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          Analysis of the design and precautions of electrical fire monitoring system

          Abstract: In recent years, electrical fires have remained high, and electrical fire monitoring systems have gradually become an important part of building fire electrical design. This paper outlines the composition and principle of the electrical fire monitoring system, and gives the relevant system structure scheme in combination with engineering design examples. Finally, the problems that should be paid attention to in the application of residual current electrical fire monitoring detectors are discussed.

          Keywords : electrical fire; monitoring system; monitoring detector; ARCM; design

          1 Introduction:
          With the development of our country's love for the national economy and the improvement of people's living standards, China's industrial electricity and household electricity have increased year by year. Taking 2010 as an example, China's total electricity consumption in the country reached 419.3 billion kWh, an increase of 14.56% year-on-year . Various electrical equipment has been continuously increased and has been applied to all areas of society, which brings great convenience to people's life and production, but also leaves more hidden safety hazards. According to the statistics of the China Fire Yearbook 2008-2010, there were 429,738 fires in China from 2007 to 2009, of which 125,947 electrical fires accounted for 29.3% of the total number of fires, which was the main cause of fires. For many years, it has occupied the first place and remained high, causing huge property losses. In order to curb the occurrence of electrical fires, relevant departments have successively formulated and revised relevant national standards and specifications, emphasizing the protective role of electrical fire monitoring systems in preventing electrical fires caused by ground faults. For example, Article 9.5.1 of "Code for Fire Protection Design of High-Rise Civil Buildings" (GB50045-2005), and Article 11.2.7 of "Code for Fire Protection Design of Buildings" (GB50016-2006) all stipulate locations where electrical fire monitoring systems should be installed. The composition and design requirements of the electrical fire monitoring system are specified in the "Electrical Fire Monitoring System" (GB 14287-2005).

          2 Analysis of the causes of electrical fires There are many reasons for electrical fires, such as short circuit, insulation aging, overcurrent, ground fault, poor contact, household appliances or electric heating equipment igniting combustibles, etc. Essentially, in all causes of electrical fires are ultimately caused by short circuits, and 90% of short circuit fires are caused by gradual failure. Gradual faults have a development process, and the ability to release in this process is directly expressed as temperature or arc, which is actually an increase in leakage current. Overcurrent load, poor contact, household appliances or electric heating equipment and other failures are also accompanied by an increase in leakage current. Because the leakage current is relatively small, it is often not enough to operate the overcurrent protection equipment (circuit breaker, fuse). The local high temperature of the leakage arc of several hundred milliamperes is above 2000 ?° C, which is enough to ignite the surrounding combustibles and cause a fire. Concealment often leads to fire.
          This type of electrical fire is caused by abnormal leakage current (even if it is a small value). It is a hidden fault. If it is not known without using certain technical detection methods, the continuous occurrence of the fault may eventually occur. Can cause electrical fire accidents. Therefore, it is necessary to have a device that specifically detects the residual current (also known as leakage current or leakage current) in the line to measure the electrical insulation between metal parts with mutual insulation, or between live parts and grounded parts in the absence of applied voltage. Through the current formed by the surrounding medium or the insulating surface, early warning of this type of electrical fire is achieved, and the purpose of eliminating hidden fire hazards is achieved.

          3 Electrical fire monitoring system

          The electrical fire monitoring system refers to a system that can issue an alarm signal, a control signal, and can indicate an alarm location when the detected parameter in the protected circuit exceeds the alarm setting value. The electrical fire monitoring system is a means of electrical fire prevention, a real-time monitoring system that acts before an electrical fire occurs, and the "fire automatic alarm system" that acts after a fire takes on tasks, functions, and technical measures taken The other aspects are different, and the two cannot be confused and replaced.

          "ARCM" electrical fire monitoring system is independently developed and produced by Shanghai Ankerui Electric Co., Ltd., including Acrel-6000 electrical fire monitoring system software and ARCM series electrical fire monitoring detectors. It is a new generation of electrical fire monitoring products applied in the dual fields of civil buildings and industrial buildings. It has the characteristics of super early, high intelligence, miniaturization, multi-function, high reliability, simple and practical. The device was developed based on many years of valuable experience accumulated by the company in system integration projects in domestic and foreign industrial and civil construction fields. The monitoring equipment is designed and developed by an industrial computer, which has good reliability, and at the same time optimizes the human-computer interaction function, making the entire system easy to install, debug, and maintain. The host uses a bus-based data transmission method and is equipped with residual current electrical fire monitoring. The detector, which constitutes a large-capacity electrical fire monitoring system, is suitable for real-time monitoring and prevention of early electrical fires in high-rise buildings and various industrial sites.

          3.1 Basic principle of monitoring system

          The electrical fire monitoring system integrates monitoring, alarming, control, and centralized management. The bus is generally connected to a monitoring detector or monitoring unit, communicates with the host, and transmits all data collection information. The host computer collects the received data, monitors the changes of the three-phase current, residual current, temperature, voltage and other parameters of the detected electrical circuit, and feeds back the current loop status and other information. When an abnormality occurs in the tested line, the electrical fire monitoring detector collects and processes signals through instruments such as transformers and temperature sensors. When the monitored value exceeds the set threshold and the trigger time is reached, an alarm signal is issued, and the alarm signal is uploaded to the monitoring equipment In the middle, after further identification and judgment, the monitoring host sends out a fire alarm signal, the alarm indicator is on, the alarm sound is stuck, and the alarm information is prompted on the display screen, the alarm location is designated, the on-duty personnel quickly check and process, and send the alarm information to the centralized control At the same time, the duty personnel can also cut off the power of the faulty loop through the monitoring equipment to control other fire fighting equipment, so as to prevent the occurrence of electrical fires.

          3.2 Basic components of the monitoring system The definition and basic components of the electrical fire monitoring system are given in "Electrical Fire Monitoring System" (GB14287). The electrical fire monitoring system is a system that can send out alarm signals, control signals and indicate alarm locations when the detected parameters in the protected circuit exceed the alarm setting value. It consists of electrical fire monitoring equipment and electrical fire monitoring detectors.
          The electrical fire monitoring system produced and developed by Shanghai Ankerui Electric Co., Ltd. mainly includes: Acrel6000 electrical fire monitoring equipment and ARCM series electrical fire monitoring detectors. According to the size of the project, different system solutions are provided to achieve better and better goals. Generally we are divided into: small single building; large single building; large group building. Figure 1 shows the basic system structure of various types of projects.

          Small single building

          Large single building

          Large group buildings

          Figure 1 Basic system structure diagram

          Taking the Shanghai Municipal Mass Transit Line 11 site electrical fire monitoring system as an example, the project has 5 districts, and 196 ARCM200 residual current electrical fire detectors are distributed, communicating with 2 monitoring devices via RS485 bus 4 ports, 1 16 ports), combined with the monitoring center duty room to form a fire monitoring system (Figure 2), using Acrel-6000 system configuration software, with centralized scheduling, control, protection, monitoring, display and other functions The advantages of electrical safety management, analysis, and recording in one integrated intelligence can greatly reduce the incidence of electrical fires in applications.

          Figure 2 System topology

          3.3 Basic functions and features The Acrel-6000 electrical fire monitoring system is used to receive signals from field devices such as residual current electrical fire detectors to realize alarm, monitoring, control, and management of protected electrical circuits. Hardware / software system. It mainly includes the following six functions: monitoring alarm; fault alarm; control output; self-test; alarm record; operation classification. The monitoring alarm function mainly includes: the real-time status of the monitored circuit switch; fault signal indication; linkage input; monitoring alarm response time of a‰¤30s; monitoring alarm signal indication and operation. The fault alarm function includes: monitoring of the disconnection and short circuit of the connection line between the monitoring equipment and the detector; monitoring of the main power supply undervoltage (a‰¤80% main power supply voltage) or overvoltage (a‰¥110% main power supply voltage) of the monitoring equipment. Control output functions include: remote control operation of opening and closing of individual or all monitored units; alarm control output; trip control output. Self-inspection functions include: open circuit and short-circuit inspection of communication lines; manual inspection or system self-inspection. Alarm recording functions include: alarm, fault event type, occurrence time recording; query according to recording date, fault type and other conditions; alarm record query and printing function. Operation classification includes: daily duty personnel, can enter the software interface to view the real-time monitoring situation, eliminate the alarm sound and query the alarm records; monitor the operator, can operate other operations than the information maintenance for the system itself; system management personnel, can operate the system Any functional module.
          In addition, the ARCM series electrical fire monitoring detectors matched with electrical fire monitoring equipment are responsible for uploading the parameters of the monitored lines. The monitoring detector mainly separates the voltage signal, the current signal and the residual current signal through the transformer isolation and conditioning, and then enters the electrical parameter acquisition module, uploads the sampled calculated data to the MCU processing unit, and the MCU processing unit further processes the received data , And collect the temperature signal of the temperature detection module, compare the protection parameters set before the data processing, determine whether there is a fault, and display the data on the LCD or LED display in real time. If a fault occurs, it will be handled according to the previously set fault handling method to protect the power supply safety, and trigger an audible and visual alarm to notify the staff, and store the fault type, time and parameters in the memory chip. In addition, the MCU processing unit also receives the fire linkage and smoke linkage signals from the linkage input module, and cooperates with the fire protection system to cut off the power remotely to prevent the occurrence of fire. The MCU processing unit can exchange data with two host computer systems through dual communication modules, which is convenient for the host computer to monitor the device and ensure that the two systems do not affect each other. At the same time, it can reduce system investment costs, facilitate later maintenance, and facilitate electrical fire systems. Popularity.

          4 matters needing attention

          4.1 Protective grounding form suitable for electrical fire monitoring system power distribution system

          1) Grounding form of power distribution system

          Definition of TN system: The power system is grounded a little, and the exposed connectable conductors of electrical devices are connected to this place through a protective wire.

          The TN system can be divided into: TN-S system, the neutral line (N line) and the protection line (PE line) of the entire system are separate; TN-C system, the neutral line and the protection line of the entire system are unified ; TN-CS system, part of the neutral line and protection line in the system are unified.

          Definition of TT system: There is a point in the power system that is directly grounded. The exposed conductive part of the electrical device has nothing to do with the ground point of the power system through the protective ground wire.

          2) Problems that should be paid attention to in different grounding systems

          When installing the residual current type electrical fire monitoring detector, the N line and the PE line must be strictly distinguished. The N line of the three-pole four-wire type or the four-pole four-wire type should be connected to the residual current transformer. The N wire through the transformer should not be used as a PE wire, and it must not be grounded repeatedly or connected to the exposed conductor of the equipment.

          In the TN system, the TN-C system must be transformed into a TN-CS, T NS system or local TT system before the residual current electrical fire monitoring detector can be installed and used. In the TN-CS system, the monitoring detector is only allowed to be used in the part where the N line and the PE line are separated.

          4.2 Installation of residual current transformer

          1) Threading the residual current transformer

          The residual current transformer should distinguish the phase line, N line and PE line in the power grid before threading. The phase line and the N line must pass through the residual current transformer together, and the PE line cannot pass through the transformer. In the system, if the N line does not pass through the transformer together with the phase line, once the three-phase load is unbalanced, there will be current flowing through the N line, and the detector detects a current signal, and a malfunction occurs. The N lines between different circuits are not connected at multiple points or are repeatedly grounded, otherwise it will cause malfunctions. During the system trial operation, the leakage current value is too large and an alarm occurs, a large part of which is caused by such situations. If the PE wire passes through the transformer together with the N wire and the phase wire, it will also cause refusal or malfunction of the monitoring detector.

          Not all residual current monitoring needs to pass the phase wire into the transformer, the total residual current monitoring method of the TN-S system can be excluded, it can only wear a cable to pass the residual current transformer. The advantage of this method is that small residual current transformers can be used to improve the measurement accuracy; if the transformer fails in the later stage, maintenance is convenient. The specific wiring diagram is shown in Figure 3.

          Figure 3 TN-S system total residual current wiring method

          2) Installation position of residual current transformer

          The residual current transformer should be installed in a place convenient for maintenance, as far away as possible from strong magnetic fields. There is no problem with the installation of the transformer. The transformer can be hung directly on the cable or fixed in the distribution box.

          The installation of the residual current transformer and the upper or lower end of the switch breaker will not affect the monitoring of the protected circuit. However, it is better to install it at the lower port of the switch for the convenience of maintenance in the future. It is not necessary to power off the upper-level switch during power-off and overhaul, and only need to disconnect this level switch to overhaul.

          4.3 Parameter setting of electrical fire monitoring detector

          The determination of the operating current value is often based on the value of the normal leakage current of the load behind the field loop. In the actual situation, the normal leakage current of different circuits in the power system is different, and the rated residual current non-operation value should not be less than 2 times the maximum leakage current of the protected electrical circuit and equipment during normal operation. The leakage current value of the protected electrical circuit during normal operation should be controlled to less than 500mA. If the leakage current is greater than 500mA, the monitoring detector should be set in the next-level power supply loop, or the line or equipment should be checked or replaced.

          If the protected line is a power line, in order to avoid the alarm action of the device caused by too large instantaneous leakage current to the ground at the start of the large equipment, the action time of the monitoring detector should be extended to avoid abnormal leakage at the start of the equipment and prevent malfunction.

          Application principle of hierarchical protection. There are often hierarchical protections in system applications, common 2 to 3, the principle of selectivity of the upper and lower levels: in terms of operating current, the setting of the upper level device must be at least twice that of the lower level device; in terms of tripping time, the delay time of the upper level device should be greater than The operating time of the first-level residual current protection device, and the operating time difference should not be less than 0.2 s.

          Residual current electrical fire monitoring detectors are generally not used alone, but are used in conjunction with electrical fire monitoring equipment. When a leakage fault occurs, no fire will occur immediately. If the power supply of the protected object is automatically cut off when the monitoring detector is protected, it may cause other unpredictable hazards. Therefore, the residual current type electrical fire monitoring detector should be used for alarm, and it should not automatically cut off the power supply. When there is electrical fire monitoring equipment, report the fault information to the host device, and the duty personnel confirm the fault information and perform the next operation according to the actual situation.

          4.4 Leakage fault check

          The residual current type electrical fire monitoring detector is generally installed at the outlet end of the power system distribution cabinet (substation) or at the entrance of the floor distribution box. When a leakage fault occurs in the protected circuit or equipment, it is not possible to determine the point or location of the specific failure. It is also a relatively tedious job to check each line and equipment after the installation point one by one.

          First of all, distinguish the cause of the leakage fault, whether it is caused by improper wiring or wiring error, or caused by equipment or line failure or new load. In the new operation project, many cases are caused by irregular wiring or wiring errors. At this time, the wiring of the protection circuit needs to be corrected one by one. After the equipment in the protected line is operating normally, if the residual current value is within the expected range, it can basically be determined that the line is normal. The residual current caused by wiring errors is often relatively large.

          Secondly, use the instrument detection method, separate power-off method (one-by-one exclusion method), power outage measurement method and other methods to detect the specific parts of the leakage. One of the most effective and convenient methods is the separate power-off method, which can narrow down the fault location one by one and finally find the cause of the fault. When power failure is not allowed, we generally use observation method and instrument measurement method. The observation method is mainly to observe whether there are obvious wiring errors or arcing phenomena in the line; in the instrument detection method, we need to have a special handheld leakage detection device to detect the size of the leakage in each circuit and locate it according to the size.

          5 Conclusion With the development of China's economy and society, electrical safety is gaining more and more attention, and people gradually realize that the electrical fire monitoring system plays a very important role in preventing the occurrence of electrical fires in the early stage. Early countries issued relevant technical standards to regulate the design and construction of electrical fire monitoring systems, which led to the current chaos in this industry and the effects of the system did not achieve the intended purpose. At present, this type of product has been confirmed to be included in the second batch of fire protection products that implement mandatory product certification, and will be included in the national CCC certified product from January 1, 2013. This shows that the relevant national departments regulate the quality of such products and market confidence. .

          The article comes from: "Intelligent Building Electrical Technology" 2012 Issue 4.

          references

          1) Ministry of Public Security of the People's Republic of China. Shenyang Fire Research Institute. GB14287-2005 Electrical Fire Monitoring System [S]. Beijing: China Standard Press, 2005.

          2) Beijing Power Supply Bureau. Shanghai Institute of Electrical Apparatus. GB13955-2005 installation and operation of residual current action protection device [S]. Beijing: China Standard Press, 2005.

          3) State Technical Supervision Bureau. Ministry of Housing and Urban-Rural Development of the People's Republic of China. GB50045-95 (2005 edition), Fire Protection Code for High-rise Civil Building Design [S]. Beijing: National Planning Press, 2005.

          4) Ministry of Public Security of the People's Republic of China. GB50026-2006, Fire Protection Code for Architectural Design [S]. Beijing: China Planning Press, 2006.

          5) Zhou Zhong. Product selection and solution of smart grid user-side power monitoring and power management system. Beijing: Machinery Industry Press, 2011.

          6) Fire Department of the Ministry of Public Security of the People's Republic of China. China Fire Yearbook (2010) [M]. Beijing: China Personnel Press, 2010.

          About the Author:

          Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system

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