GB2095880A

GB2095880A - Fire alarm circuits

Info

Publication number: GB2095880A
Application number: GB8109845A
Authority: GB
Original assignee: ELECTRONIC ALARMS Ltd
Current assignee: ELECTRONIC ALARMS Ltd
Priority date: 1981-03-30
Filing date: 1981-03-30
Publication date: 1982-10-06

Abstract

In a remote control fire detection system, sensor heads (4) are removably mounted in receptacles. When a sensor head (4) is removed from its receptacle, a switch (9) connects a resistor (8) to the remote control unit in place of the removed sensor head. Other sensor heads (4) continue their normal fire detection function and the control unit is able to detect the absence of the sensor head by detecting the small current drawn by the resistor (8) at the same time as being responsive to operation of a sensor head upon sensing a fire. <IMAGE>

Description

SPECIFICATION Monitoring circuits This invention relates to monitoring circuits comprising remote sensors, particularly but not exclusively fire detection systems.

Fire detection systems commonly have a multiplicity of sensor heads connected to a central monitoring and control unit. For ease of installation and maintenance, the heads are relatively easily removable, being for example mounted in plug, bayonet or screw sockets connected to permanent wiring. This has the disadvantage that the system can be partly or completely incapacitated by removal of the heads. This is an appreciable risk in, for example, hospitals and other institutions where disturbed or confused patients may remove the fire detection heads. It is therefore required that the system include means for indicating that a head has been removed.

Various means have been proposed for detecting the removal of a head. The commonest comprises a switch associated with the receptacle for head, so arranged that the permanent wiring is open-circuited briefly as the head is removed. As the wiring carries a standing current, the brief open circuit can be remotely sensed and used to give a fault indication.

This system has serious disadvantages. Detection of the necessarily brief open circuit is not particularly easy. False signals can easily be produced by noise induced in the wiring. The use of a switch connected in series in the wiring can cause a serious risk of unreliable operation. First, the switch may fail to give a sufficiently clear signal for detecting the removal of a head. Second, if the switch is faulty it can impair or even prevent operation of the fire detection circuit on the side of the switch remote from the power supply and control circuitry. Obviously, a prime requirement of any fire detection system is that it must remain fully operative at all times and the incorporation of switches in series in the circuit is therefore very undesirable because of the risk of accidental open-circuiting due to faulty switches.

According to the present invention, in a remote monitoring system with removable sensing elements, receptacles for such elements are provided with dummy loads for minicking the operation of a sensing element, and switching means responsive to removal of a sensing element for connecting the dummy load when the associated sensing element is removed. In such a system, the switching means which respond to removal of a sensing element are not connected in series with the rest of the system and therefore present no thread to the integrity of the system as a whole.

Preferably, connection of a dummy load does not have exactly the same effect as operation of the associated sensor, because it is desirable that the system should be able to distinguish between normal operation of a sensor, for example in a fire, and removal of the sensor, which should merely lead to a fault indication. For example, if the operation of the sensor normally leads to the flow of a specified current, removal of the sensor and connection of the dummy load should lead to a different current flow.

That is to say, the impedance of the dummy load should be different from that of the sensor when operated.

When connected, the dummy load draws current and this may reduce the line voltage in the system which could affectthe operation of remaining sensor heads. Preferably therefore the power supply and control unit, when it detects the current drawn by the dummy load, or the corresponding line voltage drop, not only produces a fault indication but also restores the line voltage and, if necessary, the sensitivity of the system, to its normal operating value so that subsequent operation of a sensorwill not be prejudiced. This can be achieved for example by suitable latching circuitry operated in response to the connection of a dummy load.The control and power supply unit is preferably adapted to perform similar operations if two or more sensor heads are removed, so as to maintain the normal operating ability of the remaining heads while providing a fault indication showing how many heads have been removed.

The dummy load can be a simple resistor, a Zener diodide, or any other suitable circuit element.

The accompanying drawing shows very schematically part of a fire detection system embodying the invention.

The illustrated system is a two-wire system with permanent conductors 1,2 connected at one end to a power supply and control unit (not shown) and at the other end to a terminating resistor 3. The control unit may for example include a series resistor, the voltage across which is monitored so as to monitor the current drawn by the detection circuitry. The latter is permanently energised and a steady current flows through the resistor 3 so that if the conductors 1,2 are broken, the resulting drop in current is detected and a fault indication is given.

The conductors 1, 2 are connected to a plurality of plug sockets or other receptacles which receive sensor heads 4, one of which is indicated by a dotted rectangle. In the illustrated circuit, the sensor heads includes a sensor element 5 which is connected between the conductors 1, 2 and normally draws only a small current. In parallel with it are a solid state switch for example a thyristor 6 in series with a resistor 7. The thyristor is normally non-conductive but its control electrode is connected to the sensor element 5. When the latter operates, it fires the thyristor and the resistor 7 carries current which is detected by the control unit which produces an alarm and may also operate further equipment for example a sprinkler system.The resistor 7 is smaller in magnitude that the resistor 3, therefore draws a much higher current so that the control unit can clearly identify the operation of a sensor head and provide an alarm.

The receptacle also contains, connected between the conductors 1 and 2, a resistor 8 in series with a normally closed switch 9. This switch is arranged so that it will be opened when the sensor head is inserted in the receptacle, and will close if the sensor head is removed. Consequently, if the head is removed the switch will close and connect the resistor 8 to the conductors 1, 2. The current drawn by the resistor 8 is detected by the control unit which produces a fault indication showing that a head has been removed. The magnitude of the resistor 8 is larger than that of the resistor 7 so that removal of a head produces a smaller current flow than operation of the sensor element, thus enabling the control unit to distinguish between the current due to head removal, requiring only a fault indication, and the current due to head operation, requiring an alarm.

The incorporation in the head receptacle of a switch and load in accordance with the invention can provide reliable and cheap warning of the removal of a head, without impairing the operation of the rest of the system. Shouid the switch fail so as to close while a head is still in place, the consequence will only be an incorrect fault indication. The worst possible failure would be for the switch to fail to respond to removal of a head. In either case, the rest of the system is not affected. Furthermore, since the response to removal of a head is not a brief signal but a steady current, the corresponding detection means that the control unit can be a simple current or voltage discriminator; noise in the conductors is no problem.

As already mentioned, depending on the nature of the control unit and power supply, the current drawn by the dummy load might affect the line voltage to some extent. Although this will in general not prevent operation of the rest of the system, nevertheless it may be desirable to design the control unit so as to restore the correct line voltage or current, and if necessary to adjust the sensitivity of the current or voltage discriminator in the control unit, after detecting removal of a head. Such operation and the maintenance of the necessary fault indication can be achieved by suitable latching circuitry, or a latching program in a micro processor. As numerous ways to achieve this will suggest themselves to the person skiiled in the art, the control and power supply unit has not been described in detail.

Claims

1. An electrical monitoring system comprising a plurality of sensor heads, a plurality of receptacles for the sensor heads, each sensor head being removably mounted to a respective one of said receptacles, and wiring permanently interconnecting said receptacles and connecting said receptacles to a common monitor control unit, said sensor heads and said receptacles having electrical contacts whereby said sensor heads are connected via the receptacles and said wiring to the monitor control unit, each receptacle having a switch responsive to removal of a sensor head from its receptacle to signal to the monitor control unit that the sensor head has been removed, each receptacle comprising an impedance, said switch being operative to connect said impedance to said wiring and hence to said monitor control unit in place of the removed sensor head, said impedance being such as firstly to enable said monitor control unit to distinguish between said impedance and the removed sensor head and secondly to enable the or each remaining sensor head to carry on its sensing function and to be monitored by the monitor control unit.

2. A receptacle for a sensor head for use in an electrical monitoring system, the receptacle comprising electrical terminals for connection to permanent wiring, electrical contacts for electrically connecting said terminals to the sensor head when mounted to the receptacle, the sensor head being removable from and mountable to the receptacle, an impedance, and a switch responsive to removal from and mounting to the receptacle of the sensor head to electrically connect said impedance to and disconnect said impedance from said terminals, respectively.