US2871466A

US2871466A - Automatic fire alarm system

Info

Publication number: US2871466A
Application number: US556710A
Authority: US
Inventors: Theo N Vassil; Francis C Evans
Original assignee: American District Telegraph Co
Current assignee: American District Telegraph Co
Priority date: 1955-12-30
Filing date: 1955-12-30
Publication date: 1959-01-27
Anticipated expiration: 1976-01-27

Description

Jan. 27, 1959 T. N. VASSIL. ETAL AUTOMATIC FIRE ALARM SYSTEM Filed Dec. 30, 1955 CENTRAL STATION TRANSMITTER United States Patent AUTOMATIC FIRE ALARM SYSTEM Theo N. Vassii, Flushing, and Francis C. Evans, Richmond, N. Y., assignors to American District Teiegraph Company, Jersey City, N. J., a corporation of New Jersey Application December 30, 1955,.Serial No. 556,710 19 Claims. (Cl. 340-233) The present invention relates to automatic fire alarm systems and more particularly to systems arranged to give an alarm indication in response to the occurrence of a fire within a protected space.

Resistance bridge fire detection systems have hereto fore been suggested, for example, in Donovan Patent 2,217,797 issued October 15, 1940, and in Bridges Patent 2,236,891 issued April 1, 1941. The present invention is concerned with an improved system of this general type.

A principal object of the invention has been the provision of a novel and improved fire alarm system which operates primarily in response to a rate of rise of ternperature in the protected space but which can include fixed temperature alarm elements.

Another object of the invention has been the provision of a novel and improved resistance bridge fire detecting system in which one or more arms of the bridge may each contain a resistance wire as the fire detecting element.

A further object of the invention has been the provision of a fire detecting system which exhibits high sensitivity and yields freedom from false alarms due to transient heat conditions.

Still another object of the invention has been the provision of a fire alarm system of the rate of rise type which is especially adapted for the detection of slowly burning fires which sometimes go undetected by rate of rise systems.

Another object of the invention has been the provision of a fully supervised resistance bridge fire detection system.

A feature of the invention has been the provision of a resistance bridge fire detection system employing a fire detecting wire as a part of the bridge and in which uniform response to fires is achieved independent of the length of the fire detecting wire circuit or circuits.

Other and further objects, features and advantages of the invention will appear more fully from the following description of the invention.

An automatic fire alarm system, in accordance with the invention, comprises a bridge circuit, a conductor having a temperature coefiicient of resistance and being included in an arm of the bridge, a source of potential coupled to one pair of opposite terminals of the bridge,

an indicating circuit coupled to the other pair of opposite 7 terminals of the bridge, the indicating circuit including, in series connection, a sensing element operative in response to a predetermined current flow in the indicating circuit and a capacitive element for limiting current flow in the indicating circuit to transient currents produced by changes in the condition of balance of the bridge, and a resistance-capacitance circuit coupled in parallel with said sensing element and having a time constant sufiiciently great to provide an appreciable time delay in the operation of the sensing element. Other more fixed temperature detecting elements may be included in the conductor, and their operation will be equivalent to a resistance increase in the conductor. The integrity of ice the system may be supervised constantlyby including relays in the bridge arms.

The invention will now be described in greater detail with reference to the appended drawing, which illustrates, in schematic form, one embodiment of the invention.

Referring now to the drawing, the fire detecting systern, in accordance with the invention, comprises generally a resistance bridge 10 having

arms

11, 12, 13, and 14, a pair of

fire detecting loops

15 and 16. included in arms 12 and 14-, respectively, of the bridge 10, an indicating circuit 17 included in the neutral leg of the bridge 10, ar1d an alarm circuit 18.

The bridge arrn11 is formed, by a resistor 19 and a supervisory relay-coil 20. The diagonally opposite arm 15 of the bridge is formed by a similar resistor 21 and supervisory relay coil 22. The bridge arm 12 is formed by detecting loop ld and a potentiometer 23. The bridge arm 14 is formed by detecting loop 16 and a potentiometer 24. p

The detecting loop 15 is formed bya resistance wire 25 whose resistance changes withchanges in temperature. Generally, the wire 25will experience an increase in resistance proportional to increases in temperature. The wire 25 is suitably disposed about the space to be protected and may be of any suitablelength, from a few feet to 1,000 feet or more. The wire 25 will preferably extend relatively close to any areain which a fire might be anticipated. It has generally been found satisfactory to extend the wire 25 along the ceiling of the protected premises. As will be explained hereinafter, satisfactory system operation requires compensating features so that under some circumstances a very slow burning fire may go undetected in so far as the wire 25 is concerned. In order to guard against this possibility, various fixed temperature elements may be introduced in series in the wire 25. Three of these elements, 26, 27 and 28, are shown interposed in the conductor 25,, although it will be understood that as many of such elements as are desired may be employed. The element 26, for example, comprises a resistor 2) in series with the wire 25, and shunted by a pair of normally closed thermostatic contacts 30 whichare arranged to open at a fixed temperature. Opening of the contacts so will act to insert the resistance 29 in the loop 15 and will be comparable in action to a considerable increase in resistance of the wire 25 produced by a rapid rise in temperature. The potentiometer 23, which is included in the arm 12, is provided in order to permit easy balancing of the bridge. 7

The loop 16, which is included in the arm 24, may be identical with the loop 15 and comprises a detecting wire 31, which has the same characteristics as the wire 25. Fixed temperature elements 32, 33 and 3,4,fwhich correspond to the fixed temperature elements included in the loop 15, may be provided. It will be understood that the number of fixed temperature elements, if any, provided in the loops will depend upon the characteristics of the premises to be protected. The potentiometer 24, like the potentiometer 23, is provided for balancing purposes. These potentiometers may be adjusted to balance the bridge, thereby compensating for differences in length and ambient temperature of the fire detecting loops and insuring a uniform response to fire independent of the length of the detecting loops. Presence "of the

potentiometers

23 and 24 also guards against development of unusually high voltages across the neutral leg of the bridge.

The neutral leg 17 extends from the junction point of arms 11 and 12 to the junction point of

arms

13 and 14, and comprises a capacitor 35, a sensitive (preferably galvanorneter type) relay 36, apotentiometer 37,

all connected in series, and a resistor 38 and capacitor 39 connected in series with each other and in parallel with the relay 36 and the potentiometer 37. The potentiometer 37 is provided to supplement the internal resistance of the relay 36.

The operating potential for the bridge, e. g., 12 volts D. C., is applied across the opposite leg of the-bridge, namely, to the junction of arms 11 and 14 and the junctron of

arms

12 and 13. As illustrated, the D. C. potential is derived from a positive terminal 40 and a negative terminal 41.

The alarm circuit 18 is coupled between the termi- 36A of relay 36 and the coil will be evident that relay 42 relay 36 is energized. Since relay 36 is preferably of the very sensitive galvanometer type, it is desirable that its contacts operate a separate alarm relay rather than diof an alarm relay 42. It will be energized whenever rectly control an alarm indicating circuit. The relay 36 is preferably provided with a magneticlocking arrangement so that it must be manually reset to release the circuit after the fire alarm has been given.

As in the case of most automatic fire alarm systems, the system of the invention achieves its greatest utility 1n c0nnection with a central station connection. For thls purpose, there is illustrated in block form a transmitter 43 shown as being connected over a pair of wires to a central station. The transmitter, which may be of any suitable type, may be caused to give a fire alarm indication upon the closing of normally open contacts 42A of relay 42. In order to provide constant supervision over the integrity of the system, the transmitter 43 may be arranged to give a trouble indication u on the opening of any one of normally closed contacts 20A of relay 20, normally closed contacts 22A of relay 22, i

and normally closed contacts 44 of a tamper switch which may be provided to guard against unauthorized opening of the cabinet containing the control portions of the system, i. e., all the parts except the detecting

loops

15 and 16.

As in most Wheatstone bridge circuits, when the bridge 1 0 is balanced by appropriate adjustment of the

potentiometers

23 and 24, no current will flow through the neutral leg 17. When a fire occurs, the exposed fire detecting wire (either wire 25 or wire 31) will be heated and Will exhibit a rapid increase in resistance causing an unbalance in the arms of the resistance bridge. This unbalance creates a potential difierence across the neutral leg sufficient to cause the capacitor 35-to charge. During this charging action, a current will flow through the galvanometer relay 36, causing its contacts 36A to close, thereby energizing the alarm relay 42, whose contacts 42A close and initiate an alarm signal to the central station or, if desired, to a local alarm bell or other suitable device. Presence of the RC circuit composed of resistor 38 and capacitor 39, galvanometer relay coil 36, introduces a time delay into the energization of the relay 36 dependent upon the time constant of the RC circuit. This time delay, which might be, for example, a few seconds, is a desirable feature, since it guards against transient or relatively rapid increases in temperature not resulting from a fire and which would, if allowed to energize the relay 36, produce a false alarm.

Slow changes in temperature in the protected premises will tend to cause an unbalance in the system which will result in the flow of a small charging current for the capacitor 35. However, this charging current will not be suflicient to operate the galvanometer relay 36, so that false alarms will not be given with changes in ambient temperature which might result, for example, from turning on a heating or air conditioning system. Any unbalance of the bridge which is more or less permanent will not give an alarm since only changes in potential across the neutral leg of the bridge cause current flow nals 40 and 41 and includes normally open contacts which are shunted across 4 through the galvanometer relay, because of the pres ence of capacitor 35.

The fixed temperature elements which are connected in series with the main fire detecting wires are constructed so as to insert their full resistance value in the fire detecting wire when a certain predetermined temperature is reached. The construction is preferably that of a resistor shunted by a fusible alloy arrangement which melts to remove the shunt when the predetermined temperature is reached or a bimetallic element which opens its contacts when the predetermined temperature is reached. Removal of one of these shunts causes a rapid increase in the resistance of the corresponding bridge arm, which results in an unbalanced condition equivalent to that created by the rapid heating of a portion of the wire by a fire. The rate of change of resistance will be sufiicient so that the charging current of the capacitor 35 will be adequate to operate the galvanometer relay 36. Operation of the relay 36 will nevertheless be delayed by the requirement that the capacitor 39 be charged before the relay 36 can be eneregized.

The capacitor 39, which discharges through the galvanometer relay, aids in detecting smoldering and slow burning fires since the discharge current of capacitor 39 will add to the charging current of capacitor 35.

The system including the fire detecting wire and fixed temperature elements is continually supervised so that a break in any of the bridge arms, including the tire detecting loops, a power failure ora reduction in operating voltage below a certain level, causes a trouble signal to be transmitted. 1

A break in either detecting loop 15 or detecting loop 16, or at any place else in the bridge arms, will cause either relay 20 or relay 22 to be de-energized,.thereby opening

contacts

20A or 22A, as the case may be, and resulting in the transmission of a trouble signal to the central station. Such a circuit break will also create a current surge through the galvanometer relay 36 ,so that contacts 36A will close and the alarm relay 42 will be energized, closing contacts 42A thereof. However, in order to prevent the transmission of an alarm signal, normally closed contacts 20B and 22B are connected in series with contacts 42A. De-energization of either relay 20 or relay 22 will open the corresponding contacts 203 or 2213 and prevent transmission of an alarm signal.

A relay 45 may be connected across the power supply and have a pair of contacts 45A included in series with the

contacts

20A, 22A and 44. The relay 45 may be constructed so as to drop out when the supply voltage decreases to some predetermined amount, e. g., 10 volts. De-energization of relay 45 will then cause contacts 45A to open, transmitting a trouble signal. This is a desirable safety feature since the sensitivity of the system is directly proportional to the supply voltage,an d when this voltage decreases appreciably, system sensitivity is correspondingly decreased. Although a trouble signal will be transmitted when relay 45 drops out, the system is still capable of transmitting an alarm if a fire occurs (provided the supply voltage has not decreased too far).

Since the system is intended to operate under a variety of conditions and with considerable lengths of detecting wire loops, it is desirable that the bridge circuit be ungrounded so that a permanent or temporary ground applied to the fire detecting wire will not affect system operation.

While two fire detecting loops are shown, it will be evident that the system can be balanced and operate in the same manner it only one loop is provided. In such case, the bridge arm corresponding to the omitted loop will be adjusted to a resistance value appropriate for balancing the bridge. Similarly, three or four loops may be provided.

While the invention has been described in a particular embodiment thereof and in a particular use, various modifications thereof will occur to those skilled in the art without departing fromthe spirit and scope of the invntion, as set forth in the appended claims.

What is claimed is:

1. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about a space to be protected and'having a temperature coefiicient of resistance and being included in an arm of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of oppositetermin'als of said bridge, said indicating circuit including, in series connection, a capacitive element and a sensing element operative in response to a predetermined current flow in said indicating circuit, and a series resistance-capacitance circuit connected in parallel with said sensing element, said 'resistance-capaci tance circuit having a time constant sufiiciently great to provide an appreciable delay in the operation of said sensing element.

2. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about a space to be protected and having a temperature coefiicient of resistance and being included in an arm of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals or" said bridge, said in- (heating circuit including, in series connection, a capacitor and a sensitive relay operative in response to a predetermined current flow in said indicating circuit, a series resistance-capacitance circuit connected in parallel with said relay, said resistance-capacitance circuit having a time constant sufficiently great to provide an appreciable delay in the operation of said relay, an alarm giving element, and a pair of contacts operated by said relay for causing said alarm giving element to produce an alarm indication upon operation of said relay.

3. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about a space to be protected and having a temperature coefficient of resistance and being included in an arm of said bridge, a source of potential coupled to one pair of opposite terrninals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge, said. indicating circuit including, in series connection, a first capacitor and a galvanometer type relay arranged to be energized in response to a predetermined current flow in said indicating circuit, and a series circuit connected in parallel with said relay, said series circuit comprising a resistor and a second capacitor and having a time constant sufficiently great to provide an appreciable delay in the operation of said galvanometer type relay.

4. An automatic fire alarm system, comprising a bridge circuit, two conductors each adapted to be disposed about a space to be protected, each having a temperature coefiicient of resistance, and each being included in a respective arm of said bridge, a source of potential coupled I to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge, said indicating circuit including, in series connection, a capacitor and a sensing element operative in response to a predetermined current flow in said indicating circuit, and a series resistance-capacitance circuit connected in. parallel with said sensing element, said resistance-capacitance circuit having a time constant sufficiently great to provide an appreciable delay in the operation of said sensing element.

5. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about a space to be protected and having a temperature coefficient of resistance and being included in an arm of said bridge, an adjustable resistance element connected in series with said conductor and being included in said arm of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge, said indicating circuit including, in series connection, a

capacitor and a sensitive relay operative in response-to a predetermined current flow in said indicating'circuit, a series resistance-capacitance circuit connected in-parallel with said sensitive relay, said resistance-capacitance circuit having a time constant sufliciently great to provide an appreciable delay in the operation of said sensitive relay, an alarm signal transmitter, an alarm relay arranged, when energized, to cause said transmitter to transmit an alarm signal, and a set of contacts controlled by said sensitive relay and arranged to energize said alarm relay when said sensitive relay is operated.

6. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about a space to be protected and having a temperature coefiicient of resistance and being included in an arm of said bridge, a relay included in an arm of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge, said indicating circuit including, in series connection, a capacitor and a sensing element operative in response to a predetermined current flow in said indicating circuit, a series resistance-capacitance circuit connected in parallel with said sensing element, said resistance-capacitance circuit having a time constant suificiently great to provide an appreciable delay in the operation of said sensing element, transmitting means adapted to transmit an alarm signal and a trouble signal, first circuit means operated in response to operation of said sensing element for causing said transmitting means to transmit an alarm signal, and second circuit means including a set of contacts operated by said relay for causing said transmitting means to transmit a trouble signal when said relay is de-energized.

7. An automatic fire alarm system, comprising abridge circuit, two conductors each being adapted to be disposed about a space to be protected, each having a temperature coefficient of resistance and each being included in a respective diagonally opposite arm oi said bridge, a pair of supervisory relays each included in a respective one of the other diagonally opposite arms of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge and including, in series connection, a first capacitor and a sensitive relay arranged to be energized in response to a predetermined current flow in said indicating circuit, a circuit connected in parallel with said sensitive relay and including, in series connection, a second capacitor and a resistance element having a time constant sufiiciently great to provide an appreciable delay in the operation of said sensitive relay, transmitting means adapted to transmit an alarm signal and a trouble signal, first circuit means operated in response to energization of said sensitive relay and arranged, when operated, to cause said transmitting means to transmit an alarm signal, second circuit means operated upon de-energization of either of said supervisory relays and arranged, when operated, to cause said transmitting means to transmit a trouble signal, and means to prevent transmission of an alarm signal upon de-energization of either of said supervisory relays.

8. An automatic fire alarm system, comprising a bridge circuit, two conductors each being adapted to be disposed about a space to be protected, each having a temperature coefficient of resistance, and each being included in a respective diagonally opposite arm of said bridge, two adjustable resistance elements each connected in series with a respective one of said conductors and each being included in the corresponding arm of said bridge, a pair of supervisory relays each included in a respective one of the other diagonally opposite arms of said bridge, a source of potential coupled to one pair of opposite terminals of said bridge, a voltage sensitive relay coupled to said source of potential and arranged to be a resistance element having a time constant sufficiently great to provide an appreciable delay in the operation of said sensitive relay, transmitting means adapted to transmit an alarm signal and a trouble signal, first circuit means operated in response to energization of said sensitive relay and arranged, when operated to cause said 4 transmitting means to transmit an alarm signal, second circuit means operated upon de-energization of either of said supervisory relays or said voltage sensitive relay and arranged, When operated, to cause said transmitting means to transmit a trouble signal, and means to prevent transmission of an alarm signal upon de-energization of either of said supervisory relays.

9. An automatic fire alarm system, comprising a bridge circuit, a conductor adapted to be disposed about the space to be protected and having a temperature coefiicient of resistance and being included in an arm of said bridge, a fixed temperature detecting device interposed in said conductor and arranged to insert a resistance value in said conductor when subjected to a predetermined temperature, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of opposite terminals of said bridge, said indicating circuit including, in series connection, a capacitor and a sensing element operative in response to a predetermined current flow in said indicating circuit, and

a series resistance-capacitance circuit connected in parallel with said sensing element, said resistance-capacitance circuit having a time constant sutficiently great to provide an appreciable delay in the operation of said sensing element.

10. An automatic fire alarm system, comprising .a bridge circuit, two conductors each being adapted to be disposed about a space to be protected, each having a temperature 8 coefficient of resistance and each being included in a respective diagonally opposite arm of said bridge, at least one fixed temperature detecting device interposed in at least one of said conductors and being arranged to introduce a resistance value therein when subjected to a predetermined temperature, two adjustable resistance elements each connected in series with a respective one of said conductors and each being included in the corresponding arm of said bridge, a pair of supervisory relays each included in a respective one of the other diagonally opposite arms ofsaid bridge, a source of potential coupled to one pair of opposite terminals of said bridge, an indicating circuit coupled to the other pair of OPPOSite terminals of said bridge and including, in series connection, a first capacitor and a sensitive relay arranged to be energized in response to a predetermined current flow in said indicating circuit, a circuit connected in parallel with said sensitive relay and including, in series connection, a second capacitor and a resistance element having a time constant sufficiently great to provide an appreciable delay in the operation of said sensitive relay, transmit ting means adapted to transmit an alarm signal and a trouble signal, first circuit means operated in response to energization of said sensitive relay and arranged, when operated, to cause said transmitting means to transmit an alarm signal, second circuit means operated upon deenergization of either of said supervisory relays and arranged when operated to cause said transmitting means to transmit a trouble signal, and means to prevent transmission of an alarm signal upon de-energization of either of said supervisory relays.

References Cited in the file of this patent UNITED STATES PATENTS 1,951,035 Parker Mar. 13, 1934 2,478,000 Miller Apr. 2, 1949 2,688,740 Merrill Sept. 7, 1954 2,696,603 Ekman Dec. 7, 1954 2,728,904 Schafer Dec. 27, 1955 2,737,642 Schafer Mar. 6, 1956