Sensor thermal cables for fire alarms.
Cable sensors are a very interesting class of detectors. In this article, we will consider several completely different products from this category, with different characteristics and intended for different applications. The main thing that unites them is the shape of the sensing element. A sensing cable is pulled from the processing unit (or even directly from the control panel) through the protected room without installing additional sensors. Cheap and cheerful. This installation option is especially convenient for protecting cable channels, air ducts and other very elongated rooms (including many industrial installations), in which tight narrow volumes do not allow installing and servicing conventional detectors or if there is an inappropriately large number of them to be installed there.
The installation of such a cable is significantly cheaper than the installation of multiple point detectors, and in some cases the cable even has increased sensitivity due to the summation of the signal along its length — in fact, it works as an intelligent system with addressable analogue detectors and makes a decision based on information from many sections of the cable.
If we look at history, the first attempts to combine detectors and cables were fairly simple disposable detectors (similar to the popular disposable destructible thermal maximum detectors of the times of historical materialism). The operating principle was based either on a break or a short circuit. Unfortunately, cables made of a material with a melting point below 100 degrees are unreasonably expensive, so cables in which the response temperature is determined by the melting point of the insulator between elastic steel cores became widespread (and are still widely produced, for example, by Protectowire, Kidde, ProLine). A simple cable contains only two cores, more complex cables can have several cores, so that different pairs of cores will respond at different temperatures. Finally, since the cable has steel cores with a fairly high resistance, it is easy to measure its resistance to the short circuit point and thus determine the distance to the fire detection point (of course, only to the point closest to the device). The great advantage of such a cable is that it can be connected (of course, without measuring the distance) to any receiving device capable of registering the closure of contacts. Note that these cables are often called digital, although they do not have any computer innovations, on the contrary, they are capable of giving out only one bit of information.
Such cables are essentially disposable, in the event of a trigger they are destroyed, but this is not so bad — if a fire occurs, replacing the destroyed section of the cable will be the least of the problems.
More advanced sensor cables are reusable, but this is not their main property. The principle of their operation is based not on the destruction of insulation, but on the change in its resistance. Thus, the control device can measure the temperature, and it will sum up the excess of the normal temperature at all points. The algorithm for making a decision on a fire can be either maximum or differential (by the rate of temperature change). This algorithm is completely determined by the cable controller. Of course, the controller of such a cable must be special, and at its output have a dry contact for connection to a conventional control panel along with other conventional (point) detectors. Such a cable does not deteriorate and is again ready for duty after eliminating the source of high temperature (unless, of course, the temperature during the fire exceeds the melting point of copper or the cable sheath).It is worth noting that such a cable is usually 4-wire, so that the integrity of the cable and the resistance between the wires can be independently monitored. And now a fly in the ointment: the ability to sum the temperature along the length of the cable is usually advertised as an advantage, supposedly increasing sensitivity. But, on the other hand, this means that sensitivity cannot be specified in degrees unless the length of the cable section on which the temperature has increased is also specified. In particular, a kilometer-long cable will not be able to distinguish between heating the entire cable to 30 degrees and heating a 1 m piece to 60 degrees. Such cables are often called analog, as opposed to «digital» cables, which are only capable of producing a short circuit.
A very specific cable sensor is a multi-point cable, offered, for example, by LISTEC, Securiton, etc. In fact, this is not a real sensor cable, but just a regular flat ribbon cable, to which miniature addressable thermal sensors are occasionally soldered. Structurally, it resembles common LED cables — it also looks like a cable on the outside, but in fact, there are quite a lot of discrete semiconductor devices inside. In some products, these sensors do not measure their own temperature, but, like systems with an analog cable, the temperature of a nearby section of the cable — in this case, it is truly a sensor cable.
It is clear that, unlike those described above, such a cable is much more complex, more expensive, and failure of a separate «micro-detector» inside such a cable is more likely than damage to the so-called digital cable with destructible insulation. Please note that such a parameter as the distance between the measuring sensors may be important for this type of cable. According to regulatory documents, it should be considered not as a linear cable sensor, but as a set of point detectors with the resulting requirements for the distance between individual detectors.
By the way, I have seen how such a «sensor» is made manually: ordinary point detectors are connected with measured pieces of cable, and then the finished «garland» is pulled through a narrow space and left like that, without fixing the detectors. Not very correct, inconvenient for maintenance, but sometimes this is the only possible option if you have not taken care of buying a real sensor cable in advance.
The most technically advanced among fire thermal sensor cables is the fiber optic sensor. The sensor itself is a regular multimode fiber cable. All the interest is in the signal processing unit connected to it. Complex electronics allow measuring the scattering spectrum and, thus, measuring the temperature, and even measuring the temperature distribution along the cable with an accuracy of better than 1 m along its entire length, and the permissible length reaches several kilometers. I will not try to describe the physical principles underlying the measurement (Raman scattering, etc.), I will only emphasize that the discreteness of temperature measurement along the cable length of 1 m means the discreteness of the reflected signal measurements in time of 10 nanoseconds. I think you understand: the cost of the system is not small. However, if you consider that such a sensor cable 8 km long is completely equivalent to 8,000 addressable analog point detectors, and the installation is completed after the cable is laid, the price may not seem so high. And, by the way, there is no electricity in the cable — like all fiber optic systems, this system has no restrictions on use in hazardous areas. If you are interested in this cutting-edge technology, contact manufacturers such as LIOS, AP-sensing, etc. for details.
In addition to sensor cables, there are also similar air «tubular» sensors. Moreover, they are also of two types: also «digital» and «analog». «Digital» are plastic tubes inflated with a special pump, they are destroyed when overheated, air comes out, the pressure drops, there is a fire alarm. By the way, they are sometimes suggested to be used as an alarm combined with a pipeline for supplying a fire extinguishing agent. Like sprinkler systems, they automatically supply a fire extinguishing agent exactly to the fire site — it is enough to turn on another pump when a fire is detected, which will supply not air, but an extinguishing agent to the pipeline. The second type of pneumatic cable sensors is «analog», when heated, the pressure in them changes. However, they must also have a pump that will regularly pump air there. This is the main disadvantage of pneumatic systems. The complexity of servicing a vacuum-sealed pipeline, repairing and restoring connections in case of damage, the need for a serviced mechanical pump for regular inflation of the system. And searching for a leak in a pipeline can become entertainment for several months. But such a system is also absolutely explosion-proof and can be used in any conditions.
A similar system of pipelines, but in reverse, is used in aspiration smoke detectors, in which a pipe with holes is located in the protected area, and through it a pump pumps air samples from the protected area into one high-quality, high-precision meter. In terms of installation and application features, aspiration detectors are also similar to sensor cables, but it should be noted that, like all smoke detectors, they are more stringent in terms of correct installation: so that the smoke intake device is located where smoke accumulates, i.e. in the upper part of the protected area, the smoke intake holes should not be blocked, etc. But aspiration systems are perhaps the most sensitive of all and provide an alarm signal much earlier, before the cable temperature rises significantly. And again, aspiration detectors can be used in many cases in explosive areas.
In conclusion, let me remind you that in addition to linear thermal and aspiration smoke detectors, linear smoke detectors can sometimes be used in similar conditions, in which the sensitive area is formed by an infrared beam between the emitter and the receiver up to a hundred meters long. Thus, we also get a linear sensitivity zone, although it must be straight, but for this you do not need to lay anything at all, not even a cable. And again, in some cases, such a detector can be mounted outside the explosive zone, and it will control smoke inside this zone.
In general, please remember that the most common point smoke detectors, although they are indeed the most affordable and reliable of those produced today (it is not for nothing that they are clearly recommended by standards for all premises where their use is permitted), but this is not the only option. In some cases, the use of somewhat unusual cable (or «pipeline») detectors can be much more effective, and at the same time much cheaper to install.