Coal mine apartments are highly populated areas, serving as crucial spaces for employees to rest, study, and live. The safety management of fire in these apartments is vital to the safety and property of the workers, and it affects the normal production and living order of the coal mine, as well as the stability of both the mine and society. In recent years, with the continuous development of society, the rapid advancement of science and technology, and the gradual improvement of people's living standards, there has been a significant change in the electrical usage of coal mine apartments. The variety and load of electrical appliances have surged, leading to an increase in potential electrical fire hazards. Should an electrical fire occur in these apartments, its severity and危害 are far greater than in other locations. Therefore, using high-tech methods to achieve early warning and forecasting of electrical fires in apartments is crucial for targeted prevention. This approach is essential for preventing accidents and holds significant practical and social importance.
The primary forms of electrical line fires in coal mine apartments
1.1. Short Circuit Fire
A short circuit occurs when the insulation of bare conductors or insulated wires in electrical circuits is damaged, leading to connections between phase lines, phase and neutral lines, and phase and protective lines, causing an instantaneous and abrupt increase in current within the circuit. During a short circuit, a strong spark and arc are produced at the short-circuit point. Due to the sudden decrease in system impedance and the abrupt increase in current, a significant amount of heat is generated in a very short time, far exceeding the heat generated during normal operation of the circuit. This heat can not only burn through the insulation but also melt metal, igniting nearby flammable or combustible materials and potentially causing a fire.
Overload Fire
When current flows through a conductor, it causes the conductor to heat up and increase in temperature. In electrical circuits, the amount of current that can continuously flow without overheating the conductor is known as the safe current carrying capacity. When the current passing through a conductor exceeds this safe capacity, it is referred to as overload. Generally, conductors are allowed to operate at a temperature of 65 degrees Celsius. If the current passing through the conductor exceeds the safe carrying capacity, it can easily lead to a continuous rise in temperature, accelerating the aging and potentially damaging the conductor's insulation. In severe cases of overload, the insulation of the conductor may even ignite, igniting flammable materials nearby, and could further lead to a short circuit, potentially causing a fire.
1.3. Leaking Electricity Fire
When the insulation of a live conductor in an electrical system is damaged due to some cause, resulting in a significant drop in insulation resistance, the phenomenon of abnormal current flow between conductors at different potentials, such as between wires or between wires and the ground, is called leakage. Leakage can cause localized charging of the conductor, posing severe or fatal electric shock hazards to people; at the same time, when leakage occurs, the leaked current, upon encountering areas with higher resistance, can generate local high temperatures, leading to fires caused by nearby flammable materials. Additionally, the leakage sparks, arcs, and overheated high temperatures produced at the leakage point can also trigger fires.
April 1.4. Poor Contact Fire Incident
When conductors are interconnected, there are junctions at the connection points. The resistance formed on the contact surface of the junctions is known as contact resistance. The contact forms of junctions are generally categorized into point contact, line contact, and surface contact. On one hand, due to the unevenness of the contact surfaces or insufficient contact pressure, the actual contact area between the metal conductors is reduced, which significantly narrows the effective conductive cross-section near the contact surface, thereby causing the contact resistance to increase. On the other hand, a layer of poor electrical conductivity oxide film may form on the contact surface of the metal conductors in the air, which can also increase the contact resistance. After the circuit is energized, the current passing through the conductors, junctions, and equipment will generate heat, which is a normal phenomenon. If the junctions are well made with minimal contact resistance, the heat generated at the connection points will be low, maintaining a normal temperature. However, if the junctions are poorly made, causing increased contact resistance at the connection points, a large amount of heat will be produced under certain currents, resulting in high temperatures. Therefore, connection locations with higher contact resistance will strongly heat up, causing a rapid increase in temperature that can lead to metal conductors discoloring, melting, or even ignite the insulation layer of the conductors, potentially causing nearby combustible materials to burn and resulting in a fire.
May 1st - Harmonic Fire Incident
In an ideal electrical power system, both current and voltage are pure sine waves. Harmonics refer to the distorted waveforms of voltage and current in AC power grids, which occur due to the operation of numerous nonlinear electrical devices, such as energy-saving lamps, fluorescent lights, computers, ballasts, and UPS power supplies, among others. These waveforms are no longer purely sinusoidal but are distorted to varying degrees. In a three-phase balanced low-voltage distribution system, when the current carried by each phase is equal, there is no current in the neutral line. When the three-phase load is unbalanced, the current, after vector addition, flows through the neutral line, typically less than the current in the phase lines. Utilizing this characteristic, the cross-sectional area of the neutral line conductor is usually reduced by half or kept the same as the phase line to save materials. When the phase lines contain the third harmonic, the harmonic currents are superimposed on the neutral line rather than canceled out. In severe cases, the harmonic currents may even exceed the phase line currents, with the neutral line carrying up to three times the current of the phase lines, significantly surpassing the safe current value. In the event of an unbalanced load, the neutral line overload becomes more severe. In such a state, overheating of the conductors may occur, igniting flammable materials around the lines or causing the neutral line to fuse, resulting in a shift of the neutral point and unbalanced phase voltages. This can lead to the burning of electrical equipment connected to the lines and potentially cause fires.
2. Introduction to Electrical Fire Monitoring System
2.1 System Features
Electrical fire monitoring systems are a new type of real-time surveillance system characterized by prevention. Unlike traditional fire automatic alarm systems, which are activated after a fire to minimize losses, electrical fire monitoring systems are designed to early alert in order to prevent losses. Their key feature is their action before the occurrence of electrical fires, detecting abnormal parameters such as leakage currents and abnormal temperatures in the power distribution system. When these parameters reach the set limits, an alarm is triggered, eliminating risks or cutting off faulty circuits before an electrical fire can form, significantly reducing the likelihood of such fires.
2.2. Basic Composition
In accordance with the National Standard GB 14287-2005 "Electrical Fire Monitoring System" and relevant specifications "Design Methods for Electrical Fire Monitoring Systems," the electrical fire monitoring system primarily consists of an alarm monitoring host, residual current-based electrical fire monitoring detectors, temperature detectors, and sensors. The alarm monitoring host is located in the fire monitoring center or on duty room, while the residual current-based electrical fire monitoring detectors and sensors are installed in the on-site distribution cabinets and boxes. The residual current-based electrical fire monitoring detectors are further composed of monitoring detectors and residual current transformers. The temperature-based electrical fire monitoring detectors are made up of monitoring detectors and temperature sensors.
2.3. Working Principle
Electrical fire monitoring systems are based on monitoring detectors and software/hardware systems running on computers. They implement monitoring and management of fire hazard parameters such as leakage current, overcurrent, and temperature rise in distribution circuits, aiming to prevent electrical fires. The fundamental principle is that when there are abnormal or sudden changes in parameters like current and temperature in electrical equipment, terminal detectors collect this information using the principle of electromagnetic induction and changes in temperature effects, then transmit it to the monitoring detector. This information is compared with the alarm set values; if it exceeds the set values, an alarm signal is triggered. The signal is also sent to the monitoring equipment, where it is further identified and judged. Once it is confirmed that a fire may occur, the monitoring host emits a fire alarm signal, illuminates the alarm indicator lights, sounds the alarm, and displays the fire alarm address and other information on the screen. In case of necessity, the system can also disconnect distribution circuits with excessive residual current or abnormal temperature, and it can also exchange and share data with fire automatic alarm systems or distribution monitoring systems. The on-duty personnel then notify professional personnel to quickly arrive at the accident site for inspection and handling based on the displayed information.
2.3.1 Remaining Current Transformer Working Principle
Residual current refers to the vector sum of the instantaneous value of the main circuit current passing through the residual current-operated protective device. The residual current transformer is a basic module of the residual current-type electrical fire detection sensor. When the vector sum of the current in the circuit is not zero, an alternating current signal is generated on the secondary side of the transformer. By collecting and processing this signal, the actual residual current value in the circuit can be obtained. Considering the unbalanced current of electrical lines, the natural leakage current of lines and electrical equipment, all actual electrical lines have normal residual currents. An alarm is only triggered when the detected residual current reaches the set alarm value.
2.3.2 Principle of Temperature-Sensing Electrical Fire Monitoring Detector
The Platinum Resistance Temperature Sensor utilizes the property of metal platinum, where its resistance value changes with temperature variations, to measure temperatures. It is suitable for high-precision temperature measurement in various confined spaces and can continuously monitor on-site temperatures, effectively preventing electrical fires caused by overheating wires and cables.
3. Precautions for the Application of Electrical Fire Monitoring Systems in Coal Mine Dormitories
3.1. Determine the grounding type for the low-voltage distribution system in the coal mine apartments and the protection levels for the electrical fire monitoring system.
Grounding forms for low-voltage distribution systems include TN-S, TN-C, TN-C-S, TT, and IT types. According to relevant regulations, electrical fire detection and monitoring detectors are suitable for installation in TN-S systems or local TN-C-S systems, as well as in TT systems.
Based on the configuration of the low-voltage distribution system in coal mine apartment buildings and the specific distribution of equipment, the electrical fire monitoring and alarm system can employ either three-level or two-level protection. The three-level protection is divided into terminal protection, middle-end protection, and initial-end protection; while the two-level protection consists of terminal protection and initial-end protection.
End-of-Line Protection: The primary goal of end-of-line protection is to prevent electrical shock accidents, serving as a supplement to fire protection. End-of-line protection typically employs non-delayed leakage circuit breakers, which are not included in the centralized monitoring and control of the electrical fire monitoring and alarm system.
Mid-Range Protection: Mid-range protection primarily focuses on safeguarding the terminal lines and equipment within the low-voltage distribution system. Typically, electrical fire alarm systems are used to detect leakage currents and abnormal temperatures.
Front-end Protection: Front-end protection refers to the protection of incoming lines, main distribution feed lines, and equipment in a low-voltage distribution system. This level of protection is also achieved through an electrical fire monitoring and alarm system.
The principle for setting residual current electrical fire monitoring detectors in coal mine apartments typically involves the following: in newly constructed high-rise coal mine apartments, a three-level protection system should be implemented, with primary protection configured on each outlet circuit of the low-voltage distribution room; intermediate protection should be installed at floor distribution cabinets, regional distribution cabinets, and similar locations; and terminal protection should be set up at the end distribution boxes.
3.2. Analyze the power distribution system diagram to determine the installation location of monitoring detectors.
The monitoring objects of the electrical fire monitoring system are the power supply and distribution systems, with the power supply and distribution diagrams serving as the foundation for the system's design. Analyzing the relevant drawings of the low-voltage distribution system in coal mine dormitories can provide insights into the specific power supply and distribution methods, the number of circuits, the electrical characteristics and power consumption of each area, the specifications and models of the main circuit breakers in each distribution cabinet, as well as the cross-sectional dimensions and load capacity of cables or copper bars. By investigating and verifying the distribution of electrical equipment in coal mine dormitories, the location of distribution equipment is determined. Each monitoring detector is then assigned to the corresponding distribution equipment based on system design requirements, and the number of detectors to be installed is accordingly determined.
3.3. Information Detection and Installation Principles of Electrical Fire Monitoring System Sensors
Information detection for electrical fire monitoring systems primarily involves two detection methods: leakage current and temperature.
For AC single-phase power supply systems, the detection of leakage current only requires the two power lines L and N to pass through the residual current transformer. For AC three-phase power supply systems, lines L1, L2, L3, and N must all pass through simultaneously. Subsequently, the neutral line is not allowed to be grounded, and the protective PE line must not pass through the residual current transformer.
Temperature detection is based on the principle of inspecting abnormal overheating in power distribution equipment. When it is necessary to monitor the temperature of the internal compartments and conductor connections in critical locations, it is advisable to install temperature-sensing electrical fire detectors. When the object being detected is an insulator, a contact-based arrangement should be used, placing the detector directly on the surface of the object being detected. When monitoring the internal temperature changes of a power distribution cabinet, the detector should be positioned close to the heating components, utilizing a non-contact-based arrangement.
3.4. Estimation of inherent natural leakage current and coordinated grading of alarm values
The alarm setting values for electrical fire monitoring systems should take into account the natural leakage current of the distribution system and electrical equipment, while adhering to the principle that the residual current alarm setting should be greater than the natural leakage current value of the measured circuit. The natural leakage current value of the protected electrical lines and equipment during normal operation can be roughly determined based on the empirical calculation formula and the typical natural leakage current values of the lines and equipment. Using this value as a basis, the initial alarm current value is designed. To determine the actual alarm setting value, on-site data measurement and testing are required during the specific installation and debugging. If the measured leakage current is significantly higher than the designed normal natural leakage current, attention should be paid to potential issues such as improper construction or substandard equipment quality.
The alarm value range for residual current electrical fire monitoring systems, as stipulated by national standards, should be between 20mA and 1000mA, with the alarm value set between 80% and 100% of the set value. Therefore, in accordance with the requirements, the residual current trip value at the main power entry point is generally set between 400mA and 800mA, while the residual current trip value on the branch power supply lines is set between 100mA and 400mA. The alarm setting value for electrical fire monitoring detectors should not be less than twice the natural leakage current of the protected electrical lines and equipment, but should not exceed 1000mA. In two-level or multi-level monitoring detection systems, the alarm current setting value should be designed to be selective, meaning the alarm current setting value of the upper-level detector should be at least 1.5 times the largest alarm current setting value of the lower-level detector, but should not exceed 1000mA.
