Often an electrical contractor will be asked to install a fire alarm system in what some would define as a “harsh” environment. In these cases, the contractor must be ready to satisfy his or her customer’s request for an adequate, code-complying system. For most knowledgeable contractors and with most detection devices on the market today, complying with a customer’s request to protect a harsh environment should not present an insurmountable problem.
Electrical contractors already are comfortable following the requirements of the NEC when installing electrical equipment in hazardous or other harsh environments. For fire alarm systems, the National Fire Alarm Code, NFPA 72-2002, cautions users to avoid harsh environments, but when these challenging environments must be protected, it provides the necessary installation requirements.
As with all fire alarm system designs, the owner must define his or her fire protection goals so that the system designer will be able to choose the type of detection best suited to meet those goals.
If heat detection will best serve the owner’s needs, NFPA 72-2002 requires that the designer ensure that heat detectors having fixed-temperature or rate-compensated elements be selected with the maximum expected ambient ceiling temperature in mind. The temperature rating of the detector must always be at least 11 C (20 F) above the maximum expected ambient temperature at the ceiling.
In environments containing chemical fumes or cleaning fluids where heat detection must be installed, rate-compensated heat detectors with coatings that can resist chemical attack but still operate within their UL-listed temperature range are available. Other heat detectors are available in explosionproof models for environments where the device must be sealed to ensure that an electrical spark within the detector will not initiate an explosion. In addition, line-type heat detection is available for use in certain chemical, saltwater or oil spray environments.
The National Fire Alarm Code requires that the selection and placement of smoke detectors take into account both the performance characteristics of the detector and the areas into which the detectors are to be installed to prevent nuisance alarms or improper operation after installation.
Luckily, there are smoke detectors on the market that are designed to work in harsh environments. Specially designed and listed harsh environment spot-type and beam-type smoke detectors offer solutions. For instance, there are linear-beam smoke detectors that are listed for temperature extremes as low as -40 F.
There are also smoke detectors that are designed with high-density filters that remove airborne dust particles but still allow smoke entry. However, even with detectors that have filters, there are important considerations that must be followed such as keeping the them out of direct airstreams.
There are product-listing standards that include tests for temporary excursions beyond normal limits. In addition to temperature, humidity and velocity variations, smoke detectors should operate reliably under such common environmental conditions as mechanical vibration, electrical interference and other environmental influences. The testing laboratories in their listing program also conduct tests for these conditions. In those cases where environmental conditions approach the limits shown in the manufacturer’s data sheet, the detector manufacturer should be consulted for additional information and recommendations.
Unless specifically designed and listed for the expected conditions, smoke detectors must not be installed if any of the following ambient conditions exist: temperature below 0 C (32 F) or above 38 C (100 F), relative humidity above 93 percent, or air velocity greater than 1.5 m/sec (300 feet/min).
The Code further requires that the location of smoke detectors be based on an evaluation of potential ambient sources of smoke, moisture, dust or fumes, and electrical or mechanical influences to minimize nuisance alarms.
However, the early warning provided by smoke detectors is still desired in buildings such as paper, lumber and cotton mills, and even woodworking shops. All of these environments inherently have a great deal of dust or other ambient aerosols that can cause a false alarm in a standard smoke detector. Other buildings may not be heated and thus fall in the temperature ranges outside the limits of normal smoke detectors. Smoke detectors can also be affected by electrical and mechanical influences found in these buildings. The detectors should be located so particulate matter from these sources is minimized.
The goal is to balance the need for early warning while maintaining a reliable and stable fire alarm system. A competent and Code-savvy electrical contractor will know what products will best serve his or her customer’s harsh environment detection needs and will be able to meet those needs with a code-compliant fire alarm system installation. EC
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.