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Talk about the origin of -48V communication power supply

The -48V power supply is used to feed power to the fixed telephone at the central office. Usually, the link from the convergence point of the telephone office to the user needs to be hung on telephone poles, walls, and pass through underground pipelines. There may be poor insulation of various termination points and cables themselves. If the line is located in some wet 48V from a telephone line is enough to electrolyze water in a pipe or hole. The process of electrolyzing water is oxygen evolution at the positive electrode and hydrogen evolution at the negative electrode, and the oxidation reaction of the positive electrode metal material will be quite serious. Assuming that the negative electrode is grounded, there is no potential between the negative electrode and the ground, and the positive electrode has a +48V potential and then rapidly oxidizes after oxygen evolution. On the contrary, the positive electrode is grounded, and there is no potential difference between the positive electrode and the ground, and the hydrogen evolution after the negative electrode electrolyzes water will not increase the degree of oxidation. As for the return path of the leakage current, it can go to the ground or the positive cable, depending on the return resistance. In short, the corrosion of metal conductors such as cables by positive grounding is far less than that of negative grounding, which can prolong the life of equipment. Another way of saying is that early telephone switches used relays or dry reed switches to switch circuits. If the pull-in coil of the relay is grounded, it will reduce the corrosion of copper wires but also increase the corrosion of the iron core. Larger ones are more resistant to corrosion, so the service life of the entire relay will still be improved. Of course, the current switch is not purely in this problem, but due to the continuity of the system, the positive grounding system is still used.

Then why use 48V voltage?

In the -48V power supply system, the nominal voltage is 48V, but the actual voltage has a certain range, and 48V is just a general term. In practical applications, AC mains power is converted to DC -54.5~-55V by a rectified step-down power supply (generally a high-frequency switching rectifier) through a low-voltage power distribution device, and the battery is float-charged, and at the same time output to supply power to equipment.

The voltage source of about 54V is mainly determined by the floating charge voltage of the lead-acid battery, which is about 2.2V, which is the working voltage of 24 single batteries connected in series. Considering the charging and discharging characteristics of the battery, the actual allowable input voltage range of the device is generally relatively wide, at least -43.5V~-57.5V.

On the other hand, the use of higher DC voltage is not safe enough, and the requirements for cable insulation level are also high, and breakdown problems are prone to occur in humid environments. However, when the voltage is low, the current will also increase, and the loss will increase, which is not suitable. At the same time, considering the uninterruptible power supply system using lead-acid batteries, 48V is just 24 2V storage battery units; and for small capacity systems, only 4 12V batteries are used. A battery pack with a reasonable number of strings is easy to balance. In addition, the specified safe voltage value in buildings without high risk of electric shock is not more than 65V. In short, the use of 48V DC voltage is the result of considering all parties.

In fact, the basic power supply of the early communication system, and 60V, 50V, 24V, and 130V levels corresponding to different communication equipment, are now unified into a -48V system.