I. Introduction As is well known, with the rapid development of the telecommunications industry, there are hundreds of millions of MDFs (Multi-line Distribution Frames) in operation online. Especially in rural areas, the environment around the external cables is relatively complex, leading to various external hazards on the cable lines, with power line contact causing the most serious consequences. If an MDF fire occurs, not only will a large amount of communication equipment be damaged and scrapped, but communication cables may also be damaged, posing a significant threat to communication security. To ensure the safe operation of communication systems, a two-tier protection system is generally adopted: primary protection via the main distribution frame and secondary protection via the program-controlled exchange. However, the core responsibility for protection still rests with the main distribution frame; therefore, the safety of the main distribution frame becomes the first line of defense for the exchange's security. II. Working Principle of the MDF Main Distribution Frame The MDF main distribution frame is one of the main pieces of equipment in the local telephone measurement room. All external lines are connected to the main distribution frame, and then from the main distribution frame to the relevant mechanical equipment. External cables cannot be directly connected to the exchange; they must pass through a junction device, which is the main distribution frame. Main Distribution Frame The basic functions of a main distribution frame are as follows: 1. It has a wiring function, allowing any internal line to be connected to any external line via patch cords. 2. It has protective devices, which, together with the protective facilities on the external lines and inside the switch, form a protection system to prevent damage and injury to equipment and operators caused by overvoltages and overcurrents from external lines. 3. It provides locations for testing internal and external lines. 4. It has an alarm function, emitting audible and visible signals when an alarm is triggered, enabling timely detection of activated security units. As a protective device for switch protection, the MDF main distribution frame plays a crucial role in protecting against lightning overvoltages, power frequency induced overvoltages, and power frequency contact overcurrents. However, it also has its obvious weaknesses. In recent years, due to the large number of domestic MDF manufacturers with varying technical levels, the types of distribution frame protection devices and security units used in various communication stations are numerous and of inconsistent quality. The installation and use of some products have created fire hazards for communication stations. Accidents of MDF main distribution frames catching fire due to high-voltage intrusion occur frequently. It can be argued that the safety hazards of MDF main distribution frames have seriously threatened the safety of communication operations. The biggest safety risk of MDF main distribution frames is the fire hazard, and the direct source of fire is high-voltage intrusion and the resulting high temperature, which can lead to fires in security units, security terminal blocks, or external cables. Since high-voltage intrusion is mainly caused by the contact between high-voltage power lines and telephone cables, what happens to the MDF security units and the line voltage and current after a high-voltage intrusion occurs? We analyze this as follows: Figure 1 shows a typical schematic diagram of an MDF security unit. Generally speaking, the overcurrent protection of the MDF will activate first when a high-voltage intrusion occurs, limiting the intrusion current to a very low level. If the intrusion voltage exceeds 190V, the overvoltage protection devices (solid-state semiconductor tubes, gas discharge tubes) of the MDF main distribution frame's protection unit will activate. Because the overvoltage protection releases heat during activation, the protection unit will temporarily fail due to the spring force, directly short-circuiting the high-voltage intrusion to ground. Simultaneously, the alarm device will activate, alerting maintenance personnel to replace the protection unit. If the intrusion current exceeds a certain value, the protection unit will open-circuit, thus ensuring the safety of the PBX. III. MDF Main Distribution Frame Status Analysis The protection unit is a protective device plugged into the protection terminal block to prevent personnel and equipment from being harmed by overvoltage and overcurrent. It is an important component of the main distribution frame. According to the international standard "CCITTK.20 Recommendation" and the actual conditions of China's line environment, the protection unit installed on the main distribution frame must have three functions: lightning protection, protection against high-voltage induction, and protection against AC mains power. Especially for PBXs with poor secondary protection against overvoltage and overcurrent, the primary protection function of the protection unit is even more important. (1) Lightning strike: refers to the surge voltage generated by lightning strike. It has the characteristics of instantaneous occurrence and completion. Some voltages are as high as tens of thousands of volts or thousands of volts, depending on the distance of occurrence. (2) Strong current induction: When cables pass through certain special places, such as electrified railways, substations or power plants, sometimes induced current is generated due to electromagnetic fields. According to the distance of the induced current, there are long-line induction and short-line induction. Generally speaking, it can be summarized as follows: long-line induction has the characteristics of large voltage and relatively small current; short-line induction has the characteristics of large voltage and large current. (3) Power line contact: In terms of cable laying, in my country, the actual situation is that telephone cables and mains cables or tram cables are laid in parallel or crisscross in the air. Due to wind, rain, sun exposure, lightning strikes, animal damage, and cable aging, power line contact still occurs relatively often. The above three situations can cause open flames, burn the distribution frame module or the user circuit board of the switch in serious cases. Although MDF (Medium-Difference Distribution Filter) protection units can ground external power intrusion protection, thus providing a certain level of protection and alarm function, from a safety perspective, traditional MDFs have significant safety hazards and shortcomings in practical applications for high-voltage intrusion alarms. A brief analysis is as follows: 1. Short-circuit protection may cause large short-circuit currents on the line . The basic design principle of current MDFs is to short-circuit intrusive overvoltages and overcurrents to ground. Therefore, in a typical high-voltage intrusion situation, the line current initially decreases due to the current limiting of the PTC (Power Transmission Control Unit). However, as the overvoltage protection device and subsequent failover protection device are activated, the line current will continuously rise, resulting in a very large line current. This may cause the line cables to melt and stick together, potentially leading to melting and deformation of the protection module and the protection unit. Furthermore, the extensive cable sticking and cross-connection could cause multiple user lines to be subjected to high current surges, potentially causing open flames. 2. Grounding Issues at Each Level of the MDF Main Distribution Frame Protection units are inserted into the protection terminal blocks. They provide protection when communication lines are subjected to high voltage, latent current, or large current surges. When a protection unit is not inserted into the protection terminal block, the internal and external lines are disconnected. Inserting the protection unit connects the internal and external lines. After the protection unit is inserted, its grounding pin passes through the grounding hole of the protection terminal block and connects to the grounding strip. The grounding strip of each protection terminal block is connected to the grounded frame, forming the entire distribution frame's grounding system. We know that due to limitations in design concepts and raw materials at the time of their existence, traditional protection terminal blocks are highly susceptible to poor grounding connections between protection units and the protection terminal blocks, leading to a lack of ground. This can directly cause power intrusion into the PBX, burning out the distribution frame module or the user circuit board of the PBX, and potentially causing an open flame. 3. Issues with Key Components of the Safety Protection Unit 3.1 Due to the chronic leakage of gas discharge tubes, their breakdown voltage continuously rises, leading to a gradual deterioration in protective performance. Furthermore, the safety protection units using gas discharge tubes have been in use for an excessively long period, and their protective performance has seriously exceeded standards. To eliminate the hidden dangers of these safety protection units and ensure they truly provide protection against overvoltage and overcurrent in the circuit, it is recommended to replace all gas discharge tube safety protection units. 3.2 Early safety protection units used thermal coil protection for overcurrent applications. Due to their slow response speed, they failed to meet the requirements of first-order protection and were therefore abolished by the China Post and Telecommunications Administration in the mid-to-late 1990s. 3.3 Early push-rod type safety protection units, limited by the design concepts and outdated components of the time, had significant structural defects. Moreover, with the rapid development of communication and broadband services, shortening the fault recovery time and improving maintenance efficiency are increasingly valued by operation and maintenance departments. Safety units that use indicator rods for alarm indication present significant inconvenience in maintenance. [align=center] [/align] 3.4 Some security unit alarms use an external line operation method, which makes it very easy to damage the security unit alarm light during FS operation. The total alarm outputs an alarm signal, but the security unit cannot be located. In addition, this type of security unit has defects in material control, and is prone to ringing interruption during normal use, outputting false alarms. After the ringing flow passes, it returns to normal, interfering with normal maintenance work. 4. Problems with the security terminal block itself Since the earliest introduction of process control switches in China was in the 1990s, due to the limitations of design, manufacturing, process and raw materials at that time, most security units now have problems such as plastic aging/broken wire clamp springs, which seriously waste port resources. At the same time, due to the limited understanding of security units at that time, many security units only served a connection function and did not really meet the protection requirements, causing fires. IV. Wiring method for retrofitting in-line security units Security units can be added to the existing in-line rack without adding any new racks. Taking the original distribution frame with a vertical capacity of 1200 lines as an example, there are a total of 12 protection racks. Using the original wound-type protection rack with a length of 305mm, and adopting the Zhongda high-density protection rack with a length of 185mm, 8 new protection racks can be added without changing the original ground clearance. Theoretically, each row can reach a capacity of 2000 lines. V. Advantages of Vertical Protection Rack Upgrades 5.1 Theoretically, as long as the original protection racks in the vertical distribution frame are replaced with Zhongda high-density protection racks, the capacity expansion can be achieved. In terms of both workload and investment, it is more economical than adding a new modular exchange, requiring only the same amount of installation work. However, it is important to note that adding new protection racks inevitably requires additional external cables. 5.2 If a modular exchange is added, new premises must be found, and a new main distribution frame (vertical, horizontal, row alarm, main alarm, etc.) must be added, greatly increasing personnel and workload. This method results in huge investment costs for the exchange. The solution provided by Delta only costs 1/4 of the original investment (the installation process of the protection unit is the same as the workload of adding a new module). VI. Features of Delta's in-line protection unit: a. High density, front-facing snap-fit, high efficiency. b. Excellent materials and sophisticated craftsmanship (polycarbonate, tin-phosphor bronze). c. Extremely low spring contact resistance, suitable for future broadband development. d. Extremely high airtightness and pull-out force design. e. All jumper springs are double-snap, which is beneficial for the exchange to reconnect. f. Free replacement of damaged snap-fit ​​springs to ensure that the signal source is not lost. g. Significantly reduced connection rate and workload. Delta's high-density protection unit uses front-facing snap-fit ​​operation and can complete the reconnection function without disconnecting the protection unit (without interrupting the call), thus reducing the workload of maintenance personnel. VI. Additional benefits of in-line upgrade: The operational safety of MDF equipment rooms and communication lines is an important part of improving the maintenance level and ensuring network security of communication stations. It is also an urgent issue that should be addressed when communication systems further promote unmanned equipment rooms. Improving the operational reliability of MDF equipment is an urgent issue that needs to be addressed. To further ensure that high-voltage intrusions will not cause catastrophic damage to the MDF equipment room and communication lines, a phased upgrade of the main distribution frame should be implemented, building upon the replacement of security units. This will guarantee the operational safety of the communication equipment room and create safe conditions for the future full implementation of unmanned operation. Considering both the cost of the upgrade and its effectiveness, this type of direct-line upgrade will bring considerable benefits to the authority in terms of security, economy, and operability.