GB2176600A - Fire hazard detection system - Google Patents

Abstract

The system has at least one fire hazard detector device 10 eg a smoke detector or heat detector, for providing an alarm signal in response to the existence of a fire hazard, and simulation means 12 which is operable on the detector device to simulate the effects of a fire hazard, and means 15 for controlling operation of the simulation means and for sensing the response of the detector device. The controlling means is preferably arranged to control operation of the simulation means at selected, regular, and frequent intervals, thereby enabling a faulty detector device to be identified early, and replaced quickly, and generally ensuring reliable operation of the system. In order to achieve high integrity of verification, the simulated hazard may cause all the components of the detector device associated with hazard detection to be exercised in the same manner as when a real hazard exists. <IMAGE>

Description

SPECIFICATION Fire hazard detection systems This invention relates to fire hazard detection systems, and especially to hazard detection systems having at least one fire hazard detector device. The invention is particularly concerned with fire detection systems which include at least one smoke or heat detector device, though in practice it is usual to employ many detector devices.

It is common nowadays for fire protection systems to employ smoke or heat detector devices for detecting the presence of a fire and providing an alarm. Such systems, if they are to operate efficiently, must be properly maintained to ensure reliability. To this end, many fire protection systems incorporate means for detecting faults in the systems wiring, for example discontinuities, between one or more detector devices and a central monitoring unit typically located remotely from the detector device(s). In addition the condition of wiring to annunciators and irregularities in power supply to the system, for example, power failures, may also be monitored. This fault detection capability is sometimes referred to as the system watchdog.

Such fault detection means are naturally very useful in helping to ensure reliable operation of the system but the protection they afford is limited. Whilst they are capable of sensing and indicating certain types of faults, as mentioned above, they are unable to determine faults in the detector devices themselves. Detector devices can and do fail from time to time for a variety of reasons. This failure cannot therefore be easily predicted. If a detector device should malfunction in the event of a hazard occurring and fail to generate an alarm signal, the consequences obviously could be serious. Thus there is a real risk with the known kind of systems that, since faults in the detector devices can remain undetected, the system may not be able to meet the demands placed upon it.

For this reason, it is customary for the detector devices in the system to be manually tested at intervals to establish their operational status, for example by deliberately injecting smoke into the device- or subjecting it to heat inthe case of smoke and heat detector devices respectively and observing the device's response. The frequency of this manual testing may be decided by the system operators or, as is more usual, set either by the relevant fire prevention authorities or in accordance with fire insurance regulations. It is typical for such manual testing to be carried out once every few months or even only once a year.

Because the manual testing procedures are time consuming and inconvenient it is all too easy for the task to be delayed and there may be a tendency for operators to ignore advisable regular testing and limit testing to longer intervals laid down by the fire authorities or insurance regulations.

As the probability of a detector device being serviceable decays exponentially from the time it was last tested, failure to carryout these tests on schedule can significantly increase the risk to the building and its occupants in which the system is installed.

If, on the other hand, the test interval is minimised, the probability of the system being available to meet a genuine demand is greatly increased.

It is an object of the present invention to provide a fire hazard detection system, having at least one hazard detector device, which overcomes at least to some extent the failings of the aforementioned kinds of systems.

According to the present invention, there is provided a fire hazard detection system having at least one hazard detector device for providing an alarm signal in response to the existence of a hazard, wherein the system includes stimulation means associated with the or each detector device which is operable on the detector device to simulate the effects of a hazard, and means for controlling operation of the stimulation means and sensing the detector device's response thereto.

Thus, the system according to the invention is capable of self testing automatically the operational status of its one or more detector devices. This self-verification avoids the need for the one or more detector devices to be manually tested in order to ascertain any device malfunction. The controlling means may be arranged to control operation of the stimulation means at selected, preferably regular, and frequent intervals, say every day, thereby enabling a faulty detector device to be identified early, and replaced quickly, and generally ensuring reliable operation of the system. The risk of system failure in the event of a genuine hazard is thus minimised.

In order to achieve high integrity of verification, the simulated hazard causes all the components of the or each detector device associated with hazard detection to be exercised in the same manner as when a real hazard exists.

Where the system has only one detector device, the device may be integrated with the controlling means and stimulation means to form a self-contained, self-testing, unit.

However, in a preferred arrangement, the controlling means may be located remote from the one or more detector devices and their associated stimulation means. Detector devices are sometimes located at positions not readily accessible and the ability to locate the control means at a convenient, accessible, point is therefore advantageous.

The controlling means is preferably arranged to monitor continuously the one or more de tector devices for alarm signals indicative of the existence of a hazard and provide an indication, visual and/or aural, in accordance therewith.

The stimulation means may be operable to simulate the effects of a hazard for a predetermined period and if at the end of that period the associated detector device has failed to provide an alarm signal, the controlling means generates a warning indication.

The controlling means may communicate with the one or more detector devices and associated stimulation means over signal lines, and in the case of the system having a plurality of detector devices, the plurality of devices conveniently share the same signal lines.

The one or more detector devices may have associated therewith, in addition to the stimulation means, a line interface unit which interconnects the detector device and stimulation means with the signal lines and through which the controlling means communicates with the stimulation means.

In a system having a plurality of detector devices and associated stimulation means connected to the controlling means along common signal lines, the controlling means may supply command signals for controlling operation of each of the stimulation means, preferably in turn, which includes identification characteristics associated with the particular stimulation means to be controlled. The command signals may comprise an identification signal unique to respective stimulation means. In this case, the line interface unit associated with each detector device may include a circuit means for recognising the unique respective identification signal and permitting operation of the stimulation means in accordance therewith.

The controlling means may also be arranged to detect wiring faults in the system and provide a warning indication upon such a fault being detected.

The one or more detector devices of the system may be smoke or heat detector devices.

In one embodiment of the system, the or each detector device comprises an ionisation type smoke detector device having a first chamber open to ambient air, a second, closed chamber, a source of ionisation for ionising air within both chambers, respective electrodes within the chambers between which electrical current can flow as a result of ionisation and circuit means for monitoring these current flows and providing an analogue signal which is compared with a limit to produce an alarm signal in accordance with predetermined changes therein as a result of smoke entering the open chamber.

In this case, the stimulation means may be operable to apply a potential to the electrode in the open chamber in response to a control signal from the controlling means so as to simulate the effects of smoke within that chamber.

Alternatively, the stimulation means may include a shutter situated in the open chamber and may be operable in response to a control signal from the controlling means to move the shutter, for example by means of an electric motor or a microammeter movement, adjacent the source of ionisation to affect ionisation within the chamber thereby simulating the effects of smoke.

In another embodiment of the system, the or each detector device comprises a reflectooptical type smoke detector device having a light emitter for emitting a beam of light which may be modulated, through a space open to ambient air towards a light receiver, the light beam and light receiver being arranged such that light is reflected and/or refracted by smoke within the space onto the light receiver, and circuit means for producing an analogue signal from which is derived an alarm signal in response to a predetermined output from the light receiver.

In this case, the stimulation means preferably includes means operable to selectively reflect light from the light beam into the light receiver so as to simulate the effects of smoke.

In one example, the stimulation means may include a liquid crystal device positioned adjacent the light beam path which is switchable from a substantially non-reflecting state in response to a control signal from the controlling means to reflect light onto the light receiver.

In another example, the stimulation means may include a reflective element physically moveable into the light beam path to reflect light onto the light receiver, thereby, again simulating the effects of smoke.

The reflective element may be shaft, a part of whose surface is reflective, rotatable, for example by means of an electric motor or a microammeter movement, within a sleeve having a window in its side wall to bring the reflective surface of the shaft behind that window.

In another embodiment of the system, the or each detector device comprises a heat detector device having a heat sensitive element exposed to ambient air, and circuit means connected to the heat sensitive element for monitoring its output and providing an analogue output from which an alarm signal in accordance with a predetermined temperature level and/or rate of change, being sensed is produced. In this case, the stimulation means may comprise a heating element disposed near the heat sensitive element, the stimulation means being operable to energise the heating element in response to a control signal from the controlling means so as to simulate the fire hazard.

Fire hazard detection systems, in accordance with the invention will now be described, by way of example, with reference to the accom panying drawings in which: Figure 1 is a schematic representation of the circuit of the hazard detection system; Figure 2 shows schematically in cross-section a form of smoke detector device which may be used in the system of Figure 1; Figures 3a and 3b shows respectively a schematic cross-sectional view and a detail of a further smoke detector device which may be used in the system of Figure 1; Figure 4 shows schematically in cross-section another form of smoke detector device which may be used in the system of Figure 1; Figures 5a and 5b shows respectively a schematic cross-sectional view and a detail of a further smoke detector device for use in the system of Figure 1; and Figures 6 & 7 show schematically in crosssection two heat detector devices which may be used in the system of Figure 1.

Referring to Figure 1, the detection system includes a hazard detector device 10, for example a smoke or heat detector device, which is connected electrically to a line interface unit 11. Also connected to the line interface unit 11 is a stimulation device 12 associated with the detector device 10. The line interface unit 11 is in turn connected across a pair of signal lines 14 which are coupled to a remotely located command and monitoring unit 15 and through which the unit 15 communicates with the detector device and stimulation means via the line interface unit. As is customary, the signal lines 14 remote from the unit 15 terminate at an end of line resistor 16 connected so as to terminate the lines.

Although only one detector device 10 is shown in Figure 1, the system includes a plurality of such devices connected in parallel across the lines each having a respective line interface unit 11 and associated stimulation device 12. The lines 14 serve both to transmit signals to and from the detector devices 10 and to provide power for the devices 10, line interface units 11 and stimulation devices 12.

The command and monitoring unit 15 may form part of a Fire Detection panel, one of which would be located in each fire area, e.g.

one floor of a building, together with associated detector devices. Each of these Fire Detection Panels is in this case linked to a building control centre, usually a room adjacent to the main building entrance. At the building control centre, facilities would be provided for annunciation of alarms and detected faults.

Each Fire Detection Panel is autonomous and capable, as will be described, of interrogation and testing of its associated detector devices, as well as local alarm annunciation.

In practice therefore, with the system installed in a building, the command and monitoring unit 15 is located at any convenient position (say one per floor). The lines 14 are routed around the building with the detector devices 10 situated at suitably appropriate areas for detecting hazards. The command and monitoring unit 15 interrogates its associated interface units in turn using a respective unique address, each line interface unit 11 being equipped with a means (not shown) for designation of the unique address to which it will reply, and in response to a hazard detector device 10 detecting a hazard, an encoded analogue signal is provided upon interrogation, which is transmitted via the line interface unit 11 along the lines 14 to the unit 15.The unit 15, which is microprocessor controlled, continuously monitors the lines 14 and upon sensing a signal from a detector device 10 indicative of a hazard having been detected generates a visual and/or aural alarm, and may operate to activiate safety systems, for example smoke extraction, emergency lighting and sprinklers, and perhaps also to communicate automatically with the local fire authorities. In addition, and in accordance with known practice, the unit 15 which operates a a system watchdog and serves to detect wiring faults in the signal lines 14, e.g. discontinuities or short circuit, and provides warning indications as necessary in this respect using conventional techniques.

In the system's quiescent state, that is with none of the detector devices 10 activated by the effects of a hazard, the components 10, 11 and 12 draw negligible current from the command and monitoring unit 15 and current flow in the lines is dominated by the amount drawn by the end of line resistor 16. Thus current flows continuously along the lines 14 and in the event of a discontinuity or short circuit occurring in these lines the resulting cessation or increase of current flow respectively is readily detected by the unit 15 which reacts to produce the aforementioned warning.

Typically the end of line resistor 16 draws ten times the sum of the currents drawn by the total number of detector devices 10 in the signal line in their non-activiated, quiescent, state.

Besides being capable of interrogating line interface units 11 and detecting wiring faults and the like, the command and monitoring unit 15 is, in accordance with the invention, arranged also to carry out detector device verification/test routines automatically at regular intervals. Detection of a genuine hazard by one of the detector devices automatically inhibits the operation of the unit 15 in this respect.

Providing therefore none of the detector devices 10 has been activated, the command and monitoring unit 15, at the expiration of the test interval, operates to command one of the detector devices to go to test mode. The control signal sent by the unit 15 to achieve this is transmitted along the lines 14 and so is received by all line interface units 11. However, each line interface unit 11 has associated with it a recognition circuit containing a unique identification code. The command and monitoring unit 15 selects the particular detector device to be tested and includes in its transmitted control signal the identification code unique to that device so that only that line interface units 11 associated with the selected detector device recognises the address code and can respond to cause the detector device to undergo testing.

In response to the appropriate address code being received, the unit 11 of the selected detector device activates its associated stimulation device 12 which then subjects the detector device to a simulated hazard, or more precisely simulated effects of a hazard. The nature of the hazard effect simulation is such as to cause all the components associated with hazard detection to be exercised in the same manner as would occur if a real hazard were to exist, thus ensuring that high integrity of verification is achieved. Providing the detector device is functioning correctly, it will respond after a short delay to the simulated effects of a hazard and be triggered to produce an analogue output indicating an alarm state to generate an encoded signal which is fed via the line interface units 11 and lines 14 back to the command and monitoring unit 15.

Upon sensing this alarm signal, and thus recognising the healthy, operative status of the detector device concerned, the unit 15 sends a further signal to the line interface unit 11 of that detector device, again with the appropriate address code, to de-activate the stimulation device and reset the detector device. Alternatively, de-activation could be accomplished by the stimulation means itself following a preset period of effect simulation. The unit 15 then checks to ensure that the detector device has returned to its non-alarm state.

If the detector device fails to return to its inactive, non-alarm, state' the unit 15 is programmed to assume that a real hazard, for example a fire, has actually occurred during the test and generates an alarm accordingly.

If, however, for some reason the detector device fails to respond to the hazard simulation test to provide an alarm signal within a given time period, the unit 15 identifies the device as faulty and provides a suitable warning indication to alert an engineer.

So that alarm signals from other detector devices sharing the signal lines 14 are not delayed by the test routine, the status of the remaining detector devices are continually monitored by the command and monitoring unit 15.

The time taken to interrogate a detector device or command it to undergo self-testing would be around 100 m secs. Therefore, the command and monitoring unit 15 can continue to scan the line interface units 11 throughout the test period in its normal manner.

Typically, the self-testing of a detector device by the system in the described manner will take around 5 seconds.

The command and monitoring unit 15 operates automatically so as to subject each of the plurality of detector devices 10 in turn to a self-test routine with a predetermined delay between the testing of each individual detector device. A testing sequence of all the detector devices may be carried out in this way continuously, or alternatively at regular intervals, say once every day.

The detector devices 10 of the system are adapted to respond to fire hazards and may comprise either smoke or heat detector devices.

With regard to smoke detector devices, it would not be practical to provide a stimulation device in the form of a smoke generator or aerosol for each detector device in the system for hazard simulating purposes. However, stimulation devices suitable for use with smoke detectors have been devised which lend themselves to automated operation and which do not rely on consumables and which do not cause undue wear. These will now be described in greater detail.

With regard to Figure 2, there is shown a generally conventional ionisation type smoke detector device 20 which is directly connected through a mating plug and socket arrangement to a housing 22 containing the line interface unit 11 and part of the associated stimulation device 12. The assembled parts are mounted with the detector device projecting through an opening in a ceiling panel 21 and the housing 22 located partly above the panel 21. The housing 22 is provided with connector terminals to which the signal lines 14, on the one side either from the next, upstream, line interface unit 11 or the remote command and monitoring unit 15 and on the other side either from the next, downstream, line interface unit 11, or the end of line resistor 16 as the case may be, are connected.

The circuit board comprising the line interface unit 11 is connected by leads to the connector terminals, these leads having been omitted from Figure 2 for simplicity. In turn, the line interface unit 11 is connected separately to the part of the stimulation device 14, again comprising a circuit board, contained in the housing 11 and to the circuit board 25 of the detector device 20 by power/signal leads.

The construction of ionisation type smoke detector devices is well known. Briefly, the ionisation type smoke detector device 20, as is customary, has two chambers, a first of which, 26, the "reference" chamber, is partially sealed, ie it has a pin hole allowing it to "breathe" the local atmosphere, and the second of which, 27, is open to ambient atmosphere, and thus smoke ingress through vents in its conical side wall. An ionisation source 23 is mounted on a partitioning wall separating the two chambers and projects into both chambers. The partitioning wall constitutes a common electrode for both chambers. Electrodes are carried on opposing wall surfaces of each chamber, in this case a parallel upper wall surface of the chamber 26 and the inner surface of an opposed, parallel bottom wall of the chamber 27.The lower electrode of chamber 27 also includes a perforated metal screen which defines the conical side wall of the chamber.

In use, a small electrical current flows between the pair of electrodes in each chamber due to the ionisation source 23. The circuit 25 of the detector device 20 continuously monitors these currents and is calibrated such that if a predetermined relationship exists between the magnitudes of the currents normal in the quiescent state, a low value analogue signal is supplied to the line interface unit 11.

If however smoke enters the chamber 27, a reduction in the current flow between the electrodes of this chamber occurs. The devices's circuit 25 responds to the resulting imbalance of current flows in the two chambers 26 and 27 in the presence of a hazard, and causes an increased analogue signal, proportional to smoke density, to be fed to the line interface unit 11. When next interrogated by the command and monitoring unit 15 the line interface unit 11 transmits the encoded analogue signal along the lines 14 to the command and monitoring unit 15.

In order to test operational status of the detector device 20, the stimulation device 14 includes an electrical lead 24 which is connected to one of the electrodes of the chamber 27. In response to a test command control signal being received from the unit 15 by the line interface unit 11, having for this purpose the previously mentioned address code identification circuit, the unit 11 triggers the stimulation device 14 to apply an electrical potential via the lead 24 to the one electrode of the chamber 27. This simulates the effect of smoke within the chamber 27 and, providing the device is functioning correctly, causes an enclosed analogue value representing an alarm signal to be supplied to the unit 15.

Conversely, if the device is not functioning correctly, the unit 15 recognises this by virtue of the absence of an alarm signal from the device and provides a warning indication identifying the particular device concerned as faulty.

With regard to Figure 3a, an alternative form of stimulation device for use with the aforementioned ionisation type smoke detector device is shown. In this embodiment, the operation of the stimulation device 14 is mechanical rather than electrical. The device 14 includes an electric motor 30 which drives through a reduction drive mechanism 31 a shaft 32 extending into the chamber 27 of the device carrying a shutter 33. Upon testing of the detector device, the motor is actuated so as to rotate the shaft 32 and move the shutter 33 into the position shown in Figure 3a between the ionisation source and the electrode of the chamber 27 carried on the lower wall thereof.This has the effect of obscuring the ionisation source from the lower electrode and/or absorbing ions thereby blocking current flow between the two electrodes of the chamber 27, and thus simulating the effects of smoke within the chamber, so that, if the device is functioning correctly, the detector device should be triggered to generate an analogue signal indicating an alarm. If not, the absence of an alarm signal causes the unit 15 to provide a warning indication that the device is faulty.

Upon termination of the test routine, the stimulation device 14 responds to the "reset" signal from the unit 15 to cause the motor 30 to rotate shaft 32 and move the shutter 33 to an inoperative position away from the ionisation source and bottom electrode. Alternatively, the stimulation means may itself return the shutter after a predetermined time.

Figure 3b shows in plan view the shutter drive mechanism in greater detail. The electric motor 30 drives a wheel 35 which bears on the surface of a disc 36 so that rotation of the motor's shaft rotates the disc 36. The disc 36 is attached to the shaft 32 carrying the shutter 33, which, as shown dotted in Figure 4, is of circular form. The stimulation device 14 may further include a position sensor 37 for sensing the position of the disc 36 and connected in a feedback circuit to the motor 30 to ensure that the shutter is positioned accurately adjacent to, and away from, the ionisation source during the testing routine.

An alternative method of providing motive power for shutter movement is to employ a microammeter movement. This has the advantage of having only one moving part.

Another kind of smoke detector device which may be used in the system is the socalled "reflecto-optical" smoke detector device. Such a device is shown in Figure 4, referenced at 40. The device 40 is coupled to a housing 22 containing the line interface unit 11 and part of the stimulation device 14 as in the previous arrangements and for simplicity and ease of understanding therefore like parts are designated by the same reference numerals. The detector device 40 is generally of conventional form and similar in many respects to the device 20 except that it has only one sensing chamber and employs a different kind of sensing arrangement. More particularly, the detector device is provided with a non-reflective chamber 41 which opens through vents in its side wall to permit the ingress of smoke and which has upper and lower walls.Mounted centrally on the upper wall is a light emitter 43 which emits modulated light in the form of a hollow, slightly diverging, beam 42 projected downwardly to wards a bottom wall of the chamber 41 on which is mounted a light detector 44. The light emitter 43 is arranged with respect to the light detector 44 such that, in the devices quiescent, non-active, state the hollow light beam 42 forms a light curtain closely surrounding the light detector 44 but not actually falling thereupon. With the chamber containing relatively clean air, little or no light from the light emitter 43 is reflected or refracted so that only an insignificant amount of light can reach the light detector 44. However, smoke particles entering the chamber 41 reflect and/or refract light onto the detector element 44.The resulting signal from the light detector 44 is amplified by the associated circuitry on circuit board 25 and, provide an analogue signal level indicative of the degree of hazard; this is encoded for transmission, via the line interface unit 11, to the command and monitoring unit 15 in the same manner as with the aforementioned ionisation type detector device. The unit 15 sequentially interrogates the line inteface units 11, therefore it knows which is replying with its status.

As with the ionisation type detector device, the stimulation device 14 associated with the reflecto-optical detector device of Figure 4 may operate either electrically or mechanically to simulate the effect of a hazard for the purpose of testing the device.

Referring to the Figure 4, the stimulation device 14 in this example includes a liquid crystal device 45 located at the edge of the hollow beam 42 of light projected from the light emitter 43 which is switchable electrically between an energised state in which it acts as a black, light-absorbing, substantially non-reflecting, body and a non-energised state in which it acts to reflect light. Operation of the liquid crystal device 45 is controlled by the circuit of the stimulation device 14 and in dependence upon test control signals supplied to the device 24 via the line interface unit 11, with its address code recognition capability, from the unit 15.The liquid crystal device 45 is arranged with respect to the light beam and the light detector 44 such that in its normal, energised state, it does not affect the light detector but when it is switched to its unenergised state in response to a test command from the command and monitoring unit 15 it becomes reflective and reflects some of the light emitted from the emitter 43 into the light detector 44 thereby simulating the effects of smoke and, providing the detector device is functioning correctly, causing the device to produce an alarm signal. If no alarm signal is provided, the unit 15 recognises this fact and generates a warning indication identifying the device concerned as faulty.

In an alternative arrangement, the stimulation device 14 may be arranged to simulate the effects of smoke in a mechanical manner using an electric motor drive shutter combination substantially similar to that shown in Figure 3b except that in this case the shutter is formed of reflective material and arranged so as to be moved into the path of the beam of light to reflect light onto the light detector 44 during detector device testing and away from the beam of light during normal operation of the detector device. A slightly modified form of this arrangement is depicted schematically in Figure 5a where like components have been designated by the same reference numerals.

Figure 5b illustrates a convenient manner in which an alternative form of shutter may operate. In this case, the shaft 32 driven by the motor 30 via the disc 36 is provided with a reflective finish 50 around part of its periphery, the remainder being non-reflective. The shaft is disposed in a stationary, non-reflective sleeve 51 which has a rectangular window 52 in its side wall adjacent the path of the light beam 42. As the shaft 32 rotates therefore within the sleeve 51, the reflective surface and the non-reflective surface of the shaft are brought in turn in front of the rectangular opening 52 by rotation of the shaft forwards and backwards, the reflective surface when testing is being carried out and the non-reflective surface during normal operation of the detector device.

As before, the unit 15 may operate to reset the stimulation means following a testing routine or this may be done by the stimulation means itself. Other sources of motive force may be employed, for example, a microammeter movement.

Turning now to Figure 6, there is shown an example of the other kind of detector device which may be used in the system, namely a heat detector device, referenced at 60. Again, those parts of the system corresponding to parts described above are referenced with the same reference numeral. The detector device 60 includes a heat sensor sensitive to absolute and rate of change of temperature. The sensor comprises two sensing elements, a first, 61, being exposed in a chamber 62 of the device which is open through vents to allow free passage therethrough of ambient air and being responsive to the temperature of that air, and a second, 63, disposed adjacent the first sensor but which is partially insulated such that it responds only to long term, gradual temperature changes. The two sensing elements 61 and 63 are electrically connected differentially. If the ambient air temperature changes only slowly, for example as room temperatures normaliy fluctuate during the day, the output of the connected sensing elements, being the difference in the amplitudes of the signals from the elements, is small.

When the ambient air temperature changes rapidly, as would occur in the case of fire, the difference between the two element signals constituting their output would be much greater. If the polarity of this difference corre sponds to a temperature increase rather than decrease and its magnitude signifies a rate of temperature change indicative of a fire, the circuit of the detector device 60 responds to close an electronic switch to supply a signal to the line interface unit 11 where it will be encoded and passed along the lines 14 to the command and monitoring unit 15 in response to interrogation by unit 15.

In addition, the output signal from the first, exposed sensing element 61 is continuously compared with an absolute temperature limit by the device's circuitry to detect slowly developing fires. On detection the operation is the same as for a high rate of temperature change.

The stimulating device associated with this detector device includes a electrical heating element 65 situated in the chamber 62 below the first sensing element 61. During testing of the device, the line interface unit, in response to the appropriate command signal from the unit 15, activates the stimulation device 14 which then supplies electrical current to the heating element 65 to heat the first sensing element 61, thus simulating-the effects of a fire hazard and, if the device is functioning normally, causing an alarm signal to be generated.

Regardless of the actual kind of detector device employed, it will be appreciated that the system according to the invention offers significant advantages over earlier systems. By the frequent automatic testing and selfverification of each of the detector devices, in contrast with the irregular, infrequent, manual testing in the earlier systems, the probability that the detector devices will be able to perform their desired function to detect a real hazard is greatly improved as detector devices developing a fault are rapidly identified and the fault can be remedied, for example by replacing the device without undue delay. Moreover, a high integrity of verification is achieved as the simulated hazard in each case exercises all the components required to detect and respond to a real hazard.

Obviously the higher the frequency of testing, the better. With these detector devices having associated therewith stimulation devices simulating hazard effects mechanically, the frequency of testing has to be a compromise which takes into account the wear induced by testing and the time the device is effectively out of service whilst being tested.

The detector devices themselves, in all described versions, contain no moving parts and are thus not subject to wear considerations.

The only components of the system likely to be harmed by wear are therefore the stimulation devices and, of course, these do not form an operative part of the actual detector device. Should they fail at any time, this will be detected at the next automatic test of the device concerned.

As each command and monitoring unit 15 is autonomous, it can test its associated detector devices at any rate up to a maximum of one per 5 seconds.

For reasons of fault tolerance, it is unlikely that more than 250 detector devices would share the same circuit. As complete test of these would take a little over 20 minutes. It is considered desirable, bearing in mind that whilst a detector device is undergoing testing it is not available to detect a hazard and therefore detection of a hazard could be delayed slightly, that the time spent in testing the detector devices should not exceed around 0.1% of any 24 hour period.

Another type of heat detector contains only one temperature sensor which is exposed to the ambient air. The construction is as per Figure 6, but omitting item 63.

With this arrangement the ambient temperature sensor 61 causes the electronic circuit 25 to produce an analogue signal proportional to temperature and pass it to the interface unit 11. By interrogating unit 11, unit 15 can determine the temperature. Unit 15 will use this information to detect high rates of change of temperature or when a fixed limit is exceeded.

Using a heating element to verify a heat detector may require too high a power consumption level for some applications. In these situations the following method will be employed.

Heat detectors of the type using only one sensing element, item 61 of Figure 6, which produce an analogue output to unit 11, will be used, see Figure 7.

The stimulation device 12, on command from item 15, feeds an appropriate signal into the detector electronics 25 to cause it to react as though a genuine increase in temperature had occurred.

Operation and validation of the detector is achieved in the same way as with the previous examples.

Claims (24)

1. A fire hazard detection system having at least one hazard detector device for providing an alarm signal in response to the existence of a hazard, wherein the system includes stimulation means associated with the or each detector device which is operable on the detector device to simulate the effects of a hazard, and means for controlling operation of the stimulation means and sensing the detector device's reponse thereto.

2. A system according to claim 1, wherein the controlling means is arranged to control operation of the stimulation means at selected, preferably regular, and frequent intervals, thereby enabling a faulty detector device to be identified early, and replaced quickly, and generally ensuring reliable operation of the system.

3. A system according to claim 1 or 2, wherein, in order to achieve high integrity of verification, the simulated hazard causes all the components of the or each detector device associated with hazard detection to be exercised in the same manner as when a real hazard exists.

4. A system according to any one of claims 1 to 3, comprising only one detector device, wherein the device is integrated with the controlling means and stimulation means to form a self-contained, self-testing, unit.

5. A system according to any one of claims 1 to 3, wherein the controlling means is located remote from the one or more detector devices and their associated stimulation means whereby the detector device(s) can be located at positions not readily accessible whilst the control means is located at a convenient, accessible position.

6. A system according to any one of claims 1 to 5, wherein the controlling means is arranged to monitor continuously the one or more detector devices for alarm signals indicative of the existence of a hazard and provide an indication, visual and/or aural, in accordance therewith.

7. A system according to any one of claims 1 to 6, wherein the stimulation means is operable to simulate the effects of a hazard for a predetermined period and if at the end of that period the associated detector device has failed to provide an alarm signal, the controlling means generates a warning indication.

8. A system according to any one of claims 1 to 7, wherein the controllilng means communicates with the one or more detector devices and associated stimulation means over signal lines, and in the case of the system having a plurality of detector devices, the plurality of devices share the same signal lines.

9. A system according to claim 8, wherein the one or more detector devices have associated therewith, in addition to the stimulation means, a line interface unit which interconnects the detector device and stimulation means with the signal lines and through which the controlling means communicates with the stimulation means.

10. A system according to any one of claims 1 to 3, having a plurality of detector devices, wherein said devices and associated stimulation means are connected to the controlling means along common signal lines, and the controlling means supplies command signals for controlling operation of each of the stimulation means, preferably in turn, which includes identification characteristics associated with the particular stimulation means to be controlled.

11. A system according to claim 10, wherein the command signals comprise an identification signal unique to respective stimulation means, and the line interface unit associated with each detector device includes a circuit means for recognising the unique respective identification signal and permitting operation of the stimulation means in accordance therewith.

12. A system according to any preceding claim, wherein the controlling means is also arranged to detect wiring faults in the system and provide a warning indication upon such a fault being detected.

13. A system according to any preceding claim, wherein the one or more detector devices of the system are smoke or heat detector devices.

14. A system according to claim 1, wherein the or each detector device comprises an ionisation type smoke detector device having a first chamber open to ambient air, a second, closed chamber, a source of ionisation for ion ising air within both chambers, respective electrodes within the chambers between which electrical current can flow as a result of ionisation and circuit means for monitoring these current flows and providing an alarm signal in accordance with predetermined changes therein as a result of smoke entering the open chamber.

15. A system according to claim 14, wherein the stimulation means is operable to apply a potential to an electrode in the open chamber in response to a control signal from the controlling means so as to simulate the effects of smoke within that chamber.

16. A system according to claim 14 or 15, wherein the stimulation means includes a shutter situated in the open chamber and operable in response to a control signal from the controlling means to move the shutter, eg by an electric motor, adjacent the source of ionisation to affect ionisation within the chamber, thereby simulating the effects of smoke.

17. A system according to claim 1, wherein the or each detector device comprises a reflecto-optical type smoke detector device having a light emitter for emitting a beam of light which may be modulated, through a space open to ambient air towards a light receiver, the light beam and light receiver being arranged such that light is reflected and/or refracted by smoke within the space onto the light receiver, and circuit means for producing an alarm signal in response to a predetermined output from the light receiver.

18. A system according to claim 17, wherein the stimulation includes means operable to selectively reflect light from the light beam into the light receiver so as to simulate the effects of smoke.

19. A system according to Claim 18, wherein the stimulation means includes a liquid crystal device positioned adjacent the light beam path which is switchable from a substantially reflecting state in response to a control signal from the controlling means to reflect light onto the light receiver.

20. A system according to Claim 18, wherein the stimulation means includes a reflective element physically moveable into the light beam path to reflect light onto the light receiver, thereby, again simulating the effectos of smoke.

21. A system according to Claim 20, wherein the reflective element is a shaft, a part of whose surface is reflective, rotatable, e.g. by an electric motor, within the sleeve having a window in its side wall to bring the reflective surface of the shaft behind that window.

22. A system according to Claim 1, wherein the or each detector device comprises a heat detector device having a heat sensitive element exposed to ambient air, and circuit means connected to the heat sensitive element for monitoring its output and providing an alarm signal in accordance with a predetermined temperature level and/or rate of change, being sensed.

23. A system according to Claim 21, wherein the stimulation means comprises a heating element disposed near the heat sensitive element, the stimulation means being operable to energise the heating element in response to a control signal from the controlling means so as to simulate the fire hazard.

24. A fire hazard system adapted to operate substantially as herein before described with reference to Figure 1, including a smoke detector device constructed and arranged substantially as hereinbefore described with reference to Figure 2, 3, 4 or 5 and/or a heat detecting device as hereinbefore described with reference to Figure 6 or 7.