FM Approved Heat Detectors

  • The Hazard

    Automatic fire detection and alarm signaling is a critical part of almost every industrial, institutional and commercial facility in the developed world. There are many types of automatic fire detection systems, including smoke, heat and flame detectors. Each of these detectors is triggered by a different byproduct of combustion and, when used in combination with sprinklers or other automatic fire extinguishing systems, can help minimize property damage in the event of a fire.

    While smoke detectors are an essential part of many automatic fire alarm systems, they are not appropriate for many industrial applications that may be dusty, steamy, hot, exposed to weather, contain aerosols, or include expansive or elongated areas.

    In these and many other situations, the right choice for automatic fire alarm signaling is often heat detectors.

    Heat detectors are primarily intended for property protection and have a broad range of applications, including:

    • Mining and material processing
    • Warehousing and storage
    • Power generation
    • Transportation
    • Painting/spray booths
    • Process industries
    • Kitchens
    • Boiler rooms
    • Metal working
    • Cold storage facilities

    Heat detectors are available in two fundamental forms: spot detectors and linear or line detectors. Heat detectors use one or a combination of detection principles, including fixed temperature, rate-of-rise, and rate compensated.

    A fixed temperature detector is designed to respond when the operating element reaches a predetermined temperature (e.g., 135oF, 200oF, etc.). Rate-of-rise detectors respond when the rise in temperature exceeds a predetermined value (e.g., 15oF/min rate-of-rise). Rate compensated detectors are designed to accurately sense the surrounding air temperature, regardless of fire growth rate, and thereby compensate for the time lag associated with fixed temperature and reduce the chance of a false alarm possible with rate-of-rise detectors.

    Spot detectors are often similar in appearance to smoke detectors and use one or more of the three detection principles noted above. Line detectors are fixed temperature systems that employ a thin cable containing wire or optical fiber to detect abnormal temperatures anywhere along the length of the cable. Linear heat detector cables can be used in lengths up to 10 kilometers, making it ideal for rail or highway tunnels and other long distance applications.

  • Testing Required for FM Approval

    FM Approved heat detectors are evaluated according to Approval Standard 3210, Heat Detectors for Automatic Fire Alarm Signaling. The heat detector samples are subjected to up to 18 different performance tests, including:

    • Various oven tests – varied tests depending on the method of detection (i.e., fixed, rate compensated, rate-of-rise).
    • Humidity, dust and vibration.
    • Sensitivity to spacing test -- installation spacing limitations of spot type heat detectors is determined through a series of plunge tunnel tests to arrive at an RTI (Response Time Index); linear heat detectors and other non-spot-type detectors that cannot be subjected to the plunge tunnel test will undergo full-scale fire testing.
    • Voltage, dielectric strength, bonding and reverse polarity.
    • Static discharge, extraneous transients, surge
    • Jarring and durability

    NFPA 72, the National Fire Alarm Code, specifies that the response time index of heat detectors must be listed along with the operating temperature. The RTI is a measure of how quickly a detector’s thermal element will respond when exposed to a gas temperature at or above its alarm threshold. To be able to adequately predict the response of a heat detector, the RTI of the device must be known.

    The RTI concept was developed in the early 1970s by FM Global researchers and helped revolutionize the accuracy of sprinkler sensitivity testing and classification. Based on the subsequent research, the plunge test and RTI concept were incorporated into Approval Standard 3210 in 2007.

    In addition to performance testing, FM Approved heat detectors must be backed by a quality assurance program to ensure that every subsequent detector provides the same quality and reliability as the detectors that were examined. The manufacturer must demonstrate a quality control program, including documentation, records, drawings and change control. An initial audit of the manufacturing facility is part of the Approval process, and includes a review of the manufacturer’s equipment, procedures and quality program. Follow-up audits are conducted at least annually, or more often if necessary.

  • Benefits and Availability

    The determination of an RTI rating to ensure proper spacing limitations for heat detectors is unique to the FM Approvals evaluation program. FM Approved heat detectors are certified to deliver the performance promised by the manufacturer specifications. This level of detailed testing enables specifiers, designers and others to choose the FM Approved heat detector that best meets the application requirements.

    It should be noted that an FM Approved heat detector will operate at least as quickly as an FM Approved one, comparably rated automatic sprinkler on 10 by 10 ft (3 by 3 m) spacing under the same conditions of heat exposure. An FM Approved heat detector is also designed to operate within 3% of its intended fixed temperature in oF or, if a rate-of- rise type, to operate at an ambient temperature increase of between 15oF and 25oF (8.3oC and 13.9oC) per minute.

    While FM Approvals testing is considered among the most rigorous in the world it is also a sign of a manufacturer’s confidence and a quality indicator that will help differentiate a product from the competition.

    FM Approvals partners with its customers, providing insights that can strengthen product design and by providing ongoing support. FM Approved products are not only listed in the Approval Guide but, just as importantly, FM Approved products are specified and accepted by building owners, designers and code authorities.

  • Success Stories

    LHD Prevents Devastating Damage

    In the pre-dawn hours of New Year’s Day, a hidden fire almost caused devastating damage to a freezer warehouse located in the Pacific Northwest. The freezer, which is owned by an independent operator, stores supplies and products that are shipped to a major fast food restaurant chain. The freezer portion of the warehouse is equipped with a double-interlock pre-action system that uses linear heat detector (LHD) as the system’s detection device. The LHD was located on the ceiling grid as well as in the racking systems. During the night, a rooftop refrigeration unit fan motor caught fire, melting a small section of the adjacent wall insulation. Although the fire did not generate enough heat to fuse a sprinkler head, there was sufficient heat to activate the LHD. Upon actuation, the control panel sounded the alarm, and turned off the refrigeration unit, thereby preventing further damage to the cooling unit and the structure. That same day, the fan motor was repaired and a small section of the LHD was quickly replaced, bringing the system back to full operation.