Linear heat fire detector “SafeCable LHD”. As always all the genius is simply.

3 July 2018

This article attempts to explain in as much detail as possible the device and the principle of operation, as well as the methods and scope of the linear thermal fire detector (thermal cable) in automatic fire alarm systems and in automatic fire extinguishing installations.

Chief engineer of the project
“ASPT Spetsavtomatika” VP. Sokolov

At oil and gas complex enterprises, in metallurgical and chemical production, in cable collectors and channels, transport and technological tunnels when creating automatic fire alarm systems and fire extinguishing systems, one often has to deal with difficult operating conditions of this equipment. Explosive and fire hazardous areas, the presence of moisture, abrasive dust, increased pollution, low temperatures or a sharp temperature drop, as well as an aggressive environment dictate strict requirements for automatic fire detectors and their selection.

According to the operating conditions of the equipment of automatic fire alarm systems, all protected objects can be conditionally divided:

– on objects with normal operating conditions;

– on objects with severe operating conditions;

– on special objects.

To normal operating conditions, it is possible to refer to the internal premises of the protected object, which are heated during the cold season. Dustiness, the presence of aggressive media and unregulated heat sources are absent.

Objects with severe operating conditions are objects with negative temperature gradients, both negative and high positive, with a constant condensate due to temperature and humidity changes, with increased dustiness (hard, abrasive and water suspension) and objects with aggressive media .

Special objects are objects having explosive operating conditions.

The uniqueness of the design of the linear thermal fire detector (SafeCable LHD) allows it to be used to protect all of the above objects without exception. It is in these conditions that a linear thermal fire detector (Therm-mocable SafeCable LHD) has invaluable advantages.

The principle of the SafeCable LHD.

The linear thermal fire detector (SafeCable LHD) consists of two steel conductors, manufactured using a special technology, each of which has an insulating coating of heat-sensitive polymer. Steel conductors with an insulating coating of heat-sensitive polymer are twisted to create a spring between them, then wrapped in insulation and placed in braid to protect from exposure to adverse environmental conditions. A linear thermal fire detector is a cable that allows you to detect a heat source anywhere in its entire length, that is, it is a single sensor (sen-sor) of continuous operation. When the critical temperature is reached, the thermistor material softens, the metal conductors begin to contact each other, thereby triggering a fire alarm signal. To operate the thermal cable, you do not need to wait for the heating of a certain length of the section. The SafeCable LHD is the maximum thermal detector and therefore allows the generation of an alarm when the temperature threshold is reached at any point throughout the entire length of the linear thermal fire detector.

The device of the SafeCable LHD thermal cable (see Fig-1).

Fig. 1.

Metal veins with special coating:

– steel provides tensile strength;

– copper increases electrical conductivity;

– tin for corrosion resistance.

Sensitive polymer:

– a heat-sensitive shell.

Outer coating:

– General purpose;

– polypropylene;

– nylon.


– the shell, depending on the type of thermal cable has different colors


– outer diameter (3.2 mm);

– Flexible enough for mounting.

There are five types of a linear thermal fire detector (SafeCable LHD), which is distinguished by a threshold of temperature triggering and has three variants of an external protective coating, differing in physical and chemical properties.

Technical characteristics of the external coating (shell) of the SafeCable LHD:

– a general-purpose thermal cable has a very strong extrusion outer protective PVC jacket, which provides reliable protection of the thermal cable in operation in almost all environmental conditions. The jacket of the thermal cable has properties of fire resistance and moisture resistance, and also has sufficient flexibility at low ambient temperatures. Thermocable with a general-purpose sheath is well suited for the protection of residential and commercial buildings, as well as industrial facilities;

– Thermal cable with polypropylene coating marked with the letter “P”, has a strong outer shell resistant to ultraviolet radiation, is characterized by high elasticity, abrasion resistance, atmospheric conditions and high reliability of operation at high ambient temperatures. Resistant to acids, corrosive media, oils and oil products. It is intended for wide application in the industry;

– Thermal cable marked with the letter “N” with a coating consisting of a two-layer shell, an inner PVC layer and an outer layer of nylon. This thermal cable is specially designed for industrial use, for example, for the protection of conveyors, where abrasion resistance is of the greatest importance. In principle, protection from abrasive dust is provided mainly by the outer protective layer of nylon, while retaining electrical and mechanical properties.

Specifications – SafeCable LHD

  1. The diameter of the thermal cable
  2. Weight
  3. Radius of bending, not less than
  4. The maximum voltage
  5. Resistance of the thermal cable (R)
  6. Response temperature (° C):
  7. he breakdown voltage (Uv)
  8. Change of resistance of a thermal cable from temperature
  9. The minimum working length of the thermal cable
  10. The maximum working length of the thermal cable
– 3.2 mm.
– 6,8kg / 305m.
– 76,2mm.
– ~ 30V, = 42V.
– 0.164 Ohm / m.
– 68°, 78°, 88°, 105°, 180°
– 1000 V.
– 1% on 5 degrees.
– 0.5 m.
– 3000m.


Warning: The SafeCable LHD is a fire detector with a normally open contact. All the rules and norms of SP 5.13130.2009 for a point thermal fire detector with a normally open contact in accordance with Table 13.5 automatically apply to a thermal cable.

Copy from the code of regulations SP 5.13130.2009.

13.6 Point thermal fire detectors.

13.6.1 The area controlled by a single point thermal fire detector and the maximum distance between the detectors, the detector and the wall, except for the cases specified in clause 13.3.7, shall be determined according to Table 13.5 but not exceeding the values specified in the specifications and passports for detectors.

Table 13.5

The height of the protected room, m Average area, controlled by a single detector, m2 Maximum distance, m
between the detectors from the detector to the wall
Up to 3.5 Up to 25 5,0 2,5
St. 3.5 to 6.0 Up to 20 4,5 2,0
St. 6.0 to 9.0 Up to 15 4,0 2,0

13.6.2 Thermal fire detectors should be located taking into account the exclusion of influence on them of thermal influences not related to fire

13.7. Linear thermal fire alarms.

13.7.1 The sensing element of linear and multi-point thermal fire detectors are located under the ceiling or in direct contact with the fire load.

13.7.2. When installing non-cumulative detectors under an overlap, the distance between the axes of the detector’s sensing element shall satisfy the requirements of Table 13.5.

The distance from the sensitive element of the detector to the ceiling should be at least 25 mm.

At the moment in the Russian market there are several types, structurally different from each other, linear thermal fire detectors:

– The first type of semiconductor is a linear thermal fire detector, in which a wire coating with a negative temperature coefficient is used as a temperature sensor. This type of thermal cable works only in conjunction with an electronic microprocessor control unit. When the temperature is applied to any part of the thermal cable, the resistance at the points of impact changes. Using the control unit, you can set different thresholds for temperature operation. After short-term exposure to heat, the cable regains its operability. The design of the thermal cable functionally does not have the ability to measure the distance to the start-up point. The maximum working length of this type of thermal cable is about 300 m.

– The second type of mechanical is a linear thermal fire detector, which uses a sealed copper tube Φ = 6 mm as a temperature sensor. (capillary) filled with an inert gas and connected to a pressure sensor. When the temperature is applied to any part of the sensor tube, the internal pressure of the gas changes. The pressure sensor registers this change and transmits the signal to the microprocessor electronic unit for processing. This type of linear thermal fire detector is reusable. The structurally thermal cable of this type is the maximum-differential fireman from the broadcaster. The length of the working part of the copper tube of the sensor has a length limit of 20 to 130 meters.

– The third type of multipoint thermal fire detector is a linear thermal fire detector that uses a twisted pair of wires with thermocouples included in it at a distance of about 50 cm from each other as a temperature sensor. The principle of operation of a thermal cable of this type is based on the summation of the emf . from individual thermocouples. Due to the spread of heat in the volume of the protected room under fire conditions, temperature increase will be observed at the location of each thermocouple. Thus, the sensor provides a summation of the heat dissipated over the room. The receiving unit converts the received signals and compares them with the alarm parameters set in its memory set by the thresholds for the temperature response. If these limits are exceeded, the device gives an alarm to the fire panel. Sensitivity of the sensor depends on the number of sensitive elements located in the same room. Therefore, when designing fire alarm systems, it is necessary to take into account that the sensitivity of the detector depends on the length of its sensor. This type of linear thermal fire detector of multi-time action. Structurally, this type of thermal cable is a maximum-differential fire detector. The length of the working part of the multi-point sensor has a length limitation of more than 300 meters.

– The fourth type of optical is a linear thermal fire detector, which uses a fiber optic cable as a temperature sensor. The principle of the optical linear sensor is based on a change in the optical transparency of the sensor, depending on the temperature change. When the light from the laser hits the fire, some of it will be reflected. The processing device determines the power of direct and reflected light, the rate at which it changes, and calculates the value of the temperature change, and the place where it happened. This type of linear thermal fire detector is reusable. It works only in a complete set with an electronic microprocessor control unit and data processing. The maximum length of an optical sensor can reach up to 10 kilometers or more (depending on the quality of the optical fiber). This type of thermal cable requires qualified experts for installation and maintenance.

– The fifth type of electromechanical is a linear thermal fire detector, which uses a thermosensitive material applied to two mechanically stressed wires (twisted pair) as a temperature sensor. Under the influence of temperature, the heat-sensitive layer softens and the two conductors short-circuit. A variation of this thermal cable is a linear thermal fire detector with three heat-sensitive conductors having different thresholds for operation under the influence of temperature (68.3 ° C and 93.3 ° C). The thermal cable of different manufacturers can have different internal resistance of steel conductors from 0.164 Ohm / m. up to 0,75 Ohm / m. The internal resistance of the steel conductors determines the maximum possible working lengths of the thermal cable, this dimension corresponds to a length of 1500 m. up to 3000m. Due to the internal resistance of the conductors, it became possible to measure the distance to the point of operation of the thermal cable under the influence of temperature. Structurally, such a device is a very sensitive electronic digital ohmmeter. But if you do not need this option, the thermal cable can work with all fire control devices that work with normally open point hot detectors. It is this type of linear thermal fire detector (thermal cable) that we consider with you in this article.

Any point taken on a thermocable electromechanical type is an independent point thermal normal open fire detector. Thus, on one meter of a cable, we conditionally have dozens, if not hundreds of point thermal firemen from broadcasters. If you follow the requirements of the technical specification of the SafeCa-ble LHD thermocouple, the minimum lengths for which the thermal cable can be divided must be equal to 0.5 m. Take as an example 10m. Thermal cable and divide into 20 segments of 0.5 m. We receive a fire alarm loop with twenty linear thermal fire alarms (in the form of small segments). The only question is why it should be divided into segments, and then connected to each other in whole, if the thermocable itself carries two functions, is a linear (multi-point) thermal fire detector (sensor) and itself connects a linear cable. Maybe this is more expensive, but the reliability of its operation without connections will be an order of magnitude higher.

At the ends of the thermal cable must necessarily retreat to 10cm. This is the zone of incorrect operation of the thermal cable due to the partial dissolution of the twisted steel conductors of the linear thermal fire detector. There is a very high likelihood that for the closure of the conductors between themselves there is not enough mechanical twisting force.

For large lengths of the used linear thermal fire detector (thermocouple SafeCable LHD), for example, more than six hundred meters, it is necessary to take into account the internal resistance of the thermal cable itself, which must be subtracted from the termination resistor in the fire loop. So the internal resistance of one meter of the SafeCable LHD is 0.164 Ohm, and six hundred meters is 98.4 Ohm. If the difference in the nominal value of the terminating resistors is 10-15%, which we use for the installation and the terminating resistor, for example 2.4 kΩ, the value of which depends on the design of the device, plus the resistance of the thermal cable, we can get a break signal. If the resistance of the thermal cable is large, you must honor it from the terminating resistor.

Thermocable SafeCable LHD when closing the initial section, when exposed to the source of fire burns out, gives a dry contact without resistance, so that in order to control the If the signal is not shorted, an additional resistance is necessary. Dependent on the additional resistance at the beginning of the section can be from 500 to 1200 ohms. The additional resistor “Rd” must necessarily be subtracted from the termination resistor of the alarm loop.

Let’s consider some features of mounting an electromechanical linear thermal fire detector (thermal cable):

  • When it is installed in rooms on the ceiling and walls, the thermal cable must be at least 25 mm away from any surface, excluding fixing points. so that the mounting surface does not work as a cooling radiator.
  • In the case where the thermal cable is used to protect motors, transformers and power distribution of cable manifolds, the cable must be attached as close as possible to the surface to be protected. Surfaces must be in contact.
  • When installing a thermal cable on the street, it is necessary to arrange a protection in the form of a canopy from a 5x5mm angle. from metal or PVC for protection from rain, snow, icicles, wind and direct sunlight, especially in summer.
  • • In the protection of steam rooms and saunas, hide the thermal cable in special open spaces, protecting it from direct exposure to hot steam or air at the time of application of the heat.
  • The temperature threshold for the operation of the thermal cable is 35 degrees higher than the operating temperature in the protected room and the maximum possible positive temperature on the street. For saunas for reliability it is necessary to take 60 degrees higher than the operating temperature because the heat generation in the sauna is cyclic.
  • To prevent false alarms, protect the ends of the thermal cable from moisture and other solvent or conductive fumes by using suitable protection boxes.
  • Fastening of a thermal cable with a simple connecting wire or with a terminal resistor due to design features is carried out through the terminal connections. And the terminal in the junction box should be deployed and be at an angle of 45 degrees to the axis of the input hole of the terminal box (see Figure-2). This position prevents the pulling out of the steel wires of the thermal cable from the terminals of the terminal when the thermal cable is swung or twisted along the axis.

  • The most reliable connection of the thermal cable in the junction box is the twisting of the steel ends of the thermal cable into rings of a certain diameter under the screw of the terminal (see Fig. 3). After that, this junction box is filled with a special plastic mastic to protect the clamps of the terminal from the aggressive environment. The plasticity of the mastic must correspond to the climatic conditions of operation. In case of need of repair, the mastic coating should be easily removed from the mounting box.

Fig. 3.

  • When attaching the thermal cable, do not make a strong mechanical tightening, so as not to mechanically trigger the tripping, that is, the short-circuit of the thermal cable.
  • When protecting rooms with a ceiling height of more than 9 meters, the distance between the parallel cables of the thermal cable is reduced to two meters (manufacturer’s recommendation). This deviation from SP 5.13130-2009 requires obligatory coordination in the form of special technical conditions (STU) with local fire inspection authorities. Depending on the functional purpose of such facilities for fire safety requirements, additional compensation measures for fire protection may be laid.

Once the only supplier of the electromechanical linear thermal fire detector (thermal cable) to the Russian market was the company “Protectowire”. At the moment, there are several such companies, including our own manufacturers of this type of fire equipment. One meter of a thermal cable depending on the manufacturer costs from 200 to 600 rubles and above. If we consider the thermocouple meter as a point thermal fire detector, then the price is not so big. But, the design of the thermal cable is original and that is not only a thermal linear sensor, but also a cable that connects itself. Hence the thermocable has its own niche in the automatic fire alarm system, where it is possible to use only a thermal cable as a thermal fire detector.

Here are some interesting solutions for using a thermal cable.


Technological and transport tunnels are extremely complex engineering and technical complexes and make special requirements to active fire protection systems. To ensure normal operation and maintenance of the tunnel, as well as to create conditions for effective fire suppression in emergency situations (emergency situations) and emergency evacuation of people, a whole complex of fire fighting measures is being created in the active fire protection system. The road transport tunnel is the extreme conditions for the operation of fire equipment, the large accumulation of people and cars (the human factor), low temperatures in winter, variable humidity, dustiness, aggressive environment from exhaust gases, vibration and other man-made impacts. Therefore, the best solution for any transport tunnels is a thermal cable. As an example, you can take the “Lefortovo” and “Gagarin” tunnels of Moscow, which are already protected by an electromechanical thermal cable. In automobile tunnels, a linear thermal fire detector is installed on the ceiling directly above the carriageway in accordance with the requirements of the regulations of SP 5.13130-2009. The cable manifold and cable risers are also protected by a thermal cable. The choice of the type and temperature of operation of the thermal cable is determined by the technical conditions.

Thermal cable in tunnels is fixed with the help of steel cables stretched along the carriageway. Due to low temperatures and frost formation, permanent drafts and wind, the cable with the thermal cable can swing, so special attention should be paid to the cable fixing in the mounting box. We have already spoken about this above. Depending on the season of heat or cold, the cable may sag or shorten. In order for the tension to be always the same, it is necessary to use the device in the form of a metal load, pulling the cable through a small pulley. The cargo must be in a special receiving cup, preventing accidental drop of the load downward.

Next to the “Gagarin” road transport tunnel is a railway transport tunnel. There was another problem. Locomotives run along this tunnel. You-the cotton pipe of the locomotive is located about 1.5 meters from the ceiling of the tunnel. As it turned out, the exhaust of gases from it has a sufficiently high temperature up to 400 ° С, which could lead to a false operation of the thermal cable, especially when the train slows down in a tunnel. The solution was found in the form of a metal corner 50x50mm. It was fixed at a short distance from the tunnel ceiling to the bottom. The thermocable was placed inside the corner on a special fastening so that it did not come into contact with the surface of the corner. The metal corner protected the thermal cable from the bottom, breaking the flow of hot air to the sides, but this did not prevent the operation of the thermal cable in a real fire, when the heat from the source of ignition rose up and filled the volume of the tunnel at the ceiling.

Entrance halls.

Large entrance halls of administrative buildings always cause difficulties in combining fire protection and lobby design requirements. Therefore, as a rule, false ceilings are closed, tightly with a flexible cardboard, with the inability to make special hatches in them to service fire detectors. Nevertheless, this space is filled with technological equipment and especially cable networks. The principal solution to this issue was the use of a linear thermal fire detector (thermal cable) to protect the false ceiling space closed with a continuous layer of flexible cardboard. The ends of the cut-outs of the thermal cables of the protecting false ceilings are reduced to a special place where a maintenance hatch is made, in the same place the thermal cables are connected to the fire alarm system. The thermal cable does not require maintenance and can be behind the false ceiling for a dozen years, carrying out its main functions of fire protection.

Hangars for aircraft parking.

Hangars for parking and maintenance of large aircraft have a complex engineering design with huge pro-years, are unique and expensive objects. To protect these structures from overheating in case of fire, water is used. As a stimulus system for incorporating water irrigation of metal structures and trusses, a thermo-cable is used. The thermocable is located in metal pipes, and the pipes themselves are tightly pressed to the surface of the farms or welded to them. In the event of a fire, the water supply for cooling the ceiling structures will be provided if the metal trusses warm up to the operating temperature of the thermal cable, which is maximum 180 ° C. There is a critical temperature for the durability of the metal under load, after which the metal is released and the structure begins to deform, and then under its own weight break down. This decision to use a linear thermal fire detector (thermal cable) in the pipe does not meet the requirements of SP 5.13130.2009 to the fire alarm system. This decision is more likely to relate to the technology of protecting the structures of the ceiling trusses and the way the thermal cable is used, as a thermal sensor.

Electrical schemes for connecting an electromechanical thermal cable to fire alarm devices.

As a fire alarm control panel, any device using thermal fire detectors with normally open contacts can be used. In projects where a thermal cable with lengths up to 3000 meters (for example, cable collectors or conveyors) is used, it is effective to use special instruments with digital indication of the distance to the point of operation.

If an electromechanical linear thermal fire detector is used in potentially explosive areas, an intrinsically safe barrier must be installed in accordance with existing regulations between the receiver and the thermal cable. The optimal solution for protecting such premises will be to lay a thermal cable from a room with normal conditions to the protected room and exit back. Thus, we take out the installation of electrical connections in a neutral room.

There are three options for connecting an electromechanical thermal cable to fire alarm loops:

– for two-level fire alarm loops;

– for single-level fire alarm loops;

– for polar fire alarm loops (type PPK-2, SIGNAL, etc.).

After the operation of the electromechanical linear thermal fire detector under the influence of a fire or mechanical damage, it is necessary to restore the efficiency of the thermal cable. This is achieved by biting the damaged area and replacing it with a conventional wire. To find the short-circuit point, special instruments are used. The thermal cable is disconnected from the control panel and connected to the sound generator. Then the specialist, using a special sensor, walking along the linear thermal fire detector (thermal cable), removes the sound signal. At the point of short closing, the sound becomes solid. Accuracy of short circuit detection up to 1cm. A less accurate way of finding a short circuit in a thermal cable, but also the most accessible is this resistance measurement by a conventional digital ohmmeter. The accuracy of the determination in this case is within five meters.

Figures Fig-4, Fig-5, Fig-6 show typical electrical wiring diagrams for a thermal cable to fire alarm devices.

Fig. 4.

Scheme connecting a thermal cable to a two-level loop fire alarm.

Fig. 5.

Scheme of connecting a thermal cable to a single-layer loop  fire alarm.

Fig. 6.

Scheme of connecting a thermal cable to a bipolar one-level fire alarm loop.

Linear thermal fire alarm (SafeCable LHD) easy to design, install, operate and maintain. The thermocable showed its reliability in working under difficult conditions and in time. It should be noted that the need for a linear thermal fire detector (thermal cable) in the Russian market is determined by its unique capabilities in the field of fire safety.

And finally, if you have any questions about using the thermal cable or you want to get more detailed information, the specialists of ASPT Spetsavtomatika LLC are always ready to provide assistance, as well as to conduct trainings and individual accompaniment of projects.

Reliability and high quality is our top priority.