Linear Heat Detection Cable and Fibre Systems Explained

Linear heat detection is a class of fire detection in which a cable, run continuously through the protected area, acts as the sensor along its entire length. Heat anywhere along the cable triggers an alarm, with modern systems also locating the heated point along the cable to within a metre or two. Linear heat detection wins in spaces where point detectors are impractical: cable trays, tunnels, conveyors, car parks, cold stores, and outdoor process plant. The technology divides into a small number of distinct families, each with its own physics and its own application sweet spot.

This article covers digital heat-cable, analogue resistive cable, fibre-optic distributed temperature sensing, and pneumatic tubing systems. It explains the principle of each, the spaces where each wins, the location and sensitivity options, and the pitfalls that catch out installers used to point detection.

Digital heat detection cable

Digital heat-cable consists of two insulated conductors twisted together, with the insulation chosen to melt at a calibrated temperature. Below that temperature, the insulation keeps the conductors apart and the cable acts as an open circuit. When the surrounding temperature reaches the calibrated melt point at any location along the cable, the insulation softens, the spring-tensioned conductors short, and an alarm signal propagates back to the controller.

The digital cable is simple, robust, and well suited to dirty industrial environments. It does not locate the heated point along the cable; the controller knows only that some part of the cable has alarmed. Adding location requires resistive analogue cable instead. Digital cable is one-shot at the alarm point: once the insulation has melted, the cable section must be cut out and replaced. That is a deliberate design feature, not a flaw, because alarm-then-replace gives a clear audit trail.

Analogue resistive heat-detection cable

Analogue resistive cable changes its resistance continuously as temperature rises along its length. The controller measures the cable resistance and applies an algorithm to estimate both the temperature profile along the cable and the location of any hot spot. Multiple alarm thresholds can be set: a fixed-temperature threshold for absolute alarm, a rate-of-rise threshold for fast-developing fires, and a cumulative threshold for prolonged elevated temperature.

The advantage over digital is the location and the resettable nature of the cable: a brief temperature excursion that triggers a rate-of-rise alarm does not destroy the cable. The disadvantage is cost and complexity, and the requirement for periodic calibration. Analogue resistive is the dominant technology for tunnel, cable-tray, and conveyor protection in markets where location of the alarm is operationally important.

Fibre-optic distributed temperature sensing

Fibre-optic distributed temperature sensing, or DTS, takes location to a different level. A standard optical fibre is run through the protected area as the sensor cable. The controller injects a laser pulse and measures the back-scattered light continuously along the fibre using either Raman or Brillouin scattering analysis. The result is a complete temperature profile along the fibre at one-metre resolution or better, sampled every few seconds.

DTS gives temperature data along kilometres of fibre, with selectable alarm rules at every metre point. It can distinguish a hot spot from generally elevated ambient, can run multiple alarm zones along a single fibre, and is essentially passive at the sensor end so it works in environments that would destroy electronic detectors. Tunnels, long conveyors, electrical cable trays, and large transformers are typical applications. The cost of the controller is high but the per-metre cost over long distances is competitive against analogue resistive cable.

DTS also gives diagnostic temperature data that has value beyond fire detection. Many transformer monitoring schemes use DTS for both early-warning fire detection and routine thermal monitoring of windings and bushings, with the same fibre serving both functions.

Pneumatic tubing

The oldest linear heat technology is pneumatic. A continuous tube filled with gas runs through the protected area; heat anywhere along the tube increases gas pressure, and a pressure switch at one end signals alarm. Pneumatic tubing is cheap, simple, and immune to electrical interference, but it does not locate the alarm and has limited tube lengths.

Pneumatic remains in use in vehicle engine compartments, certain machinery enclosures, and some legacy industrial installations, and is occasionally specified for explosion-proof applications where electrical detection would not be permitted. For most modern installations, the digital and analogue cable families have replaced it.

Where linear heat detection wins

Linear heat detection wins in several recognisable spaces. Enclosed car parks, particularly multi-storey and basement decks, use linear heat cable along the ceiling because point detectors at the ceiling are vulnerable to vehicle exhaust contamination and impact damage from delivery vehicles. The cable, run in a steel tray, is robust and easy to inspect.

Cold-storage warehouses use linear heat in racking and along ceiling structures because point detectors struggle at low temperatures and condensation degrades them. Heat cable rated for the temperature range works reliably where smoke detection cannot.

Conveyor belts in industrial and mineral plants are protected by linear heat cable run along the belt, often the full length of the conveyor including transfer chutes. A bearing failure or belt-rubbing incident produces localised heat that triggers alarm before flaming combustion. Cable trays in power stations, data centres, and large industrial sites are protected by heat cable laid alongside or above the cable bundle. Tunnels for road, rail, and metro use DTS or analogue resistive for length and location.

Sensitivity and threshold setting

Linear heat detection is set to alarm at a temperature appropriate to the protected space, not at a default value. A car park might use a 68 degree fixed-point cable; a cold store might use 88 degrees because routine ambient is below freezing and minor warm spells could otherwise nuisance-alarm. A boiler room might use a 105 degree cable because routine ambient is high.

Rate-of-rise detection on analogue cable adds another dimension: a slow-developing space-heating excursion does not alarm but a fast localised temperature climb does. The combination of fixed and rate-of-rise thresholds is a powerful fire-versus-process discriminator in industrial environments.

Pitfalls and limitations

The first pitfall is mounting position. Linear heat cable detects the temperature at the cable; a fire at floor level whose heat does not reach the cable will not trigger detection. The cable must be mounted in the path of the rising hot gases from the design fire, which usually means at ceiling level or directly above the protected machinery, not below it.

The second is mechanical damage. Cable run in inaccessible voids or above ceilings is vulnerable to damage during subsequent works: a tradesperson stripping cable insulation off a heat cable to make a connection breaks the supervision and may not realise it. Mechanical protection through conduit or tray, in addition to careful labelling, is essential.

The third is cable length limits. Each system has a maximum cable run per zone or per controller channel, set by signal margin and by location accuracy where applicable. Long protected areas may need multiple controllers or multiple zones rather than one heroic run.

The fourth is integration with the wider fire alarm system. Linear heat controllers are usually separate panels with relay outputs that interface to the main fire alarm panel via input modules. The integration must preserve supervision: a fault on the heat-detection controller must propagate as a fault to the main panel, and a sounder activation must be triggered from the main panel's cause-and-effect rather than independently from the heat-detection controller. A common audit finding is heat-detection controllers wired directly to local sounders without main-panel integration, defeating system-wide cause-and-effect.

Comparison with point and aspirating

Point heat detectors cover a limited area each and need many devices for a long protected area. Linear heat covers an arbitrary length with a single cable. Aspirating smoke detection responds to an earlier stage but at higher cost; the two are complementary rather than competing. In car parks, aspirating in the entrance and exit zones combined with linear heat across the bulk of the deck is a common arrangement.

What this article does not cover

This article does not give specific cable mounting heights, alarm temperature values, maximum run lengths, or controller-specific configuration. Those are product- and standard-specific. EN 54-22 covers linear heat detection in Europe; UL 521 and FM 3210 cover similar ground in North America. The fundamentals pillar provides the broader detection context.

Linear heat detection is the right tool for spaces where heat is the most reliable signature, the protected length is long, and point detection is not practical. Used well, it gives robust, low-maintenance fire detection across spaces that no other technology covers economically.

Integration and routine testing

Linear heat detection integration with the main fire alarm system is a frequent weak point. The heat-detection controller, often supplied as a standalone product, presents alarm and fault states through relay contacts that connect to the main panel through input modules. The integration must preserve full supervision: a fault on the heat controller propagates as a fault to the main panel, and an alarm triggers main-panel cause-and-effect rather than acting locally only.

Common integration errors include heat controllers wired to local sounders only, with no main-panel integration; heat-controller fault outputs that loop back to the main panel's alarm input rather than its fault input, producing alarm conditions when the heat system is in fault; and zone information that does not map cleanly between heat-controller channels and main-panel zone numbering, producing alarms that the panel cannot localise correctly.

Routine testing of linear heat systems requires methods that suit the cable type. Digital and analogue resistive cables can be tested by applying heat to a discrete cable section using a manufacturer's test kit, or by induced fault tests at the controller end. Fibre-optic DTS systems are tested by remote spot-heat application at known cable positions, with the controller's reported alarm location compared against the actual heat location.

The test schedule covers all installed cable sections over a defined period, with the test results recorded against the cable position. Cable damage from subsequent building work is the most common in-service failure mode and is identified by routine continuity testing rather than waiting for a real heat event. The cable run should also be physically inspected at each service visit, looking for impact damage, abrasion at support brackets, and tampering, all of which produce supervision-correct but functionally degraded sections of cable.

Applied design rules, calculations, and worked examples for linear heat detection are covered in the courses on this site.