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Linear heat detection

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Typical Detecting Temperature 85, 105, 180, 185 °C
Maximum Length 3000m

Linear Heat Detection (LHD) (also known as Linear Detection Wire or Linear Heat)is a very commonly used method of fire detection. It can detect a fire any where along the length of the cable, and can be of lengths in excess of a kilometer.

Applications can range from building fire alarm systems to mobile plant machinery.

Operation

Linear Heat Detection (LHD) cable is essentially a two-core cable terminated by an end-of-line resistor (typically 4,700Ω). The two cores are separated by a polymer plastic, which is designed to melt at a specific temperature (commonly 180°C), and without which causes the two cores to short. This can be seen as a change in resistance in the wire.

There are a limited states the LHD cable can be in:

  1. Open-circuit - Effectively an infinite resistance
  2. Normal Operating Condition - Apparent resistance will be the same as the end-of-line resistor
  3. Fire Detection - zero resistance (0Ω)

Advantages and Disadvantages

Advantages

  • Simple in construction
  • Controller complexity is low
  • Easily deployed
  • Can cover a large area
  • Small and Compact
  • Supported by the majority of fire controller panels

Disadvantages

  • Very slow response time - up to 30 seconds
  • Lack of information about fire
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