GB2307763A

GB2307763A - Fire alarm system

Info

Publication number: GB2307763A
Application number: GB9623659A
Authority: GB
Inventors: Fariz Khellaf
Original assignee: JSB Electrical PLC
Current assignee: JSB Electrical PLC
Priority date: 1995-11-28
Filing date: 1996-11-13
Publication date: 1997-06-04
Anticipated expiration: 2016-11-13
Also published as: GB2307763B; FR2741737A1; FR2741737B1; GB9623659D0

Abstract

A fire alarm system can determine whether a detection event on a zone circuit (14) occurred as a result of actuation of a manual call point (24) or an automatic detector (22). In normal operation, the zone circuit (14) is in a high-impedance state, current flowing only through an end-of-line unit (26). On actuation of a manual call point (24), the resistance is reduced to a first value (eg. 1200* capital Greek omega *) and on actuation of an automatic detector, the resistance is reduced to a second value (eg. 680* capital Greek omega *), the nature of the actuated device being thus determinable. Once a detection event has occurred, the system preferably does not respond to further changes in resistance.

Description

FIRE ALARM SYSTEM The present invention relates to fire alarm systems.

A typical fire alarm system comprises a control panel comprising electronic circuits which control the operation of the system as a whole. Connected to the control panel is at least one sounder circuit, to which audible, visual and other warning devices are connected, and at least one zone circuit to which detection devices such as smoke and heat detectors and manual call points are connected.

In such a conventional system, the control panel monitors the resistance of the zone circuit. Under normal circumstances, the resistance is a predetermined high value, the circuit being completed only by an endof-line unit which has a high resistance. When any one of the various detection devices connected to the zone circuit is actuated, the resistance of the circuit falls very significantly, this indicating to the control panel that a detection condition has occurred.

A disadvantage of such conventional systems is that it is not possible for the control panel to determine whether the alarm condition has arisen as a result of manual actuation of a call point or by automatic detection of potentially dangerous condition. A typical arrangement as described above is shown in Figure 1 of the drawings.

Where such differentiation is essential, a solution has been adopted as illustrated in Figure 2 of the accompanying drawings. In such an arrangement, two separate zone circuits are provided, one for automatic detectors and the other for manual call points. However, as will be apparent, this results in a substantial increase in the amount of wiring which must be provided within the system.

It is an aim of the present invention to provide a fire alarm system in which differentiation between manual and automatic detectors can be achieved without the need to provide multiple zone circuits.

According to a first aspect of the invention, there is provided a fire detection system comprising a control panel and a zone circuit, detection devices comprising at least a manual call point and an automatic detector being connected to the zone circuit, wherein on actuation, the manual call point presents a first resistance value on the zone circuit and the automatic detector presents a second resistance value on the zone circuit, the control panel having an input circuit operative to detect actuation of any detector and to determine whether a manual call point or an automatic detector has been actuated by the resistance value presented on the zone circuit.

Thus, the detection system of the present invention when used in a fire alarm system can determine whether an alarm condition was generated by a manual call point or an automatic detector.

Typically, a plurality of manual call points and of automatic detectors will be provided on the zone circuit.

In such cases, it is preferable that each of the manual call points presents the same first resistance value to the zone circuit and each of the automatic detectors presents the same second resistance on the zone circuit respectively when actuated.

It has been found to be advantageous to arrange for the first resistance value to be greater than the second resistance value, preferably by a factor of more than 150%. Often in a fire condition, a number of automatic detectors will be actuated in quick succession. This will cause the resistance of the zone circuit to drop in steps. This ensures that the automatic and manual detector can be distinguished, no matter how many automatic detectors are triggered.

In particularly preferred embodiments, provision may be made to allow variation of the fist and second resistance values to be varied in accordance with operating conditions of the system. Advantageously, such variation is caused to happen automatically by detection of the operating conditions of the system.

In a second of its aspects, the invention provides a fire alarm system incorporating a detection system according to the first aspect of the invention. In such an alarm system, at least one suitable indicator is typically provided operable to indicate to the user whether an alarm condition was occasioned by actuation of a manual call point or of an automatic detector.

In a third of its aspects, the invention provides a method of operation of an input circuit in a fire detection system for use in a fire alarm. The method provides that the input circuit detects the resistance on a zone circuit and, on detection of the resistance falling from a normal value to a first resistance value, indicates a fire condition determined by a manual call point; or on detection of a fall of the resistance value from the normal value to a second value indicates a fire condition determined by an automatic detector, detection of subsequent falls in resistance subsequent to either such detection even not causing the indication to change.

By this method, the input circuit is latched into indication of a particular condition which cannot be changed by subsequent events such as further detectors being actuated or by the zone circuit being damaged by fire.

The method preferably further provides for indication of a short circuit in the event that the resistance of the zone circuit should fall from a normal value to a short circuit value which is less than the first or second resistance values, without an intermediate step at either the first or second values.

In any system embodying the invention, the normal resistance of the zone circuit may be substantially opencircuit or may be of some predefined high value. An endof-line unit is typically provided which either is a passive resistance which is always present in the circuit, or is an active circuit which generates a signal detectable by the control panel.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 illustrate prior art arrangements, and have already been discussed; Figure 3 is a diagram of the fire alarm system embodying the present invention; Figure 4 shows an input circuit of a control panel of the system of Figure 3; Figure 5 is a diagram of resistance and voltage levels operative within the control panel of Figure 4; and Figure 6 is an illustration of changes which might occur in the levels of Figure 5 to accommodate changes in the system.

With reference to Figure 3, a fire alarm system embodying the invention comprises a control panel 10 which contains electronic circuits for controlling operation of the system, including an input circuit 12.

Connected to the panel 10 through the input circuit 12 is a zone circuit 14. Also connected to the panel 10, but not shown in Figure 3, is at least one sounder circuit to which or to each of which is connected one or more audible or visual warning devices, these being operable under the control of the panel 10. The panel 10 also contains indicators, in this embodiment in the form of individual lights 16,18, which are used to indicate the reason for occurrence of a fire condition.

The zone circuit 14 comprises a wire pair 20, which extends from terminals 30 of the input circuit 12. At intervals, automatic detectors 22 (for example, operative to detect heat or smoke) interconnect the wire pair 20.

Additionally, a plurality of manual call points 24 also interconnect the wire pair 20. The positions of the automatic detectors 22 and the call points 24 within the zone circuit 14 is entirely at the discretion of the system designer, the operation of the system does not place limitation on the order or position of these devices 22,24 on the zone circuit 14.

The zone circuit 14 is completed by an end of line unit 26 which interconnects the wire pair 20 at the end of the wire pair 20 remote from the panel 10. The end of line unit 26 may comprise a passive, known resistance or an active circuit for generating pulses. In either case, its presence on a complete circuit can be detected by the control panel.

At any one time, the net resistance Req appearing across the the'wire pair 20 of the zone circuit 14 is determined principally by whether any one or more of the devices 22,24 has been actuated. However, the particular value of Rtg is dependant upon many factors. For example, the number of devices on the zone circuit 14, and the particular type of detectors present can affect the magnitude of the current required to power the devices, and so will affect the apparent resistance Req of the circuit 14. Furthermore, if the end-of-line unit 26 fails, this may cause a change in the resistance of the zone circuit 14. However, none of these changes should cause the system to give a false indication.

In normal operation, the automatic detectors 22 and the manual call points 24 are open circuit. The end of line unit 26 has a high, known resistance. In this state, the control panel 10 applies to the zone circuit 14 a constant voltage which serves to power the detection devices 22,24 connected thereto. Also, this voltage causes a small current to flow in the zone circuit 14 through the end-of-line unit 26. By monitoring this current, the control panel 10 can ensure that the resistance of the circuit remains at a level which corresponds to the end-of-line unit 26 and can take appropriate action if the current rises sharply or ceases indicating, respectively, that a short circuit or open circuit failure has occurred in the zone circuit 14.

If any of the manual call points 24 is actuated, it operates to electrically interconnect the wire pair 20 of the zone circuit 14 with a first resistance, in this embodiment 1200Q. If, on the other hand, one of the automatic detectors senses a danger condition, it electrically interconnects the wire pair 20 with a second resistance value, in this case 680Q. Thus, it can be seen that it is a simple matter for the input circuit 12 of the control panel 10 to determine whether a detection event on the zone circuit 14 is due to a manual call point 24 or an automatic detector 22, simply by detecting the resistance of the zone circuit 14.The input circuit 12 can then generate a signal which is interpreted by other electronic circuits within the control panel 10 to indicate an alarm condition, activate warning devices as required, and indicate on one of the indicators 16,18 the nature of the detection device which has caused the alarm condition.

As will be appreciated, the input circuit 12 has to be responsive to other possible events occurring in the zone circuit 14. Firstly, the zone circuit may change from its normal state, in which current flows in the endof-line unit 26, to either an open circuit or short circuit state. Provided that this happens without an intermediate state corresponding to the first or second resistance levels, the input circuit 12 will then generate a signal to indicate that a fault has occurred.

An alternative condition which can arise is that multiple automatic detectors 22 on the zone circuit 14 are activated. This will cause successive stepwise reduction in the resistance of the zone circuit 14 seen by the input circuit 12. A similar condition can occur if multiple manual call points 24 or a combination of the two types of device 22,24 are actuated. It is important that the input circuit 12 reacts only to the first step change in resistance of the zone circuit 14 that it sees.

If this does not happen, a detection event which was initiated by a manual call point presenting 1200Q to the panel could then later be mistaken for an automatic detection event, or even a short circuit if further manual call points 24 were to be actuated causing the resistance in the zone circuit 14 to fall below 680n.

In practice, the first and second resistance values are each defined as ranges of resistance values, separated from one another. As illustrated in Figure 4, three ranges may be defined. A high quiescent range is defined at 46, a lower resistance range defining at 48 defining a first resistance value, and a yet lower resistance range 50 defining a second resistance value.

Each of the ranges 46,48,50 is separated from the next by a clear band 52.

In the present embodiment, the zone circuit 14 is monitored by the circuit as shown in Figure 4. A source 40 of voltage V1 is connected in series with a diode D1 and a resistor R1. The terminals 30 are connected to opposite ends of the series-connected component chain.

A voltage divider formed by two series-connected resistors R3,R4 is connected across the terminals 30. An output 42 of the voltage divider is derived from a common point of connection between the resistors R2,R3. The output 42 is fed to an analogue-to-digital converter, the output of which is processed by programmed logic circuitry within the controller.

The voltage at the output 42 is designated Va/d, the voltage across the terminals 30 is designated Vzone, the voltage of the source is Vi and the voltage across the diode D1 is Vd.

Where Res is the equivalent loading on R1.

The voltage fed to the a/d converter is:

where

However, R2 Req and R3 Req so Res - Req.

Hence, (1) becomes:

Internally, this analogue value is converted into an 8bit digital value Zcoun:

Where Vref is the reference voltage of the a/d converter.

From (2) and (3):

Equation 4 shows that the value of Zcount is dependant on Req, and so can be used as an input to the controller from which a value of Req can be derived.

A threshold value is fixed, this being the value achieved when only the end-of-line device is connected to the zone circuit 14 with no extra load from connected devices. However, the value of Req will change as detectors are added to the zone circuit, even when such detectors are all in a quiescent state. This means that the absolute values of the various resistance bands may change, although their separation will be maintained.

Such bands are shown at 46',48',50' in Fig 6, for four different configurations A,B,C,D.

Thus, in a preferred modification of the abovedescribed embodiment, a further value Zadi is defined, where Zadj = count + B. B is a value variable within an acceptable range to ensure that the value of Zad; is substantially constant despite changes in the value of Count in the quiescent state. Thus, when a change is made to the configuration of the system, the value of B can be recalculated in order to restore the value of Z adj to a predetermined, fixed value. In this way, the three resistance bands 46,48,50 continue to be valid, irrespective of the particular configuration of the system.

Claims

CLAIMS:

1. A fire detection system comprising a control panel and a zone circuit, detection devices comprising at least a manual call point and an automatic detector being connected to the zone circuit, wherein on actuation, the manual call point presents a first resistance value on the zone circuit and the automatic detector presents a second resistance value on the zone circuit, the control panel being having an input circuit operative to detect actuation of any detector and to determine whether a manual call point or an automatic detector has been actuated by the resistance value presented on the zone circuit.

2. A fire detection system according to claim 1 in which a plurality of manual call points are connected to the zone circuit.

3. A fire detection system according to claim 2 in which each of the manual call points on actuation presents substantially the first resistance on the zone circuit.

4. A fire detection system according to any preceding claim in which a plurality of automatic detectors are connected to the zone circuit.

5. A fire detection system according to claim 4 in which each of the automatic detectors, on actuation, present substantially the second resistance value on the zone circuit.

6. A fire detection system according to any preceding claim in which the first resistance value is greater than the second resistance value.

7. A fire detection system according to claim 6 in which the first resistance value is more than 150% of the second resistance value.

8. A fire detection system according to any preceding claim in which the first and second resistance values are each defined to lie within a respective first and second range of resistances, the ranges being separated from one another by a finite resistance increment.

9. A fire detection system according to claim 8 in which the first and second range or resistances are defined with reference to a threshold resistance value.

10. A fire detection system according to claim 8 in which the first and second range or resistances are defined with reference to a threshold resistance value to which an offset value is added, the offset value being adjusted in response to changes in configuration of the system.

11. A fire detection system according to any preceding claim in which a short circuit resistance value is defined, the system being operative to indicate that a short circuit has occurred in the zone circuit in the event that the resistance of the zone circuit changes from an initial high resistance value to a value not greater than the short circuit resistance value with there being an intermediate resistance state corresponding to either of the first or the second resistance values.

12. A fire detection system substantially as described herein with reference to Figures 3 to 6 of the drawings.

13. A fire alarm system comprising a fire detection system according to any preceding claim, and a sounder circuit, the control panel being operative to actuate the sounder circuit in response to a signal generated by the detection system to indicate that a manual call point or an automatic detector has been actuated, the panel having one or more indicators to inform an operator whether a manual call point or an automatic detector was actuated.

14. A method of operation of an input circuit in a fire detection system according to any one of claims 1 to 8 in which the input circuit detects the resistance falling from a normal value to the first resistance value indicates a fire condition determined by an automatic detector; and on detection of a fall of the resistance value from the normal value to the second value indicates a fire condition determined by a manual call point, detection of subsequent falls in resistance not causing the indication to change.

15. A method according to claim 14 in which on detection of a fall in resistance of the zone circuit from a normal value to a short circuit below both the first and the second resistance values, a short circuit condition is indicated.

16. A method according to claim 14 or claim 15 in which the normal value is calculated as a value of resistance when the system is in a quiescent state plus an offset, the offset being such that the normal value is substantially constant, irrespective of changes made to the configuration of the system.

17. A method of operating a fire alarm system substantially as herein described with reference to Figure 3 to 6 of the drawings.