Technical·4 min read
What Is Linear Heat Detection (Cable and Fiber Optic), and Where Is It Used?
Linear heat detection solves a problem that point detectors physically cannot cover: continuously monitoring, with exact location, where heat is rising along a cable tunnel, conveyor line, road tunnel, or car park that stretches for kilometres. Along the line, a single sensing cable (digital heat cable or fiber-optic DTS) tells you at which metre the temperature rose; so a single continuous sensor line replaces thousands of point detectors. This article explains, from an engineering viewpoint, exactly what the system solves, how it works, which areas use it, and where it is not suitable.
A-Pro Engineering
Linear heat detection solves a problem that point detectors physically cannot cover: continuously monitoring, with exact location, where heat is rising along kilometres of cable tunnel, conveyor line, road tunnel, or car park. Along the line, a single sensing cable (digital heat cable or fiber-optic DTS) tells you at which metre the temperature rose. This article explains, from an engineering viewpoint, what the system solves, how it works, where it is used, and where it is not suitable.
What does it solve? (the problem)
- Long/linear asset — point detectors over kilometres are impossible and hard to maintain,
- Dusty/humid/EMI/explosive environment — a point smoke detector will not work anyway,
- The system tells you at which metre it heated — fast response to the correct zone.
How does it work? (heat cable and fiber-optic DTS)
- Digital heat cable — insulator melts at the threshold, conductors touch, location is found,
- Fiber-optic DTS — laser backscatter (Raman) measures temperature at every metre of the fiber,
- Absolute temperature + rate-of-rise metre by metre; unaffected by dust/humidity/EMI.
Where is it used?
- Cable galleries/tunnels, road and metro tunnels,
- Conveyor belts (coal/material), enclosed car parks, transformer/power lines,
- Underground mine galleries, tank rim-seal, warehouse rack lines — linear asset + harsh environment.
Where is it NOT suitable? (its limits)
- Volumes needing very early warning (data centre, museum, server) — aspirating/point smoke needed,
- Normal-ceiling room/office — a point detector is already ideal; linear is unnecessary,
- Smoke stage — being heat-based, it does not see early smoke and can be late,
- Does not suppress — only detects; suppression is done by sprinkler/deluge/water mist/foam.
What to look for when selecting
Correct linear heat detection selection depends, beyond the type (heat cable / fiber DTS), on several critical technical criteria:
- Ex-Proof (ATEX) certification — in explosive/methane-risk zones such as tunnels, mines, and cable galleries, the sensing cable and interface must have the correct Ex class,
- FM / UL / VdS approval — the device must be tested and certified under real conditions (e.g. FM 3210); an unapproved system is unreliable in the field,
- Alarm temperature class and detection logic — fixed threshold (68°C, 88°C, 105°C…) or rate-of-rise; a threshold that will not false-alarm at the ambient temperature is chosen,
- Coverage / location resolution — in DTS, with what accuracy (± metres) the hot spot is located and the zoning resolution; this determines whether response is directed to the correct point,
- Maximum cable/loop length — how many kilometres of line a single channel/unit can monitor; coverage in long tunnels/galleries is planned with this,
- Mechanical and environmental durability — the cable’s ambient-temperature tolerance, chemical/mechanical resistance, and IP rating; preserves lifespan on harsh routes.
Suitability summary
| Situation / environment | Is linear heat detection suitable? |
|---|---|
| Cable gallery, tunnel, conveyor line | Yes — its primary application |
| Enclosed car park, transformer, mine gallery | Yes — linear asset |
| Data centre, museum (very early warning) | No — aspirating/point smoke needed |
| Normal-ceiling room, office | No — a point detector is economical |
| Suppression need | No — it only detects |
We covered the field counterparts in our cable gallery and conveyor articles, and the enclosed car park risk in our car park article.
Summary
Linear heat detection is a solution that, with a single sensing element running along the line (heat cable or fiber-optic DTS), continuously monitors at which metre heat is rising: it is ideal for linear assets like cable gallery, tunnel, conveyor, and car park; it is not suitable for volumes needing very early warning and for normal rooms, and does not suppress on its own. It is based on NFPA 72 / EN 54-22 (+ NFPA 502 in tunnels) + ATEX + BYKHY. At A-Pro Engineering we design linear heat detection integrated with building detection and suppression systems; contact us for your project.
This content is for information purposes. Binding design must be done on a project basis based on the protected risk, route geometry, environmental conditions, and the actual conditions of the facility, together with NFPA 72, EN 54-22, NFPA 502, ATEX legislation, and the current edition of BYKHY.
© 2028 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
What exactly does linear heat detection solve?+
It solves the protection of long/linear assets that point detectors cannot cover physically and economically: continuously monitoring where heat rises along kilometres of cable tunnel, road/metro tunnel, kilometres of conveyor belt, or a large open car park. On such lines, placing a point detector every few metres is both impossible and a maintenance nightmare; moreover, in very dusty, humid, electromagnetically noisy, or explosive environments a point smoke detector will not work anyway. The linear system fills this gap with a single sensing element running along the line and, by giving the exact location of a temperature rise, enables response to the correct zone.
How does it work? (heat cable and fiber-optic DTS)+
There are two main types. Digital heat-sensing cable: two conductors are separated by an insulator that melts at a specific temperature; when the threshold is exceeded, the conductors touch and trigger an alarm, and the location is found from the short-circuit position. Fiber-optic distributed temperature sensing (DTS): by analysing the backscatter (Raman) of laser light sent along a single fiber-optic cable, the temperature at every metre of the fiber is measured; thus a continuous temperature profile and heating rate along the line is obtained. DTS is preferred in critical infrastructure such as tunnels and galleries because it gives both absolute temperature and rate-of-rise metre by metre. Both types are unaffected by dust, humidity, and electromagnetic noise.
In which areas is it used?+
Linear heat detection is used where there are long, linear assets and point detectors are inadequate: cable galleries and cable tunnels, road and metro/rail tunnels, conveyor belts (especially coal/material handling), enclosed car parks, transformer and dry-type transformer windings, pipe bridges and power lines, underground mine galleries, tank roof rim-seals, and warehouse/rack lines. The common thread is that the asset to protect is linear and the environment is dusty/humid/harsh. We covered the field counterparts of these areas in our cable gallery and conveyor articles. Correct application requires selecting the cable type (heat threshold or DTS) and its route according to the risk.
Where is linear heat detection NOT suitable?+
The system is heat-based; therefore it catches, too late, early-stage fires that begin with smoke and have not yet raised the temperature. In a data centre, museum, or server room that needs very early warning, linear heat detection is inadequate; aspirating (VESDA) or point smoke detection is needed there. In volumes where point detectors are already ideal — an open, normal-ceiling room or office — a linear system is unnecessary. Also, heat cable/DTS is not a suppression system; it only detects and locates — suppression is done by sprinkler, deluge, water mist, or foam. The sensing element must be routed at the right distance and path along the protected asset; if wrongly placed it sees heat late. So the tool must be selected by the fire's early signature (smoke or heat) and geometry.
What is the difference between linear heat detection and a point detector?+
A point detector protects a single location; it is distributed across a ceiling in a volume and waits for smoke or heat at that point — very effective and cheap in normal rooms, but neither possible nor sensible for kilometres of line. A linear system runs along the protected asset itself; with a single continuous sensor line it monitors every metre, works in dusty/humid/EMI environments, and gives the exact location of heating. In return, being heat-based, it does not see smoke and may be later than a point smoke detector in the early stage. The two are not rivals; the point detector protects the volume, the linear system the linear asset. In critical facilities they are used together, integrated with building detection.
Which standards govern its design?+
Detection design is based on NFPA 72 (and the EN 54 series; relevant parts such as EN 54-22 for linear heat detectors); for tunnel applications, tunnel fire safety guides and references such as NFPA 502 come into play. In explosive environments, equipment is selected per ATEX/Ex classification. The system's detection temperature threshold, rate-of-rise, and location resolution are calculated from the protected risk and route; zoning is set up in DTS. In Türkiye, design is based on these standards together with BYKHY. Correct type (heat cable / fiber DTS) and route selection is the key both to reducing false alarms and to seeing the fire early at the correct location.
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