In linear heat detection the cable itself is the detector. Instead of installing spot detectors in long and narrow spaces such as conveyor belts, cable galleries, shelf lines and tunnels, a single line is run. Its ability to remain operational in environmental conditions such as dust, moisture, vibration and washing that would disable spot devices is the main reason for this technology.
LHD and fiber optic DTS are not the same thing
These two technologies are often mentioned interchangeably in the field; however, they are designed for different applications.
| Linear heat cable (LHD) | Fiber optic DTS | |
|---|---|---|
| What it does | Triggers an alarm when a predetermined temperature threshold is reached | Measures temperature every meter along the line |
| Position information | Zone-based; fault location can be measured in analog types | Meter-level accuracy |
| Typical length | Hundreds of meters | Kilometers |
| Trend monitoring | None — silent until threshold is reached | Available; heating trend is visible before alarm |
| Cost level | Significantly low | High; requires a central measurement unit |
| Suitable for | Cable gallery, conveyor, shelf racking, parking garage, cooling tower | Long tunnel, high-voltage line, massive conveyor, power generation facility |
In short: if budget and scope are routine for an industrial line, LHD, if there is an operational need for the answer "where is the where" then DTS is the correct answer.
Two fundamental types
Digital (fixed-temperature, non-resettable) cable
Two conductors separated by a polymer that melts at a specific temperature. When ambient temperature reaches that value, the polymer melts, the conductors make contact, and the circuit shorts to produce an alarm. Simple, robust, and economical. The cost is permanent damage to the triggered section: the alarmed portion must be cut out and replaced with new cable. EN 54-28 covers non-resettable line-type heat detectors.
Analog (resettable, linear) cable
A quantity such as resistance or capacitance along the cable is measured as it changes with temperature. When ambient cools, the cable returns to its original state and continues in service. Additionally, in some systems, the distance in meters from the cable start to the fault or alarm can be measured; this significantly reduces search time on long lines. EN 54-22 covers resettable adjustable line-type heat detectors.
The selection criterion is clear: if the probability of line activation is low and access is easy, digital type; if there is frequent activation probability, the line is long and access is difficult, or the alarm location must be known, analog type is selected.
Temperature class selection
LHD cables are manufactured at different triggering temperatures. The correct class is selected not according to the average temperature of the environment, but according to the highest normal operating temperature that may be encountered with a safety margin applied. Items to be considered:
- Temperature under a sun-exposed roof in summer — can exceed 60 degrees in the field.
- Local heating around furnaces, dryers, boiler rooms, and cooking lines.
- Point temperature in conveyor drive units and bearing areas.
- Heat radiated by the loaded cable bundle itself in a cable gallery.
Selecting the class too low produces seasonal false alarms; selecting it too high delays detection. In uncertain cases, recording temperature for several days is the most reliable method.
Where it is used and how it is installed
| Application | Route | Caution points |
|---|---|---|
| Cable gallery, cable duct | On top of the cable bundle, in contact with the bundle | Cable bundle routed to ceiling sees heating late; contact is essential |
| Conveyor belt | Along the belt, extra sensitivity over drive and tensioning unit | Heating from bearing and slip typically starts at the drive end |
| High-bay warehouse | Along the shelf row, at shelf-top level | Protected route against forklift impact; cable not under tension, suspended by supports |
| Enclosed parking garage | Over parking rows | Rapid temperature rise in vehicle fire; wet-dry cleaning should not affect the cable |
| Cooling tower, filter unit | On top of fill material | Continuous moisture; jacket material must be selected suitable for the environment |
| Transformer and generator area | Around the body and cable entry region | Electromagnetic interference; shielded type or fiber alternative is evaluated |
Three rules in installation make a difference in the field: cable is not pulled tight, suspended at regular intervals by supports; at sharp corners, the minimum bending radius permitted by the manufacturer is observed; splices and termination elements are made with the manufacturer's own components. Splice points are the most common source of faults in these systems.
Jacket material
The outer jacket of LHD cable is selected according to the chemistry of the protected environment. Standard jacket is sufficient in many locations; however, where acid or alkali vapor, oil, solvent, or continuous washing is present, durable jacket types are used. Wrong jacket selection leads the system to develop insulation faults along the line within one or two years, and the cause is usually recorded as "cable degraded."
Panel connection and fault finding
LHD line is connected via an interface module on an addressable panel or to a conventional zone input. Two points are observed in connection design:
- Line-end supervision. Cable break should produce a fault not an alarm; if the termination element is not installed correctly, a broken line leaves no trace on the panel.
- Zoning. The longer the line connected to a module, the wider the search area at the moment of alarm. In long galleries, the line is divided into sections of reasonable length; each section receives a separate address.
In fault search, if analog type is used, distance measurement gives direct results. In digital type, the line is halved for narrowing; for this reason, placing splices and junction boxes at accessible points is the most valuable decision made from day one.
Maintenance
In annual maintenance, the line undergoes visual route inspection (sag, crushing, condition of junction boxes), line resistance or system parameter is measured and compared to commissioning value, the termination element is verified, and a heat test is performed at one point using the manufacturer's recommended method. For digital-type cables, testing is done with the manufacturer's test apparatus, not on the cable itself; heating for a "test" permanently consumes that section.
For our maintenance scope is part of detection system maintenance, for introductory-level information, please refer to our linear heat detection cable articleA zone monitoring module connects an entire conventional detector line to the addressable loop. Critical areas are renewed with addressable devices while secondary zones continue to operate with their existing wiring. This provides significant cost advantage in capacity upgrade and renovation projects.
Project and product
For digital and analog linear heat detection cables, accessory elements, and interface modules the LHD product group consult; for your line's temperature class and route plan contact usA zone monitoring module connects an entire conventional detector line to the addressable loop. Critical areas are renewed with addressable devices while secondary zones continue to operate with their existing wiring. This provides significant cost advantage in capacity upgrade and renovation projects.
Frequently Asked Questions
What is a linear heat detection cable?
It is a line-type heat detection method in which the cable itself functions as the detector. When the temperature at any point along the cable reaches a preset value, an alarm is generated. It is used in place of point detectors in long and harsh environments such as conveyors, cable galleries, rack lines, and parking garages.
What is the difference between digital and analog linear heat cable?
In digital (non-resettable) cable, insulation conductors that melt at a specific temperature make contact and trigger an alarm; the section that alarmed must be cut out and replaced. In analog (resettable) cable, a quantity that varies with temperature is measured; when the environment cools, the cable continues to be used, and in many systems the alarm location can be measured in meters.
Are linear heat cable and fiber optic DTS the same thing?
No. LHD cable generates an alarm when a temperature threshold is reached and location information is limited. Fiber optic DTS, on the other hand, measures temperature at every meter along the line, shows heating tendency before the alarm, and reports location at meter-level accuracy. DTS is suitable for lines spanning kilometers, and its cost is significantly higher.
Where is the linear heat cable routed in cable galleries?
Not on the ceiling, but on top of the cable bundle and in contact with the bundle. In cable fires, heat first rises within the bundle itself; a line routed on the ceiling detects this heating late. In long galleries, the line is divided into sections and each section is given a separate address to narrow down the search area when an alarm occurs.
How is the temperature class of linear heat cable selected?
It is selected based not on average temperature, but on the highest normal operating temperature that may be encountered in the environment plus a safety margin. The summer temperature under the roof, the vicinity of ovens and dryers, the conveyor drive unit, and the self-heat of loaded cable bundles must be taken into account; in case of doubt, temperature records are obtained over several days.