Fire alarm systems with incipient hyper-sensitivity monitoring

Abstract

 A method of and apparatus for monitoring for incipient hyper-sensitivity of any detector station in an alarm system comprising a plurality of detector stations in a spur loop-ring between an alarm unit and an end-of-line unit.
The detector stations are each provided with means for triggering an alarm, and with self-monitoring means for detecting any excessive increase in sensitivity due to contamination of a detector, which means initiate a fault indication, thus enabling remedial action to be taken before a false alarm is given.

Description

[0001] The invention relates to fire alarm systems of the type having a plurality of detectors connected to a common central unit that is capable of monitoring for the integrity of the line connection, the valid presence of all detectors, and for the presence of an alarm state signal from any connected detector.

[0002] Our United Kingdom Patent No. 2,014,782B describes and claims a preferred form of ionisation smoke detector which can be used to advantage in such systems, being provided with means in the form of an outer guard electrode to inhibit the entry of unwanted ions.

[0003] If a spur line is used for a plurality of detectors, then it is necessary to provide means for monitoring the integrity of the line, and our co-pending U.K. Application No 2 134 680A of even date, claiming priority of our U.K. Application No. 83.02891 dated 2nd February 1983, relates to a method of maintaining effective monitoring along the full length of a spur loop-line having a plurality of detector stations between an alarm unit and an end-of-line unit, where the detector stations have detachable head units to facilitate servicing or the like, and the alarm unit provides a fault indication when the normal operation is interrupted, whilst producing an alarm signal when a detector is activated. The proposed method, system and design of station ensures that all other stations of a line-loop remain effective when any head unit is removed for service or inspection.

[0004] Claim 1 of this co-pending application is directed to a method of maintaining effective monitoring in an alarm system of the type in which an alarm unit incorporating power supply and monitoring circuits is connected to a spur in the form of a line-loop having a plurality of detector stations in cascade between said alarm unit and an end-of-line unit powered by the supply from said alarm unit and generating a modulated signal to be fed back to said monitoring circuit in said alarm unit, the arrangement being such that an alarm state is triggered by any surge in the current supply to said line above a given value, and a fault condition is triggered in the event of said modulated signal failing to be received at the alarm unit, said detector stations being designed to apply a low-impedance between the line conductors in the event of a disturbance being detected, and at least one said detector station having a detachable head unit whose removal from its base member causes a modulated signal suppression circuit to inhibit the return of the modulated signal to the alarm unit, whilst leaving all the remaining detector stations effectively connected for alarm initiation purposes.

[0005] Further details will be given hereunder with reference to Figure 1 of the following drawings, which corresponds to Figure 1 of the co-pending Application.

[0006] One of the most difficult problems presented to the designer of these alarm systems is the question of false-alarms, and there are references in the technical literature revealing that less than one in twenty alarm calls prove to be genuine, with a result that Governments have called for researchers to find means for reducing this serious problem. Several designs have been proposed in which the central unit becomes a sophisticated interrogation and decision-making unit, using computer technology to cyclically monitor each detector, not only for its operation state at that instant, but also to analyse its condition in terms of sensitivity.

[0007] Particularly in the case of ionisation type smoke detectors, a hyper-sensitive condition may be reached in time, due to increasing contamination, which ultimately increases the sensitivity of a detector to such an extent that it may trigger an alarm in normal ambient conditions. This problem is also present in other types of detector, and as a result it has been proposed that a central unit be provided with means to check individual detectors to remotely measure their sensitivity, and so obtain advance warning of any incipient hyper-sensitivity.

[0008] However, such a solution is inherently expensive, and involves complex circuitry which is itself increasingly liable to failure as its complexity is increased.

[0009] One object of the present invention is to provide a method of monitoring for incipient hyper-sensitivity and give a fault-condition signal in good time before any false alarm call is initiated, so that restorative action can be taken to prevent false alarms being initiated by any hyper-sensitive detector.

[0010] In accordance with one aspect of the present invention there is provided a method of monitoring for incipient hyper-sensitivity of any detector station in an alarm system of the type in which an alarm unit incorporating power supply and monitoring circuits is connected to a spur in the form of a line-loop having a plurality of said detector stations in cascade between said alarm unit and an end-of-line unit powered by the supply from said alarm unit and generating a modulated signal to be fed back to said monitoring circuit in said alarm unit, the arrangement being such that an alarm state is triggered by any si.rge in the current supply to said line above a given value, and a fault condition is triggered in the event of said modulated signal failing to be received at the alarm unit, said detector stations each being provided witt first means to apply a low-impedance between the line conductors in the event of a disturbance being detected, and second means comprising a modulated signal suppression circuit to cyclically inhibit return of the modulated signal to the alarm unit and provide a faul: indication if the pre-set sensitivity of any detector station has risen toward an alarm initiation threshold by an excessive degree which leaves an unacceptable safety margin.

[0011] Thus, each detector station is provided with in- built sensitivity monitoring, and a fault call made, leading to inspection and restoration action, without requiring complex and expensive circuitry. Where detector stations are of the type described in our co-pending U.K. Patent Application No. 2 134 680A, then the one fault condition signal can be used to advantage for a dual function. The removal of any detector head, or any damage to the line results in the fault indication which requires an engineer to conduct an inspection.

[0012] It is perfectly acceptable to utilise a pulsing indication to indicate that a detector station has reached a level of incipient hyper-sensitivity, as there will normally be a period of many hours, or even days, before that detector station becomes so contaminated that it triggers a false alarm, and the engineer, in conducting his inspection, will automatically check the sensitivity of the detector station that has been found to trigger a fault signal, and can take the necessary remedial action.

[0013] The invention will now be described with reference to the drawings, in which:-

Figure 1 is a block schematic circuit diagram of one exemplary system in accordance with the invention;

Figure 2 is a block schematic circuit diagram showing the general lay-out of the stages in each detector station; and

Figure 3 is a detailed circuit diagram of one preferred detector station

[0014] In the system shown in Figure 1 an alarm unit 1 incorporates alarm signalling means 2, an alarm monitoring circuit 3, a fault monitoring circuit 4, and a power supply circuit 5. A spur line-loop, formed in this embodiment by coaxial cable 6, extends from terminals 1A and 1B provided in the alarm unit 1 for connection of the line-loop, to terminals 7A and 7B of an end-of-line unit 7. This end-of-line unit 7 incorporates a switching stage 8 powered by the supply circuit 5 via the line 6, and operative to periodically short-circuit the line conductors for a brief time, typically 100 micro-seconds, and remove the short-circuit for a time interval, typically 20 milli-seconds, so that the d.c. line potential has a modulated signal in the form of a rectangular pulse waveform superimposed thereon. The mark-space ratio of the waveform, and its repetition frequency may be selected to suit the requirements of any particular system, and should be selected to facilitate recognition of the modulated signal at the alarm unit, allowing for cable distortion and any other factors that may be anticipated to adversely effect positive operation.

[0015] The end-of-line station also includes manually or remote-controlled test-facilities 9 for simulating a fault condition by suppressing the modulated signal and so trigger the alarm unit fault monitoring circuit 4, or by introducing a low-impedance between the terminals 7A and 7B for a prolonged period to trigger the alarm monitoring circuit 3.

[0016] Along the line 6 there is a plurality of detector stations 11, each comprising a permanently connected base member 12 and a detachable detector head unit 13 incorporating a conductive link 14 connected between a pair of terminals 13A and 13B, which contact respective terminals 15A and 15B of a modulated signal suppression circuit 15 provided in the base member 11, which has terminals 11A and 11B for connection to the line conductors.

[0017] Figure 2 shows in simplified form the basic stages that are used in each detector station. At the right hand side of the drawing there are shown line terminals LT1 and LT2, a head-link terminal HLT, that is connected in this simplified example to the terminal LT2 via a link HL in the detector head (not shown). A test point TP is provided on the detector head so that an engineer can attach a measuring instrument to ascertain the threshold sensitivity when the detector is set, as determined by a threshold detector TD and a preset potentiometer or the like provided in an adjustable setting up stage SU.

[0018] A stabilised power supply stage PS is connected between the line terminals, and provides the operating supply to the other stages in the detector station.

[0019] If the detector head is removed in this embodiment the head-link HL removes the earthing connection normally obtained from LT2, and an enabling gate EG causes a switching stage ST to energise an oscillator, in this case an astable multivibrator AM, a fault indicator FT and an energy sink ES. The multivibrator AM feeds pulses to the fault indicator FT via a buffer B1, and to the energy sink ES via a further buffer B2.

[0020] The fault indicator FT in this embodiment enables a visual indicator LED to flash intermittently, periodic charging and discharging of a drive pulse unit DPU being utilised to give a high energy flash of brief duration. The energy sink intermittently absorbs all of the modulated signal from the end-of-line unit, and so passes a pulsed signal onward to the alarm unit, which will cause the latter to indicate that there is a fault condition, although all detector stations are still effectively connected to trigger an alarm signal, should a detector be caused to respond by heat or smoke, as the case may be.

[0021] A more detailed circuit is shown in Figure 3, in which the stabilised power supply PS comprises a transistor stage controlled by a zener diode and having adequate smoothing capacitors at its output. The astable multivibrator is formed by two NOR gates, one with resistive feed-back and the other with capacitive feed-back, one output being fed via a differentiating network to a NOR gate buffer B1, and the other being connected directly to a further NOR gate buffer B2, and thence via a potential-level setting zener diode to the input of the energy sink ES, which allows a storage capacitor to remain fully charged at all times, either via a permanently connected diode D4, or by the switching on of a transistor ^T5 to periodically eliminate any modulated signal present between the terminals LT1 and LT2, and so produce a pulsing fault signal.

[0022] The threshold detector TD comprises an FET having an adjustable potentiometer in its current path to facilitate pre-setting of a desired trigger potential which is significantly lower than the alarm trigger potential. When the threshold is reached a thyristor T6 is turned on, and a temperature- compensating network is provided to maintain the circuit operating potentials substantially constant. The thyristor is connected to supply terminal 7 of the semiconductor chip containing the NOR-gates, and supply terminal 14 is connected the output of the power supply P5.

[0023] Various modifications can be utilised. For example, separate pairs of line conductor terminals may be provided for the incoming and outgoing lines, provided a robust and reliable connection is provided in the station base member to ensure loop continuity.

[0024] If the power supply circuit 5 is designed to feed an alternating current supply to the line, the detector station design can be readily adapted by the insertion of a bridge rectifier in each base member, and in such a system, the modulated signal could be a d.c. current flow or an a.c. waveform of significantly different frequency. If a d.c. current is used as the signal to be monitored, each base member can incorporate a blocking capacitor adequate to pass the alarm surge, and by-passed by a shorting link, electronically or mechanically completed when a detector head is inserted.

[0025] Instead of coaxial cable, parallel-twin line or any pair of conductors suitable for the environment can be used.

[0026] It will be understood that the removable detector with its head-link is not an essential feature, and various forms of detector may be incorporated.

[0027] Similarly, the fault indication given by the LED on the detector station could be replaced by an acoustic signal from a transducer such as a PZT device. In one normal mode of use the alarm unit will be adapted to indicate the zone in which a fault has occurred, and the engineer can rapidly identify the relevant detector station by seeking out the detector station that is producing an optical or acoustic signal.

[0028] In an alternative mode, the LED may be retained to flash periodically to indicate that all is well, and an acoustic transducer or LED of different colour can be pulsed to give the fault indication.

[0029] Finally, it should be mentioned that the public-address features of our co-pending U.K. Patent Application No. 82.24781 of 28th August 1982 could be incorporated by fitting a loudspeaker in each detector station, to give verbal instructions, these loudspeakers being matched to a hybrid electrical supply line via respective transformers in the manner set out in the last mentioned Application.

[0030] In that Application, continued operation of loudspeakers connected in a two-wire ring driven from an a.c. power output via a pair of series limiting resistors is ensured by connecting the individual stations into the live conductor of the ring via respective pairs of terminals, and arranged to operate in a parallel mode when normal operation is effective, each station having a transformer primary winding connected between the ring conductors by an associated mode-converting resistor. The loudspeaker is connected to a secondary winding. A tertiary winding is connected between the line terminals in each station to assume operation in a series mode in the event of a short circuit occurring between the ring conductors.

[0031] Thus, control announcements can be made to direct any persons in a danger zone, and these will be reliably reproduced, even if the fire or any other hazard causes a break in the wiring of the hybrid ring, which is separate from the detector station spur loop-line.

[0032] To summarise the distinction between the earlier proposed detector stations and the newly described embodiments of this present invention, in the first case the fault indication was given by a constant interruption of the modulated signal from the end-of-line unit. In the present case there are three conditions. A broken or short-circuited line will produce a constant fault indication for that zone at the alarm unit. A detector station in an incipient hyper-sensitive state, or a station whose detector head has been removed will produce a pulsing fault indication at the central alarm unit, the timing being determined by the time constant of the drive capacitor circuit. At each detector station an indication of a fault condition is provided, optically or acoustically, and a constant pulsing signal can be provided to confirm that all functions are operating normally.

[0033] According to another aspect the invention consists in a fire alarm system for carrying out the method as claimed hereinafter, comprising in combination:-

an alarm unit incorporating power supply and monitoring circuits;

a spur line connecting a plurality of detector stations in cascade between the alarm unit and an end-of-line unit powered by said alarm unit and generating a modulated signal to be fed back to the respective monitoring circuit;

triggering means in said monitoring circuits to respond to any current surge above a given level or to the absence of a said modulated signal;

each said detector station having first means to apply a low-impedance between the line conductors in the event of a disturbance being detected, and second means to cyclically inhibit return of the modulated signal by a respective modulated signal suppression circuit, said second means being dependent upon the sensitivity of the respective detector station.

Claims

1. A method of monitoring for incipient hyper, sensitivity of any detector station in an alarm system of the type in which an alarm unit incorporating power supply and monitoring circuits is connected to a spur in the form of a line-loop having a plurality of said detector stations in cascade between said alarm unit and an end-of-line unit powered by the supply from said alarm unit and generating a modulated signal to be fed back to said monitoring circuit in said alarm unit, the arrangement being such that an alarm state is triggerel by any surge in the current supply to said line above i given value, and a fault condition is triggered in the event of said modulated signal failing to be received :t the alarm unit, said detector stations each being provided with first means to apply a low-impedance between the line conductors in the event of a disturbaice being detected, and second means comprising a modulatel signal suppression circuit to cyclically inhibit return of the modulated signal to the alarm unit and provide . fault indication if the pre-set sensitivity of any detector station has risen toward an alarm initiation threshold by an excessive degree which leaves an unacceptable safety margin.

2. A method as claimed in Claim 1, in which said alam unit supplies d.c. power to said spur line-loop, and said modulated signal is produced in said end-of-line unit by applying a short-circuit of very brief duration at relatively long intervals.

3. A method as claimed in Claim 1 or Claim 2, in which each said detector station absorbs said modulated signal when its detector head is removed or said detector has reached a state of incipient hyper-sensitivity, and causes a fault indicator provided at said detector station to give an intermittent signal indicating that it is that station that is at fault.

4. A method as claimed in Claim 3, in which said fault indicator is a light-emitting diode.

5. A method as claimed in Claim 3, in which said fault indicator is an electro-acoustic transducer.

6. A method as claimed in any preceding Claim, in which signalling means are provided at each detector station to provide a verification that normal functioning prevails.

7. A fire alarm system for carrying out the method as claimed in any preceding Claim, comprising in combination:-

an alarm unit incorporating power supply and monitoring circuits;

a spur line connecting a plurality of detector stations in cascade between the alarm unit and an end-of-line unit powered by said alarm unit and generating a modulated signal to be fed back to the respective monitoring circuit;

triggering means in said monitoring circuits to respond to any current surge above a given level or to the absence of a said modulated signal;

each said detector station having first means to apply a low-impedance between the line conductors in the event of a disturbance being detected, and second means to cyclically inhibit return of the modulated signal by a respective modulated signal suppression circuit, said second means being dependent upon the sensitivity of the respective detector station.

Drawing