GB2293257A - Fire alarms - Google Patents

Abstract

Warning devices such as sounders 36 are connected in a common circuit with fire detection devices 34. The detection devices operate within a first voltage range and the warning devices operate within a second voltage range, the two ranges having the same polarity. There is a non-sounding voltage within the first range but outside the second range which does not operate the warning devices. In use the system normally supplies power to the circuit at the non-sounding voltage to power the detection devices. During an alarm condition the circuit is supplied with a voltage within the second range, this causing the warning devices to activate. Preferably there is an alarm voltage which is within both ranges thus enabling the detection devices to remain active during an alarm condition. <IMAGE>

Description

FIRE ALARMS The present invention relates to fire alarms.

In a conventional fire alarm system there is typically provided a control unit, a plurality of detection devices (for example, heat and smoke detectors, manual call points, and so forth) connected to the control unit in a zone circuit, and a plurality of warning devices (bells, sirens, strobe lights, and so forth) connected to the control unit in a sounder circuit. In any system, there may be one or more zone and/or sounder circuits.

A disadvantage to this type of system is that at least two separate circuits must be provided. It would clearly be advantageous if it were possible to use just a single circuit (or one circuit per zone) since this would result in a reduction in the amount of wiring required and also a reduction in time and effort required to install the system.

In GB-A-1491222, there is disclosed an arrangement by which detection devices and warning devices may be connected to a control unit in a single circuit. The system operates by applying to the circuit electrical power in order to power the detection devices. When it is required to activate the warning devices, the polarity of the power applied to the circuit is reversed, the detectors remaining active due to a bridge rectifier and the warning devices receiving power as well as the detectors.

The system described above has several disadvantages.

Firstly, the requirement that the control unit must generate electrical power with two opposed polarities increases the complexity of its construction.

Additionally, all detectors connected to the circuit must be provided with a bridge rectifier circuit so that they respond to electrical power of both polarities. This means that all detectors for connection to the circuit are specialised and also that additional components must be provided at the detectors.

It is an aim of the present invention to provide an improved fire alarm system in which detection devices and warning devices may be connected in a single circuit.

According to a first aspect of the invention, there is provided a fire alarm system comprising a zone circuit carrying a supply of electrical power and to which is connected a detection device and a warning device, the detection device being operative within a first range of supply voltages and the warning device being operative within a second range of supply voltages, wherein there is a non-sounding voltage within the first range but out with the second range at which the system operates for fire detection, all voltages within the first and second ranges being of the same polarity.

Thus, the system, in use, normally supplies power to the circuit at the non-sounding voltage to power the detection devices. However, during an alarm condition, the zone circuit is supplied with the voltage within the second range, this causing the warning devices to activate.

Preferably, there is an alarm voltage which is within both the first and second ranges. In this way, the detection devices can remain active during an alarm condition and their status may therefore be periodically rechecked.

As an example, typical detection devices operate over a first voltage range of 17v to 35v. If a warning device is selected having a second operating voltage range of 24v to 30v, then the non-sounding voltage may be set at 20v and the alarm voltage set at 27v. In this way, the control unit can change the action of the system from a normal, detecting mode to an alarm mode by simply raising the voltage on the zone circuit by 7 volts.

Most typically, fire alarm systems are powered from means electricity transformed and rectified to provide a 24 volt DC power supply, together with a back up source of power, to keep the system running in the event of failure of the mains supply, comprising two lead acid 12 volt batteries connected in series.

It is found in practice that some disadvantages are associated with the batteries. In small systems, the batteries may be of larger capacity (and are therefore more expensive and heavier) than required by the system, and because of the need to provide two batteries, there is a greatly increased risk that incorrect connections will be made with consequential damage to the batteries and/or fuse in the fire alarm control unit. Ameliorating these disadvantages would be advantageous, but it is not considered practical to change to an output of 12 volts since this would require modification of all devices connected to the system, nor is it possible to substitute a single 24 volt battery, since these are unavailable in anything other than very large capacities.

From a second of its aspects, the invention provides a power supply unit for a fire alarm system comprising a source of DC power, derived from the mains, at 12 volts, a back up battery of 12 volts, and a step up converter operative to receive a 12 volt DC supply and generate therefrom a DC output at or around 24 volts for powering a fire alarm system.

As well as addressing the disadvantages outlined above, it has been found that providing a primary source of power at 12 volts has the additional effect of reducing the cost of the mains power supply since its rectifying and smoothing components can be provided with a lower voltage rating. Furthermore, most fire alarm systems require a 5 volt supply to operate their electronic components. This 5 volts supply may be derived from 12 volts using components of lower cost and specification than are required for 24 volts.

Preferably, the step up converter is of a programmable type, such that its output voltage may be varied as required. A power supply of this type is particularly useful in a system of the first aspect of the invention in which it is necessary to vary the voltage applied to a zone circuit.

Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of a conventional fire alarm system; Figure 2 is a diagram of a fire alarm system embodying the invention; Figures 3 and 4 show two alternative circuits for use in a warning device of an alarm system embodying the invention; Figures 5 and 6 are diagrams of further embodiments of the invention being, respectively, single and multiple zone fire alarm systems; and Figure 7 is a circuit diagram of a fire alarm power supply system embodying the invention.

With reference to Figure 1, a fire alarm system comprises a control unit (not shown) connected to a zone circuit 10 and two sounder circuits 12,14.

The zone circuit comprises a wire pair 16 to which a DC signal is applied by the control unit. In the zone circuit 10, various detection devices 18 are connected between the wire pair 16. These devices 18 may include fire, smoke and heat detectors and manual call points.

The final device on the zone circuit is an end of line monitor 20 which sends signals to the control unit whereby a fault in the zone circuit 10 may be detected.

The sounder circuits 12,14 each comprises a wire pair 22,24 across which is connected a plurality of warning devices 26 including such things as bells, electronic sounders, and strobe lights. The last component of each sounder circuit 12,14 is an end of line resistor 28 which interconnects the wires of the wire pair, whereby the control unit can detect open or short circuit conditions within the sounder circuit 12,14.

In use, the control unit monitors the current flowing in the zone circuit 16 and if this changes due to one of the detectors 18 detecting an emergency condition, the control unit applies electrical power to one or both sounder circuits 12,14 to activate the warning devices connected thereto.

In the present invention, as illustrated in Figure 2, there is provided a control unit (not shown) to which is connected a zone circuit 30. The zone circuit 30 comprises a wire pair 32 across which is connected a plurality of detection devices 34 and warning devices 36.

The last device connected to the zone circuit 32, remote from the control unit, is an end of line monitor 38, functioning as described above.

The detection and warning devices 34,36 may be connected to the zone circuit 30 in any order - they may be mixed or separated, as shown in Figure 2. However, the end of line monitor 38 must always be the last device connected to the zone circuit.

Each warning device 36 is connected to the zone circuit 30 through a hold off circuit, alternative arrangements of which are shown in Figures 3 and 4. In each case, the function of the hold off circuit is to ensure that the warning device is not activated until the voltage on the zone circuit exceeds a predetermined threshold.

With reference to Figure 3, within a warning device 36 there is warning means 50 which, on supply of electrical power, emits a warning and might, for example, be constituted by an electric bell, an electric sounder, or an electric light.

The warning device comprises terminals 52 for connection to the zone circuit. The sounding means 50 is connected to the terminals 52 through a hold off circuit, to be described below.

The hold off circuit comprises two resistors R1,R2, a zener diode and an npn by polar transistor.

The first of the terminals 52 is connected to the more positive wire of the zone circuit and a positive supply line 54 of the sounder is connected thereto. A negative supply line 56 of the sounder is connected to the other terminal. The first resistor R1 is connected to the positive supply line 54, the second resistor R2 is connected to the supply line 56 and the zener diode is connected between the resistors, arranged such that it is reversed biased. The base of the transistor TR1 is connected to a circuit point 58 between the zener diode and the second resistor R2, the emitter of the transistor is connected to the negative supply line 56. The warning means 50 is connected between the positive supply line 54 and the collector of the transistor TR1.

In operation, when the voltage in the zone circuit (and therefore between the positive and negative supply lines 54,56) is less than the breakdown voltage of the zener diode ZD, substantially no current will flow through R1,ZD,R2. Thus, the circuit point 58 will be substantially the same voltage as the negative supply line 56, the transistor TR1 will be switched off, so no current may flow through the warning means 50. When the voltage in the zone circuit rises to exceed the breakdown voltage of the zener diode ZD, the zener breakdown within the diode ZD will occur and current will begin to flow through R1,ZD,R2. This will cause the voltage at circuit point 58 to rise, so turning on the transistor TR1, and allowing current to flow through the warning means 50.

As soon as the voltage in the zone circuit falls below the breakdown voltage, the diode ZD will drop out of conduction, circuit point 58 will return to the voltage of the negative supply line 56, and the transistor TR1 will once again turn off.

Thus, it can be seen that the sounder described can be arranged to be controlled simply through varying the voltage in the zone circuit, the warning device being activated simply by exceeding a predetermined voltage ?? determined by the breakdown voltage of the zener diode ZD.

In the hold off circuit of Figure 4, components R1,ZD,R2 and TR1 are arranged with respect to positive and negative supply lines 54-56- and terminals 52substantially as in the circuit described with reference to Figure 3. However, in the hold off circuit of Figure 4, the collector of the transistor TR1 is connected to the positive supply line 54- through two further, series connected resistors R3,R4, resistor R3 being connected to the collector of the transistor TR1 and resistor R4 being connected to the positive supply line 54-, there being a circuit point 60 between the two resistors R3,R4. A second, pnp by polar transistor is connected with its emitter to the positive supply line 54- and its base to the circuit point 60. Warning means 50- is connected between the collector of the second transistor TR2 and the negative supply line 50-.

In operation, the transistor TR1 is turned on and off by the voltage on the zone circuit, as described with reference to Figure 3. In the circuit of Figure 4, the transistor is turned off, no current flows through the resistors R3 and R4. This means that circuit point 60 is at substantially the same voltage as the positive supply line 54-, so turning the second transistor TR2 off, and preventing current flowing through the warning means 50-.

When the transistor TR1 is turned on, the voltage at circuit point 60 will fall, and the second transistor TR2 will begin to conduct. This allows current to flow through the warning means 50-. It will be seen that the circuits of Figures 3 and 4 perform in substantially the same way. The circuit of Figure 4 is advantageous where large currents must be switched since the base current of the switching transistor (TR2 in Figure 4) does not flow through the zener diode ZD.

With reference to Figure 5, there is shown an alternative embodiment of the present invention.

In some fire alarm systems, for example, those intended to comply with British Standard BS5839, Part 1, there is a requirement for two sounder circuits arranged such that if one sounder circuit should fail, the other will still operate.

With reference to Figure 5, a zone circuit 80 comprises a plurality of detection devices 82, a plurality of warning devices 84 and an end of line monitor 86, arranged substantially as described above.

By varying the voltage in the zone circuit, either the detectors or the sounders may be collectively operated by control of the voltage applied to the zone circuit 80, as described above.

There is additionally provided a secondary sounder circuit 90 comprising a wire pair across which a plurality of sounders is connected, each having a holdoff circuit as described above. The secondary sounder circuit is terminated with an end of line resistor 92.

The more negative wire of the zone circuit 80 and the sounder 90 (indicated at 88 and 94 respectively) are each connected directly to a low voltage output of the control unit.

A high voltage output of the control unit V1 is connected through a first diode D1 and a first resistor R1 to the more positive wire of the zone circuit 80 and is also connected through a second diode D2 and a second resistor R2 to the more positive wire of the secondary sounder circuit 90. A warning output V4 of the control unit is connected to a circuit point 94. The circuit point 94 is connected through a third diode D3 and a first fuse Fl to the more positive line of the zone circuit 80 and through a fourth diode D4 and a second fuse F2 to the more positive wire of the sounder circuit 94.

In normal conditions a voltage of around 20 volts is applied at V1. This powers the detection devices 82 and the zone circuit 80 with a slightly lower voltage, going to the drop in resistor R1. The current flowing in the circuit can thus be monitored in an entirely conventional manner for activation of any of the detection devices 82.

Additionally, the voltage V2 between the first fuse F1 and the third diode D3 can be monitored for pulses sent by the end of line monitor 86, again in the conventional manner. The sounder circuit 90 is also supplied with a voltage just less than 20 volts. This is insufficient to activate any of the warning devices 82. The presence of this voltage serves to generate a further voltage V3 at a circuit point between the second fuse F2 and the fourth diode D4 which can be monitored for changes as might be caused by an open or short circuit occurring in the secondary sounder circuit 90. It will be appreciated that, even when the warning devices 92 are not active, a current will flow in the secondary sounder circuit 90 through the end of line resistor 94.

During normal operation, surge in force diodes D3,D4 are reversed bias so the circuit point 94 is effectively isolated from the zone and secondary sounder circuits 80,90. However, when an alarm condition occurs, the voltage V4 is raised to 27 volts. This causes the surge in force diodes D3,D4 to be forward biased, so causing current to flow through them into the zone and secondary sounder circuits 80,90, thus activating the warning devices 84,92 on the two circuits 80,90. In this condition, the first and second diodes D1,D2 are reverse biased so that the voltage V4 is not fed back to the control unit at V1.

In the alarm condition, if either of the zone or secondary sounder circuits 80,90 should fail such that a short circuit is caused, then the current in that circuit 80,90 will rise causing the respective fuse F1,F2 to blow. Thus, the faulty circuit will be isolated while the warning devices on the other circuit remain powered.

Should either of these circuits 80,90 fail such that an open circuit occurs, some of or all of the warning devices 84,92 on that circuit will cease to operate, but others on the other circuit will be unaffected.

With reference to Figure 6, a multi-zone fire alarm system (in this case with two zone circuits 100,102) can be provided. The two zone circuits 100,102 are connected through diodes D1,D2, resistors R1,R2, fuses F1,F2 and diodes D3,D4 substantially identically to the connections of the two circuits described with reference to Figure 5.

In this case, both circuits 100,102 have a mixture of detection devices 104 and warning devices 106, and each circuit 100,102 is provided with an end of line monitor 108.

In normal use, the circuits 100,102 are provided with independent voltage supplies V1,V1' which are monitored for activation of the detection devices. Additionally, signals from each end of line monitor 108 are independently monitored at circuit points V2 and V3. As described above, in order to activate the warning devices 104, voltage V4 is raised. Then, in the event that either circuit 100,102 should fail, the warning devices 104 on the other circuit are unaffected.

With reference to Figure 7, a power supply for a fire alarm system comprises a means input 110 which is fed to the primary winding of the step down transformer 112.

The output of the transformer 112 is fed to a bridge rectifier 114 to generate a DC output +V,-V.

A smoothing capacitator 116 (which may typically comprise a plurality of electrolytic capacitators) is connected across the output +V,-V of the bridge rectifier 114 to provide a smooth low-ripple DC supply.

The DC supply +V,-V is fed to a switching and charging circuit 116, which is also connected to a 12v lead-acid battery 118 through a fuse 120. The switching and charging circuit 116 has a 12v DC output at +V,-V.

During normal operation, when main power is available, the output at +V,-V of the switching and charging circuit 116 is fed from its input, and a charging current is fed to the battery 118. If the mains supply should fail, the switching and charging circuit connects to battery 118 to its output +V,-V, so maintaining a 12v power supply +V', V during power failures.

A voltage regulator 122 is connected across the 12v power supply +V',-V, having an output of 5v at 124 with respect to -V. This is used to power electronic circuits 126 within the control unit, many of which are design for operation at 5v DC.

A step-up converter 128 is also connected to the power supply +V',-V, the output from which is at a voltage +V' ', higher, with respect to -V, than +V', suitable for driving sounder and zone circuits of a fire alarm system, typically in the range 17v to 35v.

Most preferably, the step-up converter 128 is of a programmable type, its output voltage +V'' being controlled by a voltage Vc placed on a control input 130 of the convertor 128. The control voltage Vc may suitably be generated by an electronic circuit 126 within the control unit.

In use with embodiments described with reference to Figures 2 to 7, the output voltage +V'' of the step-up convertor 128 can be set, in normal use, to a nonsounding voltage, for example 20v. When an alarm condition is detected, the electronic circuits 126 cause the control voltage Vc to charge such that the output voltage +V" rises to an alarm voltage, for example 30v, to activate the warning devices in a manner described above.

Claims (9)

CLAIMS:

1. A fire alarm system comprising a zone circuit carrying a supply of electrical power and to which is connected a detection device and a warning device, the detection device being operative within a first range of supply voltages and the warning device being operative within a second range of supply voltages, wherein there is a non-sounding voltage within the first range but outwith the second range at which the system operates for fire detection, all voltages within the first and second ranges being of the same polarity.

2. A fire alarm system according to claim 1, in which there is an alarm voltage which is within both the first and second ranges.

3. A fire alarm system according to claim 1 or claim 2, in which detection devices operate over a first voltage range of 17v to 35v, a warning device has a second operating voltage range of 24v to 30v, and the non-sounding voltage is set at 20v and the alarm voltage set at 27v.

4. A fire alarm system according to any preceding claim having a power supply unit comprising a source of DC power, derived from the mains, at a first voltage below the operating voltage of the detection and warning devices of the system; a back up battery of voltage equal to or close to the first voltage; and a step up converter operative to receive a DC supply at or around the first voltage and generate therefrom a DC output at a suitable output voltage for powering the fire alarm system.

5. A fire alarm system according to claim 4, in which the first voltage is-at or around 12v while the output voltage is at or around 24v.

6. A fire alarm system according to claim 4 or claim 5, in which the output voltage is variable, being capable of being raised to around 30v for operation of warning devices in the system at an alarm voltage.

7. A fire alarm system according to any one of claims 4, 5 or 6, in which the step up converter is of a programmable type, such that its output voltage may be varied as required.

8. A fire alarm system substantially as herein described with reference to Figure 2, Figure 5 or Figure 6, together with one or both of Figures 3 and 4 of the accompanying drawings.

9. A fire alarm system according to claim 8 substantially as further described with reference to Figure 7 of the accompanying drawings.