Flame Detection & Heat Detection

Flame Detection, Photoelectric cells detect the emission of infra‐red /ultra-­violet radiation.

Flammable Gas Detection, Flammable gas detectors measure the amount of flammable gas in the atmosphere and compare the value with a reference value. When this is exceeded the gas supply valve is closed an alarm is actuated.

Heat Detection, Heat detection can be used as a means of actuating a mechanical control or sprinkler system. The disadvantage of heat detection is that toxic smoke can be evolved from burning materials long before sufficient heat is generated to trigger the mechanism. These systems are rarely relied upon as a means of raising the alarm, however where fires raise little smoke, e.g. solvent fires, fixed temperature or rate of rise detectors can be used.

Fusible links are sensors operated by the melting of a metal (fusion detectors), e.g. used to open automatic smoke vents and close ventilation system dampers and shutters; or expansion of a solid, liquid or gas (thermal expansion detectors), e.g. to actuate sprinkler systems.

Figure: Thermal Expansion, Fusible Solder Link Sprinkler Head

Figure: Thermal Expansion, Quartzoid Bulb Sprinkler Head

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Fire Detection & Smoke Detection

Fire Detection
The earlier a fire can be detected, the more time there is for occupants to escape and the better are the chances of extinguishing or at least containing the fire. A wide range of devices exists for detecting either heat or smoke.

Smoke Detection
Smoke is usually the first detectable sign of any carbonaceous fire, therefore, smoke detectors are most frequently used unless there are problems with steam or dust, etc., which may trigger false alarms.

Ionising detectors operate when smoke enters the detector and it absorbs ions, reducing the flow of electrical current. These devices are very sensitive to smoke and may be triggered by cigarette smoke and dust, etc.

Light scatter detectors operate when smoke scatters a beam of light.

Obscuration detectors operate when smoke enters the detector preventing light from reaching a photoelectric cell.


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Common Fire Detection and Alarm System

MANUAL SYSTEMS:
Manual systems are suitable for small workplaces and involve the use of the following basic devices:

• Rotary Gongs – which are sounded by turning a handle around the rim of the gong.
• Hand Strikers – for example, iron triangles suspended from a wall accompanied by a metal bar which is used to strike the triangle manually.
• Hand bells.
• Whistles.
• Air-­horns.

These devices are normally located on the walls of corridors, entrance halls and staircase landings, etc., where they are readily available to anyone who may have to raise an alarm. While these types of manual systems provide for an alarm over a limited area, operation of one of them is rarely adequate to give a general alarm throughout the premises. Also, as a person is required to operate them, a continuous alarm cannot be guaranteed for as long as may be necessary.

In order to raise a more general alarm it is also possible to use facilities which may already be installed in a building for other purposes – for example, a telephone or public address system. With automatic telephone systems, arrangements can be made for a particular dialling code to be reserved for reporting a fire to a person responsible for calling the fire brigade and sounding the general alarm. Alternatively it can be arranged that use of the code automatically sounds the general alarm.

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Means of Fighting Fire

And also providing fire extinguishers for hot work, fire extinguishers should be located at determined fire points surrounding the site. The extinguishers should be appropriate to the size of the potential fire:

• wood, paper and cloth – water extinguisher;

• flammable liquids – dry powder or foam extinguisher;

• electrical – carbon dioxide (CO2) extinguisher

Nominated people should be trained in the way to use extinguishers.

If the building being worked in is occupied (eg an office, hotel or hospital), make sure the work does not interfere with the escape route from the building, or any type of fire splitting up, alarms, dry risers, or sprinkler systems. Check this with the building occupier or the Fire Atelier.

Fire doors should never be locked, left wide open or removed. Keep existing wet and dry risers ready for use and install any new ones as soon as possible.

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Precautions to prevent fires

The following precautions should be taken to prevent fires:

• use less-easily ignited and fewer flammable materials, eg use water-based or low-solvent glues and paint;
• keep the quantity of flammables at the workplace to a minimum;
• always keep and carry flammable liquids in suitable closed containers;
• have an extinguisher to hand when doing hot work such as welding or using a disc cutter that produces sparks;
• check the site at lunch time and at the end of the day to see that all plant and equipment that could cause a fire is turned off. Stop hot working an hour before people go home, as this will allow more time for smouldering fires to be identified; and
• provide closed metal containers to collect rubbish and remove them from the site regularly. Collect highly flammable waste such as solvent-soaked rags separately in closed fire-resisting containers.

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Ability to Survive an Electric Shock

The ability to survive an electric shock will depend on:

• The current is amperes: Anything above 25 mA(twenty five thousand of an amp) could be lethal.
• Length of contact time: (Speed of action of fuse or circuit breaker).
• The path through the body: The two most common paths are hand to foot and hand to hand.
• The conductivity / resistance of the body: This varies among individuals and circumstances e.g. sweating will increase conductivity of the skin.
• The voltage: The higher the voltage the easier it is for the current to overcome the resistance. Voltages below 50V and preferably below 25V are preferred.
• The conductivity of  the environment: A shock received in a wet area would probably be more severe than in a dry area.
• The nature of the contact: A small contact area will tend to resist flow whist full hand contact can lead to lock-­on which means the hand is unable to let go.
• Age and health status of victim.

In the event of some receiving an electric shock the following points should be observed:

• Do not touch injured person until the current is switched off.
• If the current cannot be switched off stand on some dry insulating material and use a wooden or plastic implement to remove the injured person from the electrical source.
• If breathing has stopped give mouth to mouth resuscitation.
• Ensure professional help has been called for.

Common causes of electric shock include:

• Work on electrical circuits by unqualified persons.
• Work on live circuits.
• Replacement of fuses and light bulbs on supposedly dead circuits.
• Working on de-­energised circuits that accidentally become re-­energized.
• Using electrical equipment in a wet Environment.
• Faults in electrical systems, which energize parts that are not normally conductors, for example, the casing of electrical equipment.

HEALTH EFFECTS

The various health effects of electric shock include: damage to nervous system; irregular heartbeat (fibrillation); internal burns; muscular contractions; physical trauma; stopping breathing (respiratory paralysis); stopping the heart (cardiac arrest); and surface burns to the skin.

Such health effects may be so severe that they ultimately result in death. They may also result in secondary effects such as being thrown off a ladder.

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Principles of an Alarm System

Sounding an alarm in the event of fire and fighting the fire arrangement must be made at all the workplaces.

There are many systems for raising an alarm on detecting an outbreak of fire, ranging from simple hand bells (or even just shouting) to sophisticated electronically-­triggered systems. However, whatever system is employed, all staff must know how to raise an alarm on discovering fire and what to do when the fire alarm sounds.

The general principles of an alarm system are shown below.

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Good Housekeeping - Control Measures to Minimise the Risk of Fire in a Workplace

The control of highly combustible materials and sources of ignition must be coupled with high standards of housekeeping to ensure that combustible materials in any form, but particularly waste, do not present a fire risk. This applies to all areas of work since waste materials can accumulate in any circumstances and a build-­up of dust and dirt can easily provide the fuel for fire in the right circumstances. 

The accumulation of process waste, contaminated rags, packing materials and other paper products, and general refuse and dust must be prevented Oily, flammable and combustible rags are particular problems in many workplaces as they are easily ignited and may even ignite spontaneously; they should be placed in metal containers with fitting lids.

Routine Housekeeping should ensure that:

• Waste bins are emptied regularly so that there is no accumulation of combustible materials.
• Cupboards, liftshafts, spaces beneath conveyors, stairs, benches and gratings are regularly cleaned and kept free of litter and rubbish.
• Safe disposal of all waste materials is arranged. “Unofficial” rubbish burning must be banned.

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