US3882476A

US3882476A - Alarm system with periodic test section

Info

Publication number: US3882476A
Application number: US349989A
Authority: US
Inventors: Stig Tore Johannes Lofgren
Original assignee: IMPULS REGULATOR LYCKSELE AKTI
Current assignee: IMPULS REGULATOR LYCKSELE AB; IMPULS REGULATOR LYCKSELE AKTI
Priority date: 1972-04-13
Filing date: 1973-04-11
Publication date: 1975-05-06
Anticipated expiration: 1992-05-06
Also published as: FR2180085B1; DE2318803C3; SE375395B; FR2180085A1; GB1433172A; DE2318803A1; DE2318803B2

Abstract

An alarm system includes detecting means having conductors. The impedance between the conductors is at least partly dependent on the alarm state to be detected. An alarm state detection portion includes a trigger amplifier in current circuit with said conductors. An alarm signaling portion indicates actuation of said trigger amplifier. A test circuit coacts with the normal non-alarm impedance between said conductors automatically in spaced test intervals for changing the trigger level of the trigger amplifier so as to operate same. The input level of said trigger amplifier, in periods between test intervals and in the absence of an alarm state, normally is of an alarm preparatory level below the normal trigger level thereof.

Description

United States Patent [191 [111 3,882,476 Liifgren 1 May 6, 1975 ALARM SYSTEM WITH PERIODIC TEST Primary Examiner-John W. Caldwell SECTION TEX TILE OR PAPER STRIP Assistant ExaminerDanie1 Myer Attorney, Agent, or FirmWoodhams, Blanchard and Flynn [5 7 ABSTRACT An alarm system includes detecting means having conductors. The impedance between the conductors is at least partly dependent on the alarm state to be detected. An alarm state detection portion includes a trigger amplifier in current circuit with said conductors. An alarm signaling portion indicates actuation of said trigger amplifier. A test circuit coacts with the normal non-alarm impedance between said conductors automatically in spaced test intervals for changing the trigger level of the trigger amplifier so as to operate same. The input level of said trigger amplifier, in periods between test intervals and in the absence of an alarm state, normally is of an alarm preparatory level below the normal trigger level thereof.

l7 Claims, 3 Drawing Figures PATENTEDHAY 3,882,476

SHEET 1 5 TEX TILE OR PAPER STRIP HHTMHHNH PATENTED MAY 5 I975 SHEET lEkw tmlql ALARM SYSTEM WITH PERIODIC TEST SECTION The present invention refers to an alarm system.

The demand for efficient and reliable alarm systems is progressively increasing, especially with regard to socalled flood alarms. When leakages occur in enclosed spaces for example, it is imperative that they are detected within a short space of time, i.e. before actual flooding takes place. In some instances, it is required that such an alarm is triggered when the humidity of an enclosed space increases. Hitherto, it has been difficult to satisfy this requirement, since the majority of flood alarms are based on making of a switch by means of a float and like devices. If liquid, such as water, is allowed to escape from pipes etc. unattended to such an extent that the flow on an enclosed space becomes covered with water, the amount of damages caused thereby may be quite considerable. It is therefore desirable that an alarm is signalled as soon as the liquid begins to escape onto or moisten surfaces located beneath the alarm monitor. In this way, preventative action can be taken at an early stage.

It is known to the art to use alarm signalling devices comprising a textile material on which electrical conductors connected to an electric alarm device are arranged. The mode of operation of this kind of alarm is such that when it is subjected to moisture, the electrical transition resistance is considerably reduced, causing the alarm to be signalled. The alarm system of the present invention is based on the same principle, but affords an improvement to known systems, whereby it is possible to control effectively the operating condition of both the device detecting the presence of moisture and embodied in the alarm together with the extent to which said device is activated.

Accordingly, there is provided an alarm system having a moisture detecting means in the form of electric conductors, between which is located an impedance, the value of which is completely or partly dependent on the state to be detected, said electrical conductors being connected to an electrical alarm signalling device comprising an alarm state detecting portion, and an alarm signalling portion, the first mentioned portion being constructed as a trigger amplifier, the input of which forms a current circuit with said electrical conductor. The alarm system of the present invention is mainly characterized by means which control at intervals amplification of the trigger amplifier to trigger operating levels by coaction with the impedance present between said conductors, wherewith the trigger amplifier in the periods between said intervals has such amplification that in cooperation with said impedance it takes an alarm preparatory state at levels beneath said trigger operating level.

In order that the invention will be readily understood and further features made apparent, an alarm system constructed in accordance with the invention will now be described with reference to the accompanying drawing, in which FIG. 1 illustrates diagrammatically an alarm system according to the invention and constructed to detect the presence of moisture and flooding in an enclosed space. FIG. 2 is a fractional view of the actual moisture detecting device used with the alarm system shown in FIG. 1, and FIG. 3 is a circuit diagram of the illustrated alarm system, including necessary electronic amplifying components.

The alarm system illustrated in FIG. 1 is installed in a room 1, on one wall of which is mounted an alarm signalling device 2. Extending from the device 2 is a moisture detecting strip-like device 3, the device 3 being placed against the floor 4 of the room. In practice the moisture detecting device may be placed beneath wallto-wall carpeting, when fitted, or similar floor coverings, or may be placed adjacent to or on the ceiling of a room, or in other places where leakages may occur.

With the embodiment illustrated in FIG. 2, the moisture detecting means 3 comprises a strip 5 of textile material, having woven therein two

electrical conductors

6,7 which are constantly current carrying electrically connected to the alarm signalling device 2. The

conductors

6,7 are connected together at the end of the strip 3 through a current-loop forming resistance 8, which will be described hereinafter with reference to the mode of operation of the alarm signalling device. The arrangement is such that if liquid should run out onto the floor 4, it will be absorbed by the strip 5 of textile material, whereupon the conductivity between the

conductors

6,7 will be increased, as a result of the electrical bridging effect afforded by the strip. This causes a change in the current conditions of the respective circuits in the alarm signalling device 2, whereupon the alarm is triggered, as will be explained hereinafter.

In order to obtain different alarm conditions and in order to increase the conductivity when the strip 5 is moistened, the strip may be impregnated with substances, such as salts, which are capable of producing such conductivity conditions between the

conductors

6,7 that an alarm can be triggered even when the increase in humidity of the room is only slight.

The actual moisture detecting means 3 may also have the form of a strip of paper, for example, on which the

conductors

6,7 are formed by applying conductive paint in lines on said strip. The moisture detective device may also comprise a composite device having several layers with which each alternate layer is conducting and each other layer insulating.

As will be seen from FIG. 3, the alarm signalling device 2 is constructed in three different sections which include electronic components, i.e. an alarm signalling section I, a test section II and an alarm condition detecting section III. This latter section is the input section to whichis connected a strip of liquid absorbing material, such as that shown in FIG. 2, provided with

conductors

6,7 and forming a current loop through the resistance 8, hereinafter called the terminal resistance. To each loop 5 there is alotted an electronic section 111. Only one electronic section III is illustrated in the drawing, and in practice the components forming part of said section are suitably formed on insert-type circuit cards. The electronic section III is constructed as an amplifier, and the input comprises resistances R1 and R2, of which R1 is connected to the conductor 6 in strip 5, while the conductor 7 is earthed (grounded) or connected to a terminal. The connecting point between the resistances R1 and R2 is connected to the base of a transistor T1, which together with a transistor T2 forms a Smith-trigger circuit via a resistance R3. A resistance R5 is connected in the Smith-trigger circuit as a feed back circuit. This latter circuit in turn operates a transistor T3 via a resistance R4. For the purpose of changing the trigger level, or threshold of the Smithtrigger, a resistance R6 is arranged in the connecting point between the base of the transistor T1 and the resistance R5. The free end of the resistance R6 is connected with corresponding free ends of the equivalent resistances in remaining alarm condition detecting units equivalent with the illustrated unit [11. The hereinbefore mentioned free end of the resistance R6 is connected with the collector of a transistor T12 in the test circuit ll, which will be described below. The collector of the transistor T3 is connected via a diode D1 to an alarm line and to illuminated diodes D3. As explained hereinafter, when alarm conditions are detected, the alarm circuit 1 is continuously supplied with current through the diode D1. As previously mentioned, the collector of the transistor T3 is connected to an illuminated diode D3, which is connected to an RC-mains C1,R8,R9, which via a diode D2 are arranged to transmit a pulse to the alarm circuit I when alarm conditions are detected. The illuminated diode D3 is fed via a resistance R7 and is also connected to a diode D4, the free end of which is connected to corresponding diodes in circuits equivalent to the circuit 111. The diode D4 is also connected to a resistance R17 in the test circuit II, as hereinafter described.

The diode D2 is connected with corresponding diodes in the remaining circuits lll, and is connected to the alarm section I via a resistance R10 and to the base of a transistor T4. This latter transistor is connected,

via its collector, to a thyristor T5 and to a multivibrator coupling formed of two transistors T6 and T7 and the RC-mains Rl2-15 and C2,C3. The emitter of the transistor T6 is connected via a diode D11 to the base of a transistor T8, the collector of which is connected with an alarm bell K, and the emitter of transistor T7 is connected with the base of a transistor T9, the collector of which is connected to an alarm lamp L1. The free end of the alarm lamp L1 and the free end of the alarm bell K are connected together and also to a rectifier circuit DC for current supply, fed by a transformer TR. The current supply circuit DC also feeds a signal lamp L2, which in turn is connected with the collector of a transistor T11 in the test circuit 11.

The base of the transistor T11 is connected with a thyristor T and a current supply circuit including the resistance R19. The thyristor T10 is also connected via a capacitor C6 to a circuit including the current supply resistance R to the transistor T12. A trigger circuit, comprising transistors T13 and T14 with associated resistances R21 and R22 and capacitor C7 connected to the base of the transistors T13, feeds the base of the transistor T12. The latter circuits are, inter alia, connected to a current supply circuit which includes diodes D7 and D8, which are of opposite polarity. The latter diodes thus connect to the connecting terminals of the resistances R19, R20, R21, and R2, and of transistors T1 and T3 in section III. Such diodes D7, D8 are connected to each of the ends of the secondary winding of the transformer TR. Connected to the diode D8 is a diode D9, which is connected with a circuit including the illuminated diode D10 and the resistance R18 and the capacitor C5. The resistance R17 is connected from diode D9 via a zener diode D6 to the current circuit of the thyristor T10, as well as to the aforementioned diode D4 in the alarm condition detecting circuits 11].

Testing of the system, which is effected automatically and periodically, takes place in the following manner. When testing the alarm condition detecting amplifier circuits III, the amplification (the trigger level) in all amplifying circuits is changed via resistances R6. This is effected by the fact that when the capacitor C7 is charged via the resistance R21, which is connected to the double unijunction transistor T13, a short pulse is transmitted to the transistor T12. The collector in the transistor T12 will thus obtain briefly a low voltage. As a result of this, the amplification on the alarm condition detecting amplifiers III is increased via resistances R6. Current will then pass via the

conductors

6,7 in the strip 5 and through the terminal resistance 8. When the relevant amplifying circuit 111 comes into operation, there first occurs a voltage via the diode D1, on all detector sections 111, which feed the multivibrator T6,T7. At the same time, however, a current is obtained on all alarm state detecting amplifiers 111, via the diode D2. The pulse from D2 also passes via R10 to the transistor T4 and actuates the transistor, which holds down the collector on the transistor T6, thereby preventing the transistor T8 from becoming conductive. The transistor T4 also blocks the transistor T7 via the capacitor C3. By means of the aforedescribed method, the transistors T8 and T9 are prevented from becoming conductive when relevant amplifying circuits etc. are being tested, i.e. when the operation of the strip loop is tested. As a result of the hereinbefore mentioned operation, i.e. the

increase in the amplification of the amplifiers lII, there is obtained a response pulse from relevant diodes D4, provided that the terminal resistances 8, and the circuits in general, are fault free.

If there should be a break on any of the loops 5 with an incoming test pulse the resistance R7 will not obtain a high current, since one of the amplifiers shunts down the resistance R17 via the diode D4 therein. Consequently, the thyristor T10 will not begin to operate, but the capacitor C6 will switch off the thyristor T10 which, however, can not be reenergized. The circuit thus operates in a manner such that at the same time as a pulse is obtained on the transistor T12, which pulse shall provide a test of the alarm state detecting amplifier III, the capacitor C6 will transmit a pulse and switch off the thyristor T10. The result is that current through the resistance R19 will pass through the base of the transistor T11 and switch on the lamp L2. The sequence of the events is thus as follows: the thyristor T10 is never activated by the capacitor C6 when a test pulse is transmitted and the transistor T11 is not conductive until a fault signal is obtained from one of the amplifiers. On the other hand, in order for the thyristor T10 to be conductive and hold off transistor T11 and lamp L2, a control current must be obtained from the resistance R17, requiring no fault in any circuit III or loop 5. g

7 If water, for example is present on a loop 5, the relevant alarm state detecting amplifier circuit III will come into operation. This means that current will flow through the transistor T3, via the diode D1, and energize the circuit of alarm section 1. This will cause the multivibrator circuit T6, T7 to begin to operate, since it is not blocked, whereby the bell K and the lamp L1 will be activated at each alternate pulse from the multivibrator circuit. The alarm can be switched off by means of a button TK, which closes the control circuit of the thyristor T5 to positive, via the resistance R11. The thyristor T5 then becomes conductive and holds the multivibrator circuit T6, T7 in a locked state, which causes the collector of the transistor T6 to obtain a low voltage and the transistor T8 to become nonconductive, whereupon the bell K is stopped. On the other hand the transistor T7 is conductive and the lamp is continually illuminated via the transistor T9.

If a further alarm is given during an alarm state. a pulse is obtained from another alarm state detecting amplifying circuit III via the capacitor C1 and the diode D2 in respective amplifiers, this pulse causing the thyristor T5 to become inoperative, whereupon the multivibrator circuit T6, T7 begins to operate again. In this way, a signal is given that a further alarm state has occured.

When an alarm state occurs, a constant voltage is passed to the multivibrator T6, T7 and also the resistance R22, via the diode D1. The base of the transistor T14 is thus energized, thereby preventing charging of the capacitor C7. In this way, the circuit which tests the alarm state detecting circuits ceases to operate while a state of alarm exists. When a state of alarm exists, testing of the alarm state detecting amplifiers is not carried out, since in such a case amplification of the amplifiers in question change the correct operation of the other amplifiers.

When an alarm state exists, the capacitor C4 is also charged via the resistance R16, by the voltage applied to the multivibrator circuit. When this takes place, control current is continuously applied to the thyristor T10, which then shunts the base on the transistor T11 and switches off the lamp L2. This last mentioned operation means that an alarm is always given priority and interrupts a fault signal.

A practical example will now be described with reference to a system provided with an emergency battery unit (not shown). The system normally functions so that a short test pulse is transmitted at second intervals. During this time all connected alarm state detecting amplifiers III operate, said amplifiers being energized owing to the fact that a terminal resistance 8 is located at the end of each loop 5. Should any of the amplifiers not operate, this indicates that there is a break on one ofthe loops 5 or that a fault has occurred on the amplifier or that there is a fault on the illuminated diode D3. Thus, a pulse is transmitted which switches on the fault signal lamp L2. At the same time as the test pulse is transmitted, all illuminated diodes are switched on for a short time interval, and it is thus possible for a supervisor to observe whether any of said diodes have not been illuminated and if the fault lamp is switched on. The fault may also be the result of a drop in the mains supply, wherewith it would be noticed that the illuminated diode D10 is not switched on. The diode D10 should thus normally be continuously illuminated (the system in this latter case being battery operated). The system is also provided with so called priority states. If a loop 5 is moistened or flooding conditions occur, the alarm signal is given priority over the fault signal and the fault signal lamp L2 is deenergized and the test function ceases on all amplifiers at the same time as the alarm lamp L1 is illuminated and flashes alternately with the bell K. At the same time a signal passes to a relay (not shown) which is energized during the time the alarm is signalled. When the system is cleared, the flashing lamp L1 changes to a steady light and the bell K ceases to ring. Should, at the same time, a new alarm occur on one of the other amplifiers, the lamp will begin to flash again and the system must be cleared again. To check the state of the system, it can be seen by looking at the same that the illuminated diodes D3 in the amplifiers concerned are illuminated with a fixed light and that all other diodes are switched off. As soon as the loops 5 have dried, the system will automatically return to its monitoring state, and blocking function of the test portion will cease and checking of the state of the loops will recommence.

With another, not previously mentioned operation of the claimed system, a special functionof each alarm state detecting amplifying block can be used, which means that a signal can be obtained directly behind the transistor T3, which can start up a relay function which can be used to control an emergency pump or the like directly. The relay can also be constructed to send a signal voltage to ignite pyrotechnically charged valves, which are at present commerically available to close off water conduits etc. It is also possible by these means to release a mechanically locked check valve arranged to close water conduits and the like. An alarm may also be signalled via the alarm section I at the same time as these latter measures are taken.

To enable the system to be used, for example, as a fire or burglary alarm, the conductor 7 may be connected to a positive voltage whereupon the base of the transistor T1 is set via a resistance in permanent connection with a negative or corresponding terminal. If one of the

conductors

6 or 7 is connected in series with a breaker contact means, the transistor T1 will be conductive when a break occurs in the circuit, owing to the fact that the blockage voltage from the conductor 7 ceases and the base is connected with a negative terminal, whereupon an alarm is signalled.

For the purpose of triggering a fire alarm, a solution is conceivable whereby the actual strip 3 is treated with a substance which increases the conductivity between the conductors to trigger an alarm when the surrounding temperature increases.

Furthermore, by means of a system according to the invention to operating lines of, for example, an emergency pump or a pyrotechnically charged shut-off valve can be connected to one of the trigger amplifiers inputs so that a check can be made through the test function of the system that the relevant equipment operating lines are intact.

If different forms of detecting means are connected to the inputs of the trigger amplifiers, it may be expedient to control the amplification during the test interval at different levels, depending on the characteristics of the different members. In such a case, the amplification control can be brought in stages to different control levels, wherewith also the degree in the initial alarm state or changes in impedance of the differenct, connected elements can be detected and indicated.

What is claimed is:

1. An alarm system, comprising:

detecting means for producing a change in an electrical parameter from a normal non-alarm value in response to the occurrence of an alarm state; an alarm state detecting circuit including trigger means connected to said detecting means for providing an output when said electrical parameter has a value beyond a predetermined threshold value;

an alarm signalling circuit for producing an alarm signal in response to the output of said trigger means;

test means for automatically changing said predetermined threshold value to a value beyond said normal electrical parameter value.

2. A system according to claim 1 in which said detecting means comprises a sensing device having at least two different impedance states, a first being a normal non-alarm state incapable of actuating said trigger means except upon said change of threshold value by said test means, and a second corresponding to a preselected alarm state and hence to an electrical parameter value beyond said predetermined threshold value of said triggermeans.

3. An alarm system comprising in combination: detecting means including electrical conductors having an impedance therebetween of value at least partly dependent on the alarm state to be detected;

an electrical alarm signalling device connected to said electrical conductors and including an alarm state detecting portion and an alarm signalling portion, said alarm state detecting portion comprising a trigger amplifier having an input in current circuit with said electrical conductors; and

test means connected to said alarm state detecting portion for automatically at intervals changing the trigger level of said trigger amplifier such that the trigger amplifier operates in response to the normal non-alarm impedance between said conductors, said trigger amplifier in the periods between said intervals having a different trigger level, said different trigger level being that wherein said normal non-alarm impedance places said trigger amplifier in an alarm preparatory state but does not operate said trigger amplifier.

4. A system according to claim 3, in which said test means comprises a periodically operating test circuit,

and including test disabling means connecting said test circuit to said alarm signalling portion and responsive to detection of an alarm state by said detecting portion for rendering said test circuit inacting during said intervals, and indicating means in said detecting portion for indicating operation of the corresponding trigger amplifier.

5. A system according to claim 3 in which the impedance between the electrical conductors in the detecting means comprises the electrical transition resistance of a supporting material shunting the conductors, which together with the electrical conductors form the detecting means.

6. A system according to claim 5 in which the impedance between the electrical conductors in the detecting means comprising said transition resistance together with a bridging impedance member, connected between the electrical conductors.

7. A system according to claim 5 in which said material comprises a textile material impregnated with a substance sensitive to and activated by the vaporying state of liquids such that the transition resistance is changed between the electrical conductors.

8. A system according to claim 5 in which said material is impregnated with a substance sensitive to and actuated by heat such tht the transition resistance between the electrical conductors is changed upon heating.

9. A system according to claim 3 in which the test means is arranged to control in stages the level of amplification of said trigger amplifier during said intervals.

10. A system according to claim 9 in which said test means comprises pulse circuit means including timing means for producing pulses of length corresponding to said intervals and the ends of said periods separating said intervals, and a current path connecting said pulse circuit means to the input of said trigger amplifier in common with the connection of said detecting means thereto, whereby said pulses each produce said change of trigger level for the duration thereof.

11. A system according to claim 10 in which said test means further includes a fault indicator and elctronic switch means for actuating said fault indicator in the event of a fault in said detecting means and alarm state detecting portion, said test means further including switch disabling means connected between an output of said trigger amplifier and said electronic switch means (1) for disabling said electronic switch means and fault indicator upon production of an output signal from said trigger amplifier but (2) for permitting actua tion of said fault indicator by said electronic switch means upon failure of said trigger amplifier to produce such output during said intervals.

12. A system according to claim 11 in which said disabling means comprises a thyristor gated on by said trigger amplifier output for blocking conduction of said electronic switch means and further including coupling means momentarily conductive at the onset of said pulse for momentarily disabling said thyristor, whereby failure of said trigger amplifier to produce said output thereof results in continuation of the disabled state of said thyristor during said interval, permitting said electronic switch means to actuate said fault indicator.

13. A system according to claim 11 including illuminated diode means connecting said output of said trigger amplifier to said switch disabling means for indicating operation of the corresponding trigger amplifier.

14. A system according to claim 10 in which said alarm signalling portion include a free running multivibrator and alternative alarm indicators coupled to opposite sides of said multivibrator for alternative actuation thereby upon detection of an alarm state, said trigger amplifier having a continuous supply output, means connecting said continuous supply output to a supply line of said multivibrator and responsive to actuation of said trigger amplifier for normally energizing said free running multivibrator, said trigger amplifier further having a pulse output energizeable at the time of initiation of trigger amplifier operation for a time period substantially corresponding to said interval, said alarm signalling portion including multivibrator disabling means coupled between said multivibrator and trigger amplifier pulse output (1) for disabling the former and thereby both said alternate alarm indicators during said intervals but (2) for permitting operation of said multivibrator to alternately actuate said alarm indicators in the presence of said trigger amplifier continuous supply output and absence of said trigger amplifier pulse output.

15. A system according to claim 14 in which said test means includes a fault indicator and indicator disabling means responsive to an output of said trigger amplifier during said intervals for disabling said fault indicator, said alarm signalling portion including alarm priority means responsive to energization of the multivibrator supply line and coupled to said fault indicator disabling means for then actuating the latter to disable said fault indicator, whereby an alarm is always given priority over and interrupts a fault indication.

16. A system according to claim 14 in which said timing means produces a pulse per said interval, said multivibrator supply line being coupled to said timing means conductive state thereof, whereby one said alternate alarm indicators is energized and the other deenergized during an alarm state of one of said alarm state detecting portions, said paralleling means including means connecting the pulse output of another alarm signalling portion trigger amplifier through said first mentioned multivibrator disabling means for deactivating said further multivibrator disabling means, whereby ifa second alarm is given during a first alarm state, said multivibrator will resume free running signal such second alarm state has occurred.

Claims

1. An alarm system, comprising: detecting means for producing a change in an electrical parameter from a normal non-alarm value in response to the occurrence of an alarm state; an alarm state detecting circuit including trigger means connected to said detecting means for providing an output when said electrical parameter has a value beyond a predetermined threshold value; an alarm signalling circuit for producing an alarm signal in response to the output of said trigger means; test means for automatically changing said predetermined threshold value to a value beyond said normal electrical parameter value.

2. A system according to claim 1 in which said detecting means comprises a sensing device having at least two different impedance states, a first being a normal non-alarm state incapable of actuating said trigger means except upon said change of threshold value by said test means, and a second corresponding to a preselected alarm state and hence to an electrical parameter value beyond said predetermined threshold value of said trigger means.

3. An alarm system comprising in combination: detecting means including electrical conductors having an impedance therebetween of value at least partly dependent on the alarm state to be detected; an electrical alarm signalling device connected to said electrical conductors and including an alarm state detecting portion and an alarm signalling portion, said alarm state detecting portion comprising a trigger amplifier having an input in current circuit with said electrical conductors; and test means connected to said alarm state detecting portion for automatically at intervals changing the trigger level of said trigger amplifier such that the trigger amplifier operates in response to the normal non-alarm impedance between said conductors, said trigger amplifier in the periods between said intervals having a different trigger level, said different tRigger level being that wherein said normal non-alarm impedance places said trigger amplifier in an alarm preparatory state but does not operate said trigger amplifier.

4. A system according to claim 3, in which said test means comprises a periodically operating test circuit, and including test disabling means connecting said test circuit to said alarm signalling portion and responsive to detection of an alarm state by said detecting portion for rendering said test circuit inacting during said intervals, and indicating means in said detecting portion for indicating operation of the corresponding trigger amplifier.

11. A system according to claim 10 in which said test means further includes a fault indicator and elctronic switch means for actuating said fault indicator in the event of a fault in said detecting means and alarm state detecting portion, said test means further including switch disabling means connected between an output of said trigger amplifier and said electronic switch means (1) for disabling said electronic switch means and fault indicator upon production of an output signal from said trigger amplifier but (2) for permitting actuation of said fault indicator by said electronic switch means upon failure of said trigger amplifier to produce such output during said intervals.

16. A system according to claim 14 in which said timing means produces a pulse per said interval, said multivibrator supply line being coupled to said timing means for disabling same while an alarm state exists, to prevent changing of said trigger level of said trigger amplifier by said test means while an alarm state exists.

17. A system according to claim 14 including paralleling means for coupling several alarm state detecting portions in parallel to a single alarm signalling portion and test means, said alarm signalling portion including further multivibrator disabling means including a manual switch and means responsive to actuation of said manual switch for locking said multivibrator in one conductive state thereof, whereby one said alternate alarm indicators is energized and the other deenergized during an alarm state of one of said alarm state detecting portions, said paralleling means including means connecting the pulse output of another alarm signalling portion trigger amplifier through said first mentioned multivibrator disabling means for deactivating said further multivibrator disabling means, whereby if a second alarm is given during a first alarm state, said multivibrator will resume free running signal such second alarm state has occurred.