CA1225447A - Vigilant fire alarm system - Google Patents

Abstract

ABSTRACT

VIGILANT FIRE ALARM SYSTEM

This invention relates to a communications system comprising central control means and a plurality of remote stations capable of communicating with said control means wherein the communication between said central control means and each remote station by means of at least one tone burst whereby data passing between control means and the remote station is determined by the duration and/or the number of tone bursts transmitted.

Description

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TITLE
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DESCRIPTION
. =.. . _ The present invention relates to a communicator system and has particular reference to a communication system for use with fire, intruder and access systems incorporating the advantages of a microprocessor and associated technology~ Many proposals have been advanced for the use of microprocessor technology in the use of fire intruder and access systems; In particular, proposals have been submitted for a pair of ring circuit conductors to be placed in a building in which alarm indications are desired, the ring conductors being connected to central processing means either within the - building or via modems externally therefrom~
Detectors/sensors are connected at spaced intervals along said conductors; such a system generally operates on the basis that the central processing system will sequentially or systematically interrogate each detector/sensor connected across the pair of ring conductors~ The ring conductors in this case are used as a source of both general power and as a communication means to the central processor~

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Such a system operates by the transmission of digital data between the central processor and the various sensors and detectors in the ring circuit~
While basically satisfactory and capable of extensive development towards automated alarm systems with a reduced tendency towards false alarms, many difficulties are associated with such an arrangement~
Because of the nature of the digital information transmitted, the quality of the conductors forming the ring circuit to the central processor needs to be of a high standard and this in turn, having regard to the size of buildings, such as factories, normally protected, involves a very considerable penalty in terms of the cost of wiring~ Secondly, because of the quantity of digital information transmitted there is always a tendency for errors to occur either on trasmission or reception, thus giving rise to an alarm/fault condition~
For this reason, although such ring circuit/central processor systems have been proposed extensively in practice, the price and technical disadvantages have not been overcome~
Furthermore, to provide a comprehensive system all feeding central information back to one station in a large factory a very complexed wiring system becomes necessary and in practice several conductor rings of high gL4~

quality are thus further increasin~ -the cost.
The present applican-ts have devised a general improvement in communications systems which seeks to overcome the problem of using high quality electrical conductors in such fire alarm systems.
According to the present invention there is provided a communication system for a fire alarm, in-truder or access system comprising:
(i) a control station;
(ii) a main conductor loop connected with thc control station for the supply of power and the transmission of signals, each end of the loop being connected with the control station; and (iii) at least one nodal unit provided in the main conductor loop;
characterised in that a subsidiary conductor loop is operatively connected with at least one nodal unit, at least one sensor is operatively connecte~ with each subsidiary conductoE loop and in that the control station includes means ~or sequentially monitorin~ the status of a first nodal unit and the sensors in the sub-sidiary loop associated therewith, and therea~ter to monitor the second and subsequent nodal units- an~ the sensors associated therewith in sequence whereby in the event of a break or a short in the main and/or sub-sidiary loop, the nodal units and/or the sensors cause or allow communication with the units and/or sensors adjacent the break or short without loss of performance therefrom wherein the monitoring means effects communi-cation between the central control means and the plur-ality of remote nodal units and associated sensors by means of at least one tone burst whereby data passing between control means and the remote station is deter-mined by the duration and/or the number of tone bursts transmitted.

The present invention has the advantage, therefore, that the units react to a break or short in the main and/or subsidiary loops to switch out or isolate the break or short and communications are provided around a loop, this means that the breaking of that loop allows communication from either direction; The control ~eans preferably permits reading of data from and to each nodal unit and sensor in either direction either in the main and/or subsidiary conductor loops~
The communication system in accordance with the present invention may be a burglar alarm system, a fire alarm system an access control system, a system for monitoring and controlling the energy requirements of a building or a combination of some or all of these~ The control means may include a microprocessor capable of reading analog data and/or digital data supplied from a sensor to the processor in the control means. The said processor may then examine, store and collate the information received to identify whether a situation is normal or whether there is a potential alarm condition, or if there are any abnormal signals as a result of a system fault~ The control means may drive peripheral devices such as visual display units and printers and may provide automatic communications via telephone and/or radio in the event of a fault or alarm being diagnosed 5~

and may be provided with means selectively for calling police fire or maintenance services depending upon the nature of the alarm that is raised; In a typical example of the present invention the sensors may be adapted to sense a fire or incipient fire condition and changes in sensor performance can be identified and the sensors themselves interrogated by the microprocessor provided in the control means~
The present invention also includes a communications system comprising central control means and a plurality of remote stations capable of communicating with said control means wherein the communication between said central control means and each remote station by means of at least one tone burst whereby data passing between control means and the remote station is determined by the duration and/or the number of tone bursts transmitted~
The control means may provide a trigger signal for response by remote stations and a trigger signal may preferably be of a different sense to the sense of a tone burst from the remote station The remote station may be nodal unit or sensor unit of the fire alarm system of the present inventionv In another embodiment of the present invention in order that the unit may react to a break or short to switch out or isolate the break or short in both the main ~2~S~7 power loop or the subsidiary power loops, each nodal unit and/or detector may be provided with a fault indication and control circuit; In a particular embodiment, a control circuit for switching a device to an alternative supply in response to a fault in a ring conductor loop including a pair of supply conductors may comprise transistor means for monitoring a potential difference between said pair of supply conductors, switching means capable of switching to isolate a portion of one of said supply conductors, said portion being connected via an operating device to the other of said supply conductors and diode means connecting said portion to said one supply conductor to bridge the or each switching means, the arrangement being such that in the event of a fall in the potential difference between said conductors below a thresh-hold due to a fault, the transistor causes or allows the switching means to act to isolate said portion from the fault whereby the operating device maintains its function via said diode-connection to a non-faulted portion of said supply conductor. The switching means may be either electromechanical or may be a field effect transistor~ Other switching means may also be employed such as transistor switches etc~
A communication system in accordance with the present invention providing for interrogation of the 5~

sensors associated therewith may, in the case of a fire alarm, have the following features and advantages:-1. Early warning of a fire or an incipient fire situation from the processed information available from the fire sensor~

2~ The system will be self-monitoring to identify any system or detect fault or deviation from normal~

3~ The ability to call up a system as required for routine service calls~

4. The ability to ignore infrequently occuring transient faults and to filter out the effects of such information errors caused by these faults~

5. The information can be stored in the system`log which will then provide an audit trail relating the past history of the equipment and further may provide an historical record or log of the maintenance carried out~
The sensor in accordance with the present invention may be a fire sensor which will operate on one or more of the following principles:-thermal detection, optical flame detection, ionisation smoke, light scatter smoke obscuration smoke ~%S44~

The sensors of the invention may include access control sensors, ultrasonic detectors, passive infra red detectors and microwave Deppler sensors;
The system in accordance with the present invention may be associated with an audible alarm such as an alarm bell or a siren.
In a particular embodiment of the present invention, a fire sensor may sense the environmental phenomenon to which it is sensitive, convert this to a digitally coded analogue signal for onward transmission to the control unit; The control unit interprets the sequence of signals and communicates any deviation from normally expected trends, activate an audible alarm and where appropriate, activate suitable digital communications lS equipment for communication with fire, police and/or caretaking authorities~ ~t the same time the control means may act to operate a range of devices such as door release units, emergency lighting units, ventilators and so on~
Although described with respect to fire alarm systems it is to be understood that the system of the invention may be used with fire intruder and access systems~
Following is a description by way of example only and with reference to the accompanying informal drawings ~5447 of a method of carrying the invention into effect~
In the drawings:-Figure 1 is an informal sketch of a fire alarm system in accordance with the present invention~
Figure 2 is a diagram of a tone burst communication for use in the fire alarm system of Figure 1 Figure 3 is a detail of Figure 2;
Figure 4 is a circuit diagram of the switch mechanism for the nodal units and detectors of Figure 1 Figure 5 is a block diagram of the line drive circuitry for a control unit of the alarm of Figure 1.
Figure 6 is a diagram showing the build up of the tone pulse of Figures 2 and 3 with timing of the switches in Figure 5.
The system illustrated in the accompanying drawing comprises a control unit indicated generally at 10 having a power supply 11 and a general output bus 12 for output equipment such as VDU's, printers, digital communicators and the like, all of known construction, configuration and function; The control unit 10 may be supplied with an alarm line 13 having an audible alarm means 14, such, for example, as a bell. The control unit 10 is provided with a main conductor ring 20 consisting of a pair of conductors 40+, 40~ at a first end of loop 20 and 41~, 41~ at a second end which extends as a closed ring around ~25~9L7 a building or area to be protected, the arrangement being such that the ring 20 is provided with power and/or signal input outout in each direction therearound; The ring 20 is provided with a plurality of nodal units 21, 21', 22" coupled in series within main loop 20.
Each nodal unit 21 is provided with a pair of subsidiary ring conductors 22, each subsidiary ring conductors 22 having a plurality of sensors 23 which in the case of a fire alarm will be fire detectors, the fire detectors 23 being connected between the conductors consisting subsidiary ring 22 at spaced intervals therearound~ Each nodal unit 21 incorporates a series of switches which are capable, in response to a fault or a short circuit signal in the main ring 20, of opening switches to provide an open circuit.
In the event, therefore, of a short circuit at point A in the main ring, the short circuit would be detected between nodal units 21 and 21' and each of nodal units 21 and 21' would operate to dis-enable main loop between nodal units 21 and 21' thereby isolating the short circuit. Each of nodal units 21 and 21' would still be operative due to the fact that nodal units 21 would be fed direct by main ring 20 from control unit 10 and nodal unit 21' would be similarly served by the other portion of main ring 20~ In the event of an open circuit at ~2S~L~7 point A, the system would operate in precisely the same way except there would simply be a discontinuity in a conductor between nodal unit 21 and nodal unit 21'~
Each of the detectors 23 in a subsidiary ring 22 are S similarly provided with a switching means whereby in the event of a short circuit in a subsidiary ring at, for example point B, between detectors 23 and 23' of subsidiary ring 22 the detectors 23 and 23' would react to provide an open circuit each side of the short circuit at point B and yet each of detectors 23 and 23' would remain operative, since the current can flow and signals can pass from and to nodal unit 21'' each way around the subsidiary ring 22~
The control unit lO includes means for addressing sequentially nodal units 21 and then in sequence each of the detectors in the subsidiary ring 22 associated therewith and then proceed to the next nodal unit 21' and sequentially to each of the detectors in the subsidiary ring associated therewith and so on~
Figure 5 is a block diagram of the line drive circuitry of control unit 10~ In the diagram, switches L, S, D~ and Delta (~ ) are operated from a microprocessor board forming part of control unit 10~
From the diagram, it will be seen that the closing of switch L will apply full voltage across the primary ~ZZ5~4~7 loop conductor 40+ and 40-, while the closing of switch S
will apply a full voltage across conductors 41+ and 41-.
Closing of switch D causes the voltage across the detector loop to drop to zero. The detector loop components are preset by a long (in Figure 2, a 2~8 millisecond (ms)), positive pulse applied to the detector loop by closing switches L and S. This ensures that the first nodal unit 21 in the loop will be addressed first.
Subsequent negative going pulses 32 of duration 1.5 ms (see Figure 2) will address in sequence the individual detectors 23 associated with that particular nodal unit (21~. The next positive going pulse 33, of 100 ms duration addresses the next nodal unit 21' and the sequence is repeated for all the detectors 23' associated with that nodal unit. This is repeated until all the nodal units in the system have been addressed~
In all but the first lOOms of long positive pulse 31, switches L and S are synchronous. Du~ing this initial lOOms period, however, switch L is on and S is off and during this period only switch delta (Q ) is open.
In normal operating conditions the detector will see the full length of pulse 31 since it will pass completely around main ring 20 from 40+, 40-, to 41+, 41-~ If the ring 20 is open-circuited, then nodal units in certain sections will not receive the Eirst lOOms of the pulse 31; this omission is registered by each of such nodal units to give a subsequent indication of the position of the open circuit fault when the said affected nodal units are addressed~
When switches L, S and D are open and switch delta (~ ) is closed, the main loop 20 is being supplied from both ends 40 and ~1 with one half of the supply voltage via impedance 220, thus constituting the datum voltage level 30 of the signal pulse;
Typically the address is effected by means of tone burst which is shown in some detail in Figures 2 and 3 of the accompanying drawings. The power supply on the pair of conductors of ring 20 is at a steady state indicated at 30. At the commencement of the addressing sequence, control unit 10 outputs a positive pulse or tone burst 31 of a duration in the specific example illustrated of 288 milliseconds. This pulse serves to zero sequentially all the nodal units and in their associated detectors in the ring 20; At the same time the first nodal unit (21) in the ring 20 is activated to respond to an input pulse~
The initial pulse 31 causes nodal unit 21 to address sequential~y each detector in it's subsidiary ring~ A
pause in signal transmission of 202.5 milliseconds is allowed for nodal unit to reply and complete it's address of all the detectors 23 in its subsidiary ring B; The first 12 8 milliseconds following the end of the tone burst 31 is permitted for a reply by the nodal unit itself as to the status of the node including any fault indications which are determined in the manner hereinafter described; The return information is given by way oE a negative pulse relative to the trigger pulse for the nodal unit 21 and the length of that tone burst determines the status of that node~
Thereafter, a period of approximately 5 milliseconds is allowed as a "break glass" window as hereinafter described.
The nodal unit itself is then caused to transmit a 1.5 millisecond negative pulse to address the first detector in the circuit. The cletector then has a period of 10.5 milliseconds within which to reply. The length of the tone burst transmitted in that 10~5 millisecond period determines the nature of the reply~ A second pulse 32' triggers the response from the second detector in the subsidiary circuit to nodal unit 21 and so on~ At the completion of the sequence aftér 202~5 milliseconds nodal unit 21 will have completed it's address and will not be responsive to further short trigger pulses shorter than the 288 millisecond trigger pulse which initiates the sequence, ~22~47 At the completion of the first sequence of information from nodal unit 21, the control unit 10 provides a further pulse 33 of approximately 96 millisecond duration which then triggers the next nodal unit (21') in ring 20. Again, this nodal unit 21' repeats the sequence of addressing each detector in its circuit and at the completion of that address and response by the detectors, a nodal unit 21' is disenabled and a further 96 millisecond pulse activates the next nodal unit 21'' in the circuit for response in a similar way~ This sequence continues until all the nodal units in circuit have addressed sequentially each of the detectors in their subsidiary circuit, all the responsive tone bursts - having been fed back to the central control unit 10. In the particular embodiment in accordance with the present invention, the period for complete address of a full system is 4~968 seconds.
It will be appreciated by a man skilled in the art that the information collected in one complete sequence around the detection and nodal circuits will provide information which can be compared with datum information held in the control unit 10. Variations of any responses in terms of length of response pulses from both nodal unit status and detector units will indicate a change in status and will enable conditional information to be ~a~2591~4~

prepared by the computer or central control unit 10 as to the status of any or all of the detectors and any or all of the nodal units~ The detectors 23 can be the same or different and once the steady state parameters have been determined any variation from that steady state parameter can be used and interpreted to produce a number of conditions between fault, caution, abnormal heating to full alarm~
Each or some of the nodal units 21 may include a manual alarm point such, for example, as a manual alarm of the break glass type;
Because of the extensive period of the 4~968 second addressing sequence, in the event of a manual alarm, it is necessary to provide in each nodal address sequence a lS short period in which the break glass units can superimpose their pulses within the circuit A break glass unit once triggered will give something approaching a pulse long enough to provide a tone burst during the gap referred to as the break glass window in the nodal circuit described above~
~hen a nodal unit has a manual call point signal latched to it, it produces a 300 microsecond negative going response pulse to main loop 20 for detection by the detector loop at a time 17~5 ms after the next nodal address pulse 34 (see Figure 3). It should be noted that ~Z~49~7 this oeeurs whether the partieulae nodal unit is being addressed or not; The deteetion of this pulse at the control unit 10 will result in a full alarm being transmitted by eontrol unit 10 irrespective of the nodal status and detector status given elsewhere~
On receipt of a manual alarm pulse described above, the control unit 10 initiates a search procedure; a presetting positive pulse is transmitted and all the nodal units are sequenced in rapid suecession; no deteetors are addressed at this stage~ The total cyele time is 820ms and during this eyele the manual eall point loeation ean be determined and verified~ 3 seeonds after initiation oE the manual call point signal, the said signal deeays, thus allowing the system to revert to normal operation~ The partieular nodal unit connected with the tripped manual alarm point will eontinue to give an alarm status signal during address by control unit 20 until sueh time as the eall point and the assoeiated nodal unit are reset~
The tone burst in this speeifie example operates at a frequency of 10 KHz and the nodal unit response is arranged io give a speeifie number of eycles at a nominal frequency of 10 KHz, the number of cycles being dependant on the appropriate eonditions assoeiated with that particular nodal unit, for example:-4~7 Normal condition 128 cycles Detector loop fault 96 cycles Open circuit 80 cycles Short circuit 64 cycles "Break-glass" line fault 48 cycles Remote input fault 32 cycles BG or input alarm 16 cycles In addressing individual detections 23 within the nodal circuit or subsidiary ring 22, the frequency response from individual detectors is such that the duration of the reply pulse train is an analogue representation of the sensed fire condition level~ Under normal conditions, the duration is of the order of 3 ms;
an increasing fire signal level will increase the duration of the pulse train~
Different types of detector 23 are arranged to have different nominal response frequencies so that each nodal unit 21 can distinguish different detector types within its suosidiary loop 22 as well as its location therein~
In this embodiment the nominal response frequencies are as follows:-Photoelectric sensors 6 KHz Ionization Sensors 8 ~Hz Thermal Sensors 10 KHz Other unspecified type(s)12 KHz 5~4~

Figure 6 shows the timing sequence of switches L, S, ~, and D to produce the tone burst of Figures 2 and 3~
The sequential address of each nodal unit provides for the status of the detectors in each nodal unit loop 22 and the presence of faults to be communicated to the control unit 10~ The control unit operates with hexadecimal numbering with the nodal units being numbered zero to a maximum of 15 in the main ring and the detectors on each subsidiary ring being numbered 1 to lS
around the subsidiary ring~ The control unit lO
commences a sequence of address and provides a continuous scheduling address sequentially for each of the nodal units in the main ring~ Thus, in the embodiment illustrated in Figure 1 of the accompanying drawings, the control unit will initially address nodal unit 21 ~numbered zero) and then sequentially each of the detectors on each subsidiary ring 1 to F associated ~herewith~ The control unit lO will then seek to address nodal unit 21' (numbered 1) and then each of detectors 0, 1, 2~ F in the subsidiary ring 22 associated therewith and so on completely around the main ring 20~ The period for a complete address will depend on the number of nodal units within a circuit and the number of detectors in each subsidiary circuit~ .
In the event of a fire alarm condition being sensed ~%2S~L4~

by detector 23' in the subsidiary ring 22 associated with nodal unit 21'', a signal would be despatched around the main ring to the control unit lO whereby the alarm condition would be registered and output would be transmitted along output 12 to digital communicator or the like, and at the same time, audible alarm 14 would be rendered operative~ The system described above provides all the advantages of the normal intelligence system and providesl therefore, for the elimination of a number of false alarm circumstances~ For example, excessively rapid or abnormally slow rise in sensor signals can readily indicate a false condition and in these circumstances the system can contain its own built in check~
In the event of a fault, the switching circuit shown in Figure 4 becomes effective~ The main ring 20 comprises a pair of conductors 40 and 41 which form part of the main loop A+ and A- being on one side of the nodal unit 21 and B+ and B- being on the other~
The main ring circuit in this particular embodiment is provided with a take-off 42 from main conductor 41 and the main conductor 41 is capable of being provided with an isolated portion 43 juxtaposed the junction 42 with the main conductor 41 by means of a field effect transistors, 44/44' on each side of junction 43~ A load ~2254L~7 - 21 ~
resistor 45 bridges main loop 41 A-, B- between field effect transistors 44/44' (see Figure 4). Connection 42 is connected via the operating main body oE the nodal unit to the positive conductor 40~ Positive conductor 40 is provided with a line connection 45 which i.s connected to provide a positive supply via 46 to the base of PNP
transistor 47~ This enables current flow via line connection 48 and transistor 47 and the associated circuitry to maintain the field effect transistors 44/44' closed thus connecting the main conductor A- and B- with isolatable portion 43 and nodal unit connection 42 In the event of a fault, the reduction in the potential difference between conductor 40 and conductor 41 below a threshold will result in a corresponding reduction in the positive voltage applied to the base of transistor 47 and will thus stop the flow of a substantial current through the transistor 47 thus causing or allowing the field effect transistors 44/44' to open~ In these circumstances, the junction 43 is isolated from main loop A- and the connection 42 is then connected to the main loop 41 by means of diodes 50 and 51 respectively~ If, therefore, the fault is in the main conductors A- A+, then the diode connection will be effected via diode 51 and nodal unit will rendered operative again and the fault indication can be included ~ZZ5~7 in the nodal reply as described with respect to Figures 2 and 3~
The particular advantage of the system described above is that it provides automatic switching in the event of a fault, it provides a clear indication of the area of the system in which the fault lies and it does not permit the fault to render any part of the circuit inoperative. Furthermore, the nature of the data transmission by means of tone bursts or time measured pulses does away with the problem of the transmission of direct digital information and the system as a whole permits the use of a much cheaper wiring installation because of the ready indication of the fault The system further reduces substantially the tendencies of such an automated fire alarm system to issue false alarms~

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A communication system for a fire alarm, in-truder or access system comprising:
(i) a control station;
(ii) a main conductor loop connected with said control station for the supply of power and the transmission of signals, each end of said loop being connected with said control station; and (iii) at least one nodal unit provided in said main conductor loop;
characterised in that a subsidiary conductor loop is operatively connected with at least one nodal unit, at least one sensor is operatively connected with each subsidiary conductor loop and in that said control station includes means for sequentially monitoring the status of a first nodal unit and the sensors in the sub-sidiary loop associated therewith, and thereafter to monitor the second and subsequent nodal units and the sensors associated therewith in sequence whereby in the event of a break or a short in the main and/or sub-sidiary loop, the nodal units and/or the sensors cause or allow communication with the units and/or sensors adjacent the break or short without loss of performance therefrom wherein the monitoring means effects communi-cation between the central control means and the plur-ality of remote nodal units and associated sensors by means of at least one tone burst whereby data passing between control means and the remote station is deter-mined by the duration and/or the number of tone bursts transmitted.

2. A system as claimed in claim 1 wherein the control station transmits a trigger signal for initial-ising the response by said nodal units and their assoc-iated sensors.

3. A system as claimed in claim 2 wherein the trigger signal is of a different sense to the tone burst received from the nodal units and/or their associated sensors.

4. A system as claimed in claim 1 wherein each nodal unit and/or sensor is provided with a fault indi-cation and control circuit adapted to react to a break or short in one or both of the main conductor loops and a subsidiary conductor loop.

5. A system as claimed in claim 4 wherein the fault indication and control circuit comprises a pair of supply conductors, transistor means for monitoring a potential difference between said pair of supply con-ductors, switching means capable of switching to isolate a portion of one of said supply conductors, said por-tion being connected via an operating device to the other of said supply conductors and diode means con-necting said portion to said one supply conductor to bridge the or each switching means, the arrangement be-ing such that in the event of a fall in the potential difference between said conductors below a predeter-mined threshold due to a fault, the transistor causes or allows the switching means to act to isolate said portion from the fault whereby the operating device main-tains its function via said diode-connection to a non-faulted portion of said one supply conductor.

6. A system as claimed in claim 5 wherein the switching means is selected from the group comprising a relay, an electromechanical device or a field-effect transistor.

7. A system as claimed in claim 1 wherein the control station is adapted to drive peripheral devices selected from one or more of visual display units, print-ers, automatic communications via telephone and/or radio, and in the event of a fault or alarm being diag-nosed, to device means for selectively calling police or fire services depending upon the nature of the alarm indicated.

8. A system as claimed in claim 1 wherein the control means includes a pair of power supply conduct-tors, a pair of equivalent impedances connected in series between said power supply conductors, primary and secondary output lines connected to a junction point between said two impedances, a junction in the negative conductor provide primary and secondary negative con-ductor loops, fault detector circuits provided between the primary and secondary positive supply and the prim-ary and secondary negative supply, a first switch means between the main input and the positive primary supply downstream of a diode to the connection with the imped-ances, a second switch between the main positive supply and the secondary positive supply, a third switch means between each of said primary positive and secondary supplies incorporating diode connections therebetween and a false switch means for connecting the secondary negative supply to the primary negative supply, the arrangement being such that on closing the first switch means, full voltage supply is applied across the primary loop conductor between the positive primary supply and the negative primary supply, while closing the second switch means will apply a full voltage across the cor-responding secondary positive and negative supplies to the loop.

9. A system as claimed in claim 8 wherein closure of the third switch causes the voltage across the de-tector loop to fall to zero.

10. A system as claimed in claim 8 wherein the sensors are constructed and arranged to be preset by a long positive pulse supplied thereto, by closing said first and said second switches thus ensuring that the first nodal unit in the primary loop is subject to the first address from the control unit.

11. A system as claimed in claim 10 wherein nega-tive going pulses are applied by closure of third switch means to produce negative going pulses to address in sequence the individual detectors associated with a said nodal unit.

12. A system as claimed in claim 11 wherein a further positive pulse is applied to the primary and secondary circuit by closure of the first and second switches to address the next nodal unit in the main loop and the sequence of switching is repeated until all the nodal units in the system have been addressed.

13. A system as claimed in claim 8 wherein the first and second switch means operate synchronously except for an initial period of the long positive pulse and during this initial period the first switch means is open thereby permitting detection of a fault within the main conductor loop.

14. A system as claimed in claim 8 wherein the first, second and third switch means are open and the fourth switch means is closed, whereby one half of the supply voltage is applied to the main conductor loop thus constituting the datum voltage level of a signal pulse.

15. A system as claimed in claim 8 wherein the control means includes signal decoding means to deter-mine the number of cycles of the duration of each tone burst received, thus indicating the condition of nodal units and their associated detectors.