Summary:Electricity plays a vital role in our daily production and life. To prevent and reduce the occurrence of electrical fires, an effective electrical fire monitoring system has been designed. The system, through various signal collection and processing methods, can effectively detect leakage risks in distribution lines and trigger alarms, reminding staff to promptly repair the distribution lines. This helps eliminate fire hazards and reduce personnel and property losses.
KeywordsElectrical Fire Monitoring: Residual Current; Current Sensor
Introduction
The primary causes of electrical fires include overloading, short circuits, lightning, leakage, harmonic currents, operator error, poor connections, mismatches between the design load and usage load of electrical circuits, and inherent equipment factors. Among these, electrical fires caused by leakage account for a significant portion. To effectively monitor leakage currents, an electrical fire monitoring system must be installed. The system should immediately sound an alarm when it detects leakage currents reaching the warning level, preventing them from causing electrical fires.
1. Overview of the Composition of Electrical Fire Monitoring System
The "Code for Design, Construction, and Acceptance of Electrical Fire Monitoring Systems" (DBJ 202012) defines an electrical fire monitoring system as a system that can emit an alarm signal, control signal, and indicate the alarm location when the detected parameters of the protected circuit exceed the alarm setting value. The electrical fire monitoring system is mainly composed of three parts: the electrical fire monitoring system host, the electrical fire monitor, and the electrical fire sensor. The electrical fire monitoring system host is responsible for receiving and recording the alarm signals uploaded by the electrical fire monitor, while also releasing alarm information to remind on-duty personnel to inspect. The electrical fire monitor is responsible for receiving signals from the electrical fire detector, conducting on-site alarms, and simultaneously uploading the collected signals and alarm information to the electrical fire monitoring system host. The electrical fire sensor is tasked with collecting the residual current, temperature, fault arc, and overcurrent signals of the monitored circuit and transmitting these signals to the electrical fire monitor via the system bus. The electrical fire monitoring system primarily collects signals from the monitored circuit using the electrical fire sensor, which can be categorized into residual current-type electrical fire sensors, temperature-measuring electrical fire sensors, fault arc detectors, and current sensors. Utilizing Kirchhoff's Current Law, the residual current-type electrical fire sensor measures the residual current, with its working principle illustrated in Figure 1.
According to Figure 1, Cables A, B, C, and N simultaneously pass vertically through the same residual current transformer. Under normal conditions, the vector sum of the A, B, and C phase currents is equal in magnitude but opposite in direction to the N phase current. At this point, the current in the PE line is 0, indicating a leakage current of 0. According to the law of electromagnetic induction, the induced currents of phases A, B, C, and N in the residual current transformer cancel each other out, resulting in a current of 0 passing through the transformer. When, due to certain reasons, there is a leakage current from phase A, B, C, or the neutral line into the ground within the distribution line, the vector sum of the A, B, and C phase currents will not be equal to the N phase current. In this case, the vector sum of the current passing through the residual current transformer is not 0, meaning the residual current is not 0. Based on this principle, the leakage condition within the distribution line can be indirectly analyzed by the size of the current in the residual current transformer. In our country's low-voltage distribution systems, different grounding systems exist. Due to the limitations of the detection principle, residual current-type electrical fire sensors are not suitable for all situations. When setting up residual current-type electrical fire detectors, they should be treated differently according to the characteristics of the distribution system. The grounding systems suitable for residual current-type electrical fire sensors are shown in Figure 2.
Due to the temperature rise caused by leakage current in the circuit, a fire can occur once the temperature reaches a certain level. Therefore, electrical fires can be prevented by monitoring the circuit temperature. Temperature-sensing electrical fire sensors measure the circuit temperature through their internal thermosensitive elements and transmit signals to the electrical fire monitoring system. The main monitored areas for these sensors are the equipment casing, cable distribution cabinets (boxes), etc., with temperature monitoring conducted through direct contact. Faulty arcs in phases A, B, C, and N are primarily caused by equipment aging, loose wiring connections, and damaged insulation layers, often accompanied by a large amount of heat, which can easily lead to a fire. Faulty arc detectors monitor the occurrence of faulty arcs by real-time monitoring changes in voltage and current waveforms within the circuit. In practical applications, there is always some inherent leakage in the power distribution lines. Since the presence of inherent leakage can directly affect the accuracy of the alarm, it can even cause false alarms in the system when the leakage is significant. Therefore, it is advisable to set the measurement points at the end or adjust the leakage current alarm threshold based on calculations to eliminate the impact of inherent leakage.
2. Design and Implementation of Electrical Fire Monitoring System
2.1 Design of Electrical Fire Monitoring System Composition
In practical applications, electrical fire monitoring systems are mainly composed of the following configurations.
(1) Electrical Fire Monitoring System Main Unit, Residual Current-based Electrical Fire Detector, Temperature-based Electrical Fire Detector. This method allows for simultaneous monitoring of both the residual current and temperature within the circuit.
(2) Electrical Fire Monitoring System Host, Electrical Fire Monitor, Temperature-Based Electrical Fire Detector, Arc Fault Detector – This method can monitor the temperature and fault arcs within the circuit.
(3) When the number of points to be monitored does not exceed 8, a centralized host may not be required. Instead, use a standalone monitor as a standalone system or connect to the fire automatic alarm system.
(4) Different electrical fire detector combinations can be used for the same project based on the type of load, such as arc fault detectors and temperature sensors for lighting loads, and residual current and temperature sensors for power loads.
2.2 Electrical Fire Detector Location Design
Electrical fire detectors can be installed at locations such as the output of low-voltage switchgear, floor distribution boxes, and end distribution boxes. During actual design, the following options are available: (1) Install only at the output of the low-voltage switchgear in the distribution room, which is cost-effective but can only identify a main circuit leakage when a leakage current is present, failing to pinpoint the general location of the leakage fault or high-temperature hazard. (2) Install only at floor distribution boxes, which can locate leakage faults and high-temperature hazards within the floor. This method is more accurate than the first but cannot determine which specific circuit is leaking or detect leaks between the distribution room and vertical shafts. (3) Install only at end distribution boxes, which can pinpoint leakage faults and high-temperature hazards in branch circuits. This approach is more precise than the previous two but cannot detect leaks between the distribution room and vertical shafts or between vertical shafts and branch circuits. (4) Install electrical fire detectors at the output of low-voltage switchgear, floor distribution boxes, and end distribution boxes to form a tiered protection system. This comprehensive solution can identify fire hazard locations but is cost-prohibitive. During actual design, it is common to flexibly combine the first three options to achieve the best balance between cost and measurement accuracy.
2.3 Electrical Fire Monitoring System Host Location Design
As the electrical fire monitoring system is part of the fire alarm system, the main unit of the electrical fire monitoring system can be installed in the fire control room. The main unit of the electrical fire monitoring system uploads alarm and fault information to the graphic display device or fire alarm controller in the fire control room. There is a staff member on duty 24/7 in the fire control room, facilitating the timely detection of alarm information and the prompt handling of faults.
In addition, since electrical fire monitoring systems primarily monitor leakage conditions in distribution lines, and the detectors and sensors for electrical fire monitoring systems are generally provided as a complete set by switchgear manufacturers, the main unit of the electrical fire monitoring system can also be installed within the substation, where it is managed by the on-duty personnel. 2.4 Design of Alarm Thresholds for Electrical Fire Monitoring Systems
In accordance with national standards, the alarm values of electrical fire monitoring systems must be continuously adjustable. In actual design, the residual current's action alarm threshold is generally set at 300 mA, while the temperature alarm threshold is typically set at 70% to 80% of the cable's highest耐受 temperature.
2.4 Electrical Fire Monitoring System Alarm Threshold Design
In compliance with national standards, the alarm values for electrical fire monitoring systems must be continuously adjustable. In actual design, the residual current action alarm threshold is typically set at 300 mA, and the temperature alarm threshold is generally set at 70% to 80% of the highest allowable temperature of the cable.
Achievement of the 2.5 Electrical Fire Monitoring System
The electrical fire monitoring system enables centralized management and visual monitoring, allowing for remote control and telemetry operations of monitors and detectors dispersed throughout the building. When the detected parameters in the on-site power distribution lines exceed the alarm threshold, the electrical fire monitoring system triggers an alarm signal and control signal, and indicates the alarm location on the host alarm interface, as shown in Figure 3. At the same time, the system automatically saves the alarm information for subsequent analysis and summary.
Ankore Electric Fire Monitoring System
(1) Overview
The Acre1-6000 Electrical Fire Monitoring System is a fully digital, independently-operated system developed by AnkoRui Electrical Co., Ltd. in accordance with the current national standards. It has passed the fire product testing certification by the National Fire Product Quality Supervision and Inspection Center and has all passed stringent EMC electromagnetic compatibility tests. This ensures the safe and normal operation of the series in low-voltage distribution systems. It is now in mass production and widely used across the nation. The system achieves early prevention and alarm of electrical fires through the collection and monitoring of residual current, overcurrent, overvoltage, temperature, and fault arc signals. It can also disconnect the distribution circuits with excessive residual current, temperature, and fault arc when necessary. Additionally, it can meet the needs of data exchange and sharing with the AcreIEMS enterprise microgrid management cloud platform or fire automatic alarm systems, as per user requirements.
(2) Application Scenarios
Applicable to intelligent buildings, high-rise apartments, hotels, restaurants, shopping malls, industrial and mining enterprises, as well as the petrochemical, cultural education, health, finance, and telecommunications sectors.
(3) System Architecture
(4) System Features
The monitoring equipment can receive residual current and temperature information from multiple detectors. When an alarm is triggered, it emits both sound and light alarm signals. At the same time, the red "ALARM" indicator light on the device illuminates, the display indicates the location and type of the alarm, records the alarm time, and the sound and light alarm persists until the "RESET" button on the device or the "RESET" key on the touch screen remotely resets the detector. The sound alarm signal can also be manually silenced using the "Mute" key on the touch screen.
When the monitored loop alarms, the control output relay closes to control the protected circuit or other equipment. Once the alarm is cleared, the control output relay releases.
Communication Fault Alarm: When a communication failure occurs between the monitoring equipment and any connected detector, or when the detector itself fails, the corresponding detector on the monitoring screen displays a fault alert, the yellow "Fault" indicator light on the device illuminates, and an alarm sound is emitted. Power Supply Fault Alarm: In the event of a failure in the main power supply or the backup power supply, the monitoring equipment emits an audio-visual alarm signal and displays fault information. Users can access the corresponding interface to view detailed information and deactivate the alarm sound.
When residual current, over-temperature alarms, communication, or power supply failures occur, the alarm location, fault information, and alarm time are stored in the database. Similarly, records are kept when alarms are lifted and faults are resolved. Historical data offers various convenient and quick search methods.
(5) Configuration Solutions
4. Conclusion
Electrical fire monitoring systems achieve real-time surveillance of distribution lines through various detectors, enabling early detection of electrical fire hazards to effectively prevent electrical fires and safeguard personal and property safety. With the advancement of technology, the Internet of Things (IoT) is increasingly integrated into electrical fire monitoring systems, driving their continuous智能化 evolution.
Reference
Wu Qiong, Design and Application of Electrical Fire Monitoring System [Television Technology, 2022, 46(6): 181-183.]
Ankorri Corporate Microgrid Design & Application Manual, 2022.5 Edition.
Zhang ZhenGuo. Application Research of Narrowband Internet of Things in the Construction of "Smart Firefighting" Mode [J]. Fire Science and Technology, 2020, 39(12): 1739-1742.
Guo Jiangshi. The Application of Internet of Things Technology in the Construction of "Intelligent Fire Protection." Intelligent City, 2020(6): 25-26
