Linear Heat Detection — Three Architectures, One Decision

Linear heat detection (LHD) is not a single product — it is a family of three distinct sensing architectures that happen to share a shape. A specifying engineer who treats them as interchangeable will either overspend or underprotect. This note compares the three architectures on the variables that actually matter on a real project: event resolution, location resolution, response time, cost per metre and code acceptance.

Architecture 1 · Fusible Thermosensitive Cable

This is the workhorse architecture, and the one we manufacture. Two tinned copper conductors are separated by an engineered thermosensitive compound. Below the rated activation point the compound is a stable dielectric; above it, the compound collapses and the two conductors short. The downstream panel reads a zone-level alarm.

• Event resolution: on/off. The panel knows the loop alarmed, not the exact metre.
• Location resolution: zone-level. In practice, the installer engineers zone length to match the needed response granularity (50 m zones in warehouses, 500 m zones in tunnels).
• Response time: seconds for direct flame contact; 30–60 s for slow-heating events, depending on ambient and jacket wall thickness.
• Reset behaviour: one-time-use. Activated section is cut out and replaced; the rest of the loop keeps working.
• Typical cost (cable only): low.

Architecture 2 · Digital Addressable Cable

A cable with discrete digital temperature sensors spliced in at fixed intervals (typically 5–10 m). Each sensor has an address; the panel queries them in sequence. The panel sees a temperature map along the cable, not just an alarm bit.

• Event resolution: analog-over-threshold, per sensor.
• Location resolution: address-level (5–10 m typical).
• Response time: depends on sensor dwell time; usually 10–30 s.
• Reset behaviour: reusable. Sensors survive unless the cable itself burns.
• Typical cost: 3–6× fusible cable, plus a more expensive panel.

Architecture 3 · Distributed Fiber-Optic Sensing

A passive optical fibre is pulled through the protected area. A laser interrogator at one end measures the back-scattered light (Raman or Brillouin) and reconstructs the temperature along the fibre at metre or sub-metre resolution. No electronics in the field — just glass.

• Event resolution: full temperature profile along the fibre.
• Location resolution: 1 m or finer, up to tens of kilometres.
• Response time: seconds (limited by interrogator cycle).
• Reset behaviour: reusable. The fibre is passive and survives most events.
• Typical cost: 10–30× fusible cable, dominated by the interrogator.

Side-by-Side at Project Scale

The same six dimensions, read off each architecture's spec sheet:

Fusible Cable vs Point Detectors — Where the Money Actually Goes

In large open spaces, point detectors multiply quickly — each detector protects a limited circular area, so a warehouse or hangar can easily need 200+ units. A single fusible cable run can replace the whole array. The arithmetic below explains why fusible LHD dominates in large/continuous environments:

Where Each Architecture Wins

Specify Fusible Cable When

• Budget is tight and location resolution can be managed by zone engineering.
• The protected space is large, harsh, or electrically noisy (cable trays, hangars, parking decks, tunnels). Chemical and mining service in particular carries jacket and routing constraints documented in our LHD specification note for chemical and mining plants.
• The asset is hard to access for periodic maintenance — the lifecycle case for that scenario is built out in LHD cable for inaccessible assets — the 20-year TCO argument.
• The panel expects a simple dry-contact short on activation. The wiring side of that integration sits in where the cable sits on a fire-protection panel drawing.
• Code acceptance is established (LHD fusible cable has the deepest AHJ track record).

Specify Digital Addressable When

• You need to know which 10 m section alarmed — not just the zone.
• The project funds a compatible addressable panel anyway.
• Reusability and ambient temperature trending are valuable.

Specify Distributed Fibre When

• Length exceeds a few kilometres — long tunnels, pipelines, power cables.
• Sub-metre hotspot location is a contract requirement.
• The asset owner has budget for a high-end interrogator and a commissioning team.

Fusible cable is the Toyota Hilux of heat detection — not the fastest or the most precise, but it works on every continent and it comes back for the next shipment. Most integrators should default to it unless the project explicitly demands fibre-grade resolution.

Installation Constraints That Apply to All Three

1. Mount near the ceiling or the apex of the protected volume — heat rises, and the cable needs to see it.
2. Maintain a minimum bend radius (25 mm for fusible, 50 mm for fibre).
3. Respect spacing to heat-producing equipment (motors, lighting, HVAC vents) to avoid nuisance triggers.
4. Terminate into compatible panel cards — a fusible cable on a point-detector card will be undervolted and mis-read. The end-of-line resistor has to be the value the card expects, and on a long run the cable's own resistance stacks on top of it; the EOL and loop-resistance calculation keeps the supervised loop inside the panel's window.
5. Log the lot number printed on the jacket — it is the traceability tag to the batch QC report.

Once an LHD loop is in service, the most common false-alarm cause is not a thermal event at all — it is moisture ingress, crush damage at a tray edge, or a deteriorating splice. If your panel reports a short on a fusible loop and you are not sure whether it is real, our field-diagnosis sequence for LHD short circuits walks through how to isolate the loop, run insulation-resistance and TDR distance checks, and classify the fault before replacing cable.

Closing Note

If you are at the specification stage and cannot tell whether fusible, digital or fibre is the right answer, send us the application brief — protected volume, length, location-resolution requirement, budget envelope and AHJ. We will come back with a recommendation and a draft spec sheet — turnaround scheduled subject to project scope and engineering review. Start a conversation.

FAQ — LHD Architectures

What are the three main types of linear heat detection?

Linear heat detection comes in three architectures. (1) Fusible thermosensitive cable — the workhorse: two conductors separated by a thermosensitive compound that collapses at the activation point and shorts the loop. (2) Digital addressable cable — discrete digital sensors spliced in at 5–10 m intervals, each individually addressable from the panel. (3) Distributed fibre-optic sensing — a passive optical fibre interrogated by a laser at one end, measuring temperature at metre or sub-metre resolution along the fibre. Cost rises and resolution improves left-to-right; fusible dominates by volume.

When should I specify fusible LHD cable instead of fibre-optic DTS?

Specify fusible cable when budget is tight, the protected space can be zoned at 50–500 m granularity, the panel expects a dry-contact short on activation, and AHJ acceptance is the deciding factor. Fibre-optic DTS is justified when the project explicitly requires sub-metre hotspot location, the run exceeds a few kilometres (long tunnels, pipelines, power cables), and the asset owner has the budget for a high-end interrogator and a commissioning team. For most warehouse, hangar, cable-tray and tunnel projects, fusible cable is the most practical answer.

How does fusible LHD cable compare to point smoke or heat detectors?

Point detectors protect a limited circular area each, so a large warehouse or hangar can need 200+ units, all spacing-critical. A single fusible cable run replaces the entire array — every centimetre is a sensor, no dead zones, no spacing arithmetic. The arithmetic typically lands at 1× LHD cable run versus dozens of point detectors plus zone wiring; for cable trays and conveyors longer than ~50 m, point detectors become impractical. LHD also tolerates dust, vibration and temperature extremes that derate point sensors.

Is fusible thermosensitive LHD cable reusable after activation?

No — fusible cable is one-time-use by design. Once the compound has collapsed at the activation point, that section is permanently shorted; the activated length is cut out and replaced with a new spool, while the rest of the loop continues to work. Digital addressable cable and distributed fibre-optic sensing are both reusable. The non-resettable behaviour of fusible cable is actually a feature in inaccessible infrastructure: you know an alarm is real, not a sensor that talked itself into a false trip and reset before the maintenance crew arrived.