AU437726B2 - Fire detecting system - Google Patents

Description

~This inetin This invention el 1 7 /72 rel to a fire detecting system,

especially to a fire detecting system provided with improv

disconnection detecting means.

A conventional fire detecting system comprises a sing

central unit and a plurality of fire detectors connected i parallel between a pair of conductors extending from the

central unit. The each detector includes a normally-opene

switch which is closed by heat or smoke to short-circuit

the both conductors, and the central unit includes a power supply for supplying an operation voltage between the both

conductors and an alarm device for sensing increase of cur rent flowing through the conductors due to closure of the normally-opened switch in any of the detectors to produce an alarm.

When either of the conductors is disconnected or broke

off at some position by an accident in such fire detecting

system, the detectors disposed remote from this disconn-

ected point become ineffective but such disconnection of tI conductor is difficult to determine unless a special test-

ing means is used. As a way of finding disconnection of

the conductor at the central unit, a method of providing a bridge circuit in the central unit and measuring a change c resistance between the output terminals and a method of providing a special signal source at the ends of the con- ductors and receiving the signal at the central unit has

been proposed. However, such methods have required very co

plicated and costly circuit arrangements.

Therefore, an object of this invention is to propose

a fire detecting system provided with simple disconnection

detecting means whereby any disconnection of the conductors

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3 8,01 172 is automatically detected and indicated at the central

unit.

The invention provides a fire detecting system

comprising a voltage source, a pair of conductors

source, extending from the both terminals of said voltage parallel a plurality of fire detectors connected in

between said pair of conductors, each detector being pro-

vided with switch means for short-circuiting said pair

of conductors upon actuation, an alarm device, means said of sensing increase in the current flowing through

conductors and actuating said.alarm device, a first

Zgner diode connected between the other ends of said' device pair of conductors and a disconnection detecting

for sensing increase in the voltage between said both

conductors, characterized in that said disconnection

detecting device includes a disconnection alarm, a second

Zener diode connected between said pair of conductors

and means of sensing increase in the current flowing

through said second Zener diode and actuating said

disconnection alarm, the Zener voltage of said second

Zener diode being selected to be higher than the Zener

voltage of said first Zener diode but lower than the

terminal voltage of said voltage source.

Other features and operation of the system of this

invention will be described in more detail hereinunder

with reference to the accompanying drawings.

In the drawings:

Figure 1 is a block diagram representing an example

of a fire detecting system embodying the disconnection

monitor 13, of the present invention.

3 ,01 7 /In an example Figure 2 is a circuit diagram representing

of the disconnection monitor 13 embodying the present

invention. numerals Throughout the drawings, like reference

are given to like components. includes Referring first to Figure 1, the system detectors 2 a central unit 1 and a plurality of fire

as in the prior art. Each of the detectors 2 is symbolized a as a normally opened switch since it has generally to detection switching element which is closed in response

of heat or smoke to short-circuit a pair of conductors

terminals 3 and 4 and 6 which extended from a pair of

of the central unit 1. Between the other ends of the

conductors 3 and 4,

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3 3,017 172 inserted is a Zener diode 7. a rei-din tL The central unit 1 includes a power supply 8 and a resistor 9. connected in series between the terminals 3 and 4, a voltage detector 10 which senses a voltage rise across the resistor 9 to energize a relay element included therein, and a normally-opened switch which is to be actuated by the relay element of the voltage detector 10 and an alarm gener- ator 12 such as bell or buzzer connected in series across the power supply 8. The central unit 1 further includes a

disconnection monitor 13 connected in series with a diode 14 between the both terminals 3 and 4.

3 8,017/72 A circuit configuration of the disconnection monitoi

embodying the present invention, is shown in a chained b:

in Figure 2. A Zener diode 21 and a resistor 22 are conr in series between the diode 14 and the conductor 6. A S( whose control electrode is connected to the connection pc

of the Zener diode 21 and the resistor 22 is connected ir

series in a closed circuit with a known alarm circuit 24

and a power supply 25 and the cathode electrode of the S(

is connected to the conductor 6, The Zener voltage of tl

Zener diode 21 is previously selected to be higher than I

of the Zener diode 7 but lower than the source voltage.

Therefore, in the normal monitoring condition, the Zener

diode 7 is in ON-condition but the Zener diode 21 is in

condition, and no current flows in the disconnection monj

13. When either the conductor 5 or 6 is broken or cut,

however, the Zener diode 7 is removed from the circuit ar

therefore, the voltage between the conductors 5 and 6 is

suddenly raised up to about the source voltage. Consequf

the Zener diode 21 conducts to raise the anode voltage tI

of, and the SCR 23 is thus driven into conduction, therel

the alarm circuit 24 is actuated to give an alarm.

3 8,017/72 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A fire detecting system comprising a voltage source, a pair of conductors extending from the both terminals of said voltage source, a plurality of fire detectors connected in parallel between said pair of conductors, each detector being provided with switch means for short- circuiting said pair of conductors upon actuation, an alarm device, means of sensing increase in the current flowing through said conductors and actuating said alarm device, a first Zener diode connected between the other ends of said pair of conductors and a disconnection detecting device for sensing increase in the voltage between said both conductors, characterized in that said disconnection,detecting device includes' a'disconnection alarm, a second Zener diode connected between said pair of conductors and means of sensing increase in the current flowing through said second Zener diode and actuating said disconnection alarm, the Zener voltage of said second Zener diode being selected to be higher than the Zener voltage of said first Zener diode but lower than the terminal voltage of said voltage source.

2. A fire detecting system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

DATED THIS 28th DAY OF MAY, 1973, NITTAN COMPANY LIMITED By its Patent Attorneys

CLEMENT HACK CO. Fellows Institute of Patent Attorneys of Australia.

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