Fire Alarm Systems FAQs

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Fire Alarm Control Panels

Fire Alarm Devices

Smoke Detectors

Thermal Detectors

Flame Dectector

Fire & Gas Dectector

 


 

Fire Alarm Control Panels

Conventional

Traditional fire alarm panels installed prior to 1998 were conventional zone panels. In a zoned system, fire alarm devices in a common area or floor of a facility are connected to the same alarm initiating circuit. Each zone requires its own circuit conductor. This arrangement allows alarm annunciation to be reported by areas of the building to identify which device is in alarm. Conventional panels are often used in small facilities where a few zones can provide sufficient alarm annunciation.

 Addressable

With the advent of microprocessors and digital electronics, addressable fire alarm control panels and devices have become more common than conventional systems for medium and large-sized facilities. They have become more cost effective in some small applications as well.

Addressable fire alarm systems use digital encoding and multiplex technology to more accurately identify alarm locations and device conditions. Each fire alarm device in a system is programmed with a unique address.

The fire alarm control panel is capable of communicating with a single address or a group of addresses depending on the functions required. The communication is often multiplexed over a common cable, sometimes referred to as the signaling line circuit (SLC). This arrangement significantly reduces the amount of cabling necessary to install the system. The communication channel allows two-way communication, thus enabling the fire alarm control panel to control as well as monitor fire alarm devices.

A significant component of addressable fire alarm system is the software programming necessary to make the system function correctly. The programming allows for flexible applications where you want to have specific control over the inputs and outputs.

The communication technologies employed in addressable systems allow for advanced features to accommodate sensitivity changes due to age and accumulation of dust prior to maintenance. These features are not available with the standard conventional system.

 

Fire Alarm Devices

Automatic Detection

Components of a fire consist of:

  • Smoke (particulate and aerosol)
  • Heat
  • Light Radiation
  • Fire detection devices are built to detect one or a combination of these components. While all components are necessary for a fire to exist, all components may not exist at a detectable threshold. Detectors will be selected that will detect the elements that may exist in a fire for the ambient conditions that are present. It also should be realized the similar non-fire components might exist in the same ambient conditions, which could cause unfavorable false alarm conditions.
  • Devices used for fire detection include smoke detectors, thermal detectors, flame detectors, fire-gas detectors, and other devices.
  • Smoke detectors sense visible or invisible particles of combustion generated by burning, smoldering, or the incipient stage of combustion. These devices fall into two categories -- photoelectric and ionization.
  • Thermal detectors sense the high temperature or the temperature rise caused by a fire.
  • Flame detectors sense the radiation produced by a fire.
  • Fire-gas detectors sense the gases produced by a fire.
  • Other detectors sense some phenomenon other than smoke, thermal, flame, or fire-gas to detect a fire.


Smoke Detectors

There are three types of smoke detectors: Ionization, photoelectric, and combination.

 

Ionization

The ionization smoke detector is widely used. Its capability to detect smoke originating from fire is best utilized for clean-burning fires that produce small particles during combustion.

 

The ionization smoke detector consists of an alpha particle producing a radioactive source, a smoke chamber, and charged detector plates.

  • The alpha source causes the air within the smoke chamber to become ionized and conductive
  • As smoke particles enter the smoke chamber, the smoke particles attach themselves to the ionized air molecules and the air in the chamber becomes less conductive
  • When the air conductivity within the chamber drops below a predetermined level, the alarm is triggered

Advantages of Ionization Smoke Detectors:

  • Detects invisible products of combustion -- It can detect fires that are in the incipient stage or detect other aerosol-type smoke products
  • Quick acting -- Provides for earlier detection than other types of smoke detectors or thermal detectors
  • Disadvantages of Ionization Smoke Detectors: 
  • May provide false detection if used where volatile solvents, conductive material dusts, or high humidity are present
  • Detects the presence of smoke only, not toxicity
  • Has a potential for high false alarm rate
  • Typical locations or hazards for ionization detection:
  • Clean rooms
  • Computer rooms
  • Mechanical air ducts
  • Locations where sensitive detection methods are needed

 

Photoelectric

A photoelectric smoke detector is the most common smoke detector used today. It detects smoke by using either the principle of light obscuration or light scattering. Its capability to detect smoke originating from fire is best utilized for fires that produce large particles during combustion.

Spot type photoelectric smoke detectors using the light obscuration principle have a light emitting device, usually a light-emitting diode (LED), a smoke chamber, and a photosensitive device that receives the light directly from the light source and produces a monitored current.

Smoke that enters the smoke chamber reduces the intensity of tech light reaching the photosensitive device, which reduces the monitored current. When the intensity drops below a certain level, the sensor control circuitry detects a drop in the current produced by the photosensitive device. When the current falls below a preset threshold, the smoke alarm is triggered.

Spot type photoelectric smoke detectors that use the light scattering principle are constructed similarly to the detectors that use the light obscuration principle except that the photosensitive device is set so that it cannot see the light source directly. When smoke enters the chamber, the smoke particles reflect the light from the source into the photosensitive receiver. When sufficient light intensity is detected, the alarm is triggered.

Advantages of Photoelectric Smoke Detectors:

  • Sensitive to visual particles of smoke
  • Detects smoldering low heat fires
  • Provide early warning

Disadvantages of Photoelectric Smoke Detectors:

  • Early contamination by dust causing reduced sensitivity
  • Detects presence of smoke, not toxicity
  • Must be cleaned on a regular basis
  • Has a potential for high false alarm rate
  • Typical locations or hazards for photoelectric detection:
  • Office areas
  • Clean rooms
  • Raised floor spaces
  • Atriums and corridors
  • Meeting rooms
  • Computer rooms
  • Telecommunications rooms
  • Electrical equipment rooms
  • Sleeping rooms
  • Storage closets

 

Beam Detector

Beam smoke detectors are line-type photoelectric detectors consisting of a separate light source and photosensitive receiver. These devices are usually installed in large open areas where there is an unobstructed line of sight between the light source and the receiver and where the use of spot-type detectors would be economically unfeasible due to the number of detectors required.

Advantages of Beam Smoke Detectors:

  • Cover a large area economically
  • Quick acting
  • Disadvantages of Beam Smoke Detectors: 
  • Unobstructed LoS between the light source and the receiver
  • Correct alignment needs to be maintained
  • Typical locations or hazards for beam detectors:
  • High atriums
  • Manufacturing spaces


Air Sampling Smoke Detectors

For environments where detection of smoke is most critical, an air-sampling system provides the earliest possible detection. An air sampling or aspirating type fire detection system is a self-contained smoke detection package compromised of five primary components:

  • Air-sampling system
  • Aspiration system
  • Filter assembly
  • Detector
  • Control system

It uses a network of pipes to continuously draw air samples and direct them to a central smoke detector.

 

The system operates with a network of sampling pipes that extend into the protected area. The pipes are usually made of a thermoplastic material. An internal aspirator continuously draws air into the piping network. The systems use either a filter assembly or laser particle counting technology to filter out airborne dust and debris particles, which helps to eliminate false readings.
Typical locations or hazards for Air-Sampling smoke detectors:

  • Telecommunications areas
  • Computer rooms
  • Data centers
  • Hospitals
  • Clean room environments
  • Atriums
  • Cold storage areas
  • Power stations
  • Mines
  • Paper and timber mills
  • Museums
  • Art Galleries
  • Cathedrals
 

Thermal Detectors

Fixed Temperature 

  • Fixed Temperature Thermal Detectors can respond to:
  • Fixed temperature limit
  • Rapid rate of change of the temperature in the protected area
  • Combination of these types of detection

Typical fixed temperature spot-type smoke detectors contain a bimetallic switch element that closes at a specified temperature limit. The switch is normally composed of two metals, each having a different temperature coefficient of expansion. As this bimetallic element heats the metal with higher coefficient of expansion, it causes the switch to bend or curve, closing the switch; thus indicating an alarm condition.

Line type thermal detectors are cables that detect heat along their entire length. A line type thermal detector may consist of two wires that are separated by an insulator. After the heat builds to a certain level the insulation melts, allowing the wires to touch and current to flow, initiating an alarm.

Bimetallic spot and coaxial style thermal detectors are self restoring. Fusible link and melting insulation types of line thermal detectors are not self-restoring.

Advantages of Fixed Thermal detection:

  • Lower cost than smoke detector units
  • More reliable than smoke detector units
  • Not affected by dusty or dirty environments
  • Minimal maintenance

Disadvantages of Fixed Thermal detection:

  • Slower to respond than smoke detectors
  • Will not detect products of combustion
  • Only suitable for protection of property

 

Rate of Rise

Rate-of-Rise Thermal Detectors measure the rate at which the air temperature changes during a fire event. Measuring the change in temperature provides a faster alarm response than measuring the temperature level in a space.

The rate-of-rise detector measures the change in the temperature of the space through the use of a differential pressure switch. This switch contains an air chamber separated for the air in the ambient space by a flexible diaphragm. As air in the ambient space changes temperature, the air pressure increases, creating a differential pressure across the diaphragm.

The air chamber is constructed with a calibrated leak so that normal temperature and pressure fluctuations within the room space adjust across both sides of the diaphragm and will not cause the alarm contacts to close. During a fire, the air temperature rises at a rate faster than normal, causing an increase on the room side of the diaphragme diaphragm. The leak cannot compensate, and therefore the diaphragm moves and closes the detector contacts.

Combination rate-of-rise and fixed temperature thermal detectors are also manufactured and have both technologies built in.
Advantages of Rate-of-Rise Thermal detection:

  • Responds faster than the fixed temperature detector
  • Not affected by dusty or dirty environments
  • More reliable than smoke detector units
  • Less expensive than smoke detector units
  • Minimal maintenance

Disadvantages of Rate-of-Rise Thermal detection:

  • Slower to respond than smoke detectors
  • Will not detect products of combustion
  • Only suitable for protection of property

 

Rate Compensated

Rate-compensated thermal detectors are devices that are designed to activate at a predetermined temperature in a space regardless of the rate at which the temperature in the space increases. This is accomplished by compensating for the thermal lag between the room temperature and the interior of the device.

Construction consists of an outer metal tube that expands at a fixed rate. Within this tube, alarm contacts close when a certain expansion distance is reached, but this expansion is opposed by another metal device.

At a slow rate-of-rise in temperature, the outer tube expands drawing the contacts closer together. The inner metal device exerts a counter force, keeping the contacts separated until the entire device has been heated to its rated temperature.

At a rapid rate-of-rise in temperature, the outer tube expands faster than the inner device can compensate. Therefore, the alarm contacts close when the entire device has been heated to a lower level, thus compensating for thermal lag.

Advantages of Rate Compensated Thermal detectors:

  • Responds accurately and positively to fire threats
  • Virtually eliminates false alarms
  • Not affected by dusty or dirty environments
  • More reliable than a smoke detector
  • Less expensive than smoke detector units
  • Minimal maintenance

Disadvantages of Rate Compensated Thermal detectors:

  • Slower to respond than smoke detectors
  • Will not detect products of combustion
  • Only suitable for protection of property
 

Flame Detector

Flame detectors are used to detect the light radiation component of a fire. Typical detectors of this type detect the wavelength of either IR or UV or a combination of the two. These detectors are extremely fast acting and are used in areas where rapidly occurring fires or explosions could occur.

Advantages of Flame Detection:

  • Extremely fast acting

Disadvantages of Flame Detection:

  • Narrow field of vision
  • Expensive
  • Requires unobstructed field of view
  • Difficult to maintain

Typical Uses:

  • Fuel loading docks
  • Industrial process spaces
  • Other hazardous areas where a fast developing fire could occur
 

Fire-Gas Detector

These detectors respond to the various gases produced during the combustion process.

  • Carbon monoxide
  • Carbon dioxide
  • Steam
  • Other elements

The Fire-Gas detector employs two types of technology to predict the fire. One method uses a semiconductor material that changes the metals conducting potential in a fire situation. The other method uses a catalytic element encased in an aluminum bead.

Advantages of Fire-Gas Detection:

  • Detects products of combustion
  • Sensitive enough to detect levels of gases produced between the occurrences of detectable particulate levels and detectable heat levels
  • Detects gases prior to reaching lethal levels

Disadvantages of Fire-Gas Detection:

  • Can be prone to false alarms
  • Must be mounted at a low level, leaving it susceptible to damage
  • Can be poisoned
  • Not suitable for areas where CO and CO2 and produced as part of the functions within the area
  • Cannot be considered as a universal replacement for smoke and/or thermal detectors
  • High cost
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