1. System Characteristics Electrical fire monitoring systems are early warning systems. Unlike traditional automatic fire alarm systems, electrical fire monitoring systems provide early warnings to prevent losses, while traditional automatic fire alarm systems aim to minimize losses. This explains why electrical fire monitoring systems are still necessary for both new and renovated projects, especially for units that have already installed automatic fire alarm systems. 2. System Design and Application Basis (a) The design and application of electrical fire monitoring systems are primarily based on relevant clauses in national standards: Firstly, GB50045-95 "Code for Fire Protection Design of High-Rise Civil Buildings," which stipulates in Article 9.5.1 that leakage current fire alarm systems should be installed in high-rise buildings in areas with high fire risk and dense population. Secondly, GB50016 "Code for Fire Protection Design of Buildings," which stipulates in Article 11.2.7 that residual current operated electrical fire monitoring systems should be installed in the following locations: theaters, museums, warehouses, residential areas, hospitals, shops, schools, etc. Thirdly, the relevant clauses of the national standard "Requirements and Testing Methods for Fire Prevention in Building Electrical Systems" also clearly require that "residual current operated protective devices that automatically cut off the power supply or trigger an alarm should be installed at the power input terminal." Fourthly, the products of electrical fire monitoring systems should meet the following standards: GB14287.1 "Electrical Fire Monitoring Equipment," GB14287.2 "Residual Current Type Electrical Fire Monitoring Detector," and GB14287.3 "Temperature Measuring Type Electrical Fire Monitoring Detector." Fifthly, the installation and operation of electrical fire monitoring systems should meet the requirements of GB13955 "Installation and Operation of Residual Current Operated Protective Devices." Sixthly, the power supply of electrical fire monitoring systems should meet the requirements of GB50052 "Code for Design of Power Supply and Distribution Systems." Seventhly, the design of electrical fire monitoring systems should meet the requirements of "Design Methods for Electrical Fire Monitoring Systems" (provisional regulations). (2) Understanding the word "should": The relevant clauses of the national standard "Requirements and Testing Methods for Fire Prevention in Building Electrical Systems" clearly require that "residual current operated protective devices that automatically cut off the power supply or trigger an alarm should be installed at the power input terminal." The relevant clauses in the "Installation and Operation of Residual Current Operated Protective Devices" also clearly require that "equipment and locations where residual current protection devices must be installed" are included. Specific rules are made for end-point protection and line protection. Furthermore, clause 4.6 explicitly stipulates that "alarm-type residual current protection devices should be installed" in certain locations. These are all mandatory provisions. In the national standards "Code for Fire Protection Design of High-Rise Civil Buildings" and "Code for Fire Protection Design of Buildings," the installation of electrical fire monitoring systems is indicated by the word "suitable" for installation. According to the explanation of the terminology in the standards, "suitable" installation means that it is permissible but with some selection. That is to say, installation is optional. How should we understand the meaning of "suitable" and properly design and apply electrical fire monitoring systems that are "suitable" for installation? The author believes that with the rapid development of China's modernization and the continuous improvement of people's living standards, electricity consumption has increased significantly, and correspondingly, the occurrence of electrical fire accidents remains high, causing incalculable losses. Considering the overall trend of fires in my country, the installation of electrical fire monitoring systems is very urgent and necessary. Furthermore, if we effectively predict and monitor potential fire hazards in low-voltage power distribution systems before they cause fires, such as leakage current and abnormal temperature changes, and the fires they may cause, wouldn't preventing fires be far more effective than simply reducing losses? I believe that a nationwide mandatory implementation of electrical fire monitoring systems is not far off. 3. Basic System Components and Working Principles (1) Basic Components According to the national standard GB14287-2005 "Electrical Fire Monitoring System" and related specifications "Design Methods for Electrical Fire Monitoring Systems" (Provisional), the basic components of an electrical fire monitoring system should include three basic product types: electrical fire monitoring equipment, residual current type electrical fire monitoring detectors, and temperature-measuring type electrical fire monitoring detectors. The residual current type electrical fire monitoring detector consists of a monitoring detector and a residual current transformer (available in plug-in and closed types). The temperature-measuring type electrical fire monitoring detector consists of a monitoring detector and a temperature sensor. Currently, there are about ten known manufacturers in China that design, develop, and produce electrical fire monitoring systems. However, based on available information, only a few of these manufacturers offer a complete range of the most basic product types. (2) Working Principle: The basic principle is that when parameters such as current and temperature in electrical equipment become abnormal or change abruptly, the terminal detectors (such as residual current transformers and temperature sensors) collect this information using the principle of electromagnetic field induction and temperature effect changes. This information is then transmitted to the monitoring detector, amplified, converted by an A/D converter, and analyzed and judged by the CPU. The CPU compares the amplitude of the change with the alarm setpoint. If the setpoint is exceeded, an alarm signal is issued and simultaneously transmitted to the monitoring equipment. The monitoring equipment further identifies and judges the situation. When a fire is confirmed to be possible, the monitoring host issues a fire alarm signal, illuminates the alarm indicator light, sounds an alarm, and displays fire alarm information on the LCD screen. On-duty personnel then quickly go to the scene of the incident to inspect and handle the situation based on the displayed information and send the alarm information to the central control console. 4. System Product Selection Observing the domestic electrical fire monitoring system market, the structural forms of monitoring detectors mainly fall into three categories: multi-functional residual current circuit breaker type, separate configuration type, and separate configuration integrated type. How to design and install an electrical fire monitoring system for users, ensuring its stable and reliable application in practice, makes the selection of the system's products a crucial step and a question worth exploring. ⑴ Multi-functional residual current circuit breaker type: Its characteristics include: in addition to its inherent residual current detection (or temperature detection) and alarm functions, the monitoring detector also incorporates multiple functions, such as overcurrent protection, overvoltage protection, delayed power supply, lightning protection, undervoltage protection, network-based remote centralized monitoring, and many others, all integrated into one system. This type of product appears to be boxed from the outside. Internally, it integrates monitoring detectors containing power conversion circuits, signal processing circuits, alarm circuits, display circuits, communication and linkage interfaces, along with current transformers, residual current transformers, and main circuit disconnect switches (magnetic latching relays are often used for those below 100A, and contactors or molded case circuit breakers are used for those above 100A), forming a multi-functional residual current circuit breaker. The advantages of this type of product are: multiple protection functions, built-in current transformers and residual current transformers (including temperature sensors), fewer wiring, and high overall integrity. However, the disadvantages are: complex structure, increased cost, and a higher failure rate. In particular, the close proximity of the signal monitoring, detection, analysis, processing, alarm, communication, and linkage interface circuits to the ABC three-phase main power (or single-phase) circuit makes it susceptible to strong electromagnetic interference, reducing the product's stability and reliability. Furthermore, the internal power control switch (circuit breaker) is a key power distribution product in low-voltage power distribution systems and must pass 3C certification for electrical products. Installation and use are also inconvenient. For example, in new construction projects, it is necessary to communicate and negotiate with the distribution box (cabinet) manufacturer for reasonable assembly, but it is even less suitable for distribution boxes (cabinets) that are already in use in renovation projects. (2) Separate configuration type: This type is: the electrical fire monitoring equipment and the electrical fire monitoring detector (including terminal detector head, residual current transformer, temperature sensor) are separately configured. That is, the current and leakage current signals in the conductive bus in the distribution cabinet (box) are sampled by the monitoring detector (terminal detector head), and after analysis and processing by the built-in single-chip microcomputer system, the two-bus communication protocol is used to report to 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. Generally, only residual current and temperature detection functions are available. This type of system has a clear division of labor, simple structure, low cost, and low failure rate. It does not contain a power control switch, is not connected in series with the power distribution system, and only samples the signal through the residual current transformer (or temperature probe). Its performance is stable and reliable. The drawback is that signal lines and 2-core trip control lines need to be laid between the monitoring equipment and the monitoring detectors, and between the monitoring detectors and the terminal detectors. This type of product is convenient to use in both new and renovation projects. It is foreseeable that this product structure, which is relatively independent of the low-voltage power distribution system, will be the main development direction of electrical fire monitoring system products in the future. (3) Separate Configuration Integrated Type The separate configuration integrated type is a special type of electrical fire monitoring equipment and electrical fire monitoring detectors (including terminal detectors) that are separately configured. It differs from the separate configuration type in that its bus directly uses the two-bus of the ordinary fire alarm system, eliminating the need for a centralized controller and host computer. The fire alarm controller integrates the detection and alarm functions of the electrical fire monitoring detectors (including terminal detectors) and controls them together. The advantage is that it reduces the need for a centralized controller and host computer of the electrical fire monitoring system, as well as network cabling, resulting in lower costs. The fire control center provides integrated monitoring with a unified interface and convenient management. The downside is that the original fire alarm control system needs to undergo double testing and certification of the electrical fire monitoring system again, which is not worthwhile. Furthermore, this combination, with its relatively complex fire alarm system, has a higher failure rate, directly impacting the electrical fire monitoring system and reducing its stability and reliability, potentially even causing it to malfunction. Therefore, it is best to set up an independent electrical fire monitoring system, separate from the traditional automatic fire alarm system. 5. Point Allocation in System Design According to Article 4.4 of the national standard GB13955 "Installation and Operation of Residual Current Operated Protective Devices" regarding graded protection, the steps for point allocation when installing residual current fire monitoring devices are as follows: (1) Study and analyze the relevant drawings of the controlled low-voltage AC380V/220V power distribution lines, investigate and verify the distribution of building electrical systems, determine the location of power distribution equipment (such as distribution cabinets, boxes, panels, cables, and other important equipment), and assign each monitoring detector to the corresponding power distribution equipment to determine the number of detectors and avoid waste from reinstallation. (2) Determine graded protection. To minimize the power outage range caused by electric shock accidents and grounding faults, three (or two) levels of residual current protection devices with different capacities are usually installed at different locations on the power supply line to form graded protection. Based on the electrical load and line conditions, protection is generally divided into two or three levels. This applies to the first and second levels of protection in urban and rural areas. Note: Important lines should include security, fire protection, emergency power supply, corridor lighting lines, and important locations where power outages are not permitted. (3) In the second level of protection, residual current fire monitoring detectors must be installed at all switches. That is, residual current detectors must be installed at the power supply end (first level protection) and the branch start end (second level, also known as end protection) of the line, and connected to the electrical fire monitoring system. (4) Residual current detectors and activated protection devices should be installed at switches, but not connected to the electrical fire monitoring system, to prevent electric shock. (5) Temperature detection is based on the principle of abnormal heating of power distribution equipment. Specific detection points are as follows: ⊙ Transformer low-voltage side outgoing terminals, transformer body temperature (air temperature, oil temperature, water temperature) test points, load switch contacts. ⊙ Busbar connections of each distribution cabinet (box), automatic switch (circuit breaker, knife switch) contacts, high-current conductor concentration areas, and cable splicing points. ⊙Main contacts of the bus tie cabinet and knife switch contacts. ⊙Compensation capacitor terminals and changeover switch contacts. ⑹Select the appropriate wall-mounted, floor-standing, or console type based on the total number of installation points. 6. System Design Testing Methods Terminal information testing for electrical fire monitoring systems uses two methods: leakage current and temperature. ⑴System Grounding: For users designing and installing electrical fire monitoring systems, whether it's a new project or a renovation project, it's essential to first investigate and verify the grounding type of the user's low-voltage power distribution system. Otherwise, testing the locations where residual current transformers are installed will be impossible. Low-voltage power distribution systems have IT, TT, TN-S, TN-C, and TN-CS grounding types. ⑵Detection of Total Leakage Current in Low-Voltage Power Distribution Systems: If the test is to check the overall insulation status of the user's electrical system, two prerequisites are necessary: ​​first, there must be an independent substation system; second, the grounding type of the low-voltage power distribution system must be TT or TN-S. (3) Detection of leakage current in branch lines: For AC220V single-phase power supply systems, the residual current transformer only needs to cover two power lines, but the neutral line N (i.e., the neutral wire) must not be grounded thereafter. For AC380V three-phase power supply systems, due to the use of three-phase three-wire, three-phase four-wire, and three-phase five-wire systems, the residual current transformer should cover the LA, LB, and LC three-phase power lines simultaneously, or simultaneously cover the LA, LB, LC+N lines, depending on the specific situation. Similarly, the neutral line N must not be grounded thereafter, and the protective earth (PE) wire must not pass through the transformer. When the system grounding type is TN-C, it must be converted to TN-S, TN-CS, or a partial TT type system before the residual current detection device can be installed. (4) Temperature detection is generally used in circuits with secondary protection. That is, when it is necessary to monitor the temperature of conductor connection parts in important locations, it is advisable to install a temperature-measuring electrical fire detector. It is independent of the system grounding method and mainly considers the temperature of key parts in low-voltage power distribution equipment, including cables, and a contact-type arrangement method can be adopted. When the object being detected is an insulator, the temperature sensor of the detector should be directly placed on the surface of the object being detected. When the object being detected is the temperature change inside the distribution cabinet, it is advisable to place it close to the heat-generating parts and adopt a non-contact arrangement. 7. Coordinated and graded setting of alarm values ​​(1) Alarm value setting range The alarm value setting range of residual current type electrical fire monitoring detectors, according to the provisions of 4.2.2 in national standard GB14287.2, should not be less than 20mA and should not be greater than 1000mA, and the alarm value should be between 80% and 100% of the set value. Therefore, according to this requirement, the residual current action value at the main power input line is generally set to 400-800mA, and the residual current action value on the power branch line is set to 100-400mA. The setting range is defined, but in actual field conditions, the alarm value for residual current type electrical fire monitoring detectors should specifically be no less than twice the maximum leakage current of the protected electrical circuit and equipment during normal operation, and no more than 1000mA. ⑵ The inherent leakage current estimation of the alarm setting value of the electrical fire detector should consider the normal leakage current of the power distribution system and electrical equipment (the residual current alarm setting value must be greater than the leakage current value of the circuit being measured). 8. Use of Standalone Electrical Fire Monitoring Detectors Standalone detectors can be directly connected to the electrical fire monitoring system in large venues. However, when there is no fire control room and the number of electrical fire monitoring detectors does not exceed four, standalone electrical fire monitoring detectors can be used. In buildings equipped with automatic fire alarm systems, if the alarm information from standalone electrical fire monitoring detectors is connected to the automatic fire alarm controller or the graphic display device in the fire control room, its alarm information display should be clearly distinguishable from the fire alarm information display. Strictly speaking, according to the requirements of the "Design Methods for Electrical Fire Monitoring Systems" (provisional regulations), when integrated into an automatic fire alarm system, an independent display should be used. In buildings without automatic fire alarm systems, stand-alone electrical fire monitoring detectors should be used in conjunction with fire audible and visual alarms. When a detector issues an alarm signal, the fire audible and visual alarm should be automatically activated. In non-critical power supply locations, after a delay, the power supply to the stand-alone electrical fire monitoring detector can be cut off. 9. Installation and Wiring Design of Monitoring Equipment The alarm signals for electrical fire monitoring equipment and systems should be located in the fire control room or a manned location. The main unit power supply should be taken from the fire power supply (AC220V) of the control center. 10. Installation and Wiring Design of Monitoring Detectors The power supply access point for each monitoring detector should be at the upper end of the corresponding circuit breaker. (1) Installation Design of the Internal Form of the Distribution Cabinet (Box): In general, new projects have dedicated floor distribution cabinets (boxes) on each floor. The detection controller can be placed inside the distribution cabinet (box) and installed on a rail as far away from the conductive busbar as possible. The residual current transformer is then mounted on the power busbar and securely fixed. The connection between the detection controller and the transformer should use shielded wire. (2) Installation Design of External Forms for Distribution Cabinets (Boxes): Installing detection controllers outside the distribution cabinets (boxes) is suitable for both new and renovation projects. If a dedicated fire-resistant monitoring box for installing detection controllers is available, the controllers can be placed near the distribution cabinets (boxes). Similarly, the residual current transformer should be securely mounted on the power busbar, and the connection between the detection controller and the transformer should use shielded wires with a specified range. For renovation projects, since the transformers are closed-loop, during installation, attention should be paid not only to construction safety but also to minimizing prolonged power outages that could affect users' electricity supply. (3) For the installation design of a complete set of distribution cabinets (boxes), if the detection controller is embedded in the panel of the distribution cabinet (box), and the residual current transformer remains fixed inside, without adding a fire monitoring box or altering the internal structure of the distribution cabinet (box), and the design aims for both aesthetics and convenience, this requirement should be clearly stated in the design. After approval by all relevant parties in the review of the construction drawings, the distribution cabinet (box) manufacturer can fully consider reserving holes for embedding the detection controller in the panel. (4) Installation and wiring design and precautions: ⊙ Three-proof measures should be taken when installing and wiring the product. ⊙ The product should comply with fire protection electrical regulations. All installation terminals must not be reversed during wiring, and the number of terminals must not exceed two. ⊙ When installing and wiring, the N-line and PE-line must be strictly distinguished. The N-line passing through the residual current transformer must not be used as the PE-line, and must not be repeatedly grounded or connected to exposed accessible conductors of the equipment. The PE-line must not pass through the residual current transformer. ⊙ When installing and routing two-wire lines, ensure that strong and weak current wires are routed separately, and do not allow crossing or bridging. It is strictly forbidden to run cables such as power lines, lighting lines, video lines, broadcast lines, and telephone lines into the same metal conduit. Wiring should be neat and tidy, with wires bundled together. Flame-retardant PVC conduits, metal conduits, and metal cable trays can be used for wiring. After running the cables through conduits or cable trays, the openings should be sealed. Communication lines between monitoring equipment and detectors should use twisted-pair cables, with a recommended wire diameter of at least 1.5 mm². When the system is used in environments with strong interference, shielded twisted-pair cables should be used, and the shielding layer of the shielded twisted-pair cable should be properly grounded. (Edited by: He Shiping)