United States Patent 3811126

An alarm system includes two separate electrical loop circuits and means, such as diodes, controlling the supply of electrical power to the circuits so that normally one of the circuits is energized during one part of a given time period, e.g. the positive half cycle of an alternating current source common to the two circuits, and the other of the circuits is energized during the other part of such time period, e.g. the negative half cycle of such alternating current source. Alarm-condition sensors and associated normally-open switch means provide interconnection between the two circuits in case of an alarm condition so that current flows in one of the circuits from the other during the time such one circuit is unenergized. Such one circuit has a by-pass circuit portion containing alarm means and also has means for blocking normal current flow but not current flow from the other circuit, whereby the latter current flow activates the alarm means. Means may be provided in each circuit to indicate whether or not one of the other of the circuits becomes opened or shorted. A third circuit may be provided as a ground check.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
340/16.1, 340/513, 340/590, 340/593
International Classes:
G08B17/06; G08B29/06; (IPC1-7): G08B17/00; G08B29/00
Field of Search:
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US Patent References:
3618069MOTORIZED TRANSMITTER1971-11-02Evans
3603949FIRE ALARM INSTALLATION1971-09-07Walthard
3587095SUPERVISED ANNUNCIATOR1971-06-22Earling
3521276SUPERVISED ALARM CIRCUIT1970-07-21Raber
3401384Monitoring apparatus and method for electrical protection systems1968-09-10Highstone
3103652Supervisory system1963-09-10Thorshecm
3010100Direct current operated burglar alarm system with alternating current supervision1961-11-21Muehter
2944251Fire alarm system1960-07-05Tetherow
2569338Supervisory and control circuit for coded fire alarm systems1951-09-25Richmond
1537211Signaling system1925-05-12Wootton

Primary Examiner:
Habecker, Thomas B.
Attorney, Agent or Firm:
Mallinckrodt & Mallinckrodt
1. An alarm system, including two separate electrical loop circuits; means for imposing current on said circuits alternately; means for interconnecting said circuits in response to an alarm condition; alarm means comprising an alarm current sensing means in one of said circuits; means for coupling said alarm sensing means into its circuit, said coupling means blocking normal current flow in that circuit and passing

2. An alarm system according to claim 1, wherein the means for interconnecting said circuits in response to an alarm condition comprises at least one temperature-rise sensor and normally open switch means arranged to be closed by said sensor upon the sensing of an alarm

3. An alarm system according to claim 1, wherein the means for blocking normal current flow in the alarm circuit and for passing alarm current

4. An alarm system according to claim 1, wherein the means for imposing current on said circuits alternately comprises a source of AC power, and power-blocking means operative to supply power to one circuit during the positive half cycle of power and to the other circuit during the negative

5. An alarm system according to claim 4, wherein the power-blocking means comprises at least one rectifying means in each of the two circuits, the rectifying means in one circuit being of opposite orientation to the

6. An alarm system according to claim 1, additionally including indicating means in both circuits, said indicating means being responsive to normal

7. An alarm system according to claim 6, wherein each indicating means

8. An alarm system according to claim 1, additionally including a ground circuit; means interconnecting said ground circuit with both of the other circuits; and means for indicating current flow between said other

9. An alarm system according to claim 8, wherein both the means interconnecting the ground circuit with the other circuits and the means for indicating current flow between said other circuits and said third

10. An alarm system according to claim 1, wherein the alarm means includes means in the alarm device circuit for controlling the operation of

11. An alarm system according to claim 1, wherein the alarm means includes means in the alarm device circuit for de-activating the means for

12. An alarm system according to claim 1, wherein the means for interconnecting the two circuits in response to an alarm condition are wires provided with insulation which melts at a predetermined temperature, said wires being arranged to come into electrical contact with each other and to establish a short in the system upon melting of said insulation.

13. An alarm system, including two independent loop circuits having respective loop-integrity monitors; sensing means operative to interconnect portions of said circuits in response to the occurrence of an abnormal condition; abnormal-condition-evidencing means in at least one of said circuits and responsive to the interconnection of said portions of said loop circuits; an alternating-current power source; and first and second unidirectionally conducting means connecting said source to said two loop circuits for energizing the latter, said first and second means being polarized to effect energization of said loop circuits during alternate half-cycles of said power source, said abnormal-condition-evidencing means being connected in one of said loop circuits and having polarizing means causing operation thereof upon occurrence of an abnormal condition selectively during the half-cycles of condition of said first unidirectionally conducting means and not during the half-cycles of conduction of said second unidirectionally conducting means.


1. Field:

This invention is in the field of alarm systems for detecting various alarm conditions and for warning when the alarm system itself is not functioning properly.

2. State of the Art:

Many alarm systems are presently available for detecting a variety of alarm conditions and for detecting malfunctions within the alarm system themselves. However, these systems are generally very complicated and expensive and require specially trained personnel for servicing and upkeep. Some systems also include redundant circuits to lessen the chance of malfunction. This adds considerably to the expense.


In the making of this invention it was a primary objective to provide an alarm system, without redundant circuitry, which would be reliable, relatively inexpensive, simple in construction and maintenance, and relatively fail-safe, so as to be particularly useful as an alarm system in coal mines where strict government regulations now set high standards of reliability and where very long conveyor systems extending through underground haulageways require extensive alarm installations if the required protection is to be provided.


In accordance with the invention, an alarm system has two separate electrical loop circuits and means for energizing such circuits alternately. Alarm-condition-sensing means with associated, normally open, switch means are provided to interconnect the circuits when an alarm condition occurs, such interconnection allowing current from the energized circuit to flow in the non-energized circuit. An alarm and associated current-flow-control means are provided in at least one of the circuits, so current flowing from the other circuit into such one circuit will actuate the alarm but normal current flow in that one circuit will not. Indicating means are usually provided in each circuit to show that current either is or is not flowing normally, thereby warning of any system malfunction that interferes with normal current flow.

It is preferred that the current supplied to the two electrical circuits be alternating, with the positive half-cycle going to one of the circuits and the negative half-cycle going to the other.

The system also may be provided with a ground loop for the alarm-condition-sensing means, such ground loop having indicating means operative if any ground short occurs within the system. Such ground loop is preferably separate from the other two circuits, as a third circuit, and may be provided with additional indicating means for showing whether or not the ground circuit is itself broken.


A specific embodiment presently contemplated as the best mode of carrying out the invention is shown in the accompanying drawings in which:

FIG. 1 is a simplified block diagram indicative of the inventive concepts;

FIG. 2, an abbreviated wiring diagram of a specific system corresponding to FIG. 1, with arrows showing the flow of current when an alarm condition is sensed; and

FIG. 3, a detailed wiring diagram of the system of FIG. 2 shown in detail and including a ground loop, the system being designed for use in a coal mine in connection with conveyor systems extending over long distances through underground haulageways.


As indicated in FIG. 1, the alarm system of the invention comprises two separate electrical circuits of closed loop character designated A and B, respectively. The two circuits are adapted to be electrically interconnected by a normally open switch associated with an alarm-condition sensor that acts to close the switch upon the occurrence of an alarm condition. Any number of switches and associated sensors can be utilized, depending upon the nature and extent of the area to be protected.

Under normal conditions, circuit-energizing power that varies periodically, e.g. positive and negative half cycles of AC, flows through each circuit alternately.

When an alarm-condition, such as excessive heat, is sensed by the alarm-condition sensor and the switch thereof closes and interconnects the two circuits A and B, current flows from circuit A into circuit B. That circuit is provided with an alarm device, usually a relay connected so as to activate an audio alarm such as a bell, and with associated current flow control means, such as diodes, for blocking normal current flow but for passing the opposite current flow from circuit A. Thus, when an alarm-condition is sensed, an alarm is given. The system constantly stands ready to give an alarm.

In the specific instance illustrated from an abbreviated standpoint in FIG. 2 and in full detail in FIG. 3, commercially available units 10 known as "temperature rise sensors" are utilized at intervals along the length of a coal conveyor system in an underground haulageway of a coal mine to sense temperature rise beyond a predetermined danger point and to actuate the alarm system accordingly. As shown, there are three of the units 10, but only one or many more could be employed depending upon the need.

Each of the sensor units 10 includes a normally open switch 10a, with one switch terminal in circuit A and the other in circuit B. The switch is normally maintained open in conventional manner by suitable heat-releasable means (not shown), and an electrically conductive casing, indicated at 10b, FIG. 3, is electrically connected to a ground check circuit C.

Diodes D1 and D2 and a relay 11 are provided in circuit A, and diodes D3 and D4 and a relay 12 in circuit B.

The alarm system may be supplied with 110 volt, 60 cycle, AC power from the usual power line source 13, FIG. 2, but is preferably designed to operate from a twelve volt DC storage-type battery in case of AC power line failure.

In the power supply illustrated in FIG. 3, uninterrupted AC power is provided at all times. The 110 volt AC line power is reduced to 13.9 volt DC power by a standard AC to DC converter 14. Diodes D5, D6, and D7 are provided to further reduce the DC voltage before it is applied to the terminals of storage battery 15. More or fewer diodes could be provided to adjust the voltage applied to the storage battery to any desired level. The terminals of storage battery 15 are also connected to a standard 12 volt DC to 110 volt AC inverter 16. The rectified reduced voltage power supplied to battery 15 from the line recharges the battery if it is in need of recharge and also supplies the power to operate inverter 16. If the AC line should fail, the battery maintains the power to the inverter. Thus, no interruption is caused in inverter output. The 110 volt AC output of the inverter is supplied directly to circuits A and B through electrical lines 17 and 18 and the normally closed relay contacts 19a and 19b of relay 19.

Diodes D1 and D2 are arranged to allow current to flow in circuit A when the voltage at 19a is positive with respect to 19b, i.e., during the positive half-cycle of the AC power, while diodes D3 and D4 are arranged to block current flow into and through circuit B during this time. When the voltage at 19a becomes negative with respect to 19b, i.e., during the negative half-cycle of the AC power, diodes D1 and D2 in circuit A block current flow while diodes D3 and D4 in circuit B allow current to flow into and through that circuit. Thus, circuit A will be energized during each positive half-cycle and circuit B during each negative half-cycle, resulting in each circuit being alternately energized 60 times each second.

Relay 11 in circuit A constitutes the control portion of indicator means that indicates proper current flow within the circuit. When current flows in circuit A, it energizes the coil of relay 11. Since a relay coil acts as an inductor, it will oppose any change in current flow therethrough. Thus, as the current stops flowing in circuit A, the coil of relay 11 produces a voltage pulse of similar polarity, which will be passed by a diode D8 in shunt circuit 20. This voltage pulse will maintain the relay in energized position for a fraction of a second after circuit A is de-energized. The time that relay 11 remains energized is longer than 1/120th of a second in this case, and, since the circuit is de-energized for only half a cycle on each cycle, i.e., for 1/120th of a second, the circuit is re-energized before the relay is de-energized. Relay 11 thus remains in energized position as long as circuit A is functioning properly, with the normally closed relay contacts 11a positively held open. Such contacts are connected in circuit with a signal light 21, so as to keep such light turned off when relay 11 is energized and circuit A is operating normally. When relay 11 becomes de-energized, by reason of a lack of current flow in circuit A, contacts 11a assume their normally closed positions, and signal light 21 is turned on, thereby indicating that there is a malfunction in circuit A. It should be realized that various other arrangements are possible to provide the desired indication of circuit malfunction.

Relay 12 in circuit B, normally closed relay contacts 12a, diode D10 in shunt circuit 22, and signal light 24 operate in similar manner to warn of a malfunction causing lack of current flow in circuit B.

In case of fire or other rise in temperature beyond the preset maximum, one or more of the temperature-rise sensors 10 will have its switch 10a closed, thereby interconnecting circuits A and B. During the positive half cycle, when circuit A is energized, current will flow from circuit A through the closed switch or switches 10a into circuit B, through diode D11, relay 25, and diode D12. Relay 25, which under normal conditions is prevented from being energized by blocking diodes D11 and D12, is energized in response to the current from circuit A flowing through circuit B. Relay 25 and a diode D13 in shunt circuit 26 now act similarly to relays 11 and 12 and their associate diodes D8 and D10, respectively, and remain energized, even though energizing current from circuit A occurs only during each positive half cycle. In the present case, normally-open relay contacts 25a close and cause energization of permanent-magnet-type self-locking relay 19, which, in turn, causes normally-closed relay contacts 19a and 19b to open, shutting off power to both circuits A and B and causing normally-open relay contacts 19c to close, thereby activating a bell 27 and flashing red lights 28. The energizing current for relay 19 comes directly from the AC output of the inverter 16 and is rectified by diode D14. This energizing current is only momentary, because of the opening of relay contacts 19e. Once energized, however, the relay remains in energized position because of a small permanent magnet within the relay. The self-locking of relay 19 is required because power is disconnected from circuits A and B as previously indicated, causing alarm relay 25 to become de-energized. Because it is desired to continue operation of the alarm devices and not to restore power to the circuits A and B until the proper personnel are alerted and the alarm condition corrected, the alarm devices are turned off and power restored to such circuits only by momentarily closing reset switch S1, thereby causing current to flow through the relay contacts 19d, resistor R1, diode D15, and the coil of relay 19. Diode D15 is arranged electrically opposite to diode D14, so that the rectified reset current passing through relay 19 is of opposite polarity to the energizing current. The magnetic field thus created opposes that of the permanent magnet and causes the relay to return to its de-energized position. Resistor R1 limits current flow. If the alarm condition has not been corrected in the meantime, relay 19 will immediately become re-energized due to re-energization of alarm relay 25. Relay 25 and relay 19 may be also utilized to shut off power to operating equipment, turn on fire sprinkler systems, etc.

The same results are obtained if, for some reason, the two circuits A and B become shorted. Thus, an effective fire sensing system can be provided if, instead of the temperature rise sensors 10, wires having insulation designed to melt at a predetermined temperature and arranged so that upon melting of the insulation the wires come into electrical contact and cause an electrical short circuit, are utilized.

Relay 25 and its associated diodes could be placed in circuit A rather than circuit B, so as to be activated by the current from circuit B flowing through circuit A upon interconnection of the two circuits. Also, a similar relay and its associated diodes could be included in each of the circuits A and B, so that both would be energized upon interconnection of the two circuits.

A third or ground check circuit C, FIG. 3, is advantageously provided to indicate any grounding of the circuits A and B. In the form illustrated, circuit C comprises diodes D16 and D17 and relays 30 and 31. Connections with ground are made between diodes D16 and D17 for all points of ground in the temperature rise sensors 10.

If circuit A should become grounded when energized during the positive half cycle, current would flow into ground circuit C and through diode D17 to energize relay 31, which, as associated with diode D18, would remain energized in the manner previously explained. Normally-open relay contacts 31a would close, causing light 32 to go on as a visual indication of the grounded condition of circuit A. If circuit B should become grounded when energized during the negative half cycle, current would flow into ground circuit C and through diode D16 to energize relay 30, which, as associated with diode D19, would also remain energized as previously explained. Normally-open relay contacts 30a would close, causing light 33 to go on as a visual indication of the grounded condition of circuit B.

In order to continuously check ground circuit C for continuity and to provide a warning if it is broken for any reason, an interconnection between each of the two circuits A and B and the third circuit C is made by means of neon lamps 34 and 35. During the positive half cycle, current flows from circuit A through neon lamp 34 into circuit C and through diode D17 and relay 31, thus lighting such lamp 34. The current flowing through such neon lamp 34 is smaller than the current necessary to energize relay 31, so that, even though the current flows through relay 31, it does not activate it. The voltage pulse produced by the relay during the negative half cycle not only maintains the aforementioned small current flow through the relay, in conjunction with diode D18 as previously explained, but also maintains just enough current flow through neon lamp 35 to keep it lighted. The path from ground circuit C through neon lamp 35, diode D11, relay 25, and diode D12 is not followed by the current, because the voltage drop necessary to initiate current flow along this path is large compared to that necessary to initiate flow along the path through diode D17 and relay 31, which is the path taken by the current.

If a break occurs in ground circuit C, current can no longer flow and neon lamp 34 will go out as a visual indication of that condition. In order to keep operative that portion of the alarm system which indicates electrical grounds within the system, even though a break in ground circuit C has occurred, a manual switch S2 provided for that purpose is closed. This will cause neon lamp 34 to again light and circuit C will function as before described, to indicate electrical grounds. The cause of the ground-circuit-break indication should be located and corrected and switch S2 then opened.

During the negative half cycle, current flows from circuit B through neon lamp 35, diode D16, and relay 30 in a manner similar to that described for the positive half-cycle and will cause such lamp 35 to light. Similarly, if a break occurs in ground circuit C, lamp 35 will go out.

A relay 36 is desirably provided to indicate whether the system is operating on line voltage or from battery 15. A diode D20 rectifies the line AC power applied to relay 36, and such relay, as associated with a diode D21, will remain energized in the manner previously described so long as there is line AC power. Normally-open relay contacts 36a are closed and a light 37 is energized to indicate that line voltage is present. If line voltage is interrupted for any reason, relay 36 is de-energized and normally-closed contacts 36b are closed, thereby energizing a light 38, which indicates that the system is operating from battery 15.

While a fire alarm system has been described, devices adapted to sense other dangerous conditions can be substituted for the temperature rise sensors 10 to provide other alarm systems, such as a burgler alarm. Also, other circuit arrangements can be employed to detect the current originating in one circuit and flowing in the other circuit upon interconnection of the two circuits in response to an alarm condition. Additionally, if desired, the system may be arranged for constant monitoring of both circuits by the provision of means, e.g. a DC power supply connected to each circuit, for supplying electrical power continuously to both circuits as background power for energizing the indicating means, i.e., loop-integrity monitors, in the respective circuits. If so arranged, suitable isolating means, such as capacitors and inductors appropriately interposed between such DC power supply and the AC power supply and in the means adapted to interconnect the two circuits, should be provided. Moreover, the normal energization of the two circuits alternately, can be replaced by pulse energization of the one circuit not containing the alarm means.

Whereas this invention is here illustrated with respect to a preferred specific embodiment thereof, it should be realized that various changes may be made therein and other specific forms may be constructed by those skilled in the art without departing from the invention concepts here disclosed.