A linear heat detection system is a stand-alone fire detection system used for detecting heat from fires. It consists of a length of cable connected to a control unit. The cable can be run from distances of up to 8 kilometres dependent on the type of cable installed. However, the limiting factor will always be the zone size.
Though this type of detection is not commonly used, there will be situations where the only practical and effective source of detection will be a linear heat detection system. There are two types of linear heat detection cables.
• The non-integrating heat detector cable.
• The integrating linear heat cable. There is also a fibre optic linear heat cable which forms part of the integrating linear heat cables.
The non-integrating linear heat cable
The non–integrating type consists of two wires insulated with a heat sensitive insulation. The insulation will melt at a specific, pre-determined and non-adjustable temperature. This will lead to a short circuit occurring between the two internal conductors and the system will register a fire alarm condition. This type of cable cannot be re-used once it has gone into an alarm condition and has to be replaced. This type of linear heat cable is also known as the digital type or the non-resettable type and it is a cost-effective solution compared to the integrating type.
The integrating linear heat cable
The integrating type linear heat detector relies on the electrical resistance of the conductors to detect an alarm condition. An increase in temperature will cause the resistance of the cable to change. This change is measured and analysed at the control unit where the cable is connected. A high increase of heat over a small section of the cable will subsequently produce an alarm condition.
The fibre optic heat linear heat cable is more sensitive and relatively more expensive than the previous two and can be run to distances of up to 8 kilometres. It uses an effective and advanced technology as heat sources can be identified to within a metre length of the fibre cable.
Design considerations
The designer of the fire alarm system must decide, after taking all factors into consideration, that the linear heat detection system is the best option for that application. Like in all other installations, careful consideration must be taken in order to determine which type of linear heat detection will be most suited for the application.
It is important to ascertain whether there will be any possibility of electromagnetic interference or radio frequency interference on the cable as it will determine the best type of cable to use. Linear heat cable can be run in class A or B wiring configuration depending on what will be best and most practical for that application.
Once the type of the linear heat cable has been decided and agreed upon, the installation needs to be carried out as per the manufacturer’s recommendation with the correct interfaces to monitor the cable. This will ensure that the system will function as intended.
The designer of the system will determine how the cable will be run, the spacing and the zoning of the system, bearing in mind that the installation needs to comply with SANS 10139 when run in buildings. Care must be taken when installing to ensure that the cable is not easily susceptible to mechanical damage and/or interference.
Special supports are available from the manufacturers and the cable should not be tied to metal pipes as they can act as heat shields.
Typical applications
The most common applications where linear heat cables are installed:
• Conveyor belts.
• Cable trays.
• Cable tunnels.
• Transformers.
• Ceilings.
• Pipelines
• Transformers.
We continue to encourage all end users and consultants to only use FDIA-registered contractors as they have the commitment, capability and mandate to ensure that not only is your fire detection system installed correctly, but it is serviced and maintained as per the required standards and regulations.
For comments and enquiries please contact the FDIA on fdia@fdia.co.za
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