US3821734A

US3821734A - Fire alarm system with remote central station

Info

Publication number: US3821734A
Application number: US00374795A
Authority: US
Inventors: O Herrliberg; A Scheidweiler
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1972-07-17
Filing date: 1973-06-28
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28
Also published as: FI56908C; FR2193225B1; SE390076B; FI56908B; JPS4970596A; JPS5617704B2; NO137619C; FR2193225A1; DE2328881A1; AU474604B2; GB1401146A; AU5798773A; HK55880A; CH547531A; NO137619B; DE2328881B2; CA988224A; ZA734855B; DE2328881C3

Abstract

To provide unambiguous indication of fire alarm conditions in a central station, in which a plurality of fire alarm sensing lines come together, to which sensors are connected which, in case of fire, have low electrical resistance, the central has a current limiter which limits the current in case an alarm is sensed to a first limited value (Im), the predetermined current being selected with respect to the alarm sensors such that, until the limiting current is reached, the electrical resistance of the sensor is higher than a predetermined value and, when the predetermined current has been reached, the electrical resistance drops below that resistance value; the signal central further having a voltage sensor to sense voltage drop at the connection point of the lines leading to the sensors which, when an excessive voltage drop across a certain line is sensed at the predetermined current value (Im) further causes transmission of a higher current than the predetermined current(Im), at least for a limited period of time, in order to be able to clearly differentiate between alarm conditions, possible line short circuits or interruptions, or non-alarm conditions during line trouble.

Description

United States Patent [191 Herrliberg et al.

[ June 28, 1974 FIRE ALARM SYSTEM WITH REMOTE CENTRAL STATION [75] Inventors: Otto Meier Herrliberg; Andreas Scheidweiler, Stafa, both of Switzerland [73] Assignee: Cerberus AG, Mannedorf,

Switzerland 22 Filed: June 28,1973

21 Appl.N0.:374,795

[30] Foreign Application Priority Data July 17, 1972 Switzerland 10654/72 [52] U.S. Cl 340/409, 340/412, 340/228 R,

[5]] Int. .Cl. G08b 17/10 [58] Field Of Search 340/412, 409

[56 References Cited UNITED STATES PATENTS 3,665,46l 5/l972 Gnogi 340/409 Primary Extzminer-Thornas B. l-labecker Attorney, Agent, or Firm-Flynn & Frishauf [57] ABSTRACT To provide unambiguous indication of fire alarm conditions in a central station, in which a plurality of fire alarm sensing lines come together, to which sensors are connected which, in case of fire, have low electrical resistance, the central has a current limiter which limits the current in case an alarm is sensed to a first limited value (I,,,), the predetermined current being selected with respect to the alarm sensors such that, until the limiting current is reached, the electrical resistance of the sensor is higher than a predetermined value and, when the predetermined current has been reached, the electrical resistance drops below that resistance value; the signal central further having a voltage sensor to sense voltage drop at the connection point of the lines leading to the-sensors which, when an excessive voltage drop across a certain line is sensed at the predetermined current value (I,,,) further causes transmission of a higher current than the pre' determined current(l,,,), at least for a limited period of time, in order to be able to clearly differentiate between alarm conditions, possible line short circuits or interruptions, 0r non-alarm conditions during line trouble.

14 Claims, 7 Drawing Figures I 3 L1 RL.

u l u u E FIRE ALARM SYSTEM WITH REMOTE CENTRAL STATION Cross reference to related patent and application: US. Pat. 3,767,917, Oct. 23, 1973 and US. Ser. No. 374,310, filed June 28, 1973, both assigned to the Assignee of the present application.

The present invention relates to fire alarm systems and more particularly to a central fire alarm station which is connected by means of electrical lines to a plurality of remotely located fire sensors, to provide an alarm, and preferably an indication which one of the sensors has responded.

Fire alarm sensors frequently include units which.

normally have a high electrical resistance, and, upon sensing of an alarm condition, change the electrical resistance to a very low value, change in resistance on a connected line being sensed in a central station. This central station permits constant monitoring of a plurality of sensing units. The individual sensors or sensing units can be located, as desired, in the particular locations where fire or excessive temperatures are to be monitored' The connections are usually over lines which may be quite long. These lines are of the low voltage type, for example similar to telephone lines. Suitable fire alarm sensing units are ionization-type fire alarm sensors, optical sensors, smoke detectors, temperature sensors, or various other types of sensors which react to ambient conditions indicative of fire, or results'of fire. Frequently a plurality of such fire sensors may be connected, in parallel, through a common line pair, and a number of such pairs are connected to the signal central. 1

Upon occurrence of a fire, the electrical characteristics of the fire sensor change. In one form, an abrupt change in resistance results upon transition from normal condition to alarm condition of the sensing units. The signal central is provided with units which sense the change in current flowing to the supply lines of the sensing units themselves. By means of a current sensing device, the abrupt change in current can be sensed, to obtain an alarm indication. The system as described functions well, but is subject to false alarms if there is trouble in the electrical supply line. Rapid increase in electrical current flowing through the line may be indicative of a fire, that is, a change in the sensor resistance; a similar effect, however, occurs upon short circuit, or defects in the supply line. The resistance of a short circuit connection may be in the order of magnitude of the resistance of a fire sensor, when in alarm condition. Thus, current of increased magnitude will flow through the lines, due to the breakdown in insulation resistance between the supply wires, although no fire exists, but rather, an insulation breakdown, and/or a short circuit.

In fire alarm systems it is equally important not only to signal existing fires, but also to suppress false alarms. The disadvantage of the aforementioned system could be avoided by so constructing the individual alarm sensors that the resistance of a sensing unit, when in alarm condition, has a value which is intermediate between quiescent resistance and maximum possible short circuit resistance of the line and the unit. Unfortunately,

there are limits to such resistance adjustment, since the alarm resistance of individual alarm units cannot be changed to be of any elevated desired value. The alarm resistance of any fire sensing unit must be clearly differentiated from the overall quiescent condition of all the units, connected in parallel, including a possible resistance terminating the supply lines, so that there will be a clear distinction between current flowing through the lines under alarm conditions as well as under ordinary quiescent monitoring conditions. It is also frequently customary to utilize an incandescent lamp, connected in the alarm sensing unit, to provide an indication of which unit responded; the relativelylow resistance of this lamp may, practically, be the entire alarm resistance of the sensing unit.

The order of magnitude of maximum short circuit current may reach the order of magnitude of the current flowing to a sensing unit, particularly if the sensing unit is connected to a long line and the short circuit occurs at the end of the long line. Further, if the sensing unit is connected to a long line, there will be a voltage drop, when the alarm indicator lamp is connected, due to the current flowing in the long signaling line from the sensing unit to thecentral station, so that the brightness of the alarm indicator, when illuminated, will substan-' tially depend on the line resistance, and thus on the individual location of any one of the sensing units.

It is not possible to reduce the maximum possible short circuit resistance, as seen from the central unit, that is the maximum line resistance, since the costs of increasing wire sizes rise quickly. In extensive and long fire alarm systems, in which sensing units are located at substantial distance from the signal central, the costs increase rapidly unless standard conductors of low cross sectional areas, such as telephone cables, can be used. 1

It is an object of the presentinvention to provide a fire alarm system, with a fire alarm central station, in which the foregoing disadvantages are largely avoided, and to provide a fire alarm system central which is capable of .clearly distinguishing, without ambiguity, between responseof a fire alarm sensor, and a short circuit in the connecting line between the central and the sensor itself. Further, the entire line should be capable of being monitored, up to the last sensing unit in the line, and the apparatus should be so arranged that upon either short circuit or interruption in the connecting line (for example, a wire break) a trouble indication is obtained,'even if the connecting lines are of considerable lengths.

SUBJECT MATTER OF THE PRESENT INVENTION The central station includes a circuit which limits the current going to a line connected thereto to a predetermined value. The fire alarm units or sensors areso constructed that the electrical resistance thereof is greater than a first predetermined resistance value, if this predetermined current flows. Above this limiting value of predetermined current, electrical resistance of the 'fire alarm unit drops to a second resistance'value, less than the first resistance value thereof. The signal central fur ther includes a device to determine, and evaluate the voltage drop across the line terminals leading to the fire extinguishers, and, further, a .device to at least temporarily increase the current flowing to the line, if the voltage drop, as sensed in the signal central exceeds a certain limit when the first limiting current flows.

The central thus, in case there is a change in current flow, automatically initiates a test sequence, limiting the increased current, measuring voltages, and, as a result reaching a logical decision whether a fire alarm sensor has'responded or the line became defective.

of the connection line, the signalling lamp being con n'ected to this voltage, therebyavoiding the well known difficulties arising in the use of. incandescent lamps due to the tight interrelationship between voltage and light output, as well as life of illuminating units.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a simplified highly schematic diagram of a complete fire alarm system;

FIG. 2 is a graph illustrating the voltage-current characteristics of a sensing element; 2' FIG. 3 is a circuit diagram of a complete sensing element; v a 3 FIG. 4 is a circuit diagram of another embodiment of a sensing'element; I I

' FIG. 5 is acircuit diagram, schematically, of a signal central; v

FIG. 6 is a simplified diagram of a termination unit; and

' A signal central 1 (FIG. 1) is connected over lines L L to a plurality of fire sensors. In the example shown, a first sensor F isconnected comparatively closely to the central; the lines L L continue, anda further sensor F is connected remotely from the central/The equivalent resistance of the lines is shown by resistors R The signal central 1 has a voltage supply V which supplies power to the output, to which the various sen sors are connected.

Under. ordinary, quiescent conditions, that is, when the sensors are not influenced by a fire, the resistance of all sensors is relatively high. The characteristic is shownby a dashed line D,,, FIG. 2. The lines L and- L have only a small quiescent current' flowing therein. If the results of a fire affect a sensor, then it will switch into an alarm condition. The resistance of thesensor will jump to a lower value and the current flowing in the 'lines will substantially increase. The-signal central 1 includes a current limiter 3 which permits the current toris'e to a maximum value l (FIG. 2). A current detector (Ammeter 2) detects this current inthe signal central. Current detector 2, on the one hand, controls an alarm and signalling device 5, and also triggers an alarm. If necessary, simultaneously or with some delay,

an external alarm device A can be energized. A voltage detector 4 is'connected to the terminals of lines L,, L

In accordance with the present inventionfsens'ors with a predetermined voltage-current characteristics FIG. 7'shows '5 simplified equivalent connection diagram ofa sensor. I

level. The slope, in this level'region, is so low that the sensor may be considered to have'a saturation voltage U The currentl,,,,'at the knee of the curve. is the current which is the limiting current, as determined by the current limiting device 3, FIG. 1, in the central 1.

v If one were to draw resistance lines, 'of'various slopes, and passing through the intersection between the curve D and the current line I,,,, then the behavior of the system, with various line resistance, will become clear. A

. comparatively slightly inclined line will be indicative of low line resistance, that is, R A more steeply inclined line would be representative of a much higher line resistance, R The intersection of these resistance lines with the ordinate then provides-the division of theentire voltage U being supplied by the central 1, into the voltage drop Up across the sensor, the line drop of the lines U and the voltage drop within the central 1 itself, U As seen in FIG. 2, the voltage U arising at the terminals of the connected lines L L2 is always greater than the saturation voltage U of the sensor,for'all finite line resistances R in view of the relationship:

- v f UM 0 UL The voltage-current characteristics of a short circuit in a line is entirely different from that of one of the fire sensors in accordance with the invention. The characline K. As can be seen,the intersection of current I with line K, to which a line resistance of R corresponds, will cut the ordinate belowthe saturation voltage U for a wide range of resistances. This means that, in case of a short circuit,'a measured voltage U will be apparent at the signal central, which is below that of the saturation voltage of the first sensor. Thus, by analyzing the terminal voltage in the central by means of a voltage sensor 4 (FIG. 1) such as a volt meter, it can be determined if the resulting change in the system, that is, current flow and voltage relationships, are due to a response of a sensor, that is, a real fire alarm, or are due to line trouble, for example a short circuit, de-

pending. upon whether the measured voltage U is greater than the saturation voltage U of the fire sensor, or less. It will be seen that the relationship will be true up to resistances in which R tan 7 in FIG. 2. The graphs of FIG. 2 likewise show that I,,, should, preferably, be selected to be as-low as possible, so that the system may be useful for a wide range of line resistance values, within which short circuit, and alarm conditions can be clearly and unambiguously distinguished.

For sensing an alarm, first, a comparatively low current I,,, is being used in order to be able to differentiate from a short circuit withina wide range of line resistances. After an alarm condition, that is, a true fire alarm, has been sensed, the signal central can increase the current to operate an indicator'lamp, in pulses, to an alarm current I .'In this region, the characteristics are very fiat, so that the differential resistance is low.

This enables providing sufficient current to an alarm indicator, for example an incandescent lamp, since a.

good deal of power is now available to drive the lamp.

Sensing the terminal voltages additionally can be used to localize a'short circuit. The voltage U at the terminals is a measure of the line resistance to the short circuit. Similarly, the position of a fire sensor can be 10- calized, if a plurality of fire sensors are connected to the signal central by lines of different lengths. The voltage at the terminals of the connecting lines L L provide an indication of the line resistance to the fire sensor, that is, to the unit which has responded and hence to the locality of the fire. i

The voltage sensing device 4, as seen in FIG. 1, also controls the signalling device 5.'By suitable logic, the voltage sensing device 4 inhibits generation of an alarm by unit 5, even if the current detector 2 indicates excessive current, if the voltage detector, simultaneously, detects a terminal voltage below the threshold or saturation voltage U It may even be possible to dispense with the current detector as the unit to provide an alarm, if, for example, by suitable choice of the limiting current I,,, it can be ensured that change in terminal voltage will arise upon alarm conditions pertaining at any one of the sensors. The voltage detector 4, alone, then controls the alarm signalling unit 5, capable of distinguishing between a true fire alarm condition and a line short circuit.

Besides distinguishing between a fire and short circuit, line interruption can likewise be detected. As indicated in FIG. 1, a terminating element E is connected at the end of the lines L L to form a complete sensing loop.

FIG. 3 illustrates a fire sensor, used in the system of the present invention, and operating on the ionization principle, and which has the necessary current-voltage characteristics. Supply lines 10, 11 have connected thereacross, in series, an ionization chamber 12 with two electrodes and a radioactive substance. Chamber 12 is open to ambient atmosphere. Additionally, an essentially closed ionization chamber 13, which may be termed a reference chamber, is likewise provided, also having two electrodes and a radioactive substance. Upon penetration of smoke, fire aerosols, or the like in ionization chamber 12, the resistance thereof changes so that the voltage drop over the open ionization chamber 12 increases. This changes the voltage at the junction point 14 between the two

ionization chambers

12, 13. Junction point 14 is connected to the gate or control electrode of a field effect transistor (FET) 15. The source electrode of FET 15 is connected to a voltage determined by

voltage divider

16, 17; the drain electrode is connected to control an electronic switch, formed of two complementary npn and pnp transistors l8, 19, connected in a flip-flop circuit. The collector resistors are shown at 20, 21, 22, 23.

Base capacitors

24, 25 are connectedbetween the respective bases and the emitter supply line.

Operation: If the gate voltage of F ET l5 exceeds the threshold voltage, then the flip-flop will change state, causing transistor 18 to become conductive so that resistances 22, 23 will have a voltage appear thereacross. A series circuit including an indicator, typically an indicator lamp 26, and a collector-emitter path of a further transistor is connected in parallel to resistors 22, 23. The base voltage of transistor 27 is held at the constant level by Zener diode 28. The voltage between lines 10 and 11 will thus be essentially independent of the current in lines 10 and 11.

FIG. 4 illustrates another embodiment of an ionization-type fire sensor. Two

ionization chambers

30, 31 are serially connected across

lines

32, 33. The junction between

chambers

30, 31 is connected to the control gate electrode of an FET'34, the source of which is ,flop. When FET 34 changes stage, transistor 40 becomes conductive, current will tlow through resistor 42, transistor 40, as well as

resistors

38, 39. At a predetermined voltage drop across

resistors

38, 39, transistor 43 becomes conductive. Transistor 43 has its base connected to the junction of the

resistors

38, 39 and its collector to the other terminal of resistor 38, that is, the emitter of transistor 40 and resistors 37. Transistor 44 will become conductive as well. The alarm lamp 45 is connected in the emitter-collector path of transistor 44. The voltage across

resistors

38, 39 is held at a constant level by transistor 43, in that additional current is bypassed over its collector-emitter'path.

Transistor 43 and

resistors

38, 39 form an equivalent circuit for Zener diode 28 in FIG. 3, having a certain inherent loss. It could be replaced, if desired, by a Zener diode with a resistor in parallel. This ionization sensor, like the one of FIG. 3, has the voltage saturation characteristics D illustrated in FIG. 2.

Similar voltage-current characteristics can be obtained with fire sensors having entirely different sensitive elements, that is, sensitive elements which are not ionization chambers. It is only necessary to provide a flip-flop circuit which is so arranged, that when a circuit flips into alarm condition, an equivalent constant voltage source, for example a Zener diode, or a similar circuit, such as a suitably controlled transistor or the like is connected; other constant voltage sources may likewise be provided, known in the electronics-art.

The use of a constant voltage source has the additional advantage that parallel connection with an indicator, such as an incandescent lamp, provides voltae which is voltage independent of the supply voltage, thereby ensuring uniformity in indicating quality, independently of line resistances, or line lengths.

FIG. 5 illustrates the circuit of the switching central 1 (FIG. 1). Lines L L are connected to

terminal

62, 63. A voltage divider formed of

resistors

55, 56 is connected across

terminals

62, 63, the tap point thereof being connected to the base of a transistor 53. The emitter of transistor 53'is connected over Zener diode 57 to a negative supply line, which, simultaneously, is connected to terminal 63. Under normal, quiescent, condition, that is, when the line is in order and no fire has been sensed, the voltage U across

terminals

62, 63 is roughly that of the voltage supplied to the

supply terminals

0 and 64, that is, supply voltage U This supply voltage can be derived from power mains, an auxiliary battery, or the like. The Zener voltage of Zener diode 57 is so selected, that the voltage drop across resistor 56 is sufficient in order to hold transistor 53 in conductive state, as long as the voltage U across

terminal

62, 63 is greaterthan the threshold voltage U of the alarm sensors A signal voltage appears. at the collector of transistor 53-over a resistor 58. This signal voltage is applied to an AND-gate 59 and to a trouble signalling device 61. Trouble signalling device 61 provides a signal when transistor 53 blocks, that is, when the voltage across

terminals

62, 63 dropsbelow the threshold voltage of the alarm sensor. The collector-emitter path of nected between the line terminal 62 and the supply terminal 64. The base-emitter path of a furthest transistor 47 is connected across resistor 52. The collector of resistor 47 is connected over a diode 50 with the base of transistor 46.

, Operation: Under ordinary'conditions, transistor 46 is conductive, by being biased into conductive'condition over 52. Upon passage of a predetermined current through resistor 52, transistor 47 resistor 54, connected to the base thereof. If a voltage difference arises between the terminal 62 and 64 which is substantial. for example due to response of a fire sensor, or a short circuit, then the voltage dropsacross resistor 52, transistor 47 becomes conductive, and a portion of the base current of transistor 46 will be shunted, in order to counteract further increase of the current flowing over transistor 46. As a result, a constant current circuit is provided so that between terminals 62,64 a constant current will flow. The

transistors

46, 47 thus act as current limiters, or constant current sources, in the sense ot' the' device 3 in FIG. 1.

The collector of transistor 47 is connected to a further input of the AND-gate 59. When transistor 47 becomes .conductive,- and simultaneously the terminal voltage U is higher than the threshold voltage U of the fire sensor, then both inputs of the AND-gate 59 will be energized, and a signal will be transferred to alarm device 60. The collector emitter path of a further transistor 48 is connected in parallel to the resistor 52. A collector resistor 51 isconnected in the circuit, as shown. The baseof transistor 48 can be controlled by a pulse source'schematically shown at 76. When an alarm signal is commanded, transistor 48 is controlled, in pulses, to become conductive, so that the overall resistance between

terminals

62, 64 changes in pulses. Thus, the current limiting effect also changes, in pulses, sothat a sensor which provides an alarm will have current supplied to its indicator lamp which likewise is a pulsed current, that is, the lamp will flash. This provides clearly visible indication of response, much more visible and apparent than merely uniform illumination, as would be derived from d-c or higher frequency a-c operation; further, the overall power being supplied is decreased. I

-'A fire alarm system having a signal central in accordance with FIG. 5, and tire sensors connected thereto, which have suitable voltage-current characteristics permit clear differentiation of a fire alarm from a short circuit in the line at any desired location of the line. Break of a line cannot be sensed by this system alone, however.

Line trouble is a fire alarm system with very long lines can be reliably sensed by including in the lines a terminating element E (FIG. 1) which provides an interrupted, pulsed loading when a supply voltage on the lines is applied. This permits measurement of the quiescent signal withthe. same current detectoras for an alarm, by additionally evaluating the signal in the central with respect to a-c components, or with respect to pulse components. The alarm sensing itself is delayed or arranged in such a manner that it does not respond to the short pulses of the terminating element.

An'active terminating element is seenin FIG. 6. A series circuit of a resistor 67, a capacitor 68, a four-layer semiconductive element with two control electrodes, and a resistor 70 is connected between

line terminals

65, 66. A resistor 73,and a capacitor 74 and resistor '75 are connected in parallel to the first series circuit.

The junction between resistor 73 and capacitor 74 is connected to one of the control or gate electrodes of the four-layer semi-conductor 69. The junction between capacitor 68 and the four-layer semiconductor 69 is connected to the tap point of a voltage divider formed of resistor 71, 72 and connected across

terminals

65, 66. Due to the alternate charging of

capacitors

68, 74, semiconductor 69 is opened and closed in pulsed cycles. Y

If lines L and L; from terminating element E to the signal sensor 1 are in connected, operative condition, then pulses derived from the terminating element E, connected over

terminals

65, 66, will be connected to terminals 62, 63 (FIG. 5) of the signal central. They are amplifiedby

transistors

46, 47. The pulses are applied over capacitor 78 connected to the collector of transistor 47, capacitor 78 connecting the pulses to a pulse detector 77; Pulse detector 77 is so arranged that a control signal is applied to trouble indicating means 61 if no pulses aresensed by pulse detector 77. Thus, the signal central can determine if supply voltage is applied to the terminating element E, which is an indication that the lines are operative up to the last fire sensors.

The invention was described in connection with ionization fire sensors, but it may be used with other types of tire sensors or alarm sensors, such as flame sensors, smoke detectors, or temperature sensors, or other sensing or transducer devices. FIG. 7 illustrates the equivalent circuit of such a detector. An alarm switch S(A) is connected in series with a resistor R,,, the series circuit of switchand resistor R being connected across the lines L L The alarm switch S(A) corresponds to the ionization chambers with a field effect transistor connected thereto, and the controlled electronic switching stage, as described in the preceding embodiment. Under ordinary conditions, that is, when no fire is being sensed, switch S(A) is open. Thus, under ideal conditions, no current will flow in lines L L In actual practice, however, it has been found that alarm switches also in. open condition have a certain resistance which may be very high, however. This inherent resistance is indicated in FIG. 7 by the parallel resistance R Under alarm conditions, switch S(A) closes, shunting resistance R so that an alarm current can flow between lines L, and l through resistor R,,.

In accordance with the invention, a further parallel circuit is connected to this alarm resistance R,,. The parallel circuit includes a current sensitive switch S( I in series with a further resistor R This current sensitive switch S(I) closes automatically when the alarm current through alarm resistance R, exceeds a predetermined current value. This further decreases the resistance between lines L and L by including a further parallel circuit comprising resistor R with resistor R This is seen in the voltage-current diagram of the alarm detector by the knee in the voltage-current characteristic, which can be evaluated as described in connection with FIG. 2. Resistances R R and RN need not be linear resistors. Rather,'better relationships are usually obtained, that is, better characteristics, when the resistance elements have certain non-linear characteristics. For example, the parallel resistor R may include the parallel circuit of a Zener diode, and an indicator lamp. This form provides a very flat characteristic.

The alarm system and the Sensor thus provides an indication not only of a fire, as sensed by a sensor, but is capable of clearly distinguishing, and indicating additionally short circuits in the line, or an open circuit. It is possible, therefore, to obtain a signal not only by a fire, but also by line trouble, whether short circuit or a break in the line. The terminating element is so constructed that a supervisory or monitoring current flows therethrough. In order to prevent further limiting of the system parameters by distinguishing between yet another current level, the terminating element is so constructed that it introduces an a-c component in the lines, for example by providing a pulsed load thereon. A quiescent d-c current to supervise the sensing lines is thus not necessary. This permits using alarm current from the sensors which ismuch less than in known systems in which a quiescent current must be sensed, since current levels need not be distinguished, which is difficult with long lines; The voltage sensing at the terminals is so arranged that a trouble or line fault signal will be sensed when the voltage, at the limited current being supplied to the sensor is less than the voltage which the sensor requires; and provides an alarm signal when the voltage at the'terminals of the central is between the supply voltage and the sensor voltage. This alarm signal can be further made reliable and-unambiguous by combining it with a current detector in the central.

Various changes and modifications may be made within the inventive concept, and the invention is not limited to any specific type of sensor shown, provided it has the characteristics described herein.

We claim:

1. Fire alarm systems having a central sensing station (1; FIG. and at least one remote sensor (F F FIGS. 3, 4,7)

wherein the sensor has a high electrical resistance under normal, monitoring conditions, and a low electrical resistance under fire sensing alarm conditions;

and wherein connectinglines are provided, connecting the central station to the sensors, and the central station supplied electrical power over the lines to the sensors and includes an alarm system to indicate alarm conditions said system comprising means in the central station connected into the lines limiting the current flowing to the sensors to a first predetermined limit value (1,

said sensors have the characteristic that, under alarm conditions, the electrical resistance thereof is a first value which is greater than a predetermined resistance and when current up to said first predetermined limit value (I,,,) flows to the sensor and, the electrical resistance of the sensor is at a second value which is below said predetermined resistance when current above said first predetermined limit (1, flows to the sensor;

means in said signal central (1) measuring and evaluating the voltage across the lines leading to the sensors;

and means in said signal central (1) logically controlled by said voltage measuring means at least temporarily increasing current flow to said lines upon detection of a. a voltage drop across the lines in excess of a predetermined value (Us) and t b. current flow through the lines approximately at said first limit value (1 2; System according to claim 1 wherein the sensor has a non-linear voltage-current characteristic which, when the current exceeds a predetermined level therethrough, exhibits voltage saturation (FIG. 2; D

3. System according to claim 1 wherein the sensor comprises fire detection transducer means;

and switching means connected to and controlled by said transducer means and connecting a circuit of low electrical resistance to the connection lines connecting the sensor to the signal central (1).

4. System according to claim 3 wherein the sensor further comprises means sensing current flow through said circuit;

alarm indicator means in said circuit, said alarm indicator means being connected into the circuit by said switching means. 5. System according to claim 3 wherein the transducer means comprises an ion chamber accessible to ambient atmosphere, the resistance of which increases upon presence of smoke or fire aerosols;

and said switching means comprises a field effect transistor (FET 15, 34) having an input circuit connected to and controlled by said ion chamber.

6. System according to claim 5 further comprising an electronic switch controlled by said fieldeffect transistor, establishing an alarm circuit of low electrical resistance in said sensor, said field effect transistor and said controlled switch comprising said switching means.

7. System according to claim 3 further comprising a transistor connected to sense current flow through said circuit, and indicating means connected in the collector-emitter path of said transistor, to indicate alarm conditions, the transistor being connected to be controlled to become conductive when the current in said circuit of low electrical resistance exeeds a predetermined current value. I

8. System according to claim 7 wherein the indicator comprises a lamp.

9. System according to claim 2 wherein the sensor includes a Zener diode to provide said voltage saturation characteristics, independent of current flow.

10. System according to claim 1 wherein said voltage measuring and evaluating means comprises means responsive to two or more voltage. levels across the lines leading to the sensors.

11, System according to claim 10 wherein the voltage measuring and evaluation means comprises means sensing a voltage level less than a predetermined threshold level;

and said means logically controlled by said voltage measuring means includes the line trouble indicating means and a fire alarm indicating means, the

line trouble indicating means being activated when:

a. the terminal voltage is less than said threshold level and b. the current flowing through said lines is at least at, or above said first limit current value (l,,,);

and the alarm means is activated when:

c. the voltage across the terminals is above said threshold level and d. the current flowing through the lines is approximately at said first limit value (1,

. 1 1 12. System according to claim 1 1 wherein the voltage level of the threshold level is approximately the same voltage as the saturation voltage of the current-voltage characteristic of the sensor.

13. System according to claim 1 further comprising pulse means (76; FIG. 5) connected to said central station, said pulse means being energized upon detection of an alarm condition comprising detection of a voltage drop across the lines in excess of a predetermined value s);

said pulse source being connectible to said lines and,

' in pulsed cycles, increasing the current through said lines to provide pulsed power to the sensor which changed to alarm condition to permit flashing, intermittent illumination of a sensor indicator said lines.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,82 1 734 Dated June 28 1974 '.[nventor(H) Q tto MEIER Q nd Andreas S CRPTDWEILER- It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The first inventor's name should read as follows Otto MEIER Signed and sealed this 12th day of November 1974.

(SEAL) Attest McCOY M. GIBSON JR. Y C. MARSHALL DANN Attesting Officer Commissioner of Patents F ORM PO-IOSO (10-69) USCOMM'DC 60376-P59 9 U. 54 GOVERNMENT PRINTING OFFICE i969 0-386-33L

Claims

1. Fire alarm systems having a central sensing station (1; FIG. 5) and at least one remote sensor (F1, F2; FIGS. 3, 4, 7) wherein the sensor has a high electrical resistance under normal, monitoring conditions, and a low electrical resistance under fire sensing alarm conditions; and wherein connecting lines are provided, connecting the central station to the sensors, and the central station supplied electrical power over the lines to the sensors and includes an alarm system to indicate alarm conditions said system comprising means in the central station connected into the lines limiting the current flowing to the sensors to a first predetermined limit value (Im); said sensors have the characteristic that, under alarm conditions, the electrical resistance thereof is a first value which is greater than a predetermined resistance and when current up to said first predetermined limit value (Im) flows to the sensor and, the electrical resistance of the sensor is at a second value which is below said predetermined resistance when current above said first predetermined limit (Im) flows to the sensor; means in said signal central (1) measuring and evaluating the voltage across the lines leading to the sensors; and means in said signal central (1) logically controlled by said voltage measuring means at least temporarily increasing current flow to said lines upon detection of a. a voltage drop across the lines in excess of a predetermined value (US) and b. current flow through the lines approximately at said first limit value (Im).

2. System according to claim 1 wherein the sensor has a non-linear voltage-current characteristic which, when the current exceeds a predetermined level therethrough, exhibits voltage saturation (FIG. 2; Da).

3. System according to claim 1 wherein the sensor comprises fire detection transducer means; and switching means connected to and controlled by said transducer means and connecting a circuit of low electrical resistance to the connection lines connecting the sensor to the signal central (1).

4. System according to claim 3 wherein the sensor further comprises means sensing current flow through said circuit; alarm indicator means in said circuit, said alarm indicator means being connected into the circuit by said switching means.

5. System according to claim 3 wherein the transducer means comprises an ion chamber accessible to ambient atmosphere, the resistance of which increases upon presence of smoke or fire aerosols; and said switching means comprises a field effect transistor (FET 15, 34) having an input circuit connected to and controlled by said ion chamber.

6. System according to claim 5 further comprising an electronic switch controlled by said field effect transistor, establishing an alarm circuit of low electrical resistance in said sensor, said field effect transistor and said controlled switch comprising said switching means.

7. System according to claim 3 further comprising a transistor connected to sense current flow through said circuit, and indicating means connected in the collector-emitter path of said transistor, to indicate alarm conditions, the transistor being connected to be controlled to become conductive when the current in said circuit of low electrical resistance exeeds a predetermined current value.

8. System according to claim 7 wherein the indicator comprises a lamp.

10. System according to claim 1 wherein said voltage measuring and evaluating means comprises means responsive to two or more voltage levels across the lines leading to the sensors.

11. System according to claim 10 wherein the voltage measuring and evaluation means comprises means sensing a voltage level less than a predetermined threshold level; and said means logically controlled by said voltage measuring means includes the line trouble indicating means and a fire alarm indicating means, the line trouble indicating means being activated when: a. the terminal voltage is less than said threshold level and b. the current flowing through said lines is at least at, or above said first limit current value (Im); and the alarm means is activated when: c. the voltage across the terminals is above said threshold level and d. the current flowing through the lines is approximately at said first limit value (Im).

12. System according to claim 11 wherein the voltage level of the threshold level is approximately the same voltage as the saturation voltage of the current-voltage characteristic of the sensor.

13. System according to claim 1 further comprising pulse means (76; FIG. 5) connected to said central station, said pulse means being energized upon detection of an alarm condition comprising detection of a voltage drop across the lines in excess of a predetermined value (US); said pulse source being connectible to said lines and, in pulsed cycles, increasing the current through said lines to provide pulsed power to the sensor which changed to alarm condition to permit flashing, intermittent illumination of a sensor indicator which is in alarm condition.

14. System according to claim 1 including a terminating element (FIG. 1: E; FIG. 6), said terminating element including variable resistance means cyclically changing between higher and lower resistance values, to provide for flow of an a-c component through said lines; means in said signal central (1) responsive to current flow of said a-c component; and trouble indicator means in said signal central (1) responsive to cessation of said a-c component to indicate an open circuit or break of continuity of said lines.