GB1604634A

GB1604634A - Signalling apparatus

Info

Publication number: GB1604634A
Application number: GB1720577A
Authority: GB
Original assignee: Maxted C W
Current assignee: Maxted C W
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-09

Description

(54) SIGNALLING APPARATUS (71) I, CHARLES WILLIAM MAX TED, a British subject, of 77, Church Road, London, SEl9 2TA, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: According to one aspect of the invention, there is provided electrical apparatus comprise ing: power supply circuitry for supplying electrical power between two output conductor portions thereof, the circuitry being switchable from a first supply condition to a second supply condition, by a change in external circuit conditions between the said two conductor portions; a detection device for connection between the said two conductor portions, when the apparatus is installed for use, by way of two supply lines respectively connected to those conductor portions, the detection device being operable to produce such a change in external circuit conditions between the said two conductor portions, when connected therebetween, in the event that an occurrence to which the detection device is responsive takes place while the said power supply circuitry is in its said first supply condition; and an output response device for connection between the said two conductor portions, in parallel with the said detection device, by way of the said two supply lines when the apparatus is installed for use, the output response device being such that it is changed from a first state to, a second state thereof, to provide a predetermined output response, when the said power supply circuitry is changed from its first to its second supply condition, the first and second supply conditions yielding two different voltage levels of the same polarity, which do not automatically alternate with one another to produce regular pulses.

According to a second aspect of the invention, there is provided signalling apparatus comprising: power supply circuitry for supplying electrical power between two output conductor portions thereof, the circuitry being switchable from a first supply condition to a second supply condition by a change in external circuit conditions between the said two conductor portions; a detection device for connection between the said two conductor portions, when the apparatus is installed for use, by way of two supply lines respectively connected to those conductor portions, the detection device being operable to produce such a change in external circuit conditions between the said two conductor portions, when connected therebetween, in the event that an occurrence to which the detection device is responsive takes place while the said power supply circuitry is in its said first supply condition; and a signalling device for connection between the said two conductor portions, in parallel with the said detection device, by way of the said two supply lines when the apparatus is installed for use, the signalling device being such that it is changed from a non-signalling state to a signalling state thereof, to provide a predetermined signal, when the said power supply circuitry is changed from its first to its second supply condition, the first and second supply conditions yielding two different voltage levels of the same polarity, which do not automatically alternate with one another to produce regular pulses.

According to a third aspect of the invention, there is provided a signalling device which is suitable for use in signalling apparatus accerd- ing to the second aspect of the invention, to perform the function of the signalling device specified above, the device having first and second conductor portions for connection to first and second lines, respectively, between which a first unidirectional voltage level is normally to be maintained, although a second unidirectional voltage level can exist between the lines when operation of the signalling device is required, said signalling device comprising one and only one two-terminal electric circuit element, the conductivity of which suddenly increases when the voltage across it rises above a threshold value, and one and only one switch, said circuit element and switch not being connected to any other such circuit element or switch (except via said lines when said conductor portions are connected to said lines) and said device further comprising an audible or visible alarm device such as a bell, hooter, siren, warning lamp or the like, said circuit element and switch serving to ensure that no current passes to the alarm device when the first and second conductor portions have said first unidirectional voltage level between them and to ensure that current passes to the alarm device and operates it when said second unidirectional voltage level exists between said conductor portions.Preferably, said circuit element is a Zener diode which is in series with resistance means in a circuit branch one point of which is connected to, a trigger input of said switch which is connected in series with said alarm device between said first and second conductor portions.

The switch may be a thyristor. A resistor may be connected in parallel with said alarm device.

According to a fourth aspect of the invention, there is provided a station (for example a central alarm station) for use in an electrical system (for example an alarm system), the station comprising two power supply output conductor portions, power supply means connected to said conductor portions for normally providing at said conductor portions a power supply at one voltage level other than zero, and sensing means connected to said conductor portions for sensing changes in an electrical condition in an external circuit which, in use, is connected to said conductor portions and for responding to such a change to cause said power supply means to provide at said conductor portions a power supply at another voltage level other than zero of the same polarity, which does not automatically alternate with the first voltage level to produce regular pulses.

In British Patent Specification No. 1,319,615 (see Figure 5 thereof and the related description) there is disclosed a central alarm station of a fire alarm system in which the detection of a fault by a distant alarm device results in a periodic reduction in the voltage between the output conductor portions of the central alarm station so that, in contrast to the present invention, two voltage levels between the output conductor portions automatically alternate with one another to produce regular pulses. In the system described in Specification No. 1,319,615 these pulses make a lamp in the distant alarm device blink.

For a better understanding of the invention, and to she,w how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Figure 1 is a circuit diagram of part of an alarm device, Figure 2 is a circuit diagram of an alarm switch, Figure 3 is a circuit diagram of an end-ofline unit, Figure 4 is a simplified diagram of a central alarm station, Figure 5 is a circuit diagram of part of the alarm station of Figure 4, a possible modification being shown in broken lines, Figure 6 is a simplified circuit diagram of a smoke detector, Figure 7 is a circuit diagram of a fire alarm installation, and Figure 8 shows a modification of the circuit diagram of Figure 5.

Referring to Figure 7, the fire alarm installation comprises a central alarm station 1 from which two output lines 2 and 3 emerge. Connected across the two output lines are a plurality of fire sensing devices 5, such as smoke detectors, heat detectors or flame sensors, arranged about a building as required. There are also two alarm switches 6, such as breakglass switches, and a remake alarm device 7, which includes, for example, a bell, hooter or lamp, connected across the lines 2 and 3. There may be any number of sensing devices 5, switches 6 and remote alarm devices 7 and these may be of different types. The lines 2 and 3 are terminated by an end-of-line unit 4.

The central alarm station 1 comprises a d.c.

power supply which supplies power to the lines 2 and 3 at one voltage level other than zero, i.e. 12 volts in the example which is being described. The alarm device(s) 7 is or are such as to be not operated by this voltage level. When any of the sensing devices 5 senses a condition indicative of a fire, or when a switch 6 is operated, the current drawn through the lines 2 and 3 by the relevant device or switch increases. This increase is sensed by the central alarm station and results in an alarm indication being given at the station and also causes the voltage applied to the lines 2 and 3 by the central station power supply to change without changing polarity; in the present example it increases to 24 volts. The remote alarm device or devices 7 is or are set off by this new voltage.Unless something occurs which makes it happen, the cenral power supply does not change back to supply 12 bolts to the lines 2 and 3. There is no regular fluctuation between 12 volts and 24 volts between the lines 2 and 3.

Referring to Figure 1 the remote alarm device 7 comprises an electrical warning device which in the present case is a sounder such as a bell, hooter or siren but which could instead be a lamp or the like. The warning device is not particularly illustrated. The warning device is connected to, the output terminals 8 of threshold detection means of which the input terminals 9 are connected in use to the lines 2 and 3 of Figure 7. The positive one of the terminals 9 is connected directly to the pesi- tive one of the terminals 8. Between the terminals 9 is connected a series arrangement of a Zener diode Z1 and two resistors R1 and R2.

A further series arrangement comprising a resistor R3 and the anode/cathede path of a thyristor SCAR1 is also connected between the terminals 9. The junction between the resistors R1 and R2 is connected to the trigger input of the thyristor and the junction point between the resistor R3 and the thyristor is connected to the negative one of the terminals 8. While the voltage at terminals 9 is at the lower (12 volts) level, the Zener diode Z1 and the thyristor SCR1 are both off and the sounder draws no current and is silent. If the voltage at terminals 9 rises to the higher level (24 volts), Z1 conducts and fires the thyristor SCAR1 whereby current can flow through, and operate, the sounder.The Zener diode Z1 may have a turn-on voltage of say 18 volts. Resistor R3 ensures that if the sounder device is of the kind which takes intermittent current, a small current is kept flowing through SCR1 once it has been fired. This ensures that the thyristor remains conductive when, due to continued current drain, the supply voltage, which may be from a battery, has fallen below the Zener diode turn-on voltage. An electrolytic capacitor, C e.g. a tantalum capacitor, in parallel with resistor R2 prevents the thyristor SCR1 from being triggered by transient voltages which may appear.

Referring to Figure 2, the alarm switch 6, e.g. a break-glass switch, a lever or key operated switch or the like, comprises a normally open switch 10 in series with a resistor R4. Thus, when the switch 10 is closed, increased current is drawn but the lines 2 and 3 are not directly shorted-out. Hence, the voltage thereon can rise to the higher 24 volt level as is required.

Referring to Figure 3, the end-of-line unit 4 comprises a resistor R5 in series with a Zener diode Z2 having a turn-on voltage about equal to the lower voltage level applied to the lines 2 and 3. The Zener diode Z2 ensures that, while no alarm condition is present, there can nevertheless flow through the lines 2 and 3 a small but measurable current which is monitored by a fault detector, to be described later, provided in the central alarm station 1. Then, if one of the lines 2 and 3 is broken, say, the cessation of this current is detected and indicated as a fault. The resistor R5 ensures that Zener diode Z2 does not hold the line voltage down to the lower level when it is required to rise to the higher level.

As shown in Figure 4, the central alarm station 1 comprises two output terminals 11 (referred to above as "output conductor portions") respectively connected to the lines 2 and 3 and to two respective moving contacts of an electromagnetic relay 12 comprising three changeover contact sets 12A, 12B and 12C. In the normal case, as illustrated, the two terminals 11 are connected to each other via the relay contacts, a series resistor R6 contained in a unit 13, termed a "zone board", and a d.c.

supply 42, such as a battery, of which the voltage is equal to the higher voltage to be applied to lines 2 and 3, i.e. 24 volts. The resistor R6 is such that, due to the current drawn through the lines 2 and 3 by the end-ofline unit 4 (Figure 3), it drops the difference between the higher and lower voltage levels to be applied to the lines 2 and 3 so that only the lower voltage level 212 volts) is applied to them at this time. The zone board comprises a fire alarm sensing circuit 40 connected across the resistor R6 and a fault sensing circuit 41 connected so as to be between the terminals 11 when the relay contacts are in the normal position as shown.If the current flowing into the lines 2 and 3 ceases, the fault sensing circuit 41 detects the consequent increase in the voltage between the terminals 11 and pra- duces a fault indication, for example by lighting a lamp, and if the current increases (due to detection of a fire by a fire sensing device 5 or operation of a switch 6) this is detected by the fire alarm sensing circuit 40. This circuit then causes switchover of relay 12 so that the fault sensing circuit 41 is put out of action and the terminals 11, and hence also the lines 2 and 3, are connected directly to the power supply and are thus supplied with the higher voltage level. At the same time current passes along a line 43 to an alarm (not shown), e.g.

a sounder, which is set off at the central station.

The reason why the fault indicator circuit 41 is put out of action when fire is sensed is that, when the relay contacts are switched over from the position shown to the alarm position, the terminals 11 receive the full higher level voltage and hence the circuit 41, if not put out of action, would give a fault indication which might cause confusion.

Referring to Figure 5, the zone board 13 comprises the resistor R6 to one side of which is connected one side of a capacitor 14 and to the other side of which is connected one side of a resistor 15. The other sides of the resistor and capacitor are connected together and via a Zener diode 16 to the base of a transistor 17 of which the collector is connected to the positive supply input terminal 18 by way of the coil of a relay 19 and of which the omitter is connected to, the negative supply input terminal 20. The Zener diode 16 has a turn on voltage a little greater than the normal voltage drop across resistor R6, say 15 volts if the voltage drop is 12 volts.When the current through resistor R6 increases, due to operation of a fire sensing device 5 or a switch 6, the voltage drop across it increases, so turning on the Zener diode 16 and the transistor 17 which energises relay 19. The relay contacts 19a then close, so connecting zone board terminal 26 and hence the winding of relay 12 and the central station alarm to the negative side of the power supply.

Relay contacts 19b also close, so providing a positive supply via resistor l9c to the base of transistor 17 which latches this transistor "on".

Thus, the central station alarm is set off and the relay 12 is changed over to give the higher voltage level at terminals 11. This higher vol tage level operates the remote alarm device(s) 7 (Figure 7). The capacitor 14 ensures that transient voltage increases appearing across resistor R6 do nQt turn on transistor 17 and produce an erroneous alarm. Terminal 18 of zone board 13 is connected, via board terminal 22, the contact set 12C of relay 12 and board terminal 23, to board terminal 24 so that, normally, the terminals 18 and 24 are connected together for current to flow from the power supply 42 in the positive one of terminals 11 via the contact set 12A ef relay 12.The fault sensing circuit 41 is connected between the terminals 24 and 25 so that, normally, it receives the voltage appearing between the terminals 11 but, when the relay 12 changes over, the connection between terminals 18 and 24 is broken and the fault sensing circuit 41 is put out of action as described above. The fault sensing circuit comprises a resistor 26A connected in series with a Zener diode 27 between the terminals 24 and 25. The Zener diode 27 has a turn-on voltage a little greater than the voltage normally present between the lines 2 and 3, e.g. 15 volts so that it is normally off.

A transistor 28 has its emitter connected to the terminal 24, its collector connected via a fault indicator lamp 30 to an appropriate voltage point (this could be terminal 20 say), and its base connected by way of a resistor 29 to the interconnection point between the resistor 26A and the Zener diode 27. If a fault develops in the lines 2 and 3 or in the end-ofline unit 4, leading to a cessation in current through the lines 2 and 3, the voltage between the terminals 11 starts to rise towards the higher level, i.e. towards the full output voltage of the central station supply. Before it can rise to the level required to set off the remote alarm device 7, however, the Zener diode 27 is turned on, which prevents any further rise and also initiates switch-on of transistor 28 and hence the lighting of the fault indicator lamp 30.

Figure 6 shows a smoke detector device for use as one of the fire sensing devices 5 in Figure 7. The device comprises a smoke detector 31 including a two-chamber ionisation detector and operable over an input voltage range which includes the higher and lower voltage levels applied to the lines 2 and 3 by the central alarm station. The smoke detector is connected to the lines 2 and 3 by way of a full-wave rectifier 32. A resistor 33 and a capacitor 34 are connected in series between the input terminals of the smoke detector. The smoke detector 31 comprises an indicator device such as a lamp which is lit when the smoke detector has de tected smoke.When this happens, the central and remote alarm devices connected to the system are set off as described earlier and if these produce sound it may be required to silence them from the central station by briefly interrupting the supply to the lines 2 and 3.

However, it is desirable to maintain the indicatar device in the smoke detector "on", even if the smoke has in the meantime cleared, so that subsequent inspection will reveal which of perhaps many such detectors initiated the alarm. This is the function of the capacitor 34 which is charged up via the resistor 33 when there is voltage on the lines 2 and 3 and which bridges brief interruptions in that voltage, such as the interruptions used to silence the alarm indicators.

The rectifier 32 prevents the capacitor from discharging into the lines 2 and 3. The rectifier 32, resistor 33 and capacitor 34 may be provided in the fixed connector unit of a smoke detector comprising such a fixed connector unit and the part 31 which is constructed a remov- able plug-in unit. The smoke detector may be constructed as disclosed in my Application No.

6874/76 (U.K. Patent No. 1,568,084) and/or my Application No. 17203/77 (Serial No.

1,604,633).

Instead of smoke detectors, any of the devices 5 can comprise a flame detector or heat detector or there could be a mixture of such devices. A flame detector may require to be provided with suitable current and/or voltage limiting circuitry to, enable it to operate at both the higher and lower voltage levels supplied by the central alarm station.

The central alarm station 1 could be entirely of "solid-state" construction, i.e. the relay 12 could be replaced by solid-state switch means, possibly with appropriate re-design of the rest of the circuitry of the central alarm station.

Any of the lamps which may be used in the described system, e.g. for the indicator device in the smoke detector 31 or the lamp 30 in Figure 5, can comprise an incandescent filament lamp or any other suitable electric lamp, or a solid state device such as a light-emitting diode.

The end-of-line unit 4 shown in Figure 3 could be provided in the alarm system, not as a separate unit, but instead combined with whatever one of the sensing devices, switches and remote alarm devices is the most remote from the central alarm station 1. For this, the electrical arrangement need not be any different to what is shown, i.e. the diode Z2 and the resistor R5 can still be connected in series with one another between the lines 2 and 3, but these two components are simply provided as part of the above-mentioned most remote one of the other units of the system.

It will easily be appreciated that the number of sensing devices, switches and remote alarm devices, and the order in which these are arranged along the lines 2 and 3, can be changed as required.

So that the readiness for operation of the illustrated system can be tested at any time, the central alarm station 1 may be provided with switch means (not shown) for applying such a voltage to the lines 2 and 3 as to set off the remote alarm devices 7.

It will be seen that each of the devices 5 and 6 comprises a detection member, i.e. the switch 10 in the case of the devices 6 or the fire sensing device 31 itself in the case of the devices 5, together with an associated interface circuit i.e. the resistor R4 in each device 6 or the combination of components 32, 33 or 34 in each device 5, which adapts the detection member for use in the described system. Similarly, the remote alarm device 7 also each comprise an alarm member, e.g. a sounder or lamp, and an associated interface circuit such as the circuit shown in Figure 1. In each case, the alarm or detection member may be combined with its associated interface circuit in the form of a single unit.Alternatively, however, the associated interface circuit may be comprised in a separate subsidiary or auxiliary unit with which the relevant detection or alarm member is adapted to be connected. In the latter case, each subsidiary or auxiliary unit is preferably designed so that it can be connected with the appropriate detection or alarm member in detachable manner to form a single detection or alarming unit. Such a subsidiary or auxiliary unit may then for example be designed as a mounting unit for the relevant detection or alarm member, for example, a mounting unit in the form of a socket module adapted to receive the relevant member.

A fire alarm system, such as illustrated, employing only two output lines 2 and 3 between which a set of detection and alarm devices are connected in parallel can be installed as a single zone system, i.e. a system for monitoring fire risk in a single zone within which the output lines extend. However, the present invention can also be embodied in a multi-zone system, for example by having for each additional zone to be monitored an additional pair of output lines between which an additional set of detection and alarm devices are connected in parallel.

As a pair of output lines for apparatus embodying the invention, there may be employed two individual single electrical conductors.

Alternatively, the two lines may be respectively constituted by the two lines of a two-core or twin cable, for example polyvinyl-chlorideinsulated cables, mineral-insulated cables or rubber-covered cables. Where appropriate, po,wer intake for the power supply circuitry of the apparatus or for a control unit incorporating the power supply circuitry may be by way of a three-core cable connected to a.c. mains supply.

It will be appreciated that many different types of acoustic and visual alarm devices, for example bells, hooters, sirens, buzzers or lamps, may be employed in apparatus embodying the invention.

As well as or instead of the remote alarm devices 7, there could be provided between the lines 2 and 3 automatic fire combat devices (not shown) such as the electrical control units of sprinklers, explosive devices which are arranged to release fire extinguishing gas, and so on, these devices being provided with associated circuits such as that shown in Figure 1 for making the device normally inoperable until the second power supply condition appears.

Referring back now to, Figure 5, this may be modified as shown by the broken lines and by providing the items referred to below. Referring to the modification, the direct connection between the terminals 35 and 36 is removed and the resistor R6 is connected to, the negative side of the power supply 42 by way of the normally closed side of a changeover contact set 19d of relay 19. When relay 19 is energised, resistor R6 becomes disconnected from the negative supply side at these contacts which instead connect to the negative supply side the input of a current regulator CR having an associated input resistor CRR. The output of regulator CR is connected, by way of a Zener diode ZD in series with an ordinary diode OD1, to the line between terminals 23 and 24 and, by way of a diode OD2, to the terminal 25.

Terminal 26 is connected to one side of the coil of a relay MR by way of a normally open push-button switch NOPB, the other side of the coil being connected by way of a normally closed push button switch NCPB to a positive voltage point, e.g. terminal 18. The relay MR has a normally open contact set NOC and a normally closed contact set MCC. Contact set MCC is connected between terminal 26 and the winding of relay 12 and the alarm device local to the zone board. Contact set NOC is connected in parallel with switch NOPB.

In the normal "stand-by" condition of the modified system therefore, resistor R6 is in circuit and the voltage across it is monitored as before. During an alarm condition, resistor R6 is disconnected from the supply and instead regulator CR is connected between the negative supply side and terminal 25. The regulator supplies no current, however, because terminal 25 is disconnected at the relay 12 at this time.

The system can then be put into, a third condition, ie. an "alarm silenced" condition, by pressing button NOPB. This action opens contact set NCC so that relay 12 returns to the position it has during the standby condition whereas relay 19 returns latched in its alarm position. Current is now supplied by the regulator CR to, the lines 2 and 3, the supplied current being sufficient to maintain any indicator lamps in the individual smoke detectors and such operative but not to operate the alarm sounders. Relay MR is latched into its energised position by the contact set NOC until reset by operation of push button NCPB. If in the meantime, the cause of the alarm has been removed, the operation of button NOPB returns the system to its standby condition.

The above arrangement is most appropriate for a "single zone" system wherein there is only one circuit such as is shown in Figure 5 and one pair of output lines 2 and 3. For a multi-zone system, wherein there are a plurality of zone boards and pairs of output lines, the relay 19 may be provided with a further pair of changee.ver contacts. The further pair of changeover contacts of each of the plurality of zone boards are interconnected with each other and connected to a common local alarm circuit comprising a relay, a thyristor and alarm silence and reset switches in such a way that, where an alarm signal has been received from one zone and the alarm sounders subsequently silenced, if an alarm signal is received from another zone, the alarm state is set up again and the alarms are re-sounded.

In my Patent Application No. 6874/76, there is disclosed an alarm system wherein, if a detector unit such as a smoke detector is removed from its mounting, it only briefly breaks the alarm circuit, the purpose of this being to ensure that the system as a whole remain operative with the remaining smoke detectors and such. Figure 8 of the accompanying drawings shows a modification which may be made to the circuit of Figure 5 to, enable such momentary breaks to be detected at the zone board as a fault.In the modification, the collector of transistor 28 is not connected directly to the fault indicator lamp 30 but is instead connected via a resistor 8R1 to the control input of a thyristor 8TH of which the main current path is connected between terminal 24 and one side of the fault indicator lamp 30, the other side of the lamp being connected to a negative voltage point as before. A resistor 8R2 and an electrolytic capacitor 8C are connected in parallel between the control input of thyristor 8TH and that one of its main current path terminals wihch is connected to the lamp 30. The resistor 8R2 and the capacitor 8C shunt away from the control input of thyristor 8TH any leakage current passed by the transistor 28 when it is intended to be non-conductive, and also suppress any spurious transients which may appear at this control input.However, the time con stant of the capacitor and resistor is kept small enough to ensure that the thyristor 8TH will start and continue to conduct on reception of any short current pulse from transistor 28 resulting from the aforementioned momentary break when one of the system detectors is removed from its mounting.

WHAT I CLAIM IS:- 1. Signalling apparatus comprising: power supply circuitry for supplying electrical power between two output conductor portions thereof, the circuitry being switchable from a first supply condition to a second supply condition by a change in external circuit conditions between the said two conductor portions;; a detection device for connection between the said two conductor portions, when the apparatus is installed for use, by way of two supply lines respectively connected to those conductor portions, the detection device being operable to produce such a change in external circuit conditions between the said two conductor portions, when connected therebetween, in the event that an occurrence to which the detection device is responsive takes place while the said power supply circuitry is in its said first supply condition; and a signalling device for connection between the said two, conductor portions, in parallel with the said detection device, by way of the said two supply lines when the apparatus is installed for use, the signalling device being such that it is changed from a non-signalling state to a signalling state thereof, to provide a predetermined signal, when the said power supply circuitry is changed from its first to its second supply condition, the first and second supply conditions yielding two different voltage levels of the same polarity, which do not automatically alternate with one another to produce regular pulses.

2. Apparatus according to claim 1, including one or more further detection device(s) for connection between said two output conductor portions by way of said two supply lines, the or each further detection device being operable for producing said change in external circuit conditions when connected between the two output conductor portions in the event that an occurrence to which the detection device is responsive takes place.

3. Apparatus according to claim 1 or 2, including one or mcre further signalling devices which is or each of which is, for connection between the said two conductor portions, in parallel with the or each detection device, by way of the said two supply lines and the or each further signalling device being such that it is changed from a non-signalling state to a signalling state thereof, to provide a predetermined signal, when the said power supply circuitry is changed from its first to its second supply condition.

4. Apparatus according to claim 1, 2 or 3, wherein the signalling device comprises, or the signalling devices include, an audible or visible alarm.

5. Apparatus according to any preceding claim, wherein a said detection device comprises an alarm switch, such as a break-glass switch, and an interface circuit comprising a resistor in series with the switch.

6. Apparatus according to any preceding claim, wherein a said detection device comprises an automatic fire detection component such as a smoke detector, a heat detector or a flame sensor.

7. Apparatus according to claim 6, wherein the detection device mentioned in claim 6 further comprises two input terminals for connection to respective ones of said two supply lines and, connected between these two input terminals and said automatic fire detection component, an interface circuit which includes

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. only one circuit such as is shown in Figure 5 and one pair of output lines 2 and 3. For a multi-zone system, wherein there are a plurality of zone boards and pairs of output lines, the relay 19 may be provided with a further pair of changee.ver contacts. The further pair of changeover contacts of each of the plurality of zone boards are interconnected with each other and connected to a common local alarm circuit comprising a relay, a thyristor and alarm silence and reset switches in such a way that, where an alarm signal has been received from one zone and the alarm sounders subsequently silenced, if an alarm signal is received from another zone, the alarm state is set up again and the alarms are re-sounded. In my Patent Application No. 6874/76, there is disclosed an alarm system wherein, if a detector unit such as a smoke detector is removed from its mounting, it only briefly breaks the alarm circuit, the purpose of this being to ensure that the system as a whole remain operative with the remaining smoke detectors and such. Figure 8 of the accompanying drawings shows a modification which may be made to the circuit of Figure 5 to, enable such momentary breaks to be detected at the zone board as a fault.In the modification, the collector of transistor 28 is not connected directly to the fault indicator lamp 30 but is instead connected via a resistor 8R1 to the control input of a thyristor 8TH of which the main current path is connected between terminal 24 and one side of the fault indicator lamp 30, the other side of the lamp being connected to a negative voltage point as before. A resistor 8R2 and an electrolytic capacitor 8C are connected in parallel between the control input of thyristor 8TH and that one of its main current path terminals wihch is connected to the lamp 30. The resistor 8R2 and the capacitor 8C shunt away from the control input of thyristor 8TH any leakage current passed by the transistor 28 when it is intended to be non-conductive, and also suppress any spurious transients which may appear at this control input.However, the time con stant of the capacitor and resistor is kept small enough to ensure that the thyristor 8TH will start and continue to conduct on reception of any short current pulse from transistor 28 resulting from the aforementioned momentary break when one of the system detectors is removed from its mounting. WHAT I CLAIM IS:-

1. Signalling apparatus comprising: power supply circuitry for supplying electrical power between two output conductor portions thereof, the circuitry being switchable from a first supply condition to a second supply condition by a change in external circuit conditions between the said two conductor portions;; a detection device for connection between the said two conductor portions, when the apparatus is installed for use, by way of two supply lines respectively connected to those conductor portions, the detection device being operable to produce such a change in external circuit conditions between the said two conductor portions, when connected therebetween, in the event that an occurrence to which the detection device is responsive takes place while the said power supply circuitry is in its said first supply condition; and a signalling device for connection between the said two, conductor portions, in parallel with the said detection device, by way of the said two supply lines when the apparatus is installed for use, the signalling device being such that it is changed from a non-signalling state to a signalling state thereof, to provide a predetermined signal, when the said power supply circuitry is changed from its first to its second supply condition, the first and second supply conditions yielding two different voltage levels of the same polarity, which do not automatically alternate with one another to produce regular pulses.

a capacitor for maintaining a supply of energy to the fire detection component in the event of brief interruptions of the voltage supplied along said two supply lines, and a rectifier for preventing said capacitor from discharging into said supply lines.

8. Apparatus according to any preceding claim, wherein at least one said device comprises an actual signalling or detection component as appropriate and an associated interface circuit for interfacing the component to the rest of the apparatus, the component and the interface circuit being comprised in respective detachable connectible units.

9. Apparatus according to claim 8, wherein the unit comprising the interface circuit is constructed in the form of a mounting for the unit comprising the signalling or detection component.

10. Apparatus according to any one of claims 1 to 7, wherein at least one said device comprises an actual signalling or detection component as appropriate and an associated interface circuit for interfacing the component to the rest of the apparatus, the component and the interface circuit being combined in the form cf a single unit.

11. Apparatus according to any preceding claim including an end-of-line circuit for connection between those ends of the said two supply lines which, when the apparatus is installed for use, are remote from said power supply circuitry, the end-of-line circuit comprising a resistor and a Zener diode connected in series with one another.

12. Apparatus according to any preceding claim, wherein the or each signalling device comprises an alarm device and a line to alarm device interface circuit, which circuit includes voltage threshold detection means comprising output terminals connected to the alarm device and input terminals for connection to respective ones of said two supply lines, the threshold detection means being operable to pass energy from its input terminals to the alarm device only when the magnitude of the voltage at its input terminals rises above a predetermined threshold voltage of magnitude greater than zero.

13. Apparatus according to claim 12, wherein once the magnitude of the voltage at said input terminals has risen above said threshold, the threshold detection means is operable to continue to pass energy to said alarm device until reset even if said voltage magnitude falls below said threshold magnitude in the meantime.

14. Apparatus according to claim 12 or 13, wherein the threshold detection means comprises a potential divider arrangement including a resistance and a Zener diode connected in series with one another between the input terminals of the threshold detection means, and a controllable rectifier element having two main current path terminals which are connected to respective ones of said input terminals and having a control terminal connected to said potential divider arrangement.

15. Apparatus according to claim 14, when appended to claim 13, wherein a resistor is connected to the rectifier to maintain a holding current therethrough once the rectifier been fired even if in the meantime the magnitude of the voltage across the potential divider arrangement falls below the level at which the rectifier was fired.

16. Apparatus according to any preceding claim, wherein said power supply circuitry is included in a central alarm station which comprises two po,wer supply output conductor portions, power supply means connected to said conductor portions for normally providing at said conductor portions a power supply at one voltage level other than zero, and sensing means connected to said conductor portions for sensing changes in an electrical condition in an external circuit which, in use, is connected to said conductor portions for responding to such a change to cause said power supply means to supply power at another voltage level other than zero of the same polarity.

17. Apparatus comprising a station for use in an electrical system, the station comprising two power supply output conductor portions, power supply means connected to said conductor portions for normally providing at said conductor portions a power supply at one voltage level other than zero, and sensing means connected to said conductor portions for sensing changes in an electrical condition in an external circuit which, in use, is connected to said conductor portions, and for responding to such a change to cause said power supply means to provide at said conductor portions a power supply at another voltage level other than zero of the same polarity, which does not automatically alternate with the first voltage level to produce regular pulses.

18. Apparatus according to claim 16 or 17, wherein the station comprises two source conductor portions for connection to a d.c. voltage source and a voltage dropping resistor and changeover switching means connected between said source conductor portions and said power supply output conductor portions, said sensing means being arranged to detect an increase in the voltage droop across the voltage dropping resistor to more than a predetermined level and then to cause the changeover switching means to connect said power supply output conductor portions directly to said source conductor portions.

19. Apparatus according to claim 18, wherein the sensing means comprises a resistor and a capacitor connected to series with one another across said voltage dropping resistor, and a transistor switch having a control input connected to the point cf connection between the resistor and capacitor, the transistor switch being arranged to control said changeover switching means.

20. Apparatus according to claim 19, wherein said transistor switch comprises a transistor and an electromagnetic relay of which the drive coil is connected to the collector terminal of the transistor.

21. Apparatus according to claim 18, 19 cr 20, wherein the changeover switching means comprises an electromagnetic relay.

22. Apparatus according to any one of claims 16 to 21, including fault sensing means which is arranged to sense the voltage between the power supply output conductor portions and to provide a fault singal when said voltage changes to a value indicative of a fault in said external circuit.

23. Apparatus according to claim 22, wherein the fault sensing means comprises a resistor and a Zener diode connected in series with one anther and connected, while the power supply is being provided at said one voltage level, between said power supply output conductor portions, and a transistor switch having a con trol terminal connected to the point of con action between the resistor and the Zener diode and an output connected to a fault indicator.

24. Apparatus according to claim 22 or 23, wherein the fault sensing means is put out af action when the power supply is being provided at said another voltage level.

25. Electrical apparatus comprising: power supply circuitry for supplying electrical power between two output conductor portions thereof, the circuitry being switchable from a first supply condition to a second supply condition by a change in external circuit conditions between the said two conductor portions, a detection device for connection between the said two conductor portions, when the apparatus is installed for use, by way of two, supply lines respectively connected to those conductor por tions, the detection device being operable to produce such a change in external circuit conditions between the said two conductor portions, when connected therebetween, in the event that an occurrence to which the detection device is responsive takes place while the said power supply circuitry is in its said first supply con dition; and an output response device for connection between the said two conductor portions, in parallel with the said detection device, by way of the said two supply lines when the apparatus is installed for use, the output response device being such that it is changed from a first state to a second state thereof, to provide a predetermined output response, when the said power supply circuitry is changed from its first to its second supply condition, the first and second supply conditions yielding two different voltage levels of the same polarity, which do not automatically alternate with one another to produce regular pulses.

26. Apparatus according to claim 25, wherein the output response device comprises a fire combat device.

27. Apparatus substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

28. Apparatus substantially as hereinbefore described with reference to Figures 1 to 4 and 7 of the accompanying drawings.

29. A station for use in an electrical system, the station being substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings, or with the modification shown in Figure 8.

30. A signalling device which is suitable for use in signalling apparatus according tc, claim 1, to perform the function of the signalling device specified in claim 1, the device having first and second conductor portions for connection to first and second lines, respectively, between which a first unidirectional voltage level is normally to be maintained, although a second unidirectional voltage level can exist between the lines when operation of the signalling device is required, said signalling device comprising cne and only one two-terminal electric circuit element, the conductivity of which suddenly increases when the voltage across it rises above a threshold value, and one and only one switch, said circuit element and switch not being connected to any other such circuit element cr switch (except via said lines when said conductor portions are connected to said lines) and said device further comprising an audible or visible alarm device such as a bell, hooter, siren, warning lamp or the like, said circuit element and switch serving to ensure that no current passes to the alarm device when the first and second conductor portions have said first unidirectional voltage level between them and to ensure that current passes to the alarm device and operates it when said second unidirectional voltage level exists between said conductor portions.

31. A signalling device according to claim 30 in which said circuit element is a Zener diode which is in series with resistance means in a circuit branch one point of which is connected to a trigger input of said switch which is connected in series with said alarm device between said first and second conductor portions.

32. A signalling device according to claim 30 or 31 in which the switch is a thyristor.

33. A signalling device according to claim 30, 31 or 32 in which a resistor is connected in parallel with said alarm device.

34. A signalling device substantially as described above with reference to Figure 1 of the accompanying drawings.