Proper installation and detection parameters maximize coverage and minimize false alarms.
Tenants come and go. One month, an area might be filled with a maze of cubicles, the next, it may be a suite of private offices. As each new company takes over space in a building and adapts it to their needs, there’s a completely different set of fire-protection guidelines. However, the same versatility of space that makes a building desirable for renters can also present challenges when it comes to fire protection.
To guard against hazards associated with changing uses for building space, it is important for a fire-protection system to provide “total coverage.” Defined by the National Fire Protection Association in NFPA 72 guidelines, total coverage is achieved with the proper type of detectors installed in appropriate locations.
Where to Install Detectors?
The goal of a fire-detection system is to provide an accurate, early warning of a developing fire in all areas of a building. Even pockets of unoccupied space require protection because detectors may not quickly sense a developing fire on the far side of a wall or behind a closed door, allowing damage to multiply needlessly.
The correct placement of detectors is also important for reliable operation. In general, when only one detector is required in a room or space, it is best to install a ceiling-mounted detector as close to the center of the room as possible. If a central ceiling location is not viable, for example, due to wiring constraints, the detector must have sufficient “open space,” with its edge no closer than 4 inches to a wall. Likewise, a wall-mounted detector must clear between 4 and 12 inches from the top of the detector to the ceiling.
Another consideration for a total coverage plan is the proximity of detectors to the air-handling system. NFPA 72 discusses the potential for detector malfunction if installation is in the path of an airflow supply or return duct. A smoke test to monitor particulate travel-direction and velocity is helpful in determining detector placement. Smoke tests reveal potential causes of unwanted alarms, such as an air stream directed at the detector, which could result in dust accumulation that alters sensitivity levels.
How to Space Detectors?
Spacing detectors 30 feet apart to protect 900 square feet is the NFPA 72 standard for areas with smooth ceilings and no physical obstructions between ceiling and room contents. An example of an obstruction is floor-to-ceiling shelving stacked with materials. Variables for ceiling height may also be calculated into spacing requirements based on the amount and nature of combustibles present.
To determine appropriate detector coverage for the standard 30-foot spacing, a simple technique is to map the shape and dimensions of an area. Then, draw a circle with a radius of 21 feet. A single detector may protect any square or rectangle that fits within the circumference of that circle. The same technique shows that in a hallway measuring 10 feet wide, two detectors can protect up to 82 feet of the length.
What Type of Detectors?
To answer that question is to understand the use and contents of a particular area. For example, ionization smoke detectors are quicker to detect flaming fires, such as those commonly found in chemical-storage areas, rather than slow, smoldering fires that most typically occur in offices. Ionization sensors almost immediately recognize fires characterized by combustion particles from 0.01 to 0.3 microns. However, ionization sensors offer limited or slower capabilities when installed in high-altitude locations or near high-humidity sources, such as kitchens or shower rooms.
Photoelectric smoke detectors, on the other hand, quickly respond to smoldering fires characterized by combustion particles from 0.3 to 10.0 microns. This type of detector will instantly identify visible white smoke, but will be slower to respond to black smoke produced by plastics or rubber.
A common solution to detect both types of stimuli quickly is to install a multi-criteria detector that monitors particulate detection in tandem with a thermal-sensor input. Together, the two signals are cross-referenced by an on-board microprocessor that uses algorithms to “process out” false alarms while enhancing the response time to real fires (see “NASA Researches Multi-Criteria Detectors”). By monitoring the current signal values of each sensor as well as their trends, such as increasing heat or a decreasing photoelectric signal, multi-criteria detectors actually “learn” the environment, which helps to better reject nuisance alarms and maintain heightened sensitivities.
Which Code to Follow?
The location, quantity and type of detector should be determined by the use, size and contents of the space. However, total coverage must ultimately coincide with the guidelines set by the Authority Having Jurisdiction.
Several independent organizations write model building and fire codes that are commonly adopted by local and state governments throughout the United States. While variances should be expected in individual municipalities, most regulations are based on three organizations’ codes:
• Building Officials and Code Administrators’ National Building Code (BOCA): Northeast and Midwest
• International Conference of Building Officials’ Uniform Building Code: West and Southwest
• Southern Building Code Congress International’s Standard Building Code: South and Southeast
The International Code Council Inc.’s International Building Code/International Fire Code combines the above codes into a single set of model building and fire codes. Some states have adopted the International Building Code and International Fire Code.
To receive more information about the content within this issue, fill out this Online Response Card.