Fire Alarm Systems Always Save Our life's..

 Dear All,

This article about  Fire Alarm Systems is an extract from this training module for developing your carrier in Construction Field...

Rarely in the movies do you ever see a Fire Indicator Panel (FIP), more often than not you will see an American style 'pull station' - the equivalent of a Manual Call Point or Break Glass Alarm found in India.  A Fire Indicator Panel (FIP) is probably best described as the 'brains' of a fire detection and alarm system. A fire indicator panel comprises control and indicating equipment (c.i.e) that combined together form an integrated system. These core components comprise;

1. Cabinet
2. Primary Power Supply Unit & Battery Charger
3. Secondary Power (Batteries)
4. Control Electronics
5. Visual Interface
6. Input Interface & Control
7. Input Termination & Monitoring
8. Output Termination & Monitoring

The first electric fire alarm and detection system was patented in 1890 by Francis Robbins Upton . although the significant advances were not available until the mid to late 1960?s. Today there are essentially two types of fire indicator panel; Collective (commonly known as conventional) and Addressable (sometimes known as Analogue Addressable or Intelligent).

Principle of Operation

The primary purpose of a fire indicator panel is to monitor each circuit, zone or point for any condition (alarm signal or other abnormal condition); display the status of that condition and to operate any required output or outputs according to the approved design of the system. These outputs are typically for the purpose of warning occupants on a fire alarm signal, notify the fire brigade, control the spread of heat, smoke or fire; or used for a wide variety of other purposes.

Before I go any further its important to define a couple of terms used in this article, to provide both context and and explanation of their meaning;

Circuit..

A circuit comprises an unbroken path (usually a pair of wires) along which an electric current exists or is intended or able to flow. In fire alarm terms this generally means the wiring connecting one or more detectors to the fire panel.

Zone..

A zone is a group of one or more detectors, grouped by their common location or in the case of a collective circuit all of the detectors on that circuit. Another way to describe a zone is one of the following;

1. One or more detector on a single collective circuit or;
2. One or more detectors, in a common location, defined in software on an addressable circuit.

Standards also limit the number of detectors a zone may comprise.

Types of systems..

As previously discussed, there are generally two types of fire alarm systems, Collective and Addressable;

Collective

On very simple terms, a collective fire alarm system is one that comprises one or more circuits, with detectors connected in parallel. An alarm signal is received at the fire panel when the current on a circuit increases due to the alarm operation of one of the detectors on that circuit. In modern detectors there is may also be fitted an alarm status indication, often a light emitting diode or LED.

One of the limitations of a collective systems is that the control panel is unable to identify the status of each detector of the circuit. Another way to describe this is that the control panel is only able to display the alarm or fault status of a circuit, not each detector on the circuit.

Addressable

The alternative is an addressable fire alarm system that comprises one or more circuits with detectors connected in parallel, andeach detector has a unique identification (address) on the circuit. When the conditions for an alarm signal are satisfied at the detector, an alarm signal is transmitted via the circuit to the fire indicator panel.

In an addressable system, each detector has the ability to identify itself and its current status. Over the years, the level of sophistication of detectors and systems has greatly increased. This includes collecting more information about the conditions surrounding each detector including the many bi-products of combustion and other environmental factors.

There are many advantages of addressable systems including the ability to more effectively handle contamination over time that leads to nuisance (false) alarms, improved detection of fires and the ability to identify the precise location of a detector and its current condition or state.

While we have described the two types of systems there exists variations to these basic types including fire panels that include both collective and addressable features. Generally these systems are addressable systems with additional c.i.e to provide collective functionality.

Sounders or fire alarm devices..

Sounders or fire alarm devices are the fire detection and alarm system’s means for alerting the occupants of a building to potential or imminent danger from fire.  Sounders vary in size and shape and the sounder output varies in level, frequency range and temporal pattern, but all must comply with the requirements of the same product standard.

There are two types of sounder – outdoor or indoor use. The main difference between the two being the level of protection against the ingress of water or foreign bodies by the use of rubber seals, exterior housing design and the severity level of environmental tests applied. The majority of sounders are designed for connection to fire alarm system control panels and are usually rated to operate from a 24V supply.

There is however variation across different fire alarm systems and fire alarm equipment manufacturers.

Manual Call Points Despite advanced technology, still the most reliable form of fire detection is human observation. For this reason fire detection and alarm systems always include the Manual Call Point (MCP), which allows a building occupant to raise the alarm to evacuate the building.

Whether addressable or conventional the basic principle of operating the MCP, is the breaking of a glass element and pressing a button. The Control and Indicating Equipment (CIE) interprets the signal from the button as a fire alarm signal and enters fire alarm mode. Due to the reliability of MCP use, the signal from it includes an interrupt signal, which means the CIE treats it as a priority signal and temporarily suspends all other activities.

Voice AlarmsPeople often tend to react quicker and more appropriately to clear instruction than the sound of bells or sirens. Voice Alarms incorporated into a fire detection system can relay clear and

precise speech messages, helping to minimise uncertainty and confusion in an emergency situation.

The third installment of the Principle of Fire Safety series looks at heat detectors, one of the four methods of detecting fire through the the by-products of combustion. This is an area of fire safety that has been rapidly evolving over the last 20 years.

Research and development has improved well established detection technologies and provided an array of new technologies to improve fire detection while also being less susceptible to the causes of false alarms.

Before we go any further it's important to lay the foundation for what is fire; fire also known as combustion is a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the by-products of combustion being; heat, smoke & electromagnetic radiation (light). Personally I think a illustration explains this chemical reaction in terms easier to understand.

It is also important to recognize that smoke is an aerosol or a mixture of particulates suspended in air that comprises a collection of airborne solids, liquid particulates and gases emitted when a material undergoes combustion.

This is important because each of the four detection technologies are designed to respond to one of the three by-products of combustion. There are four principal methods for detecting fire explored in this article including; Heat, Smoke, Flame and Combustion detectors.

Heat Detectors..

Heat detectors, detect changes in ambient temperature, and send an alarm state to the CIE. Rate of rise detectors which measure the speed with which the air temperature rises and fixed temperature detectors which react when a set temperature is reached.

There are two methods for detecting fire from the presence of heat;

1. Fixed temperature heat detectors operate when the ambient temperature increases sufficiently to predetermined level where the heat detector will operate; or
2. A rate-of-rise heat detector operates when the ambient temperature increases over time equal to or greater than the rate of change the detector was manufactured to operate.

Over the years there has been continuous development in the technology to detect heat. These technologies can be broken down into four main types.

Electromechanical..

Electromechanical - As the name suggests electromechanical heat detectors operate due to a combination of mechanical movement creating an electrical circuit. # There are four fundamental types of electromechanical fixed-temperature heat detector;

1. Not used in modern fire safety systems, the thermostat comprises a bi-metal strip, with one end fixed into position and the other end free to move depending to the change in its temperature. The bi-metal strip also forms one part of an electrical circuit. When the temperature increases the bi-metal strip completes an electrical circuit to actuate an alarm.
2. The second, and most common type of fixed temperature heat detector is a fusible link comprising a eutectic alloy. This type of heat detector was the most common heat detector found between the years 1970 and 2000. An eutectic alloy is mixture of two or more metals whose melting point at a lower temperature than the individual metal. When the ambient temperature increases to the eutectic temperature, the alloy changes state from a solid to a liquid, like solder. This enables a spring held under pressure to release and make an electrical circuit to actuate an alarm.
3. The third type may be found in very old fire systems and includes a length of heat sensitive wire strung between two points. This type of heat detector is a very old method of heat detection and is no longer used. The detector comprises a length of heat sensitive wire with one point fixed into position, and the second point draped over a pulley wheel. At the end of the wire is a weight to maintain the tension on the wire. As the ambient temperature increases the wire expands and the weight drops down. This system is calibrated so that at a predetermined fixed temperature the weight drops to a point where it can create an electrical circuit and actuate an alarm.
4. The fourth type of electromechanical fixed-temperature heat detector is a distributed heat detector (also known as a line-type heat detector) that comprises a twisted pair of electrical conductors separated from each other by a heat sensitive insulator, and enclosed in an protective sheath. When exposed to heat the insulator changes physical state from a hard solid to a molten state enabling the twisted conductors to create an electrical circuit to actuate an alarm.

Optomechanical..

Optomechanical - This type of heat detector is a modern variation of the electromechanical line-type heat detector and could be classified as an optomechanical distributed heat. Optomechanical heat detectors contain one or more fibre optic cables separated by a heat sensitive insulator and protected by an outer sheath. A focused light signal is passed through the fibre optic cable. When exposed to heat, the heat sensitive insulator changes state from a solid to a molten state which has the effect of degradation or discontinuation of the focused light signal. The signal change is monitored by a device which then would actuate an alarm.

Electropneumatic..

Electropneumatic - The principal for an electropneumatic rate-of-rise heat detector was first patented in 1941 and has been subject to ongoing product development since that time. Electropneumatic heat detectors comprise a controlled vented chamber containing a diaphragm that moves due to a pressure differential according to the rate of change of the ambient temperature. When the ambient temperature changes faster than the calibrated rate which the vent has been designed to release, the diaphragm moves sufficiently to create an electrical circuit to indicate an alarm. The main benefit of electropneumatic heat detectors is that they operate at a range of temperatures because they respond to the rate of change in temperature, not at a fixed temperature only.

Electronic (Thermistor)..

Electronic (Thermistor) - The most common form of heat detector used throughout Australia is electronic device with a thermistor acting as the heat sensitive element. A thermistor is a type of resistor whose resistance changes significantly according to temperature. These detectors can operate as a fixed-temperature and a rate-of-rise device or both depending on their intended design. These detectors may also include features to reduce the possibility of deceptive phenomena causing false alarms.

There are five classifications of heat detector established for use, they are classified according to their fixed temperature operation or rate-of-rise (if fitted).

Type A (white dot)

normal temperature duty, incorporating both fixed-temperature (58ºC - 88ºC) and a rate-of-rise actuation

Type B (blue dot)

normal temperature duty, incorporating fixed temperature (58ºC - 88ºC) actuation only.

Type C (green dot)

High temperature duty, incorporating both fixed-temperature (88ºC - 132ºC) and a rate-of-rise actuation.

Type D (red dot)

High temperature duty, incorporating fixed temperature (88ºC - 132ºC) actuation only.

Type E (yellow dot)

Special purpose fixed temperature. This type of heat detector is selected when the purpose of protection cannot be satisfied by types A to D

In the near future, these classification Types will be replaced by the ISO standard (AS 7240.5) heat detectors with a wider range of classes.

While heat detectors are a very reliable form of fire detection, they are not normally recommended as a life safety device. Heat detectors are often selected to detect the presence of fire where there is the potential for deceptive phenomena from one of the other by-products of combustion.

What is smoke?

Smoke is the collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion, mixed with the quantity of air that is mixed into the particulate mass.

Smoke particulates are generally grouped in three particle sizes. Depending on particle size, smoke can be visible or invisible to the naked eye.

Smoke Detectors..
Smoke Detectors or sensors, are sensitive to smoke in the surrounding atmosphere. Photoelectric/optical smoke sensors use a Light Emitting Diode (LED) and a photodiode sensing element. Smoke particles entering the chamber cause the LED light to scatter, thus triggering an alarm status.  Ionization smoke detectors use an ionization chamber. Smoke particles entering the chamber attach themselves to the ions, impeding the generated current flow and causing the alarm to trigger.

The five types of smoke detector...

1. Ionisation smoke detector...

An Ionization smoke detector consists of the following main components;

• a radioactive source, usually Americium-241 that emits alpha particles
• a ionization chamber, an air-filled space between two electrodes

The alpha particles pass through the ionization chamber, and permit a small, constant electric current between the electrodes.

Smoke particles that enter the chamber absorbs the alpha particles, which interrupts the electric current, activating an alarm.

2. Photo-electric smoke detector...

An photo-electric smoke detector is a scattered light sensor or nephelometer, that consists of the following main components;

• a light source
• a lens to focus light into a projected beam
• a sensor at an angle to the beam as a light sensor

In the absence of smoke, light passes in front of the sensor in a straight line.

When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm.

3. Projected beam smoke detector...

An projected beam optical smoke detector operates on the principle of light obscuration and comprises;

• A projected beam transmitter & lens
• A light receiver
• A light reflector (not in all cases)

The light transmitter emits an invisible beam of light that is received by a receiver in a normal condition.

The receiver is calibrated to a preset sensitivity level based on a percentage of total obscuration. When smoke obscures the beam, an alarm signal is activated.

4. Aspirating smoke detector...

An aspirating smoke detector is a very sensitive light sensor or nephelometer, that works by actively drawing a sample of air and other contaminates through a pipe network into a sensing chamber and consists of the following main components;

• a network of small-bore pipes
• a particulate filter
• a sensing chamber
• a focused light source
• a sensitive light receiver

When smoke enters the sensing chamber across the path of the light beam, some light is scattered or obscured by the smoke particles, which is detected by the sensor. The output is analogue and can trigger multiple alarms.

5. Video smoke detection...

Video Smoke Detection (VSD) is based on the computer analysis of video images provided by standard video (CCTV) cameras.

A video smoke detection system comprises the following components;

• one or more video cameras
• a computer
• software to analyse the video signal

The computer uses specialised software to identify the unique motion and pattern of smoke. This unique signal when identified triggers an alarm. 

Prepared & Collection by M.Ajmal Khan.