Detection & Fire Alarm Systems
In order to undertake the process of designing a fire alarm system for a building it is necessary to have a sound understanding of the relevant design standards, the legal framework surrounding building safety legislation and a sound working knowledge of product application theory. The following system design process is intended to give a reasonable overview of all the areas of knowledge required for the successful design of a fire alarm system. Due to the complex nature of legislation and design standards relating to fire alarm system design, this course is not intended to be a comprehensive to all aspects of fire alarm system design but rather a very useful source of background information to which further application specific detailed information can be added from other sources as required.
Why have a fire alarm system?
The answer to this question depends on the premises in question and the legal requirements. Your local fire marshal may require a fire alarm system based upon the occupancy of the building. Generally the legal requirement for a fire alarm system relates to the protection of life.
In general fire alarm systems are installed to:
1. To provide for the safety of occupants in buildings, and to make provision for their evacuation or refuge during a fire or other emergency,
2. To provide fire department with early notification of a fire in a building and to direct them to the area of risk,
3. To reduce loss of property; the property may have considerable intrinsic value and the insurers either require a fire detection system or may incentives its use,
4. To reduce building damage; the building may be unoccupied for periods where equipment is still powered and the owner wishes to ensure that if anything goes wrong the fire department is called to the scene in a timely manner. Sometimes fire detection and alarm systems are used to compensate for structural fire protection shortcomings or to give special cover for items of high value,
5. To reduce the amount of business lost, and
6. Minimize risk to the public who attend unfamiliar properties. It is often a mandatory requirement by the Building Codes.
Whatever the reason, an automatic fire detection and alarm system generally provides a network of manual call points, heat and smoke detectors, and alarm warning devices over the area covered. Once activated, the devices send signals to the fire alarm panel which in turn activates audio and visual devices including lights and sounders. The system may also send its signal to an off site monitoring station.
RISK ASSESSMENT
The first step in the design process is the risk assessment. It underpins the whole system strategy and therefore could be argued as being the most important stage. Risk assessment is the process of considering each part of a building from the point of view of what fire hazards exist within an area and what would happen in the event of fire or if explosion were to occur. This would normally be done when considering the building from the point of view of general safety. Clearly very small premises only require a first level of fire protection, such as safe construction, clear escape routes and a fire extinguisher.
Equally obviously, large hotels will require a fully automatic fire detection and alarm system, multiple sets fire protection equipment and adequate emergency lighting and escape signage. The Risk Assessment process is to help building owners of buildings between these two extremes make adequate and appropriate provision. Building owners or operators will often want to employ the services of a professional risk assessor to ensure that the building is considered impartially and in adequate detail. However there are checklists and technical advice available so that the task can be done ‘in-house’.
CONSULT WITH ALL INTERESTED PARTIES
Before embarking on a detailed design, it is highly recommended as a minimum to consult the following agencies so as to ensure that the fire detection and alarm system meets the requirements of all concerned including:
1. The authority responsible for enforcing health and safety legislation,
2. The property insurer,
3. The building user (where appropriate),
4. The proposed installer, and
5. Fire engineering specialists (where appropriate)
DESIGN PROCESS in Fire Alarm System
A fire alarm system should be designed to provide early detection and warning of a fire. The designer must consider the size, complexity and use of the building, and the degree of detection and warning desired. While the design of fire alarm systems is normally regulated by building codes, the level of protection specified is usually a minimum and the designer should consider providing higher levels of protection where circumstances indicate the need. Before looking at the details of the alarm system, it is necessary to understand some of the concepts that are used to assist the system designer. Buildings are divided up into sections in three ways as far as fire safety engineering is concerned: fire compartments, detection zones and alarm zones. Review of the Building
1. Review the physical properties of the building such as:
• Building height
• Number of floors
• Area of each floor
• Smoke compartments
• Sprinkler system, if any
2. What fire alarm equipment is required in this occupancy
3. What locations are fire alarm devices required
4. Determine if there is a special use or occupancy (refer to Model Building Code – Chapter 4, and Life Safety Code – Chapter 11)
Fire Compartments in Fire Alarm System
A fire compartment is a part of a building that is separated from the rest of the building by a fire resistant structure so as to limit the spread of fire within the building. The requirements for designing a building and hence its fire compartments, are defined in building regulations. It is necessary, however, for the designer of fire detection and alarm system to be familiar with the design of the building, in particular the position and extent of its fire compartments.
Detection Zones in Fire Alarm System
Fire detection zones are essentially a convenient way of dividing up a building to assist in quickly locating the position of a fire. The zone boundaries are not physical features of the building, although it is normal to make the zone boundary coincide with walls, floors and specifically fire compartments. The size and position of the detection zones will therefore tend to be dependant on the shape of the buildings, but will also depend on what the building is used for and to some extent the number of people the building is expected to contain at any one time.
Some specific recommendations with respect to detection zones are:
1. Zones should be restricted to single floors, except where the total floor area of a building is less than 3000 ft².
2. Voids above or below the floor area of a room may be included in the same zone as the room so long as they are both in the same fire compartment.
3. Zones should not be larger than 20,000 ft² except for manual systems in single storey open plan buildings, such as a warehouse, where up to 100,000 ft² is allowed.
4. Fire detectors in an enclosed stairwell, lift shaft or the like should be considered as a separate zone.
5. The search distance within a zone should be less than 300 ft in any direction (all possible entrance points must be considered). This can be relaxed when using addressable systems, if the information provided at the control and indicator equipment would allow fire fighters, unfamiliar with the building, to proceed directly to the location of the fire. The search distance only relates to the distance from entering a zone to being able to determine the location of the fire, it is not
necessary to travel to the fire.
6. Zones should not cross fire compartments, a fire compartment can contain several zones but a zone should not contain more than one fire compartment.
Alarm Zones in Fire Alarm System
Alarm zones are only needed in buildings where operation of the alarms needs to be different in certain parts of the buildings. If the only requirement is to activate all the alarm sounders to provide a single common evacuate signal once a fire is detected, then alarm zones are not needed as the whole building is considered one alarm zone. For more complex buildings where it is necessary to operate alarm devices differently in parts of the building, then the building should be divided into alarm zones such that all of the alarm devices in one alarm zone operate in the same way.
Here are some recommendations for alarm zones:
1. The boundaries of all alarm zones should comprise fire-resisting construction.
2. Signal overlap between alarm zones should not cause confusion.
3. The same alarm and alert signals should be used throughout a building.
4. A detection zone must not contain multiple alarm zones, alarm and detection zone boundaries should coincide. An alarm zone may contain multiple detection zones. Once the building zones and fire alarm requirements are determined, install per the applicable standards.
RELEVANT STANDARDS
The design, installation and testing of the fire detection and alarm system shall comply with all state and local codes with no exception. Standards produced by agencies such as Underwriters Laboratories (UL) and/or Approved by Factory Mutual (FM) and National Fire Protection Association (NFPA) are generally endorsed by relevant building codes. Often these standards are called up within guidance documents for pieces of legislation and since they represent best current practice, can be generally be used by building owners to demonstrate that equipment they have installed is adequate and appropriate.
Fire alarm system requirements are found in:
• NFPA 72, National Fire Alarm Code
• NFPA 101, Life Safety Code
• Model Building Codes
NFPA 72 was written to provide requirements for the installation, performance, testing, inspection, and maintenance of the fire alarm system. If you want to know if a fire alarm system is required for a given occupancy, then NFPA 101, Life Safety Code and other related codes (building codes) make that determination.
The following organizations issue guidelines and standards that relate to the USA; other countries will have their own standards.
NFPA Codes and Standards
NFPA publishes standards for the proper application, installation, and maintenance of automatic smoke detectors. The principal codes which should be reviewed before specifying or installing automatic smoke detectors are listed below:
NFPA publishes codes and standards concerning all phases of fire protection.
Among those which directly concern automatic smoke detectors are:
1. NFPA 70 National Electrical Code:
• Article 210, Branch Circuits
• Article 760, Fire Protective Signaling Systems
• Article 500, Hazardous Areas
2. NFPA 72 National Fire Alarm Code:
NFPA 72 covers minimum performance, location, mounting, testing, and maintenance requirements of automatic fire detectors.
3. NFPA 90A Standard for the Installation of Air Conditioning and Ventilating Systems
4. NFPA 92A Smoke Control Systems in Malls, Atria, and Large Areas
NFPA 90A and 92A provide information on the use of smoke detectors in ducts of HVAC systems and smoke control systems.
5. NFPA 101 Life Safety Code
NFPA 101 specifies the requirements for smoke detection in both new and existing buildings depending on the type of occupancy.
Building and Fire Codes
There are three independent regional organizations which write model building and fire codes which become law when adopted by local and state governments. These codes specify smoke detector requirements based on building type and occupancy. The organizations are:
1. Building Officials and Code Administrators (BOCA) – BOCA’s National Building Code is generally used throughout the Northeast and Midwest regions of the United States.
2. International Conference of Building Officials (ICBO) – ICBO’s Uniform Building Code is generally used throughout the West and Southwest regions of the United States.
3. Southern Building Code Congress International (SBCCI) – SBCCI’s Standard Building Code is generally used in the South and Southeast regions of the United States.
In addition these above listed organizations have formed an umbrella organization known as the International Code Council (ICC), for the purpose of combining the codes produced by the above three organizations into a single set of model building and fire codes.
Testing Laboratories
Testing laboratories test detectors, control panels and other components of fire alarm systems to verify conformance with NFPA requirements and their own standards. Equipment that passes their tests are identified by a label. Underwriters Laboratories, Inc. (UL) – UL publishes an annual report listing fire protection equipment which bear the UL label. Its standards which apply to smoke detectors are:
• UL 38 Manually Actuated Signaling Boxes
• UL 217 Single and Multiple Station Smoke Detectors
• UL 228 Door Closers-Holders for Fire Protective Signaling Systems
• UL 268 Smoke Detectors for Fire Protection Signaling Systems
• UL 268A Smoke Detectors for Duct Applications
• UL 346 Water flow Indicators for Fire Protective Signaling Systems
• UL 521 Heat Detectors for Fire Protective Signaling Systems
• UL 464 Audible Signaling Applications
• UL 864 Standards for Control Units for Fire Protective Signaling Systems
• UL 1481 Power supplies for Fire Protective Signaling Systems
• UL 1638 Visual Signaling Appliances
• UL 1971 Signaling devices for the hearing impaired
Factory Mutual Research (FM) – FM publishes an annual report listing fire protection equipment which bears its label.
• Factory Mutual Loss Prevention Data Sheets, as appropriate for the hazard
• Factory Mutual Loss Prevention Data Sheet 5-40 Protective Signaling Systems
• Factory Mutual Loss Prevention Data Sheet 5-43 Auxiliary Protective Signaling Systems
There are other testing laboratories listed here that may provide similar services:
• Industry Publications NEMA Guide for Proper Use of Smoke Detectors in Duct Applications
• NEMA Training Manual on Fire Alarm Systems
• NEMA Guide to Code Requirements for Fire Protective Signaling and Detection Systems
• NEMA Guide for proper Use of System Smoke Detectors
The final system shall receive an Underwriters Laboratories Field Certification from an alarm service company authorized to issue Underwriters Laboratories certificates. Note that there are other standards that relate to specific applications (such as hospitals or data processing installations).
FIRE ALARM COMPONENTS
A typical fire alarm system consists of a control unit, manually activated signaling boxes (pull boxes), fire detectors and audible alarm devices. There may also be visual signal devices to warn the hearing-impaired, annunciators to indicate the origin of the alarm signal, and emergency telephones and other equipment for communication between the central control panel and other parts of the building (refer to the figure below).
Figure – A basic fire alarm system
The control unit powers the fire alarm system. It transmits signals from pull boxes and fire detectors to the alarm signal devices, either audible or visual, installed at strategic locations in the building. Audible signal devices include bells, speakers, and sirens, a strobe light being a common visual signal device.
FIRE ALARM CONTROL PANEL (FACP)
The control panel is the “brain” of the fire detection and alarm system. It is responsible for monitoring the various alarm “input” devices such as manual and automatic detection components, and then activating alarm “output” devices such as horns, bells, warning lights, emergency telephone dialers, and building controls. Control panels may range from simple units with a single input and output zone, to complex computer driven systems that monitor several buildings over an entire facility. There are two main control panel arrangements, conventional and addressable, which are discussed below.
1) Conventional or “point wired” fire detection and alarm systems – In a conventional system one or more circuits are routed through the protected space or building. Along each circuit, one or more detection devices are placed. Selection and placement of these detectors is dependent upon a variety of factors including the need for automatic or manual initiation, ambient temperature and environmental conditions, the anticipated type of fire, and the desired speed of response. One or more device types are commonly located along a circuit to address a variety of needs and concerns.
Upon fire occurrence, one or more detectors will operate. This action closes the circuit, which the fire control panel recognizes as an emergency condition. The panel will then activate one or more signaling circuits to sound building alarms and summon emergency help. The panel may also send the signal to another alarm panel so that it can be monitored from a remote point.
In order to ensure that the system is functioning properly, these systems monitor the condition of each circuit by sending a small current through the wires. Should a fault occur, such as due to a wiring break, this current cannot proceed and is registered as a “trouble” condition. The indication is a need for service somewhere along the respective circuit.
In a conventional alarm system, all alarm initiating and signaling is accomplished by the system’s hardware which includes multiple sets of wire, various closing and opening relays, and assorted diodes. Because of this arrangement, these systems are actually monitoring and controlling circuits, and not individual devices.
To further explain this, assume that a building’s fire alarm system has 5 circuits, zones A through E, and that each circuit has 10 smoke detectors and 2 manual stations located in various rooms of each zone. A fire ignition in one of the rooms monitored by zone “A” causes a smoke detector to go into alarm. This will be reported by the fire alarm control panel as a fire in circuit or zone “A”. It will not indicate the specific detector type or location within this zone. Emergency responding personnel may need to search the entire zone to determine where the device is reporting a fire. Where zones have several rooms, or concealed spaces, this response can be time consuming and wasteful of valuable response opportunity.
The advantage of conventional systems is that they are relatively simple for small to intermediate size buildings. Servicing does not require a large amount of specialized training.
Disadvantages of conventional systems
• Conventional systems can be expensive to install because of the extensive amounts of wire that are necessary to accurately monitor initiating devices. Conventional systems may also be inherently labor intensive and expensive to maintain.
• Each detection device may require some form of operational test to verify it is in working condition. Smoke detectors must be periodically removed, cleaned, and recalibrated to prevent improper operation.
• With a conventional system, there is no accurate way of determining which detectors are in need of servicing. Consequently, each detector must be removed and serviced, which can be a time consuming, labor intensive, and costly endeavor.
• If a fault occurs, the “trouble” indication only states that the circuit has failed, but does not specifically state where the problem is occurring. Subsequently, technicians must survey the entire circuit to identify the problem.
2) Addressable or “intelligent” systems
represent the current state-of-the-art in fire detection and alarm technology. Unlike conventional alarm methods, these systems monitor and control the capabilities of each alarm initiating and signaling device through microprocessors and system software. In effect, each intelligent fire alarm system is a small computer overseeing and operating a series of input and output devices.
Like a conventional system, the address system consists of one or more circuits that radiate throughout the space or building. Also, like standard systems, one or more alarm initiating devices may be located along these circuits. The major difference between system types involves the way in which each device is monitored. In an addressable system, each initiating device (automatic detector, manual station, sprinkler water flow switch, etc.) is given a specific identification or “address”. This address is correspondingly programmed into the control panel’s memory with information such as the type of device, its location, and specific response details such as which alarm devices are to be activated.
The control panel’s microprocessor sends a constant interrogation signal over each circuit, in which each initiating device is contacted to inquire its status (normal or emergency). This active monitoring process occurs in rapid succession, providing system updates every 5 to 10 seconds.
The addressable system also monitors the condition of each circuit, identifying any faults which may occur. One of the advancements offered by these systems is their ability to specifically identify where a fault has developed. Therefore, instead of merely showing a fault along a wire, they will indicate the location of the problem. This permits faster diagnosis of the trouble, and allows a quicker repair and return to normal.
Each device, for purpose of testing and future reference shall have the map number identified on the device and temporary protective cover that corresponds with the address on the hard copy provided by the electrical contractor. The address for each device shall be programmed as follows: a) Building, b) Floor, c) Room/Corridor, d) additional information
Advantages provided by addressable alarm systems
Stability
stability is achieved by the system software. If a detector recognizes a condition which could be indicative of a fire, the control panel will first attempt a quick reset. For most spurious situations such as insects, dust, or breezes, the incident will often remedy itself during this reset procedure, thereby reducing the probability of false alarm. If a genuine smoke or fire condition exists, the detector will reenter the alarm mode immediately after the reset attempt. The control panel will now regard this as a fire condition, and will enter its alarm mode.
Enhanced Maintenance
With respect to maintenance, these systems offer several key advantages over conventional ones. First of all, they are able to monitor the status of each detector. As a detector becomes dirty, the microprocessor recognizes a decreased capability, and provides a maintenance alert. Advanced systems, incorporate another maintenance feature known as drift compensation. This software procedure adjusts the detector’s sensitivity to compensate for minor dust conditions. This avoids the ultra sensitive or “hot” detector condition which often results as debris obscures the detector’s optics. When the detector has been compensated to its limit, the control panel alerts maintenance personnel so that servicing can be performed.
Ease of Modification
Modifying these systems, such as to add or delete a detector, involves connecting or removing the respective device from the addressable circuit, and changing the appropriate memory section. This memory change is accomplished either at the panel or on a personal computer, with the information downloaded into the panel’s microprocessor.
The main disadvantage of addressable systems is that each system has its own unique operating characteristics. Therefore, service technicians must be trained for the respective system. Periodic update training may be necessary as new service methods are developed.
