Special Detection Challenges

Professional electrical contractors are asked to design fire alarm systems in many challenging buildings and spaces. Structures and buildings such as wharfs, power plants, aircraft hangars, museums and high-ceiling spaces require fire detection. The challenge is to find the detection devices that will work reliably in the application.

Ensuring the proper detection is chosen to meet the owner’s fire protection goals is important, but in many cases, the environment will limit the devices the contractor can use.

For example, in an office or residential building application, when an owner’s fire protection goal includes early warning and the space to be protected is a high-ceiling environment, spot-type detection typically is not used. In this application, projected linear beam smoke detection would be the better device to choose to meet the owner’s goal.

The 2007 edition of the National Fire Alarm Code provides guidance for the location of projected beam smoke detectors. It recommend smooth ceiling spacing “of not more than 60 feet between projected beams and not more than one-half that spacing between a projected beam and a sidewall (wall parallel to the beam travel) should be used as a guide.” The code also recommends that other spacing should be determined based on ceiling height, airflow characteristics and response requirements of the detection.

There are different types of projected linear beam smoke detectors to choose from. One version has the light beam projector mounted on one end wall, with the light beam receiver mounted on the opposite wall. Other versions use the transmitter and receiver mounted on the same wall with a reflector or mirror installed at the opposite wall. The second version allows for an easier installation as the wiring needs to be on only one side of the space being protected. It also is permitted to suspend the projector (transmitter) and receiver from the ceiling at a distance from the end walls not exceeding one-quarter the selected spacing. In addition to determining the spacing using performance guidelines, the code requires that the detectors be located in accordance with the manufacturer’s published instructions.

When determining the location of projected linear beam smoke detectors in high-ceiling applications, the effects of stratification also should be evaluated. These effects may require multiple levels of beam smoke detection in the space being protected.

The contractor also should ensure that the projected beam-type detectors and mirrors are mounted on stable surfaces to prevent false or erratic operation due to movement.

The transmitter’s beam also must be designed so that small, angular movements of the light source or receiver do not prevent operation due to smoke and do not cause nuisance alarms. In addition, the light path of projected beam-type detectors must be kept clear of opaque obstacles at all times to ensure that, when the light path is abruptly interrupted or obscured, the unit will not initiate an alarm. Instead, an abrupt blockage of the beam should produce a trouble signal.

When using this form of detection, the contractor should be aware that a projected beam-type smoke detector is considered equivalent to a row of spot-type smoke detectors for level and sloping ceiling applications.

Smoke on the water

When structures such as piers and wharfs require some form of fire detection, the generally accepted detection is linear heat detection. A line-type heat detector (linear-type heat detection) is a good example of a detection device that can be installed in a poor environment but continues to work reliably throughout the life of the structure. The device is still a fixed-temperature device, so it is inherently slow but in this application may be the only choice. The code requires that when line-type heat detection is used in an application other than open area protection, the manufacturer’s published instructions must be followed. Line-type heat detection also can be used in power plant applications. Typically, line-type detection will be installed in cable trays and in electrical cabinets. Some versions of line-type detection can actually detect an overheat condition and prove helpful in early detection of electrical cable faults.

All of the code requirements for spot-type heat detection apply to line-type heat detection, and it should be understood that any heat detection device is considered “slow” in comparison to smoke detection.

When contractors are called on to assist owners with their detection needs, they must know what is available and how to apply the devices and install them. The owner expects the professional contractor to be their “detection expert.”

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.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist
Wayne D. Moore, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. Moore is a vice president with JENSEN HUGHES at the Warwick, R.I., office. He c...

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