Modules for thermal cable control.

Modules for monitoring thermal cables.

Modules for monitoring thermal cables

Application area and advantages of thermal cable-based systems.
Protecting industrial facilities from possible fire hazards is a priority task that requires a competent and comprehensive analysis of both the protected facility itself and the fire alarm system (hereinafter referred to as FAS) equipment installed at this facility.

As a rule, fire alarm systems for these objects are built on the principle of a radial scheme using point fire detectors (PI). But there are also objects where such an approach to building an OPS is associated with some difficulties, or even with the impossibility of installing conventional (point) PI on them. Such objects include various types of mines, warehouses, cable routes, conveyors, aircraft hangars, etc., where a fire can start anywhere along the entire length of the protected object, and localization and determination of the location of the fire is complicated by the design.

Today, the task of fire protection of such objects is successfully solved by using fire alarm systems based on a linear fire detector (thermal cable), which allows you to determine the location of a possible fire along its entire length.

 
It should be noted separately that it is possible to use a thermal cable at explosive objects, since the thermal cable is designed to correspond to a conventional cable and can be used in explosive zones when connected to a control device that ensures intrinsic safety.

Functional features of modules for monitoring thermal cables.
It is known that the operating principle of a thermal cable is based on the short-circuiting of its cores at the point of local heating due to the melting of the insulation when a certain temperature is reached. In this case, the cores are made of a material with a certain linear resistance (usually from 0.3 to 0.7 Ohm/m, depending on the manufacturer). Based on this principle, the response of the thermal cable is controlled. Theoretically, it is possible to control the response of the thermal cable by any control panel (hereinafter referred to as the CP). However, in practice, it is more convenient and preferable to use specialized modules. They allow you to take into account all the features of the thermal cable as a linear fire detector.

As a rule, a fire alarm system based on a thermal cable includes:
• an alarm loop in which thermal cables with different response temperatures and design options, as well as conventional connecting cables, can be connected in series;
• a control module (hereinafter referred to as the module);
• communication lines and transmission of information to fire alarm recording devices (to the control panel, fire control device, warning devices).

Modules are the main link in the fire alarm system based on a thermal cable, and therefore determine the structure and characteristics of the entire system.
Today, the Russian market offers a number of modules with different technical characteristics and a set of functions, both domestic and foreign production. Some of them are listed in Table 1.

Table 1. Comparative table of functions of modules for monitoring thermal cables.

Many of the module functions are special and must be additionally considered to facilitate the selection task at the design stage. Let us dwell on some of them:
• The presence in the module of fire control panel functions(sound signaling of the mode, memorization of the fire alarm, etc.), not provided in the usual modules operating on the fire alarm control panel as an intermediate link. Allows in some cases to simplify the construction of the fire alarm system, eliminating an unnecessary link.

• Availability of the function indication of the distance to the triggering pointthermal cable. When used with such a module, the thermal cable actually turns into a multitude of addressable heat detectors distributed along the entire length. Determining the distance with a sufficient degree of accuracy (usually ±1 meter) allows you to increase the efficiency of the fire prevention system.

• The presence of the function programming the linear resistance value in modules with indication. Allows you to use thermal cables from different manufacturers together with the module. Also allows you to increase the accuracy of indication due to adjustment.

• The presence of the function control of the remote thermal cable. Allows, when connecting a remote thermal cable to the module with a regular connecting cable, to distinguish a fault caused by a short circuit of the wires in the connecting cable from a fire signal caused by a short circuit of the wires in the thermal cable. As a rule, such a function is implemented through calibration carried out at the stage of commissioning.

• The presence of a built-in interface for transmitting information about the response to the central computer, or to various AFPS and APCS systems using standard protocols.

Allows for easier integration of the module into the automation system of the facility.
• The presence of a function for ensuring intrinsic safety of the alarm loop. Allows monitoring of hazardous areas without using additional intrinsic safety barriers.

It should be remembered that the thermal cable is only a passive (non-power consuming) sensor, and ensuring explosion safety depends entirely on the control module, as associated equipment. It is the intrinsically safe module, in accordance with its explosion protection marking, that determines the class of the explosion hazardous area and the group of explosive mixtures that can be monitored using the thermal cable.

It should also be noted that both single-channel and dual-channel modules are produced. Dual-channel modules allow the implementation of the principle of double-action recognition required in automatic fire extinguishing systems. Two identical thermal cables (with the same response temperature), thermal cables with different but close response temperatures (for example, 68 and 88°C), or a dual-temperature thermal cable (a three-core thermal cable with two different response temperatures) can be connected to such modules.

Fig. 1 MIP-2I-Ex module

The use of some modules containing a set of the above-described functions, for example the MIP-2I-Ex module (Fig. 1) manufactured by the company Spetspribor, Kazan, allows you to build an intelligent addressable fire alarm system for an explosive zone with the determination of the exact coordinates of the point of possible ignition (see Fig. 2). This, in turn, allows, when using appropriate fire extinguishing systems, to quickly and at low cost prevent or localize a possible fire or even an explosion at the stage of occurrence of unacceptable heating.

Fig.2. An example of an intelligent addressable fire alarm system using a thermal cable.

Where Lm is the distance in meters to the point where the heating cable is triggered. Rок is the end-of-life resistor. x1,y1 are the coordinates of the location of a possible fire or heating.

Conclusion
Fire alarm systems based on thermal cables are increasingly are gaining deserved popularity at many industrial facilities in Russia. This is due to both a number of advantages of the thermal cable over conventional detectors, and the availability on the market of a variety of modules for monitoring the thermal cable with a set of various unique functions. Using a number of these functions allows you to build flexible and intelligent systems for early detection and prevention of fire.