GB2030336A - Improvements Relating to Emergency Alarm Systems - Google Patents

Abstract

An emergency alarm system, such as a fire alarm system, comprises a string of alarm initiation units 1 connected in parallel by a common one pair cable 2 to a monitor 3 connected at one end of the cable. Upon actuation of any one of the units 1, which may be effected automatically or manually, the unit is arranged to transmit to the monitor 3 via the cable 2 an identification signal which is unique to that particular unit, and the monitor 3 is responsive to the identification signal to initiate the alarm and to indicate, for example on a display panel 4, which particular unit 1 has been actuated. The identification signal of each unit 1 is preferably a predetermined number of pulses which are applied to the cable 2 by modulating a DC supply which is carried by the cable for powering the units 1. The system also includes a surveillance circuit for checking the integrity of the cable 2, preferably comprising a device 33 at the extreme far end of the cable 2 remote from the monitor 3 for clamping the voltage across the conductors of the cable at a reference value, and means in the monitor for detecting whether the cable voltage falls to zero (short circuit) or rises to a predetermined value above the reference value (open circuit). <IMAGE>

Description

SPECIFICATION Improvements Relating to Emergency Alarm Systems This invention relates to emergency alarm systems, principally fire alarm systems. In the past such systems have always included a separate run of cable extending from each alarm initiation unit to a common annunciation or monitor unit and, where the alarm system includes a iarge number of alarm initiation units, a large quantity of cable is necessary. Frequently cables run adjacent one another over a great part of their length, and not only is this wasteful both of the cable material and of the time taken for its installation, it is also very expensive since the cable has to comprise mineral insulated fire and damage resistant material in order to meet the standard requirement and code of practice.

With the aim of reducing the amount of cable necessary to instal an emergency alarm system having a number of emergency alarm initiation units, and reducing both the time and cost of installation, according to the present invention the emergency alarm initiation units of the system are all connected to a common cable, and each of the alarm initiation units is arranged to emit, upon its actuation, a unique identification signal which is transmitted by the cable to a monitor unit connected to one end of the cable, the monitor unit being responsive to the identification signal of each alarm initiation unit so that the monitor unit is able not only to detect that an alarm condition exists, but also to identify the particular alarm initiation unit which has been actuated.

The identification signal emitted by each emergency alarm initiation unit may be of a particular frequency, which differs from unit to unit, and in this case the monitor unit will include frequency recognition means which can recognise the frequencies which may be emitted by the alarm initiation units and can thus identify the particular unit from which a particular signal has been emitted. Preferably, however, the identification signal of each emergency alarm initiation unit comprises a predetermined number of pulses, and the monitor unit includes means for receiving and counting the pulses in order to identify the particular alarm initiation unit from which the pulses originated.

Preferably, each emergency alarm initiation unit comprises a pulse generator which, upon actuation of the unit, is arranged to emit a predetermined number of pulses, and a switching circuit which is connected across a pair of conductors of the common cable and which operates in response to the pulse generator to make and break a connection between the conductors a predetermined number of times determined by the pulses so that the identification signal pulses are applied to the cable by loop connect/disconnect signalling. The switching circuit is preferably of the solid state type, for example formed by one or more transistors.The pulse generator is preferably powered by DC supplied from the monitor unit via the common cable, and since the cable will preferably be a one pair cable as is conventional, there is preferably filter means, such as a capacitor, provided between the cable and the pulse generator whereby the pulse generator is substantially unaffected by modulation of the DC supply on the cable as a result of the identification signal pulses applied to and transmitted along the cable from any of the alarm initiation units.

Each alarm initiation unit may be arranged to be actuated by any type of automatic fire detector, for example a heat or smoke detector, or by a manually operated emergency alarm switch of the "break glass" type, or "push button" type, the actuating switch in fact forming part of the alarm initiation unit. As will be appreciated, with the alarm initiation units actuated by automatic heat or smoke detectors, the alarm system will operate pureiy as a fire alarm system, but if manual actuating switches are used, the alarm system can be operated as a fire alarm system, a bomb alert system, or for any other general alarm purposes.

The monitor unit preferably comprises display means which is responsive to identification of a signal received from any of the alarm initiation units to give a visual indication of the identity of the actuated alarm initiation unit. The display means may take any one of a number of forms.

For example, it may be arranged to display a numerical code representative of the location of the actuated alarm initiation unit, or it may comprise a display panel having a series of lights which represent the different alarm initiation units and each of which is arranged to light up when the identification signal from its corresponding alarm initiation unit has been received and identified by the monitor unit. The lights may be arranged in battery formation with information concerning the location of each alarm initiation unit beside the corresponding light, or the lights may be arranged in the form of a mimic diagram of the protected area to indicate the locations of the various alarm initiation units.The lights are preferably provided by LED's to avoid lamp failure, and preferably the display means includes latching circuits so that information displayed thereby will remain on display until the system is re-set. In other words, if a number of alarm initiation units are actuated one after another, the display means will indicate when each has been actuated and will continue to show that these units have been actuated until the system is reset.

If desired, the system may include at least one additional visual display means located at a position remote from the monitor unit and the first display means, the additional display means being connected to the monitor unit so that the information indicated by the first display means is repeated by the additional display means. The additional display means may be identical to the first means, or it may take any other suitable form as desired.

The monitor unit is preferably arranged to sound an alarm automatically upon detecting that an alarm initiation unit has been actuated. This alarm may be merely a local alarm to attract the attention of an operator to the fact that an alarm condition has been signalled, but preferably it is a general alarm sounded throughout the area protected by the system. In some cases, however, such as where the system covers a number of separate buildings of wings of a building, the monitor unit may be arranged to sound the alarm only in the appropriate building or wing from which the alarm was raised.

Preferably the emergency alarm system includes a surveillance circuit for checking the continuity and integrity of the common cable along its entire length. The surveillance circuit may comprise a device which is located at the far end of the common cable remote from the monitor unit and which is arranged to emit a regular test signal along the cable, and means in the monitor unit for identifying the test signal and indicating a fault condition if the regular test signal is not received. The provision of such a surveillance circuit provides a regular check on the common cable to ensure that neither an open circuit, nor a short circuit, fault condition exists.

However, when the monitor unit is arranged to feed a DC power supply to the cable for powering the alarm initiation units, the surveillance circuit preferably comprises a voltage reference device, for example, a zener diode, which is connected to the common cable at its far end remote from the monitor unit and which is arranged to clamp the voltage of the common cable at a level below that provided by the DC power supply of the monitor unit, and voltage sensing means in the monitor unit for indicating a fault condition if the voltage of the cable at the monitor end rises (indicating an open circuit) or falls (indicating a short circuit) from the level set by the voltage reference device.

This construction of the surveillance circuit is preferred since it means that the check on the cable is continuous.

If desired there may be a further series of alarm initiation units connected by a further common cable to the monitor unit. In this case, each series of alarm initiation units and its common cable should have its own surveillance circuit, when such is included in the system. In this way a further economy in cable can be achieved when it is convenient to have a centrally located monitor unit, such as in the case of a winged building, each wing being protected by a separate series of alarm initiation units connected by a common cable to the monitor unit.

With an alarm system constructed in accordance with the invention, besides saving considerably on the use of expensive specially insulated one pair cable, further alarm initiation units may be added to the system at a later stage without difficulty and without running a further cable back to the monitor unit. The additional alarm initiation units can either be connected to an extension of the common cable at its far end remote from the monitor unit, the remote surveillance device being moved accordingly to the new end of the cable, or each new alarm initiation unit may be connected to the existing common cable at an appropriate point by means of a four core cable so that the alarm initiation units of the system are effectively maintained in a string and the surveillance circuit covers the whole system.

A particular example of an emergency alarm system in accordance with the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic diagram illustrating the layout of a fire alarm system constructed in accordance with the invention; Figure 2 is a block diagram of each of the alarm initiation units of the system shown in Figure 1; and Figure 3 is a block diagram of the monitor unit of the system shown in Figure 1.

With reference to Figure 1 the fire alarm system comprises a long string of alarm initiation units 1 positioned strategically throughout the building or other complex to be protected, the units 1 being parallel connected by a common one pair cable 2 to a monitor unit 3 at one extreme end of the cable 2. Each alarm initiation unit 1 comprises an alarm switch which, when operated, causes the unit to send a signal which is unique to that unit via the one pair cable 2 to the monitor unit 3. The monitor unit identifies the alarm initiation unit 1 from which the signal was sent and gives a visual indication of the identity of a display panel 4.As described earlier the display panel 4 may take various forms, but in the present case it comprises a series of lights 5 representing the different alarm initiation units 1 and each arranged to be illuminated when the identification signal of its associated unit 1 is received by the monitor 3. The lights 5 are numbered or otherwise identified according to the positions of their associated alarm initiation units along the common cable 2. Upon receipt of an identification signal following actuation of one of the alarm initiation units 1, the monitor unit 3 is also arranged to provide a signal 6 for initiating an audible alarm. This may take the form of a broadcast warning or an alarm bell or klaxon, and may be sounded generally throughout the whole area or in specific zones only, depending on the layout of the area protected and the location of the actuated alarm initiation unit 1. As illustrated the monitor unit 3 is also arranged to operate repeat alarm displays 7 and 8, via suitable cables 9 and 10 respectively, at required stations remote from the unit 3. The repeat display 7 is identical to the display panel 4, whereas the repeat display 8 is arranged to give a simple numerical identification of the actuated alarm unit 1.

Figure 2 illustrates in block diagram form the construction of each alarm initiation unit 1. The common cable to which the unit is connected is indicated by a pair of conductors 2a leading into a filter and modulator section 11 of the unit and the pair of conductors 2b leading out of the section 1 The cable 2 carries a DC power supply from the monitor unit 3 to power each alarm initiation unit, as well as being arranged to carry the identification signal of any particular unit upon actuation of the unit. The alarm switch of the unit is a manually operated break glass switch 12, the contacts 13 of which are normally closed. When the glass of the switch 12 is broken the contacts 1 3 open, and this is detected by a switch detector 14, for example in the form of a nand gate.The switch detector 14 then sends an enabling signal 1 5 to a clock pulse generator 16 which in this example is arranged to oscillator at 300 Hz. The pulses emitted by the pulse generator 1 6 are fed to the supply filter and modulator section 11 by which the DC power supply on the cable 2 is modulated in synchronism with the pulses, for example by means of a darlington pair transistor switching circuit which is arranged to make and break a connection between the conductors of the cable 2 in response to the pulses received from the pulse generator 1 6. The supply filter and modulator section 11 is also arranged to shield the rest of the alarm initiation unit from the effects of the modulation applied to the line power supply (i.e. on the cable 2) by the unit, or indeed by any other alarm initiation unit following its actuation. For this purpose the supply filter and modulator section 11 may include a smoothing capacitor which is arranged to maintain the DC power supply to the components of the unit 1 during modulation of the line supply. The pulses emitted by the pulse generator 1 6 are also fed to a counter 1 7 and after a predetermined number of pulses have been counted the counter 1 7 emits a signal 1 8 which stops the pulse generator 1 6.

This of course stops the modulation of the line supply, and the predetermined number of pulses which have been generated on the line supply constitute the identification signal of the unit 1.

The predetermined number of pulses allowed by the counter 1 7 is determined by a position programmer 1 9 which in this case is an adjustable eight way DIL switch. This enables the counter to be set to count any number of pulses up to about 250, and it will be understood that by adjusting the programmer 1 9 of each alarm initiation unit 1 to a different setting, each unit 1 will have a different number of pulses constituting its identification signal. It will be appreciated that with this arrangement the alarm system can accommodate up to 250 alarm initiation units 1.

If more are required then a different programming switch must be used as the programmer 1 9. It will also be appreciated that since the pulse generator 1 6 operates at 300 Hz, the time taken to send the identification signal from any of the units 1 in the system will be less than a second, even if there are the maximum number of possible units 1 in the system.

Once the unit 1 has been actuated and its identification signal transmitted to the monitor unit 3 along the common cable 2, the unit is inoperative until re-set. As will be explained later, this is effected by actuation of a re-set switch in the monitor unit 3 which is arranged temporarily to shut off the power supply to the cable 2. This allows various capacitors of the unit 1 to discharge, and upon reestablishment of the power on the cable 2, a programme set circuit 20 is operative to re-set the counter 1 7 so that it is again ready to count pulses from the generator 1 6 up to the predetermined number set by the programmer 19. At the same time a switch detector delay circuit 21 is operative to prevent the switch detector 14 from providing an enabling signal 1 5 for a short fixed period.This avoids erroneous triggering of the unit 1 during the initial power on period. It will of course be appreciated that the broken break glass switch 12 of the unit must be replaced or repaired before the system re-set switch is actuated otherwise the unit will again trigger the alarm as soon as the switch detector delay 21 becomes inoperative following the system re-set.

Preferably the alarm initiation units 1 include CMOS elements since these have a very low quiescent current which results in the system having a very low drain current. This is of great advantage for an emergency alarm system since it is the usual requirement for such a system to be capable of operating for a period up to 48 hours in the event of a mains power failure.

Turning now to Figure 3 which is a block diagram of the monitor unit 3, the unit comprises a power circuit 22 for providing a DC output 23 from an AC mains source 24 or, in the event of mains failure, a DC power source 25. The DC power output 23 is applied to the conductors 2a of the common cable 2 through a power supply control 26 which is provided to shut down the power supply to the cable 2 when the system is to be re-set, a constant current source 27, for example a resistor, for protecting the power supply circuit 22 in the event of partial or full short circuit on the cable 2, and a peak volt limiting device 28, for example a diode, which limits the voltage applied to the cable 2 to a maximum which in the present case is 1 5 volts.

The DC power output 23 from the power circuit 22 is also used to power the display panel 4 of the monitor unit 3 and the alarm signal initiation circuit 6 via a line 29 which leads from between the power supply control 26 and the constant current source 27.

The monitor unit 3 is arranged to monitor the condition of the DC power supply on the cable 2, and for this purpose the conductors 2a of the cable are tapped by conductors 30 adjacent the peak volt limiter 28. The conductors 30 lead firstly to an open circuit detector 31 and a short circuit detector 32 which operate in conjunction with a Zener Diode 33 (Fig. 1) which is located across the conductors of the one pair cable 2 at its far end, and form a surveillance circuit for continuously checking the condition of the cable 2 for faults. The Zener Diode 33 is arranged to clamp the voltage on the cable 2 to a value (in the present case 5 volts) which is below that otherwise applied to the cable by the monitor unit.In the event of an open circuit occurring in the cable 2 the voltage on the cable will rise to about 15 volts and this will be detected by the open circuit detector 31. In the case of a short circuit fault in the cable 2 the voltage across the cable will fall to zero and this will be detected by the short circuit detector 32. The detectors 31 and 32 are each arranged to cause the monitor unit to provide a suitable warning in the event of a fault being detected.

The conductors 30 also serve to pick up the identification signal pulses which are applied to the cable 2 by modulation of the power supply thereon following actuation of an alarm initiation unit 1. The detectors 31 and 32 are designed to be insensitive to the identification signal pulses, and these pulses are fed by the conductors 30 to a modulation detector 34, which may comprise a Schmidt trigger, and which is intended to define clearly the presence of pulses against signal noise and has a fixed sensitivity ratio to provide rejection of unwanted signals. The output from the modulation detector 34 is fed to a voltage interface 35 which converts the normal peak to peak voltage of 5 volts to 1 5 volts to suit the subsequent counting and detection circuits. From the voltage interface 35 the pulses are fed to a counter 36 and also to a modulation width detector 37.The counter 36 counts the pulses and converts the information into binary form. The modulation width detector 37 is operative on receipt of the first of a series of pulses from the modulation detector 34 to prevent information from being passed from the counter 36 through a binary to decimal decoder 38 to activate the display panel 4 for as long as a series of pulses continues to be received. The termination of the series of pulses is detected by the modulation within detector 37 which accordingly sends a signal 39 to the binary to decimal decoder 38 to enable it receive the binary coded information from the counter 36 representing the identification signal constituted by the received series of pulses.The decoded signal is passed from the decoder 38 to the display panel 4 where the light 5 representing the alarm initiation unit which corresponds to the received identification signal is illuminated. The output from the modulation width detector is also fed to a counter re-set device 40 which, after a short predetermined delay is operative to re-set the counter 36 so that it is ready to receive and identify any further alarm signal which may be triggered.

As mentioned earlier, the monitor unit includes a system re-set control 41 operable manually by a re-set switch 42. The re-set control 41, upon actuation, causes the power supply control 26 to shut off the power supply to the cable 2 and to the display and alarm units 4 and 6. This is detected by a display re-set 43 which becomes operative to clear the display panel 4 and alarm signal 6. The re-set control 41 is also operative to inhibit the binary to decimal decoder 38 from signalling the display panel 4 immediately on reconnection of the power supply.

If the system includes remote repeat display panels, such as indicated at 7 and 8 in Figure 1, the requisite signals indicated at 44 are fed to each repeat display along a suitable cable, indicated at 9 and 10 in Figure 1. Fourteen core cable will be sufficient for this purpose.

Claims (21)

Claims

1. An emergency alarm system comprising a number of emergency alarm initiation units connected to a common cable, and a monitor unit connected to one end of the common cable, each of the emergency alarm initiation units being arranged to emit, upon its actuation, a unique identification signal which is transmitted by the cable to the monitor unit whereby the monitor unit is able not only to detect that an alarm condition exists but also to identify the particular alarm initiation unit which has been actuated.

2. An alarm system according to claim 1, in which the unique identification signal of each alarm initiation unit comprises a predetermined number of pulses, and the monitor unit includes means for receiving and counting pulses on the cable to identify the alarm initiation unit from which the pulses were sent.

3. An alarm system according to claim 2, in which the unique identification signal of each alarm initiation unit is generated by means of a pulse generator which is arranged to emit a predetermined number of pulses following actuation of the unit, and a switching circuit which is connected across a pair of conductors of the common cable and which operates in response to the pulse generator to make and break a connection between the conductors a predetermined number of times determined by the pulses so that the identification signal pulses are applied to the cable by loop connect/disconnect signalling.

4. An alarm system according to claim 3, in which each alarm initiation unit includes a pre-set counter which counts the pulses emitted by the pulse generator and stops the generator after the predetermined number of pulses have been emitted.

5. An alarm system according to claim 3 or claim 4, in which the pulse generator of each alarm initiation unit is powered by DC supplied by the monitor via the common cable.

6. An alarm system according to claim 5, in which the common cable is a one pair cable, and each alarm initiation unit includes filter means between the DC power supply and the pulse generator whereby the pulse generator is substantially unaffected by modulation of the DC supply on the cable as a result of the identification signal pulses applied and transmitted along the cable from any of the alarm initiation units.

7. An alarm system according to claim 5 or claim 6 in which the monitor comprises means for providing a DC power supply from either an AC power source or a DC power source, and the DC power supply is fed to the one pair cable through a constant current source and a peal volt limiting device.

8. An alarm system according to any one of claims 2 to 7, in which the means for receiving and counting pulses received by the monitor unit from the common cable includes means for detecting the presence of pulses on the cable and transmitting the pulses to the counter substantially divorced from signal noise.

9. An alarm system according to claim 8, in which the monitor includes means for detecting when a group of pulses received by the monitor finishes and for re-setting the counter a predetermined time after the pulses have finished.

10. An alarm system according to any one of the preceding claims, in which the monitor unit comprises display means which is responsive to identification of a signal received from any of the alarm initiation units to give a visual indication of the identity of the actuated alarm initiation unit.

11. An alarm system according to claim 10 which includes additional visual display means which is located at a position remote from the monitor unit and the first display means, and which is connected to the monitor unit so that the information indicated by the first display means is repeated by the additional display means.

12. An alarm system according to claim 10 or claim 11, in which the or each visual display means includes latching circuits so that information dispiayed thereby remains on display until the system is re-set.

13. An alarm system according to any one of the preceding claims, in which the monitor unit is arranged to sound an alarm automatically upon detecting that an alarm initiation unit has been actuated.

14. An alarm system according to any one of claims 10 to 13 when dependent upon claim 7, in which the monitor unit includes a manually operable re-set switch which cuts out the DC power supply means whereby the or each visual display means is cleared, the audible alarm is cut off, and the or each actuated alarm initiation unit is re-set in response to the loss and the subsequent return of the DC power supply to the cable.

15. An alarm system according to any one of the preceding claims, which includes a surveillance circuit for checking the continuity and integrity of the common cable.

16. An alarm system according to claim 1 5 in which the surveillance circuit comprises a device which is located at the far end of the common cable remote from the monitor unit and which is arranged to emit a regular test signal along the cable, and means in the monitor unit for identifying the test signal and indicating a fault condition if the regulat test signal is not received.

1 7. An alarm system according to claim 15 when dependent upon claim 7, in which the surveillance unit comprises a voltage reference device which is connected to the common cable at its far end remote from the monitor unit and which is arranged to clamp the voltage of the common cable at a level below that provided by the DC power supply of the monitor unit, and voltage sensing means in the monitor unit for indicating a fault condition if the voltage of the cable at the monitor end rises or falls from the level set by the voltage reference device.

18. An alarm system according to any one of the preceding claims, in which each alarm initiation unit is arranged to be actuated by an automatic heat or smoke detector.

19. An alarm system according to any one of claims 1 to 17, in which each alarm initiation unit is arranged to be actuated by a manually operated switch of the "break glass" type.

20. An alarm system according to any one of the preceding claims, in which there is a further series of alarm initiation units connected by a further common cable to the monitor unit.

21. An alarm system according to claim 1, substantially as described with reference to the accompanying drawings.