GB1589869A

GB1589869A - Fire detectors

Info

Publication number: GB1589869A
Application number: GB4454476A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1977-10-26
Filing date: 1977-10-26
Publication date: 1981-05-20

Description

(54) FIRE DETECTORS (71) We E M I LIMITED, a British company of Blyth Road, Hayes, Middlesex.

do hereby declare the invention, for which 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 statement:- This invention relates to a fire detector.

Fire detectors are known in which two temperature responsive sensing elements are used to provide a so-called rate of rise detector, the elements being mounted so that their thermal time constants are distinct and connected in a suitable electrical circuit.

Typically one element is inside the detector housing and preferably thermally insulated, while the other is adjacent to the thin metal sheet one side of which is exposed to ambient temperatures.

It is an object of the invention to provide an improved fire detector.

According to the invention, there is provided a fire detector comprising a housing having a thermally conductive member defining a wall portion of the housing and having a first surface portion outside the housing and second surface portion inside the housing, a temperature sensitive circuit within the housing having a temperature sensitive element and a resilient member engaging the second surface portion and resiliently maintaining the element in thermaily conductive connection with the said second surface portion.

In a preferred embodiment the housing comprises a metal shell constituting the said thermally conductive member, and a body portion sealingly engaged with the shell to form the sealed housing.

It is desirable that the circuit has a light source for indicating the existence of an alarm condition and a window in the housing which is transparent to light from the source so that the alarm condition is discernable by examination of the detector from several metres away. In the preferred embodiment the shell is symmetrical about an axis, and the window is on the axis and arranged so that light transmitted through it from the light source is visible, when the detector is mounted on a ceiling, all around the detector.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a cross-section of a fire detector and Figure 2 shows a clip for use in the fire detector of Figure I, line B-B of Figure 2 indicating the line of the section used in Figure 1 through the clip of Figure 1.

The detector has a body portion 10 which in the illustrated embodiment has two parts 11 and 12, which may be plastics mouldings, engaged at screw thread 13.

Part 12 can be attached to a support surface such as a ceiling, or engaged with a suitable socket in any suitable manner, e.g. using screws in bores 14, or plugged in. Body part 11 supports a printed circuit board 20 on which electronic components are mounted in known manner. Preferably body part 11 has mounting posts 15 and 16 on to which the board is mounted and secured, e.g. by soldering the eyelets inserted in the track on the card. Posts 15, 16 extend to terminals 17, 18 by which connections are made to the detector. Terminals 17, 18 may be of any suitable form, e.g. screw connections, pushfit connectors, plug pins, etc.

The housing is closed by a thin cylindrical metal shell 30. This shell comprises a piece of thermally conductive material having an external surface 31 exposed to ambient conditions and an internal surface 32 within the housing. A window 33 is formed in the shell 30 on the axis A-A of the housing.

When the volume 34 enclosed by the shell 30 is to be sealed from theambient atmosphere, the window is sealingly closed by a light transmitting element 35 which may be a lens. The lens may be plastics sealed into the window by adhesive. Shell 30 is retained to the housing by a suitable lip 19 on body part 12. A sealing ring 37 completes the sealing of volume 34, body part 11 being without apertures in the preferred illustrated embodiment.

A clip 36 (see Figure 2) is provided to retain a temperature dependent resistor or other temperature sensitive sensing element 21 in thermal contact with shell 30 by the action of a limb such as 36a. Clip 36, when in the shell 30, is convex in shape. When not in the shell and thus unstressed, it is planar, as shown in Figure 2, having a length greater than the diameter of the shell. It is resilient so that it is a push-fit in the cup formed by shell 30 and springs outwardly to grip the inside of the shell and hold the element 21 in place on surface 32 against thermal movement, vibration and impact. It is of thin material so it tends to cut into the shell.

In this way a repeatable thermal characteristic is obtained from the combination of element 21 and the shell 30 which is maintained during the operational life of the detector.

As secn in Figure 2, the clip which is formed from thin planar spring steel, is of generally arcuate shape with opposite edge portions sheared out and crimped to form two side limbs 36a and 36b as shown. The size of the clip is such that it becomes curved on pushing it into shell 30. The resilience causes the rounded ends 36d and 36c, which are thin and relatively sharp, to grip the shell. Similarly the natural position of limbs 36a ana 36b is such that they are flexed by the presence of element 21 and sheathed leads 38 and when thc clip is pushed fully home into the shell 36 the flexing of the limbs 36a, 36b holds the element 21 and leads 38 in place against impact etc. The flexible nature of the limb 36a in particular ensures that the contact between element 21 and shell 30 is of a constant repeatable nature.There is an aperture 36c in clip 36 corresponding to window 33.

Flying leads 38 connect element 21 to board 20 via plug and socket means on soldered joints. A further such element 22 is retained to the underside of board 20 to have a different thermal time-constant from element 21.

If required, element 22 can be lagged or otherwise insulated to increase the difference in time-constant. The close fit of board 20 in shell 30 may act as a baffle and obstruct gas movement and thus the rate of change of temperature of element 22. Clip 36 may alternatively conform to the inside of shell 36, of suitable form, and act as the baffle.

Suitable circuit means are provided to detect and indicate an excessive rate of rise of ambient temperature by the different response of the elements by virtue of their different thermal time-constants. Such circuits form no part of the present invention, save as referred to below, and will not be described, suitable ones being well-known in the art. A preferred circuit is described and claimed in our application 45836/76 (Serial No. 1589870).

A light source 23, which may be a light emitting diode, is preferably provided in the detector circuit to be illuminated when the circuit responds to indicate a fire condition (whether real or false). Preferably the detector circuit latches "ON" after responding to maintain the indication even when the condition no longer exists.

In distinction from earlier proposals in which an indicator light has been placed at one point on body portion 10 and is therefore clearly visible in one direction only, the placing of the window 33 on the axis A-A permits the window, and any light transmitted through it, to be seen from any direction forward of the mounting plane substantially transverse to Axis A-A.

The provision of a sealed housing for the detector circuit improves reliability by keeping out dust, insects, corrosive gases or vapours, etc. which could impair the operation of the circuit, particularly as low currents in the order of one or two milliamps or hundreds of microamps are involved, without adversely affecting the ability of the element 21 to respond to external temperature with which it is in thermal contact via shell 30. However, sealing is not essential to the operation of the detector. Furthermore, as the element 21 can be secured to the shell 30, e.g. by the use of a clip such as 36, before connection to the circuit on board 20, a consistent thermal arrangement can be obtained, whereas if the element 21 is secured to the board and the shell placed over it, the nature of the thermal contact is uncertain and could vary the response of the detector.

WHAT WE CLAIM IS: 1. A fire detector comprising a housing having a thermally conductive member defining a wall portion of the housing and having a first surface portion outside the housing and a second surface portion inside the housing, a temperature sensitive circuit within the housing having a temperature sensitive element and a resilient member engaging the second surface portion and resiliently maintaining the element in thermally conductive connection with the said second surface portion.

2. A detector according to Claim 1, wherein the housing is sealed.

3. A detector according to Claim 2, wherein the housing comprises a metal shell constituting said thermally conductive member, and a body portion sealingly engaged with the shell to form the sealed housing.

4. A detector according to Claim 3, comprising a retainer member which retains the shell to the body.

5. A detector according to Claim 4, wherein the retainer has provision for fixing the detector to a support surface.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. A clip 36 (see Figure 2) is provided to retain a temperature dependent resistor or other temperature sensitive sensing element 21 in thermal contact with shell 30 by the action of a limb such as 36a. Clip 36, when in the shell 30, is convex in shape. When not in the shell and thus unstressed, it is planar, as shown in Figure 2, having a length greater than the diameter of the shell. It is resilient so that it is a push-fit in the cup formed by shell 30 and springs outwardly to grip the inside of the shell and hold the element 21 in place on surface 32 against thermal movement, vibration and impact. It is of thin material so it tends to cut into the shell. In this way a repeatable thermal characteristic is obtained from the combination of element 21 and the shell 30 which is maintained during the operational life of the detector. As secn in Figure 2, the clip which is formed from thin planar spring steel, is of generally arcuate shape with opposite edge portions sheared out and crimped to form two side limbs 36a and 36b as shown. The size of the clip is such that it becomes curved on pushing it into shell 30. The resilience causes the rounded ends 36d and 36c, which are thin and relatively sharp, to grip the shell. Similarly the natural position of limbs 36a ana 36b is such that they are flexed by the presence of element 21 and sheathed leads 38 and when thc clip is pushed fully home into the shell 36 the flexing of the limbs 36a, 36b holds the element 21 and leads 38 in place against impact etc. The flexible nature of the limb 36a in particular ensures that the contact between element 21 and shell 30 is of a constant repeatable nature.There is an aperture 36c in clip 36 corresponding to window 33. Flying leads 38 connect element 21 to board 20 via plug and socket means on soldered joints. A further such element 22 is retained to the underside of board 20 to have a different thermal time-constant from element 21. If required, element 22 can be lagged or otherwise insulated to increase the difference in time-constant. The close fit of board 20 in shell 30 may act as a baffle and obstruct gas movement and thus the rate of change of temperature of element 22. Clip 36 may alternatively conform to the inside of shell 36, of suitable form, and act as the baffle. Suitable circuit means are provided to detect and indicate an excessive rate of rise of ambient temperature by the different response of the elements by virtue of their different thermal time-constants. Such circuits form no part of the present invention, save as referred to below, and will not be described, suitable ones being well-known in the art. A preferred circuit is described and claimed in our application 45836/76 (Serial No. 1589870). A light source 23, which may be a light emitting diode, is preferably provided in the detector circuit to be illuminated when the circuit responds to indicate a fire condition (whether real or false). Preferably the detector circuit latches "ON" after responding to maintain the indication even when the condition no longer exists. In distinction from earlier proposals in which an indicator light has been placed at one point on body portion 10 and is therefore clearly visible in one direction only, the placing of the window 33 on the axis A-A permits the window, and any light transmitted through it, to be seen from any direction forward of the mounting plane substantially transverse to Axis A-A. The provision of a sealed housing for the detector circuit improves reliability by keeping out dust, insects, corrosive gases or vapours, etc. which could impair the operation of the circuit, particularly as low currents in the order of one or two milliamps or hundreds of microamps are involved, without adversely affecting the ability of the element 21 to respond to external temperature with which it is in thermal contact via shell 30. However, sealing is not essential to the operation of the detector. Furthermore, as the element 21 can be secured to the shell 30, e.g. by the use of a clip such as 36, before connection to the circuit on board 20, a consistent thermal arrangement can be obtained, whereas if the element 21 is secured to the board and the shell placed over it, the nature of the thermal contact is uncertain and could vary the response of the detector. WHAT WE CLAIM IS:

1. A fire detector comprising a housing having a thermally conductive member defining a wall portion of the housing and having a first surface portion outside the housing and a second surface portion inside the housing, a temperature sensitive circuit within the housing having a temperature sensitive element and a resilient member engaging the second surface portion and resiliently maintaining the element in thermally conductive connection with the said second surface portion.

2. A detector according to Claim 1, wherein the housing is sealed.

6. A detector according to any preceding

claim wherein the circuit includes a light source and the housing has a window which is transparent to light from the source.

detector according to Claim 6, wherein the light source is a light emitting diode.

7. A detector according to Claim 6, wherein the light source is a light emitting diode.

8. A detector according to Claim 6 or 7, wherein the window comprises a lens in an aperture in the housing.

9. A detector according to Claims 6, 7 or 8 wherein the window is in the metal shell.

10. A detector according to Claims 6, 7 or 9 wherein the thermally conductive member is symmetrical about an axis and the window is on the axis.

11. A detector according to any preceding claim wherein the resilient member is a clip of spring material.

12. A detector according to Claim 11 wherein the thermally conductive member is cup-shaped and the clip comprises a curved, resilient bridge portion which bridges opposite sides of the cup-shaped member and resiliently engages them to resiliently maintain the connection of the said element with the second surface portion.

13. A detector according to Claim 12, wherein the clip comprises a resilient arm integral with the bridge portion which urges the said element into contact with the thermally conductive member.

14. A detector according to Claim 12 or 13 when indirectly appended to Claim 10, wherein the thermally conductive member is radially symmetrical about the axis and the lens is arranged at the point of intersection of the member and axis.

15. A detector according to Claim 14 wherein the clip has an aperture aligned with the window and light source.

16. A detector according to any preceding claim, comprising another temperature sensitive element.

17. A detector according to Claim 16, comprising a baffle within the housing positioned between the temperature sensitive elements to obstruct gas flow therebetween.

18. A detector according to Claim 17, wherein the baffle is constituted by a printed circuit board supporting the other element and further components of the circuit.

19. A detector according to any of Claims 16 to 18 wherein the other of the elements is covered with thermally insulative material.

20. A fire detector substantially as hereinbefore described with reference to the accompanying drawings.