GB2075265A - Solid state break glass call point - Google Patents

Abstract

A fire alarm break glass call point device comprises a box-like enclosure including a cover with a frangible glass portion 35 provided with an electrically conductive path 36. Resilient contact arms or helical springs, 45, 46 cooperate with the electrically conductive path 36 and a solid state detector arrangement mounted on board 40 within the enclosure detects when the circuit between the contacts 45, 46 and the conductive path 36 is interrupted due to breakage of the frangible portion 35 of the cover to instigate an alarm condition. One of the resilient arms 45, 46 can be moved away from the conductive path 36 to test the circuit operation without requiring opening of the device or breaking of the glass. <IMAGE>

Description

SPECIFICATION Solid state break glass call point This invention relates to a fire alarm device of the kind having a front plate of glass or other frangible material, breakage of which causes the alarm to be operated. Each such device in an alarm system being commonly known as a break glass call point.

According to the invention, there is provided a fire alarm device comprising a box-like enclosure including a cover with a frangible portion provided with an electrically conductive path, resilient contact means cooperating with the electrically conductive path and solid state detector means mounted within said enclosure for detecting when the circuit between the contact means and the conductive path is interrupted due to breakage of the frangible portion of said cover.

The invention will be further described by way of example with reference to a constructional form thereof shown in the accompanying diagrammatic drawings, wherein: Figure 1 is a front elevation of the break glass fire call point assembly, Figure2 is a vertical central sectional view through the assembly with some detail omitted for clarity, Figure 3 is a rear elevational view of the circuit support, Figure 4 is a sectional view taken on the plane 4-4 in Figure 3, Figure 5 is a sectional view taken on the plane 5-5 in Figure 3, Figure 6 is a front elevational view of the circuit shield, Figure 7 is a vertical sectional view taken along the plane 7-7 on Figure 6, Figure 8 is a sectional view taken on the plane 8-8 on Figure 6, Figure 9 is a rear elevational view of the circuit board, Figure 10 is a side elevational view of the circuit board, Figure 11 is an end elevational view of the circuit board, and Figure 12 shows a circuit for use with the alarm of the invention.

The break glass fire assembly shown in the drawings comprises a housing made up of four mouldings i.e. a box-shaped base 11, a circuit shield 15, a cover 14 and a circuit support 13.

The base 11 has holes 10 to receive screws for attaching the base to a wall.

The parts 11, 13, 14 have the same external front elevational shape which is rectangular so that the outer walls of the base 11 are contiguous with those of the support 13 and the outer walls of the support 13 are contiguous with those of the cover 14. The walls fit together with suitable lap joints at 16, 17 with part 13 between the parts 11, 14. The cover 14 is attached to a boss 18 on the circuit shield 15 by screws 19. The circuit shield 15 has wings 22, 23 (Figure 6) provided with apertured bosses 20 attached by screws which enter threaded holes 21 in the support 13. The shield 15 also has two wings 25, 26. The wings 22, 23, 25, 26 have ribs 30 for location on the cover. The shield 15 also has six bosses 31 (Figure 8) for terminal screws 32 (Figure 2). These bosses locate in six apertured bosses 24 in the support 13.The wings 22, 23 are separated from the wings 25, 26 to leave gaps 27, 28.

A glass plate 35 is located in rebates in the cover 14 and seats against the boss 18. This glass plate has on its rear surface a metal strip film 36 which forms part of the fire alarm circuitry so that the alarm is energized by breaking the glass.

The circuit support 13 is in the form of a tray having a central recess 38 deeper than the surrounding recess 39. The recess 38 contains a circuit board 40 locating on studs 41 on the support 13. Openings 42 serve for entry of the terminals 32. The recess 38 also receives the central plate part of the shield 15 and the bosses 31 while the boss 18 and wings 22, 23,25, 26 are contained in the recess 39.

The board 40 carries the main parts of the electrical circuitry 44 and has two conductive spring arms 45,46 which pass through the gaps 27,28 and engage the aforesaid metal strip 36 on the glass plate 35 (see Figure 11). Thus the metal strip completes the circuit through the arms 45,46 and breakage of the glass plate causes the metal strip to fall away from the arms 45, 46 thereby breaking the electric circuit to energize the alarm.

The support 13 is provided with an opening 50 in its side wall through which an insulated rod can be inserted to separate one of the arms 45,46 from the metal strip to start the alarm for the purpose of testing.

Although the glass or other frangible material has been described as having a metal strip on its rear face, other methods of treating the glass could be used to provide a conductive path between the arms 45,46.

Although the contact arms have been described as comprising resilient metal strips, these could be formed in other ways such as rigid metal arms cooperating with helical spring contacts interfaced between the arms and glass contact strip to provide the desired circuit path and maintaining the desired resilience.

A circuit suitable for incorporation on board 40 is shown in Figure 11. It is to be appreciated that a number of break glass call points will be provided within a system each with its internal circuit as shown. The call points will be connected back to the central sensing control in a mannersimilarto a 'ring-main' type configuration. When current is drawn by the circuit of Figure 11 in the 'alarm' condition this will cause the central sensing control to instigate an audible or other alarm indication.

In the normal or non-alarm condition the metallic strip 36 will form a conductive path so that the biasing of transistor VTR by R1 and R2 will result in it being in an 'off' condition. The transistor is expediently a VMOS device (e.g. VN1OKM) as shown as this type of semiconductor has almost no current drain in the off condition. On breakage of the glass the biasing conditions change causing the transistor to conduct and the current drain therethrough (via R3) is detected by the central control in known manner. The VMOS device is very suitable as it is a FETwith a very low resistance in the 'on' condition and capable of handling relatively high currents.

The transistor in the 'on' condition also allows current flow through light emitting diode D1 via R4 so that a visual remote indication of an alarm condition is available. This is useful in checking out the circuit operation when periodically testing the system. Each individual call point can thus be checked in this way.

Claims (15)

1. A fire alarm device comprising a box-like enclosure including a cover with a frangible portion provided with an electrically conductive path, resilient contact means cooperating with the electrically conductive path and solid state detector means mounted within said enclosure for detecting when the circuit between the contact means and the conductive path is interrupted due to breakage of the frangible portion of said cover.

2. A device as claimed in claim 1, wherein said resilient contact means comprise a pair of resilient arms extending between said detector means and said conductive path.

3. A device as claimed in claim 1, wherein said resilient contact means comprise a pair of arms each cooperating with a spring element.

4. A device as claimed in claim 1,2 or 3, wherein said conductive path comprises a metal strip or conductive layer.

5. A device as claimed in any one of claims 1 to 4, wherein said solid state detector means comprises a semiconductor switching circuit.

6. A device as claimed in claim 5, wherein said semiconductor circuit includes a field effect transistor.

7. A device as claimed in any one of claims 1 to 6, wherein the solid state detector means is provided on a circuit board and including a circuit support having walls contiguous with those of the enclosure base and having a recess containing the circuit board, a shield covering said recess but having gaps for passage of said resilient contact means, means to fix the cover to the support, and means to fix the support to the enclosure base.

8. A device as claimed in claim 7, wherein the support provides an additional recess between the shield and the cover containing said contact means and apertured bosses for attachment of the cover to the shield and for attachment of the shield to the support.

9. A device as claimed in any one of claims 1 to 8, wherein means are provided for testing the operation of the device without the requirement of breaking the frangible portion of the cover.

10. A device as claimed in claim 9, wherein the means for testing comprises an aperture in the enclosure through which a separator can be displaced to cause separation of the contact means from the conductive path to instigate an alarm condition.

11. A device as claimed in claim 9 or 10, including the indicator means provided within or on the housing for providing indication of the correct functioning of the alarm.

12. A device as claimed in claim 11, wherein the indicator means comprises a visual indicator.

13. A device as claimed in claim 12, wherein the visual indicator comprises a iight emitting diode.

14. An alarm system including a plurality of the devices as claimed in any one of claims 1 to 13.

15. A fire alarm device substantially as described herein and as illustrated in the accompanying drawings.