It is impossible to be prepared for a fire; it is always sudden and uncontrollable. But it is possible to minimize the risk of its occurrence by significantly reducing predictable material damage. For this purpose, experts have invented fire detectors, which are currently the only means capable of detecting a fire without a person. One of these is a thermal fire sensor or detector, briefly TPI.

The name itself - thermal - explains the principle of operation of the device. It contains one or more transducers - sensitive elements, which, sensing a temperature increase in the environment, lead to the activation of a loud identification signal through an audible alarm.

There is another type of detector - a fire smoke detector. It triggers on aerosol combustion products, in other words, smoke, or more precisely, its color. The advantage of fire-fighting smoke detectors is that it is allowed in administrative buildings, unlike a heat detector, but the disadvantage is that it will wake everyone up not because of a fire, but, for example, a large accumulation of dust or steam. Moreover, strictly speaking, calling it a sensor is incorrect, because it is only an integral part of the detector.

Main types

Based on the type of the main component of the TPI - the sensitive element or controller, there are four main types:

• Contact TPI. When the temperature changes, the installed contact or electrical circuit opens, a special cable breaks and causes an audible signal. The simplest, usually domestic models, are a closed contact of two conductors, packaged in a plastic container. More complex ones have a temperature-sensitive semiconductor with negative resistance. If the ambient temperature increases, the resistance will drop and a controlled current will flow through the circuit. As soon as it reaches a certain point, the alarm will go off.
• IN electronic sensor sensors are installed that are located inside the cable; as soon as the temperature reaches a certain threshold, the resistance of the electric current in the cable changes, which is transmitted to the control of the control device. Highly sensitive. The principle of the device is quite complex.
• Optical detector works on the basis of fiber optic cable. As the temperature increases, the optical conductivity changes, which leads to an audible warning.
• A metal tube with gas, hermetically filled, is necessary for mechanical TPI. The effect of temperature on any part of the tube will lead to a change in its internal pressure and trigger a signal. Deprecated.
• Other types. Semiconductor ones have a special coating with a negative temperature coefficient, electromechanical ones consist of wires under mechanical tension, coated with a heat-sensitive substance.

Types of fire detectors

Firefighters respond to different parameters of fire spread. Hence the classification into types.

The absolute value threshold is set in the maximum fire sensor:

• pressure,
• temperature - as soon as the environmental indicator reaches it, people will be notified.

Domestic devices with an operating temperature of 70-72 degrees are produced en masse. They are also very popular due to their financial accessibility.

For a differential fire alarm sensor, the rate of change of the sign that it monitors is important.

Such devices are recognized as more effective than maximum TPI -

• give the alarm earlier
• They are stable in operation, but due to two elements installed at a distance, they are higher in price.

Maximum differential devices combine both parameters.

When planning to purchase this type of fire-fighting devices, keep in mind that their temperature threshold must be at least 20 degrees higher than the permissible temperature at the facility.

Thus, technical specialists divide modern fire alarm systems into discrete (based on threshold) - they are discussed above - and analog. Analog thermal fire sensors, in turn, are divided into non-addressable and addressable. The latter transmit not only information about the fire, but also their address code.

Both discrete and analog measure the characteristics of fire factors; the fundamental difference is in the method of signal processing.

For analogues it is more complicated and its essence lies in special systematic algorithms.

• Analog Addressable Thermal Devices regularly collect information about the condition of the premises. They can produce the data they are programmed to collect in real time.
• Explosion-proof thermal fire detectors are needed where the risk of fire is high and explosive substances may be present in the air. They seem to be armored, as they are located on various power units, oil pipelines, etc. They differ in the degree of protection, the number of sensors and different set temperature thresholds.
• U linear heat detectors a cable with a heat-sensitive polymer is used - a thermal cable - it records any changes along its entire length as a single fire sensor. Used where the ceiling is large, such as an indoor stadium. In addition to the ceiling, you can also mount it on the walls.
• Multipoint thermal devices opposed to inherently linear. They are part of a single system that controls several zones and is combined into an electrical circuit. Signals received from fire sensors are processed in a single unit.

Operation and Installation

The connection diagram for thermal sensors is given in the operating instructions, however, difficulties may arise.

The requirements of GOST R 53325-2009, paragraph 4.2.5.1, require thermal detectors to be equipped with a built-in or remote optical indicator.

When calculating the values ​​of additional resistors, take into account the electrical components of the connected LED indicators.

Look in the device passport for the typical and maximum voltage drop, which indicate the limit of the parameters. For ease of installation, it is better to use LED non-polar indicators.

Normally closed contacts of thermal devices are connected to the loop in the same way as for smoke devices. The difference is that in the standby state, thermal sensors do not consume electric current, and in active mode it is less than that of smoke sensors.

Fire alarm thermal sensors have the following resistances in the connection diagram:

• Rbal.,
• Rok.,
• Radd.

We study the operating instructions for the monitoring device and take into account the resistor values.

Rbal. similar to Radd., but it is not included in the control device kit; you will have to buy it additionally.

In normal mode, the sensors are short-circuited, which means that resistance Rbal will appear only if one or two of the devices operate. And then an “Alarm” signal can be generated.

For controllers “ Mirage” there is the following diagram. If one is triggered, the “Attention” signal will be received, if the second one is triggered, the “Fire” command will follow.

The designation of the heat detector in the diagram, as well as other components, is as follows:

• Shs– alarm loop,
• IP— thermal fire detector,
• YPRES– manual fire detector,
• DIP– fire smoke detector.

Conventional graphic designation of an automatic heat detector according to the requirements of regulatory documentation - .

Standards and features of installation/connection of thermal sensors are regulated by water rules of fire protection systems 5.13.130.2009 with the latest amendments from 06/20/2011.

From Table 13.5, the distance between the thermal point devices, as well as between them and the wall, becomes known (do not forget about the exceptions specified in paragraph 13.3.7).

Source: SP5.13.130.2009.

It is not difficult to guess that the area covered by the sensor depends on the height of the room. At the same time, many install two devices in each room in case one sensor fails to work.

The distance from one to another should be limited to half the recommended one. But this works with point non-addressable sensors. Addressable analogue ones do not need duplication, since they have a completely different operating principle.

• When placing sensors in rooms, it is necessary to take into account the characteristics of the distribution of combustion products in them.
• It is ineffective to install heat sensors in “dead” zones, where hot air is the last to reach, and the fire protection device will work too late.
• So, when laying the thermal cable of a linear heat detector, there is no need to do this 15-20 cm from the corners along the ceiling and walls.
• Don’t forget about hoods and air conditioners - place the device at least a meter away from them.

Physical laws give rise to the principles that underlie the installation of fire detectors:

• a flat ceiling is protected along a circle lying in a horizontal surface;
• you need to take into account the distance from the floors of the room.

Malfunctions and ways to eliminate them

First of all, we read about them in the operating manual in a specially dedicated section. The description indicates what may not work and what method will help fix the problem.

The classic reasons are unprofessional installation and manufacturing defects. A detected defect leads to a warranty period, which averages from 18 to 36 months, but sometimes 12 months.

• Experienced engineers also point out a false fire alarm in the case of repairs, when dust gets into the device and it goes off.
• Sometimes insects also cause unjustified anxiety. Rubbing with alcohol and blowing helps.
• The loops can periodically notify of a fire when the wires are twisted and the contact is unstable.
• Electromagnetic interference from devices has also not been canceled, so they must be taken into account. Seasonal changes, acoustic vibrations and aggressive environments also influence malfunctions.
• False alarms often do not indicate high sensitivity of detectors, but low quality. Experts also warn that all cheap developments lose sensitivity over time. And only replacement will help here.

To solve most problems due to a malfunction, checking the connections, the correct location of the detectors and the normal operation of the contact connections will help.

Also, high-quality detector components will help prevent undetected fire.

Manufacturers and popular models

Fire detectors are produced by Russian and foreign manufacturers. Among them

• oldest Japanese company Hochiki,
• most popular Siemens, into which the Swiss manufacturer Cerberus joined.
• The fire detectors of the British company have proven themselves well Appolo.
• Also well known System Sensor, whose products are manufactured in 8 largest countries - from the USA to Russia.

In our country it specializes in fire heat detectors

• company “Argus-Spectrum”, located on the basis of the scientific and industrial complex in St. Petersburg.
• Komplektstroyservis is one of the leaders in domestic developments.
• Magneto-Contact produces sensors based on sealed contacts,
• wide range of products from “ Siberian Arsenal”,
• research and production enterprise “ Special Informatics-SI”.
• Private enterprise also offers its products “ Arton" And " Special automation”.

Prices

The simplest maximum fire-fighting heating devices are domestic, their price ranges from 40 to 150 rubles.

• Additional options, for example, memory for a triggered device, a light and/or remote indicator, an increase in their number entails a doubling in price, range from 270 rubles. and up to 600.
• Maximum differential sensors can be purchased for a price starting from 500 rubles. up to 900.
• One of the best selling models Aurora TN (IP 101-78-A1), its price is on average 700 rubles.
• The most popular model of explosion-proof detector due to its affordability IP 101-3A-A3R will cost 200 rubles on average, although most stores offer explosion-proof devices from 800 to 1,000 rubles.

Foreign addressable maximum differential devices

• cost from 1000 rubles per piece and higher.
• Among the addressable analog maximum differential ones - best selling model S2000 IP-03, She is standing from 500 to 800 rubles, and in general the range of addressable detectors reaches 2,000 and even higher.
• thermal sensors - thermal cables - depending on the characteristics (cable resistance, maximum permissible length, current voltage, etc.) are sold on average from 300 to 700 rubles.

Conclusion

Information about the principles of operation, design features, types and types of heat fire detectors will help you choose the most suitable model carefully and without unnecessary financial costs. Installation rules and regulations are not that complicated, and if you treat them responsibly, you can prevent many malfunctions. It is best to carry out installation under the strict guidance of experienced electricians.

Simple smoke detector

Smoke indicators used in fire protection devices: when smoke occurs, an actuator is activated - a sound siren, for example, or an extinguishing device.

The most important thing about smoke detectors This is, of course, the sensor itself.
Smoke detectors They are different in design:
Thermal, chemical (recognizing an increase in carbon monoxide in the environment), ionization, and so on, but the simplest version of a smoke sensor that can be made on one's own It's photovoltaic.

Operating principle of a photoelectric smoke detector is simple: a beam of light is received by a photocell. When smoke occurs, the light beam is distorted and the sensor is triggered.

The light source can be located anywhere - inside the sensor itself or even pass through the entire room and be reflected from a system of mirrors

You can use a simple circuit as an actuator:

The light control in this device occurs as follows. In the standby state, transistor T1 is illuminated, current flows through it, but no current flows through transistor T2 and relay winding P1. Dimming the light output reduces the current through the phototransistor. Transistor T2 goes into saturation mode, its collector current causes the relay to operate and close the contacts in the power circuit of the signaling device.

As for the phototransistor: nowadays you can buy almost anything, but in principle you can make a phototransistor yourself:

For this we need any Soviet transistor in a metal case. For example, such “ancient” ones as MP41 or more powerful ones are suitable, but it is still better to use them with the highest gain.
Useful addition:
The thing is that the crystal from which the transistor is made is sensitive to external influences: temperature, light. So in order to make a phototransistor from a simple transistor It’s enough to simply cut off part of the metal case cover (without damaging the crystal itself, of course!).

If you haven’t found a suitable transistor with the required conductivity (P-N-P is indicated on the diagram), then it doesn’t matter - you can use N-P-N, but then you will need to use transistor E2 of the same conductivity, change the power polarity and “unfold” all the diodes in the circuit.

Another diagram of a smoke photosensor (more complex but also more sensitive) is shown in the figure below:

Light from LED D1 illuminates phototransistor Q1. The phototransistor turns on, and a positive voltage appears at its emitter, which is then supplied to the inverting input of the operational amplifier. At the second input of the amplifier, the voltage is removed from the slider of the variable resistor R9. This resistor sets the sensitivity of the alarm/

In the absence of smoke in the air, the voltage at the emitter of the QL phototransistor is slightly higher than the voltage removed from the sensitivity control slider, while a small negative voltage is present at the output of the operational amplifier. LED D2 (can be any) does not light up. When smoke appears between the sensors, the illumination of the phototransistor decreases. The voltage at its emitter becomes less than that at the slider of the variable resistor R9. The voltage that appears at the output of the operational amplifier turns on the D2 LED and the PZ-1 piezoceramic buzzer.

Fire alarm sensors detect a fire and transmit it to the control panel. The sensor connection diagram depends on the number and degree of response of the sensors located in the structure. Based on this, it is customary to classify sensors according to three principles.

Detector types:

1. Spot - have one sensor and are sensitive in compact areas.
2. Multipoint - have several sensors (two, three).
3. Linear - react to changes along the line and are divided into two types:
   • single (two blocks on one wall and a reflector on the opposite);
   • two-component (two blocks located on opposite walls).

The most effective are heat and smoke detectors.

Smoke detectors

Smoke detectors are the most popular and have a high fire detection rate. The operating principle is based on determining the amount of smoke in the air.

Detector types:

Thermal sensors

Heat detectors respond to changes in ambient temperature. It is most effective in rooms where fuels and lubricants are stored.

Types of heat detectors:

1. Threshold thermal sensors have a set temperature standard and react when it is exceeded. Divided:
   • An electromechanical thermal sensor is a single-use device that contains a special plate. When the temperature rises, it melts and breaks the electrical circuit. The process turns on the alarm. The threshold temperature in sensors of this type is 75C.
   • Semiconductor threshold sensors are a device that uses semiconductors coated with a special substance. When the set temperature increases, an electronic circuit transmits a signal to the panel. Devices respond to changes faster and do not break down like electromechanical ones. The sensors are triggered by the temperature set by the user.
2. Differential thermal sensors are sensitive to the rate of temperature increase. The operating principle of detectors is based on a change in the external current from the internal circuit (temperature difference). The housing is designed using two thermoelements that form electrical circuits (inside and outside). The current from the circuits is supplied to a differential amplifier, which records the temperature relationship between the external and internal circuits. The alarm is triggered if the difference between the temperatures of the internal and external circuits begins to increase.

Installation of smoke and heat sensors

Installation of detectors is carried out by engineers, according to the calculations and plans. The sensor connection diagram is carried out according to two principles.

Connection diagram:

• square;
• triangular.

The most common and simplified connection type is the square diagram.
The distance between the sensors and the walls must also be maintained. Calculations are given in tables.

The surface for mounting sensors must have a coating that provides protection from damage.

Smoke detectors are a more effective fire alarm tool because, unlike traditional heat detectors, they are activated before an open flame forms and a noticeable increase in room temperature. Due to the comparative simplicity of implementation, optoelectronic smoke sensors have become widespread. They consist of a smoke chamber in which a light emitter and a photodetector are installed. The associated circuitry generates a trigger signal when significant absorption of the emitted light is detected. This is the operating principle that underlies the sensor in question.

The smoke detector shown here is battery powered and should therefore consume very little microampere current on average to increase practicality. This will allow it to work for several years without the need to replace the battery. In addition, the actuator circuit is supposed to use a sound emitter capable of developing a sound pressure of at least 85 dB. A typical way to ensure very low power consumption of a device that must contain sufficiently high-current elements, such as a light emitter and a photodetector, is its intermittent operation mode, and the duration of the pause should be many times greater than the duration of active operation.

In this case, the average consumption will be reduced to the total static consumption of inactive circuit components. Programmable microcontrollers (MCs) with the ability to switch to a micro-power standby mode and automatically resume active work at specified time intervals help to implement this idea. These requirements are fully met by the 14-pin MK MSP430F2012 with a built-in Flash memory of 2 kbytes. This MK, after switching to LPM3 standby mode, consumes a current of only 0.6 μA. This value also includes the current consumption of the built-in RC oscillator (VLO) and timer A, which allows you to continue counting time even after the MK is switched to standby mode. However, this generator is very unstable. Its frequency, depending on the ambient temperature, can vary within 4...22 kHz (nominal frequency 12 kHz). Thus, in order to ensure the specified duration of pauses in the operation of the sensor, it must be equipped with the ability to calibrate VLO. For these purposes, you can use the built-in high-frequency generator - DCO, which is calibrated by the manufacturer with an accuracy of no worse than ±2.5% within the temperature range of 0...85°C.

The sensor diagram can be found in Fig. 1.

Rice. 1.

Here, an LED (LED) and an infrared (IR) photodiode are used as elements of an optical pair located in the smoke chamber (SMOKE_CHAMBER). Thanks to the operating voltage of the MK 1.8...3.6 V and proper calculations of other stages of the circuit, it is possible to power the circuit from two AAA batteries. To ensure the stability of the emitted light when powered by an unstabilized voltage, the operating mode of the LED is set by a 100 mA current source, which is assembled on two transistors Q3, Q4. This current source is active when output P1.6 is set high. In the standby mode of operation of the circuit, it is turned off (P1.6 = “0”), and the total consumption of the IR emitter cascade is reduced to a negligible level of leakage current through Q3. To amplify the photodiode signal, a photocurrent amplifier circuit based on an op-amp is used TLV2780. The choice of this op amp was based on cost and setup time. This op-amp has a settling time of up to 3 μs, which made it possible not to use the ability it supports to switch to standby mode, and instead control the power of the amplifier stage from the output of the MK (port P1.5). Thus, after turning off the amplifier stage, it does not consume any current at all, and the current savings achieved are about 1.4 µA.

To signal the activation of a smoke sensor, a sound emitter (S) P1 is provided ( EFBRL37C20, Panasonic) and LED D1. ZI belongs to the piezoelectric type. It is supplemented with components of a typical switching circuit (R8, R10, R12, D3, Q2), which ensure continuous sound generation when a constant supply voltage is applied. The type of ZI used here generates sound with a frequency of 3.9±0.5 kHz. To power the ZI circuit, a voltage of 18 V is selected, at which it creates a sound pressure of about 95 dB (at a distance of 10 cm) and consumes a current of about 16 mA. This voltage is generated by a step-up voltage converter assembled based on the IC1 chip ( TPS61040, TI). The required output voltage is specified by the values ​​of resistors R11 and R13 indicated in the diagram. The converter circuit is also supplemented with a cascade for isolating the entire load from battery power (R9, Q1) after the TPS61040 is switched to standby mode (low level at the EN input). This makes it possible to exclude the flow of leakage currents into the load and, thus, reduce the total consumption of this cascade (with the ignition switched off) to the level of its own static consumption of the IC1 microcircuit (0.1 μA). The circuit also provides: button SW1 for manually turning on/off the RF; “jumpers” for configuring the power supply circuit of the sensor circuit (JP1, JP2) and preparing the RF for operation (JP3), as well as external power connectors at the debugging stage (X4) and connecting the adapter of the debugging system built into the MK (X1) via a two-wire interface Spy- Bi-Wire.

Rice. 2.

After resetting the MK, all necessary initialization is performed, incl. calibrating the VLO generator and setting the frequency of resuming active operation of the MK, equal to eight seconds. Following this, the MK is switched to the LPM3 economical operating mode. In this mode, the VLO and Timer A remain running, and the CPU, RF clock, and other I/O modules stop working. Exit from this state is possible under two conditions: generation of an interrupt at input P1.1, which occurs when the SW1 button is pressed, as well as generation of a timer A interrupt, which occurs after the set eight seconds have passed. In the P1.1 interrupt processing procedure, a passive delay (approximately 50 ms) is first generated to suppress bounce, and then changes to the opposite state of the RF control line, making it possible to manually control the activity of the RF. When an interruption occurs on timer A (interrupt TA0), the procedure for digitizing the output of the photocurrent amplifier is performed in the following sequence. First, four digitizations are performed with the IR LED turned off, then four digitizations are performed with the LED turned on. Subsequently, these digitizations are subject to averaging. Ultimately, two variables are formed: L is the average value with the IR LED turned off, and D is the average value with the IR LED turned on. Quadruple digitization and their averaging are performed in order to eliminate the possibility of false alarms of the sensor. For the same purpose, a further chain of “obstacles” to false triggering of the sensor is built, starting with a block for comparing the variables L and D. Here the necessary triggering condition is formulated: L - D > x, where x is the triggering threshold. The x value is chosen empirically for reasons of insensitivity (for example, to dust) and guaranteed operation when smoke enters. If the condition is not met, the LED and RF are turned off, the sensor status flag (AF) and the SC counter are reset. After this, timer A is configured to resume active operation after eight seconds, and the MK is switched to LPM3 mode. If the condition is met, the state of the sensor is checked. If it has already worked (AF = “1”), then no further actions need to be performed, and the MK is immediately switched to LPM3 mode. If the sensor has not yet triggered (AF = “0”), then the SC counter is incremented in order to count the number of detected trigger conditions, which further improves noise immunity. A positive decision to trigger the sensor is made after detecting three consecutive trigger conditions. However, in order to avoid excessive delay in response to the appearance of smoke, the duration of the standby mode is reduced to four seconds after the first trigger condition is met and to one second after the second. The described algorithm is implemented by a program available at the link http://www.ti.com/litv/zip/slaa335 .

In conclusion, we determine the average current consumed by the sensor. To do this, Table 1 contains data for each consumer: consumed current (I) and duration of its consumption (t). For cyclically operating consumers, taking into account the eight-second pause, the average current consumption (μA) is equal to I ґ t/8 ґ 106. Summing up the found values, we find the average current consumed by the sensor: 2 μA. This is a very good result. For example, when using batteries with a capacity of 220 mAh, the estimated operating time (excluding self-discharge) will be about 12 years.

Table 1. Average current consumption taking into account an eight-second pause in sensor operation

Current consumer	Duration, μs	Current consumption, µA	Average current consumption, µA
MSP430 in active mode (1 MHz, 3 V)	422,6	300	0,016
MSP430 in LPM3 mode	8.10 6	0,6	0,6
Operational amplifier	190,6	650	0,015
ION ADC	190,6	250	0,006
ADC core	20,8	600	0,0016
IR LED	100,8	105	1,26
TPS61040 in shutdown mode	continuously	0,1	0,1
		Total:	2

Obtaining technical information, ordering samples, delivery - e-mail:

Even in ancient times, people used the transmission of information about the beginning of some events over a distance in the form of light signals or clearly audible sounds, when bonfires were lit on hills or bells were rung.

The life of a modern person is associated with the operation of a large number of various equipment, the operation of which is often monitored remotely using various types of alarms. Among them, information about the start of a fire at critical industrial facilities and inside multi-storey buildings with a large number of people is given the utmost importance.

Purpose of fire alarm

Its main task is to promptly transmit information at the first signs of fire to the duty service, which can quickly arrive at the scene of the incident and take emergency measures to extinguish the fire that has arisen and prevent its spread.

Additional tasks of fire alarm systems (FAS) can be:

remote activation of pre-positioned fire extinguishing means - various types of fire extinguishers created in relation to specific conditions of production or facility;

ensuring the unlocking of access control systems to facilitate the mass evacuation of people from a dangerous place;

transfer of information to additional dispatch control points;

other functions.

Fire alarm composition

The fire alarm system is considered as a specific electrical control system, the circuit of which consists of various parts:

special sensors - detectors that indicate the start of a fire;

channels for transmitting signals about sensor activation;

control panels, reception (RCP) and display of information for operating personnel;

public warning systems.

How fire detectors are designed and work

The occurrence of the first signs of fire can be assessed by the appearance of smoke, rapid heating of the environment, or a strong flash of light. These three factors are incorporated into the operating principle of various technical devices.

In the industrial and residential sectors, four types of sensors operating on different principles are most widespread:

1. detecting the beginning of smoke spread - smoke detectors;

2. the appearance of sudden heating indoors - thermal;

3. separation of electromagnetic waves in the optical range of the visible, ultraviolet or infrared spectrum - flame;

4. simultaneous exposure to heat and smoke, and often in combination with the appearance of bright light - combined.

Fire alarm sensors can only monitor the state of the monitored parameter or respond to its change by issuing a signal to an external system. According to this principle, they apply not only to passive, but also to active devices. Detectors can be created to monitor a specific local area or an extended, elongated area. The latter constructions are called linear.

How smoke detectors work

The sensor is placed on the ceiling in the place where smoke rises and begins to concentrate when a fire starts.

Structurally, the smoke detector consists of:

1. split housing;

2. electronic board;

3. optical system.

These parts are individually assembled on automated production lines and, after passing various tests and inspections, are assembled manually into a single module.

The operation of the sensor is based on recording the moment of smoke appearance in its housing due to the activation of an optical system, which includes:

Emitting a strictly directed beam of light;

Which converts the light flux incident on it into an electrical signal.

Structurally, the light beam from the source is directed slightly away from the photocell. Under normal operating conditions with normal indoor air conditions, light cannot reach the surface of the photocell, as shown in picture No. 1.

If smoke appears in the sensor housing, light rays begin to be reflected in all directions. They hit the photocell and it fires. This moment is controlled by an electronic circuit. It generates an information command and transmits it via communication channels to the fire alarm receiving device.

If water vapor or gases that deflect the light flux begin to penetrate into the sensor cavity, the photocell will also work, and the logic circuit will provide false information about the occurrence of a fire.

For this reason, smoke detectors are not installed in areas where they are likely to operate incorrectly. These include kitchens, bathrooms, showers. Installing smoke detectors in places where smokers gather will also cause them to operate frequently and falsely.

Such a fire detector will not respond to an increase in temperature and a flash of light from an open fire. Therefore, such modules are installed in those rooms where a fire is associated with smoke in the environment due to temperature damage to the insulation of electrical wires, fabrics, and other similar materials.

They are installed in places with a large number of operating electrical equipment in industrial production, material storage warehouses, electrical substations and laboratories.

Operating principle of heat detectors

They are also placed on the ceiling, where the heat generated by an open fire rises. They can work according to the factor:

1. achieving the maximum permissible heating value;

2. rate of temperature increase.

Threshold devices

Sensors of this type were the very first to be created. At first, they worked due to the flow of an easily molten alloy from a fuse installed at the point of contact of two conductors. Due to this, when the environment heated to 60–70 degrees, the electrical circuit broke and a signal was issued about the start of a fire.

The operating principle of one of these designs of a disposable, non-renewable heat detector type IP-104 is shown in the picture.

Inside the housing there are spring contacts, which are retracted from each other by mechanical tension forces, and are held in place by Wood's alloy, consisting of low-melting metals. The sensor is triggered when heated to 68 degrees, and the circuit is broken by charged springs.

Such designs are constantly being improved. Now they are produced with replaceable fuse inserts or elements controlled from a distance. The logic circuit can be made using different principles and electronic components.

Integral detectors

The sensor's operation is based on measuring the rate of change in the electrical resistance of metals when they are heated.

A stabilized voltage is supplied to the terminals of the thermal control element from the power source. Under its action, a current determined by Ohm's law flows in an electrical circuit through a wire resistor and a measuring device. Its value strictly depends on the resistance.

When exposed to normal room temperature, its value remains virtually unchanged. With a stabilized voltage, the current does not change either.

When the control element begins to be affected by the temperature of the open fire from the emerging flame, the resistance of the sensor begins to quickly increase and the current begins to change according to the same law. The rate of its deviation from the previously established value is fixed by an electronic circuit, which is usually set to increase by 5 degrees per second.

When a critical heating rate is reached, the sensor’s logic circuit sends a signal to the receiving module via communication channels.

This circuit does not have devices that react to smoke, and it will not work on it.

Such structures work most effectively in fires caused by the ignition of flammable liquids from petroleum products, carbon fuels, and flammable solid materials. They are installed in storage areas for containers with flammable liquids, warehouses for building materials and in similar industrial buildings.

Operating principle of flame detectors

A fairly large class of these sensors reacts to an open fire or a smoldering fire without producing smoke.

A sensitive photocell detects the appearance of one of the optical wave spectra or its full range. At the same time, the design turns out to be quite complex and expensive. For this reason, they are not used in residential buildings, but are used in the oil and gas industry.

The simplest models of this type are capable of being triggered by exposure to a welding arc, bright sunshine, fluorescent lamps, and electromagnetic interference in the optical spectrum. Various filters can be used to eliminate false operation.

Operating principle of combined detectors

All designs of fire detectors that operate based on one sign of fire may trigger falsely. To expand the reliability of transmitted information, devices are created that immediately combine the capabilities of smoke and thermal models, or are supplemented with a flame reaction function.

To do this, they immediately include an infrared, thermal and optical sensor. They can, in most cases, be configured to trigger from each input parameter separately or only when they appear simultaneously.

For critical industrial premises, there are four-channel combined detectors that additionally take into account the appearance of carbon monoxide.

Operating principle of manual fire call points

The simplest designs of an ordinary button with a self-resetting spring are used to manually notify operational workers about the start of a fire. To do this, personnel who notice the beginning of signs of fire need only open the protective cover and press the button.

This action closes the contacts of the circuit and turns on the “Fire Alarm” notification. When the button is released, the signal is not interrupted: its power supply chain is automatically set to self-locking. People will be warned about the fire hazard until the responsible employee uses a special key to unlock it.

Such hand-held sensors are installed in all rooms where masses of people gather (shops, hospitals, cinemas, industrial facilities) at a height of one and a half meters and at a distance between them of up to 50 m.

Brief conclusions on choosing fire detectors

The design and principle of operation of the sensor must comply as much as possible with the conditions ensuring fire safety of the controlled premises.

In large industrial buildings with different equipment, it is not always advisable to use the same types of detectors, and their number, even with limited financial resources, must cover all dangerous fire zones in accordance with the requirements of regulatory documents.

Channels for transmitting signals about detector activation

After the types and number of fire sensors have been determined for installation in the premises, they are connected by wires into loops, which are assembled into a control panel in the operational security service.

For loops, wires with copper cores are selected and laid with the ability to monitor the technical condition. SNIP and GOST impose requirements on them regarding methods of separate installation with other cable lines and to ensure protection from mechanical damage.

Instruments for receiving and monitoring signals

Control panel panels are created by manufacturers of varying degrees of complexity for professional, semi-professional or household use.

Professional devices are designed to solve not only fire safety issues, but also the protection of facilities. They:

monitor the state of multipath circuits and are capable of simultaneously processing analog and digital signals;

allow cascade combination into blocks to create a complex hierarchy of control circuits;

connect to the computer of the fire and security service;

record in time and transmit all information occurring at the controlled object;

are used only at critical industrial facilities.

Semi-professional devices work with digital signals. They are manufactured in a single building that combines:

power supply from a stationary electrical network;

backup power supply - a powerful battery capable of ensuring autonomous operation of the system from several hours to a day;

electronic control unit;

CPU.

At critical facilities, the processor is protected from unauthorized access by placing it in hard-to-reach places with complete shielding, preventing hacking attempts using a special remote scanner, and complex coding of processed and transmitted information.

Such models are capable of processing signals from two hundred and fifty sensors. They can already be used in the residential sector.

Multi-beam household control panels

They are designed to work in private households with various outbuildings.

Capable of processing signals from electrical contacts of reed switches or electronic circuits, as well as information received via wireless channels from two to eight different sources.

The simplest apartment control panels

They are represented by the simplest models, operating in single-channel mode, which is quite sufficient for the apartment owner. Even such a device is capable of transmitting information about sensor activation to the owner’s mobile phone in the form of SMS.

Control panel panels intended for domestic purposes are accompanied by detailed technical documentation from the manufacturer with instructions and connection diagrams. The European standard EN54 has been introduced for them.

Fire warning systems

In crowded buildings, a light and sound warning system is used to warn personnel and visitors using the “Alarm” command. At the same time, information is transferred to the management of the enterprise and duty services for taking emergency measures.

An example of the distribution of various fire alarm devices and the organization of a warning system is shown in the picture.

Like all technical devices, fire alarm systems require periodic monitoring and performance checks, a set of maintenance measures, settings, and adjustments. In this case, it is necessary to follow the rules of their operation.

I would like to express confidence that the initial information presented about the design of modern fire alarms will prompt the reader to think: in practice, create for yourself an optimal system that excludes fire in the event of an accidental fire or deliberate arson.