GB1558471A - Fire detectors - Google Patents

Description

(54) FIRE DETECTORS (71) We, CHUBB FIRE SECURITY LIMITED, a British company, of Pyrene House, Sunburyon-Thames, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The problem in setting the response point of a fire detector is that if the detector is set to respond to conditions corresponding to an early stage in the development of a fire, it will give too many false alanns because the same conditions may arise from causes other than an outbreak of fire; and if the response point is adjusted to reduce the number of false alarms, the provision of an alarm signal in the case of a fire will be delayed.Examples of conditions which may unintentionally simulate fire conditions and cause a fire detector to give a false alarm are heavy concentrations of tobacco smoke in the neighbourhood of a smoke detector; and the moving of a portable heating appliance to a position under a heat detector.

To solve this problem, it has been proposed to use combinations of detectors, for example to provide two detectors of the same type in the same space to be protected, the composite alarm cicuit giving an alarm signal only when both detectors give an alarm output. Alternatively, the two detectors may be of different kinds, i.e., responsive to different phenomena associated with the outbreak of fire. In each case, the provision of two detectors permits each to be set to respond to conditions corresponding to an earlier stage in the development of a fire than would otherwise be possible.

In one prior proposal, a third detector was provided in the same space to be protected, this third detector being set to respond to conditions representing a more advanced state of development of a fire than the first two detectors, and an alarm signal was given if this third detector gave a fire-representing output, irrespective of the state of the first two detectors.

A system according to the present invention comprises fire detecting means giving two output signals, each corresponding to a different variable, each of the said variables being such that it changes in value with the onset and development of a fire and such that each of the said signals from the detecting means passes through a low-level value and rises to a high level value as the fire develops, and an alarm circuit connected to receive the said output signals and adapted to provide an alarm output (a) in the presence of a predetermined high level of either of the said output signals from the detecting means, and (b) in the presence of predetermined low levels for both of the said output signals.The fire detecting means may provide more than two output signals, each varying with a different parameter, the alarm circuit being adapted to provide an alarm output (a) in the presence of a predetermined high level from any of the output signals and (b) in the presence of predetermined low levels of more than one of the output signals.

A system embodying the present invention thus reduces false alarms due to each of two causes. In the first place, because signals due to conditions of smoke or heat, for example, which do not result from a fire (such as the tobacco smoke concentration or local heat source referred to above) are usually of a low value and in any case rarely occur in combination, the system will not usually generate an alarm signal in such cases. In the second place, whilst with single-signal alarm systems, the setting of the alarm system at too sensitive a point causes false alarms due to equipment variations, in the multi-signal system embodying the present invention equipment variations producing low-level signal modifications are required to affect two or more signals before an alarm is given, thereby permitting advantage to be taken of higher sensitivity settings.

It will be appreciated that the terms "low" and "high" do not necessarily indicate larger and smaller numerical values of an output signal but always represent less advanced and more advanced stages in the development of a fire. In other words, a low level signal is intended to indicate a signal obtained in conditions which typically arise at an early stage in the development of a fire and a high level signal for the same parameter is intended to indicate conditions which arise at a later stage in the development of the fire.

As an example, the fire detecting means may incorporate a smoke detector and a heatdetector.

If desired, a single detector can be used for both purposes, in combination with a circuit for recognising a characteristic in the output from the single detector which arises from smoke conditions, for example, and a circuit for recognising in the output of the detector a characteristic which arises from conditions of excessive heat, for example.

In order that the invention may be better understood, one example will now be described with reference to the accompanying drawing.

In the drawing, a detector 10 is shown as providing two output signals, a heat signal on line 11 and a smoke signal on line 12. The detector 10 may, for example, be of the kind described in our British patent specification No. 1,405.615..

The heat level signal is connected to the first input of each of two comparators 13 and 14. The second input of comparator 13 is derived from the wiper of a potentiometer 15 and the second input of the comparator 14 is derived from the wiper of a potentiometer 16.

The positions of these wipers are such that the comparator 13 acts as a high level heat comparator and the comparator 14 as a low level heat comparator.

In a similar way, the smoke signal is applied to the first inputs of each of two comparators 17 and 18, which derive their second inputs respectively from potentiometers 19 and 20, the settings of which are such that the comparator 13 acts as a low level smoke comparator and the comparator 18 as a high level smoke comparator.

The four comparator circuits are arranged to give "1" outputs in the presence of alarm conditions. Signals from the two low-level comparators 14 and 17 are applied to a dual input NAND gate which produces an "O" output in the presence of a "1" on both of its inputs. The output from the NAND gate 21 passes through an inverter stage 22, constituted by a 2-input NAND gate with inputs coupled, to a 3-input NOR gate 23. The other two inputs of the NOR gate 23 are connected directly to the outputs of the high level comparators 13 and 18 respectively.

In the presence of an alarm signal from either of the high level comparators or from both of the low level comparators, the NOR gate 23 acts through a transistor inverter circuit 24 to energise a relay 25 and thereby to generate an alarm signal. A diode D1 and a resistor RL permit latching of the output.

Thus, the invention is concerned with a fire detection system including fire detecting means giving a number of output signals, two or more of these corresponding to different variables. However, in such a system, the fire detecting means may additionally provide one or more additional output signals for one or more of these variables, if desired. As an example, the fire detecting means might include first and second temperature detectors and a smoke detector and the alarm circuit would give an alarm indication in the presence of a high level value of the corresponding signal from any of these three detectors.

and also in the presence of low level signals from any two detectors representing different variables.

In some cases it may be advantageous to provide one reference signal whose value corresponds to a rate of rise of a detector output signal. The rate of rise of the detector output signal may be derived from a feedback circuit of an amplifier receiving this signal, as in our British patent No. 1,405,615. The use of such a rate-of-rise reference assists in distinguishing abnormal environmental conditions from true alarm conditions and in permitting the safe use of detectors with higher sensitivity.

WHAT WE CLAIM IS: 1. A fire detection system comprising: fire detecting means giving two output signals each corresponding to a different variable, each of the said variables being such that it changes in value with the onset and development of a fire and such that each of the said signals from the detecting means passes through a low-level value and rises to a high level value as the fire develops, and an alarm circuit connected to receive the said output signals and adapted to provide an alarm output (a) in the presence of a predetermined high level of either of the said output signals from the detecting means, and (b) in the presence of predetermined low levels for both of the said output signals.

2. A fire detection system in accordance with claim 1, in which the fire detecting means provides more than two output signals, each corresponding to a different variable, each of the said variables being such that it changes in value at the onset and development of a fire and such that each of the said signals from the detecting means passes through a low level value and rises to a high level value as the fire develops, and in which the alarm circuit is adapted to provide an alarm output (a) in the presence of a predetermined high level from any of the said output signals and (b) in the presence of predetermined low levels of more than one of the said output signals.

3. A system in accordance with claim 2, in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. fire. In other words, a low level signal is intended to indicate a signal obtained in conditions which typically arise at an early stage in the development of a fire and a high level signal for the same parameter is intended to indicate conditions which arise at a later stage in the development of the fire. As an example, the fire detecting means may incorporate a smoke detector and a heatdetector. If desired, a single detector can be used for both purposes, in combination with a circuit for recognising a characteristic in the output from the single detector which arises from smoke conditions, for example, and a circuit for recognising in the output of the detector a characteristic which arises from conditions of excessive heat, for example. In order that the invention may be better understood, one example will now be described with reference to the accompanying drawing. In the drawing, a detector 10 is shown as providing two output signals, a heat signal on line 11 and a smoke signal on line 12. The detector 10 may, for example, be of the kind described in our British patent specification No. 1,405.615.. The heat level signal is connected to the first input of each of two comparators 13 and 14. The second input of comparator 13 is derived from the wiper of a potentiometer 15 and the second input of the comparator 14 is derived from the wiper of a potentiometer 16. The positions of these wipers are such that the comparator 13 acts as a high level heat comparator and the comparator 14 as a low level heat comparator. In a similar way, the smoke signal is applied to the first inputs of each of two comparators 17 and 18, which derive their second inputs respectively from potentiometers 19 and 20, the settings of which are such that the comparator 13 acts as a low level smoke comparator and the comparator 18 as a high level smoke comparator. The four comparator circuits are arranged to give "1" outputs in the presence of alarm conditions. Signals from the two low-level comparators 14 and 17 are applied to a dual input NAND gate which produces an "O" output in the presence of a "1" on both of its inputs. The output from the NAND gate 21 passes through an inverter stage 22, constituted by a 2-input NAND gate with inputs coupled, to a 3-input NOR gate 23. The other two inputs of the NOR gate 23 are connected directly to the outputs of the high level comparators 13 and 18 respectively. In the presence of an alarm signal from either of the high level comparators or from both of the low level comparators, the NOR gate 23 acts through a transistor inverter circuit 24 to energise a relay 25 and thereby to generate an alarm signal. A diode D1 and a resistor RL permit latching of the output. Thus, the invention is concerned with a fire detection system including fire detecting means giving a number of output signals, two or more of these corresponding to different variables. However, in such a system, the fire detecting means may additionally provide one or more additional output signals for one or more of these variables, if desired. As an example, the fire detecting means might include first and second temperature detectors and a smoke detector and the alarm circuit would give an alarm indication in the presence of a high level value of the corresponding signal from any of these three detectors. and also in the presence of low level signals from any two detectors representing different variables. In some cases it may be advantageous to provide one reference signal whose value corresponds to a rate of rise of a detector output signal. The rate of rise of the detector output signal may be derived from a feedback circuit of an amplifier receiving this signal, as in our British patent No. 1,405,615. The use of such a rate-of-rise reference assists in distinguishing abnormal environmental conditions from true alarm conditions and in permitting the safe use of detectors with higher sensitivity. WHAT WE CLAIM IS:

1. A fire detection system comprising: fire detecting means giving two output signals each corresponding to a different variable, each of the said variables being such that it changes in value with the onset and development of a fire and such that each of the said signals from the detecting means passes through a low-level value and rises to a high level value as the fire develops, and an alarm circuit connected to receive the said output signals and adapted to provide an alarm output (a) in the presence of a predetermined high level of either of the said output signals from the detecting means, and (b) in the presence of predetermined low levels for both of the said output signals.

2. A fire detection system in accordance with claim 1, in which the fire detecting means provides more than two output signals, each corresponding to a different variable, each of the said variables being such that it changes in value at the onset and development of a fire and such that each of the said signals from the detecting means passes through a low level value and rises to a high level value as the fire develops, and in which the alarm circuit is adapted to provide an alarm output (a) in the presence of a predetermined high level from any of the said output signals and (b) in the presence of predetermined low levels of more than one of the said output signals.

3. A system in accordance with claim 2, in

which a first of the output signals from the fire detecting means varies with smoke conditions and a second of the said signals from the fire detecting means varies with heat.

4. A system in accordance with claim 3, in which the smoke and heat signals are derived from the output of a single detector.

5. A system in accordance with any one of the claims 1-4, in which each detector output is compared with a reference value by a triggering circuit providing a first output signal value or a second output signal value according to the result of the comparison, the said output signal values from the comparators being selectively applied to NAND and NOR gates, the outputs of which control the alarm ciruit.

6. A fire detection system, substantially as herein described with reference to the accompanying drawing.