Linear Heat Detection (LHD) (also known as Linear Detection Wire or Linear Heat Detection Cable or Linear Heat) is a very commonly used method of fire detection. It can detect a fire anywhere along the length of the cable, and can be of lengths in excess of a kilometer.
Linear Heat Detection (LHD) is a line-type form of fixed temperature heat detection used in common commercial and industrial environments. This linear cable can detect a fire anywhere along its entire length and is available in multiple temperatures.
Linear Heat Detection (LHD) cable is essentially a two-core cable terminated by an end-of-line resistor (resistance varies with application). The two cores are separated by a polymer plastic, that is designed to melt at a specific temperature (commonly 68 °C for building applications), and without which causes the two cores to short. This can be seen as a change in resistance in the wire.
Linear Heat Detection Cable

Digital linear heat detection (LHD) cable is a combination of advanced polymer and digital technologies that can detect heat anywhere along its entire length. LHD cable is also compatible with any listed addressable or conventional panel. For addressable panels use addressable contact monitor modules. At the core of SafeCable is a twisted pair of extremely low resistance trimetallic conductors, sheathed in new advanced thermal polymers. These polymers are chemically engineered to break down at specific fixed temperatures allowing the twisted conductors to make contact and initiate an alarm at the control panel without any calibration for changes in the ambient temperature. The distance locating module (DLM-Z2) has One or Two Zones, which can be cross-zoned, built-in. It also allows you to identify and display the location, in feet or meters from the panel, where heat was detected.

There are 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 – Resistance of the Linear Heat Cable to the short circuit

The linear heat detectors shall sense an overheat condition at any point along its length and pinpoint its location. The heat detection system shall comprise a heat sensing cable and a monitoring /control unit.
A change in temperature along the sensing cable shall cause a relative change in resistance of the cable. This change shall be detected by the monitoring /control unit which shall signal an alarm at the preset temperature.

The sensing cable may be multicore l. One end shall be connected to the monitoring unit. The other end shall be terminated so as to create a loop which shall be continuously monitored for open and short circuit fault, and alarm condition.
Selection Criteria for Linear Heat Detector.

Following are some minimum requirement applicable for linear heat detector, however requirements vary from industry to industry.
The outer jacket shall be made of material that resists corrosion, chemicals, moisture etc.

The linear heat detector cable and monitoring system shall be suitable for operation on a nominal 24V DC supply.
The alarm condition shall be latched at the control unit, with reset facility from the Fire and Gas panel, and locally on the control unit.
The unit shall provide outputs for fault and alarm conditions that are compatible with the main Fire and Gas panel.
Sensing cable and any termination units required shall be certified for use as part of an IS circuit when installed in hazardous area.
Sensing cable and any termination units required shall be certified for use as part of an IS circuit when installed in hazardous area.

Benefits of liner heat detection
There are many benefits to LHD. One of which is a continuous source of heat detection up to 10,000 feet. LHD cables are also very durable and long-lasting. Conversely, spot heat detectors do not offer the continuous field of detection of LHD cables, they don’t hold up very well, and can be easily damaged.

Fike, one of the many manufacturers of linear heat detectors, notes many benefits of LHD. Some of these benefits pertaining to their detector called Protectowire include:

Fewer false alarms and false suppressant discharge with Confirmed Temperature Intiation (CTI).
Detection device works with fire alarm system interface and modules that identify source and temperature of hazard
Easy installation and maintenance
Provides hazard coverage at every point on the cable for maximum protection
Available in a variety of lengths, cable coatings and alarm temperatures for maximum flexibility
Wide zone area, up to 10,000 feet of continuous detection
Common applications

LHD is ideal for areas where electronics may not be the best choice. These devices are particularly appropriate for areas where installation or maintenance access is in a physically demanding or hazardous area. LHD is also perfect for when fire detection is required in close proximity. For example, in closed-in areas where it’s easier to run a cable than place a thermostat.

Some of the most applications for LHD include:
APPLICATION
Freezers
Printing presses
Vehicle suppression systems
Warehouses
Oil and Gas
Coal Storage
High Temperature Vessels
Fuel Storage
Conveyor System
Cable Tray
Tunnels
High Speed Train
Transformers
Production machine
Critical facilities, etc.
Installation, testing, and maintenance

LHD cables should be installed by a licensed fire protection professional. Linear heat detectors are easy to maintain and require little to maintenance making them the perfect heat detection solution for many building owners.

There are many guidelines and parameters as set by NFPA regarding the installation, testing, and overall maintenance of LHD devices. A licensed professional will follow this protocol for installation which include testing the continuity of the linear heat detector with an ohm meter to ensure it is in proper working order.

Fire Systems Inc. has experienced fire protection technicians who are specifically trained in Linear Heat Detection. We will ensure the proper installation of your linear heat detector and keep you on track with scheduled testing and maintenance. Contact us today to find out if LHD is the best choice for your facility. Visit our website or call us at 080-383-68255 for more information.

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