【 Weak current College 】 fire detectors cause interference and improve methods

Summary: this article from the fire detector works, analysis of fire detectors and major sources of interference, and to make some improvements.
Keywords: fire detectors; disturbance; improved methods
I. preface

With economic development, a large number of buildings constructed and used for the protection of personal and property safety of automatic fire alarm system is becoming more and more necessary. But the automatic fire alarm system setting, often find that some of the system is not satisfactory. Such as: fire detectors often failure or damage, increased maintenance cost; detector are often misused, making fire alarm watch troubled. Excludes equipment quality does not cross the border and other circumstances, we find that these situations are often due to the disruption of detection equipment. The following is from the fire detector works, step by step analysis of probes of parts and the main source of interference, and to make some improvements.

Second, the fire detection system works

Fire detection devices as the name suggests is that if there is fire detection is undergoing of the device, according to the characteristics of fire and its objects (for example: smoke, heat, flames, etc), can be divided into: smoke detector and heat detector and three flame detector.

Fire detectors in general working principles: sensors detect fires or fire occurs in the values, transmitting circuit will detect components of the original signal is converted to current/voltage signal or pulse, switch data, into the automatic fire alarm controller, controller received signal to calculate analysis, and determine if there is fire is occurring, is an alarm signal. Some systems to mitigate fire auto-alarm controller burden in fire detection devices installed inside the CPU, and assumed the controller portion of data calculation and analysis tasks. Now, briefly three detector works.

1, photoelectric smoke detector

Photoelectric smoke detector has a labyrinth smoke detection room, inside there is a light and a PC components. As is the labyrinth design, light rays usually does exposure to the PC component, but when there is smoke enters the light scattering in the smoke generation, thus some rays to the PC component, smoke more dense, scattering to the PC component of light on the more sensitive components and optical signal is converted to an electrical signal is output.

2, ion sensing smoke detector

Ion sensing smoke detector consists of a radioactive source (such as Am241), external sample room and a built-in ion reference sample room. When radioactive irradiation of substances in the air, some substances become positively charged ions, with other substances become negatively charged ions. With the positive ions and negatively charged ions in the electric field of the role of an electric field. When the smoke into the sample Chamber later and charged ions, reduction in the number of charged ions makes electric voltage change, smog, the more concentrated the greater the voltage changes.

3. fixed temperature-a temperature probe and temperature-a temperature probe

Temperature probe is the use of heat component to temperature values passed to the controller, or using two metal sheets produces a switch quantity and transmitted to the controller. -Fixed temperature heat detector is the first detection of high and low temperature, when the temperature reaches a set temperature (e.g. 58 ℃) alarm signal; and temperature-sensing temperature detector is the slope of the measuring temperature rise to a certain standard and generates alerts (such as 10 minutes, the temperature has risen by 6 ° c).

III. the main sources of interference

By fire detector composition, structure and working principle, we can easily figure out which parts of the fire detectors are vulnerable to outside interference, but these outside interference factors is the source of interference.

We believe that fire detectors are most vulnerable to interference of the following sections:

(1) sensing component

Sensing element is fire detector "eyes", but it's "vision" limited. For example: when smoke some characteristics of similar substances (such as dust, water spray) was it "sees", considered to be smoke, resulting in false alarms. On the other hand, because the sensing element is exposed to the outside world in General, the more susceptible to outside influence of dust, moisture and reduce sensitivity.

(2) detection circuit

Detection circuit is including the sensing element, a set of circuit, the main function is to transfer the sensing element of the "information" is converted to our required voltage or current signals. When subjected to external electromagnetic interference, it will produce false alarm or alarm, or even cause damage to the device.

(3) communication and communication lines

Communication circuits and communication line is responsible for the detection of the "information" to the fire alarm control unit, but in this process are likely to be affected by interference from false alarms. For example: detection circuit routing information from the a/d conversion into a digital signal, and then the communication circuits and communication lines to the fire alarm control unit, but may be subject to interference and signal "0" to "1" signal or low level to high level, so that the fire alarm controller receives a false information and false alarms.

Therefore, interfere with fire detectors work interference factors, we believe there are the following:

(1) dust

The destruction of the natural environment, the green space reduction, urban construction, plant litter row misplaces, resulting in a dust content in the air, and dust on fire detectors (especially the smoke detector) is a "killer". Because excessive dust over the detector of optical components, will lead to reduced sensitivity of the probe or causing a failure; on the other hand, excessive dust room stay in sampling will result in a large number of scattering of light, so that the PC components receive too many false alarms cause detector light. For ion sensing probes, because large amounts of dust entering the sample Chamber and charged ions causing false alarm detectors.When corrosive dust over the detector circuit board, circuit, corrosion wet weather will become more serious.

(2) moisture

Moisture, especially in the South of the rainy weather, humidity is usually greater than 95 per cent, which will probe the electronic panels and detection components have a great impact. First of all, will cause a short circuit electronic panels moisture damage or make insulation reduces the resulting system grounding; Secondly, when moisture into the detector detection room, on the probing components cause interference. Especially on the smoke detector, when moisture enters the detection room, a large number of scattering detector light sources emit light, resulting in false alarm detectors.

(3) electromagnetic fields

Fluorescent lamp ballasts, high-voltage motors, communication transmitters and other devices emit electromagnetic interference, when the detector is installed in the vicinity of these devices, more or less will be electromagnetic interference. When the fire detector circuit or communication line interference, you may generate false alarms.

(4) high speed air flow

When the detector is installed in a high-speed air flow position, such as ventilation and air conditioning of air near the, wind room, duct, etc; gas will smoke blown off probe detector alarm slowly, on the other hand high-speed airflow into the smoke detector sample Chamber, the air is formed, to remain in a swirl sampling room of dust raised resulting in false alarms.

4. several improved method

To completely resolve the interference problem, the best method is to remove sources of interference. But in most cases, the source of interference is not excluded, even some interference sources or of the protected object. Therefore, we can only reduce interference point of view. The following is my experience, some of the improvements:

1. reasonable installation, standardized wiring

According to incomplete statistics, 70 per cent of the false alarm of fire detectors are installed because it is not reasonable. Therefore, the correct selection of the type of fire detectors, as well as in accordance with the relevant technical descriptions and specifications for proper installation and wiring is the key to reducing interference effects.

(1) the correct selection of the type of fire detector

In the select the type of fire detector should refer to the national fire alarm system design specifications (GBJ116-88).

For example: relative humidity greater than 95% RH for long-term, airflow speed, greater than s, massive 5m/dust, water spray stranded, potentially corrosive gases would not be appropriate for the site set ion sensing fire detectors; in may produce smoke, a large number of accumulation of dust, the usual case where there's smoke there stranded, high-frequency electromagnetic interference would not be appropriate for setting photoelectric establishments fire detector; relative humidity greater than 95% RH for long-term, large amounts of dust, the usual case where there's smoke, and vapor stranded, kitchen, boiler room and generator room, tea boiler room, garage and other places is not appropriate to set the temperature sensor detector and so on.

And the product installation instructions also have similar descriptions. For example, Simplex company installation instructions described in the "PeripheralEquipment: avoid the boiler room, kitchen, garage, food storage or freezing rooms, heat treatment shop, welding shop, near high voltage electric motor or motor control spare place install ion sensing probes.

In a real application that we are not difficult to find some examples of the error, such as: smoke detectors installed in electric water heater or above installed in indoor air duct, the wind. These conditions caused by false alarm rate is very high, so according to the actual location of the correct choice of fire detectors are very important.

(2) install location

In national fire alarm system design specifications (GBJ116-88), there is a lot about the correct setting fire detectors provisions. For example: the detector to the wall the horizontal distance beam edges should not be less than 0.5m; detector around 0.5m there should not be screened from within; detector to air-conditioning supply outlet side of the horizontal distance should not be less than 1.5m, to porous air ceiling orifice horizontal distance should not be less than 0.5m etc. These provisions will not only ensure that probes can effectively and quickly detect fires occur, but is also an effective way to avoid interference.

In the United States Electrical Manufacturers Association NEMA published GuideforProperUseofSystemDetectors "cover: If the detector is installed in an air supply and return air ducts in the room, the detector should be installed in a outlet air flow back through the path.

Conduct smoke test will help determine the location of the detector is installed correctly, you need to pay special attention to the smoke of flows and paths passing through.
When the probe must be installed near the diffuser, there will be a need to keep a certain distance, as shown in Figure 5.

The correct mounting position can make rapid, accurate detector detects a fire. On the contrary, incorrect installation location, the detector alarm slow and probes also vulnerable to the outlet of the gas stream interference which produces false positives.

(3) reasonable standard wiring

Reasonable standard wiring is effective to prevent electromagnetic interference. Wiring specification in the national fire alarm system design specifications (GBJ116-88) and related electrical design specification has an associated provisions.

For example: fire control, communication and alarm circuits shall take the wear metal pipe protection, and should be concealed in the non-burning body structure?? ; Different systems, different voltage, current categories of line should not be put to the same root canal or the same slot trunking; weak line cable shaft Yi and strong electric wiring cable shaft set. If unconditional, it must be combined with weak and strong electric lines should be arranged on either side of the shaft and so on.

Reasonable standard wiring can effectively reduce the intensity of electrical interference to the weak electricity system. The wear metal line pipe protection or use of a screen wire require wire pipe or wire screen must be continuously and single-ended afterThe General requirements for the grounding resistance is less than 1 Ω or less than 0.5 Ω, can effectively prevent electromagnetic interference effect on communication lines.

In General, reasonably installation equipment and standardized wiring can effectively reduce EMI, high-speed airflow disturbances, etc.

2. optimizing fire auto-alarm controller method to reduce false alarms

Currently more advanced automatic fire alarm controller are on fire detectors from field acquisition of data back to calculate analysis to determine if there is fire. Practice has proved that by optimizing for disaster alarm controller of calculation method can effectively reduce interference.

(1) change the threshold method

Change threshold method is primarily used for smoke detector, which is delivered to the smoke detector alarm controller values for storage, computing, and concluded that the real-time values, average (the last one-hour average of the values in real-time), alarm thresholds (alarm threshold = set threshold + average). Real time values and alarm threshold, when real time values greater than or equal to the alarm threshold system to produce fire alarm as in Figure 6 (Note: ordinate indicates: detector detection value through A/D transformation of digital, horizontal representation: detection time (in seconds) = coordinates × detection cycle (S))

This method has the advantage that the detector alarm threshold by fixed values become follow detector mean changing the Variant value. As the probe after the long-term use, it is difficult to avoid dust accumulation in the detector probe indoor and dust will be scattered light source of light, resulting in the value of the detection time gradually increased. If the detector of alarm threshold is a value, then the detector allows fluctuations will become smaller and smaller. When the real-time value close to alarm thresholds, as long as the small fluctuations can make detectors produce a beep, but no fire occurred. Therefore, we take the average of the detector as the base, plus the set threshold as alarm threshold. Since the average is the recent fires one hour average of the values in real-time, so it eliminates the real-time values of fluctuation detectors is to probe recently status reflects that and real-time value as detector and gradually increased. Alarm threshold to it as the base, just like all boats, solves dust accumulation in the detector alarm threshold becomes smaller, but also eliminates dust accumulation and the detector of false positives.

Although the average value of detector can eliminate the effects of dust accumulation, but cannot grow indefinitely. There is also a limit on average, this limit is exceeded, the automatic fire alarm controller can have fault alarm, display the detector too dirty, needs cleaning.

(2) delay method

Delay method is the use of delay due to elimination of fire detectors method of instantaneous interference resulting from false alarms, but also guarantees a real fire detector alarm of real-time. This method applies to resolve the interference of instantaneous. Probe specific work flow diagram shown in Figure 7.

We have used this method solves the case due to the high-speed airflow causing smoke detector instances of false positives. Specific situation: in Guangzhou metro line, since the rail line area and platform waiting areas without blocking other devices when trains enter or a platform that can produce high-speed air flow, causing the installation in platform near the tunnel of the mouth of the false alarm smoke detector, so often watch very disturbing. Using delay method, effectively reduce the bulk of the false alarm. By: when the fire detector real-time value greater than or equal to the alarm threshold alarm controller does not have a fire alarm, but the current status of the detectors for storage of records, and then reset the detector, delay, at the next polling cycle (typically 4s ~ 6s), then read the status of the detectors. Repeat 5 times ~ 10 times, if the value of the real time is greater than or equal to the alarm threshold, this shows that the likelihood of fire is high, automatic generation of fire alarm alarm controller. On the contrary, the real-time values of a return to normalcy, the description is a false alarm, the detector's verification score plus 1. If the verification scores greater than 10 will result in a fault alarm, the alarm detectors are often misused, required attention.

(3) analogy analysis method

An analysis method is the analogy of a time period to receive real-time values of the detector and fire alarm controller database material combustion data model to determine whether a fire broke out. Due to the different material combustion will produce different curves of the smoke or heat, smoke or heat curve analysis, processing, you can establish that the substance burns the smoke or heat data model (Note: ordinate indicates: detector detection value through A/D transformation of digital, abscissa: detection time (s) = coordinates × detection cycle (s)).

Specific method is:

The first step in investigating combustible material within the protection zone, and in the laboratory on these items do the combustion experiment these materials in combustion fumes or temperature curves. By integrating these curves into time-change curve function range, slope change curve, allowed fluctuation range, and the establishment of these substances in combustion processes in the data model. Use the same method, we can build including dust interference, moisture interference, high-speed airflow disturbance, interference data model.

The second step, these combustion data model, interference data model input automatic fire alarm controller as samples.

The third step, the detector over a transfer time of real-time value and controller data model for comparison, analysis, to determine whether there is a fire.

The advantage of this approach is more accurately identify the real fire, effectively reduce environmental factors interfering, the detector more reliable.

3. to increase the protection device

For detectors with some additional protective device, or you can play a role in reducing interference effects. For example,For detectors detect circuit retrofitting shielding to reduce the effects of electromagnetic interference; to probe connection box or cover with damp-proof rubber mats to reduce the effects of moisture on the circuit; suitable for detectors with pore size of filter to block the larger dust particles and small animals, and so on.

In the installation of protective devices, you should be aware of the installation of the device cannot affect or impair detector detection feature, the probe will lose their effectiveness.

4, the introduction of new technologies

As technology progresses, people in anti-jamming on the constant exploration and research, there have been some modern times with high immunity to the new technologies, new products. Including air sampling and analysis system and linear optical fiber sensing temperature detection system is a fairly high immunity to the new technology products. These new technologies can effectively resist dust, EMI, anti-jamming high-speed airflow.

(1) air sampling and analysis system

Air sampling and analysis system is a proactive detection system. System built-in pump through the laying of the pipeline from the protection zone to extract air samples to detect Chamber for testing. To prevent the air of dust or other particles on the detection cause interference, air samples collected after a filter for filtering. In detection room, laser emission device emits parallel laser beam, laser light smoke particles caused by small angle scattering, scattered light line through concave mirror reflection to the high sensitivity laser receiver. By transmitting circuit to sink the received light signals into electrical pulses, controller after signal processing calculations determined whether the smoke concentration is the alert level of the alarm signal. Other clutter light was Centre of plane after light bar mirror reflecting detection room, to prevent causing interference. Apart from mesh filter for dust, the system also has a laser filter functionality, you can judge the laser on the smoke particles and dust particles with different pulse dust particles jamming, removal.

Because air sampling and analysis system probes sample consists of PVC pipes and PVC tube sampling hole, no need to go all electronic components are the cable, in the case of the detector, so vulnerable to external electromagnetic interference. On the other hand, air sampling and analysis system of detectors is using Active extract air samples of work, high air flow of environmental impact on the system.

(2) linear optical fiber sensing temperature detection system

Linear optical fiber sensing temperature detection system is the use of lasers in the fiber transmission of the signal back Raman scattering and optical time domain reflection to obtain the space temperature distribution information detection system. Because the system of detection circuit is the fibre, so the system with anti-burning, explosion-proof, corrosion resistance, high temperature resistance, resistance to electromagnetic interference, etc.

Linear optical fiber sensing temperature detection system of basic working principle is: when a certain wavelength in the implantation of optical fiber and energy of the laser pulse, laser pulses in the fiber transmission while constantly arise after the Raman scattering waves. These backward Raman scattering light waves pass optical filter, photoelectric conversion, magnification and mold number converted into/signal processing system, the system can be displayed in real time temperature signal, and based on the light propagation speed and back light echo time to locate the temperature information.

5. conclusions

The above describes several methods that can to a certain extent reduce interference effects of the fire detectors. Although these methods are not necessarily the best, most complete. But what is certain is that as technology technology of growing, anti-interference methods and technologies are becoming more and more perfect.