7/28/2019 Automatic Alarm System (Detector Circuit)

1/3

223

FT 37AUTOMATIC ALARM SYS

AUTOMATIC FIRE DETECTORS(Detector Circuits)

OBJECTIVE

1. To explain the fundamentals of detection, types of detectors, types of detectors theircircuits function and operation.

REFERENCE

2. a. Manual of Fireman ship Book 9.b. Fire College Notes.

CONTENTS

3. The function of the detector circuit in an automatic detection system is to transmit thesignal given by the activated detector head (or manual call point) to centrally situated controland indicating equipments from which the alarm is raised. In practice those processes occursimultaneously. Basically there are two types of detector circuit:

a. Open circuit.b. Closed circuit.

4. Open Circuit the theory. In an open circuit system detectors or call points arewired in parallel and can be regarded as switches in the off position i.e. there is no currentflow in the circuit when on standby. The operation of a detector effectively closes the

contacts and activates the alarm system.

Diagram of an Open Circuit System

5. As there is no current flow in the circuit when on standby it is not self monitoring as aresult however it does not consume as mush electricity as a closed circuit and is thereforerelatively cheaper to run. A short circuit in the detector wiring will rise on alarm, as it

effectively closes. A broken circuit, on the other hand will not end if unidentified could rendersome detector heads inoperative.

7/28/2019 Automatic Alarm System (Detector Circuit)

2/3

224

6. It is important to remember that in all except the simplest, system the detector andalarm circuits are separate meeting only in the alarm control unit.

7. Closed Circuit The Theory. In a closed circuit the detectors (or call point) canbe regarded as a series or switches whose contacts are normally closed when the system ison standby thus allowing current to flow in the detector circuit. This current energizes therelay which holds contact (1) against contact (3) and keeps the alarm circuit inoperative.

Once a detector (or call point) operates the detector circuit is broken interrupting current flowto the relay.

Diagram ofClosed Circuit System

8. The relay therefore de-energized releasing contact (1) which spring back to contact,(2), completing the alarm circuit and sounding the bells.

9. The main advantage of this type of circuit is that the continuous current flow onstandby makes it self monitoring. Any break in the circuit will cause the alarm to ring which,at though it may be false, does at least draw attention to the fault. The fact that the circuit isdrawing current from the supply on standby however can be regarded as a disadvantage the size and const of the battery and charger will be increased. In a basic closed circuitsystem a short circuit could remain unnoticed as it simply completes a separate path forcurrent flow. In doing this it could by-pass some detectors and call points rendering theminoperative.

10. Detector and Alarm Circuits The Practice. It is essential that detector and alarmcircuit are above all reliable. The diagrams in are basic illustration of open and closed

principles, is more sophisticated than this. Ring circuits and other refinements can be usedto achieve, as far as possible, a fail-safe situation and to overcome the disadvantagementioned earlier. Resistors and/or additional relays are incorporated to reduce false alarmsby providing for separate signaling of fault conditions (e.g. a broken circuit or a short circuit).

11. Closed circuits have the advantage mentioned earlier that they are continuouslyunder test i.e. current flows in the circuit on standby. If desired, however, the continuity of anopen circuit can be tested by incorporating an end-of-line resistor in the circuit.

7/28/2019 Automatic Alarm System (Detector Circuit)

3/3

225

An Open Circuit System with End-Of-Line Resistor

12. This allows a continuous but reduced current to flow through the detector circuit. The

continuity of the flow will be mentioned at the control unit. The resistor incorporated in thecircuit reduces the current sufficiently to prevent it activating the/alarms.

13. The more components there are introduced into a circuit or course; the more there isto fail. With this in mind the British Standard Code of Practice 1019 states. In the interestsof reliability it is desirable that the number of circuit elements in the systemshould be kept to a minimum ..Every additional component willinevitably Add some risk. Never the less with care taken to mini bike these a fullydiscriminating system can be well justified. In deed the Fire Officers Committee rulesrequire some element of discrimination (e.g. fault signaling or end-of-line testing) inapproved systems. Some detectors and manual call points can be used either open orclosed circuit systems.

14. Wiring and Power Supplies:

a. Wiring. It is essential for reliability that the wiring in automatic fire alarmsystems should be of high standard and be suitably protected against the possibilityof accidental damage. The degree of protection needed in an open circuit willgenerally be greater than that needed in a closed circuit in which any break in thewiring will automatically sound an alarm.

b. The thermostatic cable described can be used for wiring detector circuits itis mainly used with heat detectors.

c. Power Supplies. Obviously an adequate and reliable power supply must be

available to automatic detection systems.d. This can be a mains supply or a battery supply. In either case it is advisable(Fire Officers Committee rules require) that a standby supply must be au tomaticallyavailable in the event of a failure in the primary supply. The standby will normally bea battery maintained in a fully charge state. This supply can be automatically broughtinto operation by incorporating a changeover relay in the circuit.

e. The preferred operating voltage is 24 V d.c. A transformer will therefore benecessary to reduce the voltage to installations using a mains supply.