GB2269255A

GB2269255A - Fire alarm system

Info

Publication number: GB2269255A
Application number: GB9215938A
Authority: GB
Inventors: Nicholas O'brien; Eric Macdonald; Steven Paul Maynard
Original assignee: Thorn Security Ltd
Current assignee: Thorn Security Ltd
Priority date: 1992-07-27
Filing date: 1992-07-27
Publication date: 1994-02-02
Also published as: GB9215938D0

Abstract

A fire alarm system Fig 1 (not shown) comprises a voltage source (14) having an internal resistance (15), which source feeds a shunt regulator end-of-line device (16) via a pair of conductors (10, 12). A plurality of fire detector assemblies (18) are distributed along the conductors. Each assembly comprises a removable fire detector unit (18a) and a mount 18b therefor. Each mount has a first terminal 44 for connection to one conductor and second 42 and third 43 terminals for connection to upstream and downstream portions respectively of the other conductor. When present the corresponding fire detector unit completes a conductive path between the second and third terminals. In order to maintain power downstream when a detector unit is removed while allowing the removal to be detected by a voltage change occurring at the voltage source, an auxiliary shunt regulator 20 is connected across the break which occurs in the other conductor when the detector unit is removed. In order that removal of more than one detector should not result in more than one auxiliary voltage regulator being brought into circuit, a respective controllable switch 22 is connected across each auxiliary regulator, this switch being controlled by means of a voltage threshold detector 24 to the open and closed states when the voltage between the first and second terminals lies above and below a predetermined threshold respectively. If desired a further threshold detector 46 and controllable switch 45 may be provided to translate a short circuit between the conductors to an open circuit as seen at the voltage source. <IMAGE>

Description

FIRE ALARM SYSTEM This invention relates to a fire alarm system comprising voltage supply means for generating an open-circuit voltage V1 with a source resistance R, a shunt voltage regulator for limiting the voltage thereacross to a value V2, first and second conductors coupling respective output terminals of said voltage supply means to respective terminals of said shunt voltage regulator, monitor means for monitoring the output current of said voltage supply means, and first and second fire detector assemblies each connected to said first and second conductors, the first assembly comprising a first removable fire detector unit and a mount there for and the second assembly comprising a second removable fire detector unit and a mount therefor, the first fire detector unit being connected between the first and second conductors via the corresponding mount and also forming, via the corresponding mount, a conductive bridge across a first break in a said conductor, the second fire detector unit being connected between the first and second conductors, via the corresponding mount, downstream from the first fire detector unit as seen from the voltage supply means and also forming, via the correspond mount, a conductive bridge across a second break in a said conductor. The fire detectors may, for example, take the form of smoke, flame or heat detectors and the system may further include a plurality of manually operable call points each connected between said first and second conductors.

The invention also relates to a mount for a removable fire detector unit for use in such a system.

Figure 1 of the accompanying drawingseis 9 diagram of a known fire alarm system of this general,-kind. The system of Fig. 1 comprises first and second conductors 10 and.l2 extending from voltage supply means 14 to an end of line device in the form of a shunt voltage regulator 16. The voltage supply means 14 applies a supply voltage across the conductors 10 and 12 from a voltage source 40 having a terminal voltage V1 via a series resistor 15. A monitor 13 monitors the output current of the supply 14 by monitoring the voltage between the conductors 10 and 12. (As an alternative the voltage across the series resistor 15 could be monitored directly.) A number of call points 17 and fire detector assemblies 18 are connected in parallel across the conductors 10 and 12.The assemblies 18 each comprise a detector unit 18a and a mount 18b therefor. Each unit 18a is connected between the conductors 10 and 12 via the corresponding mount 18b and also forms a conductive bridge 41 across a respective break in the conductor 10 via the corresponding mount. To this end each mount 18b is provided with connection terminals 42,43 and 44.

Each detector unit 18a is removable from its mount 18b.

The end of line device 16, which may comprise, for example, a Zener diode, clamps the voltage between the ends of the conductors 10 and 12 at a predetermined value V2 which is less than the open-circuit voltage V1 of the voltage supply means 14. The resulting current through the series resistor 15 gives rise to a voltage drop which produces a default voltage level at the monitor position. If there is an open circuit fault only a small current at most will flow through the series resistor, and the voltage level at the monitor position will be abnormally high. Conversely, if there is a short circuit fault a large current will flow and the voltage will be abnormally low. If a detector 18 or call point 17 is operated, it draws a current to produce a lower than normal voltage.

The monitor 13 includes means (not shown) for indicating whether the circuit is in the normal condition, has a short or open circuit fault or is in the alarm condition, depending on the voltage sensed at the monitor position. For example, the ranges of voltages between conductors 10 and 12 applicable to each condition may be as follows: Open circuit 20-24V Normal 16-20V Alarm 5-16V Short Circuit 0-5V In this prior art system, if a detector is removed, an open circuit fault is indicated by the monitor, and any call points or detectors further downstream towards the end of line device 16 will not function.

However, it is desirable that operation of a call point or detector will cause the system to indicate an alarm condition even if a fire detector unit upstream has been removed, while maintaining the property that the system should indicate a fault condition when a detector has been removed in a nonalarm condition of the system.

Known systems for achieving this requirement have necessitated specialised controller devices, or 'active' end of line devices in association with Schottky diodes provided on detector base units or mounts. Alternatively, all call points can be connected upstream of all fire detectors. These solutions are expensive and/or impose limits on the number of detect or assemblies which can be provided and/or are inconvenient.

The present invention aims to alleviate the problems of the known systems described above.

According to one aspect of the present invention a fire alarm system as defined in the first paragraph is characterised in that it further comprises first and second auxiliary shunt voltage regulators connected across said first and second breaks respectively for limiting the voltages across said first and second breaks each to a value which is less than V1, and which, when subtracted from V1, produces a result which is less than V2,a controllable switch connected across said second break, and a voltage threshold detector having an input connected across said conductors downstream of said first break but upstream of said second break and having an output connected to a control input of said switch, for controlling said switch to its open and closed states when the voltage at the detector input lies above and below a predetermined threshold respectively, which predetermined threshold lies below V2 but above the result obtained by subtracting the voltage limited by the first auxiliary regulator from V1.

According to another aspect the invention provides a mount for a fire detector unit for use in such a system having a first terminal for connection to one of said first and second conductors, a second terminal for connection to an upstream portion of the other of said first and second conductors and a third terminal for connection to a downstream portion of said other of said first and second conductors, said mount comprising first, second and third contacts coupled to said first, second and third terminals respectively for cooperating with corresponding contacts in a removable fire detector unit, characterized in that said mount further comprises a shunt voltage regulator connected across said second and third contacts, a controllable switch connected across said second and third contacts, and a voltage threshold detector having its input connected across said first and third terminals and an output connected to a control input of said controllable switch for controlling said switch to its open and closed states when the voltage at the detector input lies above and below a predetermined threshold respectively.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram of a known fire alarm system as previously discussed; and Figure 2 is a diagram of a circuit associated with each detector in the system of Figure 1 in an exemplary embodiment of the present invention.

In a particular embodiment of the present invention, the fire detector base units or mounts 18b of Fig. 1 each include a circuit as shown in block diagram form in Figure 2. An auxiliary shunt voltage regulator 20, for example a Zener diode, is connected across the mount terminals 42 and 43, i.e.

across the break in the conductor 10, to limit the voltage thereacross to a value V3 which is less than the source opencircuit voltage Vl but which, when subtracted from Vl, produces a result which is less than the limiting or clamping voltage V2 of the end of line shunt regulator.

A controllable switch 22, for example the source-drain path of an enhancement-type FET, is also connected across the break.

The switch 22, e.g. of the gate of the FET, is controlled by the output of a voltage threshold detector 24, the input of which is connected across the conductors 10, 12 immediately upstream of the break. The voltage threshold detector, which may take the form of a comparator which compares the voltage at its input with a reference voltage, determines whether its input voltage exceeds a predetermined threshold, the threshold being below V2, but above V1-V3. If this is the case, it opens the switch, and if this is not the case, it closes the switch.

Because the threshold lies below V2, in the normal or default condition of the system all switches 22 will be open. If a single detector unit 18a is now removed, then the voltage at the corresponding threshold detector 24 will still exceed the threshold and the corresponding switch 22 will remain open.

The corresponding shunt regulator 20 will apply a voltage drop V3 across the corresponding (now open) break in the conductor 10, such that the input voltage of any threshold detector further downstream from the supply 14 will be below the predetermined threshold, and the corresponding switch 22 will close. If another detector unit further downstream is now also removed, the voltage in the system will not be affected since the corresponding switch 22 will already be closed, bridging the new break in the conductor 10.

Thus even if several detector units are removed, the voltage across the conductors 10 and 12 drops no lower than V1 -V3, which can be chosen to be a sufficient supply voltage to maintain correct functioning of detector assemblies and call points further towards the end of the line.

The values of the voltages V1, V2 and V3 may be, for example, 21.4V, 18.65V and 4.85V respectively with a threshold voltage for the detectors 24 of 17.15V and a value of the internal resistance 15 of 330 ohms. The detector units 18a may effectively constitute smoke, flame or heat-operated switches, which, when closed, connect resistors of, for example, 270 ohms across the conductors 10 and 12. Similarly the call points 17 may comprise manually operated switches which, when closed, connect similar resistors across the conductors 10 and 12.

Although, as described, each mount 18b is provided with a circuit arrangement as shown in Fig. 2, it will be appreciated that this is not essential in respect of the detector assembly 18 nearest the voltage supply means 14. As far as this particular assembly is concerned the controllable switch 22 and the associated detector 24 may be omitted.

It will also be appreciated that the number of detector assemblies and/or call points provided may be chosen at will, and that one or more of the detector assemblies may be included between breaks in the conductor 12 rather than between breaks in the conductor 10.

If the known system of Fig 1 is modified in accordance with the invention in the manner described so far with reference to Fig 2 a problem may arise in the case of a short circuit occurring between the conductors 10 and 12 downstream of a detector assembly 18 which has had its detector unit 18a removed and the switch 22 of which is open.In such a case the voltage at the terminal 42 of the corresponding mount 18b will be pulled down to a value (relative to that on conductor 12) substantially equal to the limiting or clamping voltage V3 of the corresponding shunt voltage regulator 20. (Although in theory this should result in the corresponding voltage threshold detector 24 closing the corresponding switch 22, thereby bringing the potential on terminal 42 further down to substantially zero, this may not happen in practice, due to the threshold detector 24 then being deprived of operating voltage). There is a possibility that this situation will be interpreted by the monitor 13 as being an alarm condition rather than a fault condition.

In order to overcome this problem the two components 45 and 46 shown in dashed lines in Fig 2 may also be included in each mount 18b, if desired. (It has been assumed up until now that component 45 is replaced by a simple through-connection). If present, component 45 is a further controllable switch and component 46 is a further voltage threshold detector which controls switch 45 (which may be an enhancement-type FET) to the closed state if the voltage between conductors 10 and 12 lies above a predetermined threshold and to the open state otherwise. This predetermined threshold is chosen to be low, for example about 1V, so that the switch 45 is closed under all conditions other than when a short-circuit occurs between the corresponding terminals 43 and 44. If and when such a short-circuit occurs switch 45 opens, thus isolating the upstream portion of the system from the short-circuit and causing monitor 13 to "see" a fault condition, albeit an open circuit rather than the short circuit actually present.

A similar result may be obtained by, as an alternative, including a controllable switch in series with the conductor 10 or 12 upstream of the most upstream detector assembly 18 of the system, this switch being controlled in a manner similar to switch 45 by a voltage threshold detector connected across the conductors 10 and 12 downstream of the most downstream assembly 18 of the system.

The components 45 and 46 may be provided in a fire detector mount 18b from which the components 20, 22 and 24 have been omitted, i.e. in a system other than in accordance with the present invention, if desired.

Claims

1. A fire alarm system comprising voltage supply means for generating an open-circuit voltage V1 with a source resistance R, a shunt voltage regulator for limiting the voltage thereacross to a value V2 less than V1, first and second conductors coupling respective output terminals of said voltage supply means to respective terminals of said shunt voltage regulator, monitor means for monitoring the output current of said voltage supply means, and first and second fire detector assemblies each connected to said first and second conductors, the first assembly comprising a first removable fire detector unit and a mount there for and the second assembly comprising a second removable fire detector unit and a mount therefor, the first fire detector unit being connected between the first and second conductors via the corresponding mount and also forming, via the corresponding mount, a conductive bridge across a first break in a said conductor, the second fire detector unit being connected between the first and second conductors, via the corresponding mount, downstream from the first fire detector unit as seen from the voltage supply means and also forming, via the corresponding mount, a conductive bridge across a second break in a said conductor, characterized in that the system further includes first and second auxiliary shunt voltage regulators connected across said first and second breaks respectively for limiting the voltages across said first and second breaks each to a value which is less than Vl and which, when subtracted from V1, produces a result which is less than V2, a controllable switch connected across said second break, and a voltage threshold detector having an input connected across said conductors downstream of said first break but upstream of said second break and having an output connected to a control input of said switch, for controlling said switch to its open and closed states when the voltage at the detector input lies above and below a predetermined threshold respectively, which predetermined threshold lies below V2 but above the result obtained by subtracting the voltage limited by the first auxiliary regulator from Vl.

2. A system as claimed in Claim 1, wherein said controllable switch comprises an enhancement-type field effect transistor.

3. A system as claimed in Claim 1 or Claim 2 including a further controllable switch in series with a said conductor, and a further voltage threshold detector having an input connected across said conductors downstream of both said first break and said further controllable switch and having an output connected to a control input of said further controllable switch for controlling said further controllable switch to its closed and open states when the voltage at the input of the further voltage threshold detector lies above and below a further predetermined threshold respectively, said further predetermined threshold lying below the predetermined threshold specified in Claim 1.

4. A system as claimed in Claim 3, wherein said further controllable switch comprises an enhancement-type field effect transistor.

5. A system as claimed in any preceding claim, including a callpoint connected between the first and second conductors downstream of said second break.

6. A system as claimed in any preceding claim, wherein each said shunt voltage regulator comprises a Zener diode.

7. A system as claimed in any preceding claim, wherein the input of said monitor means is connected across the output terminals of said voltage supply means.

8. A fire alarm system substantially as described herein with reference to Figures 1 and 2 of the drawings.

9. A mount for a removable fire detector unit for use in a system as claimed in any preceding claim, said mount having a first terminal for connection to one of said first and second conductors, a second terminal for connection to an upstream portion of the other of said first and second conductors and a third terminal for connection to a downstream portion of said other of said first and second conductors, said mount comprising first, second and third contacts coupled to said first, second and third terminals respectively for cooperating with corresponding contacts in a removable fire detector unit, characterized in that said mount further comprises a shunt voltage regulator connected across said second and third contacts, a controllable switch connected across said second and third contacts, and a voltage threshold detector having its input connected across said first and third terminals and an output connected to a control input of said controllable switch for controlling said switch to its open and closed states when the voltage at the detector input lies above and below a predetermined threshold respectively.

10. A mount as claimed in Claim 9, wherein said controllable switch comprises an enhancement-type field effect transistor.

11. A mount as claimed in Claim 9 or Claim 10, including a further controllable switch connected in series with the path formed between said second and third terminals by said shunt regulator, and a further threshold detector having its input connected between said first terminal and a point on said path lying between said further controllable switch and said third terminal and an output connected to a control input of said further controllable switch for controlling said further controllable switch to its closed and open states when the voltage at the input of the further threshold detector lies above and below a further predetermined threshold respectively, said further predetermined threshold lying below the predetermined threshold specified in Claim 8.

12. A mount as claimed in Claim 11, wherein the further controllable switch comprises an enhancement-type field effect transistor.

13. A mount for a removable fire detector unit, substantially as described herein with reference to Figure 2 of the drawings.