Fire Alarm Systems Equipped With Circuit Monitoring Devices

Abstract

A fire alarm system including a plurality of fire alarms arranged in groups. Alarm-simulating conditions are produced in each fire alarm by an electric test signal, the response of each fire alarm being sent to an evaluation device at a central signal station through a circuit coupling the fire alarms together. Each alarm generates an oscillation in normal condition which is suppressed in the alarm state.

Parent Case Text

The instant application is a divisional application of our commonly assigned, co-pending U.S. application, Ser. No. 636,174, filed May 4, 1967 and entitled "FIRE ALARM SYSTEM HAVING A DIODE COUPLED CHECKING CIRCUIT.", now U.S. Pat. No. 3,564,524.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an improved fire alarm system comprising a central signal station to which electric fire alarms are connected in groups, wherein the number of conductors between the central signal station and the fire alarms is independent of the number of fire alarms. Furthermore, the inventive fire alarm system additionally includes an apparatus for producing electrical test signals by means of which alarm-simulating conditions are produced at the fire alarms which, in turn, bring about an electrical change in condition at an intact fire alarm, and further, the inventive system includes an apparatus for evaluating such changes and conditions.

In fire alarm installations, there generally exists the requirement of checking the operational reliability of the individual fire alarms at regular intervals. To this end, and in accordance with a workable method, alarm-simulating conditions are manually delivered to the individual fire alarms one after the other or in series. The response of the relevant alarm is controlled at its locality or at the central station. However, this technique is above all, extremely time-consuming and uneconomical, if the installation consists of a larger number of fire alarms, for instance, one hundred fire alarms or more. In such case, a regular manual monitoring at shorter time intervals can no longer be considered for practical reasons.

It is for these reasons that methods have been developed in which it is possible to achieve from the central station alarm-simulating conditions at the fire alarms through use of special electronic means, and to check the response conditions of all fire alarms at the central station. The difficulty with such method, above all, resides in the reliable determination of whether all fire alarms of a group have actually responded.

In a known system, a separate conductor is lead from each fire alarm back to the central station. During the checking operation, a signal appears at such conductor which is characteristic of the response condition of the relevant fire alarm. This system has the drawback that it requires extensive additional installations, which becomes of particular importance if such a monitoring device should be subsequently installed in an already existing fire alarm system.

Consequently, it is desirable to provide a monitoring device in which the number of conductors leading from the central station to the fire alarms is independent of the number of connected fire alarms. In a known arrangement of this type, all of the fire alarms are simultaneously caused to respond from the location of the central station. Further, by means of a current measuring device at the central station which is provided at one of both current delivery conductors of the fire alarm, the response of all fire alarms is monitored, whereby there is checked the coincidence of the total current with some predetermined reference value. Apart from the great current consumption which has an adverse effect upon the choice and dimensioning of the conductors, the network devices, the emergency power groups, etc., this arrangement is limited to groups having relatively few fire alarms. Because of the diversity of the electrical characteristics of the structural elements or components in the fire alarms as well as because of the limited measuring accuracy during comparison of the total current with the reference value based upon an analogous measurement, this technique is unreliable with a larger number of fire alarms and oftentimes results in false alarms.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide an improved monitoring device for a fire alarm system of the previously mentioned type in which it is possible to reliably check or monitor an optional large number of fire alarms per group.

It is another object of the present invention to provide a monitoring system which may be installed in existing fire alarm systems.

It is a further object of the present invention to provide a monitoring system which requires low additional power.

It is an additional object of the present invention to provide a monitoring system which makes use of digital techniques.

Generally speaking, the present invention is characterized by the features that the fire alarms of a group are coupled with one another by means of a logic circuit and that the logic circuit, during the checking operation, then delivers a signal to the evaluation device when all fire alarms of a group have responded or at least one fire alarm has not responded.

A number of different embodiment of the invention as well as further inventive features will be described in greater detail hereinafter in conjunction with the figures of the drawing. In this regard, it will be appreciated that while the exemplary fire alarm installations depicted herein are in each instance equipped with ionization fire alarms, it would be equally well possible to also employ optical flame detecting alarms, smoke detecting alarms or temperature detecting alarms.

BRIEF DESCRIPTION OF THE DRAWING

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 drawing wherein:

FIG. 1 illustrates a circuit diagram of a first embodiment of the invention in which the individual fire alarms have associated therewith diode means for coupling the individual fire alarms to one another; and

FIG. 2 is a circuit diagram of an exemplary embodiment of the invention in which the individual fire alarms have associated therewith oscillation generators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Describing now the drawing, in FIG. 1, there are depicted two fire alarms 39 and 40 of identical construction. These fire alarms 39 and 40 are connected parallel to one another via the supply conductors or leads 35 and 36 as well as being connected via an additional signal conductor 29 with a central station 41. As already remarked, these fire alarms 39 and 40 are of identical construction and preferably consist of two ionization chambers 21 and 22, the air content of which can be rendered completely or partially conductive through an appropriate radioactive preparation as is known in the art. Furthermore, these fire alarms 39 and 40 each incorporate a switch element 23 which, in this case, is a field-effect transistor. The ionization chamber 21 operates as a measuring chamber, whereas the ionization chamber 22 is used as a reference chamber. Chamber 22 can be replaced by a high-ohm resistor or an equivalent high-ohm element. Likewise, in lieu of a field-effect transistor 23 it is possible to use other semi-conductor devices or a different switch element with high-ohm input resistance or impedance, for instance a cold-cathode tube.

The drain or anode of the field-effect transistor 23 is connected via a resistor 25 with the negative supply conductor 35, the source or cathode via the tap of a potentiometer 24 with the positive supply conductor 36. Furthermore, at the drain or anode of the field-effect transistor 23 there is connected the gate of a controlled rectifier 26, the cathode of which is coupled with the negative supply cathode 35 and the anode of which, on the one hand, is connected via a resistor 44 to the negative supply conductor 35 and, on the other hand, via a resistor 45 with the positive supply conductor 36. The anode of the controlled rectifier 26 is furthermore connected with the signal conductor 29 through the series circuit or connection incorporating the indicating lamp 31 and the Zener diode 28. A resistor 34 is connected in parallel with the indicating lamp 31. This resistor 34 maintains the connection between the controlled rectifier 26 and the Zener diode 28 during breakdown of the indicating lamp 31.

The potential at the tap of the potentionmeter 24 is adjusted in such a manner that the field-effect transistor 23 and thereby the controlled rectifier 26 block in the normal condition. The signal conductor 29 is connected at the central station 41 via a contact of the switch 43 and a resistor 30 with the positive pole of the voltage source designated by reference numeral 100. The zener voltage of the diode 28 is selected in such a manner that when the rectifier 26 is blocked, in which case its anode has a potential determined by the voltage divider 44, 45, this diode 28 likewise blocks. The signal conductor 29 is then without current and the voltage at points 200 equals the voltage of source 100.

Now if combustion gas enter the measuring chamber 21, then the field-effect transistor 23 and the controlled rectifier 26 change their switched condition. More precisely, the potential at the anode of the controlled rectifier 26 practically drops to that of the supply conductor 35 (ground), so that the voltage across the diode 28 now exceeds the Zener voltage and the diode begins to conduct. At the central station 41 the voltage at point 200 or the contact of the switch 43 drops from the voltage of source 100 value to a new value approximately corresponding to the Zener voltage of the diode 28. In so doing, the resistor 30 is selected in such a manner that the current flowing through the indicating lamp 31 initially does not cause such to light up or indicate. By closing a switch 33 arranged parallel the resistor 30, the indicating lamp 31, in the case of the alarm, can be caused to ignite from the central station, whereby the fire alarm receives a receipt or acknowledgement of the proper reception of the alarm signal at the central station.

The Zener diodes 28 of the individual fire alarms enable an alarm signal of the one or other fire alarm arrive via the common conductor at the central station without having any effect upon the remaining fire alarms. In accordance with the invention, the OR-gate is coupled or switched during the checking operation in such a manner that a signal only appears at the central station if at least one fire alarm has not responded. This can be realized in that, the conductor 29 is supplied by a voltage source 300 by means of the switch 43. The value of voltage source 300 is smaller than the Zener voltage of the diodes 28. Since the voltage of source 300 is smaller than voltage of source 100, the Zener diodes 28 are poled in the forward direction via the resistor 45 and the indicating lamps 31 and a current flows through the signal conductor. The alarm-simulating conditions are produced through opening for a short time the switch 37 to thus introduce a voltage reducing Zener diode 38 in the supply conductor 36 which initially causes the capacitor 42 to partially discharge. Upon closing of the switch 37, a momentary over-voltage is produced between the gate and cathode of the field-effect transistor 23. The alarm-simulating conditions thus bring about a response of the field-effect transistor 23 and thereby the controlled rectifier 26. Then, as a practical matter, the voltage of source 300 appears across the Zener diodes 28 of the intact fire alarms, which voltage by definition is smaller than the Zener voltage. If all of the fire alarms of a group have properly responded then the conductor 29 is without current. On the other hand, a signal appearing at the conductor 29 means that one or more fire alarms have not responded.

The return of the fire alarm can likewise take place by means of the signal conductor 29. To this end, the switch 43 is thrown or applied for a short time to a negative voltage source 400.

Naturally, it is also possible to apply numerous modifications to the circuit of FIG. 1. Thus, for instance, it is possible to employ conventional diodes instead of Zener diodes, whereby the measures which must be undertaken in such case will be apparent to those skilled in the art.

FIG. 2 depicts a circuit diagram of an embodiment of the invention which is essentially based upon the same principles as the circuit diagram of FIG. 1. In this circuit arrangement the individual fire alarms are connected with one another into a logical circuit and an additional signal conductor is dispensed with.

By referring to FIG. 2 in greater detail it will be recognized that the individual fire alarms 50 embody a measuring ionization chamber 52 and a reference ionization chamber 54, the common connection point of which is coupled with the starter electrode of a cold-cathode tube 56. The anode of the cold-cathode tube 56 is coupled via a resistor 58 with the supply conductor 60 and the cathode is directly connected with the supply conductor 62. Furthermore, between both of the conductors 62 and 60 a series connection of a capacitor 64 and a resistor 66 is disposed. At the common juncture point or point of connection of the capacitor 64 and the resistor 66 one electrode of the measuring ionization chamber 52 is connected. The diode 68 is connected in parallel with the resistor 66. Finally, the anode of the cold-cathode tube 56 is connected via a resistor 70 with the connection point of a series connection of a further capacitor 72 and a glow-discharge tube 74 disposed between the supply conductors 60 and 62. Glow-discharge tube 74, capacitor 72 and resistors 58 and 70 form an impulse transmitter. A current flows through the glow-discharge tube 74 which charges the capacitor 72 for such length of time until the voltage is dropped via the glow-discharge tube 74 to the extinguishing voltage, whereafter the capacitor 72 discharges via the resistors 58 and 70. As a result, the voltage across the glow-discharge tube 74 again increases until it reaches the ignition voltage, and a new cycle is initiated. The individual current surges can be registered at the secondary winding of a transformer 76 provided in the supply conductor 62 at the central station 78.

If combustion gases enter the measuring ionization chamber 52, then the cold-cathode tube 56 ignites because of an increase of the starter potential and draws a current through the resistor 58. In the event that operating voltage of the cold-cathode tube 56 is larger than the ignition voltage of the glow-discharge tube 74 then the oscillations of the associated impulse transmitter 74, 70, 72 stop. The current surge during ignition of the cold-cathode tube 56 can be employed in known manner for triggering a relay 80 or another alarm-transmitting device.

The checking operation is initiated by dropping the supply voltage 500 for a short period of time. In so doing, the capacitor 64 discharges during the reduction in voltage across the diode 68 to the new supply voltage. Now, if the supply voltage 500 is instanteously increased again to the old value, then the diode 68 once again blocks and the capacitor 64 slowly charges via the high-ohm resistor 66. In so doing, however, the potential at the starter of the cold-cathode tube 56 initially is at an increased value determined by the capacitor 64 and the series connection incorporating the ionization chambers 52-54 and the resistor 58. The value of this potential is set in such a manner that the cold-cathode tube ignites. The sawtooth oscillations stop in the intact fire alarms, so that a signal only appears at the secondary winding of the transformer 76 -- which at the same time forms the input of an evaluation circuit or sensing device 19 -- when one or more fire alarms have not responded.

After completion of the checking operation, the supply voltage 500 is dropped slightly below the operating voltage of the tube 56 and thereafter slowly again increased to the full value. In each case, the checking operation is repeated at appropriate time-intervals.

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,

Claims

What is claimed is:

1. A fire alarm system comprising:

a plurality of fire alarms connected in parallel together in at least one group;

a central signal station;

conductor means connecting said central signal station with said plurality of fire alarms, the number of said conductors being independent of the number of said fire alarms of each group;

means for generating an electric test signal during an alarm checking operation;

said fire alarms including means responsive to said electric test signal to cause each of said fire alarms which are in a functionally ready state for detecting a fire condition to respond the same as if an actual fire alarm condition were present;

electrical circuit means for said plurality of fire alarms for producing no alarm-checking response signals when all fire alarms of a group have responded to said electric test signal and producing an alarm-checking response signal when at least one fire alarm of a group has not responded to said electric test signal; and

means for sensing at least one of said respective response signals.

2. A fire alarm system comprising:

a plurality of fire alarms connected in parallel together in at least one group;

a central signal station;

conductor means connecting said central signal station with said plurality of fire alarms, the number of said conductors being independent of the number of said fire alarms of each group;

means for generating an electric test signal during an alarm checking operation;

said fire alarms including means responsive to said electric test signal to cause each of said fire alarms which are in a functionally ready state for detecting a fire condition to respond the same as if an actual fire alarm condition were present;

an electrical circuit means for said plurality of fire alarms for producing no alarm-checking response signals when all fire alarms of a group have responded to said electric test signal and producing an alarm-checking response signal when at least one fire alarm of a group has not responded to said electric test signal, each fire alarm includes an oscillator means, the oscillations of which are suppressed when each said fire alarm is in its fire alarm condition; and

means for sensing at least one of said respective response signals.

3. A fire alarm system as claimed in claim 2, wherein said conductor means comprises two power supply conductors connected to each fire alarm, said oscillator means of each fire alarm being connected together by one of said power supply conductors, a transformer having a primary winding and a secondary winding electrically connected with said electrical circuit means, said primary winding being connected to said one of said power supply conductors, and said secondary winding providing the input of said means for sensing said response signals.

4. A fire alarm system as claimed in claim 3, wherein said oscillator means comprises a saw-tooth generator.

5. A fire alarm system as claimed in claim 4, wherein said two power supply conductors comprise a positive conductor and a negative conductor.

6. A fire alarm system as claimed in claim 5, wherein said saw-tooth generator includes a glow-discharge tube having an anode and a cathode, said cathode being connected to said negative conductor; a capacitor and a resistance means connected in parallel with said capacitor, one terminal of said capacitor being connected with said positive conductor, the other terminal of said capacitor being connected to said anode of said glow-discharge tube.

7. A fire alarm system as claimed in claim 6, wherein said resistance means comprises a first and a second resistor connected in series; each said fire alarm further comprising a cold-cathode tube having an anode, a cathode and a starter electrode, said anode electrode being connected to the junction point of said first and second resistors, said cathode electrode being connected to said negative conductor; a third resistor; each of said fire alarms comprising a measuring ionization chamber and a reference ionization chamber connected in series between said junction point of said first and second resistors, and said third resistor; said third resistor being connected to said negative conductor; and means connecting said starter electrode of said cold-cathode tube to the junction point between the measuring ionization chamber and the reference ionization chamber.

8. A fire alarm system as claimed in claim 7,each of said fire alarms further including a second capacitor connected to said positive conductor at one terminal thereof and connected to said third resistor at the other terminal thereof; a diode connected in parallel with said third resistor; and wherein said electric test signal comprises a reduction in potential of said positive conductor.