GB2309109A

GB2309109A - Abnormal condition detecting unit and a circuit comprising a plurality of such units

Info

Publication number: GB2309109A
Application number: GB9700318A
Authority: GB
Inventors: William Alexander Heffron
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-01-10
Filing date: 1997-01-09
Publication date: 1997-07-16
Anticipated expiration: 2017-01-09
Also published as: GB2309109B; IES73218B2; GB9700318D0; IE970008A1

Abstract

A detecting circuit 1 comprises a plurality of detecting units 4 which operate under the control of a control unit 2 and monitor for the presence of a toxic gas. The detecting units are connected in series on a monitoring wire 10 and a test wire 11, and each detecting unit comprises a first switch 43 which is normally closed, and opens on a gas sensor 40 of the detecting unit sensing the presence of toxic gas. This causes an open circuit condition to prevail on the monitoring wire which is detected by the control unit. The detecting units are tested by applying a test signal to the test wire which causes the gas sensor in each unit on receiving the test signal to operate a second switch 45 indicating a functional gas sensor. In the event of the second switch remaining open, a faulty sensor is detected.

Description

"A detecting unit and a detecting circuit comprising a plurality of the detecting units" The present invention relates to a detecting unit for detecting the occurrence of an abnormal condition, and to a detecting circuit comprising a plurality of the detecting units for detecting the occurrence of an abnormal condition at any one or more of a plurality of remote locations, and in particular, the invention relates to a detecting unit for detecting the presence of a toxic gas.

There are many instances where it is essential to be able to detect the occurrence of an abnormal condition, for example, the presence of a toxic gas, and to provided a rapid indication of the presence of such a toxic gas, for example, in the intensive rearing of animals, such as, for example, in the intensive rearing of cattle, which typically, is carried out in slatted floor houses, where urine and faeces from the animal is collected in a slurry pit beneath the slatted floor. In such cases, toxic gases emitted from the slurry pit may accumulate in the slatted floor house with detrimental results to the animals. It is therefore important to be able to accurately monitor for the presence of toxic gases, for example, hydrogen-sulphide gas, at a number of different locations in the slatted house.It is also important that the presence of such gases should be detected rapidly, and on their detection, an indication of the presence of the gases should be rapidly given. Monitoring for such toxic gases is, in general, carried out using a plurality of gas detectors and associated electronic circuitry located at suitable locations within the animal house. The environment in such slatted houses is particularly harsh, and corrosive, and can be detrimental to electronic and electrical circuitry and gas detection apparatus. It is therefore also important to be able to regularly test any such electrical and electronic circuitry and apparatus to ensure that the circuitry and apparatus is functional.

There is therefore a need for a detecting unit and a detecting circuit for monitoring for the presence of a toxic gas, and also for monitoring for the occurrence of any abnormal condition which overcomes the problems of known detecting units and detecting circuits.

The present invention is directed towards providing such a detecting unit and a detecting circuit.

According to the invention, there is provided a detecting unit for detecting an abnormal condition, the detecting unit comprising a sensing means for detecting the abnormal condition and for outputting an alarm signal on detection of the abnormal condition, an input monitoring terminal and an output monitoring terminal for connecting the detecting unit in series with other detecting units in a one-wire monitoring circuit for monitoring the status of the detecting unit, an input test terminal and an output test terminal for connecting the detecting unit in series with the other detecting units in a one-wire test circuit for testing the detecting unit, a supply voltage terminal and a ground voltage terminal for connecting the detecting unit to a supply voltage rail and a ground voltage rail, respectively, an activating circuit connected to the input test terminal, the activating circuit being responsive to a signal on the input test terminal for activating the sensing means for outputting an alarm signal, a normally closed first switch means connecting the input and output monitoring terminals, the first switch means being responsive to an alarm signal from the sensing means for opening the first switch means so that on the first switch means being opened, an open circuit condition prevails in the monitoring circuit for indicating the detection of an abnormal condition by the detecting unit, and a normally open second switch means connecting the input and output test terminals, the second switch means being responsive to an alarm signal from the sensing means for closing the second switch means so that on the second switch means being closed a test signal on the input test terminal of the detecting unit is connected through the second switch means to the output terminal of the detecting unit for relaying through the test circuit to the next detecting unit in the test circuit for testing thereof.

In one embodiment of the invention, a first indicating means for indicating the detection of an abnormal occurrence by the sensing means is provided, the first indicating means being responsive to an alarm signal from the sensing means.

In another embodiment of the invention, a second indicating means for indicating the presence of a test signal on the input test terminal is provided, the second indicating means being responsive to a test signal being applied to the input test terminal.

Preferably, each indicating means comprises a visual indicating means. Advantageously, each visual indicating means comprises a light emitting diode.

In one embodiment of the invention, the light emitting diode of the second indicating means is connected between the test input terminal and one of the supply voltage terminal and the ground terminal.

In another embodiment of the invention, the light emitting diode of the second indicating means is connected between the input test terminal and the ground terminal.

Preferably, the first and second switch means comprise respective relay contacts of respective poles of a double pole relay. Advantageously, the double pole relay is a double pole changeover relay which is configured as a double pole double throw switch in which the contacts of the poles are in respective opposite states at the same time. Preferably, the relay is powered by a supply voltage received on the voltage supply terminal and the ground terminal.

In one embodiment of the invention, the light emitting diode of the first indicating means is connected in series with the activating coil of the relay.

In another embodiment of the invention, the relay is operated under the control of an interface circuit, which is in turn responsive to the sensing means.

Preferably, the activating circuit activates the interface circuit with a simulated alarm signal.

Preferably, the activating circuit activates the sensing means to cause the sensing means to output an alarm signal.

In one embodiment of the invention, the sensing means is a gas sensor.

In another embodiment of the invention, the gas sensor is adapted for sensing a specific gas, and is responsive to sensing the specific gas for outputting an alarm signal.

Preferably, a source of the gas to which the gas sensor is responsive is provided, and the gas source is responsive to the activating circuit for releasing a quantity of the gas to the sensing means for activating the sensing means to output an alarm signal.

Advantageously, the detecting unit comprises a housing, and the sensing means is located within the housing, an inlet and an outlet being provided in the housing for accommodating the passage of air therethrough past the sensing means. Preferably, a fan is located in the housing for drawing air through the housing past the sensing means.

Additionally, the invention provides a detecting circuit for detecting the occurrence of an abnormal condition at any of a plurality of remote locations, the detecting circuit comprising a control unit, and a plurality of detecting units according to the invention located at the respective remote locations and connected to the control unit by a four-wire circuit, the four wire circuit comprising a monitoring wire, a test wire, a voltage supply wire and a ground wire, each wire extending from the control unit, the supply voltage and ground wires being connected to the supply voltage terminals and ground terminals of the respective detecting units, and the detecting units being connected in series by the monitoring wire and the test wire by connecting the respective output monitoring terminal and output test terminal of each detecting unit to the corresponding one of the input monitoring terminal and input test terminal of the next detecting unit so that for so long as no alarm condition is detected, the first switch means of the respective detecting units remain closed, and a closed circuit condition prevails on the monitoring wire, and on a test signal being applied to the test wire, the second switch means of the respective detecting units sequentially close as the test signal is relayed from one detecting unit to the next, on the sensing means of the respective detecting units being functional, thereby causing a closed circuit condition to prevail on the test wire.

In one embodiment of the invention, the monitoring wire from the output monitoring terminal of the last detecting unit on the monitoring circuit most remote from the control unit is connected to one of the supply and ground wires for facilitating monitoring of the first switch means of the respective detecting units by the control unit.

In another embodiment of the invention, the test wire from the output test terminal of the last detecting unit on the test circuit most remote from the control unit is connected to the other of the supply and ground wires for facilitating monitoring of the second switch means of the detecting units on a test signal being applied to the test wire.

Preferably, the monitoring wire is connected to the voltage supply wire at the end detecting unit.

Advantageously, an alarm circuit is provided for activating an alarm in response to the control unit detecting the occurrence of an abnormal condition having been sensed by one of the detecting units in the detecting circuit.

The invention will be more clearly understood from the following description of a preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a detecting circuit according to the invention, Fig. 2 is a circuit diagram of a detecting unit, also according to the invention for use in the detecting circuit of Fig. 1, and Fig. 3 is a schematic view of a housing of the detecting unit of Fig. 2.

Referring to the drawings, there is illustrated a detecting circuit according to the invention indicated generally by the reference numeral 1 for detecting the occurrence of an abnormal condition, in this case the presence of a toxic gas, typically, hydrogen-sulphide gas at a plurality of remote locations, typically at remote locations in a slatted house of the type used for intensive rearing of animals. The detecting circuit 1 comprises a control unit 2, and a plurality of detecting units 4 for mounting at the respective remote locations for detecting the presence of the toxic gas. An alarm circuit 5 operating under the control of the control unit 2 comprises an audible alarm, typically, a siren which is activated on the control unit 2 determining that any one or more of the detecting units 4 have detected the presence of the toxic gas.The main control unit 2 is powered from an AC mains supply 6.

The detecting units 4 are connected to the control unit 2 by a four-wire cable 7 which comprises a voltage supply wire 8, a ground wire 9, a monitoring wire 10 which forms a monitoring circuit and a test wire 11 which forms a test circuit. The voltage supply wire 8 and ground wire 9 extend from the control unit 2 and provide power to the respective detecting units 4. A continuous 12 volt nominal DC supply is applied to the voltage supply wire 8 by the control unit 2. The monitoring wire 10 extends from the control unit 2 and connects the detecting units 4 in series to the control unit 2 for monitoring the status of the detecting units for determining if any one or more of the detecting units 4 have detected the presence of the toxic gas, as will be described below.

The test wire 11 extends from the control unit 2 and connects the detecting units 4 in series to the control unit 2 for facilitating testing of the detecting units 4 by the control unit 2 to establish if a fault exists in any of the detecting units, as will be described below. The monitoring wire 10 terminates at 15 at the end of the last detecting unit 4 in the monitoring circuit 10, most remote from the control unit 2, and is connected to the voltage supply wire 8 by a connection 16 for facilitating monitoring of the detecting units 4 by the control unit 2, as will be described below. The test wire 11 terminates at 17 at the end of the last detecting unit 4 most remote from the control unit 2, and is connected to the ground wire 9 by a connection 18, for facilitating testing of the detecting units 4.

Turning now to the detecting units 4, each detecting unit 4 comprises a housing 20, typically of steel, and defining a hollow interior region 21 for housing components and circuitry of the unit 4 as will be described below. An air inlet 22 is provided to the hollow interior region 21, and an air outlet 23 is provided from the hollow interior region 21 for accommodating air into and out of the hollow interior region 21. A fan 25 located in the hollow interior region 21 draws air to be tested for the presence of the toxic gas through the hollow interior region 21 in the direction of the arrows A for monitoring thereof.

The fan 25 is powered by the 12 volt DC supply across the voltage supply wire 8 and the ground wire 9.

An input monitoring terminal 30 and an output monitoring terminal 31 are provided on each detecting unit 4 for connecting the detecting unit 4 in series in the monitoring wire 10. An input test terminal 32 and an output test terminal 33 are provided for connecting each monitoring unit 4 in series in the test wire 11. A voltage supply terminal 35 and a ground terminal 36 are provided for connecting each detecting unit 4 to the voltage supply wire 8 and the ground wire 9, respectively.

A sensing means, in this case a gas sensor 40 for sensing the presence of hydrogen-sulphide gas is located in the hollow interior region 21 in the path of the air being drawn through the hollow interior region 21 by the fan 25 for detecting the presence of hydrogen-sulphide gas therein. The gas sensor 40 is a typical gas sensor for detecting hydrogen-sulphide gas, and will be well known to those skilled in the art. A sensor support and interface circuit 42 which is located in the hollow interior region 21 and is powered by the 12 volt DC supply from the voltage supply terminal 35 and the ground terminal 36 controls and monitors the gas sensor 40, so that on the gas sensor 40 detecting hydrogen-sulphide gas an alarm signal issues from the sensor support and interface circuit 42. Such sensor support and interface circuits 42 will be well known to those skilled in the art.

A first switch means, namely, a normally closed first switch 43 connects the input monitoring terminal 30 with the output monitoring terminal 31 so that when all the first switches 43 of the detecting units 4 are closed, a closed circuit condition prevails on the monitoring wire 10. A second switch means, namely, a normally open second switch 45 connects the input test terminal 32 to the output test terminal 33. The first switch 43 and the second switch 45 are formed by the contacts of respective poles of a double pole changeover relay 47 which is configured as a modified double pole double throw switch in which the contacts of the poles are in respective opposite states at the same time. In other words, when the first switch 43 is closed, the second switch 45 is open, and vice versa.

Energising coils 48 for operating the respective poles of the relay 47 are powered by the 12 volt supply from the voltage supply terminal 35 and the ground terminal 36.

The relay 47 operates under the control of the interface circuit 42, and is responsive to the alarm signal from the interface circuit 42 which issues in response to the gas sensor 40 detecting hydrogensulphide gas, so that on the detection of hydrogensulphide gas, the first switch 43 is opened and the second switch 45 is closed. On opening of the first switch 43, an open circuit condition prevails on the monitoring wire 10, which is in turn detected by the control unit 2, which activates the alarm circuit 5 for sounding the alarm to indicate the presence of hydrogen-sulphide gas.

A first indicating means, namely, a first light emitting diode 49, is connected in series with the relay coils 48 of the relay 47, so that on energising of the relay coils 48 by the interface circuit 42 in response to the detection of hydrogen-sulphide gas by the gas sensor 40, the first light emitting diode 49 glows for indicating that the detecting unit 4 has detected the presence of hydrogen-sulphide gas.

An activating circuit 50 which is located in the hollow interior region 21 is provided for facilitating regular testing of the gas sensor 40. The activating circuit 50 is powered by the 12 volt DC supply from the voltage supply terminal 35 and the ground terminal 36. The activating circuit 50 is connected to the input test terminal 32, and is responsive to a test signal being applied to the input test terminal 32 for activating the gas sensor 40. A reservoir 53 of hydrogen-sulphide gas is located in the hollow interior region 21 of the housing 20, and a nozzle 54 from the reservoir 53 is directed towards the gas sensor 40 for discharging hydrogen-sulphide gas towards the gas sensor 40 under the control of the activating circuit 50.A valve (not shown) in the nozzle 54 releases a test quantity of gas through the nozzle 54 to the gas sensor 40 under the control of the activating circuit 50 in response to a test signal appearing on the input test terminal 32. On the gas sensor 40 detecting the hydrogen-sulphide gas from the nozzle 54, the interface circuit 42 outputs an alarm signal for activating the relay 47 as though hydrogensulphide gas in the stream of air being drawn through the housing 20 had been detected.

A second indicating means, namely, a second light emitting diode 55 connected between the input test terminal 32 and the ground terminal 36 gives a visual indication of the presence of a test signal on the input test terminal 32.

In use, in normal operation, the 12 volt DC supply is continuously applied to the voltage supply wire 8, and the ground wire 9 is continuously held at earth or zero potential. For so long as none of the gas sensors 40 detect the presence of hydrogen-sulphide gas, the relays 47 in the detecting units 4 are in the state shown in Fig. 2. In other words, the first switch 43 is closed, and the second switch 45 is open.

Accordingly, a closed circuit condition prevails on the monitoring wire 10 which is connected to the voltage supply wire 8 at 15. Thus, the control unit 2 detects the 12 volt DC supply on the monitoring wire 10. On any one of the detecting units 4 detecting the presence of hydrogen-sulphide gas in the airstream passing through the detecting unit 4, the interface circuit 42 in response to the gas sensor 40 of that unit 4 detecting hydrogen-sulphide gas activates the relay 47 for changing the first and second switches 43 and 45. Thus, on the first switch 43 opening, an open circuit condition prevails on the monitoring wire 10, and the supply voltage from the supply wire 8 is no longer available on the monitoring wire 10 at the control unit 2. Thus, the control unit 2 determines detection of hydrogen-sulphide gas by one of the detecting units 4. The control unit 2 delivers a signal to the alarm circuit 5 for activating the audible alarm. For so long as hydrogen-sulphide gas is detected by the gas sensor 40, the interface circuit 42 continues to power the coils 48 of the relay 47, thereby maintaining the first switch 43 open and the second switch 45 closed. The first light emitting diode 49 for so long as power is applied to the coils 48 of the relay 47 continues to glow, thereby giving a visual indication of the detecting unit 4 which has detected the hydrogen-sulphide gas.

To test the detecting units 4, the following procedure is employed. During testing of the detecting units 4, the 12 volt supply is continuously applied to the voltage supply wire 8 and the ground wire 9 is continuously held at zero volts for powering the detecting units 4 during the test. A test signal, typically, a 12 volt DC signal is applied by the control unit 2 to the test wire 11. On the first detecting unit 4, in other words, the detecting unit 4 first in test wire 11 from the control unit 2 receiving the test signal on its input test terminal 32, the second light emitting diode 55 commences to glow, thereby indicating the fact that the test signal has been applied to the input test terminal 32 of the first of the detecting units 4.The test signal on the first of the detecting units 4 activates the activating circuit for releasing a test quantity of hydrogen-sulphide gas from the reservoir 53, which if the gas sensor 40 is functional, is detected by the gas sensor 40. Detection of the hydrogen-sulphide gas by the gas sensor 40 causes the interface circuit 42 to activate the relay 47 for opening the first switch 43 and closing the second 45. On closing of the second switch 45, the test signal is applied to the output test terminal 33 of the first unit 4, and in turn the test signal is applied to the input test terminal 32 of the next of the detecting units 4 on the test wire 11.The test signal thus causes the second light emitting diode 55 in that next detecting unit 4 to glow, thus indicating that the test signal has passed through the previous detecting unit 4, and confirming that the gas sensor 40 and the other components of the previous detecting unit 4 are functional. The test signal thus causes the activating circuit in the detecting unit 4 which has just received the test signal to release a test quantity of hydrogen-sulphide gas thereby causing the interface circuit 42 to activate the relay 47 for opening the first switch 43 and closing the second switch 45 provided both the gas sensor 40 and the interface circuit 42 are functional. In this way, the detecting units are sequentially tested, and as each detecting unit 4 receives the test signal, its second light emitting diode 55 glows, indicating that the previous detecting unit 4 is functional.In the event of a faulty detecting unit, the second switch 45 of such a faulty detecting unit 4 remains open, and accordingly, the test signal fails to travel beyond that detecting unit. Thus, the faulty detecting unit 4 is the last of the detecting units on the test wire from the control unit 2 having its second light emitting diode 55 glowing.

On all the detecting units 4 being functional, the test signal appears on the ground wire 9, and is detected by the control unit 2, thus indicating all detecting units 4 are functional.

A particularly important advantage of the detecting circuit according to the invention is that a single four wire cable can be run over relatively long distances, typically, over several hundred metres for connecting a relatively large number of detecting units, and the single four wire cable can power all the detecting units, monitor their respective status, and test for faulty detecting units without the need for any additional cables or circuitry.

It will be appreciated that the sensing means may be provided by any other type of sensing means besides a gas sensor. The type of sensing means used in each detecting unit will depend on the type of abnormal condition to be monitored. It will be appreciated that a detecting circuit may comprise a number of detecting units for detecting different types of abnormal conditions. In which case, the respective detecting units would comprise the appropriate sensing means suitable for monitoring for the relevant abnormal condition. Indeed, it is envisaged that in certain cases all the detecting units may have different types of sensing means for detecting different types of abnormal conditions.

It will also be appreciated that other gas sensors for sensing gases other than hydrogen-sulphide gas may be provided, depending on the type of gas the presence of which is to be detected.

It is also envisaged that instead of providing a reservoir of the gas being monitored by the gas sensor in the housing of each detecting unit, the test circuit may be adapted for activating the sensor support and interface circuit. In which case, the gas sensor itself would not be tested during a test.

It will be appreciated that other suitable switch means besides the contacts of the poles of a relay may be provided.

It is envisaged that in certain cases the fan may be omitted.

While the supply voltage has been described as being a 12 volt DC supply, any suitable supply voltage may be applied to the voltage supply wire, for example, a higher or lower voltage, and indeed, it is envisaged in certain cases that a 24 volt nominal DC supply may be used. Indeed, it is envisaged that the voltage supply may vary over a range of 10 volts to 25 volts.

While the first light emitting diode has been described as glowing when hydrogen-sulphide gas has been detected, it is envisaged that the sensor support and interface circuit and the first light emitting diode may be arranged to glow continuously during normal operating conditions, and not to glow when gas has been detected, and obviously when no power is available at the detecting unit. In which case, the relay coil would be energised under normal conditions, thus, holding the first switch closed and the second switch open during normal conditions, and on hydrogensulphide gas being detected, the relay coil would be de-energised by the sensor support and interface circuit, thus causing the first switch to open and the second switch to close.

Claims

1. A detecting unit for detecting an abnormal condition, the detecting unit comprising a sensing means for detecting the abnormal condition and for outputting an alarm signal on detection of the abnormal condition, an input monitoring terminal and an output monitoring terminal for connecting the detecting unit in series with other detecting units in a one-wire monitoring circuit for monitoring the status of the detecting unit, an input test terminal and an output test terminal for connecting the detecting unit in series with the other detecting units in a one-wire test circuit for testing the detecting unit, a supply voltage terminal and a ground voltage terminal for connecting the detecting unit to a supply voltage rail and a ground voltage rail, respectively, an activating circuit connected to the input test terminal, the activating circuit being responsive to a signal on the input test terminal for activating the sensing means for outputting an alarm signal, a normally closed first switch means connecting the input and output monitoring terminals, the first switch means being responsive to an alarm signal from the sensing means for opening the first switch means so that on the first switch means being opened, an open circuit condition prevails in the monitoring circuit for indicating the detection of an abnormal condition by the detecting unit, and a normally open second switch means connecting the input and output test terminals, the second switch means being responsive to an alarm signal from the sensing means for closing the second switch means so that on the second switch means being closed a test signal on the input test terminal of the detecting unit is connected through the second switch means to the output terminal of the detecting unit for relaying through the test circuit to the next detecting unit in the test circuit for testing thereof.

2. A detecting unit as claimed in Claim 1 in which a first indicating means for indicating the detection of an abnormal occurrence by the sensing means is provided, the first indicating means being responsive to an alarm signal from the sensing means.

3. A detecting unit as claimed in Claim 1 or 2 in which a second indicating means for indicating the presence of a test signal on the input test terminal is provided, the second indicating means being responsive to a test signal being applied to the input test terminal.

4. A detecting unit as claimed in Claim 2 or 3 in which each indicating means comprises a visual indicating means.

5. A detecting unit as claimed in Claim 4 in which each visual indicating means comprises a light emitting diode.

6. A detecting unit as claimed in Claim 5 in which the light emitting diode of the second indicating means is connected between the test input terminal and one of the supply voltage terminal and the ground terminal.

7. A detecting unit as claimed in Claim 6 in which the light emitting diode of the second indicating means is connected between the input test terminal and the ground terminal.

8. A detecting unit as claimed in any preceding claim in which the first and second switch means comprise respective relay contacts of respective poles of a double pole relay.

9. A detecting unit as claimed in Claim 8 in which the double pole relay is a double pole changeover relay which is configured as a double pole double throw switch in which the contacts of the poles are in respective opposite states at the same time.

10. A detecting unit as claimed in Claim 8 or 9 in which the relay is powered by a supply voltage received on the voltage supply terminal and the ground terminal.

11. A detecting unit as claimed in any of Claims 8 to 10 in which the light emitting diode of the first indicating means is connected in series with the activating coil of the relay.

12. A detecting unit as claimed in any of Claims 8 to 11 in which the relay is operated under the control of an interface circuit, which is in turn responsive to the sensing means.

13. A detecting unit as claimed in Claim 12 in which the activating circuit activates the interface circuit with a simulated alarm signal.

14. A detecting unit as claimed in any of Claims 1 to 12 in which the activating circuit activates the sensing means to cause the sensing means to output an alarm signal.

15. A detecting unit as claimed in any preceding claim in which the sensing means is a gas sensor.

16. A detecting unit as claimed in Claim 15 in which the gas sensor is adapted for sensing a specific gas, and is responsive to sensing the specific gas for outputting an alarm signal.

17. A detecting unit as claimed in Claim 15 or 16 in which a source of the gas to which the gas sensor is responsive is provided, and the gas source is responsive to the activating circuit for releasing a quantity of the gas to the sensing means for activating the sensing means to output an alarm signal.

18. A detecting unit as claimed in any preceding claim in which the detecting unit comprises a housing, and the sensing means is located within the housing, an inlet and an outlet being provided in the housing for accommodating the passage of air therethrough past the sensing means.

19. A detecting unit as claimed in Claim 18 in which a fan is located in the housing for drawing air through the housing past the sensing means.

20. A detecting unit substantially as described herein with reference to and as illustrated in the accompanying drawings.

21. A detecting circuit for detecting the occurrence of an abnormal condition at any of a plurality of remote locations, the detecting circuit comprising a control unit, and a plurality of detecting units as claimed in any preceding claim located at the respective remote locations and connected to the control unit by a four-wire circuit, the four wire circuit comprising a monitoring wire, a test wire, a voltage supply wire and a ground wire, each wire extending from the control unit, the supply voltage and ground wires being connected to the supply voltage terminals and ground terminals of the respective detecting units, and the detecting units being connected in series by the monitoring wire and the test wire by connecting the respective output monitoring terminal and output test terminal of each detecting unit to the corresponding one of the input monitoring terminal and input test terminal of the next detecting unit so that for so long as no alarm condition is detected, the first switch means of the respective detecting units remain closed, and a closed circuit condition prevails on the monitoring wire, and on a test signal being applied to the test wire, the second switch means of the respective detecting units sequentially close as the test signal is relayed from one detecting unit to the next, on the sensing means of the respective detecting units being functional, thereby causing a closed circuit condition to prevail on the test wire.

22. A detecting circuit as claimed in Claim 21 in which the monitoring wire from the output monitoring terminal of the last detecting unit on the monitoring circuit most remote from the control unit is connected to one of the supply and ground wires for facilitating monitoring of the first switch means of the respective detecting units by the control unit.

23. A detecting circuit as claimed in Claim 22 in which the test wire from the output test terminal of the last detecting unit on the test circuit most remote from the control unit is connected to the other of the supply and ground wires for facilitating monitoring of the second switch means of the detecting units on a test signal being applied to the test wire.

24. A detecting circuit as claimed in Claim 22 or 23 in which the monitoring wire is connected to the voltage supply wire at the end detecting unit.

25. A detecting circuit as claimed in any of Claims 21 to 24 in which an alarm circuit is provided for activating an alarm in response to the control unit detecting the occurrence of an abnormal condition having been sensed by one of the detecting units in the detecting circuit.

26. A detecting circuit substantially as described herein with reference to and as illustrated in the accompanying drawings.