Fluid handling – Ambient condition change responsive – Atmospheric
Reexamination Certificate
2000-06-21
2001-09-11
Chambers, A. Michael (Department: 3753)
Fluid handling
Ambient condition change responsive
Atmospheric
C137S343000, C251S068000, C454S369000
Reexamination Certificate
active
06286537
ABSTRACT:
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.
REFERENCES:
patent: 744404 (1903-11-01), Reno
patent: 3580321 (1971-05-01), Root
patent: 5533929 (1996-07-01), Attridge Jr.
patent: 5787919 (1998-08-01), Pyle
Backham Michael
Munson Kevin
Neal Alan
Shillitoe Charles
Akin Gump Strauss Hauer & Feld L.L.P.
Chambers A. Michael
Ruskin Air Management Limited
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