Fire Alarm Expert: Advancements in Linear Heat Detection Technology

The oldest form of electrical fire detection is the heat detector. Over the years, these devices have had many improvements in their design and operation. The basic method of heat detection still uses a fusible link developed to respond to a specific (high) temperature. This melting or softening of the alloy element allows a spring to expand in the detection device, which causes a contact closure (short) on an initiating device circuit.  These commonly used mechanical devices protect the area or “spot” in which they are installed, and are thus called spot-type detectors.

Thinking outside the box, a linear heat detection (LHD) cable was invented by Gerald Holmes in 1938, where the metal alloy wasn’t used to allow a spring to cause an alarm signal through contact closure but rather the use of plastic insulation separated the spring-action of two wires. The two wires in Holmes’ device were made of steel and had a stiff, spring-like quality to supply the movement needed to cause the shorting action. One insulated wire was wrapped spirally around the other straighter wire. The resulting tension caused them to come in contact with each other when their temperature sensitive insulation softened or melted. The advent of this linear detection method marked the point when regular heat detectors became known as spot-type detectors and the new devices as line-type heat detectors, or “linear” heat detectors, since a fire could be detected all along its length.

The Protectowire® Company, Plymouth, Mass., originated this type of electrical conductivity heat detection cable with a listed spacing of 15 to 50 feet and still sells it today. Protectowire now has a fiber optic version of this product which uses a quartz glass fiber optic conductor system and works on several known light scattering principals. Their special optical cable is connected to their proprietary control unit which transmits the light, analyzes the refracted signal and can indicate up to nine levels of alarm signal through traditional dry contacts. The fiber optic cable can be configured either with a single end (think Class B), or in a closed loop configuration that has both ends connected to the controller for a fashion of Class A redundancy.

Like their original heat sensitive steel wire product, this fiber optic version can detect heat anywhere along its length, which can be up to 5,000 feet. Both the steel wire type and the newer fiber optic version require connection to their proprietary equipment. This mini control module is installed at the Fire Alarm Control Panel (FACP) and the lengths of fiber optic cable originate from it following the specific manufacturer’s instructions. The Protectowire panel uses built-in resistance calculation software to determine the distance from the panel to the fire, measured in feet and therefore, can determine the location of the fire, its intensity and direction it is spreading.

Thermocable® is another brand of LHD manufactured by the company of the same name and works similarly to the original Protectowire product. Thermocable seems slightly more flexible in its use considering the distance locating module is optional. Of course, without this module the benefit of knowing how far away or where the fire is located is also lost.  However, it also means that you can connect this cable to any new or existing FACP or one of your fire alarm system’s monitor modules if using an addressable panel. Leader wires can be run in conduit from the panel to the detection area where the runs of Thermocable can then begin, or the Thermocable can be tied right into the FACP or the monitor modules.  The manufacturer states the spacing of their product is 35 feet but runs can be as long as 10,000 feet.

A third kind of LHD option is manufactured by Alison Control. The Alison Control Thermal Sensor is comprised of a sensing element inside an Inconel® tube, which is a specialized aerospace alloy manufactured to survive in extreme temperatures. The center conductor is electrically insulated from the outer sheath by a ceramic thermistor material. Ceramic materials have an electrical resistance that decreases as the temperatures rises, which a control module uses to determine heat levels present. It works on the averaging principal so it not only initiates an alarm when a predetermined fixed-temperature is reached, but is able to sense the build up of heat along its length and initiate an alarm when the average temperature reaches a predetermined point. The proprietary control module can provide several independently adjustable fixed-temperature alarm points, be used as a rate-of-rise detector, identify the location of a hot spot on the detector and can also provide temperature measurements in that room or space during a fire.

This product is manufactured in lengths of 25 and 50 feet and longer lengths up to 1,000 feet may be assembled from a number of shorter lengths. Since each section may be coupled together using the hermetically sealed connectors on each end, it is also possible to assemble several lengths having different temperature ranges so that hazards with different ambient temperature or heat conditions can be protected more economically than using several single lengths. 
Yet another type of LHD, which uses the electrical conductivity method to determine dangerous heat levels and is also suitable for harsh environments, is Fenwal’s line-type heat detector. The Fenwal Continuous Fire Detection System® has “the unique ability to detect specific overheat conditions at any point along the entire length of its sensing element run without regard for rate of temperature rise or average ambient temperature.” According to the manufacturer, this capability offers greater sensitivity and response than systems relying upon an “averaging” technique. The system can also connect several sensing elements with different temperature ratings and protect several areas having different temperature detection needs. The coaxial sensing element of this device is comprised of a slender Inconel® tube packed with a thermally-sensitive eutectic salt surrounding a small center wire made of nickel.

Varying lengths of these sensing elements, programmed to various temperature settings, are connected in series to a control unit which provides a small electrical charge into the sensing element. When high heat is applied to any point along the detector’s length, the resistance of the eutectic salt drops and causes current to flow between the outer covering and the center conductor.  This current flow is sensed by the control unit, which produces a signal to actuate an output relay to cause an alarm on an IDC. Unlike wire and fiber optic cables, the Inconel® tube protects these last two detectors allowing them to be re-set and re-used after a fire.

Although not as common as, and much more expensive than our basic spot type heat detectors, the LHD is an important tool in our arsenal of fire detection products and has many specialized commercial applications. All the products mentioned above are suitable for use in very cold environments and therefore are often installed in cold storage areas. Due to their physical characteristics these products provide a practical alternative when protecting voluminous cable trays, long tunnels and hard to access shafts.  If you’ve never used these products why not contact any of the mentioned manufacturers or your local distributor today to find out more.

Greg Kessinger, SET, CFPS, can be reached at [email protected] or


Basic Construction of Linear Sensing

1. Various outer jackets suitable for different environmental conditions
2. Material ‘wrap’ which provides additional protection for the inner conductors of the detector
3. Heat-sensitive polymer which coats the inner conductors
4. A twisted pair of steel conductors to comprise the core