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FT 38AUTOMATIC ALARM SYS

AUTOMATIC FIRE DETECTORS(Detector Electrical Circuits)

OBJECTIVE

1. To explain the fundamentals of detection, types of detectors electrical circuitsfunction and operation.

REFERENCE

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

CONTENTS

3. Basic Electrical Circuits. In practical situations it will be normal to have a numberof detectors all connected to one or more indicator units. The basic electrical circuits usedwill depend upon whether the detectors are conducting or non-conducting in the non-alarmsstate. Simplified circuits for the two types are shown below.

a. Closed Circuits. Heads normally conducting are connected in series. Whena detector goes to the Alarm state the circuit is broken. This de-energies the relayand closes the alarm circuit contacts.

Series (Closed) Circuit

b. This has the general advantage that any circuit fault will operate the alarm. Itis therefore a self monitoring system. Because it is continuously drawing current itspower consumption tends to be high.

c. Open Circuit. Heads normally non-conducting and are connected inparallel. When a detector goes to the Alarm state a current flows in the detectorcircuit and energies the relay which then closes the alarm circuit.

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Open Circuit

d. Since no current flows until an alarm operates the power consumption is low.However the basic system is not self monitoring.

e. Most AFD circuits employ a combination of open a closed circuit elements to

secure the necessary monitoring together with low power consumption. Fig 2 showsa monitoring resistance connected in an open circuit system.

4. Indicator/Control Units (BS 3116 Part 4). The detectors may be used to operatesound, visual and external alarm system. It may also be necessary to have separatefault/alarm, of testing the system and resetting the system. These functions are performedby the Indicator/Control unit.

Control Unit

5. The control unit shown is connected to an open circuit detector system with amonitoring resistance. One an alarm/fault is indicated it will remain until the appropriatereset button is operated. The external alarm is signaled by reversing the polarity.

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Combined Indicator/Control Unit with Zone and Sector Indication

Fig 4 shows is in the form of a block diagram how control and indicating unitsare linked into the fire alarm system.

6. Fire Hazard Detection Situation. Strictly a fire detector can only operate after a firehas started. However it is better to consider all situations where appropriate detectionequipment may help prevent a fire starting or developing. Listed below are seven suchsituations for which automatic detection devices have developed.

a. Dangerous environment is developing but ignition has not yet taken place.

b. Ignition has taken place and the material is generating invisible products ofcombustion.

c. Visible smoke is being produced.

d. Visible flame is being produced.

e. Temperature is rising at a dangerous rate.

f. Temperature has reached a pre-determined danger figure.

g. Heat generated is giving raise to lifting an oscillating thermal air current.

7. Detectors appropriate to each situation are described in subsequent paragraphs.