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Copyright International Fire Protection 2016
One of the biggest up and coming uses for linear heat detection is the protection of the renewable energy industry.

Understanding digital linear heat detection

Linear heat detection has been around for almost 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 – Linear heat detection 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 initiating an electrical short resulting in the monitoring device receiving it as 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 replaced by splicing. Zone lengths are directly related to the monitoring equipment and can extend up to 10,000 feet (3048 meters).

Proper Installation and Testing

Protectowire Digital Linear Heat Detectors are approved as a heat actuated automatic fire detector and are intended to be used on a supervised initiating circuit of an approved fire protective signaling control unit. 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 2.5” (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 2016 Section 14 Table Test Methods Initiating Devices – (d) Heat Detectors section 2. 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.

Linear heat detection is highly versatile and can be used in a variety of applications.

Linear heat detection is highly versatile and can be used in a variety of applications.

Five Simple Questions – a Guide to Digital Linear Heat Detection

Question 1: What type of application are you protecting?

Digital linear heat detection is used in commercial and industrial applications. The detector is predominantly used within the special hazards fire protection industry designed to protect property or processes and is not considered to be life safety.

Question 2: What environmental conditions are present?

It is common to employ digital linear heat detection in applications with extremely harsh environmental conditions. Detectors are available with a ambient operating temperature range of -60°F to 250°F (-51°C to 121°C) and can be placed directly into environments containing bases, acids, solvents, hydrocarbons, dust, oil, and water.

Question 3: Interior or exterior application?

The outer jacket of any digital linear heat detector provides protection from the surrounding environment. Knowing whether the location is indoor or outdoor will allow you to choose the correct protective jacket. Most PVC, nylon, polypropylene, or other jackets have UV additives designed to slow UV degradation and should be used for interior applications. Fluoropolymer jackets offer superior UV resistance as this is the only jacketing compound that is naturally UV resistant making it ideal for exterior applications.

Question 4: Is this a classified hazard?

The use of intrinsically safe and explosion proof are often mistakenly interchanged:

Explosion proof devices are designed to contain explosions or flames produced within them without igniting the surrounding flammable gases or vapors (source – businessdictionary.com).

Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy, electrical and thermal, available for ignition (source – Wikipedia).

There are no explosion proof digital linear heat detectors. Digital linear heat detectors can be made intrinsically safe when using an intrinsic safety barrier (ISB). When using an ISB, the designer and installer should use good engineering practice, follow all local codes/standards and receive the approval of the local authority having jurisdiction.

Question 5: What will the maximum ambient temperature be?

Every digital linear heat detector has a maximum ambient operating temperature. Knowing the maximum ambient temperature, including seasonal temperature fluctuations, will allow the designer to determine the correct alarm temperature detector for the application.

CTI Series Linear Heat Detector

While there have been significant developments involving the polymers employed within the detector, 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.

One of the biggest up and coming uses for linear heat detection is the protection of the renewable energy industry.

One of the biggest up and coming uses for linear heat detection is the protection of the renewable energy industry.

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:


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.

TransportationAircraft 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’s 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.

Power Generation

Conveyers, 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.


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.

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. For over 75 years, many of the world’s largest companies have chosen Protectowire to protect their valuable assets and business operations.

For more information, go to Protectowire.com

Brenton Harris is the Director of Sales at Protectowire FireSystems and a frequent speaker at industry events.

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