GB2320121A - Alarm system - Google Patents

Abstract

An alarm system comprises at least one detector (3, Fig 1) and at least one alarm device (4) mounted connected by a common circuit to a control unit (1), the control unit being adapted to normally provide a voltage of a single polarity (ie DC) to the circuit and, upon detection of an alarm condition, to produce an voltage of alternating polarity (ie AC) to the circuit. The DC voltage powers only the detector(s) whereas the AC voltage powers both detector(s) and the alarm device(s). As shown, a DC source 7 and an AC source 6 are provided. In alternative embodiments, two switched DC sources (9,10 Fig 3) and a single switched AC source (13,14,15, Fig 4) are provided.

Description

ALARM SYSTEM This invention relates to alarm systems. In particular, it relates to alarms systems of the type in which a plurality of detectors (smoke detectors, fire detectors and so on) are arranged on the same circuit as a plurality of alarm indicators, typically sounders but which may also be visual devices. The circuit on which they are mounted may typically be a two wire circuit and it is required both to power and monitor the detectors for the presence of an alarm situation and also to power and operate the indicators (sounders) to indicate an emergency situation.

Previously, separate systems of wires were commonly used to connect fire detectors and alarm devices to a control panel. The voltage of the detector wires was monitored and, if the voltage decreased due to an increased current taken by the fire detectors, the control panel acted upon this as a fire signal and went into an alarm state. The control panel also monitored the voltage on the separate alarm wires (to check for wiring faults) and, when the panel went into an alarm state, served to actuate the alarms by reversing the voltage on the alarm wires, which in turn operated the alarm devices.

More recently, systems have been developed which use detectors and alarm devices connected to the same pair of wires. In one such system, the voltage of the single pair of wires is monitored and, if a fire signal is detected the voltage is reversed, operating the alarm devices.

In an alternative system, instead of the voltage being reversed, when an alarm situation is indicated the voltage on the wires is increased. The detectors in this case are only arranged to respond when they receive a voltage of the higher amount. However, with such a system, a separate sensing circuit is required in each alarm device which may be expensive.

According to the present invention there is provided an alarm system comprising at least one detector and at least one alarm device connected by a common circuit to a control unit, the control unit being adapted to normally provide a voltage of one polarity to the circuit and, upon detection of an alarm condition, to produce an voltage of alternating polarity to the circuit.

The one polarity voltage may be a DC voltage, and the alternating voltage an AC voltage.

Alternatively, the one polarity voltage may be part (ie the half cycle having positive, or negative, polarity) of an AC voltage.

Embodiments of the invention therefore have the advantage that in an emergency situation not only are the alarm devices actuated but also the detectors may still be monitored. Typically, when the voltage is changed to an AC one, the part of this having a first polarity (eg the positive cycle) powers the detectors and the part having the reverse polarity (eg the negative cycle) powers the alarm devices.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 shows schematically an alarm system of the two wire type; Figure 2 shows a power supply/switching apparatus of one embodiment of the present invention; Figure 3 shows a power supply/switching arrangement of a second embodiment of the present invention; and Figure 4 shows a power supply/switching arrangement of a third embodiment of the present invention.

Referring to Figure 1, an alarm system comprises a control unit 1 mounted at a central location and a two wire power supply/signal line 2 extending from the control unit over a building or defined region to be monitored.

A plurality of detectors 3 and alarm devices (eg sounders) 4 are connected at selected positions along the wires 2. A sensing circuit 5 is included in the control unit and this is used to sense the outputs of the detectors and, when an alarm situation is sensed, to operate the desired one or more of the alarm devices 4. A plurality of such circuits may be connected to one control equipment.

The detectors and sounders each contain polarity setting devices, such as polarity diodes, which enables that device to respond and be powered by supply signals having a particular polarity. Thus, the detectors 3 may be arranged to utilize voltages having a positive polarity whilst the sounders 4 can be arranged to utilize voltages having a negative polarity. In a first mode, the apparatus is set in a detection mode and provides positive polarity signals around the circuit. If an alarm situation occurs, then the voltage on the wires alters, and this is sensed by sense circuit 5. This then switches the supply 1 so that it provides a signal of both polarities. This may be done by supplying an AC signal, which of course varies between positive and negative polarity, whereas, in the detection mode, only a DC signal, or only the positive cycle of the AC signal is supplied. The AC signal is applied to the circuit 2 so that the detectors 3 are responsive to the positive part as before and can continue to be monitored and the sounders 4 can respond to the negative part of the signal.

In systems with a plurality of circuits, the alarm may be detected on one circuit but signalled on all or more than one, with the other detection circuits in the 'not alarm' state. That is, with a 'normal' voltage on the positive half cycle and an alarm voltage on the negative half cycle.

Figure 2 shows one embodiment of a power supply circuit. The control unit 1 is provided with both an AC supply 6 and a DC supply 7. The DC supply may be derived from an AC (mains) input or may be separately generated.

One or other of the DC and AC supplies is applied to the circuit 2 by means of switch 8, which as shown takes the form of a relay but may be of any other type. The relay 8 is arranged to switch between the DC and AC sources when an alarm condition is sensed by sense circuit 5 The sensing circuit operates all the time when the DC supply 7 is switched on but only in the positive half cycle when the supply AC 6 is switched on. When the AC supply is switched in, no detection is carried out in the negative half cycle, during which time power is supplied to the polarised sounders 3 only.

Figure 3 shows an embodiment in which two DC supplies of respective reverse polarities 9 and 10 are provided. Two respective transistor switches 11, 12 are driven to switch between the two supplies 9 and 10 at a switching frequency which supplies enough power to operate the sounders and yet keeps the detectors powered and operating. Thus, in this embodiment, two DC supplies of alternate polarity are switched at such a frequency that, in essence, both the detectors and the sounders are substantially continually powered.

During a non-alarm state only one supply is used, to power the detectors only. When the sense circuit 5 detects an alarm situation or where the system is put into an alarm state, the switches begin to be operated, to switch alternately between the sources to generate a supply of alternating polarity.

Figure 4 shows an alternative embodiment in which only a single AC source 13 is used to supply power.

AC source 13 is supplied all the time but its output is normally restricted by a diode 14 which only allows the positive half cycle to be passed to the line 2 under nonalarm conditions. When the system is an alarm condition, a controlled rectifier 50 is switched on, which allows the negative part of the AC cycle 13 to also pass. Thus, under alarm conditions, AC is supplied to the detector line 2. Whilst under AC operation, detection is only done in the positive cycle and the sounders are powered during the negative cycle. The frequency is desirably set so that the detector sand sounders receive adequate power; as if they were both operated from separate DC sources.

Thus, in embodiments of the invention, a system may be provided in which detectors and alarm devices can be connected to one pair of wires and in which the voltage on that pair of wires can be sensed in the normal way.

During the sensing mode a DC voltage, or at least a voltage having a positive polarity, is applied to the circuit so that sensing can be achieved in a conventional manner. If a fire (or other alarm) signal is detected, then the DC voltage is changed to AC, or in more general terms the voltage having only positive polarity is changed to one which has elements of positive and negative polarity (ie alternating polarity) so that the positive cycle powers the detectors and the negative part of the cycle powers the alarm devices. Alternatively, of course, the system may be arranged so that the negative cycle powers the detectors and the positive part of the cycle powers the alarm devices.

Since presently existing standard detectors and alarm devices tend to be fitted with polarity and supply smoothing devices, which serve as polarity protection devices, then embodiments of the present invention may be used with currently available standard equipment.

If a particular zone or installation is in an alarm state then the detectors may still continue to be monitored.

Claims (14)

1. An alarm system comprising at least one detector and at least one alarm device connected by a common circuit to a control unit, the control unit being adapted to normally apply a voltage of one polarity to the circuit and, upon detection of an alarm condition, to apply a voltage of alternating polarity to the circuit.

2. An alarm system as claimed in Claim 1, wherein the one polarity voltage is a DC voltage and the alternating polarity voltage is an AC voltage.

3. An alarm system as claimed in Claim 1, wherein the one polarity voltage is a half cycle of the AC voltage.

4. An alarm system as claimed in any of the preceding claims and comprising a sense circuit for sensing an alarm or fault condition and a switch means for selecting the alternating polarity voltage upon sensing of the alarm or fault condition.

5. An alarm system as claimed in Claim 4, wherein the switch means is a relay.

6. An alarm system as claimed in Claim 4, comprising two DC sources of respective opposite polarity, and switch means arranged so that the two DC sources are alternately supplied to the circuit to provide the alternating polarity voltage.

7. An alarm system as claimed in Claim 6, wherein, during normal operation, only one of the sources supplies power to the circuit.

8. An alarm system as claimed Claim 1, 3 or 4, comprising a single AC voltage source and switch means for allowing only the positive or negative half cycles, or parts thereof, to be applied to the circuit during normal operation, and both half cycles to be applied when an alarm or fault is detected.

9. An alarm system as claimed in Claim 8, comprising a diode allowing one half cycle to pass, and a controlled rectifier switchable to allow the other half cycle to also pass.

10. An alarm system substantially as hereinbefore described with reference to and as illustrated in any of the accompanying drawings.

11. A method of powering an alarm system comprising at least one detector and at least one alarm device connected by a common circuit to a control unit, which method comprises normally applying a voltage of one polarity to the circuit and, upon detection of an alarm condition, applying a voltage of alternating polarity to the circuit.

12. A method as claimed in Claim 11, wherein the one polarity voltage is a DC voltage and the alternating polarity voltage is an AC voltage.

13. A method as claimed in Claim 11, wherein the one polarity voltage is formed by half cycles of the AC voltage.

14. A method of powering an alarm system substantially as hereinbefore described with reference to the accompanying drawings.