Signalling device and monitoring system

Abstract

 A signalling device, such as a fire detecting device, has a head (2) which is removable from a base (1). The head has circuit means (4,5,6) for sensing and signalling a change in given condition. The base is connected to supply lines and has fault signalling means (3) for generating a fault signal due to head removal or some other device fault. As the same signal is generated in each case, the invention provides an economic, simple and reliable solution to the problem of signalling fault conditions. Preferably, the fault signalling means includes a first circuit (3) in the base (1) which responds to an output from a second circuit (7) in the head (2) so that, if the head is not fitted to the base, the first circuit (3) responds to the absence of said output from the second circuit (7) in order to apply the fault signal to the lines.

Description

[0001] This invention relates to a signalling device and to a monitoring system. The signalling device has a head which is removable from a base and circuit means for applying a device fault signal to power supply lines.

[0002] The invention can be used in the field of fire detection where, for example, fire detectors (i.e. signalling devices) are placed in different locations in a fire detecting system (i.e. monitoring system) for detecting a fire, thereby causing an alarm signal to be given. The invention can be used, for example, to generate a device fault signal when the head of a fire detector has been removed from its base, and not refitted after maintenance. Alternatively, the signalling device may be an alarm device having a sounder head which can be removed from a base, the invention similarly being used to generate a device fault signal due to head removal. The term "signalling device" is therefore used broadly to cover any kind of unit which can be used for signalling and where some function may be impaired by head removal. Although the invention is particularly useful in the field of fire detection, references to such use are not to be construed as limiting.

[0003] Where a fire alarm system is used to protect life or property, national regulations or codes of practice usually require the supply lines connecting the detection and alarm devices to a CCU (Central Control Unit) to be monitored for supply line faults, e.g. open circuit and short circuit conditions and for these to be indicated and a supply line fault warning given at the CCU.

[0004] Fault signalling is used in a minority of fire systems, most notably those installed in France, and in such systems a detecting device signals a detector fault by switching either a near short circuit, or an impedance about twice that used to signal detection of fire, across the supply lines. A disadvantage of the short circuit fault signal is that detectors which are not faulty are rendered inoperative while a fault is being signalled by a faulty detector. A disadvantage of the other method of fault signalling is that if two faulty detectors are present the control unit interprets the combined signals as a fire alarm. In other systems, such as the one described in GB 2178878 and generally known as analogue addressable fire detection systems, a digital communications protocol is used to transmit fire and fault signals from detector to control unit, but such systems require more complex and more expensive electronic circuits in detectors and control unit than in the conventional, i.e. non addressable, systems.

[0005] In our copending UK application No. 9810900.2, to which reference is made for further details, a "head removal signal" is generated in the form of a pulsating signal. This device fault signal represents only that a head is not fitted to its base.

[0006] The present invention seeks to provide a signalling device which reveals a device fault that could be due to some problem with the circuitry of the device, as well as when a head is not fitted to its base, and to provide a cost-effective and reliable solution.

[0007] The present invention provides a signalling device in which a head is removable from a base, and in which:

the head has circuit means for sensing and signalling a change in given condition and for supplying detection signals to the base (when the head is fitted thereto),

the base has device fault signalling means for supplying a device fault signal in the event of either the absence of the head on the base, or some other fault which would impair the function of the signalling device, the base also has terminals for connection to supply lines to which the detection signal and/or the device fault signal are applied; the arrangement being such that:

(i) when the head is fitted to the base and no said other fault is present, no device fault signal is supplied to the supply lines, but

(ii) when the head is removed from the base, or when said other fault is present, the device fault signal is supplied to the supply lines.

[0008] Since only one device fault signal is generated, either for head removal or some other fault, the invention provides an economic, simple and reliable solution. It is of no consequence that either is indicated by the same device fault signal, because as long as the device fault signal is present, inspection of the system would be necessary before it is put back into use and this would reveal (inter alia) a missing head.

[0009] The "other fault", which is not head removal, could be a predetermined circuitry fault occurring in the circuitry of the signalling device. For example, in a fire detecting device, the device fault signalling means may include a microprocessor which monitors the operation of the circuitry which senses a fire and supplies a fire detection signal to the supply lines. A device fault signal would be generated for any condition that has, or could degrade the performance of the fire detection device. For example, a condition where a compensation limit has been reached in a detector which compensates for long term drift in the output of its fire sensor. Other examples of a device fault include failure of a voltage regulator supplying an ionisation chamber in an ionisation smoke detector; open or short circuit of a temperature sensitive device, such as a thermistor in a heat detector, and stopping of an oscillator that times the application of pulses to an LED in a photoelectric smoke detector. Known circuit means may be included for monitoring and or responding to the device fault so as to trigger the device fault signal. These means may include a microprocessor.

[0010] Suitably, the device fault signal is not of a value or kind where it would be confused with say, a fire detection signal. The invention can use the pulsating signal, which represents that a head is not fitted to its base, as described in our copending UK Application No. 9810900.2, since this can be easily recognised by, for example, a central control unit (CCU) which continuously monitors the supply line for a fire detection signal. The fire detection signal may be one where the detecting device changes from a high to a low line impedance state, so that the current drop on the lines can be detected. Suitably, the device fault signal is also selected so as not to impose any serious current drain on the supply lines, nor otherwise impair the performance of other signalling devices connected to the same lines. The invention does not essentially require a pulsating signal, since another kind of recognisable signal could be used instead. The invention generally makes use of the same signal to indicate either head removal, or other device fault.

[0011] A monitoring system includes a plurality of the signalling devices according to the invention and monitoring means for monitoring the latter devices for responding to the detection or fault signal. The monitoring means may be part of a CCU, or an end-of-line device (EOL), or some other means.

[0012] Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Fig.1 is a block diagram of a fire detecting device with a base and a detachable head;

Fig.2 is a more detailed circuit diagram of the base; and

Fig.3 is a more detailed circuit diagram of the head.

[0013] Generally speaking, a centrally controlled fire alarm system usually comprises a "central control unit" (or CCU) for monitoring different groups of fire detecting devices located in different parts of a building. Each group of fire detecting devices can be connected across a common pair of power supply lines which are linked back to the CCU which normally applies say 12 volts to the lines to operate the detecting devices. Alarm devices, which are triggered by a higher voltage, can be connected across the same pair of common supply lines and the CCU can respond to a "fire detection signal", signalled on the supply lines, so that CCU then causes say 24 volts to be applied the lines so as to operate the alarm devices to give "alarm signals". This system avoids an excessive amount of wiring compared with a system where detecting devices and alarm devices are connected to respective dedicated supply lines. When such a two-wire system is in a standby condition, the supply voltage is below a threshold of, for example, 18 volts, and each fire detection device has a high line impedance, thereby drawing little or no current from the supply. On detecting a fire, the fire detecting device produces a detection signal by changing from a high line impedance to a low line impedance, for example, by switching a known resistance across the supply lines. The detection signal is detected by a control unit which then applies the higher voltage (24v) to the lines. As the threshold voltage is then exceeded, the alarm devices are activated to produce warning signals. In order to prevent a large current drain at the higher voltage (24v), the detecting devices include means for limiting the current they draw from the lines. Such a system is disclosed in our copending UK Application No. 9808094.8 to which reference may be made for further details. Alternatively or additionally, an alarm device may produce a warning signal when the polarity of the supply is reversed.

[0014] Referring to Fig.1, a fire detecting device comprises a mounting base (1) with terminals L1 IN and L2 for connection to supply lines and a detachable detector head (2) . The base contains signalling circuitry (3) as described in co-pending patent application GB 9810900.2. When the detector head is removed, the circuitry (3) senses the absence of an input signal on terminal L1 OUT. In the absence of an input signal the circuitry (3) periodically applies a predetermined impedance across supply lines L1 IN and L2 thereby signalling a fault condition (head removed) to a central control unit. The central control unit detects the fault condition signal either directly or via an end of line device as disclosed in GB 9810900.2.

[0015] The head contains circuitry (4) for sensing a fire, circuitry (5) for processing the output of the fire sensing circuit, and circuitry (6) for generating a fire detection signal (to cause the operation of alarm devices, not shown) when the processing circuitry resolves a fire condition.

[0016] The head also contains signalling circuitry (7) which applies a signal to the head terminal corresponding with base terminal L1 OUT, the signalling circuitry being responsive to the processing circuitry so as to remove the signal from terminal L1 OUT when the processing circuitry resolves a fault condition, thereby making the signalling circuitry (3) in the base responsive to removal of the head and to another fault condition. In both instances the same device fault signal is produced by the base circuitry.

[0017] In a minimal embodiment of the invention, the processing circuitry (5) may comprise threshold-sensing means responsive to the output of the fire sensing circuitry (4). In preferred embodiments of the invention the processing means is a device such as a National Semiconductor COP 8ACC5 microcontroller or a microprocessor.

[0018] In a further embodiments of the invention the processing circuitry (5) or fault signalling circuitry (7) or both are located in the base. The output of the signalling circuitry (7) is arranged by logic or switching circuits (not shown, but which can be of known construction) to cause operation of signalling circuitry (3) when a fault is resolved and the head is still applying a signal to terminal L1 OUT.

[0019] Preferably, the detector is provided with an indicator connected to switching means responsive to processing circuitry (5) such that in the event of the processing means resolving a fault condition, the indicator changes state or produces a coded signal whereby the detector that has developed a fault can be more easily identified.

[0020] The signalling circuitry (3) in the base, preferably includes means for applying a pulsating signal to the terminals L1 IN and L2 when a device fault condition exists; the signalling means also being operative, when another pulsating signal is already on the supply lines, substantially to preserve the waveform of the pulsating signal on the supply lines, whereby the pulse waveform is independent of the number of heads removed from respective bases. This is described in our copending UK Application No. 9810900.2.

[0021] According to another embodiment of the invention, signalling circuitry (3) in the base includes additional means for inhibiting a pulsating signal (which is the device fault signal) when a voltage applied to the detection device either exceeds a predetermined threshold, or changes in polarity or both.

[0022] Figure 2 shows a diagram of signalling circuitry (3). A full description of the operation of the circuit is given in GB 9810900.2. In embodiments of this invention the pulsing output of this circuit is inhibited when a voltage close to the negative of the rectified supply is imposed on terminal L1 OUT (pulled down).

[0023] Figure 3 shows in detail the circuit arrangement in the head of a detecting device according to the invention. Terminal L1 OUT shown in this Figure engages with base terminal L1 OUT. In the absence of a fault, the output designated FAULT of the processing circuit (5) is arranged to be high so as to turn-on the open collector transistor (8) thereby pulling terminal L1 OUT negative (active pull down) when no fault is present. When a fault is resolved, transistor (8) is turned off thus removing the negative signal from L1 OUT and thereby causing the signalling circuit in the base to signal a fault. The circuit arrangement including transistors (9) and (10) and indicator (11) show simple means for visibly indicating that a detector is faulty. More complex fault indication means may be used e.g. means in which a single indicator flashes when the detector is operating normally, stops flashing or changes frequency or changes colour when the detector is faulty, and is on continuously when a fire is detected.

Claims

1. A signalling device in which a head (2) is removable from a base (1), and in which:

the head has circuit means for sensing and signalling a change in given condition and for supplying detection signals to the base (when the head is fitted thereto),

the base has device fault signalling means (3) for supplying a device fault signal in the event of either the absence of the head (2) on the base (1), or some other fault which would impair the function of the signalling device, the base also has terminals (L1,L2) for connection to supply lines to which the detection signal and/or the device fault signal are applied; the arrangement being such that:

(i) when the head (2) is fitted to the base (1) and no said other fault is present, no device fault signal is supplied to the supply lines, but

(ii) when the head (2) is removed from the base (1), or when said other fault is present, the device fault signal is supplied to the supply lines.

2. A signalling device according to claim 1, in which the fault signalling means includes a first circuit (3) in the base (1) which responds to an output from a second circuit (7) in the head (2) so that, if the head is not fitted to the base, the first circuit (3) responds to the absence of said output from the second circuit (7) in order to apply the device fault signal to the lines.

3. A signalling device according to claim 1 or 2, in which the circuit means (4,5,6) for sensing and signalling a change in given condition is also capable of detecting a predetermined fault in the circuitry of the signalling device in order to apply the device fault signal to the first circuit (7).

4. A signalling device according to any preceding claim, in which the circuit means (4,5,6) for sensing and signalling a change in given condition includes threshold sensing means (5), which respond to an output of a sensing circuit (4) above a predetermined threshold, due to the change in the given condition, for driving a third circuit (6) so as to apply a detection signal to the supply lines.

5. A signalling device according to any preceding claim, in which the base (1) includes processing circuitry (5), for causing the device fault signal to be applied to the lines whilst the fault signalling means (3) is in a condition which normally represents the head fitted to the base, but there is some other device fault.

6. A signalling device according to any preceding claim further including indicator means which provides a warning that the device fault signal has been supplied to the lines, whereby the faulty signalling device can be identified.

7. A signalling device according to any preceding claim wherein the device fault signal is suppressed if the line voltage exceeds a predetermined threshold, or changes in polarity.

8. A signalling device according to any preceding claim wherein the device fault represents that a compensation limit has been reached in compensating means for compensating for drift in means (4) for sensing the change in said given condition.

9. A monitoring system includes a plurality of the signalling devices according to any preceding claim and monitoring means for monitoring the latter devices for responding to the detection or fault signal.

10. A monitoring system according to claim 9, wherein the monitoring means is part of a CCU, or an end-of-line device (EOL).

Drawing