GB2363271A - A fire door release circuit including a latching relay - Google Patents

Abstract

A circuit is described which can be used, e.g. in a fire detection system, to release a fire door normally held open by an electromagnet. The circuit includes a relay connected in series with a capacitor, regenerative means being provided for charging the capacitor so that the relay receives a current pulse sufficient to set the relay contacts, switching means also being provided which respond to either a detection signal, or a power failure, to discharge the capacitor, whereby a pulse through the relay is sufficient to reset the contacts hence releasing the fire door. Either a single coil bi-stable relay, or a twin coil bi-stable relay is used to maintain the set state without drawing current.

Description

2363271 FIRE ALARM SYSTEM INCLUDING RELAY FOR ACTUATING AN EMERGENCY

DEVICE 5 This invention relates to a fire alarm system including a relay for actuating an emergency device so that it will move from a standby position to an emergency position in the event of the emergency.

The invention can be used, for example, for automatically closing a fire door or doors 10 either when a fire detector responds to a fire, or when a power supply fails. Instead of a door, the emergency device could be a fire damper, smoke vent, or similar device which is maintained in the standby state (open or closed) unless actuated so as to move into the emergency state (closed or open) when released. Such devices are normally held in the standby state by an energised electromagnet which attracts a magnetic plate attached to 15 the device, the device being biased so that it moves automatically into its emergency position when the power to the electromagnet is cut off. The device can be biased by a spring, which is tensioned or compressed, to provide the driving force to move the device into its emergency position. Alternatively, it may be biased by gravity, e.g. suspended so that it moves under gravity when released into position. In any event, the device is 20 arranged to move into its emergency position, without requiring any external power assistance (like a mains supply), when actuated.

A conventional fire alarm system for protecting a large building typically includes at least one detector and alarm device installed in each room. All of the detectors and alarm 25 devices are connected to a central control unit (CCU) which then takes appropriate action in the event of a fire. For example, when any detector responds to a fire, all of the alarm devices generate warnings for evacuating the building. At the same time, the CCU disrupts the power to electromagnets which hold open the fire doors, so that they all close 2 automatically. This does not prevent escape, because the doors can still be opened to leave the building. The doors are released by causing the contacts of a relay to interrupt the power supply to the coil of the electromagnet. The power supply for the electromagnet is normally independent of that of the fire alarm system, because the 5 operation of the normally energised relay is failsafe in the event of a power failure, e.g.

a mains failure due to a fire. (The fire alarm system normally derives power from the mains, as well as having batteries to provide a backup supply).

In one prior art system, shown schematically in Fig. 1, the coil 1 of a monostable relay

10 is controlled by a signal from a fire detector 2 in the room in which the fire door 3 is installed. In order to conserve power, the relay coil 1 is normally de- energised, and is energised when the fire detector 2 detects a fire. The power for energising the relay coil 1 is derived from the same monitored supply lines L that power the fire detector 2. This has the disadvantage that in the event of a failure in the power supply to the fire detector 15 2, such as an open circuit the supply line, the relay coil 1 cannot then be energised and hence the operation of the fire door 3 is not failsafe. Normally closed contacts 4 make a circuit with the coil 5 of the electromagnet 6, which attracts a steel plate 7 attached to door 3, and with a local power supply 7. When released, the fire door 3 will automatically close against the jamb 8 (as shown by the arrow). An end-ofline resistor 20 9 is also connected across the supply lines L.

In another prior art system, shown schematically in Fig. 2, relay coils 1 a, 1 b are normally held in an energised state by the CCU which monitors one or more fire detectors D. In this case, the relay coils la,lb are energised from the supply powering the CCU and 25 hence, either in the event of a fire alarm signal, or a power failure, the relay coils la,Ib are de-energised hence breaking the contacts 4a,4b which supply current to the electromagnets 5a,5b normally holding the fire doors 3a,3b open. This system is failsafe, but it has the disadvantage that additional wiring must be installed between the CCU and 3 the relay coils la,lb, because the relays are normally located adjacent to the electromagnets 5a,5b on the fire doors 3a,3b. Further disadvantages are that the relay coils 1 a, 1 b continuously consume power in a standby condition and the supply lines LR to the relay coils l a, lb are not monitored. Fig. 2 also shows supply lines LA connected 5 to alarm devices A each having a diode.

One aspect of this invention is directed towards providing a system in which (a) the power supply for the relay can be derived from the same supply lines across which are connected detecting and/or alarm devices, (b) no significant current is taken to maintain 10 the relay circuit in a standby condition (to avoid unnecessary current drain on a battery), (c) a plurality of electromagnets can be controlled by a plurality of relays connected to the same supply lines and (d) the relay(s) will operate in a failsafe manner, either in the event of an emergency, or in the event of a power failure.

According to the invention, a fire alarm system comprises:

a Central Control Unit (CCU) for monitoring supply lines for detecting devices and/or alarm devices, the detecting devices normally operating in a standby mode but having an active mode, in an emergency, which changes the state of the lines; the CCU 20 being responsive to said emergency, so as to actively change the state of the line in order to cause emergency action to be taken; an emergency device normally maintained in a standby position by an energised electromagnet so that it will move to an emergency position either as a result of the emergency action, or a failure of the power supply energising the relay, and 25 a relay having a coil or coils and contacts which can be placed in either a set state for energising the electromagnet, or a reset state for de-energising the electromagnet; characterised in that:

4 (a) the relay is of a construction which enables the contacts to remain in a set state, or a reset state, with no current drain; and (b) circuitry is provided having terminals for connection to said supply lines 5 in order to derive power for the relay coil or coils; the circuitry also including charge storage means in series with said coil or coils, and switching means for charging the charge storage means from the supply lines by subjecting said coil or one of said coils to a current pulse having sufficient power to place the relay contacts in the set state; 10 said switching means also being responsive to the actively changed state of the line, when the emergency action has been taken, or to a power failure, or to a direct signal, so as to discharge the charge storage means, whereby said relay coil or other one of said coils is subject to a current pulse having sufficient power to place the relay contacts in the reset state.

The term "detecting device" is used broadly to cover anything that will cause the CCU to respond to the emergency and it would include manually operable call points (which are pressed when an an occupant of a building detects a fire) as well as devices such as smoke and flame detectors.

The detecting devices normally operate in a standby mode where they draw a very low current (e.g. less than 100 microamps) from the lines, but in the active mode, they can change the state of the lines, for example, by placing an impedance across current limited lines which causes a voltage drop to be detected by the CCU. However, in other systems 25 the CCU can change the state of the lines differently. This does not matter as long as the switching means can respond to this active change to initiate emergency action. The switching means can also optionally respond to a direct signal from a detecting or alarm device (as explained below).

The relay is of a construction which enables the contacts to be maintained in the set state with no current drain, i.e. it is stable in each of the set and reset states. It could be either a bi-stable relay with one coil, or a bi-stable relay with two coils to achieve the same effect. With a single coil bi-stable relay a current pulse is applied to the coil in one 5 direction to place the relay contacts in the set state, and in the opposite direction to place the relay contacts in the reset state. Only a pulse of current is required to latch the contacts, since they will then remain set mechanically without drawing current. In the case of a bi-stable relay with two coils, one acts to set the relay and the other to reset the relay, and each coil is connected in series with a respective diode, the diodes being 10 oppositely poled; the series connected coils and diodes being connected in a parallel arrangement which is connected to the charge storage means. In the latter arrangement, a pulse of current through one coil sets the relay contacts which remain in that state without either coil afterwards drawing current. A pulse of current in the opposite direction through the other coil will similarly reset the contacts without either coil 15 afterwards drawing significant current.

An advantage of the invention is that power can be initially derived from the same supply lines as the fire alarm system, since only a pulse of current is necessary to set the relay contacts. Similarly, a pulse of current is all that is necessary to change over the relay 20 contacts into the reset state. This power is derived from the charge storage means so that a power failure does not have any adverse effect since it will cause a drop in voltage which can be used to trigger discharge of the charge storage means. In order to maintain a charge on the charge storage means, which is typically a capacitor, leakage is kept to a minimum and the circuitry is preferably designed so that it will periodically or 25 continuously top up charge on the capacitor. However, the current for this is negligible.

A disadvantage of conventional relays is that they need to be energised in the normal state to operate electromagnets and therefore will consume current. They cannot be connected 6 across the monitored supply lines of fire alarm system where they would be seen, by the CCU, as a fault or an alarm signal (because conventional relays may draw something of the order of 5mA).

5 As mentioned above, the fire alarm system can be based on different known types and hence it can vary in the way it responds to the state of the supply lines in order to take emergency action. One known type comprises detecting devices and alarm devices connected across the same supply lines, whereby the CCU normally applies a line voltage (of say 12v) for operating the detecting devices in their standby state, but applies a higher 10 voltage (say 24v) to the supply lines to operate the alarm devices after responding to the emergency condition. If any detecting device responds to fire, it will apply a low impedance across the supply lines and this is seen by the CCU as an emergency and hence it will then apply 24v to operate the alarms. In this system, the switching means will respond to the rise in line voltage in order to discharge the charge storage means whereby 15 the relay contacts are changed from the set to the reset state to de- energise the electromagnet and thus allow the emergency device to move into its emergency position.

Another known type employs alarm devices which operate on a different polarity to that which is normally applied to the lines for operating the detecting devices. In the 20 emergency, the CCU responds to a detection signal and reverses the polarity of the supply lines, whereby the alarm devices produce warnings. In this case, the switching means responds to the change in polarity on the supply lines in order to discharge the charge storage means and change the relay contacts from the set to the reset state.

25 As the CCU monitors the line continuously, if any fault develops in the system, this can also trigger the CCU into varying the state of the supply lines as if it were an emergency condition.

7 The state of the supply lines can be varied by a reduction in line voltage, and, in any event, if the line voltage falls below a predetermined value due to, for example, an open or short circuit or a detection device applying an low impedance to the supply lines. This causes the switching means to respond so as to discharge the charge storage means and 5 change over the relay contacts. In the event of a power failure, the line voltage will usually reduce, decay, or fall to zero, whereby the switching means is triggered to discharge the charge storage means.

Therefore, the invention can be applied to any fire alarm system where the state of the 10 supply lines is continuously monitored by a CCU which then carries out some appropriate emergency action. The system is failsafe, because a power failure, or a fault (producing either a false detection signal, or a short circuit, or an open circuit) will cause the state of the lines to vary and hence the switching means will then respond so as to change over the relay contacts, which will result in de-energising the electromagnet holding the 15 emergency device in its standby position.

As mentioned above, the invention has a charging mode in which the charge storage means is charged by a pulse to set the relay contacts, and a discharge mode in which the charge storage means is discharged to reset the relay contacts.

In the charging mode of a preferred embodiment of the invention, where the switching means includes first and second switching means, the first switching means operates regeneratively, when triggered, so as to provide the current pulse which sets the relay contacts. For example, on start-up of the fire alarm system, operating current is supplied 25 to the CCU whereby capacitors in the CCU charge up and the supply line voltage progressively increases. The first switching means operates regeneratively in response to this increase in line voltage when the line voltage exceeds a first predetermined level so that a rapid transition from a non-conducting to a conducting state occurs. The charge storage means is thereby connected to said lines and a first current pulse causes charging to occur with a pulse having a fast leading edge and of sufficient power to set the relay contacts. During the current pulse and after the circuitry has reached a fully powered conducting state, the first switching means controls the second switching means, which 5 is in parallel with the charge storage means and the relay coil or coils, so as to cause the second switching means to take up a non-conducting state, whereby the charge is retained on the charge storage means. When the alarrn system has reached this state, it is in the standby or passive mode, i.e. where the detecting devices are simply monitoring the zones in which they are located.

In the discharge mode of the latter preferred embodiment, the emergency or fault will cause at least one detecting device to change from a passive to an active state whereby the CCU will then actively cause a change in the state of the line. As noted above, this can occur in a variety of different ways, for example, reduced line voltage, increased line 15 voltage, reversal of polarity of the voltage on the lines, power failure, alternating or pulsating voltage, etc. In any of these circumstances, the first switching means is responsive to the actively changed state of the line, so as to act regeneratively and thereby trigger the second switching means which causes the charge storage means to discharge rapidly, creating a second current pulse with a fast leading edge and of opposite polarity 20 to the first pulse, and of sufficient power to reset the relay contacts.

Alternatively, the relay can be set by an initial pulse and reset by a subsequent pulse, the pulses being of the same polarity. This can be achieved with a relay which has mechanically reversible contact sets, whereby the first pulse sets the contacts and the 25 second pulse resets them, or rotatably selectable contacts, whereby contacts can be set and reset by consecutive pulses. Such relays also have bistable states which avoids drawing current when the contacts are set.

9 Whilst the circuitry can be designed or adapted for use with a particular kind of fire alarm system, e. g. where the line changes state by an increase in voltage, the circuitry preferably has an inbuilt response to different line states, so that it can be provided as a unit for use with various systems. In this way, it will respond automatically to the respective state of 5 the line in order to activate the emergency device. This has the advantage of facilitating installation, thereby saving time and expense. It could also avoid problems of installing an incorrect relay operating unit in different systems.

Generally speaking, the switching means can be triggered by more than one emergency 10 line state, such as when:

(a) the line voltage is increased (say from 12v to 24v to operate alarms) (b) the line voltage falls below a predetermined level; (c) the power is cut off (which will incorporate the latter); (d) the line voltage changes in polarity (e.g. to operate alarms in series with 15 diodes); The circuitry can include appropriate and respective means which is triggered by the line state so as to discharge the charge storage means and reset the relay contacts.

20 Where the circuitry includes means responsive to a drop in voltage below a predetermined threshold level, the charge storage means will be discharged if the line voltage falls below this level for any reason, including power failure and reverse polarity.

The circuitry can include voltage responsive means, such as a zener diode, which responds 25 to an increase in line voltage above an upper threshold, for causing the voltage applied to the first switching means to be reduced below a lower threshold to cause the charge storage means to discharge. This feature is useful with the type of fire alarm system which operates in response to an emergency condition by increasing the line voltage.

Preferably, the circuitry includes second charge storage means, connected to the first switching means, and which charge up when the circuitry is connected to the supply lines.

In this case, the first switching means switches regeneratively when the voltage on the second charge storage means exceeds the first predetermined level, so as to cause the first 5 charge storage means to be charged by a pulse of current from the second charge storage means. This causes the relay contacts to be set. In this case, in the event of the emergency, the second charge storage means is arranged to discharge so that, when the voltage on the second charge storage means falls below a second predetermined level, the first switching means regeneratively switches to a non-conducting state whereby the 10 second switching means switches to a conducting state, so as to discharge the first charge storage means by a pulse of current in order to reset the relay contacts.

Preferably, the second charge storage means is connected to the supply lines by current limiting means, which limit the charging current to a level that does not interfere with 15 monitoring of the supply lines by the CCU.

The second charge storage means preferably has discharge current limiting means so as not to discharge sufficiently to cause the relay contacts to return to the reset state, if the current supply is temporarily interrupted; provided that the current supply is not 20 interrupted for longer than a pre-determined interval. This interval is long enough to allow fire detecting devices in a system to be reset (for example), and/or for other signalling to take place, without initiating closure of the fire door or doors. However, this interval is brief enough not to have any disadvantageous effect on the fire detection system.

Preferably, diode means are included in the circuitry for responding to a change in supply polarity, which occurs in the emergency condition, the diode acting to block the supply of current from the supply line to the switching means in the emergency condition.

Preferably, the supply lines have connected thereto alarm devices. In one arrangement, each alarm device is fitted with a series diode and operative in one polarity only, the other polarity being used for line monitoring purposes.

5 Preferably, in another arrangement the circuit is incorporated in a detection device or the mounting base of a detection device so as to reduce the cost of the fire alarm system.

Preferably, the circuitry can include means for responding directly to a detection signal, from at least one of the detection devices, in order to create the effect of reducing the 10 voltage applied to the first switching means, to a value which is less than the second predetermined level. This will then trigger the first switching means into regeneratively switching to a non-conducting state, thereby triggering the second switching means to discharge the first charge storage means. However, this feature is optional. For example, a terminal of the circuitry can be connected to a terminal of the detecting device normally 15 used to operate a remote alarm indicator, whereby when the detection device responds to the emergency condition, it causes the voltage applied to the first switching means to be reduced to less than the second predetermined level, hence triggering the second switching means as noted above. This feature is considered to be optional, because the installation may or may not require such a direct connection to the detecting device. Otherwise, other 20 emergency conditions can be detected by simply monitoring the state of the supply lines, e.g. a rise or a fall in voltage, a polarity reversal, a cyclically varying signal, etc.

Embodiments of the invention will now be described with reference to Figs. 3-5 of the accompanying drawings, in which drawings:

Figures 1 and 2 illustrate prior art

Figure 3 is a circuit diagram of an embodiment of the invention.

Figure 4 is a circuit diagram of a modification of Fig. 1.

Figure 5 is a schematic diagram of a system in which the invention applied.

5 Referring to Fig. 3, which shows one embodiment of the invention, terminals TB1 and TB3 are connected to the negative and positive rails of supply lines (not shown) connecting fire detecting devices and/or alarm devices to a CCU. The system normally includes a control unit, a plurality of fire detectors, and/or alarm devices (none of which are shown, but are of known construction). Terminal TB2 or TB4 is optionally connected 10 to a "remote" output of a fire detector in the system, which output changes state, from positive (or open circuit) to negative in the case of TB4, or from negative (or open circuit) to positive in the case of TB2, when a fire is detected Zener diodes ZDI and ZD2 have predetermined breakdown voltages for the purposes 15 explained below. Diodes D1, D2, D3, and D4 conduct current with the polarity shown in the diagram. Transistors TR1, TR2, TR3, TR4 and TR5 act as switching devices as explained below. The circuit also includes resistors R1-R10 and capacitors Cl-C3 connected as shown. The circuit also incorporates a bi-stable relay RLY in series with capacitor C3. Relay RLY is of known construction and includes a relay coil for operating 20 contacts which are connected in series through terminals TB5 and TB6 with an electromagnet (not shown, but of known construction) for holding a fire door (not shown, but of known construction) normally open.

On connection to the supply capacitor C2 charges through R2 until a first predetermined 25 voltage, determined in large part by ZD2, is reached. At this voltage TR2 and TR3 conduct regeneratively and transfer charge from C2 to C3 and therefore a current pulse through the bi-stable relay RLY and "set" the relay. TR2 and TR3 are then maintained in the non-conducting state by current through R9 and TR 4 and TR5 are non-conducting.

The current consumed by the circuit then falls to a very low value, typically less than 20uA, a level low enough not to disturb any supply line monitoring or to require any significant increase in standby battery capacity.

5 The value of R2 is selected to limit the current taken from the supply lines to a level that does not interfere with the monitoring of the supply lines by the CCU.

If the supply fails, is disconnected or reversed by a control unit in response to the operation of a fire detecting device, C2 discharges through R6, TR2, TR3, R8 and R9.

10 Transistors TR4 and TR5 partially turn on discharging C3 as diode D4 becomes reverse biased. When the supply voltage falls below a second predetermined level, lower than the first predetermined level, the voltage across R8 becomes too small to maintain TR3 in the conducting state. Transistors TR3 and TR2 then turn off regeneratively and increase the reverse voltage across D4 to sufficiently to causes transistors TR4 and TR5 to tum hard 15 on, rapidly discharging C3 with a current pulse of opposite polarity, and thereby resetting the relay contacts so as to de-energise the electromagnet and thereby release the fire door.

Because C2 is arranged to discharge slowly through R6, TR2, TR3, R8 and R9 on the loss of supply voltage, the relay will not reset if the supply is removed for a short period, e.g.

20 up to 5 seconds, as may occur when detectors are reset or other signalling is present on the supply lines as in the case of analogue addressable detecting devices.

In systems where the supply voltage is arranged to increase upon an alarm such as those described in GB2336455 and other related copending applications, components ZD1, R3, 25 R4, R5 Cl, and TR1 are incorporated in the circuit. When the supply voltage is raised above a third predetermined voltage, which is higher than the first predetermined voltage and substantially set by ZD1, TR1 conducts, discharging C2 and again resetting the relay.

The terminal T134 is optionally connected to the remote alarm output of a fire detector that goes negative when it detects a fire. When the fire is detected capacitor C2 is discharged through Dl and R1 so resetting the relay and causing the door to close.

5 The terminal TB2 is optionally connected to the remote alarm output of a fire detector that goes positive when it detects a fire. When the fire is detected capacitor C2 is discharged through TR1 so resetting the relay and causing the door to close.

The circuit may also be connected to supply lines connected to monitored alarm devices 10 in which one polarity is applied to the lines in the normal condition and the other when a warning is to be given. In the standby condition the relay will be in the set state but in the warning condition the supply to circuit will be blocked by D2 and the relay will be in the reset state. Again, this saves wiring and power over prior art.

15 Fig. 4 shows a modification where the single coil bi-stable relay of Fig. 3 has been replaced by a bi-stable relay having two coils. The coils Ll, L2, which are respectively used to set and reset the relay contacts, are in series with respective oppositely poled diodes D5, D6, the parallel arrangement being connected to capacitor C3. When a current pulse is sent in one direction this will set the relay contacts and they will remain in this 20 state until a current pulse in the opposite direction resets the relay contacts.

Fig. 5 shows a system in which the circuit of Fig. 3 is applied. The system includes a circuit according to Fig. 3 connected to the same monitored supply lines Ll, L2 as a fire detector. The relay contacts are set in the standby condition when power is applied to the lines, and reset when power is terminated or an alarm signal from the fire detector is present on terminal TB4.

The system shown in Fig. 5 also shows an arrangement where a circuit according to Fig. 3 is connected to the same monitored supply lines L3, L4 as alarm devices. The relay contacts are set in the standby condition when power is applied to the lines, and reset when power is terminated orthe CCU energises the alarm devices by reversing the polarity applied to the supply lines. As the circuit according to Fig. 3 is arranged to 5 consume significantly less current than the line monitoring current, which is determined by the end of line resistor, more than one circuit according to Fig. 3 can be connected across the same supply lines as the alarm devices and hence more than one fire door can be released 10 The system shown in Fig. 5 also shows an arrangement where two circuits according to Fig. 3 are connected to the same monitored supply lines L5, L6 as alarm devices and fire detectors. In this case the detectors are of a type which normally operate at a first voltage, e.g. 12 volts, and the alarm devices operate at a second voltage which is higher than the first voltage, e.g. 24 volts. The relay contacts are set in the standby condition at 12 volts 15 when power is applied to the lines, and reset when power is terminated or the CCU energises the alarm devices by increasing the voltage applied to the supply lines to 24 volts.

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Claims (21)

1. A fire alarm system comprising:

a Central Control Unit (CCU) for monitoring supply lines for detecting devices 5 and/or alarTn devices, the detecting devices normally operating in a standby mode but having an active mode, in an emergency, which changes the state of the lines; the CCU being responsive to the emergency, so as to actively change the state of the line in order to cause emergency action to be taken; an emergency device normally maintained in a standby position by an energised 10 electromagnet so that it will move to an emergency position either as a result of the emergency action, or a failure of the power supply energising the relay, and a relay having a coil or coils and contacts which can be placed in either a set state for energising the electromagnet, or a reset state for de-energising the electromagnet; 15 characterised in that:

(a) the relay is of a construction which enables the contacts to remain in a set state, or a reset state, with no current drain; and (b) circuitry is provided having terminals for connection to said supply lines in order to derive power for the relay coil or coils; 20 the circuitry also including charge storage means in series with said coil or coils, and switching means for charging the charge storage means from the supply lines by subjecting said coil or one of said coils to a current pulse having sufficient power to place the relay contacts in the set state; said switching means also being responsive to the actively changed state 25 of the line, when the emergency action has been taken, or to a power failure, or to a direct signal, so as to discharge the charge storage means, whereby said relay coil or other one of said coils is subject to a current pulse having sufficient power to place the relay contacts in the reset state.

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2. A fire alarm system according to claim 1, where the relay is a bistable relay.

3. A fire alarm system according to claim 1, where the relay has a first coil for setting the relay contacts and a second coil for resetting the relay contacts, each coil being 5 connected in series with a respective diode, the diodes being oppositely poled, the series connected coils and diodes being connected in a parallel arrangement which is connected to the charge storage means.

4. A fire alarm system according to any preceding claim, where said switching means 10 includes first and second switching means, and first switching means (R6, R7, ZD2, TR2, TR3, R8, and R9) operates regeneratively when the voltage applied to the first switching means exceeds a first predetermined level so as to provide a rapid transition of the first switching means from a non-conducting to a conducting state and hence connect the supply line to the charge storage means (CI) to produce a first current pulse to set the 15 relay, and to switch the second switching means (R9, D4, TR4, TR5 and R10) to a non conducting state; the first switching means also operating regeneratively in response to the voltage applied to the first switching means falling below a second predetermined level, which is lower than the first predetermined level, so as to provide a rapid transition from a conducting to a non-conducting state to disconnect the charge storage means from the 20 line, and to cause the second switching means to change form a non- conducting to a conducting state, so producing a second current pulse in the opposite polarity to the first current pulse that discharges the charge storage means, to reset the relay contacts in the event of a power failure, or an alarm signal generated by a detecting device.

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5. A fire alarm system according to claim 4, including second charge storage means (C2) connected to the first switching means which charge when the circuitry is connected to the supply lines (for detecting devices and/or alarm devices), the first switching means then switching regeneratively when the voltage applied to the first switching means 18 exceeds the first predetermined level to cause the first charge storage means to be charged by a pulse of current from the second charge storage means; and the first switching means switching regeneratively when the voltage applied to the first switching means falls below the second predetermined level to reset the relay contacts.

6. A fire alarm system according to claim 5, in which the second charge storage means is connected to the supply lines by current limiting means (R2) which limit the charging current to a level that does not interfere with the monitoring of the supply lines by the CCU.

7. A fire alarm system according to claim 4 or 5, in which a conducting path (R1, R5, TR3A, TR3B and R3) is provided for discharging the second charge storage means (C2) when the supply line power is terminated, so that, when the voltage on the second charge storage means (C2) falls below the second predetermined level, the first switching means 15 regeneratively changes from a conducting to a non-conducting state causing the second switching means to change from a non-conducting to a conducting state so as to discharges the first charge storage means (0) to return the relay to the reset state.

8. A fire alarm system according to any of claims 5-7, wherein the second charge 20 storage means (C2) is connected to discharge current limiting means (R6, RTR2, TR3, R8, and R9) so as not to discharge sufficiently to cause the relay contacts to return to the reset state, when the current supply is temporarily interrupted; provided that the current supply is not interrupted for longer than a pre-determined interval.

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9. A fire alarm system according to any of claims 4-8, including means for ope a conducting path (RI and DI) for reducing the voltage applied to the first switching means below the second predetermined level when an alarm or detection signal is present.

10. A fire alarm system according to any of claims 4-9, including a terminal (TB4) for connection to a remote output terminal of a detecting device, which remote output terminal provides a signal when the detecting device responds to the emergency, and a further conducting path allowing the voltage applied to the first switching means to fall 5 below the second predetermined voltage in response to the signal.

11. A fire alarm system according to any of claims 4-10, including voltage responsive means (ZD1, R3, R4, R5, Cl, and TRI) which respond to an increase in supply line voltage above a third predetermined level for causing the voltage applied to the first 10 switching means to fall below the second predetermined voltage, whereby the relay contacts are returned to the reset state.

12. A fire alarm system according to any of claims 5-11, including diode means (D2) responsive to a change in current supply polarity, which occurs in the emergency state, 15 the diode acting to block the supply of current from the supply lines to the first switching means in the emergency condition, whereby the relay contacts to return to the reset state.

13. A fire alarm system wherein circuitry according to any preceding claim is incorporated in a detection device or the mounting base of a detection device.

14. A fire alarm system wherein a plurality of circuitries according to any preceding claim is connected to the same supply lines.

15. A fire detection system wherein a plurality of circuitries according to any 25 preceding claim is connected to the same supply lines as a plurality of detection devices and/or alarm devices.

16. A fire alarm system according to claim 1, wherein the pulse for setting the relay contacts is opposite in polarity to the pulse for resetting the relay contacts.

17. A fire alarm system according to claim 1 or 2, wherein the charge storage means remains charged whilst the system is in a passive or standby state, and is discharged when 5 the switching means is triggered by change of state of the lines.

18. A fire alarm system according to any preceding claim, wherein the circuitry includes means for topping up the charge on the charge storage means.

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19. Circuitry for use with a fire alarm system wherein a Central Control Unit (CCU) monitors supply lines for detecting devices and/or alarm devices, and actively changes the state of the line in order to cause emergency action to be taken, which emergency action includes causing an emergency device to move from a standby position to an emergency position by de-energising an electromagnet via relay contacts, the relay being of a construction which enables the contacts to remain in a set state, or a reset state, with no current drain; and the circuitry comprising terminals for connection to said supply lines in order to derive power for a coil or coils of the relay; charge storage means in series with said coil or coils, and switching means for charging the charge storage means from the supply lines by subjecting said coil or one of said coils to a current pulse having sufficient power to place the relay contacts in the set state; said switching means also being responsive to the actively changed state of the line, when the emergency action has been taken, or to a power failure, or to a direct signal, so as to discharge the charge storage means, whereby said relay coil or other one of said coils is subject to a current pulse having sufficient power to place the relay contacts in the reset state.

20. Circuitry according to claim 19, wherein the switching means can be triggered by more than one emergency line state, the switching means incorporating respective means which is triggered by the line state so as to discharge the charge storage means and reset 21 the relay contacts.

21. A fire alarm system substantially as herein described and illustrated with reference to the accompanying drawings.