GB2237461A

GB2237461A - Monitoring faults in electric circuit arrangements

Info

Publication number: GB2237461A
Application number: GB8922685A
Authority: GB
Inventors: John George Hampson; David Brown
Original assignee: Scantronic Ltd
Current assignee: Cooper Security Ltd
Priority date: 1989-10-09
Filing date: 1989-10-09
Publication date: 1991-05-01
Anticipated expiration: 2009-10-09
Also published as: GB2237461B; GB8922685D0

Abstract

A method of monitoring faults in circuit arrangements consisting of a plurality of remote units connected to a central unit comprises sending signals to the central unit via at least one of the remote units. The remote units may be smoke detectors (SD1-SDn) connected in parallel between lines (10, 11) coupled to a battery (13) in central unit (12). Signals from a pulse generator (14) normally reset a counter (16) periodically. If one of the detectors is removed, a corresponding diode (D1-Dn) blocks the signals and a circuit (17) produces an alarm. A line discontinuity or short circuit changes the voltage across an end resistor (R1) producing another alarm. <IMAGE>

Description

Monitoring Faults in Electrical Circuit Arrangements The present invention relates to a method of checking faults in an electrical circuit arrangement and an electrical circuit arrangement provided with means for checking faults. The invention is particularly, but not exclusively, applicable to fire detection systems.

A typical fire detection system consists of a central unit and several interconnected remote units.

The remote units may comprise smoke detectors of various types, sprinklers, temperature sensors, manually operable alarms and so on. Usually the units in one "zone", eg: floor of a building, are interconnected to the central unit and the central unit monitors their operation. The central unit may monitor remote units in several zones. An important feature of such a system is that a check of the continuity of the wiring between remote units should be possible from the central unit.

Many conventional fire detection systems include several remote units connected in parallel across a pair of lines connected to a central unit including a d.c. power supply. In such systems continuity checking has been done by connecting a resistor across the lines at their ends remote from the panel. This causes a standing current to flow in the lines and any break in the lines will cause this current to cease and indicate a lack of continuity.

An alternative method of checking line continuity is disclosed in GB 2101784A. In this proposal a capacitor is connected across the lines at their ends remote from the panel. The applied d.c.

voltage across the lines is monitored. If all is well the voltage across the lines is maintained, or modified by the capacitor. If there is a break in the circuit the voltage across the lines will drop quickly each time the applied voltage is reduced.

It is desirable for the remaining units in a zone to still be operational if one of the units is removed, with removal of the unit being detectable from the central unit. This can be achieved using an end of line capacitor as described above, and providing a bypass diode in the positive supply line which is normally shorted by the presence of a unit. Thus, if the unit is removed the diode prevents current flowing from the end of the capacitor to the central unit when the applied voltage is reduced. This indicates that all is not well whilst still allowing power to be provided to other units and maintain the rest of the system operational. However, with such an arrangement it is not possible to distinguish between a fault due to a discontinuity in the line and the absence of a unit.

This problem has been overcome in the past by the use of so-called "addressable units", eg. smoke detectors provided with sophisticated electrical circuitry enabling them to be addressed and send back data indicating their current status. Also, smoke detectors have been manufactured which present a different resistance in the system depending on whether they are operational, in an alarm condition, or their operating parts have been removed.

It would be advantageous to provide a fire detection system which was capable of remote monitoring for line continuity and unit removal using conventional fire detection units without the aforementioned refinements.

The present invention provides an electrical circuit arrangement comprising a plurality-of remote units connected to a central unit; a signal generator arranged to send signals to the central unit via at least one remote unit; the central unit including means for detecting said signals. The signal generator may be arranged to transmit a signal to the central unit in response to an address signal, eg. from the address unit, so that faults in the circuit can be monitored at times determined from the central unit. However, in the preferred embodiment of the invention the signal generator periodically sends signals to the central unit without the need for an address signal.

The line resistance between the signal generator and the central unit will depend on how many remote units are present between the signal generator and the central unit. Therefore, the time taken for signals to travel from the remote unit to the central unit will depend on the number of remote units present.

Thus, the presence or absence of remote units could be determined by timing signals travelling from the signal generator to the central unit. However, in the presently preferred embodiment of the invention each remote unit is connected to a bypass diode which prevents pulses travelling from the signal generator to the central unit. With the remote unit in place, connections are provided to short out the diode so that signals can pass from the signal generator to the central unit. In this way, the absence of a smoke detector can simply be determined from the absence of signals from the signal generator.

The continuity of the wiring may be monitored using a conventional end of line resistor. Thus, a fault due to the absence of a remote unit can be distinguished from a fault due to a discontinuity of the wiring.

A conventional circuit arrangement can be readily modified to provide a circuit arrangement of the type described above. Thus, the present invention also provides a method of monitoring faults in an electrical circuit arrangement comprising a central unit connected to several remote units, the method comprising sending signals to the central unit via at least one of the remote units and detecting said signals at the central unit.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing which is a schematic block diagram of a fire detection system.

The illustrated fire detection system includes several remote units in the form of smoke detectors SD1....SDn connected in parallel between positive and negative lines 10 and 11 respectively. A central unit 12 remote from the smoke detectors includes a power supply shown as a battery 13.

A resistor R1 connects the end of the lines 10 and 11 remote from the control unit 12. Also connected across the remote ends of the lines, in parallel with the resistor R1, is a pulse generator 14.

Each smoke detector SD1....SDn is provided with two terminals for connection to the positive supply line 10 across a respective diode D1....Dn. The internal construction of the smoke detectors is conventional and will not be described further herein.

With a smoke detector in place, the smoke detector connection line short circuits the corresponding diode so that voltage pulses can be transmitted in both directions along the lines 10, 11. If a smoke detector is removed pulses can only pass one way along the line.

The pulse generator is arranged to periodically output pulses of a few volts onto the lines 10 and 11. With all' smoke detectors in place these pulses are detected by circuitry within the central unit 12 to be described below. If one of the smoke detectors is removed, power is supplied to smoke detectors downstream of it ie. on the pulse generator side via its corresponding diode. However, the diode will appear to the pulse generator 10 as an open circuit and therefore voltage pulses from the pulse generator will not reach the central unit 12. Thus, whilst the overall system is still operational by virtue of the diode corresponding to the removed detector, the absence of a detector can be ascertained at the central unit.

In this embodiment of the invention the pulse generator comprises a multivibrator which outputs low voltage pulses at a frequency of about 1 Hz. The pulses are supplied to a comparator Al in the central unit 12 which passes only pulses above a certain voltage level, to eliminate noise. The resulting pulses are then applied to the reset terminal of a counter 16. The counter 16 is continuously cycled by an oscillator 15 so that if it is not regularly reset it will overflow. In the event of an overflow a signal is applied to logic circuitry 17. The logic circuitry provides a signal indicating that a smoke detector has been removed. This may be used to provide a visual warning to an operator, such as lighting an LED.

The oscillator 15 is suitably operated at 2Hz and the counter 16 may be a four bit counter so that it overflows at a count of 16. Thus, in normal operation the counter 16 will be reset by a pulse from generator 14 each time it reaches a count of typically 4 or 5. As a result of this arrangement if a single pulse from the generator fails to reach the central unit and reset the counter, this will not result in overflow of the counter. The counter will not overflow until 3 to 5 pulses in succession have failed to reach it. This eliminates to some extent possible erroneous indication of the absence of a smoke detector.

In this embodiment of the invention a resistor at the end of the line is used to monitor discontinues in the line. A second resistor R2 provided in the central unit between the supply line 11 and the earth terminal of the battery 13. Resistors R1 and R2 form a voltage divider. Disregarding pulses from the generator 14 for the moment, under normal operation the voltage across the resistor R1 and hence resistor R2 will maintain a steady state value which will drop in the event of an open circuit or short circuit fault. The voltage across the resistor R2 is maintained by bias circuitry 18 within the central unit to detect such faults.

In practice, the voltage across R2 will vary due to the pulses from generator 14 and so the voltage is averaged by a further resistor signal to a second comparator A2. The component values are chosen such that normally the averaged voltage will maintain a steady level at about 2 volts. The comparator A2 provides a signal to the logic circuitry 17 when the applied voltage drops below approximately 1 volt. Again the logic circuitry may light a diode or provide some other visual indication of a circuit fault.

It will be appreciated that the functions of the central unit 12 could be incorporated in a single integrated circuit. Also, although the central unit has been illustrated as discrete hardware components, the signals it recites could equally well be analysed by a suitably programmed microprocessor.

Claims

CLAIMS:

1. An electrical circuit arrangement comprising a plurality of remote units connected to a central unit, and a signal generator arranged to send signals to the central unit via at least one of the remote units, the central unit including means for detecting said signals.

2. A circuit arrangement as claimed in claim 1 in which the signal generator is arranged to send signals in response to address signals from the central unit.

3. A circuit arrangement as claimed in claim 1 in which the signal generator is arranged to periodically send signals to the central unit.

4. A circuit arrangement as claimed in claim 3 in which the signal generator includes a multivibrator.

5. A circuit arrangement as claimed in any preceding claim in which the central unit is arranged to time the arrival of signals from the signal generator.

6. A circuit arrangement as claimed in any preceding claim in which said at least one remote unit is connected to a bypass diode arranged to prevent passage of signals from the signal generator to the central unit.

7. A circuit arrangement as claimed in any preceding claim further comprising a resistor arranged in the circuit such that any discontinuity in the circuit will cause a change in voltage across the resistor, and means for monitoring the voltage across the resistor.

8. A method of monitoring faults in an electrical circuit arrangement comprising a central unit connected to several remote units, the method comprising sending signals to the central unit via atleast one of the remote units and detecting said signals at-the central unit.

9. A method as claimed in claim 8 in which the signals are sent to the central unit in response to address signals from the central unit.

10. A method as claimed in claim 8 in which said signals are periodically sent to the central unit.

11. A method as claimed in claim 8, 9 or 10 comprising timing the reception of signals at the central unit which have been sent via a remote unit.

12. An electrical circuit arrangement substantially as hereinbefore described with reference to the accompanying drawing.

13. A method of monitoring faults in an electrical circuit substantially as hereinbefore described with reference to the accompanying drawing.