GB2305284A

GB2305284A - Warning device for an alarm system

Info

Publication number: GB2305284A
Application number: GB9518566A
Authority: GB
Inventors: Manhar Amlani
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-09-12
Filing date: 1995-09-12
Publication date: 1997-04-02
Also published as: GB9518566D0

Abstract

A warning device or sounder for an alarm system comprises a bridge rectifier 24 having its a.c. terminals 23, 23 connected to respective wires 16, 17 of the alarm warning circuit. An audible transducer 31 is connected across the d.c. terminals 26, 27 of the bridge rectifier 24. A diode D1 is connected between one of the a.c. terminals 23 of the bridge rectifier 24 and the wire 17. A switch 25 is connected in parallel with the diode. In a standby mode of the device a reverse current I Q flows, which is blocked by the diode D1. The warning device can be tested by closing the switch 25, so that current can flow through the bridge rectifier 24 to the transducer 31. In an alarm mode of the device a forward current I A flows through the bridge rectifier 24 and through the diode D1, which is forward biassed. In this manner individual devices on an alarm circuit can be individually and single-handedly tested.

Description

Alarm Warning Device This invention relates to an audible and/or visible warning device for an alarm system and more particularly but not solely to a warning device for a fire alarm system.

Fire alarm systems generally comprise a control panel connected to a detector circuit and a warning device circuit.

Typically the warning device circuit comprises a plurality of audible and/or visible warning devices connected in parallel across two wires extending from the control panel. When a fire is detected, a positive voltage is applied across the wires to trigger the warning devices.

The British Standards Institute (BSI) requires that the continuity of the warning device circuit should be continuously monitored in order to check for open and/or short circuits.

This is generally achieved by applying a reverse voltage across the two wires of the circuit in the stand-by mode of the alarm.

A so-called end-of-line resistor is connected across the free ends of the two wires such that a reverse current flows around the circuit. The control panel checks that this reverse current is within predetermined limits, and thus determines the continuity of the two wires. When a fire is detected, the control panel triggers the alarm by reversing the voltage across the wires, such that current flows in the forward direction.

A disadvantage of this arrangement is that two people are required to test the operation of each warning device.

Typically one person is required to switch the control panel into TEST mode whilst the other person stands adjacent each warning device in turn to check its operation and to make any necessary adjustments. Obviously in large buildings two-way radios are required, so that the person operating the control panel knows when to start and stop the test.

Another disadvantage of this arrangement is that each time an audible warning device is tested in a building, each of the other audible warning devices connected to the circuit also operate. Thus any persons using the building will be disturbed by the alarm, especially if each warning device is to be tested in turn.

We have now devised an alarm warning device which alleviates the above-mentioned problems, and which can be used with conventional control panels.

In accordance with this invention there is provided a warning device for an alarm system, the device comprising first and second input terminals for respectively connecting to a pair of wires extending from an alarm control panel, sensing means for sensing the voltage across said terminals and for activating a warning transducer when the potential on either one of the said input terminals is greater than the potential on the other input terminal, and switch means for normally disabling the transducer when the potential on the second input terminal is greater than the potential on the first input terminal during a stand-by mode of the alarm system.

In use, the warning device is connected to the wires extending from the control panel in the conventional manner.

In the stand-by mode of the alarm system, the switch means is arranged to disable the transducer, so that the transducer does not operate. However, in order to test the warning device, the switch means on the device can be actuated, such that the sensing means activates the transducer. Thus, it will be appreciated that the device can be tested single-handedly by a person who is standing adjacent the device. If any adjustments are required, say to the volume of an audible transducer of the device, then these can easily be made by the person who is testing the device.

When an alarm is triggered, the potential in the first terminal becomes positive with respect to the second terminal and thus the sensing means activates the transducer.

Preferably the sensing means comprises current steering means, which is arranged to steer current, flowing in either direction, between said input terminals in one direction only through means for activating the transducer, the switch means being arranged to normally block current flow from said second terminal to said first terminal during the stand-by mode of the alarm system.

Preferably the current steering means comprises diodes, which are preferably configured in the manner of a conventional full-wave bridge rectifier.

In one embodiment the switch means comprises a switch connected in series with a diode of the bridge rectifier, which diode conducts when current flows between the second and first input terminals.

In an alternative embodiment the switch means comprises a switch connected in parallel with a diode having its anode and cathode connected to the first and second input terminals respectively.

The switch may comprise a reed relay or an optoelectronic switch.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an fire alarm system incorporating a plurality of warning devices in accordance with this invention; and Figure 2 is a schematic diagram of a warning device of the alarm system of Figure 1.

Referring to Figure 1 of the drawings, there is shown an alarm system comprising a control panel 10 which is connected to a detector circuit 11 and a warning device circuit 12. The detector circuit 11 comprises a plurality of heat and/or smoke sensors 13 and a plurality of call points 14 connected in parallel across a pair of wires that extend from the control panel 10. The warning device circuit 12 comprises a plurality of audible sounders 15 connected in parallel across a pair of wires 16, 17 that also extend from the control panel 10. The two wires 16, 17 are interconnected at their free ends by an end-of-line resistor 18.

Each of the sounders 15 are arranged to operate when a positive voltage is applied to wire 16 with respect to wire 17, i.e. so that a forward current 1A flows around the circuit 12.

During the stand-by mode of the alarm, the voltage across the wires 16, 17 is reversed, i.e. so that a reverse current flows around the circuit 12 and through the end-of-line resistor 18. The control panel monitors this reverse current and and triggers a fault alarm if the current is outside predetermined limits.

It will be appreciated that the system hereinbefore described is the same as a conventional fire alarm system.

Referring to Figure 2 of the drawings, there is shown the schematic diagram of a sounder 15. The sounder 15 comprises a pair of input terminals 20, 21 for respectively connecting to the wires 16, 17 that extend from the control panel 10. The two terminals are connected to respective a.c.

input terminals 23 of a conventional full-wave diode bridge rectifier 24. A diode D1 is connected in series between the terminal 21 and the bridge rectifier 24. The diode D1 is arranged such that it is reverse biassed during the stand-by mode of the alarm, i.e. when the wire 17 is positive with respect to the wire 16. A normally-open switch 25 is connected in parallel with the diode D1.

The positive and negative d.c. output terminals 26, 27 of the diode bridge rectifier 24 are connected to respective inputs 28, 29 of a sounder driver circuit 30. An audible transducer 31, such as a piezo-electric horn or a siren, is connected to output terminals of the driver circuit. The driver circuit 30 is arranged to activate the audible transducer 31 when its input terminal 28 goes positive with respect to its terminal 29.

In the stand-by mode of the alarm, the reverse current cannot flow through the sounder 15 owing to the reversebiassed diode D1. However, when an alarm is triggered, the forward current 1A flows through the diode bridge rectifier 24, driver circuit 30, and diode D1 such that the audible transducer 31 is activated. It will be appreciated that the diodes in the bridge rectifier 24 steer the forward current I,, such that the terminal 28 of the driver circuit 30 goes positive with respect to the terminal 29.

In order to test the sounder 15 during the stand-by mode, the switch 25 is closed and current thus flows to the bridge rectifier 24. The diodes in the bridge rectifier 24 steer the current IQ, such that the terminal 28 of the driver circuit goes positive with respect to the terminal 29, thereby causing the audible transducer 31 to activate. When the switch is opened, the current IQ cannot flow through the sounder 15, owing to the diode D1, and thus the audible transducer 31 de activates.

Each of the sounders can thus be tested in turn by a single person, simply by closing the switch 25. The switch 25 may comprise a reed relay mounted adjacent a side wall of the housing of the sounder 15. Thus, the sounder can be tested by placing a magnet in the vicinity of the reed relay. The magnet may be mounted on a pole, so that the person testing the sounders does not have to climb a ladder to reach sounders that are mounted on the ceiling.

In an alternative embodiment the switch may comprise an optical device which shorts out the diode when a bright or infra-red light is detected. Thus, the beam from a torch or from an infra-red transmitter can respectively be shone at the sounder in order to test its operation.

Claims

1) A warning device for an alarm system, the device comprising first and second input terminals for respectively connecting to a pair of wires extending from an alarm control panel, sensing means for sensing the voltage across said terminals and for activating a warning transducer when the potential on either one of the said input terminals is greater than the potential on the other input terminal, and switch means for normally disabling the transducer when the potential on the second input terminal is greater than the potential on the first input terminal during a stand-by mode of the alarm system.

2) A warning device for an alarm system as claimed in claim 1, in which the sensing means comprises current steering means, which is arranged to steer current, flowing in either direction, between said input terminals in one direction only through means for activating the transducer, the switch means being arranged to normally block current flow from said second terminal to said first terminal during the stand-by mode of the alarm system.

3) A warning device for an alarm system as claimed in claim 2, in which the current steering means comprises diodes.

4) A warning device for an alarm system as claimed in claim 3, in which the diodes are arranged as a bridge rectifier.

5) A warning device as claimed in claim 4, in which the switch means comprises a switch connected in series with a diode of the bridge rectifier, which diode conducts when current flows between the second and first input terminals.

6) A warning device as claimed in any of claims 1 to 4, in which the switch means comprises a switch connected in parallel with a diode having its anode and cathode connected to the first and second input terminals respectively.

7) A warning device as claimed in claims 5 and 6, in which the switch comprises a reed relay.

8) A warning device as claimed in claims 5 and 6, in which the switch comprises an opto-electronic switch.

9) A warning device substantially as herein described with reference to the accompanying drawings.