What are Linear Heat Detectors?

Linear heat detectors detect only heat. They do not detect smoke. On the other hand, a fire alarm system consists of detection devices that detect smoke and/or heat, depending on the type of detector. The linear heat detectors are ideal for use in areas where other detection methods may be prone to damage or inefficient.

What are Linear Heat Detectors?

Linear heat detectors (LHD) are also known as linear detection wires or cables. They can detect fire anywhere along the length of the cable. The length of the cable can be more than a kilometer also. There are various types of heat detection devices. The linear heat detectors have a wire or cable for detecting heat from fire instead of the thermostat. These can be customized according to applications.

Linear heat detectors are a line-type form of fixed-temperature heat detection. They are used commonly in commercial and industrial environments. This linear cable is able to detect fire anywhere along its length. They are available in multiple temperatures.

The linear heat detector’s cable consists of a two-core cable terminated by an end-of-line resistor. The resistance of the wire varies according to application. The two cores in the LHD cable are separated by a polymer plastic. The plastic is designed to melt at a particular temperature. In the case of building applications, the polymer plastic is designed to melt at 68°C. When the polymer plastic melts, it causes the two wires to short. This is observed as a change in the resistance of the wire.

Linear Heat Detector Cable

As mentioned above, the linear heat detector cable consists of two insulated conductors covered by a heat-sensitive polymer plastic. The heat-sensitive polymer plastic has an outer jacket for its protection. So the pair of steel conductors are beneath the jacket and heat-sensitive polymer coating.

Linear Heat Detector Cable

A linear heat detector cable can have three states, namely:

  • Open circuit

In this state, the detector’s cable has infinite resistance.

  • Normal Operating Condition

In the normal condition of operation, the detector’s cable has an apparent resistance that is equal to the end-of-line resistor.

  • Fire  Detection

In the fire detection state, the resistance of the linear heat detector’s cable is zero because it gets short-circuited at this time.

The linear heat detector’s cable is capable of sensing an overheating condition at any point along its length and pinpointing its location. The heat detection system will also have a monitoring/control unit along with the heat sensing cable.

When there is a change in the temperature along the sensing cable, a relative change in the resistance of the cable is observed. The change in resistance is detected by the control/monitoring unit. The control unit then generates an alarm at the pre-set temperature.

The sensing cable can be multicore or coaxial. One cable end should be connected to the monitoring unit and the other end should be terminated. This would create a loop that will continuously monitor open and short-circuit fault and alarm conditions.

Working of Linear Heat Detectors

In the event that the ambient temperature meets or exceeds the detector’s fixed temperature, the polymer melts. As a result, the steel conductors make contact and initiate an electrical short circuit. This will send a signal to the fire alarm panel for an alarm condition. The panel releases the fire suppression agent or the set of the fire alarm system after the sending of the signal.

Let us consider a case in which the linear heat detector is connected to a fire alarm which initiates a fire sprinkler system upon alarm. Now, when the heat from the fire melts the polymer plastic, the wires are short and the system is alerted.

The fire sprinkler valves open so as to allow water to flow in. The linear heat detectors are quite versatile and work with most of the fire alarm systems (like wet, dry, etc.).


Linear heat detectors have many advantages. They are:

  • They provide continuous heat detection up to 10,000 feet.
  • The cables of detectors are durable and long-lasting.
  • Very easy to install and maintain.
  • Provide hazard coverage at every point on the cable for maximum protection.
  • They are available in different lengths, alarm temperatures, and cable coatings for maximum flexibility.

Selection Criterion

For using linear heat detectors in industries there are some minimum requirements that are applicable. These requirements may also vary from industry to industry. Some of them are:

  • The outer jacket of the heat detectors should be made of a material that resists corrosion, chemicals, moisture, etc.
  • The heat detector’s cable and monitoring system must be able to operate on a 24 V DC supply.
  • Whenever any alarm is generated, the alarm should be latched in the system. Only pressing the reset button should reset the alarm.
  • The control unit must have defined outputs for fault and alarm conditions that are compatible with the main Fire and Gas panel.
  • The sensing cable or the termination units which are required must be certified for use as a part of an IS circuit if it is installed in a hazardous area.

Example of Linear Heat Detector

We are taking an example of Honeywell’s linear heat detectors which are available in analog and digital forms.

The analog linear heat detector’s cable has insulators and their resistance varies in proportion to the changes in temperature. It is a self-restoring cable. So, after the fire event when the temperature does not exceed the temperature limit of 120°C.

The cables are an overall insulated twisted pair of tri metallic core type wires. The cables also have a 1kV tested protective outer coat. The maximum zone length is 200 meters. They also have multiple alarm temperature ratings. 

Example of Linear Heat Detector

The digital linear heat detector cable has a fixed temperature for breakdown. It is a non-self-restoring cable. These cables have low resistance. The cables are twisted pair tri metallic conductors, having sheathed thermal polymers.

These polymers are chemically engineered to break down at a specific fixed temperature. When this polymer breaks down, then the twisted conductors will make contact and initiate an alarm. There is no need of calibrating ambient temperature changes. The system is monitored using an electronic interface.

This would provide a fault indication of an open and short circuit. These have multiple alarm temperature ratings. The digital linear heat detectors have a maximum zone length of 3000 meters. They are compatible with both conventional and intelligent fire alarm panels.

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