Title:
IONIZATION FIRE ALARM WITH INSULATION MONITORING SYSTEM
United States Patent 3676680


Abstract:
An ionization fire alarm comprising a measuring ionization chamber which is connected in series with at least one resistance element, preferably constructed as a reference ionization chamber, at a voltage source. An electrical amplifier and/or threshold value detector circuit is also provided, the input of which is electrically coupled with the junction point of the measuring ionization chamber and the resistance element. According to an important aspect of the invention the electric circuit of the ionization fire alarm system is constructed such that a signal is released when the input potential exceeds an upper threshold value or drops below a lower threshold value.



Inventors:
Scheidweiler, Andreas (Stafa, CH)
Meier, Otto (Herrliberg, CH)
Application Number:
05/044091
Publication Date:
07/11/1972
Filing Date:
06/08/1970
Assignee:
CERBERUS AG.
Primary Class:
Other Classes:
250/381, 250/384, 250/385.1, 250/388, 340/629
International Classes:
G08B17/11; (IPC1-7): G01T1/18; H01J39/28
Field of Search:
250/44,43
View Patent Images:
US Patent References:



Primary Examiner:
Lindquist, William F.
Claims:
WHAT IS CLAIMED IS

1. An ionization fire alarm comprising an electrical circuit containing a voltage supply, a measuring ionization chamber including a pair of electrodes, and a series connected resistance element coupled to said voltage supply, electrical response means having its input coupled with the junction point of said measuring ionization chamber and series connected resistance element, said electrical response means including means for monitoring the insulation resistance between the electrodes of said ionization chamber, and further including means for releasing a signal when the input potential at said junction point exceeds an upper threshold value or drops below a predetermined threshold value which is below the normal operating potential at said junction point caused by a decreasing insulation resistance.

2. An ionization fire alarm as defined in claim 1, wherein said series connected resistance element is defined by a reference ionization chamber.

3. An ionization fire alarm as defined in claim 1, wherein said electrical response means comprises amplifier means.

4. An ionization fire alarm as defined in claim 1, wherein said electrical response means comprises threshold value detector circuit means.

5. An ionization fire alarm as defined in claim 1, wherein said signal releasing means delivers a disturbance signal similar to the normal alarm signal.

6. An ionization fire alarm as defined in claim 5, wherein said electrical circuitry comprises a four junction semi-conductor possessing two control electrodes.

7. An ionization fire alarm as defined in claim 5, wherein said electrical response means comprises two components, the first component releasing a signal when the voltage drop across the measuring ionization chamber exceeds a predetermined value, and the second components releasing a signal when the voltage drop across the measuring ionization chamber falls below a different predetermined value, and wherein said electrical circuit a four junction semi-conductor with two control electrodes, each respective control electrode of said four junction semi-conductor being electrically coupled with the output of a respective one of said two components.

8. An ionization fire alarm as defined in claim 1, wherein said signal releasing means releases a disturbance signal which is different from the normal alarm signal.

9. An ionization fire alarm as defined in claim 8, wherein both of said signals consist of direct-currents of different magnitude.

10. An ionization fire alarm as defined in claim 8, wherein one signal is a direct-current signal and the other signal is composed of an alternating-current signal.

11. An ionization fire alarm as defined in claim 8, wherein one signal is a direct-current signal and the other signal is an interrupted direct-current signal.

12. An ionization fire alarm as defined in claim 1, having a second threshold value above the normal operating potential, the current flow through said electric circuitry is almost zero when the input potential is between said two threshold potentials.

13. An ionization fire alarm as defined in claim 1, wherein said electrical circuitry comprises only two conductors for coupling the ionization fire alarm with a central signal station.

14. An ionization fire alarm as defined in claim 1, wherein said electrical response means comprises two components arranged in said electrical circuit such that the first component releases a signal when the voltage drop across said measuring ionization chamber exceeds a predetermined value, and said second component releases a signal when the voltage drop across said measuring ionization chamber falls below a different predetermined value.

15. An ionization fire alarm as defined in claim 14, wherein said first component is constructed as a cold cathode tube having a control electrode electrically coupled with the input side of said electrical circuitry, and said second component is constructed as a glow discharge lamp connected in parallel to said series connected resistance element.

16. An ionization fire alarm as defined in claim 14, wherein both of said components are combined into a mechanical unit in the form of a cold cathode-glow thyratron possessing two ignition paths, one ignition path being coupled in parallel to the measuring ionization chamber and the other ignition path being connected in parallel to said series connected resistance element.

17. An ionization fire alarm as defined in claim 14, wherein both of said components are constructed as field-effect transistors.

18. An ionization fire alarm as defined in claim 17, wherein both field-effect transistors are complementary type transistors, each of said field-effect transistors possessing a gate electrode, said gate electrodes of both field-effect transistors being electrically coupled with the input side of said electrical circuitry, and both field-effect transistors being blocked when the input potential is between the threshold values.

19. An ionization fire alarm as defined in claim 18, wherein in addition to both of said complementary field-effect transistors, said electric circuit incorporates a third transistor as well as an additional resistance element which is connected in series with said measuring ionization chamber and said first-mentioned series connected resistance, the collector-emitter path of said third transistor being arranged parallel to said measuring ionization chamber and said first-mentioned series connected resistance, said third transistor being in circuit with and operatively controlled by one of both field-effect transistors.

20. An ionization fire alarm as defined in claim 19, wherein said electrical circuit is additionally provided with an emitter resistance in circuit with said one field-effect transistor operatively controlling said third transistor and a fourth transistor, the collector-emitter path of which is disposed parallel to said emitter resistance of said field-effect transistor operatively controlling said third transistor, said fourth transistor being in circuit with and operatively controlled by said third transistor.

21. An ionization fire alarm as defined in claim 20, wherein the base of said fourth transistor is additionally in circuit with and operatively controlled by the other field-effect transistor.

22. An ionization fire alarm as defined in claim 20, wherein said other field-effect transistor is connected in series with a switching element, said switching element triggering a switching operation when the input potential at said junction point falls below a predetermined value.

23. An ionization fire alarm as defined in claim 22, wherein said switching element comprises a relay.

Description:
BACKGROUND OF THE INVENTION

The present invention relates to an improved ionization fire alarm comprising a measuring ionization chamber connected in series at a voltage source with at least one resistance element, preferably constructed as a reference ionization chamber, there also being provided an electric amplifier-and/or threshold value detector circuit, the input of which is electrically coupled with the junction point of the measuring ionization chamber and the resistance element.

Ionization fire alarms make use of the fact that the resistance of a measuring ionization chamber increases when smoke or combustion aerosols enter the measuring chamber through chamber openings or are delivered into the measuring chamber by appropriate conduits or the like. Prior art ionization fire alarms utilize for the purpose of determining such resistance change a circuit in which the measuring ionization chamber is coupled in series with at least one resistance element, forming together with the measuring ionization chamber a voltage divider. With a resistance change of the measuring chamber the voltage at the junction point of the measuring chamber and resistance element changes. The change in potential at this location is determined by utilizing known amplifier-and threshold value detection circuitry.

Since the resistance of an ionization chamber is very high, generally being greater than 1011 Ω, it is advantageous to construct the resistance element, the resistance of which must be in the same order of magnitude, also as an ionization chamber which, in this case, serves as the reference ionization chamber.

Insulation problems play a decisive role because of the high resistance of the ionization chambers. It is extremely important to maintain a sufficiently high insulation or dielectric resistance and to monitor such at the essential locations of a fire alarm during the entire operating period. However, known ionization fire alarms possess only a threshold value detector element which triggers an alarm signal via the electric circuit when the resistance and therefore also the voltage drop of the measuring ionization chamber exceeds a predetermined value. The same effect of triggering an alarm is produced by a reduction in the resistance of the resistance element, that is to say, a decrease of the dielectric or insulation resistance between the electrodes of the reference ionization chamber. In this case, the potential at the connection or junction point between the measuring- and reference chamber changes in the same manner as with an increase in resistance of the measuring ionization chamber.

With these known types of electric circuitry it is thus possible to monitor the insulation or dielectric resistance between the electrodes of the reference ionization chamber, not however the insulation resistance of the measuring ionization chamber.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide an improved ionization fire alarm enabling monitoring of the dielectric or insulation resistance both of the measuring ionization chamber as well as also the resistance element, and upon decrease of one of the insulation resistances an alarm or disturbance signal should be triggered.

Since the susceptibility to breakdown or malfunctioning of an electronic device increases with the number of components, it is a further objective of the present invention to provide a complete monitoring of the insulation or dielectric resistance with as few additional components as possible.

Additionally, it is extremely advantageous if these additional components can be installed in the known circuits without requiring additional installation work at the entire fire alarm installation and without varying or changing the characteristics of the fire alarm, for instance the rest- or alarm current. It is especially advantageous if no additional consumption of current occurs with the additional insulation monitoring mechanism. Hence, another significant object of the present invention is to fulfill these requirements as just stated above.

Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the ionization fire alarm of the present invention is generally manifested by the features that the electric circuit is constructed in such a way that when the potential at the input side of the circuit deviates from a standard value by a certain amount either positively or negatively, then a signal is released.

The electric circuit can be constructed either such that a first signal, for instance an alarm signal is triggered, when the potential at the input of the circuit exceeds an upper threshold value, and that a second signal, for instance a disturbance signal is triggered, when the input potential drops below a lower threshold value. With reversed polarity then, naturally, the first signal is to be considered as the disturbance signal and the second signal as the alarm signal. In such case, the electric circuit possesses three stable conditions or states.

On the other hand, the electric circuit can be designed such that in both instances a similar type signal is released, independently of whether one is concerned with a disturbance or an actual alarm. In this case the circuit only possesses two stable conditions.

Physical constructions of the invention employing a common alarm- and disturbance signal possess the advantage that no additional installation expenditure is required. They can be easily constructed as modifications of known circuits, only a few additional components being required.

Yet, oftentimes it is desirable to indicate a disturbance independent of the indication of an alarm. A very simple solution to meet this purpose, wherein the input potential, possibly with the interconnection of a linear amplifier, is delivered to two threshold value-detector elements with different threshold values, however, has the drawback that there are required too many components which are susceptible to malfunction, and furthermore, separate conductors are required for the indication of a disturbance and the indication of an alarm. Hence, when utilizing this type system it is impossible to construct a fire alarm installation employing only two conductors. Additionally, since the rest current of the circuit must lie between the lower and upper threshold values, it is not possible to construct a fire alarm installation with a number of fire alarms connected in parallel, since the total rest current can no longer be differentiated from the alarm current.

Hence, particularly advantageous physical embodiments of the invention provide circuitry where the rest current of the fire alarm is almost null or zero and a separate alarm- and disturbance indication occurs via only two conductors. This can be attained, for instance, if the circuit is provided at its input side with two different amplifiers or switch elements, for instance two glow paths or two field-effect transistors which in the rest state are both blocked, and wherein upon dropping of the input potential one such switch or switching element conducts and upon increase of the input potential the other switch or switch element conducts. Furthermore, with this system the alarm- and disturbance signals differ, for instance are different direct-currents or direct- and alternating current signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 is a circuit diagram of an ionization fire alarm having an additional discharge path;

FIG. 2 is a circuit diagram of an ionization fire alarm using cold cathode tubes and possessing two control paths;

FIG. 3 is a circuit diagram of an ionization fire alarm with two complementary field-effect transistors;

FIG. 4 is a circuit diagram of an ionization fire alarm with two complementary field-effect transistors and SCS;

FIG. 5 is a circuit diagram of an ionization fire alarm possessing two independently switched field-effect transistors and SCS;

FIG. 6 is a circuit diagram of an ionization fire alarm with two complementary field-effect transistors and an SCR;

FIG. 7 is a circuit diagram of an ionization fire alarm with two similar field-effect transistors and SCR;

FIG. 8 is a circuit diagram of an ionization fire alarm with a field-effect transistor and SCS;

FIG. 9 is a circuit diagram of an ionization fire with two complementary field-effect transistors and a third transistor;

FIG. 10 is a circuit diagram of an ionization fire alarm with two complementary field-effect transistors and two further transistors;

FIGS. 11-13 are respective circuit diagrams of ionization fire alarms similar to the systems of FIGS. 4, 9 and 10 respectively, however equipped with a separate disturbance indication mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Describing now the drawings, in the exemplary embodiment of electric circuit of ionization fire alarm depicted in FIG. 1 a measuring ionization chamber 1, containing two electrodes 1a, 1b and a radioactive preparation or substance 1c, is connected with a resistance element 2 in series at the supply conductors 3 and 4. The junction point 100 of the measuring ionization chamber 1 and the resistance element 2 is coupled to the gate of a cold cathode tube 5, whose anode 5a is connected to the positive supply conductor 3 and the cathode 5b of which is coupled with the negative supply conductor 4.

Now if smoke enters the measuring chamber 1, then its resistance increases, and consequently, also the voltage which is applied to the gate 5c of the cold cathode tube 5. If the gate voltage exceeds the ignition voltage of the tube 5, then current flows between the cathode 5b and the anode 5a of the cold cathode switching tube 5. This flow of current causes a relay 6 to respond and thus trigger an alarm. Of course, the resistance element 2, the cold cathode switching tube 5 and the relay element 6 can be replaced by other components which function in the same manner to achieve substantially the same result.

Additionally, between the control electrode or gate 5c of the cold cathode tube 5 and the positive supply conductor 3 there is arranged a glow discharge tube 7. The ignition voltage of this glow discharge tube 7 is selected such that it does not ignite during normal operation. Only when the voltage drop across the tube 7 exceeds the ignition voltage, that is to say, when the potential at the junction point 100 of the measuring ionization chamber 1 and the resistance element 2 has dropped sufficiently, does the glow discharge tube 7 ignite and thereby bridge or shunt the resistance element 2 so that the control voltage for the cold cathode tube 5 is raised to such a degree that also the cold cathode tube 5 ignites and an alarm current flows between the supply conductors 3 and 4 and through the relay 6. Now if the insulation or dielectric resistance of the measuring ionization chamber 1 becomes poorer, then there likewise increases the voltage drop across the resistance element 2 and between the electrodes of the glow discharge tube 7. If the resistance of the measuring chamber 1 exceeds a predetermined value, then the glow discharge tube 7 ignites and as a consequence thereof also the cold cathode tube 5 and in this case also an alarm current flows through the relay 6. On the other hand, impairment of the insulation or dielectric resistance of the resistance element 2, just as if smoke were to enter the measuring chamber 1, similarly causes ignition of the cold cathode tube 5 and the flow of an alarm current. In this way there can be achieved that, in every case where there is a worsening or impairment of the insulation resistance of any high ohm component of the ionization fire alarm, then an alarm will be released.

As a matter of convenience it is here mentioned that wherever desirable similar reference characters may be used throughout the other embodiments to denote the same components. Now in the circuit diagram of FIG. 2 the measuring ionization chamber 1 is in series with a reference ionization chamber 8 which, in this instance, serves as the resistance element and is not accessible to the smoke or aerosol particles resulting from combustion or is constructed so as to be insensitive to smoke or the like. The junction point 100 of both chambers 1 and 8 is electrically connected to the control electrode 9a of a special cold cathode switching tube 9. This tube 9 possesses two different control paths, and specifically, a control path between the gate and anode 9b and a further control path between the gate and cathode 9c. If the ignition voltage of the gas fill at one of these two control paths is exceeded, then the tube 9 ignites and an alarm current flows between the cathode 9c and the anode 9b through the relay 6. Instead of using a tube 9 having a control electrode 9a and two ignition paths, it would naturally also be possible to employ a cold cathode tube with two different control electrodes and appropriate ignition paths within the tube.

FIG. 3 illustrates a transistorized circuit. Here again there is provided an open measuring ionization chamber 1 connected in series with a closed or smoke-insensitive reference chamber 8. The junction point 100 of both chambers 1 and 8 is electrically connected with the gate-electrodes 10a, 11a of the two complementary field-effect transistors 10 and 11, respectively, connected in series to the supply conductors 3 and 4 through the agency of the load resistors 12 and 13, as shown. The operating point of these transistors is adjusted such that during normal operation both are without current flow, and one becomes conductive above an associated threshold value and the other below an associated threshold value. The voltage drop across both field-effect transistors 10 and 11 is delivered to a discriminator D, transmitting a signal when one of both voltages drops below or exceeds a predetermined value. This discriminator D can be constructed and adjusted such that it delivers a different signal, depending upon which transistor is conductive, that is, exhibits a low voltage drop. In the central signal station both of these possibilities can be recorded in the form of an alarm signal and a disturbance signal. It is still to be mentioned that the measuring ionization chamber 1 and the reference ionization chamber 8 can be interchanged since the circuitry is constructed to be completely symmetrical. The disturbance signal then becomes the alarm signal and vice versa.

FIG. 4 illustrates circuitry for an especially simple embodiment of discriminator circuit of the type shown in FIG. 3. Here the discriminator D will be seen to consist of a voltage divider formed of the two resistances or resistors 15 and 16 for regulating the reference voltage for both complementary field-effect transistors 10 and 11 and further embodies a controlled switch 17 possessing two different control electrodes 17a, 17b, for instance constructed as a four-layer diode. Control switches of this type are known to the art as silicon-controlled switches (SCS). Upon falling below or exceeding the control voltage at one of both control electrodes 17a, 17b of this controlled switch 17 by a certain amount such causes the switch to become conductive and current flows through the relay element 18.

FIG. 5 shows two complementary field-effect transistors 19 and 20, whose control electrodes or gates 19a and 20a, respectively, are again electrically coupled with the junction point 100 of the measuring ionization chamber 1 and the reference ionization chamber 8. These transistors 19 and 20 are coupled independently of one another via the load resistors 21 and 22 to the supply conductors 3 and 4 in order to prevent influencing of one of the transistors by the other. Both field-effect transistors are partially conductive in the rest state.

Just as with the previously discussed circuitry, here also there is used as the switching element a controlled switch 17 possessing two control electrodes 17a and 17b. In order to regulate the required pre-bias there are provided two Zener diodes 23 and 24. The relay element 18 for releasing a signal for sounding an alarm is again connected in series with the SCS between the supply conductors 3 and 4. However, the relay element 18 can also be arranged at one of the supply conductors.

FIG. 6 illustrates a circuit which provides the possibility of employing instead of a four-layer diode a simple controlled rectifier, for instance of the SCR-type. Both of the complementary field-effect transistors 25 and 26 are once again connected independently of one another via the load resistor 27 and the resistance 28 and a Zener diode 29 with the supply conductors 3 and 4. The control electrode of the controlled rectifier 30 (SCR) is coupled both with the drain electrode of the transistor 26 as well as via a Zener diode 31 with the source electrode of the transistor 25. Consequently, with current change in one of both transistors 25 or 26 the controlled rectifier 30 is switched into its conductive state and the relay 18 is actuated, and specifically without any affect upon the other field-effect transistor.

In the circuit arrangement of FIG. 7 there are not employed two complementary field-effect transistors, rather two similar field-effect transistors 31 and 32 of an N-type channel. Both transistors 31 and 32 in the rest state are partially conductive. Adjustment of the requisite pre-bias of the control electrodes is undertaken with the aid of the resistors 33 and 34 and the Zener diode 35. With current change in one of both field-effect transistors 31 or 32, the control electrode 30a of the controlled rectifier 30 is supplied via the Zener diodes 36 or 37 with a voltage which renders the controlled rectifier 30 conductive and causes the relay 18 to respond. Furthermore, it is possible to construct a circuit with a single transistor which when there is a deviation of the potential at the control electrode positively as well as negatively by a certain amount causes the triggering of a signal. FIG. 8 shows an embodiment of circuitry in which the control electrode or gate 38a of a field-effect transistor 38 of the N-type channel is coupled with the connection or junction point 100 of a measuring ionization chamber 1 and a resistance element 2. The voltage drop at the load resistor 39 of the source electrode of the field-effect transistor 38 is delivered to a discriminator circuit formed of two anti-parallel Zener diodes 40 and 41 and the resistors 42 and 43. Each branch of this discriminator circuit controls via one of both control electrodes a controlled switch 17. Upon exceeding or dropping below the corresponding threshold value this controlled switch 17 becomes conductive and the relay 18 delivers a signal.

It is to be remarked that the employed relay element 6 or 18 can be constructed both as an electro-mechanical relay, or also can consist of controllable tubes or semi-conductor elements, or complicated assembled electronic circuits with similar switching characteristics.

In many cases it is desired that after a fire alarm has responded it assumes a self-holding condition and therefore stores the alarm condition. When using controlled rectifiers as the switch element of the ionization fire alarm, this is indeed possible, yet controlled rectifiers are exceptionally sensitive to voltage surges and switch very easily back into their non-conductive state. By using a circuit arrangement as shown in FIG. 9, it is possible to obtain a monitoring of the insulation or dielectric resistance of the ionization fire alarm while simultaneously achieving a pronounced self-holding effect.

FIG. 9 illustrates circuitry in which the measuring ionization chamber 1 and the reference chamber 8 are connected in series with a resistor 47 to the supply conductors 48 and 49. Here also, the circuitry embodies two complementary field-effect transistors 44 and 45, the control electrodes of which are coupled with the junction point 100 of both ionization chambers 1 and 8, and furthermore includes an additional transistor 46. The pre-bias for the field-effect transistor 45 is formed with the aid of the voltage divider from the resistors 59 and 50. The resistor 51 serves as the load resistance which, in turn, is connected between the drain electrode of the field-effect transistor 45 and the supply conductor 48 and, on the other hand, serves as the base resistance of the transistor 46, the collector-emitter path of which is situated parallel to the ionization chambers 1 and 8. In the normal condition both field-effect transistors 44, 45 are blocked. If a current flows in the field-effect transistor 45, then the transistor 46 is likewise rendered conductive and therefore the ionization chambers 1 and 8 are practically shunted or bridged, so that the voltage at the control electrode of the field-effect transistor 45 is still further increased and the entire system is placed into a self-holding condition. The source electrode of the other field-effect transistor 44 is connected via a Zener diode 52 with the base of the transistor 46 whereas the drain electrode is arranged at the collector of the transistor 46. If a current flows through the field-effect transistor 44, then the base of the transistor 46 receives such a voltage that the transistor 46 becomes conductive and the system again is placed into self-holding state.

FIG. 10 illustrates a further embodiment of circuitry of an ionization fire alarm, which in contrast to the circuit of FIG. 9, exhibits an additional increased self-holding. Analogous to the circuitry shown in FIG. 9, here also the circuit of FIG. 10 possesses two complementary field-effect transistors 44 and 45 and a further transistor 46. The voltage applied to the source electrode of the transistor 45 is, once again, regulated by means of a voltage divider consisting of the resistors 59 and 50. If the transistor 46 becomes conductive then the voltage across the ionization chambers 1 and 8 is reduced to such an extent that the system becomes self-holding. Additionally, the circuitry contains a still further or fourth transistor 53, the collector-emitter path of which bridges the resistor 50 of the voltage divider. The base 53a of this transistor 53 is coupled with a further voltage divider, composed of the resistors 54 and 55 bridging the series resistance 47. With appropriate design or dimensioning of the resistors 54 and 55 it is possible to dispense with the use of this resistor 47. On the other hand, the base or gate of the transistor 53 is controlled by the drain electrode of the field-effect transistor 44. The mode of operation of this further transistor 53 is as follows: If the transistor 46 becomes conductive, then the voltage drop across the resistor 47 and therefore also across the voltage divider 54 and 55 is markedly increased. As a result, the transistor 53 becomes conductive and short-circuits the voltage divider resistor 50 of the field-effect transistor 45. The voltage at the source electrode of the field-effect transistor 45 is therefore practically brought to the potential of the supply conductor 49. Resetting of a fire alarm which has once responded is therefore only possible if the supply conductor 48 is brought to the same potential, that is to say the fire alarm is switched out. In this manner it is possible to prevent with the greatest degree of security that a fire alarm which has responded will be reset by coincidental voltage surges or pulses.

The alarm condition of the fire alarm can occur in the usual manner by observing the alarm current flowing through the transistor 46 by means of the relay element 6. Additionally, it is possible to provide separate indication of the switching condition of each fire alarm with the aid of an additional individual indication device, for instance a lamp 57, which for example is electrically coupled in series with a Zener diode 56 parallel to the collector-resistor 47 of the transistor 46.

With the previously described circuits the threshold value detector element(s) act upon a common switching element. Therefore, it is not possible to determine at the fire alarm itself or at the central signal station whether there has occurred an actual alarm or a disturbance. However, in many instances it is desirable or necessary to be able to differentiate between an alarm and a disturbance.

Simple solutions of this function can be realized, for instance, by arranging at the input of the circuit two different threshold value detectors which act upon two separate switching elements and cause such to respond, or utilizing for the purpose of threshold value detection an impedance converter or amplifier element arranged at the input which in the normal condition possesses a rest current differing from null or zero. This element acts at least at the direct region of the rest value as a linear amplifier. When the input voltage drops or raises, then the current flowing through this element correspondingly drops or increases and triggers two switching elements set to respond to a lower and upper threshold value, for instance relays or thyristors.

However, such circuits possess the drawback that disturbances and alarm conditions must be indicated along separate paths at the central signal station, so that the use of a two-conductor system is impossible. Since two separate threshold value detectors and switch elements are required, there is a considerable increase in the number of components which are subject to disturbances or breakdown. Furthermore, since in the last-mentioned example the rest current additionally is relatively large, it is impossible to couple a plurality of fire alarms in parallel across common conductors to a central signal station since, in this case, the total rest current of the fire alarm group would already exceed the alarm current with but a few of the fire alarms.

FIG. 11 illustrates a circuit which overcomes these drawbacks and possesses separate disturbance- and alarm indication. The circuitry contains basically the same components as the circuit of FIG. 4, so that conveniently the same elements have been designated with like reference characters. Of course, the circuit of FIG. 11, in order to fulfill the aforementioned functions and provide the aforementioned advantages, has been modified through the addition of some further components as will be explained below. At the input of the circuit there are the two complementary field-effect transistors 10 and 11 which in the rest state of both are blocked. Whereas the transistor 10 in the same manner as its corresponding component in the circuit of FIG. 4 controls a SCS 17, the transistor 11 controls a SCR 60. The free electrode of the SCS 17 is coupled via a resistor 61 to the supply conductor 3. The current flowing through the SCS 17 and the SCR 60 during an alarm- or disturbance condition is determined by the load resistors 62 and 64. The resistances are chosen in such a way that it is possible to clearly distinguish from one another the disturbance current and the alarm current.

Continuing, it will be recognized that in this circuit arrangement there is arranged a visual indication device, for instance a lamp 63, between the load resistors 62 and 64 and the supply conductor 4, which visual indication device 63 permits ready determination whether one of both switching elements of the fire alarm have responded. At the central signal station there are arranged two different current detectors, for instance relays, which in the presence of the alarm- or disturbance current place into operation independently of one another an alarm- or disturbance indicating mechanism. In the illustrated embodiment both the alarm current as well as the disturbance current are conducted over the same two conductors 3 and 4 to the central signal station. Of course, if there is no limitation upon the use of conductors it would be possible to employ separate conductors. Likewise, the common indicator mechanism 63 can also be formed by two separate indicating devices for the alarm state and for the disturbance condition. Further, it is conceivable that the switching element associated with the alarm indicator possesses self-holding characteristics or is arranged in a self-holding circuit, however the switching element serving to indicate a disturbance will be switched back into its rest state upon disappearance of the disturbance. If necessary, however, the disturbance indicator can also be constructed to be self-holding.

FIG. 12 illustrates a different possibility of differentiating an alarm from a disturbance by means of an electric circuit which is a modification of that shown in FIG. 9, yet derived therefrom and operating according to the same principle, wherein a fire alarm is electrically coupled via only two conductors with the central signal station. Both of the field-effect transistors 44 and 45 which are arranged at the input side are chosen and coupled such that with normal input potential both transistors block, in other words, the rest current of the fire alarm is exceptionally small. Just as was the case with the circuitry of FIG. 9, here also the transistor 45 controls in the same manner a transistor 46, which when conductive causes the system to become self-holding and delivers an alarm current via the conductors 48 and 49 to the central signal station. However, in contrast with the embodiment of circuitry of FIG. 9, here the other transistor 44 does not control the transistor 46, rather its electrodes are coupled via a Zener diode 65 and a resistor 67 with the supply conductors 48 and 49, as shown. If the input voltage of the gate or control electrode of the field-effect transistor 44 exceeds a predetermined threshold value, then the Zener diode 65 ignites and a current likewise flows via the supply conductors to the central signal station. However, this disturbance current is chosen to be such that it can be clearly distinguished from the alarm current and can be evaluated at the central signal station by means of a separate disturbance relay 66. Of course, with this embodiment of circuitry there can also be employed individual indication devices, and specifically, either for disturbance and alarm conjointly or separately. Also in this case the disturbance indicator is not self-holding and resets upon elimination of the disturbance in contrast to the alarm indicator.

Continuing, it should be understood that it is possible to differentiate between an alarm and disturbance, not only by using two different current magnitudes, but through the use of different types of current. For instance, it is possible to indicate an alarm through the use of a predetermined direct-current, whereas a disturbance can be signalled or indicated by the appearance of an alternating-current voltage signal or a chopped or discontinuous direct-current signal. In order to achieve such, it is naturally possible to use the most different types of circuitry. For instance, the threshold value detector, responding in the presence of a disturbance by a reduction of the insulation of the measuring ionization chamber, can be used to control a switching element which connects with the supply conductors an alternating-current voltage generator of optional construction.

FIG. 13 illustrates an embodiment of circuitry which is a modification of the circuitry depicted in FIG. 10. The two field-effect transistors 44 and 45, here also, are coupled to the input of the circuit and are blocked in the rest condition, that is to say, when the input potential corresponds to a standard or normal value. Whereas the transistor 45 controls the transistor 46 bridging the ionization chambers 1 and 8, the transistor 44 does not, as in the case of the circuitry of FIG. 10, control the transistor 53 which changes the threshold value of the detector 45, rather controls an additional relay 69 which is shunted by a capacitor 68. As soon as a current flows through the field-effect transistor 44 which is normally blocked, the relay contact 70 is closed and short-circuits the supply conductors 48 and 49. As a result, the firm alarm is switched-out and the relay is de-energized. As long as the disturbance continues by virtue of reduced insulation or dielectric resistance of the measuring chamber 1, this process repeatedly continues and a chopped direct-current flows through the supply conductors to the central signal station. On the other hand, in the case of an alarm, the transistor 46 controlled by the field-effect transistor 45 becomes conductive and draws a direct-current. At the central signal station the alternating-current voltage component and the direct-current voltage component of the alarm current are separated and detected in known manner. The same effect, namely, generation of an alternating-current voltage component, can naturally also be achieved, instead of using the short-circuit contact 70, by switching-in a suitable transmitter, for instance a multivibrator or sweep generator.

Furthermore, the switching element used for disturbance indication can also be employed for other switching functions. For instance, instead of a short-circuit switch it is possible to actuate through the switching mechanism a switch arranged at the supply conductors, so that a conductor is interrupted or opened. If the central signal station is provided with a suitable device for indicating an interruption in the conductor, for instance in a system monitored during rest current, the same effect occurs during a disturbance because of an insulation defect as for an open conductor. In both instances, a disturbance is indicated or signalled.

The heretofore described embodiments of circuitry illustrate that it is completely possible to achieve completely or for the most part the objectives of the invention with the use of only one or two additional components.

Furthermore, it is to be readily understood that all of the components can be replaced by different components performing the same function, or by assembled or composite circuits, for instance in integrated format.

Furthermore, the invention is in no way to be limited to cold cathode tubes or field-effect transistors serving as high ohm amplifier elements. The voltage supply can be either constructed so as to deliver the power via conductors from the central signal station or as a separate supply. Transmission of the signal to the central signal station can be undertaken directly via conductors or in coded form, or else wireless with conventional remote transmission systems. Furthermore, means can be provided at the central signal station which monitor or check at any time the functional readiness of all of the fire alarms or continuously monitor such or are able to determine interruptions in the conductors and short-circuits. In this way it is possible to design a fire alarm system which provides the highest degree of reliability and functional integrity of the installation.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims. Accordingly,