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:
- Open-circuit - Effectively an infinite resistance
- Normal Operating Condition - Apparent resistance will be the same as the end-of-line resistor
- 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