Linear heat detection

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 - can be cheaply built.
• Controller complexity is low - due to few possible states
• Easily deployed
• Can cover a large area
• Small and Compact
• Supported by the majority of fire controller panels

Disadvantages

• Very slow response time - As the plastic as to be melted by the fire, it can mean there is a considerable time delay (30 seconds)
• Lack of information