Find the Line

When you, as an installing contractor, get a telephone call from a property owner asking you to propose a fire alarm and detection system for his or her building, you have more on your plate than just adding up equipment and labor costs. Certainly, both types of costs have relevance, but ensuring you choose the right type of detection to meet the owner’s fire protection goals has equal importance.

Fire alarm systems come in two basic varieties: manual and automatic. Most jurisdictional codes require at least a manual fire alarm system for the greatest number of occupancies. For some occupancies, the jurisdictional codes add automatic detection. In every case, a smoke detector must protect the fire alarm control equipment.

Obviously, if the jurisdictional code requires a manual fire alarm system, you will need to follow the manual fire alarm box location requirements in NFPA 72-2007, National Fire Alarm Code. The code requires that the installing contractor locate manual fire alarm boxes within 5 feet of the exit doorway opening at each exit on each floor. The boxes must remain conspicuous, unobstructed and accessible. The installer must pay attention to the door swing to ensure that when the door opens it will not obstruct the manual fire alarm box. In addition to one at each exit on each floor, the installer must provide additional manual fire alarm boxes to ensure that the travel distance to the nearest fire alarm box in any area will not exceed 200 feet measured horizontally on the same floor.

In special cases, such as a large store entry where “grouped openings” extend more than 40 feet in width, the installer must mount manual fire alarm boxes within 5 feet of each side of the opening. The installer must securely mount each manual fire alarm box so that the operable part of the box will not measure less than 3½ feet or more than 4½ feet above floor level.

Computing the cost estimate for the fire alarm system will determine how many and which type of automatic detection devices to choose to meet the owner’s needs.

Heat detection generally protects contained locations, such as rooms, closets, etc. Heat detection provides property protection, not life safety protection. The National Fire Alarm Code offers straightforward requirements for the use and location of heat detectors. You must know the physical dimensions, ceiling height, ceiling configuration and the ambient temperature of the space where you intend to install the heat detectors.

The code further requires that you carefully select detectors that have fixed--temperature or rate-compensated elements where the temperature rating of the detector has a value at least 20°F above the maximum expected temperature at the ceiling of the protected space. You should select the heat detector temperature rating that will best minimize this temperature difference. This will help minimize response time.

Additionally, the specific environment of the space may dictate if you use a line-type or spot-type heat detector. You may often choose to use line-type detection for harsh environments, such as the protection of a wharf or similar outside area.

When using a rate-of-rise type heat detector, you need to take note of any potential temperature fluctuations in order to avoid false alarms. In all cases where you apply heat detection in your design, you should choose the most sensitive heat detector while still using the environmental guidelines outlined in NFPA 72-2007.

Underwriters Laboratories Inc. or FM Global determines each detector’s spacing by test. The National Fire Alarm Code contains all other heat detector installation requirements.

If the owner has concerns with regard to life safety, you will more likely choose to install smoke detection. Jurisdictional codes will tend to require partial smoke detection in a building, often protecting only the means of egress, such as corridors and stairwells. However, if the owner wishes to ensure the detection of a small fire anywhere in his or her building, you will need to provide additional smoke detection.

Smoke detectors demand more extreme environmental concerns than heat detectors. Ceiling height, ceiling configuration, HVAC system operation and dust all can have a negative effect on the ability of a smoke detector to perform its intended function. In addition, the nationally recognized testing laboratories do not determine smoke detector spacing requirements. The code requires that “in the absence of specific performance-based design criteria, smoke detectors shall be permitted to be located using 30 foot spacing.” The 30-foot spacing offers a minimum spacing guideline. But this guideline has become accepted as the normal spacing of smoke detection.

Choosing spot-type or line-type smoke detection will depend on the space you must protect and if the space has a high or unusual ceiling configuration.

Selecting the right detection for the job consists of many factors to ensure your response to the owner’s request offers both an economical solution and sufficient protection.

MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.

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