1 The necessity of applying electrical fire monitoring system
1.1 Causes of electrical fire
In fires caused by electrical accidents, there are four main reasons: leakage causes fire, short circuit causes fire, overload causes fire, and excessive contact resistance causes fire. When a short circuit occurs, due to the large change in current, ordinary current protectors can act to cut off electrical circuits. The fault current generated by excessive leakage and contact resistance is very small, which makes general protective switch electrical appliances unable to operate effectively and in a timely manner. Experiments have shown that the local high temperature generated by leakage arcs slightly larger than 0.5A can reach 2000℃, which has a high fire risk.
1.2 Characteristics of electrical fire monitoring system
Compared with other protection systems, the electrical fire monitoring system has the following characteristics: leakage monitoring is an early pre-alarm system. Early warning before a fire occurs is to avoid losses, while the traditional automatic fire alarm system calls the alarm when a fire occurs to reduce losses; the traditional leakage circuit breaker focuses on the protection of personal safety, and its main function is to protect personal safety. It cuts off the power supply immediately when the leakage current of the power line exceeds the personal safety value. electrical fire monitoring focuses on preventing electrical fires. It can detect the tiny leakage current in the circuit, understand the changes in the insulation state of the circuit, and make predictions for electrical fires; the electrical fire monitoring system can detect the temperature in the circuit, and when it is found that the local contact resistance is too large, it can alarm in time, and the detection content is more comprehensive. Therefore, electrical fire monitoring systems and other protection systems can cooperate with each other in preventing electrical fires, but cannot replace each other. Requirements for the relevant specifications of
1.3
In the 1990s, my country has begun to prevent prevention regulations for grounding faults electrical fires, such as JGJ/T16-1992 "Electrical Design Code for Civil Buildings", GB50054-1995 "Low Voltage Power Distribution Design Code", etc. From 2005 to 2006, the provisions of leakage fire alarm system were added to the GB5004-95 (2005 edition) "Fire Protection Code for High-rise Civil Building Design" (hereinafter referred to as "High-rise Regulations") and GB50016-2006 "Fire Protection Code for Building Design" (hereinafter referred to as "Cultural Regulations"). Article 9.5.1 of the "High-level Regulations" stipulates that electric leakage fire alarm systems should be set up in places such as high-rise buildings with high-rise fire hazards and crowded personnel. Article 11.2.7 of the "Construction Regulations" stipulates that a residual current operation electrical fire monitoring system should be set up in cinemas, museums, warehouses, residential communities, hospitals, shops, schools and other places.
In order to further clarify the system composition, system design, equipment selection, system power supply, line laying and other requirements of the leakage fire alarm system, the national standard GB14278-2005 "Electrical Fire Monitoring System" clearly put forward the definition of "electrical fire monitoring system" and refined the electrical fire monitoring system into three parts: electrical fire monitoring equipment, residual current electrical fire monitoring detector , and temperature measurement electrical fire monitoring detector. The mandatory national standard GB13955-2005 "Installation and Operation of Residual Current Action Protection Device" emphasizes the protective role of the residual current action protection device in preventing electrical fires caused by grounding faults, and requires the installation of the residual current action fire monitoring system in the building. In addition, if the installation, use of electrical products and gas utensils and the design, laying, maintenance and inspection of lines and pipelines does not comply with fire protection technical standards and management regulations, the Fire Protection Law also stipulates corresponding laws to ensure the implementation of relevant specifications of the electrical fire monitoring system.
2 Basic composition and working principle of the electrical fire monitoring system
2.1 Basic composition
The basic composition of the electrical fire monitoring system includes: electrical fire monitoring equipment, residual current electrical fire monitoring detectors, and temperature measurement electrical fire monitoring detectors. Among them, the residual current electrical fire monitoring detector is composed of a monitoring detector and a residual current transformer, while the temperature-measuring electrical fire monitoring detector is composed of a monitoring detector and a temperature sensor. The system can monitor the current, residual current and temperature in the protected electrical circuit, and promptly detect electrical fire hazards to prevent electrical fires.
2.2 Monitoring Principle
electrical fire monitoring system integrates monitoring, alarm, control, and centralized management. The monitoring detector is generally attached to the branch of the bus, accepts commands from the main controller, and transmits all information. The main controller processes the received data and monitors the changes in parameters such as single-phase and three-phase currents of the detected electrical circuits, leakage current , temperature, etc. When the parameters are abnormal, the remaining current transformer, temperature sensor and other terminal detectors collect information and send it to the monitoring detector. When the set value exceeds the setting value, an alarm signal will be issued and sent to the monitoring equipment. After further identification, when it is confirmed that a fire may occur, the monitoring host will send a fire alarm signal, the alarm indicator light will be on, an alarm sound will be issued, and an alarm information will be displayed on the LCD screen. The on-duty personnel will quickly check and process it, send the alarm information to the centralized console, and can also be connected to the fire fire monitoring equipment and the 119 fire alarm command center.
Take the residual current electrical fire monitoring detector as an example. The leakage monitoring alarm is mainly achieved by using the leakage current on the residual current transformer installed on the monitored distribution line to detect the leakage current. The residual current transformer is essentially a highly sensitive current transformer that can detect tiny leakage currents as small as a few milliamps or more than ten milliamps on the distribution line, while the leakage current detection is determined based on the principle of residual current. When three phase lines and one neutral line in the distribution line pass through the iron core coil of the residual current transformer , under normal circumstances, their current vector sum is zero, and the coil of the transformer will not induce any current; but when a part of the current flows to the ground after any phase line or neutral line is shunted to the ground (i.e., leakage), at this time, the sum of their current vectors at the transformer is not zero, and the coil of the transformer also induces the corresponding leakage current, so it is called the residual current. After the detected residual current is processed, the monitoring device set value is compared. Once the set value is reached and exceeded, the electrical fire monitoring detector connected to the residual current transformer will issue a leakage alarm and output a trip trip signal at the same time cut off the power supply of the corresponding fault line.
3 Issues that should be noted in the application of electrical fire monitoring systems
3.1 Strengthen the application of electrical fire monitoring technology
In new construction, reconstruction, expansion or building interior decoration projects, reasonably design, select and install products of electrical fire monitoring systems in accordance with relevant regulations, and promote electrical fire monitoring technology. The fire protection agencies of the public security organs strictly check when conducting filing random inspections, fire inspections, and project acceptance. In the special fire protection rectification work, the supervision unit sets up electrical fire monitoring devices to rectify electrical fire hazards, orders to replace electrical circuits that are aging, and orders to correct those who are overloaded to eliminate electrical fire hazards, and improves the operation safety of electrical circuits and equipment.
3.2 Pay attention to the selection of electrical fire monitoring system
When installing electrical fire monitoring systems for different building sites, the first thing to do is to solve the selection problem of one product. The products of the electrical fire monitoring system should meet the relevant requirements in: GB14287.1 "Electrical Fire Monitoring Equipment", GB14287.2 "Residual Current Electrical Fire Monitoring Detector", GB14287.3 "Temperature Measuring Electrical Fire Monitoring Detector". From the current domestic product market of fire monitoring systems, there are three main types of structural forms of its monitoring detectors: multifunctional leakage switch type, separate configuration type, and separate configuration integrated type.
3.2.1 Multifunctional leakage switch type
Multifunctional leakage switch type electrical fire monitoring system monitoring detector not only has its own residual current detection (or temperature detection) and alarm functions, but also expands a number of functions, such as: overcurrent, overvoltage, delayed power transmission, lightning protection, undervoltage, networking to achieve remote centralized monitoring, etc. This type has the advantages of more protection functions, fewer wiring and high overall degree.However, the disadvantages arise from this are: complex structure, increased cost, and high failure rate. In particular, the distance between the signal monitoring, detection, analysis, processing, alarm, communication, linkage interface and other circuits and the ABC three-phase main power (or single-phase) circuit is too close to the ABC three-phase main power (or single-phase) circuit, which is prone to interference from strong electromagnetic fields, reducing the stability and reliability of product performance. In addition, it is not very convenient to install and use. For example, a new project requires communication and negotiation with the distribution box (cabinet) manufacturer to assemble reasonably, but it is even less applicable to distribution boxes (cabinets) that have been formed and in use in the renovation project.
3.2.2 Separate configuration type
In this type, the electrical fire monitoring equipment is separated from the electrical fire monitoring detector (including the terminal detector remaining current transformer and temperature sensor). That is, through the monitoring detector (terminal detector) sampling the current and leakage current signals in the conductive busbar in the distribution cabinet. After analysis and processing by the built-in microcontroller system, the second bus communication agreement is applied to report the electrical fire monitoring equipment in the fire control room or duty room. After further analysis and processing, the required linkage control is performed to complete the functions of the system. This system has clear division of labor, simple structure, low cost, low failure rate, does not include power control switches, does not enter the distribution system in series, and only samples the signal through the residual current transformer (or temperature measuring probe), and has stable and reliable performance. The disadvantage is that signal lines and 2-core tripping control lines need to be laid between the monitoring equipment and the monitoring detector, the monitoring detector and the terminal detector. This type of product is more convenient to use in new projects or renovation projects.
3.2.3 Separation and configuration integrated type
Separation and configuration integrated type is a special type of separation configuration of electrical fire monitoring equipment and electrical fire monitoring detectors (including terminal probe heads). Its different from the separate configuration is that its bus directly uses the second bus of the ordinary fire alarm system, eliminating the centralized controller and the upper computer, and the fire alarm controller integrates the detection and alarm function of the electrical fire monitoring detector (including the terminal detector head) and controls it together. The advantages of this type are cost reduction, integrated monitoring of fire centers, unified interface, and convenient management. However, this combination, the relatively complex fire alarm system has a high failure rate, which directly affects the electrical fire monitoring system, reducing the stability and reliability of the system, and even causing paralysis.
3.3 Precautions for installing an electrical fire monitoring system
First of all, the monitoring equipment and the alarm signals of the system should be set up in a fire control room or a place where people are on duty to facilitate people to manage and maintain and discover fire hazards in a timely manner. Secondly, when installing, wiring, and setting the alarm value of the residual current electrical fire monitoring detector of the distribution cabinet, it must follow the requirements of relevant technical regulations to prevent the system from operating normally due to installation errors and fail to achieve the purpose of preventing electrical fires. Finally, the setting of detector detection points of the electrical fire monitoring system is also crucial. If it is unreasonable, it will cause a high false alarm rate, which will seriously weaken the effectiveness of the electrical fire monitoring system. The location of the detectors of the electrical fire monitoring system should first make reasonable arrangements and designs for the fire protection zones in the building, and then divide the appropriate detection area and protection range for the electrical fire monitoring system based on the fire protection zone layout.
4 Conclusion
With the continuous advancement of technology, the functions of the electrical fire monitoring system have become more complete. It has already possessed the intelligent advantages of power safety management, control, protection, analysis, and recording, and has been widely used in smart communities, large shopping malls, hospitals, and public high-rise buildings for leakage fire monitoring. At the same time, in order to ensure the stable operation of the residual current electrical fire monitoring system, the characteristics of the power distribution system should be considered during design and application, and a monitoring system with reliable performance and advanced technology should be selected, so as to greatly reduce the incidence of leakage fires in related applications.
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