对MNS低压抽屉柜在额定负荷运行条件下出现起火燃烧故障进行了描述，分析故障发生的相关原因，提出了隐患的应对措施。关键词：MNS低压抽屉柜；单相接地短路；弧光放电；透明软管MNS低压抽屉柜以检修方便、出线回路多、占地面积小等优势受到广大电力用户的青睐。行业内虽然生产厂家众多，但是技术及生产能力参差不齐，很多小型开关厂由于技术力量薄弱，对低压配电设备相关的标准和规范没有的了解，在抽屉开关柜设计和装配方面存在隐患，再加上用户运维情况差等因素，随着运行时间增加柜内零部件逐渐老化，将导致开关柜事故频发以致发生烧损等严重事故。

　　This article describes the occurrence of fire and combustion faults in MNS low-voltage drawer cabinets under rated load operating conditions, analyzes the relevant causes of the faults, and proposes countermeasures to eliminate hidden dangers. Keywords: MNS low-voltage drawer cabinet; Single phase grounding short circuit; Arc discharge; The transparent hose MNS low-voltage drawer cabinet is favored by a large number of power users due to its advantages of easy maintenance, multiple outgoing circuits, and small footprint. Although there are many manufacturers in the industry, their technology and production capabilities vary greatly. Many small switch factories, due to weak technical strength, do not have a comprehensive understanding of the standards and specifications related to low-voltage distribution equipment. There are safety hazards in the design and assembly of drawer switch cabinets. In addition, factors such as poor user operation and maintenance will gradually age the components inside the cabinet as the operating time increases, leading to frequent switch cabinet accidents and serious accidents such as burning.

　　1 故障经过2024年7月28日凌晨，某配电室正常运行的一台MNS低压抽屉柜突然发生燃烧起火故障，为防止故障扩大，现场运维人员将上一级10kV开关分闸后进行灭火，因救火及时未造成大面积起火，经确认起火源为低压侧D28馈线柜，整柜烧损严重，已完全丧失运行功能。相邻柜未受影响，灭火后将重要负荷投入备用线路，继续投入运行。经与现场工作人员确认，此配电室于2015年整体投入运行，期间运行正常，无维护保养记录。火灾报警发生前，各出线开关、负载未出现开关故障、跳闸、停电及大规模启、停设备（设施）现象，同时经检查运行记录确认，各单元进线开关电流显示正常，无过负载情况。

　　On the early morning of July 28, 2024, a MNS low-voltage drawer cabinet that was operating normally in a certain distribution room suddenly caught fire. In order to prevent the fault from expanding, on-site operation and maintenance personnel opened the 10kV switch of the previous level and extinguished the fire. Due to timely firefighting, no large-scale fire was caused. It was confirmed that the ignition source was the D28 feeder cabinet on the low-voltage side, which was severely burned and had completely lost its operating function. The adjacent cabinets were not affected, and after extinguishing the fire, the important load was put into the backup line and continued to operate. After confirmation with on-site staff, this distribution room was put into operation as a whole in 2015 and operated normally during this period, with no maintenance records. Before the fire alarm occurred, there were no switch faults, trips, power outages, or large-scale start/stop of equipment (facilities) in each outgoing switch and load. At the same time, after checking the operation records, it was confirmed that the current display of each unit's incoming switch was normal and there was no overload situation.

　　2故障分析

　　2 Fault analysis

　　2.1现场故障状况如图1所示，D28柜完全烧毁，柜内元器件呈焦炭状，金属壳架已完全变形，所有元件、标识严重烧毁无法辨认。经对现场局部烧损痕迹的比对分析，如图2所示，第8回路出线单元（下数一）金属壳架烧损情况为严重，烧损程度由下往上逐级降低。图1D28柜体烧损情况

　　2.1 The on-site fault condition is shown in Figure 1. The D28 cabinet is completely burned out, and the components inside the cabinet are in the shape of coke. The metal shell frame has been completely deformed, and all components and markings are severely burned out and cannot be identified. After comparing and analyzing the local burn marks on site, as shown in Figure 2, the metal shell frame of the 8th circuit outgoing unit (bottom one) suffered the most severe burn, with the degree of burn decreasing step by step from bottom to top. Figure 1D28 Cabinet burning situation

　　2.2故障原因分析通过对D28柜现场情况的详细勘测、取样，并对样件进行检测及综合分析，运用仿真技术模拟还原运行状态，并得出结论：开关柜起火的主要原因是第8回路开关进线端导体单相接地短路，引发弧光放电，起火燃烧。具体分析如下：经对D28柜第8回路进、出两端插接件检查，插接件无明显烧损痕迹，同时检查内部断路器情况，内部导体未发现异常，可排除插接件和断路器内部短路情况。经对D28柜第8回路烧损样件检查发现，第8回路开关进线端B相铜导线为双股70mm且已完全熔断，AC相也存在不同程度的大面积烧熔情况。第8回路开关进线端B相铜导线为铜编织线，熔点温度为1083℃，柜内其它主要可燃物主要为PVC塑料，其燃烧温度为350℃，参考ISO834国际标准中火灾温度-时间特性曲线，燃烧1h后的温度约为950℃，小于导体的熔点温度，并且开关出线侧电流互感器测量线为仅4mm却未熔断。经上述分析推断，引起导体熔断的原因，并非起火后持续燃烧导致，通常此类情况的发生是由于接地故障或者相间短路造成。经对烧损件观察，D28柜第8回路开关进线端导体及出线端导体均为铜编织线，查看其它柜体发现，导体外部均套有透明软管（图3）。经现场查验临柜情况，此透明软管由于长时处于高温环境运行，相关标识已腐蚀模糊，无从查证其功能特性及合格性，经现场取样进行了绝缘测试，发现其并不具备相应的绝缘性能。

　　2.2 Fault Cause Analysis: Through detailed investigation and sampling of the D28 cabinet site, as well as testing and comprehensive analysis of the samples, simulation technology was used to simulate and restore the operating status. The conclusion was drawn that the main cause of the switchgear fire was a single-phase ground short circuit in the conductor of the 8th circuit switch inlet, which caused arc discharge and ignition. The specific analysis is as follows: After inspecting the connectors at both ends of the 8th circuit of D28 cabinet, there were no obvious burn marks on the connectors. At the same time, the internal circuit breaker was checked and no abnormalities were found in the internal conductor. Therefore, it can be ruled out that there is a short circuit between the connectors and the circuit breaker. After inspecting the burnt sample of the 8th circuit of D28 cabinet, it was found that the B-phase copper wire at the inlet of the 8th circuit switch was a double stranded 70mm wire and had completely melted, and the AC phase also had varying degrees of large-area melting. The B-phase copper wire at the input end of the 8th circuit switch is a copper braided wire with a melting point temperature of 1083 ℃. The other main combustible materials in the cabinet are mainly PVC plastic, with a combustion temperature of 350 ℃. Referring to the fire temperature time characteristic curve in the ISO834 international standard, the highest temperature after 1 hour of combustion is about 950 ℃, which is lower than the melting point temperature of the conductor. In addition, the measuring wire of the current transformer on the output side of the switch is only 4mm but not melted. Based on the above analysis, it can be inferred that the cause of conductor melting is not due to continuous combustion after ignition. Usually, such situations occur due to grounding faults or phase to phase short circuits. After observing the burnt parts, it was found that the incoming and outgoing conductors of the 8th circuit switch in D28 cabinet were both copper braided wires. Upon inspection of other cabinets, it was found that the conductors were covered with transparent hoses on the outside (Figure 3). After on-site inspection of the cabinet situation, it was found that the transparent hose had corroded and blurred its relevant markings due to long-term operation in a high-temperature environment, making it impossible to verify its functional characteristics and qualification. After on-site sampling and insulation testing, it was found that it did not have the corresponding insulation performance.

　　现场随机取样后，对其进行化验分析，发现透明软管主要成分为聚氯乙烯，是一种不稳定的聚合物，在光、热、机械力、氧及某些活性金属离子存在时会发生分解，放出氯化氢，随着氯化氢分解的数量增加，软管会老化，由原来透明逐渐发黄，逐渐变薄、变脆，容易断裂；同时，其介电性能与受热情况有关：当加热使PVC分解释放出氯化氢，氯离子的存在会导致其电绝缘性能明显下降，因此此透明软管通常不作为主回路导体绝缘通流使用。与此同时，对相应临柜进行检查，这种透明软管均已发黄老化，且这种导体安装在柜体中易紧贴金属外壳，承受壳体内凸出的金属固定件的挤压，呈明显受力状态，这些情况都极易造成老化的软管破裂。这种安装方式不符合GB/T7251.1-2013《低压成套开关设备和控制设备》第1部分总则中8.6.3条裸导体和绝缘导线的规定。经现场调查，D28柜第8回路抽屉柜所带负载为大容量空调，时值夏季高温，且长期处于较高负载运行，配电室风扇、空调等环控降温设施配备不足，导体承受内外双重高温，致使处于挤压、受力状态的导体外部包覆的透明软管在高温下加速老化、开裂，造成单相导体对地短路，弧光放电，产生高温起火燃烧，由下向上蔓延，逐级引燃各出线回路，所有的导体包覆的透明软管成为大火燃烧的助推器，导致D28整柜完全烧损。从图4可以看出，熔断点在导体与母线连接端、非导体与断路器连接端，从多个临柜验证，此位置紧贴金属外壳，且受力、挤压严重，已出现不同程度的划伤、破裂，短路风险极大。

　　After random sampling on site and laboratory analysis, it was found that the main component of the transparent hose is polyvinyl chloride, which is an unstable polymer that decomposes in the presence of light, heat, mechanical force, oxygen, and certain active metal ions, releasing hydrogen chloride. As the amount of hydrogen chloride decomposition increases, the hose will age, gradually turning yellow from its original transparency, becoming thinner, brittle, and prone to breakage; Meanwhile, its dielectric properties are related to the heating conditions: when heating causes PVC to decompose and release hydrogen chloride, the presence of chloride ions will significantly reduce its electrical insulation performance. Therefore, this transparent hose is usually not used as a main circuit conductor for insulation and current flow. At the same time, inspections were conducted on the corresponding adjacent cabinets, and it was found that these transparent hoses have yellowed and aged. Moreover, these conductors are easily attached to the metal shell when installed in the cabinet, and can withstand the pressure of protruding metal fasteners inside the shell, resulting in obvious stress. These situations can easily cause the aging hoses to rupture. This installation method does not comply with the provisions of Article 8.6.3 for bare conductors and insulated conductors in Part 1 of GB/T7251.1-2013 "Low voltage switchgear and control equipment". After on-site investigation, it was found that the load carried by the drawer cabinet of the eighth circuit of D28 cabinet is a large capacity air conditioner, which operates at high temperatures in summer and has been running at high loads for a long time. The environmental control cooling facilities such as fans and air conditioners in the distribution room are insufficient, and the conductor is subjected to dual high temperatures inside and outside. This causes the transparent hose wrapped around the conductor, which is in a compressed and stressed state, to accelerate aging and cracking at high temperatures, resulting in a single-phase conductor short circuit to ground, arc discharge, and high-temperature ignition. It spreads from bottom to top, igniting each outgoing circuit step by step. All the transparent hoses wrapped around the conductor become boosters for the large fire, causing the entire D28 cabinet to be completely burned. From Figure 4, it can be seen that the melting point is located at the connection end between the conductor and the busbar, and the connection end between the non conductor and the circuit breaker. Through multiple cabinet verifications, this position is tightly attached to the metal shell and is subjected to severe force and compression, resulting in varying degrees of scratches and ruptures, posing a great risk of short circuit.

　　2.3故障其它原因分析柜内温度过高。高负荷运行，导体长时通流，而且柜体缺乏维护，导致柜体积灰严重，整体散热不良，配电室无、无空调降温设施，开关本体及导线高温运行，整体性能下降，寿命降低。设备及线路老化。该配电室于2005年正式投运，截止目前连续运行时间近14年，因运行工况不具备停电运检条件，连续长时间运行，核心部件未得到及时的保养、检测、更换，加速设备及线路老化，使用寿命降低。

　　2.3 Analysis of Other Causes of Malfunctions: The temperature inside the cabinet is too high. High load operation, prolonged current flow of conductors, and lack of maintenance of the cabinet body result in severe dust accumulation and poor overall heat dissipation. The distribution room lacks dehumidification and air conditioning cooling facilities, and the switch body and wires operate at high temperatures, leading to a decrease in overall performance and lifespan. Equipment and circuit aging. The distribution room was officially put into operation in 2005 and has been in continuous operation for nearly 14 years. Due to the operating conditions not meeting the conditions for power outage operation and inspection, the core components have not been maintained, tested, and replaced in a timely manner, accelerating the aging of equipment and lines and reducing their service life.

　　3应对措施抽屉柜中进出线导体应选用正规低压电缆或者带有绝缘热缩管包覆的铜排，并按照相关标准进行安装；配电室应确保开启设备监控系统，以便于调取、查阅设备运行状态。针对历史数据做好统计分析，并对标实际运行数据，及时发现异常并隐患。同时配电室应加强送排风系统的管理，送排风功率及效能应满足正常运行需求；配电室内的运行低压柜应定期进行质量排查，尤其针对内部开关、铜导线、插接件等核心部件，必要时进行更换。配电室所带负载应进行分级分类并逐一进行标识，对非常重要及重要负荷应增加必要的保护装置，制定针对性运检计划并落地实施；定期组织运维人员及配电室管理人员进行有效的技能培训，比如配电室运维知识、突发异常紧急处理、防护等课题培训，并定期开展应急演练及实操考核，提升人员技能水平和应对突发事件的应变能力[6]。

　　The incoming and outgoing conductors in the drawer cabinet should be selected from regular low-voltage cables or copper bars with insulation heat shrink tubing, and installed in accordance with relevant national standards; The distribution room should ensure that the equipment monitoring system is activated for easy retrieval and viewing of equipment operating status. Conduct statistical analysis on historical data and benchmark it against actual operational data to promptly identify anomalies and eliminate potential hazards. At the same time, the management of the air supply and exhaust system in the distribution room should be strengthened, and the power and efficiency of the air supply and exhaust should meet the requirements of normal operation; The low-voltage cabinet in the distribution room should undergo regular quality checks, especially for core components such as internal switches, copper wires, and connectors, which should be replaced if necessary. The loads carried by the distribution room should be classified and labeled one by one. Necessary protective devices should be added for very important and critical loads, and targeted operation and inspection plans should be developed and implemented; Regularly organize effective skill training for operation and maintenance personnel and distribution room management personnel, such as training on distribution room operation and maintenance knowledge, emergency handling of sudden abnormalities, safety protection, and other topics. Regularly conduct emergency drills and practical assessments to comprehensively improve personnel skill levels and their ability to respond to emergencies.

　　4 结语MNS低压抽屉柜是配电室低压系统的重要配电装置，通过此次事故希望能引起开关柜生产厂家的重视，应严格按照标准进行设计、生产和安装，作为电力用户也应该对设备进行定期检查、维护和保养，才能共同保证设备正常运行。

　　Conclusion: The MNS low-voltage drawer cabinet is an important distribution device in the low-voltage system of the distribution room. Through this accident, we hope to draw the attention of the switchgear manufacturer, who should strictly follow national standards for design, production, and installation. As a power user, we should also conduct regular inspections, maintenance, and upkeep of the equipment to ensure its normal and safe operation.

　　附：MNS 单元抽屉开关分励脱扣后的复位操作1、出线单元内的主开关（塑壳断路器）因故障电流分励脱扣后，操作机构的把手仍然处于“∣”（合闸）位置。1单元以上（抽屉）应将合闸机构把手（逆时针方向）旋转左下方45°RESET位置，然后再将把手（顺时针方向）旋转“○”（分闸）位置。此时断路器回到正常分闸位置（热备用状态），完成复位操作。2. 复位操作完成后，才可再次进行合闸操作。

　　Attachment: Reset operation after the MNS unit drawer switch is tripped due to fault current. 1. After the main switch (molded case circuit breaker) in the outgoing unit is tripped due to fault current, the handle of the operating mechanism is still in the "∣" (closed) position. For units above 1 (drawer), the closing mechanism handle (counterclockwise) should be rotated to the 45 ° RESET position at the bottom left, and then the handle (clockwise) should be rotated to the "○" (opening) position. At this point, the circuit breaker returns to its normal opening position (hot standby state), completing the reset operation. After the reset operation is completed, the closing operation can be performed again.

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