AU2003204675B2 - Monitored alarm circuit with reduced quiescent drain - Google Patents

Description

MONITORED ALARM CIRCUIT WITH REDUCED QUIESCENT CURRENT DRAIN This invention relates to battery backed monitored alarm circuits and more particularly to a method for reducing the quiescent current drain of these alarm circuits so as to decrease the size of stand-by battery back-up required. Alternatively, the current reduction method of this invention allows monitored alarm circuits to operate for a much longer period from a given size of stand-by battery back-up supply.

BACKGROUND

It is good practice to continuously monitor the integrity of alarm circuits of security and fire alarm systems so that open circuit and short circuit wiring faults are immediately detected to provide a warning or indication of the fault condition. Thus, wiring faults can be promptly repaired to maintain the reliability of the security or fire alarm system. However, monitoring methods currently used have the disadvantage of being a burden on the stand-by battery backup supply of security and fire alarm systems as they are heavy on. current. This is best explained by referring to the following analysis and examples FIG. 1 shows a typical monitored fire alarm circuit comprising of a number of smoke detectors connected between the positive and negative conductors of the circuit. The circuit originates at one end from the fire alarmn panel, from which all the smoke detectors are powered, and terminates at the other end with an end of line resistor connected between the positive and negative circuit conductors. Typical parameters of a fire alarm circuit exemplified by FIG. 1 are: Quiescent current of smoke detector 0.10 milliamp.

Maximum number of smoke detectors per circuit Maximum total smoke detector quiescent current 4.0 milliamps Supervisory current due to end of line resistor 10.0 milliamps Circuit alarm current 35.0 milliamps Circuit short circuit current (current limit of circuit) 100.0 milliamps From the above, it is clear that the circuit quiescent current, depending on how many smoke detectors are on the circuit, would lie between 10.0 milliamps, with no detector 2 connected to the circuit, and 14.0 milliamps when the circuit is at maximum capacity with smoke detectors connected. It is to be noted that the circuit supervisory current due to the end of line resistor needs to be greater than the total of the quiescent currents of the circuit's maximum number of detectors, that is 4.0 milliamps, in order to detect an open circuit fault condition on a circuit with a low number of detectors. For example, the detector circuit of FIG.

1 would be connected to an electronic circuit at the fire alarm panel that would detect and indicate an open circuit fault if the circuit current was to drop below say 8.0 milliamps, detect and indicate an alarm condition if the circuit current was to rise above say 25.0 milliamps whilst remaining under the current level that would generate a short circuit wiring fault, detect and indicate a short circuit wiring fault if the circuit current was to exceed say 90.0 milliamps.

The difficulty with alarm circuits similar to that exemplified by FIG. 1 is that the high circuit current due to the supervisory current in the circuit end of line resistor represents a considerable drain on the back-up battery supply of the fire alarm system. For example, a 10.0 milliamps supervisory current of an alarm circuit that is required to be backed by a stand-by battery supply for 10 days after loss of mains normal supply, would require a sizeable 2.4 amphour battery capacity just for the monitoring of the circuit over the 10 day period. The problem is further compounded where multiple alarm circuits are connected to a single standby battery back-up supply.

FIG. 2 shows another typical monitored fire alarm circuit requiring a 4-conductor wiring system. Two conductors supply power to the circuit smoke detectors and each of the detectors includes a relay that is energised when the detectors are powered. The normally open contacts of the smoke detector relays are closed when the detectors are in the energised state, and are connected in series with an end of line resistor using the remaining two conductors of the 4-wire wiring system.

The operation of the circuit of FIG. 2 is as follows: The circuit end of line resistor establishes a supervisory current through the closed contacts of the smoke detector relays when the smoke detectors are powered and the relays energised. The fire alarm panel contains an electronic circuit that detects the circuit's supervisory current to recognise that the circuit is operating normally without any fault or alarm.

The relay of each of the circuit's smoke detectors is de-energised when the smoke detector is in alarm condition to interrupt the circuit supervisory current. The interruption of the 3 supervisory current is detected by the fire alarm panel which registers and indicates an alarm.

With the fire alarm circuit of FIG. 2, the circuit will also alarm on any of the following conditions: 1. Loss of power to the smoke detectors resulting in the de-energisation of the smoke detector relays which in turn causes the circuit supervisory current to be interrupted.

2. An open circuit on any of the two smoke detector power wiring conductors causing one or more smoke detector relays to drop out to interrupt the circuit supervisory current.

3. An open circuit on any of the two conductors forming a closed loop through the closed contacts of the circuit's smoke detectors and the circuit's end of line resistor. The open circuit thus interrupts the circuit's supervisory current to cause an alarm to be detected and indicated.

4. When the electronic circuit of the fire alarm panel detects a higher current than the supervisory current consistent with a short circuit fault on the supervisory loop.

When a short circuit condition exists on the 2-conductor power wiring to the smoke detectors, condition that causes the circuit's smoke detector relays to drop out to interrupt the supervisory current in the supervisory loop.

With the circuit of FIG. 2 any abnormal circuit occurrence as a result of a wiring fault, an interruption of power to the circuit's smoke detectors, or the detection of a fire condition by one or more of the circuit's smoke detectors, will have the same effect of raising and indicating an alarm. With regards to current consumption of the fire alarm circuit of FIG. 2, it is to be noted that it is even higher than that of the fire alarm circuit exemplified by FIG. 1 as the circuit quiescent current depends not only on the end of line supervisory current, but also on the current drawn by the normally energised relay of each of the circuit's smoke detectors.

It is therefore realised, from the above descriptions, that the known methods of monitoring alarm circuits using end of line resistors is disadvantageous because of the requirement for a supervisory current that adversely affects battery capacity required and/or battery back-up time provided.

OBJECTS OF THE INVENTION It is therefore an object of this invention to overcome the disadvantage of prior art monitored alarm circuits that use end of line devices and/or relays by providing a new circuit 4 monitoring method that does not impose a heavy current drain on the stand-by battery back-up supply of the alarm system. Thus, the battery back-up supply lasts for a much longer period after the normal supply has failed. Alternatively, the current reduction method of this invention can be used to reduce the size of the stand-by battery back-up supply necessary to provide a required back-up time.

It is another object of this invention to provide one or more ancillary alarm devices on circuits using the monitoring method of this invention and to automatically operate the ancillary alarm devices when any of the circuit detectors is in alarm condition.

BRIEF SUMMARY OF THE INVENTION This invention in one aspect resides broadly in a method of monitoring the integrity of an alarm circuit comprising of one or more detectors powered from a normal power supply and.

a stand-by battery power supply, the method including: supplying power to all detectors on the alarm circuit by connecting the normal power supply and the stand-by battery power supply to the first and subsequent detectors on the alarm circuit using a first 2-conductor detector wiring section without branching; connecting the last detector on the alarm circuit to a high impedance voltage sensitive electronic circuit powered by the nonnal power supply and the stand-by battery power supply using a second 2-conductor return path wiring section; detecting if the voltage of the 2-conductor return path wiring section, as sensed by the high impedance voltage sensitive electronic circuit, drops below a threshold voltage level in accordance with whether a fault condition exists on the alannrm circuit, and providing a signal to indicate a fault condition when it is detected that the voltage of the 2-conductor return path wiring section has dropped below the fault threshold voltage level.

The drop in voltage of the 2-conductor return path wiring section may be caused by either an open circuit in any one or more of the circuit conductors, or by a short circuit in the circuit wiring.

The method may also include:providing one or more ancillary alarm devices on the 2-conductor return path wiring section without branching, and switching the ancillary alarm devices so that they operate when any of the circuit detectors is in alarm condition.

The ancillary alarm devices may be switched by various means to achieve the desired result. In a preferred embodiment the ancillary alarm devices are switched by:providing each of the ancillary alarm devices connected to the 2-conductor return path wiring section with an electronic control circuit to operate the alarm device; disabling the electronic control circuit of each of the ancillary alarm devices when the alarm circuit is powered from the 2-conductor detector wiring section end only, and not when the alarm circuit is powered from both the 2-conductor detector wiring section end and the 2conductor return path wiring section end, and powering the alannr circuit from both the 2-conductor detector wiring section end and the 2-conductor return path wiring section end, when any of the circuit detectors is in alarm condition, to enable the electronic control circuit of each of the circuit's ancillary alann devices so that they all operate.

The new method of this invention is best explained by referring to the following embodiments. However, the embodiments described in the following subsections of this specification, and illustrated by the accompanying drawings, are merely illustrative of how the invention might be put into effect and are not to be understood as being limiting on the invention.

FIRST EMBODIMENT In the first embodiment the smoke detectors of a monitored fire alarm circuit are wired as shown in FIG. 3. The circuit consists of a first 2 conductor wiring section connecting all smoke detectors to the fire alarm panel, and a second 2 conductor return path wiring section starting from. the last smoke detector on the fire alarm circuit and terminating at the fire alarm panel. Alternatively a single four core cable could be used wired from detector to detector and wiring links CD and AB used to connect the first smoke detector wiring section to the second return path wiring section.

The operation of the first embodiment is best explained by referring to FIG. 4 which is a circuit diagram of the electronic monitoring circuit of the fire alann panel. Referring to FIG.

4, the circuit current flows through resistor R3, diode D3 and fuse F. The resistance value of resistor R3 sets the current level at which transistor TI turns on while diode D3 caps the voltage drop between the base and the emitter of transistor T1 to a safe value of around 0.60 volts. R3 is sized so that the higher circuit current, when one or more smoke detectors are in alarm, causes transistor T1 to conduct to charge capacitor CI through resistor R4. After a period of time dependent on the values assigned to capacitor C1, resistor R4 and resistor capacitor Cl acquires enough charge to raise the voltage at the gate of the FET transistor T2 to above approximately 2 volts. Transistor T2 therefore conducts to operate the alarm LED LI and to energise the alarm relay coil Cl. The diode connected across the relay coil C1 serves to protect the circuit against back emf when the current through the relay coil. is interrupted.

The alarm relay provides contacts that may be used to operate specific equipment such as a siren or bell under alarm condition.

The circuit of the first embodiment also includes a FET transistor T4 and resistors R8 and R9 connected between the positive and negative conductors of the circuit return path wiring section. Resistors R8 and R9 have resistance values such that the voltage of the gate of FET transistor T4 is above approximately two volts so that transistor T4 conducts under normal operating conditions. Thus the gate of FET transistor T3 is grounded through diode D4 to keep FET transistor T3 turned off Should any of the circuit wiring conductors develop an open circuit fault, the FET transistor T4 is either turned off or the path to ground from the source terminal of transistor T4 is interrupted. Any of the two possible occurrences will cause grounding of the gate of FET transistor T3 to cease so that transistor T3 conducts to operate the fault LED L2 and to energise the fault relay coil C2. The diode connected across the relay coil C2 serves to protect the circuit against back emf when the current through the relay coil is interrupted. The fault relay provides contacts that may be used to operate specific equipment such as a local fault buzzer under fault condition.

The fuse F of the circuit of the first embodiment serves to interrupt the circuit current when a short circuit exists in the fire alarm circuit wiring, thus preventing damage to the wiring or to the monitoring circuit of the fire alarm panel. Therefore when the fuse F operates to interrupt a short circuit current, resistor R9 is no longer connected to circuit positive so that FET transistor T4 is turned off to cause the fault relay and the fault LED to operate. Ilt is to be noted that the fault relay only operates when a fault exists and therefore does not contribute to the circuit quiescent current.

Transistors T2, T3 and '1T4 are FET transistors with high input impedances and corresponding resistor pairs R4/R5, R6/R7, and R8/R9 are assigned very high values of resistances so that the effect of the transistors and resistors on the circuit current consumption is negligible. Thus, it is realised that the circuit of the first embodiment of this invention is of very low quiescent current to overcome the disadvantages of prior art monitored alarm circuits.

SECOND EMBODIMENT The second embodiment makes use of the circuit return path wiring section, described in the first embodiment, to operate one or more ancillary alarm devices such as a relay, a sounder, or a flashing light in the event of an alarm condition being detected. The second embodiment is best explained with reference to FIG.5 and FIG. 6.

FIG. 5 showing a fire alarm circuit connected to a fire alarm panel is essentially the same as FIG. 3 of the first embodiment except for the following.

The first detector wiring section of the circuit supplying power to the circuit's smoke detectors is connected to the second return path wiring section of the circuit through diode and zener diode Zi.

The return path wiring section of the circuit is connected to one or more ancillary alarm devices such as sounders, relays, or flashing lights which operate under alarm condition when the control circuits of the ancillary alarm devices are enabled. For convenience and ease of reference, only one ancillary alarm device is shown. in FIG. 5, and its switching circuit consists of transistor T5, resistors RIO and R 1, and zener diode Z2.

Similarly, FIG. 6 showing the control circuit of the fire alarm panel is essentially the same as FIG. 4 of the first embodiment except that a normally open contact AC of the alarm relay is used to connect the positive conductor of the circuit return path wiring section to the positive supply at the fire alarm panel. Thus, under fire alarm condition when the normally open contact of the alarm relay closes, the alarm circuit is powered from both the 2-conductor detector wiring section end and the 2-conductor return path wiring section.

To better explain the operation of the second embodiment, it is assumed that the supply voltage of the fire alarm panel is 12 volts. The alarm and fault detecting and indicating functions of the fire alarm panel are identical to those described for the first embodiment.

Similarly, the smoke detecting functions of the alarm circuit are also identical to those of the first embodiment. The operation of the additional components of the second embodiment is as follows.

Referring to FIG. 5, each ancillary alarm device incorporates an electronic circuit comprising of zener diode Z2, resistors RI 0 and R1 1, and transistor T5. The positive conductor of the 2-conductor detector wiring section is connected to the positive conductor of the 2- 8 conductor return path wiring section using diode D5 and zener diode Z1. Zener diodes Z1 and Z2 are assigned values such that under normal circuit quiescent condition, the voltage across resistor RIO is negligible. As the fire panel supply voltage is 12 volts, a suitable zener voltage rating for Z1 and Z2 is 8 volts. Thus resistor RO10, zener diodes Z1 and Z2, and diode are all connected in series between the circuit negative and the circuit positive conductors.

Because the sum of the rated voltages of zener diodes Z 1 and Z2 exceeds the supply voltage of 12 volts, all the supply voltage is dropped across the zener diodes resulting in the base of transistor T5 being at ground potential. Therefore under normal conditions transistor T5 is turned off as the electronic control circuit of each of the ancillary alarmnn devices is disabled so that the ancillary alarm devices are turned off Referring to FIG. 6, under fire alarm condition the alann relay contact AC closes to connect the positive conductor of the circuit return path wiring section directly to the full 12 volts supply of the fire alarm panel. with the result that resistor RIO and the 8 volt zener diode Z2 of FIG. 5 are now connected in series directly between circuit negative and circuit positive, that is, across the full. fire panel supply voltage of 12 volts. Therefore approximately 4 volts are dropped across resistor RO10 under fire alarm condition and base current flows through resistor RI to turn transistor T5 on. Thus, the electronic circuit of each of the ancillary alarmn devices is enabled to operate the ancillary alarm devices when a fire alarm condition exists. It is to be noted, however, that the current to operate ancillary alarm devices on the return path wiring section of the fire alarm circuit does not flow through diodes D3 and resistor R3 of the circuit of FIG. 6 so that the operation of the ancillary alarm devices does not cause the fire alarm panel to latch into alarm. Furthermore, the diode D5 of FIG. 5 prevents the circuit smoke detectors from being powered from the positive conductor of the return path wiring section, under fire alarm condition, so that the fire panel continues to register an alarm condition for as long as any of the circuit's smoke detectors is in alarm condition.

Claims (4)

1. A method of monitoring the integrity of an alarm circuit comprising of one or more detectors powered from a normal power supply and a stand-by battery power supply, the method including: supplying power to all detectors on the alarm circuit by connecting the normal power supply and the stand-by battery power supply to the first and subsequent detectors on the circuit using a first 2-conductor detector wiring section without branching; connecting the last detector on the alarm circuit to a high impedance voltage sensitive electronic circuit, powered by the normal power supply and the stand-by battery power supply, using a second 2-conductor return path wiring section; detecting if the voltage of the 2-conductor return path wiring section, as sensed by the high impedance voltage sensitive electronic circuit, drops below a threshold voltage level in accordance with whether a fault condition exists on the alarm circuit, and providing a signal to indicate a fault condition when it is detected that the voltage of the

2-conductor return path wiring section has dropped below the fault threshold voltage level. 2. A method as claimed in claim 1, wherein the fault indicating signal is provided when an open circuit exists in any one or more of the conductors of the alarm circuit, or when the alarm circuit wiring is short circuited.

3. A method as claimed in claim 2, and including:- providing one or more ancillary alarm devices on the 2-conductor return path wiring section without branching, and switching the ancillary alarm devices so that they operate when any of the circuit's detectors is in alarm condition.

4. A method as claimed in claim 3, wherein the ancillary alarm devices are switched by:- providing each of the ancillary alarm devices connected to the 2-conductor return path wiring section with an electronic control circuit to operate the alarm device; disabling the electronic control circuit of each of the ancillary alarm devices when the alarm circuit is powered from the 2-conductor detector wiring section end only, and not when the alarm circuit is powered from both the 2-conductor detector wiring section end and the 2- conductor return path wiring section end, and powering the alarmn circuit from both the 2-conductor detector wiring section end and the 2-conductor return path wiring section end, when any of the circuit detectors is in alarm condition, to enable the electronic control circuit of each of the circuit's ancillary alarm devices so that they all operate. An alarm circuit powered from a normal power supply and a stand-by battery power supply, and including:- a plurality of detectors; supply means for supplying power to all detectors on the alarm circuit by connecting the normal power supply and the stand-by battery power supply to the first and subsequent detectors on the alarm circuit using a first 2-conductor detector wiring section without branching; connecting means for connecting the last detector on the alarm circuit to a high impedance voltage sensitive electronic circuit, powered by the normal power supply and the stand-by battery power supply, using a second 2-conductor return path wiring section; detecting means detecting if the voltage of the 2-conductor return path wiring section, as sensed by the high impedance voltage sensitive electronic circuit, drops below a threshold voltage level in accordance with whether a fault condition exists on the circuit, and signalling means for providing a signal to indicate a fault condition when it is detected that the voltage of the 2-conductor return path wiring section has dropped below the fault threshold voltage level. 6 An alarm circuit as claimed in claim 5, wherein the signalling means provide a warning when an open circuit exists in any one or more of the circuit's conductors, or when the circuit wiring is short circuited. 7 An alarm circuit as claimed in claim 6, and including:- one or more ancillary alarm devices provided on the 2-conductor return path wiring section without branching, and switching means for switching the ancillary alarm devices so that they operate when any of the circuit's detectors is in alarm condition. 8 An alarm circuit as claimed in claim 7, wherein the switching means include:- means for providing each of the ancillary alarm devices connected to the 2-conductor return path wiring section with an electronic control circuit to operate the alarm device; means for disabling the electronic control circuit of each of the ancillary alarm devices when the alarm circuit is powered from the 2-conductor detector wiring section end only, and 11 not when the alarm circuit is powered from both the 2-conductor detector wiring section end and the 2-conductor return path wiring section end, and means for powering the alarm circuit from both the 2-conductor detector wiring section end and the 2-conductor return path wiring section end, when any of the circuit's detectors is in alarm condition, to enable the electronic control circuit of each of the circuit ancillary alarm devices so that they all operate.