Linear Heat Detection System (LHD)

System Overview

The AGNI Linear Heat Detection (LHD) System is an advanced fire detection solution that provides continuous, real-time temperature monitoring along the entire length of a sensor cable. Unlike conventional point-type detectors that monitor specific spots, the LHD system detects heat anywhere along its installed path making it ideal for applications where traditional detectors are impractical due to harsh, confined, or extensive environments.
It is commonly used in tunnels, cable trays, conveyor systems, warehouses, power plants, oil and gas facilities, and other high-risk or outdoor installations.

Operating Principle

The AGNI LHD cable is a heat-sensitive device consisting of two twisted metallic conductors insulated with a heat-sensitive polymer.
When the temperature at any point along the cable reaches its rated alarm temperature, the polymer insulation softens and allows the conductors to short-circuit, triggering an alarm signal in the control panel.

Two main detection technologies are used:
Digital Cable (Fixed Temperature) LHD Non Resettable
Analog Cable  ( variable temperature sensor with pre alarm technology ) Resettable  LHD

Activates an alarm when the cable reaches a predetermined temperature (e.g., 68°C, 88°C, 105°C, 138°C, 180°C).

Provides precise alarm location via a digital interface module.
Suitable for general applications requiring reliable fixed-point activation.

Advantages

  • Continuously monitors temperature changes along the entire cable length.
  • Provides early warning (pre-alarm) and alarm stages based on rising temperature profiles.
  • Can pinpoint the exact alarm location (in meters) using a compatible addressable controller.
  • Can be reset after activation (non-destructive).
  • Ideal for environments with gradual temperature changes or long-distance monitoring.

System Components

Component

Description

Linear Heat Detection Cable

Two-conductor cable with heat-sensitive insulation; available in various alarm temperature ratings and outer sheath types (PVC, Nylon, Stainless Steel braid, etc.) depending on environmental conditions.

End-of-Line Unit (EOL)

Installed at the cable’s end to complete the detection circuit and provide supervision.

Junction / Terminal Boxes

Weatherproof enclosures for connecting and branching LHD cables.

Interface / Control Module

Converts the LHD cable’s electrical signal into a readable alarm or fault condition; communicates with the Fire Alarm Control Panel (FACP).

Mounting Accessories

Stainless steel clips, saddles, or straps to secure the cable along trays, ceilings, tunnels, or equipment.

Control Panel (optional) 

Dedicated LHD control unit for alarm monitoring, distance localization, and system fault reporting.

Technical Features

  • Continuous detection along the entire cable length no blind spots.
  • Fast response to heat build-up, even in dirty or dusty environments.
  • Long detection range typically up to 3,000 meters per zone (depending on model).
  • Alarm and fault supervision to detect open circuit, short circuit, or cable damage.
  • Resistant to corrosion, moisture, vibration, and mechanical stress.
  • Compatible with conventional and addressable fire alarm systems.
  • Suitable for indoor, outdoor, and hazardous area installations (optional ATEX-rated versions available).
  • Low maintenance, no calibration required.

Technical Specifications

Parameter

Typical Specification

Alarm Temperature

68°C, 88°C, 105°C, 138°C, 180°C (custom available)

Max. Ambient Temperature

20°C below alarm point 

Cable Length

Up to 3,000m (per zone)

Operating Voltage

24V DC (typical)

Response Time

≤ 10 seconds at rated temperature

Construction

Twin-conductor with heat-sensitive polymer insulation

Outer Jacket

PVC, Nylon, or Stainless Steel braid

Compliance

UL 521, FM Approved, EN 54-28, LPCB Certified

Battery Backup

24V sealed lead-acid battery

Communication Protocol

Digital proprietary / open protocol

Network Connectivity

RS-485 / Ethernet / Fiber optic

Alarm Outputs

Programmable relays (Fire, Fault, Supervisory)

Operating Temperature

0°C to 55°C 

Relative Humidity

Up to 95%, non-condensing

Compliance

EN 54-2, EN 54-4, UL 864, LPCB Certified

Advantages

  • Continuous coverage — detects heat at any point along its length.
  • Highly reliable — unaffected by dust, humidity, or corrosive gases.
  • Easy installation — can be routed around complex geometries.
  • Cost-effective — covers large or remote areas with minimal components.
  • Versatile — suitable for industrial, commercial, and outdoor applications.
  • Low maintenance — simple visual inspection and functional testing.
  • Compatible — integrates seamlessly with any fire alarm system.

Typical Applications

  • Electrical cable trays and tunnels
  • Conveyor belt systems
  • Oil and gas pipelines and refineries
  • Vehicle and train tunnels
  • Parking garages and basements
  • Escalator shafts and machinery spaces
  • Power distribution rooms and substations
  • Warehouses and cold storage facilities

Standards and Compliance

The Linear Heat Detection System and its components are typically designed and tested in accordance with the following standards:

01.

UL 521

Standard for Heat Detectors for Fire Protective Signaling Systems

02.

FM Approved / LPCB Certified

Fire Detection Equipment

03.

EN 54-28

Fire Detection and Fire Alarm Systems, Linear Heat Detectors

04.

NFPA 72

National Fire Alarm and Signaling Code

05.

BS 5839 Part 1

Fire Detection and Alarm Systems for Buildings