor
 stops and a mechanism holds the spring in its tensioned condition
 providing the supply of the electrical power to the actuator is
 maintained. If the supply of electrical power is cut off, the spring is
 released and the damper shaft is driven to a position in which the damper
 blades extend transversely relative to the ducts, that is the damper
 blades are in the closed position.
 In the known damper a thermal fuse is incorporated in the electrical supply
 to the actuator, the thermal fuse being mounted on either the damper
 casing or one of the ducts connected to the damper casing. Generally the
 thermal fuse incorporates a single fuse element which is mounted on a
 probe that extends through an opening cut in the casing or duct. In some
 arrangements however two thermal fuses are provided, one mounted in use
 within the duct and one mounted external to the duct. If one of the fuses
 is exposed to a temperature in excess of a pre-determined limit the fuse
 assumes an open-circuit condition, thereby cutting off the electrical
 supply to the damper and causing it to move to a duct closed position.
 The electrical supply to the damper may be cut off by other means, for
 example by switching power off at a control panel to which the fire
 service might have access, or a simple mains failure. The present
 invention is concerned however with situations in which a switch assembly
 incorporating for example a fuse is incorrectly positioned such that the
 fuse is not exposed to the environment the temperature of which it is
 intended to sense. This can occur for example because an installer makes a
 simple error on installing for example a damper system. The error could be
 as simple as a failure to mount a switch assembly correctly on for example
 a duct.
 SUMMARY OF THE INVENTION
 According to the present invention, there is provided a thermally-actuated
 switch assembly comprising a housing which in use is mounted on a support
 in a pre-determined orientation, a first switch carried by the housing,
 the first switch being arranged such that it is actuated if the
 temperature to which it is exposed exceeds a pre-determined threshold, and
 a second switch carried by the housing, the second switch being arranged
 such that it is actuated if the housing is mounted on the support in the
 predetermined orientation, and the first and second switches being
 connected to an output and arranged such that the output indicates an
 alarm condition if either the first switch is actuated or the second
 switch is not actuated.
 The first switch may be a fuse which defines an open circuit if the
 pre-determined temperature is exceeded, or any other heat-sensitive
 component an electrical characteristic of which changes if the
 predetermined temperature is exceeded. The second switch may be connected
 in series with the first switch. The second switch defines a closed
 circuit if the housing is mounted in the pre-determined orientation. The
 second switch may comprise an actuator button which is depressed when the
 actuator housing is mounted in the pre-determined orientation.
 Alternatively, the housing may comprise first and second sections, at least
 one fastener adapted to secure the housing to the support in the
 predetermined orientation such that two sections are held together, and
 means for applying a biasing force between the sections such that if the
 sections are not held together they move part, wherein the second switch
 is arranged to be actuated when the sections are held together and not
 actuated when the sections are moved apart by the biasing means.
 Preferably, the second switch comprises first contacts mounted on the
 first housing section and second contacts mounted on the second housing
 section, the first and second contacts being interengaged when the
 sections are held together and separated when the sections are moved apart
 by the biasing means. Each fastener may extend through and be retained in
 engagement with both the housing sections so as to limit the maximum
 spacing between the sections.
 A switch assembly in accordance with the present invention may be used to
 control for example a damper arranged to either close or open a duct in
 the event of an excess temperature being sensed. If an installer fails to
 mount the switch assembly in an appropriate way on for example a duct to
 which the damper is connected, for example by simply leaving the switch
 assembly unattached to the duct, the second switch indicates an alarm
 condition. Thus the installation error which resulted in the switch
 assembly not being correctly located is indicated and therefore the
 overall system performance in the conditions for which the damper was
 designed will be achieved in a fail-safe manner.

DETAILED DESCRIPTION OF INVENTION
 Referring to FIG. 1, the illustrated damper comprises a casing 1 supporting
 axially aligned spigots 2 and 3 to each of which square section ducts will
 in use be connected. The outline of one such duct is indicated by broken
 lines 4. An aperture in that duct is also shown by a broken line 5. A
 switch assembly 6 in accordance with the present invention is shown
 located above the aperture 5. When fully installed a probe 7 extending
 from a housing of the switch assembly will be inserted through the
 aperture 5 so as to project within the duct 4. A damper of the type
 illustrated in FIG. 1 may be obtained from Actionair Equipment Limited, of
 Whitstable, Kent, England.
 The casing 1 houses a series of parallel horizontally extending damper
 blades 8 one of which is shown in the horizontal section of FIG. 2. The
 damper blades are omitted from FIG. 1 to clarify the general structure of
 the assembly. The damper blades can be held in the open position as shown
 in FIG. 2 or rotated through approximately 90 degrees to a position in
 which each of the blades extends transversely with respect to the aperture
 defined by the casing 1, adjacent blades overlapping so as to effectively
 close the opening defined by the casing 1. The blades 8 are mechanically
 coupled to a mechanical drive arrangement housed within a casing 9 on
 which an actuator casing 10 is supported. The actuator incorporates a
 spring return mechanism of known type which is effective to rotate the
 blades 8 through 90 degrees from the position shown in FIG. 2 in the event
 of a power supply to the actuator being interrupted. Such actuator and
 damper blade spring-retuni mechanisms are well known, for example the
 BF230-T actuator available from Belimo. Given that such actuator
 assemblies are well known, they will not be further described here.
 FIG. 3 illustrates the circuit diagram of the arrangement illustrated in
 FIG. 1. Mains power is delivered via cable 11 to a transformer 12. An
 electrical motor 13 is connected in series with a first switch 14 and a
 thermally actuated fuse 15 across the output of a transformer 12. In a
 conventional system, the switch 14 would be omitted, and accordingly the
 power supply to the motor 13 would only be interrupted if the fuse 15 was
 ruptured as the result of for example being exposed to a temperature above
 a pre-determined limit. In the illustrated arrangement which is in
 accordance with the invention however the switch 14 must be in a
 closed-circuit condition for the mains supply to the motor 13 to be
 maintained. Given that the damper blades 8 will be moved from the position
 shown in FIG. 2 if the electrical supply to the motor 13 is interrupted,
 it will therefore be appreciated that both the switch 14 and the fuse 15
 must define closed circuits if the damper blades are to be maintained in
 the open condition.
 The switch 14 is provided with an actuator button 16 which projects from a
 housing of the switch assembly 6. The button 16 projects from a surface 17
 defined by the switch assembly. The probe 7 into which the fuse 15 is
 incorporated also projects from the surface 17. If the switch assembly 6
 is mounted on the duct 4 of FIG. 1 with the probe 7 extending through the
 aperture 5 and the surface 17 of the switch assembly housing from which
 the probe 7 projects bearing against the surface of the duct 4, the button
 16 of the switch 14 is depressed. In that condition, the switch 14 defines
 a closed circuit and therefore it does not interrupt the electrical supply
 to the motor 13. If however the switch assembly 6 is either not correctly
 fitted to the duct 4, or over time is displaced form the position in which
 it was originally correctly fitted as the result of for example vibration,
 the surface 17 of the switch assembly housing will no longer be secured
 against the surface of the duct 4 and as a result the button 16 will move
 to the position shown in FIG. 3. In those circumstances the switch 14 goes
 open circuit, thereby interrupting the electrical supply to the motor 13.
 Thus the damper will be closed if either the fuse 15 is ruptured or the
 switch 14 goes open circuit as a result of a failure to correctly secure
 the switch assembly housing to the duct 4.
 Referring to FIGS. 4 to 7, an alternative switch assembly to that described
 with reference to FIGS. 1 to 3 is illustrated. This alternative assembly
 comprises a housing made up of a base section 18 and a cover section 19. A
 probe 220 projects from an underside 21 of the base section 18 and
 supports at its tip a sensor 22 which houses a thermally responsive device
 23 such as a fuse which will go open circuit if the temperature to which
 the sensor is exposed exceeds a predetermined threshold. The device 23 is
 connected to a pair of socket connectors 24 each of which is carried by
 the base section 18.
 The cover 19 carries a pair of pins 25 which are connected to a lead 26
 which enables the assembly to be electrically connected to associated
 circuitry. The cover 19 and the base 18 are interengaged by a pair of
 captive screws 27 which extend through respective openings in the cover 19
 and the base 18. The ends of the fasteners 27 which project beyond the
 under surface 21 of the base 18 carry retaining washers (not shown) which
 prevent the fasteners 27 from being pulled out of engagement with the base
 18. Thus the maximum spacing between the base 18 and the cover 19 is
 determined by the length of the fasteners 27 between the fastener heads
 and the retaining washers.
 A set of four springs 28 are supported on posts 29 projecting from the base
 section 18 of the housing. The springs 28 bear against the underside
 surface of the cover 19 and thus bias the housing sections 18 and 19
 apart. If for example the switch assembly is simply left unattached to any
 associated surface, the housing sections 18 and 19 will move apart to the
 extent permitted by the captive fasteners 27. As a result a spacing
 indicated by the spacing between lines 30 will be established between the
 two housing sections. In that configuration the pins 25 will be spaced
 from the sockets 24 as shown in FIG. 5. Any electrical signals on the
 cable 26 will accordingly be exactly the same as would appear in the event
 of the sensing device 23 going open circuit.
 The illustrated switch assembly is in normal use mounted on a wall 31 of a
 duct as shown in FIG. 7. Openings (not shown) will be drilled in the wall
 31 to receive the fasteners 27. As the fasteners are tightened down, the
 cover 19 will be pressed down against the bias force presented by the
 springs 28 until the cover 19 and base 18 are held together as shown in
 FIG. 7. In that condition, the pins 25 are in engagement with the sockets
 24 and thus the cable 26 is in direct contact with the sensing device 23.
 The switch assembly will go open circuit in this condition only if the
 sensing device 23 goes open circuit. Thus if a fitter fails to tighten
 down the fasteners 27 sufficiently, or the fasteners become sufficiently
 loose due to for example vibration for the pins 25 to be pushed out of the
 sockets 24 by the springs 28, the output from the switching assembly will
 be identical to that which indicates the ensing device 23 going open
 circuit.
 It will be appreciated that although the described embodiments of the
 invention are arranged such that a duct is closed in the event of a fault
 condition being indicated, a damper assembly could be arranged to open
 only in the event of an alarm condition being indicated. The term "alarm
 condition" is used to indicate a condition in which either the thermally
 actuated switch such as fuse 15 of FIG. 3 or the mechanically actuated
 switch 14 of FIG. 3 is open circuit. It will of course be appreciated that
 the two switches need not be connected in series but could be connected in
 an alternative configuration provided associated circuitry indicates an
 alarm condition in the event of either of the switches sensing a condition
 other than normal.