Linear heat detection has been around for over 80 years but is still relatively unknown to many in the Fire Protection Industry. In this article, we will delve into the construction and operating principles of digital linear heat detection, applications and guidelines for fire protection engineers, the use of NFPA codes and standards as well as FM and UL guidelines.
Construction and Operation
LHD or LHDC – Linear heat detection or linear heat detection cable is a continuous heat detector capable of detecting heat anywhere along its length and is often described as a continuous chain of individual spot heat detectors. Digital linear heat detection is comprised of two metal conductors, both individually insulated by heat sensitive polymers designed to soften at a specific temperature. When the surrounding ambient temperature meets, or exceeds the detector’s fixed temperature, the heat sensitive polymer melts. Allowing the twisted spring metal conductors to make contact creating an electrical short resulting in the monitoring device initiating an alarm condition. The detector will short at the first heated point anywhere along its length, it does not require a specific length to be heated to initiate an alarm. Standard digital linear heat detectors do not require a proprietary interface unit, the detector is a contact device.
Digital linear heat detection does not require calibration for ambient temperature changes and alarm point location is possible with the proper control equipment. This type of digital linear detector is non-restorable but damaged or activated sections can be easily replaced by splicing. Zone lengths are directly related to the monitoring equipment and can extend up to 3000+ meters.
Proper Installation and Testing
Protectowire Digital Linear Heat Detectors are defined as a heat actuated automatic fire detector and
are intended to be used on a supervised initiating circuit. The detector must be installed in continuous runs without taps or branches in accordance with applicable sections of NFPA 70 National Electrical Code, NFPA 72 National Fire Alarm Code, or as determined by the local authority having jurisdiction.
The listed spacing shall be used as a guide or starting point in a detector installation layout. Reduced spacing is required based upon factors such as ceiling height and construction, physical obstructions, air movement, or the authority having jurisdiction.
When installing digital linear heat detectors, one must avoid mechanical stress of the detector such as pinching, crushing or bending. Do not make 90º bends, all bends must be made by hand with a minimum of 6.4cm radius. The installer should use recommended fasteners only and not overtighten the fasteners. UL Listing and FM Approval both mandate that the detector cannot be painted. Thermal coefficient (expansion and contraction) is a consideration that must be allowed for during installation. The detector should never be stored above its maximum ambient temperature.
Lastly, all linear heat detection should be tested prior to starting installation – to assure the detector is not already shorted. Testing digital linear heat detection is both quick and simple. Per NFPA 2022 Section 14 Table 14.4.3.2 Test Methods Initiating Devices – 17.4.b Fixed- Temperature, Non- restorable Line Type (Digital): a Heat test shall not be performed. Functionality shall be tested mechanically and electrically. Loop resistance shall be measured and recorded. Changes from the acceptance test shall be investigated.
CTI Series Linear Heat Detector
While there have been significant developments involving the polymers employed within digital linear heat detectors, their overall operating principle have remained relatively unchanged until now. Protectowire’s Confirmed Temperature Initiation (CTI) Series Linear Heat Detector directly addresses the issue of false alarms resulting from mechanical damage. The CTI Series Detector is available in the same jackets and fixed temperature ratings and have the same installation practices and listed spacing (FM and UL) as all Protectowire Digital Linear Heat Detectors.
Standard digital linear heat detectors offer a single detection criteria. Monitoring equipment initiates an alarm condition when the detector is shorted. This can happen from heat actuation (true alarm) or from physical damage (false alarm). Using thermocouple technology, the CTI Series can distinguish between a short caused by physical damage or a short caused by heat actuation and is the only multi-criteria Digital Linear Heat Detector. Dissimilar metal conductors enable the CTI Series to determine the exact alarm point location as well as the temperature of any short. If the temperature of the short is below the linear heat detectors fixed temperature, a short fault or trouble condition is initiated. If the temperature of the short is above the linear heat detectors fixed temperature an alarm condition is initiated. This makes
the CTI Series an ideal Digital Linear Heat Detector for extinguishing systems. The CTI Series works in conjunction with the CTM-530 interface module. This interface module can be connected to any monitoring device using relay contacts. Each CTM-530 control module can support up to 4000 feet (1220 meters) of CTI Series Detector per zone.
Common Applications
Digital linear heat detection’s most notable attributes are the ability to detect heat anywhere along its length, extended service life, ease of testing, consistent reliable performance, and ability to offer proximity detection. These attributes encourage the use of digital linear heat detection in a core group of industries and applications which are, but not limited to, the following:
Power Generation
Conveyors, Cable Trays, Transformers, Switch Gear, Electrical Equipment, Cooling Towers:
Large drive motors, belts and roller bearings are stressed to the limit in dirty, dusty, and sometimes hazardous environments present in the mining industry. Protectowire Digital Linear Heat Detectors can be installed over the belt or alongside the conveyor near the bearings for overheat detection. Protectowire’s versatile construction enables it to be installed in, around, or on controllers, switches, motors, or other conveyor components where the risk of overheat or fire is greatest. Protectowire also offers intrinsically safe solutions for classified hazards.
Petrochemicals
Fixed and Floating Storage Tanks, Pipelines, Offshore Platforms, Refineries:
Storage tanks, pipelines, offshore platforms, and refineries utilize numerous processes involving flammable liquids and toxic chemicals. Every facet of these processes must be monitored regardless of the hazardous challenges they present. Customized control panels and specialty detectors allow us to fully serve the petrochemical industry.
Transportation
Aircraft Hangars, Vehicle Systems, Tunnels, Bridges, Metro Systems, Rolling Stock:
The highest risk of fire danger within airport facilities is fuel storage, cargo materials and from the aircraft themselves. Digital linear heat and the CTI Series Detectors both offer minimal maintenance and are not adversely affected by temperature fluctuations making them ideal detectors for the transportation industry. When compared to spot-heat and infrared detectors, linear heat detection can greatly reduce installation costs while making annual testing easy and simple.
Warehousing
Freezers, Refrigerated Storage, Self-Storage Units, Stables, Barns:
CTI Series Linear Heat Detector provides the most dependable and cost-effective fire detection option for refrigerated storage applications. Unlike standard digital linear heat detectors, damage to the CTI Series Linear Heat Detector will not cause a false alarm. Most freezers and cold storage facilities utilize double interlocked pre-action systems, which makes the CTI Series an ideal Digital Linear Heat Detector for this particular type of suppression system.
Renewable Energy
Solar Panel Arrays, Wind Turbines, Battery Storage:
Protectowire CTI Series Linear Heat Detectors robust design is the ideal choice for solar panel installations. Unlike traditional spot heat detectors, the CTI Series Detectors can be placed directly onto the underside of solar panels providing immediate proximity detection. Since solar panels are installed outdoors, this means they are subjected to hail, high winds, lightning, ice, bird nests, rodents as well as other natural forces that can damage the panels or its components. The CTI Series Detectors will only issue an alarm condition when its fixed alarm temperature has been reached.
Protectowire FireSystems
Protectowire FireSystems is the originator of Linear Heat Detectors and the leading manufacturer of Special Hazard Linear Heat Detection Systems. Our solutions include complete alarm systems for fire protection suitable for a vast array of industrial and commercial applications. Our products are made
in the USA at our factory located in Pembroke, Massachusetts. Since 1938, many of the world’s largest companies have chosen Protectowire to protect their valuable assets and business operations.
