Temperature Measurement


Cable-type heat sensing systems

May 2020 Temperature Measurement

Alien Systems & Technologies (AST) offers a wide range of automatic fire detection systems from advanced ultra-smart addressable point-type detection systems to regular conventional systems. It also offers specialised conveyor belt/moving fire black body infrared detectors, along with the ProReact line-type heat sensing cable – otherwise known as linear heat detection (LHD) cable.

Sensing fire conditions using cable

Broadly speaking there are two methods to sense fire conditions using cable. Most commonly used in the South African market is the digital type method: essentially this type of cable looks for a short circuit as a means of triggering an alarm condition. The cable is comprised of two conductors, each surrounded with an insulator that is designed to melt at a certain temperature. Both these conductors are then twisted and remain under torsion. This twisted pair of conductors is then sheathed either in a standard PVC sheath or with options of UV protection (nylon) and/or mechanical protection (stainless steel braid) or chemical resistant (polypropylene) sheaths.

Digital types

The standard digital-type linear heat sensing cable comes in a range of temperature trigger point options from 68°C to 185°C. These cables are known as ‘digital’ because they are ‘off’ below the activation temperature and irreversibly switch ‘on’ when the activation temperature is reached. The primary advantage of using line-type detection is that the mechanism can be spread over an application, bringing it closer to the source of the fire and thereby improving response times over traditional point-type heat detecting devices. The entire linear detection range is UL listed UL521 and FM approved.

A UL approved digital interface monitor module (DIMM) is available to provide accurate alarm location along the LHDC cable, as well as additional benefits when used in conjunction with ProReact Digital LHD cable. The ProReact DIMM is designed to enhance the functionality of existing or new digital LHD systems by monitoring up to two zones. A unique interlock/coincidence detection mode eliminates the possibility of false alarms by requiring both LHD cables to trigger before an alarm is transmitted. A built-in digital display shows the state of each zone, including the exact distance in metres to the alarm point. It also includes an RS-485 Modbus RTU output for integration with a PLC or scada system.


AST also offers specialised digital-type cable of the low smoke zero halogen type. This variant is UV stable with a hydrocarbon resistant coating that minimises the release of smoke and harmful emissions in the event of a fire. The cable is also flame retardant and oil and fuel resistant, ensuring it can be used in harsh environments. Inherent UV stability means it is suitable for indoor and outdoor applications. This cable is also UL/FM approved.

In addition, another variant known as ProReact very high temperature (VHT) digital LHD cable is designed to detect overheating and fires in environments that are normally exposed to extremely high temperatures. The cable has an activation temperature of 230°C and can withstand continuous ambient operating temperatures of up to 170°C. ProReact VHT digital LHD cable is approved in line with UL521 and FM Class 3210 standards and is fully RoHS compliant. It is also used in conjunction with automatic fire extinguishing systems in engine compartments and floating roof tanks.

Analog types

The second method of sensing a fire with cable is to use analog linear heat detection cable. This method continuously responds to changes in temperature and the technology allows the facility to programme an early warning pre-alarm as well as the specified alarm temperature. If the temperature surrounding the ProReact analog LHD cable reaches the pre-alarm point, the control unit triggers a warning, giving the user time to survey the area at risk. Only when the temperature reaches the specified set alarm point will the control unit trigger full alarm. The optional two-stage programmable alarm setting makes this method of overheat detection flexible and ideal for use in a variety of environments and applications. The technology automatically compensates for changes in ambient temperature to maintain the accuracy of the alarm temperature as well as offering up to 500 metres of continuous detection per control unit.

The ProReact analog LHD cable is resettable which means it is not always necessary to replace the cable after an incident. Once the alarm has been triggered, and depending on the severity of the incident, the system can simply be reset with minimum disruption and inconvenience.

Applications for linear heat sensing cable include conveyor belts, in-rack sensing for warehousing storage, both floating roof and fixed roof petrochemical tanks, cold storage areas, tunnels, electrical cable runs, car parks and escalators.

Rate-of-change types

AST also offers a unique ProReact linear rate-of-change cable sensing system that has been specifically designed to detect liquefied natural gas leaks. The combination of a highly sensitive cable and control unit uses advanced digital signal processing to detect a rapid change in temperature. The nature of its intended application demands that the system is sufficiently robust to withstand a range of hazardous environments. This system is simple to install and designed to integrate into any building management system. It continually adjusts to changes in the surrounding environment to maintain a high level of sensitivity, which eliminates false alarms and guarantees rapid response to an incident. The technology offers a versatility and functionality not readily available in liquid natural gas facilities and is designed to enhance existing detection systems and provide early warning of abnormal situations. The cable is also self-restoring after an incident, allowing the system to be reset rather than replaced. It is qualified for use in hazardous areas (ATEX/IECEX zones 0, 1 and 2; Gas Group IIC; T Class T5).


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