US3603949A

US3603949A - Fire alarm installation

Info

Publication number: US3603949A
Application number: US738046A
Authority: US
Inventors: Beat Walthard
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1967-06-26
Filing date: 1968-06-18
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07
Also published as: AT275365B; CH468682A; NL165311C; SE328225B; DE1766440B2; DE1766440A1; NL165311B; DE1766440C3; NL6808477A; FR1569514A; GB1228931A

Abstract

A fire alarm installation is disclosed which provides positive detection of the response or nonresponse of an individual fire alarm during the normal operational condition thereof as well as during a checking operation. Each of the individual fire alarms are provided with circuits which exhibit a high impedance for supply voltages of a first polarity and a low impedance for supply voltages of a second polarity during normal operational conditions. During alarm conditions, however, the impedancepolarity relationships reverse and such is detected at a remote central station.

Description

inventor Beat Walthard {56] References tilted l N ggfi jg UNITED STATES PATENTS P 3.029.420 4/1962 Bagno 340/214 Filed June 18, i968 3,176,284 3/1965 Jones. 1 1 340/409 Pawn Sept W71 3 351 934 l1/1967 v" 1 340 409 Assignee Cerberus AG 16 Z Medan-l, Switzerland Primary ExaminerThomas Bi. Habeclcer Priority June 26, 1967 Anomey--W.W1l(leeman Switzerland Will/67 1.

ABT1RACTz A fire alarm installation is disclosed which provides positive detection of the response or nonresponse of an individual fire alarm during the normal operational condition izgg fiyg T k thereof as well as during a checking operation. Each of the inrawmg dividual fire alan'ns are provided with circuits which exhibit a US. Cl MAD/2M, high impedance for supply voltages of a first polarity and a low 340/409 impedance for supply voltages of a second polarity during norlnt. Cl GllBb 29/00 mal operational conditions; During; alarm conditions, howlField of Search 1. 340/214, ever, the impedance-polarity relationships reverse and such is 409 detected at a remote central station.

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ATTORNEYZ lFlIIIlE ALARM INSTALLATION BACKGROUND OF THE INVENTION The present invention relates to an improved fire alarm installation of the type incorporating a plurality of two-conductor, parallely connected fire alarms operably coupled with a central signal station.

Fire alarm installations of the type wherein fire alarms are arranged together in groups and are parallely connected and coupled via only two conductors with a central station have proven to be particularly advantageous because of their low installation cost. Known installations of this type are supplied with direct current. Moreover, the individual fire alarms, during their normal state or condition, exhibit a high impedance and, during their alarm state or condition, exhibit a low impedance, whereby the current flowing through the groups of fire alarms during response ofone or a number of fire alarms is directly or indirectly utilized for triggering an alarm at the central station. In this instance, the criteria for triggering of the alarm at the central station is designated current in contrast to no current.

It is necessary to periodically check fire alarm installations at regular intervals to determine their operational reliability so as to insure reliable response in the case of a fire. In modern fire alarm installations, in which up to several hundred fire alarms are connected to each central station, it is only possible to carry out one remote triggering of a fire alarm by delivering alarm-simulating conditions from the central station. The difficulty with this system resides in the inability to positively determine that all fire alarms of the group have actually responded during the checking operation. In a known fire alarm installation, in which, for the purpose of carrying out a checking operation, electrical signals periodically cause all of the fire alarms to simultaneously respond, a separate conductor is lead from each fire alarm back to the central station. Signals appear on each conductor during the checking operation regarding the response condition of the relevant fire alarmv However, this installation is technically far too com plicated and costly for use in larger installations. According to another known two'conductor fire alarm installation, all fire alarms are likewise simultaneously activated or alarmed by the central station. The total current flowing through the group of fire alarms is then measured by means of an ampere meter and the measurement result is compared with a reference value. Consequently, this system is also suitable only for installations which have relatively few fire alarms per group. The total current varies because of the tolerances associated with the response currents of the individual fire alarms, a consequence of the quite considerable fluctuations or variations of the electrical characteristics of different components which occur in practice. This variation is of such a degree that with a larger number of fire alarms per group, even when using extremely accurate measuring devices, a positive detection of the threshold value is not possible.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide an improved fire alarm installation of the aforementioned type wherein the response of an individual fire alarm can be just as positively detected during the normal operational condition thereof as the nonresponse of an individual fire alarm during the checking operation.

Further objects of the subject invention are:

a. To provide a fire alarm installation having high operational reliability;

b. To provide a fire alarm installation in which all major components of the system can be easily checked for operabiliy;

c. To provide a fire alarm installation having low power consumption; and,

d. To provide a fire alarm installation of relatively low cost.

These and other objects of the subject invention as will become apparent are implemented in that, generally speaking, the invention is characterized by the features that the individual fire alarms are provided with circuits which, for the normal operational condition, provide a high impedance for supply voltages of a first polarity and provide a low impedance for supply voltages of a second polarity. In the case of an alarm, these impedances reverse. Furthermore, means are provided at the central station which deliver a voltage of alternating polarity to the supply conductor and, by carrying out impedance measurements, determine the following three operating conditions of the group of fire alarms:

a. No fire alarm is alarmed (high impedance during the first polarity of the supply voltage);

b. At least one fire alarm is alarmed and at least one fire alarm is not alarmed (low impedance during both polarities of the supply voltage); and,

c. All fire alarms are alarmed (high impedance during the second polarity of the supply voltage).

According to a preferred embodiment of the invention, the circuits for the individual fire alarms, which, as stated above, provide a high impedance for supply voltages of the first polarity and a low impedance for supply voltages of the second polarity during the normal operational condition and, in the case of an alarm reverse these impedances, consist of at least two parallely arranged current paths which, on the one hand, can be connected via a diode of different polarity with one supply conductor and, on the other hand, can be connected via a reversing or selector switch to the other supply conductor, whereby the reversing switch changes its position during response of a fire alarm.

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 block diagram of the invention fire alarm installation;

FIGS. 2a to 22 are diagrams which serve to explain the mode of operation of the inventive fire alarm installation;

FIG. 3 is a first embodiment of a fire alarm suitable for use in conjunction with the inventive fire alarm installation;

FIG. 4 is a second embodiment of the inventive fire alarm;

FIG. 5 is a third embodiment of a tire alarm designed in accordance with the invention and employing individual optical indicator means;

FIG. 6 is a variant of the portion of the fire alarm of FIG. 5 appearing at the left of the line AA thereof;

FIG. 7 is an embodiment of a conductor terminal element of FIG. ll;

FIGS. 81: to lie are diagrams serving to explain the mode of operation of the conductor terminal element shown in FIG. 7; and,

FIG. 9 is a block diagram of a central signal station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings, it will be understood that the fire alarm installation depicted by way of example in FIG. it consists of a number n of two-poled fire alarms M M M, which are arranged parallel to one another and connected via two supply conductors l and 2 with a central signal station 82. The individual fire alarms M M M are of identical construction and essentially consist of a circuit which, in the normal operational condition, exhibits high internal resistance for supply voltages U of a first polarity and a low internal resistance for supply voltages of a second polarity and which, in the event of an alarm, reverses this resistance relationship. A circuit which fulfills these conditions is illustrated in a simplified form in the block of the fire alarm designated M The circuit comprises two

diodes

3 and 43, each of which is provided with a respective series connected resistor and 6. The

diodes

3 and 4 are oppositely poled, whereby the anode of diode 3 and the cathode of diode 4 collectively form one pole of the fire alarm and are connected to the supply conductor 1. A switch 7, depending upon its position is connecting the resistor 5 or 6, respectively, with the other pole of the fire alarm or the supply conductor 2. The position of switch 7 is determined by a nonillustrated apparatus which, as a function of the measured concentration of smoke, of changes in the ambient temperature, or of different criteria indicative of a fire, triggers a switching operation upon exceeding a threshold value. Apparatus which are contemplated for this purpose, among others, are those equipped with ionization chambers, thermoelemcnts, optical smoke-measuring devices, or flame alarm devices.

During the normal operational condition, switch 7 is located in the indicated full-line position. During positive supply voltages U (supply voltages of the second polarity), diode 3 is conductive and the current flowing through the fire alarm is essentially determined by the resistor 5. During negative supply voltages (supply voltage of the first polarity), diode 3 blocks and practically no current flows through the fire alarm. Thus, viewed from the central station 82, the fire alarm M comprises a relatively low impedance determined by the resistor 5 for positive supply voltages U, and, on the other hand, for negative supply voltages -U, the fire alarm M comprises a high impedance. In the case of an alarm, the exact opposite behavior of the fire alarm is determined, wherein the switch 7 assumes the phantom line position of FIG. 1. In other words, for positive supply voltages U, the alarmed or activated fire alarm M exhibits a high impedance and for negative supply voltages, fire alarm M exhibits a relatively low impedance.

This specific behavior of an individual fire alarm with regard to positive and negative supply voltages, respectively, during the normal operation and during a alarm, can be util ized in accordance with the teachings of the invention with a number of parallely connected fire alarms M M M which are coupled via only two common conductors to a central station, to determine the following conditions:

1. During normal operation:

All fire alarms are in a state of rest;

2a. During normal operation:

One or more fire alarms have responded (alarm);

2b. During a checking operation:

One or more alarms have not responded (disturbance);

3. During a checking operation:

All fire alarms have responded (installation intact).

In order to determine these conditions, a pulse-shaped voltage u, as shown in FIG. 2a, is delivered to the supply conductors l and 2. As is apparent, the pulse-shaped voltage u possesses positive components 11 (positive pulses) and negative components 12 (negative pulses) with respect to the zero line 10. Instead of utilizing a rectangular shaped voltage, it is basically possible to also choose a different voltage shape exhibiting both positive and negative amplitudes, such as a trapezoidal-shaped voltage, sinusoidal oscillations, and so forth. If all of the fire alarms M M M assume a normal operating condition or state, then a low impedance is exhibited for the positive pulses 11, the low impedance being practically determined by the parallel connection of all resistors 5 in FIG. 1, and a very large or high impedance is exhibited for negative pulses 12. In FIG. 2b, this normal operating condition is illus' trated whereby, instead of the total impedance of the group of fire alarms, the total fire alarm current is plotted which is reciprocal to the impedance. As the criterium for the normal operating condition or state there is only determined at the central station whether the negative current pulses 14 are practically equal to zero; the positive current pulses 13, on the other hand, not being evaluated for the time being.

Now, if one or more fire alarms respond, then, during the negative voltage pulses, a current flows through the group of fire alarms which differs from zero and which has a magnitude determined by the number of fire alarms which have responded and the resistors 6 which during these phases are located in parallel. This situation is represented in FIG. 2c, whereby during phase A, for instance, one fire alarm has responded, and during phase B, a second fire alarm has responded. As the criterium for the alarm condition of at least one fire alarm, there can thus be employed at the central station the noticeable deviation of the current i from the null or zero value 10 for negative pulses 14'.

In modern fire alarm installations, it has become conventional, and in certain countries even required by law, that the operational reliability of the fire alarm be checked at regular time intervals. For this purpose, alarm-simulating conditions are periodically delivered to the fire alarms in order to be able to thereafter check at the central station whether or not all fire alarms have responded. The delivery or transmission of the alarm-simulating conditions can take place in a standard manner known to the art, a particularly advantageous technique being described hereinafter in greater detail.

The purpose of checking with regard to functional or operational reliability is to determine whether all fire alarms have responded or whether one or a number of fire alarms have not responded. To this end, the behavior of the group of fire alarms with respect to the positive voltage pulses is observed at the central station. If during the checking operation all of the fire alarms have responded, then during the positive voltage pulses 11 (FIG. 2a) practically no current 13" (FIG. 2d) flows through the group of fire alarms, whereas there can be observed a clear deviation of the positive current pulses 13" (FIG. 2e) in the case of one or a number of defective fire alarms. Analogous to FIG. 2c, it will be observed from FIG. 2e that during the phase A, a fire alarm has not responded, whereas during the phase B, a second defective fire alarm appears.

A deviation of the negative current pulse amplitudes from zero is, therefore, used for determining a fire alarm which has responded or been alarmed during a normal operation, a deviation of the positive current pulse amplitudes from null or zero being used for determining a defective fire alarm during a checking operation. For the time being, the behavior of the group of fire alarms to positive voltage pulses during a normal operating condition and to negative voltage pulses during a checking operation has not been evaluated. It would therefore also be basically possible to employ a series of pulses which, in the normal case, only exhibit negative amplitudes and, in the case of testing or checking, exhibit only positive amplitudes. Thus, further functions can be carried out by the pulse portions of the supply voltage which are not employed for evaluation, such as will be explained hereinafter in conjunction with the example of the individual indicated responding or defective fire alarm.

FIG. 3 illustrates a first embodiment of a fire alarm which possesses the specific behavior or characteristics described in conjunction with the fire alarm M of FIG. 1. Two

ionization chambers

20 and 21 are connected in series with one another. Their point of connection or junction, generally formed by a common electrode in practice, is coupled to the control or gate electrode of a field-effect transistor 22. Generally, ionization chamber 20 normally consists of a compartment which is closed at all sides and serves as a reference ionization chamber, whereas ionization chamber 21, functioning as a measurement chamber, is accessible to the surrounding air. The second electrode of the reference ionization chamber 20 which is not connected with the gate electrode of the field-effect transistor 22 is connected via a junction or switching point 23 and a diode 3 with the supply conductor 1. The source electrode of field-effect transistor 22 is coupled via a Zener diode 24 to the junction 23, whereas the drain electrode of field-effect transistor 22 is connected via a voltage

divider comprising resistors

25 and 26, and the diode 4 with the supply conductor 1. The base of a transistor 27 is connected to the top of

voltage divider

25,26. The emitter of transistor 27 is connected via a resistor 28 with the drain electrode of field-effect transistor 22 and is also connected via a capacitor 29 with the diode 4, as shown. The emitter of transistor 27 is further coupled via a conductor 311 to the supply conductor 2, and via a capacitor 32 to junction 23. The collector of the transistor 27 is connected via a resistor 30 with diode 4 and, via a voltage

divider comprising resistors

33 and 34, with the junction 23. The tap of the

voltage divider

33,34 is coupled with the second electrode of the measurement ionization chamber 2i.

Now, in order to explain the mode of operation of the exemplary fire alarm illustrated in FIG. 3, it should be initially assumed that the potential of the supply conductor l is positive with respect to that of the supply conductor 2 (positive volt age pulse llll, FIG. 2a). As will be demonstrated shortly, the capacitor 29 is charged to the negative voltage 12 (FIG. 2a).

Since the diode 3 conducts during positive voltage pulses and therefore charges capacitor 32, the, sum of the negative and positive supply voltage U (FIG. 2a) appears across the

voltage divider

30, 33, 34. Capacitor 29 is designed or dimensioned to be so large that its voltage only slightly drops during the negative pulses, and thus can be considered to be constant. A potential is applied to the gate electrode of field-effect transistor 22 which, on the one hand, is determined by the relationship of the

resistors

30, 33 and 34 and, on the other hand, by the relationship of the internal resistance of the ionization chambers and 2ll. The gate supply voltage of the field-effect transistor 22 is chosen in such a manner and is accommodated to the Zener voltage of Zener diode 24 such that field-effect transistor 22 conducts. Thus, a current 13 (FIG. 2b) flows during the positive pulses from the supply conductor l via the diode 3, Zener diode 24, field-effect transistor 22 and resistor 23, to the supply conductor 2.

Resistors

25 and 26 are dimensioned such that during the normal condition of operation, transistor 27 blocks.

During the negative voltage pulses 112 (FIG. 2a), the diode 3 blocks. Capacitor 32 slightly discharges via the field-effect transistor 22 and transistor 27 continues to remain blocked. Only a relatively small current 114 (FIG. 2b) flows between the supply conductor 1 via diode 4,

resistors

26,25 and 28, conductor 31, and the supply conductor 2. During this phase, capacitor 29 is charged to the negative supply voltage.

Now, if smoke enters into the measurement ionization chamber 2]., then its internal resistance increases. The voltage across reference ionization chamber 2th and thus between the gate electrode and source electrode of field-effect transistor 22 drops until it reaches a value which causes field-effect transistor 22 to block. As a result, the voltage at the base of the transistor 27 is also reduced and its emitter-base voltage is increased, whereby transistor 27 begins to conduct. Thus, for negative voltage pulses, a current 114 (FIG. 20) flows between the supply conductor ll, via diode 4 and collector resistor 30, and the supply conductor 2, which enables the detection of an alarm at the central station. A large voltage drop appears across resistor 30 which, via both of the voltage dividers,

resistors

33, 34 and

ionization chambers

20, 21, additionally reduces the voltage between the gate electrode and the source electrode of fieldeffect transistor 22, whereby an increased smoke effect is simulated in the measurement ionization chamber. Accordingly, the circuit is positively fedback and shifts from the rest condition in self-holding fashion into the alarm condition. The alarm condition is maintained even after disappearance of the alarm-triggering conditions.

The remote triggering of the tire alarm or the attainment of alarm-simulating condition from the central station necessary for checking operational reliability can be undertaken in a number of different ways. A simple possibility resides in reducing, for a short period of time, the positive pulse amplitudes of the supply voltage. The reduction of voltage, because of the voltage-stabilizing properties of the Zener diode 24, acts completely upon the source electrode of the field-effect transistor 22, whereas the potential at the gate electrode is only partially reduced or lowered due to the

voltage dividers

33,34 and 20,21. Field-effect transistor 22 now blocks and simulates the presence of smoke. During the positive voltage pulses 13 (FIG. 2d) there is, with the fire alarm intact, aninterruption ofcurrentflow between the supply conductor l and the supply conductor 2 on the one hand, through the blocked field-effect transistor 22 and, on the other hand, through the diode 4. With defective fire alarms such as those having a short circuited field-effect transistor 22, there is maintained, on the other hand, the current flow l3 (FlG. 2e) from the supply conductor l to the supply conductor 2, which can be used at the central station for triggering a disturbance alarm.

By increasing the amplitudes of the positive voltage pulses for a short period of time, it is possible to reset a responsive or alarmed fire alarm back into the rest condition state.

The fire alarm circuit of HO. 3 althoughbeing particularly simple in construction, has the drawback that different components therein are not supervised or monitored. For example, an interruption in diode 4 results in capacitor 29 discharg ing via the

resistors

33,34, whereby, the potential at the gate electrode of field-effect transistor 22 shifts towards positive values and causes such to block. Although the fire alarm is therefore apparently alarmed, an alarm function is nevertheless suppressed because of the interruption at the diode 4. During the checking operation, field-effect transistor 22 is nonetheless blocked in spite of this defect so that, during the positive voltage pulses, there is no current flow. During cutout of the diode 4, it is neither possible to determine the defect for itself nor an eventually occurring alarm.

The circuit arrangement shown in FIG. 4 overcomes these disadvantages, yet requires a greater expenditure because it uses a further transistor. The gate electrode of field-effect transistor 22 is again connected to the common electrode of both ionization chambers 23 and 2t. Zener diode 24 in series with the source electrode of the field-effect transistor 22 is now directly coupled with the supply conductor 2. The drain electrode of fieldeft"ect transistor 22 is coupled via the voltage

divider comprising resistors

25 and 26, to the junction or switch point 40, at which there is also connected the second electrode of the reference ionization chamber 20 and the emitter electrode of transistor 27. The collector electrode of transistor 27 is coupled at the junction or switching point 41 with the second electrode of the measurement ionization chamber 211 and is connected via a resistor 46 with the supply conductor 2 as well as via a resistor 42 with the base electrode of a transistor 43. The base electrode of transistor 43 is connected via a resistor 44 and the collector electrode via a re sistor 45 to the cathode of the diode 3, whereas the emitter electrode of transistor 43 is directly connected with the supply conductor 2. Once again, the capacitor 2) is situated between the anode of diode 4 and the supply conductor 2. W hereas the field-effect transistor 22 in the circuit arrangement of FIG. 3 was conductive in the rest condition, now it blocks during the normal operating condition. Capacitor 29 is again charged to the negative operating voltage. The voltage divider relationship: reference ionization chamber 2tl-series connection of the measurement ionization chamber 21, resistor 46 and the value of the Zener voltage of the Zener diode 24 are accommodated to one another such that the gate-source voltage of field-effect transistor 22 is below the response voltage thereof.

During the positive voltage pulses a current flows via the diode 3 and the series connection of

resistors

44, 42 and 46. A voltage appears between the base and the emitter electrodes of transistor 43 which causes such to conduct. A current which is essentially dependent upon the resistor 45 flows via the diode 3, resistor 45 and transistor 43 from the supply con' ductor l to the supply conductor 2. On the other hand, the diode 3 blocks during the negative voltage pulses. Since transistor 27 is likewise not conductive due to the field-effect transistor 22 which still remains blocked, during this phase only a negligibly small charging current flows in the capacitor 29, the current through the ionization chamber being of the order of magnitude of 10" amperes.

Now, if smoke enters into the measurement ionization chamber, then the voltage between the gate electrode and the source electrode of field-effect transistor 22 increases,

whereby such begins to conduct. A current flows in the base of the transistor 27 which therefore likewise conducts.

A voltage forms across the resistor 46, whereby thejunction point 41 becomes more negative. The voltage drop at the resistor 46 is transmitted via the measurement ionization chamber 21 to the gate electrode of field-effect transistor 22, so that such becomes more conductive. The field-effect transistor 22 and the transistor 27 therefore conjointly form a complementary switching stage with positive feedback. The fire alarm which has responded remains self-holding. At the fire alarm which has responded, current flows during the negative voltage pulses on the one hand via diode 4,

resistors

26 and 25, field-effect transistor 22 and Zener diode 24, and, on the other hand, via diode 4, transistor 27 and resistor 46 between the supply conductor 1 and the supply conductor 2 and serves for evaluation of the alarm at the central signal station.

During the checking of the fire alarms, they are again caused to respond from the location of the central station by means of alarm-simulating signals, for example, by increasing the negative voltage amplitudes for a short period of time. When the fire alarms are intact or operable, the field-effect transistor begins to conduct and maintains this condition or state because of the positive feedback and the charge stored in the capacitor 29 also during the positive voltage pulses. The negative voltage drop occurring across resistor 46 is transmitted via the voltage divider 42,44 to the base of transistor 43, so that transistor 43 is also blocked during the positive voltage pulses. Consequently, during checking of the fire alarms, a current flows during the positive voltage pulses from the supply conductor 1 to the supply conductor 2, if the fire alarm had not responded, especially also if the diode 4 is defective. In this case, capacitor 29 finally discharges and the negative voltage drop across resistor 46 responsible for blocking transistor 43 for positive voltage pulses disappears after several periods. On the other hand, what have not been supervised or checked at c the transistor 43 and the diode 3 itself, yet the disadvantages associated therewith are much less than those present with the circuit arrangement of FIG. 3, since even during interruption or failure of the elements, the sounding of an alarm is possible. In order to reset the fire alarm which has responded, it is sufficient to suppress the negative voltage pulses.

A circuit arrangement in which all of the essential components of the fire alarm are checked with respect to their operational reliability is illustrated in FIG. 5. The fire alarm circuit shown by way of example in such Figure is additionally designed particularly for low power consumption, contains protective devices for the most sensitive circuit components, and is additionally equipped with an optical indicating device for fire alarms which have responded or which are defective.

Once again, the gate electrode of field-effect transistor 22 is connected with the common electrode of the measurement ionization chamber 21 and the reference ionization chamber 20. A resistor 34 is situated in parallel to the series connection of both ionization chambers and 21. Resistor 34, together with the resistor 33, forms a voltage divider. The drain electrode of field-effect transistor 22 is coupled with the second electrode of the reference ionization chamber 20 and is connected via a resistor 60 with the anode of the diode 4. The source electrode of field-effect transistor 22 is connected to the emitter electrode of a transistor 58, the collector electrode of which is connected via a resistor 59 to a conductor 57. Conductor 57 forms the second output of the fire alarm and provides the connection with the supply conductor 2. The base electrode of transistor 58 is connected at the tap of a voltage divider formed by

resistors

54 and 55, the voltage divider being directly connected between both supply conductors 1 and 2. A capacitor 56 is connected in parallel with a resistor 55. The capacitor 29 once again is located between the anode of diode 4 and the supply conductor 2, and the conductor 57, respectively. The second electrode of the measurement ionization chamber 21 is connected via a diode 50 with conductor 57, whereas the second electrode of the reference ionization chamber 20 is coupled by means of a Zener diode 51 to conductor 57.

Transistors

64 and 68 collectively form a first complementary switching stage, whereby the voltage appearing across the resistor 59 is applied to the base electrode of transistor 68. The emitter electrode of transistor 68 is directly connected with conductor 57. The collector electrode of transistor 68 is coupled via the voltage divider, resistor 62 and resistor 63, to the anode of diode 4. The base electrode of transistor 64 is connected at the tap of the voltage divider, the emitter electrode of which is directly connected with the anode of diode 47 The collector electrode of transistor 64 is lead back via the resistor 61 to the base electrode of transistor 68, and is further connected via a resistor 66 and glow lamp 67 with supply conductor 1 and finally via a voltage divider comprising resistor 65 and resistor 70, to the cathode of diode 3. At the tap of the

voltage divider

67,70, the base electrode of transistor 69 is coupled, the emitter electrode of which is coupled with conductor 57. The collector electrode of transistor 69 leads via a voltage divider, comprising resistor 71 and resistor 72, to the cathode of diode 3. The base electrode of transistor 73 is situated at the tap of the

voltage divider

71,72, and its emitter electrode is directly connected with the cathode of diode 3, whereas the collector electrode, on the other hand, is connected via a resistor 74 to the base electrode of transistor 69 and, on the other hand, via a diode 63 to the resistor 33 of the

voltage divider

33,34.

Transistors

69 and 73 collectively form a second bistable switching stage which is controlled by the first switching stage (transistors 64, 68) in a master-slave technique. A capacitor 52 is connected between the cathode of diode 53 and the supply conductor 57.

In order to explain the operation of the circuit arrangement of FIG. 5, it is initially assumed that capacitor 29 has been charged to the negative supply voltage and capacitor 52 to the positive supply voltage. A voltage appears across the series connection of both

ionization chambers

20 and 21 which is determined by the voltage divider relationship of the

resistors

33,34 and 60. The working or operating point of the gate electrode of field-effect transistor 22 is, analogous to the circuit of FIG. 4, chosen such that field-effect transistor 22 and therefore also transistor 58 block in the rest condition. The Zener voltage of Zener diode 51 is greater than the voltage appearing during normal operational condition across the series connection of both ionization chambers, so that the Zener diode also blocks. Only upon the appearance of possible overvoltages at the supply conductor 1 does the Zener diode 51 begins to conduct and produce a corresponding voltage drop across the resistor 60. Diode 50 insures that the second electrode of the measurement ionization chamber 21 starting from zero potential can only assume negative values.

A small leakage current flows only during the normal operation condition through field-effect transistor 22 and through transistor 58. A voltage of practically zero appears across the collector resistor 59 and therefore also between the base and emitter electrode of transistor 68, so that transistor 68 also blocks. The

voltage divider

62,63 is without current, whereby transistor 64 likewise blocks due to the absence of a baseemitter voltage.

During the positive pulse phases, diode 3 conducts and draws a current through

resistors

70, 65, 61 and 59. A voltage drop appears between the base and the emitter electrode of transistor 69 so that such begins to conduct and draws a collector current through the

voltage divider

71 and 72, which, in turn, causes transistor 73 to conduct. Thus, for positive supply voltage pulses, a current flows from the supply conductor 1 via diode 3 and essentially via transistor 69 to the supply conductor 2. At the same time, capacitor 52 is positively charged by means of diode 53. On the other hand, during negative pulses, diode 3 blocks and since the field-effect transistor 22 remains blocked, only a relatively small current flows in the capacitor 9 as well as across the

voltage divider

54,55 to the supply conductor 2.

Now, if smoke enters the measurement chamber 2ll, then field-effect transistor 22 begins to conduct in the manner already described above. During the negative pulse phases or cycles, a current flows between the supply conductor ll, via diode 4i, resistor Ml, field-effect transistor 22, transistor Ell, transistor till and the supply conductor 2. Transistor 5% serves to form a response threshold for field-effect transistor 22 and replaces the Zener diode 2d of FIGS. 3 and l. Its base electrode is prebiased by the charge stored in capacitor 56, whereby the voltage represents an average value of the negative and positive voltage pulses alternately appearing across resistor 55. Because of the current flowing from transistor 58 into the base electrode of transistor ss, transistor '68 begins to conduct, and because of the current which now would flow via resistor 62 into the base electrode of transistor 6i, transistor 64 also begins to conduct, so that at the fire alarm which has been alarmed, a larger or greater current flows via both of these transistors between the supply conductor ii and the supply conductor 2, such being used in the central signal station for triggering an alarm. The first bistable switching stage formed by both transistors d d and oil remains in a conductive state or condition until it is reset.

The possibility of an individual optical indication of an alarmed fire alarm will now be briefly considered. For this purpose, an indicating glow lamp 67 is employed which is connected in such a manner that it basically only tends to ignite during the positive pulses of the supply voltage. With afire alarm which is not alarmed, the voltage applied across glow lamp 67 during the positive voltage phases, is determined by the

voltage divider resistor

59,61 and

resistors

65,70 and is smaller than the response voltage of glow lamp 467. On the other hand, when a fire alarm has responded, the potential at the collector electrode of transistor 64 becomes more negative, thus the voltage across glow lamp s7 is increased during the positive pulse phases. However, if glow lamp s"! should first illuminate after the alarm signal has been properly received at the central station, whereby the illumination of the glow lamp at the same time can be evaluated as indicative of the reception of an alarm signal at the central station, or if the optical indicator should be completely suppressed for example during a subsequent checking operation to be described, then the response voltage of the glow lamp s1 is chosen such that the glow lamp, both when a fire alarm has responded or when transistor 64! is conductive, still does not ignite. By increasing the amplitude of the positive supply voltage pulses it is then possible to cause the glow lamp to burn or ignite during the positive pulse phases; the optical indicator therefore functions as a blink light.

The remote triggering of the tire alarm for the purpose of checking can take place in a number of different ways, for example, by increasing the positive voltage pulse amplitudes. One very sophisticated technique is to increase the repetition frequency of the positive pulses or to decrease their periods, whereby the average value of the direct'current voltage across capacitor 56 increases, which once again brings about a increase of the source-gate voltage of field-effect transistor 22, and such responds. As a result, as described above,

transistors

58,64 and Mt begin to conduct. The collector electrode of transistor o l becomes strongly negative, so that with a suitable voltage divider relationship of the resistors as and 7d during the positive pulse phases, the base electrode of transistor as becomes more negative than its emitter electrode and transistor 69 remains blocked. As a result, transistor 73 also remains blocked so that during the positive impulse phases with the tire alarm intact, current can no longer flow from the supply conductor l to the supply conductor 2. 0n the other hand, the capacitor 52 does not receive any new charge during the positive pulse phases, so that it discharges to the null value. The potential of the gate electrode of field-effect transistor 22 becomes more negative due to the discharging of capacitor 52. This corresponds to the positive feedback already described in connection with FiGS. 3 and i, but now the self-holding of an alarmed fire alarm is brought about in the Ml first instance by the bistable switching circuits Mott and 15%,73, respectively, which have switched into their other strible state or position.

The circuit arrangement according to FIG. 5 manifests it self, among other things, by the fact that it provides a high degree of supervising or monitoring possibilities for the in dividual components. A defect in any one of the structural components at the actual region or part of the fire alarm (components above the conductor 57) brings about a blocking of transistors did and 45%}, whereby during the checking phase, transistors 69 and. "73 further conduct and therefore deliver a disturbance signal ll3' (FIG. 2e) to the central station. An interruption at the actual monitoring circuit (elements beneath the conductor 57) on the other hand, causes a discharge of capacitor 52 and therefore a response of field-effect transistor 22, which, during the normal operating condition, indicates a (false) alarm. Naturally, it is conceivable that defects can also simultaneously occur in both circuit portions which, under these circumstances, will not be determined by the remote checking operation. However, this type of situation is highly improbable and can be taken into consideration without any great drawback.

A practical disadvantage of the circuit arrangement of FIG. 5 resides in the fact that the second electrode ofthe measurement ionization chamber 2ll is connected via the diode 54) with the supply conductor 2 which is generally grounded. Experiments have shown that it is possible to fulfill the desire of the designer to directly connect the measurement ionization chamber 21 with the supply conductor 2 in that the substrate electrode tit) of field-effect transistor 22 is connected instead with the tap of the voltage divider comprising resistors 33 and 34-, rather than with the source electrode. Such an arrangement is shown in FIG. 6 which is a variation of the portion of the circuit arrangement to the left of the section line A-A of FIG. 5.

Now, even if the fire alarms of FIG. 5 are supervised or monitored to a high degree with regard to their operational reliability, with the previously described measures it is Still not possible to detect an eventual break or interruption in the supply conductors. To this end, there is utilized a conductor terminal element LE (FIG. 11) which. is connected after the last fire alarm ll/i, and parallel to such between both supply conductors l and 2 and whose internal resistance previously was impliedly presupposed to be infinitely large. if the internal resistance of the conductor terminal element is chosen to be very large but still finite, then during the normal operating condition a current will flow through the groups of tire alarms during the phases of negative voltage pulses, and this current will be greater than zero (llil, FIG. 2c), yet smaller than the response current M of an individual fire alarm.

FIG. '7 depicts an embodiment of a conductor terminal element which can be used to particular advantage in conjunc tion with the inventive fire alarm installations. The function of this conductor terminal element resides in indicating at the central station a possibly occurring break or interruption at the conductor means of the groups of tire alarms. The exemplary illustrated conductor terminal] element here shown consists of an asymmetrical astable multivibrator which is connected via a diode ill to the supply conductor l as well as directly to the supply conductor 2. Both of the transistors $2 and M are coupled with the supply conductor 2 through the agency of a respective collector resistor as and $9. The base electrodes of both transistors $2 and 83 are coupled or connected with the junction or tap of a respective voltage divider circuit incorporating resistors 84.285 and 87b8, respectively. The emitter electrode of transistors b2 and 83 are directly connected with the anode of a diode 81. A capacitor W is ar' ranged between the base electrode of transistors 83 and the collector electrode of transistor 82, and, between the base electrode of transistor $2 and the collector electrode of transistor $3, a capacitor 91 is placed.

The mode of operation of an as'table multivibrator is well known to the art. During those phases when the potential of the supply conductor 1 is negative with respect to that of the supply conductor 2,

transistors

82 and 83 alternatingly conduct, whereby with appropriate variable section of the

collector resistors

86 and 80, respectively, a pulse-shaped current which varies between two values flows between the supply conductor 1 and the supply conductor 2. This behavior is illustrated in FIGS. 8a to 80. FIG. 8a initially shows once again the pulse-shaped voltage 14 applied between the supply conductors 1 and 2 and appearing at both inputs of a conductor terminal element. It will be recognized that FIG. 8a is identical to FIG. 2a. In FIG. 8!), there is shown the current i flowing through the astable multivibrator, whereby the pulses 92 of smaller amplitude signify the conduction of one transistor and the pulses 93 of larger amplitude signify the conduction of the other transistor. Finally, FIG. 8c shows, at the left of the line SS, the current i flowing through the group of fire alarms during the normal operating condition and represents a superimposition of FIGS. 2b and 8b, whereas at the right of the line SS there is indicated the current i received at the central station when a fire alarm is alarmed or has responded, and which results from superimposing the current shown in FIGS. 20 and 8b. Accordingly, pulses continually appear at the central station, the repetition frequency of which is considerably greater than that of the supply voltage pulses and which therefore can be easily separated by means of a frequency branching or selective network. An absence of the pulses of the conductor terminal element signifies a conductor break or interruption or a possible defect in the conductor terminal element itself.

Finally, FIG. 9 illustrates a block diagram of a central station 52 in which there is shown only the elements or components necessary for carrying out the previously described functions. A function transmitter 100 delivers the supply voltages with positive and negative amplitudes according to FIG. 2a. Changes in the shape and amplitude of the different phases of the supply voltages, remote triggering of the fire alarm, triggering of the individual indicators, and so forth, are delivered or fed into the function transmitter 100 by means ofa control device 101. The supply conductor 1 is directly connected with the function transmitter 100 whereas the supply conductor 2 is connected via a measurement resistor 102 with such function transmitter 100. Two

threshold value detectors

103 and 104 received the voltage drop appearing across the resistor 102, whereby by switching in

diodes

105 and 106 of different polarity, the threshold value detector 103 measures the positive voltage drops and the threshold value detector 104 measures the negative voltage drops. The threshold value detector 103 is controlled by the control device 101 in such a manner that, during the checking operation, a disturbance alarm is triggered when the positive voltage drop across the resistor 102 exceeds a certain threshold value. On the other hand, the threshold value detector 104 is controlled by the control device 101 in such a manner that, at its output 107, only those negative signals appearing across the resistor 102 are transmitted which, during the normal operation exceed a certain negative threshold value. These signals are subsequently divided at the

units

108 and 109 in accordance with their repetition frequencies, whereby unit 108 measures the signals with high frequency derived or coming from the conductor terminal element and, in the absence thereof, triggers a disturbance signal, and unit 109 measures the signals with low frequency and, during their occurrence, triggers a smoke or fire alarm.

At this point, it is again mentioned that the circuit arrangements illustrated in FIGS. 3 to 6 are only preferred, exemplary embodiments of fire alarms which are particularly well suited for use in a fire alarm installation of the inventive type shown in FIG. 1. It is quite readily possible to use other circuits and, in particular, individual components or groups of structural components can be replaced by equivalent different components, such as replacing transistors by tubes and the like.

It should now be apparent from the foregoing detailed description that the objects set forth at the outset of the specification have been successfully achieved.

What is claimed is:

. A fire alarm installation comprising:

. a central station;

. a plurality of fire alarms;

. a pair of common conductors for connecting said plurality of fire alarms in parallel to one another and to said central station;

d. each of said fire alarms comprising a detector element sensitive to fire phenomena and electric circuit means controlled by its associated detector element;

e. each of said circuit means possessing two stable conditions, defining a normal condition when the detector element assumes a normal operating state in a condition of readiness for detecting a fire and an alarm condition when said detector element assumes a response condition when it has been subjected to fire phenomena f. said circuit means possessing two parallel current paths for the respective flow of currents of different polarity;

g. each of said circuit means including switching means for connecting one of said parallel current paths to said two common conductors when the circuit means is in said normal condition and for connecting the other of said current paths across said two conductors when the circuit means is in said alarm condition;

h. said central station embodying means for generating a supply voltage of continuously and periodically alternating polarity which is delivered to said two conductors;

i. said central station further including means for generating a test signal applied to said two conductors; said test signal controlling said electric circuit means in like manner as said detector elements; and

j. said central station further comprising measuring means for separately detecting the current in said two conductors during the phases of different polarity of said supply voltage.

2. A fire alarm installation as defined in claim 1, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere, at least one resistance element connected in series with said measuring ionization chamber, the current flowing through said measuring ionization chamber changing when the atmosphere contains combustion aerosols or smoke, each of said circuit means being responsive to the thus produced voltage changes appearing across its associated measuring ionization chamber.

3. A fire alarm installation as defined in claim 1, wherein said parallel current paths of each of said circuit means comprises two circuit branches, each circuit branch being connected across said supply conductors via respective diode means of opposite polarity and via controllable semiconductor means, said controllable semiconductor means being connected in said circuit means such that one of said semiconductor means is conductive and the other semiconductor means is blocked during said normal condition of said electric circuit means, whereas said one semiconductor means is blocked and said other semiconductor means is conductive during said alarm condition of said electric circuit means.

4. A fire alarm installation as defined in claim 3, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a common electrode, said one controllable semiconductor means being a field-effect transistor having gate, source and drain electrodes, the gate electrode of which is electrically con nected with said common electrode of both said ionization chambers, said other semiconductor means being a further transistor, said circuit means possessing voltage divider means for coupling both saidionization chambers with said further transistor such that said field-effect transistor is positively fed back by means of said further transistor as well as by said ionization chambers.

5. A fire alarm installation as defined in claim 4, wherein a Zener diode is connected in series with said source electrode of said field-effect transistor, said test signal generating means at said central station including control means for reducing iii the amplitude of said supply voltage during one polarity for a short period of time.

b. A fire alarm installation as defined in claim 1, wherein said parallel current paths of each of said circuit means comprise diode means of opposite polarity, at least one of said cur rent paths possessing a bistable element containing at least two transistors and wherein the other current path possesses; at least one further transistor, said transistors being coupled together to provide a switching arrangement controlling either both transistors of said bistable element to be conductive or said further transistor in said other current path to be conductive, said bistable element being connected with and controlled by its associated detector element, said further transistor being connected with and controlled by said bistable element.

7. A fire alarm installation as defined in claim a, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a common electrode, one of said two transistors of said bistable ele' ment being a field-effect transistor having gate, source and drain electrodes, the gate electrode of which is connected to said common electrode of both ionization chambers, said further transistor having a base electrode, the second transistor of said bistable element possessing a collector elec trode connected with a second electrode of one of the ionization chambers and with said base electrode of said further transistor in the other current path and via a resistor with one of the supply conductors, said bistable element being connected via one of said diode means with the other of said supply conductors, one said electrode of said field-effect transistor being connected via a Zener diode with said one supply conductor, said further transistor in said other current branch being connected via the other of said diode means with said other supply conductor.

lit. A fire alarm instaHation as defined in claim t, wherein one of said current paths comprises a first diode and a first bistable element containing at least two transistors, said second current path comprises a second diode connected electrically antiparallel to said first diode and a second bistable element comprising at least two further transistors, said second bistable element being connected with and controlled by said first bistable element such that only one of said bista ble elements is conductive and the other one is blocked.

9. A fire alarm installation as defined in claim b, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a com mon electrode, a field-effect transistor having a gate, drain and source electrode and controlling said first bistable element, said gate electrode of said field-effect transistor being connected to said common electrode of said two ionization chambers, 21 still further transistor providing an alarm threshold means connecting said field-effect transistor with said first bistable element, said field-effect transistor and said first bistable element being blocked when said circuit means is in said normal condition, said second bistable element being further connected via a diode with a capacitor and with a voltage divider for connecting said second bistable element with the series connection of both of said ionization chambers, said further capacitor during the normal condition of said circuit means being charged and in said alarm condition discharges due to blocking of said second bistable element and thereby id stores said alarm condition by displacing the potential at said gate electrode of said field-effect transistor.

iii. A fire alarm installation as defined in claim ll, wherein said central station comprises control means for increasing as well as for decreasing the supply voltage during the phases of one polarity for short periods of time, said electric circuit means including circuitry for maintaining said electric circuit means in a self-holding state during the presence of. said alarm condition.

ii. A fire alarm installation as defined in claim i, further including a conductor terminal element connected in parallel to the last fire alarm of said plurality of fire alarms, said conductor terminal element comprising an asymmetrical astable multivibrator having a higher pulse repetition frequency with respect to the repetition frequency of said supply voltage.

ll2. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a conductor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal having an alternating voltage component for periods during which said central station is delivering a supply voltage of a certain polarity to said conductors and different from the signals drawn by said plurality of alarms.

13. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a concluctor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal different from the signal delivered by said supply voltage means to said plurality of fire alarms and different from the signals drawn by said plurality of alarms, said supply voltage means delivering signals of alter nating polarity, said terminal element means applying to said conductors a signal of a different: frequency than the signal frequency of said supply voltage means.

M. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a con ductor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal different from the signal delivered by said supply voltage means to said plurality of fire alarms and different from the signals drawn by said plurality of alarms, said supply voltage means delivering a signal of altcr nating polarity, said means of said conductor terminal element means for applying said signal to said conductors comprises an asymmetrical astable multivibrator having a higher pulse repetition frequency than the repetition frequency of said supply voltage means.

Claims

1. A fire alarm installation comprising: a. a central station; b. a plurality of fire alarms; c. a pair of common conductors for connecting said plurality of fire alarms in parallel to one another and to said central station; d. each of said fire alarms comprising a detector element sensitive to fire phenomena and electric circuit means controlled by its associated detector element; e. each of said circuit means possessing two stable conditions, defining a normal condition when the detector element assumes a normal operating state in a condition of readiness for detecting a fire and an alarm condition when said detector element assumes a response condition when it has been subjected to fire phenomena f. said circuit means possessing two parallel current paths for the respective flow of currents of different polarity; g. each of said circuit means including switching means for connecting one of said parallel current paths to said two common conductors when the circuit means is in said normal condition and for connecting the other of said current paths across said two conductors when the circuit means is in said alarm condition; h. said central station embodying means for generating a supply voltage of continuously and periodically alternating polarity which is delivered to said two conductors; i. said central station further including means for generating a test signal applied to said two conductors; said test signal controlling said electric circuit means in like manner as said detector elements; and j. said central station further comprising measuring means for separately detecting the current in said two conductors during the phases of different polarity of said supply voltage.

4. A fire alarm installation as defined in claim 3, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a common electrode, said one controllable semiconductor means being a field-effect transistoR having gate, source and drain electrodes, the gate electrode of which is electrically connected with said common electrode of both said ionization chambers, said other semiconductor means being a further transistor, said circuit means possessing voltage divider means for coupling both said ionization chambers with said further transistor such that said field-effect transistor is positively fed back by means of said further transistor as well as by said ionization chambers.

5. A fire alarm installation as defined in claim 4, wherein a Zener diode is connected in series with said source electrode of said field-effect transistor, said test signal generating means at said central station including control means for reducing the amplitude of said supply voltage during one polarity for a short period of time.

6. A fire alarm installation as defined in claim 1, wherein said parallel current paths of each of said circuit means comprise diode means of opposite polarity, at least one of said current paths possessing a bistable element containing at least two transistors and wherein the other current path possesses at least one further transistor, said transistors being coupled together to provide a switching arrangement controlling either both transistors of said bistable element to be conductive or said further transistor in said other current path to be conductive, said bistable element being connected with and controlled by its associated detector element, said further transistor being connected with and controlled by said bistable element.

7. A fire alarm installation as defined in claim 6, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a common electrode, one of said two transistors of said bistable element being a field-effect transistor having gate, source and drain electrodes, the gate electrode of which is connected to said common electrode of both ionization chambers, said further transistor having a base electrode, the second transistor of said bistable element possessing a collector electrode connected with a second electrode of one of the ionization chambers and with said base electrode of said further transistor in the other current path and via a resistor with one of the supply conductors, said bistable element being connected via one of said diode means with the other of said supply conductors, one said electrode of said field-effect transistor being connected via a Zener diode with said one supply conductor, said further transistor in said other current branch being connected via the other of said diode means with said other supply conductor.

8. A fire alarm installation as defined in claim 1, wherein one of said current paths comprises a first diode and a first bistable element containing at least two transistors, said second current path comprises a second diode connected electrically antiparallel to said first diode and a second bistable element comprising at least two further transistors, said second bistable element being connected with and controlled by said first bistable element such that only one of said bistable elements is conductive and the other one is blocked.

9. A fire alarm installation as defined in claim 8, wherein each of said detector elements comprises a measuring ionization chamber accessible to the surrounding atmosphere connected in series with a reference ionization chamber by a common electrode, a field-effect transistor having a gate, drain and source electrode and controlling said first bistable element, said gate electrode of said field-effect transistor being connected to said common electrode of said two ionization chambers, a still further transistor providing an alarm threshold means connecting said field-effect transistor with said first bistable element, said field-effect transistor and said first bistable element being blocked when said circuit means is in said normal condition, saId second bistable element being further connected via a diode with a capacitor and with a voltage divider for connecting said second bistable element with the series connection of both of said ionization chambers, said further capacitor during the normal condition of said circuit means being charged and in said alarm condition discharges due to blocking of said second bistable element and thereby stores said alarm condition by displacing the potential at said gate electrode of said field-effect transistor.

10. A fire alarm installation as defined in claim 1, wherein said central station comprises control means for increasing as well as for decreasing the supply voltage during the phases of one polarity for short periods of time, said electric circuit means including circuitry for maintaining said electric circuit means in a self-holding state during the presence of said alarm condition.

11. A fire alarm installation as defined in claim 1, further including a conductor terminal element connected in parallel to the last fire alarm of said plurality of fire alarms, said conductor terminal element comprising an asymmetrical astable multivibrator having a higher pulse repetition frequency with respect to the repetition frequency of said supply voltage.

12. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a conductor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal having an alternating voltage component for periods during which said central station is delivering a supply voltage of a certain polarity to said conductors and different from the signals drawn by said plurality of alarms.

13. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a conductor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal different from the signal delivered by said supply voltage means to said plurality of fire alarms and different from the signals drawn by said plurality of alarms, said supply voltage means delivering signals of alternating polarity, said terminal element means applying to said conductors a signal of a different frequency than the signal frequency of said supply voltage means.

14. An alarm installation comprising a central station, a plurality of alarms, at least one pair of conductors for electrically connecting said plurality of alarms in a group in parallel with said central station, said central station including supply voltage means for said group of parallely connected alarms, a conductor terminal element means connected in parallel to the last alarm of said plurality of alarms for monitoring possible interruptions at the conductors of said plurality of alarms, said conductor terminal element means including means for applying to said conductors a signal different from the signal delivered by said supply voltage means to said plurality of fire alarms and different from the signals drawn by said plurality of alarms, said supply voltage means delivering a signal of alternating polarity, said means of said conductor terminal element means for applying said signal to said conductors comprises an asymmetrical astable multivibratOr having a higher pulse repetition frequency than the repetition frequency of said supply voltage means.