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FT 40AUTOMATIC ALARM SYS

AUTOMATIC FIRE DETECTORS(Summary Automatic Fire Detection)

OBJECTIVE

1. To explain the summary of automatic fire detection.

REFERENCE

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

CONTENTS

3. A summary is produced on the following pages providing a cays reference to the

suitability and main points of smoke, radiation and heat detectors. This is followed by a briefguide to detector selection and some of the principles contributing to effective detection.

4. The Summary.

Types and summary of main points Suitability

SMOKE

a. Ionization.

Sensitive in early stages of a fire when smokeparticles are small. Sensitivity tends to dropas particles to grow in size.

b. Optical.Most effective in situations where theprotected risk is likely to give rise to denseSmoke (i.e. large particles)

Areas having a controlled environment i.e.free from airborne dust etc. and generallyhousing complex of a high intrinsic valuee.g. computer installations.

Normally used as point detectors but havebeen develop to from zone samplingsystems by monitoring air samplesdrawn through tubes.

Types and summary of main points Suitability

RADIATION

a. Infra-redRapid detection because of almostinstantaneous transmission of radiation to thedetector head. This is dependent, however, onthe detector having a clear view of theprotected area.

b. Ultra-VioletAs for infra-red.

Warehouses of storage areas etc.Detectors are available which can scanlarge open areas and will respond only thedistinctive flame flicks. Can be used todetect certain chemical fires. The ultra-violet detector tends to be used mainly forspecialized purpose.

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Types and summary of main points Suitability

HEAT

a. Fusible AlloysAlloy will need replacing each time detectorsoperates.

b. Expansion of metal, air and generally selfresetting.

c. Electrical effect.

Not widely installed some specialist use.

May be used as point or line detectors and aredesigned to operates at a pre-selectedtemperature (fixed temp type) or on a rapid rise

in temp (rate-of-rise) or both. With all heatdetectors (particularly fixed temp. types)thermal lag needs to be considered whenchoosing the operating temperature.

Areas or general risk where Vaporparticles are normal present. So it isrelative low compared to other types ofdetectors.

Both Fix temp and rate-of-rise areequally efficient but: fix temp arepreferred in areas where a rapid rise intemp is a likely result of the normal workprocesses.

Rate-of-rise types will compensate for

gradual rises in ambient temperature andare more efficient than the fixed temptype in low temperature situations. (Asmention in the text) rate-of-rise detectorsgenerally incorporate a fixed temperaturedevice.

5. Detector Selection Type of Detector. It is clear from the earlier table thatalthough all available types of detector will in the main, detected fire, not all will be equallysensitive in very possible situation. In some case a combination of various detectors may berequired.

6. In general terms, smoke and heat detectors are suitable for meat buildings.Radiation detector is particularly useful for high-rooted buildings, e.g. warehouse, etc, andsituations in which clean burning flammable liquids are kept.

7. Little experience is available it the present time on laser/infra-red beam detectors butappear to have advantages there are tall compartments or long cable tunnels for example.

8. Such generalizations, however, need to be considered in conjunction with otherfactors the nature of the risk to be protected the process that are normally carried on andthe plan of the area to be protected. These considerations enable further decisions to bemade:

a. The reliability required from the detector.b. The sensitivity required from the detector.c. The location of detectors.

(1) Detector Reliability. Obviously a more robust detector is needed inan industrial setting than is required for hotel purpose. Dusty or dameatmospheres will affect some dot eaters more than others; (mention haspreviously been made of the advantage of aircrew switches in this type ofsituation. Micro-switches can be even reliable).

(2) Detector Sensitivity. This is an important consideration. It wouldobviously be undesirable to install a smoke detector set at high sensitivity in a

normally crowded hotel bar (or similar conditions).

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In this connection it is interesting to note that in research into falsealarms from automatic fire detector systems the greatest percentage of falsealarms (17.5 percent) were due to extraneous heat and smoke in the ambientconditions. To quote from the report* This could arise from carelessnesswithin the premises, from unforeseen circumstances of from insufficientattention being paid to the conditions by the designers and installers of thesystem.

(3) Detector Location. The fire Officers Committee rules and the BritishStandard Code of Practice 1019 lay down certain rules covering the spacingof detectors. These rules are for guidance and while the maximumrecommended spacing should never be exercised it is important that the rulesare thoughtfully applied in relation to the geography of the area to beprotected-having regard to its shape etc. The detectors should be so locatedthat they are in the best possible position to perform their function.

CONCLUSION

9. . If proper consideration is given to these various factor a more effective detectionsystem is likely to result. No hard and fast rules can be drawn up on detector selectionhowever. In the final analysis it needs to be based on experience and a careful assessmentof all the known factors pertaining to the protected area.