Ch8 Fire Alarm System

CHAPTER(8)

Fire Alarm System

Chapter (8) Fire Alarm System

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Chapter (8)

Fire Alarm System

8.1. Introduction 8.1.1. Why we need fire alarm system? The goal of Fire Safety plans and equipment is to detect and announce fire,

heat or smoke in real time, so that people may reach safety and buildings

and information may be protected.

8.1.2. About the fire Generally, fires are initiated within a single fuel object. The smoke

produced from the burning objects transported by a smoke plume and

collects in the upper portion of space as layer. The smoke plume also

transports the heat produced by the fire into the smoke layer, causing the

smoke layer to increase in depth and temperature. This smoke layer

radiates energy back to unburned fuels in the space, causing increase in

temperature.

The main combustion products are:

1. Flame and heat

2. Smoke

Smoke considered the most dangerous and is responsible for death

Cases because it is mainly make oxygen insufficient in indoor fires.

8.1.3. Classes of fire

1. Class A fire

"Class Afire," means a fire involving ordinary combustible as paper,

wood, cloth, and some rubber and plastic materials.

2. Class B fire


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"Class B fire" means a fire involving flammable or combustible liquid

flammable gases, greases and similar materials, and some rubber

and plastic materials.

3. Class C fire

"Class C fire" means a fire involving energized electrical equipment

where safety to the employee requires the use of electrically

nonconductive extinguishing media.

4. Class D fire

"Class D fire" means a fire involving combustible metals such as

magnesium, titanium, zirconium, sodium, lithium and potassium.

8.2. Classification of fire alarm system:

8.2.1 Household fire alarm system Its tools and instruments that used in houses to give alarm signal for people

to leave the house in case of fire. In this case, we use a smoke detector, which

has alarm at the same time. This type operates by batteries and AC current

and mostly used in kitchens and children rooms.

8.2.2 Local fire alarm system (for local buildings) The tools and instruments give alarm in manual cases in order to alarm

people in this building to make the suitable requirements or leaving the

building.

8.2.3 Auxiliary fire alarm system It is the system that connecting buildings with fire fighting communication

center. Moreover, by control circuits we can know place of fire, its

happening time, and other information. This type widely used in America

and European countries


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8.3. Basic components of fire alarm system

1. Initiating devices

2. Control panel

3. Signaling devices

4. Power supply

These components must be compatible electrically with each other and

connected by suitable wires to complete system operation as shown in

figure (8.1);

Figure (8.1). ''Connection method of fire alarm system components''


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8.4. Types of connecting fire alarm system

components: 8.4.1. Wire connection

Class ''A'' type (loop connection); this mean that the wires loop

connecting control panel to other equipments return to control panel as

shown in figure(1).

Class ''B'' type (Zone connection); this mean that the wires loop

connecting control panel to other equipments don't return to control

panel, but ended by resistance called ''end line resistance''. The value of

this resistance do not exceed (1-kilo ohm) and changes due to as shown in

figure (8.1).

8.4.2. Wireless connection This type is the latest type of connecting fire alarm components without

using wires. It has some advantages and disadvantages such as other

connecting types. However, it has very low economic cost when compared

with wire connection type

In our project, we use wire connection classA type because it is

the most used and available type in wide range.

8.5. General components (parts) of fire

alarm system:

8.5.1. Initiating devices:

The initiating devices (initiators) have many types we will take about these

types.


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8.5.1.1. Automatic initiating detectors

fire alarm system generally depend on area we want to cover it with fire

alarm system grid .where each fire detector operate approximately in circle

has radius (8m) , hence we distribute detectors in required area . The

automatic detectors classified as follows:

First: Smoke detectors:

These detectors give alarm when it detects any smoke cutting its

photoelectric cell .and it has two types;

Types of Smoke Alarm Sensors:

There are two primary types of smoke alarm sensors:

Ionization sensors

Photoelectric sensors

Some alarms also include other types of safety sensors, such as heat sensors,

or carbon monoxide sensors

1. Ionization Sensors

Best at detecting fast, flaming fires like grease fires

Detect combustion particles of .01 to 3 microns (an average human

hair is about 90 microns)

Most sensitive to dark or black smoke

Sensitive to steam, so they may produce false alarms if installed near

kitchens or bathroom

Ionization Senor Illustration


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How Ionization Sensors Work?

Use a weak radiation source (Americium 241) to ionize the air in a

detector chamber

The ionized air conducts an electrical current

The detector circuit senses this current; if it is present, the alarm does

not sound

Smoke particles interfere with the current flow; when the current is

reduced, the alarm sounds

2- Photoelectric Sensors

Best at detecting slow, smoldering fires like furniture ignited by a

cigarette

Detect combustion particles of .3 to 10 microns

Most sensitive to light gray smoke

Not very sensitive to steam, so they are better for use near kitchens or

bathrooms

Higher power requirements than ionization

How Photoelectric Sensors Work?

An LED creates a beam of infrared light in the detector chamber

The detector circuit senses this light; if it is present, the alarm does

not sound

Figure (8.2). ''different shapes of smoke detectors


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Smoke particles scatter the light, and reduce the amount that reaches

the detector; when the amount of light is reduced, the alarm sounds

There is some types of smoke detectors according to kind of building

,some of this detectors are:

I. addressable smoke detector

This detector is the most type used in buildings, small and large

rooms, and passes. This type should not use near air soaker or fan

II. smoke addressable ducts detector

This type placed on return duct of central air conditioning in order to

detect a smoke communing from fire area. If it placed on supply duct,

it does not detect any smoke because all air communing from air

conditioner is fresh air .and it is shown in figure (8.3);

III.receive and transmitter laser beam detector

This type contains two detectors opposite to each other and third

apparatus analogy the alarm signal and transmitted it to control panel.

It used in long paths closed with glass (sky light way).

Smoke addressable air duct detector multi use detector

Figure (8.3) ''other shapes of smoke detectors''


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Second: Heat detectors

The detectors give alarm with increase of temperature rise above certain

limit. This happened by expansion of a material inside the detector

therefore, close the fire circuit and giving alarm. it has two types;

I.addressable fixed heat detector:

This type depends on fixed temperature. it do not operate unless

temperature exceed its setting value, so if there is a gradually rise in

temperature, it do not give alarm. This type is not secure 100%.

II.Addressable rate of rise heat:

This type depends on rate of heat change. Moreover, it is the best type

because it gives alarm with any up normal change in temperature.

We use the two types of heat detectors in our hotel project when we

designed fire alarm system

Third: multi use detectors These detectors operate as heat and smoke detectors in the same time. This

type used in active places that need the two types of detectors (heat and

smoke). One detector of this type showed in figure (8.3)

Fourth: gas detector These types of detectors operate in case of gas pipes. It used on pipes in

order to detect any Infiltration in gas

8.5.1.2. Manual initiating detectors;

These devices operate manually to make fire alarm when any person see

afire or feel it, or when automatic detectors have any fault or not operate.

From these devices, manual pull station (break glass) and they have many

types and shapes as shown in figure (8.4);


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Figure (8.4)''two types of break glass manual detectors ''

This type operates by giving alarm signal the glass on it is broken by any

one. It has many types such as firefighter telephone.

8.6. Control panels of fire alarm system: It is the device that analysis the signals coming from initiating detectors and

give instructions of (alarms not alarm)to signaling devices. Some of control

panels have displays from it we can know all information about date and

time of fire and incorrect alarms. Some shapes of control panels shown in

figure (8.5)

There are many methods of connecting control panel with other devices.

The following figures (6, 7, and 8) show how to connect fire alarm system

components;

Figure (8.5)some shapes of control panels


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8.7. Fire Alarm system Power Sources: AC power (hard wired) linked to normal AC wiring system

Most have a battery backup in case AC power fails

Batteries

9-volt carbon zinc (general purpose)

9-volt alkaline

9-volt lithium (ten year battery)

Large rechargeable lead-acid or gel cells

AC Power

Most dependable (at least if backup batteries are maintained

properly)

Cost-competitive with battery power for new construction but

expensive to retrofit in older buildings

Normally used as the primary power source for centrally controlled

alarm systems


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Battery (DC) Power

Fairly dependable if batteries are checked and replaced consistently

Inexpensive and easy to install, even in older buildings

Often used as the primary power source for self-contained

independent alarms

Limited power for horns and auxiliary function

8.8. Fire alarm system Maintenance: Fire alarm system requires regular maintenance, which includes:

Maintenance of Power Supply

Cleaning of Sensor and Air Passages

Regular Testing

Replacement of outdated Sensors or Alarms

8.8.1. Power Supply Maintenance:

For AC powered alarms:

Check AC power supply monthly (or more often)

Replace backup batteries as recommended by the alarm

manufacturer

For battery powered alarms:

Test alarm weekly (or as directed by manufacturer)

Replace batteries:

Every six months for general purpose or alkaline batteries

When alarm signals low battery or fails test for lithium batteries

8.8.2. Fire Alarm system Testing:

AC powered alarms should be tested monthly, or more often if the

manufacturer or codes require

Battery powered alarms should be tested weekly, unless the

manufacturer recommends otherwise

One of the most common reasons for failed smoke alarms is a lack of

regular testing

8.8.3. Replacing Outdated Alarms:


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The recommended service life for most smoke alarms is ten years

After that point, electronic failure becomes likely

If an alarm system has separate sensors, the sensors and other

manufacturer-recommended components should be replaced

If the alarm is self-contained, the entire alarm should be replaced

8.8.8. Nuisance Alarms:

Nuisance alarms occur when the alarm sounds without a fire being

present

Often called false alarms, but in most cases theyre not the alarm

does detect something

Usually caused by exposing the alarm to smoke, combustion products

or steam

From tobacco smokers

From wood-burning stoves or fireplaces

From kitchens and bathrooms

8.8.6. Reducing Nuisance Alarms:

Locate alarms and sensors away from areas where they will be

exposed to smoke, other combustion products or steam

Clean the alarm regularly

Maintain the alarm power supply (low power can sometimes trigger

a true false alarm)

Avoid activities that trigger the alarm

8.8.7. Other Problems with fire alarm system:

Lack of Power

Usually due to failure to test alarm and replace battery as needed

Electronic failure

Rare, but it happens testing is important!

Deliberately disabled alarms

Usually due to nuisance alarms, but may also be done to get alarm

battery


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Type of Alarm Selection:

If you want to use an alarm with a long life lithium battery, you will

have to use an alarm with an ionization sensor. because Photoelectric

alarms require more power and are not currently offered with lithium

batteries

If the power source is not a limitation, use the type of alarm most

suitable for the site

Photoelectric alarms work best in or near kitchens, bathrooms,

laundry rooms and other areas where steam may be present

Both sensor types work well in other areas

Use hardwired (AC powered) linked alarms in new construction

8.9. How to select the suitable alarm system? If you want to use an alarm with a long life lithium battery, you will

have to use an alarm with an ionization sensor

Photoelectric alarms require more power and are not currently

offered with lithium batteries

If the power source is not a limitation, use the type of alarm most

suitable for the site

Photoelectric alarms work best in or near kitchens, bathrooms,

laundry rooms and other areas where steam may be present

Both sensor types work well in other areas

Use hardwired (AC powered) linked alarms in new construction

8.10. Alarm Location: Installing smoke alarms in proper locations is important

Alarms that are installed in the wrong location may:

Not provide adequate warning of fire or smoke

Fail prematurely due to heat, cold, etc.

Produce nuisance alarms

You may need to install alarms inside a bedroom in special

circumstances, such as where a resident smokes in bed


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Install at least one alarm on each occupied level of the house

including the basement, if it is regularly occupied

Do not install smoke alarms

In or near kitchens, bathrooms or laundry rooms, if the alarm has an

ionization sensor

In attics, cellars or other areas that become very hot, cold or dusty

On un-insulated exterior walls or ceilings that are not insulated from

the roof (the alarm will get too hot and/or cold)

Do not install smoke alarms

In the dead air zones that occur within two feet of any corner of a

room or hallway

8.11. An Example showing distribution of

fire detectors on basement floor;


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Ch8 Fire Alarm System